The BGS Lexicon of Named Rock Units — Result Details

Gelli-Grin Calcareous Ash Formation

Computer Code: GEL Preferred Map Code: notEntered
Status Code: Full
Age range: Longvillian Substage (OL) — Woolstonian Substage (OS)
Lithological Description: Calcareous volcanaclastic sandstone, tuffs and tiffites.
Definition of Lower Boundary: Conformably overlies the upper part of the Ceiswyn Formation in the Bala district, but might pass into the latter formation in the Cadair Idris district, where its correlative is the Craig Hen-gae Member (of the Ceiswyn Formation).
Definition of Upper Boundary: Conformably overlain by the black mudstones of the Nod Glas Formation, but also passes laterally into its lower part (Lockley, 1980). Also overlain unconformably by the Rhiwlas Limestone Formation.
Thickness: 60m
Geographical Limits: Bala and Corwen districts, North Wales.
Parent Unit: Ogwen Group (OGW)
Previous Name(s): Gelli-Grin Calcareous Ashes (-924)
Gelli-Grin Calcareous Ash Series (-4674)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Exposures at Gelli-grin. Bassett and others, 1966. 
Type Area  Area about 350m westsouthwest of Gelli Grin Farm. 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. 
Alder, K E, 1976. The geology of an area around Bettws Gwerfil Goch and Melin-y-Wig. Unpublished PhD Thesis, University of Cambridge. 
Elles, G L. 1922. The Bala country: its structure and rock succession. Quarterly Journal of the Geological Society of London, Vol.78, 132-175. 
Lockley, M G. 1980. The geology of the Llanuwchllyn to Llanmawddwy area, south Gwynedd, North Wales. Geological Journal, Vol.15, 21-41. 
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:
E120 E136 E150