CAPE COD GEOLOGY 



39 



temporaneous Jerseyan drift of New Jersey. The deposits of this stage include 

 the following beds, which are named in ascending order: (1) the Dukes boulder 

 bed; (2) the marine Sankaty formation and the contemporaneous glacial sand 

 and gravel here named the Weyquosque formation; and (3) the stony blue clay 

 and overlying gravel tentatively correlated with the Mannetto gravel of Long 

 Island. 



Dukes Boulder Bed 1 (First Glacial Substage) 



At the base of the glacial deposits in the section at Gay Head cliffs, and ap- 

 parently at the base in Clay Head, on Block Island, there are lenticular patches 

 of boulders and cobbles of glacial origin derived from the mainland on the north. 



The deposits have yielded only one striated stone, which was found in 

 1889 in a huddle of small boulders resting on Cretaceous deposits in the highly 

 folded section at Gay Head just east of the quadrangular fold. The same huddle 

 of boulders contained rolled fragments of the near-by Aquinnah conglomerate, 

 of earlier Pleistocene age. Elsewhere in the section at Gay Head the basal gravel, 

 which rests on the Aquinnah conglomerate, carries isolated angular boulders of 

 granite, many of them deeply disintegrated, and at one place a boulder weighing 

 about eight tons was seen in 1889. 



On Block Island a notable deposit of rounded and waterworn boulders lies 

 at the base of the Pleistocene section in the overturned iolds at Clay Head. 



The Dukes boulder bed is evidently a discontinuous deposit of glacial origin, 

 somewhat rearranged by the action of water. The deposit is of small extent, and 

 is found only where the upturning of the beds has brought the basal Pleistocene 

 above sea level. The underlying beds were apparently not disturbed by glacial 

 thrust at the time these boulders were laid down. Probably the ice sheet did not 

 invade the district in which the boulders are now seen; otherwise the beds con- 

 taining them should have been pushed up and folded as they were during later 

 ice advances. 



The ferruginous conglomerate at the southwest end of the Gay Head section, 

 heretofore referred by me to this horizon, is now regarded, on structural grounds, 

 as a deposit laid down after the close folding occurred in that cliff. 



The Dukes boulder bed is succeeded above by beds of glacial gravel and 

 sand and, where boulders are absent, these form the base of the Pleistocene de- 

 posits at Gay Head and Clay Head. This gravel and sand form the next higher 

 formation, here known as the Weyquosque. 



1 Named from the "county of Duke's County," the quaint legal designation of the county comprising 

 Marthas Vineyard, No Mans Land, and the Elizabeth Islands, so-called in honor of the Duke of York, 

 to whom the islands were at one time granted. 



