The Lost Land of the Bassian Plain

The Bassian Plain:

Beneath the restless waters of Bass Strait, between Victoria and Tasmania, lies a forgotten world — a vanished land known as the Bassian Plain. For tens of thousands of years during the last ice age, this vast landscape was not sea, but land: a sweeping expanse of grasslands, wetlands, and granite hills where rivers flowed, animals grazed, and people lived. It was a vital land bridge that connected Tasmania to the mainland, but more than that, it was a thriving ecosystem and a homeland in its own right — a place as real and rich in life as any part of Australia today.

Today, only the mountain peaks of the Bassian Plain remain above the waves as islands – the Furneaux Group and King Island were once highlands rising over a coastal plain. Up until about 10,000–8,000 years ago, this now-submerged landscape connected Tasmania to mainland Australia, forming a land bridge teeming with life. The Bassian Plain supported a mosaic of grasslands, wetlands, and sparse woodlands where humans and animals roamed. This video explores the geological and environmental history of that vanished plain, painting a picture of what the landscape looked and felt like before the sea claimed it at the end of the last ice age.

Geological Origins of a Sunken Plain

Bass Strait – the waterway that now separates Tasmania from the Australian mainland – is geologically a shallow depression in the continental shelf. Its average depth is only about 60 m (with a maximum around 85 m), a clue that it hasn’t always been filled with ocean. Over the past several million years, global sea levels have repeatedly risen and fallen with ice ages, alternately submerging and exposing the Bassian Plain. During glacial periods, when vast amounts of water were locked up in ice, sea-level dropped enough to reveal a continuous land bridge. The most recent emergence began roughly 43,000 years ago, and by the peak of the last ice age (~20,000 years ago) sea-level was 125–130 m lower than today – fully exposing a broad plain between Victoria and Tasmania.

Geologically, this plain was framed by higher ground to the east and west. The eastern side was a granite highland known as the Bassian Rise, which now underlies islands like Flinders Island and the ridges of Wilsons Promontory. To the west, another uplifted block (the King Island High) ran through what is now King Island toward the Victorian coast. Between these uplands lay the low Bass Basin – essentially a shallow bowl in the centre of Bass Strait. This basin became the heart of the Bassian Plain. As long as the ocean remained below the basin’s rim, the area stayed dry land; any rivers flowing into it would form lakes or marshes. In effect, the Bassian Plain was a flat, low-lying valley floor surrounded by gentle rises and occasional hills that once were coastal headlands. It was into this landscape that humans and animals ventured as the ice age lowered the seas.

An Ice Age Climate and Landscape Unveiled

At the Last Glacial Maximum (LGM) around 20,000 years ago, the Bassian Plain stretched out as a windswept expanse of land. Granite hills and ridges on the eastern side of the plain jutted up as islands of rock, marking what are now the Furneaux islands and Wilsons Promontory. In today’s terms, the dry land extended from Wilsons Promontory across all the Bass Strait to north-east Tasmania, forming a continuous bridge between the two landmasses. This was not a tiny land bridge, but a “big country” in its own right, as one researcher put it.

Though today it's hidden beneath the waves of Bass Strait, the geology of the Bassian Plain is surprisingly well understood thanks to offshore drilling, seismic surveys, and detailed geological studies of the surrounding landmasses. Far from being a featureless valley, this drowned land was underlain by a complex mixture of ancient granite, sedimentary basins, and metamorphic ridges — each with its own story and mineral potential.

At the heart of the plain lies the Bass Basin, a broad, low-lying sedimentary trough that formed during the Cretaceous Period, around 100 million years ago, as Australia began to rift away from Antarctica. This basin filled with up to 10 kilometers of sediment over tens of millions of years. These sediments include alternating layers of sandstone, shale, siltstone, and coal-like lignite, deposited in river deltas, floodplains, and shallow seas. The Bass Basin remains a geologically active area today and is known as a modest hydrocarbon province. Natural gas has been successfully extracted from the Yolla Gas Field, while other parts of the basin may host untapped coal seams or geothermal resources due to the thickness of its insulating sediment cover.

To the east, the Bassian Rise forms the granitic backbone of the plain. This region is geologically continuous with the granite belts of northeastern Tasmania and the Furneaux Islands, made up mostly of coarse-grained Devonian biotite granite and surrounding metamorphic rocks like schist and phyllite. These types of rocks are significant because, in exposed parts of Tasmania, similar granites are associated with tin and gold deposits, often formed by hydrothermal activity along fault zones.

To the west, the King Island High marks a structural ridge composed of much older rocks — ancient Precambrian and Cambrian formations that include high-grade metamorphic rocks like granulite and gneiss. Gold and Tungsten has been found on King Island, and the submerged rocks hint at a more rugged, mineral-rich terrain beneath what is now King Island and the western Bass Strait.

