804 ROLLIN D. SALISBURY 



the terminal layers of ice. The same explanation would apply 

 to the lateral margins of a glacier, where the upturning, so far 

 as due to this factor, should be less than at the end, as is the 

 fact. 



Superglacial material. — Apart from the debris which gathers 

 on the surface of glaciers near their ends and edges, and apart 

 from that which gathers on the surface of the ice-cap near its 

 edge, superglacial drift is generally wanting on the Greenland 

 ice. Except in situations just below nunataks, stony debris was 

 not seen at any point on the surface of the Greenland ice-caps, 

 whether main or local, more than a fraction (rarely so much as 

 one-fourth) of a mile back from their edges. On the surface of 

 differentiated glaciers debris was often seen on the surface far- 

 ther back from their ends, but this, in general, was in the form 

 of medial moraines, marking the approximate line of contact 

 between confluent glaciers, or representing material derived from 

 nunataks, or from elevations which projected well up into the ice, 

 but not through it, and therefore did not constitute nunataks. 

 Occasionally there was debris in the form of medial moraines 

 along the anticlinal part of the structure of the ice, as shown in 

 Fig. 24. 



At the very edges of the ice-sheets, and at the ends and 

 edges of the glaciers, and in the position of medial moraines, 

 superglacial drift was abundant, but apart from these situations 

 there was no superficial drift on either ice-caps or glaciers, 

 except the small amount of dust which had reached its position 

 by the help of the wind. The wind-blown dust diminishes in 

 quantity with increasing distance from the edge of the ice-cap, 

 and from the ends and edges of glaciers. It is generally incon- 

 spicuous a mile or two back from the edge of an ice-cap, and 

 inconspicuous on glaciers at any point as much as two or three 

 miles from the nearest land surface. 



That the dust was of wind origin could not be doubted, 

 since it was absolutely free from all material which might not 

 be readily transported by the wind, and did not affect the ice 

 except at its surface. Furthermore, it contained, not infre- 



