614 



D. W. JOHNSON BEACH CUSPS 



The direction of the wind seems to have little effect on the formation 

 of cusps. They have been observed in process of formation during on- 

 shore, offshore, and longshore winds^ both gentl}'' and fairly strong. 

 Under ordinary conditions the only result noticed was a more or less 

 marked cliffing on one side of the cusps when the wind produced small 

 waves at an angle oblique to the beach. The cusps thus cliffed may 

 have been partially developed before the oblique waves began their work. 

 If the wind is strong enough and from such a direction as to combine 

 with the breakers in producing a very irregular wave attack, the forma- 

 tion of cusps is probably interfered with, since numerous observations 

 tend to show that a fairly regular advance and retreat of the water is 

 essential to their development. 



Beach cusps are formed at all stages of the tide. It is probable that 

 the greatly elongated type is produced when the waves remain of approx- 

 imately the same size during a falling tide, but the development of this 

 type has not been observed throughout the entire process. 



The direction of wave advance has been carefully noted wherever 

 cusps were being formed. On the basis of numerous observations on all 

 kinds of beaches and of extended experimentation, it may be confidently 

 stated that the best conditions for cusp formation exist when a single 

 series of waves advances parallel with the beach. It is possible that 

 cusps may be produced by waves striking the shore at a markedly oblique 

 angle, but no satisfactory evidence that such is the case has been secured. 

 On the other hand, the progressive destruction of cusps by oblique waves 

 has been repeatedly observed. Such partially destroyed forms are shown 

 in the lower left-hand corner of figure 3. I am inclined to think that 

 the asymmetrical "cusplets" reported by Wilson (122) were formerly 

 symmetrical beach cusps of the ordinary type, which were later cliffed by 

 the oblique waves shown in a photograph reproduced in his paper. Inter- 

 secting waves of the type hypotheticated by Branner have been seen in a 

 number of cases, but no cusps have been observed to develop under the 

 action of such waves. 



The periodicity of the waves does not appear to be a significant factor 

 in beach cusp formation. Varying the period with artificial waves pro- 

 duces no apparent effect on the cusps. 



Jefferson says (1903, 124) : "The cusps seem related to a longshore 

 current, their precise cause not being evident;" but he does not indicate 

 in what manner the cusps seemed related to the current. In most of our 

 observations no evidence of a longshore movement of the water was 

 found. In the few cases where a distinct drift or current in one direc- 

 tion was apparent there seemed to be no relation between the current 



