THE OCEAN AND ITS WORK 



223 



whether or not the supply of sand is too great for the waves to 

 remove. 



The lagoons back of sand reefs are gradually filled by the sediment 

 carried in by streams from the mainland, by the sand blown in by 

 the winds, and by the accumulation of marsh vegetation. Back of the 

 sand reef on which Atlantic City, New Jersey, is situated, peat has 

 accumulated to a depth of one or more feet over a wide extent. In 

 time a marsh-filled lagoon will become dry land, and the sand reef 

 will be joined to the mainland. 



Sand reefs are sometimes hardened by the deposition of lime car- 

 bonate between the sand grains until they form rock reefs. A no- 

 table case of this kind occurs off* the coast of Brazil. 



Tied Islands. — Islands are sometimes tied to the mainland by 

 sand and gravel brought by littoral currents. This is accomplished 

 in one of two ways. (1) If 

 littoral currents exist which 

 move parallel to the shore in 

 opposite directions, some- 

 times simultaneously and 

 sometimes successively so 

 that they carry material to the 

 same point, which is gener- 

 ally a strait separating an 

 island from the mainland, a 

 tongue of land consisting of 

 sand and gravel may unite 

 the island to the mainland. 

 (2) Islands are also tied to 

 the mainland (Fig. 216) by 

 the extension of sand spits 

 from either the mainland or 

 the island or from both. 

 Many examples of islands 

 tied to the mainland in one 



of these ways might be cited. Gibraltar, an island tied to Spain by 

 a narrow sand beach called the " neutral ground," and Nahant, off" 

 the coast of Massachusetts, are familiar examples. 



Examples of the Constructive Work of the Sea. — The work of 

 the sea, as we have seen, is constructive as well as destructive. It is 

 stated that on one portion of the coast of England (the estuary of 



CLELAND GEOL. — IS 



Fig. 216. — Tied island, southern Italy. 



