OR/13/006 Geology

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Entwisle, D C, Hobbs, P R N, Northmore, K J, Skipper*, J, Raines, M R, Self, S J, Ellison, R A, and Jones, L D. 2013. Engineering geology of British rocks and soils - Lambeth Group). British Geological Survey. (OR/13/006).

* Geotechnical Consulting Group (GCG)

This chapter provides information on the lithologies and lithostratigraphy of the Lambeth Group and those deposits below and above that may be confused with the Lambeth Group, that is, the Thanet Formation beneath and the Harwich Formation above.

The term Lambeth Group has been in the public domain only since 1995 (see Ellison et al., 1994[1]) and replaces the formerly used Woolwich and Reading Beds. The term was introduced in order to clarify the stratigraphy shown on British Geological Survey maps, initially in the London area. The change in name was necessary for two reasons. Firstly, the strata within the group exhibit considerable lateral and vertical lithological variation, and the new classification helps to constrain some of the variation within the formations. Secondly, the Lambeth Group is within 50 m of the surface beneath large tracts of London and, therefore, has been, and continues to be, an important issue in many engineering projects. Much of the drive for the improved understanding of the complexity of the Lambeth Group has been from major infrastructure projects in London and the information derived from high quality ground investigations, including the development of London Docklands, new underground services including the London Underground and the Channel Tunnel Rail Link.

In the following account the principal constituent lithological units of the Lambeth Group are defined. Their correlation and three-dimensional relationships are demonstrated in a series of cross sections and maps. For each unit, aspects of the geology that may pose problems for engineering are identified. Accounts are also given of the underlying Thanet Formation and the overlying Harwich Formation.

Regional setting[edit]

After the deposition of the Chalk across much of Europe, increased tectonic activity relating to the opening of the Atlantic Ocean and building of the Alps in southern Europe in the late Cretaceous and early Tertiary resulted in a period of uplift and tilting and a global fall in sea level. This produced newly emergent landmasses in the north and west of Britain and the Amorican Massif (Skipper, 1999[2]) and the erosion of the youngest part of the Chalk from southern England. The depositional hiatus between the Chalk and the sediments of the Palaeocene in southeast England lasted about 10 to 15 Ma. During this hiatus, many major plate tectonic changes occurred worldwide. In the west was the widening proto-Atlantic Ocean, the development of which was associated with rifting and volcanic activity that culminated in early Eocene times about 55 million years ago (Knox, 1994[3]). This led to much smaller depositional areas and a change from carbonate sedimentation to increased clastic sedimentation (Chadwick, 1986[4]), followed by the intermittent deposition of Tertiary sediments about 58 Ma when a shallow sea extended from the North Sea across the south east of England. The Lambeth Group forms part of this sedimentary sequence deposited around 55 to 56 Ma. About 20 Myr later, during the Miocene, the outcrops of the London and Hampshire Basins were separated by gentle folding.

During the Palaeocene the climate warmed, with short-term rapid increases in temperature of 4 to 6°C (Beck et al., 1995[5]). Evidence of broad-leaved evergreen vegetation suggests a mean annual land temperature of 10–20°C with abundant precipitation. However, the rainfall was probably seasonal particularly during the deposition of the upper Lambeth Group.

Volcanic activity to the north west of Britain provided ash, which can be seen in layers in the Ormesby Clay Formation (Cox et al., 1985[6]) below the Lambeth Group and in the Harwich Formation and London Clay above. The reworked and weathered (argillized) ash was subsequently deposited as smectite throughout most of the Palaeocene. However, ash bands have not been reported in the Lambeth Group and are very rare in equivalent deposits in the North Sea Basin, currently thought to be Sele Formation (King, 2006[7]). At the beginning of Palaeogene time the south east of England lay on the edge of a sedimentary basin that included much of the present North Sea, and extended eastwards at least as far as Poland. The Palaeogene deposits were laid down during alternating transgressions and regressions driven by global sea level changes and this broad pattern was overprinted locally by changes in ground level from tectonic activity. The general succession (Figure 2.1) is divided into major depositional sequences (Knox, 1996a[8]).

Deposition of the Thanet Formation, Lambeth Group and Harwich Formation occurred in embayments on the western margin of a deep-water marine basin of the North Sea. These marginal deposits were very sensitive to relatively minor changes in sea level. This resulted in alternating incursions and recession of the sea and migration of depositional environments, followed by erosion, changes in groundwater levels, soil formation and down-cutting by rivers that contributed to the development of complex lithologies. Rising sea level led to rapid inundation and a new phase of sedimentation.

Figure 2.1    The sequence stratigraphy of the middle part of the Palaeogene, with the depositional sequence and tectonic activity (after Knox 1996a). (Vertical stripes indicate no deposition).

Deposits beneath the Lambeth Group[edit]

The main deposits immediately beneath the Lambeth Group are listed below and their distribution is shown in Figure 2.2.

  • White Chalk Subgroup in the Hampshire Basin and the west of the London Basin.
  • Thanet Formation in the middle and east of the London Basin.
  • Ormesby Clay Formation to the north east of the London Basin.
Figure 2.2    The strata underlying the Lambeth Group.

The surface of the chalk can be karstic and undulating. However, it is very easy to differentiate between the Chalk and the lowest part of the Lambeth Group comprising the sandy facies of the Upnor Formation.

The boundary of the Upnor with the underlying Ormesby Clay Formation is relatively simple to identify as the latter is predominantly clay as opposed to sand, but there is little available information. Restricted to eastern East Anglia, the Ormesby Clay Formation is not found at surface and is only known in a limited number of boreholes. It consists generally of very stiff glauconitic clay or very weak mudstone with sporadic, thin ash bands occurring in the lower part of the formation. The clay mineral assemblage is dominated by smectite, derived from weathered and redeposited volcanic ash that results in extremely high clay/mudstone plasticity (liquid limits ranging from 98 to 172% and plastic limits from 36 to 78%, Cox et al., 1985[6]). The unit is over 25 m thick in east Norfolk thinning to about 10 m in Suffolk and less than 10 m in Essex.

Differentiating between the Upnor Formation and the underlying Thanet Formation can be more difficult. It is usually possible in boreholes as the sand at the top of the latter is generally denser and finer grained than the coarser clayey sand of the Upnor Formation. However, when field mapping, this boundary can be extremely difficult to identify particularly where the two formations are weathered and the base of the Upnor Formation does not contain flint gravel. The distribution, lithological characteristics and boundary of the Thanet Formation with the overlying Lambeth Group Upnor Formation is described in more detail below.

Thanet Formation[edit]

Introduction[edit]

The Thanet Formation (Aldiss, 2012[9]) deposits are interpreted as inner shelf to coastal in origin. In general, they are well sorted, indicating considerable winnowing and reworking prior to deposition. Grain shape suggests a rather juvenile origin with only minor recycling from older sedimentary formations.

The base of the Thanet Formation is unconformable on the eroded surface of the Chalk Group. The unconformity is not caused by a single event but is attributed to erosion and reworking during two or more depositional sequences (Knox, 1996a[8], 1996b[10]). The base of the Thanet Formation is marked by the unconformable contact between the bouldery, cobbly gravelly sand of the Bullhead Bed and the Chalk, and the top by the unconformable boundary marked by an upward change from the fine-grained grey sands of the Thanet Formation to grey to greenish grey, commonly gravelly, clayey, fine to medium sand of the base of the Upnor Formation of the Lambeth Group. At outcrop and, where weathered, the Upnor Formation sands are speckled with dark green grains of glauconite.

Distribution[edit]

The Thanet Formation occurs in the east and middle part of the London Basin but is absent from the Hampshire Basin. The principal outcrops are in south-east London and north Kent including outliers around Dartford [TQ 520 730], Swanley [TQ 510 690], Southfleet [TQ 614 711] and Cobham [TQ 671 685]. The Thanet Formation is also preserved locally in dissolution pipes and hollows in the Chalk peripheral to these outliers, in some cases beneath a cover of Head or Clay-with-flints. In central London it is typically 10 to 15 m thick and in west London and Surrey it thins westwards where it is below the Lambeth Group. In the eastern parts of the London Basin it is generally thicker being thickest in north Kent, where it generally 20 to 30 m but as much as 37 m in the Canterbury district. Pockets of Thanet Formation occur in dissolution cavities within the surface of the chalk, but the majority of them do not form features. At surface, the formation forms positive, well-drained features with convex slopes. The basal contact with the chalk often forms a pronounced concave break of slope. The top of the formation is generally located by augering or placed on the evidence of borehole data. In the north east of the London Basin the Thanet Formation has not been mapped separately from the Lambeth Group as, in the field, it is not possible to distinguish consistently between the top of the Thanet Formation and the base of the Upnor Formation. Here, they are mapped as the Lower London Tertiaries on the current England and Wales 1:50k geological maps of Sudbury (206) (Pattison et al., 1993[11]), Braintree (223) (Ellison and Lake, 1986[12]), Ipswich (207) (Boswell, 1927[13]), Great Dunmow (222) (Lake and Wilson, 1990[14]), Epping (240) (Millward et al., 1987[15]) and Woodbridge and Felixstowe (204) (Boswell, 1928[16]). However, with good quality borehole data it is possible to differentiate between the Thanet Formation and Lambeth Group in this area.

