However, these fabled outcrops are just the tip of the iceberg in terms of the presence of limestone in central Canberra.

"Limestone in central Canberra is even more prevalent underground, where over time, slightly acidic groundwater has created caves as well as many smaller limestone cavities," reveals retired Geoscience Australia earth scientist Dr Doug Finlayson.

According to Dr Finlayson, "many Canberrans would be surprised to know that large areas beneath the city, including parts of the Parliamentary Triangle, [are] pockmarked with cavities created by dissolving limestone".

Due to their depth beneath the surface, these cavities are usually only discovered during construction of significant infrastructure projects.

Take for example, Kings Avenue Bridge.

"Oral history has it that in 1959-60 when driving steel tube piles for the Kings Avenue Bridge, one suddenly disappeared into the ground, with workers reporting that they heard it 'hitting the sides on the way down'... but did not hear it hit the bottom," reveals Canberra-based freelance consultant and writer Mark Butz, in his recently published tell-all Where has all the Limestone Gone? (Canberra Historical Journal #88, March 2022).

Meanwhile, in nearby Parkes, a leaky budget would have been the least of the federal treasurer's concerns in the early 1960s when construction began on the Treasury Building.

Although preliminary testing identified underlying rock as basalt, according to Butz, "after construction began, major problems arose with sinking foundation piers".

"Drilling revealed that the whole site was actually underlain by limestone with caverns and cavities 'at all elevations' and with their location, size and shape 'impossible to predict'. Further, this tapped a significant aquifer with an 'uncontrollable flow of groundwater at the foundation level'."

Engineers urgently ordered water be pumped out of the aquifer at the alarming rate of 180,000 litres per hour - that's about two Olympic swimming pools per day. Heck.

"But it apparently made little difference," reveals Butz. "The base of the limestone was more than 50 metres below the surface in places, with cavities up to three metres high." As a result, extensive alterations to foundation design were required. I bet.

Similar hidden limestone caves also caused engineering challenges at many other construction sites in Canberra including the (since demolished) Macarthur House in Lyneham and the Edmund Barton Building in Barton.

During his earlier career at the Bureau of Mineral Resources, Dr Finalyson also discovered a similar cavernous void near the Parkes Way underpass adjacent to Blundells Cottage, not far from where the ASIO Headquarters was subsequently built.

"I hope they know about it," he laughs. Me too. That's not a place where you want substantial leaks of any kind.

While there are still caves with openings to the surface near the Cotter (unfortunately closed for several reasons, including public safety), sadly there are no longer any limestone caves in central Canberra that provide access from the surface.

In Canberry Tales: an informal history (Arcadia, 2012), Allen Mawer reports "local legend had it that there were inhabited caves along the Molonglo but at Acton only a pothole set back from the river was large enough to permit human access in historical times". Ann Gugler in True Tales from Canberra's Vanished Suburbs of Westlake, Westridge and Acton (GPN Publications, Fyshwick, 1999) reports "the Kaye boys dropped willow branches through the fissures and it took some seconds to hit the bottom".

MORE YOWIE MAN:

Despite stories of several other small limestone caves with openings to the surface, only one of these was formally recorded before they were destroyed or filled in - an eight-metre-deep cave leading beneath the Acton outcrop. After the kiln and associated quarry closed, the cave was purportedly unceremoniously used as a rubbish dump, and in the early 1960s the flooding waters of newly created Lake Burely Griffin sounded its death knell.

Meanwhile, the Wells Limestone Quarry, about 13 kilometres north of the city, was also filled with rubbish and is now buried under suburban Throsby, ironic given it was Dr Charles Throsby's (after whom the suburb is named) first printed account of the area in 1821 that described "very fine limestone, in quantities perfectly exhaustible".

This week marks 200 years since explorers Mark Currie, John Ovens and Joseph Wild and their Aboriginal guides passed through the Limestone Plains and became the first Europeans to set eyes on the main range of the Snowy Mountains.

On the return trip, on June 8, just south of present-day Queanbeyan, they were "met with large rocks of limestone" and what Currie described in his journal as "a natural bridge of one perfect Saxon arch, under which the water passed".

