The Myddfai Steep Belt is a

geological structure Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover informatio ...

which affects rocks of

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

and

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, whe ...

age in mid

Wales Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...

. It extends for tens of miles across country from near Carmarthen northeastwards via Mynydd Myddfai,

Mynydd Bach Trecastell Mynydd Bach Trecastell is a hill on the border between the counties of Carmarthenshire and Powys in southwest Wales. It lies within the Black Mountain (range) of the Brecon Beacons National Park and Fforest Fawr Geopark. Its summit is plateau-li ...

and

Mynydd Epynt Mynydd Epynt () is a former community and upland area in Powys, Wales. The Ministry of Defence controversially evicted the community of Mynydd Epynt in 1940, creating what is now the core of the Sennybridge Training Area (SENTA), the largest mil ...

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

to early

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 ''See also Line (geometry)'' A lineament is a linear feature in a landscape which is an expression of an underlying geological structure such as a fault. Typically a lineament will appear as a fault-aligned valley, a series of fault or fold-aligne ...

in aerial and satellite views of Wales. The Steep Belt effectively marks the southeastern edge of the intensely folded region of the Caledonian Orogenic Belt in central Wales. A series of major folds and faults affect the Ordovician and Silurian rocks to its northwest whilst folding and faulting of the Devonian rocks to its southeast is much more subdued. The Steep Belt forms a part of a much longer feature which extends from

Pembrokeshire Pembrokeshire ( ; cy, Sir Benfro ) is a Local government in Wales#Principal areas, county in the South West Wales, south-west of Wales. It is bordered by Carmarthenshire to the east, Ceredigion to the northeast, and the rest by sea. The count ...

as far as

Shropshire Shropshire (; alternatively Salop; abbreviated in print only as Shrops; demonym Salopian ) is a landlocked historic county in the West Midlands region of England. It is bordered by Wales to the west and the English counties of Cheshire to th ...

and

Cheshire Cheshire ( ) is a ceremonial and historic county in North West England, bordered by Wales to the west, Merseyside and Greater Manchester to the north, Derbyshire to the east, and Staffordshire and Shropshire to the south. Cheshire's county t ...

and is sometimes referred to as the Pontesford Lineament. The Lineament includes the Pontesford-Linley Fault, the Clun Forest Disturbance and the Red Rock Fault Zone which marks the eastern edge of the Cheshire basin.

References

{{Reflist Geography of Carmarthenshire Geography of Powys Geology of Wales