Claypan, a dense layer in the subsoil with high clay content
They live in the subsoil of blue, grey or red clay soils along stream banks and some south or west facing hills of their remaining habitat which is in Gippsland in Victoria, Australia.
Columnar structure is common in the subsoil of sodium affected soils and soils rich in swelling clays such as the semectites and the kandite Halloysite.
The depth of an STP depends on the depth at which either the bedrock or the sterile subsoil is found.