814 GEOGRAPHY OF BLUE MTS. AND SYDNEY DISTRICT, 



they had gradually cut a host of valleys to base-level along their 

 lower courses. The coast may at this advanced period of marine 

 erosion be considered from two standpoints : firstly, as being 

 minutely irregular ; secondly, as being broadly very regular. 

 The latter is the result of long-continued marine erosion, the 

 former as being due to the action of streams on weak structures. 

 A regular network of dykes exists in the Sydney sandstones, and 

 these quickly became guiding lines for streamlets. The larger 

 irregularities were the canons of the Georo^e's, Parramatta, and 

 Hawkesbur}'- Rivers. 



A measure of the amount of shoaling for the shore of this 

 period is supplied by a calculation of the amount of material lost 

 to the mountains by the excavation therein of valleys such as 

 those of the Hawkesbury and its tributaries. 



Post-Tertiary Subsidence. 



After corrasion had reached the stage when the coast-line was 

 regular and harbourless, and the numerous streams discharged 

 into the shallow sea at the shore-line, the coast and shore began 

 to sink very gradually, the sea trespassed over the sandy shoals 

 and ran far into the drowned river valleys, converting them into 

 magnificent harbours (Plates xlii.-xliii.). The movement was 

 differential, and probably resulted in a slight elevation for the 

 central areas. Rivers like the Hawkesbury had cut their channels 

 down almost to base-level for long distances^ yet the sea trespassed 

 over such areas only. At various points along the shore-line or 

 thereabouts an estimate can be obtained of the amount of drown- 

 ing. At Peat's Ferry, where the river original!}^ flowed at sea- 

 level, the depth of the old valley floor below the present water- 

 way would give the amount of subsidence at that point. The 

 amount of drowning was probably about 200 feet in the neigh- 

 bourhood of the shore-line.^ 



Sydney Harbour was formed by the drowning and hetrunking 

 of the Parramatta and Lane Cove Creeks. North and South 



T. W. E. David, Journ. Proc. Roy. Soc. N.S. Wales, xxx. 1896, p. 57. 



