304 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
The depth to which a glacier may cut its channel when it enters 
the sea is of particular importance. If the glacier is a thousand feet 
thick at its end, it must continue to press upon and scour its bed 
until only about 140 feet of ice remain above seadevel; its channel 
will thus be worn more than 800 feet beneath sea-level. Truly, the 
latter part of this work will be performed with increasing slowness; 
but if time enough be allowed the work must be accomplished, just 
as is the case with rivers. If a glacier should melt away from its 
deep entrenchment, its channel would be occupied by an arm of the 
sea or fiord, reaching many miles into the land. The fiord might be 
shallower at its mouth than further inland, if differential erosion and 
deposition had occurred along its channel. Yet even this result is 
analogous to the case of a river; for if the Mississippi were to dis¬ 
appear in a prolonged drought, a slender arm of the sea would invade 
the river channel many miles up-stream from the delta front. Indeed, 
the Mississippi offers an excellent example of a channel that is ba- 
sined inward from the river mouth: for while it is only a score of 
feet deep at the passes where most of its sediment is deposited, it is 
several score of feet in depth further up-stream; and the slender 
arm of the sea that would occupy its channel if it should disappear 
by climatic change, would be truly fiord-like in having a less depth 
at its mouth than further inland. 
An important corollary from this conclusion — perhaps not so much 
of a novelty to glacial erosionists as to their confreres of the opposite 
opinion — is that the depth of water in the fiords of a strongly gla¬ 
ciated coast is not a safe guide to the movement of the land since 
preglacial time. If there had been a still-stand of the earth’s crust 
through the whole glacial period, the preglacial river channels that 
were graded down a little below sea-level at their mouths would be 
replaced by glacial channels that might be eroded hundreds of feet 
below sea-level. The depth of fiords thus seems to depend on the 
size of their ancient glaciers, on the height of the mountain back¬ 
ground, and on the duration of the glacial period, as well as on 
movements of the land. If liberal measures of glacial erosion and 
glacial time are allowed, no depression of glaciated coasts since pre¬ 
glacial time is needed to account for their peculiar features. The 
glacial channels may have been simply invaded by the sea, as the 
ice melted away, without any true submergence. 
Even the advocates of strong glacial erosion do not seem to have 
explicitly recognized the full importance of this possibility. James 
Geikie, for example, writes: “ The fiords of high latitudes and the 
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