AND THEIR HISTORY
is the lowest, flattest and, apart from Antarctica, the driest of the
continents. Unlike Europe and North America, where some landscapes
date back to 'only' 20 000 years ago, when great ice sheets
retreated, the age of landforms in Australia is generally measured in
many millions of years. This fact gives Australia a very distinctive
physical geography. Map 1 shows the elevation of the Australian
1 AUSTRALIA ELEVATION.
continent can be divided into three parts:
- the Western Plateau
- the Central Lowlands
- the Eastern Highlands.
Western Plateau consists of very old rocks (some over 3,000 million
years old), and much of it has existed as a landmass for over 500
million years. Several parts have individual plateau names (e.g.
Kimberley, Hamersley, Arnhem Land, Yilgarn). In the Perth area,
younger rocks along a coastal strip are separated from the rest by
the Darling Fault escarpment. The Nullabor Plain is virtually an
uplifted sea floor, a limestone plain of Miocene age (about 25
Central Lowlands stretch from the Gulf of Carpentaria through the
Great Artesian Basin to the Murray-Darling Plains. The Great Artesian
Basin is filled with sedimentary rocks which hold water that enters
in the wetter Eastern Highlands.
of the centre of Australia is flat, but there are numerous ranges (eg
Macdonnells, Musgrave) and some individual mountains of which Uluru
(Ayers Rock) is probably the best known. Faulting and folding in this
area took place long ago. The area was worn to a plain, and the plain
was uplifted and then eroded to form the modern ranges on today's
plain. In looking at Uluru, one remarkable thing is not so much how
it got there, but that so much has been eroded from all around to
leave it there.
the South Australian part of the Central Lowlands, fault movements
are more recent, and the area can be considered as a number of blocks
that have been moved up and down to form a series of ranges (Mt
Lofty, Flinders Ranges) and hills (such as the Adelaide Hills), with
the down faulted blocks occupied by sea (e.g. Spencer Gulf) or
lowlands including the lower Murray Plains.
Eastern Highlands rise gently from central Australia towards a series
of high plateaus, and even the highest part around Mt Kosciuszko
(2,228 metres) is part of a plateau.
are a few younger faults and folds, such as the Lake George Fault
near Canberra, and the Lapstone Monocline near Sydney.
plateaus in the Eastern Highlands are dissected by erosion into
rugged hills, and the eastern edges of plateaus tend to form high
escarpments. Many of these are united to form the Great Escarpment
that runs from northern Queensland to the Victorian border.
highest waterfalls (Wollombi on the Macleay, Wallaman Falls on a
tributary of the Herbert, Barron Falls near Cairns, and Wentworth
Falls in the Blue Mountains) all occur where rivers flow over the
Great Escarpment. For most of its length the Great Dividing Range
(separating rivers flowing to Central Australia from rivers flowing
to the Pacific) runs across remarkably flat country. In eastern
Victoria, however, the old plateau has been eroded into separate High
Plains (such as Dargo High Plain).
present topography results from a long landscape history which can be
started in the Permian, about 290 million years ago, when much of
Australia was glaciated by a huge ice cap. After the ice melted,
parts of the continent subsided and were covered with sediment to
form sedimentary basins such as the Great Artesian Basin.
early Cretaceous times, about 140 million years ago, Australia was
already so flat and low that a major rise in sea level divided it
into three landmasses as the shallow Cretaceous sea spread over the
the following Tertiary times, Australia can be regarded as a
landscape of broad swells varied by a number of sedimentary basins
(Murray, Gippsland, Eucla, Carpentaria, Lake Eyre and other basins).
These slowly filled up and some are now sources of coal or oil. The
Eastern Highlands were uplifted about this time.
the Tertiary, volcanoes erupted in eastern Australia. Some individual
volcanoes were the size of modern Vesuvius, and huge lava plains
covered large areas. Volcanic activity continued up to a few thousand
years ago in Victoria and Queensland. Australia's youngest volcano is
Mt Gambier in South Australia, about 6,000 years old.
55 and 10 million years ago, Australia drifted across the surface of
the earth as a plate, moving north from a position once adjacent to
Antarctica. There have been many changes in the climate of Australia
in the past, but oddly these do not seem to be due to changing
latitude (associated with global scale plate movements).
when Australia was close to the South Pole, the climate was
relatively warm and wet, and this persisted for a long time despite
changes in latitude. It was probably under this climate that the deep
weathered, iron-rich profiles that characterise much of Australia
were formed. Aridity only seems to have set in after Australia
reached its present latitude, and the northern part was probably
a large part of Australia is arid or semi-arid. Sand dunes are mostly
longitudinal and are aligned with dominant wind directions associated
with the regular passage of high pressure cells (anticyclones).
'highs' rotate anticlockwise and track at about 28°S in winter and
38°S in summer, resulting in predominantly south-east to easterly
flows in the north and north-west to westerly flows in the south.
