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.f 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 f 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. 



t Dana's Manual 0/ Geology, p. 441; new edition, p. 422 ; and Popular Science Monthly, vol. x, p. 444. 



