Protecting the geodiversity of the capital

Evolution 7 and 8). The London district lies at the junction of two deep-seated geological 4.1 this chapter sets the geological scene for terranes within the pre-Mesozoic a discussion of the geodiversity of Greater basement. A third terrane lies just to the London, summarising the geological east (Pharaoh et al, 1993). These can be evolution and development of the seen on the Bouguer gravity anomaly map landscape. of the region Figure 9).

Geological time 4.5 the north and west of the district is underlain by the Midlands Microcraton, 4.2 geological time is divided into Eons, Eras, an area where Proterozoic and Palaeozoic Periods and Epochs (Figure 6). Although rocks occur at relatively shallow depths. the Earth is almost 4600 million years Structural trends are complex. The old, events from only the last 100 million Midlands Microcraton extends west as years or so are represented in the surface far as Worcestershire and north beneath of London. The coloured bands Leicestershire. It formed part of the Early in Figure 6 indicate those periods of Palaeozoic continent of Avalonia. In mid- geological time represented in the district’s times, when it lay at a latitude of rocks, from the mid- to the about 30° south, the northern edge of present, with their age in millions of years. Avalonia was driven at a shallow angle Also indicated are events that occurred beneath Laurentia, during the Caledonian during the long periods of time for which Orogeny. Since then, the Midlands no record remains; their presence has been Microcraton has been relatively tectonically inferred from evidence in adjoining areas. stable. The terrane to the east of London is part of a Caledonide fold belt that 4.3 the extent of geological time may be underlies eastern . appreciated by considering the whole of Earth history as a single day. On this 4.6 evidence from the English Midlands scale, the oldest rocks in the London area suggests that the Proterozoic strata formed around 11.30 pm, and the London beneath London include volcanic rocks that underlies most of the metropolis between about 600 and 700 million about 11.45 pm. The Quaternary ice years old. Deep boreholes in the region ages began less than one minute before show that the Early Palaeozoic strata are midnight and human-like creatures first dominated by mudstones and sandstones, strolled by the River Thames at less than with some limestones, laid down in shallow one second to midnight. seas. Most of the , however, was a period of non-deposition in this region. 4.4 to understand the surface geology, however, it is also necessary to consider the 4.7 during the , following the older rocks that form the local geological Caledonian Orogeny, the London region basement. In much of the district these lie formed part of a broad low-lying area with between only 250 and 400 metres below mountains of the Anglo-Brabant Massif the surface, but in the south they are much to the north and an ocean to the south. deeper, at more than a kilometre (Figures Devonian strata occur at depth throughout 25

Figure 6 A timescale and summary of geological history for the area Protecting the geodiversity of the capital

most of the region, although varying in interpretation, the Variscan Front (the thickness considerably. They are dominated northern limit of the main fold belt) is by sandstones, with siltstones and locally marked by the Addington Thrust, a mudstones, deposited in desert, lacustrine major fault detected by seismic reflection and fluvial environments. at more than a kilometre deep beneath the North Downs. To the south of this thrust, 4.8 the southern part of the area includes a borehole at Warlingham, , found the northern margin of a Variscan fold early limestones and shales. belt, formed towards the end of the Rocks of Carboniferous age have not so far Carboniferous and in early been proved in the London district to the times during the Hercynian Orogeny, a north of the Addington Thrust. mountain-building event that can be traced across southern Europe. This terrane 4.9 the presence of a thick wedge of folded is represented by arcuate structural trends, Late Palaeozoic sedimentary rocks oriented approximately east-west, as seen (probably Devonian in age), deep beneath in the Bouguer anomaly map. In the usual south London, is shown by a large negative

Figure 7 Palaeozoic basement of Greater London (from Ellison, 2004) 27

Figure 8 Geological section showing the syncline (from Sumbler, 1996)

Bouguer gravity anomaly. This wedge may Caledonide fold belt of eastern England also be part of the Variscan fold belt. It remained relatively stable, together is faulted against the older, denser rocks forming the London Platform (Figure 10). of the Midlands Microcraton, as shown No beds of Permian or age are by the curvilinear traces in the gravity known in the London area. map, extending from the south-west in a north-eastwards and then eastwards 4.11 The northern edge of the Weald Basin direction. These curvilinear fault traces occurs in the south of the London district. represent a northwards splay from the main Movement on major faults bounding Variscan fold belt. This fault zone is likely this basin controlled the distribution and to be quite complex at depth, becoming thickness of strata. For example, simpler upwards. Faults and folds seen in about 1050 m of Jurassic mudstones with the bedrock under south-west and south interbedded limestone and sandstone London (the Wimbledon to Greenwich occur at Warlingham, but only 12 m occur tectonic zone) formed by later movement in a borehole at , just in this structural zone. 15 km to the north. In general, younger Jurassic formations overlap further onto Mesozoic and Cenozoic the London Platform. Some may have extended right across it but were removed 4.10 The Variscan fold belt was the site of basin by erosion during periods of low sea level, subsidence during the Mesozoic and basin and uplift in the late Jurassic or early inversion during the Cenozoic. During that Cretaceous associated with the opening of time, the Midlands Microcraton and the the North Atlantic Ocean. The London area Protecting the geodiversity of the capital

appears to have been dry land during this grained sandstone and siltstone and less time, with no deposition. than about 15 m thick. It is absent in the north-west of the district, passing laterally 4.12 In mid-Cretaceous times, however, about into the Gault or having been removed by 120 million years ago, rising sea levels erosion. began to flood the London Platform and by about 105 million years ago, deep 4.13 The Late Cretaceous, from about 100 water marine mudstone of the Upper million to 65 million years ago, was a Gault were being deposited throughout period of ‘greenhouse Earth’, with a the area. Beneath London, the Gault is much warmer global climate than at typically between about 50 and 70 m in the present, and globally very high sea thickness. In most of the London area, levels, possibly the highest of the past the Gault passes upwards into the Upper 500 million years. A warm epicontinental Greensand, composed of glauconitic fine- sea extended across Europe, and at least

Figure 9 Gravity map of London and surrounding area

Colour shaded relief image of variable density Bouguer gravity residual of upward continued field to 10 km. Dotted lines indicate the approximate boundaries of the regional basement terranes: the ‘deep geological structure’. Red: gravity ‘high’ (mass of underlying rock is greater than average); Blue: gravity ‘low’ (mass of underlying rock is less than average). Outline of the GLA in magenta. 29

Figure 10 Mesozoic structural setting (from Sumbler, 1996)

as far east as Kazakhstan. The gradual region, probably from volcanoes to the accumulation of the calcareous, siliceous west of Britain. Although this was a period and phosphatic remains of marine of tectonic stability in the region, limited organisms of many kinds led to the earth movements led to some localised formation of the Chalk. With nearby areas compositional variations in the Chalk. of dry land diminishing and ultimately confined to the highlands of Wales and 4.14 Greater, more widespread earth movements Scotland (and possibly even further afield), in the latest Cretaceous and early and the onset of a predominantly arid Palaeogene, associated with the early climate, input of terrigenous sediment phases of the Alpine Orogeny of southern was minimal, although occasional clouds Europe, caused the onset of basin inversion of volcanic ash were blown across the and uplift of the Weald. There was gentle Protecting the geodiversity of the capital

folding and some erosion of the Chalk, Syncline extends from east-north-east to and the formation of a broad basin of west-south-west between the Chiltern Palaeogene deposition that extended from Hills in the north-west and the North eastwards into the North Sea and Downs in the south: its axis passes through across Europe. The portion now preserved central London. Smaller folds, notably the in England is known as the London Basin. Greenwich and the Millwall anticlines, were caused by more localised ‘up-to-the-south’ 4.15 The Palaeogene sediments of the London movements on deep-seated faults amongst area, which lie across the centre of the those bounding the Midlands Microcraton. London Basin, were deposited in shallow Some of these folds have faulted axial marine, coastal and fluvial environments zones, within a zone of faulting extending between about 58 million and roughly 50 from Wimbledon north-eastwards across million years ago. At this time, Britain lay the Thames near Greenwich, and beyond at about 40° N, and ‘greenhouse’ global towards . climate conditions continued. Generally high, but fluctuating sea levels, driven by 4.18 For most of the period since the late global sea level changes coupled with local , the London district has been tectonic movements, brought a series of land, and the existing deposits have been cyclical marine transgressions and retreats. gradually weathered and eroded. Sparse To begin with, this generated a varied remnants of coastal deposits on and in places rather complex sedimentary the highest ground suggest that the sea sequence with evidence of erosion covered the area relatively briefly in the between some of the formations. The Pliocene. prevailing climate was generally warmer than at present, with evidence of mangrove Quaternary swamps and tropical-style weathering and soil formation on emergent coastal 4.19 In the early Quaternary, rivers flowed plains. However, the greater part of the across the London area from the south and Palaeogene deposits in the London area south-west towards the ‘proto-Thames’, a belongs to the , which was laid river larger than the modern Thames that down in fully marine conditions. flowed out of Wales, through the Midlands and southern East Anglia, into the North 4.16 The youngest Palaeogene sedimentation Sea (Figure 11). The Quaternary saw in the London area is represented by the the onset of a series of extreme climatic Bagshot Formation, deposited in fluctuations, with glacial periods much a shallower sea than the London Clay, or colder than at present alternating with possibly a coastal environment. interglacial periods that were in general somewhat warmer. During the Anglian 4.17 The main period of Alpine earth glaciation, about 450 000 years ago, the movements occurred in the mid-Miocene, ice sheets that covered northern Britain when further basin inversion caused uplift reached their most southerly extent, in of the Weald, and concomitant down- the north of the London district (Figure warping of the London Syncline, in which 12). This caused a diversion of the River the London Basin is preserved. The London Thames to its present-day valley. In the 31

Figure 11 Reconstruction of the pre-Anglian drainage system (from Sumbler 1996)

