The BGS Lexicon of Named Rock Units — Result Details

Rhiw Bach Volcanic Formation

Computer Code: RBVO Preferred Map Code: RBV
Status Code: Pending Upgrade
Age range: Llanvirn Series (OE) — Llanvirn Series (OE)
Lithological Description: Felsic ash-flow tuffs, fine-grained dust tuffs and tuffites with interbedded silty mudstones and, locally, conglomerates, as well as intercalated high-level intrusions and extrusions of both dioritic and rhyolitic composition.
Definition of Lower Boundary: Conformably overlies or is intruded into undivided mudstone of the Nant Ffrancon Subgroup immediately above the top of the Allt Lwyd Formation, and passes laterally into them.
Definition of Upper Boundary: Overlain conformably by undivided mudstones of the Nant Ffrancon Subgroup.
Thickness: To 640m
Geographical Limits: Southern Snowdonia and northeast of the Harlech Dome, North Wales.
Parent Unit: Aran Volcanic Group (AVG)
Previous Name(s): Rhiw Bach Formation (-1786)
Alternative Name(s): Serw Formation
Serw Formation [Obsolete Name And Code: See Supplementary Information]
Stratotypes:
Partial Type Section  Rhiw Bach Quarry, ezast of Blaenau Ffestiniog, Snowdonia, North Wales. Howells, M F and Smith, M. 1997. 
Reference(s):
Lynas, B D T, 1973. The Cambrian and Ordovician rocks of the Migneint area, North Wales. Journal of the Geological Society, London, Vol.129, 481-503. 
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. 
Zalasiewicz, J A. 1992. Graptolite dating of Ordovician vulcanicity in the Arenig area, North Wales. Geological Journal, Vol.27, 379-389. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E119