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found in the vicinity of all the mountain ranges of Central Europe, and they undoubtedly demonstrate the fact of a recent change of climate, from a time when all our higher mountains were covered with perpetual snow, and the adjacent valleys were filled with glaciers at least as extensive as those now found in Switzerland.
But this conclusion, marvellous as it is, by no means affords us an adequate conception of the condition of our islands during the whole duration of the glacial epoch; for there are other phenomena, best developed in Scotland, which show that what we have hitherto described was but its concluding phase, and that during its maximum development the mountainous parts of our islands must have been reduced to a condition only to be now paralleled in Greenland and the Antarctic regions. As few persons besides professed geologists are acquainted with the weight of the evidence in support of this statement, we will here briefly summarize it, referring our readers to Mr. Geikie's volume for fuller details.
Over almost all the lowlands, and in many of the highland valleys of Scotland, there are immense superficial deposits of clay, sand, gravel, or drift, which can be traced more or less certainly to glacial action. Some of them are moraines, others are lacustrine deposits in temporary lakes formed by moraines or glaciers, while others have been formed or deposited under water during periods of submergence. But below them all, and often lying directly on the solid rock, there are extensive layers of a very tough clayey deposit called 'till.' It is very fine in texture, very tenacious, and often of a stone-like hardness; and it is always full of rocks and stones, which are of rude sub-angular forms, rubbed smooth and partially rounded, and almost always covered with scratches or deep striæ, often crossing each other in various directions. Sometimes the stones
are so numerous, that there seems to be only just enough clay to unite them into a solid mass; and they are of all sizes, from mere grit up to rocks many feet in diameter. The till' is found chiefly in low-lying districts, where it covers extensive areas, sometimes to the depth of a hundred feet; while in the highlands it occurs in much smaller patches, but in some of the broader valleys it forms terraces which have been cut through by the streams. Occasionally it is found as high as two thousand feet above the sea in hollows on hillsides, or in other situations where it seems to have been protected from denudation.
The 'till' is totally unstratified, the stones it contains being found all mixed together and evidently unsorted by water, while the rock surfaces on which it rests are invariably worn smooth,
and greatly grooved and striated when the rock is hard, while when it is soft or jointed it frequently has a greatly broken surface, as if it had been subjected to enormous pressure. The colour and texture of the 'till' and the nature of the stones it contains all correspond with the character of the rock of the district, so that it is clearly a local formation. It is often found underneath moraines, drift, and other glacial deposits, but never overlies them, so that it was certainly formed at an earlier date. Throughout Scotland where 'till' is found, the glacial striæ, perched blocks, roches moutonnées, and other marks of glacial action, occur very high up the mountains, to at least 3000 and often even to 3500 feet above the sea; while all lower hills and mountains are rounded and grooved up to their very summits, and these grooves always radiate outwards from the highest peaks and ridges towards the valleys or the sea.
Now these various phenomena taken together are only explicable on the supposition that the whole of Scotland was once buried under a vast ice-sheet, above which only the highest mountains reared their summits. There is absolutely no escape from this conclusion, for the facts which lead to it are not local, found only in one spot or one valley, but general throughout the entire length and breadth of Scotland; and these facts correspond so wonderfully in every detail to this conclusion, and this only, as to amount to absolute demonstration. The weight of this vast ice-sheet, at least three thousand feet in maximum thickness, and continually moving seaward with a slow grinding motion like that of all existing glaciers, must have worn down the whole surface of the country, especially all the prominences, leaving the grooves and striæ we still see, marking the direction of its motion. All the loose stones and rocks which lay on the surface would be pressed into the ice, the harder ones would serve as scratching and grinding tools, and would thus themselves become worn, scratched, and striated as we find them, while all the softer masses would be ground up into impalpable .mud along with the material worn off the rock-surfaces of the country.
The peculiar characters of the 'till,' its fineness and tenacity, correspond closely with the fine matter which now issues from under all glaciers, making the streams milky white, yellow, or brown, according to the nature of the rock. The sediment from such water is a fine unctuous ooze, only needing pressure to form it into a tenacious clay; and when 'till' is exposed to the action of water it dissolves into a similar soft, sticky, unctuous mud. The present glaciers of the Alps, being confined to valleys which carry off a large quantity of drainage water, lose this mud per
haps as rapidly as it is formed; but when the ice covered almost the whole country, there would be comparatively little drainage water, and the mud and stones would collect in vast compact masses in sheltered hollows, and especially in the lower flat valleys, which would necessarily be ground into hollows or basins where the pressure of the ice was greatest. As the ice retreated, the areas of greatest pressure would retreat also, and the hollows would be left full of the stones and glacier-mud which was continually being formed. At a later period it was greatly denuded by rain and rivers, but, as we have seen, large quantities remain to this day, to tell the wonderful story of its formation. It was at this time that the glaciers of Wales and of Ireland acquired their greatest extension, and there is clear evidence that the ice on the west of Scotland extended far out to sea, overspreading all the islands, and connecting itself in one unbroken mass with the almost equally extensive ice-sheet that covered Ireland.
