272 



GEOLOGY, 



Debris below the surface. — The lower part of a glacier, as well as 

 the upper, carries rock debris. This debris is sometimes so abundant, 

 especially near the ends and 

 edges of the ice, that it is diffi- 

 cult to locate the bottom of 

 the glacier; for between the 

 moving ice which is full of 

 debris, and the stationary debris 

 which is full of ice, there seems 

 to be a nearly complete grada- 

 tion. The debris in the lower 

 part of arctic glaciers, and to 

 some extent of others, is often 

 disposed in thin sheets sand- 

 wiched in between layers of 

 clean ice. These debris sheets 

 are often numerous and usually 

 discontinuous, though groups 

 of such sheets often persist for 

 considerable distances. Debris 

 also occurs to some extent in 

 the ice well above its base. It 

 is sometimes in belts, as seen in 

 section, and sometimes in 

 bunches. These various rela- 

 tions are illustrated by Figs. 

 227, 229, and 246-249. 



Another characteristic of the 

 basal debris-laden part of some 

 glaciers is the foliation of the 

 ice (Figs. 248, 249, etc). This is especially well shown in the arctic 

 glaciers, the ends and sides of which have steep or vertical faces. 

 The foliation is best developed in the debris zone, though often shown 

 above. The foliation is sometimes minute, consisting of layers of clean 

 ice, an inch or less in thickness, separated by mere films of earthy 

 matter. In extreme cases there are a score or more of laminae within 

 a foot. Locally, and especially where debris is abundant, the laminae 



Fig. 247. — Profile of the lower part of the 

 lateral margin of a glacier. Southeast side 

 of McCormick Bay, North Greenland. 



