AUSTRALIAN QUATERNARY CLIMATES AND MIGRATION 75 



south of the latitude of Grafton, was covered by it including the 

 drainage systems of the Darling (V) and the Murray (VI). 



About 15,000 years ago, when the tropical rain-forest reached 

 Cape York, the rainfall-reliability belt had somewhat contracted. 

 It then extended as far north as Georgetown in Queensland, and 

 as far west as Lake Eyre, covering the eastern half of the Diaman- 

 tina Basin (III) and the whole of the Cooper Basin (IV) ; the 

 rest of its cover was much the same as in the glacial period. 



Approximately 8,000 years ago, when Torres Strait was formed, 

 the climate was as it is now. The line of maximum aridity (Fig. 

 1 ) passed from a little south of N.W. Cape in Western Australia, 

 about 50 miles north of Lake Eyre, to near Walgett in New South 

 Wales. Theoretically, the coincident line of the northern front of 

 the arid belt j)assed through Boulia in Queensland and its south- 

 ern front was covered by the westward extension of the belt of 

 rainfall-reliability. North of the arid belt was savannah and 

 south of it steppe. The rivers of the Diamantina Basin (III), 

 had their sources in the savannah and outlets into Lake Eyre in 

 the arid belt; the Cooper Basin (IV) was wholly in the arid belt. 



At the Postglacial Optimum, 4,000 years ago, the northern peak 

 of the rainfall-reliability belt had receded south to about the 

 latitude of Maryborough in Queensland; the line of maximum 

 aridity then passed approximately through Marree south of Lake 

 Eyre, the northern front of the arid belt through Alice Springs, 

 and its southern front was covered by the rainfall-reliability belt. 

 The headwaters of the Darling — the upper reaches of the Con- 

 damine and some of its southern tributaries, were during the 

 Postglacial Optimum within the rainfall-reliability belt. The 

 rest of its valley (V), almost to its confluence with the Murray, 

 was in the arid belt which began to encroach on it about 5,000 

 vears ago. The take-off or orographic rainfall in the Darling- 

 Basin has been, during this period, small ; it has been estimated 

 that now, 4,000 years after the Postglacial Optimum, due mainly 

 to evaporation, 'less than 2 per cent of the rain falling in the 

 upper Darling valley passes the town of Bourke on its middle 

 reaches. The headwaters of the rivers of the Diamantina and 

 Cooper basins were in the savannah, but their middle and lower 

 roaches in the arid belt. It has been already pointed out that 

 since the early part of the Postglacial, the rivers of the Diaman- 

 tina Basin (111) have not been channels for orographic rainfall, 

 and their drainage has been, for the most part, the scanty tropical 

 rainfall of the sub-tropics. 



From the climatic standpoint, the problem of migration along. 

 the west coast of Australia is a difficult one. In Fig. 13 delimiting 



