OCEANS AND LAKES 251 



the shore against wave erosion, and certain plants which live 

 in shallow salt water aid effectively in the extension of the 

 land seaward. 



Along a rocky shore one may often see at low tide that the 

 rocks are covered with a mat of seaweed and other vegetation 

 which, during storms, acts as a buffer to deaden the force of the 

 waves (Fig. 254). 



FIG. 270. Mangroves on shore of Biscayne Bay, near Lemon City, Florida. 

 (R. M. Harper.) 



The mangrove flourishes on shallow, muddy bottoms off 

 many low-latitude coasts not exposed to strong surf. Florida 

 furnishes good examples (Fig. 270). The many widely spread- 

 ing roots start from above the surface of the water and even 

 from the lower limbs, forming a tangle which serves to catch 

 and hold the sediment washed from the land. The effect of 

 great numbers of trees is to occasion the lodgment of large 

 quantities of sediment. Certain low, marshy coastal plains 

 appear to have originated in this way. A similar work is done 

 by grasses which grow in coastal lagoons and marshes. 



STAGES IN SHORE-LINE DEVELOPMENT 



Shore lines tend to pass through regular cycles of develop- 

 ment. A coastal cycle is begun by diastrophism, emergence 



