The hills are formed from early Carboniferous sedimentary rocks overlying Devonian sandstones and intruded by sills and volcanic plugs during late Carboniferous and Permian times. The lower ground to the north and west is formed from late Devonian rocks of the Glenvale Sandstone Formation traditionally ascribed to the Old Red Sandstone. Overlying this and forming the lower slopes of the scarp are the early Carboniferous sandstones of the Knox Pulpit and the Kinnesswood formations. Next in succession are the Pathhead Formation rocks which include cycles of sandstones, siltstones, mudstones, ironstones and limestone, formerly referred to as the Calciferous Sandstone Measures. These in turn are overlain by the varied cyclic sequences of the Lower Limestone Formation.