THE ORIGIN OE BEACH CUSPS 



483 



right, as shown along the beach D E. The distance between the 

 cusps is equal to the spaces, measured on the beach, between the 

 radii along which the wave interference approaches the shore. 



It is noticeable in California that the cusps are not permanent 

 features of a given beach, but that they are sometimes very pro- 

 nounced, at others but feebly developed, and at still others alto- 

 gether obliterated or scarcely perceptible. The accompanying 

 illustration (Fig. 3) is made from a photograph taken by the 



Fig. 3. — Cusps on the beach at Santa Cruz, Cal. From a photograph taken from 

 the Sea Beach Hotel, June 14, 1900. 



writer June 14, 1900, from the Sea Beach Hotel at Santa Cruz. 

 These particular cusps were 60, 69, 78, and 81 feet apart. They 

 are not always visible on that beach, however. The beach of 

 Half Moon Bay, twenty-five miles south of San Francisco, is 

 sometimes perfectly smooth and sometimes beautifully notched. 

 These variations are due to the changes of the relations of the 

 waves to each other, and of the relations of the radii of the 

 points of interference to the beach (if there are still two sets 

 of waves). It is evident that a variation in the depth of the 

 water off shore would retard or hasten the advance of the waves, 

 and would consequently produce a variation in the direction 

 of these radii and of the distance between the cusps on the 

 beach. 