Figure 12 The extent of the Anglian glaciation in the Thames Valley (from Sumbler 1996) Protecting the geodiversity of the capital

north of the district, where tributaries of within ice, global sea level was considerably the Thames had once flowed north, they lower than at present. Down-cutting in the now flowed south. Lower Thames, which then extended far into what is now the offshore area, created 4.20 Cyclical deposition and downcutting by a deep channel. As the climate ameliorated the Thames, chiefly following changes and sea level rose once more, this channel in sea level caused by climatic change was then infilled with and sands. and crustal adjustments to glaciation, These deposits now form a ‘buried channel’ has left a complex series of river terrace beneath the river alluvium, peat, salt-marsh deposits. These form a discontinuous and and estuarine sediments, which have been rather irregular ‘staircase’ down the valley lain down within the past 10 000 years or sides. They are dominated by gravels so, since the end of the last glacial period. and sands laid down under cold climate conditions by large braided river systems, 4.22 Upstream of its confluence with the River overlain in many places around London Lea, the width of the Thames flood plain by fine-grained ‘brickearths’, whose major decreases abruptly, so creating the most component is wind-blown silt, or loess. natural place to found a crossing point and Pockets of interglacial deposits also exist, settlement. The rest, as they say, is history typically at the ‘back edge’ of a river (!). terrace. These deposits mark periods of warm climate, when the Thames would have followed a single meandering channel with a broad flood plain much as at Bedrock geology present, although at times with much more lush vegetation and a fauna with some 4.23 Bedrock formations predate the Quaternary exotic elements reminiscent of modern Sub-Era: they define the local geological Africa. Interglacial deposits dating from structure (Figure 13). The bedrock may about 400 000 years ago, overlying the be overlain by natural superficial deposits, Boyn Hill Terrace at Swanscombe (just east by artificial deposits, or by both; or it may of the London district), include the remains crop out at the surface, perhaps covered of one of the oldest hominids found in only by soil. Parts of the bedrock close Britain. to the surface have generally suffered the effects of weathering; they are then 4.21 During the coldest part of the last glacial typically weaker and more oxidised stage, the Devensian, between about 25 compared with fresh, unweathered 000 and 13 000 years ago, ice-sheets material from the same formation, and are covered northern Britain, reaching as far commonly of a different colour. south as north Norfolk. The London area was subject to frigid, periglacial conditions. 4.24 The exposed geological succession is Peat with plant remains indicating the described in order of formation age, severity of the climate has been found from the oldest to youngest. The Chalk close to the River Lea at Ponders End, represents the youngest part of the in the north-east of the district. With Mesozoic. The rest of the bedrock enormous volumes of water captured sequence dates from the Palaeogene 33

Figure 13 Bedrock geology of Greater London

Period. The Palaeogene and the England, East Anglia and northwards succeeding Neogene together used to into Lincolnshire and Yorkshire. It also be known as the ‘Tertiary’ period. The occurs widely outside the UK, reflecting Tertiary and Quaternary periods are now the remarkable continuity of depositional referred to as the Cenozoic Era (Figure 6). conditions produced by the exceptionally Quaternary and possible Neogene deposits high sea levels in the Late Cretaceous. The are described in later in this chapter. UK Chalk succession was deposited in a A summary of the geological strata of few hundred metres of water, and probably London is given in Figure 14. blanketed much of the British Isles. Chalk deposition largely ceased at the end of the Chalk Group Cretaceous when there was a major fall in sea level that transformed much of the UK 4.25 The Chalk, of Late Cretaceous age (100 into land and exposed the newly-formed to 65 million years ago), is the oldest Chalk to erosion. bedrock unit seen at outcrop in the district, and underlies the whole area at 4.26 In the London area the Chalk crops out in depth. The Chalk is a very widely occurring the Chiltern Hills and in the North Downs, geological unit, extending across southern and also in the core of the Greenwich Protecting the geodiversity of the capital

Figure 14 Summary of the geological strata of London (adapted from Ellison, 2004) 35

Anticline and related fold structures. Its produced by regular changes in the Earth’s maximum preserved thickness is about 200 attitude and orbit that in turn altered the m, which is quite small compared to the c. amount and distribution of solar radiation. 400 m present in the – Sussex area. This contrast partly reflects deeper 4.29 The higher part of the Chalk Group is erosion of the Chalk in the London area almost pure calcium carbonate in the form prior to deposition of Cenozoic sediments, of low magnesian calcite, and is the typical and partly differences in tectonic white chalk with which most people are environment. The relatively shallow familiar. Flints are a conspicuous feature occurrence of ancient basement rocks in of this part of the succession, occurring the London area may have influenced the as bands at regular intervals and giving extent of this erosion as well as limiting the an indication of the natural bedding. space available for Chalk accumulation. Flint is composed of silica, in the form of ultramicroscopic quartz crystals, derived Lithology from the dissolved skeletons of siliceous sponges and microfossils (radiolarians 4.27 In southern England, the Chalk is typically and diatoms) that inhabited the Chalk a very fine-grained, relatively soft, white sea. The complex chemical process of limestone, predominantly composed of flint formation occurred at some distance the disaggregated skeletal remains of tiny below the sea bed whilst the Chalk was still planktonic algae called coccolithophores. being deposited, often as replacements of These flourished in the seas of the Late burrow systems formed by organisms living Cretaceous, and today remain an important in the seabed sediment (Clayton, 1986). source of biologically produced marine Such flints typically have irregular nodular limestones. The Chalk is compositionally or elongate forms. Laterally continuous similar in northern England, but much tabular flints are typical of homogeneous, harder due to greater cementation. well-bedded sediments in which burrows are either absent or poorly defined; lack of 4.28 The lower part of the Chalk Group contains these preferred sites for silica replacement up to 30 per cent clay, and comprises a promoted the formation of more evenly decimetre-scale alternating succession developed flint horizons (Clayton, 1986). of hard limestones and soft mudstones. Some flint bands have a distinctive Individual limestones and mudstones are appearance and are geographically widely traceable across the UK and far extensive, making them valuable for beyond, and individual beds have been correlation. Locally, thin sheet-like flints, identified using an alphanumeric code. The with a distinctive hollow centre, are found rhythmic cycle of limestone and mudstone cross-cutting parts of the Chalk succession. has been interpreted as reflecting regular These flints are inferred to have grown climatic oscillations, with the limestones along fractures or shear-planes in semi- accumulating in warmer phases and the consolidated chalk. mudstones in colder periods. Estimates of the frequency of these changes, 4.30 The thin marls that are first seen in the every 20 000 years or so, suggests that lower part of the Chalk Group continue to they correspond to Milankovitch Cycles, occur at intervals through the higher part Protecting the geodiversity of the capital

of the succession. These clay-rich horizons marine fauna. The formation of nodular have higher water retention than the chalks and hardgrounds has been related adjacent chalk, and are often preferentially to reductions in sedimentation rate by vegetated in outcrops. Where visible in current-winnowing across submerged weathered exposures, marls may appear massifs, in proximity to basin margins, or as shaly horizons and can be preferentially associated with eustatic fall in sea level eroded, but in fresh exposures their darker (Hancock, 1989). colour is usually their key distinguishing feature. Stratigraphy

4.31 Some aspects of the formation of the 4.33 Traditionally, a three-fold classification was marl seams in the higher part of the Chalk applied to the Chalk Group everywhere in remain unknown, but some have been the UK. This simple subdivision into Lower, interpreted as decomposed volcanic ashes Middle and Upper Chalk was based on the and others as the result of an enhanced development of beds of hard chalk which, influx of land-derived sediment (Wray through their topographic expression, and Wood, 1995; Wray, 1999). Many marl could be traced across chalk downlands. seams are geographically extensive marker In the last 20 years, research has shown beds and are important for correlation. that there is much greater variation within the Chalk than is suggested by the 4.32 Decimetre-scale beds of characteristic traditional classification. Separate modern hard, nodular chalk occur at some levels in classifications are given to the Chalk of the Chalk. These have a distinctive knobbly north-eastern England and southern appearance produced by preferential England, based on differences in the removal by weathering of the softer character of the chalk and distribution chalk that surrounds the harder nodules. of flint and marl. Between these two Thinner horizons of very hard chalk, areas is a region, extending from the known as hardgrounds, also sometimes London area across most of East Anglia, occur. These beds, generally less than a where classification of the chalk becomes metre thick, have typically been bored less straightforward because of subtle and encrusted by marine organisms and changes in its character (Mortimore et al., stained orange, green or brown with iron 2001). Investigations of the outcropping and phosphate minerals. Both nodular succession around and buried chalk and hardgrounds reflect primary successions seen in boreholes suggest hardening of chalk caused by enhanced that in general, the southern England sea floor cementation associated with classification can be applied to the Chalk periods of non-deposition (Hancock, of the London area. The southern England 1989). Cementation occurred just below scheme follows Rawson et al. (2001); it the sea floor, initially producing nodules recognises two subgroups and up to nine of harder chalk in softer sediment (Gale, formations within the Chalk Group. 2000). Local exposure by sea floor erosion facilitated further cementation, 4.34 The two major subdivisions are the and allowed the hard chalk pavement Grey Chalk Subgroup and White Chalk to become bored into and encrusted by Subgroup. The boundary between them is 37

an erosion surface at the base of a clay- Marl of the traditional scheme. The West rich unit named the Plenus Marls Member. Melbury Marly Chalk is unusually thin in The top of the Plenus Marls marks the top the Fetcham Mill Borehole [TQ 1581 5650] of the traditional Lower Chalk, but in the at Leatherhead, where it is less than 20 m new scheme this unit marks the base of the thick, but it shows evidence of thickening White Chalk Subgroup. to perhaps 30 m or more elsewhere within the region. The Zig Zag Chalk is more 4.35 In the London area, where Cenozoic uniform, at about 40 m thickness. erosion has removed part of the succession, a maximum of even formations White Chalk Subgroup can be recognised. 4.37 The change to the much purer chalks Grey Chalk Subgroup that characterise the White Chalk is thought to be related to a major sea 4.36 The Grey Chalk Subgroup is typically level rise, following the short-lived sea 60 – 70 m thick in the London area, and level fall that led to the erosion surface generally comprises less pure, more clay- at the base of the Plenus Marls. This sea rich chalk than the White Chalk Subgroup. level rise curtailed the supply of detrital Unlike the latter, it does not contain flint. material from land areas and probably The two formations that make up the Grey fundamentally changed the circulation Chalk Subgroup, the West Melbury Marly pattern on the drowned continental Chalk Formation and overlying Zig Zag margins, allowing the low-nutrient oceanic Chalk Formation, are not seen at outcrop water favoured by coccolithophores to in the London district, although they are flood across these areas. penetrated by boreholes. These boreholes, and outcrops in quarries south of London, 4.38 Five formations are recognisable in the show that the base of the West Melbury London area, namely (from oldest to Marly Chalk, as elsewhere in southern youngest) the Holywell Nodular Chalk, the England, comprises a sandy clay, rich in the New Pit Chalk, the Lewes Nodular Chalk, green-coloured iron mineral glauconite, the Seaford Chalk and the Newhaven named the Glauconitic Marl Member. Chalk. The lowest three of these are Above this are regularly alternating hard seldom seen at outcrop in the London limestones and mudstones, each typically area, except where exposed in deep quarry a few tens of centimetres thick. This or mine workings. The Seaford Chalk interval was traditionally referred to as the has the greatest surface exposure and Chalk Marl. Alternating limestones and underlies most of the Cenozoic sediments. mudstones continue up into the lower The Newhaven Chalk has a restricted part of the Zig Zag Chalk, but eventually occurrence along parts of the southern these give way to thick beds of creamy margin of the London area. grey chalk with thin interbeds of marl, traditionally known as the Grey Chalk. 4.39 The Holywell Nodular Chalk Formation is The base of the Zig Zag Chalk is marked hard, nodular, and distinctively shell-rich by a silty chalk horizon, named the Cast in the upper part. It is typically 14 to 18 Bed, within the upper part of the Chalk m thick in the London area, and contains Protecting the geodiversity of the capital

