CAPE COD GEOLOGY 



143 



RELATION OF THE CRETACEOUS TO OLDER DEPOSITS 



The contact between the Cretaceous and the basement rocks is nowhere 

 seen on Marthas Vineyard, and the relations between the two must be inferred 

 from their relations at other places. The lowest Cretaceous deposit doubtless 

 lies almost undisturbed upon the eroded surface of older rocks; it has probably 

 not been dislocated by the shove and drag of the Pleistocene ice sheets. In other 

 words, the lower Cretaceous beds lie on the older rocks here just as they do in 

 the coastal plain to the south, although the Upper Cretaceous beds have been 

 more or less disturbed by the action of the ice. 



STRUCTURE 



The structure of the Cretaceous beds is well disclosed only at Gay Head, 

 but traces of similar structure may be seen throughout the western area. At 

 Gay Head (Plate 19, fig. 2), where the details of the structure have been care- 

 fully worked out by Woodworth, 1 the beds lie in a series of closely folded anti- 

 clines and synclines and are strongly overthrown to the southwest. The beds 

 have not only been folded but have been overthrust, so that the structure is 

 very complicated, as shown in Figure 10. At and near Gay Head the dip of the 

 beds is fairly high, averaging over 45°, and is almost uniformly to the northeast. 

 In the region between Menemsha Pond and Chappaquonsett Pond the dip is 

 not easily determined, but the strike is always northeast, so that the general 

 dip is probably northwest. In this region the dip appears to be very steep, but 

 in the eastern part of the island it is generally slight. 



The Upper Cretaceous beds seem to have been deformed by the thrust and 

 drag of the earlier ice advances. The idea that glacial thrust was competent 

 to deform them was first suggested by F. J. H. Merrill 2 in 1886, and is now 

 generally adopted instead of the mountain-building theory advocated by Shaler. 

 Merrill thought that the high elevation of some of the ridges of the terminal 

 moraine on Long Island could be accounted for only by supposing that some 

 of the underlying older beds had been raised to their present positions by glacial 

 action. These older beds are thrown into a series of folds, which run at a right 

 angle to the line of glacial advance. Hollick 3 held the same view in regard to 

 the structure of these beds and has summed up his reasons for holding it as follows : 



'Woodworth, J. B., Unconformities of Marthas Vineyard and of Block Island: Geol. Soc. Am. 

 Bull, 8, pp. 197-212, 1897. 



2 Merrill, F. J. H., On the geology of Long Island, Ann. New York Acad. Sci., 3, pp. 341-364, 1886. 



3 Hollick, Arthur, Dislocations in certain portions of the Atlantic coastal plain strata and their 

 probable causes, Trans. New York Acad. Sci., 14, pp. 8-20, 1895. 



