THE NATURE AND ORIGIN OF GTLGAI COUNTRY. 353 



when occasional heavy downpours occurred in the Warrum- 

 bungles water would come down in a sheet, carrying with 

 it the fine silt now forming the Black Soil Plain. After a 

 period of great aridity, in which wind blown detritus only 

 accumulated, moister conditions again allowed flood waters 

 to come down from the mountains. The larger floods carved 

 shallow beds in the level country. These were promptly 

 infilled with sand by smaller floods, while the finer silts 

 settled on the flooded plains adding to the black soil deposits. 

 The old stream beds formed in this way are known as 

 ■ monkeys.' The country being very level, the rivers would 

 continually change their courses as it was easier for fresh 

 floods to sweep across the black soil than to sweep over 

 the old sand beds which were often higher than the 

 intervening country, owing to the greater subsidence on 

 drying of mud than of sand. The continual change of course 

 by the Oastlereagh River in the early part of the present 

 cycle was probably aided by the fact that the drainage 

 system had not yet become reintegrated. Occasionally 

 the flood waters would sweep westwards from Coonamble 

 and empty into the Macquarie marshes. At other times 

 they would sweep north-west into marshy country near the 

 present junction of the Oastlereagh with the Barwon. 



That conditions are moister now is shown by the fact 

 that the present bed of the Oastlereagh is a definite U- 

 shaped hollow containing little or no sand. 



The Givydir and Moree Plains. — Similar observations 

 were made on the Gwydir River near Moree. 



The Bogan and Bogan Plains. — At Nyngan on a visit 

 about two years ago, I noticed similar evidence of a recent 

 arid period. The red soil plains on the western side of the 

 Bogan are fine grained windblown soils derived from the 

 Oobar massive of metamorphic rocks. The black or rather 

 brown soils on the eastern side of the Bogan consist of a 



W— Dec. 6, 1911. 



