The BGS Lexicon of Named Rock Units — Result Details

Lower Chalk Formation [Remains In Informal Use But Generally Regarded As Obsolete Onshore]

Computer Code: LCK Preferred Map Code: LCk
Status Code: Pending Upgrade
Age range: Cenomanian Age (KE) — Cenomanian Age (KE)
Lithological Description: A grey marly chalk with marl content decreasing upwards. No flint. Comprises a thin basal bed of glauconitic marl (Cambridge Greensand) overlain by more typical Lower Chalk sequence that is usually divided into a lower "Chalk Marl" with rhythmic alternations of chalk and marl, and an upper "Grey Chalk" separated by a distinctive hard band. (Totternhoe Stone).
Definition of Lower Boundary: In Hitchin area, unconformity. Base of Cambridge Greensand resting on Gault Clay.
Definition of Upper Boundary: Sharp contact with overlying Middle Chalk the base of which is marked by Melbourn Rock, a hard nodular chalk. Top of Lower Chalk marked by a thin but distinctive yellowish marl (Plenus Marl).
Thickness: 45-80m in Hitchin area.
Geographical Limits: As defined above it is restricted to East Anglia, but the term can be applied more widely if defined differently. Limits necessarily vague.
Parent Unit: Chalk Group (CK)
Previous Name(s): Chalk, Lower (Now Lower Chalk Formation) (-2893)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  In Hitchin district (221) to show Lower Chalk. Open farmland to the south of Shillington. Aspley End south-south-east for 2.1km. 
Reference Section  In Biggleswade district (204) to show the Upper Junction. Ashwell Quarry. Disused and mainly backfilled quarry - now a private garden on southern side of Ashwell Village 400 metres south east of the main church. (Smith, 1992). 
Reference Section  In Hitchin district (221) to show Lower Junction. Arlesey Brickpit. Active clay pit with changing positions of working faces. To west of mainline railway. 
Reference(s):
Penning, W H and Jukes-Browne, A J, 1881. Geology of the neighbourhood of Cambridge. Memoir of the Geological Survey, Old Series Sheets 51SW and part 51NW (England and Wales). 
Rawson, P F, Curry, D, Dilley, F C, Hancock, J M, Kennedy, W J, Neale, J W, Wood, C J and Worrsam, B C. 1978. A correlation of Cretaceous rocks in the British Isles. Geological Society of London, Special Report No.9. 
Smith, A, 1992. Geology of the Henlow, Stotfold and Ashwell districts. British Geological Survey Technical Report, WA/92/22. 
Jukes-Browne, A J and HILL, W, 1887. On the lower part of the Upper Cretaceous series in west Suffolk and Norfolk. Quarterly Journal of the Geological Society of London, Vol.43, p.544-598. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E221 E220 E204 E266 E282 E284 E298 E299 E327 E129 E314 E283 E237 E064 E072 E160 E146 E145 E161 E162 E132 E148 E130 E173 E175 E187 E188 E189 E190 E205 E206 E207 E222 E238 E239 E240 E252 E253 E254 E255 E256 E257 E258 E259 E268 E267 E269 E270 E271 E272 E273 E274 E281 E343