UNCONFORMITY FOLLOWING GAY HEAD DIASTROPHE. 207 



At several points where these sands have been folded or thrust under 

 the lignitic series, as in the Gay Head section, they have become firm 

 red sandstones, as at locality 28, 24, and again at 42. 



Unconformity following the Gay Head diastrophe. — Diastrophism has been 

 defined in the abstract as the folding and faulting of strata. The Gay 

 Head diastrophe is a particular instance of folding and faulting. There 

 was, as above noted, some folding before the deposition of the lower 

 boulder bed, but the principal diastrophe on Marthas Vineyard took 

 place after 25 or more feet of gravelly and sandy deposits had been laid 

 down upon the marine beds of the coast-plain. 



By the folding and uplift of this time the present island areas appear 

 to have been brought above the marine limit. The tops of the folds were 

 truncated, but the precise nature of the erosion is not well displayed 



Figure i. — Cross-section of Gay Head. 



Showing the attitude of the strata between the northwest and southwest points of the cliffs. 

 A , Potomac and overlying Cretaceous strata ; B, Neocene, mainly Miocene with the probable 

 Pliocene ; C, Boulder bed and Sankaty sands of Weyquosque or Lower Columbia age ; D, Thrust- 

 planes and faults. The morainal deposits which mantle the surface of the Head are omitted. 



upon Marthas Vineyard. Westward on Block island, at Clay Head, the 

 facts are more clearly shown (see page 210). It is evident, however, on 

 Marthas Vineyard that the next succeeding deposits, those of the Tis- 

 bury beds, are separated from the folded series beneath by an uncon- 

 formity. 



Tisbury beds. — Along the north shore of Marthas Vineyard is a bench 

 of clays and sands with scattered boulders disposed horizontally on the 

 flanks of the highly inclined beds involved in the Gay Head diastrophe. 

 These beds attain a thickness of about 150 feet above the present sealevel 

 in the town of West Tisbury. 



The beds are locally cemented by oxide of iron and are evidently older 

 than the last glacial drift not only by reason of their greater ferrugina- 

 tion, but also by reason of their erosion and stratigraphic relations to the 

 moraine. They are not well exposed at Gay Head. 



Moraine of Marthas Vineyard and Block island. — This outer moraine of 

 southern New England is composed on Marthas Vineyard of bouldery 

 accumulations and gravelly till over the uplands of the island. In places 



