Australia: The Land Where Time Began
Water in this part of the continent tends to arrive from great distances, in huge quantities, and be dispersed over a vast area, and the rest of the year the land surface is very dry, with the only standing surface water being found in the refugia, such as sheltered places among the central ranges.
Landforms of the central Australian floodplains suggest that over the last 10,000 years there have been 3 scales of activity involved in the formation of these landforms.
At Alice Springs, in the foothills of the Macdonnell Ranges, the yearly rainfall averages 250 mm, though it ranges from 82 to 783 mm/year. In the Simpson Desert, to the south, it is 125 mm/year. In the central Lake Eyre Basin, the end of the line for the south-flowing drainage systems of the region, it varies over a wide range. When the monsoon storms reach the ranges they can produce some spectacular floods among the ranges, but once the flood leaves the ranges it spreads out in a shallow sheet across the landscape, but eventually dissipated in the central deserts.
The Central Ranges
These ranges are surrounded by wide plains that have surface characteristics of different origins and history. At the base of the ranges are pediments (steep slopes with fans of rock debris), sandplains, windblown dunefields, floodplains, stony deserts, calcrete rises, claypans, ephemeral lakes, as occur in other arid parts of Australia.
In this area not all rivers are constrained by the topography. The Finke River and Hugh River, e.g., are exceptions. They cut across the structural grain of the ranges, crossing several ridges, breaching the snouts of some folds.
Dune fields near Finke
The dunefield near Finke are linear and were mostly reworked about 30,000-12,000 BP, during the last glacial stage. They form part of the large anti-clockwise ring of dunes in central Australia. Paler source-bordering dunes now cover the oldest, bright red, source-bordering dunes from the Finke River that were originally deposited about 100,000 BP. The uppermost, paler, dunes are composed of 2 units. The lower one dating to 17,000-9,000 BP is of unknown orientation. The younger, paler dunes seem to have a different origin, and are believed to have been recently moved into the dunefield.
The regional dunes near Finke, dating to 30,000-18,000 BP, are aligned about due north. A response to south-westerly to westerly winds between 18,000 and 10,000 BP is suggested by steeper eastern slopes of the cross-sectional asymmetry. It is during the last 5,000 years that the winds seem to have shifted to their present south-east direction. It has been suggested that during the last glacial period there may have been a shift in the wind pattern at Finke of about 100-150 km to the north. But the dune patterns have remained very stable.
The fluvial landscapes of Australia are mostly in the driest, inland parts of the continent. The rivers of this arid region differ from those of the coastal strip in a number of ways, one of the main ways they differ from the coastal rivers is that they are inward-draining. They have much longer courses, are of much lower gradient, as they cross vast areas of very flat ground, they spread over huge distances from the main channels during the infrequent but very heavy floods. A feature of these inland rivers is that they are very ancient. The Finke River, eg., is probably the oldest river in the world.
Most river systems of central Australia arise in the central Australian ranges. In these ranges the river patterns are constrained by the ridge and valley topography of the ranges that are constructed of resistant, ancient rocks.
Ancient River Systems
35 m below the Simpson Desert a river system has been found. It is believed to have formed about 50 Ma in the Late Cretaceous to early Tertiary, during a wetter phase in the area than that of the present. As with much of the ancient land surfaces that have been preserved in Australia, its preservation is the result of a stable continent and a continuous arid climate in the area since about 15 Ma in the Middle Miocene.
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|