Through this valley, probably much deeper than now, during the period of elevation the Hudson flowed on its way to the ocean. Had it been, as it now is, deeply submerged, no river could have flowed in it, nor could it have been maintained as a valley while the deposit of drift was going on. The facts imply elevation of the region to an extent of from 300 to 400 feet above the present level of the sea. This would change in a marked manner the aspect of the coast. The site of the city of New York would be inland and greatly elevated, while the gorges of the Hudson and East Rivers would be deepened and widened by glacial torrents and ice. The ocean-border would be from 70 to 80 miles southward from the present shore of Long Island, and the deepest point attained in the artesian well on Barnum's Island would be above the level of the sea.
There is reason to conclude that the entire subsidence of our coast, from its greatest elevation in the glacial age to the greatest depression of the Champlain period which followed it, was from 600 to 700 feet, possibly much more than that. The elevation which followed carried its stratified deposits not only to their present height above tide, which, as we have seen, is about 260 feet, but at least 63 feet more than that when the buried marsh at Fort Lafayette was formed at the surface. By the present subsidence it is submerged or buried to that depth.
Here we pause. Further observations may confirm or correct our conclusions. Geology has not a more tangled skein than is presented in the structure of Long Island. There is evidence of minor oscillations, and pauses of movement, during which great clay deposits formed in depressions upon the surface, now deeply covered with drift or stratified sands, affording also some evidence that interglacial periods, perhaps of mild climate, occurred, but more observations and more facts are needed to justify a definite judgment on the subject.
Underneath the glacial drift, and underneath the sands which we refer to the advent of the ice, are beds of clay and colored sands which appear to be independent of the drift, and are referred to Tertiary or Cretaceous periods. They appear at the surface along the north side of the island, and are found buried by both unmodified drift and by coarse glacial rubble.
We present in tabular form the series of deposits which seem well defined on Long Island, and which represent the probable order of events, but they are fragmental, and perhaps do not occur anywhere in a continuous vertical series:
2. Stratified gravels and sands of the Champlain subsidence with fossil shells of clam, oyster, and scallop; also wood and lignite.
3. Coarse glacial rubble in deep beds without fossils, representing floods at the close of the Ice period, chiefly on the north side of the island.
4. Unmodified bowlder-drift without fossils.