regularly developed thin beds of marl London area the top of the Lewes Chalk throughout and an interval with several is marked by the Rochester Hardground, beds of greenish-grey marl at the base, up but elsewhere this is replaced by a pair of to several metres thick, named the Plenus marl seams separated by sponge-rich chalk. Marls Member. Much research has focussed Marls occur at intervals through most of on the Plenus Marls and immediately the succession. They include important overlying strata in recent years, since marker horizons such as the distinctively evidence from their carbon and oxygen thick (about 0.1 m) and plastic-textured isotope geochemistry points to a series Southerham Marl, which chemical analysis of major environmental perturbations at has shown to be a decomposed volcanic this level, including oxygen-starvation of ash. The base of the traditional Upper the Late Cretaceous oceans and a major Chalk falls within the lower part of the change from global climatic warming to Lewes Chalk, but in the London area the global cooling. The base of the traditional beds that define this boundary are not well Middle Chalk Formation was mapped developed. using a hard bed of chalk that immediately overlies the Plenus Marls, named the 4.42 The Seaford Chalk Formation is typically Melbourn Rock. soft, smooth-textured chalk, with shell-rich beds and common horizons of nodular and 4.40 The New Pit Chalk Formation is firm, tabular flints. Some flints are widespread smooth-textured chalk, 40 to 46 m thick marker beds in the Chalk of southern where recognised in boreholes in central England, such as the Seven Sisters Flint London. It lacks the nodularity and in the lower part of the succession, and abundant shell remains of the Holywell Bedwell’s Columnar Flint and Whittaker’s Nodular Chalk, and in its upper part, bands 3-inch Flint in the higher part of the of nodular flint appear. Marls continue to Seaford Chalk. Marls are restricted to occur in the succession, and are generally the lower part of the formation, and in slightly more noticeable than in the the higher part of the succession is an Holywell Chalk. Several thick marl beds, interval of iron-stained chalk with common more than 0.1 m thick, occur in the upper sponge remains (Barrois Sponge Bed) or a part of the New Pit Chalk. hardground (Clandon Hardground). Over most of the area the preserved thickness is 4.41 The Lewes Nodular Chalk Formation, affected by the depth of Early Palaeogene up to 40 m thick, marks a return to erosion (where overlain by Palaeogene hard, nodular chalk, but unlike the rocks) and Quaternary erosion (where underlying Holywell Chalk, flint occurs exposed). The complete thickness of fairly regularly throughout the formation. Seaford Chalk is proved in the Fetcham Mill Some of the flints are very distinctive, Borehole at Leatherhead, but the c.33 m such as the Lewes Tubular Flints, which there assigned to the formation is probably can be traced across southern England atypical because part of that succession and northern France. Hardgrounds are shows unusual local thinning. relatively common, with at least four major intervals containing closely spaced 4.43 The Newhaven Chalk Formation is very hardground surfaces. In some parts of the soft, smooth-textured chalk with marl 39

seams, and less abundant flint than the relative abundance of flint, marl, nodular underlying Seaford Chalk. Only the lower chalk and hardgrounds, as well as variable and middle parts of the formation are chalk composition, cementation and preserved locally, mainly on the southern style of fracturing. Differences in these and south-western fringes of the London characters between formations subtly district. The thickest development is alter the way each weathers, which in probably in the Fetcham Mill Borehole turn produces changes in slope profiles. at Leatherhead, just south-east of the It is this feature that has allowed the new London district, where nearly 20 m of Chalk Group stratigraphy to be mapped out Newhaven Chalk were proved. Sparsely across often poorly exposed terrains. flinty chalk belonging to this formation also occurs around , Purley, Ewell Economic Importance and . 4.46 Water supply and cement manufacture are Landscape the two most important economic uses of the Chalk. In the past chalk was also 4.44 Where it crops out, Chalk produces exploited in small workings as a source attractive undulating downlands, seen in of agricultural lime, and has been mined the North Downs to the south of London, extensively in several places in south- and the Chiltern Hills to the north-west. east London as a raw material for brick The undulating character is controlled by manufacture, or for flint. geological structure and by the different character of each of the formational 4.47 The Chalk is crucially important for water subdivisions of the Chalk. Where not supply, being one of the UK’s major overlain by younger strata, the gentle dip aquifers. Boreholes drilled into the Chalk of the Chalk produces a broad tract of are a source of water for large parts of outcrop, successively revealing each of south-east England, including the London the different formations to the effects of area. The dense urban development that surface weathering. covers much of the exposed Chalk in south-east England also means that this 4.45 Both ancient and modern weathering aquifer is vulnerable to contamination from has shaped the Chalk landscape. In the surface pollutants. Often a considerable Quaternary, the development of permafrost interval of Chalk occurs above the water during glacial epochs and/or the presence table (the ‘unsaturated zone’), where of an elevated water table, allowed streams pollutants may build up over many to flow across the normally permeable years, and then be rapidly distributed Chalk and develop valley systems. These into the aquifer by a rise in the water are now visible as networks of dry valleys. table. In recent years there has also been Weathering of the Chalk has also been concern about maintaining supplies of affected, and continues to be influenced, ground water during prolonged droughts; by the unique combination of physical mitigating the negative effects of excessive features that characterise each of the groundwater abstraction on chalk streams formational subdivisions. These features and wetlands, and assessing the risks of include the presence or absence and flooding due to sudden rises in the water Protecting the geodiversity of the capital

table. Consequently much recent work has wide and can be up to 9 m high, generally focussed on understanding pathways of narrowing upwards for stability. They can ground water movement in the Chalk, and extend more than 100 m from the access how this is influenced by stratigraphical point. Some of the largest were dug within variation and geological structure. brick works, as a source of chalk to be added to the brick clay prior to firing. 4.48 Chalk is an important source of raw The Pinner chalk mine, dug through the material in the manufacture of cement, and Lambeth Group in north-west London, is has been extensively worked in quarries one of the deepest, at about 35 m. Most close to the London area, especially along other examples are in south-east London, the Chalk outcrop flanking the Thames around Woolwich and Erith, or in south estuary. Some of these quarries have a long London, most notably at Chislehurst. history of development and have furnished important geological data that has greatly Wider Geological Importance contributed to our understanding of the Chalk of the London area. Some of the 4.51 The Late Cretaceous is thought to older quarries, especially those adjacent to have had a ‘super greenhouse’ climate, built-up areas, were extended underground produced by large amounts of submarine by driving adits from the faces. and continental volcanic activity that pumped huge volumes of carbon dioxide 4.49 Chalk mines are found in places in the into the atmosphere. Consequently there south-east of the London district, and at has been considerable interest in studying Pinner in the north-west. The oldest and the Chalk in order to gain insights into smallest types of mine are the ‘dene hole’ how biological systems responded to this and related forms of hand-dug mine used environment, and what the implications to win agricultural lime in areas of acidic or might be for modern global warming. There clay-rich soils. Some may be of pre-Roman has been particular focus on the strata at age but most seem to date from the 13th the base of the White Chalk Subgroup, to the 19th centuries. They typically consist represented by the Plenus Marls and of a shaft of about 1 m in diameter sunk immediately overlying succession of the through a cover deposit of Thanet lower Holywell Nodular Chalk. This interval or of clay-with-flints, plus enough Chalk appears to show evidence of a period of to make a safe roof. Chalk was excavated global oceanic oxygen starvation that is by expanding the shaft, commonly along broadly coincident with a sudden climatic adits of various patterns and lengths, over reversal, from global warming to global a radius up to about 12 m. Dene holes cooling. can occur singly or in groups of up to 70, or more. They are common in parts of the 4.52 Milankovitch cycles are regular climatic Cray valley in south-east London (see also fluctuations produced by cyclical changes paragraphs 5.9-5.10) in the Earth’s orbital parameters. Evidence of the presence and impact of these 4.50 Pillar and stall chalk mines mostly date changes has been gained by studying the from the 19th century, with some earlier or sediments of the Grey Chalk Subgroup, and later. Their passages are typically 2 to 5 m it has been suggested that some aspects 41

