The Adaminaby Beds were formed from quartz turbidite capped with black shale. It consists mainly of fine to medium grained quartz rich sandstone and some mostly thin beds of siltstone, shale and slate. The sediments occur in a band on the west side of the ACT on Bulls Head, Mount Franklin, Mount Ginini and along the upper parts of the Cotter River. A separate outcrop extends into the ACT from the south in the area of the Gudgenby River. Yet another outcrop is on the Bullen Range.