Nod Glas Formation
| Nod Glas Formation | |
|---|---|
| Stratigraphic range: Hirnantian Stage | |
| Type | Formation |
| Underlies | Broad Vein Mudstone Formation |
| Overlies | Ceiswyn Formation |
| Thickness | Typically 20 metres (66 ft) to 30 metres (98 ft), up to 400 metres (1,300 ft) at the northern end |
| Lithology | |
| Primary | Mudstone |
| Location | |
| Coordinates | 53°10′37″N 3°50′27″W / 53.1770°N 3.8407°W |
| Region | Mid Wales |
| Country | Wales |
| Type section | |
| Named for | Adapted from "Blue Mark" a quarryman's term for the strata |
| Named by | W. J. Pugh |
The Nod Glas Formation (also known as the Nod Glas Black Shale, the Cadnant Shale, the Penarwel Mudstones or just Nod Glas) is an Ordovician lithostratigraphic group (a sequence of rock strata) 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.