The BGS Lexicon of Named Rock Units — Result Details

Ceiswyn Formation

Computer Code: CSW Preferred Map Code: notEntered
Status Code: Full
Age range: Costonian Substage (OO) — Longvillian Substage (OL)
Lithological Description: Interbedded silty-mudstones and siltstones with subordinate sandstones and tuffs. Anoxic mudstones common in upper part. Disturbables occur throughout formation, but are more common in lower part.
Definition of Lower Boundary: Formation is underlain by volcanic rocks of the Craig-y-Llam Formation. Boundary placed at top of air fall tuffs or bedded tuffaceous mudstones. Abrupt transition from volcanic rocks to mudstones but contact appears conformable.
Definition of Upper Boundary: Formation is overlain by anoxic, black, graptolitic mudstone of the Nod Glas Formation with little or no siltstone or sandstone content. The boundary is conformable/transitional over 1.5m and is marked by the disappearance of mudstone turbidites of the Ceiswyn Formation.
Thickness: Approximately 1400m in type locality area. Exceeds 1550m at western end of outcrop.
Geographical Limits: Occupies the central part of Sheet 149. Passes west into Cardigan Bay and east into a group of distinguishable rock units collectively termed the "Lower Bala Group" (Bassett, and others, 1966. Quarterly Journal of the Geological Society, Vol.122).
Parent Unit: Ogwen Group (OGW)
Previous Name(s): Nant Hir Mudstones [Obsolete Name and Code: Use CSW] (-2128)
Talyllyn Mudstones [Obsolete Name and Code: Use CSW] (-2129)
Allt Ddu Mudstones Formation [Obsolete Name And Code: Upper Part Of CSW] (ADM)
Ceiswyn Beds (-826)
Allt Ddu Mudstone Formation [Obsolete Name and Code: Use CSW] (-3966)
Ceiswin Mudstone [Obsolete Name and Code: Use CSW] (-173)
Nant Hir Mudstone [Obsolete Name And Code: Use CSW] (NHM)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Forestry track section (managed by the Economic Forestry Group, Macchynlleth) at the head of the Ceiswyn Valley. The track section, c.1km in length, displays the lower 700m of the Formation and the lower boundary is exposed. 
Reference Section  The upper part of Formation is well seen in a Forestry Commission track section at Craig Hen-Gae. The contact with the overlying Nod Glas Formation is exposed. 
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 
Bassett, D A, Whittington, H B and Williams, A. 1966. The stratigraphy of the Bala district, Merionethshire. Quarterly Journal of the Geological Society of London, Vol.122, 219-271. 
Rushton, A W A and Howells, M F. 1998. Stratigraphical framework for the Ordovician of Snowdonia and the Lleyn Peninsula. British Geological Survey Research Report, RR/99/08. 
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. 
Jehu, R M. 1926. The geology of the district around Towyn and Abergynolwyn (Merioneth). Quarterly Journal of the Geological Society of London, Vol.82, 465-489. 
Pratt, W T, Scott, M M, Leng, M J. 1990. Geological notes and local details for sheets SH71SE and SH71NE, Cadair Idris. British Geological Survey Technical Report WA/90/41. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E149 E136