BEACH CUSPS 239 



of the waves varies primarily with the wind. The great rollers 

 that tumble up a beach some days of calm are due simply to a 

 distant wind whose effect is transmitted faster through the water 

 than through the air. For beach work, however, we must dis- 

 tinguish two orders in the magnitude of waves that follow each 

 other even in a brief period. Anyone who visits a beach may 

 satisfy himself that at fairly regular intervals there occurs a great 

 wave, far overtopping the average in height and extent of 



Fig. 2. — Diagram of Beach Cusps. 



AA Zone of Cobbles. CC Stony Cusps. 



BB Zone of Seaweed. DD Gravel Cusps. 



EE Flat Beach of Sand. 



advance up the beach. Now this great wave is as much more 

 efficient than its fellows for beach work in ordinary weather, as 

 the work of a single storm outweighs months of normal tides in 

 building and modifying features of shore topography. As 

 regards the stage of the tide, in a similar way, spring tides are 

 the occasions of maximum beach work in average weather. This 

 is especially true in all that concerns the upper beach line, where 

 our gravel cusps are situated. All the waves, great and ordinary, 

 have an excess of shoreward over off-shore movement during 

 rising tide. Generally speaking it is thus during the rising of 

 the tide that objects are driven up the beach, and during the fall 

 that they are drawn out seaward, unless left stranded, as must 

 happen in most cases, since both forces, though opposite in 

 direction, have least intensity at the shoreward margin of the 

 beach. 



