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Geology of the Ipswich district — brief explanation of the geological map Sheet 207 Ipswich

S J Mathers, M A Woods, and N J P Smith

Bibliographic reference: Mathers, S J, Woods, M A, and Smith, N J P. 2007 Geology of the Ipswich district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 207 Ipswich (England and Wales).

Keyworth, Nottingham: British Geological Survey. 2007.

Printed in the UK for the British Geological Survey by B&B Press Ltd, Rotherham

© NERC 2007 All rights reserved. Copyright in materials derived from the British Geological Survey's work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail ipr@bgs.ac.uk. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

(Front cover) The Orwell bridge across the estuary south of Ipswich, looking north. (Photograph: P. Witney; P626235)

(Rear cover)

(Geological succession) Geological succession in the Ipswich district.

Notes

The word 'district' refers to the area of Sheet 207 Ipswich. National Grid references are given in square brackets; unless otherwise specified they lie within 100 km square TM; specific locations and boreholes are accompanied by their National Grid reference at their first mention within the text. Borehole records referred to in the text are prefixed by the code of the National Grid 25 km2 area upon which the site falls, for example TM14SE, followed by its registration number in the BGS National Geological Records Centre. Lithostratigraphical symbols shown in brackets in the text, for example (Tms) are those shown on the published map. Numbers (following the letter P) included with the plate captions refer to the BGS Photographic archive.

Acknowledgements

This Sheet Explanation was compiled by S J Mathers; M A Woods contributed information on the Cretaceous strata; N J P Smith provided the Structure section and data for (Figure 1). Series editor is A A Jackson: cartography by S Ward: page-setting by A Hill. The authors' thanks are due to the many landowners, local authorities, utility and site investigation companies for access to land and provision of geological information. Anglian Water and A F Howland Associates generously provided site investigation data on the Chalk of the Ipswich district. C J Wood and T Wright provided data and advice on the biostratigraphical and lithostratigraphical classification of the Chalk of the Ipswich district. Thanks are also due to Antonia Weston for help with visits to local quarries.

The grid where it is used on figures, is the National Grid taken from Ordnance Survey mapping.

© Crown copyright reserved Ordnance Survey licence number 100017897/2007.

Geology of the Ipswich district (summary from the rear cover)

(Rear cover)

(Geological succession)

Geology of the Ipswich district. An explanation of sheet 207 (England and Wales) 1:50 000 series map.

Ipswich, the county town of Suffolk and a major port, lies in the Gipping valley as it opens out into the Orwell estuary In the hinterland, rich loamy soils are developed on glacial and fluvial deposits that include those of the ancestral River Thames. The area is given over mainly to arable farming.

Outcrop of bedrock is limited in the district: the Cretaceous Chalk Group underlies most of the district but is largely concealed by Palaeogene and Quaternary strata. The lowest Palaeogene deposits are the marine Thanet Sand Formation, succeeded by the Reading Beds of the Lambeth Group deposited in a marginal marine, swampy environment. The overlying Thames Group reflects fully marine deposition. Oligocene Miocene sediments are absent here, probably due to uplift and folding associated with the Alpine orogenic events farther south. The Crags are the most extensive bedrock present; these shallow water Pliocene marine sediments include the Coralline Crag, Red Crag and Norwich Crag formations. Quaternary deposits fall into three categories.

The Kesgrave Formation predates the Anglian glaciation and is part of deposits of the ancient River Thames before it was diverted south to its present course by the Anglian ice sheet. The Lowestoft Formation includes till and glaciofluvial deposits of the Anglian glaciation; it is absent south of Ipswich. Periglacial conditions prevailed here during the last glaciation (Devensian), and the type-site for the last Ipswichian Interglacial is located at Bobbitshole. The postglacial deposits are not part of a named formation but include alluvium, peat, river terrace and estuarine sediments.

Geological factors have an influence on ground conditions and hence on planning and development. Important factors are discussed including water resources, mineral workings, the risk of flooding and chalk dissolution.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the geological 1:50 000 Series Sheet 207 Ipswich. It is written for the professional user, and those who may have limited experience in the use of geological maps and may wish to be directed to further geological information about the area.

The Ipswich district comprises a low-lying coastal part of Suffolk. The highest ground lies in the north-west at around 70 to 90 m above OD and is underlain by loamy soils developed on glacial deposits. The ground falls away south and east, and here extensive heathlands are developed on broad plateaux underlain by the predominantly sandy fluvial deposits including those of the ancestral River Thames and the shelly sand 'crag' deposits for which this area is famous.

The main drainage is that of the River Gipping flowing south-eastwards through the district and opening out below Ipswich into the tidally dominated Orwell estuary, which is flanked by mudflats. Farther west, the River Brett flows in a similar direction through Hadleigh towards its confluence with the River Stour, south of the district. In the east of the district, the drainage is by the eastward flowing tributaries of the River Deben including the River Fynn. To the south of the district the Tattingstone Brook has been dammed flooding the lower parts of the valley within the district, to produce the Alton Water Reservoir.