Looking down from above, the south-east Trade Winds or 'Trades' would
be those winds in the top right hand quarter of a hypothetical,
stationary 'high' centred on the Australian continent.
dunes are mostly fixed now. Stony deserts or gibber plains (covered
with small stones or 'gibbers') are areas without a sand cover and
occupy a larger area than the dune fields. Salt lakes occur in many
low positions, in places following lines of ancient drainage. They
are often associated with lunettes, dunes formed on the downwind side
of lakes. Many important finds of Aboriginal prehistory have been
made in lunettes. Despite the prevalence of arid conditions today,
real aridity seems to be geologically young, with no dunes or salt
lakes older than a million years.
past few million years were notable for the Quaternary ice age. There
were many glacial and interglacial periods (over 20) during this
time, the last glacial period occurring about 20,000 years ago. In
Tasmania there is evidence of three different glaciations: the last
glaciation, one sometime in the Quaternary, and one in the Tertiary.
On the mainland there is evidence of only the last glaciation, and
the ice then covered only 25 square kilometres, in the vicinity of Mt
broad shape of Australia has been influenced over long periods by
earth movements associated with large tectonic processes. However,
much of the detail has been carved by river erosion. A significant
number of Australia's rivers, like the Diamantina River, drain
inland. While they may be eroding their valleys near their highland
sources, their lower courses are filling up with alluvium, and the
rivers often end in salt lakes which are dry for most of the time.
Other rivers reach the sea, and have dissected a broad near-coast
region into plateaus, hills and valleys. Many of the features of the
drainage pattern of Australia have a very long history, and some
individual valleys have maintained their position for hundreds of
millions of years.
salt lakes of the Yilgarn Plateau in Western Australia are the
remnants of a drainage pattern that was active before continental
drift separated Australia from Antarctica.
the last ice age, sea level was more than 100 metres lower than it is
today; the current outer reef area of the Great Barrier Reef would
have been the coast at that time. The rivers tended to cut down to
the lower level, especially towards the sea. When the sea level rose
again, some of the lower valleys were drowned, making fine harbours -
like Sydney Harbour - while others tended to fill with alluvium as
the sea rose - making the typical lowland valleys around the
geomorphology is also largely the result of the accumulation of
sediment in drowned coasts. In some areas, such as Ninety Mile Beach
(Victoria) or the Coorong (South Australia), there are beaches made
simply from this accumulation. In much of the east there is a
characteristic alternation of rocky headland and long beach, backed
by plains filled with river and marine sediments.
offshore shape of Australia, revealed in isobath contours, results
mainly from the pattern of break-up of the super-continent of which
Australia was once a part. In some areas, such as the Great
Australian Bight, there is a broad continental shelf bounded by a
steeper continental slope. In other areas, like south-east New South
Wales around Merimbula and much of the Tasmanian coastline, the
continental shelf is very narrow, sometimes coming to within 20
nautical miles of the coast. The Queensland coast is bounded by a
broad plateau on which the Great Barrier Reef has grown in only the
last two million years. In South Australia, the continental shelf is
grooved by submarine canyons.
Australian landforms of today are thus seen to result from long
continued processes in a unique setting, giving rise to typical
Australian landscapes, which in turn provide the physical basis for
the distribution and nature of biological and human activity in
can be inferred from the elevation and relief map (DIAGRAM
1), the rivers of Australia may be divided into two
major classes: those of the coastal margins with moderate rates of
fall, and those of the central plains with very slight fall. Of the
rivers of the east coast, the longest in Queensland are the Burdekin
and the Fitzroy, while the Hunter is the longest coastal river of New
South Wales. The longest river system in Australia is the
Murray-Darling which drains part of Queensland, the major part of New
South Wales and a large part of Victoria, finally flowing into the
arm of the sea known as Lake Alexandrina, on the eastern side of the
South Australian coast. The length of the Murray is about 2,520 kms,
and the Darling and Upper Darling together are also just over 2,000
kms long. The rivers of the north-west coast of Australia, for
example the Murchison, Gascoyne, Ashburton, Fortescue, De Grey,
Fitzroy, Drysdale and Ord, are of considerable length.
also are those rivers in the Northern Territory, for example the
Victoria and Daly, and those on the Queensland side of the Gulf of
Carpentaria, such as the Gregory, Leichhardt, Cloncurry, Gilbert and
Mitchell. The rivers of Tasmania have short and rapid courses, as
might be expected from the configuration of the land.
are many types of lake in Australia, the largest being drainage sumps
from the internal rivers. In dry seasons, these lakes finally become
beds of salt and dry mud. The largest are Lake Eyre 9,500 square
kilometres, Lake Torrens 5,900 square kilometres and Lake Gairdner
4,300 square kilometres.
lake types are glacial, most common in Tasmania; volcanic crater
lakes, predominantly in Victoria and Queensland; fault angle lakes,
of which Lake George near Canberra is a good example; and coastal
lakes formed by marine damming of valleys.