It is obvious that sand-paper applied "irresistibly" and long enough, must gradually wear away and lower the surface. I cannot therefore resist the conclusion that glaciers have taken an important part in the formation of lakes.
The question has sometimes been discussed as if the point at issue were whether rivers or glaciers were the most effective as excavators. But this is not so. Those who believe that lakes are in many cases due to glaciers might yet admit that rivers have greater power of erosion. There is, however, an essential difference in the mode of action. Rivers tend to regularise their beds; they drain, rather than form lakes. Their tendency is to cut through any projections so that finally their course assumes some such curve as that below, from the source (a) to its entrance into the sea (b).
Fig. 46.—Final Slope of a River.
Glaciers, however, have in addition a scooping power, so that if similarly a d b in Fig. 47 represent the course of a glacier, starting at a and gradually thinning out to e, it may scoop out the rock to a certain extent at d; in that case if it subsequently retires say to c, there would be a lake lying in the basin thus formed between c and e.
Fig. 47.
On the other hand I am not disposed to attribute the Swiss lakes altogether to the action of glaciers. In the first place it does not seem clear that they occupy true rock basins. On this point more evidence is required. That some lakes are due to unequal changes of level will hardly be denied. No one, for instance, as Bonney justly observes,[55] would attribute the Dead Sea to glacial erosion.
The Alps, as we have seen, are a succession of great folds, and there is reason to regard the central one as the oldest. If then the same process continued, and the outer fold was still further raised, or a new one formed, more quickly than the rivers could cut it back, they would be dammed up, and lakes would result.
Moreover, if the formation of a mountain region be due to subsidence, and consequent crumpling, as indicated on p. 217, so that the strata which originally occupied the area A B C D are compressed into A' B' C' D', it is evident, as already mentioned, that while the line of least resistance, and, consequently, the principal folds might be in the direction A' B', there must also be a tendency to the formation of similar folds at right angles, or in the direction A' C'. Thus, in the case of Switzerland, while the main folds run south-west by north-east there would also be others at right angles, though the amount of folding might be much greater in the one direction than in the other. To this cause the bosses, for instance—at Martigny, the Furca, and the Ober Alp,—which intersect the great longitudinal valley of Switzerland, are perhaps due.