South Western Slopes - landform
The South Western Slopes Bioregion is a large area of foothills and ranges comprising the western fall of the Great Dividing Range to the edge of the Riverina Bioregion. A very wide range of rock types is found across the bioregion, which is also affected by topographic and rainfall gradients that decrease toward the west. These physical differences have an impact on the nature of the soils and vegetation found across the bioregion.
Inland streams pass across the slopes in confined valleys with terraces and local areas of sedimentation. Geology, soils and vegetation are complex and diverse but typified by granites and meta-sediments, texture contrast soils and a variety of eucalypt woodlands, making this bioregion the southern equivalent of the Nandewar Bioregion.
Geology and geomorphology
The bioregion lies wholly in the eastern part of the Lachlan Fold Belt which consists of a complex series of north to northwesterly trending folded bodies of Cambrian to Early Carboniferous sedimentary and volcanic rocks. Granites are common and mostly located in large scale upfolded bodies of rock. Granite landscapes occur either as central basins surrounded by steep hills formed on contact metamorphic rocks, or as high blocky plateau features with rock outcrops and tors.
Hilly landscapes developed on the sedimentary and volcanic rocks are controlled by structural features (bedding and faults) and typically form lines of hills extended along the strike of more resistant rocks such as quartzite. The valleys between ranges are either in granite or generally softer rocks such as shale, phyllite or slate.
Limited areas of Tertiary basalt with underlying river gravels and sands occur, and as the country becomes lower to the west and north, wide valleys filled with Quaternary alluvium and occasional lakes become the dominant landscape form.
At the western edge of the bioregion the alluvial fans of the Riverine Plain have largely buried bedrock forms. Remnants of earlier gravel deposition on these fans, indicative of higher river discharges than today, are found as terrace features in the valleys and as gravel outwash plains.
Some rock types and landscape features deserve special mention. Several limestone outcrops are known, all of which have developed karst topography and carry locally different vegetation. A narrow belt of serpentinite with chemically distinctive soil runs northwest from Tumut to Cootamundra. A very large number of mineral deposits have supported the mining industry over the past 150 years.
Perhaps the greatest significance of this bioregion is the very diversity of its geology, geomorphology and biota. In addition to this there are a number of special features to be noted, as follows:
- several occurrences of limestone with well-developed karst landscape and rich fossil assemblages. Wellington Caves, for example, contains an abundance of extremely important Tertiary and Quaternary vertebrate fossils, the systematic study of which has only just begun despite 170 years of collecting;
- numerous fossil occurrences in other locations;
- the serpentinite belt with its unusual mineralisation, soils and vegetation;
- a very large range of economic mineral occurrences with their attendant mining heritage; and
- numerous sites exhibiting important structural features of folds and faults in the bedrock.
The overall pattern of soils in these landscapes is one where shallow, stony soils are found on the tops of ridges and hills. Moving downslope, texture contrast soils are the norm with subsoils derived from the underlying weathered rock and the topsoils being an homogenised surface mantle of coarser material derived from all parts of the slope.
On valley floors subsoils have drabber colours indicative of poor drainage and they may accumulate soluble salts. Dryland salinity is widespread. Alluvial sands and loams are more common than clays in most parts of the landscape but alluvial clays become more important nearer to the Riverine Plain. Over the Quaternary, soils in these landscapes have accumulated a considerable quantity of wind-blown silt and clay from western NSW.
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Page last updated: 27 February 2011