The 290-Million-Year-Old Hill That Rode an Ice Sheet

One hundred and fifty kilometres north of Adelaide, a ridge of quartzite rises from the wheatbelt like a frozen wave. It is called the Hummocks, and it has no business being there.

The Glacial Subway

The Hummocks are a glacial erratic — not a boulder, but an entire hill. During the Permian glaciation, 290 million years ago, an ice sheet kilometres thick scraped across the Gawler Craton and planed off a slab of billion-year-old quartzite. That slab, weighing perhaps millions of tonnes, was carried southward inside the ice like a pebble in a shoe. When the ice melted, it dropped the block where it stood.

The result is a geological contradiction. The Hummocks are made of quartzite that matches bedrock found only in the Eyre Peninsula, 150 kilometres north. The surrounding plain is soft limestone and shale from the Jurassic and Cretaceous. The hill is a stranger, a piece of the north that was dragged south by ice.

The hill is a stranger, a piece of the north that was dragged south by ice.

The Map in the Ice

Australia was not always at its present latitude. In the Permian, the continent was part of Gondwana and sat near the South Pole. The ice that carried the Hummocks was part of a vast ice sheet that covered much of southern Gondwana. Striations — grooves carved by rocks embedded in the ice — are still visible on the quartzite surface at the Hummocks. They point north-northwest, the direction the ice was moving.

These scratches are the only record of the flow direction of an ice sheet that no longer exists. By mapping them across South Australia, geologists have reconstructed the Permian ice sheet's behaviour: it flowed from a centre near the present-day Great Australian Bight, 500 kilometres west, and moved across the continent toward the east coast. The Hummocks are one of the largest known glacial erratics in the world, and one of the few that is a hill rather than a boulder.

The Landscape That Refused to Leave

The Hummocks did not stop at being carried. After the ice receded, the quartzite block stood above the plain. It was harder than the surrounding rock, so it eroded more slowly. Over 250 million years of weathering, the softer limestone around it dissolved away, leaving the Hummocks as a raised ridge. The erratic became a hill because of its own stubbornness.

Today the ridge is covered in native cypress pine and sheoak. The wheat paddocks stop at its base. It is a remnant of an ice age that ended before the first dinosaurs appeared — a chunk of northern South America (which was then adjacent) carried south by a vanished ice sheet, then left behind as the continent drifted north into warmer latitudes.

What Survives

The Hummocks are not famous. There is no visitor centre, no walking trail, no plaque. They are listed on geological maps as a minor feature. But they preserve something rare: a direct physical link to the Permian ice age, a time when Australia was buried under kilometres of ice and connected to Antarctica, India, Africa, and South America. The ice is gone. The continent has moved thousands of kilometres. The hill remains.

Glacial erratics are common in the Northern Hemisphere, where the last ice age deposited boulders across Europe and North America. But in Australia, most erratics from the Permian have weathered away. The Hummocks survived because of their size and hardness. They are a monument to a climate regime that no longer exists, carried by a force that has vanished, stranded in a landscape that has forgotten the cold.