A narrow outcrop of Carboniferous Limestone extends east-west across the southern part of the park from Blorenge in the east through Gilwern Hill, Mynydd Llangatwg, Mynydd Llangynidr, Cefn yr Ystrad, Cefn Cil Sanws and Moel Penderyn to Dinas Rock. West of here its outcrop is offset to the north by the Neath Disturbance as it wraps around the north of Waterfall Country through the Ogof Ffynnon Ddu national nature reserve to Cribarth and the Black Mountains. Both the thickness and details of the sequence vary from east to west with dolomites and palaeokarsts more common in the east.