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| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:22:20 -0700 |
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diff --git a/old/3772-h/files/ch12.html b/old/3772-h/files/ch12.html new file mode 100644 index 0000000..74d029e --- /dev/null +++ b/old/3772-h/files/ch12.html @@ -0,0 +1,740 @@ +<!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 174">[ 174 ]</a></p> + +<p> </p> + +<center><b>Chapter XII</b><br> +<br> +POST-PLIOCENE PERIOD, continued.—GLACIAL CONDITIONS, +concluded.</center> + +<p class="intro">Glaciation of Scandinavia and Russia. — +Glaciation of Scotland. — Mammoth in Scotch Till. — +Marine Shells in Scotch Glacial Drift. — Their Arctic +Character. — Rarity of Organic Remains in Glacial Deposits. +— Contorted Strata in Drift. — Glaciation of Wales, +England, and Ireland. — Marine Shells of Moel Tryfaen. +— Erratics near Chichester. — Glacial Formations of +North America. — Many Species of Testacea and Quadrupeds +survived the Glacial Cold. — Connection of the Predominance +of Lakes with Glacial Action. — Action of Ice in preventing +the silting up of Lake-basins. — Absence of Lakes in the +Caucasus. — Equatorial Lakes of Africa.</p> + +<p><b>Glaciation of Scandinavia and Russia.</b>—In large +tracts of Norway and Sweden, where there have been no glaciers in +historical times, the signs of ice-action have been traced as high +as 6000 feet above the level of the sea. These signs consist +chiefly of polished and furrowed rock-surfaces, of moraines and +erratic blocks. The direction of the erratics, like that of the +furrows, has usually been conformable to the course of the +principal valleys; but the lines of both sometimes radiate outward +in all directions from the highest land, in a manner which is only +explicable by the hypothesis above alluded to of a general envelope +of continental ice, like that of Greenland (<a href="ch11.html#page 170">page 170</a>). Some of +the far-transported blocks have been carried from the central parts +of Scandinavia towards the Polar regions; others southward to +Denmark; some south-westward, to the coast of Norfolk in England; +others south-eastward, to Germany, Poland, and Russia.</p> + +<p>In the immediate neighbourhood of Upsala, in Sweden, I had +observed, in 1834, a ridge of stratified sand and gravel, in the +midst of which occurs a layer of marl, evidently formed originally +at the bottom of the Baltic, by the slow growth of the mussel, +cockle, and other marine shells of living species, intermixed with +some proper to fresh water. The marine shells are all of dwarfish +size, like those now inhabiting the brackish waters of the Baltic; +and the marl, in which many of them are imbedded, is now raised +more than 100 feet above the level of the Gulf of Bothnia. Upon the +top of this ridge repose several huge erratics, consisting of +gneiss for the most part unrounded, from nine to sixteen feet in +diameter,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 175">[ 175 ]</a></p> + +<p>and which must have been brought into their present position +since the time when the neighbouring gulf was already characterised +by its peculiar fauna. Here, therefore, we have proof that the +transport of erratics continued to take place, not merely when the +sea was inhabited by the existing testacea, but when the north of +Europe had already assumed that remarkable feature of its physical +geography which separates the Baltic from the North Sea, and causes +the Gulf of Bothnia to have only one-fourth of the saltness +belonging to the ocean. In Denmark, also, recent shells have been +found in stratified beds, closely associated with the boulder +clay.</p> + +<p><b>Glaciation of Scotland.</b>—Mr. T. F. Jamieson, in +1858, adduced a great body of facts to prove that the Grampians +once sent down glaciers from the central regions in all directions +towards the sea. “The glacial grooves,” he observed, “radiate +outward from the central heights towards all points of the compass, +though they do not always strictly conform to the actual shape and +contour of the minor valleys and ridges.”</p> + +<p>These facts and other characteristics of the Scotch drift lead +us to the inference that when the glacial cold first set in, +Scotland stood higher above the sea than at present, and was +covered for the most part with snow and ice, as Greenland is now. +This sheet of land-ice sliding down to lower levels, ground down +and polished the subjacent rocks, sweeping off nearly all +superficial deposits of older date, and leaving only till and +boulders in their place. To this continental state succeeded a +period of depression and partial submergence. The sea advanced over +the lower lands, and Scotland was converted into an archipelago, +some marine sand with shells being spread over the bottom of the +sea. On this sand a great mass of boulder clay usually quite devoid +of fossils was accumulated. Lastly, the land re-emerged from the +water, and, reaching a level somewhat above its present height, +became connected with the continent of Europe, glaciers being +formed once more in the higher regions, though the ice probably +never regained its former extension.