Set in Stone

The 2.7 billion year old stromatolite juxtaposed by the modern loop of the National Museum of Australia
The 2.7 billion year old stromatolite juxtaposed by the modern loop of the National Museum of Australia Photo: Tim the Yowie Man

I'm sure you've heard the saying that Canberra is built on the limestone plains. Since setting up camp in our fair city some 22 years ago I've traipsed over countless granite tors in our parks, dug through seemingly impenetrable layers of clay in order to establish a vege patch and even slid down the shaley slopes of Black Mountain. But I can't say I've ever come face-to-face with the greyish rock for which our city is renowned. It's akin to living in the ''harbour city'' and never having set eyes on Sydney's sparkling waterfront.

In fact, I recently began to think that perhaps our treasured limestone was as elusive as the fabled Brindabella yowie. So when I saw a notice for a walk led by Dr Doug Finlayson of the ACT Geological Society that promised a visit to the limestone outcrop on Acton Peninsula, I jumped at the opportunity. Finally, I'd get to see the mysterious greyish rock on which our city was founded.

Doug's geology walk is one in a series of fascinating rambles around Acton, hosted by Acton Walkways, a local community initiative, established in 2009 to raise awareness of the Acton region for its natural beauty, history, culture, intellectual and artistic highlights. It's a sunny but cool weekend morning and 20-or-so walkers meet Doug at Regatta Point. Before long Doug is pulling out maps of the Mount Painter volcanics and bamboozling us with all sorts of geomorphological lingo. While everyone else listens intently to Doug's revelation that Canberra once was on the beach, albeit 420 million years ago, I'm focused on one thing only - the elusive limestone outcrop. I'm like a kid having to get through the plate of brussel sprouts before the prized chocolate ice-cream for dessert.

The enticing path that leads through Acton Peninsula's limestone outcrop
The enticing path that leads through Acton Peninsula's limestone outcrop Photo: Tim the Yowie Man

Next stop is the cutting near the Archbishop's House on Commonwealth Avenue. Here, while Doug crouches among a wedding cake of sedimentary layers bedded down during the Silurian age describing its formation, I pretend I'm interested by snapping some random photos.

Finally, we cross Commonwealth Avenue and head towards the National Museum of Australia. Still no sign of the limestone. Perhaps Doug's just going to produce a map of where an outcrop once was? Then just before we round the bay towards the museum foreshore, Doug pulls away some low hanging willow branches and we enter a darkened grove of trees and weeds. It's like entering another world. A tiny dirt track winds its way a few hundred metres along the shore and ahead of us hiding in the dappled light in an area not much bigger than a couple of tennis courts are dozens of smooth grey rocks. It's my holy grail! The fabled limestone outcrop does exist after all.

''These outcrops, and similar ones found along the Queanbeyan and Molonglo rivers, prompted British explorers to name the locality, the limestone plains,'' exclaims Doug to my repetitive camera flashes.

A sculptural view of the Acton Peninsula Limestone Outcrop. Bradley Cummings.jpg = a sculptural view of the Acton Peninsula Limestone Outcrop
A sculptural view of the Acton Peninsula Limestone Outcrop. Bradley Cummings.jpg = a sculptural view of the Acton Peninsula Limestone Outcrop Photo: Bradley Cummings

Sure it's not as impressive as the glistening waters of Sydney Harbour but these rocks tell a fascinating tale about our heritage. Allen Mawer in his recently published book, Canberry Tales: An Informal History (Arcadia, 2012) explains that this limestone is literally older than the hills, having been laid down as deep sea sediment some 420 million years ago. ''There was life here even then, as fragmentary coral and shell fossils found in the limestone show, but the here is misleading because at that time the rocks were located north of the equator, slowly being carried south to join the supercontinent Gondwana.''

Doug explains that when the first British explorers and would-be settlers came to this area they often brought acid with them to pour on rocks to check they were limestone [acid will react with the limestone, dissolving it]. For apart from good pastures, water supply, they also needed limestone to make mortar [lime mixed with sand and water] for construction.

''Although the subsequent quarry and kiln that operated here never reached any large scale, the presence of limestone here was one of the determining factors in the settling of the area we now call Canberra,'' he says.

