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Australia: The Land Where Time Began |
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Heading into the Big Dry 130,000-18,000 Australia was already drying before the world began cooling at the
start of the latest glacial phase. As increasing amounts of water were
being locked up in the expanding glaciers, the sea around Australia was
lowered, exposing large areas of the continental shelf. The
flora and
fauna adapted
as the climate cycled from wet to dry and back again a number of times
in the previous 100,000 years. The coming fluctuations were much more
extreme than those that occurred during previous cycles. In the period 128,000 - 124.000 years ago, there is evidence that the
sea levels were higher than at present, some coastal lowlands being
inundated, as happened in the Richmond River Valley in New South
Wales. The sediments deposited at this time in the continental seas
formed the Gundurimba Clay. The extent of this coastal inundation
at this time is indicated by the extent of the Gundurimba Clay, it
reached as far are 160 km inland from the present mouth of the Richmond
River at
Boatharbour,
upstream from Lismore, and to as far as a point 4 km east of Casino up
another river valley. The clay deposits show that the inundation of the
continental margins extended further inland than the present coastline.
During this wetter phase of the
Pleistocene this epicontinental sea had an irregular shape,
extending up tributary valleys, and some highlands being isolated as
islands. Ostracods were found in the marine clay deposit. Along the
coastal fringe, the Gundurimba Clay is overlain by dunes that extend
beneath the sea as sand bars. The ostracod species found in the Clay indicate that the
shallow water the Clay was deposited in was probably a protected bay,
with a nearby hinterland with mostly low relief. As the bay appears to
have been open to the sea, it implies that the Inner Barrier Beach
ridges, that are characteristic of the present-day area, were deposited
at a later time of lower sea level when sand from the exposed
continental shelf was redistributed. This barrier of dune sand is
composed of the Woodburn Sand. As well as ostracods, a coral
fauna from the Late
Pleistocene
at Evans Head, which is beyond the limits of the Clay, indicate that the
environment was more exposed to the open sea than it is at present,
because the corals found require open marine circulation. At the time
the corals were living, the Barrier Beach ridges had not been formed. It
is believed that water temperature at the time may have been slightly
higher than at present. Parabolic
dunes of quartz sand formed during
the Pleistocene stretching for 350 km along the coast of southeast
Queensland north of the mouth of the Richmond River. Their orientation
is northwest-southeast, they run obliquely to the present coastline,
rising in places to 200 m. Off the coast lie the greatest sand deposit
in the world. There are 5 barrier islands - North Stradbroke Island,
South Stradbroke Island, Moreton Island, Bribie Island and Fraser
Island. The dunes at Stradbroke Island are vegetated and fixed, the sand
extending to 60 m below sea level. The sand, strongly pigmented in
places, contains layers of sand rock and peat. The Great Sandy Region comprises the
Cooloola Sands, Fraser Island, the largest sand island in the world,
and the strait separating
Fraser Island
and the mainland. The sand came originally from erosion along the
eastern coast that was carried north along the coast by along-shore
currents, and then deposited on the continental shelf and rocky ledges.
The accumulation of sand that led to the formation of
Fraser Island has been occurring
throughout the Pleistocene. It is believed the dunes on its eastern
side, the oldest, appear to have begun forming at times of low sea level
during the Late Pliocene when the continental shelf was exposed. The
reason that
Fraser Island formed is that
the small area of rock of the present island blocked the passage of the
sand north when the sea level was high enough for the northward-flowing
shore current to carry sand to that point where it was deposited in the
lee of the rocky ridge. North of Fraser Island, the
Capricorn Basin
prevents sand being deposited further north on the continental shelf.
Dunes along the northern and eastern margins of
the continent are composed of quartz; those on the southern,
south-western, and north-western are composed of calcareous sand. This
type of sand forms coastal limestone, only the youngest remaining as
mobile dunes. Beginning about 120,000 years ago, environments began to resemble
those of the present. Sea level hasn't changed much since then, and the
coastal features and shorelines were similar. The continental interior
was dry, with stabilised dunes that had formed in previous dry phases
during earlier glacial phases. During the first 40,000 years of the
cycle there appear to have been only minor climatic changes. The spread
and diversification of eucalypts was being encouraged by the increasing
fire incidence. The large amount of variation within species and species
groups of
Eucalyptus
indicates a relatively recent radiation and massive expansion of range.
