The Cwm Llwyd Fault is a fault in the west of the Black Mountain of

South Wales South Wales ( ) is a Regions of Wales, loosely defined region of Wales bordered by England to the east and mid Wales to the north. Generally considered to include the Historic counties of Wales, historic counties of Glamorgan and Monmouthshire ( ...

. It runs north, parallel to the A4069 road, for over 4 km from near Brynaman to meet the

Carreg Cennen Disturbance The Carreg Cennen Disturbance is a zone of geological faults and folds in south and mid Wales which forms a part of both the Church Stretton Fault Zone and the Welsh Borderland Fault System. To the southwest it is known as the 'Llandyfaelog Dis ...

near Brest Cwm Llwyd. It moved as a

sinistral Sinistral and dextral, in some scientific fields, are the two types of chirality ("handedness") or relative direction. The terms are derived from the Latin words for "left" (''sinister'') and "right" (''dexter''). Other disciplines use different ...

(left lateral)

strike-slip In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectonic ...

fault during the

Variscan Orogeny The Variscan orogeny, or Hercynian orogeny, was a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica (Laurussia) and Gondwana to form the supercontinent of Pangaea. Nomenclature The name ''Varis ...

. Together with the Llwyn Celyn Fault it formed a left-stepping offset that created a pull apart structure, which preserved the Cwm Llwyd

Outlier In statistics, an outlier is a data point that differs significantly from other observations. An outlier may be due to a variability in the measurement, an indication of novel data, or it may be the result of experimental error; the latter are ...

Namurian The Namurian is a stage in the regional stratigraphy of northwest Europe, with an age between roughly 331 and 319 Ma (million years ago). It is a subdivision of the Carboniferous system or period, as well as the regional Silesian series. The Na ...

rocks.

Description

The Cwm Llwyd Fault is a steep, north-to-south-trending

fracture Fracture is the appearance of a crack or complete separation of an object or material into two or more pieces under the action of stress (mechanics), stress. The fracture of a solid usually occurs due to the development of certain displacemen ...

that slices through the Black Mountain escarpment near Nant Oesglyn in

Carmarthenshire Carmarthenshire (; or informally ') is a Principal areas of Wales, county in the South West Wales, south-west of Wales. The three largest towns are Llanelli, Carmarthen and Ammanford. Carmarthen is the county town and administrative centre. ...

, Wales. It separates older

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

and

Dinantian The Dinantian is a series or epoch from the Lower Carboniferous system in western Europe between 359.2 and 326.4 million years ago. It can stand for a series of rocks in Europe or the time span in which they were deposited. The Dinantian is eq ...

limestones on its east from progressively younger

sandstones and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

s on its west, giving a total vertical mismatch (known to geologists as throw) of roughly 200–225 m where the fault crosses the escarpment. Field mapping shows that

displacement Displacement may refer to: Physical sciences Mathematics and physics *Displacement (geometry), is the difference between the final and initial position of a point trajectory (for instance, the center of mass of a moving object). The actual path ...

increases north-wards, so that below the mountain road the Lower Limestone Shale is dropped against much older

Old Red Sandstone Old Red Sandstone, abbreviated ORS, is an assemblage of rocks in the North Atlantic region largely of Devonian age. It extends in the east across Great Britain, Ireland and Norway, and in the west along the eastern seaboard of North America. It ...

; south-wards, the sense of movement fades out. Because the rocks on either side stand nearly vertical at the fault contact, the structure acts like a wall, and the stream of Nant Oesglyn exploits the resulting line of weakness to carve a narrow

gorge A canyon (; archaic British English spelling: ''cañon''), gorge or chasm, is a deep cleft between escarpments or cliffs resulting from weathering and the erosion, erosive activity of a river over geologic time scales. Rivers have a natural tend ...

. Although many Welsh faults are drawn as simple cracks on maps, detailed work on the Cwm Llwyd Fault reveals it is one strand in a small pull-apart system created by sideways (

) movements during the Variscan mountain-building episode around 300 million years ago. The key clue is a left-stepping overlap between the Cwm Llwyd Fault and the parallel Llwyn Celyn Fault 200 m to the east; where the two overlap, the ground between them dropped out to form a narrow, trench-like hollow (a

graben In geology, a graben () is a depression (geology), depressed block of the Crust (geology), crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German language, German, meaning 'ditch' or 't ...

). This sunken strip now preserves the Cwm Llwyd Outlier—an isolated mass of soft Namurian "Middle Shales". Because the shales are far less resistant than the surrounding sandstones and limestones, they have been whittled into a conspicuous grassy re-entrant in the otherwise craggy Black Mountain face, making the fault zone easy to pick out in the landscape. The fault's importance lies in the insight it gives into the later tectonic history of South-West Wales.

Petrographic Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The classi ...

and

weathering Weathering is the deterioration of rocks, soils and minerals (as well as wood and artificial materials) through contact with water, atmospheric gases, sunlight, and biological organisms. It occurs '' in situ'' (on-site, with little or no move ...

evidence from the outlier suggest the fracture system was re-activated long after the Variscan orogeny—probably during early

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

times when the

North Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for ...

was opening—and this second phase of movement may have freshened an ancient weathering profile now preserved along the fault's eastern wall.

References

{{coord , 51, 50, 21, N, 3, 52, 22, W, type:landmark_region:GB-CMN, display=title Geology of Wales

Description

See also

References