BEACH CUSPS 



243 



a heap of weed that may while fresh considerably modify the 

 waves, may become quite inconspicuous when dry. Consider- 

 able quantities of weed are also carted from the beaches for 

 manure. The only sure way to determine the presence or 

 absence of seaweeds on a beach is to visit it immediately after 

 on-shore storms have torn up the bottom just beyond the low 

 tide mark. The residual cusps in Fig. I are rendered unusually 



Fig. 4. — Residual and Delta Cusps. 



visible by the wetting of a portion of their line in what seems to 

 be an ordinary wave of a rising tide. The original photograph 

 shows the delta cusps distinctly alternating with the residual 

 cusps above. At Gay Head I have seen only the residual cusps, 

 which are there constructed of pebbles an inch or two in diameter 

 and derived from the till topping the cliff above. At that time 

 the form was unintelligible to me, but now I am sure that at 

 lower water the delta cusps will be seen at Gay Head too. 



In Fig. 4 both series are well shown on Lynn Beach. The 

 water stands in the hollows between the delta cusps, which show 

 only their points on the left. The view was taken about two 

 hours after high water. Half an hour later the waves break far 



