Wilsons Promontory Geology

Leader: Gary Wallis; Facilitators: Estelle & Bruce Adams

Wind and showers of rain sweeping across the bay didn’t dampen our enthusiasm as we looked at the granites and rock formations on Wilson’s Promontory. In the car park above the mouth of Tidal River, we examined the granite rocks and Gary ensured we knew the common minerals in granites as they could be easily identified in the rocks there – glassy quartz, feldspar and black mica. Gary commented that mica is flexible and used to be mined in great sheets in a mine near Alice Springs.

Down in the bay, Peter waded Tidal River to indicate the various layers of rocks as Gary described them to us. The pale granite has less black mica in it and xenoliths were darker. A “log-jam” of xenoliths were like smarties in a rock cake which would have formed during an interruption to the normal flow of the magma. Three different forms of granite have been identified in this area with differing colours and sizes of crystals.

Those without gumboots removed our shoes and socks and we waded through the very cold water of the river to study the rocks on the far side of the bay. Sands on the east side of the ‘Prom’ are silica-rich, while the sands on the west side are lime-rich. Pillar Point has limestone in its cliffs; nutrient-rich currents coming up from Antarctica resulted in a proliferation of shell-fish to create the limestone.

Coastal erosion here has given us a unique view of the granites which would have taken about a million years to cool, far below the surface. Mt Oberon could have been up to five kilometres high before erosion began.
As we clambered over the rocks and into the next two small bays, we were shown xenolith sheets sloping down, circles of xenoliths in the granite, ‘pock’ marks which would have been bubbles of gas, granite rocks containing tiny garnets and a black line which was a very narrow dyke. A tourmaline and quartz vein filled cracks after the granites had cooled. A dyke of micro-granite with very fine crystals possibly headed up to the surface; then, when it broke through, it cooled very quickly. Up in the rocks was a cavity of tourmaline; the dark crystals shone brightly within the cavity despite the dull light.

Before heading back to the relative warmth of the sheltered car-park and vehicles, Gary explained what happens when the granite breaks down. e.g. feldspar becomes clay. This excursion was enlightening to the participants and, in future, will result in a far greater understanding of any granite rocks we see.

Estelle Adams