The Narrow Vein Mudstone Formation (commonly known as the Narrow Vein) is an

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

lithostratigraphic Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily igneous ...

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic iden ...

(a sequence of

rock strata In geology and related fields, a stratum (: strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ...

) in

Mid Wales Mid Wales ( or simply ''Y Canolbarth'', meaning "the midlands"), or Central Wales, is a region of Wales, encompassing its midlands, in-between North Wales and South Wales. The Mid Wales Regional Committee of the Senedd covered the unitary autho ...

. The rock of the formation is silty, homogeneous or finely-laminated

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

. It generally a medium blue colour. This formation has been commercially quarried as

slate Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism. It is the finest-grained foliated metamorphic ro ...

in several locations along its length. The formation is between and thick and runs from

Dinas Mawddwy Dinas Mawddwy (; ) is a town in the Community (Wales), community of Mawddwy in south-east Gwynedd, north Wales. It lies within the Snowdonia National Park, but just to the east of the main A470 road, A470, and consequently many visitors pass the ...

south-west to

Cardigan Bay Cardigan Bay () is a large inlet of the Irish Sea, indenting the west coast of Wales between Bardsey Island, Gwynedd in the north, and Strumble Head, Pembrokeshire at its southern end. It is the largest bay in Wales. Geography Cardigan Bay ha ...

Tywyn Tywyn (; ), formerly spelled Towyn, is a town, community, and seaside resort on the Cardigan Bay coast of southern Gwynedd, Wales. It was previously in the historic county of Merionethshire. It is famous as the location of the Cadfan Stone, a ...

Outcrops

The formation is exposed in a number of locations in Mid Wales where glacial valleys cut across it. It is especially visible in the quarries along its length

Commercial quarrying

The Narrow Vein is one of the two major slate veins in Mid Wales that were commercially quarried. Narrow Vein rock is generally more splittable than Broad Vein rock and was often used to make roofing slates. The Narrow Vein was quarried in the following locations: *

Bryn Eglwys Bryn Eglwys quarry was a Slate industry in Wales, slate quarry and mine near Abergynolwyn, in Merionethshire (now part of Gwynedd), Wales. More than 300 men worked at the site, making it the principal employer in the area. Two veins of slate, k ...

where the Broad Vein was also worked, connected to the

Talyllyn Railway The Talyllyn Railway () is a narrow-gauge railway in Wales running for from Tywyn on the Mid-Wales coast to Nant Gwernol railway station, Nant Gwernol near the village of Abergynolwyn. The line was opened in 1865Drummond 2015, page 17 to carr ...

* Gaewern and Braichgoch quarries in Corris Uchaf, connected to the Upper Corris Tramway * Aberllefenni Quarry which worked from the 14th. century until 2002. Also worked the Broad Vein. Connected to the

Corris Railway The Corris Railway () is a narrow gauge railway based in Corris on the border between Merionethshire (now Gwynedd) and Montgomeryshire (now Powys) in Mid-Wales. The line opened in 1859 as a horse tramway, running from quays on the River Dyfi ...

* Cymerau Quarry, connected to the Ratgoed Tramway * Ratgoed quarry, which mainly worked the Narrow Vein, connected to the Ratgoed Tramway * Hendre Ddu quarry a smaller operation connected to the

Hendre-Ddu Tramway The Hendre-Ddu Tramway was a narrow-gauge industrial railway built in 1874 in Mid-Wales to connect the Hendre-Ddu slate quarry to Aberangell station on the Mawddwy Railway. It consisted of a main line long and several branch lines and s ...

Gartheiniog quarry Gartheiniog quarry was a slate quarry served by the Hendre Ddu Tramway. It is located about a mile west of Aberangell in Merioneth (now Gwynedd), North Wales near the confluence of the Nant Maesygamfa and the Afon Angell. It worked the Narrow ...

, also connected to the Hendre-Ddu Tramway * Minllyn quarry connected to the

Mawddwy Railway The Mawddwy Railway was a rural line in the Dyfi Valley in mid-Wales that connected Dinas Mawddwy with a junction at railway station on the Newtown and Machynlleth Railway section of the Cambrian Railways. Despite being only 6 miles 63 cha ...

References

{{Reflist Ordovician System of Europe Upper Ordovician Series Rock formations of Wales Slate industry in Wales