CAPE COD GEOLOGY 



189 



West Chop and East Chop cliffs are probably composed largely of beds of 

 Hempstead gravel and overlying Wisconsin till. These beds have been only 

 slightly disturbed. As the Montauk is nowhere exposed, and as it appears at 

 successively lower levels east of Norton Point, it probably lies below these beds 

 of gravel. It is possible, however, that these beds are Wisconsin deposits that 

 accumulated in the interlobate area as the ice front was retreating. The cliffs 

 on both sides of Lagoon Pond are also probably composed of beds of Hemp- 

 stead gravel, although the beds are overlain by considerable sandy and gravelly 

 till. This till is of Wisconsin age and seemed to be composed largely of reworked 

 Manhasset gravel, but the contact between the two is obscure. 



The cliff on the north shore of Chappaquiddick, about 2 miles east of 

 Edgartown, shows thick beds of gravel that are here also regarded as Hemp- 

 stead. 



Age. — The Hempstead gravel represents the closing stages of Manhasset 

 deposition, when the Montauk ice front was retreating. Its deposition followed 

 closely that of the Montauk till, but antedated by a long time the deposition 

 of the Wisconsin. According to the correlation indicated previously in this 

 report, the Manhasset is Illinoian; and as the Hempstead is the uppermost 

 member of the Manhasset it is therefore late Illinoian. 



VINEYARD INTERGLACIAL STAGE 



At the end of Manhasset deposition, before the advance of the Wisconsin 

 ice sheet, there was a long period of normal subaerial erosion, during which a 

 mature system of drainage was developed on Marthas Vineyard. The topography 

 of that time is still preserved in the western area, and, though less completely, 

 in the northeastern area. During this period of erosion the land evidently stood 

 higher than it stands now, for its eroded surface now lies below sea level. This 

 period was obviously long, much longer than the time that has elapsed since 

 the Wisconsin ice invasion, for the erosion then was much greater than that 

 which has taken place since Wisconsin time. 



Shaler 1 presented the evidence of this period of erosion in 1888 and again 



in 1890, and Woodworth 2 called the period the Vineyard interval in 1896. In 



regard to its age Woodworth 3 writes: 



This erosion epoch, which may be known as the Vineyard interval, corresponds to the 

 long inter-glacial epoch which has been so often pointed out by various evidences outside of 



1 Shaler, N. S., Report on the geology of Marthas Vineyard, Seventh Ann. Rept., U. S. Geol. Survey, 

 p. 345, 1888; Tertiary and Cretaceous deposits of eastern Massachusetts, Bull. Geol. Soc. Am., 1, p. 

 448, 1890. 



2 Woodworth, J. B., in The glacial brick clays of Rhode Island and southeastern Massachusetts, by 

 N. S. Shaler, J. B. Woodworth, and C. F. Marbut, Seventeenth Ann. Rept., TJ. S. Geol. Survey, pt. 1, 

 pp. 979-980, 1896. 



3 Woodworth, J. B., op. tit., pp. 208-209. 



