Stronend, the summit, is not formed from the stratigraphically uppermost member but from the Shelloch Burn lavas since the entire pile is tilted. The northern cliffs and Double Craigs are formed by the Spout of Ballochleam lavas. Substantial amounts of landslipped material are arrayed beneath these cliffs. The volcanic sequence sits upon a plinth of sedimentary rocks which from the low ground to the north and west, together with the lower parts of the north and west facing scarp. These are the Clyde Sandstone Formation (sandstone, siltstone, mudstone) and the underlying Ballaggan Formation (mudstone, sandstone, limestone), both of which form part of the Inverclyde Group of the Carboniferous system. In the vicinity of a dun and southeast to Spittalhill by an intrusion of basalt/microgabbro. Dykes of similar composition intrude the lava sequence, notably on the hills' southern flanks.