The Bako sandstone is rich in iron. When it rains, the iron is dissolved by the water, which percolates into the porous sandstone. This percolating solution re-emerges at the edges of the plateau. The iron ore then precipitates when water evaporates. The result is a resistant iron skin which forms a protective cover on the older sandstone surfaces.
Iron reacts with oxygen to form iron oxide which is brown.
That is why iron skin is dark-reddish brown. On older iron skins, they are black because they are covered with algae. Copyright 1996 Raffles Girls School.