|
Top. Rock domes in Hidden Valley near Kununurra. Spinifex and Turkey Bush grow in permeable
layers in the sandstone.
Bottom. Orange and dark grey bands indicate sandstone layers with different permeability. The
mobile phone indicates the thickness of the layers.
|
Hidden Valley (Mirima National Park), immediately adjacent to Kununurra township, is sometimes described as the convenient
alternative to the Bungle Bungle Ranges because it has similar rock structures but is much easier to reach. Both parks are
famous for domed rock structures containing horizontally banded orange and black/grey strata.
Hidden Valley is on a much smaller scale than the Bungle Bungles and the orange banding is not as prominent or widespread.
Rock domes characterise both Bungle Bungles and Mirima (Hidden Valley) formations. About 360 million years ago sand
deposited in layers up to several hundred metres thick formed the quartz sandstone in this area.
Subsequently (70 to 50 million years ago) the area was subjected to deep weathering leading to the formation of a hard
laterite cap on the surface. Deep weathering also dissolved the silica cement binding sand grains and weakened
sandstone beneath the surface but the weight of overlying rock held sand grains in place. Eventually the overlying laterite cap was
removed by weathering exposing the sandstone. Water flowing over the surface was concentrated by cracks or
joints in the rock and rapidly eroded the edges of soft sandstone blocks to form rounded towers and domes.
Orange and grey/black banding is on a large scale at the Bungle Bungle and is also present, on a smaller scale, at Mirima.
Colour bands on the surface are caused by different permeability of rock layers arising from differences in particles
laid down when the sandstone formed millions of years ago. In more permeable layers moisture seeps outwards to the rock
surface where it promotes the growth of black-grey algae and lichens. In between these dark grey
permeable layers, less permeable layers are covered by a patina of iron and
manganese staining giving an orange colour to the whitish rock. Plants can sometimes take root in the more permeable layers.
|