* After all these changes, +there were some minor oscillations in the level of the land, on +which, although they have had important geographical consequences, +separating Ireland from England, for example, and England from the +Continent, we need not here enlarge.</p> + +<p><i>Mammoth in Scotch Till.</i>—Almost all remains of the +terrestrial fauna of the Continent which preceded the period of</p> + +<p class="fnote">* Jamieson, Quart. Geol. Journ., 1860, vol. xvi, +p. 370.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 176">[ 176 ]</a></p> + +<p>submergence have been lost; but a few patches of estuarine and +fresh-water formations escaped denudation by submergence. To these +belong the peaty clay from which several mammoths’ tusks and horns +of reindeer were obtained at Kilmaurs, in Ayrshire as long ago as +1816. Mr. Bryce in 1865 ascertained that the fresh-water formation +containing these fossils rests on carboniferous sandstone, and is +covered, first by a bed of marine sand with arctic shells, and then +with a great mass of till with glaciated boulders.* Still more +recent explorations in the neighbourhood of Kilmaurs have shown +that the fresh-water formation contains the seed of the pond-weed +<i>Potamogeton</i> and the aquatic Ranunculus; and Mr. Young of the +Glasgow Museum washed the mud adhering to the reindeer horns of +Kilmaurs and that which filled the cracks of the associated +elephants’ tusks, and detected in these fossils (which had been in +the Glasgow Museum for half a century) abundance of the same +seeds.</p> + +<p>All doubts, therefore, as to the true position of the remains of +the mammoth, a fossil so rare in Scotland, have been set at rest, +and it serves to prove that part of the ancient continent sank +beneath the sea at a period of great cold, as the shells of the +overlying sand attest. The incumbent till or boulder clay is about +40 feet thick, but it often attains much greater thickness in the +same part of Scotland.</p> + +<center><img src="../images1/fig107.jpg" width="395" height="307" alt= +"Figs. 107-112: Northern shells common in the drift of the Clyde, in Scotland."> +</center> + +<p><i>Marine Shells of Scotch Drift.</i>—The greatest height +to which marine shells have yet been traced in this boulder</p> + +<p class="fnote">* Bryce, Quart. Geol. Journ., vol. xxi, p. 217, +1865.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 177">[ 177 ]</a></p> + +<p>clay is at Airdie, in Lanarkshire, ten miles east of Glasgow, +524 feet above the level of the sea. At that spot they were found +imbedded in stratified clays with till above and below them. There +appears no doubt that the overlying deposit was true glacial till, +as some boulders of granite were observed in it, which must have +come from distances of sixty miles at the least.</p> + +<center><img src="../images1/fig113.jpg" width="410" height="155" alt= +"Fig. 113: Leda truncata; Fig. 114: Tellina calcarea, Chem."> +</center> + +<p>The shells figured in Figs. 107 to 112 are only a few out of a +large assemblage of living species, which, taken as a whole, bear +testimony to conditions far more arctic than those now prevailing +in the Scottish seas. But a group of marine shells, indicating a +still greater excess of cold, has been brought to light since 1860 +by the Reverend Thomas Brown, from glacial drift or clay on the +borders of the estuaries of the Forth and Tay. This clay occurs at +Elie, in Fife, and at Errol, in Perthshire; and has already +afforded about 35 shells, all of living species, and now +inhabitants of arctic regions, such as <i>Leda truncata, Tellina +proxima</i> (see Figs. 113 and 114), <i>Pecten Grœnlandicus, +Crenella lævigata, Crenella nigra,</i> and others, some of +them first brought by Captain Sir E. Parry from the coast of +Melville Island, latitude 76° N. These were all identified in +1863 by Dr. Torell, who had just returned from a survey of the seas +around Spitzbergen, where he had collected no less than 150 species +of mollusca, living chiefly on a bottom of fine mud derived from +the moraines of melting glaciers which there protrude into the sea. +He informed me that the fossil fauna of this Scotch glacial deposit +exhibits not only the species but also the peculiar varieties of +mollusca now characteristic of very high latitudes. Their large +size implies that they formerly enjoyed a colder, or, what was to +them a more genial climate, than that now prevailing in the +latitude where the fossils occur. Marine shells have also been +found in the glacial drift of Caithness and Aberdeenshire at +heights of 250 feet, and in Banff of 350 feet, and stratified drift +continuous with the above ascends to heights of 500 feet. Already +75 species are enumerated</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 178">[ 178 ]</a></p> + +<p>from Caithness, and the same number from Aberdeenshire and +Banff, and in both cases all but six are arctic species.</p> + +<p>I formerly suggested that the absence of all signs of organic +life in the Scotch drift might be connected with the severity of +the cold, and also in some places with the depth of the sea during +the period of extreme submergence; but my faith in such an +hypothesis has been shaken by modern investigations, an exuberance +of life having been observed both in arctic and antarctic seas of +great depth, and where floating ice abounds. The difficulty, +moreover, of accounting for the entire dearth of marine shells in +till is removed when once we have adopted the theory of this +boulder clay being the product of land-ice. For glaciers coming +down from a continental ice-sheet like that which covers Greenland +may fill friths many hundred feet below the sea-level, and even +invade parts of a bay a thousand feet deep, before they find water +enough to float off their terminal portions in the form of +icebergs. In such a case till without marine shells may first +accumulate, and then, if the climate becomes warmer and the ice +melts, a marine deposit may be superimposed on the till without any +change of level being required.</p> + +<p>Another curious phenomenon bearing on this subject was styled by +the late Hugh Miller the “striated pavements” of the boulder clay. +Where portions of the till have been removed by the sea on the +shores of the Forth, or in the interior by railway cuttings, the +boulders imbedded in what remains of the drift are seen to have +been all subjected to a process of abrasion and striation, the +striæ and furrows being parallel and persistent across them +all, exactly as if a glacier or iceberg had passed over them and +scored them in a manner similar to that so often undergone by the +solid rocks below the glacial drift. It is possible, as Mr. Geikie +conjectures, that this second striation of the boulders may be +referable to floating ice.*</p> + +<p><i>Contorted Strata in Drift.</i>—In Scotland the till is +often covered with stratified gravel, sand, and clay, the beds of +which are sometimes horizontal and sometimes contorted for a +thickness of several feet. Such contortions are not uncommon in +Forfarshire, where I observed them, among other places, in a +vertical cutting made in 1840 near the left bank of the South Esk, +east of the bridge of Cortachie. The convolutions of the beds of +fine and coarse sand, gravel, and loam, extend through a thickness +of no less than 25 feet vertical, or from <i>b</i> to <i>c</i>, +Fig. 115, the horizontal stratification being resumed very abruptly +at a short distance, as to the right</p> + +<p class="fnote">* Geikie, Trans. Geol. Soc. Glasgow, vol. i, part +ii, p. 68, 1863.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 179">[ 179 ]</a></p> + +<center><img src="../images1/fig115.jpg" width="397" height="216" alt= +"Fig. 15: Section of contorted drift overlying till, seen on left bank of South Esk, near Cortachie, in 1840."> +</center> + +<p>of <i>f</i>, <i>g.</i> The overlying coarse gravel and sand, <i> +a</i>, is in some places horizontal, in others it exhibits cross +bedding, and does not partake of the disturbances which the strata +<i>b</i>, <i>c</i>, have undergone. The underlying till is exposed +for a depth of about 20 feet; and we may infer from sections in the +neighbourhood that it is considerably thicker.</p> + +<p>In some cases I have seen fragments of stratified clays and +sands, bent in like manner, in the middle of a great mass of till. +Mr. Trimmer has suggested, in explanation of such phenomena, the +intercalation in the glacial period of large irregular masses of +snow or ice between layers of sand and gravel. Some of the cliffs +near Behring’s Straits, in which the remains of elephants occur, +consist of ice mixed with mud and stones; and Middendorf describes +the occurrence in Siberia of masses of ice, found at various depths +from the surface after digging through drift. Whenever the +intercalation of snow and ice with drift, whether stratified or +unstratified, has taken place, the melting of the ice will cause +such a failure of support as may give rise to flexures, and +sometimes to the most complicated foldings. But in many cases the +strata may have been bent and deranged by the mechanical pressure +of an advancing glacier, or by the sideway thrust of huge islands +of ice running aground against sandbanks; in which case, the +position of the beds forming the foundation of the banks may not be +at all disturbed by the shock.