218 



CAPE COD GEOLOGY 



between similarly overturned anticlines of Cretaceous beds. The isoclinal struct- 

 ure has a northerly dip of about 45°, and the bottoms of the synclines, or their 

 turns, were not exposed above the beach. In 1892 the bottom turn of a syn- 

 cline then exposed stood a few feet above the beach. It appears that as the 

 cliff retreats these folds, which dip steeply inward, carry their inferior turns 

 below sea level. The beds may have been overthrust on each fold, but they 

 are so greatly crushed as to prevent a satisfactory determination of details as 

 well as of thickness. No traces of fossils were seen. 



A mass of rather consolidated gravel and boulders, which appears to be an 

 extension of the gravel bed shown in the syncline figured by Woodworth * in 

 1897 was exposed here in 1915. This mass as now exposed is a boulder bed of 

 glacial origin. It lies in such a position that it may be interpreted as a basal bed 

 in the syncline in which it is evidently infolded. It shows a gradation from large 

 boulders at the bottom to gravel at the top, upon which lies finer gravel and 

 sand. The bed contains numerous ironstone concretions derived from the 

 Cretaceous formations and small boulders of hardened drab clay derived from 

 near-by deposits. This bed has evidently been eroded by an ice sheet, but 

 practically every stone is waterworn, as if it had been exposed to the action 

 of sea waves. Similar boulder beds are involved in the early highly folded de- 

 posits at Gay Head. This bed may be modified drift of the earliest ice advance, 

 a local deposit at the base of the Pleistocene deposits. It is older than the 

 Gardiners clay, the Jameco gravel, and the icesheet whose thrust folded the 

 beds that included these early Pleistocene deposits. It is the local representative 

 of the Dukes boulder bed. 



A few rods north of the section just described the blue Gardiners clay 

 may be seen in an anticline, but its contact with the highly folded mass is not 

 exposed. Perhaps it may be wrapped unconformably around an uplift composed 

 of the highly folded beds of the older series. The more significant part of the 

 section, so far as it could be understood in 1915 and 1916, is shown in fig. 19. The 

 section is not less important in the investigation of the first glacial invasion 

 in southern New England than that at Gay Head cliffs. It should be examined 

 from year to year as Clay Head is cut back by the sea, and the parts now covered 

 by a deep talus are exposed to view, in order to obtain a more complete record 

 of an obscure group of deposits, which have been seen only at three or four 

 places in the New England Islands. 



The beds of boulders, gravel, and sand that lie north of the overturned 



1 Op. cit. 



