78 



CAPE COD GEOLOGY 



on bare flats and the brows of gravelly cliffs, pebbles and fragments have been 

 shaped and polished by wind-blown sand. Fantastically carved forms, combina- 

 tions of water-worn and glaciated surfaces, with sand-carved facets or spoon- 

 shaped depressions, abound along the tops of the bluffs of the Cape, along the 

 north coast of Nantucket, and on lag gravel tracts on Marthas Vineyard. On Gay 

 Head boulders of granite and diorite 3 feet across show sand-blasted facets. 1 



Bluffs of gravel and sand are eroded by wind in dry weather when the sand 

 is not held together by films of moisture. At such times the wind removes a con- 

 tinuous stream of sand and pebbles from the high bluffs on the ocean side of 

 Cape Cod or on the south side of Marthas Vineyard, and high tides sweep away 

 the sand and pebbles that reach the base of the talus along the bluff. Violent 

 gusts carry sand over the top of the bluff to form an eolian layer near its edge. 

 This layer gradually becomes thinner inward but may be as much as 6 feet thick 

 at the edge of the bluff. 



Of quite different type are the sand dunes along the beach. Though some 

 are carried half a mile inland, most of them are formed immediately back of the 

 beaches from whose sand they are fed. They reach an altitude of nearly 100 feet 

 on the tip of Cape Cod back of Provincetown, where the most extensive tract 

 of dunes in the district is found. Other notable dunes are seen in the southeastern 

 part of Cape Cod, at Nauset and Monomoy; about the south shore of Cape Cod 

 Bay off Barnstable; on the north coast of Nantucket; and east of Menemsha 

 Inlet, on Gay Head. 



The sand of these dunes consists mainly of grains of quartz but includes 

 particles of magnetite, as well as garnet, which at some places forms thin layers 

 between layers of grains of white quartz. These layers indicate the action of 

 winds strong enough to blow away the light quartz but not strong enough to 

 move heavier material. The dunes on the east coast of Block Island are darkened 

 by grains of iron ore derived from the magnetite in the schist of the Narragansett 

 Bay region and probably also from ilmenitic iron ore in the glacial drift derived 

 from the stock of peridotite at Cumberland Hill, in northeastern Rhode Island, 

 3 miles east of Woonsocket. 



On many parts of the coast the wind during dry weather blows back into the 



1 For Shaler's original account of the sand-blasted pebbles on Nantucket see U. S. Geol. Survey Bull. 

 53 pp. 23-26, pi. x, 1889. For a statement of the sand blast theory of origin of glyptoliths see J. B. 

 Woodworth, Post-glacial aeolian action in southern New England, Am. Jour. Sci., 47, pp. 61-71, 1894. 

 For notes on sand-blasted pebbles on Cape Cod see W. M. Davis, Facetted pebbles on Cape Cod, Mass., 

 Proc. Boston Soc. Nat. Hist., 26, pp. 166-175, pis. 1-2, 1894, including figures of pebbles from Cape Cod 

 and Martha's Vineyard collected by Davis and Woodworth. See also E. E. Free, The movement of soil 

 material by wind, U. S. Bureau of Soils Bull. 68, 1911. 



