CAPE COD GEOLOGY 



247 



The Bed Rock Basement 



The bed rock of the mainland of southeastern Massachusetts passes below 

 present sea level along a curved line drawn from Brant Rock, on the open coast, 

 in the town of Marshfield, southward through Plymouth to the head of Buzzards 

 Bay and thence southwestward on the south side of the ledges skirting the 

 coast of the Bay about New Bedford. There are two means of determining the 

 depth of the bed rock basement beneath Cape Cod. One, the direct method, 

 depends on deep borings; the other depends on projecting the slope of the surface 

 of the bed rock on the mainland beneath Cape Cod. The first method, by reason 

 of the lack of more than one boring on which to rely for evidence, offers little 

 more than a check on the other. 



In 1905 the water commissioners of Provincetown caused to be put down 

 in the dunes near the eastern limit of the town on ground then occupied by the 

 pumping station, a driven well which, according to the report of the commission, 

 reached a depth of 420 feet. According to Mr. J. Henry Blake, of Cambridge, 

 Mass., who visited the site in June and August, 1905, the well was put down 

 to the depth of 450 feet from the surface, where the drill encountered a rock, 

 a fragment of which Mr. Blake collected at the time. This specimen, which is 

 19 mm. long, is a fresh looking grayish-green coarsely crystalline hornblende 

 syenite, unlike any syenite exposed between Newport, R. I., and Portland, 

 Maine. Near Portland, however, as I am informed by Professor John E. Wolff, 

 a piece of drift rock of the same facies was found some years ago by Dr. George P. 

 Merrill and is now in the collections of the petrographical department of Harvard 

 University. 



It was the opinion of the well borers, so far as could be learned by inquiries 

 made at Provincetown in 1915, that the rock was a boulder, because the well 

 borers stated that it moved when struck by the drill. Thus the only well that 

 has been put down on Cape Cod to a depth near the presumed position of the 

 bed rock floor afforded no decisive information as to the character or depth of 

 the rock. The depth of the floor can be inferred only from borings made at widely 

 separated points on the Cape, and the experience in boring deep wells for potable 

 water does not afford promise that such evidence can be had. 



Any attempt to determine the depth of the bed rock floor by projecting 

 seaward the slopes of the neighboring land surface back of Cohasset and Plymouth 

 is beset with doubts and difficulties as to the surface that represents that floor. 

 The eastern coast of Massachusetts, from Cohasset northward at least, appears 