Description[edit]

Most of the Thanet Formation is composed of very dense, grey or greenish grey, slightly clayey or slightly silty fine sand. In the London area, it is generally coarsens upwards with larger proportions of silt and clay in the lower part. Figure 2.3 shows the particle size envelopes based on over 700 analyses of the Thanet Formation, not including the Bull Head Beds, and the more restricted slightly silty fine sand within 2 m of the base of the Lambeth Group. The sand grains consist almost exclusively of quartz, and are mainly angular and subangular, with only a small proportion of subrounded grains and flint chips (Figure 2.4). Smectite is generally the dominant clay mineral, a weathering product of penecontemporaneous volcanic ash falls (Knox, 1994[3]). Although predominantly sand, this is not always the case and in Kent a number of named units are described (Aldiss, 2012[9]). These are the Base Bed Member, which includes the Bullhead Bed, the Stourmouth Silt Member, the Kentish Sands Member, the Pegwell Silt Member and Reculver Sand Member.

The basal unit of the Thanet Formation is commonly represented by the Bullhead Bed. This distinctive unit generally comprises very dense, greenish grey, glauconitic, slightly gravelly, silty fine to medium SAND with low cobble and boulder content. The gravel is of fine well-rounded flint, whilst the larger gravel, cobbles and occasional fine boulders (up to 0.3 m in diameter) are unworn nodular flint, which can have large protuberances similar to a bull’s head, hence the name. This unit is generally up to 0.5 m thick but in parts of north London it is sometimes up to 1.5 m thick.

Figure 2.3    The particle size distribution of the Thanet Formation. Dark blue represents the Thanet Formation within 2 m of the base of the Upnor Formation (over 100 samples) and pale blue all the Thanet Formation not including the Bullhead Beds (over 700 samples).
Figure 2.4    Typical example of Thanet Formation lithology, comprising well sorted fine-grained sand, locally weakly cemented by detrital clay. The large secondary voids are due to feldspar dissolution. Jubilee Line Extension borehole 404T (BGS borehole TQ37NW/2118 [TQ 33638 79604]), 52.80 m (BGS Photomicrograph No. D789P101).

The unweathered sediments are pale to medium grey to brownish grey but weather at surface to pale yellowish grey. Figure 2.5 is an example of borehole core from the Thanet Formation in south central London. Contemporary weathering and pedogenic processes locally give rise to a typical podsol profile, with purplish brown weak ferruginous cement developed within 0.8 m of the surface.

Figure 2.5    Typical Thanet Formation core sample, showing grey, slightly silty fine SAND; top to left. (CTRL (Union Rail), borehole 1112B.

The sediments are intensely bioturbated so that primary sedimentary structures such as lamination are generally missing. Bioturbation structures are identified as wisps of relatively dark grey clay in hand specimen and in exposures. Dark grey to black manganese-rich silt may occur in the linings of sinuous burrows up to 8 mm in diameter. Scattered oblique and near-vertical burrows also occur in the top 1 to 1.5 m. These are usually filled with glauconitic sand derived from the overlying Upnor Formation (CTRL borehole 1131, Figure 2.6a). This mixing contrasts with the sharp contact between the Thanet Formation and the base of the Upnor Formation comprising clayey rounded flint gravel, as shown in CTRL borehole 2112 (Figure 2.6b).

Figure 2.6    Two different forms of the interface between the Thanet and Upnor formations.
(a) The pale yellow brown of the Thanet Formation has been mixed (bioturbated) with the dark green of the overlying Upnor Formation. (CTRL (Union Rail) Borhole 1131).
(b) the very dense, pale green fine sand of the Thanet Formation has a sharp contact with the slightly clayey, black, sub rounded to rounded, fine to coarse flint gravel of the overlying Upnor Formation (CTRL (Union Rail) Borehole 2112).

In places, faint bedding is seen in weathered exposures, and some fine lamination is recorded near the top of the Thanet Formation in the Crystal Palace Borehole at 152.2 m to 145.8 m depth (BGS borehole TQ37SW/671 [TQ 3379 7082]). Glauconite grains and flakes of white mica are sparsely disseminated throughout. Beds weakly indurated by iron oxide have been described in north Kent. Irregular to oblate masses of siliceous sandstone (colloquially known as ‘doggers’) have been recorded in the vicinity of Thurrock and Grays. Irregular nodules of pyrite less than 5 mm across occur rarely, and Prestwich (1852) described gypsum at Blackheath, presumably derived from the oxidation of pyrite by weathering and the reaction of the products with calcium carbonate.

Thin sections indicate the presence of corroded feldspar, minor randomly orientated white mica, chlorite and ilmenite. Authigenic pyrite and glauconite clasts are rare. Some larger voids may be caused by dissolution of framework grains. Apatite occurs as fine sand grade detrital grains in samples from central London, but is absent in the Crystal Palace (BGS borehole TQ37SW/671 [TQ 3379 7082]) and Stanford-le-Hope (BGS borehole TQ68SE/35 [TQ 6965 8241]) boreholes, presumably due to dissolution by acidic groundwater (Hallsworth, 1994[17]).

Representative sections[edit]

There are few natural exposures. The basal few metres of the Thanet Formation can be seen in the top part of exposures in former Chalk quarries at Grays [TQ 609 792] and Swanscombe Western Quarry [TQ 606 728]. The top part of the formation is exposed in sand pits near Orsett [TQ 673 806]. There are a number of coastal sections including Pegwell Bay ([TR 355 644] and [TR 350 642]), Herne Bay ([TR 193 685] to [TR 224 693]) (Daley and Balson, 1999[18]).

Almost complete sequences through the entire formation are held by the BGS from Borehole CTRL A2 (BGS borehole TQ38SW/2201 [TQ 32096 80510]) in southeast London and Jubilee Line Extension Borehole 404T (BGS borehole TQ37NW/2119 [TQ 3376 7956]).

Lambeth Group[edit]

Exposures of deposits that constitute the Lambeth Group were first described in pioneering work by Prestwich (1854) and systematically mapped by the Geological Survey in the late 19th century. The findings of these surveys were published in Geological Survey Memoirs covering the London Basin (Whitaker, 1872[19]) and much of the Hampshire Basin (Reid, 1897[20], 1898[21], 1899[22], 1902[23], 1903a[24] and 1903b[25]). Subsequently, memoirs covering the whole of the London and Hampshire Basins have been published. The most recent of these, in which there are accounts of the Lambeth Group, cover Southampton (Edwards and Freshney, 1987[26]) and Bournemouth (Bristow et al., 1991[27]) in the Hampshire Basin, and Chelmsford (Bristow, 1985[28]), Braintree (Ellison and Lake, 1986[12]), Sudbury (Pattison, 1993[11]), Great Dunmow (Lake and Wilson, 1990[14]), Epping (Millward et al., 1987[15]) and London (Ellison et al., 2004[29]) in the London Basin. The distribution and relationships of the Lambeth Group in the London and Hampshire Basins are illustrated in Figure 2.7 and Figure 2.8. A full list of memoirs and map explanations covering the Lambeth Group is given as a supplement to the Reference list.

Figure 2.7    The surface and subsurface distribution of the Lambeth Group (Quaternary deposits are not shown).

The formal term Lambeth Group has been adopted in recent years (Ellison et al., 1994[1]) to replace the Woolwich and Reading Beds of earlier authors (see for example Whitaker, 1899[30]; Hester, 1965[31]). The group is divided into three formations and several informal lithological units (Table 2.1). The relationship between these informal units is most complex in the central part of the district, coincident with central and south-east London.

Table 2.1    Lambeth Group nomenclature used in this report.
Formation Previous nomenclature Informal units used in this account
Woolwich Formation Woolwich Beds Upper Shelly Clay
‘striped loam’*
Laminated Beds
Lower Shelly Clay
Reading Formation Reading Beds Upper Mottled Clay
Lower Mottled Clay
Upnor Formation Bottom or Basement Bed

† Ellison et al., 1994[1]      * Dewey et al., 1924[32]      ‡ This report.

The base of the Lambeth Group is represented by the Upnor Formation, formerly known as the ‘Bottom’ or ‘Basement Bed’ of the Reading or Woolwich Beds. Above it, the Reading Formation (formerly the Reading Beds) is predominant in the Hampshire Basin and in the north and west of the London Basin. In the extreme east of the Hampshire Basin and the south and east of the London Basin, deposits above the basal Upnor Formation are those of the Woolwich Formation (Formerly the Woolwich Beds). The distribution of formations is shown in Figure 2.8. Distribution of the Lambeth Group: (a) Woolwich Formation, (b) Reading Formation, (c) Upnor Formation. Figure 2.8. Distribution of the Lambeth Group: (a) Woolwich Formation, (b) Reading Formation, (c) Upnor Formation.

Figure 2.8    Distribution of the Lambeth Group: (a) Woolwich Formation, (b) Reading Formation, (c) Upnor Formation.

In an area coincident more or less with central and south London, Hester (1965)[31] identified a transition zone between what he termed the ‘Reading type’ and ‘Woolwich type’ strata where both types occur and in some places interdigitate. It is principally in this zone where a new classification of the Lambeth Group was devised (Figure 2.9). Initially, the interpretation of borehole records, the results of drilling by BGS of cored boreholes and detailed examination of exposures in Essex and Suffolk led Ellison (1983)[33] to recognise a relationship between several units of the Lambeth Group.

Figure 2.9    Schematic diagram showing the relationship of the informal lithological units in the Lambeth Group in central London (after Ellison, 2004[29]).

The Reading Formation comprises the Upper and Lower Mottled Clay (Ellison et al., 1994[1]; Ellison et al., 2004[29]). Industry practice in the London area is based on the work by Skipper (Page and Skipper, 2000[34]), and uses a slightly different classification. Pedogenically altered deposits are included within the Reading Formation either as the Lower or Upper Mottled Beds. The Lower Mottled Beds, therefore, includes the pedogenically altered, Upnor Formation as defined by Ellison et al. (1994)[1] and Aldiss (2012)[9] and may be called mottled Upnor Formation.