of the flint distribution in the White Formation in the east, and by a complex Chalk Subgroup might be a response to interdigitation of both in the centre of the environmental changes produced by these district. The consists mostly cycles. of the London Clay Formation, which is by far the most widespread single bedrock 4.53 The details of the Chalk succession in the formation in the London district, with the London area, particularly the nature of any Harwich Formation, previously known as local variations, are likely to be pertinent the ‘London Clay basement bed’. to an understanding of the relationship between sedimentation and structure Thanet Sand Formation during the Late Cretaceous, particularly the timing of tectonic movement at the 4.57 The Thanet Sand is the oldest Palaeogene southern margin of the London Platform formation in Essex and , with London and along the Wimbledon – Greenwich marking its north-western extent. It forms tectonic zone. a fairly narrow outcrop along the west side of the Cray valley and westwards at the 4.54 Exposures of the local Chalk will aid foot of the North Downs, with a few broad understanding of its behaviour in outliers just to the east of the Cray at the engineering and hydrogeological contexts, south-eastern boundary of the district and including (for example) the likely pathways some narrow inliers in south-east London. taken by pollutants in the groundwater It also crops out around the Greenwich system. and Millwall anticlines but is there almost entirely covered by superficial deposits. Palaeogene strata The Thanet Sand and the Lambeth Group were together previously known as the 4.55 Most of the London district is underlain ‘Lower London Tertiaries’. by an intricate succession of Palaeogene strata within the London Basin. This 4.58 The Thanet Sand was deposited in a was laid down during a series of cyclical shallow marine environment, following fluctuations in sea level between about 58 a global rise in sea level after about 58 and 50 million years ago, and represents million years ago in the late Paleocene (the deposition in environments ranging from earliest series of the Palaeogene). It rests terrestrial to fully marine. unconformably on the Chalk, on what was originally an approximately planar erosion 4.56 Four major divisions are present. The surface, although that surface is now oldest, the Thanet Sand Formation, is folded, and typically highly convoluted overlain in turn by the Lambeth Group on a small scale following dissolution (with 3 formations), the Thames Group of the Chalk. The unconformity surface (with 2 formations) and the Bracklesham is attributed to erosion and reworking Group, of which the Bagshot Formation during two or more depositional cycles. is the only representative in the London Where it is more fully developed to the district. The Lambeth Group comprises the east, up to nine onlapping sequences have Upnor Formation, overlain by the Reading been recognised in the Thanet Sand but Formation in the west, the Woolwich the bulk of the deposits (and perhaps Protecting the geodiversity of the capital

all) in the London area represent only about 10 per cent at the base, to about one. Deposition of the Thanet Sand was 60 per cent at the top. The unweathered followed by further cycles of falling and strata are pale grey to brownish-grey, and rising sea level prior to the deposition of weather to a yellowish-grey. They are often the succeeding Lambeth Group, which intensely bioturbated, shown by wisps of overlaps the Thanet Sand towards the relatively dark silt and clay, but few primary west. Consequently, the Thanet Sand has sedimentary structures remain. The sands been truncated, diminishing in thickness may locally contain irregular masses of from about 22 m in east and south-east much harder siliceous sandstone known as London to where it is cut out almost ‘doggers’. completely along a line between and Borehamwood. To the west of this 4.60 At the base of the Thanet Sand Formation line, sparse remnants that occur in some there is typically a distinctive unit up solution cavities in the Chalk demonstrate to 0.5 m thick called the Bullhead Bed. the greater original extent. This is usually a conglomerate with flints ranging in size from small well-rounded 4.59 In the London district, the Thanet Sand pebbles to large unworn flint nodules (the Formation consists predominantly of ‘bull heads’). The flints typically have a a coarsening-upwards sequence of distinctive green coating of glauconite fine-grained sands which is clayey and and are set in a matrix of dark greenish- silty, particularly in the lower part. The grey clayey, fine- to coarse-grained sand proportion of fine-grained sand varies from containing glauconite pellets. Some fossils

Figure 15 Relationship of Lambeth Group informal lithological units in central London 43

derived from the underlying Chalk are under conditions of a warm climate with present. a distinct dry season. It is associated with duricrust formation and soil formation. Lambeth Group 4.64 Three formations are present in the 4.61 The Lambeth Group (formerly known Lambeth Group, each of which varies as the ‘Woolwich and Reading Beds’) significantly in composition both laterally succeeds the Thanet Sand, overlapping and vertically. They together make up it to rest on the Chalk in the north-west four depositional sequences separated by of the district. Like the Thanet Sand, it unconformities (Knox, 1996). The basal forms a narrow outcrop on the south-east Upnor Formation occurs throughout the side of the London Basin, but with only area. In the west of the London Basin it is a few small outliers. It likewise delineates overlain by the Reading Formation, and the Greenwich and Millwall anticlines and in the east by the Woolwich Formation. forms scattered inliers, where it is mostly In the centre of the London area, both covered by superficial deposits. Some of the Reading and Woolwich formations the inliers have arisen through incision in are present in a complex interbedded the Ravensbourne River and its tributaries, relationship, in which a number of informal but others (such as those in the Lower Lea subdivisions have been recognised (Figure Valley and nearby) are probably controlled 15). These divisions are described in more by a local geological structure. Unlike the detail in the London Memoir (Ellison et al., Thanet Sand, the Lambeth Group also 2004). crops out at the north-western edge of the London Basin, in north-west London. 4.65 The Lambeth Group was deposited during the youngest part of the Paleocene and 4.62 The Lambeth Group is of rather more the oldest part of the Eocene (Collinson uniform thickness than the Thanet Sand. It et al., 2003; King, 2006). The Upnor is generally thickest in south-west London Formation is of marine origin, and rests and reaches as much as 30 m at Southwell, unconformably on a erosion surface. but in most of the district it is between Deposition of the upper part of the Upnor 10 and 20 m thick. It is least in south- Formation followed a lowering of sea east London, in large part due to erosion level that may have led to the removal or prior to the deposition of the Harwich reworking of earlier deposits. The Upnor Formation, and in places east of the Cray Formation as a whole was followed by a valley, near Swanley, it has been removed further fall in sea level. Emergence of the entirely. Upnor Formation is indicated by the local presence of silcretised sediments (some 4.63 The Lambeth Group typically comprises as clasts in the pebble beds at the top of interbedded colour-mottled clays and the formation) typical of ‘Hertfordshire fine silty sands, with occasional shell puddingstone’ and ‘sarsen stone’. beds, thin limestones and some beds of sandy gravel containing black, very well-rounded flint pebbles. The colour- mottling is a consequence of weathering Protecting the geodiversity of the capital

4.66 The succeeding Reading Formation unworn flint nodules, as found in the represents deposition in an area of marshy Bullhead Bed. At the base, burrows extend coastal plains crossed by rivers, and the as much as 2 m below the contact. Woolwich Formation in estuarine, lagoonal or nearshore marine conditions. These 4.68 The pebbles that occur throughout the two formations are divided by a sequence formation are generally less than 30 mm boundary, marking a period of sea level in diameter but may exceptionally reach fall, emergence of the lower part of the 200 mm. They are typically well-rounded Lambeth Group and their consequent and elongate, spheroidal to flattened weathering and local erosion. This spheroidal. Many have small surface boundary marks onset of the ‘Paleocene- crescentic percussion marks. Pebble- Eocene Thermal Maximum’, a relatively dominated beds occur principally at the short-lived period of global warming (King, base and top of the unit: in central and 2006). The corresponding stratigraphic south-east London, there is a persistent surface (which seems to disappear pebble bed up to 3 m thick at the top of westwards within the Reading Formation) the formation. has been named the ‘mid-Lambeth Group hiatus’ (Hight et al., 2004). This 4.69 The Upnor Formation was previously is commonly characterised by pedogenic treated as the ‘basal bed’ of either the effects such as calcrete formation in the Woolwich Formation or the Reading underlying strata, which may be part Formation, respectively. It underlies the of the Reading Formation or the Upnor mid-Lambeth Group hiatus and so is of Formation, and by lignite deposits. Paleocene age.

Upnor formation 4.70 Pit SSSI, Hillingdon, exposes the Upnor Formation overlying the Chalk 4.67 This formation comprises fine- to medium- (Figure 4). grained sand with variable glauconite, beds and stringers of flint pebbles and minor Lower Mottled Clay (Reading Formation) thin clay interbeds or intraclasts. It may be bioturbated, or well-bedded or laminated. 4.71 The Lower Mottled Clay generally consists Oyster shells occur. Rare fragments of of purple, red, green, blue-grey and brown carbonaceous material occur also. It is mottled clays, with some silty or sandy, mainly dark grey to greenish-grey in colour, and fine- to medium-grained sands. The but the highest part (and locally the whole clays are generally unbedded. The sands thickness) has been oxidised to brown, tend to be brown. Lenticular bodies of orange, red, and purple-brown. It tends to cross-bedded, medium-grained sand be made up of slightly coarser sand than (commonly containing layers of mud the Thanet Sand, and the lower beds may flakes) are interpreted as the deposits in be gritty. A basal bed of rounded flint river channels. pebbles up to 1 m thick is usually present, but is not persistent. Where the Thanet 4.72 The Lower Mottled Clay is present Sand is absent, the Upnor Formation rests throughout the Lambeth Group in the directly on the Chalk and can contain London district, but is thin or absent in 45

the north-east. In the west of the district, 4.79 The Laminated Beds generally rest it has not been differentiated from the conformably on the Lower Shelly Clay and Upper Mottled Clay (if that is present). The have a similar distribution. The unit reaches Reading Formation as a whole is as much as much as 5 m thickness in east London. as 20 m thick in the south-east of London, decreasing eastwards where it interfingers 4.80 A second unit of similar composition occurs with the Woolwich Formation. higher in the sequence (above the Upper Shelly Clay) in places, notably in south-east 4.73 SSSI, Hillingdon, exposes the London. This has been called the ‘Striped whole of the Reading Formation (Figure 4). Loams’.

Lower Shelly Clay (Woolwich Formation) 4.81 The Laminated Beds (these also known as the ‘Leaf-Bed of Lewisham’) are exposed in 4.74 This is a very uniform unit composed of Gilberts Pit SSSI, Greenwich (Figure 4). dark grey to black organic clay with shells. Some brownish grey clay, partly cemented Upper Mottled Clay (Reading Formation) with siderite, can be present, especially in the upper part. Sand or shell beds can be 4.82 This unit comprises clay and sands, very present, especially in the north-east of the occasionally with gravel. It is typically district. Lignite is commonly seen at the grey-brown in colour. It is generally less base of the unit, especially in the south- variable than the Lower Mottled Clay, and east, where it marks the mid-Lambeth can be difficult to differentiate from the Group hiatus. The thickest such lignite bed Harwich Formation. In places it is known to (up to 2 m) occurs at Shorne, in Kent. interdigitate with the Laminated Beds.

4.75 In general this unit thickens from central 4.83 The Upper Mottled Clay occurs only London towards the south-east, reaching between Walthamstow and Merton. To as much as 6 m. It is absent in the west of the east its distribution is limited by non- the district. deposition or erosion before the deposition of the Harwich Formation, whereas to the 4.76 The Lower Shelly Clay overlies the mid- west (where the Woolwich Formation is Lambeth Group Hiatus and so is of Eocene absent) it is not differentiated from the age. Lower Mottled Clay.

