356 



GEOLOGY. 



teach. The lower margin is beneath the water, a Uttle beyond the Une 

 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. To the beach, 

 material is brought from seaward 

 by the in-coming waves, and 

 from it detritus is carried out 

 by the undertow. The cross- 

 section of a beach is shown in Fig. 312. In horizontal position the 

 beach follows the general boundary between water and land, though 

 it does not conform to its minor irregularities (Fig. 313). The beach 

 or barrier ridge often causes the deflection of the lower cou^^ses of streams 

 descending to it (PL XXI). 



Fig. 312.- 



-Cross-section of the beach. 

 (Gilbert.) 



Fig. 313. — A lake-beach (barrier) ; GrifRn's Bay, Lake Ontario. 



The barrier. — ^AVhen the agitation of the water along shore becomes 

 insufficient to carry the material, it is dropped. In its deposition it 

 assumes various forms. Where the bottom of the lake or sea near 

 shore has a very gentle inchnation, the in-coming waves break some dis- 

 tance from the shore-hne, and it is here that the most violent agitation 



