relations' to other beach forms 613 



projDer. The breaching of the ridge by ^vater confined between pre- 

 viously formed cusps has been repeatedly observed in the laboratory ex- 

 periments. 



There are abundant instances of cusps unrelated to any beach ridge. 

 Cusps of gravel are often formed at vs^idely separated intervals with 

 smooth, sandy beach between ; the points of old cusps are nipped off and 

 respaced without the development of a ridge. One must conclude that 

 cusps may develop as the serrate seaward margin of a beach ridge and 

 may determine the places where it will be breached by the waves, but 

 that there is no necessary relation between the two. 



The return current of water flowing down the beach after the wave 

 has ended its advance, sweeps seaward more or less fine material which is 

 fashioned into a deposit analogous to a delta ; but since the current which 

 transports and finally deposits this material has, despite its very short- 

 length, an almost indefinite width, the deposit is not shaped like the ordi- 

 nary stream delta, but is more in the nature of a narrow subaqueous 

 platform merging with the beach near the shoreline and dropping oS 

 abruptly to deeper water at its seaward margin. When cusps have not 

 formed, the margin of this platform is relatively straight; but after 

 cusps have developed, the greatest amount of water returns down the 

 slope from the intercusp spaces, building the subaqueous platform seaward 

 more rapidly than does the smaller amount of water returning from 

 around the apices of the cusps. In this way the margin of the platform 

 becomes scalloped, each intercusp space having a scallop or miniature 

 delta to correspond with it. It is evident that the scalloping of the 

 platform presents no difficulty if the origin of the cusps is understood. 



RELATION OF BEACH CUSPS TO OTHER FACTORS OF SHORE ACTIVITY 



In collecting data concerning beach cusps some attention has been 

 given to several other factors of shore activity, in view of the possibility 

 that they might exert some influence on cusp formation. Several of 

 these factors are briefly treated below. 



It was thought at first that the angle of beach slope might exert an 

 important control over the spacing of the cusps, inasmuch as the slope 

 affects both the volume and velocity of the water advancing and retreat- 

 ing over the zone of wave attack. It soon became apparent, however, 

 that if the inclination of the beach does influence the spacing, the effect 

 is largely masked by the far more important factor of wave size. I still 

 think it probable that the slope of the beach plays a small part in the 

 spacing of cusps, but have not sufficient data on this point to demonstrate 

 the truth of the theory. 



XLIII— BtjLI.. Geol. Soc. Am., Vol. 2X, 1909 



