The igneous rocks are relatively resistant to erosion and form the main scarp and two summits. A quartz-dolerite sill of probable Permo-Carboniferous age, forming a part of the Midland Valley Sill Complex intrudes the early Carboniferous sedimentary rocks of the Lower Limestone Formation. The peaks of West Lomond and Green Hill are nepheline-basanite intrusions whilst East Lomond is a teschenite/olivine dolerite intrusion and vent agglomerate. A few faults affect the escarpment vertically offsetting the crags on either side. The Coul Fault is a northerly downthrowing fault aligned WNW-ESE running through the range and beneath Ballo Reservoir.