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diff --git a/old/60143-0.txt b/old/60143-0.txt deleted file mode 100644 index 747571c..0000000 --- a/old/60143-0.txt +++ /dev/null @@ -1,5019 +0,0 @@ -The Project Gutenberg EBook of Prairie Peak and Plateau, by -John Chronic and Halka Chronic - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Prairie Peak and Plateau - A Guide to the Geology of Colorado - -Author: John Chronic - Halka Chronic - -Release Date: August 21, 2019 [EBook #60143] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK PRAIRIE PEAK AND PLATEAU *** - - - - -Produced by Stephen Hutcheson and the Online Distributed -Proofreading Team at http://www.pgdp.net - - - - - - - STATE OF COLORADO - John A. Love, _Governor_ - - DEPARTMENT OF NATURAL RESOURCES - T. W. Ten Eyck, _Executive Director_ - - COLORADO GEOLOGICAL SURVEY - John W. Rold, _State Geologist and Director_ - A. L. Hornbaker, _Mineral Deposits Geologist_ - Richard H. Pearl, _Ground Water Geologist_ - William P. Rogers, _Engineering Geologist_ - Antoinette M. Ray, _Secretary_ - -MISSION OF THE COLORADO GEOLOGICAL SURVEY - -The Colorado Geological Survey was legislatively re-established in -February 1969 to meet the geologic needs of the citizens, governmental -agencies, and mineral industries of Colorado. This modern legislation -was aimed at applying geologic knowledge toward the solution of today’s -and tomorrow’s problems of an expanding population, mounting -environmental concern, and the growing demand for mineral resources. - -SPECIFIC LEGISLATIVE CHARGES: - - “Assist, consult with, and advise state and local governmental - agencies on geologic problems.” - “Promote economic development of mineral resources.” - “Evaluate the physical features of Colorado with reference to present - and potential human and animal use.” - “Conduct studies to develop geologic information.” - “Inventory the state’s mineral resources.” - “Collect, preserve and distribute geologic information.” - “Determine areas of geologic hazard that could affect the safety of or - economic loss to the citizens of Colorado.” - “Prepare, publish, and distribute geologic reports, maps, and - bulletins.” - - - - - PRAIRIE - PEAK and - PLATEAU - - - A GUIDE TO THE GEOLOGY OF COLORADO - - _by John and Halka Chronic_ - -[Illustration: Relief image of Colorado] - - COLORADO GEOLOGICAL SURVEY BULLETIN 32 - 1972 - - Available from Colorado Geological Survey - 1845 Sherman Street - Denver, Colorado 80203 - Price—$2.00 - - - ACKNOWLEDGMENTS - -This guidebook was written at the request of the Colorado Geological -Survey to fulfill a long-felt need for a popular account of the state’s -geology and its relationship to Man. - -The authors wish to thank those of their colleagues who have assisted at -various times in the preparation of this book. John Rold, Colorado State -Geologist, and William Weber, of the University of Colorado Museum -staff, made many helpful suggestions concerning the manuscript. John -Schooland, vice president of the Colorado Historical Society, generously -provided several pictures of early mining activities in Colorado. -Permission to reproduce drawings and paintings of fossils and -reconstructions of past environments was granted by the American Museum -of Natural History and the University of Colorado Museum. Drawings, -maps, and diagrams are largely the work of Robert Maurer, who also -designed the cover and title page. - -[Illustration: Tilted dark red sedimentary rocks of the -Pennsylvanian-Permian Maroon Formation are well exposed in the cliffs of -Maroon Bells, southwest of Aspen. (Photo courtesy Hydraulic Unlimited -Mfg. Co.)] - - - CONTENTS - - - _Page_ - Introduction 1 - I Colorado’s Three Provinces 3 - The Prairies 8 - The Peaks 10 - Front Range 11 - Wet Mountains 16 - Sangre de Cristo Range and Spanish Peaks 17 - Park Range and Rabbit Ears Range 19 - Gore Range 20 - Tenmile and Mosquito Ranges 21 - Sawatch Range 22 - Elk Mountains and West Elk Mountains 24 - San Juan Mountains 25 - Uinta Mountains 26 - The Plateaus 28 - II Geologic History of Colorado 32 - Precambrian Era 33 - Paleozoic Era 38 - Cambrian Period 39 - Ordovician Period 40 - Silurian Period 42 - Devonian Period 42 - Mississippian Period 43 - Pennsylvanian Period 44 - Permian Period 48 - Mesozoic Era 51 - Triassic Period 51 - Jurassic Period 52 - Cretaceous Period 56 - Cenozoic Era 59 - Tertiary Period 59 - Quaternary Period 68 - III Geology and Man in Colorado 74 - Gold, Silver, and Other Metals 77 - Boulder County 79 - Central City and Idaho Springs 80 - Georgetown, Empire, and Silver Plume 81 - Leadville 82 - Breckenridge 83 - Fairplay 84 - Silverton 85 - Ouray 87 - Aspen 88 - Creede 89 - Cripple Creek 90 - Climax 91 - Radium, Uranium, and Vanadium 93 - Oil, Natural Gas, and Oil Shale 94 - Coal 96 - Construction Materials 97 - Sand, Gravel, and Clay 97 - Stone 99 - Lime and Gypsum 101 - Gems 102 - Water 103 - Surface Water 103 - Groundwater 105 - Caves 106 - Springs 109 - Environmental Geology 111 - Glossary 114 - Suggested Reading 119 - Index 121 - - - ILLUSTRATIONS - - - _Page_ - Colorado’s three geologic provinces 2 - Pikes Peak, seen from the Garden of the Gods 4 - Rock classification (chart) 5 - Stratigraphic column (chart) 7 - Jurassic rocks in Colorado (map) 9 - East face of Longs Peak 11 - Rocky Mountain National Park (east-west profile) 12 - Big Thompson Canyon, west of Loveland 13 - Red Rocks Amphitheater, west of Denver 14 - Colorado Springs area (map and cross section) 15 - Joint systems in Precambrian rocks, Boulder Canyon 15 - Spanish Peaks, southwest of Walsenburg 18 - Hahn’s Peak, north of Steamboat Springs 19 - Gore Range from the east 20 - Aspen Mountain geology (map) 23 - Mt. Sopris, south of Glenwood Springs 24 - Ouray, in the San Juan Mountains 25 - Steamboat Rock, Dinosaur National Monument 27 - Grand Hogback, near Rifle (block diagram) 28 - Mt. Garfield, near Grand Junction 30 - Precambrian-Cambrian unconformity south of Ouray 34 - Geologic map of Colorado 35 - Black Canyon of the Gunnison National Monument 36 - Precambrian-Cambrian unconformity, Glenwood Canyon 38 - Cambrian fossils 39 - Ordovician fossils 41 - Devonian fossils 43 - Mississippian fossils 44 - Pennsylvanian paleogeography (map) 45 - Fountain Formation northwest of Denver 45 - Pennsylvanian fossils 46 - Contorted Pennsylvanian rocks near Gypsum 46 - Balanced Rock, Garden of the Gods 48 - Permian reptile tracks 49 - The Flatirons, near Boulder 50 - Colorado National Monument 51 - Morrison Formation, west of Denver 53 - Dinosaur bones, found near Morrison 54 - Dakota Sandstone hogback 56 - Cretaceous fossils 57 - Wolford Mountain, north of Kremmling 60 - Eohippus, the “Dawn Horse” 61 - Golden and South Table Mountain 62 - Devil’s Staircase, near Spanish Peaks 63 - Green River oil shale, west of Rifle 64 - Florissant Fossil Beds National Monument 65 - Pawnee Buttes, north of Fort Morgan 66 - Fossil mammals, northeastern Colorado 67 - Glacial lakes in Rocky Mountain National Park 68 - Arapaho Glacier, west of Boulder 70 - Pleistocene mastodons 72 - Great Sand Dunes National Monument 73 - Colorado Mineral Belt (map) 78 - Sluicebox mining in early Colorado 81 - Early-day Leadville 82 - Gold dredge, Fairplay 84 - Silverton, in the San Juan Mountains 86 - Abrams Mountain, south of Ouray 87 - Creede and its mines (map) 89 - Cripple Creek, near Pikes Peak 90 - Climax molybdenum mine (cross section) 91 - Rampart Range quarry, near Colorado Springs 98 - Yule Marble quarry, near the town of Marble 99 - Lyons Sandstone quarry 100 - University of Colorado Museum 100 - Colorado-Big Thompson Project (cross section) 103 - San Luis Valley (cross section) 105 - Cave of the Winds, near Manitou 107 - Mesa Verde cave and Indian dwellings 108 - Glenwood Hot Springs 109 - - - - - PRAIRIE PEAK and PLATEAU - - - - - Introduction - - -Gold was discovered in the bed of the South Platte River in 1858. -Prospectors flocked to Colorado as they had flocked only a few years -before to California. They worked the sands and gravels of Cherry Creek, -Clear Creek, Boulder Creek, and California Gulch. Exhausting the placer -sands of the stream bottoms, they moved higher to mine gold-bearing -veins at Central City and Blackhawk. Mining camps sprang into existence -overnight, each heralding some new “strike,” each populated by a new -rush of fortune seekers. As lower areas were mined out, prospectors -moved yet higher—to Breckenridge, Gold Hill, and Empire, Aspen, -Leadville, and Cripple Creek. Silver was found as well as gold, then -iron, and later tungsten and molybdenum. The metallic ring of mining -tools echoed from Colorado’s peaks. Fortunes were made here. Legends -were born. - -Prospectors and miners were not, however, the first people interested in -the rocks of Colorado. Earlier, bands of nomadic Cheyenne and Arapaho -Indians had searched Colorado’s hills for flint for arrowheads and -brightly colored clays for warpaint. Cliff-dwelling Pueblo Indians in -southwestern Colorado sought clay for their pottery and fossil seashells -for the magic of their medicine men. And from farther to the southwest, -Navajo tribesmen came to Colorado for turquoise. - -From clay to gold, much of Colorado’s wealth has come from her -mountains. But after the rush to the mines, as veins were mined out and -placers worked over, as values and prices changed, her population sought -the riches of the prairies: fertile lands for agriculture, and in the -rock layers below, black gold—vast accumulations of oil and natural gas. -The tablelands and plateaus west of the mountains yield their wealth, -too. Here are valley farms, fed often by irrigation water, and ranch -country. Here is more oil, and in some areas precious metals and -uranium. - -In recent years Colorado’s prairies, peaks, and plateaus have brought -new meaning to all America: the state now provides an attractive -playground for state residents and their visitors. Campgrounds, streams, -lakes, and high trails beckon in summer; barren slopes deep in winter -snow attract the skier. More and more, those who live in Colorado and -those who visit her seek to understand these mountains and hills and -prairies, to learn of her geologic origins and her far distant past. For -tourist and resident, casual visitor, ski enthusiast, Sunday picnicker, -for all those who have met Colorado and enjoyed her, this book is -written. - -[Illustration: Topographically, scenically, and geologically, Colorado -can be divided into the three provinces shown here.] - - PLATEAUS - UINTA MTS. - GREEN RIVER BASIN - Yampa River - Steamboat Springs - UINTA BASIN - White River - WHITE RIVER PLATEAU - ROAN PLATEAU - Glenwood Springs - Colorado River - Grand Junction - GRAND MESA - Gunnison River - UNCOMPAHGRE PLATEAU - Dolores River - PARADOX BASIN - MESA VERDE - MOUNTAINS - NORTH PARK - RABIT EARS RANGE - PARK RANGE - MIDDLE PARK - GORE RANGE - FRONT RANGE - ELK MTS. - Aspen - SAWATCH RANGE - Leadville - MOSQUITO RANGE - Fairplay - SOUTH PARK - WEST ELK MTS. - Gunnison - Salida - WET MTS. - SANGRE DE CRISTO RANGE - SAN LUIS VALLEY - Rio Grande - Alamosa - SAN JUAN MTS. - Ouray - Silverton - Durango - MESA DE MAYA - PLAINS - Fort Collins - South Platte River - Denver - GREAT PLAINS - Colorado Springs - Arkansas River - WET MT. VALLEY - HUERFANO PARK - La Junta - Walsenburg - - - - - I - Colorado’s Three Provinces - - -Scenically, Colorado is divided into three provinces: the Plains or -Prairies on the east, the Rocky Mountains bisecting the state from north -to south, and the Colorado Plateaus on the west. There are a number of -local variations of course, but by and large the provinces are clearly -defined. These three divisions will form the basis for our discussion of -the geology of Colorado, for the scenic differences are almost exactly -paralleled, and usually controlled, by differences in geologic -structure. - -The Plains rise gently from an elevation of about 3350 feet at the -eastern border of the state to 5000 feet where they meet the mountains -150 miles further west. - -Two major rivers cross the Colorado Plains: the South Platte River, -flowing northeastward from the Denver region, and the Arkansas River, -which leaves the mountains at Canon City south of Colorado Springs and -travels eastward across the southern portion of the state. Tributaries -of these two main river systems have etched the prairie surface, so that -much of eastern Colorado has a gently rolling, hilly appearance. - -The Mountains rise abruptly along a north-south line at about 105° west -longitude. They reach elevations of over 14,000 feet at Pikes Peak, -Mount Evans, Longs Peak (all visible from far out on the plains), and -fifty other peaks further west. The ranges of the Colorado Rockies form -rank upon rank of ridges and peaks, roughly north-south in trend, about -100 miles across from east to west, extending from the northern to the -southern border of the state. Here, in mountain springs and lakes, are -born the rivers of Colorado: the Platte, the Arkansas, the Yampa, the -Colorado. Crags and cliffs tower above tree-covered slopes, the rocks -always a dominant part of the landscape. The continental divide runs -through the state along the summit ridges. West of the divide, all -streams flow to the Colorado River and the Pacific; east of it, streams -flow into the Mississippi or the Rio Grande, and thence to the Gulf of -Mexico. - -West of the highest ranges, the country flattens out once more into the -Plateaus, which extend across western Colorado, southern Utah, and -northern Arizona. Here, the predominant land forms are flat-topped mesas -and deep canyons. Redrock walls shimmer in the brilliance of the western -sun, offset by deep purple shadows sometimes hiding ancient cliff -dwellings. Fragrance of pine and juniper mingles with the pungency of -sage. Narrow tracks lure the explorer. Despite the canyons, water is -scarce except along major river systems, for this is the beginning of -the desert west. - -The scenic and geologic division of the state into three north-south -strips is not everywhere clearly defined. In southwestern Colorado, the -San Juan Mountains and the complicated uplifts surrounding Ouray and -Silverton are out of key with either mountain or plateau. They are best -considered part of the Mountain Province, however, although they extend -it far to the west. Other exceptions to these divisions occur also. The -Mountain Province is interrupted by four broad high-altitude valleys: -North Park, Middle Park, South Park, and the San Luis Valley. The Uinta -Mountains jut into the northwest corner of Colorado from adjacent Utah. -And the Paradox, Uinta, and Green River Basins protrude into the Plateau -Province, modifying its topographic character. - -[Illustration: Pikes Peak rises to an elevation at 14,110 feet. Composed -of Pikes Peak Granite, the mountain is almost surrounded by younger -sedimentary rocks, including those of the Garden of the Gods, in the -foreground. (Floyd Walters photo)] - -Before discussing the geologic nature of the three provinces, let us -review briefly two sets of geologic terms. The first set has to do with -the rocks themselves—What kind of rock is that?—but serves also to tell -something about the origin of the rocks. The second set is concerned -with time—When was that rock formed? Is it older or younger than -adjacent rock? How does it relate, time-wise, to geologic events in -other parts of the world? - -These two sets of terms are presented in the charts that follow. If you -are unfamiliar with geologic terminology, refer to these charts as often -as you need to while you read this book, as well as to the glossary on -pages 114-118. - - Geologists divide rocks into three main groups, depending on their - modes of origin. - - _Igneous rocks_ originate from molten material, cooling deep below the - surface of the earth (intrusive igneous rocks) or flowing out and - hardening at the surface (extrusive igneous rocks). - - _Sedimentary rocks_ are formed from broken or dissolved bits of other - rock, washed by wind and water and deposited as layers of fragments or - as chemical precipitates. They often contain fossil plants or animals. - - _Metamorphic rocks_ are pre-existing rocks (igneous or sedimentary) - changed by heat, pressure, or chemical action. - - Examples of these three classes of rocks are given in the accompanying - figure. Many varieties of all three classes occur in Colorado. - - Class Example Occurrence in Colorado - - Sedimentary Sandstone Plains, plateaus, flanks of mountain - areas - Shale - Conglomerate - Limestone - Igneous Extrusive: Volcanic areas such as San Juan - Basalt Mountains, Spanish Peaks - Intrusive: Pikes Peak, Longs Peak, and most - Granite central mountain areas - Diorite - Metamorphic Marble Mountain areas - (from - limestone) - Quartzite - (from - sandstone) - Gneiss - (from - granite or - sandstone) - Schist - (from shale - or basalt) - - Geologists arrange rocks in their chronologic sequence by studying the - fossils and minerals which they contain. The age of some rocks can be - determined with reasonable precision from ratios of radioactive - minerals and their fission products. The relative age of others can be - determined from their position, the fossils enclosed in them, and many - minor details of their structure. - - The _stratigraphic column_ shown opposite may be thought of as a - calendar by which geologic events in Colorado can be arranged in their - proper order and related to events in the rest of the world. - Mississippian and Pennsylvanian Periods are American divisions; - elsewhere this time interval is known as the Carboniferous Period. - Other time terms are in worldwide use. - - In the generalized geologic map of Colorado which accompanies Chapter - II, rocks are identified by the era in which they were formed. A more - detailed geologic map can be obtained from the U.S. Geologic Survey - map distribution center in the Federal Building, Denver. - -[Illustration: Stratigraphic Column] - - ERA Period Millions Distinctive fossils Events in Colorado - of years - ago - - CENOZOIC - (Age of Mammals) - Quaternary Modern types of Development of present - animals and plants topography; glaciation in - mountains - 3 - Tertiary Mammals, flowering Uplift and mountain - plants building - 70 - MESOZOIC Dinosaurs and other - (Age of Reptiles) reptiles - Cretaceous Submergence, then uplift - 135 - Jurassic Desert, then submergence - 180 - Triassic Widespread floodplains - and deserts - 225 - PALEOZOIC - (Age of Fishes) - Permian First reptiles Widespread floodplains - and deserts - 270 - Pennsylvanian Swamp and forest “Ancestral Rocky - plants Mountains” - 310 - Mississippian Reef corals, sharks Partial submergence - 350 - Devonian Armored fish, first Probable submergence - insects - 400 - Silurian Corals and shellfish Probable submergence - 440 - Ordovician First fish Submergence - 500 - Cambrian First hard-shelled Gradual encroachment of - animals sea from west - 570 - PRECAMBRIAN “Lipalian Interval” Erosion to almost flat - surface or peneplain - Primitive Alternate episodes of - soft-bodied marine mountain building and - organisms erosion - 3,600 plus - - - THE PRAIRIES - -Beneath the flat prairies of eastern Colorado, sedimentary rocks form a -series of layers. Those near the surface are among the youngest rocks in -Colorado. We know this from the fossils they bear, fossils of large -mammals such as the hairy mammoth, which lived in early Quaternary time, -the bison, and many smaller mammals living today. - -The layers below—sandstones, shales, and limestones—become progressively -older as one goes deeper. Most of them were formed originally on the -bottoms of shallow seas that covered this part of North America several -times during the history of the continent. In most places the layers are -horizontal or nearly so, but westward, as they approach the mountains, -they bend upward, gently at first and then more steeply. At the very -edge of the mountains, where they were dragged upward when the mountains -rose, their eroded edges appear at the surface. - -The entire sequence of flat-lying rocks can be studied where they are -exposed along the mountain front or where streams and rivers have -dissected them. They are also known from cuttings and cores of oil and -water wells. Some parts of Colorado’s eastern plains have been drilled -so intensively in the search for oil and gas that we know a great deal -about the subsurface sedimentary rock and can even make maps showing the -distribution and character of the individual rock layers. From such -maps, the history of the region can be deduced. We know, for example, -that the area around Denver has subsided more in the past than has the -area near La Junta or Lamar; it is called the Denver Basin because of -its past history and not because it is a basin at present. - -Although the plains of Colorado appear flat, they really slope gently -eastward. The rock layers near the surface slope eastward also, but the -deeper rock layers may not. - -Near the western edge of the Plains Province, hills and valleys are -formed by differential erosion of hard and soft rock layers. Some hills, -such as Castle Rock, are topped with resistant sandstone; others, like -Mesa de Maya south of Trinidad and Table Mountain near Golden, are -capped with layers of basalt. Close to the mountains flat-topped -foothills result from partial dissection of former erosion surfaces as -the mountains, stabilized for a time, rose again, or as climatic cycles -changed. Examples of these dissected erosion surfaces can be seen north -and south of Boulder. - -Far east of the mountain front, near the northern border of Colorado, -remnants of another, higher prairie surface stand as Pawnee Buttes. -Torrential erosion—spring floods and summer thunderstorms—has deeply -furrowed the prairie surface here and left these buttes as lonely -sentinels. - -[Illustration: This map shows the distribution, character, and thickness -of certain Jurassic rocks in Colorado. These rocks are deeply buried -beneath the plains and are known there only from well samples. They have -been eroded from most mountain areas. They come to the surface along the -edges of the mountains and in the deeply incised canyons of the Plateau -Province.] - - PRECAMBRIAN ROCKS - PALEOZOIC ROCKS - JURASSIC ROCKS - SANDSTONE - SHALY SANDSTONE - SANDY SHALE - SHALE - JURASSIC ROCKS COVERED WITH VOLCANICS OR NEVER DEPOSITED. - -What lies below the sedimentary layers of the plains? The sedimentary -rocks are 5,000 to 10,000 feet thick. They lie on an almost horizontal -surface of much, much older rock, the Precambrian or “basement” rock. -This is igneous and metamorphic rock, much crumpled and folded, the -roots of long gone mountains which were beveled and leveled to an almost -flat surface or _peneplain_ perhaps a billion years ago. - -We know little of the ancient basement rocks below the sedimentary -layers of the plains, for few wells penetrate this deep. What we do know -indicates that they are similar to rocks of the mountain masses to the -west, and are composed of granite, schist, and gneiss. They probably are -not rich in valuable minerals, however, for the mineral-rich veins of -the mountains came about as a result of uplift of the mountain areas. - - - THE PEAKS - -Most of the individual ranges making up the Rocky Mountains in Colorado -are the result of highly localized movements of the crust as the entire -region was thrust upward from below. These movements broke the deep, -massive igneous and metamorphic rocks of the Precambrian basement, and -bent the more flexible Paleozoic and Mesozoic layered rocks above them -until they arched upward in a series of corrugations. The mountains thus -formed are known to geologists as _faulted anticlines_. - -As the mountains rose, they were of course attacked by the forces of -erosion. The sedimentary layers were completely stripped from the crests -of many of the uplifts, so that Precambrian rocks were exposed. It is -these rocks which form the summits of the highest peaks of Colorado. As -with all rules, there are exceptions: the Spanish Peaks are volcanic, -and the crest of the Sangre de Cristo Range is composed of sedimentary -rocks. - -The trend of most of the ranges in Colorado is north-south, swinging to -northwest-southeast near the southern end. Surprisingly, in the -northwestern corner of the state there is an east-west trending range, -the Uinta Mountains. - -Fifty or more mountain ridges in Colorado have been named as separate -ranges. Of these, the most prominent, frequently visited ones will be -discussed here. - - - Front Range - -The easternmost range of the Rocky Mountains is the longest continuous -uplift in the state. It is a relatively simple faulted anticline -extending from Canon City northward to the Wyoming border, where it -splits into two ridges, the Medicine Bow Mountains and the Laramie -Range. - -[Illustration: Longs Peak challenges technical climbers with its -2000-foot vertical east face, the Diamond. This magnificent cliff is the -result of glacial action and freezing and thawing in homogeneous but -fractured granite. The small remnant of ice and snow at the lower left -is all that remains of the glacier. The flat summit may be part of an -ancient erosion surface formed toward the end of Precambrian time. (Jack -Rathbone photo)] - -Along the highest portion of the range, from Pikes Peak to Rocky -Mountain National Park, the Paleozoic and Mesozoic sediments formerly -draped over the top of the range have long since been washed away, -leaving only the gneiss, granite, and schist of the mountain core. The -almost flat tops of Longs Peak, Mt. Evans, and Pikes Peak, and the -rolling upland