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diff --git a/old/3772-h/files/ch11.html b/old/3772-h/files/ch11.html new file mode 100644 index 0000000..87b630e --- /dev/null +++ b/old/3772-h/files/ch11.html @@ -0,0 +1,407 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Lyell/The Student's Elements of Geology --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Student's Elements of Geology: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv= +"Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel=stylesheet href="geology.css" type="text/css"> +</head> +<body> +<p><b>The Student’s Elements of Geology</b></p> + +<hr> +<p class="page"><a name="page 166">[ 166 ]</a></p> + +<p> </p> + +<center><b>Chapter XI</b><br> +<br> +POST-PLIOCENE PERIOD, continued—GLACIAL CONDITIONS.*</center> + +<p class="intro">Geographical Distribution, Form, and Characters of +Glacial Drift. — Fundamental Rocks, polished, grooved, and +scratched. — Abrading and striating Action of Glaciers. +— Moraines, Erratic Blocks, and “Roches Moutonnees.” Alpine +Blocks on the Jura. — Continental Ice of Greenland. — +Ancient Centres of the Dispersion of Erratics. — +Transportation of Drift by floating Icebergs. — Bed of the +Sea furrowed and polished by the running aground of floating +Ice-islands.</p> + +<p><b>Character and Distribution of Glacial +Drift.</b>—In speaking of the loose transported matter +commonly found on the surface of the land in all parts of the +globe, I alluded to the exceptional character of what has been +called the boulder formation in the temperate and Arctic latitudes +of the northern hemisphere. The peculiarity of its form in Europe +north of the 50th, and in North America north of the 40th parallel +of latitude, is now universally attributed to the action of ice, +and the difference of opinion respecting it is now chiefly +restricted to the question whether land-ice or floating icebergs +have played the chief part in its distribution. It is wanting in +the warmer and equatorial regions, and reappears when we examine +the lands which lie south of the 40th and 50th parallels in the +southern hemisphere, as, for example, in Patagonia, Tierra del +Fuego, and New Zealand. It consists of sand and clay, sometimes +stratified, but often wholly devoid of stratification for a depth +of 50, 100, or even a greater number of feet. To this unstratified +form of the deposit the name of <i>till</i> has long been applied +in Scotland. It generally contains a mixture of angular and rounded +fragments of rock, some of large size, having occasionally one or +more of their sides flattened and smoothed, or even highly +polished. The smoothed surfaces usually exhibit many scratches +parallel to each other, one set of which often crosses an older +set. The till is almost everywhere wholly devoid of organic +remains, except those washed into it from older formations, though +in some places it contains marine shells, usually of northern +or</p> + +<p class="fnote">* As to the former excess of cold, whether brought +about by modifications in the height and distribution of the land +or by altered astronomical conditions, see Principles, vol. i, +(10th ed., 1867), chaps. xii and xiii, “Vicissitudes of +Climate.”</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 167">[ 167 ]</a></p> + +<p>Arctic species, and frequently in a fragmentary state. The bulk +of the till has usually been derived from the grinding down into +mud of rocks in the immediate neighbourhood, so that it is red in a +region of Red Sandstone, as in Strathmore in Forfarshire; grey or +black in a district of coal and bituminous shale, as around +Edinburgh; and white in a chalk country, as in parts of Norfolk and +Denmark. The stony fragments dispersed irregularly through the till +usually belong, especially in mountainous countries, to rocks found +in some part of the same hydrographical basin; but there are +regions where the whole of the boulder clay has come from a +distance, and huge blocks, or “erratics,” as they have been called, +many feet in diameter, have not unfrequently travelled hundreds of +miles from their point of departure, or from the parent rocks from +which they have evidently been detached. These are commonly +angular, and have often one or more of their sides polished and +furrowed.</p> + +<p>The rock on which the boulder formation reposes, if it consists +of granite, gneiss, marble, or other hard stone, capable of +permanently retaining any superficial markings which may have been +imprinted upon it, is usually smoothed or polished, like the +erratics above described, and exhibits parallel striæ and +furrows having a determinate direction. This direction, both in +Europe and North America, agrees generally in a marked manner with +the course taken by the erratic blocks in the same district. The +boulder clay, when it was first studied, seemed in many of its +characters so singular and anomalous, that geologists despaired of +ever being able to interpret the phenomena by reference to causes +now in action. In those exceptional cases where marine shells of +the same date as the boulder clay were found, nearly all of them +were recognised as living species—a fact conspiring with the +superficial position of the drift to indicate a comparatively +modern origin.