224 



CAPE COD GEOLOGY 



pebbles now on the beaches of the island were brought in during one ice advance 

 or another, for the material in all the boulder beds and pebble beds is very similar, 

 the chief difference being that many kinds of rocks in the older boulder beds 

 are much disintegrated, although their disintegration is not everywhere apparent. 

 Boulders found on the beaches, therefore, even if they certainly came from 

 the cliffs, have little significance in determining the direction of movement 

 or the ice sheet during any particular epoch. The following observations were 

 made with this precaution in mind. 



DIRECTION OF DRIFT SHOWN BY BOULDERS 



Some boulders seen on Block Island can be traced to well-defined outcrops 

 on the mainland. 



CUMBERLANDITE BOULDERS 



One of the most distinctive rocks of southeastern New England is the so- 

 called Cumberland iron-ore or peridotite of northeastern Rhode Island. The dis- 

 tribution of boulders and fragments of this rock in areas south of its outcrop 

 was described in 1893 by Shaler, who had not at that time extended his obser- 

 vations to Block Island. In August of that year a boulder of this rock weighing 

 ten pounds was found on the shore of the island on the west side of the point 

 on Mohegan Bluffs, south of Fresh Pond. Another boulder, 2 feet long, 18 inches 

 wide, and 1 foot thick was found near by. The smaller of these two boulders 

 may have been brought to the island by fishermen as ballast in a boat, but the 

 larger boulder was probably not transported by human agency. Boulders of this 

 rock are found as far east as Gay Head, on Marthas Vineyard. The boulders 

 on Gay Head lie 30 miles east of the meridian passing through the place of their 

 origin, but those found on Block Island lay scarcely 10 miles west of that meri- 

 dian. As the ice moved, at least during the Wisconsin stage, nearly southward 

 through Narragansett Bay, deploying lobewise to Block Island on the southwest 

 and toward Gay Head on the southeast, with an axis of southward striation on 

 the site of Newport, it is evident that Block Island lay well within the limit of 

 the fan of erratics that were spread southward by the lobes of ice that moved 

 at different times from the Narragansett Bay region. 



Glacial striae on the west shore of Narragansett Bay north of Point Judith 

 indicate the direction of transportation of drift to Block Island, as follows : 



Narragansett Pier, on high granite bluff south of railroad 



terminus S. 19° W. 



Rose Hill, Wakefield, R.I S. 21° W. 



