612 D. W. JOHXSON BEACH CUSPS 



and while tlieir action may be influenced within certain limits by natural 

 or artificial trenches on the beach, they refuse to be controlled by such 

 depressions unless these are themselves appropriately spaced. 



RELATION OF CUSPS TO OTHER BEACH FORMS 



A strongly marked ridge of sand, gravel, or cobblestones rising above 

 the general surface of the beach, and known as the beach ridge, is a well 

 recognized product of wave activity. The bases of the cusps may merge 

 with this ridge in such a manner as to leave no doubt that they constitute 

 an integral part of it. The ridge may or may not be breached opposite 

 the intercusp spaces; but it should be noted that with the progressive 

 concentration of the water in the intercusp spaces, which converge shore- 



^^^/- e ///7 e 







__ — - — — — — 



— 



^. - 



~ 



Figure 6.- — Normal and inverted Beach Cusps 



ward, the parts of the ridge most likely to be broken through are the 

 parts opposite these spaces. It is, therefore, not necessary to regard the 

 intercusp spaces as the product of erosion by water which was imprisoned 

 back of the ridge and broke through it, either at the lowest places or at 

 points of weakness. Conclusive evidence that the ridge may be breached 

 from the seaward side is found in the gravel or cobblestone deltas which 

 are sometimes built landward from the gap in a ridge at the head of an 

 intercusp space (figure 6). It seems clear that the water concentrated 

 between cusps broke through the ridge and carried gravel and cobbles 

 into the area back of it. In one case observed at N'ahant the landward 

 projection of cobblestone accumulations was so systematic as to give a 

 series of "inverted cusps" alternating regularly with the beach cusps 



