Narrow Vein Mudstone Formation

The Narrow Vein Mudstone Formation (commonly known as the Narrow Vein) is an Ordovician lithostratigraphic group (a sequence of rock strata) in Mid Wales. The rock of the formation is silty, homogenous or finely-laminated mudstone. It generally a medium blue colour. This formation has been commercially quarried as slate in several locations along its length. The formation is between 400 metres (1,300 ft) and 560 metres (1,840 ft) thick and runs from Dinas Mawddwy south-west to Cardigan Bay at Tywyn.[1]

Narrow Vein Mudstone Formation Stratigraphic range: Hirnantian Age
The Aberllefenni Quarry showing the main working of the Narrow Vein
Type	Group
Unit of	Abercorris Group
Underlies	Garnedd-Wen Formation
Overlies	Broad Vein Mudstone Formation
Thickness	400 metres (1,300 ft) to 560 metres (1,840 ft)
Lithology
Primary	Mudstone
Other	Slate
Location
Region	Mid Wales
Country	Wales
Extent	Dinas Mawddwy to Tywyn

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.[2]

The Narrow Vein was quarried in the following locations:

• Bryn Eglwys where the Broad Vein was also worked, connected to the Talyllyn Railway
• 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
• 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
• Gartheiniog quarry, also connected to the Hendre-Ddu Tramway

References

1. "Narrow Vein Mudstone Formation". BGS on-line lexicon of rock units. British Geological Survey. Retrieved 11 September 2019.
2. Richards, Alun John (1999). The Slate Regions of North and Mid Wales and Their Railways. Gwasg Carreg Gwalch. ISBN 0-86381-552-9.