310 GEOLOGY 



waves, and is usually a few feet above the level of still water. Ma- 

 terial is brought to the beach from seaward by incoming waves, 

 and from it detritus is carried out by the undertow. The cross- 

 section of a beach is shown in Fig. 255. The beach follows the 

 general boundary between water and land, though it does not con- 



Fig. 255. Cross-section of a beach. (Gilbert.) 



form to its minor irregularities (Figs. 240 and 254). The beach 

 (or barrier) often causes the deflection of the lower courses of 

 streams descending to it. 



In its deposition, the shore drift assumes various forms. Where 

 the bottom near shore has a very gentle inclination, the incoming 

 waves break some distance from the shore-line, and it is here that 

 the most violent agitation occurs when the waves are strong. To 

 this line of breakers, material is shifted from both directions. Accu- 



Fig. 256. Section of a barrier. (Gilbert, U. S. Geol. Surv.) 



mulating here, it builds up a low ridge, called the hairier (Fig. 256) . 

 If it is built up above the surface of the water by stonn-\\ avcs, it 

 may shut in a lagoon behind it, and this may ultimately be filled 

 by sediment washed down from the land. At one stage in the 

 filling, the lagoon becomes a marsh (Fig. 258). 



The disposition of shore deposits depends largely on the currents 

 at and near shore. If the coast-line is deeply indented, the littoral 

 current usually fails to follow the re-entrants. In holding its course 

 across the mouth of a small bay, a shore-current usually passes into 

 deeper water. Here its velocity is checked because its motion is 

 communicated to the water beneath it, and, a larger amount of 



