CORRELATION OF MOHEGAN BLUFF BEDS. 211 



Clay Head the beds were overturned as if by a push from the north, so 

 as to give dips toward the mainland. 



MOHEGAN BLUFF BEDS THE EQUIVALENT OF THE TISBUBY BEDS. 



Block island is thickly coated with a nearly horizontal group of sands 

 and clays with boulders and cobbles, which rest on the truncated folds 

 of the Clay Head section and bear upon their uneven eroded surface the 

 moraine and sands of the last ice advance. These beds vary greatly in 

 texture from point to point, and are equally varied in structure. They 

 are typically exposed in the upper portion of Mohegan bluffs, where they 

 rest on dark blue clays probably of the Cretaceous series. There is a 

 lower bouldery clay, overlaid by gravels and sands, which are in turn 

 succeeded by dark bluish clays carrying striated glacial boulders. The 

 entire series is evidently of glacial origin, and is composed of some 

 locally derived material at Clay Head, but more largely of debris from 

 the mainland about the western margin of Narragansett bay. 



LAST GLACIAL DRIFT. 



The last glacial drift mantles Block island in the form of a moraine 

 with local sand plains as near the center. It is a relatively thin deposit 



Figure 4. — Section in the Interior of Block Island. 

 Showing last glacial drift resting on eroded members of the Mohegan Bluff series. 



at Clay Head, and in the interior of the island is found at different ele- 

 vations up to nearly 200 feet, resting on eroded sections of stratified 

 sands and clays of the Mohegan Bluff series. 



UNCONFORMITY BETWEEN LAST % GLACIAL DRIFT AND EARLIER PLEISTOCENE 



BEDS. 



The Mohegan Bluff beds where exposed are usually horizontal, but in 

 the south side cliff foldings on a small scale are recognizable ; but these 

 dislocations do not account for the very different altitudes of the upper 

 surface of the formation as it now exists. Standing upon the higher 

 parts of Block island and looking down the slopes toward the sea, one 

 can recognize the lines of a drainage system carved in the Mohegan 

 Bluff beds, but now embarrassed by irregular accumulations of the last 

 glacial drift. This is particularly true of the slopes descending from 

 Beacon hill. This sculpturing marks the duration of the Vineyard 



