CAPE COD GEOLOGY 



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or eroded at the present surface, which bears a thin coating of till, some rubbly 

 drift, and here and there beds of sand or clay, as well as embedded boulders, 

 most of them presumably brought here during the last glacial invasion. An 

 examination of the interior of the island gives some clue to the nature of this 

 erosion prior to the last glacial stage, after the Manhasset beds were laid down, 

 for the massive contours of the higher ground show traces of slopes and masked 

 gullies running down in the manner of the vales left in incoherent strata by the 

 action of running water. The structure of the Manhasset beds, as shown in 

 the cliffs, particularly at Clay Head and north of Balls Point, indicates that 

 the Wisconsin ice sheet deformed them and pushed up, at least to a certain 

 extent, the highland on Corn Neck; but despite this action and the masking 

 effect of the glacier upon the antecedent topography, there remains here and 

 there the appearance of a land surface typical of the Vineyard interval, the 

 full nature of which is well shown in the upland part of Marthas Vineyard. 



The traces of old drainage lines are best seen around the northern slopes 

 of Beacon Hill, especially on the northeast versant of the hill, where a well- 

 defined groove widens out downward and the contours of stream work are 

 slightly masked by subsequent glaciation. Over the greater part of the island 

 all semblance of the sculpture accomplished by streams is entirely obliterated 

 by glacial erosion and the deformative work of the Wisconsin ice sheet. (Plate 

 31, fig. 1.) Owing to this defacement of the pre-Wisconsin topography it is not 

 possible to say whether the depression between the northern and southern 

 parts of the island represents the depth of erosion during the Vineyard interval 

 or whether it was made by the Wisconsin ice sheet. 



The flatfish areas in the southern part of the island north and west of 

 Sands Pond, which stand at an elevation of about 120 feet and another area 

 north of Fresh Pond, standing above 100 feet, appear to be remnants of a 

 plain or terrace, which was probably larger during the Vineyard interval. Traces 

 of a pre-Wisconsin surface at about this elevation (100 to 120 feet) are also found 

 in the eastern part of Marthas Vineyard. 



NANTUCKET MORAINE 



CHARACTER AND EXTENT 

 Aside from the beaches and dunes along the coast and the marshes formed 

 in the shallow, protected embayments, chiefly between the northern and south- 

 ern parts of the island, the entire surface is covered with a thin mantle of 

 bouldery drift of the Nantucket moraine of the Wisconsin stage of glaciation. 



