GLACIAL FEATURES OF THE ALPS g 



ence of the depths of those lakes is partly caused by this difference 

 of accumulation. 



The rule of the cross-sections helps us to understand 'not only 

 the formation of the lake basins in Alpine valleys, but also how the 

 trough's upper end was formed. We usually find it where there 

 was a confluence of the glaciers in the upper parts of a valley, where 

 the slope glaciers united to form the valley glacier. The mass of 

 ice, coming from this semi-circular head, was pressed here into the 

 diameter of the same circle. Now, in order to maintain a continuous 

 movement, an increase of velocity was necessary at this place. This 

 increased velocity must act on the bed of the glaciers until a sufficient 

 depth is attained. Theoretically this depth must be 57 per cent. 



(that is, — — 1) greater than at the semi-circle from which the glaciers 



came. 



The cross-sections of glacier-beds are in general, as was recognized 

 long ago, U-shaped, which indicates that a certain relation between 

 width and depth is the most appropriate one for the glacier's move- 

 ment. This U-form is, however, only constant in homogeneous 

 rocks. In places where there are sudden changes in the nature of rocks, 

 we meet with changes in the shape of the trough. At those places we 

 observe that the glaciers exercise a very strong selective erosion on 

 their bottoms. Some rocks resist more than others, and here the 

 glacier bed shows a remarkable adjustment to the nature of the 

 rocks. Many steps in glaciated valleys are caused by highly resistant 

 rocks. Now and then, but not at all regularly, an increase of width 

 corresponds here to the decrease of depth of the glaciated valley. 

 Conversely, a sudden increase of width in a glaciated valley is often 

 connected with a diminution in the depth of the trough. 



The study of the old glaciers of the Alps reveals that, as far as their 

 movement is concerned, they consist of two parts. In their upper 

 parts, where they were fed by numerous affluents, there was a con- 

 fluence of ice in the main valleys corresponding to the confluence of 

 waters which occurs there now. In their lower parts, however, they 

 no longer received lateral affluents. Here they spread out fan-like 

 on the plains at the foot of the mountain chain, or even in its interior, 

 penetrating into those valleys which afforded them no affluents. 



