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CAPE COD GEOLOGY 



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the shore, a zone of gravel and sand that was swept seaward by the undertow 

 and by offshore currents. Any such broad zone of gravel and sand laid down 

 upon the lowland of eastern Massachusetts and southern Rhode Island during 

 Gardiners time would have been eroded and masked by the action of succeeding 

 ice sheets, but thick beds of gravel and sand may have been here and there rede- 

 posited in kames and sand plains at later stages of the ice. A great mass of strati- 

 fied sand remains on the irregular slope between the upland at Worcester and the 

 lowland along the coast between the 200-foot and the 400-foot contour line, but 

 nothing is known to warrant the reference of any part of this sand to the Gardiners 

 stage. In the Champlain valley, at the end of the last or Wisconsin glacial stage, 

 beds of marine clay were laid down beneath as much as 400 feet of water, or, if 

 Fairchild's estimates are correct, under more than 600 feet. The distribution 

 of the Gardiners clay indicates that the marine invasion then may have covered 

 the eastern lowland of Massachusetts as far west as longitude 71° 30'. 



Where the Gardiners clay lies between beds of gravel and sand, the fall of 

 the clay in cliffs where it is undercut by the sea presents for a time a steep escarp- 

 ment, and landslides carry great masses of clay out over the beach, leaving scars 

 in the cliff. 



JACOB TRANSITIONAL STAGE 

 Jacob Sand 



Upon the Gardiners clay in the New England Islands lie beds of fine sand 

 that range in thickness from 5 to 50 feet. This sand was derived from granitic 

 rock and in mineral composition is like the finer wash found at some places on 

 the mainland, except that here and there it has assumed yellow and orange colors, 

 due to the decomposition of iron oxides and their redeposition in another form in 

 lower parts of the bed, particularly just above the impervious Gardiners clay, 

 where some crusts of iron were formed and are exposed at the edges of the beds 

 in sea cliffs. The type locality of this deposit, described by Fuller, is at Jacob, 

 on Long Island. Good sections of it are exposed in Mohegan bluffs, on the south 

 side of Block Island, as well as on the south shore of No Mans Land and at 

 Nashaquitsa cliffs, on Marthas Vineyard. Much of the fine sand that lies beneath 

 the moraines on the island of Naushon may occupy this horizon. The Jacob sand 

 is exposed also above a small head of Gardiners clay at Sachem Spring, in Sher- 

 burn bluffs, west of the town of Nantucket. Beds of precisely similar sand overlie 

 a bed of blue clay, tentatively referred to the Gardiners formation, in the bluffs 

 at Highland Light, on Cape Cod. On the mainland farther north, in Third Cliff, 



