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CAPE COD GEOLOGY 



Some of the boulders in the terminal moraine on the islands and in the 

 moraine on Cape Cod are more than 25 feet long, and one boulder measured 

 34 feet. The largest boulders on Marthas Vineyard and Naushon measure 29 

 feet. Enos Rock, in the Nauset kame fields, on Cape Cod, measures 34 feet. 



Many large erratics have broken up along joint planes into a mass of smaller 

 blocks. Here and there an erratic has been split along a single nearly central 

 joint and the divided halves have moved apart, leaving a narrow passage between, 

 giving rise to the name "split rock." Such split rocks are seen on the beaches 

 within reach of the tide, where the difference in strain at the top and the bottom 

 of the boulder may cause the fracture. Some large jointed rocks have been broken 

 into fragments by frost and by the growth of trees. Such rocks are common on 

 Naushon, and a big boulder described by Hitchcock in Brewster is of this sort. 



Pairs of large boulders that stand in the sounds, particularly in navigable 

 water, have been called by such names as "Rose and Crown," "Bishop and 

 Clarks " (clerks, in modern spelling), ' 'Hen and Chickens," all well-known danger 

 points in Vineyard and Nantucket sounds. 



POST-GLACIAL CHANGES 



The postglacial changes in this district consist of slight weathering of the 

 surface rocks, the formation of soils, the growth of swamps and marshes, insig- 

 nificant erosion by small streams, notable work done by the wind in shifting sand, 

 and the irresistible and rapid erosion of the coast by the sea. Among the effects 

 produced by these agencies the dunes and lag gravel tracts formed by the wind 

 and the swamps and marshes formed by the growth of plants are the most con- 

 spicuous. The soils of the district are proverbially poor, owing to the wide extent 

 of sand in the level tracts that are suitable for agriculture. Certain areas on the 

 upland of Marthas Vineyard and Block Island that are underlain by clay have 

 better soils. 



SWAMPS AND MARSHES 



The swamps on the islands of the district are small. Most of them border 

 or occupy small lakelets in the glacial drift, the waters of which are in different 

 stages of displacement by the growth of water-loving plants. Hundreds of 

 small swamps that lay in depressions on the sandy plains of Cape Cod and on 

 Marthas Vineyard have been converted into cranberry bogs, and some depres- 

 sions that contained no natural vegetation have been by means of barriers to 

 the drainage artificially turned into similar bogs. Cranberry bogs are particu- 