4.77 The Lower Shelly Clay is exposed in Upper Shelly Clay (Woolwich Formation) Gilberts Pit SSSI, Greenwich (Figure 4). 4.84 This comprises grey shelly clay, with thinly Laminated Beds (Woolwich Formation) interbedded grey-brown silt and very fine- grained sand with glauconite grains. Lignite 4.78 The Laminated Beds are mostly composed is locally present. The unit is lithologically of fine- to medium-grained, laminated the same as the Lower Shelly Clay, but sands, silts and clays, which can contain the shelly fauna can be more diverse. It shells. These sediments are typically pale can include ‘Paludina’ freshwater biogenic greenish grey to brown in colour. limestone, especially in south London. Protecting the geodiversity of the capital

4.85 The base of the unit is generally sharp considerably. Fossil shells occur in places. and rests on the Upper Mottled Clay, the The formation is locally cemented. This Laminated Beds or, probably, the Lower type of deposit, known as the Blackheath Shelly Clay. Beds, where at it’s thickest in south-east London. Elsewhere, the Harwich Formation 4.86 It is up to about 3 m thick, but tends to was previously known as the ‘basement have a patchy development, and is inferred bed’ of the London Clay. to be preserved in depressions below the erosion surface at the base of the Thames 4.91 A sandy shelly bed at Abbey Wood SSSI, Group. Bexley, is part of the Harwich Formation (Figure 4). The Harwich Formation is Thames Group exposed in Elmstead Pit SSSI, Bromley, Gilberts Pit SSSI, Greenwich and Harefield 4.87 The Thames Group underlies the large Pit SSSI, Hillingdon (Figure 4). majority of the London district. The basal unit, the Harwich Formation, is present London Clay Formation throughout, but is mostly too thin to show separately on geological maps. Only in 4.92 The London Clay typically comprises south-east London, where it attains as bioturbated or poorly laminated, slightly much as 12 m in thickness, has its outcrop calcareous, silty to very silty clay. It been delineated from the London Clay. The commonly contains thin courses of London Clay is up to 130 m thick. carbonate concretions (‘cementstone nodules’) and disseminated pyrite. At 4.88 The Thames Group is entirely of marine depth, where fresh, it is grey, blue-grey or origin, and was deposited during the grey-brown in colour. Near the surface the Early Eocene, between about 55 and uppermost metre or few metres typically 51.5 million years ago. It everywhere weathers to clay with a distinctive brown overlies the Lambeth Group on an erosion colour produced by the oxidation of pyrite. surface. In places considerable portions of The London Clay may contain thin beds of the Lambeth Group have been removed shells, and fine sand partings or pockets prior to the deposition of the Harwich of sand, which commonly increase towards Formation. the base and towards the top of the formation. Glauconite can be present in the 4.89 Several subdivisions of the London Clay sands and in some clay beds. White mica can be recognised in borehole core but grains may be present. At the base, and at only the topmost, the Claygate Member, some other levels, there may be a thin bed can be mapped on the ground. of black well-rounded flint pebbles.

Harwich Formation 4.93 Five sedimentary cycles have been recognised in the London Clay, each 4.90 In the London area, this is typically recording an initial sea level rise and composed of glauconitic fine-grained marine transgression, followed by gradual sand with beds of rounded black flint shallowing of the sea (King, 1981). pebbles. The proportion of pebbles varies The base of each cycle of deposition is 47

typically marked by a sparse pebble bed. Bagshot Formation This is covered by thick clays, which become progressively more silty and 4.98 The Bagshot Formation is the lowest part sandy upwards. An alternative scheme of of the Bracklesham Group in the London subdivision has been recognised in the Basin, and the only part to occur within the London area, from the study of borehole London area. It is typically composed of core (Ellison et al., 2004). brown to white-coloured quartzose sands, with occasional thin beds of clay, and some 4.94 The Claygate Member is the uppermost pebbly layers. A coarse iron-cemented flint part of the London Clay Formation, and gravel is commonly found at the base. corresponds to the upper part of the last of the five sedimentary cycles. It typically 4.99 The Bagshot Formation was deposited in a comprises interbedded fine-grained sands, shallow marine and estuarine environment. and silty clays and silts. The proportion of sand tends to increase upwards. The clays Landscape are generally blue-grey where fresh, and brown where weathered. 4.100 in southern and south-west London, the older Palaeogene strata form a gentle 4.95 The Claygate Member deposits are of tidal escarpment rising up to about 20 m from marine origin, and represent a transition the Chalk dip slope of the North Downs, to the overlying Bagshot Formation. They or (from Erith westwards) up to about 35 occur only as scattered outliers, throughout m from the river terraces adjacent to the the Thames Group outcrop. River Thames. The Palaeogene outliers to the south and east tend to form low, 4.96 The sandy basal beds of the London Clay, well-drained hills with convex slopes. The there known with the Harwich Formation inliers tend to occur in the floors of shallow as the Harefield Member, are exposed at valleys. Harefield Pit SSSI, Hillingdon (Figure 4) (Daley, 1999b). 4.101 the main outcrop of the Thanet Sand lies at the foot of gently rising ground Bracklesham Group formed by the Lambeth Group, or is covered by superficial deposits. No 4.97 Only the oldest part of the Bracklesham distinctive landforms are associated with Group, the Bagshot Formation, is the Lambeth Group, which crops out in represented in the London district. It concavo-convex slopes or locally on hill- occurs only as a few scattered outliers, tops in some outliers. Minor springs occur forming high ground (most notably at the top of clay-rich units, particularly ). It is much more the Lower Shelly Clay. extensive to the north-east in Essex and to the south-west in Surrey. The maximum 4.102 the London Clay gives rise to a broad thickness in the outliers in London ranges swathe of low-lying, subdued topography up to about 18 m. Local erosion at the in the Thames Valley. Even where base has removed the top part of the dissection has been most pronounced (as Claygate Member. between Stanmore and Hampstead) only Protecting the geodiversity of the capital

gently rolling ground with convex slopes of 4.107 the variability of the Palaeogene generally less than 4° is found. deposits, and in particular the presence of poorly cemented sand bodies (some 4.103 the Bagshot Formation underlies the of irregular and unpredictable shape and prominent hills of Hampstead Heath and distribution), have important implications Highgate in north London, Harrow On The for the ground conditions experienced Hill in the north-west, and Havering-atte- by the construction industry. Exposures Bower in the north-east. It gives rise to of the Lambeth Group are of great relatively steep convex slopes up to 12°. potential value to its understanding by It is characteristically free-draining and in the engineering geology community, a natural or semi-natural state supports through which the information provided typical heathland vegetation. The base of by ground investigation boreholes can the formation is commonly marked by a be better appreciated. On the other spring line. hand, the London Clay is an excellent medium in which to drive tunnels: this has 4.104 there are few natural exposures of the greatly facilitated the development of the Palaeogene formations in the London metropolis. district, where it is usually studied in borehole samples or in temporary 4.108 however, the Palaeogene clay deposits, excavations, especially those associated especially the London Clay, are known with construction projects. Some exposures for a relatively high content of the clay occur in nearby parts of Essex and mineral smectite. This is especially prone to Kent (Ellison et al., 2004, pp.24, 26). change volume with moisture content. The Publications of the Geologists’ Association resulting ground movement is a common and of local societies such as the Croydon cause of structural damage in the London Natural History Society contain a wealth of district. Steep slopes in the London Clay description of pits and cuttings that are no are prone to landslide. This has occurred longer available for study. particularly in places in north and north- east London that were once adjacent to Economic significance the Anglian ice sheet.

4.105 regionally, the clays of the Reading Wider geological significance Formation and of the London Clay have been important historical sources for the 4.109 parts of equivalent Palaeogene local production of bricks and tiles. Local successions offshore beneath the North examples include those on the Woolwich Sea are up to 2 km thick and include sands side of . Sand and that are important hydrocarbons reservoirs. gravel for construction purposes has Study of their onshore correlatives in the also been taken from the Palaeogene London Basin has helped understand these formations in places. strata.

4.106 Fragments of silcrete and ferricrete from 4.110 the Palaeogene is particularly relevant the Lambeth Group have been used locally to studies of global climate. It contains a as building stone. variety of fossil indicators of environment 49

Figure 16 Summary of the Quaternary strata of London

Glacial Deposits Till Glaciofluvial Deposits Glaciolacustrine Deposits Pre-Diversionary Thames River Clay-with-flints Terrace Deposits Stanmore Pre-Diversionary Kempton Park Gravel Gravel Thames River Taplow Gravel Head Terrace Deposits Head Gravel Dollis Hill Gravel Brickearth Langley Silt Landslip Interglacial Deposits Alluvium Peat Tidal River of Creek Deposits

Figure 17 Chronology of the main Quaternary deposits of London (from Ellison, 2004) Protecting the geodiversity of the capital

Figure 18 Superficial deposits of Greater London

and climate, and encompasses a variety generally dips north-west at 2° or less (and of climatic conditions, including a short in general somewhat more steeply than to thermal maximum at the Paleocene- the north-west), but this regional pattern is Eocene boundary, and a longer period of distorted by a series of elongated periclinal climatic warming in the early Eocene (King, folds. The largest of these is the Greenwich 2006). Anticline. Some of these subsidiary folds are asymmetrical, with steeper north- Geological Structures facing limbs, in which dips are generally up to about 7°. 4.111 the axis of the London syncline runs in an east-north-east to west-south-west 4.113 a belt of faults, extending from direction across the middle of the London Wimbledon to Greenwich, lies parallel to area, from the vicinity of Romford in the the axes of these periclinal folds, some north-east to Surbiton in the south-west. cases breaching the fold noses. Close to these faults, the dip of the strata may 4.112 north-west of that axis, the bedrock increase and in some places is vertical. The dips gently south or south-east at largest faults have displacements of as less than about 2°, decreasing to near much as about 30 m. horizontal in the middle of the basin. To the south-east of the axis, the bedrock 4.114 structure contours on the Palaeogene 51