The chains of playa lakes extending from
Lake
Hopkins in Western Australia to the
Finke
River in the Northern Territory, via
Lake Neal
,
Lake Amadeus and many small playas. The
remains of a
palaeodrainage system,
that in the past flowed into the
Finke River,
is evidenced by these salt lakes, then on to
Lake Eyre. Groundwater calcrete deposits, that are often silicified,
as well as gypsum and glauberite deposits at places along the
palaeodrainage have been dated. They come from a very arid phase. Evidence of a relatively long arid phase is found at
Curtain
Springs in the Northern Territory, which lasted from 75,000 to
35,000 years ago. At this time, slow moving groundwater formed a valley
floor type silicified calcrete deposit beneath the water table. This is
an aquifer at the present time. The latest glacial maximum initiated a
shorter arid phase, from 16,000 to 8,000 years ago. Between 27,000 and 22,000 years ago, water seeping through soil
layers above the water table, formed vadose calcrete, a characteristic
of this phase that is indicative of a wet phase. This time period
coincides with a lake-full stage of
Lake
George. At this time the lakes of the Centre were full. In all the
arid regions, much of the continent, the dry periods became seasonal
instead of the constant dry times that often characterise the arid
regions, though droughts were probably still common. The landscape
features that had already been well established, such as high
evaporation, wind-blown sands, salinity, swales with a clay substrate
and gibber plains, still exercised a control of the vegetation type that
was possible. Though there were some areas of
mesic vegetation, even
during these wetter phases, there were no corridors that would allow the
spread of vegetation to the newly wetter areas, so vegetation patterns
tended to remain largely unchanged. The landscape of the central areas
had been altered to such an extent by long and recurring periods of
aridification that several thousand years of wetter conditions were
unable to return the vegetation to the type that had been there prior to
the spread of aridification. The main change of vegetation that would
have occurred was the fluctuating margins of the various plant
communities. Near Newcastle Waters in the Northern Territory,
Lake Woods is a good example of a lake
presently set in a semi-arid region, that during the wet phase 22,000
years ago was much larger and 6 m deeper. The beginning of the arid interval in central Australia that covered
the period from 75,000 to 35,000 years ago coincides with the start of
the cold interval at
Lake George that
began 75,000 years ago and continued until 64,000 years ago, as
determined by pollen cores from the lake. At this time Lake George was a
usually dry lake surrounded by treeless grassland. The fire frequency
decreased because there wasn't sufficient vegetation to feed a fire.
Between 64,000 and 22,000 years ago, the area around Lake George was
warmer and wetter, dominated by eucalypt woodland. Between 26,000 and
23,000 years ago the lake was overflowing. The oldest aeolian sands at
Lake George contain stone tools of the
Ngunawal Aboriginal People, dated to
between 23,000 and 17,000 years ago. The beginning of an arid phase is
indicted by these beds, when the lake was dry. The
Murray Basin was another area that
didn't experience the constant dryness that affected central Australia
between 75,000 and 35,000 years ago. From about 60,000 years ago, the
catchments of the Murray Basin on the southern half of the
Great Divide that supplies water
to the
Murrumbidgee and
Murray, as well as the south-eastern
rivers, had plenty of water. All the lakes of the Murray Basin were full
by 55,000 years ago. At this time the
Willandra Lakes that are fed by a tributary of the Lachlan River
covered a total area of about 1000 sq km. One of these lakes, Lake Mungo, is of
interest because of its archaeological evidence of Aboriginal
occupation. There was a relatively short dry phase about 36,000 years
ago when the lakes shrank, becoming increasingly saline, most drying up
completely. At this time
lunette dunes formed
on the eastern side of the lakes, and a lot of salt was carried from the
saltpans over the surrounding land, causing soil salination and
resulting eventually in a very saline water table. After this dry phase,
wetter conditions returned, and the lakes filled, remaining at moderate
