The BGS Lexicon of Named Rock Units — Result Details

Frondderw Tuff Member

Computer Code: FRT Preferred Map Code: FRT
Status Code: Full
Age range: Soudleyan Substage (OY) — Soudleyan Substage (OY)
Lithological Description: Coarse- to fine-grained rhyodacitic lithic tuff and tuffaceous sandstones and siltstones with rare fossils.
Definition of Lower Boundary: Lithic tuff at base conformably overlies the siltstones of the Glyn Gower Siltstone Member, within the Ceiswyn Formation.
Definition of Upper Boundary: Tuffaceous sandstones are conformably overlain by mudstones of the Ceiswyn Formation.
Thickness: 2 metres. Lateral variations 0 - 17 metres.
Geographical Limits: Corwen and Bala districts, North Wales.
Parent Unit: Ceiswyn Formation (CSW)
Previous Name(s): Frondderw Ash [Obsolete Name And Code: See FRT] (FRA)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Disused quarry, 200m northnorthwest of Frondderw House (now Maes Awelon). Bassett et al, 1966. 
Reference(s):
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. 
Campbell, S D G. 1983. The geology of an area between Bala and Betws-y-Coed, North Wales. Unpublished PhD Thesis, University of Cambridge. 
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. 
Elles, G L. 1922. The Bala country: its structure and rock succession. Quarterly Journal of the Geological Society of London, Vol.78, 132-175. 
Schiener, E J. 1970. Sedimentology and petrography of three tuff horizons in the Caradocian sequence of the Bala area (North Wales). Geological Journal, Vol.7, 25-46. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E120 E136