The 300-Million-Year-Old Ice That Scoured a Continent's Spine

It sounds wrong to say that ice shaped Australia, but 300 million years ago the continent sat at the South Pole. Sheets of ice kilometres thick ground across what is now New South Wales and Victoria, carving the skeleton of the Great Dividing Range into the shape we recognise today.

The Permian Glaciation

During the Permian period, Australia was locked inside the supercontinent Gondwana, its eastern margin pressed against Antarctica. Ice sheets flowed from a highland centre toward the sea, gouging valleys that ran north–south along the continent's eastern flank.

The evidence is written in the rock. At the Hunter Valley's Seaham Quarry, you can see striated pavements—parallel scratches cut into bedrock by rocks embedded in the base of moving ice. The direction of the scratches matches the ancient ice flow, toward the north-west.

The same ice that scratched the Hunter Valley also dropped boulders the size of cars into the sea, where they sank into soft mud and are now preserved as dropstones in the Permian coal measures.

Valleys Carved by Ice

The glaciation lasted from about 300 to 270 million years ago. During that time, ice sheets advanced and retreated multiple times, each cycle deepening the valleys that today form the drainage of eastern Australia.

In the Blue Mountains west of Sydney, the deep gorges of the Grose and Coxs rivers follow paths first cut by Permian ice. Later, the rivers simply inherited the valleys the glaciers had carved—a landscape shaped by processes that ended before the first dinosaurs appeared.

Further south, the Monaro region's U-shaped valleys and glacial lakes, such as Lake George and the Blue Lake in Kosciuszko National Park, are direct products of Pleistocene ice. But those younger glaciers were small and localised. The real sculpting happened during the Permian, when the ice was continental in scale.

The Legacy in the Coal

Permian glaciation did more than carve valleys—it created the conditions for the coal that powered Australia's industrial age. The ice sheets ground down mountains into fine sediment, which filled river valleys and shallow seas. When the ice retreated, swamps formed on this flat, fertile surface.

Those swamps became the Gunnedah Basin's coal seams, the Sydney Basin's Illawarra coal measures, and the huge deposits of the Bowen Basin in Queensland. The plants that formed the coal were Glossopteris, a Gondwanan tree that grew in the cold, wet conditions left behind by the retreating ice.

The coal seams sit directly above glacial tillites—rock made of unsorted, ice-ground debris. You can walk from glacial sediment into coal in a single outcrop, the transition from ice to forest recorded in a few metres of strata.

A Continent's Spine

The Great Dividing Range is not a single mountain range but a complex of plateaus, escarpments, and highlands that runs 3,500 kilometres from Queensland to Victoria. Its eastern scarp, the Great Escarpment, is one of the longest continuous landforms on Earth.

Geologists once thought the range was built by tectonic uplift. But the evidence now suggests that much of its topography is inherited—a landscape exhumed from beneath Permian ice, then dissected by rivers over tens of millions of years.

The ice sheets of the Permian did not build mountains. They revealed them, stripping away softer rock and leaving the hard, ancient cores exposed. What we see as the Great Dividing Range is partly the ghost of a landscape the ice chose to preserve.