216 



CAPE COD GEOLOGY 



presents more local detail. The succession of deposits is shown in the following 



list: 



Wisconsin till 



Submarginal ground moraine of Nantucket substage 

 Vineyard erosion interval or interglacial stage 

 Manhasset formation: 



Hempstead gravel member 



Montauk till member 



Herod gravel member 

 Jacob sand 

 Gardiners clay 

 Jameco gravel 



Post-Mannetto unconformity 

 Weyquosque formation 

 Dukes boulder bed 



CHARACTER OF THE FORMATIONS 



The Dukes boulder bed consists of waterworn, rearranged, partly cemented 

 boulders and stones found on Block Island only at Clay Head. The type locality 

 of this bed is at Gay Head, Marthas Vineyard. 



The post-Manetto unconformity is an irregular surface of contact between 

 the beds just described and the overlying Pleistocene formations. This incon- 

 gruity is shown on Block Island at Clay Head, on Corn Neck, but is well dis- 

 played at Gay Head, on Marthas Vineyard. 



The Jameco gravel is glacial outwash that in places carries boulders at its 

 base, if it has been correctly identified at Clay Head. It is believed to be the 

 product of glaciation that profoundly disturbed and folded the older Pleistocene 

 and Cretaceous beds at Clay Head. 



The Gardiners clay is typically displayed on Gardiners Island, where it 

 carries marine fossils in its lower part. The beds are extensively exposed in the 

 lower part of Mohegan bluffs, on Block Island. 



The Jacob sand is nearly everywhere a fine sand composed of grains of 

 quartz, feldspar, and other minerals, such as are produced by the levigation of 

 glacial material by off-shore waves or gentle currents. In places, beds of fine 

 gravel lie at the base of the formation. 



The Manhasset formation consists of glacial gravel below, known as the 

 Herod gravel member, followed above by a very thick bed of boulder clay (the 

 Montauk till member) , which in places on Block Island forms two beds separated 

 by beds of gravel. Upon the Montauk lie the gravel and sand of the Hempstead 

 member, less well preserved on the island as the result of their erosion. 



