The southern shores of the bay are largely exposed Lewisian gneiss. Immediately to the west of and east of the Walls Fault the exposures are of Dalriadan gneisses, and to the north and east of the bay the bedrock consists mainly of intrusive Devonian age igneous rocks. These include Muckle Roe granite and granophyre, Mangaster Voe gabbro and Ronas Hill granite.