606 



D. W. JOHKSON BEACH CUSPS 



are so great that their form is often not as sure a guide to their detection 

 as is their systematic recurrence at fairly uniform intervals. One or two 

 indefinite heaps of gravel on a beach would escape notice, but a hundred 

 such heaps, evenly spaced, attract attention. 



A cusp may rise from an inch or less to several feet above the general 

 level of the beach. Many are relatively low and flat, others high and 

 steep-sided. Sometimes the highest part is comparatively near the apex ; 

 at other times the highest part is far back, and from it a long, sloping 

 ridge trails forward toward the water. As a rule, the cusps appear to 

 point straight out toward the water — that is, the axis of the cusp is at 

 right angles to the shoreline — and neither side of a cusp is steeper than 

 the other, except where oblique, wind-made waves have eroded one side 

 only, a condition observed in a few cases. 



^ 









_ — - — 



_ _-. _ ___-.^-. - _r --I- 



~ - - r :- 



Figure 4. — Partially eroded older Cusps and respaced later Series 



An interesting variation in form is found where old cusps terminate 

 abruptly in little "cliffs'^ instead of in sharp points. It is plain that 

 after the old cusps had been formed they were clifled by waves under 

 changed conditions and their apices cut away. From this eroded mate- 

 rial later series of cusps may form, unrelated in position to the original 

 series. Figure 4 represents a case of this kind as observed in cobble- 

 stone and gravel cusps on a gravel beach at Winthrop. Sometimes the 

 cusps are more completely eroded than in the case figured, and remnants 

 of three or four distinct sets, of different sizes and spacing, may often be 

 observed on a beach at one time. 



MATERIAL 



As in the form of cusps, so in the material of which they are com- 

 posed, is there the widest variation. In building them the waves make 

 use of everything, from the finest sand to the coarsest cobblestones. 



