Fig. 6.—A diagrammatic cross-section of a recently glaciated valley. AB, the mountain slope which rose above the ancient glacier and has therefore retained the sharp, unrounded forms due to ordinary weathering. BC, the shelf or shoulder, formerly covered by the ice, and therefore strewn with glacial débris. It now usually forms a pasture or alp. The dotted line connecting CC shows the probable form of the pre-glacial valley; CD, the rocky wall of the existing U-shaped valley on whose floor the river now flows.
Associated with the hanging valleys of Alpine regions is the presence of a curious shelf, shoulder, or “bench,” which frequently lies on the top of the cliff from which the lateral streams spring (see [figs. 6] and [7]). Any one who has done some walking in the Alps, must have noticed a peculiar and often trying feature of any walk which leads up the side of the valley. This is that the walk begins with a very steep ascent, where the road or track zig-zags to and fro. After this steep and trying climb the walker reaches a broad shelf (BC in figs. 6 and 7), where the slope is much less, and where the extent of relatively level ground gives room for the erection of a huge hotel, or perhaps only of a group of chalets. This shelf is covered with fine herbage, destined to be cropped by the cows of the community.
Fig. 7.—An actual cross-section of the Lauterbrunnen valley. The vertical and horizontal scales are the same. B marks the edge of the cliff wall over which the streams leap in cascades. A is the position of the stream at the bottom of the U-shaped valley. BC marks the position of the shelf, largely occupied by the pastures or alps. Above them are rocky, unsmoothed slopes.
If the traveller continue his walk he will find that above this pasture ground or alp the slopes are again steep up to the mountain summits. Possibly, however, his walk has been to see a famous waterfall from above, and he will find that the streams which flow with relative slowness over the comparatively gentle slopes of the alp or shelf, will at some point tumble over the region up which he climbed, probably in a series of leaps or cascades.
The U-shaped valley, the “hanging” tributaries, the shelf or shoulder running along the upper part of the cliff wall which bounds the main valley, all these are striking features of glaciated regions. We shall not here discuss the probable causes of this striking “break of slope,” so different from the characteristically continuous slopes of an ordinary mature river valley. As has been indicated, it is here that active controversy rages. It is, however, important to note that the shoulder or bench of which we have spoken was almost certainly once covered by the ice, its gentle slope indicating the original valley floor, before over-deepening took place.
The reason why pasture now grows upon it is that it is covered with fine glacial débris, which makes fertile soil. The fertile soil, which is often irrigated by milky water from existing glaciers, combined with the effect of altitude upon the plants, produces rich pasturage, and makes cattle-rearing an important alpine industry.