The oldest bedrock of Kerrera is black slate ascribed to the Easdale Subgroup of the Dalradian Argyll Group. Overlying this across half of the island are conglomeratic sandstones and basalt and augite-andesite lavas of early Devonian age, the former being the local representative of the Old Red Sandstone and the latter constituting part of the ‘Lorne Plateau Lavas'. Ripple marks and sun cracks are preserved in shale strata on the south coast, belying the subaerial environment in which deposition of these sediments took place. There are thin limestones within both the Easdale Slate Formation and the younger sandstones. All of these strata are cut by NW-SE aligned igneous dykes forming part of the North Britain Palaeogene Dyke Suite associated with early Palaeogene volcanism. A number of geological faults cut across Kerrera, the majority are aligned NE-SW.