l6O 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 