</p> + +<p>There are indeed many signs in Scotland of the action of +floating ice, as might have been expected where proofs of +submergence in the Glacial Period are not wanting. Among these are +the occurrence of large erratic blocks, frequently in clusters at +or near the tops of hills or ridges, places which may have formed +islets or shallows in the sea where floating ice would mostly +ground and discharge its cargo on</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 180">[ 180 ]</a></p> + +<p>melting. Glaciers or land-ice would, on the contrary, chiefly +discharge their cargoes at the bottom of valleys. Traces of an +earlier and independent glaciation have also been observed in some +regions where the striation, apparently produced by ice proceeding +from the north-west, is not explicable by the radiation of land-ice +from a central mountainous region.*</p> + +<p><b>Glaciation of Wales and England.</b>—The mountains of +North Wales were recognised, in 1842, by Dr. Buckland, as having +been an independent centre of the dispersion of +erratics—great glaciers, long since extinct, having radiated +from the Snowdonian heights in Carnarvonshire, through seven +principal valleys towards as many points of the compass, carrying +with them large stony fragments, and grooving the subjacent rocks +in as many directions.</p> + +<p>Besides this evidence of land-glaciers, Mr. Trimmer had +previously, in 1831, detected the signs of a great submergence in +Wales in the Post-pliocene period. He had observed stratified +drift, from which he obtained about a dozen species of marine +shells, near the summit of Moel Tryfaen, a hill 1400 feet high, on +the south side of the Menai Straits. I had an opportunity of +examining in the summer of 1863, together with the Reverend W. S. +Symonds, a long and deep cutting made through this drift by the +Alexandra Mining Company in search of slates. At the top of the +hill above-mentioned we saw a stratified mass of incoherent sand +and gravel 35 feet thick, from which no less than 54 species of +mollusca, besides three characteristic arctic varieties—in +all 57 forms—have been obtained by Mr. Darbishire. They +belong without exception to species still living in British or more +northern seas; eleven of them being exclusively arctic, four common +to the arctic and British seas, and a large proportion of the +remainder having a northward range, or, if found at all in the +southern seas of Britain, being comparatively less abundant. In the +lowest beds of the drift were large heavy boulders of +far-transported rocks, glacially polished and scratched on more +than one side. Underneath the whole we saw the edges of vertical +slates exposed to view, which here, like the rocks in other parts +of Wales, both at greater and less elevations, exhibit beneath the +drift unequivocal marks of prolonged glaciation. The whole deposit +has much the appearance of an accumulation in shallow water or on a +beach, and it probably acquired its thickness during the gradual +subsidence of the coast—an hypothesis which would require us +to ascribe to it a high antiquity,</p> + +<p class="fnote">* Milne Home, Trans. Royal Soc. Edinburgh, vol. +xxv, 1868-9.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 181">[ 181 ]</a></p> + +<p>since we must allow time, first for its sinking, and then for +its re-elevation.</p> + +<p>The height reached by these fossil shells on Moel Tryfaen is no +less than 1300 feet—a most important fact when we consider +how very few instances we have on record beyond the limits of +Wales, whether in Europe or North America, of marine shells having +been found in glacial drift at half the height above indicated. A +marine molluscous fauna, however, agreeing in character with that +of Moel Tryfaen, and comprising as many species, has been found in +drift at Macclesfield and other places in central England, +sometimes reaching an elevation of 1200 feet.