Small areas of coalfield rocks occur within the park. At the base of the succession is the Farewell Rock, the faulted outcrop of which occurs along the park's southern margin. It forms the lip over which the Nant Llech drops to form Henrhyd Falls at Coelbren. Succeeding parts of the lower Coal Measures extend into the park in the Amman valley and the lower parts of the Giedd and Twrch valleys as at Henllys Vale as well as sections of the Tawe and Pyrddin valleys. Further east Coal Measures sandstones and mudstones form large parts of the upper surfaces of Mynydd Llangatwg and of the hills which now form a part of the Blaenavon Industrial Landscape World Heritage Site including Blorenge.