PHYSICAL FEATURES
Location
The ACT lies at a latitude of between 35 degrees and 36 degrees south of the equator. Elevations range from 450 m above sea level to 1,911 m at the top of Mount Bimberi in the south. These features, plus the territory's distance of about 130 km from the coastline, are the principal determinants of the area's climate.
Topography
The total area of the ACT is 2,351.6 km2 (about 235,000 ha), of which 60% is hilly or mountainous. The highest peak in the ACT is Mount Bimberi in the south (1,911 m). The ACT's main physical features are timbered mountains (located in the south and west), and plains and hill country (in the north).
The ACT is situated within the upper Murrumbidgee River catchment, in the Murray-Darling Basin. The Murrumbidgee flows through the Territory from the south, and its tributary, the Molonglo, from the east. Other tributaries of the Murrumbidgee include the Cotter, Paddys, Naas and Gudgenby Rivers. The Molonglo River was dammed in 1964 to form Lake Burley Griffin.
The Palaeozoic Lachlan Fold Belt stretches from central New South Wales (NSW) to Victoria. The ACT is sited on the belt and underlain by sandstone, limestone, siltstone and shale, all of which were formed from sediments deposited 460 million years ago, when the area was under sea. The ACT does not have any mineral-bearing rocks of economic significance, though deposits of base metals and gold are known to occur. A number of quarries provide materials such as crushed granite, gravel and sand for building.
Landscapes
The landscapes of the ACT reflect geological events, the long-term effects of weather and climate, and the influences of human settlement. The ACT has three broad landscape types: uplands, rolling or undulating country, and plains.
Uplands are areas at altitudes above 800 m. They occur mainly west of the Murrumbidgee River and consist of a series of north-south parallel ridges, separated by stream valleys. The uplands lie mainly on erosion-resistant, ancient, sedimentary rocks and granites.
Rolling or undulating country occupies the north-eastern area of the ACT and the eastern parts of the Murrumbidgee Corridor to the south. These areas are formed across moderately weathered rocks at varying elevations, generally 600-900 m above sea level.
Plains occur at elevations of 550-650 m along many of the streams in the north of the ACT. The plains formed on top of readily weathered rocks and on stream sediment. They surround isolated hills and ridges of erosion-resistant rock, such as Black Mountain and Mount Ainslie.
Soils
The soils of the ACT are highly variable and generally infertile. The ACT has three principal types of soils: lithosols, gradational soils and texture contrast soils.
Lithosols are soils which are nutritionally poor and, on the steeper slopes, they are subject to erosion should native vegetation be removed. They occur mostly in the south and west of the ACT.
Gradational soils are deep, relatively fertile soils which are the principal soil types under the Limestone Plains of Canberra. Gradational soils occurring on the north-east plain are relatively resistant to erosion, however in the western ranges, where slope is a factor, they will erode if the land is cleared. They are characterised by a gradual increase in clay content with depth.
Texture contrast (or duplex) soils dominate the ACT and its settled areas. They are particularly vulnerable to erosion on the slopes in the south of the ACT and in areas of urban development, where native vegetation has been cleared leaving the surface unprotected. The surface structure of these soils is fragile and becomes relatively impermeable and hard setting if they are cultivated or overgrazed. Much of the turbidity of Canberra's lakes after heavy rain is caused by erosion of these soils.