</p> + +<p>Professor Ramsay* estimated the probable amount of submergence +during some part of the glacial period at about 2300 feet; for he +was unable to distinguish the superficial sands and gravel which +reached that high elevation from the drift which, at Moel Tryfaen +and at lower points, contains shells of living species. The +evidence of the marine origin of the highest drift is no doubt +inconclusive in the absence of shells, so great is the resemblance +of the gravel and sand of a sea beach and of a river’s bed, when +organic remains are wanting; but, on the other hand, when we +consider the general rarity of shells in drift which we know to be +of marine origin, we can not suppose that, in the shelly sands of +Moel Tryfaen, we have hit upon the exact uppermost limit of marine +deposition, or, in other words, a precise measure of the +submergence of the land beneath the sea since the glacial +period.</p> + +<p>We are gradually obtaining proofs of the larger part of England, +north of a line drawn from the mouth of the Thames to the Bristol +Channel, having been under the sea and traversed by floating ice +since the commencement of the glacial epoch. Among recent +observations illustrative of this point, I may allude to the +discovery, by Mr. J. F. Bateman, near Blackpool, in Lancashire, +fifty miles from the sea, and at the height of 568 feet above its +level, of till containing rounded and angular stones and marine +shells, such as <i>Turritella communis, Purpura lapillus, Cardium +edule,</i> and others, among which <i>Trophon clathratum</i> +(=<i>Fusus Bamffius</i>), though still surviving in North British +seas, indicates a cold climate.</p> + +<p><i>Erratics near Chichester.</i>—The most southern +memorials of ice-action and of a Post-pliocene fauna in Great +Britain is on the coast of the county of Sussex, about 25 miles +west of Brighton, and 15 south of Chichester. A marine deposit +exposed between high and low tide occurs on both sides of the</p> + +<p class="fnote">* Quart. Geol. Journ., 1852, vol. viii, p. +372.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 182">[ 182 ]</a></p> + +<p>promontory called Selsea Bill, in which Mr. Godwin-Austen found +thirty-eight species of shells, and the number has since been +raised to seventy.</p> + +<p>This assemblage is interesting because on the whole, while all +the species are recent, they have a somewhat more southern aspect +than those of the present British Channel. It is true that about +forty of them range from British to high northern latitudes; but +several of them, as, for example, <i>Lutraria rugosa</i> and <i> +Pecten polymorphous</i>, which are abundant, are not known at +present to range farther north than the coast of Portugal, and seem +to indicate a warmer temperature than now prevails on the coast +where we find them fossil. What renders this curious is the fact +that the sandy loam in which they occur is overlaid by yellow +clayey gravel with large erratic blocks which must have been +drifted into their present position by ice when the climate had +become much colder. These transported fragments of granite, +syenite, and greenstone, as well as of Devonian and Silurian rocks, +may have come from the coast of Normandy and Brittany, and are many +of them of such large size that we must suppose them to have been +drifted into their present site by coast-ice. I measured one of +granite, at Pagham, 21 feet in circumference. In the gravel of this +drift with erratics are a few littoral shells of living species, +indicating an ancient coast-line.</p> + +<p><b>Glacial Formations of North America.</b>—In the western +hemisphere, both in Canada and as far south as the 40th and even +38th parallel of latitude in the United States, we meet with a +repetition of all the peculiarities which distinguish the European +boulder formation. Fragments of rock have travelled for great +distances, especially from north to south: the surface of the +subjacent rock is smoothed, striated, and fluted; unstratified mud +or <i>till</i> containing boulders is associated with strata of +loam, sand, and clay, usually devoid of fossils. Where shells are +present, they are of species still living in northern seas, and not +a few of them identical with those belonging to European drift, +including most of those already given in Figs. 107 to 112, p. 176. +The fauna also of the glacial epoch in North America is less rich +in species than that now inhabiting the adjacent sea, whether in +the Gulf of St. Lawrence, or off the shores of Maine, or in the Bay +of Massachusetts.</p> + +<p>The extension on the American continent of the range of erratics +during the Post-pliocene period to lower latitudes than they +reached in Europe, agrees well with the present southward +deflection of the isothermal lines, or rather the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 183">[ 183 ]</a></p> + +<p>lines of equal winter temperature. It seems that formerly, as +now, a more extreme climate and a more abundant supply of ice +prevailed on the western side of the Atlantic. Another resemblance +between the distribution of the drift fossils in Europe and North +America has yet to be pointed out. In Canada and the United States, +as in Europe, the marine shells are generally confined to very +moderate elevations above the sea (between 100 and 700 feet), while +the erratic blocks and the grooved and polished surfaces of rock +extend to elevations of several thousand feet.</p> + +<p>I have already mentioned that in Europe several quadrupeds of +living, as well as extinct, species were common to pre-glacial and +post-glacial times. In like manner there is reason to suppose that +in North America much of the ancient mammalian fauna, together with +nearly all the invertebrata, lived through the ages of intense +cold. That in the United States the <i>Mastodon giganteus</i> was +very abundant after the drift period, is evident from the fact that +entire skeletons of this animal are met with in bogs and lacustrine +deposits occupying hollows in the glacial drift. They sometimes +occur in the bottom even of small ponds recently drained by the +agriculturist for the sake of the shell-marl. In 1845 no less than +six skeletons of the same species of Mastodon were found in Warren +county, New Jersey, six feet below the surface, by a farmer who was +digging out the rich mud from a small pond which he had drained. +Five of these skeletons were lying together, and a large part of +the bones crumbled to pieces as soon as they were exposed to the +air.</p> + +<p>It would be rash, however, to infer from such data that these +quadrupeds were mired in <i>modern</i> times, unless we use that +term strictly in a geological sense. I have shown that there is a +fluviatile deposit in the valley of the Niagara, containing shells +of the genera <i>Melania, Lymnea, Planorbis, Velvata, Cyclaz, Unio, +Helix,</i> etc., all of recent species, from which the bones of the +great Mastodon have been taken in a very perfect state. Yet the +whole excavation of the ravine, for many miles below the Falls, has +been slowly effected since that fluviatile deposit was thrown down. +Other extinct animals accompany the <i>Mastodon giganteus</i> in +the post-glacial deposits of the United States, and this, taken +with the fact that so few of the mollusca, even of the commencement +of the cold period, differ from species now living is important, as +refuting the hypothesis, for which some have contended, that the +intensity of the glacial cold annihilated all the species in +temperate and arctic latitudes.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 184">[ 184 ]</a></p> + +<p><b>Connection of the Predominance of Lakes with Glacial +Action.</b>—It was first pointed out by Professor Ramsay in +1862, that lakes are exceedingly numerous in those countries where +erratics, striated blocks, and other signs of ice-action abound; +and that they are comparatively rare in tropical and sub-tropical +regions. Generally in countries where the winter cold is intense, +such as Canada, Scandinavia, and Finland, even the plains and +lowlands are thickly strewn with innumerable ponds and small lakes, +together with some others of a larger size; while in more temperate +regions, such as Great Britain, Central and Southern Europe, the +United States, and New Zealand, lake districts occur in all such +mountainous tracts as can be proved to have been glaciated in times +comparatively modern or since the geographical configuration of the +surface bore a considerable resemblance to that now prevailing. In +the same countries, beyond the glaciated regions, lakes abruptly +cease, and in warmer and tropical countries are either entirely +absent, or consist, as in equatorial Africa, of large sheets of +water unaccompanied so far as we yet know by numerous smaller ponds +and tarns.</p> + +<p>The southern limits of the lake districts of the Northern +Hemisphere are found at about 40° N. latitude on the American +continent, and about 50° in Europe, or where the Alps intervene +four degrees farther south. A large proportion of the smaller lakes +are dammed up by barriers of unstratified drift, having the exact +character of the moraines of glaciers, and are termed by geologists +“morainic,” but some of them are true rock-basins, and would hold +water even if all the loose drift now resting on their margins were +removed.