Mynydd Bach Trecastell

It lies within the Black Mountain range of the Brecon Beacons National Park and Fforest Fawr Geopark.

[1] The summit of the hill is formed in sandstones and siltstones of the Cae'r Mynach Formation which like all of the rocks are tilted steeply to the southeast as part of a geological structure called the Myddfai Steep Belt.

To the northwest are the various sandstones, mudstones and siltstones of the Aberedw, Hafod Fawr, Cwm Graig Ddu and Halfway Farm Formations.

A series of northwest to southeast aligned faults cut through this succession and give rise to minor features in the landscape.

[2] There are several round barrows and a stone circle on Mynydd Bach Trecastell dating to Bronze Age Britain.

On the slopes of Mynydd Bach Trecastell
Y Pigwm, Trecastle, Breconshire. Plan of the Roman Marching Camps at Y Pigwm [The Beacon], drawn by William Rees in 1854