138 



CAPE COD GEOLOGY 



LACK OF BORINGS 



Well borings have been found at only two places on the island. A group 

 of wash borings were made in the Upper Cretaceous beds near Prospect Hill 

 to find white kaolin sand. The records of these borings were not of much assis- 

 tance, especially as they were made in an area where the lower beds of the 

 moraine have been greatly folded and probably overthrust. Borings were made 

 near Norton Point by the Clay Products Company. The records of these bor- 

 ings show the presence of Cretaceous beds, but they are of no value to the 

 geologist, for the notes showing exactly where the borings were made cannot 

 now be found. Since the field work for this report was completed, a deep well 

 has been sunk just east of Vineyard Haven, at the head of the harbor. This 

 well had reached a depth of 160 feet, but no record of it is available except 

 that it encountered no hard rock. 



NATURE AND AGE OF THE PRE-PLEISTOCENE DEPOSITS 



The Cretaceous and Tertiary deposits constitute all the pre-Pleistocene 

 formations found on Marthas Vineyard. They are all unconsolidated, but 

 otherwise they vary considerably in character. These older deposits are ex- 

 posed only at Gay Head and at a few places in the western morainal area, both 

 along the shore and inland. The beds seen are much folded and overthrust. 

 The Cretaceous deposits comprise variegated clays and arkosic sand in which 

 the feldspar has been altered to kaolin. According to Berry 1 these Cretaceous 

 beds belong to the lower part of the Upper Cretaceous system. The Tertiary 

 deposits are much less extensive and have been clearly recognized only in the 

 Gay Head cliffs. They have been usually described as osseous conglomerate 

 (a semi-consolidated quartzose conglomerate containing numerous cetacean 

 bones) and greensand, though they possibly include some other sand. These 

 beds have been much folded and overthrust and are separated from the over- 

 lying Pleistocene beds by a marked unconformity. The fossils in the sand 

 show that it is almost entirely Miocene, though it includes some Pliocene patches. 

 The osseous conglomerate, however, is now regarded as of early Pleistocene age. 

 No Eocene or Oligocene deposit has been found, except possibly a few fossilifer- 

 ous fragments in the glacial drift on Chappaquiddick Island. 



The Cretaceous and Tertiary deposits are of interest because they are 

 the most northerly and easterly known remnants of the beds of those ages in the 



1 Berry, E. W., The age of the Cretaceous flora of southern New York and New England: Jour. 

 Geol., 20, pp. 608-618, 1915. 



