BEACH FORMATIONS AT BOTANY BAY. 183 



closer inshore and excavates the accumulated sand mass 

 in some measure, while the undertow action piles up some 

 of the excavated sand behind it (that is upon the submarine 

 portion of the old storm profile behind the new wave base). 

 Landward of the new wave base it now excavates a narrower 

 and steeper beach out of the storm beach. At its upper limit, 

 however, it proceeds to pile up a small beach crest upon 

 the storm beach. The next point to consider is the manner 

 in which this newer beach profile will be excavated. The 

 breaking wave, now reduced in power, does not pare off 

 slices from the beach after the manner of a sharp knife, 

 because of the nature of water to advance in undulations, 

 such undulations being accentuated in proportion to the 

 lack of adjustment of wave strength and surface passed 

 over. In breaking upon the storm beach the reduced wave 

 is opposed by the returning wave and the resultant of these 

 is broken into pulses which attempt to spread out in circles 

 from the breaking line. Against the on-coming water from 

 the bay such movement is inappreciable, but it is never- 

 theless appreciable and pronounced in the direction of 

 wave motion up the beach. The size of the undulations 

 moreover is in direct relation to the strength of the break- 

 ing wave minus the strength of the backwash. The 

 undulations or pulses thus advance up the beach in the 

 form of modified arcs of circles which mutually interfere, 

 and these imperfect interferences are again arranged in 

 groups until a more perfect interference of waves occurs. 

 At these points the cuspate forms commence to form. The 

 localisation of these cusps is governed in some measure by 

 the salients of the beach, but is effected practically simul- 

 taneously along the beach, and the operation once started 

 the interfering waves in their return to the bay form the 

 scallops or cusps. Perhaps in ninety-nine per cent, of the 

 occurrences the undulations which form the cusps by their 

 interference are due to the shape of the front upon which 



