572 THE GEOLOGY OF THE WARRUMBUNGLE MOUNTAINS, 



England area at the end of the Permo-Carboniferous gave the 

 Upper Coal Measures a S.W. dip. 



(b) Str earn- Development. — In Cretaceous times the present 

 drainage system commenced, the rivers like the Namoi, Castle- 

 reagli (lower part) and the Macquarie taking a consequent direc- 

 tion, and flowing, therefore, N.N.W. The uplift continued for 

 some time, but the rainfall being good, on account of a Cretaceous 

 sea lying to the N.W., erosion almost kept pace with the uplift. 

 Tv'xhwt^v J subsequent streams like the Talbragar River, Baradine 

 Creek, etc., now developed, and low watersheds like the Warrum- 

 bungle Range were formed by erosion. A stationary period 

 following, allowed most of the country to be reduced to a pene- 

 plain, at present marked by the 2,000 feet level mesas all round 

 the Warrumbungles. In the centre of the group there was a 

 sandstone area which had not yet been quite reduced to a level, 

 but was diversified with ridges and valleys. This takes us to 

 early Tertiary (Eocene) times. Now volcanic action commenced, 

 and the lavas built up the central mass to a great height; whilst 

 subsequent outpourings not only filled up any valleys in the 

 country around, but covered the peneplain over a considerable 

 area with a lava-sheet thinning out away from the central mass. 

 In this way it is possible to explain that some lava hills like 

 Nandi near Coonabarabran, Yarrighnan and Yarabala near 

 Bugaldi, rest on sandstone at a level of from 1,500 to 1,700 feet, 

 being relics of flows filling valleys; whilst in most cases the lavas 

 rest on the sandstone at a level of from 1,900 to 2,000 feet, being 

 mappings on the old peneplain level {e.g.^ cappings around Coona- 

 barabran on the Warrumbungle Range, around Bugaldi, on the 

 *'Dillys," etc., etc.). In fact there seems to have been a slight 

 uplift and recommencement of stream-dissection in the area 

 before the eegirine trachytes, phonolites and basalts were poured 

 out. This uplift was probably due to the injection of sills at the 

 period of eruption of the arfvedsonite- trachytes. 



The drainage of this area was now altered. The waters had 

 to find their way round a great effusive pile. In this wa}' the 

 Castlereagh developed. 



