806 GEOGRAPHY OF BLUE MTS. AND SYDNEY DISTRICT, 



Very probably the old Hawkesbury River bed of Lapstone 

 Hill belonged to the early canon cycle, and the post-basaltic 

 period of elevation revived the old stream which had previously 

 been flowing over an area of but slight elevation. 



The movement w^as not uniform, since we find on studying the 

 original slopes of the surface (by connecting different portions of 

 the Lithgow Plain across the gullies cut into them during a later 

 period) that : — 



1. At Sydney the general surface is 300 feet above sea-level. 



2. ,, Hornsby „ GOO „ 



3. ,, Glenbrook ,, 600 ,, ,, 



4. ,, Lithgow, Rydal, and Oberon ,, 3100 ,, ,, 



5. „ Hill End and Wattle Flat „ 3000 „ 



6. ,, Orange and Blayney ,, 2900 ,, ,, 

 6. „ Clifton „ 1200 „ 



8. ,, Kiama Mountains and Moss Vale 2200 ,, ,, 



B}' following the uppermost beds of the Haw^kesbury Sandstone 

 south of Sydney, a splendid idea of the gradual southward rise 

 of the Lithgow Plain is obtained. The heights progressively 

 obtained are, approximately, Sydne}" 300, Port Hacking 600, 

 Waterfall 700, Helensburgh 900. Bulli Pass 1350, Robertson 

 2200, and Moss Vale 2200 feet. From all points the observer 

 appears to stand on a limitless plain broken here and there 

 (especially westward) by flat-topped hills. 



Thus it appears that there has been an even vertical lift of 

 3000 feet between Orange and Lithgow^ thence towards Sydney 

 a gradual decrease in height, interrupted, however, localty by a 

 large fold and fault,* wdiile northw^ards and southwards of Sydney 

 the coast has been gradually elevated to maxima of 600 and 2200 

 feet respectively. Sydney thus occupies the centre of a warped 

 area. 



* T. W. E. David, "An important Geological Fault at Kurrajong Heights' 

 (and references). Journ. Proc. Roy. Soc. N.S. Wales, xxxvi., 359. 



