The hills are composed of volcanic rocks formed in the Carboniferous period and which have proved more resistant to erosion than the surrounding sedimentary rocks. The bedrock across the larger part of the area is the basaltic Gargunnock Hills Lava Member. In stratigraphic order (i.e. youngest/uppermost first), the remaining subdivisions of the Clyde Plateau Volcanic Formation which outcrop on the steep northern scarp of the hills, are: