616 D. W. JOHNSON BEACH CUSPS 



of tlie swash is thin as well as irregular and variable, and under these 

 conditions must be very ineffective in developing intercnsp spaces or 

 "bays/' Nor does the theory as stated by its author explain the regular- 

 ity in spacing of the cnsps nor their respacing consequent upon a change 

 in size of waves. It would seem that Professor Shaler's theory does not 

 go far enough adequately to explain the observed phenomena. 



CORNISH'S THEORY 



In the account of "ridges and furrows" (cusps and intercusp spaces) 

 given by Cornish, it is stated that the water washes depressions at selected 

 places because neither the force of the water nor the resistance of the 

 beach material to erosion are absolutely uniform. The regular spacing 

 of the cusps is not explained, nor does the author appear to have recog- 

 nized this character of their distribution. E'either does he recognize the 

 fact that gentle waves build cusps. The erosion which produces the 

 "furrowing" is related by him to a change from small to large waves 

 only. But we have seen that cusps form under the reverse conditions as 

 well. It thus appears that Cornish points out certain causes of the un- 

 equal erosion of beaches, but does not throw much light upon the origin 

 of the cusps. 



JEFFERSON'S THEORY 



The seaweed barrier theory of Jefferson, advanced to account for the 

 occurrence of cusps on a beach where there happened to be considerable 

 accumulations of seaweed at the time, breaks down under the test of a 

 broader application. There are also serious objections to the theory 

 aside from the fact that cusps are abundantly developed on beaches free 

 from seaweed and other similar material. Even if we admit that a strip 

 of seaweed might form an effective dam behind which considerable masses 

 of water would be imprisoned, we must regard it as in the highest degree 

 improbable that this water would break through the seaweed barrier at a 

 large number of rather evenly and often closely spaced intervals. The 

 degree of regularity in beach cusp spacing is wholly incompatible with 

 the seaweed barrier theory. 



On the other hand, it should be remembered that after the cusps have 

 once formed, a seaweed barrier, as well as a barrier of sand or gravel, may 

 be breached by the waves where their water is concentrated for the attack 

 in the intercusp spaces. Thus an observer might find breaches in the 

 barrier corresponding with the intercusp spaces. As shown more fully 

 on a preceding page, both theoretical considerations and the field evi- 

 dence support the view that the breaching is effected by direct wave 



