250 



PHYSICAL GEOLOGY 



shore. By this means, and also by the addition of material 

 washed outward by the undertow, a ridge may be built above 

 the sea surface, forming a long, narrow sand island called a 

 barrier (Fig. 267). Dunes may presently be built upon its 

 surface, and vegetation may obtain a foothold. Barrier 

 islands extend along much of the coast of the United States 

 from New Jersey to Texas. They inclose shallow-water areas 



called lagoons, which are being 

 filled gradually by wash from 

 the mainland and the islands, 

 by migrating sand dunes and 

 wind-borne dust, and by en- 

 croaching vegetation (Figs. 

 268 and 269). Thus lagoons 

 may become marshes and 

 finally be added to the main- 

 land area. If, on the other 

 hand, a barrier comes to re- 

 ceive less sand from the bot- 

 tom or from other places 

 alongshore than is carried 

 away by waves and currents, 

 it may be destroyed, together 

 with the lagoon marsh it in- 

 closed. Occasional breaks (in- 

 lets} in barriers are kept im- 

 perfectly open by tidal scour, 

 by the outflow of waters from 

 the mainland, or by both. 

 The tides of the Gulf of Mexico being weak, the barrier 

 islands off the coastal plain of Texas have few breaks (Fig. 

 274), a fact which has retarded the commercial development 

 of the region. Most lagoons are accessible only to boats of 

 light draft, and are not frequented by extensive commerce. 



Influence of plants and animals upon shore lines. Plants 

 affect shore lines in two important ways ; they often protect 



FIG. 269. A portion of the New 

 Jersey coast. 



