BY E. C. ANDREWS. 823 



gradually from near sea-level to a height of over 2,000 feet some 

 30 miles in a westerly and south-westerly direction, while 

 immediately to the north, across the Shoalhaven, rises the great 

 escarpment of the Lithgow Plain some 2,000 feet in height. 



By the removal of the land waste, the former smooth and later 

 warped plain was roughened into valleys and ridges, reduction 

 almost to base-level occurring along the lower stream courses. 

 The recent subsidence converted these valleys to bays and 

 harbours, as explained earlier in these notes. The sea dashing 

 into bays of open type like the original Narrabeen and Rock Lily 

 indents formed huge bars across their entrances, and impounded 

 the inner waters to form lagoons. Bars]: of the Narrabeen type 

 show several aggradational lines of beach-growth according to 

 the observations of Mr. M. Morrison and myself. In all cases 

 the influence of a dominant current from the south is inferred 

 from the occurrence of wedge-shaped bars attached by their bases 

 to headlands forming the southern boundaries of the indents, 

 while their northern ends are free, the lagoon outlets being in all 

 cases jammed closely up to the headlands on the northern aspect 

 of the lagoons. No dominant overlaps occur, however, bottom 

 action being more pronounced. With the formation of the 

 lagoons the force of the sea was able more rapidly to steepen the 

 shore-slope instead of being fruitlessly expended in the smooth 

 waters of an inlet. The waste brought down by the coastal 

 streams and the sand blown over the bar by the dominant wind 

 were now carried into the lagoons and redistributed by the tides 

 to form shoals. Before complete silting up ensued the recent 

 vibration of elevation occurred, and the old bays w-ere transformed 

 into huge flats, backed up by high broad belts of sand dunes 

 (beaches), with small residual lagoons representing the still 

 unshoaled portions. Se(iuential stages in the shore-line topo- 

 graphy will be the complete filling of the lagoons, the formation 



X The explanation of the cause of these bay bars will be dealt with in a 

 future note on the submarine platform of New South Wales. Along-shore 

 currents, combined with maximum wave action to form a steepening of the 

 shore by accumulation, is doubtless the explanation. 



