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Development of continental aridity has been linked to late Cenozoic global cooling, but the evidence is indirect, based on terrestrial loess deposits and eolian silt in marine sediments, whereas direct dating of the inception of and landforms has been frustrated by a lack of suitable methods. Here we report the first age determination (if a major arid-zone dune field, based on cosmogenic Be-10 and Al-26 measurements of drill cores from dunes in the Simpson Desert, central Australia. Results show that the dune field began to form ca. 1 Ma, whereas dating using quartz optically stimulated luminescence indicates episodic dune building during late Quaternary ice ages. Less intense desertification began earlier; the previous cosmogenic exposure dating showed that neighboring stony deserts began to form at the onset of Quaternary ice ages 2-4 Ma. Aridity deepened and the dune field formed when ice age cycles increased their amplitude and switched their periods from 40 k.y. to 100 k.y. ca. 1 Ma.