332 SURFACE GEOLOGY. 
When the ice-sheet over New York and southern New England melted, 
the sea stood, at least for part of this time, about 200 feet higher than 
now.* At this time the extensive plains of modified drift, forming the 
south side of Long Island and the submarine plateau that extends fully 
fifty miles south and south-west to the New Jersey shore, were deposited, 
being spread nearly level by the waves and currents of the sinking ocean. 
The valley of the Hudson river was also filled with modified drift to a 
height at Albany of 330 feet above the sea. The submarine channel 
proves that after this the sea-level was depressed at least 600 feet lower 
than now, while immense floods pouring down the Hudson valley exca- 
vated these deposits below our present sea-level from Albany southward. 
This channel at Haverstraw bay and the Tappan Zee is two to five miles 
wide. Its south-east portion with the areas on each side is now covered 
by the sea, but it is plainly traceable by soundings for more than a hun- 
dred miles south-east from New York bay. This channel must have 
been excavated, as we have shown, after the melting of the ice-sheet 
over southern New England and New York, for otherwise it would have 
been filled with the modified drift which forms submarine plains on each 
side. 
Although the deposition of modified drift seems to have ended in this 
region before the sea was thus depressed and this channel of the Hud- 
son was formed, it still appears that very immense floods were discharged 
here. The ice had probably retreated from the most of New York state, 
and mainly from the basin of the great lakes, but still obstructed the St. 
Lawrence valley, turning a large part of the floods of this basin into the 
Mohawk and Hudson. This submarine channel thus appears to belong 
to the same epoch in which the beach-ridges about the great lakes were 
being formed. 
At the east edge of the sketch map? representing this former ex- 
tension of Hudson river, may be seen (south-south-east from Montauk 
point) the similar channel of Connecticut river in the same period, less 
notable than that of the Hudson, because the latter discharged vastly 
greater floods. A difficult point in our surface geology, which was be- 
fore unexplained, is made clear by this depression of the ocean below its 
* A bed of marine shells at this height in the modified drift of Long Island is described by Mr. Elias Lewis, Jr., 
in Popular Science Monthly, vol. x, p. 440. 
+ Dana’s Manual of Geology, p. 441; new edition, p. 422; and Popular Science Monthly, vol. x, p. 444. 