The area was settled in Roman times with the major settlement of Combretovium located on the floor of the Gipping valley near Baylham [TM 102 515] from which roads diverged across the region. Today, Ipswich, the county town of Suffolk, is the main settlement in the district and a flourishing port dealing with imports of timber, fertilisers and construction materials and exports of the region's agricultural products. Outside Ipswich, much of the district is rural, given over to arable farming with cereals and sugar beet as the principal crops. The other major settlements are the market towns of Hadleigh [TM 026 425] in the Brett valley in the west, Needham Market [TM 086 553] in the Gipping valley in the north and Woodbridge [TM 270 490] in the Deben valley straddling the eastern edge of the district.

During the Early Palaeozoic, this district lay on the margin of the Tornquist Ocean in which thick marine sequences accumulated at least until the Devonian. These rocks comprise part of the East Anglian Caledonides and were probably deformed in the Acadian Orogeny. No proven late Devonian or Carboniferous rocks are known from a wide area surrounding the district and it is probable that rocks of this age were never deposited. The Palaeozoic rocks underlying the district form the core of the London–Brabant Massif, that extends into the southern North Sea in a south-easterly direction. This massif was a positive feature throughout the Late Palaeozoic and much of the Mesozoic. It was submerged again in the Early Cretaceous and then continued to subside slowly as the thick Chalk sediments accumulated during the Late Cretaceous (Plate 1).

Uplift, tilting and erosion at the end of the Cretaceous was followed by renewed sedimentation in the Palaeogene, initially marginal marine in character, but fully marine during the deposition of the Thames Group. The absence of in situ Oligocene–Miocene sediments in the region reflects the uplift and folding associated with the Alpine Orogeny. Shallow water Pliocene marine sediments (Crags) were then deposited in the district prior to gradual emergence and easterly down-tilting in response to subsidence in the adjacent Southern North Sea Basin. The succeeding Quaternary, proto-Thames, fluviatile sediments of the Kesgrave Formation were predominantly deposited in cold stages as the climate began to fluctuate markedly. In the ensuing Anglian glaciation, an ice sheet reached the district and deflected the lower part of the Thames drainage to the south to follow its present course through London. During the succeeding cold stages, downcutting continued to base levels well below OD, and aggradation led to the deposition of river terrace deposits. During the ultimate glacial stage (Devensian) the area experienced prolonged harsh periglacial conditions. The following climatic amelioration, starting about 16 000 years ago, resulted in the gradual rise in sea level leading to deposition of the postglacial (Holocene) fluvial and estuarine sediments of the district during the last 7000 to 8000 years.

Chapter 2 Geological description

The geological succession present at outcrop and in boreholes beneath the district is shown on the inside of the front cover. The deepest unit encountered is the Gault Formation. Much of the evidence of the concealed strata is drawn from regional geophysical interpretation and deep boreholes sited adjacent to the district.

Concealed geology

Silurian

There are no boreholes within the district that encounter the Palaeozoic basement, however seven deep boreholes in the surrounding area enable some general conclusions about the basement structure to be deduced. The deep boreholes at Harwich [TM 2593 3278], Stutton [TM 1500 3340] and Weeley [TM 1474 2183] to the south of the district, Lake Lothing in Lowestoft [TM 5380 9260] to the north-east, Clare [TL 7834 4536] to the west, and Stowlangtoft [TL 9475 6882] and Four Ashes [TM 0223 7186] to the north-west, all reached Palaeozoic basement beneath the sub-Mesozoic unconformity (Figure 1). These basement rocks include grey sandy shales and slates with a well-developed cleavage (Whitaker and Thresh, 1916) and are tentatively dated as Silurian, based on acritarchs, chitinozoa, and nautiloid and eurypterid fragments (Molyneux, 1991), except at Four Ashes, where the strata may be early Devonian in age. About 160 m of these steeply dipping rocks were drilled at Stutton. The thickness in this district is unknown, but probably amounts to several kilometres.

Cretaceous

The sub-Mesozoic unconformity deepens from about 200 m below OD, just west of the district to about 325 m below OD in the east (Figure 1). The overlying rocks in the Harwich, Stutton, Weeley, Clare, Four Ashes and Stowlangtoft boreholes are all of the Gault Formation (G), which represents a marine transgression at the end of the early Albian. The Gault does not crop out in the Ipswich district, but occurs at depth in a single borehole at Combs [TM 0427 5625], which was terminated within the Gault Formation after 3.35 m had been proved. Preserved thicknesses in adjacent deep boreholes are as follows: Harwich 6.7 m, Stutton 15.4 m, Weeley c.23 m, Clare 11 m and Stowlangtoft 13.5 m. In the Weeley Borehole, the lower part is a smooth grey mudstone, overlain by a very thin green pebbly sand, with black phosphatic nodules and capped by grey laminated mudstone. Mudstones and subordinate sands were encountered in the Harwich (Whitaker and Thresh, 1916) and Stutton boreholes (Whitaker, 1906) whilst at Clare and Stowlangtoft mudstones predominate (Bristow, 1990; Pattison et al., 1993).

Cretaceous

In the Late Cretaceous, large rises in sea level allowed marine deposition to extend across nearly the whole of the UK, represented by the deposits of the Chalk Group (Ck). The group underlies the whole of the Ipswich district, but only crops out in the flanks of the Gipping–Orwell valley mainly to the north of Sproughton Manor [TM 127 456], and in two tributary valleys, one between Bosmere Hall [TM 103 545] and Coddenham [TM 133 542], and the other around Offton [TM 066 496] and Somersham [TM 083 488]. The Chalk also crops out in the Brett drainage system south of Chelsworth Common [TL 986 472], and at Bildeston [TL 992 494]. Across much of the district the Chalk is concealed by Palaeogene and Quaternary deposits, but data from boreholes and sporadic outcrops permit investigation of its stratigraphy. About 250 m of Chalk were proved in the Combs Borehole (in Figure 1), and to this a further 45 m of stratigraphically higher strata can probably be added based on outcrops in the Gipping valley and site investigation boreholes beneath Ipswich (Figure 2).

Chalk is typically a very fine-grained, white limestone, and composed predominantly of the disaggregated skeletal remains (coccoliths) of tiny planktonic algae that flourished in the seas of the Late Cretaceous. The Chalk Group is composed of almost pure calcium carbonate in the form of low-magnesian calcite, except in the lower part, that contains up to 30 per cent clay. Flints, clay-rich horizons (marls), beds of indurated, mineralised chalk (hardgrounds), and coarsely bioclastic chalk horizons also occur, and some of these form geographically extensive marker-horizons that can be recognised on the resistivity log of the Stowlangtoft Borehole in the adjacent Bury St Edmunds district (Bristow, 1990; (Figure 2)).

Traditionally, a tripartite classification has been applied to the Chalk Group (Lower, Middle and Upper Chalk), based on the development of feature-forming beds of hard chalk. In southern England this classification is now superseded by that of Rawson et al. (2001), which recognises two subgroups and up to nine formations within the Chalk Group. East Anglia belongs to the so-called 'Transitional Province', which shows features that are intermediate between the distinct Chalk Group stratigraphies of southern and northern England (Mortimore et al., 2001; Rawson et al., 2001). The extent to which the southern England Chalk Group nomenclature can be applied to East Anglia is still unclear. In the Ipswich district, it seems likely that the stratigraphy of the concealed succession and some of the exposed succession is analogous to southern England. However, there are lithostratigraphical differences in part of the exposed succession that warrant a modified nomenclature ((Figure 2); see below).

The probable stratigraphy of the unexposed part of the Chalk Group can largely be determined from the BGS Stowlangtoft Borehole that lies about 12 km north-west of the district. Many features of its core and resistivity log can be compared with logs from boreholes in southern England. On this basis, it seems likely that the Ipswich district is underlain by representatives of the following formations (in ascending stratigraphical order): West Melbury Marly Chalk, Zig Zag Chalk, Holywell Nodular Chalk, New Pit Chalk, Lewes Nodular Chalk and Seaford Chalk (Figure 2).

Outcrops in the Gipping valley show that the exposed part of the Chalk Group can be divided into two formations, the Newhaven Chalk (NCk) overlain by the Culver Chalk (CCk), these are shown on the map sheet where the chalk is present at rockhead. The Newhaven Chalk Formation forms the bulk of the outcrop succession (Figure 3), and consists mainly of very poorly flinty chalk, lacking conspicuous marl seams (Plate 1). Macrofossils are generally rare, except for a few specimens of the belemnite Gonioteuthis, and an oyster-rich bed with abundant Pseudoperna boucheroni (Woods non Coquand, 1859) and inoceramid shell fragments (Platyceramus and Sphenoceramus) seen near the base of the succession at Needham Quarry [TM 0940 5395] and in one of the two pits at Little Blakenham [TM 1086 4910]. Outcrops in the Gipping valley belong mainly to the O. pilula and basal G. quadrata zones (Wilkinson, 2004; (Figure 3)), but in Ipswich, site investigation boreholes proved up to 55 m of the Formation, including the U. socialis and M. testudinarius zones. Because the Newhaven Chalk of the Ipswich district appears so distinct from the typical flinty and marl-bearing Newhaven Chalk of southern England, it is here named the Blakenham Member, with its stratotype section in the large quarry at Great Blakenham [TM 1161 4986]. The member is similar to the Margate Member of Robinson (1986), but much less flinty, and future work in East Anglia might justify designation of formational status.

The Culver Chalk Formation forms the top of the succession at Claydon Quarry [TM 1363 4966] and is present in many of the Ipswich town site investigation boreholes. The formation contains regularly developed medium and large nodular flints with moderately common remains of the echinoid Echinocorys. The belemnite Belemnitella is associated with the formation in the Ipswich boreholes, and also recorded in historical accounts of outcrops at Coe's Pit, Bramford [TM 1292 4814] and Claydon [TM 1319 4943] (Boswell, 1927). The locally abundant record of Belemnitella was the basis for the historical identification of B. mucronata Zone chalk in the Ipswich district (Boswell, 1927). However, new biostratigraphical data shows that these occurrences of Belemnitella are within the lower part of the G. quadrata Zone, and are most probably correlative with the local abundance of this belemnite at the base of the Culver Chalk Formation in southern England (e.g. Bailey et al., 1983, fig. 3; Mortimore, 1986, fig. 20).

Palaeogene

The lowermost Palaeogene strata belong to the Lambeth Group and Thanet Sand Formation (LT). They are Palaeocene in age and rest unconformably on the Chalk, which dips south-east at less than 1˚. These deposits are up to 20 m thick and form discontinuous outcrops along the flanks of the Gipping–Orwell, Belstead Brook, and Brett valleys, extending northwards as far as Semer in the Brett valley, and to Bramford [TM 126 463] in the Gipping valley. Away from the main valleys, their extent at rockhead can be traced in boreholes. Their extent is restricted by the widespread Red Crag deposits that overstep the Palaeogene deposits to rest directly on the Chalk in the north-western parts of the district. A simplified map showing the complex distribution of these bedrock units at rockhead is shown in the margin of Sheet 207.

Boreholes indicate that complete preserved sequences beneath the overlying Thames Group deposits vary from about 12 to 20 m in thickness but no systematic pattern in thickness variation can be discerned. In many boreholes and in accounts of sections in former quarries (Whitaker 1885; Boswell, 1927) the base of the sequence is a bed of glauconite-coated flints overlain by 1 to 2 m of glauconitic sands and silts. These marine deposits probably comprise the northernmost extensions of the Thanet Sand Formation with the basal unit being the Bullhead Bed (Morris, 1876). The overlying strata comprise green, grey and red mottled silts and clays with brown sand beds in the upper parts; locally these sand beds have become cemented into hard silcretes or sarsen. By comparison with surface exposures farther inland in Suffolk, beyond the district, these strata are mainly included in the Reading Formation of the Lambeth Group. Studies of the Reading Formation elsewhere in the region suggest much of it is nonmarine (Hester, 1965), possibly deposited in a paludal environment traversed by sluggish streams (Ellison, 1983). The complex colour mottling and reddening of the clay-rich sediments is thought to result from multiple cycles of pedogenesis in a warm climate (Buurman, 1980). The detailed logs of many boreholes within the district indicate beds of presumably marine green sands and silts at varied levels within this sequence, thus showing that the two-fold classification of a marine Thanet Sand Formation overlain by the nonmarine Reading Formation is a considerable over-simplification.

Conformably overlying the Palaeocene deposits are the Eocene Thames Group (Tms) with a maximum preserved thickness in the district of about 40 m. The deposits crop out along the flanks of the Gipping–Orwell and Brett valleys and tributaries extending northwards in the Brett valley to Kersey [TM 000 441] and to Bramford in the Gipping valley. The deposits also crop out along the Fynn valley and its tributaries, in the north-east of the district. The extent of the Thames Group at rockhead can be traced in boreholes although over much of the district they are overlain by the widespread Red Crag deposits that overstep the Palaeogene deposits to rest directly on the Chalk in the north-western parts of the district.

The term Thames Group is here used in the sense of King (1981) and Ellison et al. (1994), it corresponds to the London Clay, including the basement bed, of the older literature. It is now defined as comprising a lower Harwich Formation (HAR) and an upper London Clay Formation (LC). It was not practicable to separate these two formations during the survey although a substantial portion of the deposits present in the district belong to the Harwich Formation.

Five main transgressive-regressive cycles (A–E) have traditionally been recognised in the Thames Group (King, 1981 and references therein). Some, however, are composite: for example the basal A1 subcycle that corresponds to the Harwich Formation. Each cycle comprises, at its fullest development, a basal pebble bed and/or glauconitic horizon overlain by a coarsening-upward sequence of clays, silts and sands.

Studies of boreholes and cliff sections in adjacent districts indicate that the Harwich Formation in this area comprises 15 to 20 m of silty and sandy clays, clayey silts and thin glauconitic sand beds. Beds of volcanic ash are common in the upper parts of the sequence but more sporadic in the lower parts. Over 30 individual layers have been identified ranging from 10 to 80 mm in thickness: some contain evidence of reworking by currents. The ash is these layers is mainly altered to bentonite. Most of the layers are blue-grey in colour on fresh surfaces but weather to a distinct cream-brown colour. About 6.5 m from the base of the sequence is the resistant Harwich Stone Band (Knox and Ellison, 1979), 20 to 30 cm thick, and comprising a concretionary cementstone layer with a substantial ash content (Elliott, 1971). The type sections for the formation (Ellison et al., 1994) lie just beyond to the southeastern corner of the district. They include the Shotley Gate Borehole [TM 2439 3460] (Knox and Ellison, 1979), and the cliff and foreshore exposures at Wrabness [TM 171 323] to [TM 174 324] and Walton on the Naze [TM 264 230] to [TM 267 245] (King, 1981; Daley and Balson, 1999).

Detailed studies of the coastal outcrops south of the district, principally at Harwich, Wrabness and Walton on the Naze, have provided considerable data on the age and genesis of the Harwich Formation and possible schemes for its subdivision: these include palynological studies that recognised pollen-spore assemblages (Jolley and Spinner, 1991). Later studies were by Jolley (1996), Aubry (1986) and Powell et al. (1996). Work on magnetostratigraphy indicated Chron C24R (Ali et al., 1996).

The overlying London Clay Formation as defined by Ellison et al. (1994) mainly comprises blue-grey, silty clay and clayey silts that weather to a characteristic chocolate-brown colour. Subordinate coarser sand beds, pebble seams and some glauconite are also present, marking the base of trangressive events.

Neogene

Miocene

Strata of Miocene age are very rare in Britain and none are preserved in situ within the district. However, the basal pebble seams and conglomerates of the Red Crag Formation contain abundant phosphatic material. Much is reworked from the underlying London Clay Formation, but there are also bones and teeth of Miocene age and concretions of cemented sand containing a late Miocene, mainly molluscan, marine fauna. These concretions have been found mainly in the south-east of the district and the informal term 'Trimley Sands', after the twin villages of Trimley [TM 277 368] just beyond the south-east of the district, was proposed by Balson (1990).

The concretions are composed of carbonate fluorapatite- and calcite-cemented sand grains and are up to about 20 cm in diameter. These have traditionally been called 'Suffolk box-stones'. A full description of these concretions and their contained faunas was given in Boswell (1928) and extended by Balson (1990). The sand is moderately well sorted, medium grained and predominantly quartzose, with a rich accessory mineral assemblage including minor glauconite. The concretions are authigenic, and are believed to have formed around sites of organic decay, notably with large molluscan shell moulds in their core.

The fauna of these box-stones has been studied in considerable detail, a Syltian (latest Miocene) age is probable for most of the concretions (Balson, 1990, and references therein). However, some of the phosphatised material associated with the concretions is of an older, middle Miocene age, most notably the teeth of the extinct giant shark Carcharocles megalodon. So a prolonged period of reworking and possibly multiple phosphogenic episodes is indicated by these diverse remanié deposits.

Pliocene

Three small outliers of the Pliocene Coralline Crag Formation (CCg) are present on the shores and beneath Alton Water south of Tattingstone White Horse [TM 136 383]. The deposits here were formerly exposed in a small pit, and belong to the Ramsholt Member (Balson et al., 1993). This is the lowest part of the formation comprising shelly mud-rich calcareous sands with a basal phosphorite lag gravel. Investigations including the drilling of a borehole (Balson 1981) indicate the deposits are about 6 m thick; they are thought to be of early Pliocene age (Balson et al., 1993).

The Red Crag Formation (RCg) forms discontinuous crops along the sides of Brett, Gipping–Orwell valleys, and along the tributary valleys of the River Deben, in the east of the district. The Red Crag Formation is present at rockhead across much of the southern and eastern parts of the district where it rests unconformably on the Palaeogene strata. In the north and north-west the deposits overstep the Palaeogene strata to rest directly on the Chalk, here subsequent erosion has lead to a more patchy distribution. A map showing the complex distribution of these bedrock units at rockhead is shown in the margin of the Sheet 207.

The thickness of the Red Crag sediments is largely a reflection of the morphology of its very undulating basal contact. The maximum thickness within the district is probably about 25 m but preserved thicknesses of 5 to 15 m are more typical. The Red Crag is locally overlain, without marked discontinuity, by remnant patches of the Chillesford Sand in several parts of the district (Plate 2).

The Red Crag predominantly comprises medium- to coarse-grained, poorly sorted, shelly sands that exhibit an overall fining-upward trend. They are commonly decalcified in their upper parts. Near the surface the sands have a distinct red coloration, but at depth they are green, commonly with a glauconitic content. Two main sedimentary units are present (Dixon, 1979; Balson et al., 1991). The lower unit comprises up to 15 m of medium- to coarse-grained, very shelly sands with discontinuous pebble seams near, and at the base ('Coprolite' or 'Nodule' bed). The pebbles are predominantly rounded phosphorite derived mainly from the London Clay, box-stone nodules, rounded flint, quartz and exotic components (Boswell, 1928). These pebbly deposits were the primary source of material for the once-flourishing phosphate fertilizer industry around Ipswich and Felixstowe. Much of the lower unit contains finely comminuted shell debris and the sands show large planar cross-stratified bedforms (Megaripple Facies of Dixon, 1979) interpreted as subtidal sandwaves deposited in an open marine embayment (Balson et al., 1991).

The upper unit, up to 10 m thick, comprises medium-grained sands with some shell fragments, but it is commonly extensively decalcified and shows an extensive burrowing infauna. Horizontal bedding, shallow trough cross-stratification, ripple cross-stratification and mud drapes encrusted with iron hydroxides are present. These deposits were regarded as intertidal sandflat deposits (Dixon, 1979) whilst Balson et al. (1991) suggested deposition under the influence of rectilinear tidal currents in a constricted estuary.

Micropalaeontological studies identified climatic oscillations within the Red Crag and overlying Norwich Crag sediments (Funnell, 1961; West, 1961) and led Funnell and West (1977) to conclude, based on foraminifera, that the Red Crag within the district was predominantly Pre-Ludhamian in age by comparison with the type section in the Stradbroke Borehole (Beck et al., 1972).

Faulting has been invoked to explain abrupt thickness variations and changes in level of the basal contact of the Red Crag Formation in the area immediately north of the district (Bristow, 1983). However, the abrupt thickness variations of crag deposits are mirrored elsewhere in southern East Anglia where sea-bed scouring by tidal currents (Funnell, 1972, Dixon, 1979, Zalasiewicz et. al 1988) or fluvial action (Carr, 1967) has been thought adequate to explain such pronounced variations (Mathers and Zalasiewicz, 1988). Furthermore, there is a lack of any significant faulting in the systematic sparker profile coverage of the equivalent sediments in the adjacent southern North Sea Basin (Cameron et al., 1992, and references therein).

