124 ALASKA GLACIERS 



range from 2,000 to 2,500 feet above tide, and near the 

 mountain rise above 3,000 feet. 



Viewing these hills collectively, one can hardly fail to 

 be impressed with the appearance of system in their 

 crest lines. Some of the hills have broad and flattish, or 

 gently arched, backs; and all the higher parts of their 

 profiles seem to belong to a single gently sloping plane. 

 Should the valleys between them be rilled up even with 

 the crest lines, the group would become a plateau with 

 undulating surface. It is natural for the geologist, when 

 he sees such a harmonious arrangement of hill tops, to 

 seek an explanation in the structure of the rocks, but in 

 this case a structural explanation can not be found. The 

 rock layers are not horizontal but nearly vertical, and 

 they have been cut across in the shaping of the land. 

 The local elements of the upland forms are due purely to 

 erosion. The most probable explanation of the phenom- 

 ena is that the area was first worn down to a plain at or 

 near sea-level, afterward raised so as to be a plateau, and 

 then dissected into a group of hills. The habit of the 

 hills indicates that the principal work of dissection was 

 by streams, but there was also glacial sculpture. They 

 were overrun by the ice-sheet, and the glacial rounding 

 of summit angles helped to obscure, though it failed to 

 destroy, the evidence of the old base-level plain. 



Yet other evidence of the geologic history is connected 

 with the trends. The rock structure strikes northwest, 

 or from the foreground toward the mountains, and this is 

 also the trend of the upland as a whole. But the crests 

 of individual hills trend east of north, making angles of 

 50 to 60 with the strike; and the separating valleys 

 have the same trend. The valleys do not head against 

 high summits among the hills, but traverse the plateau 

 from side to side. They seem to be the work of a system 

 of streams whose courses across the plateau were deter- 



