140 



CAPE COD GEOLOGY 



earlier Paleozoic rocks, which may be accompanied by intrusive rocks. These 

 Paleozoic rocks are probably beds of more or less metamorphosed conglomerate, 

 sandstone, and shale. The surface of the basement series was probably pene- 

 planed during Cretaceous time. The depth at which this basement series lies 

 is also unknown and can only be roughly inferred. By calculating the average 

 slope of the hard rock surface from northern Rhode Island to the northwest 

 shore of Buzzard's Bay and projecting this to Marthas Vineyard we get a 

 depth of 500 to 600 feet. Only deep borings can give the depth accurately. 



Cretaceous System 



NAMES OF THE BEDS 



No geologic name has been proposed for the series of Cretaceous beds 

 exposed on Marthas Vineyard, but locally and commercially a part of the 

 series has been called the "Gay Head clays." Shaler applied the term "Vine- 

 yard series" to the greater part of these beds and to the associated Tertiary 

 beds, but he thought the whole series was of Tertiary age. Ward called them 

 the "Island series," but this term has not been used by later writers. 



SUBDIVISIONS 



The beds of Cretaceous clay are so greatly folded and overthrust at the 

 only place where they are well exposed that their subdivisions and relations 

 cannot easily be determined. A paper by Shaler 1 , published in 1890, con- 

 tains a section of the Gay Head cliffs prepared by Woodworth, in which the 

 following beds are assigned to the Cretaceous system: (1) lignite beds; (2) 

 noduled clays and leaf beds; (3) white micaceous sand and clay. In a later 

 paper Woodworth 2 describes all three of these beds as ' 'non-marine lignitic and 

 leaf-bearing clays," and added "marine sands and clays of Upper Cretaceous 

 age." These beds of sand and clay were not clearly exposed at Gay Head, 

 but numerous fragments of a micaceous hematitic sandstone containing Creta- 

 ceous fossils had been found at Indian Hill, 10 miles to the northwest. 



Fuller 3 gives a more complete columnar section of the Cretaceous on Long 

 Island but adds that the deposits "differ extremely in composition within short 

 distances." Similar differences would probably be found on Marthas Vineyard 



1 Shaler, N. S., Tertiary and Cretaceous deposits of eastern Massachusetts, Bull. Geol. Soc. America, 

 1, pp. 443-452, 1890. 



2 Woodworth, J. B., Unconformities of Marthas Vineyard and of Block Island, Bull. Geol. Soc. 

 America, 8, pp. 197-212, 1897. 



3 Fuller, M. L., Geology of Long Island, New York, U. S. Geol. Survey Prof. Paper 82, p. 67, 1914. 



