LAKES AND SHORES 



155 



Deposition by waves, shore currents, etc. The material worn 

 from the land by waves, or brought to the shore by rivers, is shifted 

 about by waves, undertow, and shore currents, but it finally comes 

 to rest. If left at the shore-line it makes a beach (Fig. 157). If 

 carried farther out into the water it takes on other forms. 



Fig. 157. A barrier beach, shutting in a marshy tract behind it. Lasells 

 Island, Penobscot Bay, Me. 



Waves often build reefs or barriers a little out from the shore- 

 line. They are developed near the line of breakers, where the in- 

 coming wave leaves much of the sediment which it is moving in 

 toward the shore. The undertow may contribute sediment to the 

 reef by carrying it out from the shore. There are sometimes 

 several such reefs along a coast, parallel to one another and to the 

 shore. They are, in some cases, troublesome to navigation. 



After a reef is developed, waves may build its crest above the 

 surface of the water, converting it into land (Fig. 158). This seems 

 to have been the origin of many of the low, narrow belts of sandy 



Fig. 158. Section of a barrier. (Gilbert, U. S. Geol. Surv.) 



land parallel to coasts, with marshes and lagoons behind them. 

 This type of irregularity is illustrated by the coast of the United 

 States at various points between New York and Texas. 



Shore currents move sediment along the coast, but w%here such 

 a current reaches a bay, it does not, as a rule, follow the outline of 



