96 



CAPE COD GEOLOGY 



and adopted for the deposits at this horizon the name Sankaty beds, because the 

 beds at Sankaty were fossiliferous and were well known. In Fuller's report 

 on Long Island the Sankaty beds were described as ' 'separable into two distinct 

 units, the Jacob sand and the Herod gravel member of the Manhasset formation," 

 each of which had been traced by him from Long Island to Nantucket. 1 



According to Fuller the beds at Sankaty Head lie below the Montauk till 

 and are well up in the Pleistocene series, having below them the Gardiners clay, 

 about 40 feet thick, the Jameco glacial gravel (with possibly lateral changes to 

 a glacial boulder clay), a lower Pleistocene glacially derived bed of gravel and 

 sand, and finally, as the sections on Marthas Vineyard and Block Island would 

 lead us to expect, a more or less continuous bed of washed glacial boulders at or 

 near the base of the series. 



In a doctorate thesis published by Dr. J. Howard "Wilson 2 the beds at 

 Sankaty Head are regarded as deposits laid down in a lagoon. A lower bed of 

 shells is regarded as well protected from the sea. The upper bed of shells was 

 laid down under colder water, supposed to be due to the readvance of the ice 

 sheet from the north. The fossils in this bed are of more northern types than those 

 in the lower bed. A noticeable unconformity is observed between the fossiliferous 

 beds and overlying fine sand, which contains only minute fragments of shells 

 and is regarded as probably wind-blown material. 



Wilson made very thorough collections of the fossil shells in this bed, and 

 found in the upper bed Serripes laperousii Deshayes and Macoma incongrua 

 von Martens, species belonging to the fauna of the Pacific coast, not found 

 heretofore, according to Dall, east of Point Barrow. He also found Pandora 

 crassidens Conrad, a form common in the Miocene of Maryland but not known 

 at higher horizons northeast of that district. Pandora crassidens and a single 

 specimen of Chrysodomus stonei Pilsby were the only extinct forms discovered 

 in the beds. Wilson regarded the fauna of both beds as Pleistocene, thinking 

 the underlying "clay" of Desor and Cabot a ground moraine of this period. 



In discussing the deposit carrying Macoma incongrua, Dall stated his inclina- 

 tion to "regard that horizon as probably Upper Pliocene." He writes: 



I take it that the glacial period cut off the intercommunication between the two sides 

 of the continent, but there is no reason why some of the Pliocene species should not have 

 lingered on into the Pleistocene on the Atlantic side. But I am confident from the association 

 of species that the period of close intercommunication was Pliocene, because the fauna shows a 

 milder climate than subsequently. This is true all around the northern hemisphere. 3 



1 Fuller, M. L., Geology of Long Island, U. S. Geol. Survey Prof. Paper 82, p. 92, 1914. 



2 Wilson, J. Howard, The glacial history of Nantucket and Cape Cod, New York, the Columbia Uni- 

 versity Press, 1906. 



3 Official letter. 