Dr Doug Finlayson gets up close and personal with the limestone on Acton Peninsula
Dr Doug Finlayson gets up close and personal with the limestone on Acton Peninsula Photo: Tim the Yowie Man

But it's not just the limestone that makes this place special. Due to the dense and lush vegetation punctuated by the smooth rocks it's a secret haven on the fringes of the city. Kirsty Guster, the brainchild behind Acton Walkways, recalls the first time she stumbled on the site: ''I remember being really taken by the beauty and sculptural features of the rocky setting - having no idea at the time of its geological, historical and social significance.

''Given that these rocks are registered on the Commonwealth Heritage List, their position en route to many of the cultural attractions in Acton, and their scenic bush setting [with views towards the lake and Commonwealth Bridge], the limestone outcrop and the trail leading to it could become a significant feature of the landscape for residents and tourists alike.''

I tend to agree. We should be promoting our city's heritage, especially when it is in such an inviting setting. As part of celebrating our city's centenary next year it would be wonderful if this site was recognised for its historical significance and sculptural beauty, while maintaining the area's appealing bush setting.

Enjoying the view from amongst Acton’s limestone outcrop
Enjoying the view from amongst Acton’s limestone outcrop Photo: Kirsty Guster

FACT FILE

Acton Geology Walk: As part of the October program of Acton Walkways, tomorrow (10.30am-12.30pm), Dr Doug Finlayson will lead a stroll around the scenic foreshores of Lake Burley Griffin's West Basin with insights into the ancient and modern geology of Canberra, including a visit to the limestone outcrop and the stromatolite. Cost: free. Bookings essential on 0437 301 390 or actonwalkways.com.

Did You Know? Marble from the Acton limestone quarry was used to decorate the foyer of the Institute of Anatomy building, now home to the National Film and Sound Archive. Lime from the disused kiln was used to mark the lines on Acton's tennis courts.

The lower level of the lake has exposed more of Acton Peninsula’s limestone outcrop (centre right)
The lower level of the lake has exposed more of Acton Peninsula’s limestone outcrop (centre right) Photo: Tim the Yowie Man

LOCAL SECRETS

Not far from Acton's limestone outcrop is another rock with an even more remarkable story concealed in its layers. At first it looks like a boulder dumped in the carpark of the National Museum as part of the current construction work. But this isn't any old boulder, it is arguably the most significant rock on public display in the whole of the ACT.

The 2.7-billion-year-old rock is actually a stromatolite (a high-rise colony of algae) that was transported from a quarry in the Pilbara region of Western Australia to the museum a decade ago to feature in the museum's temporary exhibition, To Mars and Beyond. Stromatolites are recognised as one of the earliest forms of life on the planet.

Dr Doug Finlayson and Kirsty Guster of Acton Walkways check out the 2.7 billion year old stromatolite near the National Museum of Australia bus stop
Dr Doug Finlayson and Kirsty Guster of Acton Walkways check out the 2.7 billion year old stromatolite near the National Museum of Australia bus stop Photo: Tim the Yowie Man

What the stromatolites do (like any plant) is take in carbon dioxide and excrete oxygen. They did this to such great effect that they ''oxygenated'' the world, creating the atmosphere that allowed life to flourish and diversify. The oxygen not only flooded the atmosphere but also dissolved in the oceans, changing their chemical composition and causing iron in the water to precipitate out to form massive beds of iron oxide around the world, including vast deposits in the Pilbara.

So, in short, this very stromatolite is one of a number of colonies that transformed the world around 3 billion years ago - creating the iron ore deposits that underpin our civilisation and creating the atmosphere that sustains life.

A museum spokesperson advises that when the exhibition, To Mars and Beyond, left the museum, the stromatolite remained behind ''due to logistical complications and transport costs''. Somehow it ended up outdoors near the museum's bus stop, without any signage and where over the last 10 years I've seen it used by visitors as a bike stand and a picnic table, and I've even seen dogs relieve themselves on it, oblivious to its pedigree.

A museum spokesperson advises that they are ''discussing a plan to find it a permanent home in Canberra''. I hope it ends up in a more prominent position as befits such an object, perhaps near the lake's edge (perhaps ironically near current algal blooms) with some interpretive signage, for it really does tell an incredible story.

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