traversed by Trail Ridge Road in Rocky Mountain National -Park are thought to be remnants of the 600-million-year-old erosion -surface that once existed at the top of the Precambrian rocks, and that -still exists below the sedimentary rocks of the Plains Province. This -surface, formed near sea level, has been raised 12,000 to 14,000 feet -within the Mountain Province. - -Throughout most of its length, the Front Range displays some of the most -striking high-altitude scenery in the world. Particularly accessible -areas, well worthy of visits, are Rocky Mountain National Park, Berthoud -and Loveland Passes, Mt. Evans, and Pikes Peak. In these areas the -Precambrian rocks can be seen and studied, and the effects of glaciation -observed. - -The granite, gneiss, and schist of the mountain core are shattered and -broken into blocks of various sizes. The breaks between the blocks are -called _joints_ if there is no apparent displacement between adjacent -blocks, and _faults_ where there is obvious displacement. The joints -frequently appear in parallel arrays or sets; there may be two or more -intersecting sets, giving a cross-hatched appearance to large exposures. - -[Illustration: East-west profile across Rocky Mountain National Park, -through Grand Lake and Longs Peak, showing the inferred position of the -original surface of the anticlinal uplift of the Front Range. This -diagram is generalized, and faults are not shown. (USGS Bull. 730a)] - - Restoration of surface which emerged from Cretaceous sea - Restoration of Dakota sandstone - MIDDLE PARK - _Grand Lake_ - Longs Peak - Foothills - GREAT PLAINS - Sedimentary rocks - Granite and schist - Sedimentary rock of plains - _South Platte R._ - -[Illustration: Big Thompson Canyon, west of Loveland on U.S. highway 34, -is carved in almost vertical layers of Precambrian metamorphic rocks. -Gently dipping Late Paleozoic and Mesozoic sedimentary rocks of the -Fountain, Lyons, Lykins, and Morrison Formations can be seen in the -distance, capped by the Cretaceous Dakota Sandstone. (Floyd Walters -photo)] - -The Precambrian rocks vary from place to place. Several irregular masses -of granite, called _batholiths_, make up portions of the range. -Batholiths are large intrusions of molten rock that cooled slowly at -great depth. The minerals in them form distinct crystals, often quite -large. The Pikes Peak Granite and the Boulder Creek Granite are -examples. Highly contorted and banded gneiss and schist are well exposed -elsewhere, particularly in the Idaho Springs-Central City-Black Hawk -region. - -Along the flanks of the Front Range, the eroded edges of the sedimentary -rocks which once covered the range are exposed. These rocks are usually -tilted sharply against the mountains, as at Garden of the Gods, Denver’s -Red Rocks Park, and the Flatirons near Boulder. The Rocky Mountain -Association of Geologists has erected a plaque explaining the geology of -the Red Rocks area; look for it about half a mile northeast of the Red -Rocks Amphitheater. Tilted layers of Paleozoic and Mesozoic sandstones -form hogback ridges along the mountain front, and stand out clearly on -aerial photographs. - -In some areas, particularly near Boulder, Coal Creek, and Golden, the -tilting of the sedimentary layers has been so extreme that the layers -are upside down. Basement rocks may even be thrust out above them. - -[Illustration: Sandstones and conglomerates of the Pennsylvanian -Fountain Formation dip steeply toward the plains along the eastern edge -of the Rockies. Near Denver, erosion has carved these rocks into a -natural amphitheater, now the site of Red Rocks Amphitheater. -Precambrian granite forms the hill in the background. (Jack Rathbone -photo)] - -Further north, near Loveland and Lyons, as well as further south at -Colorado Springs, irregularities in the uplift have caused abrupt breaks -in the generally smooth eastern edge of the range. Folds and faults in -these areas trend northwest, cutting across and offsetting the mountain -front. - -[Illustration: South of Colorado Springs, between Fort Carson and the -NORAD installation in Cheyenne Mountain, Mesozoic rocks are faulted -against the mountain front. Paleozoic rocks are deeply covered by as -much as 3000 feet of Mesozoic sediments. They come to the surface about -10 miles further south.] - - RAMPART RANGE - Garden of the Gods - Ute Pass Fault - MANITOU SPRINGS - PIKES PEAK MASSIF - CHEYENNE MOUNTAIN - COLORADO SPRINGS - CROSS SECTION - Ute Pass Fault - Rampart Fault - Tertiary - Mesozoic - Paleozoic - Precambrian - -[Illustration: West of Boulder, several intersecting sets of joints -pattern the Precambrian rocks above Boulder Creek. (John Chronic photo)] - -The west margin of the Front Range is not as sharply defined as the -eastern margin. Prominent faults edge North, Middle, and South Parks, -however. The northern end of the range merges with the Medicine Bow -Mountains, where dips of sedimentary rocks seldom exceed 30 to 40 -degrees. At its southern end, the Front Range plunges into the plains, -although a southwest-trending ridge connects it with the Wet Mountains. - -Within the Precambrian core of the Front Range, many economic mineral -deposits have been found. These are discussed in Chapter III. Glacial -features of the Front Range are discussed in Chapter II in the section -on the Quaternary Period. - - - Wet Mountains - -The Wet Mountains are the easternmost range of the Rockies south of -Canon City. Their crest has a distinct northwest-southeast trend, with -the north end offset about 25 miles westward from the south end of the -Front Range. The Canon City Embayment lies at the junction between the -ranges. - -Though smaller and lower than the Front Range, the Wet Mountains include -many pleasant and easily accessible recreation areas and a number of -attractive streams and reservoirs. Greenhorn Peak, the summit of the -range, is 12,334 feet high. It is formed of Precambrian granite, as is -most of the crest of the range. - -The structure of the eastern side of the Wet Mountains is similar to -that of the Front Range, except that there are more faults in the -sedimentary layers. The southern end plunges southeastward into the -plains. On the western side, westward-dipping sediments are completely -submerged in Cenozoic lava flows and debris from the mountains. Ore -minerals very like those of the Front Range occur near Silver Cliff, but -they have so far proved to be of little economic importance. - - - Sangre de Cristo Range and Spanish Peaks - -The Sangre de Cristo Mountains are visible from many parts of -southeastern Colorado as a jagged, sawtoothed, snow-crested ridge on the -western skyline. They extend about 150 miles from the Arkansas River -near Salida southward into New Mexico. - -Few mountain ranges form so impassable a barrier as the Sangre de -Cristos. Only at La Veta Pass does a highway cross the range. However, -old wagon roads, passable now by jeep or on foot, once existed across -Hayden, Music, Mosca, and Whiskey Creek Passes. - -Often no more than twenty miles wide, the central portion of the range -is composed largely of red Late Paleozoic sediments like those exposed -in the Garden of the Gods and Red Rocks Park. These rocks are -intricately folded and faulted, but not metamorphosed. They include -sandstones, shale, conglomerates, and fossil-bearing limestones. The -northern end of the range is formed of Precambrian igneous and -metamorphic rocks. - -Just west of La Veta Pass, Sierra Blanca stands as an outpost of the -range where its continuity is interrupted and its structure changed. -Huge blocks of Precambrian granite were here pushed upward and thrust -westward to form a cluster of peaks, several of which are over 14,000 -feet in elevation. - -Many prominent rock glaciers are present in the Sangre de Cristo -Mountains. They are composed of fragments of rock, lubricated by snow -and ice, creeping almost imperceptibly down the steep flanks of the high -peaks. One of these rock glaciers can be seen on the slope of Mt. Mestas -east of La Veta Pass; others are visible from Great Sand Dunes National -Monument. - -South of La Veta Pass, an igneous intrusion along the axis of the range -changes the character of the Sangre de Cristos. This intrusion is harder -and has weathered more slowly than the rest of the range, and forms a -group of prominent peaks known as the Culebra Range. - -On the west flank of the Sangre de Cristo Range, east of Villa Grove, a -prominent iron-mineralized area can be seen. Here the ghost mine of -Orient marks the site where iron ores were mined in the early days of -the Colorado Fuel and Iron Company. Nearby, an abrupt terrace along the -edge of the valley marks the position of a fault. Recent gravels are -involved in this fault, indicating that movement has taken place here -within the last few hundred years. A number of hot springs occur along -the base of the mountains nearby. - -The Spanish Peaks, not structurally related to the Sangre de Cristos, -are visible from La Veta Pass highway. These two peaks represent a pair -of Cenozoic volcanoes, now deeply eroded and much reduced from their -former height. Numerous dikes radiating from the bases of these peaks -represent fissures which were filled with lava as the peaks formed. - -The Great Sand Dunes, close to the Sangre de Cristo Mountains north of -Sierra Blanca, are discussed in Chapter II in the section on the -Quaternary Period. - -[Illustration: Spanish Peaks, south of Colorado Springs and southwest of -Walsenburg, are twin mountains of volcanic and intrusive rock, the roots -of Tertiary volcanoes greatly worn down and reshaped by erosion. This -view looks southeast from near La Veta Pass, on U.S. Highway 160. (Jack -Rathbone photo)] - - - Park Range and Rabbit Ears Range - -Bordering the western side of North, Middle, and South Parks, another -long north-south trending ridge extends from the Wyoming border toward -the center of Colorado. The northern part of this ridge, forming the -western boundary of the main mountain mass in the state, is called the -Park Range. - -The structure of the Park Range is similar to that of the Front Range: a -huge linear corrugation in the earth’s crust, bounded by faults. Because -this area has fewer resistant sedimentary rock layers above the -Precambrian basement rocks, it is not prominently edged with upturned -sedimentary layers. - -[Illustration: Hahn’s Peak, a highly eroded laccolith of rhyolite -porphyry, lies on the west side of the Park Range, along the eastern -margin of the Plateau Province. Placer gold was discovered here in 1865, -but the bedrock source of the gold was never found. (Jack Rathbone -photo) A geologic section shows the structure of the area.] - - TERTIARY - RED BEDS - JURASSIC - DAKOTA - MANCOS - DAKOTA - Hahn’s Peak - PORPHYRY - MANCOS - DAKOTA - PORPHYRY - JURASSIC - RED BEDS - RE-CAMBRIAN - -[Illustration: Hahn’s Peak] - -The range is crossed by Rabbit Ears Pass in the north; Gore Pass near -Kremmling marks its southern end. Mt. Zirkel (12,180 feet) and Flattop -Mountain (12,118 feet) are the two high points of the range; these and a -number of unnamed peaks over 11,000 feet high are upward-faulted blocks -of Precambrian granite. - -A rough ridge of volcanic country joins the Park Range with the Front -Range and effectively separates North Park and Middle Park. This is the -Rabbit Ears Range, named for a double-eared knob of Precambrian granite -near Rabbit Ears Pass on U. S. highway 40. Many Tertiary volcanic -features, including dikes and lava flows, can be seen along this ridge, -which is also traversed by Colorado state highway 125 between Granby and -Walden via Willow Creek Pass. - - - Gore Range - -The Gore Range lies south of Gore Pass, along the Park Range trend. The -ridge of this range is low for about 15 miles south of Kremmling, but -the southern part of the range forms a spectacular high cluster of peaks -with many relatively inaccessible and rugged summits. Many of the peaks -in this remote country are as yet unnamed; the area has been set aside -as the Gore Range-Eagle’s Nest Wilderness Area. The Colorado River cuts -directly across the northern part of the Gore Range just west of -Kremmling, in a steep-walled canyon that is one of the wild scenic spots -of Colorado. - -[Illustration: The southern part of the Gore Range, viewed from the -east, shows Precambrian granite and metamorphic rocks rising above -Cretaceous shale hills. The nearly horizontal crest of the range -probably represents the Precambrian erosion surface. (Jack Rathbone -photo)] - -The Gore Range is, like the Front Range, a faulted anticline with -Precambrian rocks at its core. The red sedimentary rocks on the west -flank of the range, visible at Vail Pass and Vail ski area, are of the -same age as those in Red Rocks Park near Denver and the Garden of the -Gods near Colorado Springs. Paleozoic rocks are absent on the east flank -of the range, having been eroded from that area before Mesozoic -deposition. South of the Colorado River and north of the Wilderness -Area, Mesozoic rocks extend over the crest of the range. - -The south end of the Gore Range is marked by Tenmile Gorge (U. S. -highway 6 between Frisco and Vail Pass). This gorge is a glacial valley, -carved during the Ice Age by a glacier more than 1,000 feet thick, along -a weak faulted zone in the range. A fault surface can be seen on the -east side of the valley. - -From Vail Pass, or from the top of the Vail ski lift, other evidences of -glaciation can be seen—cirques and U-shaped valleys—testifying to the -former presence here of many large valley glaciers. - - - Tenmile and Mosquito Ranges - -With scarcely a break, the Park Range-Gore Range structure continues -southward into the Tenmile and Mosquito Ranges. These high ridges -separate South Park from the upper Arkansas Valley, and include a -cluster of very high peaks, Quandary, Mt. Lincoln, Mt. Democrat, and Mt. -Bross, all over 14,000 feet in elevation. - -Structurally, both the Tenmile Range and the Mosquito Range are highly -asymmetrical anticlines, gentle on the east and steeply faulted on the -west. Paleozoic sedimentary rock layers containing many fossils cover -large portions of the higher parts of these ranges, but two of the -highest peaks, Mt. Bross and Mt. Lincoln, are capped by the Lincoln -Porphyry, a Tertiary intrusive, while Quandary Peak is Precambrian -granite. - -These mountains are highly mineralized, and have been extensively -explored and mined. The Climax Molybdenum Corporation operates an -especially large mine at Climax, and the New Jersey Zinc Company has a -large underground mine and mill at Gilman, on the western slopes of -Tenmile Range. - -Buffalo Peaks, two highly eroded volcanic mountains near the south end -of Mosquito Range, are extrusions of lava and ash which have buried the -axis of the Mosquito uplift. They are major volcanoes related to a group -of small volcanic cones near Antero Junction, in South Park. - -South of Buffalo Peaks, near Trout Creek Pass, the Mosquito Range loses -altitude rapidly and merges with the rough country called the Arkansas -Hills. Cinder cones, dikes, and other evidences of Tertiary volcanic -activity can be seen between Trout Creek Pass and Salida. - - - Sawatch Range - -Bordering the Arkansas River valley on the west, the Sawatch Range -includes Colorado’s highest mountain, Mt. Elbert (14,417 feet). With -several other 14,000-foot summits, this range is the highest in the -state. One group of peaks, known as the Collegiate Range (Mts. Harvard, -Yale, Columbia, and Princeton) forms a particularly imposing vista from -U. S. highway 24 between Trout Creek Pass and Buena Vista. The -Independence Pass highway (Colorado 82) between Leadville and Aspen -penetrates the heart of the Sawatch high country. - -The Sawatch Range as a whole is about 100 miles long (north to south) -and 40 miles wide. It is a great faulted anticline intruded by igneous -rocks. The high area north of Leadville shows that the Sawatch and -Mosquito Ranges are in reality one huge dome with a slight sag in the -middle. The ranges, though, are sharply separated topographically by the -deep valley of the Arkansas River. Precambrian rocks are near the -surface between the ranges, hidden only by a thin cover of stream -gravels. Near Leadville, some complexly faulted Paleozoic limestones lie -in the sag between the ranges. - -At Mt. Princeton Hot Springs there is evidence of repeated faulting and -igneous activity. The rocks are strongly altered by hot water coming to -the surface through fissures and cracks. - -On the west side of the Sawatch range, the old mining towns of Tincup -and Aspen grew up where limestone and sandstone layers, broken and -crumpled as the Sawatch Range rose, were mineralized by solutions rich -in gold and silver. The Aspen Mining District was studied extensively by -geologists of the U.S. Geological Survey, and their maps show almost -unbelievable complexity in the faulting of the rock layers which exist -there. - -The north end of the Sawatch Range plunges under shales and sandstones -along the Eagle River east of Wolcott. Gypsum in the sediments here has -acted like putty: the layers of rock in which it was deposited have -become peculiarly crumpled, making the area along the Eagle River -(visible from U. S. Interstate 70) between Avon and Edwards hummocky and -irregular. Vegetation is unusually sparse here because of gypsum in the -soil. - -About midway between Edwards and Wolcott, the Eagle River suddenly -changes direction and flows northward for about a mile before resuming -its former westward course. This sudden change is caused by a sharp -north-south fold in the sedimentary rocks on the northwestern flank of -the Sawatch Range. A magnificent series of roadcut and hillside -exposures along the highway here illustrates the close relation between -rock layers and river course. Within about a mile, the highway cuts -through rocks of Pennsylanian, Permian, Triassic, Jurassic, and -Cretaceous age, spanning a geologic time interval of more than 200 -million years. - -The south end of the Sawatch Range, at Monarch Pass, contains steeply -dipping Late Paleozoic limestones and coal beds. The coal has been mined -on a small scale; the limestone is now quarried for use as a flux in -iron smelters at Pueblo. - - [Illustration: The area below the Aspen Mountain ski lift is highly - complex geologically. It is particularly well known because of - extensive prospecting and mining activity in the region.] - - - Elk Mountains and West Elk Mountains - -The Elk Mountains and West Elk Mountains appear to be westward -continuations of the Sawatch Range. Structurally, however, they are not -faulted anticlines like most of the other ranges in Colorado, but are -composed of a series of layers of Paleozoic sediments thrust westward -over one another. These rocks, often crumpled and highly metamorphosed, -are cut by numerous sills, dikes, and other intrusions, many of which -have caused mineral enrichment locally. - -At Maroon Bells, in the canyon of Maroon Creek, and at Redstone on the -Crystal River, these metamorphosed sediments are well exposed. Here, red -sandstones and shales have been altered to quartzites and slate. At -Marble, metamorphism of a thick limestone bed has produced white marble -of great beauty, known as Yule Marble. This decorative stone was -quarried extensively until about 1940. It was used in the Lincoln -Memorial and several other monumental structures; in the town of Marble -it has been used for the doorsteps of log cabins! The largest block -quarried, for the Tomb of the Unknown Soldier in Arlington National -Cemetery, measured 14 by 7.4 by 6 feet in the rough, and weighed 56 -tons. - -[Illustration: Mt. Sopris, south of Glenwood Springs, is an igneous -intrusion. (Jack Rathbone photo)] - -Crested Butte, at the south end of the Elk Mountains, is a small -intrusive igneous mass called a _laccolith_. Hard and resistant to -erosion, it stands over 2,000 feet above the adjacent valley floor. - - - San Juan Mountains - -The San Juan Mountains are the most extensive range in Colorado, and -also the most heterogeneous. Covering more than 10,000 square miles of -the southwestern part of the state, these mountains are formed mostly of -Tertiary volcanic rocks, the result of repeated outpourings of lava and -ash from a cluster of volcanoes. Water-laid gravels composed of volcanic -sand and pebbles are interlayered with basalts and ash beds; the total -thickness of these beds reaches many thousands of feet. - -[Illustration: The mining town of Ouray, now also a tourist haven and -summer resort, nestles below Pennsylvanian sedimentary rocks of Ouray -Canyon. At the north end at town can be seen the Ouray Hot Springs -swimming pool. Gold, silver, lead, and zinc are still mined in this -area. (Jack Rathbone photo)] - -The widespread volcanic activity which formed most of the range began in -mid-Tertiary time and continued for several million years. A few -Quaternary volcanic flows are known in the region, but there is no -active volcanism there at present. - -The western side of the main range, including some of the highest peaks, -consists primarily of uplifted and faulted Paleozoic sedimentary layers. -These layers, highly dissected by erosion, can be seen near Ouray, at -Molas Lake, and at Durango. Large patches of Precambrian granite and -metamorphic rocks protrude through the sediments, as in the Needle -Mountains; they indicate that this part of the range is a faulted -anticline like many other Colorado ranges. - -Early Cenozoic glacial deposits occur in some parts of the San Juans. -These are unusual features, as glaciation of this age is unknown -elsewhere in Colorado. - -Three small ranges rise just west of the San Juans: the San Miguel, -Rico, and La Plata Mountains. Each consists of several small masses of -Tertiary igneous rock intruded into Paleozoic conglomerates, shales, and -limestones. - -Mineralization has been intense in the San Juans; most of it took place -during the Late Tertiary volcanic period. Rich veins penetrate -Precambrian gneiss and granite, and Paleozoic limestones are often -enriched also. Several mines are still active near Ouray, Silverton, -Telluride, and Rico. - - - Uinta Mountains - -The eastern end of Utah’s Uinta Mountains extends into Colorado. Unlike -other ranges in Colorado, these mountains trend east-west. Structurally, -the range is a faulted anticline. It is quite asymmetrical, however, and -is tilted and folded upward on the south, and overturned or -thrust-faulted on the north. Steeply dipping Mesozoic and Paleozoic -sediments on the south side of the range, sparsely vegetated and often -thrown into spectacular folds, are a prominent feature of northwest -Colorado scenery. - -In Colorado the crest of the Uintas reaches an elevation of about 8,500 -feet. It consists of Precambrian rocks, but these are not the igneous -and metamorphic rocks that characterize the Precambrian core of other -Colorado mountains. They are easily recognized as sediments—dark red -conglomerates, sandstones, and mudstones—virtually unmetamorphosed -though they were deposited nearly a billion years ago. Called the Uinta -Mountain Formation, these rocks are found only in this part of Colorado -and adjacent areas of Utah. They are probably related to similar -Precambrian rocks found in Montana and Canada. - -At the east end of the Uintas two isolated uplifts, Cross Mountain and -Juniper Mountain, are faulted blocks of Paleozoic rocks standing like -islands in a sea of Cenozoic valley fill. They are the last outposts of -the Uinta anticlinal pattern as it wanes toward the southeast. - -Dinosaur National Monument, a Uinta Mountain tourist attraction, -encompasses a vast area of wilderness on both sides of the Yampa River -in Colorado. Here many of the features of the east end of the Uinta -Mountain structure can be seen. A unique display of the world’s largest -fossils can be visited in the Utah portion of the Monument. - -[Illustration: At their confluence in Dinosaur National Monument, the -Yampa and Green Rivers have carved Late Paleozoic sandstone into the -precipitous cliffs of Steamboat Rock. (William C. Bradley photo)] - - - THE PLATEAUS - -The western quarter of Colorado is a region of flat-lying Paleozoic, -Mesozoic, and Cenozoic sedimentary rocks which have not been bent up -into mountains except in a few isolated instances. This area lies more -than a mile above sea level, however, and because of the gradient such -an elevation affords, it is deeply sculptured. The Colorado River and -its tributaries have sliced into the plateau surface, separating it into -many isolated tablelands or mesas. Some are capped with sedimentary -rock, others with Tertiary basalt. - -[Illustration: The Grand Hogback is a good example of the type of -geologic structure known as a _monocline_. The hogback ridge is formed -by differential erosion, where soft layers wear away more easily than -hard layers.] - -Simple folds and faults have given the mesas different elevations. Thus -the average elevation of the White River Plateau is 11,000 feet, that of -the Roan Plateau 9,500 feet, and that of Mesa Verde only 7,000 feet. -West of Durango the plateaus dip gently southward, as can be seen at -Mesa Verde. Igneous intrusions and extrusions have altered plateau -topography in some areas. West of Mesa Verde, for instance, an intrusive -stock forms a prominent dome in the Southern Ute Indian Reservation. - -West of the northern Colorado mountains, and north and west of the White -River Plateau, a rolling upland extends from Colorado into Utah and -Wyoming. It is interrupted by the Uinta Mountains and a