Geology - Recent lacustrine (lake or swamp) deposits.
Landform - Lacustrine plain.
Goudie (1942) defines three phases of this soil type: normal phase, burnt peat phase and the deep peat phase. He mapped two of these, the normal phase (which includes the burnt phase) and the deep peat phase. The normal phase has a peaty clay surface layer about 9 inches (230 mm) thick overlying a gritty clay. The deep peat phase has an additional peat layer up to 12 inches (300 mm) thick below the peaty clay surface.
During the course of this survey all of these phases were found but they could not be mapped separately. The depth of surface soil in the normal phase was found to be slightly deeper than that reported by Goudie and generally was between 250 and 400 mm deep. Only small scattered occurrences of the other two phases were found.
The main type, Koo-Wee-Rup peaty clay, has about 300 mm of dark greyish brown to dark brown loam or light clay surface soil with high amounts of peat or organic matter. This peat, which was originally deeper, accumulated in the marshes and lagoons fed by slow-moving water. Since drainage, much peat has disappeared through shrinkage, consolidation, burning and blowing.
Below the peat layer, dominantly dark grey medium or heavy clays occur, passing at about 500 mm to mottled paler grey and yellow-brown mottled medium to heavy clays. Clays continue to at least 1.8 m. Variable amounts of gritty sand to gravel may occur throughout the soil profile.
These soils can usually be referred to as Humose-Acidic, Eutrophic, Grey DERMOSOLS using the Australian Soil Classification. Soil site information is available for Soil Pit Sites GP26 and GP28.
Typical profile of Koo-Wee-Rup peaty clay
0 - 300 mm. Grey-brown to brownish grey peaty clay, clear transition to:
300 - 500 mm. Dark grey gritty clay*, diffuse transition to:
500 - 900 mm. Grey mottled with yellow-brown gritty clay*, diffuse transition to:
900 mm continuing to 1000 mm +. Grey or pale grey mottled with yellow-brown gritty clay*.
*Grittiness varies considerably.