The Lambeth Group, formerly known as the Woolwich and Reading Beds,
occurs in the London and Hampshire Basins, where it directly overlies
the Chalk or Thanet Sand Formation, and is succeeded by the Harwich
and London Clay Formations.
Although generally less than 50 m thick,
its lithological variability and position beneath much of London
has concerned tunnelling engineers since the early nineteenth century.
The complex stratigraphy of the Lambeth Group was clarified during
the 1990s. Recent infrastructure development in central and eastern
London has provided a great deal of data on engineering characteristics.
The sediments of the Lambeth Group were laid down in one or more
embayments on the western margin of a deep-water marine basin centred
on the present North Sea.
The period was characterised by small
but significant variations in sea level producing periodic migration
of depositional environments resulting in complex lithological
variation. The environments include near-shore marine, lagoonal,
estuarine settings and wide floodplains.
The relationship between
the different depositional environments is seen in central and
south-east London. Here, deposits of fine-grained sand, flint gravel
beds, mottled clay, shell beds and altered beds form a complex
interdigitating sequence, which is divided into three formations
and several informal lithological units.
The engineering challenge
Horizontal and vertical lithological variability is characteristic
of the Group and a source of continuing difficulty for the construction
industry. Excavations may encounter water-bearing sands; seepage
from perched water tables may cause slope instability; and swelling
clays may affect shallow foundations.
Hard bands occur sporadically
at some levels, which may slow drilling and tunnelling operations.
The lithological inconsistency often makes tunnelling difficult
and costly operation requiring sophisticated tunnel-boring machines.
Summary of engineering problems encountered with the Lambeth
rapid changes in lithology
clay with irregular water-bearing sand bodies
perched water tables
hard bands that obstruct boring, drilling, excavations and piling
flint gravel beds, which are water bearing, and may restrict
boring progress and increase wear of cutting equipment
lignite, which may provide differential settlement
deposits infilling dissolution features in the underlying chalk
fissuring of clay, potentially leading to instability in cuts
de-oxygenation of atmosphere in tunnels cut into Upnor Formation
Summary geological descriptions of the Lambeth Group formations
This shallow marine deposit occurs at the base of the Lambeth
It is generally a dense to very dense, glauconitic fine- to medium-grained
sand. Thin clay beds or clay stringers may be present.
Flint gravel may occur at any depth but is generally found in
courses at the base or near the top of the formation beneath central
The upper part may be pedogenically altered: calcrete nodules
(often described as 'limestone') occur beneath central London.
Silcrete ('Hertfordshire puddingstone'), is mostly found in the
north-west of the London Basin.
Stiff, fissured mottled clay (Lower Mottled Beds)
Mottled glauconitic sandy clay with irregular calcareous concretion
Very weak limestone (Upnor Formation)
Firm glauconitic sandy clay or clayey sand with calcareous patches
This is mostly a floodplain deposit altered by pedogenic and
It was deposited in two leaves, the Upper and Lower Mottled Beds,
separated in the east by the Woolwich Formation. Elsewhere, the
units are difficult to distinguish.
It is a mottled or multicoloured, stiff or very stiff fissured
clay, compact silt, and dense or very dense sand deposited in overbank
(fine-grained) or channel (sand) settings.
The Lower Mottled Beds are absent from the far eastern part of
The Upper Mottled Beds are absent from south-east London and
Pedogenic calcrete formation occurs in the upper part of the
Lower Mottled Beds in central London and scattered sites elsewhere.
This is a lagoonal and estuarine deposit found in the east of
the London Basin and eastern part of the Hampshire Basin.
It is divided into a lower part (Lower Shelly Clay and Laminated
Beds) in central London and to the east and in the east of the
Hampshire Basin, and an upper part (Upper Shelly Clay), found in
central London and sporadically to the north and east.
The Lower Shelly Clay is a dark grey to black clay with abundant
shells but may also be a shelly sand particularly in the east London
Basin. Where shells predominate, thin limestone bands are formed.
Lignite beds occur at the base in north Kent.
The Laminated Beds consists of thinly interbedded fine- to medium-grained
sand, silt and clay, with locally more extensive sand bodies and
thin shell and lignite beds.
The Upper Shelly Clay is mainly a grey shelly clay, and occasionally
sand dominated unit and shelly limestone.
Very thickly laminated grey clay, silt and fine sand with occasional
shell beds (Laminated Beds)
Very stiff, thinly laminated grey clay and brown fine sand
with occasional shell fragments( Laminated Beds)
Very stiff, thinly laminated, grey clay with partings of brown
silt, occasional thin shelly beds and scattered sand sized particles
of organic matter (Laminated Beds)
Very stiff, dark grey, organic, very shelly clay Shells locally
cemented (Lower Shelly Clay)
Very shelly dark grey fine sand. Sharp irregular base with
burrows extending 0.30 m into bed below (Lower Shelly Clay)
Presentation of the geology
The Lambeth Group is being modelled in 3D to provide better information
on the distribution of likely engineering problems. A typical section
(below) shows the relationships between lithologies in the Lambeth
Group and those in the sequences above and below. In this example,
the potential for groundwater inflow can be identified where the
lower, coarse-grained part of the Lambeth Group is in hydraulic
continuity with the underlying Chalk and Thanet Sand Formation.
Geotechnical data from over 50 ground investigations, held in the
National Geotechnical Properties Database have been analysed to
assess the spatial variation in geotechnical properties across
the main formations of the Lambeth Group. An example of the undrained
shear strength with depth is presented below.
The BGS gratefully acknowledges the contribution
made to parts of this study by Dr Jackie Skipper (Engineering Stratigrapher,
Natural History Museum) based on her extensive experience of the
Lambeth Group geology and associated engineering problems.
Jones, L D, and Hobbs, P R N. 2004. The shrinkage and swelling behaviour of UK soils: the clays of the Lambeth Group. British Geological Survey Report, RR/04/001.
Jones, L D. 2001. Determination of the shrinking and swelling properties of the clays of the Lambeth Group. British Geological Survey Report, IR/01/054.