periods of continental drought by imposing high atmospheric pressures. As for the fifth hypothesis of cold or frozen deserts, as absurd as this assertion may appear in present-day Australia, they existed several million years ago. Geomorphologists thus explain a number of spectacular rock formations, from the Mount Olga or Uluru to the over-deepened wave of the wind rocks, by involving a thaw of (peri)glacial formations followed by wind action over a long period. The sand ridges have a trend of SSE-NNW and continue parallel for great distances. Areas of the formerly desert outback, deserts such as the Simpson Desert from west to east or mountainous regions such as the Arckaringa Hills are characterized by ocean landscapes of charred rocks, called gibberss. As noted by early Australian explorers such as Ernest Giles large portions of the desert are characterized by gravel-covered terrains covered in thin desert grasses and it also contains extensive areas of undulating red sand plains and dunefields, low rocky/gravelly ridges and substantial upland portions with a high degree of laterite formation. The sandy soil of the lateritic buckshot plains is rich in iron in the Gibson Desert. Several isolated salt-water lakes occur in the centre of the region and to the southwest a system of small lakes follow paleo-drainage features. The desert proper is uninhabitable and the environment remains unmarred, while the greener fringe is used for sheep grazing. Waterbodies Lakes in the regions (most of which are dried up saline lakes), include: Rivers and creeks, which are sparse and generally ephemeral, in the Australian desert include: Darling River Alberga River Finke River Georgina River Hale River Alligator Rivers Mary River Todd River Diamantina River Gawler River Macumba River Warburton River Fitzroy River Leichhardt River Biodiversity Vegetation Two types of semi-desert, referred to as "grassland" in Australia, occur in the Australian deserts: Tussock – or Mitchell grasslands are found in the desert areas of the Northern Territory, South Australia and western Queensland. The annual precipitation that falls on these marl and alluvial soils covered with grasses of the Astrebla genus ranges from 150 to 500 mm. Trees cannot take root on the heavy clay soils, and they are riddled with bushfires. Spinifex or hummock semi-desert grows spiny- headed grasses (Spinifex) in clumps, next to free areas as green Triodia pungens and blue-grey Triodia basedowii. Zygochloa dominates on the sand dunes of the Simpson, Strzelecki and Tirari deserts. In