298 



GEOLOGY. 



near the edge of the layers, owing to the penetration of the surface-waters 

 which are much more abundant at the borders than elsewhere, and 

 which in the arctic glaciers probably do not penetrate deeply before 

 they reach a freezing temperature. 



Wear of drift in transit.— Drift carried at the bottom of the ice is 

 subject to notable wear. The materials in transportation abrade one 

 another and are abraded by the bed over which they pass. Englacial 

 drift is subject to less wear because it is commonly more scattered. 

 Superglacial drift is worn little or none while it lies on the surface of the 

 ice; but in so far as superglacial or englacial drift is derived from 

 the basal load, it may show the same evidences of wear as the basal 

 drift itself. Superglacial drift often reveals its history in this way. 



Fig, 274. — Thickening of the upturned layers of ice. 



Deposition of the Drift, 



1. Beneath the body of the ice. — During the advance of a glacier, 

 deposition may take place both beneath the body of the ice and beneath 

 its end and edges. Deposition beneath the body of the ice is Hable to 

 take place wherever the topography favors lodgment, or wherever the 

 ice is overloaded. The topography favoring deposition is much the 