number of -smaller related uplifts such as Juniper Mountain and Cross Mountain. -South of the Uinta axis the area is known as the Uinta Basin. - -The northern part of this area is structurally the south edge of the -Green River or Washakie Basin in Wyoming. The Rangely anticline, in the -northeastern corner of the Uinta Basin, is one of Colorado’s richest oil -fields; it is discussed in Chapter III. - -Although surfaced with much younger sediments than the rest of the -Plateau Province, this area is structurally similar. On the whole, -sedimentary layers are relatively flat-lying, and where they are -uplifted they are deeply sculptured by streams and rivers. The -sedimentary rocks in this region contain uranium and placer gold in -addition to great oil and gas deposits. The southeastern part of the -Uinta Basin, usually called the Piceance Basin, is the site of a great -deposit of oil shale (see Chapter III). The term “basin” may here seem -unusual to the casual observer, for the oil shales occur on the Roan -Plateau at places well over 10,000 feet in elevation. However, the -entire region was basin-like—lower than the surrounding ranges—for many -millions of years, and during Tertiary time thousands of feet of valley -and lake deposits were laid down in it. - -The White River Plateau, north of Glenwood Springs, is composed of -almost horizontal Paleozoic sedimentary rocks that fold downward sharply -along its south and west edges. The fold is 135 miles long and is -clearly marked by the Grand Hogback, the eroded edge of hard Cretaceous -and early Cenozoic rock layers. Shale and coaly layers involved in the -same fold have eroded more readily, leaving the resistant sandstone as a -prominent ridge. - -The Uncompahgre Plateau, southwest of Grand Junction, is structurally -very like the White River Plateau. Its features can be well observed in -Colorado National Monument. It has been elevated several thousand feet -more than the Book Cliffs and Grand Valley areas to the north. Sharp -folding and faulting near the Colorado River at the north boundary of -the National Monument show that differential movement between the two -regions was sharp and localized. - -A series of northwest-trending anticlines along the Utah border in -southwestern Colorado are of special geologic interest. They represent -peculiar structures in which salt and gypsum have played a major part. -These minerals were deposited in thick layers late in Paleozoic time; -subsequently they were covered by thousands of feet of sand, shale, and -limestone. Because of their low density and high plasticity they have -since crept upward along weak spots in the overlying sediments, often -contorting these rocks as they moved. Breaking through to the surface, -the salt and some of the gypsum washed away more rapidly than the -surrounding rock, leaving long faulted troughs such as Gypsum Valley and -Paradox Valley. In most of these structures the gypsum can still be -seen, although the more soluble salt has eroded away. Oil wells in this -part of Colorado and in adjacent parts of southeast Utah have penetrated -thousands of feet of evaporites, including pure salt, gypsum, and -potassium salts. - -[Illustration: In the arid climate of the Colorado Plateaus, ledges of -well-cemented sandstone stand out sharply from slopes of shale or -mudstone. The Mesa Verde and Mancos Formations, Cretaceous in age, form -the slopes and top of Mt. Garfield near Grand Junction (Jack Rathbone -photo)] - -The peculiar weathering characteristics of flat-lying sedimentary rocks -in an arid climate are well demonstrated in Colorado National Monument, -Mesa Verde National Park, and elsewhere in the Plateau Province. Those -fortunate enough to make a river trip through the Yampa or Green River -Canyons in northwestern Colorado or on the rivers of eastern Utah and -northern Arizona will have an unusually fine opportunity to observe -close at hand the weathering and erosion in this area. Resistant -sandstone and limestone layers break into sheer cliffs, often many -hundreds of feet high, while the softer layers of mudstone and shale -form gentle slopes and terraces. Vast arching caves often develop where -resistant layers are undermined—caves sometimes containing ancient -Indian dwellings. - - - - - II - Geologic History of Colorado - - -Astronomical and geologic evidence indicates that the earth was probably -formed as an immense blob of molten rock, held together and shaped into -spherical form by its own gravity. It may even have been gaseous at -first, cooling gradually to a molten state. After hundreds of millions -of years it became cool enough to begin to harden. - -As the surface cooled, a crust formed, and lay like a blanket over the -liquid mass beneath. Convection currents—large-scale boiling -movements—stirred the molten interior, thrust portions of the crust -upward, and sucked other portions downward to be remelted. Some of the -lighter components, such as compounds of silicon and oxygen and -hydrogen, accumulated on the surface like froth on a kettle: the -continents were born. With further cooling the atmosphere and oceans -came into being. - -Something can be told of the age of the continents. Measurements of -radioactivity in the most ancient rocks exposed at the surface today -indicate that the oldest known continental rock is between three and -four billion years old. Since the continents were formed, they have been -bent and shifted and broken by the pressures exerted against them by -convection in the interior. Parts of the continents at times have been -submerged below the level of the sea, even as they are today. Other -portions, lifted above sea level, were immediately attacked by the -wearing-down processes of erosion. The battle between mountain-building -forces and erosion has been a continuous one ever since the crust was -formed. Even now earthquakes give testimony to continued crustal -movement, storms still sweep across the continents and wash mud and -frost-loosened rocks into churning torrents, rivers still deposit great -floodplains and deltas, sediments accumulate slowly but persistently -upon the bottoms of the seas. - - - PRECAMBRIAN ERA - -Only part of the earth’s very early history is represented in Colorado, -where the oldest known rocks are the gneisses and schists of the Idaho -Springs Formation, at least 1,800,000,000 years old. These rocks appear -to be the remains of ancient sediments, folded and metamorphosed into -vast mountain areas long before recognizable life inhabited the earth. - -Precambrian rocks in Colorado are on the whole very poorly known. They -have, however, been studied in detail in the Front Range west of Denver -and Boulder, where they have been intensively explored for valuable -minerals. The lack of fossils in the oldest rocks makes their close -correlation difficult, but from studies of radioactive minerals -contained in these rocks, and of the relationships of the rock units -themselves, we can list them in order of their relative ages. - -Note that the rock sequence given below reads chronologically from -bottom to top—a logical pattern in geology since younger rocks, -especially those of sedimentary origin, normally lie above older ones. -Recent studies indicate that the sequence may be much more complex than -shown here. - - (youngest) Silver Plume Granite: light pinkish gray, fine-grained - granite. - Pikes Peak Granite: pink, coarse-grained granite. - Boulder Creek Granite: dark gray, faintly banded - granodiorite. - Coal Creek Quartzite: light gray quartzite with grains - ranging in size from fine sand to - boulders, with some interbedded - schist. - Swandyke Hornblende Gneiss: dark gray to black, strongly banded - gneiss. - (oldest) Idaho Springs Formation: gray to black schist and gneiss. - -From a sequence such as this, it is possible to reconstruct some -features of Colorado’s early history. The first chapter of which we have -a record is the deposition of the Idaho Springs Formation, probably as -an accumulation of mud, sand, and limy mud in an ancient sea. Swandyke -deposition followed—the sediments were iron-rich, perhaps derived from -ancient volcanic materials. The original Coal Creek sediments were sands -and gravels, some of them quite coarse and therefore indicative of -near-shore deposition. The schist layers suggest that muds must have -been deposited also. - -[Illustration: South of Ouray, Cambrian sandstones of the Sawatch -Formation lie almost horizontally across the vertical Precambrian -metamorphic rocks. (Jack Rathbone photo)] - -Together these three formations represent some 40,000 feet of -sedimentary layers. Deposition of such a great thickness of mud, sand, -and lime must have taken a very long period of time. Details of the -geography of the continent during that period have of course been -obscured by later events, when these rocks were subjected to repeated -uplift, crumpling, folding, various degrees of remelting and -recrystallization, and erosion. But the ancient sediments must have been -derived from even more ancient highlands, either folded and faulted -mountains or volcanoes, and probably were deposited under water in broad -expanses of sea that covered portions of the continent. - -[Illustration: Geologic map of Colorado. Geologic maps show the age of -rocks appearing at the surface, disregarding soil cover. A more detailed -geologic map of Colorado may be obtained from the U.S. Geological Survey -at the Federal Center in Denver.] - - PRECAMBRIAN - PALEOZOIC - MESOZOIC - CENOZOIC SEDIMENTS - CENOZOIC VOLCANICS - Yampa River - White River - Fort Collins - South Platte River - Glenwood Springs - Denver - Colorado River - Grand Junction - Aspen - Gunnison River - Colorado Springs - Gunnison - Salida - Dolores River - Rio Grande - Arkansas River - La Junta - Walsenburg - Alamosa - Durango - -The Boulder Creek, Pikes Peak, and Silver Plume Granites cut through the -metamorphic rocks, and are therefore younger. They represent pulses of -molten rock forced upward from deep within the crust, probably during -three separate episodes of mountain building. As each set of mountains -was formed, it was worn down, perhaps to low rolling hills, perhaps to -flat plains almost at sea level, and partially or entirely covered with -thick layers of sediment. Each time, another mountain building episode -followed, with new intrusions of granite and new metamorphism of the -pre-existing rocks. - -Each succeeding period of metamorphism and mountain building further -changed the nature of the rocks involved, complicating the patterns of -folding and faulting, adding recrystallization to recrystallization, -until the oldest of the rocks bore little trace of their original -sedimentary nature. In general, the rocks that are oldest were most -altered by the repeated metamorphism, while the younger rocks were less -altered. - -[Illustration: The Black Canyon of the Gunnison River is one of the -state’s deep and spectacular chasms. Canyon walls are of Precambrian -gneiss intruded by many dikes and highly fractured by later uplifts. The -flat upper surface of the Precambrian rocks represents an ancient plain -on which, during Jurassic time, the dinosaur-bearing Morrison Formation -was deposited. (John Chronic photo)] - -The Precambrian Era ended with a long period of erosion, a period known -to geologists as the Lipalian Interval. During this time, over almost -the entire world there was no mountain building. The land lay sleeping, -subject only to the forces of erosion. The last mountains were flattened -nearly to sea level. Slow, sluggish streams and rivers carried sand and -mud toward the oceans—oceans in which perhaps primitive, soft-bodied -organisms, with no hard parts to be preserved as fossils, were beginning -to evolve. - -On the continents, the time of intense metamorphism was over; most rocks -of later eras are preserved today in pretty much their original state. -The boundary between the Precambrian and later rocks is normally well -defined. It is visible at many places in Colorado: in Williams Canyon -near Colorado Springs, in Glenwood Canyon, near Red Rocks west of -Denver, just west of La Veta Pass, at the top of Royal Gorge and the -Black Canyon of the Gunnison. At most of these localities it is a -smoothly beveled surface, with highly contorted Precambrian rocks below -it and flat-lying Paleozoic sediments above it. Near Red Rocks and La -Veta Pass, the same relationship prevails, but the entire contact, and -the rocks above and below it, have been steeply tilted by the uplift of -the present mountains. - -In portions of western North America, deposition late in Precambrian -time has left a series of flat-lying rocks between the contorted -Precambrian and later Paleozoic sediments. These rocks can be seen in -northwestern Colorado, where they form the dark red sedimentary core of -the Uinta Mountains. - - - PALEOZOIC ERA - -Geologists have divided the second great era of geologic time into units -called Periods. The rocks deposited during a Period are called Systems, -but more often than not it is convenient to discuss them in terms of -easily recognized units of rock, called Formations. Formations are named -after areas in which they are well exposed. - -The stratigraphic column given in Chapter I shows the Periods and -Systems in their correct order, and gives the age in years for each, as -determined by radioactivity methods. As you read, refer as often as -necessary to this column. - -The geologic map on page 35 will help you locate areas where the rocks -discussed in the text are exposed, and will greatly facilitate your -understanding of the geology of the state. - -[Illustration: The Cambrian Sawatch Sandstone lies almost horizontally -on Precambrian granite in Glenwood Canyon. In the foreground is the -Colorado River. (Jack Rathbone photo)] - - - Cambrian Period - (500-570 million years ago) - -The first fossiliferous rocks in Colorado were deposited during the -Cambrian Period, at a time when over much of the world the seas were -creeping in across wide, level plains formed during the Lipalian -Interval. Colorado was not covered by these seas until quite late in the -Cambrian Period. Beach deposits progressively younger in age suggest -that the sea invaded from the west, and spread slowly eastward, -inundating most of the central part of the state but not the extreme -north or south. - -The beach deposits, now called the Sawatch Sandstone because they are -well exposed in the Sawatch Range, are composed mostly of fine quartz -sand. They are colored with glauconite, a green mineral, and hematite, a -dark red mineral, so that the rock has a variegated appearance. The post -office at Manitou is built of this red and green rock, and good -exposures of it exist in Williams Canyon near Manitou, along U. S. -Highway 24 northwest of Manitou, near Red Cliff and Minturn, and in -Glenwood Canyon. - -The sea which crept over Colorado at this time contained small -conical-shelled mollusks, brachiopods, and trilobites. Their shells can -occasionally be found in Cambrian rocks in Williams Canyon and in the -Sawatch and Mosquito Ranges. At two localities unusual fossils called -graptolites have been found in thin Upper Cambrian shales overlying the -Sawatch Sandstone. - -[Illustration: These fossils can occasionally be found in Cambrian rocks -in central Colorado.] - - - Ordovician Period - (440-500 million years ago) - -The sea deepened and widened as the Ordovician Period began, and a -series of limestones and dolomites was deposited, either on top of the -Sawatch Sandstone or, where the Sawatch had not been deposited, directly -on the Precambrian. These rocks are now called the Manitou Formation. - -The fossils in these rocks are much more varied than those in the -Sawatch Sandstone: snails, echinoderms, sponges, cephalopods, -brachiopods, and trilobites are common. The Ordovician sea must have -teemed with life, as many rocks deposited at this time are more than -half composed of animal remains. In addition to hard-shelled animals -which formed fossils, there were probably abundant soft-bodied animals -such as jellyfish and worms, which left no record of their presence. - -After deposition of the Manitou Formation, the seas receded slightly. A -new series of sands was deposited above the Manitou in central Colorado. -These now form the Harding Sandstone, a formation of unusual interest -because it contains remains of the earth’s earliest known vertebrates, -primitive jawless fish called Agnathids. In places in the Harding -Sandstone there are dense accumulations of the tiny polygonal armor -plates from these fish. Although no whole fish have been found, we can -reconstruct their appearance by comparing individual plates or groups of -plates with later, better known relatives. - -Also present in great quantities in the Harding Sandstone are conodonts, -peculiar tiny brown tooth-like fossils. Relationships of the conodonts -are unknown; they may be parts of the Agnathids, or perhaps they -represent some entirely different group of animals, with no living -relatives. - -After deposition of the sands of the Harding Sandstone, the sea deepened -locally and the Fremont Limestone, a massive gray crystalline limestone -containing many marine fossils, was deposited. Mollusks (some quite -large), brachiopods, and corals contributed their shells to the Fremont -Limestone. The chain coral _Catenipora_ and the horn coral -_Streptelasma_ may often be used to identify the formation. - -The Fremont Limestone was deposited very late in the Ordovician Period. -Probably the seas were much more extensive then than present deposits -indicate; subsequent erosion has at several times erased the evidence in -uplifted areas. - -[Illustration: These Ordovician fossils can be found in the Manitou -Formation in the Colorado Springs area.] - -[Illustration: The earliest known fish remains come from the Ordovician -Harding Sandstone of central Colorado. These fragments of the protective -plates have been magnified about five times.] - -[Illustration: Corals and coral-like organisms occur in the Ordovician -Fremont Limestone.] - - - Silurian Period - (400-440 million years ago) - -Until very recently, no Silurian rocks or fossils were known in -Colorado, and it was thought that seas did not extend into the state -during this period. However, a few years ago good Silurian corals and -brachiopods were discovered near the northern edge of the state. They -occur in broken blocks and patches of Silurian limestone, mingled with -blocks of other sedimentary rocks and, oddly enough, with volcanic -material. - -What seems to have happened here is that sedimentary layers of Silurian -age _were_ present over northern Colorado at one time. During some -subsequent period of volcanism, volcanic lavas penetrated these -sediments from below. Near the volcanic tubes, broken, angular fragments -of the surrounding sedimentary rocks were sometimes carried upward or -downward by the motion of the lava. - -Much later, both the volcanic outpourings (if the lavas ever reached the -surface) and the sediments were stripped away by erosion, probably at a -time when mountains were rising in the area. Only the deep portions of -the tubes that fed the volcanoes were preserved. These tubes are called -diatremes, and thanks to the blocks of sedimentary rock in them we know -that there were indeed seas in Colorado during Silurian time, seas -containing the abundant life of a shallow marine environment very much -like that existing at the same time in Illinois, Iowa, and Indiana. - - - Devonian Period - (350-400 million years ago) - -As far as we know now, Colorado was just a little above sea level during -most of Devonian time. Early and Middle Devonian deposits are lacking. -Late in the period, however, Colorado was widely inundated once more. -Embayments of a western sea covered most of the central part of the -state and an area in southwestern Colorado around Ouray. - -Deposits formed in these embayments have been given several names. -Chaffee Formation is the name most commonly used in central Colorado; -Ouray Formation identifies rocks of the same age in southwest Colorado. -The Chaffee Formation has been subdivided into two well defined units, -the Parting Sandstone or Quartzite, and the Dyer Dolomite or Limestone. -Many ore deposits are associated with these rock units—notably deposits -of lead and zinc. The Parting Sandstone is frequently so well cemented -with silica that it is actually a quartzite; thin shale beds or -“partings” make it easy to recognize. It frequently contains remains of -fossil fish and distinctive beds of algae. - -The Dyer Dolomite contains brachiopods and bryozoans, mollusks and -corals. Some of the best fossil hunting in Colorado is in Dyer beds -around the White River Plateau, where the fossils frequently weather out -of the rock as almost perfect specimens. - -[Illustration: These Devonian brachiopods come from the White River -Plateau in western Colorado.] - - - Mississippian Period - (310-350 million years ago) - -The sea continued to cover most of Colorado after the end of the -Devonian Period, well into Mississippian time. Mississippian rocks are -characteristically thick, massive gray limestones collectively called -the Leadville Limestone. This unit is well known as the host rock for -many Colorado ore deposits, notably those around the town of Leadville. - -During Mississippian time the western sea, warm and rich in organisms, -covered much of North America. Brachiopods and corals flourished, as did -many other forms of life. The seas during part of this time extended -completely across Colorado to merge with seas that covered the -midwestern part of the United States. - -Over all this vast area, as well as southwest into Arizona, the gray, -massive, fossiliferous Mississippian limestone is remarkably uniform and -easily recognized, although it is called by different names in different -areas. - -Late in Mississippian time, the Colorado area rose slightly and the sea -in which the Leadville Limestone was deposited receded. An interval of -erosion followed. The surface of the limestone was dissolved and pitted, -tunnels and caves formed where running water etched deep into the rock, -and a reddish soil formed on the surface and in the hollows. This -portion of the limestone, which in some places also contains pebbles of -chert, is named the Molas Formation. Part of the Molas may be -Pennsylvanian in age. - -[Illustration: Mississippian fossils from western Colorado show that -seas covered much of the state about 330 million years ago.] - - - Pennsylvanian Period - (270-310 million years ago) - -As the Pennsylvanian Period began, the Colorado area continued to rise. -Earliest deposits of this age are fine-grained black shales and -sands—the Glen Eyrie Formation along the southern Front Range and the -Belden Formation in west central Colorado. Then, through millions of -years, mountain-building took place. Some areas rose more than others, -so that formerly flat-lying marine sediments were bent and broken, and a -series of high mountain ridges and deep basins were formed. Geologists -sometimes call these the Ancestral Rocky Mountains. - -Although the pattern of the mountains changed repeatedly, the Ancestral -Rockies consisted principally of two large ranges. One range roughly -paralleled the present Front Range, but lay thirty to fifty miles -further west. The other extended from the San Luis Valley northwest -toward Colorado National Monument, including the area around the Black -Canyon of the Gunnison and the present Uncompahgre Plateau. Coarse -sediments washed off both sides of both ranges, and accumulated as -alluvial fans and valley fill along the mountain margins. These exist -today as the Fountain Formation of the eastern Front Range, the Minturn -Formation between the ancient uplifts, and the Hermosa Formation west of -the western uplift. - -[Illustration: This paleogeographic map reflects the distribution of -land and sea during the early part of the Pennsylvanian Period and shows -where coarse sediments derived from the Ancestral Rockies were -deposited.] - - FOUNTAIN FORMATION - MINTURN FORMATION - HERMOSA FORMATION - -[Illustration: West of Denver, the main line of the Denver & Rio Grande -Railroad tunnels beneath steeply dipping sandstones and conglomerates of -the Fountain Formation. (Jack Rathbone photo)] - -[Illustration: Corals, brachiopods, and fusulinid Foraminifurida can be -found in the Pennsylvanian Minturn Formation at many places in the -Mountain and Plateau Provinces.] - -[Illustration: In western Colorado, where vegetation is sparse, rock -structures are clearly defined. This photograph shows beds of the -Pennsylvanian Minturn Formation sharply folded, probably as a result of -the deformation of gypsum in underlying layers. (Jack Rathbone photo)] - -In the Flatirons near Boulder, Red Rocks Park near Denver, and the -Garden of the Gods near Colorado Springs we see well exposed examples of -the Fountain Formation. The Minturn Formation is visible along the Eagle -River west of Wolcott, and along Gore Creek near Vail. The Hermosa -Formation forms striking red cliffs north of Durango. In the Sangre de -Cristo Mountains area, exceptionally great and rapid deposition took -place, and the Minturn Formation is very thick. - -In west central Colorado, near the towns of Eagle and Gypsum, a large -basin formed. In it, gypsum and other salts were deposited as arms of -the sea were cut off from the main marine area. The unusual appearance -of the hills along the Eagle River, especially north of U. S. Highway -24, is caused by the presence of gypsum in the bedrock. - -In a similar manner, the