Lake George (Weereera) is an intersting place because most people who drive past see it as any ordinary lake bed when in fact it is one of the planets oldest lakes! The GZ is at the upper carpark and highlights the vast space the lake occupies and the surrounding features. The view is also awesome. At the GZ the steepness of the lake edge is visible as are the jagged fractured edges of the Lake George Fault.
The GZ is a roadside reat area with toilets, space, tables and seclusion from road noise.
Lake George is thought to be one of the world's oldest lakes; it is shallow and has a history of dramatically fluctuating water levels, despite having no outlet. Originally, there was no lake in the area at all, and water drained straight from the Great Dividing Range west into the Yass River. That was before a massive geological uplift that occurred some five million years ago. Around the Mid Miocene period, there was a major crustal movement along a strong fault line and this formed the Lake George Range - a natural dam blocking the creeks. This type of lake is known as an endorheic basin; a closed drainage basin that retains water and allows no outflow to other bodies of water such as rivers or oceans. Any rain or moisture that falls within its small catchment does not flow out naturally and so can only leave the drainage system by evaporation and seepage.
In point of fact, the water levels in Lake George depend simply on the balance between the evaporation rate on the lake surface and the rainfall and water flowing into the lake.
There is, however, a theory that saline water makes its way underground from Lake George to the Yass River which would explain the high salt levels in the Yass River, but this remains unproven.
At 25 kilometres long and 10km wide, Lake George is long, largely flat and extremely shallow, with a very small catchment. Resultant evaporation rates as well as a tendency for strong winds to blow the water back on itself explain the mysterious filling and drying episodes on both short term (hours) and long term (years) time scales that have been observed. The lake's depth when full can range from 1.5 to 4.5m, however, in many areas it is only around 0.8 to 1.0m deep. Its deepest point has been measured as 7.5m. When full, the lake holds about 500,000,000 cubic metres of water. Between the late 1980s and mid 1990s, the lake lapped the Federal Highway on its western edge, whereas at other times, it has remained completely dry. It has a catchment area of about 932km2 and Lake George occupies some 16% of the area of the catchment basin.
The thickness of sediment beneath the lake exceeds 250m, according to a Bureau of Mineral Resources Canberra drilling program conducted in the 1982/83 summer. The oldest sediments, which lie some distance above the bedrock, were dated at 3 to 5 million years using spore and pollen analysis and magnetic reversal stratigraphy. The top 8.6m of sediment, covering the last 350,000 years, - are the longest continuous record of fire and vegetation geological history in Australia. The base of the drilling core, which is 72m long, is estimated to be between 4.2 and 7 million years old. It is thought to extend down to about 134m. This would put the origin of Lake George at about 20 Ma. (Ma stands for megaannum, which is a unit of time equal to one million years.)
The sediments show alternating layers that identify eight different times when the lake was full and eight when it was dry, indicating eight glacial and eight interglacial periods in the past 750,000 years.
The groundwater below the catchment is mainly low salinity, but under the actual lake bed, it is of a much higher salinity, and is confined by a layer of clay. As the lake dries by evaporation, the salt it carries is concentrated into the remaining water. There are complicated interactions between the lake water and the pressured high saline aquifer beneath the lake bed. The result is that salt has been stored in the aquifer at 10-12m below the sediment surface, at a level of hydrostatic balance. This has been occurring continuously for the last 1-2 million years.
QUESTIONS:
At Stage 1 (GZ):
1. Measure the altitude
2. On the visible rock fractures confirm if the rock layers are horizontal or vertical.
3. Take a photo of yourself at the site with your GPSR and forward it with your answers.
At Stage 2 (Lower carpark):
1. Measure the altitude
Please submit your answers as soon as you can. It's ok to post prior, I will check your findings and respond if there are issues. Feel free to add a photo of yourself at the site if you want too. Group visits are encouraged. I hope you enjoy the experience, this is a very special area..