</p> + +<p>The term “diluvium” was for a time the most popular name of the +boulder formation, because it was referred by many to the deluge of +Noah, while others retained the name as expressive of their opinion +that a series of diluvial waves raised by hurricanes and storms, or +by earthquakes, or by the sudden upheaval of land from the bed of +the sea, had swept over the continents, carrying with them vast +masses of mud and heavy stones, and forcing these stones over rocky +surfaces so as to polish and imprint upon them long furrows and +striæ. But geologists were not long in seeing that the +boulder formation was characteristic of high latitudes, and that on +the whole the size and number of erratic blocks increases</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 168">[ 168 ]</a></p> + +<p>as we travel towards the Arctic regions. They could not fail to +be struck with the contrast which the countries bordering the +Baltic presented when compared with those surrounding the +Mediterranean. The multitude of travelled blocks and striated rocks +in the one region, and the absence of such appearances in the +other, were too obvious to be overlooked. Even the great +development of the boulder formation, with large erratics so far +south as the Alps, offered an exception to the general rule +favourable to the hypothesis that there was some intimate +connection between it and accumulations of snow and ice.</p> + +<center><img src="../images1/fig106.jpg" width="405" height="370" alt= +"Fig. 106: Limestone, polished, furrowed, and scratched by the glacier of Rosenlau in Switzerland."> +</center> + +<p><b>Transporting and abrading Power of +Glaciers.</b>—I have described elsewhere (“Principles” +vol. i, chap. xvi, 1867) the manner in which the snow of the Alpine +heights is prevented from accumulating indefinitely in thickness by +the constant descent of a large portion of it by gravitation. +Becoming converted into ice it forms what are termed glaciers, +which glide down the principal valleys. On their surface are seen +mounds of rubbish or large heaps of sand and mud, with angular +fragments of rock which fall from the steep slopes or precipices +bounding the glaciers. When a glacier,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 169">[ 169 ]</a></p> + +<p>thus laden, descends so far as to reach a region about 3500 feet +above the level of the sea, the warmth of the air is such that it +melts rapidly in summer, and all the mud, sand, and pieces of rock +are slowly deposited at its lower end, forming a confused heap of +unstratified rubbish called a <i>moraine</i>, and resembling the +<i>till</i> before described (p. 166).</p> + +<p>Besides the blocks thus carried down on the top of the glacier, +many fall through fissures in the ice to the bottom, where some of +them become firmly frozen into the mass, and are pushed along the +base of the glacier, abrading, polishing, and grooving the rocky +floor below, as a diamond cuts glass, or as emery-powder polishes +steel. The striæ which are made, and the deep grooves which +are scooped out by this action, are rectilinear and parallel to an +extent never seen in those produced on loose stones or rocks, where +shingle is hurried along by a torrent, or by the waves on a +sea-beach. In addition to these polished, striated, and grooved +surfaces of rock, another mark of the former action of a glacier is +the “roche moutonnee.” Projecting eminences of rock so called have +been smoothed and worn into the shape of flattened domes by the +glacier as it passed over them. They have been traced in the Alps +to great heights above the present glaciers, and to great +horizontal distances beyond them.</p> + +<p><b>Alpine Blocks on the +Jura.</b>—The moraines, erratics, polished surfaces, +domes, and striæ, above described, are observed in the great +valley of Switzerland, fifty miles broad; and almost everywhere on +the Jura, a chain which lies to the north of this valley. The +average height of the Jura is about one-third that of the Alps, and +it is now entirely destitute of glaciers; yet it presents almost +everywhere similar moraines, and the same polished and grooved +surfaces. The erratics, moreover, which cover it, present a +phenomenon which has astonished and perplexed the geologist for +more than half a century. No conclusion can be more incontestable +than that these angular blocks of granite, gneiss, and other +crystalline formations came from the Alps, and that they have been +brought for a distance of fifty miles and upward across one of the +widest and deepest valleys in the world; so that they are now +lodged on a chain composed of limestone and other formations, +altogether distinct from those of the Alps. Their great size and +angularity, after a journey of so many leagues, has justly excited +wonder; for hundreds of them are as large as cottages; and one in +particular, composed of gneiss, celebrated under the name of Pierre +à Bot, rests on the side of a hill about 900 feet above the +lake of Neufchâtel, and is no less than 40 feet in +diameter.