The BGS Lexicon of Named Rock Units — Result Details
Nant-Y-Big Formation
Computer Code: | NYB | Preferred Map Code: | NyB |
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Status Code: | Full | ||
Age range: | St David's Series (ED) — St David's Series (ED) | ||
Lithological Description: | Dark grey and blue-grey siltstones with thin beds of pale sandstone/quartzose siltstone in upper part, and local bentonite horizons. | ||
Definition of Lower Boundary: | Conformably overlies red-brown-green siltstones and subordinate sandstones of the Ceiriad Formation. | ||
Definition of Upper Boundary: | Unconformity defined by irregular erosion surface, commonly with stromatolites, above which lie conglomeratic sandstones of the Maentwrog Formation. | ||
Thickness: | At least 110m | ||
Geographical Limits: | St Tudwal's Peninsula, northwest, Wales. | ||
Parent Unit: | Mawddach Group (MWG) | ||
Previous Name(s): | Nant Pig Mudstone [Obsolete Name And Code: See NYB]
(NPM)
Nant Pig Mudstones [Obsolete Name and Code: Use NYB] (-4818) Upper Caered Mudstones [Obsolete Name and Code: Use NYB] (-3592) |
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Alternative Name(s): | none recorded or not applicable | ||
Stratotypes: | |||
Partial Type Section | Adjacent to faulted contact in Porth Ceiriad. At eastern end of prominent cliff of Pared-mawr, where 110m are exposed in cliff and foreshore. | ||
Partial Type Section | Foreshore, northwest part of Porth Ceiriad, which exposes the basal contact with the underlying Ceiriad Formation. | ||
Reference(s): | |||
Roberts, B and Merriman, R J. 1990. Cambrian and Ordovocian metabentonites and their relevance to the origins of associated mudrocks in the northern sector of the Lower Palaeozoic Welsh marginal basin. Geological Magazine, Vol.127, 31-43. | |||
Nicholas, T C. 1915. Geology of the St Tudwal's Peninsula. Quarterly Journal of the Gelogical Society of London, Vol.71, 83-143. | |||
Young, T P, Gibbons, W and McCarroll, D. 2002. Geology of the country around Pwllheli. Memoir of the British Geological Survey, Sheet 134 (England and Wales). | |||
1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
E134 |