TRANSVERSE FIORDS 159 



Arm, Behm Canal and Portland Canal, have the character 

 of fiords through their whole extent. Where rivers flow 

 through them, the whole width from wall to wall is 

 occupied by alluvium, and a lace-work of channels indi- 

 cates rapid deposition. The open water has the depth, 

 and the irregularity of depth, characteristic of fiords. In 

 Tracy Arm, which has a breadth of one mile and a length 

 of twenty, the soundings range from 850 to 1,150 feet. 

 The main part of Behm Canal, with a width of two to 

 three miles, has a depth ranging from 1,100 to 1,850 feet. 

 In Portland Canal, which for eighty miles has an average 

 width of two miles, the range of soundings is from 300 to 

 2,100 feet, and a depth of 1,200 feet occurs near the head. 



It is scarcely to be doubted that all these long troughs 

 were initiated by pre-glacial rivers; but the rivers could 

 not have opened their valleys to the present width with- 

 out giving time for the breaking down of the walls. If 

 the river valleys were deep, they were also narrow, and 

 their widening was the work of the Pleistocene glaciers. 

 In the work of widening, the more obstructive projections 

 were removed and the contours of the walls were simpli- 

 fied. The amount of erosion necessary to convert the 

 assumed V-gorges into the observed U-troughs is large, 

 and the ice streams by which it was done could not have 

 failed to wear down the floors of their channels at the 

 same time. While it is quite possible that the down- 

 stream parts of the river gorges had been sunk below 

 present sea-level, the greater part of the excavation below 

 sea-level was probably performed by the glaciers. 



Inequality of Glacial Erosion. The great work 

 which it has seemed reasonable to ascribe to ice in the 

 deepening and widening of fiords and other troughs stands 

 in striking contrast to the feebleness of ice erosion in 

 other places, which permitted, for example, the preserva- 

 tion of the low peneplains of Annette Island and the 



