CAPE COD GEOLOGY 



117 



CHAPTER II 



GEOLOGY OF MARTHAS VINEYARD 

 By Edward Wigglesworth 



GEOGRAPHY 



General Features 



Marthas Vineyard is the second largest of the so-called New England 

 Islands, being exceeded in size only by Long Island. It is roughly triangular in 

 shape, with the apex of the triangle to the north, and its southern shore line 

 is remarkably straight. Its greatest length from east to west is 20 miles; its 

 length from north to south is about 9 miles ; and its total area is about 96 square 

 miles. Chappaquiddick, at the east corner of the triangle, has been from time to 

 time cut off as an island. Gay Head, at the west corner, has probably long 

 been a part of the main island, with which it is connected by a more permanent 

 beach, behind which a small marsh has been formed. 



Marthas Vineyard is separated from the "heel" of Cape Cod and from 

 the Elizabeth Islands by Vineyard Sound, which ranges in width from about 

 3 miles at the apex of the triangle to more than 5 miles at its west corner. The 

 shore of the island along Vineyard Sound consists of a series of headlands and 

 intervening lower stretches, which form irregular curves. Most of the head- 

 lands are steep cliffs, some of which are more than 100 feet high. The shore 

 between the cliffs is at some places only a low bank or a rather flat beach. Fresh- 

 ly cut parts of the cliffs show that the coast facing the sound is being pushed 

 back. 



The eastward continuation of Vineyard Sound is known as Nantucket Sound. 

 The shore along this sound is a low beach except near the apex of the island and 

 on Chappaquiddick. Near the apex, between East Chop and Oak Bluffs, there 

 is a long bluff about 40 feet high, and on Chappaquiddick there are similar bluffs 

 at the entrance to Cape Poge Bay and the end of Cape Poge. Sengekontacket 

 Pond and Cape Poge Bay are lagoons formed by bars thrown up by the waves of 

 Nantucket Sound. Thus, although the headlands at East Chop and Cape Poge 

 are being cut back, the remainder of the shore has been built forward by the 

 formation of barrier beaches. 



Marthas Vineyard is separated from Muskeget and Nantucket, which lie 

 more than 5 miles to the east, by Muskeget Channel. The east shore of Chappa- 

 quiddick consists entirely of a barrier beach, which is tied at its north end to 



