812 GEOGRAPHY OF BLUE MTS. AND SYDNEY DISTRICT, 



from Wiseman's Ferry to Sydney is a narrow gorge sunken into 

 the Lithgow Plain and bordered by monstrous precipices. Every 

 particle of material lost to the plateau by the formation of the 

 mountain valleys has had to pass through the narrow defile of 

 the Lower Hawkesbury to the sea. 



Interesting in this connection, also, is the choice of course 

 adopted by the Hawkesburj^ below Penrith, where its path is a 

 caiion sunken in a plateau about 700 feet in height, while the 

 short track to the sea from Penrith lies through a plateau only 

 300 feet above sea-level and composed in the main of soft shales. 

 This Lower Hawkesbury course, then, teaches four most im- 

 portant lessons :— 



(1). That the river sawed its way through the coastal sandstone 

 as the land was warped athwart its course, otherwise, instead 

 of wandering 50 miles out of its way to attack a dense, hard 

 plateau overtopping the surrounding eastern areas, it would have 

 been forced under stress of gravity to take the line of steepest 

 descent over the deformed area to the sea, viz., by way of Sydney, 

 where the warping was but 300 feet above sea-level. 



(2), The Hawkesbury is thus seen to be a revived stream, 

 whose piratical tendencies had practically determined its present 

 direction of flow in the preceding plateau cycle. 



The wonderful "adjustment of streams to structure" (to 

 employ a phrase used by Prof. W. M. Davis) in the case of this 

 stream also points to at least a double cycle of subaerial erosion. 

 A glance at any good map will make this clear to students of 

 topography, especially if examined in connection with its piracy 

 of the Shoalhaven River. 



(3). The excessively resistant nature of the upper sandstone 

 layer as compared with the underlying soft beds. 



(4). The excessive youth of the caiion cycle as compared with 

 the time occupied in the formation of the Lithgow peneplain, for 

 in the plateau cycle great areas of hard sandstone and quartzite 

 had been cut away to sea-level, whereas canons only express the 

 loss suffered since the last great uplift. Untrained minds would 

 be liable to be overawed by the tale of erosion as revealed by a 



