Glaciers—Their nature and composition—Their unceasing motion—Powerful agents of denudation—Icebergs—Their number and size—Erratic blocks and loose gravel spread out over mountains, plains, and valleys, at the bottom of the sea—Characteristic marks of moving ice—Evidence of ancient glacial action—Illustrations from the Alps—From the mountains of the Jura—Theory applied to northern Europe—To Scotland, Wales, and Ireland—The fact of denudation established—Summary of the evidence—This fact the first step in geological theory. T The next agent of Denudation to which we invite the attention of our readers, is one of which our own country affords us no example, but which may be seen in full operation amidst the wild and impressive scenery of Switzerland. And we know not how we can better introduce the subject than by the solemn address of a great poet, in whom an ardent love of nature was blended with a deep sense of religion. As he stood in the midst of the snow-clad mountains that shut in the valley of Chamouni, his spirit, “expanded by the genius of the spot,” soared away from the scenes before him to the Great Invisible Author of all that is beautiful and sublime in nature, and he poured forth that well-known hymn of praise and worship in which he thus apostrophizes the massive glaciers of Mont Blanc:— “Ye ice-falls! ye that from the mountain’s brow Adown enormous ravines slope amain— Torrents, methinks, that heard a mighty voice, And stopped at once amid their maddest plunge! Motionless torrents! silent cataracts! Who made you glorious as the gates of Heaven Beneath the keen full moon? Who bade the sun Clothe you with rainbows? Who with living flowers Of loveliest blue, spread garlands at your feet? God! let the torrents, like a shout of nations, Answer! and let the ice-plains echo, God! God! sing ye meadow-streams with gladsome voice! Ye pine-groves, with your soft and soul-like sounds! And they too have a voice, yon piles of snow, And in their perilous fall shall thunder, God!”29 A Glacier is an enormous mass of solid ice filling up a valley, and stretching from the eternal snows which crown the summits of the mountains, down to the smiling cornfields and rich pastures of the plains. It is constantly fed by the accumulated snows of winter, which, slipping and rolling down the slopes of the mountains, lodge in the valleys below, and are there converted into ice. For it must be remembered that the Glacier properly so called does not commonly extend much higher than 9000 feet above the level of the sea. Beyond that elevation the compact and massive ice gradually passes into frozen snow, called by the French NevÉ, and by the Germans Firn. The change which takes place in the condition of the snow as it descends into the valley is chiefly owing to these two circumstances: first, it is closely compacted together by the weight of the snowy masses pressing down upon it from above; and secondly, in the summer months it is thawed upon the surface during the day by the heat of the sun, and frozen again at In Switzerland the Glacier is often two or three miles in breadth, from twenty to thirty miles in length, and five or six hundred feet in depth. Though so vast in its bulk and so solid in its character, it is not, as might be supposed, a fixed, immovable mass. On the contrary, it is moving incessantly, but slowly, down the valley which it occupies, at the rate of several inches—sometimes one or two feet, and even more—in the day. In Greenland a Glacier explored by Doctor Hayes, in his expedition to the North Pole, was found to move for a whole year at the average rate of a hundred feet a day. It may be thought, perhaps, that this fact requires further confirmation; but at all events it is certain that the language of the poet, when he addresses the Glaciers as “motionless torrents,” though it conveys an accurate and beautiful idea of the appearance they present to the eye, is not rigorously true in a scientific sense. Indeed, it is just because the Glaciers are not motionless that they serve as instruments of Denudation. Their agency in this respect “consists partly in their power of transporting gravel, sand, and huge stones, to great distances, and partly in the smoothing, polishing, and scoring of their rocky channels, and the boundary walls of the valleys through which they pass. At the foot of every steep cliff or precipice in high Alpine regions, a sloping heap is seen of rocky fragments detached by the alternate action of frost and thaw. If these loose masses, instead of accumulating on a stationary base, happen to fall upon a Glacier, they will move along with it, and, in place of a single heap, they will form in the course of years a long stream of blocks. If a Glacier be twenty miles long, “The stones carried along on the ice are called in Switzerland the moraines of the Glacier. There is always one line of blocks on each side or edge of the icy stream, and often several in the