GLACIAL FEATURES OF THE ALPS 7 



If there are no sudden changes either in the velocity of the moving 

 bodies — those changes will always disappear in the course of time — 

 or in the precipitation and evaporation or ablation of a certain region, 

 then neighboring cross-sections will increase in the same way as the 

 areas do. They will be nearly equal in size, if there is only a slight 

 increase of the catchment basins between the two sections; they may 

 be rather different if a sudden increase of the catchment basin occurs. 

 If, for example, a river or glacier gets an important affluent, there 

 will be a rapid increase in the size of its cross-section. 



The arrangement of neighboring cross-sections is controlled by 

 the fact that the surface of the moving liquid must have a slope. 

 Their surfaces must continually decrease in height when we follow 

 the direction of the movement, and where two moving fluids unite 

 their surfaces must join at the same level. The surfaces of glaciers 

 as well as the surfaces of rivers obey the law of Playfair, but their 

 channels conform at the same time to the law of cross-sections. 

 The large cross-section of a main river or a main glacier, as well as 

 the smaller cross-sections of their affluents, have accordant surface 

 junction, and therefore their bottoms must have different heights as 

 long as the cross-sections are similar to each other, which is in general 

 the case. The bottoms of side streams hang above the bottom of 

 many rivers in the same way as the bottoms of side glaciers hang 

 above the bottom of main glaciers. The hanging mouth is a feature 

 of the bottom of moving liquids; the accordance, a feature of their 

 surfaces. Since the river surfaces are the base levels of the country 

 along their sides, they govern the heights of the bottoms of the valleys, 

 which therefore, obey, the law of Playfair. 



Every river bed shows inequalities which may be compared with 

 the inequalities of the glacier bed. There is, however, one very 

 marked difference. Most rivers constantly grow, and their cross- 

 sections become therefore larger and larger. On the other hand 

 most glaciers grow only to a certain limit; then they decrease by 

 ablation until they terminate. Therefore their cross-sections will 

 increase at first and then decrease. In a simple glacier the maximum 

 cross-section will be found just at the snow-line; in a composite one, 

 it may occur farther downward. Above and below this maximum 

 cross-section the surfaces and bottoms of the cross-sections will 



