154 



WORK OF SNOW AND ICE 



Englacial drift may become superglacial by surface ablation. 

 In this case the drift does not rise, but melting brings the surface of 

 the ice down to it. This occurs chiefly at the end or edge of the 

 ice, where the surface melting is greatest. Englacial debris, es- 

 pecially that near the bottom, also may become basal by the 

 melting of the ice beneath it. 



Drift is sometimes transferred from a basal to an englacial and 

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



155. Taking debris from a protuberan 



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

 about three times that of ice, so that the normal tendency 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 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 than 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. 155). 



At the borders of many arctic glaciers the lower layers are turned 

 up, as shown in Fig. 156. Where the layers turn up at the end of a 



