The Uriarra Volcanics consists of dacite lava flows and pyroclastic deposits of tuff. A fine ashstone bed called the Tarpaulin Creek Ashstone Member outcrops in an approximate north–south line and acts as a marker within the volcanics. Tuff and flows above and below the ashstone member contain obvious pink feldspar crystals. The tuff shows bedding, and the flows have banded flow structure. The Cotter Porphyry to the north of the Cotter Dam is actually a dacite flow. There is a limestone lens north of Uriarra Crossing.