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Uluru is an composed of a coarse sandstone, arkose, that was considered to be the largest single rock in the world, though the actual largest single rock is Mt Augustus in Western Australia.
It has remained largely unchanged for the past 40 million years, just slightly smaller and more rounded than it would have been originally.
The history of the rocks comprising Uluru resulted in a very hard, extremely resistant rock.
As the surrounding softer material has been eroded away the visible part of the rock has grown, because the surrounding surface wears away faster than the rock.
The present rock is being eroded by sloughing off layers of approximately equal thickness from all parts of the rock, by a process called spalling, or sheet erosion, or exfoliation, that was believed to result from the diurnal temperature variations that can be quite large in the desert environment, maintaining the shape while slowly reducing the size (White, 1994).
On the ribbed crest there are ridges that reach up to 4 m in height, and between some of the ridges the linear depressions coincide with bedding planes, though in other cases the depressions develop along strata, bedding planes reinforced by siliceous infilling being associated with the adjacent ridges.
In many of the depressions there are rock basins (gnammas), most being shallow pans with flat floors that can be 40 cm deep, with some hemispherical basins being up to 2 m deep.
The arkose is greenish grey, but a thin patina of feldspar fragments and clay give it its distinctive red colour, deriving from small amounts of iron oxide in the form of haematite and goethite (White, 1994; Twidale & Campbell, 2005).
Because of its singular nature, being composed of hard sandstone with few cracks or faults, erosion has had little chance to grind it down as usually happens with uplifted rocks.
It has been suggested that the mechanism of sheet erosion may not be the heating and cooling of the rock as has been accepted for some time, suggesting instead strong horizontal compressive crustal stresses may be responsible for sheet fractures, based on evidence from field work quoted by Twidale & Campbell (2005).
see Physical Weathering (Twidale & Campbell, 2005).
The process that led to the formation of Uluru began about 600 Ma (White 1994) or 550 Ma in the Cambrian, (Twidale & Campbell, 2005) when material eroded from the Petermann Ranges was deposited in the shallow inland sea, later to be buried and compressed to form the the coarse sandstone, arkose.
According to Twidale & Campbell (2005), the material was deposited in the piedmont of the Musgrave Ranges.
There are annular rings around the margins of some, and in some cases the floor is covered with sand (Twidale & Campbell, 2005).