The thickness of the Lambeth Group in the London Basin ranges from less than 10 m in the southeast where much of it is eroded away beneath the Harwich Formation (Oldhaven and Blackheath Beds) to about 30 m in the central part of the basin, around Chertsey. In the Hampshire Basin it is generally around 25 m thick; on the Isle of Wight the sequence is up to 50 m thick, and it is thinnest in the far west. Figure 2.10 shows the thickness below London.

Figure 2.10    Distribution and thickness of the Lambeth Group in London (from Ellison, 2004[29]).

The Lambeth Group is overlain by sands and gravel beds of the Harwich Formation (Ellison, 1994[1]), which in turn is overlain by the London Clay Formation.

The Lambeth Group sequence[edit]

To understand the distribution and complex lithologies of the Lambeth Group it is necessary to understand the sequence of the deposition of the formations, the conditions in which they were deposited, and their alteration due to contemporaneous soil formation or pedogenesis.

Sequence stratigraphy[edit]

The Lambeth Group is made up of four depositional units, that is Lambeth sequences 1 to 4, Lmbe1-4, (after Knox, 1996a[8]), with depositional hiatus, erosion, weathering and soil formation between. Other local disruptions to deposition, indicated by erosional surfaces, with or without soil formation, are found in many sections. The sequences and the formations deposited are summarized in Figure 2.1 with the upper part of the Ormesby-Thanet sequence and the Thames Group (Harwich and London Clay Formation). The complete sequences are as follows:

Lambeth sequence Lmbe-1 comprises the lower Upnor Formation.
Lambeth sequence Lmbe-2 comprises the upper Upnor Formation and the Lower Mottled Clay (lower Reading Formation).
Lambeth sequence Lmbe-3 comprises the Upper Mottled Clay (upper Reading Formation) and the lignite, lower Shelly Clay and Laminated Beds (lower Woolwich Formation).
Lambeth sequence Lmbe-4 comprises the Upper Shelly Clay (upper Woolwich Formation).

The lower Upnor Formation of sequence Lmbe-1 is a glauconitic, calcareous, gravelly, clayey SAND with a rich and diverse marine flora and fauna. It represents a temporary transgressive phase, re-establishing open marine conditions. A relative sea level fall and a period of regression, emergence and erosion subsequently reduced its original development to its present thickness and extent. The lower Upnor Formation was first identified in central London (Ellison et al., 1994[1]; Knox, 1996a[8]) but recent work suggests that it is more widespread (Skipper, 1999[2]).

Sequence Lmbe-2 comprises the transgressive upper Upnor Formation and the lower leaf of the Reading Formation, the Lower Mottled Clay. The upper Upnor Formation generally consists of non-calcareous, glauconitic, sometimes clayey, SANDS and GRAVELS with a relatively restricted microfauna and palynoflora. The basal beds of the Lambeth Group in the west Hampshire Basin have characteristics of fluvial deposits and contain reworked glauconite derived from material similar to the lower Upnor Formation (Skipper, 1999[2]). In eastern parts of the Greater London area and in parts of Essex and Kent the upper Upnor Formation contains relatively thick accumulations of gravel, deposited from fast flowing marine or estuarine channels. High concentrations of glauconite indicate periods of maximum flood. In Central London and further east, a progressive change from shallow marine to estuarine Upnor Formation deposits becomes shallower and is replaced in some areas by water logged soils and then drier soils of the Reading Formation. Further east in north Kent shallow marine Upnor Formation deposits became emergent and pedogenically altered. The upper part of the sequence tends to be more pedogenically altered than other parts of the Lambeth Group.

The transgression of the Lower Mottled Clay terrestrial facies eastwards indicates a relative sea level fall due to further uplift in the west. The continuing fall in relative sea level resulted in a period of erosion and weathering that produced a subdued topography and resulted in widespread pedogenesis. Skipper (1999)[2] and Page and Skipper (2000)[34], have referred to the sharp boundary marking the end of this sequence as the mid-Lambeth Hiatus, which is now known as the mid-Lambeth Group Hiatus. The top of the deposits are typically pale, often bleached and contain many burrow traces, which may be filled with material from the bed above. These deposits are generally clay overbank deposits with sand filled river channels, but in the east of the London Basin they are mainly sand.

Most of the hard beds are from the upper part of this sequence and include silicate cemented beds to the north of London and south of the South Downs, calcium carbonate cemented beds in central and east London and near Arundel in the Hampshire Basin and iron oxide cemented beds in the east, most notably in north Kent.

The transition from sequence Lmbe-2 to Lmbe-3 is marked by the relative uplift of sediment sources in the west leading to an influx of sediment.

This sequence Lmbe-3 comprises the Lower Woolwich Formation (lignite, Lower Shelly Clay and Laminated Beds) and the Upper Reading Formation (Upper Mottled Clay). At the base of the sequence hydromorphic, lignitic, reduced black or dark grey soils are widespread. They are markedly different in colour and appearance to the bioturbated bleached or oxidised palaeosols at the top of sequence Lmbe-2. The lower Woolwich Formation marks a westwards extension of lagoon environments. Although these deposits are well documented in central London and the east of the Hampshire Basin they are often represented by thin lignitic or near black clay deposits above a bleached horizon of Lmbe-2 seen in borehole log descriptions as far west as Newbury in Berkshire and Alum Bay on the Isle of Wight. This represents a short high stand when lagoonal conditions prevailed on a wide, nearly flat plain. The upper Reading Formation, to the west, represents a re-establishing of mainly continental conditions with pedogenically altered, multicoloured overbank clay and river infill sands. The Upper Mottled Clay of the upper Reading Formation generally overlies the lower Woolwich in central London where the top of lower Woolwich Formation is pedogenically altered beneath the upper Reading Formation. On occasion, the two might be interleaved, because of local deposition conditions or apparently interleaved due to faulting. There followed sequences of initial water-logging, followed by upwards drying and oxidising of the Upper Mottled Clay (Skipper, 1999[2]). Isolated deposits similar to the Woolwich Formation have been found within the Lower Mottled Clay near Kings Cross, London (J. Skipper, personal communication, 2005) and in the Upper Mottled Clay.

The upper Woolwich Formation or Upper Shelly Clay of Lambeth sequence Lmbe-4 is deposited on an irregular and eroded surface. It is much more limited and patchy in extent than the other sequences and is only known in central, eastern and north eastern parts of the London Basin (Knox, 1996a[8]).

Description of the Lambeth Group Formations[edit]

Upnor Formation[edit]

The Upnor Formation was formerly known as the Bottom Bed (of the Woolwich and Reading Beds) in the London Basin, and the Basement Bed (of the Reading Beds) in the Hampshire Basin.

Distribution
The Upnor Formation is present nearly everywhere at the base of the Lambeth Group (Figure 2.8c).

Basal Boundary
Chalk
The Upnor Formation rests on the Chalk in the Hampshire Basin and in the west of the London Basin (Figure 2.2) northwest of a line from Northolt [TQ 130 840] to Borehamwood [TQ 200 950]. In the central and southern part of the London Basin the Upnor Formation lies on the Thanet Formation and in parts of Essex and Suffolk on the Ormesby Clay Formation, though this contact is at depth. A basal bed containing flint gravel is usually present.

The contact with the Chalk is unconformable and sharp and in the west Thames Basin either frequently burrowed by the ichnofossil glyphichus (Bromley and Goldring, 1992), or often undulating because of dissolution features in the Chalk. The dissolution features often have steep sides and undulating bases, and may be a few metres deep and tens of metres across (P212494). They are generally lined with clay derived from the insoluble remains of the Chalk (Figure 2.12). In some areas this process is still active. The lowest Upnor Formation bed contains nodular unworn green-coated flint gravel.

P212494    Fine-grained brown sands of the Upnor Formation filling dissolution pipes in the Upper Chalk. A27 road-cutting at Falmer, East Sussex [TQ 3541 0890]. Field of view is about 6 m wide. (BGS photograph P212494).
Figure 2.12    Upnor Formation basal gravel bed above Newhaven Chalk Formation. The contact is undulating due to solution features and bioturbation. (Newhaven, East Sussex, [TV 4459, 9990]. Field of view is about 1.5 m wide.

Thanet Formation
The junction with the Thanet Formation may be gradational because of relatively intense bioturbation, and burrows may extend 2 m below the contact (Figure 2.6). The contrasting lithological characteristics between the lowest bed of the Upnor Formation and the upper part of the Thanet Formation are:

  • Upnor Formation sands are generally slightly coarser, and more clay-rich, which can be identified when reworked, than the silty fine sands of the Thanet Formation,
  • The lowest bed of the Upnor Formation may contain some flint gravel (Figure 2.6b),
  • The Upnor Formation, when fresh, is generally green or dark green, whereas the upper part of the Thanet Formation is grey,
  • Whilst both are generally very dense the Thanet Formation is generally denser than the lowest part of the Upnor Formation,
  • The Upnor Formation may contain fossil molluscs, which are very rare in the Thanet Formation,
  • The Upnor Formation commonly contains more clay than the top of the Thanet Formation,
  • In the field there may be weak seeps at the contact due to the greater fine material in the Thanet Formation and its greater density.

Thickness
The thickness of the Upnor Formation is well documented in cored boreholes and exposed sections in the London area (Figure 2.13). Over much of the London and Hampshire Basins the formation is generally less than 3 m thick. However, in parts of central London, north Kent and Essex it may be 6 to 7 m thick. At Orsett in south Essex it is up to 9 m thick. Thicknesses may also be greater where the Upnor Formation has filled solution features in the Chalk.

