Hidden depths
South Australia’s Limestone Coast region owes its character to the mineral deposits that gave it its name.
South Australia’s Limestone Coast region owes its character to the mineral deposits that gave it its name.
As Canunda National Park’s longest-serving ranger, Glenn is used to these buffetings. For 20 years, he has watched this turbulent strip of the Limestone Coast, in South Australia’s south-east, with a keen eye. He’s not just cluey about the park’s sandy recesses, plants and animals, but as a surfer Glenn also has a sixth sense for the onrush of wind and waves. He’s seen dunes come and go, and entire headlands bust apart.
“Only 25 years ago, there was no beach at all here,” Glenn says as he gazes across the bay’s sweep of sand. “It was just water lapping up to the cliff line. And over there, a whole jetty of rock has eroded away. We used to walk out there. Now you can see there’s a gap of about 15m.” As waves slam into jagged limestone stacks offshore, it’s diff icult to believe that Glenn was surf ing this bay just three days earlier. That’s life on the riotous Southern Ocean frontline.
Canunda is but the latest of these seafront combat zones. Travel inland and every 10km or so you cross a range. All up, there are 14 of these low rises, aligned roughly parallel with the coast. Most are barely 30m high and each rib in the coastal plain marks an old dune rise, a calcified shoreline where the pounding ocean surf once dumped its payload of sand and shell. Head 80km north-east and you eventually bump into the Naracoorte Range. About 1 million years ago, this is where Glenn Jackway would have stood crouched over his board to ride waves to the shore.
As the Lucky Country, Australia abounds with mineral prodigies. Our giant monoliths and billion-year-old rocks, desert peaks and horizon-wide escarpments are among the world’s oldest and most exquisitely preserved landforms.
Yet our more recent geology is no less impressive. Take, for example, the relatively youthful limestone that encrusts the southern parts of the continent. These formations include the Nullarbor Plain – the planet’s largest limestone karst landscape – and the gargantuan procession of cliffs and headlands that runs along the southern coast from Western Australia all the way to Victoria’s Gippsland shores.
Few places on Earth have such a sprawling open-air gallery of limestone. And for farmers and communities living on the south-east apron of SA this legacy is no idle backdrop. From coastal formations, pastures and forests, through creeks, lagoons and vineyards, it’s limestone that def ines the region and drives local livelihoods. Their home patch is the embodiment of climate change – the handiwork of 1 million years of ice-age-driven sea-level f luctuations.
THIS LEGACY ISN’T a lways obv ious at f irst g lance. Anstruther, the property of local farm manager John Cooper, sits atop the Naracoorte Range at Joanna. The views are pin-up Australiana: rolling pastures, plump lambs and grand old river red gums. Geology doesn’t get much of a look-in. However, one paddock tells a different tale. After heavy winter rains, the ground is soggy underfoot and streaming water. John points to a rocky depression where run-off is disappearing as surely as if someone has pulled a plug.
“That’s it, there’s our runaway hole,” he says. “We haven’t seen it run for 15 years and it’s been going strong like this for two months now.” Never mind that his favourite barbecue spot is under water, John welcomes the recent bumper rainfall. “The good thing about this year is that all these little swamps and wetlands have filled up. It’s amazing how much water you get when everything’s f lowing.”
This runaway hole is no one-off. Its vanishing trick is replicated in hundreds of sinkholes that dimple backyards and farm paddocks across the region. Among the biggest stories here is what lies beneath: an almost limitless labyrinth of caves, slots and dark, dripping chambers. Riddled with openings, the
Glenn Jackway is squinting into the south-westerly gale hitting Cullen Bay. Whipped by wind and spray, his wavy mop of hair swirls like the grass tussocks at his feet.
surface crust of limestone serves as a colossal colander, funnelling run-off into underground streams and aquifers, making this the best-watered farmland in Australia’s most arid state.
But it isn’t just rain that takes a tumble. The district abounds with tales of tractors, farm tanks and even newly raised power poles suddenly punching through the crust into caverns below. Accounts of lambs and other livestock dropping into sinkholes are legion. The knack of this terrain to snare the unwitting is nothing new: the Limestone Coast has been amassing deposits in underground vaults for half a million years.
Just 10km north-west of John Cooper’s runaway hole is Victoria Cave, one in a cluster of larger openings into the depths of the Naracoorte Range. Although f irst discovered during the mid-19th century, it wasn’t until 1969 that the site’s buried treasures were revealed. When avid cave explorers Grant Gartrell and Rod Wells writhed their way through a narrow f issure at the back of the cave they emerged into a large chamber that contained a mountain of sediment and bones – the fossil mother lode of Australia’s long-lost megafauna.