middle, where they are arranged in long ridges or mounds of snow and ice, often several yards high. The reason of their projecting above the general level, is the non-liquefaction of the ice in those parts of the surface of the Glacier which are protected from the rays of the sun, or the action of the wind, by the covering of the earth, sand, and stones. The cause of medial moraines was first explained by Agassiz, who referred them to the confluence of tributary Glaciers. Upon the union of two streams of ice, the right lateral moraine of one of the streams comes in contact with the left lateral moraine of the other, and they afterward move on together, in the centre, if the confluent Glaciers are equal in size, or nearer to one side if unequal. “Fragments of stone and sand which fall through crevasses in the ice, and get interposed between the moving Glacier and the fundamental rock, are pushed along so as to have their angles more or less worn off, and many of them are entirely ground down into mud. Some blocks are pushed along between the ice and the steep boundary rocks of the valley, and these, like the rocky channel at the bottom of the valley, often become smoothed and polished, Fig. 2.—Iceberg seen in mid-ocean 1400 miles from any known land. Sometimes, however, it happens, especially in extreme northern and southern latitudes, that the glacier valley leads down to the sea. In such cases, huge masses of ice are floated off, and, with their ponderous burden of gravel, How enormous must be the magnitude of those ponderous masses may be learned from the fact that the bulk of ice below the level of the water is about eight times as great as that above: and in point of fact, Captain Sir John Ross saw several of them aground in Baffin’s Bay, where the water was 1500 feet deep. It has been calculated that the beds of earth and stones which they carry along cannot be less than from 50,000 to 100,000 tons in weight. Sir Charles Lyell, writing in 1865 from the results of the latest investigations on this subject, says: “Many had supposed that the magnitude commonly attributed to icebergs by unscientific navigators was exaggerated; but now it appears that the popular estimate of their dimensions has rather fallen within than beyond the truth. Many of them, carefully They have been known to drift from Baffin’s Bay to the Azores, and from the South Pole to the Cape of Good Hope.33 As they approach the milder climate of the temperate zones, the ice gradually melts away, and thus the moraines of arctic and antarctic glaciers are deposited at the bottom of the deep sea. In this way, submarine mountains and valleys and table-lands are strewn over with scattered blocks of foreign rocks, and gravel, and mud, which have been transported hundreds of miles across the unfathomable abysses of the ocean. Though we are chiefly concerned with Glaciers and Icebergs as agents of Denudation, yet we cannot pass away from the subject without referring to the Geological theory of an ancient Glacial Period. This little digression from the main purport of our present argument will not be unacceptable, we hope, to our readers. The theory is in itself interesting and ingenious; and it offers an admirable illustration of the kind of reasoning by which Geologists are guided in their speculations. It is well known that the action of moving ice leaves a very peculiar and characteristic impress on the surface of the rocks, and even on the general aspect of the country over which it passes. This is no mystery of science, but a plain fact which any one that chooses may observe for himself. Every Glacier carries along in its course a vast quantity of loose gravel, hard sand, and large angular Fig. 3.—Block of Limestone furrowed, scratched, and polished, from the Glacier of Rosenlaui, Switzerland. (Lyell.) aa, White streaks or scratches. bb, Furrows. Similar effects are produced by Icebergs; not of course when drifting about in the deep sea, but when they come into contact with a gently-shelving coast and grate along the bottom. These mountains of ice, laden with the dÉbris of the land, are often carried along with the velocity of from two to three miles an hour; and before their enormous momentum can be entirely destroyed, an extensive surface of rock must have been rounded, grooved, and scarred, pretty much in the same way as by the action of a Glacier. There can be no failure of the grinding materials. During the process of melting, the Iceberg is constantly turning over according as the centre of gravity shifts its position; and thus a new part of its surface, with fresh angular blocks of stone, together with fresh masses of sand and gravel, is constantly brought into contact with the floor of the ocean. And this is not mere theory. All these phenomena may be witnessed any day on the shores of Baffin’s Bay and Hudson’s Bay, and along the coast of Labrador. Again, the evidence of glacial action may be discovered