CAPE COD GEOLOGY 



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another. The division of the island into the three areas here discriminated is 

 chiefly topographic, but each division is composed of essentially different geo- 

 logic deposits. 



Salient Geologic Features 



Marthas Vineyard presents a wider range of geologic formations than any 

 one of the other islands considered in this report, or than Cape Cod, and is of 

 special geologic interest for three reasons: (1) It is underlain by Cretaceous and 

 Tertiary beds like those found on the Atlantic coastal plain, which are exposed 

 at several places and are not found farther north or east; (2) it is the only one 

 of these islands on which the Tertiary beds seen on the coast plain are manifestly 

 in contact with the overlying Pleistocene deposits ; and (3) the Pleistocene series 

 is more nearly complete here than at any other place in New England and is 

 comparable with the series found on Long Island (see Plate 13.) 



Marthas Vineyard is probably underlain everywhere by Upper Cretaceous 

 beds, but they are exposed only in the western half of the island. Upon these 

 beds, here and there, patches of Tertiary deposits are found. In the eastern 

 half of the island the deposits of both systems lie below sea level. Both the 

 Cretaceous and the Tertiary beds have been greatly deformed by the advances 

 of the ice sheets of the early part of the Pleistocene epoch, and the beds of both 

 systems are unconsolidated, consisting largely of clay, sand, and gravel. Uncon- 

 formablyupon the Tertiary beds lie a remarkable series of Pleistocene deposits, 

 which are similar to those of Long Island. The lowest Pleistocene beds consist of 

 a basal conglomerate overlain by gravel and bouldery till and are only obscurely 

 exposed, but wherever seen they are distorted, along with the Cretaceous and 

 Tertiary beds, and they have obviously undergone long erosion since their de- 

 position. Upon these oldest Pleistocene beds lies a thick dark clay, which grades 

 upward into a fine sand. These beds are at the geologic horizon of the Gardiners 

 clay and the Jacob sand on Long Island and are much less disturbed than the 

 older beds, but they show considerable deformation. Upon these lies a series 

 of beds of sand and gravel separated by a thick bed of till. This series corre- 

 sponds exactly to the Manhasset formation on Long Island, and its deposition, 

 like that of the Manhasset, was followed by a long period of erosion before it 

 was covered with the Wisconsin glacial deposits. The Wisconsin deposits were 

 therefore laid down on a maturely dissected land surface and they attain con- 

 siderable thickness at only a few localities outside of the great plain. At many 

 places they are only 2 or 3 feet thick. 



