134 



CAPE COD GEOLOGY 



a pond. Others of similar origin separate Sengekontacket Pond from the sea, 

 and cut off Lagoon Pond from Vineyard Haven Harbor. 



At the west end of the island, on the north and southwest coasts of Gay- 

 Head, there are two wide stretches of beach and dune. One extends from a 

 point about a mile east of Gay Head cliffs to the entrance of Menemsha Pond, 

 and its eastern part shuts off this pond from Vineyard Sound. The other extends 

 from the south end of Gay Head cliffs to Squibnocket and encloses Squibnocket 

 Pond. These beaches are being built forward, largely with material from the 

 Gay Head cliffs, and are accompanied by large dunes. 



Barrier beaches or bars, some of which convert former bays into ponds, 

 are found along the shore of Vineyard Sound, especially where valleys come down 

 to the coast. Such are the bars in front of Chappaquonsett and James ponds. 



DESTRUCTIVE ACTION 



Most of the forms produced by the destructive action of the waves are sea 

 cliffs. A large part of the shore on the two northern sides of Marthas Vineyard 

 consists of headlands that end in cliffs, between which lie stretches of beach. 

 As these cliffs are composed of unconsolidated material, and as many of them 

 are exposed to the action of strong waves, their appearance is considerably 

 altered from year to year. The two largest and most striking cliffs are the 

 Gay Head cliffs and the Nashaquitsa cliffs. Lesser cliffs are seen near Roaring 

 Brook and at Cape Higgon, Norton Point, East Chop, Stonewall Beach, and 

 Squibnocket. 



The Gay Head cliffs, long famous for their vivid colors, are about three- 

 quarters of a mile long and are 140 feet high at their highest point. They con- 

 sist of Cretaceous clay and some Miocene and Pleistocene beds, all much folded. 

 The material and the structure have determined in large part the picturesque 

 forms into which they have been cut by erosion. (See Plate 14). The more 

 resistant beds of clay form small headlands; the less resistant beds have been 

 cut back and form recesses, such as the Devil's Den. Landslides have deter- 

 mined some of the forms, although most of these are, in turn, caused by the 

 undermining action of the waves at the base of the cliffs. The colors are due 

 in part to the original materials in the beds and in part to the weathering of 

 these materials, especially those containing iron oxides. All the colors have 

 "run" more or less, for the rain washes down from the upper beds material 

 that coats the lower ones. The Gay Head cliffs evidently once extended farther 

 northwest, for the sea has long been cutting them back southeastward. The 



