It lies on the Yilgarn Craton, an ancient block of crystalline rock, which was uplifted in the Tertiary and dissected by rivers.

[4] In the south and west (the Katanning subregion), streams are mostly perennial, and feed rivers which drain westwards to empty into the Indian Ocean.

Here streams, which are remnants of ancient drainage systems, flow only during wet years, and drain to chains of salt lakes.

Mixed eucalypt woodlands with salmon gum (Eucalyptus salmonophloia) and rock sheoak (Allocasuarina huegeliana), and woodlands of jam (Acacia acuminata) and York gum (Eucalyptus loxophleba), are found on granite-derived soils and the alluvial soils of plains and stream valleys.

The hills are home to remnant woodlands of salmon gum, York gum, gimlet (Eucalyptus salubris), and silver mallet (Eucalyptus falcata), and low forest of jam (Acacia acuminata), plant communities were once widespread but now rare in the Wheatbelt.

[5][4] Toolibin Lake is an important breeding area for waterbirds in the inland drainage systems of south-western Australia, particularly freckled duck (Stictonetta naevosa).