| Narrow Vein Mudstone Formation | |
| Type: | Group |
| Age: | Hirnantian |
| Period: | Ordovician |
| Prilithology: | Mudstone |
| Otherlithology: | Slate |
| Region: | Mid Wales |
| Country: | Wales |
| Unitof: | Abercorris Group |
| Underlies: | Garnedd-Wen Formation |
| Overlies: | Broad Vein Mudstone Formation |
| Thickness: | 400m (1,300feet) to 560m (1,840feet) |
| Extent: | Dinas Mawddwy to Tywyn |
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 400m (1,300feet) and 560m (1,840feet) thick and runs from Dinas Mawddwy south-west to Cardigan Bay at Tywyn.[1]
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
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: