When in an Alpine valley we come upon a glacier, filling it from side to side, there will be noticed upon both sides a long train of rock, drift, and other _débris_ that have fallen down upon its surface from the mountain sides. If two of these ice-rivers unite to form one glacier, two of these trains will then be borne along in the middle of the resulting glacier. As this glacier continues down the valley, it at length reaches a point where a further advance is rendered impossible by the increased temperature melting the ice as fast as it advances. At this point the train of rocks and dirt are dumped, and of course form great mounds, called moraines. The glacier at times shrinks back on its rocky bed and allows explorers to examine it.
In such cases they find the rocks smoothed and polished, but here and there marked with long grooves and striæ. These points are learned from an examination of existing glaciers. Further down the valley, where now the glaciers never extend, are seen very distinctly the same signs. There are the same moraines, striated rocks, and bowlders that have evidently traveled from their home up the valley. The only explanation possible in this case is that once the glaciers extended to that point in the valley.
It required a person who was perfectly familiar with the behavior of Alpine glaciers, and knew exactly what marks they left behind in their passage, to point out the proofs of their former presence in Northern Europe and America, where it seems almost impossible to believe they existed. Such a man was Louis Agassiz, the eminent naturalist. Born and educated in Switzerland, he spent nine years in researches among the glaciers of the mountains of his native country. He proved the former wide extension of the glaciers of Switzerland. With these results before them, geologists were not long in showing that there had once been glacial ice over a large part of Europe and North America.
The proofs in this case are almost exactly the same as those used to show that the ancient glaciers of Switzerland were once larger than now. But as the great glaciers of the glacial age were many times larger than any thing we know of at the present day, there were of course different results produced.