THE 



JOURNAL OF GEOLOGY 



SEPTEMBER-OCTOBER, igoo 



THE ORIGIN OF BEACH CUSPS 



In the April-May 1900 number of this Journal, p. 237, 

 Mark S. W. Jefferson has an interesting article upon "Beach 

 Cusps. ' I have often noticed these peculiar beach forms and 

 was for some time puzzled to know how they were produced - 

 The explanation offered by Mr. Jefferson for those on the Lynn 

 Beach, Massachusetts, is that they "must be ascribed to the 

 agency of the seaweed piled up on the beach, modifying the 

 action of the greater waves." The attention I have been able 

 to give the subject leads me to the conclusion that beach cusps 

 are formed by the interference of two sets of waves of transla- 

 tion upon the beach. I know of no peculiarities of these cusps 

 that are not explained by this theory of their origin. It will be 

 understood by reference to the accompanying diagram, Fig. 1. 

 The concentric lines represent two sets of waves advancing on 

 the beach in the directions indicated by the arrows and crossing 

 each other along the broken lines. In deep water these are 

 waves of oscillation, but when they reach the shallow water on 

 the beach they become waves of translation and interfere with 

 each other where they converge upon the shore. The tendency 

 is for them to check each other along these lines of interference 

 and to heap up the sands at the points marked A, where they 

 strike the beach. At the points marked B the waves diverge 

 Vol. VIII, No. 6. 481 



