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.
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.
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'.
The ORS forms the Black Mountain scarp of which the Carmarthenshire section rises to 781m at Fan Foel, the highest point in the county.
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.
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.
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.
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.
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.
Glaciated pavements of Twrch Sandstone characterise the ridge of Garreg Las 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.
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.
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 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.
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.
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.