The Clay That Became a Fossil of a Dying Sea: South Australia's Lake Bungunnia

Three million years ago, a lake larger than all of Tasmania covered the flat heart of South Australia. Its shores lapped against what is now the Murray River cliffs. Then it vanished, leaving only a white clay that still holds the shape of the water it once held.

The Lake That Covered a Continent

Lake Bungunnia formed around 3.2 million years ago, during the late Pliocene, when tectonic uplift blocked the Murray River's outlet near the present-day Murray Gorge. Water backed up behind that natural dam, spreading across 100,000 square kilometres of the Murray Basin. It was the largest lake ever to exist on the Australian continent.

The lake persisted for nearly two million years. Its waters were fresh, fed by rainfall and rivers draining the eastern highlands. Along its margins, fine clay sediments settled in still water, layer by layer, building deposits now known as the Norwest Bend Formation. These clays are the lake's only remaining body.

A Record Written in Clay

The Norwest Bend clays are strikingly uniform — white, fine-grained, almost pure kaolinite. Geologists can read them like a sediment core. The clay's mineralogy tells of a landscape that was deeply weathered, with slow-moving rivers carrying only the finest particles. No coarse sand or gravel appears. The lake was quiet for most of its existence.

Fossil pollen grains preserved in the clay reveal what grew along those ancient shores. There were casuarinas, eucalypts, and podocarps — a temperate woodland, not the arid scrub that surrounds the same sites today. The lake's slow evaporation left behind carbonate nodules, marking the beginning of the end.

The Drying of a Continent

Around 1.5 million years ago, the tectonic dam that held Lake Bungunnia began to breach. The Murray River cut through, draining the lake in what geologists believe was a geologically sudden event. The exposed lake floor became a plain of cracking clay.

The drying of Lake Bungunnia coincides with the onset of Australia's modern aridity. As the lake disappeared, so did the temperate woodlands. The climate shifted toward the dry, erratic pattern that now defines the interior. The clay deposits that survive are the only record of that transition — a fossil of a lake that died when a continent dried.

The white cliffs along the Murray River near Blanchetown are not limestone. They are the floor of a vanished sea of fresh water.

What the Clay Still Holds

Today, the Norwest Bend clays are exposed in cliffs along the Murray River between Waikerie and Blanchetown. They are soft, easily eroded, and constantly slumping into the river. Quarries in the region extract the clay for ceramics and brick-making — the same material that once settled on a lake floor two million years ago.

Archaeologists have also found stone tools embedded in the clay, left by the first Australians who walked along the lake's shoreline. Those tools sit directly atop the same white clay that records the lake's final drying. The continuity is quiet but unmistakable: the same sediment that held water once now holds the traces of people who saw the last of it.