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, homogeneous 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
The Aberllefenni Quarry showing the main working of the Narrow Vein
TypeGroup
Unit ofAbercorris Group
UnderliesGarnedd-Wen Formation
OverliesBroad Vein Mudstone Formation
Thickness400 metres (1,300 ft) to 560 metres (1,840 ft)
Lithology
PrimaryMudstone
OtherSlate
Location
Coordinates52°40′33″N 3°49′23″W
RegionMid Wales
CountryWales
ExtentDinas 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:

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.
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