I f irst visited Victoria Cave as a young teenager a couple of years after its secrets were revealed, clad in my dad’s khaki overalls and a hard hat. With typical gusto, Grant led my boyscout mates and me underground for a quick look-see. I remember squirming f lat on my stomach with arms outstretched. It was a tight squeeze over grimy red silt, our hard hats scraping the ceiling. Never mind the bones under our boots, it was a buzz just being there. To us wide-eyed kids, entering the new cavern was like stepping into King Tut’s tomb.
Nearly half a century on, the real story of these inner sanctums is more fantastic than any boy scout could have imagined. At about 70m long and 40m wide, the cave’s huge cache of sediment is the culmination of more than 500,000 years of material spilling down through narrow openings. These same entrances also served as nature’s own pitfall traps for a multitude of unwary creatures, which became buried within subsequent loads of sediment.
Decades of painstaking study, spearheaded by one of the cave’s discoverers, palaeontologist Rod Wells, has unearthed tens of thousands of specimens belonging to about 135 animal species – everything from 2.5-tonne diprotodons and giant kangaroos to tiny frogs. As a result, the Naracoorte Caves are globally recognised as the largest, most diverse and best-preserved fossil site of its kind. In 1994 Naracoorte, together with older fossil deposits at Riversleigh in north-west Queensland, was inscribed on the World Heritage list as the Australian Fossil Mammal Sites.
WITH HUNDREDS OF other sediment-rich caves yet to be studied, the work here has only just begun to scratch the surface. For palaeontologist Liz Reed, a University of Adelaide research fellow, “Naracoorte really is the Rolls Royce fossil site for this period in time.” As well as preserving an astonishing inventory of specimens, these sediments contain a wealth of insights about climate and vegetation in the restless surface world in which the animals lived – and where many went extinct.
“Naracoorte really is the Rolls Royce fossil site for this period in time.”
Blanche Cave is one of Naracoorte’s busiest visitor attractions. Yet here, right beside the tourist path, is where Liz has been working on one of her “libraries of change”, a carefully excavated 3m-deep pit. “The age of this deposit is from 70,000 years to about 13,500 years, so it cuts right through the megafauna extinction boundary,” says Liz, in her element within this intricate layer-cake of silt and debris. To her, this trench reveals millennia of turmoil like an open book. “We can pass our eyes up along a section and see the lead-up to that extinction period – the environmental change, the time of the arr ival of
humans – and then the consequences to ecosystems of the removal of the large herbivores. It’s really remarkable.”
This tale’s language isn’t written in old bones but a host of minutiae: pollen grains, charcoal, diatoms, fossilised leaf wax, ancient DNA and even individual sand grains, dated using a technique called optically stimulated luminescence. As one of 32 senior scientists from 11 different institutions analysing this forensic stash, Liz is helping piece together a veritable Rosetta Stone of knowledge about climate f luctuations and their impact on southern Australian biodiversity. “At Naracoorte, we have the almost perfect combination of materials and preservation,” Liz says. “There’s nothing else quite like it in Australia – or the world.”
The guardians of this miracle archive are the caves themselves. Naracoorte’s limestone dates from about 15 million years ago, the by-product of marine deposits from the Southern Ocean, which had muscled its way in after Australia unzipped from Antarctica. At the same time, as sea levels have ebbed and f lowed during the past million years, the landmass of the Limestone Coast region has been gradually uplifted. This added elevation eventually left each of the old shorelines – including Naracoorte Range – high and dry. In the wake of each ice age, the next sea-level high point forged a new beachfront and dune system further to the west. The 14 relic ridges that now corrugate these coastal f lats are a result of this.
In the case of Naracoorte Range, movement along an underlying fracture in the basement rock – the Kanawinka Fault – accentuated the uplift. It cracked and buckled the limestone, opening weaknesses that allowed groundwater to percolate deep into the range, enlarging caverns and passages. Winds of change also channelled sand and sediment into the caves, including the vivid red soil that forms their most distinctive fossil stockpiles.
WITH FEW RIVERS and 14 ranges as natural barriers, the ability of this high-rainfall country to trap water is legendary. Since the 1860s, farmers have been wrestling with the challenge of sodden terrain. The long-running and often contentious South East Drainage Network manages water and salinity across the region via an elaborate 2500km network of drains. While the scheme has helped free up low-lying land and boost local agricultural production by as