The Secrets of Australian Caves and Karst

Dr Catriona Nguyen-Robertson MRSV

 This article follows the 2022 Howitt Lecture, presented to the Royal Society of Victoria and the Geological Society of Australia (Victoria Division) on 23rd June 2022, delivered by Professor John Webb (La Trobe University).

 ‘People think of Australia being an old, boring continent…but it is definitely not’. As geomorphologist Professor John Webb points out, while the continent is not well-endowed with caves on a global scale, those we do have are notable for their diversity.

Jenolan Caves, spectacular limestone caves in NSW

As you walk across the Australian landscape, perhaps unbeknown to you, there may be caves beneath your feet. Australia is endowed with a variety of caves of different sizes, scattered across the country.

The caves contain fossils that are records of animals and the environment for at least the last 25 million years and hold the memory of past climates. They also house many living creatures, including bat colonies that consume hundreds of kilograms of insects every night.

Along the Nullarbor Plain, time has stood still for millions of years. It is a former shallow seabed, as indicated by the presence of calcareous skeletons of sea invertebrates that make up the limestone. The limestone is layered as the sea retreated before coming back and stacking on more. There are also low, undulating ridges that are remnants of the sand dunes that once lined the Nullarbor, marking the previous wind directions that deposited these dunes.

Below the ground, accessible via surface collapses, are many caves. Entering one is like taking a step back in time. John has explored many of them. The largest caves are close to the coast – simple long tunnels that stretch on, perhaps with branching here and there. Even the most delicate of stalagmites, like thin straws, have survived, seemingly untouched. The area drifted into an arid climate some 2.5 million years ago, preventing the geological processes that may have otherwise obliterated these structures.

Collection of water in a cave along the Nullarbor Plain.

The Nullarbor caves are covered in sand dunes from the degraded limestone that became grain, blown about by the wind. In some chambers, beautiful, clear, blue-green water collected over time to form lakes. The lakes can stretch to depths of 100 metres and being extremely salty (as John can attest to), they hold a large amount of rock salt (halite), interesting microbes and a variety of fossilised bones.

Further inland, the caves are smaller and rounder. Rather than forming by collapses, they were made by the water from stream valleys that carved its way through the limestone. John believes that they formed across a previous shoreline, where a mix of fresh and salt water dissolved the caves.

A more secret third area of Nullarbor caves exists, only explored by divers. Lying close to sea level, the caves are completely flooded with green water. Although too salty for vegetation, when it rains, a thin layer of fresh water lies on top where tree roots can grow to form seemingly floating plants.

Two cave divers exploring completely submerged caves below the Nullarbor.
A thylacoleo (marsupial lion) skeleton reconstructed from bones found within the caves at Mt Gambier. Photo credit: Garry K Smith.

Another Australian area underlined by limestone is the region stretching west from Portland to Mt Gambier, with caves even extending underneath houses in the towns. Collapsed chambers and sinkholes, called cenotes, formed as carbon dioxide from volcanic activity rose through fractures in the earth and acidified groundwater that then dissolved the limestone. John has snorkelled in the water-filled caves south of Mt Gambier and awed at the completely clear water that allowed him to follow a wall of limestone for as long as he could hold his breath.

These caves hold the history of the land. Within the caves are 80,000-year-old stromatolites, stony structures in ancient rock that were carved by colonies of microscopic cyanobacteria. Further north in the Naracoorte caves are chambers that have acted as pitfall traps for wandering animals over hundreds of thousands of years. They now store the fossilised remains of thousands of ancient animals that roamed the area.

On the other side of our state, in East Gippsland, the Buchan Caves formed 400 million years ago. The tourist caves are a honeycomb of crystallised calcium carbonate stalagmites and stalactites, derived from the skeletons of shellfish and coral that were deposited when the sea still covered southern Gippsland. Subterranean chambers and passages were carved out by rainwater that trickled in, and some chambers even now hold large springs.

A flat-roofed chamber of the Buchan Caves

While the tourist caves are more famous, slightly north is an unusually large collection of vertical caves in The Potholes Area. So named because of their small entrances, the Potholes are a denser collection of caves compared to anywhere else in Australia. The stepping roofs of the caves suggest that they formed along the ancestral flow of the Snowy River. Where limestone met mud of the riverbank, groundwater was forced to flow up and out, carving a path as it did. While the river now flows to the east, it made its mark: as water filled the stone, the caves began to form. Then, after a major tectonic uplift 2.6 million years ago (similar to the uplifts that created the Snowy Mountains), the caves drained.

‘The landscape has changed beyond all recognition in the last ten million years,’ says John. As the sea and land shifted, it dissolved the bedrock to create karst: sinkholes, sinking streams, caves, springs – spectacular mazes that now support unique ecosystems.

Australia’s caves are nature’s time capsules, formed over millions of years, and exploring them is a journey to a hidden underworld that holds many wonders. But caves and karst landforms need our protection. They house complex ecosystems, critical habitat for plants, animals, and micro-organisms which, in many cases, cannot survive elsewhere. Mining and littering have been damaging to cave systems in the past. But now, the biggest threat is overuse from tourism – which can be positive, as it helps people to value caves, but also requires cleaning and infrastructure to be built in natural spaces.

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