Myddfai Steep Belt
Encyclopedia
The Myddfai Steep Belt is the name given to 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 lineation 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 fauting 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
as far as Shropshire
and Cheshire
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.
Structural geology
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 information about the history of deformation in the rocks, and...
which affects rocks of Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
and Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
age in mid Wales
Wales
Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...
. It extends for tens of miles across country from near Carmarthen
Carmarthen
Carmarthen is a community in, and the county town of, Carmarthenshire, Wales. It is sited on the River Towy north of its mouth at Carmarthen Bay. In 2001, the population was 14,648....
northeastwards via Mynydd Myddfai
Mynydd Myddfai
Mynydd Myddfai is a hill 4.5 miles southeast of Llandovery in the county of Carmarthenshire, southwest Wales. It lies within the Brecon Beacons National Park and Fforest Fawr Geopark. The hill is in the form of a broad ridge aligned southwest to northeast with three or four distinct tops separated...
, Mynydd Bach Trecastell
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 region of the Brecon Beacons National Park and Fforest Fawr Geopark. Its summit is plateau-like and reaches a height of 412m at OS grid ref SN...
and Mynydd Epynt
Mynydd Epynt
Mynydd Epynt is an upland area of Mid Wales within the county of Powys, Wales. It is bounded to the south by the upper section of the valley of the River Usk, to the north by that of the Afon Irfon and to the east by the valley of the River Wye. Its western boundary is less distinct but lies...
to the vicinity of Llangammarch Wells
Llangammarch Wells
Llangammarch Wells is a village in the parish of Llangammarch in Powys, within the historic boundaries of Brecknockshire, mid Wales, lying on the River Irfon....
.
Within this linear zone, the rock beds have been tilted steeply to the southeast. The folding took place in late Silurian
Silurian
The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...
to early Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
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 lineation 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
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
and Silurian rocks to its northwest whilst folding and fauting 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
Pembrokeshire is a county in the south west of Wales. It borders Carmarthenshire to the east and Ceredigion to the north east. The county town is Haverfordwest where Pembrokeshire County Council is headquartered....
as far as Shropshire
Shropshire
Shropshire is a county in the West Midlands region of England. For Eurostat purposes, the county is a NUTS 3 region and is one of four counties or unitary districts that comprise the "Shropshire and Staffordshire" NUTS 2 region. It borders Wales to the west...
and Cheshire
Cheshire
Cheshire is a ceremonial county in North West England. Cheshire's county town is the city of Chester, although its largest town is Warrington. Other major towns include Widnes, Congleton, Crewe, Ellesmere Port, Runcorn, Macclesfield, Winsford, Northwich, and Wilmslow...
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
Wem-Bridgemere-Red Rock Fault System
The Wem-Bridgemere-Red Rock Fault System is a complex zone of intersecting faults which defines the eastern boundary of the Cheshire Basin, a deep sedimentary basin in the northwest of England and extending south into the northern Midlands...
which marks the eastern edge of the Cheshire basin.