Geology of Carmarthenshire

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The bedrock geology of Carmarthenshire in west Wales consists largely of Palaeozoic age sedimentary rocks. [1] Unconsolidated (or 'superficial') deposits of Quaternary age in Carmarthenshire include a dissected cover of glacial till, valley floor alluvium and some scattered peat deposits in both upland and lowland settings. There are extensive beach and tidal flat deposits along the Carmarthenshire coast. The exploitation of the county's mineral riches, particularly coal and limestone, was a key part of the local economy through much of the nineteenth and twentieth centuries.

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Precambrian

An occurrence of extrusive igneous rocks of limited extent and of Neoproterozoic age is found to the northwest of Llanybri. Also in the vicinity are some Proterozoic age intrusive microgabbros.

Ordovician

Roughly half of the county is underlain directly by rocks of Ordovician age. Mudstones, siltstones and sandstones of Arenigian, Llanvirnian and Ashgillian age occur in a broad band from the northeast through the centre of the county and west to the Pembrokeshire border. They also extend northwards to the Ceredigion border west of Llanybydder.

Silurian

Silurian sandstones and mudstones of Llandovery age extend into the county from Ceredigion, forming a part of what was traditionally referred to as the 'Central Wales Synclinorium'. They occur too in a narrow strip running NE-SW through Llandovery towards Llandeilo and include the lowermost parts of the Old Red Sandstone. A further occurrence of Silurian strata is in the Whitland area.

Devonian

The Devonian in Wales is more or less synonymous with the Old Red Sandstone though, as noted above, the lowermost part of the 'ORS' is of late Silurian age. A band of country is characterised by the ORS from the Usk Reservoir and Fan Brycheiniog on the Powys border west to Tavernspite on the Pembrokeshire border. The ORS forms the Black Mountain scarp of which the Carmarthenshire section rises to 781m at Fan Foel, the highest point in the county.

Carboniferous

Carboniferous rocks provide the bedrock in the southeast of the county and in its extreme southwestern corner.

Carboniferous Limestone

The various limestones and shales of the Carboniferous Limestone Supergroup form an almost unbroken outcrop from the heart of the Black Mountain west via Llandyfan to the Gwendraeth estuary north of Kidwelly. To the west of Carmarthen Bay, it is seen again at Pendine where it extends west into Pembrokeshire. There are a handful of minor outliers to the north of the main outcrop, the most notable of which from a landscape perspective is that in the Brecon Beacons National Park on which Carreg Cennen Castle is built.

For much of its outcrop, the limestone forms a broken north to northwest facing scarp. Innumerable small workings opened up to provide lime for agricultural and building use are scattered along its length along with a fewer number of large quarries. Being soluble, the limestone hosts a number of cave systems, though most are short. Of particular note is Llygad Llwchwr. Water exiting the system at this resurgence has been dye-tested to sinks in the vicinity of the A4069 road high on the Black Mountain several kilometres to the east. Another major resurgence is that of Ffrydiau Twrch above the upper Afon Twrch. It discharges water that has been traced to sinks northwest of Cefn Carn Fadog and east of Foel Fraith. In the west of the county, a large resurgence on the beach near Ragwen Point discharges water from sinks on the northern margin of the limestone outcrop east of Marros.

Marros Group

The Marros Group is the modern formal name used in South Wales for what was traditionally referred to as the Millstone Grit series. The Twrch Sandstone Formation (traditionally the 'Basal Grit') overlies the limestone unconformably and is in turn overlain by the Bishopston Mudstone Formation (traditionally the 'Middle Shale'). These rocks outcrop in a band immediately south of the limestonebetween the county boundary with Powys at the Afon Twrch in the Black Mountain, west via Carmel to Kidwelly. The outcrop continues to the west of Carmarthen Bay but only a small portion lies within Carmarthenshire. Significantly this is in the vicinity of the hamlet of Marros from which place this part of the geological succession derives its name. The coastal feature of Telpyn Point near the border with Pembrokeshire gives its name to the highest part of the Marros Group succession, the Telpyn Point Sandstone. Glaciated pavements of Twrch Sandstone characterise the ridge of Garreg Las

Coal Measures and Pennant Sandstone

The northwestern corner of the South Wales Coalfield extends into the southeast of the county from Ystradowen in the east to Pembrey in the west. The rocks of the South Wales Coal Measures Group are divided into three formations (lower, middle and upper) and these are in turn overlain by the Pennant Sandstone Formation. All are found within this part of the county, the Pennant Sandstone lying to the south of an uneven line between Cwmgors and Burry Port via Cross Hands.

