BEACH FORMATIONS AT BOTANY BAY. 



175 



A 160 yds 



^W^wWPii 



Fig. 8. — General plan of cusp groups shortly after storm of July 1912. 



A A' — Approximate position of beach cliff. BB 1 — Cusps formed by decreas- 

 ing storm waves. CC — Cusps made by still weaker waves. DD' — Cusps 

 made one week after storm by ordinary waves. EE 1 — Approximate position 

 of low tide. 



near and at the breaking line to form small banks or mounds 

 while opposite the cusp points a local deep remained. Such 

 formations were observed to be associated with simple 

 undulations, in other words, they were formed when the 

 wind was absent or else straight off-shore, the waves thus 

 coming in as smooth swells. At each of these intercuspate 

 mounds near the breaking line the waves would be retarded 

 but deflected from their sides on to the cusp points. Whereas 

 at an earlier period, the waves were accustomed only to 

 interfere along the centre of the intercuspate troughs, at 

 this stage they would frequently interfere at the cusp 

 point itself, gradually cutting it back to a facet and then 

 fashioning it into cusps. Ooce a system of cusps had been 

 established upon a beach formerly smooth, it was evident 

 that the alteration in shape, size and arrangement of cusps 

 under any prescribed conditions of weather could be fore- 

 cast. The point still needing explanation was the formation 

 of cusps on a smooth and evenly sloping beach. 



February 20th -March 3rd, — Strong wind from south 

 east quadrant produced a wide flattish beach ending land- 

 wards in a step cuspate in plan. The wind moderating, a 

 beach ridge was thrown up in front of this indented step 



