The answer to this question is found in examining the rocks over which glaciers once flowed. Now, the Swiss glaciers once extended far beyond their present limits; and the rocks in the lower parts of their present valleys, now free from ice, show unmistakable signs of having been considerably worn down. The corners and angles of projecting pieces of rock have been worn away until the once rugged outline has become wavy and round, so much so as to produce more or less resemblance to the backs of sheep lying down. Hence the name _roches moutonnées_, by which rocks of this shape are known. They frequently retain on their surface peculiar markings, such as long scratches and grooves which must have been made as the old glacier, with its embedded angular fragments of rock, slowly ground over their surfaces. Such markings are called "striæ." But besides these glacial records graven on the rocks, we have other evidence, in the form of great moraines in some of the valleys of Switzerland, and especially at those places where side valleys open out into a main valley. Any one may learn by a little observation to recognise these peculiar heaps of stones, mud, and sand, deposited long ago by the old glaciers of Switzerland. It will be perceived that the evidence for the erosive power of glaciers is of two kinds,--first, there is the testimony of the smoothed and striated rocks, which is very convincing; secondly, the equally strong proofs from the moraines, both great and small. These old rubbish heaps give us a very fair idea of the amount of wear and tear that goes on under a glacier, for there we see the rock fragments that tumbled down the mountain-side onto the surface of the glacier (together with those which the glacier tore off its rocky bed), all considerably smoothed, worn down, and striated. But a still better idea of the work done is afforded by the gravel, mud, and sand in which these stones are embedded. All this finer material must have been the result of wear and tear. This kind of action may well be compared to what takes place on a grindstone as one sharpens an axe on it. The water poured on the stone soon becomes muddy, owing to the presence of countless little grains of sand worn off the grindstone. But a good deal of the mud thus formed is carried away by the little stream that runs out from the end of every glacier; so that there is more formed than we see in the moraine. [Illustration: THE MER DE GLACE AND MONT BUET. FROM A PHOTOGRAPH BY MR. DONKIN.]