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CAPE COD GEOLOGY 



This reentrant is probably a submerged valley formed before the last advance 

 of the ice. Two ponds of the same origin, though smaller, are Chappaquonsett 

 and James Ponds. Menemsha Pond, near the southwest corner of the triangle, 

 nearly separates Gay Head from the main island, and just south of Menemsha 

 Pond, beyond that part of the island known as Nashaquitsa, lies Squibnocket 

 Pond. The depressions in which these ponds lie are probably not due to absence 

 of Wisconsin morainal material but to depressions in the surface on which 

 the moraine was deposited. Both ponds would be large, open bays except for 

 barrier beaches, which entirely shut off Squibnocket Pond and would probably 

 shut off Menemsha Pond had not large jetties been built to keep the entrance 

 open. The larger bodies of water on the south shore include Chilmark Pond, 

 Tisbury Great Pond, Oyster Pond, Edgartown Great Pond, and Katama Bay. 

 All these bodies are in the great plain and all have characteristic shapes due to 

 the forms of the channelways in which they lie. With the exception of Katama 

 Bay all have long, narrow arms or branches called coves. On the northeast 

 shore the prominent bodies of water are Cape Poge Bay and Sengekontacket 

 Pond. These also owe their existence to barrier beaches. 



If we attempt to restore the shore line of Marthas Vineyard before these 

 bays and ponds were formed by barrier beaches, we shall be struck at once by 

 the extreme irregularity of the island, in contrast with its present fairly simple 

 outline, yet such an irregular shore line must have existed at some earlier stage. 



Geologic Significance of the Topography 



There is a close relation between the form of the surface and the geologic 

 structure on Marthas Vineyard, but apart from the geologic sections exposed 

 in some of the cliffs there are few places on the island where anything but the 

 surface of the uppermost formation can be seen. At most places, therefore, the 

 nature of underlying beds must be inferred from the topography. The inferences 

 thus made will generally be trustworthy if two facts are borne in mind, namely 

 (1) that in the western area all pre-Wisconsin beds were pushed or dragged up 

 by the ice and form the foundation on which the true Wisconsin morainal ridges 

 rest; (2) that the Wisconsin deposits were laid down on a surface that had reached 

 maturity. Thus the morainal material does not consist entirely of Wisconsin 

 gravel, and not all the valleys can be attributed to the action of the last ice 

 advance and subsequent causes. The topographic evidence is locally of still 

 further value, for the nature and even the structure of the material at some 

 places furnishes no aid in the task of discriminating between one deposit and 



