The BGS Lexicon of Named Rock Units — Result Details

Aran Volcanic Group

Computer Code: AVG Preferred Map Code: notEntered
Status Code: Full
Age range: Arenig Series (OR) — Caradoc Series (O4)
Lithological Description: A mixed suite of basalts, rhyolites volcaniclastic and sedimentary rocks.
Definition of Lower Boundary: Overlies the Mawddach and Rhobell Volcanic groups, with unconformity in places. The base is drawn at the base of the quartzose sandstones of the Garth Grit Member (Allt Llwyd Formation), where it rests unconformably upon bedded felsic rocks of the Rhobell Volcanic Group.
Definition of Upper Boundary: Drawn at the conformable upward passage of the Craig Cau Formation (Cadair Idris District) into turbiditic mudstones and siltstones o the Ceiswyn Formation. The top of the group is not seen in the Harlech district, where the highest formation represented is the Aran Fawddwy Formation, comprising felsic ash-flow tuffs.
Thickness: Up to 2.5km in the Cadair Idris district.
Geographical Limits: The surrounding area of Harlech known as the Harlech Dome, which includes most of the Snowdonia National Park.
Parent Unit: Not Applicable (-)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  The southern, eastern and northeastern sides of the Harlech Dome. 
Reference(s):
Allen, P M, Jackson, A A and Rushton, A W A. 1981. The stratigraphy of the Mawddach Group in the Cambrian succession of North Wales. Proceedings of the Yorkshire Geological Society, Vol.43, 295-329. 
Allen, P M and Jackson, A A. 1985. Geological Excursions in the Harlech Dome. Classical areas of British Geology. British Geological Survey. 
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 
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. 
Sedgwick, A. 1852. On the classification and nomenclature of the Lower Palaeozoic rocks of England and Wales. Quarterly Journal of the Geological Society of London, Vol.8, 136-168. 
Kokelaar, B P. 1979. Tremadoc Tollanvirn volcanism on the south east side of the Harlech Dome, North Wales. "In" Harris and others. The Caledonides of the British Isles. Special Publication of the Geological Society of London, Vol.8, p.591-596. 
Wells, A K. 1925. The geology of the Rhobell Fawr district (Merioneth). Quarterly Journal of the Geological Society of London, Vol.81, 463-538. 
Andrew, A R. 1910. The geology of the Dolgelly gold belt, North Wales. Geological Magazine, Vol.7, 159-171, 201-211, 261-271. 
Matley, C A and Wilson, T S. 1946. The Harlech Dome, North of the Barmouth Estuary. Quarterly Journal of the Geological Society of London, Vol.102, 1-40. 
Ridgeway, J. 1975. The stratigraphy of Ordovician volcanic rocks around the south and east of the Harlech Dome in Merionethshire, North Wales. Geological Journal, Vol.10, 87-106. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E149 E136 E135 E119 E120