CAPE COD GEOLOGY 



177 



Fossils and Age 



The deposition of the Jacob sand immediately followed that of the Gardiners 

 clay. The sand underlies the Manhasset formation without any sign of a break, 

 and the deposition of these three formations was apparently fairly continuous. 

 There is thus clear structural evidence that the Jacob sand is post-Gardiners 

 and pre-Manhasset, or, in other words, that it is of about mid-Pleistocene age. 

 It is a transitional formation between clay and gravel, but it is regarded by 

 Fuller x as a deposit laid down during the Illinoian stage, in spite of the long 

 time which intervened between its deposition and the actual invasion of the 

 Montauk ice. Woodworth (pp. 37, 69 of this report) correlates it with the Yar- 

 mouth interglacial stage. 



No fossils have been found in the Jacob sand on Marthas Vineyard, but 

 some have been found in it on Long Island. Nearly all the forms found are still 

 living in the waters of this region. 



MANHASSET FORMATION 



Name 



The name Manhasset formation is applied collectively to a series of glacial 

 deposits of post-Jacob and pre-Vineyard age. Although the formation is com- 

 posed of three distinct members, formed by the ice that deposited the Montauk 

 till, the deposits may be considered a unit. The top and bottom beds are of 

 essentially the same character and would constitute a continuous bed were it 

 not for the intervening bed of till. Regarding the origin of the name Fuller 1 

 writes: "The name is derived from Manhasset Neck [Long Island], along the 

 shores of which, in the immense gravel pits on the Hempstead Harbor side, are 

 to be seen exposures of these beds more than 100 feet in height. The name was 

 first applied by J. B. Woodworth, who described and mapped in detail the 

 deposits in a part of the west end of the island (Long Island)." The succession 

 and nature of these beds on Marthas Vineyard, as well as their stratigraphic 

 position, are in every way similar to those of the Manhasset formation on 

 Long Island, and the same names are therefore here adopted for them. The 

 term "Tisbury beds" was once used by Woodworth for a part of the Manhasset 

 that was found along the northwest coast and that was approximately horizontal, 

 but Woodworth's "Tisbury beds" included also the Jacob sand and Gardiners 

 clay, and the name was abandoned by him. 



Wp. cit., p. 113. 



