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 stones. A considerable proportion of these materials in course of time fall through crevasses in the ice, and become firmly embedded in the under surface of the Glacier. Then, as the moving mass slowly descends the valley, they are shoved along under enormous pressure, and the surface of the rocks beneath is furrowed, scratched, and polished, in a remarkable and unmistakable manner. The furrows and scratches are rectilinear and parallel to an extent never seen in the marks produced by any other natural agency: and they always coincide more or less in their direction with the general course of the valley. A reciprocal action often takes place: the large blocks of stone, frozen into the under surface of the Glacier, are themselves scored and polished by friction against the floor and sides of the valley.
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 altogether. When the melting of the ice at the lower extremity exactly balances its onward progress, then the Glacier seems stationary to the eye, and occupies from year to year the same position. But, when a number of hot seasons follow one another in immediate succession, the ice is melted more rapidly than the Glacier advances, and in consequence it gradually becomes shorter, and seems to the eye to recede toward the upper parts of the valley. In this case the long lines of moraines, which before had rested on the ice, are left spread out on the plains or deposited on the slopes of the mountain. Immense blocks of stone are by this means frequently set down on the summits of lofty crags, and in such like positions to which they could not be brought by any other natural agency. These Perched Blocks, as they are called, and also those long regular mounds of earth and stones abound in several of the Swiss valleys, and constitute a very striking feature of Alpine scenery.