Relief and drainage-Australia
Relief and drainage-Australia
- OVERALL CHARACTERISTICS
Australia is a land of vast plains. Only 6 percent of the island continent is above 2,000 feet (600 metres) in elevation. Its highest peak, Mount Kosciuszko, rises to only 7,310 feet (2,228 metres). This situation stems in part from the long periods of geologic time during which Australia has been subject to weathering and erosion and in part from Australia’s position at the edge of a zone of significant and geologically recent earth movement.
Patterns of faulting and folding in large measure control the distribution and attitude of rocks and thus play a significant part in determining the shape of the land surface. But the nature and intensity of the processes at work at and near the land surface also give rise to characteristic assemblages of forms. Australia is an arid continent; fully one-third of its area is occupied by desert, another third is steppe or semidesert, and only in the north, east, southeast, and southwest is precipitation adequate to support vegetation that significantly protects the land surface from weathering.
Permanently flowing rivers are found only in the eastern and southwestern regions and in Tasmania. The major exception is the Murray River, a stream that rises in the Mount Kosciuszko area in the Eastern Uplands and is fed by melting snows. As a result, it acquires a volume sufficient to survive the passage across the arid and semiarid plains that bear its name and to reach the Southern Ocean southeast of Adelaide. (In Australia, the southern portions of the Pacific and Indian oceans surrounding Antarctica are called the Southern Ocean; this body of water is also known as the Antarctic Ocean.) All other rivers in Australia are seasonal or intermittent in their flow, and those of the arid interior are episodic.
Many areas—notably the Nullarbor Plain, which is underlain by limestone, and the sand ridge deserts—are without surface drainage, but there are underground streams. A map of Australia can be misleading; though many “lakes” are depicted in the interior, the fact is that many of them are now salt lakes that contain no water for years on end.
- THE WESTERN PLATEAU
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 (those older than 540 million years). 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 (those about 490 to 540 million years old) and partly underlain by swamp deposits of Neogene and Paleogene age (i.e., about 2.6 to 65 million years old). 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.
In the far southwest of the shield, and especially in the northern areas, precipitation is sufficient to support a considerable vegetation and is regular enough for streams to flow seasonally. Here the work of rivers in shaping the land surface is more obvious and widespread; the landscape consists essentially of valleys and intervening divides, the precise form of each depending on local structure. But in such areas the rate of landscape change is more rapid than in the arid zones.
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 Paleogene and Neogene periods 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.
Finally, in several parts of the shield remnants of eroded surfaces, planed off and covered with hard, silicified crusts of weathered rock, cut across local bedrock and are either preserved high in the relief or buried beneath later sedimentary deposits. They attest to changes in the disposition of the land surface (either base-level changes or regional warping or faulting) and also indicate that, in the past, surfaces of low relief similar to present ones were widely developed. Reference has already been made to the distribution of the laterite surface. At the eastern margin of the shield there are remnants of a still older surface, of middle or late Mesozoic age (i.e., formed about 175 to 65 million years ago), which has been warped by subsequent earth movements and now disappears beneath the sediments of the Great Artesian and similar basins. Other evidence of the existence of this surface has been found in northwestern Queensland, central Australia, and South Australia.
On the southeastern extremity of the shield, the Flinders–Mount Lofty ranges occupy the site of the Adelaide downwarp in the Earth’s surface. These sediments were folded and faulted, principally in the early Paleozoic (about 540 million years ago), though recurrently since. The Flinders Ranges are a much-eroded fold mountain belt characterized by ridge and valley forms in which sandstone ridges and bluffs are dominant. The Willochra Plain occupies an elongate intermontane basin excavated from a major upwarped structure and achieved through the erosion of some 20,000 feet (6,000 metres) of sediments. There are remnants of old land surfaces of low relief, and, in the north, extremely rugged relief developed on a much-shattered granite outcrop.
