Travelling Australia - Journal 2011
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21-26 June 2011 - Hughenden - Porcupine Gorge
Porcupine Gorge
Porcupine Gorge from the Gorge Lookout; the gorge is 120 metres deep.


Porcupine Gorge is the result of natural erosive powers of water operating over many years. Millions of years ago thick layers of sand were deposited at the bottom of an extensive inland sea covering what is now this part of Queensland. In time, the pressure from overlying material converted the sand into sandstone which was then uplifted to be higher than the country to the south.

Between two and six million years ago the sandstone was covered by sheets of lava flowing from small volcanoes nearby. The lava hardened to basalt; this hard rock for a long time protected the underlying sandstone from being worn away by streams flowing over this high ground. Eventually Porcupine Creek, a tributary of the Flinders River, eroded its bed through the basalt exposing the sandstone beneath. Erosion of the underlying sandstone was then rapid (in geological terms) forming the gorge we see today but the hard basalt cap at the sides prevented the gorge widening very far.

In the photograph above the vertical cliffs are composed of rock classified as Gilbert River Formation; this is mainly sandstone made of quartz and formed from deposits in a shallow marine sea 130 to 140 million years. The sandstone contains shallow layers of conglomerate, siltstone and claystone formed from other materials. Above this vertical face lies the thin white Doncaster Member of the Wallumbilla Formation formed 110 - 115 million years ago from shale and mud deposited in an open marine environment. This layer extending under much of Northern Queensland is known to contain marine fossils. The Gorge Lookout is on this layer. At the surface, and not as easy to see but visible on the right of the photograph above is the black rock of the basalt cap which protected the underlying rock. The red soil common at the surface around the gorge is decomposed black basalt.

The grey coloured rock on the bottom of the gorge, extending up the sides as far as the top of the vegetation band, is Blantyre Sandstone. This formed in a major river system about 200 million years ago. Porcupine Creek is still eroding this rock layer.

During the considerable gaps in time sequences between various layers new rocks were not being laid down.

Travelling Australia - Porcupine Gorge - page 2
Basalt Outcrop Outcrop of black basalt on the edge of Porcupine Gorge.


Side of gorge Vertical sides of gorge with woodland covered the surrounding plain. The sharp boundary to the cliff-top vegetation is obvious.


Bottom of Gorge Porcupine Creek in its eroded bed at the bottom of Porcupine Gorge.


Travelling Australia - Porcupine Gorge - page 3
The Pyramid
The Pyramid in Porcupine Gorge.


North of the Gorge Lookout there is a camping ground, additional lookout, and an access track 1.2 kilometres down to the bottom of the gorge. The base of the gorge is white sandstone dominated at one end by the Pyramid; this is a spur of flat-bedded brown sandstone of the Blantyre Formation cut off by erosion in the gorge forming equally inclined sides giving the appearance of an Egyptian pyramid. The Pyramid rests on a pavement of bluish-white Warang Sandstone (laid down in river systems about 220 million years ago) which Porcupine Creek has eroded into waterworn patterns. The access track from the camping ground ends on the white sandstone and visitors can make their way along the sandstone to the base of the Pyramid.
Porcupine Creek
Porcupine Creek continues to erode the sandstone at the base of the gorge near the Pyramid.


Travelling Australia - Porcupine Gorge - page 4
Sandstone Porcupine Creek in the channel it has cut for itself along the white sandstone base of the gorge below the camping ground.


Plateau top woodland Woodland covering plains around the gorge taken from Bottletree lookout a few kilometres south of the lookout.


Trees and Grassland Trees and grassland extending over the plateau dissected by Porcupine Gorge. Photographed from Bottletree lookout.


INFORMATION
Rocks and Landscape Notes - Brief Backgrounds - Porcupine Gorge. By the Geological Society of Australia at www.qld.gsa.org.au/BBPorcup.pdf
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