Uluru is dominantly composed of coarse-grained
arkose (a type of sandstone characterised by an abundance of
feldspar) and some
conglomerate.
[45][50] Average composition is 50% feldspar, 25–35%
quartz and up to 25% rock fragments; most feldspar is
K-feldspar with only minor
plagioclase as
subrounded grains and highly altered inclusions within K-feldspar.
[45] The grains are typically 2–4 millimetres (0.079–0.157 in) in diameter, and are angular to subangular; the finer sandstone is
well sorted, with sorting decreasing with increasing
grain size.
[45] The rock fragments include subrounded
basalt, invariably replaced to various degrees by
chlorite and
epidote.
[45] The minerals present suggest derivation from a predominantly
granite source, similar to the
Musgrave Block exposed to the south.
[49] When relatively fresh, the rock has a grey colour, but weathering of iron-bearing minerals by the process of
oxidation gives the outer surface layer of rock a red-brown rusty colour.
[45] Features related to deposition of the sediment include
cross-bedding and
ripples, analysis of which indicated deposition from broad shallow high energy
fluvial channels and sheet flooding, typical of
alluvial fans.
