348 H. I. JENSEN. 



of the Tertiary period was decimated owing to the estab- 

 lishment of desert conditions. Drainage became disin- 

 tegrated. Arid erosion succeeded normal erosion. 



To the west of the Warrumbungle Mountains near Toora- 

 weanah, at Ooonamble, at Nyngan, and throughout the 

 Pilliga Scrub I have obtained abundant evidence of a period 

 not far removed from the present, in which drainage was 

 completely disintegrated, and erosion was wholly of the 

 arid type. 



The late Tertiary wet period leaves its insignia in the 

 form of old water channels bestrewn with boulders so large 

 that the floods of present streams are unable to account 

 for anything like them in the same districts, and in the form 

 of huge accumulations of sand and coarse gravel underneath 

 the varied soils of tlie western plains. 



The dry cycle which followed levelled up the country, 

 filling former creek beds with windblown drift, and arid 

 erosion added to the accumulation of detritus at the base 

 of the slopes. 



The present period can only be described as subarid. 

 There is a sufficient rainfall to permit erosion to take place 

 and the drainage systems have become re-integrated. 

 Additional evidence of increased rainfall is afforded by the 

 present creeks cutting V shaped valleys along their present 

 courses through the heavy thicknesses of Tertiary and 

 Quaternary drift, especially along their upper courses. 

 Along their lower courses over the plain the tendency is 

 largely to aggrade in flood time. 



In the Pilliga Scrub gilgai belt the two or three hundred 

 feet of sand, gravel and clay under the surface soil, consist 

 largely of detritus borne down from the Warrumbungle and 

 and Nandewar Mountains in the wet period of the Tertiary. 



During the succeeding dry climate the detrital deposits 

 were added to and drainage became disintegrated by the 



