46 



CAPE COD GEOLOGY 



deposit of stratified material, probably formed about the front of the ice sheet 

 before, during, and after the end of the advance of the ice. 



The beds that were at this time thrown into close folds in the high ground at 

 Gay Head on Marthas Vineyard and at Clay Head on Block Island strike gener- 

 ally from northeast to southwest, the direction of the main axis of the islands, 

 except those at Gay Head, which strike more nearly from northwest to southeast, 

 giving a northeasterly dip to the isoclinal and overturned folds in that area. The 

 low land between Gay Head and the western part of Marthas Vineyard, east of 

 Menemsha Pond, occupies an angle between the two local lines of structural 

 trend and appears to have been a place of maximum forward thrust by the ice. 

 The general direction of the motion of the ice appears to have been normal to the 

 general strike. The boulders and small pieces of Cumberland iron ore (peridotite) 

 found on Gay Head must have been carried about the time of this dislocation 

 from Iron Mine Hill, in Cumberland, R. I., along a line bearing S. 47° E., a line 

 more nearly southeast than that followed by the ice of the Wisconsin stage. 



The mass of Cretaceous beds on Marthas Vineyard that was then disturbed 

 by ice was apparently much larger than any mass subsequently disturbed. This 

 maximum effect might well be expected at the first strong advance of the ice, for 

 at the subsequent advances the deposits would have been pushed into attitudes 

 that would not strongly oppose the overriding ice. Nevertheless, the beds of 

 Gardiners clay and the later Manhasset deposits on the south shore and on Block 

 Island were notably displaced in more or less broad and open folds produced 

 either at the time the Montauk till was laid down or much later, beneath the 

 first Wisconsin ice sheet. 



POST-MANNETTO INTERGLACIAL STAGE 

 Fuller's term post-Mannetto is here applied to the interval of erosion repre- 

 sented by the unconformity between the eroded edges of the first folded beds at 

 Gay Head, on Marthas Vineyard, and at Clay Head, on Block Island, and the 

 overlying nearly horizontal beds of boulders and gravel at those places. Only 

 small exposures of this unconformity are seen, such as those in the southern part 

 of the section at Gay Head. Some of the beds on Block Island were folded after 

 the larger features of the structure had been developed and overthrust faults on 

 Gay Head broke the continuity of the plane of the unconformity. It is not known 

 whether the deformation is due to the continuance of the action of the glacier or 

 to subsequent action by the water of streams or of the sea. The immediately 

 overlying bouldery gravel beds at Gay Head and Clay Head were afterward de- 



