CAPE COD GEOLOGY 



49 



ward and are seen in the "clay heads" on Cape Cod. These beds, which are now 

 correlated with the Gardiners clay, were once regarded as Tertiary or Creta- 

 ceous, an error due to the occurrence of certain beds of lignitic Cretaceous clay 

 containing fossil plants in Nashaquitsa cliffs at places where the Gardiners clay 

 and the glacial gravel below it and the sand above it are now extensively exposed. 

 Fuller and Veatch showed that this bed of clay lies above a series of beds of gravel 

 that are not distinguishable from the beds of glacial gravel on Long Island and 

 elsewhere to the east through the New England Islands wherever the base of 

 the uppermost bed of blue clay containing no boulders or gravel is visible. 



The correlation of the beds of blue clay on Nantucket with the lenticular 

 beds of blue clay on Cape Cod is doubtful, principally because of the improba- 

 bility that a series of glacial deposits would persistently maintain parallelism, but 

 the field study has afforded no ground for a better correlation. The difficulty of 

 correlation is greatest where the clay occurs as lenses in gravel, as it does at 

 Highland Light, on Cape Cod, though it may be overlain by a bed of fine sand, 

 such as is found on the islands to the southwest. 



The Gardiners clay is dark blue or, in places on Long Island, blackish; its 

 color there being due to lignite. It contains some lignite also on Marthas Vine- 

 yard, derived from underlying Cretaceous beds, which were turned up at the 

 time of the post-Mannetto folding and erosion. Some of the dried clay has a 

 whitish cast, due to the presence in it of fine grains of quartz. Although certain 

 sections contain thin layers the clay is generally massive and was apparently laid 

 down in quiet water off the shore of a shallow sea. 



At a few localities, as on the north coast of Marthas Vineyard east of Menem- 

 sha Creek, the clay is "banded" or laminated. Sayles 1 has presented evidence that 

 such lamination is due to the changes in the conditions of deposition from winter 

 to summer. In deposits of clay laid down on the sea floor it might be expected 

 that the heavier wave action in winter would produce coarser beds in a zone 

 at a given distance from the shore than those formed in the zone in summer, 

 and that the round of the seasons would be registered in an alternation of coarse 

 and fine material, as shown by Baron Gerard de Geer. The banding, however, 

 is not generally so distinct in marine as in fresh-water deposits. 



Fossils and origin. — The marine origin of the Gardiners clay is proved by 

 fossils found in it at the type locality on Gardiner's Island. 2 According to Fuller's 



1 Sayles, R. W., Seasonal deposition in aqueo-glacial sediments, Memoir Mus. Comp. Zool., 47, pp. 

 1-67, pis. 16, February, 1919. 



2 Fuller, M. L., Geology of Long Island, U. S. Geol. Survey Prof. Paper 82, pp. 104-106, 1914. 



