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.

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] The formation is exposed in a number of locations in Mid Wales where glacial valleys cut across it.