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 170">[ 170 ]</a></p> + +<p>In the year 1821, M. Venetz first announced his opinion that the +Alpine glaciers must formerly have extended far beyond their +present limits, and the proofs appealed to by him in confirmation +of this doctrine were acknowledged by all subsequent observers, and +greatly strengthened by new observations and arguments. M. +Charpentier supposed that when the glaciers extended continuously +from the Alps to the Jura, the former mountains were 2000 or 3000 +feet higher than at present. Other writers, on the contrary, +conjectured that the whole country had been submerged, and the +moraines and erratic blocks transported on floating icebergs; but a +careful study of the distribution of the travelled masses, and the +total absence of marine shells from the old glacial drift of +Switzerland, have entirely disproved this last hypothesis. In +addition to the many evidences of the action of ice in the northern +parts of Europe which we have already mentioned, there occur here +and there in some of these countries, what are wanting in +Switzerland, deposits of marine fossil shells, which exhibit so +arctic a character that they must have led the geologist to infer +the former prevalence of a much colder climate, even had he not +encountered so many accompanying signs of ice-action. The same +marine shells demonstrate the submergence of large areas in +Scandinavia and the British Isles, during the glacial cold.</p> + +<p>A characteristic feature of the deposits under consideration in +all these countries is the occurrence of large erratic blocks, and +sometimes of moraine matter, in situations remote from lofty +mountains, and separated from the nearest points where the parent +rocks appear at the surface by great intervening valleys, or arms +of the sea. We also often observe striæ and furrows, as in +Norway, Sweden, and Scotland, which deviate from the direction +which they ought to follow if they had been connected with the +present line of drainage, and they, therefore, imply the prevalence +of a very distinct condition of things at the time when the cold +was most intense. The actual state of North Greenland seems to +afford the best explanation of such abnormal glacial markings.</p> + +<p><b>Greenland Continental +Ice.</b>—Greenland is a vast unexplored continent, +buried under one continuous and colossal mass of ice that is always +moving seaward, a very small part of it in an easterly direction, +and all the rest westward, or towards Baffin’s Bay. All the minor +ridges and valleys are levelled and concealed under a general +covering of snow, but here and there some steep mountains protrude +abruptly</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 171">[ 171 ]</a></p> + +<p>from the icy slope, and a few superficial lines of stones or +moraines are visible at certain seasons, when no snow has fallen +for many months, and when evaporation, promoted by the wind and +sun, has caused much of the upper snow to disappear. The height of +this continent is unknown, but it must be very great, as the most +elevated lands of the outskirts, which are described as +comparatively low, attain altitudes of 4000 to 6000 feet. The icy +slope gradually lowers itself towards the outskirts, and then +terminates abruptly in a mass about 2000 feet in thickness, the +great discharge of ice taking place through certain large friths, +which, at their upper ends, are usually about four miles across. +Down these friths the ice is protruded in huge masses, several +miles wide, which continue their course—grating along the +rocky bottom like ordinary glaciers long after they have reached +the salt water. When at last they arrive at parts of Baffin’s Bay +deep enough to buoy up icebergs from 1000 to 1500 feet in vertical +thickness, broken masses of them float off, carrying with them on +their surface not only fine mud and sand but large stones. These +fragments of rock are often polished and scored on one or more +sides, and as the ice melts, they drop down to the bottom of the +sea, where large quantities of mud are deposited, and this muddy +bottom is inhabited by many mollusca.</p> + +<p>Although the direction of the ice-streams in Greenland may +coincide in the main with that which separate glaciers would take +if there were no more ice than there is now in the Swiss Alps, yet +the striation of the surface of the rocks on an ice-clad continent +would, on the whole, vary considerably in its minor details from +that which would be imprinted on rocks constituting a region of +separate glaciers. For where there is a universal covering of ice +there will be a general outward movement from the higher and more +central regions towards the circumference and lower country, and +this movement will be, to a certain extent, independent of the +minor inequalities of hill and valley, when these are all reduced +to one level by the snow. The moving ice may sometimes cross even +at right angles deep narrow ravines, or the crests of buried +ridges, on which last it may afterwards seem strange to detect +glacial striæ and polishing after the liquefaction of the +snow and ice has taken place.