Pleistocene of the Champlain Valley, — Baldwin. 179 
Deltas. 
When the main preglacial stream of the Champlain valley, 
the Champlain river, as I shall call it, flowed in a channel 400 
feet or more below the present lake surface, the mouths of its- 
tributaries, the Winooski, La Moille, and others, must have 
been proportionately below their present hight. These val- 
leys were filled up, partly with drift during the Glacial period, 
then with the blue clays, brown clays, and sands, during the 
subsequent submergence. At the mouths of the rivers, gn.it 
deltas were formed which now stand not only high above the 
former deep channels, but high above the present channels. 
These tributaries now Mow entirely in clay or sand beds, ex- 
cept here and there where they have not returned to their old 
valleys, but have cut" small recent gorges of rock instead. The 
history of the delta seems to be this. The highest faint levels, 
which I believe to have been formed by a temporary glacial 
lake, were of too short duration for the accumulation of 
marked deltas. But on admission of the sea, the highest ma- 
rine level was more permanent; the rivers were backed up as 
slack water far inland, and great quantities of sand and clay 
were washed from the glacial deposits by the Hooded streams. 
the sand to be left along the upper part of the delta and 
stream, while the clays were being deposited in deeper water. 
Then, as the water fell from this highest marine level to the 
next hight, much of the highest delta sand was washed out 
to be spread as a second delta over the clays of the first bight, 
while some of this clay was in turn carried out to cover the 
finer clay of the deepest waters. Thus as the lake fell to 
lower levels, each terrace was partly cut away to form, with 
what fresh material was brought down, the next terrace. An 
interesting explanation of the derivation of the sands and 
clays may be seen, high above the highest terraces, at Kbit 
Rock, Altona, N. Y., where are immense accumulations of 
glaciated boulders, in which is no materia] of even small 
enough size to be called gravel, the smaller material having 
been entirely carried away. 
Preglacial Channels and Postglacial Gorges. 
Suppose such a high level delta formed, and that, as the lake 
falls, the river cuts down into the level-topped terrace. Then. 
