CAPE COD GEOLOGY 



17 



found farther southwest, the beds of gravel containing these boulders are of 

 Jameco age ; they were laid down in the third stage of the glacial epoch. 



Of the Oligocene series no trace has been found in the district. The Oligocene 

 epoch appears to have been a time of erosion, in which at least the coastal border 

 of the lowland was reduced to its present relief, but as the overlying marine de- 

 posits are not the earliest of that epoch, the erosion may have begun before Oligo- 

 cene time and continued after its end. 



MIOCENE SERIES 



The Miocene series is represented in this region by remnants of deposits 

 of greensand, which are seen at Gay Head, on the west coast of Marthas Vine- 

 yard; on Nonamesset Island; and at Marshfield and Duxbury, Mass., on the 

 mainland, about six miles north of Plymouth (see PL 4). There was formerly 

 a small deposit of this greensand, carrying much clay, in the Nashaquitsa cliffs, 

 but it was carried away several years ago by the retreat of the cliffs. 



EARLY INVESTIGATIONS 



Locally, at Gay Head, the greensand appears to be underlain by a deep blue 

 clay containing the same assemblage of fossils as the greensand bed. The green- 

 sand, with its fossil crabs, shark teeth, and casts of mollusks, found during the 

 first geological survey of the state at Gay Head, on the west end of Marthas 

 Vineyard, and in Marshfield on the mainland, were identified as of Miocene age 

 by Lyell at the time of his visit to Gay Head in April, 1842. Lyell also assigned 

 to the Miocene series the plant-bearing beds on Marthas Vineyard, but he greatly 

 overestimated the thickness of the beds. Shaler adopted his estimates in his 

 first report on the geology of Marthas Vineyard but modified it later. At Marsh- 

 field the Miocene beds are exposed in wells and are apparently about 30 feet 

 thick and undisturbed. At Gay Head the entire Miocene section, which varies 

 greatly in constitution as well as in thickness as seen in the cliffs, is probably 

 nowhere more than 10 feet thick. 



In 1895 Dall attempted to correlate the Miocene of this region with the 

 Miocene formations in the Atlantic coastal plain south of New York. After 

 examining the fossils collected at Gay Head and Marshfield and visiting the 

 Gay Head section he referred the greensand and the associated osseous conglom- 

 erate of President Hitchcock's report to the Chesapeake or "newer Miocene," 

 which along the Atlantic coast contains a colder water fauna than that of the 

 "older Miocene." 



