The hill of Pen-Cerig-calch, reaching to a height of 2,300 feet, on the southern borders of the Black Mountains, lies about 2 miles north of Crickhowell, and about 4 miles from the nearest part of the main Carboniferous outcrop above Llangattwg. It first attracted the attention of Murchison. Since his time, apart from incidental references, it has not been examined in detail, and as it lies just outside the borders of the Geological Survey Sheet 232, it is not included in the description of the succession of that area given by the Survey officers. The recent revision of this memoir, which includes a discussion of the zonal sequence of the Carboniferous along the main outcrop, is completed, structurally, by the following description of the sequence at Pen-Cerig-caich.