CAPE COD GEOLOGY 



139 



Atlantic coastal plain except a small amount of Miocene greensand at Marsh- 

 field, Massachusetts, and a small bed on the Elizabeth Islands. These beds 

 were probably once continuous with those of the same age on Long Island 

 and in New Jersey. 



NATURE AND AGE OF THE PLEISTOCENE DEPOSITS 



The Pleistocene beds form by far the greater part of the formations exposed 

 on Marthas Vineyard. With the exception of some cemented patches of gravel 

 these beds also are unconsolidated. They differ considerably in lithologic char- 

 acter, consisting of sand, gravel, till, and clay. They differ from the pre-Pleis- 

 tocene deposits in containing unaltered feldspar. The pre- Wisconsin beds are 

 exposed only in the western morainal area and are best displayed in the sections 

 along the shore. The oldest Pleistocene beds are greatly folded and distorted; 

 the later ones are much less so. The Manhasset beds are but slightly folded. 



These deposits range in age from early Pleistocene to Wisconsin. At times 

 deposition ceased and interglacial stream erosion was established. The deposits 

 are mostly of glacial or glacio-aqueous origin, and afford evidence of four dis- 

 tinct advances and retreats of the ice. They cannot be certainly correlated 

 with those of the Mississippi Valley or those of Europe, but the beds on Long 

 Island 1 have been tentatively correlated with those on Marthas Vineyard. 



The Pleistocene series of beds makes up the greater part of Marthas Vine- 

 yard, where it is more nearly complete than it is anywhere else in New England. 

 Together with Cape Cod, Nantucket, No Mans Land, and the Elizabeth Islands, 

 it is the only region, except a few small areas farther north, where the older 

 Pleistocene beds are well shown. Furthermore, Marthas Vineyard has the 

 finest exposures and the most complete series. This island, with Nantucket, 

 also marks the southernmost stand of the Wisconsin ice sheet. It exhibits 

 features of the terminal moraine not found elsewhere in New England, such 

 as the interlobate character of the moraines and outwash and the effects of the 

 overriding ice on older unconsolidated material. 



PROBABLE NATURE AND DEPTH OF THE PRE-CRETACEOUS BASEMENT 



The nature of the rocks composing the basement on which the beds of 

 Martha's Vineyard rest can only be inferred from what is known of the geology 

 of the southeastern mainland of Massachusetts. This basement probably 

 contains no rocks later than Carboniferous, but it may possibly include some 



1 Fuller, M. L., The geology of Long Island, New York, U. S. Geol. Survey Prof. Paper 82, p. 220, 

 1914. 



