1 84 WORK OF THE OCEAN 



The sorting of shore drift, effected while it is in transportation, 

 may be very perfect. The conditions favoring assortment are (i) 

 vigorous wave-action, (2) prolonged transportation, and (3) a 

 moderate volume of sediment. 



Deposition by waves, undertow, and shore-currents. The zone 

 occupied by shore drift in transit is the beach. Its lower margin 

 is beneath the water, a little beyond the line where the great storm- 

 waves break. Its upper margin is at the level reached by storm- 

 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 car- 

 ried out by the undertow. 

 The cross-section of a beach 

 is shown in Fig. 188. The 

 beach follows the general 

 Fig. 188. Cross-section of a beach. (Gil- boundary between water 



and land, though it does not 



conform to its minor irregularities (Figs. 174 and 187). The beach 

 (or barrier) may deflect the lower courses of streams descending to it. 

 In its deposition, 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 direc- 

 tions. Accumulating here, it 



Fig. 189. Section of a barrier. (Gilbert.)' builds up a low ridge, called 



the barrier (Fig. 189). If it 



is built up above the surface of the water by storm-waves, it may 

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

 sediment washed down from the land. At one stage in the filling, 

 the lagoon becomes a marsh (Fig. 191). 



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, the velocity of the shore-current is 

 checked because it passes into deeper water. Deposition follows. 

 The deposits are in a narrow belt which marks the course of the 

 current, and the result is the construction of a ridge beneath the 

 water. The current does not build the embankment up to the 



