Pen y Fan is formed from various units of the Old Red Sandstone which were laid down during the Devonian period. The lower slopes of the mountain are formed from the sandstones and mudstones of the Senni Formation whilst the upper slopes are formed from those of the Brownstones Formation. The very summit, like that of neighbouring Corn Du, is composed of hard-wearing sandstones of the Plateau Beds Formation. The beds of sandstone dip gently southwards towards the South Wales Coalfield basin.