Together, these regions formed a geologically diverse landscape beneath the Pleistocene grasslands of the Bassian Plain. Surface soils and outcrops likely varied widely — from sandy coastal sediments and dune fields to exposed granite ridges and weathered ironstones. While there is no direct archaeological evidence of mineral exploitation on the plain itself, it is reasonable to assume that Aboriginal people used the landscape’s natural resources. Exposed quartz veins, granite outcrops, or iron-rich weathered rocks may have been sources of stone tools or ochre pigments.

From a tectonic perspective, the Bassian Plain sits within the broader Tasman Fold Belt, a Paleozoic mountain-building zone that shaped much of eastern and southeastern Australia. The plain formed as part of a rift system that includes the Otway, Sorell, and Bass Basins, created as Australia split from Antarctica and the Southern Ocean began to form. This stretching and thinning of the crust produced fault blocks, sediment-filled grabens, and potential zones for hydrothermal mineralisation.

In short, the Bassian Plain wasn’t just a flat land bridge — it was underlain by a rich and complex geological story. Its drowned basins hold gas and coal; its ridges conceal granite-born mineralisation; and its ancient rocks hint at a deep tectonic past. Though now lost beneath the sea, the rocks of the Bassian Plain speak of a land that was once geologically alive, and potentially rich in the very materials that shaped both the landscape and the life upon it.

The climate of this exposed plain was markedly colder and drier than today. The world was in an ice age, so average temperatures in southeast Australia were several degrees lower, with Tasmania’s highlands even supporting small glaciers and permafrost in places. At lower elevations like the Bassian Plain, there was no ice sheet, but the environment was periglacial – characterized by cold winters, frost, and fierce winds. Rainfall was reduced, and for stretches of time the region became quite arid. In these especially cold, dry episodes, loose sand from riverbeds and exposed coastal sediments was blown into dune fields, forming long sand ridges across parts of the plain.

Yet the Bassian Plain was not an unrelenting desert. Shallow streams and rivers still flowed across its surface in wetter seasons, meandering through grassy valleys towards the central lake. Pollen records and other environmental evidence indicate that much of south-eastern Australia at this time was covered in open steppe vegetation – dominated by grasses (Poaceae) and daisies (Asteraceae) rather than dense forests. In fact, contrary to earlier theories that Tasmania might have been cloaked in impassable rainforest, the evidence now shows that northwestern Tasmania and the adjoining Bassian Plain were largely an expanse of sweeping grasslands. Scattered among these grasslands were patches of heathland and pockets of woodland in more sheltered or well-watered spots. Overall tree cover was limited – sparse eucalypts and wattles may have clustered in favourable areas, but vast vistas of open grass and shrubland were the norm. One ecologist imagines it as a “well-watered plain” despite the chill – a landscape with ample wetlands and moisture to support life, even if it was cold. It was a land of big skies and broad horizons, likely frosty at times, but rich with grass, flowers, and seasonal marshes glinting under the pale Ice Age sun.

Grasslands, Lakes, and Life on the Plain

The Bassian Plain’s mix of grasslands, wetlands, and sparse forest provided habitat for a variety of wildlife. Pleistocene Australia still harbored many of the animals familiar to us today (along with some now-extinct megafauna), and on this land bridge species moved freely between the mainland and Tasmania. Herds of large kangaroos grazed the open meadows, browsing on tough Ice Age grasses. Emus stalked across the steppe in great numbers – a fact confirmed by archaeology: excavations in the Furneaux Islands (which were high ground on the plain) uncovered thick layers of emu eggshell in ancient Aboriginal camp sites, indicating that emus were once abundant on the Bassian Plain. Wombats and wallabies would have been common in the scrub and dune country, and their bones have indeed been found in cave deposits from this era. Around the marshes and Lake Bass (as the central lake is sometimes called), waterbirds flourished – swans, ducks, pelicans and other waterfowl formed large populations attracted by the extensive shallows. The well-watered swales and lagoons of the plain might have buzzed with insects and frogs in summer, and seasonally attracted migratory birds.

One can imagine thylacines (Tasmanian “tigers”) trotting over the grasslands, shadowing the movements of grazing kangaroos, or Tasmanian devils scavenging on carrion – both species were present in Tasmania and the mainland in late Pleistocene times. Perhaps small bands of megafauna (like giant wombats or giant kangaroos) still lingered in refuges on the plains in the earlier millennia of human arrival, although most of Australia’s megafauna had vanished by the height of the LGM. The overall picture is of a productive if often chilly savanna: windswept tussock grass hills, heath blooms on the higher granite ridges, and life gathering around the reliable water sources. The plain was a vital ecological corridor, allowing mainland species to colonize Tasmania and vice versa – a fact reflected today in the shared wildlife (wombats, kangaroos, devils, etc.) that Tasmania retained after the land bridge disappeared.

