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 filled 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 6o° 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- 
