Detailed geological mapping of the Alum Rock area, northwest of Stanthorpe, reveals a tightly folded sequence of probably Lower Carboniferous strata unconformably overlain by a thin sequence of Lower Permian sediments and volcanics. The Permian deposits are downfaulted into the Carboniferous sequence, probably as a result of taphrogenic movement associated with the emplacement of the Stanthorpe Granite.
Two rock units are recognized in the area: the probably Lower Carboniferous Texas Beds and the Lower Permian Alum Rock Beds. The former are typified by sandstones, rhythmically alternating with mudstones; together with minor quantities of intraformational conglomerate and siltstone. The Alum Rock Beds are divided into three informal units : Unit A consisting of sandstones, conglomerates and diamictites; Unit B of ignimbrites, rhyolitic tuffs and porphyritic rhyolites; and Unit C of sandstones, limestones and diamictites.
Two fossil assemblages, containing bivalves, gastropods, brachiopods, bryozoans and fossil wood, are distinguished within the Permian sequence. Six species (of molluscs and brachiopods) are described in detail and four others are discussed briefly. The older and sparser fauna, Fauna A, is probably correlative with the Trigonotreta campbelli Zone (of the eastern Australian Permian); while Fauna B, the younger assemblage, is considered to be representative of the Martiniopsis elongata Zone. Fauna B, a cold, shallow marine biota, has close affinities with faunas from the Cranky Corner and Sydney Basins, New South Wales, and indicates an Early Permian (Sakmarian) age.
The area has been subjected to several phases of deformation. Lower Carboniferous strata were folded and faulted during Late Carboniferous diastrophism (Kanimblan Orogeny). During Middle to Late Permian times block jostling and faulting occurred, presumably associated with the emplacement of various phases of the New England Batholith.