</p> + +<p>In a paper read before the Geological Society of London in 1862, +Professor Ramsay maintained that the first formation of most +existing lakes took place during the glacial epoch, and was due, +not to elevation or subsidence, but to actual erosion of their +basins by glaciers. M. Mortillet in the same year advanced the +theory that after the Alpine lake-basins had been filled up with +loose fluviatile deposits, they were re-excavated by the great +glaciers which passed down the valleys at the time of the greatest +cold, a doctrine which would attribute to moving ice almost as +great a capacity of erosion as that which assumed that the original +basins were scooped out of solid rock by glaciers. It is impossible +to deny that the mere geographical distribution of lakes points to +the intimate connection of their origin with the abundance of ice +during a former excess of cold, but how far the erosive action of +moving ice has been the sole or even the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 185">[ 185 ]</a></p> + +<p>principal cause of lake-basins, is a question still open to +discussion.</p> + +<p>The lakes of Switzerland and the north of Italy are some of them +twenty and thirty miles in length, and so deep that their bottoms +are in some cases from 1000 to 2000 feet beneath the level of the +sea. It is admitted on all hands that they were once filled with +ice, and as the existing glaciers polish and grind down, as before +stated, the surface of the rocks, we are prepared to find that +every lake-basin in countries once covered by ice should bear the +marks of superficial glaciation, and also that the ice during its +advance and retreat should have left behind it much transported +matter as well as some evidence of its having enlarged the +pre-existing cavity. But much more than this is demanded by the +advocates of glacial erosion. They suggest that as the old extinct +glaciers were several thousand feet thick, they were able in some +places gradually to scoop out of the solid rock cavities twenty or +thirty miles in length, and as in the case of Lago Maggiore from a +thousand to two thousand six hundred feet below the previous level +of the river-channel, and also that the ice had the power to remove +from the cavity formed by its grinding action all the materials of +the missing rocks. A constant supply, it is argued, of fine mud +issues from the termination of every glacier in the stream which is +produced by the melting of the ice, and this result of friction is +exhibited both during winter and summer, affording evidence of the +continual deepening and widening of the valleys through which +glaciers pass. As the fine mud is carried away by a river from the +deep pool which is formed from the base of every cataract, so it +seems to be imagined that lake-basins may be gradually emptied of +the mud formed by abrasion during the glacial period.</p> + +<p>I am by no means disposed to object to this theory on the ground +of the insufficiency of the time during which the extreme cold +endured, but we must carefully consider whether that same time is +not so vast as to make it probable that other forces, besides the +motion of glaciers, must have cooperated in converting some parts +of the ancient valley courses into lake-basins. They who have +formed the most exalted conceptions of the erosive energy of moving +ice do not deny that during the period termed “Glacial” there have +been movements of the earth’s crust sufficient to produce +oscillations of level in Europe amounting to 1000 feet or more in +both directions. M. Charpentier, indeed, attributed some of the +principal changes of climate in Switzerland, during the glacial +period, to a depression of the central Alps to the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 186">[ 186 ]</a></p> + +<p>extent of 3000 feet, and Swiss geologists have long been +accustomed to attribute their lake basins, in part, to those +convulsions by which the shape and course of the valleys may have +been modified. Our experience, in the lifetime of the present +generation, of the changes of level witnessed in New Zealand during +great earthquakes is entirely opposed to the notion that the +movements, whether upward or downward, are uniform in amount or +direction throughout areas of indefinite extent. On the contrary, +the land has been permanently raised in one region several feet or +yards, and the rise has been found gradually to die out, so as to +be imperceptible at a distance of twenty miles, and in some areas +is even exchanged for a simultaneous downward movement of several +feet.