</p> + +<p>Rink mentions that in North Greenland powerful springs of clayey +water escape in winter from under the ice, where it descends to +“the outskirts,” and where, as already stated, it is often 2000 +feet thick—a fact showing how much grinding action is going +on upon the surface of the subjacent</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 172">[ 172 ]</a></p> + +<p>rocks. I also learn from Dr. Torell that there are large areas +in the outskirts, now no longer covered with permanent snow or +glaciers, which exhibit on their surface unmistakable signs of +ancient ice-action, so that, vast as is the power now exerted by +ice in Greenland, it must once have operated on a still grander +scale. The land, though now very elevated, may perhaps have been +formerly much higher. It is well-known that the south coast of +Greenland, from latitude 60° to about 70° N., has for the +last four centuries been sinking at the rate of several feet in a +century. By this means a surface of rock, well scored and polished +by ice, is now slowly subsiding beneath the sea, and is becoming +strewed over, as the icebergs melt, with impalpable mud and +smoothed and scratched stones. It is not precisely known how far +north this downward movement extends.</p> + +<p><b>Drift carried by +Icebergs.</b>—An account was given so long ago as the +year 1822, by Scoresby, of icebergs seen by him in the Arctic seas +drifting along in latitudes 69° and 70° N., which rose +above the surface from 100 to 200 feet, and some of which measured +a mile in circumference. Many of them were loaded with beds of +earth and rock, of such thickness that the weight was conjectured +to be from 50,000 to 100,000 tons. A similar transportation of +rocks is known to be in progress in the southern hemisphere, where +boulders included in ice are far more frequent than in the north. +One of these icebergs was encountered in 1839, in mid-ocean, in the +antarctic regions, many hundred miles from any known land, sailing +northward, with a large erratic block firmly frozen into it. Many +of them, carefully measured by the officers of the French exploring +expedition of the Astrolabe, were between 100 and 225 feet high +above water, and from two to five miles in length. Captain +d’Urville ascertained one of them which he saw floating in the +Southern Ocean to be 13 miles long and 100 feet high, with walls +perfectly vertical. The submerged portions of such islands must, +according to the weight of ice relatively to sea-water, be from six +to eight times more considerable than the part which is visible, so +that when they are once fairly set in motion, the mechanical force +which they might exert against any obstacle standing in their way +would be prodigious.</p> + +<p>We learn, therefore, from a study both of the arctic and +antarctic regions, that a great extent of land may be entirely +covered throughout the whole year by snow and ice, from the summits +of the loftiest mountains to the sea-coast, and may yet send down +angular erratics to the ocean. We may also conclude that such land +will become in the course of</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 173">[ 173 ]</a></p> + +<p>ages almost everywhere scored and polished like the rocks which +underlie a glacier. The discharge of ice into the surrounding sea +will take place principally through the main valleys, although +these are hidden from our sight. Erratic blocks and moraine matter +will be dispersed somewhat irregularly after reaching the sea, for +not only will prevailing winds and marine currents govern the +distribution of the drift, but the shape of the submerged area will +have its influence; inasmuch as floating ice, laden with stones, +will pass freely through deep water, while it will run a ground +where there are reefs and shallows. Some icebergs in Baffin’s Bay +have been seen stranded on a bottom 1000 or even 1500 feet deep. In +the course of ages such a sea-bed may become densely covered with +transported matter, from which some of the adjoining greater depths +may be free. If, as in West Greenland, the land is slowly sinking, +a large extent of the bottom of the ocean will consist of rock +polished and striated by land-ice, and then overspread by mud and +boulders detached from melting bergs.</p> + +<p>The mud, sand, and boulders thus let fall in still water must be +exactly like the moraines of terrestrial glaciers, devoid of +stratification and organic remains. But occasionally, on the outer +side of such packs of stranded bergs, the waves and currents may +cause the detached earthy and stony materials to be sorted +according to size and weight before they reach the bottom, and to +acquire a stratified arrangement.</p> + +<p>I have already alluded (p. 172) to the large quantity of ice, +containing great blocks of stone, which is sometimes seen floating +far from land, in the southern or Antarctic seas. After the +emergence, therefore, of such a submarine area, the superficial +detritus will have no necessary relation to the hills, valleys, and +river-plains over which it will be scattered. Many a water-shed may +intervene between the starting-point of each erratic or pebble and +its final resting-place, and the only means of discovering the +country from which it took its departure will consist in a careful +comparison of its mineral or fossil contents with those of the +parent rocks.</p> + +<br> + + +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch10.html"> +Chapter X</a> / <a href="ch12.html">Chapter XII</a></small> +</body> +</html> + |