Ysgyryd Fawr is the most easterly of the Black Mountains in Wales, part of the Brecon Beacons National Park. The name is often anglicised to The Skirrid or Skirrid Fawr, and the mountain is also known as Holy Mountain or Sacred Hill. The spelling Skyrrid is also encountered in older literature. The lower hill of Ysgyryd Fach or 'Little Skirrid' (270m) lies about 4.5 km / 2.5 mi to the south. It is 486 m high and lies just outside Abergavenny , Monmouthshire, about 10 miles from the English border. The Beacons Way passes along the ridge .
Its distinctive shape comprises a long ridge oriented nearly north–south, with a jagged western side resulting from ice age landslips . The upper slopes of the hill are composed of Devonian age sandstones assigned to the Brownstones Formation. These overlie weaker mudstones of the St Maughans Formation - a situation which has contributed to the instability of the hill's steep flanks.