The BGS Lexicon of Named Rock Units — Result Details

Lewes Nodular Chalk Formation

Computer Code: LECH Preferred Map Code: LeCk
Status Code: Full
Age range: Turonian Age (KT) — Coniacian Age (KO)
Lithological Description: Composed of hard to very hard nodular chalks and hardgrounds (which resist scratching by finger-nail) with interbedded soft to medium hard chalks (some grainy) and marls; some griotte chalks. The softer chalks become more abundant towards the top. Nodular chalks are typically lumpy and iron-stained (usually marking sponges). Brash is rough and flaggy or rubbly, and tends to be dirty. First regular seams of nodular flint, some large, commence near the base and continue throughout.
Definition of Lower Boundary: The lower boundary is conformable with the underlying smooth white blocky chalks of the New Pit Chalk Formation at the base of Glynde Marl 1 in Sussex, but one of the higher marls elsewhere, although invariably in the interval Glynde Marls to Southerham Marls in the Southern Province. The mapping boundary is placed at the appearance of nodular chalks and significant flint development within that range of marls. In the "Transitional" Province the formation is condensed in response to the later inception of nodularity and hardground development, such that the lower boundary is diachronous here and placed below the "Chalk Rock Member" at the Reed Marl (the lateral equivalent of the Bridgewick Marls of the Southern Province).
Definition of Upper Boundary: The upper boundary is conformable at the change from nodular flinty gritty chalks up into the base of the Shoreham Marl 2 (Seaford Chalk Formation), which is equivalent to the East Cliff Marl 2 in Kent and also identified at the Anstey Quarry [TL 395 329] south of Royston, in the "Transitional" Province. In the field, this is the most difficult boundary to place precisely because of the gradual upward transition in predominant lithology, with interbeds of hard chalk in soft chalk and vice versa. The most positive criterion is the incoming of abundant thick-shelled inoceramid debris (Platyceramus) in soft chalks, although this might not be applicable everywhere. The presence of a sequence containing carious flints within the low Seaford Chalk and high Lewes Chalk is a helpful field indicator of the general proximity of the boundary.
Thickness: The formation is thickest in the basinal successions of the Southern Province in east Hampshire and Sussex where it attains c.35 to 60m in the east and up to 80m in the west. The formation thins towards the margins of the outcrop and over synsedimentary structural highs.
Geographical Limits: The formation is known throughout the Southern Province, within the Chilterns and northward into East Anglia in the "Transitional" Province.
Parent Unit: White Chalk Subgroup (WHCK)
Previous Name(s): Lewes Nodular Chalk Member (-1018)
Lewes Nodular Chalk [Obsolete Code: Use LECH] (LENO)
Lewes Nodular And Flinty Chalk Member (-2286)
Akers Steps And St Margarets Members (-4756)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  The Aston Rowant (Stokenchurch) M40 cutting [SU 728 965 to 740 965] Oxfordshire. The whole succession is visible in the cutting. See Mortimore ety al. (2001, p.334, Figure 4.18). 
Reference Section  Compton Bay, Isle of White. The full steeply northward dipping succession is exposed in the cliffs between Compton Bay and Freshwater. Access is only at suitable states of the tide. 
Partial Type Section  Navigation Pit, Lewes, Sussex. Base of Lewes Marl to the top of the Cliff Hardground of Mortimore (1986). 
Reference Section  Akers Steps, near Dover, Kent. The section is used as the standard reference section for the foraminiferal biostratigraphy of the Turonian Stage in the UK and a full succession of the formation (see Robinson, 1986, where the formation is now known to be equivalent to his Akers Steps Member and the St Margaret's Member of his Dover Chalk Formation and Ramsgate Chalk Formation respectively). 
Reference Section  Langdon Stairs, near Dover, Kent. see Mortimore et al. (2001, p.274, figure 3.123). 
Reference Section  White Nothe, Dorset. The full succession, here about 39m thick, is exposed in numerous sections showing parts of the succession within the cliffs and correlatable slipped masses. See Mortimore et al. (2001, p. 167, figure 3.52). 
Reference Section  Whitecliff Bay, Isle of Wight. The formation is visible in full within the cliffs from Sandown Bay through to Culver Point. Access is only poissible at low tide. 
Reference Section  Kensworth Chalk Pit [TL 015 197] in Bedfordshire. The quarry faces show 20m of the formation from the base to just above the Top Rock Bed. 
Partial Type Section  Caburn Pit, Lewes, Sussex. Glynde to Kingston Beds of Mortimore (1986). The section includes the basal stratotype. 
Partial Type Section  Beachy Head to Light Point, Sussex. Hope Gap to Shoreham Marls of Mortimore (1986). The section includes the basal stratotype of the Seaford Chalk Formation. 
Reference Section  Shillingstone Chalk Pit at the foot of Shillingstone Hill. See Bristow et al. (1995). The pit exposes 35m of the full formation from a basal Okeford Marl (Glynde marl equivalent) up to a significant flint (Shillingstone Flint) and a further succession of nodular chalks up to a prominent 10cm marl (Shoreham Marl). 
Reference(s):
Mortimore, R N. 1986. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists' Association, Vol.97(2), 97-139. 
Mortimore, R N. 1987. Upper Cretaceous Chalk in the North and South Downs: a correlation. Proceedings of the Geologists' Association, Vol.98, 77-86. 
Bristow, C R, Mortimore, R N and Wood C J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists' Association, Vol.108(4), 293-315. 
Hopson, P M, Aldiss, D T and Smith, A. 1996. The geology of the country around Hitchin. Memoir of the British Geological Survey, Sheet 221 (England and Wales). 
Mortimore, R N, Wood, C J and Gallois, R W, 2001. British Upper Cretaceous Stratigraphy. Geological Conservation Review Series. No. 23. (Peterborough: Joint Nature Conservation Committee.) 
Robinson, N D. 1986. Lithostratigraphy of the Chalk Group of the North Downs, south-east England. Proceedings of the Geologists' Association, Vol.97(2), 141-170. 
Bristow, C R. 1991. Geology of Sheet ST80NW and Sheet ST80SW (Turnworth-Milton Abbas, Dorset). British Geological Survey Technical Report, WA/92/20. 
Bristow, C R, Barton, C M, Freshney, E C, Wood, C J, Evans, D J, Cox, B M and Woods, M A. 1999. The Wincanton district - a concise account of the geology. Memoir of the British Geological Survey, Sheet 297 (England and Wales). 
Hopson, P M. 2005. A stratigraphical framework for the Upper Cretaceous Chalk of England and Scotland, with statements on the Chalk of Northern Ireland and the UK Offshore Sector. British Geological Survey Research Report RR/05/01 102pp. ISBN 0 852725175 
Bristow, C R, Barton, C M, Freshney, E C, Wood, C J, Evans, D J, Cox, B M, Ivimey-Cook, H C, and Taylor, R T. 1995. Geology of the country around Shaftesbury. Memoir of the British Geological Survey, Sheet 313 (England and Wales). 
Bristow, C R, 1989. Geology of the East Stour - Shaftesbury district (Dorset) British Geological Survey Technical Report WA/89/58. 
Rawson, P F, Allen, P M and Gale, A. 2001. A revised lithostratigraphy for the Chalk Group. Geoscientist, Vol.11, p.21. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E268 E281 E300 E313 E314 E327 E341 E342 E343