Lithology
The dominant lithology of the Upnor Formation is fine to medium SAND and clayey sand with variable amounts of fine to medium sand grade glauconite grains and sporadic beds or stringers of well-rounded flint gravel and laminations of clay. At outcrop and when weathered the sediments are pale grey-brown to orange-brown and yellow-brown. The glauconite grains are dark green and impart a speckled ‘pepper and salt’ appearance. At depth, the sediments are mainly dark grey or dark greenish grey. In the central and eastern parts of the London Basin some of the sandy beds contain up to 25% glauconite. Shelly clay occurs in unweathered sections and, in places, oyster and pychnodontid shells occur near the base. In the west of the Hampshire Basin the glauconite content declines and the flints are decomposed and associated with irregularly developed ironstone.

The sands may be completely bioturbated with no primary bedding, and with vertical and subhorizontal burrows filled with sand of contrasting colour to the bioturbated matrix. Rare fragments of carbonaceous material also occur. A few impersistent seams of grey clay and angular clasts derived from such seams have been recorded within dominantly bioturbated beds. Other parts of the succession are well bedded and with horizontal planar lamination, ripple lamination, hummocky and planar cross bedding, and clay drapes. Stringers of well-rounded flint gravels occur on a few bedding surfaces and there are beds of gravelly sand, mostly less than 0.3 m thick. Thin seams of grey clay, angular clay clasts and rounded balls of clay are also present. Ophiomorpha and Macaronichnus burrows are typical in these beds (P211784) and can be seen in the quarries at Upnor and Orsett. Clay-dominated units, up to 0.3 m thick, contain relatively small amounts of sand, arranged in flaser lamination and with lenticular cross-lamination. These strata are well exposed at Lower Upnor Pit, north of Chatham, north Kent [TQ 759 711], Orsett Cock Pit, [TQ 656 811], and Orsett Tarmac Pit near Walton’s Hall Orsett, Essex [TQ 673 803].

Figure 2.13    Distribution of the Upnor Formation in London (Ellison et al., 2004[29]).
P211784    Detail of Upnor Formation. Cross-stratified fine-grained sand overlain by finely-interbedded fine and medium-grained sand and clay. Burrows and clay laminae are seen to stand out on the weathered face. Orsett Depot Quarry, West Pit. Looking south, [TQ 656 810], (BGS photograph P211784).

The flint gravels that occur in the London Basin are generally less than 30 mm in diameter, but cobbles up to 200 mm occasionally occur, for example around Gravesend. In central and south-east London there is a persistent gravel bed (Ellison, 1991[35]) up to 5 m thick at the top of the formation consisting of well-rounded flint gravel. At Orsett, south Essex, a wedge of gravel up to 9 m thick occurs at the base of the formation.

Four main facies have been identified in the Upnor Formation (Skipper, 1999[2]):

  1. Transgressive gravel and sands,
  2. Laminated silts, clays and fine sand,
  3. Upper gravelly sands,
  4. Sand beds.

These facies may be pedogenically altered beneath thin Lower Mottled Clay of the Reading Formation in north and west London leading to major lithological and other character changes.

1. Transgressive gravel and sands
The contact of the basal Upnor Formation with the underlying Thanet Formation and White Chalk Subgroup often contains a few courses of well-rounded, spheroidal black flint gravel in a matrix of very glauconitic clayey sand, up to 1 m thick. This bed sometimes fines upwards. Shells are rare but often well preserved, although frequently abraded and chaotically oriented in the gravel. The gravel may also have a green coating of glauconite and percussion marks. The basal gravel may be missing, for instance in the north east of the London Basin. Figure 2.12 shows cemented slightly clayey sandy gravel above the Newhaven Chalk Formation, west of Newhaven, East Sussex [TV 4459 9990].

2. Laminated silts, clays and fine sand
The basal transgressive gravel and sand is often succeeded by thinly laminated clay, silt or fine sand. It may contain lignite but in places is extensively bioturbated sometimes destroying the laminations. In the London area, e.g. around Islington, this bed may be up to 7 m thick.

3. Upper gravelly sands
To the east of London the upper part of the Upnor Formation comprises up to 5 m of well rounded fine to coarse flint gravel in a clayey sand matrix. This is known as the ‘pebble bed’ (Ellison et al., 1994[1] and 2004[29]) and is distinct from the basal gravels. Pedogenesis and calcrete formation have altered the matrix removing any sedimentary structure. These deposits are best seen at Orsett Cock Quarry, [TQ 657 811] in Essex (P211781) where gravel beds, up to 50 cm thick, are interbedded and interdigitated with glauconitic fine to medium sand with clay laminations.

P211781    View looking east showing the Upnor Formation at Orsett Cock Quarry [TQ 657 811] with inclined sets of well rounded flint gravel ‘pebble’ beds. (BGS photograph P211781).

Where the Upnor Formation is overlain by thin Lower Mottled Clay pedogenesis has altered the highest beds, which are mottled brown and purple-brown. In the gravel bed at the top of the Upnor Formation, a clay matrix is developed and the pebbles are brittle and red stained. Irregular-shaped carbonate concretions, up to 0.5 m in diameter, are sometimes present and are often described as ‘limestone’ in borehole logs. Pedogenic processes have also given rise to the development of silcretes (silica cementing), small ironstone nodules, and clay coatings on sand grains and. These secondary alterations may locally occur throughout the entire formation in the western part of the London Basin and Hampshire Basin.

4. Sand Beds
In the east, the sand may change from horizontal bedding to trough cross-bedding. In the west Hampshire Basin the lowermost ‘Upnor’ Formation comprises fluvial deposits of angular to well-rounded, sometimes clayey, usually fine to coarse sand, as at Studland [SZ 0416 234] and Alum Bay [SZ 3054 0852]. In places irregular hollows up to 2 m deep in the Chalk are infilled with a thin bed of red-stained, angular flints up to 100 mm. This is succeeded by lignitic, glauconitic, fine to medium sand.

Reading Formation[edit]

The Reading Formation is now considered to comprise those deposits once referred to as the Reading Beds minus the Basement or Bottom Bed, which is now attributed to the Upnor Formation.

Distribution
The Reading Formation occurs throughout the London and Hampshire Basins, reaching a maximum thickness the south-west of the district, up to 49 m thick at Whitecliff Bay [SZ 639 581] on the Isle of Wight; thinning progressively eastwards, where it passes laterally into, and interdigitates with, the Woolwich Formation.

The Reading Formation consists of two leaves, the Lower Mottled Clay and the Upper Mottled Clay. The Lower Mottled Clay was deposited on the Upnor Formation before the mid-Lambeth Group Hiatus, afterwards followed by deposition of the Upper Mottled Clay. The Lower Mottled Clay persists across the entire area of the Reading Formation, but the Upper Mottled Clay is absent from most of the eastern part of the London Basin (Figure 2.16).

Figure 2.16    Reading Formation distribution and main lithologies in London (Ellison et al., 2004[29]).

Basal Boundary
The boundary of the Lower Mottled Clay with the underlying Upnor Formation is usually diffuse and difficult to place precisely because of pedogenic alteration that may include either migration of clay particles into the Upnor Formation and/or colour mottling. The degree of alteration may be such that it is impossible to identify the boundary accurately. Examples of some of the contact variations identified in rotary borehole cores are in shown in Figure 2.17.

Figure 2.17    Examples of the variable contact between the Upnor Formation and the Lower Mottled Clay of the Reading Formation. JLE404T — top of Upnor Formation is marked by 0.06 m of oyster fragments and red, white and black sub angular to well rounded flint gravel in a clay matrix. CTRL2131 the ‘pebble bed’ tops the Upnor Formation. CTRL2141 the change from Upnor Formation to Lower Mottled Clay is taken at the sharp contact of very finely laminated pale greenish grey fine sand (Upnor Formation) and the mottled clayey fine to coarse sand (Lower Mottled Clay of the Reading Formation).

The base of the Upper Mottled Clay gives rise to a similarly diffuse contact with the Laminated Beds of the Woolwich Formation, except around Stratford where the two units sometimes interdigitate. Where both the Lower and Upper Mottled Clay are present, differentiating between the two is relatively easy in central London as they are separated by the Woolwich Formation (Figure 2.9). Elsewhere, where the Woolwich Formation is absent, it is difficult to identify the boundary, although the top of the Lower Mottled Clay is often pale or bleached and the base of the Upper Mottled Clay usually consists of a thin carbonaceous grey, blue or black clay layer. Many of the site investigation boreholes used in this study identify this darker layer or lignite that is likely to be the base of the Upper Mottled Clay. This can be seen in coastal section at Alum Bay in the Isle of Wight, and was documented during the excavations for the Newbury Bypass in Berkshire. This is best identified in section, as at Alum Bay, or in good quality core, this boundary has also been described in core retrieved from standard ground investigations using cable percussion techniques.

Thickness
The Reading Formation is over 30 m thick in the Isle of Wight where, due to folding, it is almost vertically bedded, but is generally about 15 m thick around Newbury, up to 20 m in the south-west of London and thins progressively eastwards. In the east of London, where the upper leaf is missing, it is generally less than 2 m thick and locally may be missing altogether.