levels until about 25,000 years ago. Following the wet phase, drier
conditions returned associated with a cooling. Faunas that are probably older than 30,000 years have been found in
lunettes at
Lake Tandou and
Lake Menindee, among which are
Diprotodon,
Protemnodon (the giant
kangaroo), Sthenurus the browsing kangaroo,
Thylacoleo (the marsupial
lion), and
Procoptodon
(the giant browsing kangaroo). There were also modern animals from the
arid zone - hare wallabies, nail tail wallabies, bettongs, pig-footed
bandicoots and bilbies. At
Lake Menura and
Lake Victoria, there are similar faunas that
also contain koalas, indicating that they lived in a riverine forest in
an area subject to seasonally arid conditions. They live in such forests
in present-day parts of Queensland. At sites on the Nepean River near Sydney there is evidence of
wet conditions between 60,000 and 36,000 years ago. Between 47,000 and
40,000 years ago a braided river system laid down an alluvial terrace
deposit, the Cranebrook Terrace. Aboriginal stone artefacts were
found in this deposit. Many rivers around the continent display the same
increased activity at this time. Greatly increased river activity is
indicated in Tasmania by large alluvial deposits, from the period 44,000
to 22,000 years ago, at Rocky Point and near the mouth of the
Welcome River in the northwest of the island. In the last 40,000 years, lowered sea level resulting from increased
ice accumulation, led in the northernmost pasts of Australia to the
exposure of the floor of the
Gulf of
Carpentaria. This would have required a drop in sea level of 70 m.
The same thing happened at Bass Strait, where it required a drop of 65 m
in sea level. Between about 35,000 and about 12,500 years ago there was
a large lake,
Lake Carpentaria, in
the centre of the Gulf of Carpentaria. It covered an area of about
30,000 km2 with a maximum depth of about 10 m. Cores from the
lake sediments indicate that it was surrounded by savannah-type
vegetation, similar to that found on the
black soil plains south of the
Gulf at present. Sedges and swamp plants and aquatic plants
predominated, and grasslands with some Myrtaceae and
Callitris and
Casuarina were present. At this time when there was a land bridge to New Guinea there is no
evidence from the pollen record that plants from New Guinea invaded the
newly exposed land. Between 50,000 and 30,000 years ago lake levels in southeast
Australia were high, the Mungo Lacustral phase. Following this
wet phase was a dry phase. Between 25,000 and 14,000 years ago the
drying trend on the mainland increased in severity, with maximal aeolian
activity between 18,000 and 15,000 years ago, coinciding with the peak
of ice accumulation in the northern hemisphere. The evidence from the rivers of the
Riverine Plain in the eastern part for the
Murray Basin at this time shows that the
lakes changed from full to empty. The river systems crossing the plain
during the Mungo Lacustral phase were wide and meandering. There were
dunes on their northern and southern margins. This is called the
Tallygaroopna Stage of
river evolution. Towards the glacial maximum, the rivers narrowed in the
drying phase, reducing the amplitude of the meanders, sand sheets
spreading from marginal dunes, the
Kotupna Stage. After
14,000 years ago, in the interglacial, rivers assumed their modern form
of highly sinuous channels, the modern
Goulburn-Murray Stage.
These patterns also occurred in the ancestral Darling, still
characterised by narrow, sinuous channels. The big meander stage corresponds to wetter phases when the
groundwater level was higher and lakes were full. The smaller meanders
correspond to the arid phase with lower water tables and empty lakes.
The final stage is the return to wetter conditions after the intense
aridity of the
glacial maximum.
From about 30,000 years ago to the glacial maximum, glacial
conditions in the highlands of Tasmania and the south-eastern part of
the continent were increasing. The snowline was lowered by more than 100
m at the glacial maximum. During the
Margaret Glaciation in
Tasmania, beginning about 30,000 BP, reaching a maximum about 19,000 BP,
and finishing about 10,000 BP, the west coast mountains glaciated and
there was an ice cap on the central plateau west of the
Coast Ranges, that covered an area of up to 6,000 km2
that was at least 700 m thick. The inclusion of the fringe areas would
bring the figure to 7,000 km2.
