CAPE COD GEOLOGY 



21 



a Tellina had been found in clay marl over a deposit of greensand at a depth of 

 30 feet from the surface. 



In 1890 Shaler J published a brief report on a geologic examination of the 

 greensand deposit at Marshfield, made with the help of his assistant, C. P. 

 Sinnott. Shaler remarks that the area occupied by the greensand, as shown in 

 excavations, covers more than a square mile. The material consists of layers of 

 greensand substantially like those at Gay Head. The beds appear to be horizon- 

 tal. The few molluscan remains found were regarded as possibly of Cretaceous 

 age. 



The greensand collected by Mr. Sinnott contained a few marine fossils — 

 the casts of two mollusks (see PL 5), and fragments of crabs, one like that com- 

 mon at Gay Head and the other an undescribed form with a very large telson. 



Fossils in Greensand at Marshfield, Mass. 



Mollusks: 



Panopaea sp. cf. P. goldfussi Wagner. Internal cast of both valves. 



Venus sp. cf. V. campechiensis var. mortoni (Conrad). Internal cast of right valve. 

 Crustacea : 



Cancer proavitus Packard. 2 



The fossil shell that Jackson called Tellina is probably the Macoma lyelli of 

 Dall, which is common in the greensand bed at Gay Head. The fossils found at 

 Marshfield and the bed containing them resemble so closely the fossils and the 

 bed at Gay Head that the two beds probably belong at the same stratigraphic 

 horizon. 



The deposition of the greensand at Marshfield directly on the granitic 

 surface, without any remnants of the intervening Cretaceous beds found on 

 Marthas Vineyard, shows that there is a great gap here between the Upper 

 Cretaceous and the Miocene, a gap including all of the Eocene and Oligocene 

 epochs. 



ORIGIN OF THE GREENSAND 

 Late in Miocene time the sea invaded the lowland of Massachusetts and 

 Rhode Island, as shown by the position of the beds of greensand in Marshfield 

 and Duxbury, which rest on the granite that then formed the border of the 



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

 1, p. 447, 1890. Also Report on the Atlantic Coast Division, Eleventh Ann. Rept. U. S. Geol. Survey, 

 pt. 1, p. 63, 1891. 



2 Packard, A. S., A new fossil crab from the Miocene greensand bed of Gay Head, Marthas Vineyard, 

 Proc. Am. Acad. Arts and Sci., 36, pp. 3-9, pis. 122, 1900. Cushman, J. A., Fossil crabs of the Gay Head 

 Miocene, Am. Naturalist, 3, pp. 381-390, pis. 1, 2, 1905. 