in the materials themselves which have been transported by ice. Many of the large erratic blocks, after having travelled immense distances, exhibit the same sharp angular appearance as if they had only just fallen down from the cliff on the mountain side. By this circumstance they are at once distinguished from blocks of stone transported by running water; for in these the angles are sure to be rounded off by friction. Sometimes, too, they are deposited not only far away from the same rock, but in regions where no rock of the same kind exists. In the case of Icebergs, they are not unfrequently carried many hundreds of miles before being dropped into the depths of the ocean, and, in the course of their long journey, borne over the lofty ridges of submarine mountain chains. Furthermore, it often happens that a Glacier shrinks backward up the valley, and sometimes even disappears Now, it appears that all these various characteristic marks of glacial operations can be distinctly traced in many countries where the action of moving ice has been unknown within the period of history. And on this fact is founded the Geological theory of an ancient Glacial Period. We are confidently assured that a great part of Northern Europe, including even our own islands, not to speak of America and other countries as well in the northern as in the southern hemisphere, were, in some far distant age, the scene of those same phenomena which are witnessed at the present day amid the solemn grandeur of the Alps, and in the frozen wastes of the Arctic regions. In that age enormous Glaciers moved slowly downward from the snow-clad heights over innumerable valleys now rich with the fruits of the earth; ponderous Icebergs floated over wide areas of the ocean, where now the dry land appears; and vast piles of promiscuous rubbish, with great angular blocks of stone, were deposited To illustrate this theory, we would begin with a country where the vestiges of glacial operations in past times may be studied side by side with the glacial phenomena of the present day. In Switzerland it needs but little skill to discern many marks and tokens of moving ice where moving ice is no longer found. In descending, for example, the valley of the Hasli or the valley of the Rhone, the intelligent traveller can hardly fail to observe how the rocks all around are scarred and furrowed, precisely after the same fashion as the rocks in the higher parts of the same valleys are now being scarred and furrowed by the Glacier of the Aar and the Glacier of the Rhone. At intervals, too, may be seen long mounds of unstratified gravel and mud, with large fragments of rock, in every way resembling the terminal moraines now daily accumulating at the extremities of existing Glaciers. When these facts are once distinctly brought home to the mind, it is impossible to resist the conclusion that several of the Alpine Glaciers once extended far beyond their present limits down the valleys of Switzerland. If we proceed a little distance to the mountains of the Jura, now wholly devoid of Glaciers, we shall find that the same glacial phenomena with which we have become so familiar in the Alps, are still everywhere presented to the eye. And we feel instinctively impelled to pursue the same line of inductive reasoning. Moving ice, we know from abundant observation, is capable of producing these effects: nor have we ever seen effects of this kind produced by any other cause: nay, there is no other natural agent known that is capable of producing such effects: it is therefore reasonable to infer that moving ice was the cause of these effects; and that, in some bygone age, great masses of ice moved slowly over the valleys of the Jura as they now move slowly over the valleys of the Alps. Another circumstance may here be noticed which is well worthy of consideration. The Alps are composed of granite, gneiss, and such like crystalline rocks: the Jura, of limestone and various other formations, altogether different from those of the Alps. Now, scattered loosely over the valleys of the Jura, and perched upon its lofty crests, we find immense angular blocks—some of them as large as cottages—of the Alpine rocks. The question naturally arises, how have they been transported to their present site. Certainly not by the action of water; for in that case the projecting angles would have been rounded off, and the sharp edges worn away. But the work might have been easily accomplished by the power of moving ice, and could not have been accomplished by any other natural agency with which we are acquainted. Thus we are led to conclude that the Glaciers of the Alps must, by some means or another, have once made their way northward across the great valley of Switzerland, fifty miles wide, and deposited their ponderous burdens of gravel, sand, and erratic blocks on the mountains of the Jura. It would carry us too far from our present purpose to draw out this theory in all