South East Corner - landform
The South East Corner Bioregion is shared between NSW and Victoria. It covers the eastern fall from the Great Dividing Range across the Great Escarpment to the coast. It also includes the upper catchment of the Deddick River and Lower Snowy River gorge. Most of the bioregion is underlain by folded and metamorphosed Ordovician to Devonian sedimentary rocks that have been intruded by several granite bodies. The topography runs from plateau above the escarpment across steep hills toward the coast with short, active streams. Altitude and rainfall affects the vegetation patterns across the bioregion.
Geology and geomorphology
Basement rocks are folded Ordovician slates, cherts, and quartzite of the Lachlan Fold Belt. Less deformed Devonian sandstones and mudstones overlie the Ordovician rocks and the whole sequence has been intruded by several major granite bodies (about 380 million years old) that now form the bedrock over half of the bioregion. Small areas of Tertiary basalt and local deposits of quartz gravel and sand occur along the coast. Quaternary sediments are relatively unimportant except for the beaches and dunes of small coastal barrier systems.
Different rock types have a strong influence on topography. The metamorphosed sediments are oriented north-south and this controls the overall direction of the coastal ranges. Granites in the Bega Valley weather faster than the surrounding metamorphosed sediments and are eroded into local topographic basins. Other granites with a higher proportion of quartz form steep country with areas of outcrop and rounded tors.
The most prominent feature of the region is the Great Escarpment, a line of steep hills and gorges on the coastal side of the Great Divide that is formed by headward erosion of streams into the continental flexure created at the time of rifting of the Tasman Sea. Most streams have their headwaters at the escarpment but some begin on the plateau above it and flow parallel to the coast for some distance before crossing the escarpment in a gorge with waterfalls. More detailed patterns of stream direction relate to smaller joints and faults in the bedrock and both dendritic and rectangular drainage patterns are present.
Soils vary with bedrock type and slope position and texture contrast profiles dominate. Metamorphic rocks weather to clay and granites weather to a mixture of sand and clay. Metamorphic rocks generally form steeper slopes and thus the soils on them are thin and stony and form a texture contrast profile with thin topsoil of fine sandy loam. The clay subsoil resists the penetration of water and most profiles, especially in lower slope positions, have a strongly bleached zone in the topsoil caused by lateral throughflow. Soils on granites are generally coarser, deeper and better drained and deliver more sandy sediment to the valley floors and the coastline.
The coastline is a mixture of rocky cliffs and small sand barriers built across the mouths of most streams. Unlike the north coast only one phase of barrier development is apparent and soils formed in the dunes are podsol profiles but these only have minimal profile development. Sediments in the estuaries are mainly sand.
Important features include the following:
- cliffs south of Durras have exposed glacio-marine sediments and an unconformity;
- cliffs at Twofold Bay have an excellent example of a faulted fold;
- good examples of pillow lavas can be seen on Narooma Headland;
- Silurian limestone in the upper Deua Valley contains important fossils and has a karst topography;
- the Mt Dromedary monzonite is petrologically unusual;
- Montague Island is composed of monzonite with a cover of dune sand that supports an unusual vegetation community dominated by lomandra (Lomandra longifolia) that is apparently affected by high inputs of guano from sea birds; and
- some landscape features such as Mumbulla Mountain have cultural significance to Aboriginal people and still retain story associations.
Typical soils found across the South East Corner Bioregion are texture contrast profiles with their properties differing with rock type. Well-drained coarse granite soils are found in the lower Snowy River valley.
Documents to download
Page last updated: 27 February 2011