CAPE COD GEOLOGY 



47 



posited on the eroded surface. The unconformity represents the removal of an 

 unknown thickness of folded beds of clay, gravel, and sand. The overlying beds 

 of gravel show no signs of marine action, but they contain rolled fragments of the 

 Cretaceous and the Miocene deposits of the district, together with hardened nod- 

 ules of clay such as are laid down on the beds of lignite through secular rain wash. 

 In most of the region here considered the unconformity now lies below sea level 

 or is not exposed. In some areas on Gay Head the section including the uncon- 

 formity was worn away by erosion prior to the deposition of the Wisconsin till. 



JAMECO STAGE OF GLACIATION 

 Jameco Formation 



Ferruginous Boulder Bed. The most significant bed of boulders in the older 

 Pleistocene deposits at Gay Head is exposed on the west side of Gay Head cliffs 

 from the Devil's Den southward, where it lies unconformably on eroded and 

 folded Cretaceous beds and the infolded glacial gravels of the Weyquosque for- 

 mation. The bed consists mainly of boulders of granite and diorite, 3 or 4 feet 

 in diameter, but contains a large variety of smaller stones, derived from the bed 

 rock of southeastern Massachusetts, as well as some cobbles and pebbles of clay 

 cemented by iron oxide. The iron has penetrated only the outer part of some 

 of these cobbles, so that when they are broken their central parts wash out, leav- 

 ing rounded cavities. The boulders and pebbles appear to be waterworn and 

 show no striations due to glaciation, yet the coarseness of their materials, their 

 commixture, the direction in which they have been transported, and their associa- 

 tions indicate that they were brought to their present positions by an ice sheet 

 that carried them many miles south of their parent ledges. Whether the boulders 

 and gravel were directly deposited by water that worked over deposits of till in 

 place, or whether they were washed out of more ancient beds is not known. The 

 deposit is probably equivalent to a part of the Jameco gravel of Fuller, and it is 

 so classified in this report. It forms the local base of what appears to be a less 

 disturbed series of beds deposited about Gay Head in Jameco time, prior to the 

 deposition of the Gardiners clay. At Clay Head, on Block Island, very similar 

 boulder beds lie unconformably upon high folded older Pleistocene and Cre- 

 taceous deposits; but the conglomerate there is less ferruginous than at Gay 

 Head. 



Coarse Gravel of Jameco Type. Typical glacial gravel, usually coarse, and 

 consisting of a great variety of small erratics derived from the mainland on the 

 north, is seen in the islands east of Long Island. It resembles the typical Jameco 