</p> + +<p>But, it is asked, if such inequality of movement can have +contributed towards the production of lake basins, does it not +leave unexplained the comparative rarity of lakes in tropical and +subtropical countries. In reply to this question it may be observed +that in our endeavour to estimate the effects of subterranean +movements in modifying the superficial geography of a country we +must remember that each convulsion effects a very slight change. If +it interferes with the drainage, whether by raising the lower or +sinking the higher portion of a hydrographical basin, the upheaval +or depression will only amount to a few feet at a time, and there +may be an interval of years or centuries before any further +movement takes place in the same region. In the mean time an +incipient lake if produced may be filled up with sediment, and the +recently-formed barrier will then be cut through by the river, +whereas in a country where glacial conditions prevail no such +obliteration of the temporary lake-basin would take place; for +however deep it became by repeated sinking of the upper or rising +of the lower extremity, being always filled with ice it might +remain, throughout the greater part of its extent, free from +sediment or drift until the ice melted at the close of the glacial +period.</p> + +<p>One of the most serious objections to the exclusive origin by +ice-erosion of wide and deep lake-basins arises from their +capricious distribution, as for example in Piedmont, both to the +eastward and westward of Turin, where great lakes are wanting,* +although some of the largest extinct glaciers descending from Mont +Blanc and Monte Rosa came down from the Alps, leaving their +gigantic moraines in the low country. Here, therefore, we might +have expected to find lakes of the first magnitude rivalling the +contiguous Lago Maggiore in importance.</p> + +<p class="fnote">* Antiquity of Man, p. 313.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 187">[ 187 ]</a></p> + +<p>A still more striking illustration of the same absence of lakes +where large glaciers abound is afforded by the Caucasus, a chain +more than 300 miles long, and the loftiest peaks of which attain +heights from 16,000 to 18,000 feet. This greatest altitude is +reached by Elbruz, a mountain in lat. 43° N. three degrees +south of Mont Blanc, but on the other hand 3000 feet higher. The +present Caucasian glaciers are equal or superior in dimensions to +those of Switzerland, and like them give rise occasionally to +temporary lakes by obstructing the course of rivers, and causing +great floods when the icy barriers give way. Mr. Freshfield, a +careful observer, writing in 1869, says:* “A total absence of lakes +on both sides of the chains is the most marked feature. Not only +are there no great subalpine sheets of water, like Como or Geneva, +but mountain tarns, such as the Dauben See on the Gemmi, or the +Klonthal See near Glarus, are equally wanting.” The same author +states on the authority of the eminent Swiss geologist, Mons. E. +Favre, who also explored the Caucasus in 1868, that moraines of +great height and huge erratics of granite and other rocks “justify +the assertion that the present glaciers of the Caucasus, like those +of the Alps, are only the shadows of their former selves.”</p> + +<p>It seems safe to assume that the chain of lakes, of which the +Albert Nyanza forms one in equatorial Africa, was due to causes +other than glacial. Yet if we could imagine a glacial period to +visit that region filling the lakes with ice and scoring the rocks +which form their sides and bottoms, we should be unable to decide +how much the capacity of the basins had been enlarged and the +surface modified by glacial erosion. The same may be true of the +Lago Maggiore and Lake Superior, although the present basins of +both of them afford abundant superficial markings due to +ice-action.</p> + +<p>But to whatever combination of causes we attribute the great +Alpine lakes one thing is clear, namely, that they are, +geologically speaking, of modern origin. Every one must admit that +the upper valley of the Rhone has been chiefly caused by fluviatile +denudation, and it is obvious that the quantity of matter removed +from that valley previous to the glacial period would have been +amply sufficient to fill up with sediment the basin of the Lake of +Geneva, supposing it to have been in existence, even if its +capacity had been many times greater than it is now.†</p> + +<p>On the whole, it appears to me, in accordance with the views of +Professor Ramsay, M. Mortillet, Mr. Geikie, and others,</p> + +<p class="fnote">* Travels in Central Caucasus, 1869, p. 452.<br> +† See Principles, vol. i, p. 420, 10th ed., 1867.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 188">[ 188 ]</a></p> + +<p>that the abrading action of ice has formed some mountain tarns +and many morainic lakes; but when it is a question of the origin of +larger and deeper lakes, like those of Switzerland or the north of +Italy, or inland fresh-water seas, like those of Canada, it will +probably be found that ice has played a subordinate part in +comparison with those movements by which changes of level in the +earth’s crust are gradually brought about.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch11.html"> +Chapter XI</a> / <a href="ch13.html">Chapter XIII</a></small> +</body> +</html> + |