The Simpson Desert, known to the Wangkangurru Yarluyandi people as Munga-Thirri, is a 176,500 km² (68,100 sq mi) sea of red sand in central Australia, spread across the corner where the Northern Territory, South Australia and Queensland meet. It is Australia's fourth-largest desert and an erg holding the world's longest parallel sand dunes: north-south ridges that run unbroken for up to 200 km, rising from about 3 m in the west to around 30 m in the east, with the 40 m Nappanerica, or Big Red, counted as the largest. The sand sits on top of the Great Artesian Basin and inside the Lake Eyre Basin, and since November 2021 much of the South Australian part has been protected as Munga-Thirri–Simpson Desert National Park, at 36,000 km² the largest national park in Australia. Everything visible today, from the dunes to the hot springs on its western rim, comes from a long geological sequence, read here from the bottom up.
The Eromanga Sea and the sandstone under Munga-Thirri
The oldest layer that matters here is buried kilometres down. During the Triassic, Jurassic and early Cretaceous, rivers draining higher ground spread sand across the interior of the continent. That sand hardened into a thick, porous sandstone, and in the early Cretaceous much of inland Australia sank beneath a shallow inland sea now called the Eromanga Sea, named after the Queensland town of Eromanga. The sea flooded parts of Queensland and central Australia at least four times, and each time it left a layer of marine sediment over the older sandstone. That marine sedimentary rock became a confining lid, sealing water inside the sandstone below.
The result is the Great Artesian Basin, the largest and deepest artesian basin in the world. It covers about 1,700,000 km², 22 percent of the continent, reaches 3,000 m deep in places and is estimated to hold 64,900 km³ of groundwater. When the Great Dividing Range later lifted the basin's eastern edge, rain soaking into the exposed sandstone in Queensland and New South Wales began creeping slowly south and west underground. A smaller amount enters along the arid western margin and moves through the rock at one to five metres a year. Dating with carbon-14 and chlorine-36 puts the oldest water, in the south-western discharge zones near the Simpson, at nearly 2 million years old. So the driest part of the continent sits on top of one of its largest stores of fresh water.
Sixty million years of sinking: the Lake Eyre Basin forms
The next stage built the surface the sand would later cover. About 60 million years ago, in the early Paleogene, south-eastern South Australia began to subside. Geologists link this to a remnant slab of old oceanic plate sinking in the mantle beneath, drawing the crust down above it. Rivers began dumping sediment into the shallow depression, and it is still sinking and still filling today. This is the Lake Eyre Basin, an endorheic (inward-draining) basin of about 1,200,000 km², roughly the size of France, Germany and Italy combined, with no outlet to the sea.
For most of its history the basin was green. It was largely forested, and around 20 million years ago large shallow lakes spread across it and lasted for about 10 million years. As Australia drifted north and the climate slowly dried, the lakes and floodplains shrank. The decisive change came with the ice ages of the last 2.6 million years, which brought the present climate and fairly rapid desertification. The basin's low point is Kati Thanda–Lake Eyre, 16 m below sea level, just south-west of the Simpson. Our guide to the Lake Eyre Basin follows the whole drainage system; the Simpson is its sandy western heart.
Rivers that never reach the sea: the Finke, Georgina and Diamantina
Three river systems still feed the desert from its edges, and none of them reaches the sea. The Finke River, called Larapinta in Arrernte, rises in the MacDonnell Ranges west of Alice Springs and meanders about 600 km to the desert's western edge. It is thought to have the oldest riverbed in the world, but it flows only a few days a year, and its water normally disappears into the Simpson sands. Only in extreme floods does it link through the Macumba River to Lake Eyre, a journey of about 750 km from its headwaters.
On the eastern side the Georgina River rises on the Barkly Tableland and spreads through countless channels for more than 1,000 km to Goyder Lagoon in north-eastern South Australia. The Diamantina flows about 800 km from country between Mount Isa and Winton, through Birdsville, to join it. These braided, low-gradient rivers gave the region the name Channel Country. Their floods can be enormous: in the wet seasons of 2009 and 2010 some 17 million megalitres of water poured into Queensland's western rivers, and in early March 2010 Birdsville recorded more rain in 24 hours than it usually receives in a year. That same year researchers traced the courses of ancient river systems buried beneath the dunes.
Wind, quartz and iron: how the 200 km dune ridges were built
The dunes are the youngest part of the story. In Munga-Thirri National Park on the Queensland side, the dunes are reckoned to have started forming about 30,000 years ago, long after the basin had begun to dry. Wind has piled the quartz sand into long linear ridges trending south-south-east to north-north-west, a pattern repeated across Australia's deserts but nowhere longer than here.