The sediments of the Norwich Crag Formation (NCg) are preserved as erosional remnants in the district resting conformably on the Red Crag in some areas, but in the north of the district around Great Blakenham [TM 118 508] and Baylham they overstep to rest directly on the Chalk. The deposits belong to the Chillesford Sand Member (CfS) and include deposits formerly assigned to the Creeting Formation (Allen, 1984; Gibbard et al., 1996; (Plate 3)). The Chillesford Sand comprises up to 11 m of well-sorted, fine- to medium-grained, micaceous, quartz sand (Zalasiewicz and Mathers, 1985). It is characterised by planar bedding and small-scale ripple cross-stratification. Mud laminae, flasers and intraclasts occur sporadically and much of the sequence is strongly bioturbated and contains prominent vertical burrows. A depositional environment of tidal sand-flats is probable (Dixon, 1972). Funnell et al. (1979) ascribed a Bramertonian age to these deposits. In the pit section [TM 1130 5040] at Great Blakenham, the sequence is capped by the College Farm Silty Clay Member, which is up to 3.75 m thick. This member comprises grey laminated silty clays with sand interbeds exhibiting water-escape structures. The deposits are very restricted in extent and cannot be mapped as a unit so they are included within the Chillesford Sand member on the map face. Studies of indigenous and recycled dinoflagellate cysts and other derived palynomorphs (Moorlock et al., 2002) assign the deposits to the Tiglian TC3 substage.

Quaternary

Pre-Pastonian

The Kesgrave Formation occurs extensively in southern East Anglia as a series of cold-phase braided stream terrace deposits (Plate 4). These comprise sand and gravel deposits laid down in the proto-Thames drainage system between the deposition of the Pliocene Norwich Crag and the Anglian glaciation (Wooldridge 1938, 1960; Rose et al., 1999). The deposits are characterised by an abundance of far-travelled material including abundant quartz and quartzite pebbles, and rare exotic volcanic lithologies that together indicate a derivation from the English Midlands and Wales. In the London Basin and southern East Anglia these terrace remnants are broadly aligned south-west–north-east and successively younger terraces are preserved at progressively lower levels towards the south-east. These levels can be correlated from the area around the Goring Gap to the coast of southern East Anglia (Rose et al., 1999).

Intervening periods of warm climatic conditions are indicated by the preservation on several terrace levels of a complex palaeosol unit — the Valley Farm Soil (Rose and Allen, 1977; Kemp, 1985, 1987). This clay-rich layer, typically 1 to 2 m thick, developed at the top of the sequence contains clear evidence of reddening and other processes normally associated with subtropical soils. In addition, lenses of temperate organic material have been found within some of the terrace sequences elsewhere within the region showing that some of the aggradations were complex and composite. The deposits of the Kesgrave Formation occur extensively in the district, and comprise interbedded, pale yellow, grey and white sands, pebbly sands and gravels. The formation is generally 3 to 6 m thick, although locally thicker. In some boreholes, the contact between the Kesgrave Formation and underlying sands of the Crag is hard to distinguish because of the effects of reworking. Similarly, reworking of Kesgrave-type material into younger glaciofluvial and fluvial deposits has also occurred. There are few existing exposures of the deposits within the district but details of former quarry sections including those at Valley Farm [TM 116 434], Barham [TM 133 515] and the type section at Kesgrave [TM 228 465] are given by Rose and Allen (1977); other compositional, structural, grading and palaeocurrent data for sites within the district are given in Allen (1984, 1988, 1991). The spreads of the Kesgrave sands and gravels have, in recent years, been regarded by some authors as of group status (Rose et al., 1999, and references therein), and were subdivided into two formations. The higher 'Sudbury Formation' is found across much of this district and represented by the Beaconsfield (Baylham Common) and Gerrards Cross (Moreton) members. Topographically lower and younger is the 'Colchester Formation' that includes the deposits in the south-eastern corner of the district of the Waldringfield Member(Figure 4).

The 'Sudbury Formation' is believed to have been deposited between about 1.7 and 0.9 Ma (Oxygen Isotope Stages 57–21) with the 'Colchester Formation' from about 0.9 and 0.45 Ma. (Oxygen Isotope stages 21–12), (Rose et al., 1999, and references therein).

Anglian

Glacigenic deposits of the Anglian Stage are extensively distributed throughout East Anglia and although they have been partly eroded, especially along drainage lines, the overall distribution of the Lowestoft Till (shown in pale blue on the map sheet) still broadly reflects the area of extensive ice-cover during the Anglian glaciation that extended as far south as Ipswich. The glacial deposits are collectively known as the Lowestoft Formation. Their composition shows a derivation from the northwest with abundant material derived from Cretaceous and Jurassic strata. The dominant deposit is the widespread 'chalky boulder clay' (Lowestoft) till that occurs at surface over much of southern East Anglia (Plate 5). Associated with the Lowestoft Till are spreads of Glaciofluvial Deposits and patches of Glacial Silt and Clay.

