64 



CAPE COD GEOLOGY 



North of the Eastham plain, extending through Wellfleet and Truro, there 

 is a plain that is deeply dissected by channels locally known as "hollows," the 

 bottoms of some of which lie below the present sea level. These hollows appear 

 to have been similar in origin to the channel of Agawam River, in the Plymouth 

 region. If they were occupied by remnants of the ice sheet they appear to have 

 been modified by running water from an ice mass on the east, signs of which are 

 found in kettles and boulders from Highland Light southward, as well as in ice 

 block holes now occupied by lakes. 



At Highland Light the blue clay of the so-called clay pounds, together with 

 an overlying bed of fine sand and an underlying bed of coarse gravel, with an 

 occasional erratic, reproduce in vertical section the essential features of the 

 Jameco gravel, the Gardiners clay, and the Jacob sand in the island on the south- 

 west and have been assumed by Fuller to be the same beds. 



North and west of the Truro high plain, at about the level of the Eastham 

 plain, there is a lower plain, which appears to have been cut back into the high 

 plain. 



The tip of the Cape at Provincetown is formed chiefly by marine deposits 

 and sand dunes. What appears to be an extension of the glacial deposits of 

 Cape Cod, laid down before the north end of the peninsula was cut back by the 

 sea, was seen by Dr. Vaughan in gravel exposed at low tide on the outer beach, 

 near Wood's End. 



Cape Cod Bay is in origin analogous to the New England sounds; it is a 

 depression back of a well-developed frontal moraine, and, like the sounds, it 

 appears to have been scoured and deepened by the action of ice; but drainage 

 lines like those drawn by Shaler 2 in his report on Cape Cod probably had a share 

 in the erosion of the depression in the Vineyard interval prior to the first advance 

 of the Wisconsin ice. 



CERTAIN GLACIAL FEATURES OF THE DISTRICT 



Certain features of the glacial history and the glacial deposits of Cape Cod 

 and the New England Islands may here be considered, namely: 



1. The deformation and dislocation of beds by glacial thrust 



2. The correlation of the glacial deposits of New England with those of the 

 Mississippi Valley 



3. The classification of glacial deposits 



4. The larger glacial erratics 



5. Postglacial changes 



1 Shaler, N. 8., Geology of the Cape Cod district, Eighteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 

 pp. 407-593 (see p. 516, fig. 86), 1898. 



