The BGS Lexicon of Named Rock Units — Result Details

Ffernant Formation

Computer Code: FANT Preferred Map Code: notEntered
Status Code: Full
Age range: Chadian Substage (CI) — Arundian Substage (CJ)
Lithological Description: Red,purple and variegated silty mudstones, siltstones and sandstones with lenticular bodies of conglomerate. Breccias dominantly composed of weathered Lower Palaezoic mudstone clasts occur at base. The silty mudstones locally display glaebular calcrete profiles including abundant calcareous nodules and thin units of nodular locally dolomitised limestone. The formation records the accumulation, within palaeovalleys and embayments, of screes and alluvial sediments during the initial phases of Dinantian transgression, prior to the establishment of carbonate platform conditions of the Clwyd Limestone Group.
Definition of Lower Boundary: Rests unconformably on cleaved and folded Silirian rocks (Elwy Formation).
Definition of Upper Boundary: Sharp, erosive contact with limestones at the base of the Llanarmon Limestone Formation (Clwyd Limestone Group).
Thickness: To 330m
Geographical Limits: West side of the Vale of Clwyd (SH 8976) and in the Dyserth area, (SJ 0678) North Wales.
Parent Unit: Not Applicable (-)
Previous Name(s): Basement Beds Formation [Obsolete Name And Code: See Supplementary Information] (BED)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Partial Type Section  Ffernant Dingle, 400m southeast of Ffernant formally exposed the unconformable contact with Silurian rocks [at SH 8937 7520] and still provides a discontinuous section through the lower part of the formation. Warren et al., 1984 
Reference(s):
Morton, G H 1876-8. The Carboniferous Limestone and Millstone Grit of North Wales. Proceedings of the Liverpool Geological Society. Vol.3, 190-201. 
Strahan, A and Walker, A O, 1879. On the occurrence of pebbles with Upper Ludlow fossils in the Lower Carboniferous conglomerates of North Wales. Quarterly Journal of the Geological Society of London, Vol.35, 268-274. 
Wedd, C B, Smith, B and Wills, L J. 1927. The geology of the country around Wrexham, Part 1. Lower Palaeozoic and Lower Carboniferous rocks. Memoir of the Geological Survey of Great Britain. [London: HMSO.] 179pp. 
Warren, P T, Price, D, Nutt, M J C and Smith, E G. 1984. Geology of the country around Rhyl and Denbigh. Memoir for 1:50 000 geological sheets 95 and 107 and parts of sheets 94 and 106. 
Waters, C N, Waters, R A, Barclay, W J, and Davies, J R. 2009. Lithostratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/09/01. 184pp. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable