

CORRASION BY GRAVITY STREAMS. 283 



recession in glacial valley formation, the temporary head- 

 ward limit of growth. Otherwise the glacier — or other 

 stream — occupying the valley head must be possessed of 

 sufficient strength to form the cirque and its continuation 

 below as a valley by a gouging action from behind — an 

 operation mechanically impossible for a glacier. 



Let us assume then that the cirque has been formed, and 

 that the gathering ground is small. Consider now the 

 action of the corrading glacier under conditions of undi- 

 minished volume. The snow banks pass downward into 

 ice, and as the glacier forms in the cirque, the tendency is 

 to produce increase of velocity with the progress of the 

 ice stream down the cirque declivity (thalweg), neverthe- 

 less the actual percentage of the total glacial energy that 

 is expended as corrasion decreases as the steepness of the 

 cirque slope increases. This arises from a consideration 

 of the paths of the ice particles. It is evident also, other 

 things being equal, that the maximum stream force that is 

 expended as corrasion lies at the most abrupt changes of 

 channel slope. In the case of the cirque such point will lie 

 at its base. The tendency here then is to produce a rock 

 basin, as described in Part I of this Series. See also Fig. 

 6 (c). If, however, there is a rapid increase in the valley 

 cross-section (thus reducing the glacial velocity) during 

 this corrasion at the cirque base and sides, then the basin 

 dies away and passes upstream into a more or less gentle 

 slope. This has been considered also in Part I. 



Above the basal portion of the cirque there is a dual 

 action. 



Firstly, there is the important sapping action attending 

 the formation of the basin or other form located at the point 

 where the glacier expends its greatest energy. This sap- 

 ping action it is which tends to give the characteristic 

 profiles to the typical cirque ; or an approximation to the 