Paradox Basin was formed in southwestern -Colorado. Thousands of feet of gypsum, salt, and potash were deposited -here, probably also precipitated in restricted arms of the sea. These -minerals, the so-called evaporites, have since significantly controlled -development of the landscape in Gypsum Valley and other parts of this -region. (See The Plateaus in Chapter I and the section on Gypsum in -Chapter III). - -Between the mountain masses and their surrounding alluvial deposits, -shallow seas repeatedly invaded the lowland areas of the state. Marine -fossils in some parts of the Minturn Formation bear witness to as many -as twenty marine cycles. Strangely, the Pennsylvanian Period appears to -have been cyclical in other parts of the United States as well, for -marine sediments are found alternating with nonmarine sediments in -Pennsylvania, Illinois, Kansas, Nebraska, and New Mexico. In middle -Pennsylvanian time, general uplift occurred in Colorado, and almost the -entire state was above sea level for the rest of the period. - - - Permian Period - (223-270 million years ago) - -By the end of the Pennsylvanian Period, the mountains of the Ancestral -Rockies had been almost entirely removed by erosion, and the deep basins -were filled with sediments. Colorado was once more a great plain, -sloping gently to the northeast. In eastern Colorado, a shallow sea -gradually dried up, leaving some thin limestone and gypsum beds along -its margin. The western shore of this sea was edged with beaches and -sand dunes, preserved as the Lyons Sandstone. The buildings of the -University of Colorado, as well as many homes and other structures in -the Boulder-Denver area, are faced with this beautiful salmon-colored -sandstone. - -[Illustration: Balanced Rock, in the Garden of the Gods northwest of -Colorado Springs, is an erosional remnant of iron-rich conglomerate and -sandstone. It remains while the rest of the surrounding layers are gone -because it is harder and more completely cemented together by silica. -The rock is part of the Late Paleozoic Fountain Formation. (John Chronic -photo)] - -In the western part of the state, Permian deposits consist mostly of -shales and sandstones. The red color of these rocks, and the complete -absence of fossils in them, suggest that the environment in which they -were deposited was not marine, but was a vast, level mudflat subject to -alternating wet and dry periods. The shales and sandstones collectively -are called the Maroon Formation, named for Maroon Bells, near Aspen, -where they are dramatically exposed in the mountain cliffs. - -[Illustration: Tracks of Permian reptiles called _Laoporus -coloradoensis_ occur in the Lyons Sandstone near Lyons. These are about -life size.] - -During part of Permian time, a shallow sea extended from Idaho, Utah, -and Wyoming into the northwest corner of Colorado. In this sea was -deposited the Phosphoria Formation, a highly phosphatic limestone -containing only rare, poorly preserved molluscan fossils. - -As the Paleozoic Era ended, Colorado was still flat and low-lying. By -this time land plants and animals had evolved, but if vegetation grew in -the Colorado area, or animals roamed it, they left few fossil remains. -Tracks of early reptiles have been found in the Lyons Sandstone. Dune -sandstones here and in adjacent areas suggest that desert conditions may -have prevailed, in which case Colorado would have been very similar, -scenically and climatically, to Sahara regions today. - -[Illustration: Dark red Pennsylvanian and Permian conglomerates form the -Flatirons that overlook the University of Colorado campus at Boulder. -University buildings are faced with Permian Lyons Sandstone quarried -along the foothills of the northern Front Range. (University of Colorado -photo)] - - - MESOZOIC ERA - -The Mesozoic Era, popularly known as the Age of Reptiles or Age of -Dinosaurs, is divided into three periods. The climate of the entire -earth appears to have been warmer then than it is at present, perhaps -because of a different distribution of land and sea areas, or because -continental areas were not as high and mountainous as they are just now. -Colorado was a rather low land area for most of the first two Mesozoic -periods; then a vast sea covered the entire state for the remainder of -the era. - -[Illustration: The pink cliffs of Colorado National Monument are made of -Wingate and Entrada Sandstones. Underlying them, in the valley bottom, -Chinle shales form steep red slopes. (William C. Bradley photo)] - - - Triassic Period - (180-225 million years ago) - -Saharan conditions continued to prevail in western North America during -the early part of the Mesozoic Era. In central Colorado, the lowest -Mesozoic deposits are the Triassic Lykins Formation, a series of soft, -bright red sandstones and shales. Where the Lykins is exposed along the -Front Range, its bright red color identifies it. Because of its -softness, it is often less prominent than adjacent rock layers in the -mountain foothills. The Lykins Formation includes some evaporites, -apparently derived from Permian evaporites washed into the Triassic -ponds and lakes which existed occasionally in this region. - -Over almost the entire state, the rocks deposited at this time are very -similar. Formation names may differ—Lykins, Moenkopi, Chinle, Ankareh, -Wingate—but the rocks are almost universally fine-grained sandstones and -shales with a red or pink color. They represent ancient coastal plain, -dune, or delta deposits. Toward the western edge of the state they -coarsen, and contain layers of conglomerates similar to the Triassic -conglomerates of northern Arizona and Utah. These suggest that -mountain-building was taking place west of here at that time. - -There are virtually no fossils known from Triassic rocks in Colorado, -although some fossil palm fronds have been found west of the San Juan -Mountains, in the southwestern corner of the state. - - - Jurassic Period - (135-180 million years ago) - -During the Jurassic Period, Colorado was still a low, flat desert area -with intermittent streams flowing eastward over the surface of older -sediments. The Navajo Sandstone, formed from dune sands, was deposited -in the western part of the state. Streams flowing eastward from Utah -brought fine sediments—silts and muds—to western Colorado, forming what -is now the Carmel Formation. Near Canon City, coarse gravels bear -witness to local uplift in Jurassic time. Both these gravels and the -Carmel Formation were overlain by more dune sands, now hardened into the -Entrada Sandstone. - -In Late Jurassic time the Colorado area, which had been predominantly -desert since Permian time, appears finally to have been submerged once -more. Fine calcareous muds of the Curtis Formation, containing -ammonites, belemnites, and other marine shellfish, show us that a -shallow sea transgressed from the west over the wind-blown sands. This -sea was, geologically speaking, of short duration—only a few million -years. Bounded on almost all sides by desert, it seems to have dried up, -depositing the gypsum that is now present in a thin layer along the -Front Range between Denver and Canon City in the Ralston Formation. - -At about this time, however, the climate underwent a major change. -Deposits above the Ralston indicate an increasingly moist environment, -the environment in which the Morrison Formation was deposited over most -of Colorado and parts of the adjacent states of Kansas, Arizona, Utah, -and Wyoming. The Morrison Formation is exposed in many places, and is -characteristically composed of layers of fine, limy mud, brightly -colored in streaks of red, brown, green, and blue. In most areas it is -so soft that it becomes soil-covered; it is well exposed only in -roadcuts or where it is protected from erosion by a “caprock” of harder -sediments or lava. Spectacular outcrops can be seen in new roadcuts -along U. S. Interstate highway 70 just west of Denver. - -[Illustration: In this roadcut along U. S. Interstate 70 west of Denver, -Jurassic and Cretaceous rocks are unusually well exposed in the Dakota -hogback. Green and purple shales represent the dinosaur-bearing Morrison -Formation. The Cretaceous Dakota Group forms the eastern, higher half of -the cut. Black layers are carbon-rich clays of the South Platte -Formation, frequently quarried locally for ceramic uses. (John Chronic -photo)] - -Fossil dinosaur bones occur in great numbers in the Morrison Formation -near the towns of Morrison and Canon City and at several other places in -Colorado. Those at Canon City have been quarried extensively, and are -now mounted in a number of museums in the United States. At Dinosaur -National Monument, in eastern Utah and northwestern Colorado, many -excellent remains have been found; those in Utah can be seen in place in -the rock in a striking exhibit at the National Monument. - -[Illustration: In an old painting, a paleontologist contemplates fossil -bones found near Morrison. The date is 1877. The bones are those of the -70-foot dinosaur _Apatosaurus_, more commonly known as _Brontosaurus_, -shown below in reconstruction.] - -[Illustration: Apatosaurus] - -Some of the dinosaurs known from the Morrison Formation reached 80 feet -in length. Both plant-eating and meat-eating types are known. In -addition to the bones themselves, gastroliths or gizzard stones can -frequently be found; these highly polished stones were as essential to -dinosaur digestion as gravel is to a chicken or a caged canary. - -Along with the dinosaur fossils are found abundant remains of water -plants called charophytes. These plants formed tiny spiralled balls of -calcite as part of their reproductive activities; both the little balls -and the stalks of the plants themselves occur in many parts of the -state. In western Colorado, near Grand Junction, silicified shells of -freshwater snails can also be found in the Morrison. - -Early in the 1900s vanadium, radium, and uranium were discovered in -Jurassic sandstones and mudstones of western Colorado. Extensive mining -in this area has revealed that these elements often become concentrated -by groundwater in organic material such as fossil plant stems or -dinosaur bones. The search for radioactive minerals has thus brought to -light many ancient fossil accumulations. - - - Cretaceous Period - (70-135 million years ago) - -Early in Cretaceous time, marine conditions once more prevailed in -Colorado. This is indicated by a marked change in rock types from beach -and near-shore deposits to true marine sediments. - -[Illustration: Between the Front Range and the Plains the Cretaceous -Dakota Formation forms a hogback ridge which can be traced for 200 miles -or more. The well-cemented sandstone resists erosion, and so remains as -a ridge when softer layers are stripped away. (Jack Rathbone photo)] - -The sandstones derived from beach sands sometimes include coarse pebbles -of chert which can be traced to sources in Permian rocks of Utah and -Nevada. Occasionally the beach and near-shore deposits include marine -shells like oysters, indicating that there were brackish and salt water -lagoons and marshes along the shore. The Dakota Formation represents the -beach of the transgressive or advancing sea. This formation contains oil -in eastern Colorado, Nebraska, and Wyoming; the oil itself may have been -derived from decay of organic materials in swamps behind the beaches and -bars. - -As the sea deepened in eastern Colorado, finer sediments were deposited. -These included the black muds of the Benton Shale, and the Niobrara -Limestone, a shallow-water deposit containing abundant shells of clams -(_Inoceramus_ and _Ostrea_) and ammonites and tiny one-celled animals -called Foraminiferida. Above the Benton and Niobrara Formations lie the -fine gray muds of the Pierre Shale. Several thousand feet thick, the -Pierre contains occasional beautifully preserved ammonite shells as well -as bones from fossil fish and swimming reptiles. - -[Illustration: Cretaceous rocks in Colorado are rich in fossil -pelecypods. Each of the fossils illustrated above may grow to a much -larger size than shown.] - -[Illustration: Shales of the Laramie Formation contain many recognizable -plant fossils.] - -The rocks deposited in western Colorado at this time are markedly -different from those deposited in eastern Colorado. In the east, -deposits are fine and very limy, containing abundant shells and little -in the way of coarse debris. In the west, sandstones of the Mesa Verde -Formation dominate, and coal beds suggest marshy or swampy conditions -inshore from the ancient ocean. This is just the pattern we would expect -from a low-lying region bordering a shallow sea, a region similar -perhaps to the southeastern Atlantic and Gulf coasts of the United -States today. - -Toward the end of the Cretaceous Period, the sea receded from Colorado. -Beaches and bars of the retreating sea left a sandstone layer which now -outcrops prominently east of the Front Range as the Fox Hills Sandstone. -Above lie interbedded sands and coals, the Laramie Formation. The -presence of coal above beach sands shows that the coal swamps moved -eastward as the sea retreated. - -The exact age of the shoreline deposits and coal beds varies from place -to place in such a way as to indicate that the sea withdrew slowly and -irregularly. In general the shore moved eastward, but there are -localities such as North Park where deposition lasted much longer than -elsewhere. In some places no real beach was formed at the ancient strand -line. - -In western Colorado, the end of Cretaceous time is marked by coarser -beds, indicating an increased rate of uplift in Utah. Conglomerates were -deposited in the beds of the McDermott Formation, now visible along the -Animas River south of Durango. - - - CENOZOIC ERA - -It is characteristic of earth history that the younger the rocks are, -the more we know about them. This is because younger rocks lie near the -surface, have not been disturbed as much by mountain building processes -as have older rocks, and have not been affected as strongly by repeated -erosion. Many of the events of the Cenozoic Era are documented in detail -in the geology of Colorado, and these events have intimately influenced -the scenery as we see it today. - -The Cenozoic is the Age of Mammals. How it happened that mammals -triumphed over reptiles is one of the mysteries of geology. Some -scientists think that climatic changes—dropping temperatures and -increases in rainfall—swung the balance in favor of the warm-blooded -mammals. Others believe that cosmic ray bombardment during some unusual -astronomical event may have destroyed many surface-living dinosaurs, -while small burrowing mammals (as well as many small reptiles) were able -to survive. Still others maintain that the superior intelligence and -regulated body temperatures of mammals enabled them to win out in the -battle for survival without the aid of climatic or cosmic change. - -The names Tertiary and Quaternary, used for the two Cenozoic Periods, -are holdovers from early studies in geology in which rocks were divided -into Primary (very hard, crystalline rocks such as igneous and -metamorphic rocks), Secondary (well consolidated layered rocks), -Tertiary (layered rocks which are not fully cemented but which are -nevertheless fairly well consolidated), and Quaternary (sediments in -which the grains have not become cemented together). - - - Tertiary Period - (3-70 million years ago) - -During the first part of the Tertiary Period, uplift began in earnest in -Colorado and adjacent states. This uplift was part of the great Laramide -Orogeny that built the Rocky Mountain chain from Alaska to New Mexico. -The entire area rose above the level of the sea, and mountains were -thrust up in a great series of north-south ranges that extended unbroken -almost the length of the continent. Between the ranges, thick layers of -gravel and sand, derived from the surrounding highlands, were deposited -in intermontane basins. Occasional freshwater limestones and shales -indicate the presence of lakes. - -In Colorado, many details of the formation of the Rockies stand out in -bold relief. The Front Range moved upward sharply, mostly as a linear -block broken or faulted along both edges. Paleozoic and Mesozoic -sediments along the margins of the block were steeply tipped and in some -places even overturned, while in some localities Precambrian rocks were -thrust out over the younger sediments. - -Just east of the Front Range, especially in the area around Denver, the -land remained lower and was the site of thick deposits of gravel and -sand eroded from the range. The Denver Formation, the Arapahoe -Conglomerate, and the Dawson Arkose are more than 2,000 feet thick in -this area. These are delta and river sediments, all varying a great deal -from place to place. Individual layers of sand or gravel are not -continuous over extensive areas, but some, such as the Castle Rock -Conglomerate, are very prominent locally. - -[Illustration: On Wolford Mountain, just north of Kremmling, Precambrian -granite lies on top of Cretaceous shale. The older rocks were thrust up -and over younger rocks during the Laramide Orogeny. The position of the -fault shows clearly because trees prefer the granite soil above the -fault to the shale below. (Jack Rathbone photo)] - -Along the eastern margin of the Front Range west of Castle Rock and -Sedalia, rocks deposited at this time are now folded steeply, indicating -that the mountains continued to rise even as basin sediments were being -deposited. - -In southern Colorado, the Sangre de Cristo and Wet Mountains were also -formed as upthrust blocks. Between them, the Huerfano Basin and -adjoining Raton Basin received particularly rapid alluvial deposition. -In the Raton Basin, quantities of vegetation were deposited in swamps -and marshes, forming the thick coal beds which can now be seen in road -cuts west of Trinidad and along the Raton Pass highway. Huerfano Basin -deposits contain some of the earliest known horse remains, skeletons of -a tiny four-toed horse called _Hyracotherium_ (formerly known as -_Eohippus_). - -[Illustration: Bones of _Hyracotherium_, the “dawn horse,” have been -found northwest of Walsenburg in Early Tertiary sediments of the -Huerfano Basin. (C. R. Knight painting, courtesy American Museum of -Natural History)] - -Other rising ranges provided material for alluvial deposition in North -Park, Middle Park, South Park, and the San Luis Valley. Layers of basalt -and volcanic peaks show that as the mountains rose, the crust cracked -and allowed lava to rise to the surface in great quantities. Tertiary -basalts are very much part of the Colorado landscape: some can be seen -west of Granby, others in Table Mountains east of Golden. Near Boulder, -Valmont Dike was intruded, though lava may not have reached the surface -in that area. Spanish Peaks in southern Colorado, Mesa de Maya, the -Rabbit Ears Range, Grand and Battlement Mesas, and many other volcanic -features were formed at this time. - -[Illustration: The town of Golden nestles between the Front Range and -South Table Mountain. Tertiary basalt capping South Table Mountain -covers beds of the Denver Formation. It thins to the right, or south, -indicating that its source was probably to the north or northwest. -Buildings in the right foreground are the Colorado School of Mines. -(Jack Rathbone photo)] - -[Illustration: A series of almost vertical dikes radiate from West -Spanish Peak. Surrounding sediments are Tertiary. Weathering and erosion -along sets of joints in the largest dike have shaped it into the -“Devil’s Staircase.” (Jack Rathbone photo)] - -Most of the rich mineral deposits of Colorado are thought also to have -been formed during the early part of the Tertiary Period. Solutions rich -in gold, silver, zinc, lead, copper, and sulfides of iron seeped into -joints and faults in the crust as the mountains were pushed upward. Ore -minerals crystallized out, sometimes in veins in the ancient Precambrian -igneous and metamorphic rocks, sometimes in Paleozoic sediments. These -are further discussed in Chapter III. - -[Illustration: The Eocene Green River Formation includes great -thicknesses of oil shale, an untapped petroleum reserve containing -perhaps three trillion barrels of oil. The richest part of the oil shale -is a dark brown layer called Mahogany Ledge, visible here on cliffs just -west of Rifle. If placed in a campfire, fragments of this shale release -enough oil to burn with a yellow, smoky flame. (Jack Rathbone photo)] - -Further to the north and west, the Uinta Mountains rose. They are a -fault-block range, but they lie at right angles to the general -north-south trend of the Rocky Mountains. South of them the Uinta Basin, -one of the largest of the intermontane basins, received shaly deposits -in a great lake which existed here for probably several million years. -The lake extended over some 100,000 square miles, and during its -existence great quantities of tiny organisms lived in its waters. Oily -material from these organisms was deposited in the mud of the lake -sediments, particularly in the eastern end of the basin, there to remain -trapped in a great oil-shale deposit. Fossil fish, crayfish, algae, and -many forms of insect and plant life have been found as fossils in these -lake shales. - -West of Pikes Peak, another lake formed, dammed by a lava flow from a -nearby volcanic field. Fine volcanic ash falling into this lake -preserved the trunks and leaves of many plants as well as abundant -insects, fish, and occasional mammal bones. These are now protected and -exhibited in Florissant Fossil Beds National Monument. The fossil -plants, among them redwoods, poplar, hackberry, and pine, suggest a -climate warmer than the present one, and have been taken to indicate -that regional uplift to the present altitude had not yet occurred. - -Another rich deposit of fossil insects and plants occurs near Creede. -Other lake deposits in South Park contain ash layers with fossil algae -and snails. - -[Illustration: Large petrified trunks of redwoods and other trees can be -seen at Florissant Fossil Beds National Monument, west of Colorado -Springs. (John Chronic photo)] - -In southwestern Colorado, extensive Tertiary lava flows, ash falls, and -river deposits form the eastern part of the San Juan Mountains, the -largest volcanic area in the state. Mineral collectors are attracted to -this region by the many excellent localities for agate and other -siliceous stones. - -Still another center of Tertiary volcanism was located in what is now -Rocky Mountain National Park. Specimen Mountain, northwest of Trail -Ridge, was an active volcano about 30 million years ago, shedding ash -and lava over much of northern Colorado. The rhyolite which now caps the -hill west of Iceberg Lake, on Trail Ridge Road, was derived from this -volcano, but is now separated from it by the deep glaciated valley of -the Cache la Poudre River and Milner Pass. - -Volcanic ash at times drifted far eastward and blanketed the surface of -the plains, burying specimens of many animals and plants. The White -River Formation, extending from northeast Colorado northward into South -Dakota, is formed of such drifting ash. Many now-extinct mammals have -been excavated from this formation. - -Sometime after the mid-Tertiary episode of violent volcanic activity, -Colorado was uplifted to its present altitude. This was a general -uplift, raising the plains and plateau areas as well as the mountains. -The uplift was not an abrupt process, but continued for perhaps ten -million years. It raised the entire state 3,000 to 5,000 feet above its -previous level. - -[Illustration: Pawnee Buttes, about 40 miles north of Fort Morgan, rise -like castles from the eastern Prairie Province. Remnants of Oligocene -and Miocene sedimentary rock that once covered much of northeastern -Colorado and adjacent states, they contain jaws, teeth, and other bones -of primitive mammals. (Department of Highways photo)] - -During the remainder of the Tertiary Period, Colorado was the site of -erosion rather than deposition. However, some stream material was -deposited in the mountain valleys, and on the prairies wind-blown and -stream-borne sands were spread thinly, interlayered with impure -limestones deposited in ponds and lakes. In the San Luis Valley, -deposition was probably more continuous than elsewhere, as the exit from -the valley was blocked by volcanic flows. The deposits in this valley, -sands and clays of the Santa Fe and Alamosa Formations, form a great -artesian basin. The rich agricultural development of the valley is made -possible by water wells tapping these formations. - -[Illustration: Remains of many now-extinct mammals have been found in -Tertiary sedimentary rocks of northeastern Colorado, in the general area -of Pawnee Buttes. Those illustrated are _Oreodon_ from Oligocene strata -and a “giraffe-camel” (_Oxydactylus_) from Miocene rocks.] - - - Quaternary Period - (3 million years ago to present) - -The most significant feature of the Quaternary Period in Colorado, as -elsewhere in the northern hemisphere, is the evidence of glaciation. -During the first part of the Quaternary Period, known as the Pleistocene -Epoch, great continental glaciers covered most of Canada and much of -northern United States. The ice sheets did not extend southward as far -as Colorado, but large valley glaciers developed in many of the mountain -ranges of the state and left their traces in many mountain valleys. - -[Illustration: Mills and Jewel Lakes, in Rocky Mountain National Park, -occupy small glacier-gouged basins in Glacier Gorge. The flat-topped -peak at