The BGS Lexicon of Named Rock Units — Result Details

Ffestiniog Flags Formation

Computer Code: FF Preferred Map Code: Ff
Status Code: Full
Age range: Merioneth Series (EM) — Merioneth Series (EM)
Lithological Description: Regular alternatives of quartzose siltstone and sandstone in beds up to 2m thick, interbedded with silty mudstone; abundant sedimentary structures.
Definition of Lower Boundary: Conformable and transitional, with the Maentwrog Formation. Marked by change in colour from pale to dark grey in mudstones, and an increase in sandstone.
Definition of Upper Boundary: Conformably overlain by mudstones of the Dolgellau Formation. Sharp contact locally.
Thickness: 475 to 1020m
Geographical Limits: Northern, eastern and southern flanks of the Harlech Dome, North Wales.
Parent Unit: Mawddach Group (MWG)
Previous Name(s): Ffestiniog Group (-2182)
Ffestiniog Formation [Obsolete Name And Code: Use FF] (FFB)
Lingula Flags [Obsolete Name And Code: Use LF] (LFS)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Cliffs immediately east and west of Portmeirion. Howells, M F and Smith, M. 1997. 
Type Section  Bryn-Y-Gath, Upper Mawddach Valley. 
Reference Section  Foreshore of Borth-y-gest, Porthmadog. Howells, M F and Smith, M. 1997. 
Reference Section  North of Morfa Bychan. 
Reference(s):
Ramsay, A C. 1866. The geology of North Wales. Memoir of the Geological Survey of Great Britain, Vol.3, 
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. 
Ramsay, A C, Salter, J W and Etheridge, R. 1881. The geology of North Wales. Memoir of the Geological Survey of Great Britain (2nd edition). No 3. [London: HMSO.] 611pp. 
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. 
Belt, T. 1867. On the "Lingula Flags" or "Ffestiniog Group" of the Dolgelly District. Parts I and II. Gelogical Magazine, Vol.4, 493-495, 536-543. 
Belt, T. 1868. On the "Lingula Flags" or "Ffestiniog Group" of the Dolgelly District. Part III. Gelogical Magazine, Vol.5, 5-11. 
Young, T P, Gibbons, W and McCarroll, D. 2002. Geology of the country around Pwllheli. Memoir of the British Geological Survey, Sheet 134 (England and Wales). 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E119 E135 E149 E134