THE WORK OF SNOW AND ICE 



245 



on either hand. So far does this spreading go, that in some cases 

 the lower end of a glacier is completely covered with the debris 

 whicii has' spread from the medial and lateral moraines. 



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

 the upper, carries rock debris. This debris is sometimes so abun- 

 dant, especially near the ends and edges of the ice, that it is difficult 



Fig. 201. Side view of end of glacier. Southeast side of McCormick Bay, 

 North Greenland. Shows structure of ice as well as position of debris. 



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 gradation. The debris in the 

 lower part of arctic glaciers, and to some extent of others, is 

 often disposed in thin sheets sandwiched in between layers of clean 

 ice. Debris also occurs to some extent in the ice far above its 

 base, sometimes in sheets and sometimes in bunches. These 

 various relations are illustrated by Figs. 201 and 202. 



Temperature 



In winter, the surface of the ice becomes as cold as the air 

 above it, and the cold of the surface penetrates downward. This 



