Fan Bwlch Chwyth is a peak in the Fforest Fawr section of the Brecon Beacons National Park and within the county of Powys.The hill is formed from sandstones and mudstones of the Senni Beds Formation and Brownstones Formation of the Old Red Sandstone laid down during the Devonian period. A north-northwest - south-southeast trending fault runs across its north-eastern face.
This face is much the steepest and has its origins in the ice ages when a minor glacier grew at this spot. It has left a couple of arcuate moraines which are readily seen from the track. The glaciated rock face has been quarried. The quarry is designated as a site of special scientific interest for its geological interest.