Lithology
The bulk of the Reading Formation consists of indistinctly or poorly bedded, colour mottled or multicoloured, silty clay and clay. This characteristic lithology was formerly called the ‘Reading Beds’ or ‘plastic clay’. Colours include pale brown, pale grey-blue, dark brown, pale green, red-brown and crimson, depending on the oxidation and hydration state of the iron in the sediments. The clays contain numerous fissures, many of them listric, which give rise to a blocky texture. Thin, black, carbonaceous clays are recorded locally in the west of the district in the middle of the sequence and this marks the approximate boundary of the upper and lower leaves of the Reading Formation (Upper Mottled Clay and Lower Mottled Clay). Beds of colour-mottled silt and sand constitute up to 50 per cent of the unit, particularly in the east, in the west around Newbury and the north Hampshire Basin. The colour is dominantly of brown hues, red hues being less prevalent than in the clays (Figure 2.18). These beds are thinly laminated in places with small burrows and root traces, and minor brecciation caused by soft sediment deformation. The clays are generally stiff to very stiff and occasionally extremely weak, closely or very closely sometimes extremely closely fissured, the fissures may be sub vertical or randomly oriented, striated or polished, undulating to planar and clean.

Figure 2.18    Reading Formation examples of mottled clays from Alum Bay, Isle of Wight (left), Knowl Hill Quarry, near Maidenhead (right), Berkshire [SU 8160 9770].

Beds of well-sorted sand, mainly in the west of the region, represent sand in-filled river channels (Figure 2.19).

Figure 2.19    Channel sand (below) and mottled clay (above). Newbury Bypass, Berkshire.

The Lower Mottled Clay may be purple-red. The top part of the unit contains irregular-shaped, soft and ‘powdery’ to strong ‘limestone’ nodules up to 0.5 m in diameter. Sands become increasingly dominant in an easterly direction east of central London and are sometimes referred to as lower mottled sand. In Essex and Kent, the sediments pass into pale grey-brown, turquoise to dark green and brown mottled, structureless slightly clayey sand with minor amounts of irregularly iron-cemented calcareous clayey sands. In places in north Kent these iron-cemented sands are known locally as Winterbourne Ironstone.

The Upper Mottled Clay is not distinguished lithologically either from the Lower Mottled Clay or the main bulk of the undivided Reading Formation. It is identified by its position, above the Lower Shelly Clay or dark beds associated with the Woolwich Formation. It consists largely of mottled clay, silty clay and silt with channel sand infill. The colours are similar to those of the Lower Mottled Clay, but the purple hues are absent. However, in some parts of London the colours are more limited and are mostly mottled grey and brown (Figure 2.20).

Figure 2.20    Reading Formation, Upper Mottled Clay comprising very stiff, grey mottled brown, extremely to very closely fissured, slightly sandy CLAY. CTRL borehole 2112, (top to the left).

Very occasionally, deposits similar to the Woolwich Formation occur in both the Upper and Lower Mottled Clay. For example dark grey sulphide-rich clay could be deposited in permanent vegetated ponds or small lakes, possibly ox-bow lakes, in the alluvial tract. If these deposits are thick enough they might be preserved within the mottled clays.

Origin of mottled appearance in Reading Formation
The Reading Formation sediments were deposited at a time when SE England had a sub-tropical climate, similar to that which exists currently in parts of Africa, the Far East and Central America. Although these sediments would have originally been deposited in a variety of environments from rivers to marshes, shortly afterwards they were subjected to sub-tropical weathering or pedogenesis, at or just above base water/sea level.

Tropical pedogenesis rapidly alters the mineralogy changing the colour and textures of the original material. The main processes involved in this change are hydrolysis, downward transport of fines (illuviation), chemical breakdown and dissolution, and redeposition of minerals from solution (e.g. iron, calcium, silica). Seasonal changes in the height of the water table, cause radical changes in the dominant weathering process from reducing to oxidising, and this affects the final colour of the sediments.

Other, mechanical, affects also have a huge influence during tropical pedogenesis. These include expansion and contraction during daily temperature change and shrinkage and swelling caused by drying and wetting probably caused by seasonal rains. These two processes resulted in multiple fissuring, the fissures often with polished surfaces. Finally, animals, such as burrowing crustaceans and worms, and plants all radically change the texture of the soil.

Two examples of Reading Formation sediments from a cored borehole in north London are shown in Figure 2.21. The top core run is firm to stiff, multicoloured, sandy clay, which is light blue green with large (up to 25% of the sediment) bright red and occasional smaller, orange yellow mottles, which are seen to be even more abundant in the lower core. The red is the oxidised iron mineral haematite, which is commonly formed during drier periods. The yellow coloured mottles are goethite, a hydrated iron oxide that forms in damper conditions. The lower core material is superimposed with a fine network of blue-grey to brown formed by plant roots and rootlets. The activity of bacteria during the decaying of the roots produces reduced iron minerals, which are grey.

Figure 2.21    Borehole core from St Pancras area of north London. See text for description. (Copyright Jackie Skipper).

Woolwich Formation[edit]

The Woolwich Formation rests on the Lower Mottled Clay of the Reading Formation (Figure 2.9) and is present in the east of the London Basin and the easterly part of the Hampshire Basin (Figure 2.8). There are several distinctive units within the Woolwich Formation, namely:

  1. Lower Shelly Clay (including the basal lignite),
  2. Laminated Beds,
  3. Upper Shelly Clay,
  4. ‘Striped Loam’.

1. Lower Shelly Clay
Distribution
The Lower Shelly Clay occurs principally in east and southeast London (Figure 2.22), north Kent and the eastern edge of the Hampshire Basin. It has been identified recently in boreholes near Wandsworth Bridge [TQ 260 755] but not at Putney Bridge [TQ 242 757] (Jackie Skipper personal communication June 2013). In general, the unit thickens from central London towards the southeast, reaching a maximum of about 6 m.

Figure 2.22    Distribution and thickness of the Lower Shelly Clay in London (Ellison et al., 2004[29]).

Basal Boundary
The basal boundary of the Lower Shelly Clay is sharp, well defined and disconformable on the often pale and bleached, pedogenically altered Lower Mottled Clay. Burrows containing dark clay, lignite or shells may occur in burrows extending up to 1.5 m into the Lower Mottled Clay. In some parts of central London the contrast is between multicoloured clay with calcium carbonate concretions or pale, bleached Lower Mottled Clay and overlying dark grey shelly clay (Figure 2.23).

Figure 2.23    The contact between the Lower Mottled Clay and Lower Shelly Clay. JLE404T, 33.00 to 33.90 m. Very stiff multicoloured grey CLAY with ‘powdery’ and nodular calcium carbonate (Lower Mottled Clay, 33.77 to 33.90 m.b.g.l.) below stiff highly fossiliferous grey clay with sand at the base (Lower Shelly Clay, 33.19 to 33.70 m.b.g.l.). CTRL 2131. Very stiff, multicoloured sandy clay (Lower Mottled Clay) (below) very stiff, thickly laminated, fissured gravelly clay. Gravel is of shell fragments (Lower Shelly Clay). Base irregular sharp and inclined.

Lithology
The dominant lithology of this unit is dark grey to black clay that contains abundant shells. In east London, there is a general increase of medium sand in the matrix and, in places (e.g. near Stratford) beds up to 1 m thick occur consisting almost entirely of shells forming weakly cemented limestone. The basal few centimetres of the unit also tend to be relatively sandy and commonly contain oyster shells. About 1 to 2 m above the base of the Lower Shelly Clay a bed dominated by oysters encrusted with bryozoa and cemented in places, occurs locally (Dewey and Bromehead, 1921[36]; Tracey, 1986[37]). These shell-dominated beds indicate that sediment input was low, thus allowing the development of shell banks, and they may represent a maximum flooding surface. A few beds of brownish grey clay, slightly cemented with siderite, occur sporadically throughout, particularly in the higher parts of the unit and, in places, fine carbonaceous debris occurs, some of it is pyritised.

Lignite is commonly seen at the base of the Lower Shelly Clay in southeast London. It consists of very weak to weak, extremely to very closely fractured, sometimes thinly laminated, dark brown and black, lignite with soft black and brown very organic clay. The lignite may be recovered as dark brown and black clay, angular and sub-angular fine to coarse lignite gravel or extremely to very closely fractured lignite. It is generally less than 0.3 m thick but at Shorne [TQ 678 697], in north Kent, it is up to 2 m thick and displays a cleat (closely spaced joints) similar to a sub-bituminous coal (Figure 2.24) (Collinson et al., 2003[38]). It is interbedded with pale grey, leached, medium-grained sand and pale grey clay with lignitic wood fragments and small listric fractures, similar to seatearth.

Figure 2.24    A succession of partly bleached Lower Mottled Clay sand facies overlain by a 1.5 m thick lignite bed, displaying cleat, and 1 m of shelly clay, of the Lower Shelly Clay unit. HS1 railway cut at Shorne, Kent [TQ 678 697].

Dewey and Bromehead (1921[36], p.20) recorded a ‘freshwater bed’ of limited distribution, within the middle of the ‘shell beds’ in an area around Leytonstone, Dulwich, Pechham and Brockley. The Upper Shelly Clay may rest directly on the Lower Shelly Clay in south-east London in the vicinity of Petts Wood and St Mary Cray [TQ 45 68] (Whitaker, 1872[19], p.116, Figure 14).

2. Laminated Beds
This unit is the equivalent to the ‘laminated sands and silts’ of Ellison (1991)[35].

Distribution
The Laminated Beds occur in the south-east of the London as far as Swanscombe (Figure 2.25) and in the east of the Hampshire Basin most notably at Newhaven, and reaches a maximum thickness of up to about 5 m south of Stratford.

Figure 2.25    Distribution and thickness of the Laminated Beds in London (Ellison et al., 2004[29]).

A second unit of laminated beds occurs higher in the succession and are part of the Upper Shelly Clay around Lewisham where it was formerly known as ‘Striped loams’ (Dewey et al., 1924[32]). The stratigraphical relationships of these laminated beds are uncertain but they probably have an erosive base, cutting down through the Upper Shelly Clay.

