INWARD MOVEMENT OF OUTLYING BEACHES 69 



beach-wall may in time be built to a considerably greater 

 height than the land upon which it formed. If the land 

 sinks down, the waves and winds may constantly add to the 

 height of this growing beach, so that its crown is kept well 

 above the surface of the water, while the interior land, having 

 no such means of adding to its mass, slowly subsides below 

 the level of the sea. 



These outlying beaches, especially wdierc they are swept 

 by strong, coastwise-running currents, are apt to work rapidly 

 in toward the land. Thus on the southern shore of the island 

 of Martha's Vineyard, where an extensive series of lagoons is 

 shut out from the open water by a low sand barrier, the coast 

 current constantly cuts away the sand next the shore and con- 

 veys it to the eastward until it is discharged around the end 

 of the island to form the great promontory of Cape Pogue. 

 This leads to the deepening of the water next the shore so 

 that the waves have very free access to It. In times of great 

 storms the swash from the surf sweeps clear over the beach, 

 and in its movement conveys a great quantity of sand from 

 its outer to its inner wall. In this manner a beach moves 

 inward in much the same way as a sand-dune, rolling over and 

 over itself in its forward- motion. On the southern coast of 

 Martha's Vineyard the inward march of the beach is now at 

 the rate of about three feet per annum. It is now probably 

 near a thousand feet from the place it occupied when the 

 land was first seen by the whites. 



These barrier-beaches, arising, as we have seen they may, 

 from either the uprising or the downsinking of the continent, 

 are extremely common features of the ocean coasts ; probably 

 near one-fourth of the continental shores are fringed by them. 

 They are of much interest to man, not only because they 



