THE NATURE AND ORIGIN OF GILGAI COUNTRY. 351 



a former stream. These ridges are usually only from 10 to 

 15 feet higher at the most than the intermediate gullies. 

 The present streams in this vicinity are denuding the 

 country, otherwise they would not occupy depressions, but 

 the water would come down in a sheet from the mountains, 

 and the old creek gravels as well as the intervening country 

 would be covered with a uniform level sheet of recent silt. 

 In very recent geological times (Quaternary) there was an 

 arid period in which the ancient creeks ceased to run, when 

 debris accumulated in the mountain valleys of the Warrum- 

 bungles mainly through arid erosion, when the small spring 

 fed creeks flowed only to the edge of the mountains and 

 became absorbed in the thirsty soil while they deposited 

 their fine silt in the form of small black soil plains. In 

 this period the country immediately north and west of the 

 mountains became a plain, levelled by arid agencies. When 

 again a moister climate came, the streams, though large 

 enough to cut down, were not large enough to remove the 

 boulder beds of the old stream channels, hence we find 

 present streams carving down in the old alluvial plain 

 formed by the floods of the wet cycle leaving the old chan- 

 nels as intervening ridges. This I take as clear evidence 

 of a more arid climate than the present in recent geological 

 time. 



(b) North of the Warrumbungles. — To the north of the 

 Warrumbungles on the drainage areas of Cubbo Creek, 

 Dubbo Creek, and Baradine Creek, similar facts are in 

 evidence. Great alluviation took place in the wet cycle, 

 the streams depositing most of their sand and gravel as 

 soon as they reached the level country. A dry period 

 followed in which all inequalities were levelled by wind 

 action, the courses of old streams becoming infilled with 

 drift sand. Many of these former courses are easily picked 

 out by their more sandy soil and by gum and apple trees 



