The BGS Lexicon of Named Rock Units — Result Details

Rhobell Volcanic Group

Computer Code: RHG Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Tremadoc Series (O1) — Tremadoc Series (O1)
Lithological Description: Basalt with flow autobreccia and epiclastic breccias; laharic flows at base; rare fluviatile intercalations.
Definition of Lower Boundary: Drawn at the base of the Rhobell Volcanic Group where mixed compound and multiple basaltic flows unconformably rest upon banded dark grey mudstone of the Dol-Cyn-Afon Member.
Definition of Upper Boundary: Drawn at the conformable upward passage of the Rhobell Volcanic Group where it is unconformably overlain by grey volcanic sandstone with interbedded black siltstone and sandstone with quartzose sandstone of the Allt Lwyd Formation.
Thickness: 960 metres approx.
Geographical Limits: The area from Nannau-Uwch-Afon in the south to Rhobell-y-big in the north.
Parent Unit: Gwynedd Supergroup (GWYN)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Area  All the volcanic and volcaniclastic rocks exposed on the south eastern side of the Harlech Dome. 
Allen, P M and Jackson, A A. 1985. Geology of the country around Harlech. Memoir of the British Geological Survey, Sheet 135 with part of Sheet 149 [England and Wales]. [London: HMSO.] 112pp. 
Kokelaar, B P, Not published. The Igneous history of the Rhobell Fawr area, Merionethshire, north Wales. Ph.D. Thesis, University of Wales, Aberystwyth. 1977. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E149 E135