BY E. C. ANDREWS. 811 



Tertiary deformation they kept the stream within their area, 

 while allowing great freedom of lateral movement. 



The deformation which characterised the uplift doubtless 

 modified some of the beheaded streams, as also headwater corra- 

 sion, although, as will shortly be shown, an excessively slow 

 movement is evidenced for the elevation, thus reducing the com- 

 petence of gravity as a deviating agent to a minimum. 



The significance of a weak series underlying a hard shell 

 beneath base-level during the plateau cycles, and raised thousands 

 of feet above that datum surface in the canon period, cannot be 

 overestimated in stream modification. After the initial move- 

 ments of uplift the upper streams kept gnawing their wa}'- back 

 into the sandstone, forming deejD canons therein, with alcoves 

 and recesses branching oif from the main valleys, while waterfalls 

 occupied the receding niches in the walls of masonry. Some 

 little distance west of Penrith the gradual tilting of the surface 

 had caused the underljdng shales to be exposed by the incising 

 streams. This discovery by the streams was the signal for a 

 marked change in valley-making. Instantly " sapping " was set 

 up, the soft shales were washed out by vertical and lateral 

 cutting, the weight and great vertical joints of the overlying 

 sandstone causing it to fracture and fall in wholesale manner, and 

 a wondrous recession of the canon walls took place, the V-shaped 

 trenches opening out into very broad valleys. The upper cliifs 

 now altered their steeply sloping attitude to great vertical 

 ramparts (PI. xL), Yet along their lower courses the aggregated 

 waters of the Hawkesbury streams are still forced to occupy steep 

 narrow channels only, since the soft underlying shales here lie 

 below base-level, and the canon cycle is not of suflficient age to 

 have allowed the streams to form wide valleys in the hard sand- 

 stone. Thus the upper streams of the Wollondilly, Cox and 

 Capertee valleys occupy broad canons, which open out lower down 

 into the main stream by mere "gaps."* The Hawkesbury itself 



* For a fuller description of the philosophy of this mixture of " iron and 

 clay" structures, see Memoirs of Geological Survey of N.S. Wales. Geology. 

 Vol. 3, pp. 115-120 (J. E. Came, F.G.S.). 



