226 



CAPE COD GEOLOGY 



encrusting growth of quartz that is seen on the pebbles found at Diamond Hill 

 and a few other places within a radius of five or six miles from it. On Block 

 Island there are also pebbles of vein quartz in the drift on beaches where the 

 country rock is a black schist, undoubtedly one of the Carboniferous rocks 

 that crop out in Narragansett Bay. This material confirms the impression 

 made by the agates carrying a reddish shaly trace of the country rock, namely, 

 that the source of all these veinstones lies in exposures of rock swept over by 

 glacial sheets moving southward through the Narragansett Bay region. 



CHIASTOLITE SCHIST PEBBLES 

 Pebbles and glaciated fragments of the well-known chiastolite schist of 

 George Hill, in the town of Lancaster, Mass., are found on Block Island. A 

 fragment of this rock was found on the beach under Mohegan Bluffs about a 

 mile west of Southeast Light in 1915, and pebbles of the same material were 

 found on the east beach south of Clay Head in 1916. The andalusite phyllite 

 of George Hill is known only in boulders, but these boulders may have been 

 derived from rock in place there beneath the glacial drift. The distance from 

 George Hill to the south coast of Block Island is about 91 miles, in a direc- 

 tion 3° east of south. 1 



ABSENCE OF KAMES AND ESKERS 



Sections along the shore of Great Salt Pond and elsewhere on the island 

 show stratified beds of gravel, sand, and clay, but wherever the stratigraphic 

 position of these beds has been determined they have proved to be pre-Wisconsin 

 glacial deposits. No stratified Wisconsin glacial deposits that have a distinctive 

 topography are found on the island. 



Post-Glacial or Recent Deposits 



After the Wisconsin ice sheet disappeared the vegetation of the island, in- 

 cluding that in the peat bogs and marshes, began to grow in sheltered positions 

 along the shore of the sea, but no trace of vegetation is found on the surface of 

 the Wisconsin drift of the island above its present swash line. Vegetal deposits 

 were laid down in swamps and peat bogs of fresh-water origin and in marine 

 marshes. 



1 For references to the andalusite and chiastolite phyllite of Lancaster, consult C. T. Jackson : An 

 account of the chiastolite or nacle of Lancaster: Boston Jour. Nat. Hist., 1, pp. 35-62, 1837. See 

 also Perry, J. H., and Emerson, B. K., The geology of Worcester, Massachusetts, p. 28, 1903. 



