294 



GEOLOGY, 



but rather a decline. If carried upward at all, the upward movement 

 is temporary only, and incident to the passage of the ice over the hill, 



Pig. 267. — Diagram to illustrate the taking of debris from a hill-top. It also illus- 

 trates how englacial debris may become superglacial as the result of surface ablation. 



or to other local causes. The englacial debris may be little or much 

 above the basal zone according to the height of the elevation overridden. 



Fig. 268. — Taking debris from a protuberance of the bed. 



Superglacial debris may obviously become englacial by faUing into 

 crevasses or by being carried down by descending waters. Either 

 superglacial or englacial debris may become basal by the same means. 



From their form and position, there is less ice-free land in immediate 

 association with ice-caps than with valley glaciers. Furthermore, 

 the ice-free land about the borders of an ice-cap is less Hkely to be in the 



