296 GEOLOGY. 



where the surface melting is greatest. Englacial debris, especially that 

 near the bottom, may also become basal by the melting of the bottom 

 of the ice. 



Englacial material plucked or rasped from an elevation over which 

 the ice has passed is liable to be disposed in a longitudinal belt in the ice 

 in the lee of the elevation itself. By surface ablation this material 

 may reach the surface at some point below its source, and be disposed 

 as a medial moraine. Such a moraine has an origin very different from 

 that of a medial moraine formed by the junction of two lateral moraines 

 of superglacial origin. 



Much less in the natural order of things is the transfer of material 

 from a basal to an englacial and from an englacial to a superglacial 

 position by upward movement of the debris itself. Such transfer is 



Fig. 271. — Surface terminal moraines due to upturning. Edge of the ice-sheet, North 



Greenland. 



remarkable because the specific gravity of rock is from two and a half 

 to three times as great as that of ice, so that its normal tendency is to 

 sink. 



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 hes 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. 268). 



At the borders of arctic glaciers the lower layers are not infrequently 

 upturned, as shown in Figs. 269 to 272. Where the layers turn up at 

 the end of a glacier (Figs. 269 and 270), basal and englacial debris is 



