The summit and upper slopes of Waun Fach are formed from the Early Devonian Epoch sandstones of the Brownstones Formation, a division of the Old Red Sandstone. Beneath these and forming the lower slopes are the sandstones of the Senni Formation (traditionally the Senni Beds). Mudstone layers within these sandstones are more readily eroded and have given rise to the stepped appearance of parts of the mountain. No rocks are exposed around its summit as this is covered by a thick layer of peat which has accumulated in the postglacial period, though is much reduced through erosion in recent decades.
