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. Having learned how the three agents of denudation--namely, rain, rivers, and glaciers--accomplish their work, let us now take a wider view of the subject and consider the results of their united efforts both in the present and in the past. We have already alluded to the enormous amount of solid matter brought down to the sea every year by rivers (see chap. v., pp. 166-168), and we pointed out that all this represents so much débris swept off the land through which the rivers flow; also that it comes down in three ways, one part being suspended in the water as fine mud, another part being pushed along the river-bed as gravel, etc., while a third part is the carbonate of lime and other mineral matter in a dissolved state, and therefore invisible.