its details. But we cannot for-bear briefly to touch upon some of the bold and startling conclusions to which it has led. The Geologist having, by patient and varied exercise, in the regions of existing Glaciers, trained his eye and his judgment in the observation of those phenomena that mark the action of moving ice, soon begins to discover that they are not wanting in other countries. They are not to be found, indeed, beneath the burning sun of Africa, nor on the borders of the Mediterranean Sea. But as he travels northward they begin by degrees to appear; and when at length he reaches the shores of the Baltic, they are spread out profusely before him as they were in the bosom of the Alps. All this had puzzled Geologists for years; but the clue has been found Following out this line of argument, and applying it to countries nearer home, Geologists have come to the conclusion that the Grampian Hills in Scotland, the mountains of Kerry in Ireland, the Snowdonian heights in Wales, and many other ranges of hills in these islands, were in former times subjected to the action of moving ice. Nay, it is contended, with much show of reason, that these islands must have been, for a considerable time, in great part submerged beneath the sea, and traversed by floating Icebergs. When large erratic blocks are found in the immediate neighborhood of the formation from which they have been derived, then it is easy to explain their origin and to trace their course. But it often happens that the nearest rock of the same mineral composition, and therefore, the nearest rock from which they can possibly have been derived, is separated from the site which they now occupy by a lofty chain of mountains. By what means, then, have they been transported hither? Not by moving water, for their sharp edges and projecting angles are still preserved. Not by Glaciers; for a Glacier cannot climb a steep mountain ridge. It would seem, indeed, that in the present geographical distribution of land and water, there is no natural cause which could carry them from the parent rocks to their present position. But if we suppose that in some long past age of the world, Great Britain and Ireland were submerged beneath the sea, and that Icebergs floated in the waters above, the problem is solved at once. The The presence of marine shells, belonging chiefly to species which now exist only in the arctic seas, affords a strong confirmation of this hypothesis. For they are found intimately associated with the erratic blocks, not merely in valleys, to which the sea might be supposed to have had access in times of extraordinary flood, but upon lofty mountains at a height of five hundred, six hundred, and even thirteen hundred feet above the level of the sea. There is no difficulty in accounting for this phenomenon if we suppose the country to have been at one time submerged, and the glacial drift in which the shells are found embedded to have been deposited by Icebergs on the floor of the ocean. If we refuse to make this supposition the difficulty is simply insurmountable.34 But it is somewhat beside our purpose to wander so far into the region of theory and speculation. Our main object in these chapters has been to establish the fact that Denudation is actually taking place to an almost incredible extent, in the present age of the world. For this purpose we have enumerated the principal agents by which this process is carried on; and we have endeavored to show from the authenticated researches of travellers and scientific men that they have been at work within the period of history, and are still at work around us. Our summary is, indeed, Now Geologists tell us that these are the raw materials of a new building which is going on in these latter times under the guiding hand of Nature. Indeed, they say it is not so much a new building as the uppermost story of an old building. If we descend into the Crust of the Earth we may trace this building even from the foundations, which are laid upon the solid granite, up through each successive stage of limestone, and sandstone, slate, conglomerate, and clay, until we come to the surface, where new strata, composed of the same elements, and exhibiting the same general characteristics, are slowly growing up before our eyes. Thus will the idea gradually steal upon the mind, that the works of ages long gone by are reproduced once again in our own days, and that we may study the history of the past in the mirror of the present which nature holds up to our view. This is the branch of Geological argument upon which we are now about to enter. We have visited Nature, as it were, in her quarry, and we have seen how she collects her materials, how she fashions them to her purpose, how she transports them to the place for which they are designed. If it be true, as alleged, that with these materials she is actually engaged, at the present moment, in building upon the existing surface of our Globe |