CAPE COD GEOLOGY 



217 



The Vineyard interval was an epoch of stream erosion prior to the advent 

 of the last or Wisconsin ice sheet, during which the island was considerably 

 modified in contour. 



The Wisconsin stage of glaciation is represented by the surface boulders 

 and rubbly till seen in the tops of the bluffs and by a peculiar rolling topography 

 that masks the stream lines of the Vineyard erosion interval except where those 

 features were strongly developed. Eskers, true kames, and typical frontal 

 moraines are wanting on the island. 



The distribution of these deposits is described in the text on Clay Head and 

 the Mohegan bluffs, where they form the chief features in the coastal scenery. 



BASAL PLEISTOCENE OF THE CLAY HEAD SECTION 



The oldest Pleistocene on the island is exposed in the sea cliff at Clay Head. 

 In the summer of 1892 the section showed two overturned folds of the beds of 

 white clay containing granitic gravel and sand, infolded in synclines. 1 The 

 deposits were regarded as Pleistocene because of the fresh feldspar they con- 

 tained and their lithological similarity to other beds of gravel of glacial origin, 

 either in the kames, eskers, and sand plains of the mainland or, reworked by the 

 sea, on existing beaches. No Tertiary deposits were seen between the Pleistocene 

 and the Cretaceous beds. 



In the summers of 1915 and 1916 the cliff at Clay Head had worn back 

 considerably and the beds, being comparatively free from talus, were much 

 better exposed than in 1892. The infolded oldest Pleistocene beds exposed in 

 clear relations to the Cretaceous deposits consist of fine sandy gravel composed 

 of subangular bits of feldspar-bearing rocks and quartz of vein origin, derived 

 from the mainland, along with reddish felsite — an assemblage indicating an origin 

 from the region about eastern Rhode Island and the adjacent part of Massachu- 

 setts as far northeast as the town of Attleboro, where red felsitic rocks now crop 

 out. Part of the granitic pebbles are somewhat decomposed and the entire 

 deposit has a peculiar drab color, which recalls the lower sand beds at the base 

 of the Pleistocene in Nashaquitsa and Gay Head cliffs, on Marthas Vineyard. 

 In the region to the east this drab color is usually associated with oxidized and 

 hydrated glauconite or greensand derived from the subjacent Miocene beds, and 

 it probably has the same significance at Clay Head. At least four closely ap- 

 pressed overturned synclines inclosing beds of these gravelly sands were exposed 



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

 8, 1897, pp. 197-212. (See fig. 3, p. 210.) 



