On the highway west of Nhill the road passes through a red sandstone cutting, one of few rocky features in a wide undulating landscape. This ridge is a reminder that some 4 million years ago this was the shoreline of a vast inland sea, much like the South Australian Coorong of today. This sea once reached as far north as Broken Hill, and as it retreated it left a legacy of salt and shifting sands that characterise the Mallee. Following this period of marine influence, extensive freshwater sediments were laid down in huge lakes, the result of the natural damming of the ancestral Murray River drainage system. For the last 700,000 years there has been a succession of arid and wetter phases. The arid phases were far more arid than the present climate, and the sandy surface was readily moved and shaped by winds.

The blanket of sandy soil which typifies the Mallee has created a gentle topography and superficially simple landscape, with subtle vegetation changes. Dunes are covered in a low mallee scrub: dwarf, multi-stemmed eucalypts which are long-lived and hardy opportunists, overcoming wildfire by resprouting from large underground lignotubers. The deeper sands are dominated by heathlands of Desert Banksia, Scrub-pine and Desert She-oke. In the more saline, older dune systems, low succulent saltbushes grow under the mallee. Where the saline groundwater intersects the surface, permanent saltpans are dotted between plains of spear-grasses and low rises of Slender Cypress-pine woodland.

Although the low rainfall and poor fertility of the sandy soils lead to some parts of the Mallee being called ‘deserts’, the flora and fauna are surprisingly diverse. There are many species of reptiles and a distinctive range of birds, the best known being the Malleefowl, which constructs huge mounds of sand and litter to incubate its eggs.

Parrots are also prominent, including the colourful Mallee Ringneck, Pink Cockatoo and Regent Parrot. In the understorey, diverse wattles and daisies grow amongst the widespread Porcupine Grass and low, heathy shrubs.

The more fertile areas were selectively cleared, but it is not certain whether this clearing was the main cause of the extinction of at least 12 species of mammals in the area in recent times. Remnants of native vegetation continue to be at risk from the effects of fragmentation, including environmental weed invasion, predation and competition from feral animals, and the effects of increasing salinity. Fire also presents a risk if it is managed inappropriately.

Large and significant natural areas on less fertile sands remain in the Big Desert and Sunset Country and have been set aside in conservation reserves. However, some of these areas were once extensive pastoral leases, and the recovery of perennial vegetation (particularly in woodland habitats) from the impacts of grazing and the continuing effects of rabbits, feral goats and kangaroos remains a challenge.