Dominant Dunes

If you’ve ever driven through the Australian outback it’s highly likely you will have driven over sand dunes. Not the steep faced orange/yellow hills of loose slowly moving sand, but more than likely a tree or shrub-covered hump running across the landscape that the made road simply ran up and over — something so subtle that you in your torquey, air conditioned 4WD were barely even conscious of.

Those are the vast majority of the sand dunes that you will come across in Australia, though it must be admitted they do cover a large proportion of the landscape. There are certainly the spectacular dunes, such as the famous Big Red, outside Birdsville, but most are more modest scale structures easy to explore and well worth stopping and taking a walk over the next time you’re on a journey in such regions.

Australia’s major dunefields occur in a diagonal band from the Mallee area in western Victoria, running north-west to the Great Sandy Desert in northern Western Australia, with the Strzelecki, Simpson and Great Victoria deserts as outliers. Though these represent the major areas, there are many smaller, localized dunefields over an even greater area.

When we think of deserts, we tend to think of seas of sand forever shifting with vast standing dunes rising above the horizon, such as you would see in Saudi Arabia. There really aren’t any such areas in Australia, though you will find occasional similar small patches. Our deserts almost universally have some level of vegetation covering, even if scattered, and our dunes are generally quite low. In the Middle East, dunes can reach as high as 250 metres!

Dunes form where there is a prevailing wind direction and low rainfall. The low level of vegetation covering diminishes plant matter in the soil, including roots, which leaves the soil exposed to the weather and free of bonding agents. Particles small enough to be blown away on the wind are soon carried off, leaving sand-sized grains free to be blown along the surface.

Moving sand will gather at obstructions, such as rocks, clumps of vegetation or even a small pile of pebbles, forming long lines in the lee of the obstruction. The gathered sand then becomes its own obstruction and so a dune extends. The height of the dune depends on the supply of grains, their shape (rounded or angular), size and the wind strength. Sand moves up the windward side on a gentle slope and then drops sharply over crown on the lee side. This is the reason that it is not recommended to tow camper trailers across areas such as the Simpson Desert, as the added weight creates difficulties for many travellers, obstructing the harder packed tracks and forcing others to drive around. This most frequently occurs near the softer sand the at top of dunes, creating erosional problems in the area where it can be least tolerated by the environment.

Dunes can form in a number of different ways, depending on conditions. A solitary crescent shaped dune (or lunette, such as seen at Lake Mungo, in south-west NSW), also known as a barchan dune, moving across the landscape is the result of a limited supply of sand grains. In the Lake Mungo example, this was formed from beach sand on the eastern side of a large lake. As the lake dried saline clays were blown over the top of the beach and small dune deposits, stabilising them and allowing vegetation to grow.

Where sand is more abundant, dunes can form in long wavy ridges at right angles to the direction of the wind. These are typical of the sort of dunes associated with back-of-beach environments, but scarce in Australian deserts. Where surface vegetation is disrupted — typically around developed shoreline environments — a blowout can form where the wind takes control and begins moving the sand rapidly. These can take a long time to restabilise, even when there is active work to achieve this, and it is often only where wooded or shrub-covered hills dominate old dunes that stability finally takes control. This is why the recent fires on Fraser Island represent a significant threat to that fragile environment.

Where there is a moderate supply of sand, a rough pavement (such as a gibber surface) and varying wind that usually always remains in the same general quadrant linear (or seif) dunes, which are typical of so much of the Australian outback, can be produced. The individual longitudinal dunes are joined by short lengths of dune at varying angles, similar to a fingerprint. This also explains the extensive areas of gibbers throughout the outback.

The arid Australian landscape is dominated by these longitudinal dune systems, the largest and most spectacular in the world. Many of these dunefields exceed 100km in length and have built up over the past 20,000 to 30,000 years as the Australian climate has slowly changed — though the oldest, in the Lake Amadeus region of Central Australia, exceed 900,000 years in age. The current explanation for these dune systems, to quote scientists, is that an arid climate with a dominant sub-tropical anti-cyclonic wind system has picked up dust and sand and gradually built the landscape we see today.

The basic landform and underlying geology has a strong influence on the formation of dune systems. Areas such as the Nullarbor Plain (underlain by limestone), Georgina and Carpentaria basins and the Darling and Mudgee and Murrumbidgee plains (clay subsoils), which might otherwise support dune systems, are largely free of them because the underlying soils and rocks do not suit.

Australia’s dunefields are relatively stable. While overall the continent is dry, we do not, at this time experience, hyper-aridity. The driest area of the continent, around the northern side of Lake Eyre, still averages over 100mm of rain per year, which is enough to keep dune interiors moist and sustain stabilising vegetation.

Each dune system has distinct plant communities on the dunes to that in the inter-dune areas between.

Next time you’re in the outback, look for the lines of vegetation types marking the dunes, which is usually very different to that of the surrounding plains, and take the time to stop. If you come across high dune lines, such as you might see while driving along the Birdsville Track, get out and climb to the top for a great view of the surrounding countryside, and take in the vegetation types, looking for traces of the animal life that make a speciality of those environments, and give some thought to how these landforms have impacted the development and interactions of our life with this land.