The BGS Lexicon of Named Rock Units — Result Details

Ffairfach Grit Formation

Computer Code: FFG Preferred Map Code: notEntered
Status Code: Full
Age range: Abereiddian Stage (O2) — Abereiddian Stage (O2)
Lithological Description: The Ffairfach Grits Formation comprises massive, pink-weathering, medium- to coarse-grained arkosic sandstone with scattered, impersistent pebbly horizons with felsic tuff and rhyolite locally developed toward the top of the Formation.
Definition of Lower Boundary: In the type area, the formation rests upon black, micaceous, graptolitic mudstones of the Abergwilli Formation (formerly known as Didymograptus bifidus Beds). The lower boundary is taken at the incoming of beds of indurated, medium-grained arkosic sandstone, which gradually pass up into massive sandstone (Williams, 1952). The contact is not exposed, but a near complete section is preserved in railway cuttings, small quarries, and in the banks of the River Cennen just south of the village of Ffairfach (Ordovician GCR).
Definition of Upper Boundary: The Formation is overlain by fossiliferous, buff weathering, thinly interbedded fine-grained sandstone, silty calcareous mudstone and limestone of the Llandeilo Flags Formation. The contact between the two formations is not exposed and has been argued as being disconformable (Bergstrom et al., 1987). The most intact section across the boundary is preserved in the railway cutting south of Ffairfach, which has also been taken as the type section for the Llandeilo Flags Formation itself (Rushton et al., 1999).
Thickness: To 160m
Geographical Limits: The Ffairfach Grits Formation principally crops out on the southern flank of the Tywi valley, south of the town of Llandeilo in Carmarthenshire. It is preserved in the hinges and on the limbs of a northeast trending series of tectonically disrupted anticlines and synclines between Golden Grove in the southwest and Rhyd-y-saint in the northeast. The Ffaifach Grits also form a thin unit exposed to the north of the Tywi in Dynefor Park.
Parent Unit: Not Available (!)
Previous Name(s): Ffairfach Grit (-2184)
Ffairfach Grit Member (-245)
Ffairfach Group [Obsolete Name And Code: Use FFG] (FFA)
Alternative Name(s): none recorded or not applicable
Partial Type Section  Southwest of the level crossing in the village of Ffairfach. A near complete section is preserved in railway cuttings and scattered small quarries. The top and base are not exposed. Rushton, A W A, Owen, A W, Owens, R M and Prigmore, J K, 1999. 
Williams, A. 1952. The geology of the Llandeilo district, Carmarthenshire. Quarterly Journal of the Geological Society, London, Vol.145, 915-923. 
Strahan, A, Cantrill, T C, Dixon, E E L and Thomas, H H. 1907. Geology of the South Wales Coalfield, Part VII, the country around Ammanford. Memoir of the Geological Survey of England and Wales, Sheet 230 (England and Wales). (London: HMSO.) 
Rushton, A W A, Owen, A W, Owens, R M and Prigmore, J K. 1999. British Cambrian to Ordovician stratigraphy. Geological Conservation Review Series No 18. Joint Nature Conservation Committee. 
Bergstrom, S M, Rhodes, F H T and Lindstrom, M. 1987. Conodont biostratigraphy of the Llanvirn-Llandeilo and Llandeilo-Caradoc series boundaries in the Ordovician System of Wales and the Welsh Borderland. 294-315 in Austin, R (Editor), Conodonts: Investigative techniques and applications. [Chichester: Ellis Horwood.] 
1:50K maps on which the lithostratigraphical unit is found, and map code used: