We have no measure of the amount of erosion which New York Island and the adjacent country suffered during the Ice period, but it is not improbable that a mass a hundred feet in thickness was taken from the surface of all the region occupied by the ice.
Most of the finer material ground up by the glaciers was washed out to sea and deposited as the "Champlain clays." Of these there is very little showing in the vicinity of New York, since none of the coast from this point southward has been raised to display them; but a great continental elevation has since taken place toward the north, bringing them at Croton Point 100 feet, at Albany 250, at Burlington 400, at Montreal 500, at Labrador 800, at Davis Straits 1,000, and at Polaris Bay 1,800 feet above the present sea-level.
The coarser portion of the grist ground by the glacier remains as beds of gravel and sand, or heaps of bowlders scattered over the surface of the country where they were left as local moraines, or as the gravel bars of streams flowing beneath the glacier. The greatest accumulation of material transported by the ice in all the country about New York is seen on Long Island, which is indeed a great terminal moraine heaped up along the margin of the continental glacier. As is generally known, Long Island is mostly composed of heaps of gravel and sand, which sometimes form hills from 200 to 300 feet in height, and in these no solid rock has been found in any exploration yet made. The formation of this huge gravel-bank seems to have been, in brief, as follows: The great ice-sheet, moving down from the north in Connecticut and Southern New York, passed over a region occupied mostly by hard, crystalline rocks. These were extensively worn away by it, and much of the material taken from the surface was pushed on as by a great scraper to its margin. When the ice-sheet reached the line of Long Island Sound, it passed from the area of upturned crystalline rocks on to the comparatively soft horizontal Tertiary and Cretaceous strata, which here formed a plain stretching seaward, from the highlands, just as they now do in New Jersey and more southern States. These were scooped out to form the basin of Long Island Sound, and the material excavated from it, as well as much brought from the country lying farther north, was banked up between this basin and the ocean. Thus it will be seen that, of the water-connections of New York Harbor, Long Island Sound is much the most modern; and yet, as a part of it occupies the site of the valley of a large stream—the Housatonic, with perhaps the Connecticut—which passed through the Hell Gate gorge, its formation must have been begun in preglacial times.
As has been said, the rock foundations of Long Island are almost entirely concealed, but a number of cases are reported of the penetration in wells of strata containing Cretaceous fossils, and there is little doubt that the Cretaceous series of New Jersey and Staten Island, represented by the Raritan sands, and the Amboy, Keyport, and Staten Island clays, once formed a continuous margin to the continent, all the way around