The rock of the formation is made up of pyritous, graptolitic mudstone that is generally black in colour.

[4] 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.

[3] Just north of Dinas Mawddwy, at Aber Cywarch the exposed shale beds contain thin layers of limestone.

There is one further outcropping of the formation, on the Llŷn Peninsula which is about 16 metres (52 ft) thick, and is known locally as the Penarwel Mudstones.

[3] Fossils of graptolites, conodonts and trilobites have been found in the Nod Glas Formation near Welshpool.