136 
ALASKA GLACIERS 
This interpretation by Willis seems to me reasonable. 
The glacial deposits in the southern part of the Puget 
Sound region are voluminous, and much of their material 
is of distant origin. This is the place where the ice lobe 
discharged its load, and it is not probable that in the field 
of deposition the ice also developed by erosion a system 
of narrow deep troughs. Regarded as stream valleys, the 
channels of the sound tell of a pre-glacial base-level at 
least 500 feet, and probably 1,000 feet or more, below the 
present sea surface. 
A few features seen at the north might be regarded as 
confirmatory, but their interpretation is subject to con¬ 
siderable doubt. They are apparent exceptions to the 
general rule that at all low levels the sculpture forms of 
trough walls are glacial. 
But while the existence of a pre-glacial low base-level 
is on the whole probable, its precise relation to present 
base-level and the period of its duration are altogether 
conjectural. To bring all parts of the deep channels 
within reach of stream erosion it would need to be 3,000 
feet below present sea-level in the region of the Alexander 
Archipelago. Under present climatic conditions, such a 
change would carry a very large area above snow-line, 
and would so promote the alimentation of glaciers as to 
flood the whole district with ice and abolish stream ero¬ 
sion. Stream erosion, therefore, could not have been 
carried, by lowering of base-level, to the lowest parts of 
the channel system without the aid of important climatic 
variation. Without doubting the possibility of wide range 
in independent climatic factors, it seems easier to assume 
that the lowering of base-level was comparatively mod¬ 
erate, and that a considerable part of the down-cutting of 
the channels was accomplished by Pleistocene glaciers. 
There is equal doubt as to the duration of the low 
base-level, or the extent of the erosive work it enabled 
