Large blocks of stone, quite different from the rocks on which they lie, are very numerous, and are called "erratics," since they are evidently wanderers from a distance. Sometimes such blocks can be proved to have been brought many miles from their home among the higher peaks. The long lines of stones and mud seen on the sides of a glacier are called "moraines," and at the end of every glacier we find a big heap known as a "terminal moraine." But the stones of which they are composed are probably not to be entirely accounted for in this way. Can we not conceive that the weight and pressure of a descending glacier may be sufficient to break off many protruding portions of the rocky bed over which it flows, and then to drag them along with it? This seems reasonable. Let us therefore consider the materials of which moraines are composed to be derived partly from the rocks beneath and partly from those above the glacier. But whatever their origin, such materials must inevitably find their way to the end of the glacier and be added to the big heap there. The work of transportation is then taken up by the stream which always flows from the end of a glacier. Such streams are in summer-time laden with fine sediment, which gives them a milky and turbid appearance.
Thus a glacier wears away the rocks over which it flows; rock fragments become embedded in the ice, and these are the tools with which a glacier does its work. It must be granted that the downward movement of a great mass of ice is irresistible, and consequently that as the moving glacier slowly creeps along, it must inevitably cause the stones which it thus holds to grind over the surface of the rock. It is easy to imagine the effects of this grinding action. If sand-paper, rubbed for a minute or two over wood, wears down and smooths its surface, what must be the result of all these stones, together with sand and mud, grinding over the rocky bed?
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.
THE MER DE GLACE AND MONT BUET. From a Photograph by Mr. Donkin.
We have already alluded in former chapters to the "Ice Age" in Britain, when great glaciers covered all our high mountains, and descended far and wide over the plains. Now, the evidence for the former existence of these glaciers is of the same kind as that which we have just described. In Wales and Scotland we may soon learn to recognise the roches moutonnées, the old moraine heaps, and the erratic boulders brought down by these old glaciers. Besides these proofs, there is also the evidence of the arctic plants now flourishing in the highlands (see chapter iv., pages [123]-[124]).
There can be no doubt, then, that glaciers have an erosive action, and therefore must be regarded as agents of denudation. But it is important to bear in mind that their powers in this direction are limited; for it is manifest that a mountain stream is a much more powerful agent, and will deepen its little valley much more rapidly, than a cumbrous, slow-moving glacier, advancing at the rate of a few inches a day. It has been found by careful measurements that the Mer de Glace of Chamouni moves during summer and autumn at the average daily rate of twenty to twenty-seven inches in the centre, and thirteen to nineteen and one half inches near the side, where friction somewhat impedes its course. This seems very slow compared to the rapid movement of a mountain stream; but then, a glacier partly makes up for this by its great weight.
In considering a glacier as an agent of erosion, we must not forget that probably a good deal of water circulates beneath glaciers. If this is so, the water must have a considerable share in producing the effects to which we have already alluded. It would be extremely rash to conclude, as some students of glaciers have done, that valleys can be carved out entirely by glaciers; and we must be content with believing that they have been somewhat deepened by ice-action, and their features more or less altered, but no more. The valleys of Switzerland, of Wales, and Scotland, were probably all in existence before the period of the "Ice Age," having been carved out by streams in the usual way; but the glaciers, as it were, put the final touches and smoothed their surfaces.