The Western Plateau contains about 1,500,000 square miles, or half the area of the continent. It includes essentially all Western Australia and Northern Territory along with the western half of South Australia. Most of the plateau is elevated about 1250 feet above sea level, and it retains the characteristic surface features of a very old land mass. Ancient granites and metamorphic rocks are characteristic. The margins of the plateau have undergone considerable dissection in the moist northern and southwestern sections, and hills and valleys are characteristic land forms.
At various points low mountain peaks and ranges rise above the monotonous plateau surface. The Macdonnell and Musgrave ranges rise to heights of 4800 to 5200 feet near the center of the continent.
The Hammersley and Wiluna ranges approach 4000 feet in the west. In the far north the rugged Kimberley district displays elevations between 2000 and 3000 feet. The Stirling Range, in the extreme southwest corner, rises abruptly to heights of 2000 to 3000 feet from the surrounding plain along its 50-mile length. About 100,000 square miles of the Western Plateau rise above the 2000-foot contour line. The Macdonnell Uplands, near the center of the continent, include half of this higher country; the other half is divided among the Hammersley, Flinders, Musgrave, Wiluna, and Ashburton uplands.
The Nullarbor region that borders the Great Australian Bight is distinctive because of its uniform surface, lack of streams, and absence of tree growth. Its thick limestone beds are filled with underground drainage lines and eaves. High cliffs mark the southern margin of the Nullarbor Plain either at the ocean front or at the inner edge of a narrow coastal plain. The Nullarbor Plain rises from 200 to 400 feet in the south to 1000 feet in the interior, where it merges with the ancient rocks of the plateau.
The Salt Lakes district in the southwest corner of the Western Plateau contains numerous large and small ephemeral lakes. It is an area of interior drainage, but the lakes are loosely strung together along former drainage lines. Drifting sand has blocked many of the old drainage courses, and the lake beds are covered with salt deposits and dry mud during most of the year. Low monadnocks rise above the plateau surface in places. Gravel-filled channels of ancient streams, known as "deep leads," have proved to be important sources of alluvial gold, as at Kalgoorlie.
Sand Ridge Desert
The Sand Ridge Desert occupies over 400,000 square miles of the Western Plateau region, extending across its center from Ooldea in the southeast to Ninety Mile Beach in the northwest. The ridges vary in height from a few feet to 50 or 60 feet, and they show a remarkable degree of parallelism as well as a high degree of uniformity in spacing (one-fourth mile apart, usually). Their orientation is generally from southeast to northwest, but the direction varies from place to place, depending upon the prevailing winds. The principal source of sand is the surface sandstone strata of the region. Stream patterns are lacking, and there is no permanent surface water.
Narrow coastal plains border the Western Plateau principally on its western and southern margins. The coastlines about the margins of the Western Plateau vary from a high degree of uniformity along the Nullarbor region to one of extreme irregularity in the northern Kimberly and Arnhem districts. Recent slight submergence of the deeply dissected north coast accounts for its island-fringed and indented characteristics, but high cliffs along the sound margins make them useless for harbors.
The west coast is generally smooth, but the construction of sand bars by the northward-moving currents has produced numerous bays with openings to the north. The good natural harbors at Perth and Albany are the result of submergence. Ninety Mile Beach owes its smooth curve to the abundant supply of sand made available on the beach by land winds. Submerged coral reefs border the northwest coast for hundreds of miles.