190 



CAPE COD GEOLOGY 



New England. It constitutes the last and the most marked unconformity by erosion on 

 the island of Marthas Vineyard. 



The Vineyard interval was mainly a period of erosion, but a few small beds 

 of peat were then formed. These beds seem to lie below the Wisconsin and 

 recent deposits and above the Manhasset. Such beds of peat, which contain 

 stumps of trees, have been seen at places on the southwest shore of Gay Head, 

 on the northwest shore, and on Squibnocket. All these beds except those on 

 Squibnocket lie nearly at sea level. Similar beds at this horizon on Long Island 

 are mentioned by Fuller. x 



WISCONSIN DRIFT 

 Subdivisions 

 The Wisconsin drift is the last glacial deposit laid down and is therefore 

 the highest division of the Pleistocene series. Its type area is in Wisconsin, from 

 which it was named. Chamberlin, 2 Wright, 3 and others have traced it from this 

 region to Wisconsin and have established its identity in the intervening country. 

 As it is the uppermost glacial formation it is not concealed by overlying deposits 

 and can therefore be studied much better than any of the others. Its diversity 

 here is greater than that of any of the earlier Pleistocene beds, because during 

 one stage of the Wisconsin glaciation this region was occupied by the ice front. 

 It includes three distinct types of deposits, (1) the sheet of till or ground mo- 

 raine; (2) the frontal moraine, here part of the terminal moraine, the outer- 

 most frontal moraine of this ice sheet; (3) the outwash from the ice along the 

 frontal moraine. 



The Till Sheet 



Character. — The till or ground moraine is a deposit laid down beneath the 

 ice. It is composed largely of unassorted gravel, which was of local origin and 

 was dragged along under the ice or liberated from the bottom of the ice by melt- 

 ing. The till on Marthas Vineyard contains very little clay; it is composed mainly 

 of granitic gravel, which is at many places difficult to distinguish from gravel 

 of different origin, whose bedding has been obliterated by overriding ice or 

 for some other reason is not apparent. When it is closely examined, however, 

 an occasional pebble that shows faint glacial striae may be found. To some 

 extent the Wisconsin till varies in its nature according to the deposits which it 



1 Op. tit., p. 157. 



2 Chamberlin, T. C, Preliminary report on the terminal moraine of the second glacial epoch, Third 

 Ann. Rept., U. S. Geol. Survey, pp. 291-402, 1882. 



3 Wright, 1883, G. F., The glacial boundary in western Pennsylvania, Ohio, Kentucky, Indiana, and 

 Illinois, Bull. U. S. Geol. Survey, 58, pp. 39-110, 1890. 



