The BGS Lexicon of Named Rock Units — Result Details

Nod Glas Formation

Computer Code: NOG Preferred Map Code: notEntered
Status Code: Pending Upgrade
Age range: Longvillian Substage (OL) — Onnian Substage (ON)
Lithological Description: Black, pyritous, graptolitic mudstone. Rusty weathering, soft, very well cleaved and coal-like.
Definition of Lower Boundary: As conventionally defined, lower boundary is a conformable transition over 1.5m, from paler, silty mudstones of Ceiswyn Formation (and equivalents), to coal black soft mudstones of Nod Glas Formation. However, elements of Nod Glas lithostratigraphy (anoxic pelagic beds) appear in the underlying Ceiswyn Formation (Craig Hen-gae Member) at least 200-300m below the junction. The Cadnant Shales (=Llanrychwyn Slates) of the Dolgarrog area, considered to be equivalent to the Nod Glas, are likewise up to 400m thick. See also the Supplementary Information Description.
Definition of Upper Boundary: Upper boundary is transitional or possibly a non-sequence locally, from coal black mudstones into pale-bluish grey, bioturbated silty, mudstones of the Broad Vein Formation. Transition occurs over less than 2m but is not exposed.
Thickness: Generally 20m to 30m, but up to 400m recorded in Cadnant Shales of North Wales.
Geographical Limits: Extends discontinuously from the coast of Cardigan Bay in the southwest to the Conwy area in the north.
Parent Unit: Not Applicable (-)
Previous Name(s): Cadnant Shales (*318)
Black Slates Of Dolwyddelan [Obsolete Name And Code: Use NOG] (BSD)
Cadnant Shales Formation [Obsolete Name And Code: See NOG] (CDN)
Tyn-Y-Glyn Mudstone Formation [Obsolete Name And Code: See NOG] (TYG)
Ddwyryd Shales (-3587)
Nod Glas (-3588)
Cadnant Shale Formation (-2959)
Penarwel Shales (-2960)
Llanrhychwyn Slates [Obsolete Name And Code: Use NOG] (LLR)
Llanrychwyn Slates (-1090)
Nod Glas Black Shale (-1091)
Alternative Name(s): Cadnant Shales
Type Section  Nant yr Nod stream section on flank of Cae Afon. Stratotype not defined in detail. Pugh, W J. 1928. 
Howells, M F and Smith, M. 1997. Geology of the country around Snowdon. Memoir of the British Geological Survey, Sheet 119 (England and Wales). (London: The Stationery Office). 104pp. 
Pugh, W J. 1928. The geology of the district around Dinas Mawddwy (Merioneth). Quarterly Journal of the Geological Society of London. Vol.84, 345-381. 
Howells M F, Reedman A J and Leveridge B E. 1985. Geology of the country around Bangor: Explanation for 1:50 000 geological sheet 106 (England and Wales). British Geological Survey Sheet Explanation. [London: HMSO.] 34pp. 
Howells, M F, Leveridge, B E, Evans, C D R and Nutt, M J C, 1981. Classical areas of British Geology. Dolgarrog. Description of 1:25 000 Sheet SH76. London: HMSO for Institute of Geological Sciences. 
Lockley, M G. 1980. The geology of the Llanuwchllyn to Llanmawddwy area, south Gwynedd, North Wales. Geological Journal, Vol.15, 21-41. 
Mately, C A. 1938. The geology of the country around Pwllheli, Llanbedrog and Madryn, southwest Carnarvonshire. Quarterly Journal of the Geological Society of London, Vol.94. 555-606. 
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, 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 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E149 E136 E106 E119 E094 E120 E137 E134