CROSBY, GEOLOGY OF LONG ISLAND 429 



north shore. The transverse valleys and bays were occupied by lobes of 

 ice after the uncovering of the intervening peninsulas, — chiefly irregular 

 ridg<3S of Cretaceous and Tertiary sediments and the earlier Manhasset 

 gravels. Bordering the ice-lobes and overspreading the ridges was deposited 

 a second series of deltas and outwash plains (later Manhasset gravels). 



Both the earlier and the later Manhasset gravels merge outward with the 

 moraines and the outwash plains, and, through these, are chronologically 

 as well as stratigraphically continuous, the chief structural contrast being 

 the general absence in the later Manhasset gravels of deformation due to 

 glacial thrust. 



The Manhasset was, in general, never continuous across the bays and 

 harbors, toward which it still presents in part normal ice-contact slopes, 

 and we are thus relieved of the necessity of attributing these wide and deep 

 valleys to the erosive action in post-glacial time of the wholly insignificant 

 tributary streams. 



As noted by Woodworth and others, the bowlder bed conformably 

 dividing the Manhasset Gravel on the west side of Hempstead Harbor is 

 probably best explained as iceberg drift; and to the same agency, apparently, 

 may well be referred, in general, the larger erratics scattered through and 

 over the gravel. The so-called veneer of till over the undisturbed or later 

 Manhasset Gravel, north of the moraine, seems to demand no other explana- 

 tion. It is not a continuous body of drift, but it consists chiefly of widely 

 scattering granitic bowlders devoid of clayey matrix, and is clearly recog- 

 nizable in none of the numerous borings penetrating the Manhasset Gravel. 

 In part, no doubt, it is till (ground moraine) which has not been com- 

 pletely covered by the modified drift (Manhasset Gravel). 



The later Manhasset Gravel is in general entirely undisturbed and no 

 where shows deformation that would not be readily accounted for by a 

 relatively slight movement of the ice during its deposition. In short, proof 

 that the later Manhasset is older than the moraines or was ever over-run by 

 the ice-sheet, is wanting; and hence it may fairly be regarded as the last 

 chapter in the glacial history of Long Island. The only important later 

 contributions to the geology of the island are the post-glacial beach, dune 

 and marsh deposits. It is especially notcAvorthy that there is no evidence 

 of marine deposition during the Pleistocene or between the Yellow Gravel 

 (Lafayette) and the modern shore. 



