CAPE COD GEOLOGY 



79 



sea the sand that is thrown upon the land by gales. The difference between the 

 trend of the beaches and the direction of the prevalent wind produces differences 

 in the position, distribution and shape of the dune tracts. By a singular paradox, 

 on certain beaches, such as the crescentic bathing beach on the east side of 

 Block Island, the southwesterly dry winds move the sand northward and sea- 

 ward, so that when the wind is strong the beach is lowered several inches during 

 a single low tide ; but at the same time the pebbles on the beach work up against 

 the wind in the opposite direction to that in which the sand is traveling, for the 

 wind scours out a small pit on the windward side of each pebble, into which 

 it slides under the action of gravity as the sand is blown away from around it. 

 In this manner pebbles work their way from the headlands along the wasting 

 beach independently of the longshore motion of the surf. 



The movement of the dunes has been troublesome at Provincetown, in the 

 so-called "province lands," where the state of Massachusetts has made attempts 

 to arrest the shifting of the sand by planting grasses and trees. 1 On the northeast 

 side of Gay Head a small tract of dunes has marched inland and uphill, burying 

 trees and shrubbery, and on the southwest side of Gay Head the dunes, sweeping 

 inward under the prevailing southwesterly wind, have ponded the drainage. 



CHANGES IN THE SHORE LINE 



The most effective geological work now in progress in this district is the 

 attack of the sea, which cuts away the land and forms beaches, bars, spits, and 

 hooks of gravel and sand. The outer side of Cape Cod is being cut away at a 

 rate between 5 and 8 feet a year, and the coast on the south side of Marthas 

 Vineyard is being carried into the sea at a rate almost as fast. Nantucket is also 

 being cut back rapidly, as at Wauwinet and in a stretch along the south coast 

 between Tom Never's Head and the coast guard Station. Great changes have 

 been made in a few years in the length and position of the sand bars off the em- 

 bayed parts of the coast line, as at Nauset, Chatham, and Monomoy on Cape 

 Cod, at Haulover Break, Surfside, and Smith Point on Nantucket, and south of 

 Edgartown on Marthas Vineyard. 



Within historic time several small islands near the shore have disappeared, 

 such as Webb Island, off Chatham, and a small island north of Sandy Point, at 

 the north end of Block Island. Beach flats formed at several places serve as a 

 partial and temporary compensation, of little economic value, for the loss of 



1 Report of the Trustees of Public Reservations on the subject of the province lands, February, 1893, 

 Appendix III, House No. 339, Second Ann. Rept. of the Trustees of Public Reservations, Boston 1893. 



