CAPE COD GEOLOGY 



95 



level or reported in well borings on the island. The sole trace of these Cretaceous 

 deposits appears to be a drifted fragment of fossiliferous, reddish argillaceous 

 sandstone containing poorly preserved casts of marine mollusks, the whole re- 

 sembling similar drifted fragments of the Matawan formation (Upper Creta- 

 ceous) found on Marthas Vineyard. Of the Eocene and Miocene formations 

 found elsewhere in the district, either in place or as glacial erratics, the only 

 known trace on Nantucket consists of the battered and waterworn tooth of a 

 Miocene shark found in fossiliferous sand at the base of the cliff at Squam Head 

 in 1915. This deposit is a part of the Sankaty sand, which has been variously 

 referred to the Upper Pliocene and Lower Pleistocene series. Dall held that a 

 certain part of it should be referred to the upper part of the Pliocene series, but 

 for reasons stated below it is here referred to the lower part of the Pleistocene 

 series, the term Pleistocene being used to include deposits of the first ice invasion. 



Pleistocene Epoch 



SANKATY SAND. 



The name Sankaty beds, proposed by me for the beds of fossiliferous ma- 

 rine sand under the Sankaty Head Lighthouse at the east end of Nantucket, 

 is retained in this report as Sankaty sand to designate a group of deposits con- 

 cerning which three different views have been held as to their position in the 

 time scale and their place in the succession of local deposits. These beds appear 

 to be the lowest that are exposed above sea level on the island. According to 

 Desor and Cabot they rest on a bed of ferruginous gravel, which in turn over- 

 lies unconformably a bed of light-brown sandy clay of unknown thickness. 



For a time J. D. Dana adopted the erroneous view that the shell-bearing 

 beds were laid down after the glacial epoch and put them in his "Champlain 

 epoch" of submergence. The mistake was further made of assuming that the 

 altitude of the upper edge of a set of tilted beds indicates the height at which 

 they had stood above the level of the sea. Upham has shown that the beds had 

 been disturbed and that their elevation above the present sea level does not 

 indicate their relation to the sea level at the time of their deposition. It was 

 also shown that the deposit had certainly underlain the last ice sheet, that it 

 was much eroded, and that it considerably antedated the last glacial stage. 

 For many years the assemblage of fossil mollusks in the beds was regarded as 

 evidence of their post-Pliocene or Pleistocene age. In a paper on the older Pleis- 

 tocene deposits of the islands, I tentatively placed the beds at Sankaty Head 

 at the same horizon as certain beds of sand and gravel on Marthas Vineyard, 



