i6o 
ALASKA GLACIERS 
vicinity of Sitka. In the one case the depth of the ero¬ 
sion is measured by hundreds of feet, in the other by 
tens. To a certain extent inequalities of erosion were 
determined by inequalities of resistance, but as the rock 
of the low peneplain is not of notably resistant character, 
and the rock traversed by the transverse troughs is in part 
of highly resistant character, it is evident that this is 
not the dominant factor. To an important extent also 
differences of erosion were determined by differences in 
the depth and consequent pressure of the flowing ice. 
But I conceive that the most important of the variable 
factors was the velocity of the ice currents. At the height 
of the Pleistocene flood the snow-fields were on high pla¬ 
teaus and mountain masses, from which the ice crept in 
broad sluggish streams to the preexisting channels of 
drainage. In these channels it assumed the character of 
rivers, and the lines of pre-glacial water drainage became, 
in the main, the lines of Pleistocene glacial drainage. 
Along the deeper waterways the ice could flow more 
rapidly because its depth was greater, and its ability to 
erode was correspondingly increased. Thus it was that 
the old river gorges, being adopted as lines of flow by the 
ice, were widened, straightened and deepened, while 
the adjacent uplands received comparatively little modifi¬ 
cation. The remnants of low peneplain were preserved, 
despite the softness of their rocks, because they lay out¬ 
side the lines of flow of the strong currents. The remark¬ 
able deepening of the fiords of the mainland is probably 
connected in part also with the fact that they were longer 
occupied by glaciers than were the channels of the archi¬ 
pelago. Many of them still have glaciers at their heads; 
others are flanked by glaciers which would descend and 
fill them should the climate swing but slightly toward the 
Pleistocene condition. 
The more thorough rounding of salient angles at low 
