230 GEOLOGY OF THE WARRUMBUNGLE MOUNTAINS, 



trachytes, overlain by trachydolerites, which were again capped 

 with basalt. 



Surrounding the mountains we have the "plain" country 

 through which the Castlereagh flows after leaving the mountains. 

 The plain very gradually diminishes in altitude from 1,700 feet 

 at Coonabarabran east of the group, to about 1,200 feet on the 

 west side, where it merges into the great western plains. It is 

 not a true plain or tableland, but dotted over it we meet with 

 numerous volcanic hills and sandstone residuals, which usually 

 obtain an elevation of 2,000 feet, the same as that of the 

 Warrumbungle Range. The sandstone residuals are typical 

 mesas, having steep, often precipitous, sides. The volcanic hills 

 are usually conical, and sometimes reach 2,200 feet in height. 

 From observations made at Shawn's Creek, at Tannabar, and 

 around Coonabarabran, it was ascertained that the lavas rest on 

 a dissected peneplain of Trias-Jura rock. The peneplain is marked 

 by the present 2,000 feet level, the Warrumbungle Range and 

 the sandstone mesas being relics of it. The lava flows filled up 

 its gorges and valleys, and in places increased the average 

 altitude by several hundred feet. Subsequent erosion has 

 removed the bulk of the volcanic flows as well as most of the old 

 level. 



The Coonabarabran tableland is a "peneplain" (of Post-Tertiary 

 age) bestrewn with mesas and buttes which mark two old levels, 

 the raised " peneplain " of the present 2,000 feet level, and a 

 later lava plain forming part of the conical lava mass of the 

 Warrumbungle Mountains, and reaching a higher altitude than 

 the sandstone " peneplain." 



The Post-Tertiary " peneplain " has been formed under the 

 influence of a normal cycle in Pleistocene times, modified by an 

 " arid " cycle in recent times. It is being base-levelled, not to 

 the general base-level of the sea, but to the level of the western 

 plains (1,100-1,200 feet). 



The Pilliga Scrub lies north of the mountain group. It has a 

 poor sandy soil covered with a thick pine forest. Mesas and 

 buttes like those around Coonabarabran abound in it near the 



