73 snowball nearly the hardness of ice. He could make it really ice if he possessed sufficient strength. The fact is, then, that snow under the influence of pressure passes into the form of ice. In some cases nature does this on a large scale. Where mountains are sufficiently elevated to raise their heads above the snow line we know they are white all the year around with snow. What is not blown away, evaporated, or, as an avalanche, precipitated to lower heights, must accumulate from year to year. But the weight pressing on the lower portions of this snow-field must soon be considerable, and at length become so great, that the snow changes to the form of ice. But as ice it is no longer fixed and immovable. We need not stop to explain just how this ice-field moves, but the fact is that, though moving very slowly, it acts like a liquid body. It will steal away over any incline however small, down which water would flow. Like a river it fills the valleys leading down from the mountains. But, of course, the lower down it flows the higher the temperature it meets, and it will sooner or later reach a point where it will melt as fast as it advances. This stream of ice flowing down from snow-clad mountains is called a glacier. Those we are best acquainted with are but puny things compared with those of the polar regions, where in one case a great river of ice sixty miles wide, flowing from an unknown distance, some thousands of feet in depth (or height), pours out into the sea. We at once perceive that such a mass of ice could not pour down a valley without leaving unmistakable signs of its passage. The sides of the mountains would be deeply scarred and smoothed. Projecting knobs would be worn away. The surface of the valley, exposed to the enormous grinding power of the moving ice, would be crushed, pulverized, and dragged along with it. Pieces of stone, like that here represented, 74 would form part of this moving debris, and as they were crowded along they would now and then grate over another piece of stone more firmly seated, and so their surface would be deeply scratched in the direction of their greatest length. There is always more or less water circulating under the Alpine glaciers, and the streams that flow from them are always very muddy, containing, as they do, quantities of crushed rock, sand, and clay. Illustration of Scratched Stone--------- If, for any reason, this earthy matter was not washed out it would form a bed of hard clay, in places packed with these striated stones. Such beds of clay are known as "till" or bowlder clay. This is descriptive, though in a very general way, of the glaciers as they exist to-day. Geologists have long been aware of the fact that they have convincing proofs of the former presence of glaciers in Northern Europe, where now the climate is mild. The mountains of Scotland and Wales show as distinct traces of glaciers as do those of the Alps. It is not necessary, in this hasty sketch, to enumerate the many grounds on which this conclusion rests. It is sufficient to state that by the united labors of many investigators in that field we are in possession of many conclusions relating to the great glaciers of this age which almost surpass belief; and yet they are the results of careful deductions. The former presence of this ice sheet itself is shown in a most conclusive manner by the bowlder clay formed underneath the great glacier, containing abundant examples of stone showing by their 75 scratched surface that they have been ground along underneath the glacier. The rocks on the sides of the mountains are scratched exactly as are those in the Alps. By observing how high up on the mountains the striae are, we know the thickness of the ice-sheet; and the direction in which it moved is shown in several ways. Briefly, then, the geologist assures us that when the cold of the Glacial Age was at its maximum glaciers streamed down from all the mountains of Scotland, Wales, and Northern England; that the ice was thick enough to overtop all the smaller hills, and on the plains it united in one great sea of ice some thousands of feet in thickness, that it stretched as far south as the latitude of London, England. But that
to the west the ice streamed out across, the Irish Sea, the islands to the west of Scotland, and ended far out into what is now the Atlantic. But these glaciers, vast as they were, were very small compared with the glaciers that streamed out from the mountains
of Norway and Sweden. These great glaciers invaded England to the southwest, beat back the glacier ice of Scotland from the floor of the North Sea, overran Denmark, and spread their mantle of bowlder clay far south into Germany.
While such was the condition of things to the north, the glaciers of the Alps were many times greater than at present. All the valleys were filled with glacier ice, and they spread far out on the plains of
Southern Germany and westward into France. The mountains of Southern
France and the Pyrenees also supported their separate system of
glaciers. Ice also descended from the mountains of Asia Minor and North
Africa. In America we meet with traces of glaciers on a vast scale;
but we can not pause to describe them here.
76
It need not surprise us, therefore, to learn of reindeer and musk-sheep feeding on stunted herbage in what now constitutes Southern France. When a continuous mantle of snow and ice cloaked all Northern Europe, it
is not at all surprising to find evidence of an extremely cold climate prevailing throughout its southern borders. We thus see how one piece of evidence fits into another, and therefore we may, with some confidence, endeavor to find proofs of more genial conditions when the snow and ice disappeared, and a more luxuriant vegetation possessed the land, and animals accustomed to warm and even tropical countries roamed over
a large extent of European territory. In Switzerland it was long ago pointed out that after the ancient glaciers had for a long time occupied the low grounds of that country they, for some cause, retreated to
the mountain valleys, and allowed streams and rivers to work over the
debris left behind them. At Wetzikon most interesting conclusions have been drawn. We there learn that, after the retreat of the glaciers, a lake occupied the place, which in course of time became filled with peat, and that subsequently the peat was transformed into lignite. To judge from the remains of animals and plants, the climate must have been at least as warm as that at present; and this condition of things must have prevailed over a period of some thousands of years to explain the thick deposits of peat, from which originated the lignites. But we also know that this period came to an end, and that once more the ice descended. This is shown by the fact that directly overlying the lignite beds are alternating layers of sand and gravel, and, resting on these, glacier-born bowlders. The same conclusion follows from the discoveries made at many other places. 77 In Scotland it is well known that the bowlder clay contains every now and then scattered patches of peat and beds of soil either deposited in lakes or rivers. The only explanation that can be given for their presence is that they represent old land surfaces; that is, when the land was freed from ice, and vegetation had again clothed it in a mantle of green. In this cut is shown one of these beds. Both above and below are the beds of bowlder clay. The peat in the centre varies from an inch to a foot and a half in thickness, and contains many fragments of wood, sticks, roots, etc.; and of animals, numerous beetles were found, one kind of which frequents only places where deer and ruminant animals abound. Diagram of Interglacial Bed------------ From a large number of such discoveries it is conclusively shown that, after all, Scotland was smothered under one enormous glacier, a change of climate occurred, and the ice melted away. Then Scotland enjoyed a climate capable of nourishing sufficient vegetation to induce mammoths, Irish deer, horses, and great oxen to occupy the land. But the upper bowlder clay no less conclusively shows that once more the climate became cold, and ice overflowed all the lowlands and buried under a new accumulation of bowlder clay such parts of the old land surface as it did not erode. Substantially the same set of changes are observed in English and German geology. Having thus given an outline of the climatic changes which took place in Europe during the Glacial Age, and the grounds on which these strange conclusions rest, we must now turn our attention to the appearance of 78 man. The uncertainties which hung over his presence in the earlier periods, spoken of in the former chapter, do not apply to the proofs of his presence during this age, though it is far from settled at what particular portion of the Glacial Age he came into Europe. We must remember we are to investigate the past, and to awaken an interest