Basal Boundary
The base of the Laminated Beds is sharply defined with the underlying Lower Shelly Clay (Figure 2.26) or, locally, on the Lower Mottled Clay.

Figure 2.26    The contact between the dark grey very shelly clay of the Lower Shelly Clay below, and the thinly laminated clay and silt of the Laminated Beds above. (Borehole JLE404T, (TQ (5) 3363 (1)7960, 33.00 to 33.60 m).

Lithology
The Laminated Beds consist of thinly to thickly laminated silt and clay and laminated to thinly bedded clay or silt and sand with scattered, occasionally thin beds of packed, intact bivalve shells. Beds are generally less than 50 mm (Figure 2.27). These deposits are typically pale grey to dark grey when un-oxidised and pale greenish brown, yellow to orange when oxidised. Sedimentary features include lenticular bedding; ripple lamination, burrows and some bioturbated, structureless beds. Bodies of sand are commonly present and vary in thickness from 5 mm to 1.4 m thick , but locally up to 5 m (for example Jubilee Line extension [JLE] borehole 404T [BGS borehole TQ37NW/2118 {TQ 33638 79604}]). They occur throughout London as channel fills or more extensive sheets and are best known around Lambeth and Bermondsey. Typically the sand is medium grained and cross-laminated, with some clay drapes and rare bivalves. Thin beds of colour mottled clay and silt occur within the Laminated Beds between Docklands and Stratford.

Figure 2.27    The lithologies of the Laminated Beds from Borehole JLE404T, west of Bermondsey Station (a) finely laminated clay and silt, laminae with some ripple lamination (32.20 to 33.00 m). (b) Laminated fine to medium sand with cross lamination (30.70 to 31.50 m).

At Abbey Wood [TQ 484 787], shelly medium-grained sands underlie the Harwich Formation. The strata are included in the Laminated Beds, although their age and precise stratigraphical relationship is uncertain (Hooker, 1991[39]).

3. Upper Shelly Clay
Distribution
The main occurrence of the Upper Shelly Clay is in south London (Figure 2.28). To the southeast and northwest there are isolated occurrences, proved sporadically in boreholes, preserved in shallow depressions below an erosion surface at the base of the Harwich Formation. The unit is up to about 3 m thick. It is likely that beds equivalent to the Upper Shelly Clay are present farther southeast than is shown in (Figure 2.28) but, in the absence of the intervening Upper Mottled Clay unit, they cannot be distinguished from the Lower Shelly Clay in sections or borehole cores. One exception to this is in the Crystal Palace Borehole [TQ 3379 7082] where the base of the Upper Shelly Clay is taken at a thin lignite bed. In some borehole records, the Upper Shelly Clay was formerly interpreted as the Harwich Formation.

Figure 2.28    The distribution and thickness of the Upper Shelly Clay in the London area (Ellison et al., 2004[29]).

Basal boundary
The base of the unit is sharp and it rests disconformably on the Upper Mottled Clay; where it rests on Laminated Beds the contact may be a rapid gradation.

Lithology
The Upper Shelly Clay consists mainly of stiff, brown and dark grey to black shelly clay, sandy clay and very weak to strong argillaceous limestone with fossil oysters. Thinly interbedded grey-brown silt and very fine-grained sand with scattered glauconite grains also occur and it becomes mainly sand to the south east. However, the sands may be partly incised channel-fills that post date the deposition of the Upper Shelley Clay (King, in press). Bioturbated beds, sand-filled burrows and clay rip-up clasts (less than 5 mm in diameter) are characteristic, and locally there is a weakly to strongly cemented shell bed (up to 0.43 m thick) containing Ostrea. Between Bermondsey and Lewisham is a more or less continuous bed of grey argillaceous shelly limestone known as the Paludina Limestone. The bed is generally 0.1 to 0.3 m thick, up to a maximum of 1.89 m, and contains unbroken and broken gastropods Hydrobia, Planorbis and Viviparus, which indicate deposition in a freshwater lake. A thin bed of lignite occurs locally at the base of the unit at Crystal Palace.

Most of the Upper Shelly Clay contains disarticulated bivalves of more marine-tolerant species and a generally greater diversity of fauna than in the Lower Shelly Clay.

4. ‘Striped Loam’
Distribution
South East London up to about 9 m thick.

Basal boundary
Sharply-defined contact between the shelly clay, sandy clay and muddy limestone of the underlying part of the Upper Shelly Clay to laminated and thinly-bedded fine sand, silt and clay of the Striped Loam.

Lithology
Laminated and thinly bedded fine sands, silts, clay and sandy clay. Seen in the Charlton Pit [TQ 418 786] where it comprises laminations and thin bedded fine sand and clay with lignite.

Depositional environment and processes[edit]

The regional distribution of deposits of the Lambeth Group was recognised as cyclic in nature by Stamp (1921)[40], an idea developed further by Hester (1965)[31] and Ellison (1983)[33]. Four depositional sequences separated by unconformities are now recognised (Knox, 1996a[8]; Table 6). The sediments as a whole were laid down in a coastal or possibly estuarine setting (Figure 2.29) in which small fluctuations in sea level led to marked changes in depositional environment.

Figure 2.29    Schematic block diagram to illustrate the environment of deposition of the Lambeth Group (Ellison et al., 2004[29]).

Following a period of regression and weathering of the top of the Thanet Formation, the lowest beds of the Upnor Formation were laid down in transgressive littoral to sub-littoral marine, tidal conditions.

The abundance of glauconite indicates periods of sediment starvation. The Upnor Formation as a whole is interpreted as highstand deposits in which marine flooding events are marked by pulses of glauconite deposition, winnowing and low sediment input (Ellison et al., 1986[12]).

Deposition of the upper part of the Upnor Formation followed a lowering of sea level that may have led to the removal of some of the earlier deposits. A reduction in sea level, possibly combined with uplift, led to emergence and establishment of a terrestrial environment in which the Reading Formation was laid down on marshy mudflats that formed an alluvial floodplain crossed by river channels. This period of emergence marked the beginning of deposition of the Lower Mottled Clay of the Reading Formation on an alluvial floodplain that was liable to intermittent floods and water table fluctuation. Periodic emergence led to oxidation, pedogenesis and the development of the characteristic red coloration, but the instability of this environment precluded the development of extensive colonisation by vegetation. The prevailing sub-tropical climate of high temperatures and pronounced wet and dry seasons led to subaerial weathering and soil-forming processes (pedogenesis) that affected both the Lower Mottled Clay and the Upnor Formation where they were close to the ground surface. This led to the formation of local silcretes (Kerr, 1955[41]) and clasts of silica-cemented conglomerate in gravel beds at the top of the Upnor Formation. These clasts are typical of the ‘Hertfordshire Puddingstone’ that, in association with silica cemented sandstones (sarsens) were widespread to the north and west of the district (see Section 3.6.1.2 and Potter, 1998[42]) and in the eastern South Downs. Estuarine and fresh water palynomorphs in the sandier parts of the Lower Mottled Clay in the east are evidence for intermittent encroachment by the sea onto the alluvial floodplain. This period of emergence represented the mid-Lambeth Group Hiatus and not only resulted in pedogenesis, but also erosion and low relief. A temporary rise in sea level led to the establishment of lagoonal and estuarine conditions and the deposition of the Lower Shelly Clay and Laminated Beds of the Woolwich Formation in the central and eastern parts of the district. Sand bodies in these units contain a brackish water palynomorph assemblage consistent with deposition in estuarine tidal channels. This sequence culminated with a return to continental conditions and deposition of the Upper Mottled Clay. Following a further depositional hiatus a rise in sea level and renewed flooding resulted in the deposition of brackish lagoonal and estuarine sediments (Upper Shelly Clay and ‘striped loams’). This period of deposition was terminated by uplift and erosion that removed much of the Lambeth Group sediments in the north and east of the district.

The variation of the lithology and lithostratigraphy of the London Basin and Hampshire Basin are shown in (Figure 2.30).

Figure 2.30    Schematic sections showing the lithostratigraphic variation in the Lambeth Group in the London Basin (top) and Hampshire Basin (bottom) (after Ellison et al., 2004[29]).

Post depositional features[edit]

The principal post depositional influences on the Lambeth Group are tectonic forces, glacial and periglacial processes and changing groundwater level. In areas of outcrop surface weathering effects also may have modified the deposits.

Structural features[edit]

The Lambeth Group has been gently folded on a regional scale and the beds in the majority of the London and Hampshire Basins generally dip less than 1º. Steep dips, greater than 45º, occur principally along the Hog’s Back and in the Isle of Wight. In these areas the Lambeth Group, mainly Reading Formation, may contain shear planes and minor faults. Steep dips have also been recorded close to the Greenwich Fault in south-east London (Bromehead, 1922[43]). Major fault systems with large throws are fairly well delineated and, in London, include the Greenwich Fault, Wimbledon Fault and the Streatham Fault. However, in the London area the Tertiary deposits and chalk are faulted by generally minor normal faults, generally with throws of less than 10 m. They often trend between N–S to NW–SE and ENE–WSW to E–W. In boreholes both normal and reverse faulting have been observed. Complex faulting systems have been identified during the site investigation of the Lee Valley Tunnel (Newman, 2008[44]), Thames Water Ring Main (Newman, 2009[45]), Thames Tunnel (Newman and Hadlow, 2011[46]) and in the 3D geological model for the Farringdon Station area of the Crossrail project (Aldiss et al., 2012[9]) in which seven faults of about 2 to 10 m throw were identified within a study area of about 900 m east to west and about 500 m north to south. High quality borehole description, the lithological variation and recent understanding of the units within the Lambeth Group has meant that detailed modelling of these small faults can be undertaken. These fault, although most are minor, may have implications for engineering design and construction particularly in tunnelling projects due to rapid localised changes in lithology and hydrogeology conditions.

