Purnululu NP (5)
|The Bungle Bungle Range in Purnululu NP rises up to 578 metres above sea level. The range stands 200 to 300 metres above a woodland and grass-covered plain, with steep cliffs on the western face. Elsewhere, particularly where Piccaninny Creek has formed Piccaninny Gorge, the range is cut by deep gullies and breaks up into complex areas of ridges and domes, with prominent orange and black or grey bands. |
Virtually every visitor to Purnululu asks the same question - how did this remarkable landscape come about? The distinctive beehive-shaped towers of the Bungle Bungle are made up of sandstones (rocks formed by the consolidation of sand grains) and conglomerates (rocks composed mainly of pebbles and boulders and cemented together by finer material). These sedimentary formations were deposited into the Ord Basin 375 to 350 million years ago, when active faults were altering the landscape.
Most of the rocks in the Bungle Bungle Range, however, were formed from sand deposited further from the highlands by lower-energy braided rivers flowing across broad plains in open valleys. As more sand accumulated, the older channels consolidated to form sandstone.
The distinctive beehive-shaped landforms seen today have been produced by uplift and erosion during the last 20 million years. Contrary to its solid appearance, the sandstone is extremely fragile. The weight of overlying rock holds the sand grains in place, but when this is removed, the sandstones are easily eroded and the rounded tops reflect this lack of internal strength. Water flowing over the surface will exploit any weaknesses or irregularities in the rock, such as cracks or joints, and rapidly erodes the narrow channels that separate the towers.
One of the most obvious features of the sandstones is the alternating orange and black or grey banding. The darker bands are on the more permeable layers of rock (which means water is able to move through them with relative ease). They allow moisture to seep through to the rock surface, promoting a dark algal growth.
The less permeable layers in between are covered with a patina of iron and manganese staining, creating the orange bands. These outer coatings (the rock beneath is a whitish colour) help to protect the lower parts of the towers from erosion.
About 250 million years ago, after the area was uplifted, a meteorite hit just north-east of Piccaninny Creek. All that remains today is a 10 kilometre circular structure on top of the Range. The same erosional forces that produced the Bungle Bungle and its sandstone towers have removed the crater.