184 
ALASKA GLACIERS 
low terraces observed in Prince William Sound are pos¬ 
sibly of the same order. It is, furthermore, probable that 
the pre-Pleistocene dissection of each area was continued 
in association with a comparatively low base-level. In 
view of the large number of common elements, the whole 
region, with the exception of the high mountain district, 
may be provisionally regarded as a unit in its later geo¬ 
logic history. The uplifted peneplains do not all stand at 
the same height, and there are important differences of 
altitude within individual plateaus. These local differ¬ 
ences suffice to show that not all changes can be ascribed 
to the sea, and make it probable that the plateaus were 
created by changes originating within the earth’s crust. 
The low peneplains also show minor discordances, and 
while these also must be ascribed to crustal movement, 
the fact that they are of moderate amount is, on the 
whole, indicative of general crustal stability. It is a note¬ 
worthy, and probably significant, fact that the oceanic 
base-level of the region, after resting for a long time at 
the height indicated by these low peneplains, dropped 
below them at the time of the excavation of the fiords 
and then returned to approximately the same position. If 
this oscillation was an oscillation of the land, it was of the 
broad or epeirogenic type, and the association of wide 
extent with approximate uniformity of position after the 
completion of the cycle is most remarkable. It would 
seem probable that so great a general movement of the 
earth’s crust would afford opportunity for the relief of 
local strains, and thus be accompanied by important 
differential movements. On the other hand, sensible uni¬ 
formity for any region of the magnitude here considered 
would theoretically be a characteristic of an oscillation 
of the sea. The local evidence, therefore, seems to me 
more favorable to the hypothesis that the sea was low 
during the fluvial and glacial erosion of the fiords and has 
