260 



GEOLOGY 



Drift is sometimes transferred from a basal to an englacial and 

 then to a superglacial position, by upward movement. Such trans- 

 fer is the more remarkable because the specific gravity of rock is 

 two and a half to three times that of ice, so that the normal tend- 

 ency of rock is to sink in ice. 



In arctic glaciers, and probably in others, some material which 

 has been basal becomes englacial by being sheared forward over 



Fig. 213. Taking debris from a protuberance of the bed. 



ice in front of it. So far as observed, this takes place chiefly where 

 the ice in front of the plane of shearing lies at a lower level th.-m 

 that behind, as where the surface of an upland falls off into a valley, 

 or where a boss of rock shelters the ice in its lee from the thrust of 

 the overriding ice (Fig. 213). 



At the borders of arctic glaciers the lower layers are not infre- 

 quently turned up, as shown in Fig. 214. Where the layers turn 

 up at the end of a glacier, basal and englacial debris is carried to 

 the surface by actual upward movement, and a terminal moraine 

 or a series of terminal moraines is sometimes aggregated where the 



