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CAPE COD GEOLOGY 



The boulders and pebbles on the beaches are commingled derivatives from 

 the Moshup till, from the Montauk till, and possibly from the Wisconsin de- 

 posits. No reliable conclusions can be drawn from these erratics regarding the 

 general drift of rocks at any one ice invasion. These materials may have been 

 brought to the island during several successive stages. Among the pebbles on 

 the beaches there were seen red felsite like that which crops out in the Carbon- 

 iferous area from South Attleboro northward toward North Attleboro; gray 

 Carboniferous conglomerate; red sandstone of the Carboniferous type seen in 

 North Attleboro; gray Carboniferous sandstone, probably from the northern 

 part of the Narragansett Bay region; porphyritic granite; gneiss; black schist, 

 probably from the western part of the Narragansett Bay region; and quartzite. 

 One slab of the red Cretaceous sandstone was seen. This fragment must have 

 been dragged out of Cretaceous beds that lay between the island and the eastern 

 entrance to Narragansett Bay. A piece of greenish-white slaty quartzite, re- 

 sembling certain rocks in the Blackstone valley was also seen. 



Marine Action 



The eastern and southern coasts are evidently in rapid retreat. The north 

 coast west of the sandspit appears to have become largely covered with vegeta- 

 tion owing to a cessation of the attack of the waves and currents on that side — 

 an attack which apparently went on when the island was larger and its shape 

 different. Unfortunately no accurate charts of the island are available to permit 

 a comparison of the old coast line with the present and to show the rate of loss 

 of land. The boulders derived from the beds of till greatly retard the retreat of 

 the cliffs and account in part for the preservation of this small island to the 

 present day. 



