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second stage of the glacial epoch, and must have been characterised by all that glacier on land or iceberg on water are capable of performing. The ice-sheet that now gathered over the gradually-decreasing land would push its way shoreward with its annual burden of debris, and this debris, as it was carried seaward, would be dropt in part over the clay, shingle, and boulders that had been accumulated during the first stage. Of course a considerable portion of the debris of the first stage would be removed by shore denudation as the land subsided, but a large portion was also left undisturbed ; and thus it is that we find in many places the earlier clays and angular blocks covered over by other clays, replete with boulders more worn and rounded, and more strongly marked by scratches and furrows. It was then, and during this period of subsidence, that huge boulders were carried by floating ice far from their parent rocks; and thus it is that these boulders, now hundreds of miles from their original cliffs, mark in a special manner the second stage of the glacial epoch.* During this stage, as during the preceding, we have no evidence of a terrestrial flora or fauna, the climate being evidently too rigorous for their support; and it is only towards the southern limits of the ice-field (the 40th or 42d parallel of latitude t) that we can expect to find the remains either of
* It is difficult to convey by description the difference between the clays and boulders of the first and second stages; but a few days in the field will train the eye sufficiently to mark the distinction, and this altogether independent of superposition. There is a roughness of admixture and heterogeneousness about them that never appear in those of the earlier stage. Perhaps the best test of the second stage is the number of “erratic blocks,” or boulders far removed from their parent rocks. In Europe, Scandinavian rocks are found in Central Germany and over the south-east of England ; in North America, Canadian blocks occur a hundred and a hundred and fifty miles southward of their parent sources.
+ American geologists give the southern limit of the drift in their
plants or animals, and these in all likelihood of borcal habits, like those now inhabiting the borders of the arctic regions. How long this period of descent continued relatively to the other stages, it is difficult to determine, though, on the whole, it appears to have been the longest stage, and that which has most impressed its character on the terrestrial surface. The extent to which the subsidence took place is variously estimated at from 1800 to 2000 feet, for from that limit down to the existing seashore, the land-surface is marked with rounding and smoothing, polishing and scratching, glacial moraines and clays, ice-borne blocks and boulders. One cannot turn to the higher districts of our own islands, to the north of Europe, or to Northern America, without perceiving on every hand traces of this long-continued ice-action—the bouldery clays, the rounded blocks and boulders, the scratched and polished rock-surfaces, the rounded outlines of the hills and knolls, all bespeaking its presence as incontestably as the existing surface of the Alps, the Scandinavian mountains, or the uplands and shores of Greenland.
But the forces that govern the external conditions of our planet are never at rest. Change succeeds change, and cycle follows cycle. The downward tendency of the land ceases, and an upward movement commences. Along with this gradual elevation, new distributions of sea and land begin to appear, and with these changes the intensity of the glacial epoch seems to come to a close. Glacier and ice-sheet, however, still shroud the land, and icebergs drift away from the shores. Other currents, however, are evidently setting in, a more genial climate begins to prevail, and with this higher temperature the ice disappears from the seas and lower grounds, and only clings to the higher hills in continent as 39° N. lat.; in Europe it has been variously stated at 40°, 42°, and even 44° N. lat.
shrinking and gradually lessening glaciers. Even these, too, vanish in the long-run, and the present order of things, the ordainings of the current epoch, are established. This gradual elevation of the land constitutes the third and last stage of the glacial epoch, the proofs of which are to be found in the moraines, lateral and terminal, that still linger in every glen and corrie of our island, in the reassorted clays and boulders of the two former stages, in the numerous terraces which mark the successive steps of the land's uprise, and in the fine silty clays (the “brick-clays” of some geologists), with boreal shells, star-fishes, bones of seals and whales, which fringe our bays and estuaries at various altitudes above the present sea-level. Compared with the earlier stages of the ice-epoch, the traces of this latter stage are still fresh and recent. The mounds of sand and gravel so frequent at the mouths of all our glens and upland valleys, are but the terminal moraines of the ancient glaciers; the gravelly terraces (in some instances with shells) that fringe so many of our hill-sides, are but the ancient beaches of the gradually uprising land; the great blocks and boulders so abundantly strewn over our heathy uplands, are but the denuded ice-borne blocks of the two former stages; and the fine silty “brick-clays” are but the upraised muds of the deeper sea-bed. Since then, frost, rain, and rivers have done their work on the land's surface, and obliterated many of the ice-traces, yet enough still remains to convince the unbiassed inquirer of its long reign over these northern latitudes during the three successive stages we have here endeavoured to describe.
The accumulations described in the preceding paragraphs are usually distinguished by such names as “northern drift,” “ glacial drift," " erratic blocks,” and “ boulderclay,” all conveying the idea that they have not been deposited under the ordinary conditions of water, that they belong especially to the northern hemisphere, have been drifted from a northerly source, and in all likelihood by ice partly on land and partly in water.* The older terms “ diluvium” and “diluvial drift," under the idea that they were the results of the Noachian deluge, have been long ago set aside, and geologists with one consent now look to ice, in one or other of its manifestations, as the only known agent by which they could have been accumulated. Rain and rivers can, no doubt, waste and wear down the land, but it is ice alone that can grind and smooth and confer those rounded outlines which characterise so much of the surface in the higher latitudes of the northern hemisphere. River floods and freshets can accumulate vast mounds of sand and shingle ; but it is the glacier alone that throws across the glen its dam-like moraine, and scores and polishes the pebbles of which it is composed. Running water laden with debris will wear and smooth the rock-surfaces over which it flows; but it is ice alone that can put on the glassy polish, and scratch and gouge in long straight furrows with its embedded blocks and fragments. Rivers and torrents will roll and transport blocks of considerable size ; but it is ice alone, either as the ice-sheet on land or the iceberg on water, that can transport boulders many tons in weight up and over hills, or float them away hundreds of miles from their parent precipices. Running water invariably assorts its debris in beds and layers according to its fineness; it is floating ice alone that drops its burden on the sea-bed without regard to arrangement. All these appearances—boulder
* Objection has been taken to each and all of these designations, and it must be confessed we are still in want of a good general term for the accumulations of the glacial epoch. Drifts, erratic blocks, and boulderclays, are but members of a great series, and it is for this series, taken as a whole, that we still stand in need of a comprehensive desiguation.
clays of great thickness, erratic blocks of enormous size, polished, striated, and grooved rocks, moraine-like mounds of gravel, and smoothed and rounded surfaces—are so common in Britain, Northern Europe, and North America, that geologists are driven to the conclusion of a glacial epoch; a long period intermediate between the Tertiary and the Current era, when all the northern hemisphere, down to the 40th or 42d parallel, was under the influences of an icy climate like that which now prevails within the arctic and antarctic circles.
Geologists have long been at variance, and in some instances are still at variance, as to whether the phenomena are to be attributed more to land-ice or to sea-ice, to glaciers or to icebergs. As we became better acquainted, however, with the operations of ice in such regions as the Alps, Himalayas, Scandinavia, Spitzbergen, Greenland, and the polar seas, such differences of opinion grew less, and competent authorities seem to be agreed that we must call in both agencies, and this during the successive stages of subsidence and re-elevation which we have already endeavoured to describe. Indeed it is impossible to conceive of a glacial climate over any large portion of the earth's surface without seeing that it must affect sea and land alike; and that if there is any brief period of thaw, like the existing arctic summer, the ice must be set in motion both on land and water.* Once set in motion, each would contribute its quota to the general result—the land-ice grinding and smoothing and rounding the rocky surface in its descent to the sea, and the sea-ice ploughing the shallower sea-bed as it floated away to drop its burden of boulders and debris
* Even without any great degree of summer thaw, a mobile mass like ice and snow would be urged forward by its own accumulating weight, and this over heights and hollows, so long as the head pressure remained sufficiently powerful. See preceding Sketch, p. 221.