The hill is formed from multiple layers of Pennant Sandstone with intervening mudstone layers and occasional coal seams. All are tilted to the south and southwest towards the axis of the South Wales Coalfield syncline. Parts of its slopes are mantled by glacial till. The coal seams have been worked extensively in the past. One of the last operations was that at Gleision Colliery under the northwestern side of Mynydd Marchywel.