The Myddfai Steep Belt is a geological structure which affects rocks of Silurian and Devonian age in mid Wales. It extends for tens of miles across country from near Carmarthen northeastwards via Mynydd Myddfai, Mynydd Bach Trecastell and Mynydd Epynt to the vicinity of Llangammarch Wells. Within this linear zone, the rock beds have been tilted steeply to the southeast. The folding took place in late Silurian to early Devonian times. This structure gives rise to a series of landscape features along its length, not least the steep northwest-facing scarps of the hills and ranges mentioned above. Indeed, the feature can be seen as a significant lineament in aerial and satellite views of Wales.