The Precambrian western core area, known geologically as a shield or craton, is subdivided by long, straight (or only slightly bowed) fractures called lineaments. These fractures, most obvious in the north and west, delineate prominent rectangular or rhomboidal blocks, some of which have been raised to form uplands; others have been depressed to form lowlands or topographic basins. The lineaments display strong northwest-southeast and northeast-southwest trends in the northern, northwestern, and southeastern parts of the shield, but east-west alignments are prominent in the centre, and major structural lines are more nearly longitudinal in the west and southwest. In all areas, however, trends other than those that are locally dominant can be discerned.
Within such structurally defined areas as the Kimberleys, the Mount Isa Highlands, and the Pilbara, the nature of the land surface varies according to the type and disposition of the rock outcrops. In the Kimberleys and the Mueller Range there are extensive outcrops of flat-lying massive sandstone that have been dissected to give rise to striking isolated rock features known variously as plateaus, mesas, and buttes. Under these circumstances, local joints and bedding planes in the rocks, combined with the permeable nature of the bedrock, control the local landforms.
Similar plateau forms dominate the Pilbara and Arnhem Land, though in the former region horizontally bedded or only gently warped massive ironstone formations, together with massive sandstones, give rise to prominent bluffs bordering the plateau assemblages; and in the latter karst landforms (greatly eroded) are developed where limestone occurs at the surface. At the margins of the Kimberleys (in the Fitzroy region and in the Durack Range) and in the southern part of the Pilbara, in the Ophthalmia Range, dipping rock strata have been differentially eroded to form ridges and valleys.
Such features are also extensively and well developed in the uplands of central Australia (the MacDonnell, James, and Krichauff ranges), in the Isa Highlands, and in the Stirling Range of the southwest. In all of these areas it is the sandstones and quartzites that underlie the upstanding ridges, the intervening valleys being eroded in siltstones or shales; and in all these areas the pattern in plan of ridge and valley reflects the pattern of folding in the underlying rocks.
In the far southwest, the Darling Range forms an upfaulted block underlain mainly by granite but capped by laterite, a reddish, iron-rich product of weathering rock. The Gawler block, in the southeast, is complex. There are crystalline and sandstone uplands in the east, sandstone plateaus in the northeast, and, in the centre and north, the rounded Gawler Ranges built of Precambrian volcanic rocks. Much of Eyre Peninsula is occupied by a rolling plain traversed by fixed sand dunes, but in the northwest numerous low isolated granite rocks of spectacular appearance, called inselbergs, stand above the plain. These epitomize the isolated ranges and hills widely developed in the northwest of South Australia, in the Musgrave, Everard, Birksgate, Mann, and Tomkinson ranges.
The lowlands between these raised blocks also display varied topography. The so-called Barkly Tableland is in reality a high plain of remarkable flatness, partly eroded in Cambrian sedimentary rocks and partly underlain by Tertiary swamp deposits. The Nullarbor Plain, a karst area, is approximately coincident with the Eucla Basin. Its surface is so flat that in one section the Trans-Australian Railway runs absolutely straight for some 300 miles (500 km) as it passes over the region. A vast area of the southwest of Western Australia is occupied by an extensive high plain traversed by elongate ribbons encrusted with salt, the desiccated and disrupted remnants of former river courses. The Gibson Desert consists in large part of a laterite-capped plain, but huge areas of the plains of central and northern Australia are occupied by active sand dunes, and large areas of southern South Australia and Western Australia are covered by fields of fixed dunes.
Actively developing and moving sand ridges occupy the Canning Basin, the Great Victoria Desert, the Amadeus depression, and large areas of the Arunta-Sturt Complex. The dune fields extend to the east into the Great Artesian Basin, where the dunes constitute the well-known Simpson Desert. These dune deserts reflect the prevailing aridity of most of Australia, and the dune trend displays a huge swirl around the centre of the continent. Yet, even in these most arid areas, rain falls from time to time, and the rivers run occasionally. Because of the scarcity of vegetation and the common development of impermeable rock layers of various types, runoff in the arid lands tends to be rapid and achieves dramatic and significant results. Hillslopes are scoured and washed bare of weathered debris; streams erode gullies and transport large volumes of sediment from the uplands to the plains; broad, braided river channels are developed; and extensive alluvial plains are formed. It is the alluvium, carried to the lowlands by rivers and deposited on the plains, that is, in large measure, the source of the sand out of which the desert dunes are molded by the wind.
Many of the landforms of the shield are inherited from the past, when different climatic conditions obtained. Remnants of laterite are widespread in many parts of Australia: the Darling Range, the far southwest, the Isa Highlands, and Mueller Range, near Darwin, and the southern Eyre Peninsula. The evidence indicates that during the Tertiary Period, these areas had been reduced to low relief, and humid tropical climates prevailed, for laterite is at present forming only under such conditions in such areas as Southeast Asia and the Congo River basin. The disrupted former drainage system of southwest Western Australia has already been referred to, and remnants of similar old stream networks occur in the Amadeus depression, on the Nullarbor Plain, and in the Great Victoria Desert.
A large swamp formerly occupied the south of the Barkly Tableland; and Lake Woods, near Newcastle Waters, is now dry, with a bed of some 70 square miles (180 square km) in extent, but shorelines indicate that the lake formerly occupied some 1,100 square miles (2,850 square km). Fossil remains also suggest wetter climates in the past in many parts of Australia and subsequent deterioration toward aridity. But in the south the occurrence of dunes now fixed by vegetation shows that the climate there has recently become moister. "Australia" © Emmanuel BUCHOT, Encarta, Wikipedia
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