The continent of Australia probably assumed its present topographic form about a million years ago after undergoing many significant changes in outline and elevation during the very long geologic period that preceded the Pleistocene. The ancient land mass was finally reduced over most of its area to heights not far above sea level. During the early Pleistocene period the western portion was elevated about 1000 feet and the eastern portion was raised to heights varying from 2000 to 7000 feet. The intervening land remained near sea level and thus separated the raised western plateau from the higher eastern blocks and folds.
Reduced ocean levels during the glacial epoch eliminated the straits separating Australia from New Guinea on the north and from Tasmania on the south, thus permitting the migration of fauna and flora.
These straits were subsequently restored, and the gradual dissection of the uplifted land masses has proceeded under progressive desiccation of the continent. Minor local uplifts, subsidences, and volcanic outflows are responsible for many of the minor land-form features such as marine terraces, the drowned estuaries of the south coast, and the scattered basalt flows throughout the eastern highlands. Erosion has made substantial progress in dissecting the uplifted portions of the continent only where rainfall is sufficient. Large parts of the interior plateaus have changed but slightly since their major uplift, because of aridity. Large deposits of erosional detritus have accumulated in the central lowlands, but its surface has not been changed substantially in recent geological times.
The present-day surface of Australia is generally low and monotonous in appearance, averaging less than 1000 feet above sea level, and only about 180,000 square miles, or 6 per cent of the continent, lie above the 2000-foot contour line. No continent has a form that is more compact or a coastline that is more smooth.
Despite these broad elements of uniformity in the form of the continent, significant contrasts exist among three major land-form regions, and many lesser differences occur among the score of minor divisions. Many of these land-form variations are reflected in the patterns of distribution and qualities of climates, soils, vegetation, water supply, and other physical conditions.
The three major land-form divisions of Australia are (1) the Eastern Highlands, (2) the Central Lowlands, and (3) the Western Plateau. These three major regions will be discussed in turn, and some of the characteristics of the lesser divisions will be described at the same time.