CAPE COD GEOLOGY 



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This deposit, which consists of rounded pebbles of quartz in a quartz matrix, 

 is the only one in the Pleistocene series that precedes the Jameco, from which 

 it is separated by a marked unconformity representing a long period of erosion. 

 According to Fuller, 1 the Mannetto does not exist on Marthas Vineyard, having 

 probably been removed by post-Mannetto erosion. The Jameco gravel, accord- 

 ing to Fuller, is the oldest Pleistocene deposit on Long Island. To this deposit 

 he assigned the well-exposed gravel that lies below the Gardiners clay in the 

 Nashaquitsa cliffs. The present writers, however, believe that four older Pleisto- 

 cene beds lie below the Jameco gravel. 



Sections in Gay Head Cliffs 



Aquinnah conglomerate. — In the Gay Head cliffs, directly below the light- 

 house, there is a large quadrangular fold in which the lower members are beds 

 of Upper Cretaceous sand and clay. Above these beds, according to Wood- 

 worth, there is a thin bed, formerly thought to be of Miocene age, called by 

 Hitchcock the osseous conglomerate. The larger constituents of this bed are 

 small, well-rounded, white quartz pebbles, chert pebbles, and cetacean bones. 

 The bones and some shark teeth, as well as the fact that the deposit was formerly 

 supposed to lie below undoubted Miocene greensand, led to the belief that it 

 also was of Miocene age. Later, however, a metacarpal of a Pliocene camel 

 and an astragalus of a Pleistocene horse were found in this bed 2 (Plate 22). It 

 must therefore be regarded as a Pleistocene deposit formed before the ice 

 reached the region. The presence of Tertiary fossils in the bed is due to the 

 erosion and redeposition of preexisting deposits. Woodworth has given to this 

 osseous conglomerate the name Aquinnah, the Indian name for Gay Head. 



Dukes boulder bed. — Above the Aquinnah conglomerate in the Gay Head 

 cliffs there is a remarkable bed of boulders, entirely free from clay, which 

 was described by Woodworth in 1897, 3 and by Shaler in 1889. This bed at 

 one time contained boulders as much as 8 feet long, weighing about 4 tons. The 

 boulder bed is composed of granite, diorite, and gneiss from the mainland and 

 includes also a small boulder of peridotite from Iron Mine Hill, in Cumberland, 

 Rhode Island, 60 miles away. The bed is locally cemented by limonite and 

 contains pebbles of Cretaceous sand and clay. Many of the boulders are much 

 decayed and can be broken up readily with a hammer. The nature of this bed, 



1 Op. cit., pp. 219-220. 



2 Woodworth, J. B., Glacial origin of older Pleistocene in Gay Head cliffs, with a note on fossil horse 

 of that section, Bull. Geol. Soc. Am., 11, pp. 455-460, 1900. 



3 Woodworth, J. B., Unconformities of Marthas Vineyard and of Block Island, Bull. Geol. Soc. Am., 

 8, p. 205, 1897. 