To the south, the Mount Lofty Range is a faulted and much dissected and complex horst, or ancient uplifted structural block. Bounded on both east and west by meridional or gently arcuate fault scarps, which developed initially in the Early Paleozoic but which have suffered recurrent movements since (and which indeed are still active), the ranges are surmounted in many areas by the remnants of a lateritic plain. In many other areas, such a hard capping of rock, if ever present, has been eliminated by stream erosion. Sandstones again form prominent ridges and residuals (isolated relief features), such as Mount Lofty itself; small granite outcrops give rise to boulder-strewn surfaces; and exposures of gneiss form slablike blocks known as tombstones, monk stones, or penitent rocks.
Between the Mount Lofty and the Flinders ranges is a region of broad simple folds in which the sandstone ridges run for the most part north-south and in which the broad open valleys were in some instances occupied by lakes during the Paleogene and Neogene periods. To the northeast, similar upland areas of low relief, but with domes of crystalline rock standing above the general level, dominate the Olary Spur.
- THE INTERIOR LOWLANDS
The Interior Lowlands are dominated by three major basins, the Carpentaria Basin, the Eyre Basin, and the Murray Basin. The Carpentaria and Eyre basins are separated by such minute residual relief elements as Mount Brown and Mount Fort Bowen in northwestern Queensland. The Wilcannia threshold divides the Eyre and Murray basins, and the latter is separated from the Otway Basin and the Southern Ocean by the Padthaway Ridge. The Eyre and Murray basins are entirely terrestrial, but the Carpentaria is partly inundated by the sea.
The Carpentaria plains, occupying the basin of the same name, form a narrow lowland corridor between the Isa Highlands and the Einasleigh uplands (part of the Eastern Uplands). They are drained by the Leichhardt, Flinders, and Gilbert rivers and in the south take the form of broadly rolling plains underlain by heavy gray lime-enriched (pedocalic) soils. In the north, however, there are extensive flat depositional plains, some of them related to swamps from the Pleistocene Epoch (i.e., about 2,600,000 to 11,700 years ago), some associated with the present floodplains of the braided river systems. Standing above the plains, for example around Normanton, are considerable plateau and mesa remnants of the Paleogene and Neogene laterite surface.
Similar rolling plains with laterite residuals standing above them occur in the Eyre Basin, particularly around the headwaters of the Diamantina, near Kynuna. But to the south, toward the more arid interior, the plains become flatter and are protected by a veneer of stones—the well-known stony desert with its mantle of gibber (hammada, serir, and desert armour). In many parts of southwestern Queensland, northeastern South Australia, and northwestern New South Wales, there are plateau and related relief remnants similar to those found in other parts of the lowlands, although these are capped and protected not by laterite but by silcrete, another hard rock residue. This region is folded in places, and the subsequent dissection by erosive forces has brought about disintegration of the silcrete, which is about 20 million years old and which formerly extended over vast areas of central Australia. This process provided much stony debris for the gibber plains so characteristic of much of central Australia and particularly of the Lake Eyre depression.
The catchment of Lake Eyre extends over some 500,000 square miles (1,295,000 square km) of central and northern Australia. It occupies the lowest point of the Australian continent (51 feet [15.5 metres] below sea level), and many large river systems drain into it. These rivers drain the driest part of the continent. But no desert is rainless, and floodwaters entirely cover the bed of Lake Eyre about twice each century, the waters deriving not only from central Australia but also from the higher-rainfall areas drained by the headwaters of the Georgina, Diamantina, Thomson, Barcoo, and similar rivers. It is now clear that, during the late Pleistocene, the precipitation of central Australia was heavier than it is now. The interior drainage basin has received vast quantities of sediment and salt by these rivers, past and present. This has provided ample source material for the Simpson Desert dunes, and many of the normally dry lake beds, including all the large ones, are encrusted with salt. Most of the largest salinas, or salt pans (Eyre, Frome, Torrens, Gregory, and Blanche), are, at least in part, of structural origin, having been formed by downfaulted blocks. Torrens and Gregory are surfaced mainly by gypsum, but the remainder carry a crust of sodium chloride, common salt. Around the major salinas there are extensive alluvial plains.
Under the prevailing arid conditions, fine dust is winnowed from the surface sediments and can be carried high into the air in dust storms. Some is carried long distances, even reaching New Zealand from time to time. The sand of the alluvium is molded into dune ridges.
