Geology of Long Island. 359 



deriving- strata, it evidently rode over them and continued its 

 course southward, pushing before it an immense mass of sand 

 and gravel, together with debris from the rocks of New York and 

 New England. 



The theory that Long Island Sound was a body of water pre- 

 vious to the arrival of the ice-sheet, would seem to be sustained 

 by the character of the detritus deposited by the ice on Long 

 Island. From Brooklyn to Whitestone, where the sound is nar- 

 row, the till or drift proper is quite conspicuous ; east of this it 

 becomes less noticeable, and beyond Eoslyn, as before stated, it 

 does not again occur in abundance until we reach the vicinity of 

 Greenport, where the Sound again grows narrow. This seems 

 to be due to the fact that the finer debris of the northern rocks 

 was carried along imbedded in the lower part of the glacier. 

 The channel of the East Eiver, owing to its narrowness, was 

 rilled up and passed over, the till being deposited to form the 

 range of hills near Brooklyn ; but in crossing the broader part 

 of the Sound, the ice probably lost the greater portion of its load 

 of till, and only carried over the boulders which were on the sur- 

 face or in the upper part of the glacier. On reaching the north 

 shore of the island the alluvial gravel and sands were scooped up 

 and pushed forward in front of the ice-sheet, to form the "mo- 

 raine," and the boulders, when the ice melted, w 7 ere deposited 

 on the surface. The map shows that the principal bays on the 

 north shore penetrate the land in a direction identical with that 

 of the advance of the glacier. We may reasonably infer from 

 this fact, that these indentations were ploughed out by project- 

 ing spurs of ice, and the inference is supported by the fact that 

 the bays are walled in by high ridges which have been formed 

 largely through the upheaval of the beds by lateral thrust. The 

 best example of this displacement in the formation of a bay is 

 shown in the section at Grossman's clay-pit in Huntington, (Fig. 

 2) which I have previously described. Harbor Hill, which stands 

 at the head of Hempstead Harbor, is 384 feet high, and chiefly 

 consists of gravel and sand more or less stratified. Jane's 

 Hill, four miles S.S.E. of the head of Cold Spring Harbor, is 

 383 feet high, and is composed of the same materials. In the 

 vicinity of each of these hills, moreover, there are other ridges 

 and elevations averaging about 300 feet in height. Southeasterly 



