 Like Mynydd Illtud to its east and Fforest Fach to its west, the hill is formed from sandstones and mudstones of the St. Maughans Formation of the Old Red Sandstone laid down during the Devonian period.Ths sandstones form positive if subdued relief features in the landscape.A number of geological faults run through the area forming a part of the Swansea Fault, itself a part of the Welsh Borderland Fault System.A legacy of the ice which overran the area during the last ice age is the glacial till which covers the lower flanks of Cefn Llechid.