The BGS Lexicon of Named Rock Units — Result Details

Swinley Clay Member

Computer Code: SWCL Preferred Map Code: notEntered
Status Code: Full
Age range: Ypresian Age (GY) — Lutetian Age (GL)
Lithological Description: The Swinley Clay Member consists of several metres thickness of organic laminated clay, which is white, grey, pale purple, brown or yellow in colour, with flasers and laminae of fine-grained sand and silt. In the BGS Bracknell Borehole, the Swinley Clay comprises about 4m of dominantly dark grey to brown organic-rich (lignitic) laminated clays with laminae of fine sand and thin beds of medium-grained sand. Plant debris, including lignite, is common and glauconite is almost absent. The top of the unit has been bleached and oxidised, and has root traces. It appears to represent a soil horizon (King, unpublished report to BGS, 1996).
Definition of Lower Boundary: The base of the Swinley Clay Formation is a sharply-defined, probably erosional contact with the underlying non-glauconitic sands of the typical Bagshot Formation (Ellison et al., 2002).
Definition of Upper Boundary: The top of the Swinley Clay Member is an interburrowed and brecciated surface with root traces, taken to represent an eroded emergent surface, overlain by glauconitic sands, silts and clays of the Windlesham Formation.
Thickness: Up to 8m in the Guildford district (Ellison et al., 2002). The unit varies regionally in thickness, due to its highly erosional basal contact.
Geographical Limits: The Swinley Clay Member occurs in the western part of the London Basin.
Parent Unit: Bagshot Formation (BGS)
Previous Name(s): 'Lower Unit' Of The Middle Bagshot Beds (-5095)
Alternative Name(s): Swinley Member
Type Section  The type section is between 60.05 and 63.66m deep in the BGS Mytchett (Farnborough) Borehole 2 (SU85NE185). (Ellison et al., 2002, fig. 6). 
Reference Section  The BGS Bracknell Borehole (SU86NE42) between about 28.1 and 32.27m. (Ellison and Williamson, 1999, fig. 6). 
Dewey, H, and Bromehead, C E N. 1915. The geology of the country around Windsor and Chertsey. Memoir of the Geological Survey of Great Britain, Sheet 269 (England and Wales). 
Irving, A. 1885. General section of the Bagshot strata, from Aldershot to Wokingham. Quarterly Journal of the Geological Society, Vol. 41, 492-510. 
Islam, M A. 1983. Dinoflagellate cyst taxonomy and biostratigraphy of the Eocene Bracklesham Group in southern England. Micropaleontology, Vol. 29, 328-353. 
Ellison, R A and Williamson, I T. 1999. Geology of the Windsor and Bracknell district - a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 269 Windsor (England and Wales). 
Prestwich, J, 1847. On the main points of structure and the probable age of the Bagshot Sands and on their presumed equivalents in Hampshire and France. Quarterly Journal of the Geological Society, Vol.3, p.378-409. 
Aldiss, D.T., 2012. The stratigraphical framework for the Palaeogene successions of the London Basin, UK. British Geological Survey Open Report OR/12/004. Available from 
King, C. in prep. A revised correlation of Palaeogene and Neogene deposits in the British Isles. Geological Society of London Special Report. 
King, C, 1981. The stratigraphy of the London Clay and associated deposits. Tertiary Research Special Paper No.6. (Backhuys: Rotterdam). 
Ellison, R.A., Williamson, I.T., Humpage, A., 2002. Geology of the Guildford district - a brief explanation of the geological map. Sheet Explanation of the British Geological Survey 1:50 000 Sheet 285 Guildford (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used: