622 D. W. JOHKSON BEACH CUSPS 



progressively up the beach; with a falling tide, some of the gravels may 

 be dragged downward to give much elongated cnsps. 



There are a number of considerations which appear to support the 

 foregoing theory of beach cusp formation. The theory accounts for the 

 degree of regularity observed in the spacing of beach cusps, since the 

 spacing is dependent on the development of channels which do not reach 

 equilibrium until of approximately uniform size. At the same time the 

 considerable degree of irregularity in spacing occasionally observed is not 

 incompatible with the theory, since the degree of regularity in spacing 

 depends on the progress which has been made toward the establishment 

 of perfect equilibrium. The occurrence of imperfect and compound 

 cusps is readily explained as the product of wave action in channels not 

 yet eroded to the standard size, as when two unusually small channels 

 have not yet been fashioned into a single large one, and consequently 

 give a compound cusp near their upper limits. We should expect, on the 

 basis of this interpretation, that irregular and compound cusps should be 

 most characteristic of the early stages of development, and the experi' 

 ments with artificial cusps prove most conclusively that this is the case. 

 One of the commonest occurrences in the experiments is the gradual 

 moulding of irregular and compound cusps into simple cusps regularly 

 spaced. 



The respacing of cusps with a change in size of waves may be thus ex- 

 plained : A given set is formed and driven up the beach, and then left by 

 the falling tide. The size of waves changes, and new channels appro- 

 priate to them are formed. New cusps result, and as the tide rises these 

 are in turn pushed up the beach. If the new cusps do not coincide in 

 position with the older ones, when the latter are reached their ends will 

 be eroded by the waters converging on them from between the new ones. 

 Eepetitions of this process, with waves of decreasing size, will give several 

 sets of partially preserved cusps, each set more closely spaced than the set 

 above it. On the other hand, if a big storm drives in unusually high 

 waves, big channels will be formed, older sets of cusps will be quickly 

 swept out of existence, and a single set of large, widely spaced cusps will 

 be developed. 



In the laboratory experiments difficulty was often experienced in get- 

 ting the cusps started. The artificial beach was very smooth, of fairly 

 uniform sand grains. It appeared that the difficulty was due to the regu- 

 larity of the beach, on account of which the initiation of channels was 

 delayed. In order to facilitate the process a series of closely spaced 

 creases down the beach was made, after which the cusps began to form 

 more rapidly. As already shown, the artificial creases did not control 



