Wednesday Rocks: #5

I've been in the lab so today's Wednesday Rocks is a teaser. The first photo is of the of the Brandberg, Namibia's highest mountain. The second photo is one of the perks of pottering around looking at rocks in Namibia, Desert Elephants. 

Brandberg granite.

Desert Elephants, Ugab River.

Wednesday Rocks: #4

The petrified forest in Namibia's Erongo Region is the result of glacial outwash at the end of a Carboniferous-Permian, Gondwana wide glaciation. The trees are Dadoxylon Arber, an extinct conifer and reach lengths of about 45m (Wikipedia). They are preserved in a palaeo-river channel and recent erosion has uncovered many of the fossilised conifers (Wikipedia). This is evident in the multitude of handwritten 'Petrified Forest' signs along the road from Twyfelfontein to Khorixas. The official park employs people from the area as guides who explain a bit about the geology and botany as you wonder through the silicified trees. The photos below are from the official forest park.

 

Petrified Forest - Wikipedia  

All Photos were taken by Sameer Morar.

This fragment is about 15cm across

Michael, our guide, and I for scale. I'm 1.69m

The knot Michael was pointing at with his stick

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.

Webb, J.A., Joyce, E.B. and Stevens, N.C. Lava Caves of Australia

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!! :)