The BGS Lexicon of Named Rock Units — Result Details

Clay-With-Flints Formation

Computer Code: CWF Preferred Map Code: SYMB
Status Code: Full
Age range: Pliocene Epoch [Obsolete concept for epoch] (NO) — Quaternary Period (Q)
Lithological Description: The Clay-with-Flints Formation is a residual deposit formed from the dissolution, decalcification and cryoturbation of bedrock strata of the Chalk Group and Palaeogene formations and, in the extreme west of the outcrop, the Upper Greensand Formation. It is unbedded and heterogenous. The dominant lithology is orange-brown and red-brown sandy clay with abundant nodules and rounded pebbles of flint. Angular flints are derived from the Chalk, and rounded flints, sand and clay from Palaeogene formations. There is commonly a discontinuous basal layer up to 100m thick, with dark brown to black matrix, stiff, waxy and fissured, with relatively fresh flint nodules stained black or dark green with manganese or glauconite. The deposit locally includes bodies of yellow fine- to medium- grained sand, reddish brown clayey silt, and sandy clay with beds of well-rounded flint pebbles, derived from Palaeogene formations. In the extreme west of the outcrop, in Devonshire, the deposit locally comprises sand and clayey sand containing angular, shattered blocks of chert. This is derived wholly from the Upper Greensand Formation, in areas where the Chalk Group has been removed by erosion.
Definition of Lower Boundary: The Clay-with-Flint Formation rests upon Palaeogene formations, Chalk Group or Upper Greensand. Where it rests upon a Palaeogene formation or Upper Greensand, the base is taken at the base of intense cryoturbation. The base may be flat or may be highly uneven where it rests in dissolution hollows, particularly where it rests upon the Chalk. If the material is soliflucted and comes to rest on an inclined surface on a hillside, then it is classed as Head and not as a part of the Clay-with-Flints Formation.
Definition of Upper Boundary: Commonly occurs at surface. Mainly overlain by Middle Quaternary glacial deposits, particularly in Hertfordshire. Locally overlain by Late Quaternary head or fluvial deposits.
Thickness: Up to c 10m, but very variable where let down into dissolution pipes in the Chalk Pipes and hollows may be 1 to 50m in diameter.
Geographical Limits: Co-incidental with the outcrop of the Chalk Group, along the Chiltern Hills as far as Letchworth (Hertfordshire), the North Downs as far as Dover, the South Downs as far as Eastbourne, westwards to Sidmouth (Devon), with extension across the Upper Greensand outcrop of Devon and Somerset as far as the Blackdown Hills above Wellington, Somerset.
Parent Unit: Residual Deposits Group (RESID)
Previous Name(s): CLAY-WITH-FLINTS (CWF)
Alternative Name(s): none recorded or not applicable
Type Section  Trial pit, since infilled, at Rothamsted, on the interfluve between the rivers Ver and Lea. 
Type Section  Trial pit, since infilled, at Soberton, on the South Downs in Hampshire. 
Klinck, B A, Hopson, P M, Lewis, M A, Macdonald, D M J, Inglethorpe, S D J, Entwisle, D C, Harrington, J F and Williams, L. 1998. The hydrogeological behaviour of the clay-with-flints of southern England. British Geological Survey Technical Report WE/97/5. 
Jukes-Brown, A J. 1906. The Clay-with-Flints; its origin and distribution. Quarterly Journal of the Geological Society, Vol.62, 132-164. 
Loveday, J. 1962. Plateau deposits of the Southern Chiltern Hills. Proceedings of the Geologists' Association, Vol.73, 83-102. 
Hodgson, J M, Catt, J A and Wear, A H. 1967. The origin and development of Clay-with-Flints and associated soil horizons on the South Downs. Journal of Soil Science, Vol.18, 85-102. 
Catt, J A and Hodgson, J M. 1976. Soils and geomorphology of the Chalk in SouthEast England. Earth Surface Processes, Vol.1, 181-193. 
Catt, J A. 1986. The nature, origin and geomorphological significance of clay-with-flints. 151-159 in Sieveking, G de G and Hart, M B (editors), The scientific study of flint and chert. Proceedings of the Fourth International Flint Symposium, Brighton, 1983. [Cambridge: Cambridge University Press.] 
Newell, A J, Reeves, H J, Royse, K R, and Aldiss, D T. 2003. Quaternary methods and training: Trial pit characterisation of Clay-with-Flints and Head in the Berkshire Downs. British Geological Survey Internal Report. 
Aldiss, D T, Newell, A J, Marks, R J, Hopson, P M, Farrant, A R, Royse, K R, Aspden, J A, Evans, D J, Smith, N J P, Woods, M A and Wilkinson, I P. 2005. Geology of the Newbury district and part of the Abingdon district. British Geological Survey Sheet Description, Sheet 267 and part of Sheet 253 (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E326 E340 E341 E342 E343 E314 E327 E221 E237 E285 E300 E268