BY w. !^. BE?rsoif. 279 



manner of evolution of the former river-system into the present 

 one. The Upper Manilla River being blocked by basalt, and 

 reversed by the tilting, passed east "up" its old valley, as far as 

 Barraba, the hinge-line in the tilting, and then continued "up" 

 the now reversed Anti-Manilla River (See Text-fig. 10). The 

 waters which came down through the old gaps at Woods' Reef 

 and Crow Mountain formed Ironbark and Crow Mountain 

 Creeks, and joined the Lower Manilla River, which carried 

 away the waters of the reversed Anti-Manilla River; Black 

 Springs Creek, Tea-Tree Creek, and the intervening air-gap 

 being the remnants of this old line of flow. The wavy course of 

 the Lower Manilla River is due to the fact that the bars of 

 resistant tuff run obliquely to the average direction of slope of 

 the land-surface. The relative depression of the Nanioi plains 

 allowed the dissection of the Black Mountain and Baldwin 

 Mountain regions, stripping off the soft Barraba mudstones, and 

 laying bare the Baldwin Agglomerates, of which probably onl}' 

 a small amount was exposed in Tertiary times (See Text-fig.5). 

 Upper Oakey Creek and Tarpoly Creek may be the entrenched 

 descendants of the streams which formed the gravels on Mount 

 Elijah. The lower portion of Borah Creek was deflected by the 

 southerly slope into the Manilla River, and, as its upper portion 

 cut lower and lower down into the mudstones covering the slopes 

 of Black Mountain, it was pushed to the south along the sloping 

 surface of the Baldwin Agglomerate; the point where it crossed 

 the Baldwin Range could not move laterally, but could only cut 

 deeper into that most resistant rock, and hence, perhaps, was 

 produced the sharp northerly bend in the creek at the gorge. 

 The uppermost portion of the Horton River, which flows south- 

 east from Mt Kaputar, may be a deflection and reversal of the 

 part of the Nandewar River now hidden beneath the adjacent 

 basalts. Its diversion to the north, into the Horton valley, was 

 probably the result of a capture by streams tributary to a second 

 drainage-system produced by the modification of a Tertiary 

 stream, the Proto-Horton, through aggradation, flooding with 

 basalt, and differential elevation and erosion. Information to 

 hand is insutiS.cient to permit of discussion of this. 



