The Tuff That Trapped a Rainforest: New South Wales' Miocene Chalk Mountain

About seventeen million years ago, a volcano in what is now central New South Wales erupted and sent a plume of ash across a nearby lake. The ash fell through still water, settled on the muddy bottom, and entombed every leaf, flower, and fruit that had drifted down from the surrounding rainforest. What remained, when the lake dried and the land lifted, was a white cliff of tuff—Chalk Mountain—packed with the most complete Miocene plant assemblage yet found on the continent.

The Ashfall That Stopped Time

Chalk Mountain is not a mountain of chalk. The name comes from the pale colour of the volcanic tuff, a fine-grained rock formed when hot ash and pumice fragments compacted under water. The deposit sits near the town of Gulgong, about three hundred kilometres northwest of Sydney, in what was once a shallow crater lake.

Each layer of ash records a different eruption event. Some beds are only a few centimetres thick; others are more than a metre. Between them, preserved in exquisite detail, lie the remains of a warm-temperate rainforest that has no modern equivalent.

The leaves fell into the lake one autumn, and they have not decayed in seventeen million years.

What the Leaves Reveal

More than a hundred species of plants have been identified from the Chalk Mountain tuff. There are leaves of beech, elm, and hickory; flowers with petals still attached; fruits with seeds intact. Some specimens show insect damage—neat holes chewed before the leaf dropped.

The forest was not tropical. It resembled the mixed mesophytic forests of modern eastern China or the southeastern United States: tall canopy trees, dense understorey, seasonal rainfall. But it grew at a latitude of about 35°S, where today the climate is semi-arid and the vegetation is eucalypt woodland.

This tells us something striking. In the Miocene, Australia was warmer and wetter than it is now. The continent had drifted north but was still connected to Antarctica through Tasmania. The Antarctic ice sheet was smaller, ocean currents were different, and rainforest extended across much of the southeast.

A Window Into the Great Drying

The Chalk Mountain flora belongs to a period of profound change. The Miocene epoch (23 to 5 million years ago) saw the gradual aridification of Australia as the continent moved into drier latitudes and the Antarctic ice cap expanded. Rainforests that had covered the continent for tens of millions of years began to contract.

By the end of the Miocene, most of Australia's ancient rainforest lineages had disappeared from the southeast, replaced by sclerophyll forests adapted to fire and drought. The Chalk Mountain leaves are a snapshot of what was lost: a diverse, mesic ecosystem that once stretched from Victoria to Queensland.

Some of the plant genera preserved in the tuff—*Nothofagus*, Atherosperma, Eucryphia—survive today only in small pockets of Gondwanan rainforest in Tasmania and the Wet Tropics of Queensland. Others, like Cercidiphyllum (the katsura tree), have no living relatives in Australia at all.

The Tuff as Archive

Volcanic ash is an exceptional preservational medium. The fine-grained silica particles seal organic material from oxygen and bacteria, halting decay. At Chalk Mountain, the ash was also rich in calcium carbonate, which precipitated around the plant remains and cast them in a durable cement.

The result is a compression flora of rare fidelity. Veins are visible on the finest leaflets. The cuticle, the waxy coating of the leaf surface, is often intact and can be peeled off for microscopic study. These cuticles retain the shape of individual epidermal cells and the pattern of stomatal pores—data that allow palaeobotanists to reconstruct the forest's physiology and climate.

By measuring the density of stomata on fossil leaves, researchers can estimate atmospheric carbon dioxide levels at the time the leaf grew. The Miocene was a high-CO₂ world, with concentrations of around 400 to 500 parts per million—comparable to today's levels. The Chalk Mountain leaves confirm that the climate then was several degrees warmer than the pre-industrial baseline.

The ash that fell into a lake seventeen million years ago buried a forest whole. The tuff that remains holds not just the shapes of leaves but the chemistry of an atmosphere, the behaviour of insects, the outline of a continent's turning point. It is a quiet archive, written in volcanic glass and carbon.