pleistocene of the Champlain Valley. — Baldwin. 183 
Another evidence of a glacial lake appears in the form of the 
moraines crossing the lake, if they are rightly located. I have 
elsewhere discussed the relation of the water front of a gla- 
cier to the land front under conditions of advance and retreat.* 
Evidently where no water rested against the ice-front the ice 
must have extended farther south in the valleys than on the 
highlands, and consequently the moraines crossing a valley 
will he convex. But the moraines crossing the Champlain 
valley are concave, due to rapid wasting of the retreating ice- 
front by the waters of a glacial lake. 
During or after the greatest extent of the ice. this entire 
region was depressed so that since the departure of the ice 
the valley was occupied by the sea. The depression was 
greatest toward the north, the subsequent differential uplift 
being apparently a little over three feet to the mile. From 
the fact that the shores of the glacial lake seem to show the 
same differential uplift, it appears that the depression had 
reached its maximum during the existence of the glacial lake. 
The marine limit barely, if at all, reached Whitehall, but the 
higher glacial lake probably extended across the divide and 
was continuous with the glacial lake of the upper Hudson, 
but for no great length of time. Further north, toward 
Plattsburgh and St. Albans, the faint action of the glacial 
lake can be found up to 500 or 600 feet. 
The Champlain Estuary. When the ice had retreated to 
the St. Lawrence, the Champlain valley became occupied by 
the sea. The marine clays and shells prove that the sea 
reached the upper limit of such deposits. But the shallow 
water deposits of the sea must have been coarser than the 
clays and higher. Between the limit of clays and tin- faint 
high level terraces is a limit at 75 to 100 feet above the clays, 
marked by the largest and broadest terraces and the immense 
sand deltas of the rivers. This I believe to be the marine 
limit. The shells of the brown clays bear out this supposition, 
as the brown clays are at about the depth, below this limit. ;ii 
which these marine mollusks live. Again, we must suppose 
that the movements of the earth's crust are slow. When thi- 
region reached its greatest depression, it did not pop up again 
suddenly, but must have remained at that level a considerable 
*Am. Geologist, vol: xi, p. 374, June, 1893. 