the upper left is Longs Peak, elevation 14,256 feet; Pagoda -Mountain is in the center of the skyline. Bedrock in this area is -Precambrian granite, gneiss, and schist at the Front Range “core.” (Jack -Rathbone photo)] - -The conditions leading to Pleistocene glaciation are not fully -understood. Climatic changes may have been initiated by a decrease in -solar radiation, changing patterns of ocean currents, reduction of solar -heating by volcanic dust, or an increase in general elevation of the -land. As the climate became cooler and moister, snowfall increased in -the north and at high altitudes. In areas where winter snowfall exceeded -summer melting, glaciers developed. - -In Colorado, glaciers formed along the crests of the Front Range, the -Sawatch Range, the Elk Mountains and West Elk Mountains, the Sangre de -Cristo and Mosquito Ranges, the San Juan Mountains, and the Park and -Gore Ranges. Glaciation in Colorado was selective: in many places -elevation was sufficient for glaciation, but snowfall apparently was not -great enough. Where they did occur, the glaciers extended down to -elevations of about 8,000 feet. There, temperatures became mild enough -to melt the ice. - -The mountain glaciers have left many tell-tale signs of their presence. -Valleys above 8,000 feet are U-shaped, their upper ends bounded by -horseshoe-shaped, steep-walled cirques. In the lower portions of the -valleys, at elevations just above 8,000 feet, lie long lines of glacial -debris known as moraines: terminal moraines forming crescents across the -valleys to show where melting glaciers dropped their rocky loads; -lateral moraines along the sides of valleys; medial moraines where -glaciers from two valleys met. Terminal moraines, often forming -effective barriers across the present streams, may act as dams, creating -lakes such as Grand Lake in Rocky Mountain National Park. - -There were at least three distinct glacial episodes in Colorado. This is -known because careful studies of glacial debris in moraines reveal three -different degrees of rock weathering. All three stages can be seen in or -near Rocky Mountain National Park. The oldest is represented by a -moraine about three miles west of Estes Park, where the Big Thompson -River traverses a wide U-shaped valley before entering its narrow, -unglaciated canyon. The next oldest is represented in terminal moraines -further up the valley, at Aspenglen campground. The youngest is shown in -a prominent terminal moraine about one mile west of the park entrance in -Horseshoe Park. - -A large lateral moraine separates Hidden Valley from the south side of -Horseshoe Park, and an almost equally large lateral moraine is present -on the north side of this valley. At Moraine Park, both sides of the -valley are edged with lateral moraines also. - -Studies in Rocky Mountain National Park have revealed many other details -of glaciation in this area. These are described in Park Service -brochures and guidebooks, in the museum at Park headquarters, and in -informative roadside signs. - -[Illustration: A line of hikers approaches Arapaho Glacier, west of -Boulder. Movement of the glacier is evidenced by the crevasses apparent -just below the snowfield in the dirty gray glacial ice. (H. H. Heuston -photo)] - -Several small glaciers are still present in the Colorado mountains, all -in sheltered cirques above 11,000 feet. These may be remnants of the -former larger glaciers, or new glaciers formed after a long warming -episode. A hike to one of these glaciers is a rewarding experience for -anyone interested in geology. Some of the more accessible are St. Mary’s -Glacier west of Denver, Arapaho Glacier west of Boulder (the Boulder -Chamber of Commerce sponsors a festive hike to Arapaho Glacier every -August), and Tyndall Glacier in Rocky Mountain National Park. - -The Ice Age brought drastic changes also to the landscape below 8,000 -feet elevation. Heavily loaded with glacial debris, mountain streams -disgorged coarse sands and gravels along the mountain front and in the -intermontane basins. As the glaciers melted after each period of -expansion, the swollen streams cut deeply into their former deposits and -into much older rocks as well. Royal Gorge, the Black Canyon of the -Gunnison, and many of the deep, colorful canyons of the Plateau Province -were cut or at least deepened by these waters. The canyons along the -east face of the mountains—Big Thompson, Boulder, Clear Creek, and -others—were also deepened and sharpened by the rushing ice-fed torrents. - -On the prairies, rivers dumping their loads of sand covered the older -rocks. Sand dunes developed along the river channels. Bones and huge -tusks of hairy mammoths were sometimes buried in these soft deposits; -now they are occasionally revealed as the dune and river sands are -washed or blown away by continuing erosion. - -About 20,000 years ago, man arrived in Colorado. Soon after this, the -water supply of the valleys diminished greatly, and erosion slowed down -correspondingly. The climate gradually became semiarid to arid. Many -features of the natural scene were much as they must have been a century -ago, without the highways, dams, and television aerials of today. -Buffalo and many smaller types of game roamed the plains and foothills; -deer, elk, and bighorn sheep were plentiful in the mountains. Nomadic -tribes camped and hunted in both mountain and prairie. In the western -part of the state, homes could be built in the shelter of great caves, -as at Mesa Verde, and game could be supplemented with corn and squash -planted on plateau surfaces. - -Several features of Colorado scenery changed with increasing aridity. -The glaciers of course were gone or nearly gone. Streams were no longer -the violent torrents they had been. Many mountain lakes, filled with -sediment and vegetation, became instead mountain meadows. And the once -fertile intermontane valleys became deserts. - -[Illustration: During the last Ice Age, elephant-like mastodons roamed -Colorado. As present-day erosion removes sediments, bones, teeth, and -tusks are frequently exposed, especially in the Prairie Province. (C. R. -Knight painting, courtesy American Museum of Natural History)] - -[Illustration: Mastodon] - -On the eastern side of the San Luis Valley, the Great Sand Dunes -developed at this time. These dunes nestle against the Sangre de Cristo -Range, where strong southwesterly winds blowing across the wide valley -tend to funnel toward Mosca and Music Passes. These winds lift loads of -sand from the lightly vegetated valley floor, and drop it as they rise -over the mountains. Where the sand is dropped, the dunes have formed. -They rise to about 700 feet above the valley floor, and cover about -forty square miles. The low rainfall of the area, seven to eight inches -per year, keeps vegetation from creeping over the dunes and makes them a -most distinctive feature of Colorado, a lesson in geology in the making. - - * * * * * * * * - -Geologic processes in Colorado now seem to be much reduced from what -they were a few thousand years ago. Reduction in rainfall has led to -reduced erosion. Mountain-building, having reached a climax in Tertiary -time, has declined markedly. However, we find evidence that volcanism -has occurred within the last few thousand years and faulting within the -last few hundred, and Colorado streams rise after sudden mountain storms -to approximate the violent torrents of glacial times. Colorado’s -scenery, fashioned during some three billion years of earth history, is -ever changing. - -[Illustration: The Great Sand Dunes of Colorado were formed during -Pleistocene and Recent time by deposition of quartz sand lifted from -unconsolidated alluvial deposits in the San Luis Valley. The highest of -the dunes rises 700 feet above the adjacent valley floor. (John Chronic -photo)] - - - - - III - Geology and Man in Colorado - - -Colorado’s first permanent settlers arrived in 1858, when gold was -discovered in river sands near what is now the city of Denver. The -ensuing gold rush, coming ten years after the rush to California, -rivalled it in fury and brought sudden wealth to lucky miners and the -adventurous merchants who grubstaked them. Several hundred mining towns -or “camps” sprang into existence almost overnight, their sites -determined by the geology of the mountain areas. The cities of Denver, -Boulder, and Golden were established as milling and shipping centers for -the products of the mines. In 1876 the now-wealthy area, previously part -of Kansas Territory, became the State of Colorado. - -For more than a hundred years Colorado’s minerals—products of her long -and diverse geologic history—have influenced her development in many -ways. The state’s early wealth, stemming from bonanzas in gold and -silver, is evidenced by palatial homes, hotels, and public buildings -constructed during the first few decades of mining activity. Some of -these are still standing—the opera houses at Central City and Aspen, -Central City’s famous Teller House, and the Grand Imperial Hotel at -Silverton are examples. - -Many of the stories and legends of Colorado’s gold camps are recounted -in _Stampede to Timberline_, by Muriel Sibell Wolle, delightfully -illustrated with sketches of old mining towns as they appear today. -_Mining in Colorado_, published by the U. S. Geological Survey, also -makes fascinating reading, as it contains many historical anecdotes and -eyewitness accounts of gold-rush days. - -Development of the metal-mining areas in Colorado followed a definite -sequence. Placer gold was usually discovered first. Recovery of placer -gold was followed by mining of gold from veins or “lodes.” Although at -first only native gold was mined, gold-bearing compounds such as -telluride were soon recognized as an additional source, especially at -Gold Hill, Cripple Creek, and of course the camp that came to be known -as Telluride. As gold sources were depleted, silver, first produced as a -byproduct, became of prime interest. Lead and zinc were in turn -byproducts of silver mining. Other metals, notably copper, vanadium, -tungsten, and iron, were produced later. Molybdenum is the -Johnny-come-lately of the state’s mining industry, but is now the chief -metal produced. A uranium boom in the 1950s brought a short rush to -western Colorado and new vigor to the economy. - -Oil was discovered near Canon City in 1862. The nearby Florence field -and a small, shallow field near Boulder preceded much greater -discoveries in the Denver Basin, the Uinta Basin, and southwest -Colorado. Oil reservoirs, confined to areas of sedimentary rock, are -found primarily in the Prairie and Plateau Provinces of the state, and -recovery of the oil has done much to distribute population to these -areas. - -Coal is also restricted to sedimentary rock areas. Coal production in -Colorado has waxed and waned with the years, but has provided fuel for -export, for the railroads, for the manufacture of electric power, and -for many of the state’s industries. - -A good picture of present mineral production in Colorado can be obtained -from the following summary for 1971, prepared by the Colorado Bureau of -Mines: - - Product Value - - Molybdenum $105,389,456 - Petroleum 90,494,459 - Sand and gravel 32,842,503 - Coal 30,251,443 - Natural gas 18,695,225 - Uranium 18,048,692 - Vanadium 15,863,554 - Cement 13,377,520 - Zinc 13,310,787 - Lead 6,582,025 - Tungsten 6,360,020 - Limestone and dolomite 5,397,570 - Silver 4,198,054 - Fluorspar 3,887,210 - Copper 3,875,976 - Stone 1,961,279 - Gold 1,832,791 - Clay 962,986 - Iron 880,047 - Pumice 309,370 - Tin 278,862 - Gypsum 253,856 - Pyrites 142,640 - All others 1,091,927 - Total ($376,288,252) - -Colorado is now the nation’s leading producer of molybdenum, tin, and -vanadium, and second in output of tungsten. In oil production it ranked -twelfth among the states in 1968, but ninth in reserves, with -420,000,000 barrels of proven reserves on 1 January 1969. An as yet -untapped source of oil lies in the oil shales of western Colorado. - -As part of the natural environment, water plays a major role in man’s -activities. Water problems in Colorado revolve mainly around the best -use of runoff in a state whose major catchment basins are across the -continental divide from her largest population centers and most fertile -farm land. Groundwater, closely related to surface water distribution -and movement, is a geological problem, and in Colorado as in other -states many government and private geologists serve farm and industrial -communities in the search for usable supplies. - - - - -CAUTION: Old mines are dangerous! They may contain water or deadly -gases, or be on the verge of collapse. Keep away from abandoned prospect -pits and mine shafts. WARN AND WATCH YOUR CHILDREN. - - - GOLD, SILVER, AND OTHER METALS - -Colorado’s placer and lode sources of gold, which gave first impetus to -the series of mining booms in the state, were fantastically rich. Summit -and Lake Counties, for instance, each produced more than $5,000,000 in -placer gold between 1859 and 1867. During the same nine-year period, -more than $9,000,000 in lode gold was produced from Gregory Gulch, a -tiny canyon between Central City and Black Hawk. Other districts -rivalled or surpassed these figures. - -Early in the game it was recognized that almost all the deposits -occurred along what came to be known as the “mineral belt,” a -fifty-mile-wide zone extending southwest from the Boulder region. Most -of the metals mined in the state come from this belt, but there are -three notable exceptions: Cripple Creek, Silver Cliff, and western -Colorado vanadium and uranium districts. In the first few years of the -Colorado rush, gold ores and placer gold were discovered only in the -northeastern part of the mineral belt. Gradually the belt was found to -extend further and further southwest: Tincup was discovered in 1861, -Silverton in 1870, Lake City in 1871, and Telluride in 1875. Aspen, on -the western edge of the belt, was not discovered until 1879, perhaps -because the area was difficult of access and lacked the easily -recognizable native gold. - -In the northeast part of the mineral belt, gold and other minerals occur -in veins in Precambrian granite and gneiss. In the Leadville and Aspen -areas, ores are associated with altered Paleozoic limestones. At the -southwest end of the mineral belt, in the San Juan Mountains, ore veins -are found near or in Tertiary volcanic rocks. Native gold, gold-bearing -compounds, and other metallic ores in these veins originated where -mineral-rich solutions from deep within the earth penetrated fissures -and joints in the surrounding rock. Regardless of the age of the host -rock, almost all the ores of Colorado were deposited in the early or -middle Tertiary Period, about 35 to 70 million years ago. - -Gold and silver are no longer mined extensively in Colorado, although -any summer Sunday will see weekend operators panning near mountain -streams or trundling rock from one-man mines. The recent rise in the -price of silver has encouraged many miners to reopen old shafts. The -most active mines in the state today are those producing molybdenum, -lead, zinc, and vanadium. (Vanadium, although a metal, usually occurs in -Colorado with radioactive minerals, and so is discussed with them rather -than with the metals.) - -[Illustration: The Colorado mineral belt extends from Boulder County on -the northeast to San Juan County on the southwest. Almost all of the -prominent mining districts in Colorado lie along this belt. Cripple -Creek and Silver Cliff, however, lie far to the east of the general -trend.] - - Telluride - Denver - Colorado Springs - Alamosa - BOULDER - Ward - Gold Hill - Boulder - Nederland - GILPIN - Central City - Black Hawk - JEFFERSON - Golden - CLEAR CREEK - Empire - Georgetown - Silver Plume - Idaho Springs - SUMMIT - Breckenridge - EAGLE - PITKIN - Aspen - GUNNISON - Tincup - CHAFFEE - PARK - Climax - Alma - Como - Fairplay - TELLER - Cripple Creek - FREMONT - OURAY - Ouray - Camp Bird - Ironton - SAN JUAN - Silverton - HINSDALE - Lake City - LA PLATA - Durango - MINERAL - Creede - CUSTER - Silver Cliff - -All told, some 430 metal mining districts have been established as legal -entities in the state of Colorado. Each of these districts had the right -to draw up its own regulations concerning prospecting, claims, and -mining rights, within a framework established by the Federal government. -Only a few of the districts ever became really significant producers. -The geology and history of several of the leading areas are presented in -the pages that follow. - - - Boulder County - -Gold Run, near Gold Hill, was the scene of one of the earliest strikes -in Colorado. Gold was found here in December 1858, and was sluiced from -stream sands and mined from veins early in 1859. Active placer mining -lasted only about a year, however, and lode mining dropped off rapidly -as near-surface oxidized ores were worked out. When a smelter was -erected at Black Hawk in 1868, and sulfide ores could be treated, there -was a revival of activity. In 1869 the Caribou and Poorman mines near -Nederland were discovered; they quickly became the most active mines in -the county. The Ward district opened soon after. - -In 1872, a gold-silver telluride called petzite was found in veins at -Gold Hill. Renewed prospecting in this area resulted in location of -mines near Sunshine, Salina, and Magnolia. During the years that -followed, new mines appeared almost as fast as old ones were depleted. -In 1892, the peak year, more than $1,000,000 in gold and silver was -produced; total production has been about $25,000,000. - -In 1900, a black mineral common in the Nederland area was recognized as -ferberite, an ore of tungsten, and a new rush to the area started. -During the next eighteen years Boulder County was the main tungsten -producer in the United States; about 24,000 tons of tungsten trioxide, -worth $23,000,000, were produced here. The ore was found in nearly -vertical veins six inches to three feet thick, in a lenticular area -about nine miles long extending from Nederland northeast to Arkansas -Mountain, four miles west of Boulder. - -Boulder County is characterized by an abundance of small mines. Old -shafts, pits, and mine buildings can be found throughout the central -part of the county. Little mining is done here today; many of the towns -that once peppered these hills have fallen into decay or disappeared -entirely. - - - Central City and Idaho Springs - -The Central City-Idaho Springs area was the principal metal mining -region in the state until the late 1880s. In 1858, rich placer deposits -were discovered in gravels and river terraces along both forks of Clear -Creek. Exploration upstream led to discoveries of rich oxidized quartz -veins at Central City, Black Hawk, and Idaho Springs. These veins, which -generally trend northeast-southwest, extend through the mountains in a -zone about six miles long and three miles wide between the two forks of -Clear Creek. - -The ores filled a multitude of cracks and fissures in the Precambrian -bedrock. The veins are usually less than five feet thick, and are almost -vertical and often clustered in zones up to thirty feet wide. The -position of one of the vein systems may be seen clearly between Black -Hawk and Central City—the ore-bearing rock has been mined out, but a -series of collapsed tunnels marks the line where the veins crossed the -valley. A monument here commemorates the discovery of Gregory Gulch, one -of the richest localities in the state. - -Several rich veins were mined in both directions—southwest from Central -City and northeast from Idaho Springs—until the mines met. The Argo -tunnel, marked by dilapidated buildings and extensive dumps on the north -side of Idaho Springs, connected the two districts; it was completed in -1904. - -The “Patch,” a deep crater-like hole on Quartz Hill, about one mile -southwest of Central City, is an intriguing feature in this area. It was -produced by glory-holing, a mining technique in which a deep tunnel is -deliberately caved by blasting, so that ores above the tunnel can be -removed. This glory hole was dynamited below an irregular mass of highly -broken rock where many ore-rich veins converged. After the caving, ores -were taken out through the remaining part of the tunnel. - -The principal ore minerals of Central City and Idaho Springs are native -gold, pyrite, sphalerite, galena, chalcopyrite, and tennantite. -Prospecting for uranium was carried out during the 1950s but no uranium -was ever mined here. - -The area has produced almost $200,000,000 worth of gold, silver, lead, -zinc, and copper. A few mines still operate seasonally or on a small -scale, but tourists, many of them riding Jeeps across the mountainous -terrain to visit mines and ghost towns, are often more visibly active -than the mines. - - - Georgetown, Empire, and Silver Plume - -A few miles southwest of Idaho Springs, another mining area had a -similar, though less productive, history. In 1859, placer and lode gold -were discovered near what is now Georgetown. Placer mining dominated -here between 1859 and 1863. Gravel and crushed rock from decomposed -quartz and sulfide veins were washed through sluiceboxes in the same way -as placer gravel, gold being caught in riffles or gunny sacking on the -bottoms of the troughs. The veins were found to be decomposed to depths -of about 40 feet; below this the gold occurred closely associated with -sulfides such as pyrite, sphalerite, galena, and chalcopyrite, from -which it could not easily be separated. However, smelters were developed -in 1866 for treatment of these sulfides, and gold, silver, lead, and -copper were recovered. Gradually, as the gold was worked out, silver and -lead became the important products of the mines. - -[Illustration: Sluicebox mining was a common sight near the early gold -camps, where primary recovery was from placer deposits or decomposed -quartz and sulfide veins. (State Historical Society of Colorado photo)] - - - Leadville - -Placer gold was discovered in 1859 in California Gulch, about seven -miles north of the present town of Leadville. The rush that followed was -short but sweet; the camp was called Oro—gold! About $5,000,000 was -produced from the placer mines within two years, though by 1861 the area -was all but deserted, for the easily won placer gold was gone. - -[Illustration: Early-day Leadville sprawled among its mine dumps at an -elevation of 10,200 feet. The Sawatch Range, in the background, -contained many smaller mining communities, now deserted. Mt. Massive, -the state’s second highest peak, forms the crest of the continental -divide here. (State Historical Society of Colorado photo)] - -In 1875 a smelter was erected a few miles downstream from Oro to process -cerussite—silver-rich lead carbonate—that occurred in the placer sands. -For years this mineral had been considered a nuisance because, being -much heavier than sand, it tended to separate out with the gold. The new -town of Leadville sprang up near the smelter and shortly afterward more -lode deposits were discovered south of the placer workings. From $63,000 -in 1875, production climbed to $2,500,000 in 1878 and more than -$15,000,000 in the peak year of 1882. - -Geologically, the ores of this district occur as Tertiary replacements -and veins in Ordovician, Devonian, and Mississippian limestones. The -“Blue” or Leadville Limestone, of Mississippian age, contains the -richest ore. Ore deposits were formed after the limestones had been -faulted and cracked extensively by mountain-building movements; the ores -themselves probably crystallized from molten or gaseous materials -involved in related igneous intrusions. River gravels and glacial debris -mask the true nature of the lode deposits, but studies in the mines show -that the fault systems along which ores are deposited trend north or -north-northeast. - -The Leadville district is now experiencing its third mining boom as a -newly recognized lead-zinc orebody is being developed. Production is -expected to reach 700 tons of ore per day by 1971. Total production of -gold, silver, lead, zinc, and copper in the district has reached -$500,000,000. - - - Breckenridge - -Breckenridge was also discovered in 1859, with placer gold the first -attraction. The placers gave out in 1862 after about $3,000,000 in gold -had been recovered. Earliest attempts to mine the rich silver and lead -veins of the district were in 1869. - -As at Leadville, the sedimentary rocks of the area were intruded by -granitic masses in Tertiary time, but here the sedimentary rocks are -mostly Pennsylvanian sandstones and shales. These rocks were badly -faulted and broken during the intrusion, and the ores were deposited as -the granitic material cooled. The lode deposits occur mostly in small -veins well hidden by surface sands and gravels. Some of the veins -yielded exceptionally beautiful crystallized wire and flake gold, -specimens of which are on display at the Colorado School of Mines -library in Golden and in the Denver Museum of Natural History. - -Dredging for alluvial gold was attempted in 1898 in the Breckenridge -district, but this method of extracting gold was not successful until -1905. A number of dredges operated between 1910 and 1925. These floating -behemoths shovel up gold-bearing gravels from the bottom and one side of -the pond on which they float, sort out the gold in giant sluiceboxes, -and spew out the leftover gravels in great arc-shaped heaps that can be -seen near Breckenridge and Fairplay and in a number of other valleys in -Colorado. They depend for their operation on a plentiful supply of water -and a