The sands of the Erin Formation are generally well sorted and range from fine to medium grained, the former grain size predominating. The sandstones are massive and alternate with subordinate blue-grey arenaceous clays. Bands of siltstone, lignite and porcellanite are common. Numerous references have been made to such sedimentary structures as current-bedding, graded-bedding, cross-bedding and slump structures in descriptions of outcrops.
The thickness of the formation exceeds 4,000 feet in the centre of the Erin Syncline