Sunday 17 May 2015

Freycinet Experiences 2: Rambling Along the Rift

Geology isn’t sexy, or so we’ve been told. Yet show anyone a pure sand beach and a sapphire sea, and you’ve got them, whether or not they recognise the geology behind the appeal.

[Great Oyster Bay from Hazards Beach] 
Our second day on the Freycinet Peninsula doesn’t start like a geology excursion. But as we journey south by boat, I admit to pondering the origins of the great misshapen rectangle that is Great Oyster Bay. And I wonder too why we’re occasionally seeing dolerite. I think of this peninsula as dominated by granite. Why are two rocks of such different origins rubbing up against one another here?

There are more immediate questions first. Today we have a choice: to take the high road or the low road. The former climbs from the northern end of Cooks Beach over 579m Mt Graham, then winds back to Wineglass Bay. The latter starts south of Cooks Beach, and follows the western coastline of the peninsula to Hazards Beach. From there it’s over the low-level Isthmus Track to Wineglass Bay. Jodi and four others take the high road; Eric, Lynne and I take the low road. And yes, we’ll be in Wineglass ‘afore them!

[On the low road, with the high road in the background] 
The swell is rising as we drop the “high roaders” at the beach. There are some sand-churningly anxious moments before “Shep” extricates the boat from the beach. Minutes later he deposits us in a sea-weedy bay a bit further south, and we start our foot journey north.

Above us to the east are the wooded hills that rise to the granite peaks of Mts Freycinet and Graham. To our west are the dancing blue waters of Great Oyster Bay, and at our feet there’s dolerite – as well as questions that require a geological answer.

It turns out that today we’re walking the eastern rim of a graben. This is a kind of mini rift valley which has dropped relative to the higher blocks (horst) on either side. In the case of Great Oyster Bay, the graben has been inundated by the sea, which now separates one higher block (the Freycinet Peninsula) from another (the hills above Swansea). The rift was a late consequence of the long, complicated pulling apart of the super-continent of Gondwana. This began around 200 million years ago, and precipitated the injection of huge quantities of dolerite beneath what is now Tasmania. The graben probably formed closer to the period in which Tasmania finally tore away from Antarctica, around 65 million years ago.

[She-oak woodland, Freycinet Peninsula] 
As well as pulling blocks apart, these earth movements rotated and twisted the pre-existing rocks. Older granites, which had formed a huge sub-surface bubble around 400 million years ago, were brought to the surface here. The tearing apart also caught up some younger dolerite on the Freycinet side of the rift.

What remains today is a predominantly granite peninsula – albeit with dolerite and sedimentary remnants – separated from a predominantly dolerite eastern Tasmania.

If I seem to have strayed a little too far from white sands and blue seas, let me return to them. The beaches and the sea we’re walking beside simply wouldn’t be that colour without the slowly eroding pure quartz that’s derived from the granite. Other rock types usually contain fine-grained (ie muddy) elements. This granite has very few impurities to cloud the water, resulting in a rare, translucent clarity. And of course the beaches couldn’t be here without a graben deep enough to allow ocean inundation.

[Approaching Hazards Beach] 
Knowledge of these geological coincidences may not add to every walker’s enjoyment of this place, but it does enlarge my appreciation of a superb day in a rare landscape. So too does the fact that we can amble, knowing we have ample time to make our Wineglass Bay rendezvous with the other group. We pause for coffee, take way too many photographs, enjoy a swim (in my case a very brief one) and take the time to exchange bits of our life story. We also ponder millennia of Aboriginal presence along this coast as we come across some of the many shell middens that dot the area.

[An Aboriginal shell midden, Freycinet Peninsula] 
Despite our leisurely pace, we reach Wineglass in time for a long and restful lunch. After the almost deserted western side, the bay is very busy. Groups, couples, individuals spread out and take up their own little bit of this paradise.

[The "high roaders" return along Wineglass Bay] 
Lynne and I find a driftwood log in the sun, and settle down to wait for the “high roaders”. When they get back we gather ‘round each other like a happy pack of dogs, keen to share news of the day. But there’s rain forecast, and Gil and his bus are waiting for us back at the carpark, so Jodi gently urges us to talk while we walk. After a day like this, try to stop us!

[Just beating the rain back to the carpark] 

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