1. The ancient rocks of the Lachlan Fold Belt, mainly of volcanic and marine origin, occupying the area south of an arc stretching from north of Narromine, through Toongi, Geurie and on to north-east of Dunedoo.
2. Freshwater fluvial deposits of the Sydney and Great Artesian Basins to the north of the ancient rocks.
3. Basalt flows centred around Dubbo and Mendooran.
4. Recent alluvium of the Western Plains and in areas adjacent to present rivers and streams.

Fossils in the older rocks of the Lachlan Fold Belt are all of marine origin. Along the Molong Rise during Ordovician time, lime-secreting algae and corals formed fringing reefs in the shallow water around the volcanic islands. Beds of limestone created from these reefs outcrop in the Wellington area and east of Gollan. Fossils are rare due to the limestone mostly having been recrystallised.

For a more complete description of the Geology of Dubbo, see here: Geology of Dubbo.PDF

Natural History of the Dubbo Area