CAPE COD GEOLOGY 



187 



subdivisions of the Manhasset formation, the name Hempstead gravel member 

 is here introduced, from Hempstead Harbor, along the west side of which the 

 gravel is finely exposed in the upper parts of many large gravel pits." 1 The 

 conditions of deposition and the stratigraphic position of the gravel on Marthas 

 Vineyard seem to have been the same as on Long Island, and the same name is 

 therefore retained for this deposit. 



Character. — Of the upper divisions of the Pleistocene the Hempstead is by 

 far the most indefinite. At many places it cannot be distinguished from the 

 Herod or from the Wisconsin. Over the greater part of the island it appears 

 to be lacking. A few sections, however, seem to prove its presence. 



The Hempstead gravel on Marthas Vineyard is lithologically exactly like 

 the Herod. What has been said here in regard to the character of the Herod 

 applies equally well to the Hempstead. This member, however, is not so widely 

 distributed, because much of it was removed during the following period of 

 erosion. Were it not for its position above the Montauk till this member could 

 not be distinguished from the Herod, and where the Montauk or other key beds 

 are absent or not exposed it is impossible to distinguish them. 



Thickness. — The thickness of the Hempstead gravel cannot be given 

 positively on account of the erosion it has undergone and the scarcity of the 

 exposures. The section at Cape Higgon, if correctly interpreted, shows the 

 greatest thickness exposed anywhere. Here there are about 40 feet of gravel 

 whose base is exposed only where a large amount has obviously been removed 

 from the surface. In other words, this figure may be no more than a minimum 

 one. 



Source of materials. — The source of the material of the Hempstead is essen- 

 tially the same as that of the Herod. Both were abraded from the same region 

 and by the same ice sheet. The glacial front in Hempstead time, however, 

 was retreating, whereas in Herod time it was advancing. 



Relation to other deposits. — The Hempstead gravel overlies the Montauk 

 conformably, and the contact is fairly sharp. The Wisconsin drift lies uncon- 

 formably upon the Hempstead. This unconformity is due to the normal erosion 

 of Vineyard time, which preceded the advent of the Wisconsin ice and lasted 

 long enough to remove much of the Hempstead gravel and to develop a mature 

 system of drainage. 



Structure. — The Hempstead gravel has been much less disturbed than any 

 of the preceding deposits. Indeed, owing to its thinness or its short duration, 



1 Fuller, M. L., op. tit., p. 150. 



