314 GEOLOGY AND PETROLOGY OF THE GRIiAT SERPENTIXK BELT OF N.S.W. 



Tamwoilh Common north-westwards to Moore Creek, and is continued to the 

 south-east in the ridges running out into the Common, where tlie liack slope or 

 scarp of tile tilted block has been deeply dissected by streams which have worked 

 back along- the soft elaystones and crush zones between the hard agghmiorates and 

 tuffs, and have even captured part of the drainage that previously llowed to the 

 north-west. 



Again in the Nundle District {see 28) the varying elevation of the Tertiary 

 gravels shows that warping and faulting has occurred since their formation, per- 

 haps during and certainly also after, the period of Tertiary volcanic activity. 

 The following facts will indicate this, reference being made to the geological and 

 topographical map of the Xundle District (28, Plate xxii.), the figures being based 

 <m aneroid observations. Commencing at Hanging Kock there are a number of 

 occurrences of a "deep lead" or gravel-filled valley covered over by basalt. The 

 floor of this valley descends nine hundred feet within a distance of three miles 

 in a south-westerly direction, the sharp drop including one fault of two hundred 

 feet. This steep descent is, however, only a local feature, for in an adjacent dee|i 

 lead beneath the basalts of Yerrowinn, the fall is only two hundred and ten feet 

 in a distance of two and a half miles in a north-westerly direction. In Yellow 

 Rock Hill the slope of the base of the gravel is 140 feet in a distance 

 of two miles to the north-east, but the slope of the base of the overlying 

 basalt is 160 feet in the opposite direction, the gravel being 340 feet thick at the 

 soutli-c-astern end and 40 feet only at the other.* Moreover, the lowest point of 

 these gravels is lower now than anj' possible outlet for the Tertiary stream sys- 

 tem in which they were formed. It seems, therefore, certain that the region abo\it 

 Yellow Rock Hill has been depressed relatively to the surrounding regions, during 

 later Tertiai'y or post-Tertiary crust movements. To this wari>ing' and faulting is 

 )irobably due also the sharp decrease in the height of the Liverpool Ranges soutli 

 of the head of Nundle Creek. We must therefore conclude that within the water- 

 shed cf the Peel River System, late or post-Tertiary differential crust-movements, 

 as well as differential erosion, have been significant factors in determining the 

 present topogi-apliv . This is in accord with the conclusi<m obtained from a study 

 of the western slopes of New England between the Namoi and Gwydir Rivers, and 

 in particular tlie "Nandewar Buttress" (2) . 



Probably several epochs of movement and subsequent erosion occurred, as 

 Andrews (2!)/f) has urged. Of these some evidence is afforded in the Nundle 

 district. The study of the relation between the present contour lines and the 

 boundaries of the Tertiary basalts there suggests that tliey flooded over a fairly 

 matured ])eneplain, and down into comparatively youthfid valleys filled to a con- 

 siderable deptli with gravel. Uplift of the jieneplain to iiermit dissection, and 

 subsequent depression, accounting for the great thickness of the gravel appears |o 

 have occurred. From the plateau of basalts and the older rocks, however, have 

 been caivod out broad mature valleys to a depth of about 300 feet, wliich lead into 

 the deep lanyons of the upper, but not head waters of sonir of tlie streams of th<i 

 ju'csent cycle. Thus we realise the complexity of the history of the ])resent topo- 

 graphy and drainage system. If it be indeed a superimposed system, formed by 

 the cutting down of streams through a more or less uniform Mesozoic or Permo- 

 ('arboniferous sandstone covering (now I'emoved) on to a foundation of Upper 

 Palaeozf)ic rocks of very variable hardness, it must be recognised that it has not 



"Coiiipivri.son should ho made with the facts recorded cono'rning the relative levels 

 of basalts and underlying gravels in the Nandewar Kange (2, pp.27l'>-2"8). 



