Late Pleistocene alluvial sediments (most likely outwash from nearby hills).
Stagnant alluvial plain. This is an old flood plain with now defunct, poorly defined stream channels, levees, flood plains and back swamps.
The soils may be classified into two broad groups: firstly the soils on the slightly higher ground and broad plains, and secondly, the soils in the slightly depressed parts of the plain and depressions.
The soils of the first group are the major component of the Yinnar map unit. They have a depth to subsoil clay which is generally quite deep, typically 40 cm to 60 cm. The surface soils are generally fine sandy loams to silty clay loams which are dark greyish brown or dark brownish grey, often with a sporadically bleached sub-surface. There is a clear to abrupt change to the mottled light to medium clay subsoil with brown and brownish yellow colours dominating. Pale coloured medium to heavy clays continue to at least 1 m depth. These soils are generally classified as Brown and Grey Chromosols using the Australian Soil Classification.
The soils in the slightly depressed parts of the plain and depressions are generally darker and greyer in colour. They are also heavier in texture with surface soils frequently being light clays (these soils are classified as Vertosols). Soil Pit Site GP16 is an example of a Black Vertosol in this mapping unit.
Texture contrast soil in the Yinnar