CAPE COD GEOLOGY 



159 



been inverted by folding or not. The upper part of the bed, and in places the 

 whole of it, has been made rusty red by the oxidation of the iron in the 

 original glauconite of the foraminiferal nodules, and at many places in the 

 Gay Head cliffs the so-called greensand is therefore a rusty red bed. 



Thickness 



The thickness of the greensand bed varies greatly in its exposure in 

 the Gay Head cliffs east of the lighthouse, to which part of the island it is ap- 

 parently limited. The bed is at few places more than 3 or 4 feet thick, but in 

 certain folds it appears to be as much as 10 feet thick. 



A small bed of greensand carrying much clay was found in a vertical position 

 in the Nashaquitsa cliffs in 1899, about in the position of the Pleistocene beds, 

 in contact with the Cretaceous deposits exposed there. This bed disappeared 

 in the retreat of the cliffs several years ago. 



Uncomfoemable contact with Ceetaceotjs clay 



The greensand rests with sharp unconformity upon the underlying Upper 

 Cretaceous clay, though the beds are so greatly disturbed that a change of dip 

 in the two formations is not recognizable. Fair exposures of the greensand beds 

 in the Gay Head cliffs may usually be seen at stations 13, 16, 17, 19, 20, 21, 23, 

 24, and 28, on a chart of Gay Head cliffs accompanying a paper by J. B. Wood- 

 worth. 1 



FOSSILS 



The greensand in the Gay Head cliffs has yielded several fossils, most abun- 

 dantly casts of the clam-like form Macoma lyelli Dall. Many of these casts are 

 in the attitude of growth in the foraminiferal mud. Casts of gastropods, the 

 remains of at least two species of crabs, and the teeth of sharks are also occa- 

 sionally found, along with the vertebrae of cetacea, chiefly whales. The verte- 

 brate bones include the tooth of a walrus, possibly that of a seal, and among 

 land animals that of a small rhinoceros. 



The assemblage of fossils found in the greensand suggests a division in 

 the history of the greensand bed into an earlier period, in which the greensand 

 was laid down, possibly in deep water, and a later period, when shallow-water 

 mollusks like Macoma and Panope bored into the foraminiferal mud. The 

 bones of whales found at or near the base of the bed may indicate a still earlier 

 period of shallow water. Certain cobbles or rolled stones, the largest 5 or 6 



Uncomformities of Marthas Vineyard and Block Island, Bull. Geol. Soc. Am., 8, 1896, Plate 16, p. 



197. 



