222 



CAPE COD GEOLOGY 



The glacial boulders and other drift of this sheet appear to be rather uni- 

 formly distributed over the island, the mounds, ridges, and hollows of the 

 surface being possibly due in part to irregularities in the thickness of the drift 

 but more likely to the effect of the deformation of the surface by the kneading 

 of the incoherent largely clayey beds by the overriding ice. Neither in the align- 

 ment of these ridges and hollows nor of such boulders as have not been gathered 

 into walls is there indication of any considerable frontal moraine. Such deposits, 

 if formed at all, must have accumulated south of the present land area on a 

 surface that has been cut away by the sea or that now lies below sea level. The 

 surface of the island, which may be called a corrugated moraine, is very similar 

 to that of the northern half of Nantucket, where this type of topography was 

 developed half a mile to a mile or more back of the ice front. Therefore, although 

 Block Island was moulded beneath the ice sheet in the manner described, the 

 front of the ice at the time its surface was formed may have been 1 to 5 miles 

 south of the existing south coast of the island. 



The axes of the corrugated moraine of the island in Corn Neck, north of 

 the central depression, extend nearly westward, but those in the main body 

 of the island, farther south, particularly in its western half, trend southward. 

 These features are brought out best by the detailed chart of the island published 

 by the United States Coast and Geodetic Survey. 1 



The westward trend of the ridges and hollows on Corn Neck coincides with 

 the trend of the axes of the folds seen in the cliffs at Balls Point. The beds in 

 Mohegan Bluffs, on the south coast, show several rather gentle folds, which 

 have axes nearly normal to the coast line and are essentially in agreement with 

 the trend of the formations of the inland surface. 



The corrugations on Corn Hill thus appear to be the effect of pressure 

 acting from north to south, in the general direction of the motion of the ice. 

 This direction is shown not only in the older folded beds below the Jameco 

 formation but in the broadly folded Manhasset formation. The folding in 

 the area south of the depression occupied by Great Salt Pond seems to have 

 been produced by pressure acting from the east or west, and as this folding 

 is more clearly shown" in the west half of that area the pressure probably came 

 from the west. This conclusion is borne out by some small folds in Mohegan 

 Bluffs, which have been overturned toward the east. As Upham many years 

 ago pointed out, Block Island lies in an interlobate position, in the axis between 

 the Narragansett Bay lobe and a broad lobe on the west. The moraines near 



1 TJ. S. Coast and Geodetic Survey, Chart No. 356, scale 1/10.000. 