basal unconformity (Ellison et al., 2004) studies rely on examination of coastal show that the patterns of folding and of successions, but these are not always ideal faulting are significantly more complex in terms of stratigraphical development and than shown by the geological map. accessibility. The absence of saline water Although not all structures present at the intrusion in inland Chalk successions is an base of the Palaeogene necessarily extend advantage for geochemical studies. to the surface, some probably do so within the outcrop of the London Clay, where a 4.118 each of the Palaeogene formations, lack of lithological contrast renders such especially those of the Lambeth Group, structures unmappable. will continue to be the subject of studies for regional stratigraphic correlation and Future Earth science research potential palaeoenvironment analysis. These topics are relevant both to scientific research, 4.115 the Chalk of the London area for example in sequence stratigraphy and probably has a much more complex climatic variation, and to applied studies, structure (folds and faults) than has for example in geotechnical engineering. previously been assumed. The more refined modern stratigraphy of the Chalk offers the potential to resolve previously unrecognised structures. Future Quaternary deposits understanding of the structure and related fracturing in the Chalk offers great benefits 4.119 the Quaternary deposits of London for major engineering works that impact provide a fascinating insight into the on the Chalk and understanding the capital’s Ice Age past. They overlie the hydrogeology of the Chalk. bedrock of the area and were deposited during the Quaternary Period, between 2.6 4.116 there are changes in the thickness of million years ago and present day (Figures formational subdivisions in the Chalk across 16 and 17). They are collectively known as the London area. Future studies need to ‘superficial deposits’ or, in older literature, focus on the extent to which these relate as ‘drift’ or ‘drift deposits’ (Figure 18). to the effects of long-lived geological structures and the architecture of the 4.120 The superficial deposits are thickest and sedimentary basin in which the Chalk was most extensive within a few kilometres deposited. of the River Thames, especially on its north bank, and along its main northern 4.117 the very large chalk quarries in the tributary, the River Lea. Less extensive London area provide almost unparalleled Quaternary deposits occur in the valley opportunities for examination and floors of the other tributaries. By sampling of inland chalk successions. contrast, the older deposits are confined This is important for wider Chalk studies, to interfluve areas and tend to have particularly those concerned with relatively restricted distribution on the understanding the nature of the Late higher ground. Large areas of ground, Cretaceous greenhouse climate and Chalk of intermediate elevation, are shown by depositional environment. Many of these the geological map to have no cover of Protecting the geodiversity of the capital

superficial deposits, especially in the north- 4.122 there are very few natural exposures of west and the south, but even here there is any of the superficial deposits. likely to be a thin discontinuous cover of head deposits (paragraphs 4.157-158). Clay-with-flints

4.121 The oldest superficial deposits in the 4.123 The clay-with-flints is a rather variable London district probably date from the deposit that occurs on the highest Neogene (Figure 6). In the south they interfluves of the chalk downlands in the are the clay-with-flints, a residual deposit extreme south of the district. Typically, derived mainly from a past cover of the it comprises an orange or reddish brown Palaeogene, and in the extreme north- clay or sandy clay, with abundant matrix- west the Stanmore Gravel of unknown but supported flint nodules and pebbles. In probable marine origin. At lower levels, places, however, it can comprise clean sand the interfluve deposits also include a series or gravel. The lower surface of the deposit of river terrace deposits laid down by is generally irregular, often infilling deep, southern tributaries of an ancestral River steep-sided solution pipes and fissures in Thames. During the Anglian Glaciation, the underlying Chalk. The thickness of the approximately half a million years ago, deposit is thus very variable. In general an ice-sheet extended southwards, it forms a blanket up to about 5 m thick, diverting the river system and laying locally greater, but can abruptly increase to down glacial deposits in the north of the several tens of metres in a solution pipe. district. When the climate warmed and the glacier retreated, the River Thames 4.124 The clay-with-flints is a residual deposit and its tributaries had been established formed by weathering, cryoturbation in approximately their modern positions. and solifluction of the original cover of Subsequently, the superficial deposits Palaeogene, Neogene and early Quaternary were largely confined to valley floor areas: deposits, together with the insoluble they include extensive representatives components left after dissolution of the of cold climate deposition (river terrace Chalk. This process of weathering probably deposits and ‘brickearths’) and restricted started during Neogene times, continuing occurrences of warm climate interglacial into the Quaternary. deposits. The latter, however, arguably include the alluvium (the extensive 4.125 The clay-with-flints is very widespread deposits of the modern floodplain) and on the Chalk of southern England. The estuarine deposits. The mass movement occurrences in the London district appear deposits of the intermediate slopes (head to have no special significance, other than deposits and landslides) formed mainly as local representatives of this deposit. during periods of cold climate, albeit with some development at other times. This Stanmore Gravel stratigraphy provides a remarkable record of climate change in the region, recording 4.126 Hilltop occurrences of gravel at between the oscillations between glacial and 130 and 150 m OD in the extreme north- interglacial periods. west of the London district have been named the Stanmore Gravel. This typically 53

Figure 19 River Terraces of Greater London (pre-diversionary terraces in red colours)

contains well-rounded pebbles of flint, 4.128 As such, it could yield significant with lesser proportions of quartz pebbles, information about the early Quaternary subangular to nodular flint, quartzitic palaeogeography and climate, and about sandstone, and some other types. These the long term rates and patterns of vertical are set in a clayey, sandy matrix, with some crustal movement in the London region. pockets of coarse sand. The deposit is up to about 5 m in thickness. 4.129 the Stanmore Gravel, overlying the Claygate Member, is exposed at Harrow 4.127 the Stanmore Gravel is of uncertain age Weald SSSI, Harrow (Figure 4). and origin. It has been proposed as river deposits from south bank tributaries of the Glacial deposits and landforms proto-Thames, rather like the older of the Thames terraces. However, its distribution Till suggests that it is a westwards correlative of the Red Crag of East Anglia, and that 4.130 In the north of the district, particularly it therefore comprises marine deposits of around , and in the north-east at latest Pliocene to earliest Pleistocene age Upminster, glacial till overlies the proto- (Ellison et al., 2004, p. 52). Thames river terrace deposits. The till (part of the Lowestoft Formation) was deposited Protecting the geodiversity of the capital

during the Anglian Glaciation, about half 4.134 small outcrops of glaciolacustrine a million years ago, when an ice-sheet deposits also occur near Upminster, in the extended into the region from the north. north-east of the district. They consist of The deposits in north-east London mark thinly bedded sand, clayey sand and silty the most southern ice margin in eastern clay laid down in proglacial lakes at, or England (Figure 12). close to the front of the ice-sheet. As with glaciofluvial deposits, in the GLA area, 4.131 the Anglian till of eastern England these occur only in very small areas in the was traditionally known as ‘chalky north and extreme north-east. boulder clay’, reflecting its most obvious lithological characteristics. It is a Wider geological significance heterogeneous deposit typically consisting of variably sandy, silty clay with pebbles, 4.135 the glacial deposits mostly occur cobbles and boulders. The clayey matrix is on what is now high ground, on the derived mainly from Jurassic mudstones. interfluves. Although they were therefore The clasts are mostly of flint or chalk once more extensive, their distribution but commonly include Jurassic and provides important information on the Carboniferous sandstones and mudstones maximum extent of the Anglian ice-sheet. (and fossils derived from them, such as the Jurassic oyster Gryphaea), Triassic 4.136 there are also sporadic occurrences of quartzite, as well as other rarer erratic till found in boreholes at relatively low material such as dolerite or granite. The levels in the Lea Valley, near Edmonton till is composed essentially of the material and Waltham Cross (Cheshire, 1981). These eroded, transported and subsequently provide evidence about the contemporary deposited by the ice-sheet. topography, and the pre-glacial river systems. 4.132 The Lowestoft Till (locally known as the ‘ Till’) is exposed below river 4.137 their relationship to some of the older terrace deposits in river terrace deposits is an important SSSI (Figure 4). constraint on the age and stratigraphy of those deposits. Glaciofluvial sands and gravels Fluvial deposits 4.133 Glaciofluvial sands and gravels form lenses and sheets in close association with Thames river terrace deposits the till, locally underlying it. They were deposited by meltwater streams flowing 4.138 the Thames has left a record of its from the ice-sheet. Their composition former courses in the form of a series of is thus similar to that of the coarser fluvial deposits, which together make up components of the till, chiefly flint and the most widespread suite of superficial chalk pebbles with variably clayey sand. deposits in the London area (Figure 19).

Glaciolacustrine deposits 4.139 the river terrace deposits consist primarily of sand and gravel in varying 55

proportions, most usually comprising flint Park, separated by steeper areas of terrace gravel in a matrix of medium- to coarse- front slopes, e.g. leading from the Strand grained sand or clayey sand. Beds or lenses to Temple tube. of sand, some clayey or silty, occur locally and there are rare lenses of clayey silt, silty 4.142 the river terrace deposits in the London clay or peat. district can be separated into two systems (Figures 18 and 19). The older terraces 4.140 each of the terraces was originally pre-date the Anglian glaciation. They deposited on the contemporary valley floor, mark north-eastwards-flowing tributaries typically overlying bedrock. They mostly of an ancestral River Thames, which at represent deposition under cold climate that time flowed through what is now conditions, when vegetation was sparse, Hertfordshire and Essex on its way to the freeze-thaw weathering was active, and North Sea. The advance of the Anglian seasonal run-off was great. Consequently, ice-sheet diverted the Thames southwards large braided rivers carried voluminous to approximately its present course. The sediment, which accumulated relatively younger system of river terraces follows rapidly. Subsequent uplift and erosion left the course of the post-diversionary Thames portions of each deposit some distance and its tributaries. For example, in the above the modern river, with the older London Borough of Enfield, the Dollis Hill terraces at greater altitude. Together the Gravel was deposited by a pre-diversionary Thames river terraces thus take the form tributary of the Thames, flowing towards of a disjointed ‘staircase’ which range from the north-north-east, whereas the nearby some ridge-tops down to beneath the floor portions of the post-diversionary Taplow of the main valleys, where they are overlain Gravel and Kempton Park Gravel were by alluvium and peat. Some terraces are deposited by the southwards-flowing River preferentially preserved on one side of the Lea. valley; and the older terraces have been more extensively dissected by subsequent Wider geological significance erosion. 4.143 One of the main reasons the Thames 4.141 each terrace deposit is typically a few river terrace deposits are so important metres thick, lying on a step-like surface to scientists studying the Quaternary is eroded into the underlying bedrock. their content of the remains of plants and Greater thicknesses of the terrace animals. These can give an insight into deposits occur locally associated with the types of vegetation and habitats in either channels or closed hollows in the the area at the time, potentially providing underlying bedrock. In their undisturbed information of the climate and temperature state, each river terrace forms areas of during the time of deposition. For example, almost level ground which slope gently organic deposits associated with the downstream at a similar gradient to that of Kempton Park Gravel beside the River the modern river. The importance of this is Lea in the Enfield area contain plant and that it gives the London landscape part of insect remains indicative of a sub-arctic, its topographic character of long flat areas, tundra environment. Some terraces contain e.g. underlying airport or Hyde evidence of early human activity in the Protecting the geodiversity of the capital

