CAPE COD GEOLOGY 



119 



famous for its striking array of beds of bright-colored clay. (See Plate 14). The 

 ridges in this western area attain elevations of 300 feet. They are not continu- 

 ous throughout the area but are interrupted by small valleys and near the 

 southwest end of the island by two large ponds, which stand at sea level and 

 nearly separate the township of Gay Head from the rest of the island. In ad- 

 dition to these large ponds there are a few small ponds at higher levels. The 

 materials composing the other two areas of the island are too porous to hold 

 water on the surface, and this western area is therefore the only one of the three 

 that has a drainage system sufficiently well developed to form brooks. 



THE NORTHEASTERN AREA 



The northeastern area includes the north-central part of the island, a strip 

 along its northeast shore, and the greater part of Chappaquiddick Island. This 

 area is widest at its west end, where it includes the apex of the triangle that forms 

 Marthas Vineyard. It measures nearly 4 miles from north to south. Midway 

 between its extreme ends it is not more than half a mile wide. Its surface is 

 much less rugged than that of the western area and closely resembles that of 

 the north half of Nantucket. It includes no distinct ridges, and most of it 

 stands not more than 120 feet above sea level. This area is not continuous but 

 is interrupted by Vineyard Haven Harbor and Lagoon Pond, as well as by Ed- 

 gartown Harbor and Katama Bay, which cut off Chappaquiddick Island from 

 the rest of Marthas Vineyard. 



THE GREAT PLAIN 



A great plain makes up the remainder of the island. To the eye this ap- 

 pears to be a great level stretch interrupted only by the remarkable channels in 

 whose southern parts lie the large ponds that are so conspicuous on the topo- 

 graphic map. At its north edge this plain has an elevation of 100 feet, from 

 which it slopes southward at the rate of 20 feet to the mile until it reaches 

 sea level. Most of it is barren and uninhabited and is covered with scrub 

 oaks. (Plate 15, fig. 2). 



Notable Topographic Features 



Ponds and bays are notable features of the topography. A deep reentrant 

 is formed at the apex of the triangle by Vineyard Haven Harbor and Lagoon 

 Pond, which are now nearly separated by a narrow barrier beach, but which 

 once formed a continuous body of water that extended nearly 4 miles inland. 