shallow water table, but they can sift through quantities of -gravel at relatively low cost. All told, about $7,000,000 in gold has -been dredged from this district. - - - Fairplay - -[Illustration: This gold dredge, still floating in its pond just south -of Fairplay, operated from 1941 to 1952. With chains of buckets like -those in the foreground, it dug gravel 70 feet below water level, -carving a 35-foot bank above water level; in effect it mined to a depth -of 105 feet. This dredge extracted nearly 115,000 ounces of gold from -about 33 million cubic yards of gravel (John Chronic photo)] - -Another gold field discovered in 1859 was in the northwest corner of -South Park, along the headwaters of the South Platte River. Several -mining camps were established here. After early production of rich -placer deposits, claims were consolidated and large flumes constructed -so that gold could be recovered by hydraulic mining. In this type of -mining, streams of water from high-pressure hoses are directed at gravel -surfaces. The gravels are washed into long sluiceboxes, where gold is -caught in riffles. Hydraulic mining continued upstream from Fairplay -until about 1900. - -In 1922 a dredge was constructed near Fairplay to process gravel along -the South Platte and in the valley floor. An even larger dredge, -constructed in 1941, operated until 1952, when rising labor costs -overrode the narrow margin on which it operated. At the time operations -ceased, the dredge was recovering about six cents in gold for each cubic -yard of gravel processed. - -Placer gold has always been the principal mineral product of the -Fairplay area, but native gold also occurs in the surrounding mountains -in quartz veins, and many small mines were developed to extract it. -Sulfide ores were also mined; they contained silver, lead, and zinc as -well as gold. In the Mosquito Pass and Horseshoe Amphitheater areas, -there is renewed activity now because of the recent rise in the price of -silver. - - - Silverton - -Gold was discovered in the San Juan Mountains of southwest Colorado in -1870. The earliest mine, near what is now Silverton, was located by a -group of prospectors sent out by Governor Pile of New Mexico Territory. -Since the site was on Ute Indian land, real mining did not begin until a -treaty allowing it was ratified in 1874. - -Production in the Silverton district has been from veins in Tertiary -volcanic rocks within an elliptical area known as the Silverton -cauldron. Here the volcanic rocks, part of the several thousand feet of -lava flows and ash falls of the San Juan volcanic field, were cracked -and faulted by a second period of igneous activity. Ores formed in the -cracks and fissures. - -In the 1870s the Silverton district was very remote, and difficulties -with transportation retarded activity there. In 1882, however, a -narrow-gauge railroad was built connecting Silverton with Durango, and -the problem of transporting ore out of the isolated mountain valley was -simplified. The railway still exists; a train makes daily passenger runs -during the summer—the only remaining operating narrow-gauge line in the -United States. The track follows the Animas River canyon, whose cliffs -and crags are dotted with long-abandoned mines, prospect holes, and mine -buildings, monuments to the tenacity and determination of the men who -mined here. - -Production in this district was more than $22,000,000 in gold and -$20,000,000 in silver between 1874 and 1923. New activity is evident -here, as in other silver-rich areas of Colorado, because of recent -demand for silver, lead, and zinc. - -[Illustration: Silverton lies in a remote mountain valley in the San -Juan Mountains. Silver, gold, lead, and zinc have been mined here since -1874. Storm Peak, composed of Tertiary volcanic rocks, forms the -backdrop; the narrow-gauge railroad track is visible in the foreground. -(Jack Rathbone photo)] - - - Ouray - -Ouray was settled in 1875, when gold and silver deposits were found near -Mount Sneffels. Since 1877, mines in Ouray County have produced over -$35,000,000 in gold and $32,000,000 in silver. The district is still -quite active: in 1965, mines in this area produced more than $9,000,000 -in gold, silver, copper, lead, and zinc, about a third of total Colorado -production of these metals for that year. - -[Illustration: A few miles south of Ouray, along Uncompahgre Gorge, an -old mine clings to the slope below the Million Dollar Highway (U. S. -550). Abrams Mountain rises in the background. The Precambrian -Uncompahgre Quartzite outcrops up to about the road level; Miocene -Sunshine Peak Rhyolite caps the peak. (Jack Rathbone photo)] - -A mile north of Ouray a prominent intrusive stock marks the center of -mining activity closest to Ouray. The richest deposits of the Ouray -area, however, lie about five miles southwest, near Mount Sneffels and -Red Mountain Creek. There, several large mines, including the famous -Camp Bird mine, have operated for many years, extracting ore from -hundreds of veins that underly the surface. Some of these veins are two -to four miles long. They are in Tertiary volcanic rocks of the San Juan -Formation. Quartz and calcite are the common gangue (non-economic) -minerals, and pyrite, sphalerite, galena, and chalcopyrite are the most -abundant ores. Most of the silver is in the galena; gold occurs in -streaks and nodules associated with quartz. - -About ten miles south of Ouray, along the “Million Dollar Highway” (U. -S. 550), the Red Mountain district lies on the northwest edge of the -Silverton volcanic cauldron. It contains a number of small pipelike -bodies very rich in silver-copper and silver-lead ores. Following the -mid-Tertiary volcanism and ore intrusion, surface rocks in this area -were intensely oxidized: resulting iron oxides now form the gaudy reds -and yellows of Red Mountain and the slopes near Ironton. This -alteration, as well as the fact that much of the area is covered with -fallen rock, stream gravels, or glacial deposits, compounds difficulties -of locating the small though high-grade ore deposits. - -The Idarado Mine, on the east side of U. S. highway 550 near Red -Mountain, used to produce ores from nearby volcanic pipes; now it -produces from veins some distance to the northwest. The area is -honeycombed with tunnels and shafts. - - - Aspen - -Silver was found at Castle Creek and on Aspen Mountain in 1879. A group -of prospectors from Leadville, apparently after examining maps of the -Geological and Geographical Atlas of Colorado published in 1877, -explored along the line of Paleozoic limestones encircling the Sawatch -Range. As they had hoped, they found ores similar to those at Leadville -in rocks of the same age. - -Mining began at Aspen in 1880. Here, as at Leadville, intrusion of -granite porphyry into or near the Leadville Limestone had broken and -deformed the layers, and ores were deposited in fissures and as -replacements during cooling of the intrusions. The intricacy of faulting -which controls the ore pockets in the limestone is well shown on the map -of Aspen Mountain in Chapter II. - -Glaciation occurred in this area, and glacial deposits cover most of the -ore bodies and outcrops so that little bedrock is exposed. Mapping was -accomplished by extrapolating to the surface the bedrock patterns shown -in mine pits, shafts, and tunnels. - -Aspen produced some of the richest silver ores in the world, and thrived -as a boom town for most of two decades. In 1888 the value of ores -produced reached over $7,000,000; the next year it topped $10,000,000. -After the silver crash of 1893 production declined rapidly; the last -mines were closed in the 1920s. Total production of silver, lead, zinc, -and copper reached about $100,000,000. There was virtually no gold in -the ores at Aspen. - - - Creede - -[Illustration: Creede and its mines are located in an area of Tertiary -rhyolite and dacite, light-colored volcanic rocks.] - - Happy Thought Mine - Amethyst Mine - West Willow Creek - AMETHYST FAULT - Last Chance Mine - Del Monte Mine - Commodore Mine - Jackpot Mine - Coppervein Mine - Bachelor Mine - BULLDOG MOUNTAIN FAULT - Kansas City Star Mine - Commodore Tunnel - Mustang Tunnel - Nelson Tunnel - Exchequer Mine - SOLOMON FAULT - CAMPBELL MOUNTAIN - Holy Moses #2 - Holy Moses Mine - Ridge Mine - Solomon Mine - Monte Carlo Mine - Mollie S. Mine - East Willow Creek - Ramey Tunnel - Dora Belle Mine - Mammoth Tunnel - Homestake Mine - Mammoth Mine - MAMMOTH MOUNTAIN - Nancy Hanks Mine - Pipe Dream Mine - THE NARROWS - Windy Gulch - CREEDE - Willow Creek - -The Creede district ranks as one of the most productive silver areas in -the United States. It came into being largely as a result of a discovery -by N.H. Creede in 1889. When exploring in this area, he was reported to -have exclaimed “Holy Moses!” on examining a rich piece of ore, thus -giving the name to the mine which initiated the rapid development of the -district. By the end of 1892 the Holy Moses and nearby mines had -produced ore valued at more than $4,000,000. The area was so rich that -it managed to survive 1893’s great decline in the price of silver; by -1920 almost $42,000,000 in gold, silver, lead, and zinc had been mined -there. - -The ores, silver-bearing galena, sphalerite, native gold, pyrite, and -chalcopyrite, are in quartz or amethyst veins in faulted and shattered -Tertiary volcanic rocks. Nearly all the ore deposits lie along a complex -system of vertical faults, the Amethyst fault zone, which runs more or -less northwest-southeast through this region. Both the faulting and the -enrichment of the fault fissures are believed to have taken place in -mid-Tertiary time, shortly after deposition of the volcanic host rocks. - - - Cripple Creek - -[Illustration: Cripple Creek, on the flanks of the Pikes Peak massif, -has produced more than $400,000,000 worth of gold. The Sangre de Cristo -Mountains are visible in the distance beyond the Arkansas River valley. -(Jack Rathbone photo)] - -In 1890, two sheepherders stumbled on some richly mineralized rocks near -Cripple Creek. A boom developed immediately, for the rocks contained -both gold and silver. Since then, the area has produced more than -2,000,000 ounces of silver and nearly 19,000,000 ounces of gold. - -Cripple Creek has produced almost half of all the state’s gold and -silver. The ores are located in or at the edge of a large mass of middle -Tertiary volcanic rocks which form an elliptical basin or _caldera_ -several miles across. The caldera, surrounded by Precambrian gneiss and -granite of the Pikes Peak massif, was probably formed by collapse of a -volcanic center that had erupted through the older rock. The collapse -shattered the rocks around the basin margin, and subsequent volcanic -activity introduced mineral-rich solutions into the many faults and -fissures produced by the collapse. Tellurides of gold, silver, and -copper, as well as pyrite, sphalerite, galena, tetrahedrite, and other -minerals, are characteristic. - - - Climax - -[Illustration: At Climax, the ore occurs scattered through the intrusive -Climax Granite Porphyry and the intruded Idaho Springs Formation. -Visitors can tour the surface workings during the summer months.] - - Tertiary dikes - Shell of Climax stock - Core of Climax stock - Ore zone - Precambrian granite - Fault - Dykes - -Molybdenum now ranks as the number one metal mined in Colorado. Over -$105,000,000 of “moly” was mined here during 1969, almost all of it from -the Climax Mine, the world’s largest single source of this metal. The -Climax deposit is located high on the west slope of Ten Mile Range in -central Colorado, about 100 miles southwest of Denver. It is in the -central part of the Colorado mineral belt, near the Mosquito Fault, a -prominent structural feature which extends about sixty miles along the -north-south trend of the mountains. Rocks on both sides of this fault -are intruded by Tertiary granite dikes, sills, and stocks. The Climax -Mine is in a stock just east of the fault, near the axis of a broad -anticline in Precambrian metamorphic rocks. - -Ore minerals at Climax are molybdenite, huebnerite, and cassiterite; -pyrite is recovered also for the manufacture of sulfuric acid. The ore -is very low in metal content, containing only one-third of a percent of -molybdenum, 0.005% tungsten trioxide, and 0.0001% tin. The great size of -the ore body and efficient recovery by modern methods make Climax a -profitable mine, however. Production has risen each year since the mine -began operation. - -Urad Mine near Berthoud Pass is a newly developed near-surface -molybdenum mine similar to Climax. Nearby at the Henderson Mine the ore -body is more than half a mile below the surface of the ground. - - - RADIUM, URANIUM, AND VANADIUM - -Over a large area of the Plateau Province in western Colorado, Mesozoic -sedimentary rocks are locally stained bright yellow, orange, or green. -Such staining suggests mineralization, and radioactive compounds were -recognized here before 1900. At that time, however, there was little or -no market for them or for the vanadium frequently associated with them. -When Marie Curie required radium for experiments with her newly -discovered element, the raw materials were sent from western Colorado; -by and large, though, production of radium from these ores was -prohibitively expensive. - -In 1905, vanadium was found to be effective in toughening steel. The -Vanadium Corporation of America was formed to mine the Colorado ore. -This company mines a rich zone in the Jurassic Entrada Sandstone, where -vanadinite occurs with carnotite and other uranium ores. In the early -days of vanadium mining, the uranium ores were discarded with other -gangue materials; now, of course, uranium is produced from them. - -Since 1945, uranium production has been an important Colorado industry; -in 1969 about $17,500,000 worth was produced. Uranium occurs in the -state in two very different situations. In the Plateau Province, where -it was first discovered, it occurs in sedimentary rocks as patches of -pitchblende, carnotite, and a greenish yellow mineral called -schroekingerite. It is most abundant in the Triassic Chinle Formation -and the Jurassic Entrada and Morrison Formations, where it was probably -deposited by downward movement of rainwater from overlying uranium-rich -Tertiary volcanic rocks. Concentrations of uranium often occur in or -near organic matter such as coal, fossil bone, or petrified wood, so -mines tend to be located along rock layers carrying abundant organic -material. - -Another type of uranium ore is found in the Mountain Province. Veins in -Precambrian rocks of the Front Range and several other ranges contain -pitchblende which seems to have been deposited by hot groundwater rising -through broken and fissured Precambrian rocks. Often exceedingly rich, -such ore is mined in the manner of most of Colorado’s metals. The -Schwartzwalder Mine, a few miles northwest of Golden, has produced more -ore of this type than any other mine in Colorado. - - - OIL, NATURAL GAS, AND OIL SHALE - -Petroleum and natural gas have been found in large quantities in the -Prairie and Plateau Provinces in Colorado, as well as in smaller -quantities in North Park in the Mountain Province. They generally occur -in porous sandstone and limestone layers, where they have been trapped -by overlying finer-grained, less permeable layers in or near folds and -faults. - -Several oil and gas seeps were found along the mountain front shortly -after the arrival of the earliest settlers. Near Canon City, on Oil -Creek, a plaque commemorates the first production: - - Oil Creek—site of the first oil well in the west—second place in the - United States to produce petroleum from wells. In 1862 ... A. M. - Cassedy drilled an oil well 50 feet deep. By February, 1863, - production was one barrel a day. Later, several thousand gallons of - petroleum were produced by primitive methods, and kerosene and - lubricating oil were shipped by ox team as far as Denver and Santa Fe. - -About twenty miles to the southeast, near Florence, the Cretaceous -Pierre shales were drilled in 1876. Oil was found in a system of -intersecting fractures and joints. Some of the early wells in the -Florence field are still producing, making this Colorado’s oldest and -longest producing field. It has yielded more than 10,000,000 barrels of -oil. - -Small quantities of oil have been produced near Boulder since about -1900, also from Pierre sandstones and shales. In this area, wells were -located by “dowsing” or “witching,” as was fashionable at the time. -Several old rigs can be seen near Boulder Reservoir. As at Florence, oil -has been trapped in fractures of otherwise dense and impervious shale. -Some gas is produced and is used by local farms. - -More recently, oil was found far beneath the surface in the northern -part of the Prairie Province. Here, in the Denver Basin, oil is produced -from several levels in the Dakota Sandstone. The oil has accumulated in -lenses of beach sand deposited along the shoreline of the Cretaceous -sea. The general trend of the shoreline, and of the oil fields, is -northeast-southwest. The shore appears to have been similar to Georgia’s -present coastline: a swampy tidal zone separated from open sea by -lagoons, sandy bars, and clean sand beaches. - -Individual oil pools in the Denver Basin are small, but there are many -of them; they lie nearly a mile below the surface, under much of Morgan -and Logan Counties and adjacent parts of Nebraska. Exploratory and -development drilling keeps total oil production at about 50,000 barrels -a day. Oil and gas produced here is piped to Denver and other Colorado -cities. - -In southeastern Colorado, oil and gas occur in late Paleozoic limestones -and sandstones similar to those which outcrop at the edge of the Wet -Mountains. Prospecting by geophysical methods and by drilling has -revealed several small, rich accumulations, one of which is thought to -contain about 30,000,000 barrels of oil. - -The Rangely field, in northwestern Colorado, is the most productive -field in the state. Located in the northeastern part of the Uinta Basin, -it is an outstanding example of an anticlinal field, where oil is -trapped in a large, gentle dome. The shape of the dome shows up well on -the surface; rock layers can be seen dipping outward in all directions -from the town of Rangely. Oil was found by drilling on the crest of the -dome. At first, oil was produced from fractures in the Cretaceous Mancos -Shale at less than 1,000 feet depth. Later, deeper drilling showed that -oil had also accumulated in the Permian Weber Sandstone, at 5,000 to -7,000 feet. At present this field is producing about 28,000 barrels of -oil a day, but the figure is dropping each year as the field is -depleted. - -Oil and gas are produced in southwestern Colorado from the eastern edge -of the Paradox Basin and the northern edge of the San Juan Basin. In the -Paradox Basin, oil comes from Pennsylvanian limestone mounds or reefs. -Production in the Colorado part of the basin has been at most a few -thousand barrels per day; more is produced in adjacent Utah. In the San -Juan Basin, gas and oil are trapped in thin porous layers of Cretaceous -and Pennsylvanian sandstone, between impervious layers of shale. Most of -the production is in New Mexico, although some oil comes from the -Colorado part of the basin. - -The greatest known potential oil resource in the world lies in the oil -shales of western Colorado. The richest of these shales cover an area of -1,600 square miles north of the Colorado River, south of the White -River, and just east of the Colorado-Utah line. The oil shales are part -of the Tertiary Green River Formation, which extends over much of -northwest Colorado, northeast Utah, and southern Wyoming. Oily material -called _kerogen_ is locked in these rocks, too solid to flow out of the -fine pore spaces of the shale. To free it the shale must be mined, -finely crushed, and heated until the kerogen converts to liquid oil. -This is an expensive process, and as yet production of petroleum from -the oil shale has not been possible at a cost which will compete with -production of oil and gas from wells. The United States Bureau of Mines, -as well as a number of oil companies, have sought for more than fifty -years to discover a less expensive method for extracting oil from the -shale. No doubt at some time in the future a competitive technique will -be developed, or a growing shortage of other oil will bring world prices -to a level with which present production techniques can compete. - -Oil and gas production in Colorado is decreasing at present, even though -great efforts are being made to find new oil pools. Petroleum -prospecting and wildcat drilling are carried out in as yet unproductive -basins in the Plateau Province, in intermontane basins such as the San -Luis Valley, and on the Plains. Known reserves will continue to provide -the state with significant income for many years to come, and if oil -shale recovery becomes profitable. Colorado’s hydrocarbons will become -the most prominent of her commodities. - - - COAL - -Coal resources of Colorado amount to about 60 billion tons. Only one per -cent of this has been mined. Thousands of tons are now being produced -daily from large mines in central, southern, and northwestern parts of -the state. - -Colorado’s coal deposits were formed during late Cretaceous and early -Tertiary time, when seas were receding from this region and the land was -rising. They represent accumulations of leaves and other plant material -in swamps and flood plains similar to those now found in the delta of -the Mississippi River and in the swamps of southeastern United States. -Almost all Colorado coal is bituminous or soft coal. - -Coal was recognized early in Colorado history by settlers along the -mountain front, and was mined west and north of Denver in the 1860s. -Several large underground mines still operate in this district, -supplying local power plants, but production does not compare with that -of the Walsenburg-Trinidad area in southern Colorado or the Hayden area -in northwest Colorado. - -The Walsenburg-Trinidad region, part of the Raton coal field, has -produced coal since the building of the Santa Fe Railroad in the early -1870s. For many years coal from these mines moved the Santa Fe trains -and many of the numerous smaller railroads that served Colorado’s cities -and mining camps. The location of the mines helped to determine the -location of the Colorado Fuel and Iron Company smelter in Pueblo. Now, -most southern Colorado coal is used to produce electric power. Many -small mines, miles away from the power plant west of Trinidad, are -deserted. - -A large coal-burning power plant has recently been built between Hayden -and Steamboat Springs, just west of the Yampa River. Here, some of the -extensive coal deposits can be seen in road cuts along U. S. highway 40. -Until conversion to diesel fuel became almost universal in North -American railroads, mines of this district produced coal for -locomotives. - -In the heyday of the gold and silver mines, coal was also mined near -Coalmont, in North Park, and Como, in South Park. Coal from these areas -was used for fuel in nearby mining towns and ranches, and for the -narrow-gauge railroads that penetrated the mountains here. - -At Anthracite, near Crested Butte, high-grade anthracite coal was mined -for a time. Identical in origin with other Colorado coal, the anthracite -of this region was hardened by heat and pressures from Tertiary igneous -intrusions forcing their way into local sedimentary rocks during -post-Cretaceous mountain building. - -A multitude of other coal camps are scattered about Colorado: Cokedale, -Delcarbon, Coaldale, Roncarbo, Carbondale, and Cardiff stand out because -of their suggestive names. These early small camps are, like their -metal-mine cousins, largely deserted today. - - - CONSTRUCTION MATERIALS - - - Sand, Gravel, and Clay - -Sand, gravel, and crushed rock rate high among geologic products in -Colorado; more than $27,000,000 worth of these materials were produced -in the state in 1969. Highway and construction activities have brought -recent expansion in the number and size of quarries and gravel pits. -Increasingly, Coloradoans are insisting that quarries and pits be -excavated only where they will not mar the natural beauty of the -landscape, and many old pits are now being filled in. Unfortunately, the -scars left by some quarries—such as that on the Rampart Range near -Colorado Springs—are difficult to erase. - -Clay of good quality occurs in Cretaceous deposits in many parts of -Colorado, most frequently in the Dakota or Laramie Formations. In the -area around Golden, the Coors Porcelain Company for many years mined -clay for use in pottery and low temperature ceramic ware. Scars from -this mining can be seen along the mountain front north and south of -Golden, and deep clefts within the town, just west of Colorado School of -Mines, testify to the amounts of clay that have been removed. Colorado -clay is not pure enough to be used in high temperature ceramics, and the -present use for it is in the manufacture of common tiles and bricks. - -A recent development in Colorado is the use of Cretaceous Pierre shales -in manufacturing lightweight aggregate for building. The shale is mined -between Golden and Boulder, near Colorado highway 93. In the nearby -plant, it is pulverized and then heated in a large rotating cylinder -until the surface of each particle fuses. Then the particles are quickly -cooled. The resulting product is much like cinder, light in weight and -yet strong. It can be mixed with