Cefn Cul is formed from sandstones and mudstones of the Senni Beds and Brownstones Formations of the Old Red Sandstone laid down during the Devonian period. Whilst the regional dip of the rock beds is to the south, locally they vary and a broad anticlinal structure coincides with the hill. It is within the area designated in 2005 as Fforest Fawr Geopark.