816 



GEOGRAPHY OF BLUE MTS. AND SYDNEY DISTRICT, 



and current action in the previous oscillation of subsidence. The 

 movement is extremely recent, even historically considered, since 

 very little changes have been effected b}^ the streams even on the 

 raised flats (coastal plains) of incoherent sand. Probably 100 or 

 200 years would embrace the period of time since the movement. 



A brief description of several portions of the shore-line of 

 Sydney will illustrate the significance of the recent subsidence 

 and much more recent uplift, and furnish an explanation of some 

 of the most interesting topographical features of the sea-margin. 



(a) Dominant Wind and Current. — Let b represent the 

 dominant wind of any region, the dominant current is shown 



for the coast by the arrow c acting 

 in the larger angle made by the 

 course of the wind with the shore- 

 line. This wind appears to be from 

 the south-south-east, near S3'dney, 

 as evidenced by the general direction 

 which the sand dunes of Kronulla, 

 Lady Robinson's Beach and Bondi 

 incline, or the prevailing inclination 

 of the vegetation on the exposed 

 headlands. Prevailingand dominant 

 currents must not be confused. A 

 current may set from the north for 

 B nine months in the year and 3'et not 

 accomplish a tithe of the work 

 Fig. '2.— Diagram illustrating accomplished by one acting fiercely 

 dominant wind and resulting fj,^^^ ^^le south durino the remaining 

 dommant current. ^ ■ ^ p ^ 



three months, since work performed 



varies as the sixth power of the current velocity. Thus, if a 

 current from the north perform a certain work, a current from 

 the south moving at three times the velocity will perform 729 

 times the task of the weaker current. In this connection also 

 must be considered the work performed by waves during severe 

 storms. 



