Wednesday Rocks: #3

As this Wednesday Rocks being posted 40 minutes into Australia Day (Thursday 26th January) it's fitting that the subject is Australian.

Springbrook National Park, Queensland.

Natural Bridge

In November 2011 we took a drive south of Brisbane into the SE Queensland / NE New South Wales Hinterland (-28.202768,153.249664 - just enter the coordinates into the Google Maps search bar). Our exploring took us to Natural Bridge in the Springbrook National Park (I keep seeing Springbok but that's a result of my eight years living in South Africa). The rocks at Springbrook are part of a ~20Ma shield volcano which forms the Tweed Province, a series of basalt and rhyolite lava flows. Natural Bridge is an erosional feature formed behind a waterfall (Willmott, et al. 1981). 

A river runs through it, the top of the waterfall.

I assume that the formation is the result of a pothole or kettle forming behind the lip of the waterfall, as evident in the photo above. The rocks possibly faced a four-pronged attack from above, the pothole, below, the falling water eroding the rocks at base of the falls, water eroding joints and fractures in the rocks and then continued erosion by the waterfall. However, these processes are not something I've looked at in detail for a couple of years so feel free to drop suggestions in the comments box.

The cave at Natural Bridge is 46m long, 26m wide and 6m high (Webb, et al). It is famous for glow-worms and bats neither of which were in residence on our visit.

Willmott, W.F, Webb, J.A. and Wade, M. (Eds), 1981. Geological Sites in Southeast Queensland, Report 2. Geological Elements of the National Estate in Queensland, Geo. Soc. Aust. Inc. Queensland division, Brisbane, pp 119.

Tweed Valley
Happy Australia Day!! :)

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