TRANSVERSE FIORDS 
J S9 
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 
