196 ALASKA GLACIERS 



of wear below sea-level, and (4) to the parallelism of gla- 

 ciers and rivers. 



THE SURFACE OF A GLACIER 



Evenness as Compared to Rock Floor. The parts of 

 glaciers which came under my observation were the lower 

 or distal portions, with surfaces usually less than 1,000 

 feet above sea-level. In these lower parts the master 

 characters of the surface are a forward slope in the direc- 

 tion of flow and horizontality in the direction normal to 

 the flow, the direction of flow being inferred from the 

 courses of medial moraines. 



The configuration of the rock floors beneath the glaciers 

 could not be directly observed, but it was possible to infer 

 their general characters, with high probability, from what 

 could be seen of the rock floors in front of the glaciers. 

 Such rock floors are in all cases abandoned glacier beds, 

 having been covered by the ice not only in Pleistocene 

 times but, in many instances, in historic time also. As 

 a rule these bared portions of the glacier troughs exhibit 

 much irregularity. The fiords vary rapidly in depth; in 

 places they are diversified by islands. The land troughs 

 have hills and hollows; and other hills jut through the 

 glaciers as nunataks. It is therefore believed that the 

 bottoms of the glacier channels have in general consider- 

 able inequalities. 



These irregularities are only slightly represented in the 

 configuration of the glacier surface. The greater inequali- 

 ties of the longitudinal profile of the bed are shown by 

 cascades of the glacier, but inequalities of the cross-pro- 

 file are rarely indicated by visible shapes of the ice, and 

 bosses or hills hundreds (perhaps thousands) of feet high 

 may fail to influence the surface. If the summit of a boss 

 approaches the surface it produces crevasses, but the 

 broader features of the surface contours are not changed. 



