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The Nod Glas Formation is an Ordovician lithostratigraphic group in Mid Wales. The rock of the formation is made up of pyritous, graptolitic mudstone that is generally black in colour. It weathers to a soft, very well cleaved and coal-like material. The formation runs from Conwy in the north, down to Cardigan Bay in the area around Aberdyfi and Tywyn, though it is not a continuous over this area.
The formation is the topmost of the Caradoc Series in North Wales, and the name refers to all the black shale beds in the area. In South Wales, the equivalent beds of shale are called the Dicranograptus Shales.
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