AbstractThe Silurian rocks of Cwm Graig Ddu consist mostly of blue and grey shales and mudstones, all of Ludlow age, the succession extending from thenilssoniZone of the Lower Ludlow to theHolopellaGrits and Shales at the top of the Upper Ludlow. They are overlain by the Tilestones of the Lower Downtonian. The rocks are exposed along the side of the Brecon road on the western side of the valley, in the cliffs at its head, and in the roadside quarries beyond its head. The succession, generally, closely resembles that of the Builth district, but differs in detail. The Lower Ludlow rocks contain many slumped bands which vary in thickness from several inches to many feet.