The Glacier That Dug Australia's Deepest Gorge: Tasmania's Lake St Clair

Lake St Clair, in Tasmania's Central Highlands, is Australia's deepest natural lake. At 167 metres, its floor lies below sea level. But the lake is only half the story: the same glacier that scooped it out also carved the country's deepest river gorge, the 600-metre deep Franklin River valley, whose walls are Jurassic dolerite.

The carving happened fast, in geological terms. Between 20,000 and 10,000 years ago, during the last glacial maximum, a tongue of ice moved south from the Central Plateau, following a pre-existing valley in the dolerite. As the glacier advanced, it plucked blocks of columnar basalt from the valley walls and ground them into silt. When the ice retreated, it left a U-shaped trough dammed by a terminal moraine—the pile of rock debris that now holds Lake St Clair in place.

The Rock That Resisted

The dolerite that forms the lake's basin is the same Jurassic flood basalt that caps much of eastern Tasmania. It erupted about 180 million years ago, when the supercontinent Gondwana began to split apart. Magma intruded into layers of Permian sedimentary rock and cooled slowly, forming the distinctive six-sided columns that give the Organ Pipes at Mount Wellington their name.

Dolerite is tough. Its interlocking crystals of pyroxene and plagioclase feldspar make it harder than most granites. Yet the Pleistocene glaciers of Tasmania—modest by Antarctic standards, but thick enough to flow under their own weight—managed to quarry it. The ice sheet that covered the Central Highlands reached a thickness of perhaps 300 metres, enough to exert enormous basal pressure on the bedrock.

The same rock that resisted a million years of weathering yielded to a few thousand years of ice.

What the glacier left behind is a landscape of polished pavements, striated surfaces, and erratic boulders—dolerite blocks carried kilometres from their source and dropped when the ice melted. The lake itself is a classic rock-basin lake, formed where the glacier over-deepened a section of its valley, then plugged the outlet with moraine.

The Gorge That Followed

When the ice finally withdrew from Tasmania's west coast, around 14,000 years ago, the meltwater had to find a way to the sea. It carved a new channel through the dolerite, cutting down at a remarkable rate. The Franklin River now flows through a gorge that is 600 metres deep in places, with walls that rise almost vertically from the riverbed.

The gorge exposes the full thickness of the Jurassic dolerite sill—about 500 metres of columnar basalt, underlain by Permian siltstone and sandstone. In the lower reaches of the Franklin, you can see where the hot magma baked the sedimentary rocks into hornfels, a tough, dark rock that preserves the imprints of Glossopteris leaves from the Permian coal swamps.

The Franklin Gorge is not just beautiful; it is a textbook example of how glaciers reshape landscapes. The valley above the gorge is wide and U-shaped, typical of glacial erosion. The gorge itself is narrow and V-shaped, carved by post-glacial rivers. The transition from one to the other is abrupt, visible in a single photograph from a raft on the Franklin.

A Landscape in Recovery

Tasmania's high country is still rebounding from the weight of the ice. The crust, depressed by the glacial load, is slowly rising—a process called isostatic rebound. At current rates, the Central Highlands are rising about one millimetre per year. This is barely measurable, but over tens of thousands of years it will change the drainage patterns of the entire region.

The plants that colonised the freshly scoured dolerite include some of Australia's most ancient lineages: the pencil pine (Athrotaxis cupressoides), a conifer whose fossil record goes back to the Jurassic, and the cushion plants that form dense mats on the exposed plateau. These species survived the last glacial maximum in small refugia along the coast, then moved back into the highlands as the ice retreated.

Lake St Clair itself is oligotrophic—so low in nutrients that its waters are almost as clear as distilled water. The glacier left behind no soil, only crushed rock. It will take millennia for enough organic matter to accumulate in the lake to support a rich ecosystem. For now, the water reflects the sky, and the dolerite walls stand as they have for ten thousand years: bare, striated, and patient.