808 GEOGRAPHY OF BLUE MTS. AND SYDNEY DISTRICT, 



portions to disappear first, and great plains to exist in their place, 

 while considerable Hat-topped masses occupy almost the whole of 

 the centre.* 



To return to the discussion of the elevation, we note that the 

 streams received a marked impetus therefrom. Instead of wind- 

 ing as heretofore sluggishly over the wide Lithgow Plain, the}^ 

 cascaded furiously to the sea and proceeded to entrench them- 

 selves in their old basins. They would also be confined to steep 

 narrow canons until they could saw their way through the hard 

 overl3'ing sandstone. As the consecjuent Hawkesbury cut its way 

 through the resistant sandstone layer between Richmond and 

 Broken Bay, so the soft layers of shale between Penrith and 

 Picton were rapidly attacked l)y the lengthwise course of the 

 river. It could, however, only cut downwards through the shales 

 as quickly as the hard sandstone allowed the lower portion of the 

 river to sink through its mass; thus the subsequent Hawkesbury 

 amused itself meanwhile with meandering across the soft layers, 

 scooping them out for miles in its migrations. The old plateau 

 (Miocene 1} stream once flowed through Glenbrook, traces of 

 which survive in the large conglomerates occurring there. I he 

 stream at that period doubtless ran at a point but slightly above 

 sea-level, 600 feet below its present position. Traces of lower 

 levels may be seen in the cuttings between Penrith and Glen- 

 brook and on the large flat at St. Mary's. f 



The caiion cycle was the period of exultation for the Hawke.s- 

 bury. During the previous period it had searched out the weak 

 spots then above sea-level and had developed its subsequent 

 course in great measure, capturing the smaller consequent streams 

 as they were unable to cut their caiions fast enough into the hard 

 eastern layers. The Nepean appears to have been captured in 

 this manner, and as we now know it is thus an ohsequent stream. 



* E. C. Andrews, "An outline of the Tertiary History of New England " 

 (and references). Rec, Geol. Surv. N.S. Wales, vii, 1903. 



t W. B. Clarke, Sedimentary Formations of New South Wales. 4th 

 Edition, and map. 



