In common with other peaks of the Brecon Beacons, the upper slopes of Fan y Bîg are formed from sandstones of the Brownstones Formation of the Old Red Sandstone laid down during the Devonian period. The lower slopes of the hill are formed from sandstones and mudstones of the underlying Senni Formation. The rock strata tilt gently southwards. The valleys to the northwest and east nurtured small glaciers during the last ice age. This glacial erosion resulted in the very steep northern face which the mountain possesses.