GEOLOGICAL HISTORY OF NEW YORK. 651 



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 accumula- 

 tion 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 forma- 

 tion 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 cnystalline 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 en- 

 tirely 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 