Human Footprints Across the Bassian Plain

This once-lost land was not only home to animals – it was also home to people. Aboriginal Australians discovered and settled the Bassian Plain early in their history on the continent. Archaeological and genetic evidence shows that people had reached Tasmania by at least 40,000 years ago, and possibly even earlier. They likely came southward in waves as land access opened. The Bassian land bridge was not merely a transient route for these peoples; as Professor Chris Johnson notes, calling it a “land bridge” understates its significance. “It wasn’t a crossing, it was a country,” he explains – a homeland in itself, where generations of people lived, hunted, and built their lives. Recent scientific research has even detected human environmental impacts (such as burning or vegetation change) on the Bassian Plain beginning about 41,600 years ago, hinting at a very early human presence on the landscape.

What was life like for those people on the lost plain? From the archaeological clues, we know they were adept hunter-gatherers making the most of the cool, open environment. They hunted kangaroos and wallabies, probed the wetlands for bird eggs and freshwater shellfish, and likely trapped or fished in the lakes and streams. The abundance of emu eggshells in some sites suggests that collecting emu eggs was a routine activity – providing a rich food source. Stone tools found in caves hint at the toolkit they carried: sharp flakes and points of local stone for cutting meat and scraping hides, perhaps wooden spears or throwing sticks for hunting (though wood rarely preserves). There is evidence they lit small fires in their camps – for warmth, cooking, and perhaps for managing the landscape.

Drowning of a Land – and Its Legacy

After the glacial maximum passed, the world began to warm. The ice sheets melted and sea levels rose, slowly at first and then in rapid pulses. On the Bassian Plain, people would have witnessed their world changing in real time. Around 18,000–15,000 years ago, shallow bays and tidal marshes formed at the edges of the plain as the seas crept back in. The surviving land bridge shrank to a narrower corridor in the east, perhaps a chain of marshy islands and peninsulas near present-day Wilsons Promontory and the Furneaux region. Finally, somewhere between 14,000 and 12,000 years ago, the last dry connection between Tasmania and Australia was broken as the waters surged through. The Bassian Plain, which had existed for millennia, was gradually swallowed by the sea.

For the Aboriginal peoples living on that land, this would have been a time of profound upheaval. Generation by generation, they retreated to higher ground. Families on the plains would move their camps uphill to stay ahead of the encroaching tide, perhaps season by season watching familiar hunting grounds disappear under water. Some groups likely became stranded on newly formed islands for a time, as happened in the Furneaux group – archaeological evidence shows people continued to live on those diminishing islands up until around 8,000 years ago, when rising seas finally isolated them completely. After that, there were no people living on the Bass Strait islands until modern times, suggesting that the last remaining inhabitants either evacuated (joining their kin in Tasmania or on the mainland by boat) or eventually died out as resources became scarce. By 9,000–8,000 years ago, the sea reached roughly its current level. Tasmania was now an island, home to a population of Aboriginal Tasmanians cut off from mainland Australia. The fertile Bassian Plain was no more – only the scattered mountain-tops remained as islands in the new Bass Strait.

This cataclysmic flooding of a homeland left its mark in memory. Indigenous Australian oral traditions from coastal peoples contain striking accounts that may encode these ancient events. The Tasmanian Aboriginal people, for instance, have stories of a time when Tasmania was connected to the north and of a great flood that forced people to flee in canoes as waters rose. Some stories even speak of an era when the country to the north was covered in ice – perhaps a cultural echo of the distant glacial cold. On the Victorian side, Indigenous people tell of a time when their ancestral coastline extended far out into what is now the sea, recalling that the land was much more expansive. These oral histories, passed down through millennia, align strikingly well with the scientific timeline of Bass Strait’s inundation and suggest that the memory of the Bassian Plain’s drowning persisted in Aboriginal culture for thousands of years.

Today, the Bassian Plain lies hidden under the waves, but it is far from forgotten. For scientists, it offers a fascinating case study of climate change and human resilience – a real-world “Atlantis” that shows how ecosystems and communities adapt to dramatic sea-level rise. Marine archaeologists and geologists continue to study the seabed, hoping to find submerged archaeological sites or landscape features from that lost world.

Standing on a Tasmanian beach today, gazing north across Bass Strait, it’s awe-inspiring to imagine that not so long ago (in Ice Age time) an entire country lay between here and the mainland. Grasslands ran to the horizon where now there is open sea; kangaroos hopped and emus trotted in what are now white-capped waves. Though submerged and silent beneath the strait, the Bassian Plain’s legacy endures in the biology, stories, and collective memory of Australia. It is a sunken chapter of the land’s history – one that we can now begin to visualize through scientific discoveries and Indigenous knowledge, bringing this lost Ice Age world back to life in our imagination.

Here's the video we made on the Bassian Plain on the OzGeology YouTube channel: