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CAPE COD GEOLOGY 



north and north by west under the water of Massachusetts Bay. Fragments 

 of similar rock, a ferruginous sandstone or mudstone carrying casts of marine 

 mollusks, are scattered over the surface of the island as far west as Gay Head, 

 but these fragments contain Upper Cretaceous fossils. 



MIOCENE SERIES 



Miocene fossils were seen in the first half of the nineteenth century in the 

 cliffs at Gay Head, and for a time not only all the folded and contorted beds 

 there but all the deposits on the island under the surface moraines were regarded 

 as Miocene. Later scrutiny of the beds and the discovery in them of additional 

 fossils, however, led to the restriction of the Miocene deposits to not more 

 than two distinct beds in the Gay Head cliffs, and one of these beds (the osseous 

 conglomerate), which contains Miocene fossils, is, for reasons already stated 

 in this report, now referred to the very dawn of the Pleistocene epoch, a period 

 before the advent of the glaciers. The Miocene series, therefore, is here repre- 

 sented by a single deposit, long known as the "greensand bed," which in places 

 at Gay Head appears to be underlain by a deep-blue clay that also contains 

 Miocene fossils. 



THE GREENSAND 

 Character 



The Miocene greensand is a greenish to brownish bed, in places clayey, 

 composed largely of the small nodular remains of foraminifers, many of them 

 of genera peculiar to the Cretaceous deposits of the coastal plain of Mary- 

 land. At some places the bed looks so much like certain beds of Cretaceous 

 greensand of the Atlantic coast that some geologists have suggested that it 

 may have been redeposited on Marthas Vineyard from a Cretaceous formation, 

 but no sections thus far exposed in the island have given reason to believe that 

 greensand was laid down in this area during Upper Cretaceous time. Moreover, 

 the occurrence of casts of Miocene mollusks in the greensand bed, many of them 

 in the attitude of growth, and the absence of any Cretaceous admixture of such 

 fossils make it reasonable to suppose that the greensand is a local deposit, formed 

 when the conditions favored the growth of foraminifers and the accumulation 

 of their remains on the sea floor. 



Oxidation 



The normal color of the greensand bed in this area is a peculiar shade of 

 green, which is invariable in the lower part of the bed, whether that part has 



