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CAPE COD GEOLOGY 



by glacial drift and boulders. It is mainly in the recognition of this wrinkle- 

 moraine or pseudo-moraine and its significance as to the age of the deposits 

 forming the mounds, which are scattered over the surface, that this report 

 represents an advance in the interpretation of the structure and origin of a 

 large part of the morainal belt on the outer islands. 



The nearly flat plains on Marthas Vineyard, Nantucket, and Cape Cod, 

 generally underlain by gravel and sand, consist of deposits laid down by streams 

 that flowed out from the ice front, but the plains in certain areas were leveled 

 by running water that flowed over old glacial deposits. Cliffs shaped by the 

 erosive action of sea waves in sand, gravel, clay, and till are common. The long- 

 est stretches of these cliffs are found on the east side of Cape Cod, where they 

 extend from Highland Light southward beyond Chatham. Other well-marked 

 lines of cliffs stand along the south side of Marthas Vineyard, in Chilmark; at the 

 west end of that island, at Gay Head; on the south side of No Mans Land; and 

 on the south side of Block Island. The southward-facing cliffs are composed 

 mainly of dark clays and old boulder beds. 



The relief of the district is generally low, in common with that of the Atlantic 

 coastal plain, of which it forms a peculiar part. The land about Cape Cod Bay 

 and on the islands rises slightly more than 300 feet above mean sea level at 

 three points : Manomet Hill, in Plymouth (altitude about 390 feet) ; Peaked 

 Hill, on Marthas Vineyard (altitude 311 feet); and Prospect Hill, near Beaked 

 Hill (altitude 308 feet). Manomet Hill is wholly of glacial origin, and the two 

 highest points on Marthas Vineyard are drift-capped hills formed by the erosion 

 of deformed Cretaceous and early Pleistocene beds. The highest point on Nan- 

 tucket is a rounded knob of deformed drift, 100 feet high, in the morainal belt. 

 No Mans Land has one similar high point, which stands at an altitude of 110 feet. 

 One point on Block Island has an altitude of 211 feet, and a small area in the 

 northern part of that island and a large area in its southern part rise above 100 

 feet. 



The topography of the upland and of the till-covered tracts in the northern 

 part of Nantucket, the northern and western parts of Marthas Vineyard, and the 

 whole of No Mans Land and Block Island is the net result of the following 

 processes: 



1. Deposition of beds of boulders, gravel, sand, and clay on the sand and 

 clay of the coastal plain. 



2. Deformation, dislocation, and overthrust by ice during the two or three 

 stages of glaciation prior to the last. 