Geological structure

A series of major faults and folds run across the county eastwards from Pembrokeshire before curving towards the northeast. These reflect the dynamics of the Caledonian Orogeny which marked the collision of the micro-continent of Avalonia (of which the future Wales formed a part) with the former continent of Laurentia to the north. The Carboniferous strata in the southeast are affected by numerous N-S or NNW-SSE aligned normal faults.

Quaternary

The Welsh ice sheet over-ran Carmarthenshire during the last and earlier ice ages whilst Irish Sea ice invaded the county from the west. Rivers have continued to erode valleys into the earlier upland surface both prior to and since the ice age. The sub-surface dissolution of limestone has resulted in extended areas of overlying Marros Group strata foundering, notably to the north and west of Tair Carn Uchaf and to the east of Foel Fraith in the national park section of Carmarthenshire.

Glacial legacy

There is one well-developed glacial cirque in Carmarthenshire, that which contains Llyn y Fan Fach beneath the Black Mountain scarp. A couple of less well-defined cirques lie just to its east at Pwll yr Henllyn and Pant y Bwlch both of which sport depositional features interpreted as cirque moraines though origins as protalus ramparts have also been postulated. Glacio-fluvial deposits are common in the broader parts of the Teifi valley between Lampeter and Llandysul and more patchily down to Newcastle Emlyn. They are also widespread in the Towy valley between Rhandirmwyn and Llandovery and more locally at Carmarthen where parts of the town are built upon them. Glacial till is widely encountered across the county and derives from both the Welsh ice-sheet and Irish Sea ice.

Post-glacial deposits

The floors of the major valleys are floored with alluvium (sand, silt, gravel and pebbles) built up since the last ice age. River terraces are present along the length of the Towy and Loughor valleys in particular. There are broad expanses of beach and tidal flat deposits around the coast and extending up the estuaries of the Taf, Towy and Loughor. Small areas of landslip are locally present along the side of the Teifi valley. There is an isolated slip in Carboniferous strata in Cwm Pedol north of Garnant.

Economic geology

Numerous quarries have exploited the narrow band of Carboniferous Limestone; those in continued use in 2020 include Coygen quarry near Laugharne, Torcoed quarry near Crwbi and Garn Bica quarry near Porthyrhyd, Foelfach quarry near Cynwyl Elfed, Ty Howel quarry near Lampeter, Pennant quarry at Five Roads and Coed Moelion quarry near Pontyberem each exploit sandstones. Igneous and metamorphic rocks are quarried at Garn Wen quarry near Crymych. Sand and gravel are worked at Llwynjack quarry near Llandovery whilst marine sand & gravel is landed at Burry Port.

Coal mining and opencasting for coal in the southeast of the county were formerly key parts of the Carmarthenshire economy though all activity has now ceased.

Rottenstone for use in the metal polishing industry was worked on a small scale during the nineteenth century along the outcrop of the Upper Limestone Shale in the Black Mountain area. Silica sand was worked from open pits beside the A4069 and also in the vicinity of Cefn Carn Fadog a little further east. It was used in the manufacture of refractory bricks until the middle of the twentieth century.

Building stone

Sandstone was quarried for use in buildings and walls at numerous sites around the county, virtually all of which are now abandoned. An Ordovician limestone was exploited at Llandowror at St Clears whilst the D. bifidus beds were worked at Tre-vaughan near Carmarthen. The Llandeilo flags were worked at Llandeilo and Ludlow age sandstones emerged from Cwar Glas beside the Afon Sawdde. The tilestones were quarried for use as roofing material at numerous locations along their very linear outcrop, the numerous workings giving rise to a former name of these rocks - the 'Long Quarry Formation'. Key locations included Mynydd Myddfai quarries and Cilmaenllwyd in what is now the national park [2] and also Pen-y-waen, Gelli-groes and Onen Fawr outside of it. Those opened up in the Old Red Sandstone include Pen-y-back quarry, south of Whitland, Cil-yr-ychen, Cincoed and Llyn y Beddau, southwest of Trap.