Today, we refer to this eye-catching landmark as London Bridge, and unlike its namesake in the nursery rhyme, the 34-metre-long arch is thankfully unlikely to fall down anytime soon.

The origins of the arch date back 420 million years ago when limestone began to form as sediment and coral remains were deposited on an ancient ocean floor. Over millions of years, the ocean receded and the dramatic arch was formed by water leaching through the limestone, slowly enlarging cracks until a passage became just large enough for Burra Creek to pass through. The arch reached its present form about 20,000 years ago.

In the late 1800s there were rumours that human bones, including skulls, had been crammed into caves at London Bridge - possibly part of some local burial custom. Following the apparent discovery of the bones by a local policeman, Superintendent Brennan, in January 1874, Coroner Morton from Queanbeyan was called to investigate. In his memoirs, Reminiscences of the Goldfields and Elsewhere in New South Wales Covering a Period of Forty-Eight Years of Service as an Officer of Police (Sydney: Brooks, 1907), Brennan reports that Morton "pronounced them to be skeletons of Aborigines of former times". Regrettably, exactly what happened to the bones has been lost in the sands of time.

There are various (all unsubstantiated) versions of the story, including one account that suggests it took the coroner several days to arrive and by the time he did, most of the bones had vanished. There is also speculation that the bones were removed by locals to avoid collection. Adding further intrigue to this story is that the small number of bones which were supposedly retrieved by the coroner were later buried in the Queanbeyan Pioneer Cemetery, only to have subsequently washed away in a flood during the 1970s.

When I investigated this case in detail in 2011, Helen Cooke, an archaeologist involved in several digs at the arch and nearby Douglas and Burra caves in the late 1990s, reported that "although the remains of locally extinct native rats, mice and small marsupials were found in these digs, no evidence of Aboriginal or European burials was found". She said many caves were filled in to prevent stock loss so it was possible "there were a few burials in caves we can't find now".

Check it out: A 3.4-kilometre walking circuit starts at the London Bridge car park, about 20 kilometres south of Queanbeyan via the southern entrance to Googong Foreshores. A brochure and map is available at the start of the walk. For more information, call the ranger station on 6207 2779 or visit www.parks.act.gov.au.

Rating: Medium

Clue: Looks 'nice'

How to enter: Email your guess along with your name and address to tym@iinet.net.au. The first correct email sent after 10am, Saturday June 3 wins a double pass to Dendy, the Home of Quality Cinema.

Last week: Congratulations to Ray Lindsay of Hughes who was first to identify last week's photo as an aerial view of the Tuggeranong 'Hyperdome' (now called South Point) and adjacent Tuggeranong Town Centre under construction. "The view is looking north towards Oxley, with Athllon Drive in the foreground," explains Drew McKinnie who along with Patrick Leerdam, Jo Sharpe of Moncrieff, and Mary McLaughlin of Weetangera, was amongst many readers just beaten to the prize.

In 2013, your akubra-clad columnist successfully tracked down the "long-forgotten" obelisk that was erected on private land south-east of Bredbo to mark the 150th anniversary of where some believe European explorers Currie, Ovens and Wild first set eyes on the main range of the Snowy Mountains on June 8, 1823.

I'll never forget it because the day I visited, a koala casually observed me, from just two metres away and without flinching an inch, taking photos of the obelisk. While koalas are rare in the ACT, I was subsequently advised that there is a well-recorded population of the cute critters in Bredbo/Numeralla area (and beyond).

Unlike in 1973 when the obelisk was erected, today there is widespread acceptance that the 1823 expedition heralded the dispossession of the Monaro, and the introduction of diseases that all but wiped out the traditional custodians. As such, I'm not aware of any formal event to mark the bicentenary of the expedition. I am, however, aware of at least one person who, on June 8, plans to sit on a nearby hill overlooking the Snowies and contemplate how we can make the next 200 years better for all.

CONTACT TIM: Email: tym@iinet.net.au or Twitter: @TimYowie or write c/- The Canberra Times, GPO Box 606, Civic, ACT, 2601

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