| Broad Vein Mudstone Formation | |
| Type: | Group |
| Age: | Hirnantian Age |
| Period: | Ordovician |
| Prilithology: | Mudstone |
| Otherlithology: | Slate |
| Region: | Mid Wales |
| Country: | Wales |
| Unitof: | Abercorris Group |
| Underlies: | Narrow Vein Mudstone Formation |
| Overlies: | Nod Glas Formation |
The Broad Vein Mudstone Formation (commonly known as the Broad Vein, historically known as the Red Vein[1] and in Welsh as Y Faen Goch) is an Ordovician lithostratigraphic group (a sequence of rock strata) in Mid Wales. The rock of the formation is silty mudstone, intensely bioturbated in places. It varies in colour from a pale to a medium blue. 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.[2]
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 Broad Vein is one of the two major slate veins in Mid Wales that were commercially quarried. Broad Vein rock is generally dense, with few natural joints, so most of the commercial use was for slab and products such as mantlepieces, cisterns and (later) electrical switchboards. Production of roofing slates was relatively rare in Broad Vein quarries.[3]
The Broad Vein was quarried in the following locations: