THE WORK OF SNOW AND ICE 259 



the hill the ice from either side may close in under that which came 

 over the top. The debris derived from the top of the hill by the 

 bottom of the overriding ice will then be well up in the ice. It has 

 passed from an initial basal to a subsequent englacial position; but 

 the change does not usually involve an actual rise of the material. 

 If carried upward at all, the upward movement is temporary only, 

 and incident to the passage of the ice up over the hill, or to other 

 local causes. 



Superglacial debris may obviously become englacial by falling 

 into crevasses or by being carried down by descending waters, and 

 either superglacial or englacial debris may become basal by the 

 same means. There is less ice-free land in immediate association 

 with ice-caps than with valley glaciers, and the ice-free land about 

 the borders of an ice-cap is less likely to be in the form of cliffs 

 above it. Except at their edges where the ice is thin, the surfaces 

 of ice-caps are comparatively clean, for there is little land which 

 rises above them in such position as to furnish surface debris. 



Englacial material 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 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 eleva- 

 tion itself. By surface ablation this material may reach the surface 

 at some point below its source (Fig. 212), and be disposed as a 



Fig. 212. Diagram illustrating one way in which a glacier gets englacial 



material. * 



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. Englacial debris, especially that 

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

 bottom of the ice. 



