Australia: The Land Where Time Began

A biography of the Australian continent

A lake on the Southern Tablelands, about 100 km inland. It is a fluctuating closed lake that is situated between the south-east mountain catchments and the arid inland. It has a catchment area of about 932 km². Lake George occupies about 16 % of the area of the catchment basin.

A core drilled into the sediments of Lake George found pollen and charcoal that were dated to more than 100,000 years ago. In the top 8.6 m, covering 350,000 years - are the longest continuous record of  fire and vegetation history in Australia. The base of the core, which is 72 m long, is estimated to be between 4.2 and 7 million years ago. It is estimated to extend down to about 134 m. This would put the origin of Lake George at about 20 Ma. The sediments show alternating layers that indicate 8 times when the lake was full and 8 when it was dry, indicating 8 glacial and 8 interglacial periods in the past 750,000 years.

See Lake George Project

The groundwater below the catchment is mainly low salinity, but under the actual lake bed, it is of higher salinity, and is confined by a layer of clay. As the lake dries by evaporation the salt it carries is concentrated in the remaining water. There are complicated interactions between the lake water and the pressurised high saline aquifer beneath the lake bed. The result is that salt has been stored in the aquifer at 10-12 m below the sediment surface, at a level of hydrostatic balance. This has been occurring over the last 1-2 million years.

Faulting formed the lake as a normal fault-angle basin in or before the Mid Miocene. The sediment deposits in the lake show the history of the area since it formed. Below the lake bed there is a series of river and lake sediments that reaches 150 m thick. 3 sediment layers have been identified.

The Gearys Gap Formation is the oldest, made up of deeply weathered sand and gravels. Palaeozoic bedrock underlies these Early Tertiary river sediments. The north-west flowing rivers were incised into the bedrock, so are associated with the previous drainage system. The palaeochannels can be traced below the lake bed. Between this sediment layer and the following 2 layers there is a break.

The next layer is comprised of river sands and lake clays, the Ondyong Point Formation. There is a laterally persistent layer of sand and silt that separated the upper 2 layers. There is no break in sedimentation.

Lake clays and silts of the Bungendore Formation make up the uppermost layer. The sediments from the top 2 layers represent deposition in a closed drainage basin. The differing sediment types result from climate changes.

The low watershed of the Great Divide in the east and the Lake George Range in the west form the margins of the basin. The northern and southern ends of the lake have subdued topography with low saddles and a complex of natural and artificial drainage lines. There are also swampy and lagoonal areas.

In the upper reaches of the streams in the basin occupy wide, open valleys. These streams converge towards the lake, from the north, Collector Creek, from the east, Alliianoyonyiga Creek and Butaroo Creek and Taylor Creek, from the south, Turallo Creek. The extreme variations of the lake during the Quaternary are evidenced by the ancient strand lines up to 37 m above the lake bed. Past this level the lake overflowed via Gearys Gap into the Yass River drainage system. This occurred between 27,000 and 21,000 years ago.

The faults that formed Lake George cut across pre-existing drainage lines. Prior to faulting, Taylors Creek used to flow across the area from east to west. It has been dammed to form a lake in the fault angle depression. In the process of creating the basin the fault disrupted the drainage system of the area, beheading headwater streams of the Yass. A number of creeks in the north that used to flow north-west were diverted by the fault, after which they flowed along the fault scarp base into Lake George. The fault crosses the Molongolo River to the south. This river has a much bigger catchment than the northern rivers, so was capable of continuing in is course across the rising fault block, cutting through the rock to form the Molonglo Gorge.

At the time of the arrival of Europeans in the area around Lake George it was mostly eucalypt woodland. The sediment contains pollen that shows that fire increased from about 130,000 BP, associated with an increase in eucalypts in the surrounding area. Prior to this eucalypt proliferation the vegetation was less sclerophyllous, with vine thickets and casuarinas.

The present landscape around the lake, with increased erosion, with widespread gullies, is the result of European clearing and agriculture and grazing. The land has been degraded even further by sand mining of the strand lines and aeolian deposits on the southern and northern margins of the lake. Land degradation has been increased greatly by roadworks in the area.

Lake George has been subject to cyclical wet and dry times. Salt is carried to the lake from the catchment in wet times. In dry times the lake acts as a playa, with capillary zone evaporation. It has been called a natural analogue of a waste water disposal basin.

The pollen record

Pollen cores contained pollen indicating the presence of cool temperate rainforests, especially Nothofagus, in wetter times during interglacial phases (Colhoun & Van der Geer, 1988). During the glacials the forests were replaced by open sclerophyll woodland and treeless herb fields and grasslands (Singh & Geissler, 1985). The woodlands were replaced by treeless vegetation during the coldest parts of each glacial. This process reached a peak in the second last glacial, the area being covered by almost unbroken grassland. In the LGM the vegetation became more diverse, having some grasses but also a variety of shrubs. Casuarina originally dominated the sclerophyll forests, by the start of the last glacial they were replaced by eucalypts (Johnson, 2006).

Water levels in Lake George between 1819-1990 Approximate depth in metres, estimated from a graph on p. 111 of Mary White's book, Running Down.

1819    6

1820    7

1830    2.5

1840    1.5

1850    0.8

1860    1

1870    3.5

1880    5.5

1890    2.8

1900    2.5

1910    0.5

1920    0.91

1930    0.1

1940    0.0

1950    2.9

1960    4

1970    2.1

1980    2

1990    2.9

Sources & Further reading

1. Josephine Flood, Archaeology of the Dreamtime, J. B. Publishing
2. Phillip J. Habgood & Natilie R. Franklin, The revolution that didn't arrive: A review of Pleistocene Sahul, Journal of Human Evolution, 55, 2008
3. Mary E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994
4. Mary E White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
5. Chris Johnson, Australia's Mammal Extinctions, a 50,000 year history, Cambridge University Press, 2006