156 



WORK OF SNOW AND ICE 



been observed only at or near the terminus of the ice. It perhaps is 

 due in part to the resistance of frozen morainic or other material 

 beneath and in front of the edge. To this should probably be added 

 the effect of the great rigidity of the outer part of the ice due to the 

 low external temperature during the larger part of the year, while 



Fig. 158. Diagram to illustrate one method of formation of medial and lateral 

 moraines. The horizontal line at the base represents sea-level, and the lower 

 part of the glacier is under the sea. The layers of upturning ice bring debris up 

 along the planes of movement, and it accumulates at the top as indicated. 



the interior, with its higher temperature, remains more fluent; 

 but even this probably leaves the explanation incomplete. 



Wear of drift in transit. Drift carried at the bottom of the ice 

 is much worn, for the materials in transit abrade one another and 

 are abraded by the bed over which they pass. Englacial drift is 

 subject to less wear, because it commonly is more scattered. Super- 

 glacial drift is worn little or none while it lies on the surface of the 

 ice; but in so far as superglacial or englacial drift is derived from 

 basal drift, it may show the same evidences of w r ear as the basal 

 drift itself. In many cases superglacial drift reveals its history in 

 this way. 



Deposition 



During the advance of a glacier, deposition takes place both (i) 

 beneath the body of the ice, and (2) beneath its end and edges. In 

 the former position it takes place where the topography favors 

 lodgment, or where the ice is overloaded. The topography favoring 

 deposition is much the same as that favoring erosion, but the two 

 processes are not favored at the same points. Erosion is greatest 

 on the "stoss" side (the side against which the ice advances) of an 

 obstruction, and deposition on the lee side (Fig. 159). Glacier ice 



Fig. 159. Crag and tail. The passage of glacier ice is likely to leave drift in 

 the lee of the boss of rock, C. 



