Page:The American journal of science, series 3, volume 49.djvu/32

14 14. W. TJpham — Champlain Subsidence and

in beds overlying the glacial drift on the shores of southern New England, Long Island, and New Jersey, and the water- courses which extend from the terminal moraine on Long Is- land southward across the adjacent modified drift plain and continue beneath the sea level of the Great South bay and other bays between the shore and its bordering long beaches, prove that this coast stood higher than now when the ice-sheet extended to its farthest limit. A measure of this elevation of the seaboard in the vicinity of New York during the Cham- plain epoch is supplied, as I believe, by the shallow submarine channel of the Hudson, which has been traced by the soundings of the U. S. Coast Survey from about 12 miles off Sandy Hook to a distance of about 90 miles southeastward. This submerged channel, lying between the present mouth of the Hudson and the very deep submarine fjord of this river, ranges from 10 to 15 fathoms in depth, with an average width of 1J miles, along its extent of 80 miles, the depth being measured from the top of its banks, which, with the adjacent sea-bed, are covered by 15 to 40 fathoms of water, increasing southeastward with the slope of this margin of the continental plateau. During the whole or a considerable part of the time of the glacial Lake Iroquois, this area stretching 100 miles southeastward from New York was probably a land surface, across which the Hudson flowed with a slight descent to the sea. But northward from the present mouth of the Hudson the land at that time stood lower than now ; and the amount of its depression, beginning near the city of New York and in- creasing from south to north, as shown by terraces and deltas of the glacial Lake Hudson-Champlain, which were formed before this long and narrow lake became merged in the glacial Lake St. Lawrence, was nearly 180 feet at West Point, 275 feet at Catskill, and 340 feet at Albany and Schenectady. From these figures, however, we must subtract the amount of de- scent of the Hudson river, which in its channel outside the present harbor of New York may probably have been once 50 or 60 feet in its length of about 100 miles.

Before the time of disappearance of the ice-barrier from the St. Lawrence valley at Quebec, the descent of the Hudson river beyond New York city may have diminished, or the sea- board at New York may have sunk so as to bring the shore line nearly to its present position ; but the Hudson valley meanwhile had been uplifted, so that the outflow from the Lake St. Lawrence crossed the low divide, now about 150 feet above the sea, between Lake Cham plain and the Hudson. This is known by the extension of fossiliferous marine deposits along the Lake Champlain basin nearly to its southern end, while they are wholly wanting along all the Hudson valley.