The BGS Lexicon of Named Rock Units — Result Details

Nant-Y-Big Formation

Computer Code: NYB Preferred Map Code: NyB
Status Code: Pending Upgrade
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)
Upper Caered Mudstones (-3592)
Nant Pig Mudstones (-4818)
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. 
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). 
Nicholas, T C. 1915. Geology of the St Tudwal's Peninsula. Quarterly Journal of the Gelogical Society of London, Vol.71, 83-143. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E134