U pb dating of zircon
In the later Tertiary the Basin began to subside, resulting in the internal (endogenetic) drainage system that drains much of inland Australia.
Depositional plains are also a prominent feature of inland Australia (Twidale & Campbell, Source 5).
By the time the water table gets close to the surface it has passed through these salt layers and so causes salinisation in the root zone and eventually at the surface, which makes the land useless for agriculture.
Landforms The geologic history and its climatic contrasts are reflected in the landforms.
It is only in very wet years that the water reaches all the way to Lake Eyre, especially as a lot of the water is absorbed into the dry channel beds and evaporates in the hot dry air of the interior long before it reaches Nearly all of the area inland of the peripheral drainage systems generates less the about 3 cm per year of run-off.
As aridity has increased any rivers that had existed on the low gradient sandy surfaces of the shield areas have disintegrated or disappeared entirely.
The Traps are the step-like rock made up of many layers of basalt, which were laid down by successive eruptions of magma.
On the east coast the short, steep rivers have not regressed as much as would be expected, some having eroded headward by about 100 km in about 60 My.
The authors suggest that about 30 million years would be required to base-level a continent of the size of Australia, after allowing for isostatic compensation that would result from erosional unloading, allowing plenty of time since the last major orogenesis for the formation of extensive plains by weathering and erosion.
According to Twidale & Campbell, the compact shape of the Australian continent has contributed to the tendency for widespread plantation, with few major indentations or embayments along the extensive coastline.
Another factor in the formation of fertile soils that was absent in Australia was glaciers.
During the ice ages of the last 2.6 million years, glaciers covered much of the Northern Hemisphere, grinding unweathered rock into fine particles that became soil enriched with new minerals when the glaciers retreated.
The traps are much smaller than the Siberian Traps, which occurred not long afterwards, about 251 million years ago.