During this time caves in the
Florentine River Valley and the
Shannon River Valley were
occupied by
Aboriginal People. At
Bluff Cave, occupation occurred between 31,000 and 11,500 years
ago, and at the edge of the Plateau,
cave ORS7, in the Shannon River Valley, was occupied between 31,000
and 25,000 years ago. Among the remains in these caves were emu egg
shells, wallaby bones, apparently the food eaten by the
Aboriginal People. Artefacts were also
found in the caves. The caves show no evidence of megafauna. It is not
known if that was because they were already extinct in Tasmania, or
because they were not eaten in the caves, possibly being eaten at the
site of the kill, or hunted in the summer when the Aboriginal People were most
likely to live away from the caves. The megafauna survived on the
mainland until about 15,000 years ago. During the glacial maximum the
mean temperature was at least 6.5o C lower than at present. Mt Kosciusko was glaciated, the glaciers covering about 25 km2.
Across the large periglacial area, all the mountainous country in the
southeast was affected by frost and winter snow. The sediment load of
upland rivers was increased by the material produced by the freeze-thaw
erosion of the rocks, often overflowing across the plains as alluvial
fans. Between 25,000 and 20,000 years ago, Antarctic sea ice reached its
greatest extent. Open water is thought to have been present for only
about 3 months/year south of 55o south latitude.
On a global scale, sea levels were more than 100 m lower than now,
exposing vast areas of continental shelves. At this time there was about
20 km of coastal plain between Sydney and the sea, and the
Queensland Plateau, site of the
Great Barrier Reef, was coastal lowland. At the glacial maximum,
the dry windy conditions resulted in the most severe bout of
aridification that
Australia had endured in the entire 2.4 million years of the
Pleistocene Ice Age.
At this time lakes dried up, often remaining dry ever since. Large
gypsum dunes were formed at
Lake Eyre and
Lake Frome, indicating that conditions were
among the worst ever experienced in Australia, as they were not produced
before or since the glacial maximum. To produce such dunes it was
necessary to have a high water table prior to a severe drying phase,
which mobilised the salts, forming a concentration of sulphate in
surface water. When the water evaporated, the salt crystallised and
gypsum formed with halite at the water-sediment interface. At present it
does this below the surface. In the dry, windy conditions of the glacial
maximum these crystals dried and were blown to the downwind side of the
lake where they formed very large dunes. As well as the strong winds usually associated with glaciation in the
northern hemisphere, the orientation of clay
lunettes indicates the climate of southern Australia may have
changed, the seasonal pattern of prevailing westerly winds was shifted,
as it was affected by the summer anticyclone that moved 5o
further north than at present. There was also the effect of the greater
area of land surface causing an increasingly severe continental climate,
resulting from lower sea levels exposing more of the continental shelf.
Hot dry air from the interior probably reach the coast more frequently
in summer during the glacial times, bringing aridity closer to the humid
coastal margin. There is evidence from
Kangaroo Island and Tasmania that supports this expansion of aridity
further south. In the
Seton rock shelter on
Kangaroo Island, evidence
indicates that between 20,000 and 10,000 years ago, bettongs and
wallabies inhabited open grassland. At present the area is covered by
heathland and
woodland. The
presence of big
grey kangaroos
and open dry grasslands in north-western Tasmania is more evidence for
the spread of aridity, even to Tasmania. Between 20,000 and 15,000 years ago the
Nullarbor was drier than now, the last vestiges of the
Mallee vegetation had moved onto its fringes by
20,000 years ago, and woodland had gone for the last time. Evidence from
Koonalda Cave and
Madura Cave, as well as deposits from a
cave near
Eucla and the
Norina rock shelter, also confirm
the increased aridity occurring at the
glacial maximum.
Mammoth Cave in the
Cape Leeuwin-Naturaliste
region of Western Australia has deposits with a faunal record from
37,000 years ago to the present.
Devil's Lair
covers a timespan from 35,000 to 5,000 years ago. Here there was a very
diverse fauna, including such arid land species as the bettong and
burrowing rat-kangaroo. The southeast trade winds are believed to have intensified from about
20,000 years ago onwards towards the glacial maximum in northern
Australia, but would have changed direction. The alignments of the dunes
in arid central Australia indicate that the direction of the wind was
not significantly different from what it is today. The ultimate drying
that occurred between 18,000 and 15,000 years ago, coinciding with the
peak of glaciation in the Northern Hemisphere, can be seen in the extent
of the
central dune deserts in present day
Australia.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |