■91 



GEOGRAPHY OF BLUE MTS. AND SYDNEY DISTRICT, 



Fig. 1 is a sketch section across the ordinary type of canon. 

 AA represents the high sandstone cliffs, bb the steep talus 

 slopes. The floors of these valleys, as shown at c, are usually 



Fig. 1. — Diagram illustrating the formation of the Blue Mountain canons. 



wide. A most interesting feature about them is the fact that if 

 the ordinarj^ slope of one of the sandstone beds as at a be con- 

 tinued across the valley it will coincide with a similar bed in cliff 

 a. Similarly for the shale and sandstone slopes of b and b.* 

 This fact of observation points to the obvious conclusion that at 

 some previous period the sandstone and shale beds must have 

 been continuous across the canons. 



Thus, from the foregoing paragraphs, it will be seen that by 

 filling up all the hollows for each set of flat-topped massesf we 

 shall obtain several plains rising one above the other; the highest 

 being 4,100 or 4,200 feet above sea-level, and of relatively insig- 

 nificant extent, being represented merely by peaks and a few 

 mesas; the next 3,500 feet above sea-level, represented by long 

 winding mesas; and a still lower one 3,000 or 3,100 feet in 

 absolute height, and of much greater area than the 3,500 feet 

 levf'l. In the case of the 3,100 and 3,500 feet levels we notice 

 that they slope gradually towards sea-level east of a line drawn 

 north and south somewhere between Lithgow and Blackheath, 



* C. S. Wilkinson, "Mineral Products of New South Wales," 1882. 

 t Mesas. 



