The BGS Lexicon of Named Rock Units — Result Details

Cefn-y-fedw Sandstone Formation

Computer Code: CFS Preferred Map Code: notEntered
Status Code: Full
Age range: Brigantian Substage (CX) — Marsdenian Substage (CZ)
Lithological Description: White and pale grey, fine- to medium-grained, planar- and cross-bedded, quartzose sandstones, pebbly sandstones and thin beds of quartz conglomerate; with units of calcareous sandstone, sandy limestone, chert and siliceous mudstones in lower part; locally thick beds of siltstone and mudstone, and thin, impersistent coal seams. Lithologies are commonly arranged in coarsening upwards cycles. The Formation records fluviodeltaic deposition across the former North Wales Dinantian platform. The cyclic arrangement of lithologies represent progradational sequences formed in response to contemporary movements in marine base level. The sandstones and conglomerates record the advance of delta distributary channels and mouth bars across finer pro-delta and interdistributary mud and silt facies.
Definition of Lower Boundary: Drawn at base of sandstone and conglomerate of Cefn-y-Fedw Sandstone Formation where it rests conformably on sandy limestone of the Minera Formation or Cefn Mawr Limestone Formation. Intertongues on Halkyn Mountain with Pentre Chert Formation.
Definition of Upper Boundary: Highly diachronous, but taken at conformable contacts between sandstone of the Cefn-y-Fedw Sandstone Formation and mudstones of the Bowland Shale Formation. However, the two formations exhibit complex intertonguing.
Thickness: To 585m
Geographical Limits: Northeast Wales between Halkyn Mountain [SJ2270] and Oswestry [SJ2826]. Three sandstone leaves, interpreted to belong to the Cefn-y-fedw Formation, are proved in the Blacon East No.1 Borehole (SJ36NE/23).
Parent Unit: Millstone Grit Group [See Also MIGR] (MG)
Previous Name(s): Dee Bridge Sandstone (DBS)
Cefn-Y-Fedw Sandstone [Obsolete Name and Code: Use CFS] (-1423)
Cefn-Y-Fedw Series [Obsolete Name and Code: Use CFS] (-3918)
Dee Bridge Sandstone (DBS)
Millstone Grit Series [Obsolete Name and Code: Use MORRI, MARR, CFS] (-3919)
Middle Cefn-Y-Fedw Sandstone [Obsolete Name And Code: Part Of CFS] (MCFS)
Cefn-Y-Fedw Sandstone Group (-768)
Millstone Grit [Obsolete Name and Code: Use COS, MG, CFS] (-2071)
Cherty Shale (Llangollen) [Obsolete Name And Code: See CFS] (CTSH)
Halkyn Formation [Obsolete Name And Code: See Revision Data] (HAF)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Area  Crags and stream sections on Ruabon Mountain, west of Wrexham. Wedd et al., 1927. 
Reference(s):
Wedd, C B, and others. 1928. The geology of the country around Wrexham, Part II. Memoir of the Geological Survey of Great Britain. 
Wedd, C B, Smith, B, King, W B R and Wray, D A. 1929. Geology of the country around Oswestry. Memoir of the Geological Survey of Great Britain, Sheet 137. 
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. 
Morton, G H 1876-8. The Carboniferous Limestone and Millstone Grit of North Wales. Proceedings of the Liverpool Geological Society. Vol.3, 190-201. 
Davies, J R, Wilson, D and Williamson, I T. 2004. Geology of the country around Flint. Memoir for 1:50 000 Geological Sheet 108. 
Cornwell, J D. 1987. Gravity anomalies associated with the Cefn-y-Fedw Sandstone Group, northeast Wales and their geological significance. Geological Journal, Vol.22, 261-272. 
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. 
Jackson, J W. 1946. Tylonautilus nodiferus (Armstrong) from the Cefn Fedw Series at Nant-y-Ffrith. Proceedings of the Liverpool Geological Society. Vol.19, 161. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E121 E137 E096