A quarry at Felindref near Crwbin worked the Carboniferous Limestone for building stone. Lower Carboniferous sandstones were worked for flags at Pontyates and Tan-y-lan quarries just north of Pontyates. The Twrch Sandstone was worked at Garn Bica, southwest of Trap. Pennant Sandstone was quarried at Dan-y-quarry south of Pontyates, at Craig-capel and nearby Cwm Capel quarries, at Burry Port and at Furnace and Pen-y-fan quarries at Llanelli. Other quarries working upper Carboniferous sandstones included those at Llandybie, and at Nant Melyn and Craig Derlwyn at Brynaman. [3]

Metal ores

Lead and zinc ores were mined on a small scale during the nineteenth century in the area around Myddfai. [4] Lead and zinc ore in the form of galena and sphalerite were also worked at Rhandirmwyn in the upper Towy valley. Both localities may be considered as southerly extensions of the Mid Wales orefield, centred on northeast Ceredigion. It has been suggested that the sulphur-rich mineralising fluids responsible for the formation of these orefields may have been associated with the Acadian Orogeny. Gold has been recovered from mines at Dolaucothi since at least the Iron Age and the industry was developed further by the Romans. The mine enjoyed a new lease of life from the late nineteenth century through until 1940. [5]

Geoconservation

There are a number of SSSIs in the county and numerous RIGS have been identified. The Carmel National Nature Reserve protects the only turlough in Wales. [6] Part of the east of the county falls within the Brecon Beacons National Park, this western part of which is also designated as Fforest Fawr UNESCO Global Geopark. [7]

See also

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Moel Penderyn

Moel Penderyn is a hill on the edge of Penderyn village, in the Brecon Beacons National Park in the county borough of Rhondda Cynon Taff, south Wales. It also appeared on older maps as 'Y Foel Penderyn'. Its summit at 371m is marked by a trig point.

Cribarth

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Y Garn Goch Hill in Carmarthenshire, Wales

Y Garn Goch is a hill in the Brecon Beacons National Park in the east of Carmarthenshire, Wales. The name means the 'red cairn'. It lies near the village of Bethlehem, three miles southwest of Llangadog and four miles east of Llandeilo on the southern side of the broad Towy Valley. It is also commonly known as either Garn Goch or Carn Goch. Current owners and land managers are the Brecon Beacons National Park Authority.

Garreg Las

Garreg Las is a subsidiary summit of Fan Brycheiniog in the Brecon Beacons National Park in Carmarthenshire, southern Wales. Its summit sits towards the northern end of a broad north-south ridge, Esgair Hir, at 635 metres (2,083 ft) above sea level. The hill is sometimes referred to as Twyn Swnd.

Waun Lefrith

Waun Lefrith is a top of Picws Du and is also the westernmost of the Carmarthen Fans or Bannau Sir Gaer, a group of peaks within the Black Mountain of the Brecon Beacons National Park. It lies within Carmarthenshire, Wales. The summit plateau of the mountain reaches a height of 2221 feet above sea level. Picws Du and Fan Foel are the other, higher summits of the Bannau Sir Gaer / Carmarthen Fans. The glacial lake of Llyn y Fan Fach dominates the panorama to the north of the peak. Beyond the lake to the north lies the Usk Reservoir and then the Cambrian Mountains on the horizon. Swansea Bay and the Bristol Channel are visible to the south across the undulating dip slope of the mountain. The Tywi valley lies to the west, with Llandovery and Llandeilo as important market towns nearest to the hills.

Picws Du

Picws Du is the second highest peak of the Carmarthen Fans in the Carmarthenshire section of the Black Mountain in the west of the Brecon Beacons National Park in south Wales. The highest peak is Fan Foel immediately next along the ridge and it is a subsidiary summit of Fan Brycheiniog. Picws Du falls within Fforest Fawr Geopark and its prominent summit is marked by a large Bronze Age round barrow at a height of 2457 feet above sea level. Waun Lefrith is the other, lower summit of the Bannau Sir Gâr / Carmarthen Fans situated to the west. The peak overlooks the glacial lake of Llyn y Fan Fach in the cwm below. As the peak sits on the edge of the escarpment on a ridge which juts out into the valley below, the views from the summit are panoramic and extensive. The views to the north are especially impressive when the weather is clear, looking towards the Cambrian Mountains, Mynydd Epynt and Brecon. Swansea and the Bristol Channel can just be seen on the horizon to the south, across the gently falling dip slope. Pen y Fan and Corn Du are distinctive landmarks seen directly to the east across Fforest Fawr.

Foel Fraith

Foel Fraith is a hill in the Black Mountain in the county of Carmarthenshire, southwest Wales. It lies within the Brecon Beacons National Park and Fforest Fawr Geopark. Its plateau-like summit attains a height of 602 metres (1,975 ft) above sea level. To the north the subsidiary summit of Cefn y Cylchau reaches 556 feet (169 m), and to the southeast is the subsidiary summit of Carn Fadog or Cefn Carn Fadog, which reaches a height of 512 metres (1,680 ft).

Cefn Cil Sanws

Cefn Cil Sanws is a hill in the Brecon Beacons National Park within the county borough of Merthyr Tydfil in south Wales. The summit at 460m above sea level is crowned by a trig point. The steep cliffs of Darren Fawr and Darren Fach defend its western side which drops down into Cwm Taf. A major limestone quarry is worked on the southeastern side of the hill. An unexpected feature at this altitude is Merthyr Tydfil Golf Course which extends high onto the eastern and southern slopes of the hill.

Mynydd-y-glog

Mynydd-y-glog is a hill just east of the village of Penderyn in the northern corner of the county borough of Rhondda Cynon Taff in south Wales. It lies within the Brecon Beacons National Park and Fforest Fawr Geopark. Its summit plateau rises to a height of 389m / 1277 ft at OS grid ref SN 982088, a spot marked by a trig point. To its north lies the partly dry valley of Pant Sychbant which forms the eastern extension of Cwm Cadlan. Its southern slopes are drained by various streams which feed into the Afon Cynon.

Carreg Dwfn

Carreg Dwfn is a hill four miles southeast of Llandeilo and just to the south of the village of Trap in the county of Carmarthenshire, south Wales. It lies at the western extremity of the Brecon Beacons National Park and Fforest Fawr Geopark. Its summit at 283 metres (928 ft) is towards the eastern end of a rocky east-west-aligned ridge.

Marros Group

The Marros Group is the name given to a suite of rocks of Namurian age laid down during the Carboniferous Period in South Wales. These rocks were formerly known as the Millstone Grit Series but are now distinguished from the similar but geographically separate rock sequences of the Pennines and Peak District of northern England and northeast Wales by this new name.

The geology of Monmouthshire in southeast Wales largely consists of a thick series of sedimentary rocks of different types originating in the Silurian, Devonian, Carboniferous, Triassic and Jurassic periods.

This article describes the geology of the Brecon Beacons National Park in mid/south Wales. The area gained national park status in 1957 with the designated area of 1,344 km2 (519 sq mi) including mountain massifs to both the east and west of the Brecon Beacons proper. The geology of the national park consists of a thick succession of sedimentary rocks laid down from the late Ordovician through the Silurian and Devonian to the late Carboniferous period. The rock sequence most closely associated with the park is the Old Red Sandstone from which most of its mountains are formed. The older parts of the succession, in the northwest, were folded and faulted during the Caledonian orogeny. Further faulting and folding, particularly in the south of the park is associated with the Variscan orogeny.

Mynydd Garnclochdy is a gentle summit on the long moorland ridge which extends south from Blorenge towards Pontypool and which defines the boundary between the modern county of Monmouthshire to the east and the county borough of Torfaen to the west though historically it was entirely within the traditional county of Monmouthshire. Its summit, at a height of 448 m (1407 ft), is marked by a small cairn. The summit and the eastern slopes of the hill fall within the Brecon Beacons National Park. A southern top of the hill, Mynydd Garn-wen achieves a height of 436m, and carries a trig point further south again at an elevation of 425m. To the north the broad moorland ridge runs via a poorly defined 425m top and a col at 404m elevation just south of a minor east–west road, into Mynydd y Garn-fawr, east of Blaenavon.

The geology of the Gower Peninsula in South Wales is central to its character and to its appeal to visitors. The peninsula is formed almost entirely from a faulted and folded sequence of Carboniferous rocks though both the earlier Old Red Sandstone and later New Red Sandstone are also present. Gower lay on the southern margin of the last ice sheet and has been a focus of interest for researchers and students in that respect too. Cave development and the use of some for early human occupation is a further significant aspect of the peninsula's scientific and cultural interest.

References

  1. ↑ British Geological Survey 1:625,000 scale geological map Bedrock Geology UK South 5th Edn. NERC 2007
  2. ↑ Harlow, Dilys (2014). The Land of the Beacons Way:Scenery and geology across the Brecon Beacons National Park (First ed.). Cardiff: Geologists Association - South Wales Group. pp. 106–107. ISBN   9780903222020.
  3. ↑ "Geoindex Onshore". British Geological Survey. Retrieved 31 October 2020.
  4. ↑ Schofield, David (2009). Geology of the Llandovery District. Keyworth, Nottingham: British Geological Survey. p. 29. ISBN   9780852726334.
  5. ↑ Davies, J.R. (2006). Geology of the Lampeter District. Keyworth, Nottingham: British Geological Survey. p. 21. ISBN   9780852725658.
  6. ↑ "Carmel Woods National Nature Reserve, Llandeilo, Carmarthenshire, South Wales". First Nature. Retrieved 10 April 2016.
  7. ↑ "About". Fforest Fawr Geopark. Brecon Beacons National Park Authority. Retrieved 31 October 2020.
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