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Ochre comes from soft varieties of iron oxide minerals (such as haematite - a fine-grained iron oxide which produces a strong red colour with a purple tint) and from rocks containing ferric oxide.
Nauwalabila shelter in Kakadu National park, Arnhem Land Stone tools and ochre are the toughest of this camping debris.
For an interesting discussion of issues to do with dating Arnhem Land rock art, see the article by Chippindale and Tacon.
Techniques for dating have usually involved radio-carbon dating of material associated with the art, but there are also newer techniques now available including optically stimulated luminescence (OSL) and accelerator mass spectrometry (AMS).
This is, of course, a contentious area, with recent claims for dates in southern France and northern Italy going back as far as 35 000 years.
Archaeologist Sue O'Connor at the Australian National University has found a buried fragment of rock painting preserved in the limestone rock-shelter of Carpenter's Gap in the Kimberley (near Windjana Gorge National Park) in a layer dated to 40 000 years old.
The oldest dates so far found by direct dating of art were obtained by geologist Alan Watchman for layers of pigment in two rock-shelters on Cape York in north Queensland, one of 25 000 years and one of almost 30000 years.
There is, however, indirect evidence going back a lot further, leading some archaeologists to argue that the rock art galleries in northern Australia are some of the oldest in the world by modern humans.
Archaeologists have now discovered early occupation sites at the three most probable entry areas - the Kimberley, Arnhem Land and Cape York Peninsula.In Arnhem Land, there is no certainty either that ochre was used for painting from the beginning; or that painting with ochre was on rock surfaces (rather than on perishable subjects); or that the first paintings on rock are amongst the ones that survive.However, the hardness of much of the ochre found in deposits strongly suggests that it was used on rock or other hard surfaces and the pattern of wear is totally consistent with use of the ochre in art.The red pigment seems to be the remains of paint on a rock art fragment fallen from the ceiling above.The layer containing the painted fragment yielded ochre, burnt bone, stone artefacts and charcoal with an accelerator mass spectrometry (AMS) radiocarbon date of 39,700 1,000 BP (BP means Before the Present, which in this context is 1950, when the radiocarbon dating technique was developed).