PLEISTOCENE SEA-LEVEL 
163 
ished. While I do not regard this assumption as valid, 1 
I entertain it for the moment because it gives a minimum 
estimate of the pressure of the Chatham Strait glacier. 
If the then sea plane had the same height as the present, 
the pressure of the glacier would be modified, by assump¬ 
tion, by the sustaining power of 2,900 feet of sea water. 
This sustaining power is equivalent to about 3,300 feet 
of the total ice thickness, leaving 2,700 feet of ice to press 
upon the bottom of the strait; and such a pressure would 
manifestly be ample for the work of erosion. So far as 
our numerical data go, this locality affords an extreme 
case; and as the hypothesis of glacial erosion below 
present sea-level is not barred for this locality, it is prob¬ 
ably not barred for the whole district. 
Another consideration is connected with the reaction 
of the ocean on the front of the glacier. My own obser¬ 
vations, though comparatively limited, are so accordant 
with Dawson’s generalization that I accept with confi¬ 
dence his conclusion that the Pleistocene ice front lay 
outside the present coast line throughout practically the 
whole district. If the ocean had then its present level, it 
washed the ice front for hundreds of miles. The power 
of the ocean to waste a glacier by melting is ordinarily 
greater than the power of atmospheric agents, and along 
the present coast of Alaska is much greater. As the fac¬ 
tors are complex, it would be difficult to give an analytic 
demonstration of this proposition, but it is easy to illus¬ 
trate by an example. The great confluent glacier which 
filled Glacier Bay in the eighteenth century had a general 
depth of 1,000 to 2,000 feet along the axis of the bay, and 
rested with less depth on adjacent tracts of land. All 
through the nineteenth century it was depleted, its wast¬ 
ing being brought about partly by the sea and partly by 
atmospheric agencies. The sea not only melted back the 
1 See discussion in chapter in. 
