The summit and upper slopes of Pen y Gadair Fawr 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, not least the summit section. Peat has accumulated on parts of the hill in the postglacial period, notably north towards Waun Fach.