262 GEOLOGY 



glaciers (Fig. 216) as well as their ends, and the material thus 

 brought to the surface gives rise to lateral moraines. Sometimes 

 also there is an upturning of the ice along a longitudinal zone well 

 back from the lateral margins (Fig. 216), and the material so 

 borne to the surface in such a zone gives rise to a medial moraine. 

 The upturning of ice here referred to has been observed only at 

 or near the terminus of the ice. It is perhaps 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 ice due to the low external temperature during 



Fig. 216. 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 upturn- 

 ing ice bring debris up along the planes of movement, and it accumulates 

 as indicated. 



the larger part of the year, while the interior, with its higher tem- 

 perature, remains more fluent. But even this probably leaves the 

 explanation inadequate. 



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 is commonly 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 

 the basal load, it may show the same evidences of wear MS the bns.-il 

 drift itself. Superglacial drift often reveals its history in this way. 



DEPOSITION OF THE DRIFT 



During the advance of a glacier, deposition may take place 

 both (1) beneath the body of the ice, and (2) beneath its cud and 

 edges. 



1. Beneath the body of the ice deposition takes place where. 

 the topography favors lodgment, or where the ice is overloaded. 



