ROCK LANDFORMS of Australia & NZ - TECTONIC LANDFORMS

to Tectonic Page		Tectonic features have resulted from earth movements Tectonic landforms are found across a wide range of rock types. The Lake George area makes for an interesting study; it contains evidence of subsidence, a fault-block mountain range, and a lake whose level rises and falls periodically. Land movements may be upwards or downwards, initiated through local or widespread forces, illustrate faulting or folding, reflect violent or gradual processes, and a range of other factors. In general, land movements in Australia have been gradual in comparison with most other parts of the world.
TECTONIC LANDFORMS		- Theme Study (A) © C.Grant (Lake George features Sites 1-3)

This site examines the interesting landscape of Lake George, NSW, where subsidence has taken place.
The lake level rises and falls periodically according to the balance between precipitation and evaporation.

Theme Study (A): TECTONIC SUBSIDENCE (SITE: 1) Location: Lake George, Southern Tablelands, NSW Feature: Signboard - area of internal drainage (i.e. basin)
KEY POINT: This part of the Southern Tablelands NSW is a basin, an area of subsidence surrounded by a drainage divide.
Lake George makes for an interesting study in tectonic (earth moving) processes that have resulted in a basin lower than the surrounding locality. Creeks flow into the lake, but as there is no stream exit, the area forms an internal (endoreic) drainage basin (Lake Eyre is a similar basin on a much larger scale). The signboard adjacent to the lake advises: 'The Lake occupies the floor of a rift valley and the abrupt change in terrain from the lake shore to the escarpment on its western flank marks the line of a geological fault.' It is believed that originally the drainage was to the west. These streams were blocked as land here rose, forming the Lake George Range (Cullarin Horst), an uplifted fault block. Why doesn't the basin fill up and overflow? Although it was once believed possible that a mysterious force was at work, it is now known that the lake level is a result of differences between precipitation and evaporation, along with porous silt soils on the lake floor that can absorb large volumes of water. top of page
Theme Study (A): TECTONIC SUBSIDENCE (SITE: 2) Location: Lake George, Southern Tablelands, NSW Feature: Area of internal drainage, block mountain range
KEY POINTS: Left of photo: the almost flat bed of the Lake George Basin. Right of photo: western boundary of basin, a fault-bock range.
From the northern side it is possible to view the flat basin making up the area of the Lake George depression, along with the line of the fault scarp that forms the western boundary of the basin. This scarp rises steeply on the visible eastern side by about 150m. The visible scarp edge has been dissected by stream action. The far side (western) side of the range (not visible) slopes away gently. The scarp is the eroded remnant of a high angle reverse fault, overthrust from the west (right side of picture). It is possible that the fault line erosion scarp resulted as softer rocks to the east were removed. Subsequent recent fault activity may have caused scarp renewal. This fault now appears inactive but the district still feels frequent minor tremors probably related to other nearby faults. The Federal Highway runs along the base of the range, on the lake's western edge, an example of the influence of topography upon human activity (i.e. transport and grazing). top of page
Theme Study (A): TECTONIC SUBSIDENCE (SITE: 3) Location: Lake George, Southern Tablelands, NSW Feature: Lake surface (area of internal drainage)
KEY POINTS: Lake bed almost flat. Porous sediments make up the basin floor
The area of internal drainage is defined by the Great Dividing Range which splits, passing around the basin to the east (hills in photo) and to the west (scarp summit - previous photo). The lake water level changes periodically. When dry, the grassy surface is grazed by sheep. Since records began the lake has been dry about half the time. Studies of former shorelines may reveal clues to climate change and earth movements. The lakeside signboard tells us the following: The Lake's aboriginal name is 'Weereewaa'. Governor Macquarie visited the area in 1820 and was impressed. The Governor recorded that 'after dinner we drank a bumper toast to the future settlers of the shores of Lake George - which name I have given this grand and magnificent sheet of water in honour of his present majesty' (George IV). DETAILS: Area of drainage basin: 906 sq km Surface area of lake: 155 sq km Maximum recorded depth: 7.3m Height of lake bed above sea level: 673m. top of page

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signboard basin and range the lake