Sand dunes also occupy large areas of the Murray River basin. They are, by contrast, fixed (or “fossil”) dunes, which developed at some time in the recent past and have since been stabilized by higher precipitation conditions. The eastern part of the basin, near the foothills of the Eastern Uplands, shows evidence of these former higher precipitation amounts in the numerous abandoned river channels of the Riverina. But the western Murray plains are a stony as well as a climatic desert. The plains are underlain by limestones of Miocene age (those about 23 to 5.3 million years old) and, in many areas, by calcrete, a calcareous soil accumulation. Instances of water-dissolved sinkholes and enclosed depressions can be found, and there is a lack of surface drainage characteristic of this type of topography. Only the Murray River, which originates outside the area in a different environment, crosses the basin, flowing in a narrow trench in its lower reaches.
In the east of this region there are extensive alluvial plains associated with major tributaries of the Murray. One feature of interest is the diversion of the Murray, near Echuca, by a rising structural block bounded by fault zones and known as the Cadell Fault Block.
- THE EASTERN UPLANDS
The Eastern Uplands are a complex series of high ridges, high plains, plateaus, and basins that extend from Cape York Peninsula in the north to Bass Strait in the south, with a southerly extension into Tasmania and one extending westward into western Victoria. The uplands are the eroded remnants of an ancient mountain range recently rejuvenated by block faulting. They occupy the site of the Tasman downwarp belt, the sediments of which were folded and faulted in late Paleozoic times. Granite batholiths were intruded into this region, and during the Cenozoic Era (the past 65 million years) lavas appeared extensively in areas as far apart as northern Queensland and Tasmania. Characteristic features associated with this process were lava fields, with stony rises, soil-filled depressions, and lava caves. Extinct cones and craters survive in southeastern Queensland, in the Monaro district of New South Wales, and in western Victoria.
In considerable measure the landforms reflect these various geologic events. Uplifted structural blocks, many of them trending north to south, are common in some areas, while straight river courses reflect the control exercised by fault zones. Ridge and valley forms, as found in the Grampians of Victoria, reflect the differential erosion of broken and folded rock strata. Massive domes or clusters of boulders are common on the exposed granitic batholiths. The lava plains and plateaus display stony rises, shallow alluvial depressions, and volcanic vents and plugs of various types and ages.
Other features reflect the erosional history of the region. Wide areas of the upland had been reduced to a uniform low relief by the time of the later Mesozoic Era (about 100 million years ago) and many remnants of this ancient surface, exhumed by erosive action from beneath a later Cretaceous cover (i.e., up to about 65 million years ago), survive in the landscape, notably in northern Queensland. The Cenozoic leaching of rocks by weathering in humid climates—which forms iron-rich residuals (laterization)—also affected the uplands, from northern Queensland to Tasmania.
Lastly, during the Pleistocene, small glaciers developed in the Mount Kosciuszko area of New South Wales and the central plateau of Tasmania. Small, ice-scoured hollows and small moraines (ridges of glacial debris) attest to these events, while over rather wider areas frost-shattered rocks that subsequently caused soils to flow down-slope (solifluction) have helped shape the surface. No snow normally survives through summer in either of these areas now, but in winter the snowfields of the Mount Kosciuszko area alone are more extensive than those of all Switzerland, if far less heavily supplied.
The Great Barrier Reef is related in important respects to the Eastern Uplands. Lying off the Queensland coast, this great system of coral reefs and atolls owes its origin to a combination of continental drift (into warmer waters), rifting, sea-level change, and subsidence.
- THE HUMAN IMPACT
In the context of such extraordinary environmental time frames, neither the Aboriginals nor the European settlers can be described as long-term residents, yet in their brief time they have already modified the landscape considerably and in most ways deleteriously. The Europeans in particular have been responsible for initially minor, but later significant and widespread, changes, notably considerable soil erosion. Clearing vegetation for agricultural purposes, overgrazing, introducing exotic plants and animals, making tracks and roads, even clearing stones from paddocks—all have rendered the land surface more susceptible to soil erosion. Humans have set in train their own great cycle of erosion, similar to that which beset many parts of western Europe in the 18th century and which has assailed many parts of the American West since the late 19th century.