The Myddfai Steep Belt is a geological structure which affects rocks of Silurian and Devonian age in mid Wales.
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.
The Steep Belt forms a part of a much longer feature which extends from Pembrokeshire as far as Shropshire and Cheshire and is sometimes referred to as the Pontesford Lineament.