area, such as bones and flint tools. extensively worked for brick manufacture. Large areas of London close to the 4.144 another of the values of the Thames Thames and its tributaries occupy former river terrace deposits is the continuity in brickfields. their source, and the river basin geometry, over a considerable duration of time. The Interglacial deposits continuity provided by this stratigraphy is vital for building an understanding of 4.149 there are a relatively few, small, but long term Quaternary climate change. The widely distributed examples of deposits Thames river terrace deposits are arguably from past interglacial periods, representing one of the most complete sequences of deposition in temperate climates, typically Quaternary deposits in the UK, particularly warmer than at present. These are rich in information on the cold stages associated with the river terrace deposits represented by the gravels, evidence of and the brickearths, typically towards the which is scarce in other parts of the UK. ‘back’ of the terraces, although the nature of the relationship is not everywhere clear. 4.145 the SSSI, Bexley, formerly exposed the Dartford Heath Gravel, part 4.150 they are mostly sandy to clayey in of the complex of Thames river terrace nature, representing fluvial or lacustrine deposits (Figure 4). deposition. Many have yielded significant fossil material, including in some cases the Brickearth remains of large mammals now found in Africa, and so have a greater significance 4.146 the river terrace deposits (and some of than their relative rarity might suggest. the nearby bedrock outcrops) are widely overlain by ‘brickearth’, now assigned to 4.151 the most well known of the Thames the Langley Silt or other named formations river terrace deposits are probably on a geographical basis. The London those evacuated from around Trafalgar brickearth is typically composed of very Square. These were identified in the fine-grained sand, silt and clayey silt, early 1700s, when remains of large ranging to silty clay, with some flint gravel. vertebrates were first discovered. This It is generally yellowish-brown in colour river terrace deposit is from the Ipswichian and poorly stratified. (Last) interglacial, providing a detailed insight into the animals and habitats 4.147 the silt is thought to have originated which would have been found living as a wind-blown deposit, a loess, but the close to the Thames 125 000 years ago. admixture of much sand and gravel in Mammals found include present day the local development of these deposits hippo (Hippopotamus amphibious), cave suggests they have been largely reworked lion (Panthera leo) and straight-tusked by fluvial processes or solifluction (down- elephant (Palaeoloxodon antiguus). slope mass movement under cold climate Molluscs found in the deposit suggest full conditions), or both. interglacial conditions, with a large slow flowing river containing plenty of aquatic 4.148 in the past, the brickearths were vegetation (Preece, 1999). 57

Figure 20 Soils of Greater London

4.152 The scientific importance of Alluvium the interglacial deposits is quite disproportionate to their extent. The 4.153 alluvium, composed of mainly silt and richness and variety of the contained flora clay with rare seams of sand and gravel, and faunal assemblages provide detailed forms the floodplains of the Thames information about the prevailing climate and the other rivers in the region. Also and physical environment and in some found within the alluvium are peat beds, cases can provide the basis for stratigraphic indicating periods of favourable conditions correlation from one site to another across allowing vegetation growth. Mesolithic northern Europe. For example, certain artefacts have been found in early of the Pleistocene Mammal Assemblage Holocene alluvium, providing information Zones have been named after localities in on the activities of humans in the area. the east of the London district and nearby Also abundant in alluvium are fossil Essex (Schreve, 2001). This stratigraphic molluscs, insects, ostracods and mammals, value is enhanced by the close relationship providing yet more information on the with individual river terrace deposits, which habitats and climate during the Holocene. can be traced over very wide areas. The Thames foreshore is a source of sites Protecting the geodiversity of the capital

with Holocene alluvium, such as the 4.158 Aside from the types of superficial submerged forest at Erith in east London. deposit, the past occurrence of permafrost conditions can be demonstrated by the 4.154 in most parts of the London district, presence of certain superficial structures. the floodplains are defended against The most common are cryoturbation inundation, and so alluvial deposition has structures, such as involutions. These occur there effectively ceased. Deposits below in the glaciofluvial deposits and some river the level of high water, within the modern terrace deposits. river channel, are generally referred to as tidal river or estuarine deposits. 4.159 pingo scars are thought to be present in a few places, notably at Blackwall. Mass-movement deposits Aside from their scientific interest, these structures are an important element of the Head local groundwater systems, and place a major constraint on civil engineering works. 4.155 head deposits are commonly present on slopes or on the floor of valleys. They Holocene deposits and processes formed mainly by gradual down-slope mass-movement (solifluction) under past 4.160 after the end of the last glacial period, conditions of cold climate, but can include around 11,800 years ago, river alluvium, the products of more recent soil creep or peat, salt-marsh and estuarine sediments hill wash. Their composition reflects that were laid down and soils developed. of the local materials from which they were derived, either the bedrock or other Soils types of superficial deposit, or both in combination. Head deposits typically are 4.161 soils link the underlying geology with poorly stratified and poorly sorted, and can surface habitats, biodiversity and land be variable in composition. Head deposits use. At any given location, soils reflect the may be more extensive than shown on the interaction between soil parent material, geological map. climate, topography, vegetation and fauna. Soil biodiversity is known to be greater 4.156 in some places head deposits may pass than above-ground biodiversity, but is laterally into river terrace deposits or poorly understood. London’s soils, like all landslide deposits. English soils, are young, but still represent more than 11,000 years of ecological Landslide processes and are thus irreplaceable (Figure 20). London’s soils have also 4.157 in the London area, landslides are been extensively modified by urban associated mainly with steep slopes on the development. London Clay. These occur in the Shooter’s Hill area of Greenwich, in , 4.162 chalk soils occur in a broad band across and around Romford. the south of London and include downland habitats of , High Elms, Saltbox Superficial periglacial structures Hill, Riddlesdown, Farthing Down and 59

Happy Valley biological SSSIs. In the east London’s surviving ancient woods are on of London chalk soils are confined to small the clay, including: areas in Lewisham, Woolwich, and Abbey Wood. A small area occurs in the • Queen’s Wood, much of north-west of London. biological SSSI and most of Highgate, Whitewebbs and Scratch woods in 4.163 sand and gravel soils (on river terrace north London deposits and parts of the Lambeth Group) occur extensively on the hill slopes south • woodlands National Nature of the Thames, in small areas to the north- Reserve and those on Stanmore Golf east of London, on top of the clay in south Course in west London London at Richmond Park and and in a band north of the chalk, • and all of the Oxleas which widens to the east. Sandy hilltops Woodlands occur on top of the clay across the north and west of London. Acid grasslands and • Sydenham Hill in south London. heathlands include: 4.165 most grasslands tend to be damp, • Richmond Park, Wimbledon Common, and this includes some of London’s best , Keston and Hayes ‘mesotrophic’ grasslands, which include: Commons SSSIs in south London • Fields, Edgeware Way, • , the western end of Arrandene and in north London Common and some of the northern and western outliers of • Richmond Park biological SSSI and Hampstead Heath in north London Morden Cemetery in south London

• Stanmore and Little Commons, • Bentley Priory biological SSSI, Fryent Stanmore Country Park and part of Country Park, Horsenden Hill, Bentley Priory in west London Brook Fields and Kensal Green Cemetery in west London. • Epping Forest SSSI, Hainault Country Park, and Lesnes Abbey 4.166 in the north-east there is extensive Wood in east London. agricultural land as at Fairlop Plain.

4.164 low clay hills (London Clay Formation) 4.167 loams occur south of the clay in west occupy most of the north and west London London in a wide band, in a few small areas and the north-eastern and south-eastern near the Thames and to the west of the boundaries, now very largely occupied by Lea Valley. A scattering of loam soils is also built development, except in the Green found over the low level gravels largely Belt of the far north and west. In south north of the Thames. These soils are fertile, London low clay hills occupy a band to the and in history were the focus of productive north of the chalk overlapping with the agriculture and horticulture for London’s sands and gravels in the west. Many of breadbasket. Protecting the geodiversity of the capital

4.168 extensive low level gravels (river terrace valley and areas associated with wet gravel deposits) occur near the Thames and also workings in the Colne and Crane valleys. beside tributary rivers, particularly the In places, previous use has left rubble, Wandle, and are found either side of the sand, etc, on top of the natural floodplain Lea Valley. Acid grassland and heathland surface. habitats are evident here, such as: Artificially modified ground • , Home Park, , and Barnes and Mitcham Commons in 4.171 Artificial deposits are those created south London by human activity. They comprise Made Ground (material deposited on an existing • the centre of Whitewebbs Wood in land surface) and Infilled Ground (material north London deposited in an excavation, typically a quarry or the like). For example, rubble • in east London from cleared World War II bombsites was used to fill gravel excavations on • in south London. Blackheath and large quantities were used to raise above flood level 4.169 Narrow belts of floodplain (alluvium) to create playing fields. In some instances soils occur around the Thames and also this distinction cannot be made clearly and in valleys of the tributaries (including so Made Ground and Infilled Ground can the lost rivers of central London). These together be referred to as Artificial Ground. would once have been managed as grazing Geological maps typically also include marsh, but most have been displaced by Worked Ground (excavated areas) in the development. The best surviving grasslands category of Artificial Deposits or Artificially and remnant saline grazing marsh are at: Modified Ground. Artificial Deposits may overlie either superficial deposits or • the northern edge of Ham Lands, bedrock formations. Wilderness Parkfield at the eastern edge of Hampton Court and beside the 4.172 underground chalk workings are present upper Wandle in south London in some parts of London, notably at Pinner, Chislehurst, Sidcup, Blackheath and • and Woolwich. These are of a variety of ages biological SSSIs and in wet gravel pits and types. Some are very extensive (see in west London paragraphs 4.46-4.50 and 5.9-5.10).