Features related to glaciation[edit]

Only the north-western areas of the Lambeth Group, from north London to Essex and Suffolk, have been covered by glacial deposits. There is no specific information about glacial disturbance but it is possible that release of high hydrostatic pressure beneath an ice margin may have caused disruption of bedding in sands within the Lambeth Group when near surface.

Features related to periglaciation[edit]

Periglacial conditions existed in southern England during at least two glacial episodes in the past 500 000 years. The thinly bedded nature of the Lambeth Group, in particular the presence of water-bearing sands, resulted in a relatively high susceptibility to disturbance caused by ground ice and cryoturbation.

Pingos, large dome-shaped bodies of ground ice developed below the ground surface, grow by the progressive addition of water, probably under artesian pressure. Melting of pingos is thought to be at least partly responsible for more than 25 anomalously deep subsurface depressions in the rockhead beneath London (Hutchinson, 1980[47]). Associated with some of these hollows are masses of Lambeth Group sediments that have been injected, under high hydrostatic pressure, through the London Clay into the base of the hollow. Artesian groundwater conditions occurred in much of the central part of the London Basin, and, therefore, there is a possibility that undiscovered areas of similarly disturbed Lambeth Group exist.

Modification of the Lambeth Group at outcrop by active-layer processes such as cryoturbation is largely unstudied. It is likely to have resulted in the development of small-scale structures such as involutions and the diapiric injection of sands. These features may be particularly well developed in interbedded sand and clay because of the potential contrast in the freezing point of groundwater in coarser compared to fine grained sediments, dependent on the relative pore pressures. Slopes formed of the clay-dominated Reading Formation owe their present form to periglacial slope processes and are likely to be mantled by 1 to 3 m of Head deposits containing shear surfaces aligned roughly parallel to the ground surface. Immediately beneath the Head, periglacially weathered clay is generally brecciated and softer than the unweathered clay beneath (see for example Spink, 1991[48]). Periglacial shearing of the Reading Formation is likely to be exacerbated by the presence of pre-existing shears in the mottled clays.

Valley bulging involves broad anticlinal deformation of strata underlying valley floors, under periglacial conditions. It is commonly associated with clay-dominated strata and is likely to affect all valleys that have been rapidly incised. This releases large horizontal stresses, which under favourable conditions may be sufficient to initiate lateral deformation of the deposits towards the valley axis.

Features due to chalk dissolution[edit]

The Chalk dissolves to give a karstic surface with pipes and swallow holes up to several metres deep. The most significant dissolution has occurred at the margin of overlying impermeable deposits where surface drainage is concentrated, for example close to the junction with the Lambeth Group. Although many of the dissolution features are filled with superficial deposits, the Lambeth Group and the underlying Thanet Formation, particularly close to the edge of the outcrops, may also be let down into Chalk dissolution features.

The effect of rising groundwater[edit]

Major abstraction of water from the London aquifer, starting in the early part of the 19th century, led to a fall in groundwater levels in the central region of the basin (Water Resources Board, 1972[49]). Consequently the top of the Chalk was probably dewatered over an area of several square kilometres in the centre of the basin (Lucas and Robinson, 1995[50]).

The current rise in groundwater levels in London, caused by a reduction in water abstraction, is well documented (see for example Environment Agency, 2001[51]). It has an influence on the Lambeth Group because the sandy Upnor Formation is regarded as being in hydrogeological continuity with the underlying Thanet Formation and Chalk. Historically, these sandy Tertiary beds together have been known as the ‘Basal Sands aquifer’.

The abstraction of water from the aquifer, starting in the early part of the 19th century, led to a fall in groundwater levels in the central region of the basin (Water Resources Board, 1972[49]). Consequently the Upnor and Thanet formations and the top of the Chalk were probably dewatered over an area of several square kilometres in the centre of the basin (Lucas and Robinson, 1995[50]). However, some sand beds not in contact with the main aquifer, particularly within the upper part of the Lambeth Group, still contain water and ground water lowering of these units required dewatering.

The recovery of groundwater levels in London has several implications that were considered in a report by CIRIA (Simpson et al., 1989[52]). Basements or tunnels excavated above the water table and not sealed against the ingress of water would be subject to flooding. Sealed structures submerged by rising water would become buoyant and liable to uplift pressures detrimental to stability. Structures originally below the water table might not be sufficiently watertight to contend with increased hydrostatic head and remedial sealing or continuous pumping would be required. In response, the GARDIT (general Aquifer Research Development and Investigation Team) strategy was launched to control the rising groundwater level in the lower aquifer, which are now broadly stable throughout central London (Jones, 2007[53]).

Tunnels are already suffering from increased seepage, and chemical attack. One example is on the London Underground Northern Line, where very acidic waters caused deterioration of the tunnel linings south of Old Street station. Investigations there suggested that the source of the acid was oxidised pyrite in sands in the Lambeth Group, probably in the Laminated Beds. These beds had originally been saturated, but dewatered as the water table was lowered. The pyrite was subsequently oxidised by air from the railway tunnels, in particular by the piston effect of passing trains and by changes in barometric pressure. Water seeping from the overlying London Clay, has resulted in the production of highly acidic, aggressive groundwater (Robins et al., 1997[54]). As the water table rises, increasing amounts of oxidised pyrite will give rise to potentially corrosive acidic groundwater (Rainey and Rosenbaum, 1989[55]).

During conditions of falling water table, the resultant under drainage and consolidation of strata resulted in the lowering of the ground surface in Central London by several hundred millimetres (Water Resources Board, 1972[49]). It also increased the strength of the London Clay and clays in the Lambeth Group. As a result of rising groundwater, increase in pore water pressure and the swelling of clay may result in a loss of shear strength.

Features due to burial[edit]

After deposition the Lambeth Group was buried beneath the London Clay and later Tertiary sediments that together were possibly up to 250 m thick in London. The Lambeth Group sediments have become denser, stronger and stiffer by consolidation. However, within the Lower Mottled and Upper Mottled Clay and the alter Upnor Formation pedogenic processes including desiccation and cementing will have increased these characteristic before the deposition of the London Clay Formation. This has been overprinted by relaxation because of erosion of the overburden after uplift over the last 20 million years. Both the initial burial and the subsequent swelling associated with stress relief due to erosion may have weakened cementation; particularly in the clays with higher plasticity. Additional fissuring may also have developed during unloading, although this will have been resisted by the presence of cementing and is likely to occur only near surface.

Current geological mapping[edit]

The Lambeth Group is represented on the current BGS DIGMapGB50 in a number of ways, as indicated below and in (Figure 2.31).

  • Lower London Tertiaries (Lambeth Group and Thanet Formation) in northeast London Basin,
  • Lambeth Group in the rest of the London Basin and east Hampshire Basin,
  • Reading Formation in the west central Hampshire Basin,
  • West Park Member of the London Clay Formation in the west Hampshire Basin.

The lithostratigraphical classification is further separated by lithology as below:

  • Lambeth Group, undifferentiated clay, silt and sand or clay, sand and gravel,
  • Lambeth Group or Reading Formation, sand and gravel,
  • Lambeth Group or Reading Formation, sand,
  • Reading Formation, clay, silt and sand,
  • Reading Formation, sand and gravel,
  • Reading Formation, sand,
  • Lower London Tertiaries, clay, silt and sand,
  • West Park Farm Member, clay,
  • West Park Farm Member, clay and silt,
  • West Park Farm Member, sand.
Figure 2.31    The distribution and differentiation of the Lambeth Group as recorded in the BGS DIGMapGB50.

Much of the Lambeth Group is described as clay, silt and sand on the maps. However, there are some exceptions. In east Kent, where the Upnor Formation predominates, it is mapped as sand. Three lithological units are mapped in the north and west Hampshire Basin, that is, clay, silt and sand, sand only and gravel only. In this area more of the deposits are thought to have of fluvial origin and hence the greater importance of coarse material. In the west Hampshire Basin what was mapped as the Lambeth Group is now mapped as clay and silt or sand of the West Park Farm Member of the London Clay Formation.

Locations of exposures and borehole core material from the Lambeth Group[edit]

Man-made and natural exposures are useful aids to understand the lithologies and possible variation of a deposit. The complexity of the Lambeth Group does mean that there are no ‘typical’ exposures; however, they can be used as a guide. Borehole core is available for inspection on request at the British Geological Survey in Keyworth.

Natural exposures[edit]

Daley and Balson (1999)[18], Ellison et al. (2004) [29] and Skipper (1999)[2] have reviewed the accessible exposures of the Tertiary deposits (Table 2.2). Permission may be required to access the sites.

