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CAPE COD GEOLOGY 



55 



show that one or more of the Pleistocene ice advances reached a latitude nearly 

 50 miles south of the present south coast of Marthas Vineyard. 



The boulders in the Montauk till, which were embedded in impervious clay; 

 are fairly fresh looking. Many of them are less weathered than the Wisconsin 

 boulders, but what number of them were wrested from ledges during the Man- 

 hasset ice invasion and what number were derived from older deposits of till can- 

 not be determined, for many of the larger boulders in pre-Montauk beds are 

 but slightly weathered. 



The lithologic character of the boulders in the Montauk till is practically 

 identical with that of the boulders in the Wisconsin drift. The greater part of 

 the boulders and larger cobbles on the beaches of the south coast of Block Island, ■ 

 No Mans Land, and Marthas Vineyard were probably derived from the Mon- 

 tauk till, and where that till has been folded and its beds have been eroded the 

 large boulders now found on the surface may be residual blocks that were trans- 

 ported for short distances and redistributed, especially by the later Wisconsin 

 ice sheet, into which they would have been incorporated and commingled with 

 boulders derived from the mainland. 



Nothing seen in the Montauk till indicates that the land was then submerged; 

 indeed the beds of coarse gravel immediately above and below the till indicate 

 that the places at which these deposits are found may have stood anywhere from 

 just at sea level to any elevation above the sea. In other words, the deposition of 

 the entire series of Manhasset beds appears to have involved no action of the sea. 



Hempstead gravel member.— On Long Island deposits of glacial gravel and 

 sandy gravel lie upon the Montauk till in the gravel pits at Hempstead, and 

 thinner beds of similar gravel or sandy gravel may be seen here and there above 

 the Montauk till along the coasts of the islands as far east as Marthas Vineyard. 

 If the beds of bouldery clay about Barnstable, on the bay side of Cape Cod, are 

 also of Montauk age, as seems probable, the beds of gravel overlying the de- 

 posit there are extensions of the Hempstead gravel. 



The Hempstead gravel is the natural successor of the boulder bed, for 

 during the retreat of the ice front across a field of subglacial till streams would 

 at once flow out upon the surface and deposit the drift washed out from the 

 melting ice. Farther south sand and still farther out clay might be deposited. 



The gravel at this horizon appears to be the last remaining deposit laid down 

 in this region above sea level before the Wisconsin ice advance; but in the interval 

 between the disappearance of the Manhasset ice sheet and the advance of the first 

 Wisconsin ice sheet in the outer islands there was a long period of stream erosion, 