cement for use in construction work -requiring a great strength-to-weight ratio, or made into concrete -blocks. - -[Illustration: Quarrying of Paleozoic limestones and dolomites along the -east flank of the Rampart Range northwest of Colorado Springs has badly -defaced a prominent mountain backdrop. Recent seeding efforts by quarry -operators are returning the exhausted part of the quarry to its original -lightly vegetated condition, and hopefully, as the quarry is depleted, -the scar will disappear. (John Chronic photo)] - - - Stone - -In Colorado, as in most parts of the world, building stone for local use -is quarried locally. Two of the state’s stones, however—Yule Marble from -the Crystal River Canyon, and Lyons Sandstone of the Front Range—have -been more widely used. - -The Yule Marble, or Yule Colorado Marble, was produced by metamorphism -of Leadville Limestone in an area intruded by the Treasure Mountain -Granite, thirty-five miles south of Glenwood Springs. This exquisite -marble, which has graced many famous monuments and buildings (among them -the Lincoln Memorial and the Tomb of the Unknown Soldier), is known for -its almost uniform snowy whiteness and regular, fine crystallization. -Although its beauty, massive character, and uniformity made it a -sought-after ornamental stone, quarrying was economically marginal -because of the remoteness of the site. In spite of this, nearly -$7,000,000 worth of the marble was produced before the quarry closed in -1940. - -[Illustration: Pure white marble was quarried for many years at the Yule -Colorado Marble Quarry, about three miles southeast of the village of -Marble. (U. S. Geological Survey photo)] - -The Lyons area, north of Boulder, provides pink, hard, even-grained -sandstone which splits readily into slabs or flagstones. These are used -in the Denver-Boulder area for sidewalks and patios as well as for -facing buildings. Quarries owned by the University of Colorado provide a -constant supply of handsome facing material and flagstone for new -university buildings, although in recent years the high cost of stone -construction has limited its use on the campus. - -[Illustration: Lyons Sandstone is quarried near Lyons, Colorado. The -salmon-colored sandstone splits along surfaces defined by slight -differences in size and arrangement of the sand grains. (John Chronic -photo)] - -[Illustration: Most of the buildings of the University of Colorado are -faced with Permian Lyons Sandstone, which is widely used for buildings -and flagstones throughout the Boulder-Denver area. The University -Museum, shown here, was established in 1902, and contains over a million -scientific specimens, including many Colorado fossils and minerals. -Exhibits in the Hall of Earth portray Colorado’s geologic history. -(Tichnor Bros. photo)] - -The Lyons Sandstone was deposited as beach and bar sand along the edge -of a sea which lay east of the Front Range in Permian time. After -deposition, the sand was deeply buried and compacted. Now tilted up -along the Front Range uplift, it comes to the surface along the east -side of the range. Only between Fort Collins and Boulder does the stone -have the desirable combination of hardness, thin-beddedness, and color -which makes it desirable for ornamental use. The pink color of the Lyons -Sandstone is derived from iron oxides, mostly hematite, disseminated -between the sand grains. Dendrites (often erroneously called fossil -ferns or plants) ornament some slabs; they were formed by -crystallization of manganese dioxide from groundwater as it slowly -percolated through the rock. - - - Lime and Gypsum - -Outcrops of the Cretaceous Greenhorn and Niobrara Limestones provide -most of the cement materials in Colorado. A number of plants along the -mountain front, including a completely automated and dust-free one near -Lyons, provide the major population centers with millions of tons of -cement each year. - -Colorado is richly endowed with gypsum, useful in cement and plaster -manufacture and for ornamental stone and sculpture. Along the eastern -front of the mountains, gypsum occurs in the Triassic Lykins Formation; -in the Mountain Province, it is abundant in Pennsylvanian sedimentary -rocks. Particularly high-quality Pennsylvanian gypsum is quarried at the -town of Gypsum, west of Eagle. - -The Colorado portion of the Paradox Basin, in the Plateau Province, -contains immense deposits of Pennsylvanian gypsum. Here, rocks near the -surface have been pushed up into sharp northwest-trending faulted -anticlines by upward movements of gypsum and salt from depths of several -thousands of feet. The soluble salt and gypsum cores of these structures -have been washed away more rapidly than the surrounding layers of -sandstone and shale, leaving depressions such as Gypsum Valley, Paradox -Valley, and Sinbad Valley, on the crests of the anticlines. Red and -yellow Triassic sandstones and shales, especially the Chinle Formation -and the Wingate Sandstone, dip away from these valleys. Exploratory -wells indicate that vast masses of salt and gypsum are present beneath -the surface, and may extend to depths greater than 10,000 feet. - - - GEMS AND ORNAMENTAL STONES - -More than thirty different gems and ornamental stones are known to occur -in Colorado. Amazonstone, amethyst, garnet, tourmaline, aquamarine, -topaz, lapis lazuli, quartz crystal, smoky and rose quartz, sapphire, -several varieties of agate, zircon, and other attractive stones are -gathered within the state, mainly in the Mountain Province. Turquoise is -known at several places in the volcanic area of southern Colorado. -Alabaster is mined along the northeastern mountain front near Fort -Collins and Loveland. Localities of interest to gem hunters are -described in _Colorado Gem Trails and Mineral Guide_, by Richard M. -Pearl. - -Gem Village, in southwestern Colorado on U. S. highway 160 between -Durango and Pagosa Springs, is a favorite stopping place for tourists -wishing to see or buy colorful and attractive Colorado stones such as -petrified wood, agatized dinosaur bones, chalcedony, and jasper. - - - WATER - -Although not all aspects of water and water supply are geologic, water -is an important geologic agent, determining the shape of the surface, -the distribution of minerals, and the location of caves. Water used in -Colorado comes entirely from precipitation within the state, as all of -Colorado’s rivers flow from Colorado outward toward the surrounding -lower-elevation states. - - - Surface Water - -[Illustration: A cross section through the Front Range northwest of -Denver shows the redistribution and use of western slope water in -eastern Colorado through the Colorado-Big Thompson Project. This project -has cost about $160,000,000, but it is repaying the investment many -times over by providing electric power and increasing farm production.] - -Moisture carried by prevailing westerly or northwesterly winds falls -primarily on Colorado’s western slope, although at some times of year -precipitation may come from the northeast or southeast. West of the -continental divide, where population is sparse, there is a surplus of -water. East of the divide, where more than 90 per cent of the population -lives, water is in desperately short supply. The high and largely -unpopulated Mountain Province receives by far the greatest proportion of -precipitation, while agricultural areas of the Prairie and Plateau -Provinces receive much less. Needless to say, the major problem -involving water in Colorado is how to move it from areas where it is -abundant to areas where it is needed. - -In many parts of the state, complex water laws and complicated -irrigation canals and water systems were developed soon after the area -became settled. Gradually but inevitably, water resources have been -transferred from the western slope to the eastern. However, such -transfer must be undertaken with due regard for the rights of downstream -users, notably California, Arizona, and New Mexico. - -One of the largest water movement schemes in the state is the -Colorado-Big Thompson Project. Water that otherwise would flow into the -Colorado River is piped from Grand Lake through the Alva B. Adams tunnel -under the high mountains of Rocky Mountain National Park, and into the -Big Thompson drainage near Estes Park. It then travels through a series -of reservoirs and tunnels into the South Platte River basin, where it is -used for irrigation and household water. The water is pumped up the -western gradient of this system by electric power produced as it flows -down the eastern slope. Surplus electric power serves the -Colorado-Wyoming area. - -Another large project is the Denver Water Board’s Dillon Reservoir -Project, in which western slope water collected at Dillon is pumped -twenty-three miles under the continental divide through the Harold D. -Roberts tunnel to the North Fork of the South Platte River for use by -the city of Denver. The exit point of this tunnel can be seen a few -miles west of Grant along U. S. highway 285. This project is -continuously growing as Denver’s water needs mount. - -In each of these projects, engineering geologists played a prominent -part in locating dams and tunnels that would not leak or fail, and that -could collect and transport a maximum amount of water during the -high-runoff spring season for distribution through the rest of the year. -Fortunately for geologists, the tunnels and bores necessary to the -projects allowed them to learn a great deal about the structure of the -interior of the high mountains, and helped to improve their -interpretation of earth history in this most interesting region. - -The necessity for storing irrigation water along the eastern mountain -front has led to the creation of hundreds of new lakes in the region. -Although water levels vary with the season, many of the lakes provide -opportunities for water sports and recreation for the burgeoning inland -population. - -Two large dams have recently been built in western Colorado for another -purpose: to control the flow of water in the Colorado River drainage -basin. Electric power for western Colorado also comes from these dams. -One of the dams is on the Gunnison River at Curecanti, upstream from the -Black Canyon of the Gunnison National Monument, and the other is on the -Frying Pan River near Ruedi. The latter was completed over the -objections of geologists, who believed that the extensive gypsum -deposits underlying the damsite would cause its failure. Cement pumped -deep into the rocks in the vicinity has so far prevented serious -rupture. - -There is strong resistance by conservation groups to the construction of -more dams on Colorado River drainage, primarily because the Colorado and -its tributaries pass through many irreplaceable canyons, some of them -parts of National Parks and Monuments, that are very much a part of our -western heritage. - - - Groundwater - -[Illustration: In the San Luis Valley, runoff from the San Juan and -Sangre de Cristo Mountains sinks into layers of sand in the Alamosa -Formation. Flowing along the sand layers toward the center of the -valley, it provides artesian water for irrigation of valley farmlands.] - - SAN JUAN MOUNTAINS - LIMIT OF FLOWING WELLS - HUBBARD’S WELL - OTTOWAY’S WELL - ALAMOSA WELL - GEORGE NEWSOM’S WELL - CALKIN’S WELL - LIMIT OF FLOWING WELLS - Moraine - Alluvial Slope - SANGRE DE CRISTO MOUNTAINS - Sands, lava beds, gravels, conglomerates, etc. - Alamosa formation - Granites - WEST - SANTE FE FORMATION - SANTE FE FORMATION - EAST - -Groundwater is extremely important to Colorado, especially in the -Prairie Province and the San Luis Valley. Below these two areas lie a -number of distinct and productive groundwater aquifers, several of them -artesian. In Otero County, for example, there are five major aquifers: -three separate Quaternary gravel deposits, the Cretaceous Dakota -Sandstone, and the Cheyenne Sandstone Member of the Purgatoire -Formation, also Cretaceous. All these aquifers are characterized by -their high porosity and permeability, which allow water to flow rapidly -through them. Wells in the younger, shallower aquifers produce as much -as 2,000 gallons per minute; those in the older, deeper aquifers produce -about eighty gallons per minute, some of it with an artesian “head.” - -The San Luis Valley supports intensive agriculture, made possible by a -great artesian water supply. A thick series of soft interlayered clays -and sands, the Alamosa Formation, slopes down toward the center of the -basin from the surrounding mountains. Water entering the sandstone beds -at the mountain edges flows through the sand layers held there by the -impermeable clay beds. By the time it reaches the center of the valley, -it has developed considerable hydrostatic head, and the water rises in -wells without pumping. Unfortunately, both the irrigation water and the -soils in the San Luis Valley are highly alkaline. Constant evaporation -from the irrigated fields has concentrated the alkali near and on the -surface, rendering some of the land less usable than it was originally. - - - Caves - -Colorado has many caves, most of them carved by underground water in -Paleozoic limestone. The Cave of the Winds at Manitou is the only one in -the state which has been developed as a tourist attraction. It is in -highly faulted Ordovician and Mississippian limestone near the mountain -front, where the faulting, coupled with the high relief, has accelerated -solution of the rock by allowing groundwater to percolate downward -rapidly. The cavern was probably carved during the Pleistocene Ice Age, -when surface water and groundwater were much more abundant than at -present. Deposition of stalactites and stalagmites has occurred within -the last few thousand years, as supplies and movement of water have -decreased. - -Spanish Cave, above timberline on Marble Mountain in the Sangre de -Cristo Range, is probably the nation’s highest limestone cave. It is in -thick folded and faulted Pennsylvanian reef limestone, at an elevation -of over 12,000 feet. The cave has many intricate passageways branching -from its main vertical tubes and channels. - -Fulford Cave, south of Eagle, is in the Mississippian Leadville -Limestone of the northern part of the Sawatch Range. Many other caves -are situated south of Fulford, near Woods Lake, where the limestone is -widely exposed and highly dissected. - -Fairy Cave, northeast of Glenwood Springs, is the best known of the many -caverns in the Paleozoic limestones that form the southern flanks of the -White River Plateau. - -[Illustration: In Cave of the Winds near Manitou, Paleozoic limestones, -cracked and tilted by uplift of the Front Range, have been honeycombed -by ground water. Calcite stalactites hang from the ceiling, while -stalagmites grow up from the floor. (Cave of the Winds Company photo)] - -In the Plateau Province another type of cave is formed not so much by -groundwater as by weathering of the flat-lying alternating beds of -massive resistant sandstone and less resistant, thinly bedded mudstone -and shale. Where the resistant layers are undermined, great arching -caves develop. These are best observed at Mesa Verde National Park, -where many of them once sheltered Indian communities. They can also be -seen in Colorado National Monument and along the Colorado River and -several of its major tributaries. - -[Illustration: Along the edge at Mesa Verde, caves in Cretaceous Mesa -Verde sandstone were used for shelter by Indians. Springs near the bases -of the caves, which provided the Indian communities with water, probably -contributed to the undermining of the sandstone cliffs. (Colorado -Department of Highways photo)] - - - Springs - -The multitudes of mineral and hot springs in Colorado are a fascinating -and interesting facet of the Mountain Province. Some are located along -major faults, where the rocks are so broken and shattered that -groundwater can move freely toward the surface. Colorado Springs, -Manitou Springs, and Eldorado Springs are on the fault complex that -forms the east edge of the Front Range. Glenwood, Juniper, Steamboat, -and Poncha Springs are on well defined faults also. - -[Illustration: Glenwood Hot Springs flow from Pennsylvanian shales of -the Belden Formation, where sedimentary layers are faulted by the sharp -upward tilting against the south side of the White River Plateau. Behind -the hotel and on the right can be seen the Mississippian Leadville -Limestone, cut by the Colorado River. (From a painting by William H. -Jackson, courtesy of Colorado State Archives and Public Record)] - -Many other springs do not seem to be controlled so strongly by faulting, -but owe their presence to sources of volcanic or magmatic heat which -exist near to the surface of the ground. Some springs of this type issue -from Precambrian granite, or Cenozoic volcanic rock, while others flow -from sedimentary rock layers. Waunita Hot Springs and Pagosa Springs, -although near volcanic rocks, reach the surface through porous -sandstones and shales of Cretaceous age. Mt. Princeton Hot Springs comes -from alluvium but its heat source is the intrusive igneous rock which -makes up part of the adjacent mountain. - -Springs of another general type are also present in Colorado where -aquifers, generally sandstones, are dissected by erosion. These springs, -usually not highly mineralized or warm, are most often found in the -Plateau Province. Such springs are frequent at the bases of the great -sandstone cliffs of Mesa Verde and Colorado National Monument. - -Manitou’s carbonated springs, which attract many tourists, have their -origin in the arrangement and nature of the rocks through which the -water flows. Water from the Pikes Peak region, slightly acid from its -contact with the granitic rock, flows into the Manitou limestone all -along Ute Pass fault, which extends from Cheyenne Mountain northwest to -Woodland Park. Descending through channels along the fault, the water -becomes pressurized. Because of its pressure and its acid content, it -partly dissolves the calcium carbonate of the limestone, and from then -on carries carbon dioxide in solution. As the water comes to the surface -at the low point of the fault exposure, near the west edge of Manitou, -the pressure is released and the carbon dioxide effervesces, just as a -bottle of soda water effervesces when the cap is removed. - - - ENVIRONMENTAL GEOLOGY - -The preceding part of this chapter mentions many ways in which man’s -destiny in Colorado has been shaped by geologic factors. Early -Coloradoans settled near gold and silver placers, later ones near mines -that produced ores of other metals, or in the towns that sprang up -around the mills and smelters that processed these ores. Our present -distribution of population is partly a heritage from these first -settlements, partly a result of later discoveries of oil, gas, and -radioactive minerals, and partly a response to the state’s extreme -topographic variation, which controls and delineates agricultural areas -and transportation routes. - -In recent years, man has begun to appreciate the fact that he may -benefit in other ways from knowledge about geology. A new geology has -developed—_environmental geology_—which may be defined as the total of -all geological conditions and influences affecting the life and -development of man. - -Environmental geology is a broad science, concerned not only with the -location of cities and towns, but with the uses people make of the land -and its economic products, and with the relationship between the -geological character of the land and the present and future location of -roads, dams, bridges, factories, homes, recreation facilities, sanitary -land fills, and even sewage plants. - -Two aspects of environmental geology which are particularly pertinent to -Colorado’s residents are discussed below. - - -_Landslides_ and slumping rock or earth are a frequent menace to -Colorado’s development in the Mountain Province. Often activated by -heavy rains or deep manmade cuts, they can cause—and _have_ caused—much -damage to roads, buildings, and other works of man. - -The flanks of North and South Table Mountains, near Golden, are mantled -by thick landslide debris; intermittent movement of the individual -slides has repeatedly affected the railroad, irrigation ditches, and -roads. As many as six different slides have moved within a single year. -In one slide area, asphalt road material is estimated to be thirteen -feet thick; successive layers of pavement have been laid one on top of -another to keep the street up to grade. - -Landslides and landslide-prone areas may not be obvious to the untrained -eye. Each year buildings and roads are constructed on unsuitable rock -and soil foundations, in places where some degree of land slip is almost -inevitable. Building in such areas is risky, but sometimes worth the -risk; if condition are less than ideal, risks can be reduced by -specialized types of construction. - - -_Floods_ are a perennial threat to much of the state, because of the -high relief of the drainage basins and the torrential nature of the -spring and summer rainfall. Their damaging effects were realized early -in Colorado’s history, when canyons were used as highways and railroad -routes. - -Colorado’s most expensive flood was probably the flood in the South -Platte River basin south of Denver in 1965, which caused $508,000,000 -worth of damage and drowned six people. The losses can be attributed to -man’s failure to realize the significance of the South Platte drainage -routes and flood plains. Homes, shopping centers, and many other -buildings occupied—and still occupy, as of 1971—land that has been -intermittently flooded for many years. The following description of this -flood, by H. F. Matthai of the U. S. Geological Survey, may help to -convey some warning to residents or potential residents of the South -Platte valley and other river valleys in Colorado: - -“The morning of June 16 was most pleasant, but conditions changed -rapidly shortly before noon. A tornado touched ground 15 miles south -southeast of Denver about 1 p.m. Within the next hour, another unroofed -30 homes in the little town of Palmer Lake, 40 miles south of Denver. -About 2 p.m., a dense mass of clouds descended and concealed the top of -Dawson Butte, 7 miles southwest of Castle Rock; and the little light -remaining faded until it was dark black and frightening, according to -some people. A nearby rancher’s wife described the intense quiet as -awesome, but the calm did not last very long. - -“The deluge began, not only near Dawson Butte, but also at Raspberry -Mountain, 6 miles to the south, near Larkspur. The rain came down harder -than any rain the local residents had ever seen, and the temperature -dropped rapidly until it was cold. The quiet was shattered by the -terrible roar of wind, rain, and rushing water. Then the thudding of -huge boulders, the snapping and tearing of trees, and the grinding of -cobbles and gravel increased the tumult. The small natural channels on -the steep slopes could not carry the runoff; so water took shortcuts, -following the line of least resistance. Creeks overflowed, roads became -rivers, and fields became lakes—all in a matter of minutes. - -“The flow from glutted ravines and from fields and hillsides soon -reached East and West Plum Creeks. The combined flow in these creeks -have been described as awesome, fantastic, and unbelievable; yet none of -these superlatives seem adequate to describe what actually occurred. -Large waves, high velocities, crosscurrents, and eddies swept away -trees, houses, bridges, automobiles, heavy construction equipment, and -livestock. All sorts of debris and large volumes of sand and gravel were -torn from the banks and beds of the streams and were dumped, caught, -plastered, or buried along the channel and flood plains downstream. A -local resident stated, ‘The banks of the creek disappeared as if the -land was made of sugar.’ - -“The flood reached the South Platte River and the urban areas of -Littleton, Englewood, and Denver about 8 p.m. Here the rampaging waters -picked up house trailers, large butane storage tanks, lumber, and other -flotsam and smashed them against bridges and structures near the river. -Many of the partly plugged bridges could not withstand the added -pressure and washed out. Other bridges held, but they forced water over -approach fills, causing extensive erosion. The flood plains carried and -stored much of the flood water, which inundated many homes, businesses, -industries, railroad yards, highways, and streets. - -“The flood peak passed through Denver during the night, and the -immediate crisis was over by morning; but those in the inundated areas -were faced with a Herculean task. The light of day revealed the nature -of the destruction—mud in every nook and cranny, soggy merchandise, -warped bowling alleys, drowned animals, the loss of irreplaceable -possessions, to name a few types. The colossal cleanup job, which would -take months, began.” - -Hydrogeological studies by the U. S. Geological Survey and Corps of -Engineers give knowledgeable estimates of flood danger for different -populated areas of the state, and recommend that homes, roads, and other -structures be placed above likely flood levels. - - - - - GLOSSARY - - -Alluvial fan. A cone-shaped mass of sediment built by rivers or streams - as they issue from mountains onto more level ground. - -Alluvium. Stream deposits formed in recent geologic time, composed of - sand, gravel, and stones. - -Ammonite. One of a large group of extinct mollusks related to the living - chambered _Nautilus_. Ammonite shells, usually cone-shaped or - coiled, are divided into many chambers by crenellated septa. - -Angular unconformity. A surface separating tilted or folded layers of - rock from overlying less disturbed layers. - -Anticline. An upward fold or elongated arch in rock layers. - -Aquifer. A rock layer that is water-bearing. - -Artesian water. Groundwater that is under sufficient pressure to rise - above the level at which it is encountered in a well. It does not - necessarily rise completely to the surface. - -Basalt. An extrusive igneous rock, fine-grained and dark colored, - composed mainly of calcium-rich feldspar and the black mineral - pyroxene. - -Basement. A name commonly applied to metamorphic or igneous rocks - underlying the sedimentary rock layers. - -Batholith. A large body of intrusive igneous rock, 40 square miles or - more in outcrop area, which extends downward to an unknown depth. - -Bedrock. The solid rock which underlies soil, sand, clay, or other loose - surface material. - -Belemnite. The cigar-shaped internal shell of an extinct marine mollusk - similar to a squid. - -Brachiopod. One of a large group of marine shelled animals having two - unequal, bilaterally symmetrical shells. - -Bryozoa. A large group of tiny colonial marine animals that secrete - calcareous or horny coverings in a great variety of shapes. - -Caldera. A large basin-shaped depression caused by explosion or collapse - around a volcanic center. - -Cassiterite. A heavy, brown to brownish black mineral composed of tin - and oxygen (SnO₂) that is an ore of tin. - -Cephalopod. A marine mollusk with a head surrounded by tentacles. Squids - and octupuses belong to this group, as do fossil forms having - straight or coiled shells divided into numerous interior chambers. - -Chalcopyrite. A reddish-gold colored ore of copper (CuFeS₂). - -Cirque. A deep, steep-walled recess in a mountain, caused by glacial - erosion at the head of a valley. - -Concretion. A nodular or irregular concentration of minerals such as - calcite or limonite, formed by precipitation of the mineral from - groundwater around a nucleus. - -Conglomerate. A rock containing coarse fragments of an older rock, - usually as rounded water-worn stones or pebbles. - -Conodont. One of a group of tiny dark brown tooth-like fossils thought - to be dermal or dental parts of some extinct group of fish. - -Diatreme. A volcanic vent or pipe drilled through rocks by the explosive - energy of gas-charged molten rock, now containing igneous rock and - often altered or unaltered fragments of the surrounding rock. - -Dike. A vertical or nearly vertical sheet of igneous rock which cuts - across the structure of adjacent rocks. - -Diorite. An intrusive igneous rock composed of sodium-rich feldspar and - dark minerals, with only small amounts of quartz. - -Dip. The angle at which a layer of rock is inclined below the - horizontal. - -Dome. A roughly circular upfold in which the rock layers dip outward in - all directions from the center. - -Dowsing. Searching for underground water or ore with a divining rod, - usually a forked stick supposed to locate spots where the desired - substance may be found under the surface. - -Echinoderm. One of a large group of marine invertebrate animals, most of - which have pentagonal symmetry and a skeleton of many calcite - plates. Many forms are spiny. The group includes starfish and sea - urchins. - -Evaporite. Chemical sediments precipitated when water (usually sea - water) evaporates. - -Extrusive rocks. Igneous rocks formed when molten rock material is - ejected onto the surface. Synonymous with volcanic rocks. - -Fault. A break in the rocks in which there has been displacement of the - two sides relative to each other. - -Fault block range. A mountain range bounded on two or more sides by - faults. - -Feldspar. A group of light-colored aluminum silicate minerals that are - major constituents of igneous rocks. They contain potassium, sodium, - and calcium in differing proportions. - -Fold. A bend in rock layers. - -Foraminiferida. One-celled marine animals with microscopic, perforated, - many-chambered calcium carbonate shells, often called forams. - -Fossil. The remains or traces of an animal or plant which has been - preserved in the rock. - -Fusulinid. One-celled marine animals (forams) with shells which look - like a grain of wheat in shape and size, frequently abundant in - Colorado Pennsylvanian rocks. - -Galena. A heavy gray metallic mineral (PbS), often cubic in form, that - is the most important ore of lead. - -Gangue. Nonvaluable minerals occurring in veins with ore minerals. - -Glaciation. Alteration of the earth’s surface by erosion and deposition - by glacier ice. - -Glacier. A body of ice originating on land by recrystallization of snow, - and showing evidence of movement by flowing. - -Gneiss. A coarse-grained metamorphic rock usually banded with streaks of - darker, finer-grained rock. - -Granite. An intrusive igneous rock consisting essentially of sodium or - potassium feldspar and quartz, often speckled with dark-colored - minerals. - -Graptolite. Extinct marine organisms without known close living - relatives, with small black sawblade-like chitinous hard parts - preserved as fossils. - -Hematite. A steel gray or metallic grayish black or reddish gray mineral - (Fe₂O₃) that is an important ore of iron. - -Hogback. A sharp-crested ridge formed by a resistant layer of steeply - dipping rock. - -Huebnerite. A heavy reddish brown mineral (MnWO₄) that is a major ore of - tungsten. - -Igneous rocks. Rocks formed by solidification from a molten state, - either at the surface (extrusive) or below the surface (intrusive). - -Intrusive rocks. Igneous rocks formed when molten rock material - solidifies without reaching the surface. - -Joint. A fracture in the rock, along which no discernible movement has - taken place. - -Kerogen. Solid bituminous material in oil shales. - -Laccolith. A lens-shaped mass of igneous rock intruded into layered - rocks. - -Lava. Fluid or molten rock such as that which issues from a volcano. - -Lode. A rock mass, often a vein, containing valuable minerals. - -Massif. A mountainous mass that has relatively uniform geologic - characteristics and which may embrace a number of peaks. - -Mesa. A flat-topped mountain bounded on at least one side by a steep - cliff. - -Metamorphic rock. Rock formed by alteration of pre-existing rock, - especially by great temperatures and pressures. - -Mollusk. Any one of the large group of invertebrate animals which - includes the snails, clams, octopuses, squids, and their extinct - relatives. - -Molybdenite. A soft bluish gray, metallic mineral (MoS₂) that is a major - ore of molybdenum. - -Monocline. A steplike fold in otherwise horizontal or gently dipping - rock layers. - -Moraine. An accumulation of unsorted rock material built up by the - action of glacier ice. - -Native gold. Gold occurring in nature uncombined with other elements. - -Peneplain. A land surface worn down by erosion to a nearly flat or - broadly undulating plain. - -Petzite. A heavy black or steel gray metallic telluride ore of gold and - silver (Ag₃AuTe₂). - -Placer. A sand or gravel deposit containing particles or nuggets of gold - or other heavy valuable minerals. - -Plateau. An elevated, comparatively flat surface of land, usually larger - than a mesa, sometimes composed of many mesas, and often dissected - by deep stream valleys. - -Porphyry. An igneous rock, usually intrusive, which contains conspicuous - large crystals in a fine-grained matrix. - -Pyrite. A brass-yellow metallic mineral (FeS₂) that is an important - source of sulfur. It is commonly known as fool’s gold. - -Reef. A moundlike limestone structure built in the sea by sedentary - organisms such as corals. - -Rhyolite. A light-colored volcanic rock with quartz and feldspar as the - principal constituents. - -Schist. A metamorphic rock characterized by parallel orientation of - flat-grained minerals like mica. - -Sedimentary rocks. Rocks formed of fragments of other rock transported - by wind or water, or formed by precipitation from solution. - -Sphalerite. An amber-yellow to black mineral (ZnS) that is an important - ore of zinc. - -Stalactite. A cylindrical or conical deposit of calcite hanging from the - roof of a cavern, formed by evaporation of water droplets containing - calcium carbonate. - -Stalagmite. Columns or ridges of calcite rising from the floor of a - cavern, formed by water containing calcium carbonate dripping from a - stalactite. - -Stock. A mass of igneous intrusive rock that covers less than 40 square - miles, has steep sides, and extends to an unknown depth. - -Tennantite. A metallic gray mineral that contains copper, iron, and - arsenic, and is an ore of copper. - -Tetrahedrite. A brittle, dark gray to black, metallic mineral containing - copper, iron, zinc, and silver. - -Trilobite. One of a primitive group of extinct marine crustaceans, - related to crabs and lobsters, having segmented bodies divided by - longitudinal grooves into three lobes. - -Unconformity. A surface separating layers of rock, formed by a period of - nondeposition or erosion. - -Vein. A crack or fissure filled with mineral material, often with - valuable ore minerals. - - - - - SUGGESTED READING - - -There are thousands of scientific articles and books on Colorado -geology, and many new ones appear each year. Following is a selection of -books and booklets which we believe will be most useful and interesting -in extending your knowledge of the state’s geology. - -Donnell, John R., editor, 1960, GEOLOGICAL ROAD LOGS OF COLORADO. Rocky - Mountain Association of Geologists, Denver. Itineraries for a number - of geological trips along Colorado highways and byways. - -Eckel, Edwin B., 1961, MINERALS OF COLORADO, A 100-YEAR RECORD. U. S. - Geological Survey Bulletin 1114. - -Emmons, S. F., Cross, Whitman, and Eldridge, G. H., 1896, GEOLOGY OF THE - DENVER BASIN IN COLORADO. U. S. Geological Survey Monograph 27. The - classic early treatment of the surface geology around Denver, with - many historic illustrations. - -Hansen, Wallace R., 1965, THE BLACK CANYON OF THE GUNNISON TODAY AND - YESTERDAY. U. S. Geological Survey Bulletin 1191. A readable account - of this unusual national monument near Montrose. - -Hansen, Wallace R., 1969, THE GEOLOGIC STORY OF THE UINTA MOUNTAINS. U. - S. Geological Survey Bulletin 1291. The eastern part of this range - is in Colorado. - -Henderson, C. W., 1926, MINING IN COLORADO, A HISTORY OF DISCOVERY, - DEVELOPMENT AND PRODUCTION. U. S. Geological Survey Professional - Paper 138. - -Lee, W. T., 1917, THE GEOLOGIC STORY OF THE ROCKY MOUNTAIN NATIONAL - PARK, COLORADO. U. S. National Park Service Publication. An old - report, not adequately superseded. - -Lovering, T. S., and Goddard, E. N., 1950, GEOLOGY AND ORE DEPOSITS OF - THE FRONT RANGE, COLORADO. U. S. Geological Survey Professional - Paper 223. A comprehensive study of mineral-bearing areas in the - Front Range. - -Lohman, S. W., 1965, THE GEOLOGIC STORY OF COLORADO NATIONAL MONUMENT. - Colorado and Black Canyon Natural History Association, Grand - Junction. - -Pearl, Richard M., 1956, NATURE AS SCULPTOR: A GEOLOGIC INTERPRETATION - OF COLORADO SCENERY. Denver Museum of Natural History Popular Series - No. 6, Revised Edition. - -Pearl, Richard M., 1969, EXPLORING ROCKS, MINERALS, FOSSILS IN COLORADO. - Swallow Press, Revised Edition. - -Pearl, Richard M., 1971, COLORADO GEM TRAILS AND MINERAL GUIDE. Swallow - Press, 3rd Edition. - -Powell, John Wesley, 1876, REPORT ON THE GEOLOGY OF THE EASTERN PORTION - OF THE UINTA MOUNTAINS AND A REGION OF COUNTRY ADJACENT THERETO. U. - S. Geological and Geographical Survey of the Territories. One of the - earliest accounts of geology in Colorado, written by the explorer of - the Colorado River and the father of the U. S. Geological Survey. - -Rabbit, Mary C., and others, 1969, THE COLORADO RIVER AND JOHN WESLEY - POWELL. U. S. Geological Survey Professional Paper 669. A resumé of - part of Powell’s work and a good discussion of the geologic history - of the entire Colorado River, which begins near Grand Lake. - -Richmond, Gerald M., 1965, GLACIATION OF THE ROCKY MOUNTAINS. A part of - THE QUATERNARY OF THE UNITED STATES, Princeton University Press. A - summary of current knowledge of glaciation in Colorado and - surrounding areas. - -Rodeck, Hugo G., editor, 1964, NATURAL HISTORY OF THE BOULDER AREA. - University of Colorado Museum Leaflet No. 13. Contains articles on - geology and biology. - -Untermann, G. E., and Untermann, B. R., 1954, GEOLOGY OF DINOSAUR - NATIONAL MONUMENT AND VICINITY, UTAH—COLORADO. Utah Geological and - Mineralogical Survey Bulletin 42. A detailed study of the eastern - Uinta Mountains. - -Weimer, Robert J., and Haun, John D., editors, 1960, GUIDE TO THE - GEOLOGY OF COLORADO. Geological Society of America, Rocky Mountain - Association of Geologists, and Colorado Scientific Society, Denver. - A concise summary of many aspects of Colorado geology, this guide - includes several geological itineraries and many reference listings. - -Wolle, Muriel Sibell, 1949, STAMPEDE TO TIMBERLINE, Sage Books. An - excellent account of early mining activity in the state, with many - fine drawings of the early settlements. - - - - - INDEX - - - A - Abrams Mountain, 87 - Alamosa, 35 - Alamosa Formation, 67, 105, 106 - Alma, 78 - Ancestral Rocky Mountains, 44, 45 - Animas River, 58, 86 - Ankareh Formation, 52 - Antero Junction, 21 - Anthracite, 97 - Arapahoe Conglomerate, 60 - Arapaho Glacier, 70, 71 - Arkansas Hills, 21 - Arkansas Mountain, 79 - Arkansas River, 3, 22, 35, 90 - Arkansas Valley, 21 - Aspen, _Front._, 1, 22, 35, 50, 74, 77, 78, 88-89 - Aspen Mountain, 23, 88 - Avon, 22 - - - B - Battlement Mesa, 62 - Belden Formation, 44, 109 - Benton Shale, 57 - Berthoud Pass, 12, 92 - Big Thompson Canyon, 12, 71 - Big Thompson River, 69, 103, 104 - Black Canyon of the Gunnison, 36, 37, 44, 71, 105 - Black Hawk, 14, 77, 78, 79, 80 - Blue River, 103 - Book Cliffs, 29 - Boulder, 8, 14, 33, 45, 47, 48, 50, 71, 74, 75, 94, 98, 99, 100, - 101 - Boulder County, 78, 79 - Boulder Creek, 1, 15, 71, 103 - Boulder Creek Granite, 14, 33, 35 - Boulder Reservoir, 103 - Breckenridge, 1, 78, 83-84 - Bross, Mt., 21 - Buena Vista, 22 - Buffalo Peaks, 21 - Building stone, 24, 48, 50, 99-101 - - - C - Cache la Poudre River, 66, 103 - Cambrian, 7, 34, 39 - Camp Bird, 78, 88 - Canon City, 3, 11, 16, 52, 53, 75, 94 - Canon City Embayment, 16 - Carbondale, 97 - Carboniferous, see Mississippian, Pennsylvanian - Cardiff, 97 - Carmel Formation, 52 - Carter Lake, 103 - Castle Creek, 88 - Castle Rock, 8, 61, 112 - Castle Rock Conglomerate, 60 - Cave of the Winds, 106, 107 - Caves, 31, 106-108 - Cenozoic (see also Tertiary, Quaternary), 7, 16, 18, 26, 28, 29, - 59-73, 109 - Central City, 1, 14, 74, 77, 78, 80 - Chaffee Formation, 42 - Cherry Creek, 1 - Cheyenne Mountain, 14, 15, 110 - Cheyenne Sandstone, 105 - Chinle Formation, 51, 52, 93, 101 - Clay, 75, 97-99 - Clear Creek, 1, 71, 80 - Climax, 21, 78, 91-92 - Climax Granite Porphyry, 91, 92 - Coal, 23, 75, 96-97 - Coal Creek, 14, 15 - Coal Creek Quartzite, 33 - Coaldale, 97 - Coalmont, 97 - Cokedale, 97 - Collegiate Range, 22 - Colorado National Monument, 29, 31, 44, 51, 108 - Colorado River, 3, 20, 21, 28, 29, 35, 39, 103, 104, 105, 108, 109 - Colorado Springs, 14, 15, 35, 37, 48, 97, 98, 109 - Columbia, Mt., 22 - Como, 78, 97 - Construction materials, 97-102 - Copper, 74, 75, 80, 81, 83, 89, 91 - Creede, 65, 78, 89-90 - Crested Butte, 24, 97 - Cretaceous, 7, 12, 20, 23, 29, 30, 53, 56-58, 94, 95, 97, 98, 101, - 105, 108, 110 - Cripple Creek, 1, 74, 77, 78, 90-91 - Cross Mountain, 26, 29 - Crystal River, 24, 99 - Culebra Range, 17 - Curecanti, 105 - Curtis Formation, 52 - - - D - Dakota Formation, 12, 51, 53, 56, 94, 97, 105 - Dawson Arkose, 60 - Dawson Butte, 112 - Delcarbon, 97 - Democrat, Mt., 21 - Denver, 3, 8, 14, 33, 35, 37, 45, 47, 48, 52, 53, 60, 74, 83, 94, - 96, 99, 100, 104, 112, 113 - Denver Basin, 8, 75, 94, 95 - Denver Formation, 60, 62 - Devonian, 7, 42-43, 83 - Dillon, 104 - Dinosaur National Monument, 27, 53, 55 - Durango, 25, 28, 35, 47, 58, 78, 85, 102 - Dyer Dolomite, 42 - - - E - Eagle, 47, 101, 106 - Eagle River, 22, 47 - Edwards, 22 - Elbert, Mt., 22 - Eldorado Springs, 109 - Elk Mountains, 24, 69 - Empire, 1, 78, 81 - Englewood, 113 - Entrada Sandstone, 51, 93 - Environmental geology, 111-113 - Eocene, 64 - Estes Lake, 103 - Estes Park, 69, 104 - Evans, Mt., 3, 12 - - - F - Fairplay, 78, 84-85 - Fairy Cave, 107 - Flattop Mountain, 19 - Floods, 112-113 - Florence, 74, 94 - Florissant Fossil Beds National Monument, 65 - Fort Carson, 15 - Fort Collins, 35, 101, 102 - Fountain Formation, 12, 14, 45, 47, 48 - Fox Hills Sandstone, 58 - Fremont Limestone, 40, 41 - Frisco, 21 - Front Range, 11-16, 33, 35, 44, 50, 51, 52, 56, 60, 61, 68, 69, - 70, 71, 93, 99, 101, 103, 107, 109 - Frying Pan River, 105 - Fulford, 107 - Fulford Cave, 106 - - - G - Garden of the Gods, 4, 14, 15, 47, 48 - Garfield, Mt., 30 - Gas, natural, 1, 28, 75, 94-96 - Gems, 75, 102 - Gem Village, 102 - Georgetown, 78, 81 - Gilman, 21 - Gilpin County, 78 - Glen Eyrie Formation, 44 - Glenwood Canyon, 37, 39 - Glenwood Springs, 24, 29, 35, 99, 107, 109 - Gold, 1, 22, 29, 74, 75, 77-91 - Golden, 8, 14, 62, 74, 78, 83, 93, 98, 111 - Gold Hill, 1, 74, 78, 79 - Gore Creek, 47 - Gore Pass, 19, 20 - Gore Range, 20-21, 35, 69 - Gore Range-Eagle’s Nest Wilderness Area, 20, 21 - Granby, 20, 62 - Granby Lake, 103 - Grand Hogback, 28, 29 - Grand Junction, 29, 30, 35, 55 - Grand Lake, 12, 69, 103, 104 - Grand Mesa, 35, 62 - Grand Valley, 25 - Gravel, 75, 97-99 - Great Sand Dunes National Monument, 17, 18, 73 - Green River, 27, 103 - Green River Basin, 4 - Green River Canyon, 31 - Green River Formation, 64, 95-96 - Greenhorn Formation, 101 - Greenhorn Peak, 16 - Groundwater, 76, 105-106 - Gunnison, 35 - Gunnison, Black Canyon of the, 36, 37, 44, 71, 105 - Gunnison River, 35, 36, 37, 105 - Gypsum (mineral), 22, 30, 75, 101-102 - Gypsum (town), 47, 101 - Gypsum Valley, 30, 47, 101 - - - H - Hahn’s Peak, 19 - Harding Sandstone, 40, 41 - Harvard, Mt., 22 - Hayden, 96, 97 - Hayden Pass, 17 - Hermosa Formation, 45, 47 - Hidden Valley, 69 - Horseshoe Amphitheater, 85 - Horseshoe Park, 69 - Horsetooth Reservoir, 103 - Huerfano Basin, 35, 61 - - - I - Ice Age, see Pleistocene - Iceberg Lake, 66 - Idaho Springs, 14, 78, 80 - Idaho Springs Formation, 33, 92 - Independence Pass, 22 - Iron, 1, 17, 74 - Ironton, 78, 88 - - - J - Jewel Lake, 68 - Juniper Mountain, 26, 29 - Juniper Springs, 109 - Jurassic, 7, 9, 23, 36, 52-55, 93 - - - K - Kremmling, 19, 20, 60 - - - L - La Junta, 8, 35 - Lake City, 77, 78 - Lake County, 77 - Lamar, 8 - Landslides, 111-112 - La Plata Mountains, 26 - Laramide Orogeny, 59, 60 - Laramie Formation, 57, 58, 97 - Larkspur, 112 - La Veta Pass, 17, 18, 37 - Lead, 74, 75, 80, 81, 82, 83, 85, 86, 89 - Leadville, 1, 22, 77, 78, 82-83 - Leadville Limestone, 43, 44, 83, 88, 99, 106, 109 - Lime, 75, 101 - Lincoln, Mt., 21 - Lincoln Porphyry, 21 - Lipalian Interval, 7, 36, 39 - Littleton, 113 - Logan County, 95 - Longs Peak, 3, 11, 12, 68 - Loveland, 14, 102 - Loveland Pass, 12 - Lykins Formation, 12, 51, 52, 101 - Lyons, 14, 49, 99, 100, 101 - Lyons Sandstone, 12, 48, 49, 50, 99, 100, 101 - - - M - Magnolia, 79 - Mancos Shale, 30, 95 - Manitou, 39, 106, 107, 110 - Manitou Formation, 40, 41, 110 - Manitou Springs, 109 - Marble, 24, 99 - Marble Mountain, 106 - Maroon Bells, Front., 24, 50 - Maroon Creek, 24 - Maroon Formation, 50 - Mary’s Lake, 103 - Massive, Mt., 82 - McDermott Formation, 58 - Mesa de Maya, 8, 35, 62 - Mesa Verde, 28, 29, 35 - Mesa Verde Formation, 30, 58, 108 - Mesa Verde National Park, 31, 71, 108 - Mesozoic (see also Triassic etc.), 7, 10, 11, 12, 14, 15, 26, 28, - 51-58, 60, 93 - Mestas, Mt., 17 - Middle Park, 4, 16, 35, 61 - Million Dollar Highway, 87, 88 - Mills Lake, 68 - Milner Pass, 66 - Minturn, 39 - Minturn Formation, 45, 46, 47 - Miocene, 66, 67, 87 - Mississippian, 6, 7, 43-44, 83, 106, 109 - Moenkopi Formation, 52 - Molas Formation, 44 - Molas Lake, 25 - Molybdenum, 1, 74, 75, 76, 77, 91-92 - Monarch Pass, 23 - Morgan County, 95 - Morrison, 53, 54, 55 - Morrison Formation, 12, 36, 51, 53, 55, 93 - Mosca Pass, 17, 73 - Mosquito Pass, 85 - Mosquito Range, 21, 22, 35, 39, 69 - Mountain Province, 3, 4, 10-27, 35, 46, 93, 94, 101, 102, 103, - 109, 111 - Mt. Bross, 21 - Mt. Columbia, 22 - Mt. Democrat, 21 - Mt. Elbert, 22 - Mt. Evans, 3, 12 - Mt. Garfield, 30 - Mt. Harvard, 22 - Mt. Lincoln, 21 - Mt. Massive, 82 - Mt. Mestas, 17 - Mt. Princeton, 22 - Mt. Princeton Hot Springs, 22, 110 - Mt. Sneffels, 87, 88 - Mt. Sopris, 24 - Mt. Yale, 22 - Mt. Zirkel, 19 - Music Pass, 17, 73 - - - N - Navajo Sandstone, 52 - Nederland, 78, 79 - Needle Mountains, 26 - Niobrara Formation, 57, 101 - North Park, 4, 16, 35, 58, 61, 94, 97 - - - O - Oil, 1, 29, 30, 75, 76, 94-96 - Oil Creek, 94 - Oil shale, 95-96 - Oligocene, 66, 67 - Ordovician, 7, 40-41, 83, 106 - Orient, 17 - Oro, 82 - Otero County, 105 - Ouray, 4, 25, 26, 34, 42, 78, 87-88 - Ouray Formation, 42 - - - P - Pagoda Mountain, 68 - Pagosa Springs, 102, 109 - Paleozoic (see also Cambrian etc.), 7, 10, 11, 12, 14, 15, 17, 21, - 22, 24, 26, 27, 28, 30, 34, 37, 38-50, 60, 64, 77, 95, 98, - 106, 107 - Paradox Basin, 4, 47, 95, 101 - Paradox Valley, 30, 101 - Park Range, 19-20, 35, 69 - Parting Sandstone, 42 - Pawnee Buttes, 9, 66, 67 - Peak Province, see Mountain Province - Peat, 75 - Pennsylvanian, _Front._, 6, 7, 14, 23, 44-47, 48, 50, 83, 95, 101, - 106, 109 - Permian, _Front._, 7, 23, 48-50, 95, 99, 100, 101 - Petroleum, 1, 29, 30, 75, 76, 94-96 - Phosphoria Formation, 50 - Piceance Basin, 29 - Pierre Formation, 57, 94, 98 - Pikes Peak, 3, 4, 11, 12, 15, 65, 90, 91, 110 - Pikes Peak Granite, 4, 14, 33, 36 - Plains Province, see Prairie Province - Plateau Province, 3, 4, 9, 28-31, 35, 46, 71, 75, 93, 94, 96, 101, - 103, 107, 110 - Platte River, 3 - Pleistocene, 7, 8, 25, 59, 68-73, 105, 106 - Plum Creek, 112 - Poncha Springs, 109 - Prairie Province, 3, 8-10, 12, 35, 66, 72, 75, 94, 96, 103, 105 - Precambrian, 7, 10, 11, 14, 15, 16, 17, 19, 20, 21, 26, 33-37, 40, - 60, 64, 68, 77, 87, 91, 92, 93, 109 - Princeton, Mt., 22 - Pueblo, 23, 97 - Pumice, 75 - Purgatoire Formation, 105 - Pyrites, 75 - - - Q - Quandary Peak, 21 - Quaternary, 7, 8, 25, 59, 68-73, 105, 106 - - - R - Rabbit Ears Pass, 19 - Rabbit Ears Range, 20, 35, 62 - Radium, 93 - Rampart Range, 15, 97, 98 - Rangely, 29, 95 - Raspberry Mountain, 112 - Rattlesnake Reservoir, 103 - Raton Basin, 61 - Raton Pass, 61 - Red Cliff, 39 - Red Mountain, 88 - Red Mountain Creek, 88 - Red Rocks Park, 14, 37, 47 - Redstone, 24 - Rico, 26 - Rico Range, 26 - Rifle, 64 - Rio Grande, 35 - Roan Plateau, 28, 29, 35 - Rocky Mountain National Park, 11, 12, 66, 68, 71, 104 - Roncarbo, 97 - Royal Gorge, 37, 71 - Ruedi, 105 - - - S - St. Mary’s Glacier, 71 - Salida, 17, 21, 35 - Salina, 79 - Sand, 75, 97-99 - Sangre de Cristo Range, 10, 17-18, 35, 47, 61, 69, 73, 90, 105, - 106 - San Juan Basin, 95 - San Juan County, 78 - San Juan Formation, 88 - San Juan Mountains, 4, 25-26, 35, 52, 65, 69, 77, 85, 86, 87, 88, - 105 - San Luis Valley, 4, 35, 44, 61, 67, 73, 96, 106 - San Miguel Range, 26 - Santa Fe Formation, 67, 105 - Sawatch Range, 22-23, 35, 39, 69, 82, 106 - Sawatch Sandstone, 34, 39, 40 - Sedalia, 61 - Shadow Mountain Reservoir, 103 - Sierra Blanca, 17, 18 - Silurian, 7, 42 - Silver, 22, 74, 77-91 - Silver Cliff, 16, 77, 78 - Silver Plume, 78, 81 - Silver Plume Granite, 33, 35 - Silverton, 4, 26, 74, 77, 78, 85-86, 88 - Sinbad Valley, 101 - Sneffels, Mt., 87, 88 - Sopris, Mt., 24 - South Park, 4, 16, 21, 35, 61, 65, 84, 97 - South Platte River, 1, 3, 35, 85, 103, 104, 112, 113 - Spanish Cave, 106 - Spanish Peaks, 10, 18, 62 - Specimen Mountain, 66 - Springs, 17, 22, 109-110 - Steamboat Springs, 97, 109 - Summit County, 77 - Sunshine, 79 - Sunshine Peak Rhyolite, 87 - Swandyke Gneiss, 33 - - - T - Table Mountain, 8, 62, 111 - Telluride, 26, 74, 77, 78 - Tenmile Gorge, 21 - Tenmile Range, 21, 91 - Tertiary, 7, 15, 20, 21, 25, 26, 29, 59-67, 73, 77, 83, 87, 88, - 89, 90, 91, 92, 93, 95, 96, 97 - Tin, 75 - Tincup, 22, 77, 78 - Trail Ridge Road, 12, 66 - Treasure Mountain Granite, 99 - Triassic, 7, 23, 51-52, 93, 101 - Trinidad, 8, 61, 96, 97 - Trout Creek Pass, 21, 22 - Tungsten, 1, 74, 75, 79 - Tyndall Glacier, 71 - - - U - Uinta Basin, 4, 29, 64, 75, 95 - Uinta Mountain Formation, 26 - Uinta Mountains, 4, 10, 26-27, 29, 35, 37, 64 - Uncompahgre Gorge, 87 - Uncompahgre Plateau, 29, 35, 44 - Uncompahgre Quartzite, 87 - Urad Mine, 92 - Uranium, 1, 29, 80, 93 - - - V - Vail, 21, 47 - Vail Pass, 21 - Valmont, 62 - Vanadium, 74, 75, 93 - Villa Grove, 17 - - - W - Walden, 20 - Walsenburg, 35, 61, 96 - Ward, 78 - Water, 76, 103-110 - Waunita Hot Springs, 109 - Weber Sandstone, 95 - West Elk Mountains, 24, 35, 69 - Wet Mountains, 16, 35, 61, 95 - Wet Mountain Valley, 35 - Whiskey Creek Pass, 17 - White River, 35, 95 - White River Formation, 66 - White River Plateau, 28, 29, 35, 43, 107, 109 - Williams Canyon, 37, 39 - Willow Creek Pass, 20 - Willow Creek Reservoir, 103 - Wingate Formation, 51, 52, 101 - Wolcott, 22, 47 - Wolford Mountain, 60 - Woodland Park, 110 - Woods Lake, 107 - - - Y - Yale, Mt., 22 - Yampa River, 3, 27, 31, 35, 97 - Yule Marble, 24, 99 - - - Z - Zinc, 74, 75, 80, 82, 83, 85, 86, 89 - Zirkel, Mt., 19 - - - - - Transcriber’s Notes - - ---Retained publication information from the printed edition: this eBook - is public-domain in the country of publication. - ---In the text versions only, text in italics is delimited by - _underscores_. - ---In the ASCII version only, subscripted numbers are preceded by - 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