The BGS Lexicon of Named Rock Units — Result Details

Ampthill Clay Formation

Computer Code: AMC Preferred Map Code: AmC
Status Code: Full
Age range: Oxfordian Age (JO) — Oxfordian Age (JO)
Lithological Description: Mudstone, mainly smooth or slightly silty, pale to medium grey with argillaceous limestone (cementstone) nodules; some rhythmic alternations of dark grey mudstone in the lower part; topmost beds are typically pale grey marls with cementstone.
Definition of Lower Boundary: Base of mudstones (as described under Lithology), resting on darker more silty mudstone of the West Walton Formation or on limestone/sandstone of that formation's members (Upware Limestone, Elsworth Rock) or of the Corallian Group; generally a marked colour change and break of slope at outcrop; change commonly well-marked on borehole geophysical (particularly gamma-ray) logs. Base of unit AmC1 in Fenland (Cox and Gallois, 1979).
Definition of Upper Boundary: Upward change to darker mudstone of the Kimmeridge Clay Formation; commonly an erosive non-sequence marked by phosphatic pebbles; generally a marked colour change and break of slope at outcrop; change comonly well-marked on borehole geophysical (particularly gamma-ray) logs. Top of unit AmC42 in Fenland (Cox and Gallois, 1979), but incomplete and succeeded unconformably by Cretaceous rocks in the vicinity of the type section.
Thickness: 0 to c.90m; typically c.25m in Oxon-Bucks., c.10m in the Bedfordshire type section; c.50m in Fenland, c.90m in north Lincolnshire.
Geographical Limits: Oxon/Wilts, where it passes progressively southwestwards into the Corallian Group (Horton et al., 1995; Gallois and Cox, 1994), to the Market Weighton area, where it is overstepped by Cretaceous rocks (Gaunt et al., 1980); also present in the Cleveland Basin (Powell et al., 1992).
Parent Unit: Ancholme Group (AMG)
Previous Name(s): Bluntisham Clay (-3798)
Ampthill Clay (-4426)
Corallian (-4427)
Tetworth Clay (-3164)
Gamlingay Clay (-2563)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Type Section  Ampthill railway cutting. Seeley 1869; Woodward, 1895; Shephard-Thorn et al., 1994. 
Reference Section  North Wootton Borehole (Borehole 23) TF62SW/96. Description of standard succession; Gallois and Cox , 1977; Cox and Gallois, 1979; Gallois 1994. 
Type Area  Ampthill area of Bedfordshire to "Fenland" ie. Boston-Wisbech area adjoining the Wash. Gallois and Cox, 1977. 
Reference Section  British Geological Survey Ampthill Borehole (TL03NW/4). Shephard-Thorn et al., 1994. 
Reference(s):
Seeley, H G. 1861b. On the Fen Clay Formation. Annals and Magazine of Natural History (3), 8, 503-505. 
Seeley, H G. 1867. Remarks on the Potton sands. Annals and Magazine of Natural History (3), 20, 8. 
Woodward, H B. 1895. the Jurassic Rocks of Britain. The Middle and Upper Oolitic rocks of England (Yorkshire excepted). Vol.5. Memoir or the Geological Survey of the United Kingdom. 
Seeley, H G. 1869. Index to the fossil remains of Aves, Ornithosauria and Reptilea from the secondary System of Strata, arranged in the Woodwardian Museum of the University of Cambridge. (Cambridge.) 
Gallois, R W and Cox, B M. 1977. The stratigraphy of the Middle and Upper Oxfordian sediments of Fenland. Proceedings of the Geologists' Association, Vol.88, 207-228. 
Gallois, R W, and Cox, B M. 1994. The Kimmeridge Clay and underlying strata (Upper Jurassic) at Swindon, Wiltshire. Proceedings of the Geologists' Association, Vol.105, 99-110. 
Seeley, H G. 1861a. Notice of the Elsworth and other new rocks in the Oxford Clay, and of the Bluntisham Clay above them. Geologist, Vol.4, 460-461. 
Gaunt, G D, Ivimey-Cook, H C, Penn, I E and Cox, B M. 1980. Mesozoic rocks proved by the Institute of Geological Sciences boreholes in the Humber and Acklam areas. Report of the Institute of Geological Sciences, No. 79/13. 
Powell, J H, Cooper, A H C and Benfield, A C. 1992. Geology of the country around Thirsk. Memoir of the British Geological Survey, Sheet 52 (England and Wales). 
Gallois, R W, 1994. The geology of the country around King's Lynn and The Wash. Memoir of the British Geological Survey, Sheet 145 and part of 129 (England and Wales). 
Shepard-Thorn, E R, Moorlock, B S P, Cox, B M, Allsop, J M and Wood, C J. 1994. Geology of the country around Leighton Buzzard. Memoir of the British Geological Survey, Sheet 220 (England and Wales). 
Cox, B M and Gallois, R W, 1979. Description of the standard stratigraphical sequences of the Upper Kimmeridge Clay, Ampthill Clay and West Walton Beds. 68-72 in Geological investigations for the Wash Water Storage Scheme. Gallois, R W. Report of the Institute of Geological Sciences, No.78/19. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E052 E102 E103 E104 E115 E116 E128 E129 E145 E144 E159 E172 E187 E204 E220 E236 E237 E064 E081 E114 E173 E188 E254 E219