The BGS Lexicon of Named Rock Units — Result Details

Narrow Vein Mudstone Formation

Computer Code: NRV Preferred Map Code: notEntered
Status Code: Full
Age range: Rawtheyan Stage (OW) — Rawtheyan Stage (OW)
Lithological Description: Medium blue, silty mudstone. Homogenous or finely laminated (0.5-1cm).
Definition of Lower Boundary: The lower boundary can be examined at Aberllefenni Quarry, where the contact is transitional, over 0.5m, from the pale-blue of the Broad Vein Mudstone Formation to a medium-blue, laminated silty mudstone of the Narrow Vein Mudstone Formation.
Definition of Upper Boundary: The upper boundary of the Narrow Vein Mudtone Formation is taken at the base of the first sand band seen in an air shaft on Foel Crochan (7706 1050). The boundary is transitional over 2.65m, where sandy bands 1-2cm thick, occur separated by 10-20cm silty mudstones.
Thickness: 20 to 30m
Geographical Limits: Passes west into Cardigan Bay. The outcrop of the Narrow Vein Mudstone Formation crosses Sheet 149 diagonally from southwest to northeast (Tywyn-Aberllefenni).
Parent Unit: Abercorris Group (ARO)
Previous Name(s): Narrow Vein (-4226)
Narrow Vein Formation (-1731)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Collapsed cavern at Bryn-Eglwys Quarry. 
Reference(s):
Pratt, W T, Woodhall, D G and Howells, M F. 1995. Geology of the country around Cadair Idris. Memoir of the British Geological Survey, Sheet 149. (England and Wales). (London: HMSO), 111pp 
Pugh, W J. 1923. The geology of the district around Corris and Aberllefenni, (Merionethshire). Quarterly Journal of the Geological Society, London, Vol.79, 508-541. 
Pratt, W T, 1990. Geological notes and local details for part of 1:10 000 Sheet SH60NE. British Geological Survey Technical Report WA/90/40. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E149