160 



CAPE COD GEOLOGY 



inches in diameter, composed of a quartz-pebble conglomerate, found at or 

 near the base of the bed in 1889, appear to have been derived from some part 

 of the underlying Cretaceous deposits rather than from the breaking up of the 

 osseous conglomerate, as was then thought. In the course of the reexamination 

 of the cliffs in 1915-16 no such cobbles were found in the greensand bed. 



In places at Gay Head the greensand appears to be underlain by a deep- 

 blue clay containing few foraminifera but the same assemblage of fossils. It 

 was in such a bed that the tooth of a rhinoceros was found. 



From such reconstruction of the highly distorted beds as can now be made, 

 it would seem that the Miocene sea swept over the site of the island while the 

 beds of Cretaceous clay and sand formed an undisturbed eastward extension 

 of the coastal plain like that still remaining southwest of Staten Island, and 

 that the water gradually deepened until the shore line, advancing across the 

 lowland of southeastern Massachusetts and Rhode Island, lay so far from the 

 present shore line that little or no sand and mud reached the site of Marthas 

 Vineyard. During this stage, in water that may not have been more than 

 50 or 60 feet deep, the deposits of foraminifers were laid down by the life and 

 death of these organisms in that part of the sea. The story of the retreat of the 

 sea in the succeeding Pliocene epoch belongs to the account of the immediately 

 overlying fossiliferous sand. 



PLIOCENE SERIES 



A deposit of sand carrying Pliocene fossils was found in 1899, faulted down 

 as a block less than 2 feet thick and having an exposure on the cliffs about 6 

 feet long, in the upper surface of the Miocene greensand bed near the north 

 end of the cliff, at an elevation of 80 feet. This remnant of a marine deposit 

 that was apparently widely extended is the only known Pliocene bed east of 

 Southampton, Long Island. If the interpretation here made is correct, the 

 Aquinnah or osseous conglomerate is a land deposit laid down at the very 

 dawn of the Pleistocene epoch, before the ice sheets reached the south shore of 

 New England. 



AQUINNAH CONGLOMERATE (EARLIEST PLEISTOCENE) 



In the part of the Gay Head cliffs that is described by Hitchcock as the 

 "quadrangular fold," between stations 30 and 36, there is exposed above the 

 white Cretaceous beds a bed of conglomerate, from 12 to 18 inches thick, made 

 up of rounded quartz pebbles; black and grayish, well rounded, smooth, and 

 in places highly polished chert pebbles; and an admixture of cetacean bones, 



