210 J. B. WOOD WORTH UNCONFORMITIES OF MARTHAS VINEYARD. 



leaves and fragments of sandstone carrying the marine Upper Cretaceous 

 fauna occur as drift in the overlying Mohegan Bluff beds, pointing to the 



extension, at least in former times, 

 5. N. of the Cretaceous beds north of the 



present site of Block island. 



TERTIARY WANTING AT CLAY HEAD. 



Neither the Eocene nor the Neo- 

 cene have been identified in the Clay 

 Head section or elsewhere on the 

 island. The local erosion of the Mio- 

 cene at Gay Head makes it proba- 

 ble that the Miocene, if not also the 

 Pliocene beds, were originally laid 

 down on this area but were removed 

 either in late Pliocene or early Pleis- 

 tocene time. 



PLEISTOCENE. 



The beds which on Marthas Vine- 

 yard form the lowest Pleistocene are 



Figure 3. — Section of a Portion of Clay Head, 

 Block Island. 



Showing folding and unconformity of the 

 time of the Gay Head diastrophe. A, Creta- 

 ceous clays (Potomac); B, Compound gravels 

 and sands, equivalent to the Sankaty beds ; C, 



Mohegan Biufr beds, equivalent to the Tisbury represented in a fragmentary way on 



beds on Marthas Vineyard and probably upper -p, 1 • i -1 ri-n 1 "Uj. 



Columbia ; I), Last glacial drift, separated from -olOCK island. Ihe graVClS Caught 

 C by unconformity, indicating a long erosion in the Syiicline at Clay Head are 



evidently Pleistocene for the reason 

 that the sediments are compound and not simple silicious residues such 

 as those which make up the Pliocene and older formations of the coast- 

 plain^ They rest here upon the Cretaceous and are evidently separated 

 from them by an erosion unconformity. Structurally the beds occupy 

 the place held at Gay Head by the Sankaty, the Pliocene, and the Mio- 

 cene together. On lithological grounds alone does it at present seem 

 possible to correlate them with the lowest Pleistocene, an identification 

 which carries with it the inference that the Miocene and the Pliocene 

 are wanting at Clay Head. 



The lower boulder bed is probably represented at Clay Head by a mass 

 of ferruginous conglomerate just south of the section shown in figure 3, 

 but the stratigraphic relations of this mass are not clearly revealed. 



GAY HEAD DIASTROPHE ON BLOCK ISLAND. 



The pre-Pleistocene and the earliest Pleistocene on Block island pre- 

 sent evidence of the same disturbance which at the close of Sankaty time 

 profoundly inverted the coastal plain in the Marthas Vineyard area. At 



