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The Narrow Vein Mudstone Formation is an Ordovician lithostratigraphic group 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 and 560 metres thick and runs from Dinas Mawddwy south-west to Cardigan Bay at Tywyn.
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