It is evident that the great change of climate, which produced such marvellous effects in the British Isles, could not have been confined to them; and accordingly we find that there are strikingly similar proofs that Scandinavia and much of Northern Europe have also been covered with a continuous ice-sheet. We have already seen that huge glaciers almost buried the Alps, carrying granitic blocks to the Jura and depositing them on its flanks to a height of 3450 feet above the sea-level; while to the south, in the plains of Italy, the terminal moraines left by the retreating glaciers have formed considerable hills, those of Ivrea, the work of the great glacier of the Val d'Aosta, being fifteen miles across and from 700 to 1500 feet high.
In North America the marks of glaciation are even more extensive and perhaps more remarkable than in Europe, stretching over the whole eastern part of Canada, and at least as far as the 40th parallel of latitude, south of the great lakes. There is, over considerable areas, a deposit like the 'till' of Scotland, produced by the grinding of the great ice-sheet when it was at its maximum thickness. In the eastern part of Canada and the United States, the ice appears to have risen to its greatest height over the northern watershed of the St. Lawrence near its mouth, and to have extended across to the White Mountains of New England, filling up the Gulf of St. Lawrence, overspreading Nova Scotia and Long Island, and terminating in an ice-cliff in the Atlantic. It is believed by Mr. J. D. Dana to have had a thickness of over 5000 feet in New England, and to have reached a height of 13,000 feet over the northern watershed of the St. Lawrence. At a later period the local
glaciers left moraine-matter, travelled-blocks, and striated rocks, as in Europe. There are also in North America, as well as in Britain and Scandinavia, proofs of the submersion of the land beneath the sea, to a depth of more than a thousand feet, in the latter part of the glacial epoch; but this is a subject we need not here enter upon, as our special object is to show the reality and magnitude of that wonderful and comparatively recent change of climate termed the glacial epoch.
Many educated persons, and even men of science who have not given much attention to the subject, look upon the whole of this enquiry as an elaborate but highly improbable theory, made to fit certain phenomena which are otherwise difficult to explain; and they would not be much surprised if they were some day told that it was all a delusion, and that somebody had explained the whole thing in a more simple way. The outline now given of the wide basis of facts on which the theory rests, will, it is hoped, prevent any of our readers from being imposed upon by such statements or disturbed by any sceptical doubts. There is perhaps no great conclusion in any science, which rests upon a surer foundation than this does; and if we are ever to be guided by our reason in deducing the unknown from the known, or the past from the present, we cannot refuse our assent to the reality of the glacial epoch in all its more important features. Just as surely as the extinct volcanoes of Auvergne or of Victoria demonstrate the former existence of active volcanoes and
flowing lava-streams, so surely do the striated rocks, the perched blocks, the moraines, and the 'till,' demonstrate the former existence of the glaciers, by which alone they could have been produced.
Before quitting this part of our subject, we must notice some curious facts, which seem to show that there were recurring periods of warmth during the glacial epoch itself, as they have a very important bearing on the theory by which changes of climate in general seem to be best explained.
The till,' as we have seen, could only have been formed when the country was buried under an ice-sheet of enormous thickness, and when in the parts so buried there could have been neither animal nor vegetable life. But in several places in Scotland, layers of sand and gravel with beds of peaty matter have been found intercalated between layers of 'till.' Sometimes these intercalated beds are very thin, but in other cases they are twenty or thirty feet thick, and contain the remains of the extinct ox, the Irish elk, the horse, reindeer, and mammoth. Here then we have evidence of two distinct periods of intense cold when the country was buried in ice, and of an intervening
mild period sufficiently prolonged for the country to become covered with vegetation and stocked with large animals. In some districts borings have proved the existence of no less than four distinct formations of 'till,' separated from each other by beds of sand from two to twenty feet in thickness.* In North America similar beds occur, intercalated between true glacial deposits, and containing remains of the elephant, mastodon, and great extinct beaver. In Switzerland a similar interglacial bed contains peat, and remains of pines, oaks, birches, and larch, with bones of the elephant, rhinoceros, stag, and cave-bear, and also abundance of insects.† There seems, therefore, to be ample proof that the glacial epoch did not consist of one continuous change from a temperate to a cold and arctic climate, which, having reached a maximum, then passed slowly and completely away, but that there were certainly two, and probably many, distinct alternations of arctic and temperate climates.
It is true that the evidence of such alternations is scanty, but a little consideration will show that we could not expect to find more complete evidence, because each succeeding ice-sheet would necessarily grind down or otherwise destroy most of the superficial deposits left by its predecessors, while the torrents that must have been produced by the melting of the ice would wash away most of the fragments which had escaped. It is therefore fortunate that we find any portions of interglacial deposits containing vegetable and animal remains; and, as we might expect, these seem to have been formed when each succeeding phase of the cold period was less severe than those which preceded it, in other words, when the glacial epoch was passing away. If there had been similar intercalated warm periods while it was coming on, it is hardly possible that any record of them could have been preserved, because each succeeding extension of the ice would be greater than that which preceded it, and would certainly destroy all traces of animal or vegetable remains in superficial deposits.
Now it is a very remarkable fact, that the only theory which affords a distinct explanation of the changes which brought about the glacial period, leads to the conclusion that such alternations of warm and cold climates must have occurred during its continuance; and as the same theory also explains, more or less completely, the other changes of climate of which we have geological evidence, it will be most convenient to our readers
The Great Ice Age,' p. 177.
† Heer's 'Primæ val World of Switzerland,' vol. ii. p. 296.