Table 2.2    Location of Lambeth Group exposures.
Basin Site Site type Grid reference Formations
London Hearne and Reculver Bay Cliff section TR 218 690 Upnor Formation
London Lower Upnor Sand Pit Quarry TQ 759 711 Upnor Formation,
Lower Mottled Clay(?) — sand
Woolwich Formation
London Orsett Cock Quarry TQ 657 811 Upnor Formation
London Orsett Tarmac Quarry Quarry TQ 671 805 Upnor Formation
London Swanscombe Eastern Quarry Quarry TQ 605 730 Upnor Formation
Lower Mottled Clay
Woolwich Formation
London Charlton Sand Pit
(Gilbert’s Pit)
Old quarry (SSSI) TQ 419 786 Upnor Formation
Lower Mottled Clay
Woolwich Formation
London Harefield Great Pit Old quarry TQ 050 898 Upnor Formation
Reading Formation
London Pincent’s Kiln, Theale Old quarry SU 651 721 Upnor Formation
Reading Formation
London Bolter End, Bucks. Old quarry SU 651 721 Upnor Formation
Reading Formation
London Knowl Hill Quarry SU 825 795 Upnor Formation
Reading Formation
Hampshire Newhaven Cliff section TQ 446 002 Upnor Formation
Woolwich Formation
Hampshire Whitecliff Bay Cliff section SZ 639 858 Upnor Formation
Reading Formation
Hampshire Alum Bay Cliff section SZ 305 858 Upnor Formation
Reading Formation
Hampshire Studland Bay Cliff section SZ 043 823 Upnor Formation (?)
Reading Formation (?)

Bold = type section

Reference core material[edit]

Some borehole core has been retained by the British Geological Survey as reference material that can be inspected on request (Table 2.3). (Contact: The National Geoscience Data Centre, British Geological Survey, Keyworth, Nottingham). Photographs and description of JLE404T (BGS borehole TQ37NW/2118) cores are in Appendix 1.

Table 2.3    Reference borehole core.
Basin Borehole BGS Borehole Reference National Grid Reference Formations
London JLE404T TQ37NW/2118 TQ 3363 7960 Upnor Formation
Reading Formation
Woolwich Formation

Detailed distribution and lithological cross-sections[edit]

Sediments of the Lambeth Group are lithostratigraphical and lithologically complex, being extremely variable both laterally and vertically. To illustrate this, a number of sections with schematic borehole logs are presented.

Lithostratigraphical sections[edit]

Detailed lithostratigraphical borehole logs of the Lambeth Group by Skipper (1999)[2] and Ellison et al. (2004)[29] are shown in Figure 2.32 to Figure 2.37 using the mid-Lambeth Group Hiatus as a common lithostratigraphic horizon.

Figure 2.32    Key to the lithostratigraphical boreholes shown in Figure 2.33 to Figure 2.37.
Figure 2.33    Lithostratigraphical boreholes in the London Basin, west to east, from the Newbury Bypass to Winterbourne Sand Pit. For key see Figure 2.32.
Figure 2.34    Lithostratigraphical boreholes in the London Basin, north to south from Islington to Crystal Palace. For key see Figure 2.32.
Figure 2.35    Lithostratigraphical boreholes in the London Basin, west to east, St. Pancras to Barking. For key see Figure 2.32.
Figure 2.36    Lithostratigraphical boreholes in the London Basin, west to east, London Bridge Station to Canada Water Station. For key see Figure 2.32.
Figure 2.37    Lithostratigraphical boreholes in the Hampshire Basin, west to east, Alum Bay to Newhaven. For key see Figure 2.32.

Lithostratigraphical and lithological cross section[edit]

Twenty selected cross-sections based on boreholes drilled for major site investigations (including motorways, other major roads and road and railway tunnels) are listed in Table 2.4 and presented in Appendix 2. The sections presented show both lithostratigraphy and lithology (indicated by annotated ‘borehole sticks’) and highlight the lithological variations within the lithostratigraphic divisions. The sections provide an impression of continuous lithostratigraphy and are based on more closely spaced boreholes than those shown in Figure 2.33 to Figure 2.37.

Lithostratigraphic and lithological data for the boreholes used to construct the cross sections shown in Appendix 2 were extracted from records held in the BGS National Geotechnical Database and displayed graphically, in their correct geographic position, using a 3D modelling package (GSI3D).

Table 2.4    Cross-sections in Appendix 2.
Number Figure Project name Area
1 4 A34 Newbury Bypass Curridge to Bunkers Hill
2 5 M4 J8(9)–12 Widening South Reading, Berkshire
3 6 M40 J1A-3 Widening Beaconsfield to M40/M25 junction
4 7 M25: M4 to Maple Cross
5 8 Crossrail Paddington to Bishop’s Gate
6 9 + 10 Jubilee Line Extension Green Park to Millennium Stadium
7 11 + 12 Channel Tunnel Rail Link St Pancras to A406 Barking
8 13 M11 Link — A104/A114 to A12 Hackney
9 14 Channel Tunnel Rail Link Stratford to Leyton
10 15 + 16 Channel Tunnel Rail Link A406 Barking to Rainham
11 17 A406 South Woodford to Barking Relief Road South Woodford to Barking
12 18 Docklands Light Railway:
Lewisham extension
Greenwich to Island Gardens, Isle of Dogs
13 19 A102 Blackwall Tunnel Third Bore Blackwall Tunnel
14 20 Jubilee Line Extension North Greenwich to Canning Town
15 21 A13 Orsett Cock to Stanford Interchange A13 Orsett Cock to Stanford Interchange
16 22 Stanford Le Hope Low Level Sewerage Scheme Stanford-Le-Hope
17 23 M2 widening Shorne Cut, Kent

Summary of lithostratigraphical and lithological data[edit]

Lithological variation has been identified as the main engineering problem of the Lambeth Group. To further highlight this, summaries of the lithostratigraphic units and their dominant lithologies as described in boreholes, for selected areas, are presented graphically in (Appendix 3). The graphs show percentage core lengths of the lithostratigraphic units encountered in each area and the dominant lithologies (clay, silt, sand, gravel, limestone and lignite) described for each unit and the Lambeth Group as a whole. Where ‘limestone’ lithologies are indicated, these may be either calcrete, found in the Lower Mottled Clay and the Upnor Formation, or shell limestone in the Lower or Upper Shelly Clay. The graphs also show the total number of metres of Lambeth Group core described in each area in order to give an indication of the significance of the data.

Deposits above the Lambeth Group[edit]

Harwich Formation[edit]

This term was introduced by Ellison et al. (1994)[1] to include all sediments between the Lambeth Group and the London Clay Formation. They were formerly differentiated by Prestwich (1854)[56] as the ‘Basement-bed of the London Clay’, and subsequently divided by Whitaker (1866)[57] into the Basement-bed, the Oldhaven Beds and the Blackheath Beds. Currently, the Harwich Formation is divided into the Blackheath, Oldhaven, Swanscombe, Orwell, and Wrabness members (Aldiss, 2012[9]).

Distribution[edit]

The Harwich Formation occurs in the London and Hampshire Basins but is best known and described for the latter region (Figure 2.38), where it reaches a maximum thickness of 10 to 12 m around Orpington and Chiselhurst, Kent. South and east of London, where the deposits were formerly mapped as Blackheath Beds, there are numerous descriptions of former exposures recording very variable thicknesses over quite short distances; this probably indicates an irregular base. Elsewhere in the region, most of the information on the Harwich Formation is from borehole records (for strata formerly referred to the Basement Bed of the London Clay), and recent excavations for Crossrail, which indicate that the Harwich Formation is up to 4 m thick. In the northeast, an incomplete thickness of 6.88 m was proved in the Stock Borehole [TQ 7054 0045], and in the south-east the Stanford-le-Hope Borehole [TQ 6965 8241] proved 4 m.

Figure 2.38    Ribbon diagram of the Harwich Formation showing the lithological variation and general thickness in London (Ellison et al., 2004[29]).

Basal boundary[edit]

The base is sharply defined, and forms an erosive contact on the Lambeth Group. Locally, in outliers at Kelvingtown [TQ 485 675] and Swanley [TQ 515 686], the Harwich Formation oversteps onto the Thanet Formation.

Lithology[edit]

Glauconitic fine-grained sand and gravel beds of rounded black flints are the principal lithologies with, in places, common disarticulated and broken shells of marine to brackish fauna (see Dewey et al., 1924[32]). The proportion of gravels varies considerably. Calcareous, ferruginous and siliceous cements occur locally in beds and masses up to several metres thick (for details see Dewey and Bromehead, 1921[36]; Dewey et al., 1924[32]), particularly at outcrop in the southeast of the district. Recent excavations for Crossrail and Thames Water projects have found multiple, discontinuous layers of calcite cemented sand and gravel beds in the Blackheath and Oldhaven members up to 750 mm thick in the West Ham and Isle of Dogs areas of east London. Also in these areas, weak calcareous mudstone and strong calcareous siltstone concretions up to 250 mm thick and 450 mm wide occur in the Swanscombe Member.

In the northeast the Harwich Formation is dominated by relatively fine-grained sand and is generally less pebbly. The succession is known in detail only in Stock Borehole [TQ 7054 0045] where grey-green silty fine-grained sand with scattered broken shell fragments and stringers of black flint gravels are recorded. At outcrop in south Essex, the Harwich Formation consists of green-grey, weathering to pale yellow-brown, highly glauconitic fine to medium-grained sand and gravelly sand. Calcareous mollusc fossils and scattered sharks’ teeth are typical although the shells are decalcified in places.

In areas where gravel beds dominate the sequence they consist of a series of superimposed channels as is seen, for example, in the cutting for the A2 at Shorne (Figure 2.39), with large foresets composed almost entirely of flint gravels, imbricated in places, and rare gravels of siliceous sandstone (similar to sarsenstone). The gravel beds include clasts up to 150 mm in diameter but generally less than 20 mm.

Figure 2.39    Harwich Formation sand and gravels in a cutting for the HS1 railway at Shorne, Kent.

Harwich Formation gravel beds are best exposed in former quarries, now Sites of Special Scientific Interest (SSSI), at Charlton [TQ 419 786] and Elmstead Rock Pit, Chiselhurst [TQ 423 706], and in a small pit at the Inn on the Lake [TQ 675 699] near Gravesend.

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