• Rainham biological SSSI, Dartford and Natural processes Bexley Marshes, and the best fresh water example at the Ingrebourne 4.173 london’s natural river system has largely Marshes biological SSSI. been lost due to urban development. Most smaller streams have been built over and 4.170 Woodland is naturally scarce but there channelized, or altered by the quarrying of are some valuable wet woodlands in the aggregate, for example, in the Lea Valley. SSSI, in the Ingrebourne However, over the last few years, efforts 61

have been directed at reversing this loss. Fossils and palaeontology The Blue Ribbon Network policies in the London Plan (7.24-7.30) aim to protect Late Cretaceous and improve London’s semi-natural and man-made water systems. The policies 4.177 On an ultra-microscopic scale, the Chalk take into account that this network is a is made entirely fossils. The nanometre- dynamic system and is subject to natural sized (a billionth of a metre) fragmented forces such as tides, erosion and floods. skeletons of coccolithophores, visible only Restoration of rivers to a more natural form under a scanning electron microscope, is encouraged and proposals to impound or are the major constituent of the Chalk, partially impound rivers and development but microfossils visible using conventional on functional flood plains are resisted. optical microscopes, such as foraminifera, Proposals that include the removal of radiolaria and ostracods, also occur. impounding structures in rivers are also Macrofossils, those visible with the naked welcomed. eye, have been the traditional focus of Chalk palaeontological studies, and 4.174 guidance on river restoration in London include a wide variety of forms, the most is available from: River Restoration – A important being sponges, brachiopods, stepping stone to urban regeneration, bivalves, ammonites, belemnites, crinoids highlighting the opportunities in South and echinoids. At some levels macrofossil London (Environment Agency, 2002) remains are so abundant that they are and Bringing your rivers back to life - A important rock-forming elements, such as strategy for restoring rivers in North the bivalve Mytiloides forming the shell- London (Environment Agency, 2006). rich higher part of the Holywell Chalk, or the fragmented occurrence of the bivalve Future Earth science research potential Platyceramus in the Seaford Chalk. Trace fossils, representing burrows made by 4.175 many of the Quaternary (and in the animals in the sea bed sediment whilst the case of the Stanmore Gravel, possible late Chalk was being deposited, are also locally Neogene) deposits, particularly the system abundant and extensively developed at of river terrace deposits and the associated some levels. interglacial deposits, preserve an important record of changes in sea level and 4.178 the technique of biostratigraphy palaeoenvironment, and so also of climate classifies the Chalk into intervals variation. Some of these deposits also yield (biozones) on the basis of its fossil significant archaeological information. content. Historically this methodology offered a much more sophisticated way of 4.176 the details of the relationships between subdividing up the Chalk than was possible the river terrace deposits, interglacial using the traditional scheme based on rock deposits, and glacial deposits provide composition. The standard zones of the important controls on Quaternary Chalk are based on macrofossils, but there stratigraphic correlation in southern are also microfossil schemes. In Mesozoic England. rocks, ammonites are usually the best macrofossils for biostratigraphy, as they Protecting the geodiversity of the capital

show rapid evolution, are widely distributed 4.183 the Thanet Sand is not conspicuously and are normally well preserved. The Chalk fossiliferous, containing sporadic, poorly is unusual, however, in that ammonite preserved bivalves and gastropods, but remains are only common in the lower part microfossils are important. Calcareous of the Grey Chalk Subgroup, requiring nanoplankton and palynomorphs have reliance on a variety of other macrofossil been used for regional correlation. The groups for the biozonation of the higher foraminifera and other fauna indicate that part of the Group. the greater part of the formation was deposited in a shallow sea, less than 50 4.179 Biostratigraphy remains important for m deep, and that the prevailing climate understanding the age distribution and was generally cool, turning warmer during correlation of the Chalk, but increasingly deposition of the younger sediments. attention is being focussed on the Irregular nodules of pyrite, less than 5 mm microscopic fossils, as these may offer across, are presumed to have replaced insights into fundamental environmental fragments of wood. The Bullhead Bed processes in the Late Cretaceous, such includes a variety of fossils, including as ocean circulation and global climate echinoid spines, bryozoa, pelecyod shells change. and fish vertebrae, derived from the Chalk.

Palaeogene 4.184 the marine deposits in the Lambeth Group contain molluscs and sharks’ teeth. 4.180 parts of the Palaeogene sequence in The Woolwich Formation, especially the London area, particularly but not the Shelly beds, commonly contains a exclusively the marine deposits, have a rich limited mollusc fauna indicating brackish and varied fossil fauna and flora. water conditions. However, the ‘Paludina Limestone’ of the Upper Shelly Clay 4.181 Bivalves and microfossils such as contains the remains of freshwater foraminifera and ostracods have long been molluscs, while the rest of the Upper Shelly used for stratigraphic correlation. More Clay contains a generally more diverse recently, nanoplankton and dinoflagellates fauna than the Lower Shelly Clay, including have also been found to be of value. more marine-tolerant species. The Harwich Fossil remains of many other groups of Formation is locally shelly, as at Elmstead marine organisms can be commonly found, Pit (Section 4.3), and also yields sharks’ including sharks’ teeth which in places teeth and fish scales. have been concentrated by winnowing of the sea floor sediment. 4.185 as a non-marine deposit, the Reading Formation is relatively poor in fossils, but 4.182 the Palaeogene fossil mammal fauna thin beds rich in leaf impressions and other is less important for correlation than the plant remains occur in places. Clay beds marine assemblages, but provides valuable with leaf impressions have been noted insights into evolution following the large- in the ‘Striped Loams’ of the Woolwich scale extinctions (most notably of the Formation. The lignite deposits at the dinosaurs) at the end of the Cretaceous. Paleocene-Eocene boundary arose from vegetation dominated by angiospermous 63

trees and herbs, with many ferns, which to different areas within the park and was subject to repeated fires. These fires subsequently became damaged. Funding may have been a response to climatic for the models was cut in 1855 before all events associated with the Paleocene- those planned had been completed. Eocene thermal maximum (Collinson et al., 2003; King, 2006). 4.190 By the end of the 19th Century, further research into the species on display had 4.187 the marine fossil fauna of the London revealed that the models were highly Clay includes numerous types of bivalve, inaccurate and they drew many critics. The gastropods and some brachiopods, corals, popularity of the models declined and they echinoderms such as starfish and sea lilies, fell into disrepair – this was compounded arthropods including crabs, lobsters and by the fire in 1936 which destroyed the barnacles, cephalopods and a variety of Crystal Palace. The models and much of sharks’ teeth. Bones of fish and reptiles the park became overgrown. can also be found. A variety of plant seeds and fruits, some quite large, also occur in 4.191 in the 1950s Victor H C Martin carried the London Clay. out a full restoration of the models during which time they were moved to fresh sites. Crystal Palace geological illustrations After this restoration, the models were once again left to decay with the exception 4.188 the Crystal Palace was originally built in of occasional touch ups. This was the case Hyde Park as part of the Great Exhibition for the whole park until 2002 when the of 1851; it was relocated to its current Institute of Historic Building Conservation position once the exhibition was over. As completed renovation of the models. part of the relocation and development The models were mended and repainted; of the grounds, Benjamin Waterhouse missing models were reconstructed using Hawkins was commissioned to build life- fibreglass. The vegetation around the size models of extinct animals. With the models was also restored to reflect the help of Sir Richard Owen (a biologist and plant life that would have existed when the palaeontologist), he was able to design exhibited animals were alive. dinosaurs to be included in the collection. Concurrently, a model of geological strata 4.192 there are now 29 models in the park and a lead mine were built to illustrate representing the following genera: the natural state of geological resources including coal, iron and lead. These were Anaplotherium; Dicynodon; Hylaeosaurus; constructed by James Campbell, an Ichthyosaurus; Iguanodon; Labyrinthodon; engineer and mineralogist. Irish Elk; Megalosaurus; Megatherium; Mosasaurus; Palaeotherium; Plesiosaurus; 4.189 the animal models were originally Pterodactyl; and Teleosaurus. set out so they followed a rudimentary timeline; Palaeozoic, Mesozoic and 4.193 the models are now part of a ‘Prehistoric Cenozoic eras were each represented by Monster Trail’ which leads visitors through a separate island within an artificial lake. time with information boards explaining The Cenozoic models were later moved each model and how they would differ Protecting the geodiversity of the capital

if constructed today. This trail is now Evidence of early Man available as an I-Phone Application. 4.196 palaeolithic artefacts have been found Quaternary in the river terrace deposits of the Thames and its tributaries, and in associated 4.194 A number of flint artefacts have been interglacial deposits, reflecting the found in London of Early and Late presence of human occupation along the Palaeolithic age (corresponding to Pre- water’s edge. Anglian to Late Devensian in geological time). These include hand axes, blades, and 4.197 the oldest such remains were found other small tools. in interglacial deposits dating from the Hoxnian (which immediately followed 4.195 pleistocene bone, plant and insect the Anglian glaciation) about 400 000 remains have been found at a number of years ago, in association with the post- localities in London. The main sites are diversionary Boyn Hill Gravel. Such Isleworth, Ilford, Brentford, Kew, Trafalgar deposits at Barnfield Pit, Swanscombe Square, Ponders End, Erith, East Ham and (in Kent, about 6 km east of the London Crayford. The Corbetts Tey – Mucking/ district) have yielded some of the oldest Lynch Hill – Taplow terraces have yielded hominid fossils so far found in Britain. cave lion, wolf, mammoth, rhinoceros, giant ox and red deer, with Ilford being 4.198 tools are found in deposits dating one of the best sites. The East Tilbury from the subsequent two interglacials, at Marshes/Kempton Park terrace has yielded about 320 000 years ago (associated with hippopotamus, beaver, hyaena, cave the Lynch Hill and Hackney gravels) and lion, brown bear, elephant, boar, bison, 250 000 to 200 000 years ago (associated giant ox and red deer, with Brentford and with the Taplow Gravel). The fossil record Trafalagar Square being the best examples. indicates that humans were then absent More recent deposits associated with the from Britain until about 70 000 years ago Devensian ice age have yielded lemmings, (Stringer, 2006). hyaena, cave lion, polar bears, woolley mammoth, rhinoceros, reindeer, bison, 4.199 From the late 19th century onwards, ox and antelope. Erith, Twickenam and archaeologists realised that flint weapons the Lea Valley are the best examples. The and tools were contemporary with alluvium has yielded beaver, wolf and giant the remains of many extinct animals, ox with East Ham being the best known representing a long period of time prior to site. the appearance of modern man. Gradually, distinctive assemblages of tool types were identified, coincident with the cultural evolution of man. For further details see Sumbler (1996).