The red sandstone is the toughest rock in the face of Rainbow Valley, as it has
a relatively high content of iron and silica. However, at the base lie fallen
blocks of dark-brown sandstone which have the most iron of all. These blocks
were part of the very top of the capping, and as such represented the land
surface of 20 million years ago.
The yellow and orange sandstones have less
iron and silica than the overlying red capping. They are softer and have been
washed away to form the sand dunes below.
The white bleached zone was formed by the leaching out of iron and silica,
which were deposited in the overlying capping. When erosion attacks this
relatively soft and fragile rock, the capping is undermined and will eventually
collapse, often falling off in large blocks.
Source: NT Parks and Wildlife signage