602 



D. W. JOHXSON BEACH CUSPS 



direction 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 transla- 

 tion and interfere with each other where they converge upon the shore. 

 The tendency is for them to check each other along these lines of inter- 

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

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

 the beach sands and all floating material alternately right and left." 



"In figure 2 the waves are represented as breaking on a straight beach. 

 If the water offshore were of a uniform depth and the waves were evenly 

 spaced, the cusps in this case would, for obvious reasons, be further and 

 further apart from left to right, as shown along the beach D E. The 



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Figure 1. — Diagram illustrating the Formation of Beach Cusps 



The concentric lines represent two sets of wave crests. The heavy line is the curve of 

 a beach which, with these waves, would yield cusps of uniform size. (After Branner.) 



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

 between the radii along which the wave interference approaches the 

 shore.'^ 



In an editorial note in the Journal of Geology for 1901, Professor 

 Branner briefl}^ restates his theory of cusp formation, and calls attention 

 to the fact that the "giant ripples" and similar beach structures ob- 

 served in sedimentary rocks may be fossil beach cusps. 



Among the "author's abstracts of papers read at the Washington meet- 

 ing of the American Association for the Advancement of Science, Section 

 E," published in the Journal of Geology for 1903, is an abstract of a 

 paper by Professor Jefferson entitled "Shore Phenomena on Lake 

 Huron." The abstract suggests a modification of the author's views as 

 published four years before ; for while in the earlier paper the possibility 

 of a stony barrier's playing the same part in cusp formation as a seaweed 

 barrier is considered and rejected as improbable, in the later paper we 



