BEACH FORMATIONS AT BOTANY BAY. 161 



range of 6 feet 9 inches and 3 feet 3 inches respectively. 

 For this information I am indebted to the courtesy of Mr. 

 G. H. Halligan, Hydrographer to the Public Works Depart- 

 ment. A southerly current 1 flows at a distance of a few 

 miles from the coast, and the average velocity of this 

 current varies from one mile to two miles an hour. The 

 heavy on shore gales come from the east and south-east 

 directions, and the great rollers thus generated pulse from 

 the ocean into the bay through the heads and strike the 

 northern point of the beach first, thence they travel south- 

 ward along the beach at an average rate of from three to 

 six miles an hour. This wave motion is not regularly as 

 from uorth to south, but is broken slightly at the horn of 

 each cusp, thus causing the waves often to reach the shore 

 in a series of pulses arranged approximately en echelon. 

 Generally speaking, however, the waves travel fairly regu- 

 larly along the beach from north to south. 



The beach is of fine sand. No cobbles or boulders are 

 thrown up on the beach except during record gales, say, 

 only once (July 1912) during the last thirty years at least. 

 In intertempest periods the cobbles lie buried in the sand. 



Behind the present beach lies a narrow belt of sand dunes 

 reaching a general height of twenty-five feet above present 

 low water mark. A series of beaches and lagoons lie 

 between the present beach and the Trias-Jura sandstones 

 and shales which formed the edge and base of the bay when 

 it was more extensive than at present. The beaches of 

 the series just mentioned reach a general height of from 

 17 to 20 feet above low water mark. On Plate VII and 

 Fig. 1, the main features of these beaches and lagoons 

 are shown. 



1 C. Hedley. Presidential Address, Proc. Linn. Soc, N.S.Wales, 1910> 

 p. 9. G. H. Halligan. Sand Movement along the Coast of N. S. Wales. 

 Proc. Linn. Soc. N.S.W., 1906. 



K— Oct. 2, 1912. 



