The hill is formed from the acid ash-flow tuffs of the Llyn Conwy Formation of the Ordovician age Aran Volcanic Group. These same erosion-resistant volcanic rocks are responsible for several other hills and mountains in the area. Mudstones of the overlying Nant Hir Formation occur beneath the eastern slopes of the hill. Several intrusions of quartz-latite outcrop around the lower northern and western slopes of Mynydd Nodol though both these and the Nant Hir mudstones are partly obscured by glacial till which mantles the lower ground. There are abandoned manganese mining levels and tips on the hill's northern and western flanks, recorded as being worked intermittently from 1867 for around 40 years.