70 



CAPE COD GEOLOGY 



known that the Labradorean ice sheet advanced at the lowan stage, for Iowan 

 drift has not been clearly identified east of the Mississippi River. 



Three advances of the ice prior to the deposition of the Montauk till and the 

 Gardiners clay appear to have occurred. One of these, indicated by the Moshup 

 till, probably represents Kansan glaciation. The Moshup till and the Dukes 

 boulder bed, which are exposed at but few places and have complicated structural 

 relations, are regarded as representing two advances of the Jerseyan ice sheet, 

 separated by an interval during which the Weyquosque gravel and sand were 

 laid down. 



The relations between the Jameco formation and the beds on which it rests 

 are not well shown either on Block Island or Marthas Vineyard. Many of the 

 beds at Gay Head cliffs and at Clay Head on Block Island are local or lenticular 

 and their stratigraphic order is uncertain. 



A comparison of the deposits of till in the New England Islands is hampered 

 by differences in the state of their preservation. The deposits of the Wisconsin 

 drift remain above sea level essentially as they were left on the retreat of the ice, 

 except for certain weathering. Nowhere do they show modification by marine 

 action at places above the present reach of storm waves. The great bulk of the 

 material in the so-called frontal moraines is water-laid gravel and sand; the 

 boulders, most of which are superficial, are only here and there huddled together 

 along lines comparable with a frontal moraine. The average thickness of the 

 Wisconsin till on Marthas Vineyard, Nantucket, and Block Island is probably 

 not more than 3 feet. Over a large part of the surface the Wisconsin drift is repre- 

 sented only by scattered boulders, which rest in a rubbly surface layer. The 

 thickness of the beds of water-laid gravel in the outwash plains on Nantucket is 

 measurable approximately by their elevation above sea level. Their mean thick- 

 ness in the section not yet cut away by the sea is about 25 feet. The mean thick- 

 ness of the beds of gravel and sand in the plain on Marthas Vineyard is greater, 

 though it is probably less than 50 feet, but the possible presence of pre-Wisconsin 

 beds above sea level in that tract and the absence of borings makes any estimate 

 uncertain. 



The Manhasset series of beds of gravel and gravelly sand and the intercal- 

 ated Montauk till form the thickest glacial deposit on the islands. The Montauk 

 till, which is seen in continuous exposures in the cliffs, is probably thicker than 

 any comparable bed of the Wisconsin till, which is an insignificant rubbly veneer, 

 not thick enough to bury the boulders that form its grosser members. Moreover, 

 the Montauk till is a typical boulder clay, whereas on these islands the Wisconsin 



