198 J. B. WOODWORTH — UNCONFORMITIES OF MARTHAS VINEYARD. 



Page 

 Last glacial drift 211 



Unconformity between last glacial drift and earlier Pleistocene beds 211 



Summary 212 



Introduction. 



Evidence of unconformity in the deposits of the island of Marthas 

 Vineyard has been pointed out by Professor Shaler * in two papers, to 

 which this may be regarded as a sequel. Having familiarized myself 

 with the evidence on this island, I examined Block island with the view 

 of determining the extent of these erosion phenomena and endeavored 

 to apply the methods of Irving and Van Hise in the correlation of the 

 terranes which the several unconformities divide. As the most complete 

 exposure of strata in these two islands is that of the Gay Head cliffs, I 

 have taken that as a standard. 



Gay Head Section on Marthas Vineyard. 



gay head cliffs. 



The topography of the Gay Head cliffs is exhibited on the accompany- 

 ing contoured map (plate 16), which I am able to present through the 

 courtesy of the Superintendent of the United States Coast and Geodetic 

 Survey. On this map are represented only the outcrops of the Miocene 

 where they are ascertained to be in place, and the outcrop on the cliff of 

 the more important thrust-planes. The numbers at the base of the cliff 

 on the map correspond to the stations established in 1889 and recorded 

 on the section published by Shaler as plate 9 of volume 1 of this Bui-" 

 letin. For the general structure of the cliffs, see the cross-section as rep- 

 resented in figure 1, page 207. 



The survey of the cliffs made in 1889 served to show that the Gay 

 Head section was a sedimentary complex of marine, fresh-water, and 

 probably glacio-aqueous formations, with a definite sequence, including 

 unconformities, some of which are involved in folds and faults of one or 

 more epochs. The principal advance then made was the determination 

 by physical means of a stratigraphic succession which has met the re- 

 quirements of paleontologic correlation on the basis of a fossil flora by 

 David White and of Neocene faunas by Dr Dall. Additional work upon 

 the cliffs done from time to time in the conduct of summer schools of 

 Harvard University enable me to state more clearly the existence of un- 



* N. S. Shaler : The Geology of Marthas Vineyard, 7th Annual Report of the U. S. Geological 

 Survey. Washington, 1888, pp. 297-363. 



N. S. Shaler : On the Cretaceous and Tertiary deposits of eastern Massachusetts, Bull. Geol. 

 Soc. Am. New York, 1890, vol. i, pp. 443-452, 



