The BGS Lexicon of Named Rock Units — Result Details

Lower Mottled Clay

Computer Code: LMCL Preferred Map Code: notEntered
Status Code: Index Level
Age range: Thanetian Age (GT) — Ypresian Age (GY)
Lithological Description: The Lower Mottled Clay typically consists of purple, red, green, blue-grey and brown mottled or multicoloured unbedded clays, some silty or sandy, and fine to medium-grained sands. Typically, sands in the Reading Formation are brown or grey in colour, and generally not silty. Glauconite is absent. They are either not or only weakly bioturbated, but may be cross-bedded and can form steep-sided deposits up to 5m thick. Nodular calcrete, calcrete-cemented silt, or gravel to cobble sized calcareous nodules are widespread, which may coalesce to form a local limestone bed and which has been described in some borehole logs as 'chalk'. The deposits are generally not shelly, but those that are contain a low diversity non-marine (brackish) fauna.
Definition of Lower Boundary: The Lower Mottled Clay rests on the marine deposits of the Upnor Formation. The boundary may be obscured by penecontemporary pedogenic alteration, but is typically marked by an upwards change from dark grey or greenish-coloured glauconitic sands to vari-coloured mottled clays or, locally, brown or grey non-glauconitic sands.
Definition of Upper Boundary: The top of the Lower Mottled Clay is a generally clearly marked unconformity known as the mid-Lambeth Group Hiatus (Page and Skipper, 2000).Where the Woolwich Formation is present, the upper boundary of the Lower Mottled Clay is typically marked by an upwards change from vari-coloured mottled clays or, locally, brown or grey non-glauconitic sands, in places with calcrete, below the unconformity, to a dark grey shelly clay or laminated clay and silt or fine to coarse-grained sand, commonly with lignite fragments or, locally, beds, above the unconformity. Where the Woolwich Formation is absent, the nature of the boundary between the Lower Mottled Clay and the Upper Mottled Clay is not well known. It seems to be marked by a bed of grey clay, or by lignite fragments, or both, at least locally. These presumably form the basal layer of the Upper Mottled Clay.
Thickness: Locally up to 5m in central London.
Geographical Limits: The Lower Mottled Clay is probably present throughout the Reading Formation outcrop and subcrop. It extends eastwards beneath the Woolwich Formation, where the two units co-exist (Ellison, 1983; Hooker, 1991).
Parent Unit: Reading Formation (RB)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Area  The type area of the Lower Mottled Clay is central, south and south-east London. No type section has been designated Ellison et al.,2004, fig 20 
Reference Section  BGS Crystal Palace Borehole (TQ37SW 671)138.00 TO 141.95m depth Ellison et al.,2004,p.32 
Reference Section  Jubilee Line Extension Borehole 404T (TQ37NW 2118), Bermondsey, London, 3.77 to 35.09m depth. This borehole passes through the two parts of the Reading Formation, here separated by the Woolwich Formation.(Ellison et al.,1994; Ellison et al., 2004,fig.15,p.32). 
Ellison, R A, Knox R W O'B, Jolley, D W and King, C, 1994. A revision of the lithostratigraphical classification of the early Palaeogene strata of the London Basin and East Anglia. Proceedings of the Geologists' Association, Vol.105, 187-197. 
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 
Ellison, R.A., 1983. Facies distribution in the Woolwich and Reading Beds of the London Basin, England. Proceedings of the Geologists' Association 94, 311-319. 
Page, D P, and Skipper, J A E. 2000. Lithological characteristics of the Lambeth Group. Ground Engineering, Vol. 33, 38-43. 
Goldring, R, and Alghamdi, J A. 1999. The stratigraphy and sedimentology of the Reading Formation (Palaeocene to Eocene) at Knowl Hill, near Reading (southern England). Tertiary Research, Vol. 19, 107-116. 
Hooker, J J. 1991. The sequence of mammals in the Thanetian and Ypresian of the London and Belgian basins. Location of the Palaeocene-Eocene boundary. Newsletters in Stratigraphy, Vol. 25, 75-90. 
Ellison, R A, Woods, M A, Allen, D J, Forster, A, Pharaoh, T C and King, C. 2004. Geology of London. Memoir of the British Geological Survey, Sheets 256 (North London), 257 (Romford), 270 (South London), 271 (Dartford) (England and Wales). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable