The BGS Lexicon of Named Rock Units — Result Details

Ty'r Gawen Mudstone Formation

Computer Code: TYGN Preferred Map Code: notEntered
Status Code: Full
Age range: Caradoc Series (O4) — Caradoc Series (O4)
Lithological Description: Predominantly mudstones with subordinate siltstones and sandstones, locally tuffaceous, with a distinctive phosphatic and ooidal ironstone (the Fron Newydd Member) at its base.
Definition of Lower Boundary: Regionally unconformable on the Allt Lwyd Formation, Offrwm Volcanic Formation and Cregennan Formation, and paraconformable on the Llyn y Gafr Volcanic Formation. Taken at the appearance of ooidal ironstones and phosphatic mudstones above these various formations. Passes laterally into the Pen y gadair Volcanic Formation.
Definition of Upper Boundary: Overlain by ash-flow tuffs of the Craig Cau Formation and, in the extreme west of its occurrence, by mudstones of the Ceiswyn Formation, although the nature of the latter boundary is problematical.
Thickness: To 1000m.
Geographical Limits: Cadair Idris area, North Wales.
Parent Unit: Ogwen Group (OGW)
Previous Name(s): Llyn-Y-Gadair Mudstones (-4945)
Ty'r Gawen Formation (-2509)
Llyn Cau Mudstones (-3740)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Small pits between Llyn y Gadair and Mynydd Moel (for basal Fron Newydd Member). Pratt, Woodhall and Howells, 1995. 
Partial Type Section  Exposures around Ty'r Gawen Farm, 2.5km northeast of Llanegryn. Pratt, Woodhall and Howells, 1995. 
Partial Type Section  Craig Cau, on the south side of Cadair Idris, North Wales. Pratt, Woodhall and Howells, 1995. 
Partial Type Section  Craig-y-llyn, west of Cadair Idris. Pratt, Woodhall and Howells, 1995. 
Partial Type Section  Craig Cwm-llwyd. Pratt, Woodhall and Howells, 1995. 
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 
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. 
Cox, A H and Wells, A K, 1927. The geology of the Dolgelley district, Merionethshire. Proceedings of the Geologists' Association, Vol.38, 265-318. 
Smith, M, Rushton, A W A and Howells, M H. 1995. New litho- and biostratigraphical evidence for a Mid-Ordovician hiatus in southern central Snowdonia, North Wales. Geological Journal, Vol.30, 145-156. 
Cox, A H. 1925. The geology of the Cadair Idris Range, (Merioneth). Quarterly Journal of the Geological Society of London, Vol.81, p.539-592. 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E149 E136 E135