The geology make up is of Ordovician or Tertiary sediments. The rock types are of volcanic origin, dated to the Palaeozoic era. Quaternary basalts, all of volcanic origin, were in the west of the Wombat State Forest. These lands have been cleared for farming. However, Ordovician or sedimentary soils, with poor soil conditions for farming, have remained undisturbed. Since the Pliocene the forest has changed from ash rainforest to a dry sclerophyll forest.