Like the neighbouring Gargunnock Hills they are composed of volcanic rocks, mainly basaltic lavas and tuffs erupted during the Chadian to Asbian substages of the Carboniferous period. The igneous rocks are named as the Clyde Plateau Volcanic Formation, itself a part of the Strathclyde Group. In stratigraphical succession i.e. youngest/uppermost at top, the individual members (subdivisions of a formation) are: