Rhigos lies at the apex of the Cynon Valley and the Vale of Neath and Rhigos Mountain, Craig y llyn, is the highest peak in the Glamorgan. The Llyn Fawr lies beneath the Pennant Sandstone scarp of Craig y Llyn which is evidence of the effect of the Ice Age on the landscape. The glaciers sculpted and overdeepened the valleys into characteristic U-shaped profiles, and most of the north Wales examples, such as the Llanberis, Nant Ffrancon and Conwy valleys, display successive rock basins in their longitudinal profile. The long, southerly directed, dip-slope feature of the Fans and Brecon Beacons fed ice across the uplands and valleys of the coalfield, although a small ice cap across Craig y Llyn fed ice into the Neath and Cynon valleys on its north side. All the deposits of the Welsh ice were locally derived.