Height and spacing are tied together: where five or six ridges fit into a kilometre, the dunes stand about 15 m high, but where only one or two ridges occupy a kilometre, they reach 35 to 38 m. In cross-section each ridge is lopsided. The eastern lee slope is steep, at 34 to 38 degrees, while the western slope is gentle, at 10 to 20 degrees. That asymmetry is why most drivers cross from west to east, climbing the easy side and riding the prevailing wind. Our explainer on types of dunes sets these linear ridges beside crescent barchans and star dunes.
The sand itself is mostly rounded to subangular quartz from 0.05 to 1.2 mm across, typically 0.5 mm on the crests and 0.3 mm on the flanks. Near rivers and dry lake beds it is light grey; further from them it turns pink and then brick red, because weathering releases iron oxide that stains each grain. Most ridges are now static, held in place by plants, especially the grass Zygochloa paradoxa on the crests and spinifex on the slopes, with only the crests still moving. The interdune corridors between them hold claypans and salt lakes.
Big Red sits about 35 km west of Birdsville, at the eastern edge where the dunes are tallest. The climate keeps the system dry: rainfall averages only about 150 mm a year, falling mostly in summer, and summer temperatures can reach 50 °C, with large sandstorms common.
Dalhousie Springs and the mikiri wells: where the old water surfaces
The two oldest layers of the story meet at the desert's western edge. At Dalhousie Springs, in Witjira National Park about 180 km north-east of Oodnadatta, Great Artesian Basin water rises through more than 600 natural springs in 12 major groups. The water comes out at 38 to 43 °C, highly mineralised but just drinkable, and it is recharged by water thousands of years old that seeped down through the beds of the Finke and nearby rivers. The pools hold fish found nowhere else, including the Dalhousie catfish, the Dalhousie hardyhead and the Dalhousie goby. Witjira was proclaimed on 21 November 1985 to protect Australia's largest group of artesian springs.
The springs are shrinking. In 1915 the Dalhousie complex discharged more than 23,000 litres a second; by 2000 drilling had cut that to 17,360 litres a second. Across the basin, bore output has fallen from 2,000 megalitres a day in 1915 to about 1,500 today, and some mound springs have dried up entirely.
Aboriginal people have lived in the Simpson for at least 5,000 years, and the geology shaped how. The Wangkangurru dug wells called mikiri into the interdune corridors to reach shallow water, and lived by them until the Federation Drought. Soaks in the dune hollows could be 7 m deep. Seeds of nardoo and pigweed were ground into breads and seed patties, and people hunted kangaroos, wallabies, bandicoots, lizards and snakes.
From Poeppel Corner to Birdsville: what the landscape built
European surveyors met the desert late. Charles Sturt saw it between 1844 and 1846. In 1880 Augustus Poeppel marked the point where Queensland, South Australia and the Northern Territory meet, but his surveying chain had stretched and his post stood 300 m too far west; Larry Wells moved it in 1884 to the eastern bank of Lake Poeppel. Poeppel Corner lies about 174 km west of Birdsville at 26° S, 138° E. David Lindsay crossed the central and southern desert in 1886 with a Wangkangurru guide who showed him nine native wells. Ted Colson made the first full crossing by a non-Indigenous person in 1936, with camels, and the geologist Cecil Madigan, during aerial surveys in the 1930s, named the desert after Alfred Allen Simpson, the washing-machine maker who led the South Australian branch of the Royal Geographical Society of Australasia.
The parallel dunes also decided where roads could go. No maintained road crosses the desert. Oil and gas exploration in the 1960s and 1970s cut the tracks still used today: the French Line, now the most popular crossing, the Rig Road and the QAA Line. In September 1962 Reg Sprigg, his wife Griselda and their two children made the first vehicle crossing. Birdsville, the eastern gateway with 110 residents in the 2021 census, even draws power from the basin below: a 1.2 km bore delivers water at 98 °C to a geothermal plant supplying between a quarter and a third of the town's electricity, depending on the source. Since the 2008–09 summer the South Australian parks have closed in the hottest months, after unprepared travellers got into trouble.
Protection now covers most of the dunefield. Queensland's Munga-Thirri National Park, created in 1967 around Poeppel Corner, spans 10,120 km²; as of 2019 its head ranger was the Wangkangurru elder and traditional owner Don Rowlands. In the Northern Territory, the Pmere Atherre Antenterreme Indigenous Protected Area covers 47,311 km². The ecosystem's chief threats are rabbits and feral camels rather than farming, because the dunes themselves were never ploughed. For the stony gibber plains that lie just east of the sand, continue with our guide to the Sturt Stony Desert.