The hill is formed from both Carboniferous Limestone rocks and the Namurian age Twrch Sandstone (former 'Basal Grit') of the Marros Group (former 'Millstone Grit Series'), also dating from the Carboniferous Period. It lies on the northeast-southwest aligned geological structure known as the Cribarth Disturbance and indeed gives its name to that regionally important structure. The Disturbance is characterised locally by a tight anticline which follows the alignment of the summit ridge and which is accompanied by one or two lesser folds on its northwestern side.