These Anglian glacial deposits are widespread throughout all but the south-eastern corner of the district, in addition to underlying most of the interfluvial areas they are also found at all elevations on the flanks of the main valleys down to the deeply incised buried channels (tunnel-valleys). Within the district, segments of tunnel-valley systems have been proved in boreholes beneath the Gipping valley between Bramford and Ipswich Dock [TM 169 437] and in the Brett valley around Chelsworth and Hadleigh. The occurrences in the Brett valley are relatively shallow, whilst those in Gipping are cut to at least 45 m below OD. Their infill is complex and includes sands and gravels, silts and clays and layers of till. Because of this lithological complexity they are shown in Section 2 on Sheet 207 as undifferentiated Glacial Channel Deposits. These tunnel-valleys appear to have undulating thalwegs and terminate abruptly downstream; they are believed to have been excavated by jokulhaup-like events involving meltwater acting under excess hydrostatic pressure flowing beneath the ice-sheet (Woodland, 1970).

The presence of these tunnel-valleys suggests that the main elements of modern drainage pattern were, at least in part, inherited from the patterns established by the Anglian glaciation (Boswell 1913; Woodland 1970). Other buried channels infilled only with thick Lowestoft Till deposits occur within the district but these are unrelated to the present drainage pattern and may be simply due to glacial scouring rather than meltwater.

Intense glaciotectonic deformation, involving shearing and contortion parallel to the direction of ice-flow, occurs in the superficial and bedrock strata down to the Chalk. This style of deformation has been recorded in quarries and excavations in the Gipping–Orwell valley and also in the Brett valley to the west (Boswell 1913; Boswell 1927; Slater, 1927). The established occurrences are shown on the Sheet 207 with an appropriate symbol, they tend to occur on spurs or promontories on the flanks of the major valleys; it is likely that many other similar disturbances of strata exist in comparable locations within the district. Boreholes near Elmsett [TM 056 466] also suggest that at depth rafts of bedrock lithologies have been sheared and mixed into the glacial sequence.

A large sheet of Lowestoft Till commonly 10 to 20 m thick, but locally exceeding 40 m, crops out over most of plateau of the district except in the south-eastern quadrant. Along the main drainage tracts patches of the deposit are found at lower elevations usually in complex relationships with other glacial deposits. The deposits also infill parts of buried channels and tunnel-valleys.

The till is generally a massive diamicton composed of a grey-black, sandy, silty clay matrix with abundant pebbles, cobbles and boulders, principally of chalk and flint (Perrin et al., 1979). Other clasts present in minor quantities include quartz, quartzite, ironstone, black shale and Mesozoic fossils. The upper 1 to 2 m of the deposit is typically decalcified and weathered to an oxidised yellowish brown clay with flint pebbles. At depth, the deposit is over-consolidated and massive with local subordinate interbeds of water-sorted sands and gravels and silts and clays. Lithological variation has been observed in the till between the plateau and the valley flank occurrences; in detailed studies of quarry sections at Great Blakenham (Allen, 1988), the till becomes noticeably richer in sand and gravel at the lower elevations. There is insufficient data elsewhere to judge whether this is purely a local phenomenon, or a regular pattern. The majority of the till is regarded as a lodgement till deposited subglacially and representing the unsorted detritus carried by the ice-sheet.

The Glaciofluvial Deposits occur as valley side spreads within the major drainage tracts, and as an extensive sheet of sand and gravel on the plateau beyond the main areas of till outcrop in the southeast of the district. Here, on either side of the Orwell estuary these deposits are commonly 5 to 6 m thick and are preserved in the form of large proglacial outwash fans. Glaciofluvial sand and gravel deposits are also recorded within the buried channel sequences in the Gipping valley.

The deposits comprise water-laid sands, gravels and silts of very variable character. Many of the deposits are poorly sorted and have a high clay content in their matrix, which is commonly strongly oxidised to hues of red and orange. They are usually compositionally distinct from the Kesgrave Formation and have been termed Barham Sand and Gravel (Rose and Allen 1977), in the district. They commonly contain more nodular and freshly broken flint, and proportionally less quartz and quartzite than the Kesgrave Formation. Locally chalk is also preserved within the deposits where decalcification is incomplete.

The Glacial Silt and Clay deposits are restricted mainly to isolated patches and channel infills. They comprise laminated silts and clays with thin sand partings, the deposits are usually less than 3 m in thickness and interbedded with other glacigenic sediments. Two major channels traversing the proglacial fan near Rushmere Heath [TM 200 445] are infilled with these deposits.

Post-Anglian–Devensian

The type site for the Ipswichian Interglacial is located at Bobbitshole [TM 150 415] in the valley occupied by the Belstead Brook. Here a sequence, commonly up to 4 m thick, of calcareous and in part shelly clays and silts have been encountered in boreholes, buried beneath a surficial cold-phase gravelly head deposit of a similar thickness (West, 1957). There are no surface exposures of these deposits; they occur at elevations ranging from 3 m below OD to 4 m above OD. Detailed studies of the palaeobotany and nonmarine mollusca indicate that the deposits accumulated following an initial rapid climatic amelioration, during the silting up of a freshwater lake under climatic conditions similar to, and perhaps at times warmer than, the present.

Nearby at Stoke, other important interglacial Lacustrine Deposits were first encountered around the southern end of the cutting leading into the rail tunnel [TM 162 434] when it was dug in the mid 19th century. Subsequently, more detailed investigations were carried out during widening of the cutting (Layard, 1920) revealing the presence of about 4 m of organic silts and clays beneath gravels, and resting on Anglian glacigenic deposits. The interglacial deposits include a rich bone bed in the lower part of the sequence occurring at 8 to 9 m OD. This bone bed includes remains of species such as mammoth, horse, ox, wolf, cave-bear, cave-lion, large deer and freshwater tortoise. These interglacial deposits are in turn overlain, as at Bobbitshole, by about 3 m of cold-phase gravelly head deposits. Further excavations reported by Turner (1977), revealed the deposits were more extensive and also occurred at the school [TM 161 429] at Maidenhall some 400 m south of the original occurrence. These deposits were also overlain by gravels. The assemblage of vertebrate and amphibian bones recovered, plus the freshwater mollusca and pollen suggest the deposit is fluvial-lacustrine in origin, perhaps accumulating in an ox-bow lake (Turner, 1977) or a solutional collapse at the margin of the floodplain.

The deposits at Stoke occur at significantly higher elevations (about 7 to 11 m above OD) than the type Ipswichian deposits at Bobbitshole, which occur only up to 4 m OD. Whilst it is possible that the two deposits date from the same interglacial, the higher elevation of the Stoke deposits suggests they may be part of an older interglacial stage.

It is unlikely that these two deposits are the only isolated remnants of such sedimentation within the Gipping–Orwell system, and it is probable that similar deposits will be encountered in future excavations and boreholes within the valley.

Patches of sand and gravel fringing the alluvium and intertidal deposits in the Gipping–Orwell, and Fynn valleys, are classified as River Terrace Deposits (undifferentiated). In the valley of the River Brett they have been classified as Second Terrace Deposits as a distinct higher third terrace is recognisable in the Brett system south of the district. The age of these terraces cannot be established with any certainty, but it is probable they were deposited as periglacial braided stream deposits during a cold climatic phase. Their clasts are dominated by nodular angular and rounded flint, quartz and quartzite, reflecting their derivation from earlier deposits within the drainage basin. The deposits are commonly less than 5 m thick. These deposits probably continue downstream beneath the intertidal deposits of the Orwell estuary.

Similar sand and gravel deposits are also recorded in several boreholes drilled through the Holocene alluvial and intertidal deposits in the main valleys. These deposits are classified as (First) Terrace Deposits and typically comprise a thin basal lag gravel 1 to 2 m thick, overlain by sands and gravels. They occur at lower levels than the Second Terrace Deposits and lie within channels cut to levels about 10 m below OD in the Orwell estuary. These are the upper parts of drainage systems that have been traced offshore to much lower levels. These deposits are thought to be Late Devensian in age. The deposits are not exposed and are depicted only on the cross-sections on the Sheet 207.

Head is a mass-movement deposit that has accumulated at or near the base of slopes within the district. Whilst most slopes contain some material of this kind, deposits have only been mapped where consistently in excess of 1.0 m in thickness. The deposits are not thought to exceed 5 m. Lithologically the material is of varied character reflecting the materials upslope from which it derives. For example, those located on the Thames Group outcrops commonly comprise sandy and pebbly material washed down the impermeable slope from the overlying Red Crag and Kesgrave Formation deposits. Most deposits however are diamicts such as pebbly sandy clays and pebbly clayey silts reflecting derivation from multiple sources. Where these deposits are known to be dominantly clayey gravels they are shown as Head Gravel.

The Head deposits are thought to have developed mainly under periglacial conditions during cold phases when limited vegetation cover and alternating freeze and thaw conditions aided mobility of near-surface material and its movement downslope. The head deposits are likely to be associated mainly with the last (Devensian) glaciation when the district remained beyond the ice cover.

Flandrian (Holocene)

The decay of the ice sheets associated with the ultimate Devensian glaciation commenced about 16 000 years ago, and gradually as the ice masses wasted sea level began to rise reaching levels close to those of the present day about 7000 to 8000 years BP. Within the district, the rising sea levels were accompanied by sedimentation in fluvial and coastal environments, producing the suite of Flandrian deposits that continue to form at the present day.

Along the major river valleys, Alluvium and downstream Intertidal Deposits (tidal flat, channel and saltmarsh) have accumulated, gradually infilling the incised channels of these rivers that were established by the strong downcutting during periods of low sea level in the preceding Devensian cold phase. The boundary between the freshwater alluvium and the tidally influenced deposits is gradational and has shifted through time in response to sedimentation, subsidence and changing sea level. On Sheet 207, the limit of the two deposits (at surface) are taken at the approximate limit of the tidal influence in the Gipping–Orwell valley and Martlesham Creek [TM 265 473], but at depth the deposits are known to interdigitate in a complex fashion. These sequences locally exceed 5 m in thickness.

Throughout the area, alluvium deposits are a mixture of silt and mud with subordinate thin peat layers; locally they contain shelly debris, pebbles and thin interbeds of sand. The tidal deposits are similarly varied. The dominant component is organic mud with thin peat layers; in addition lenses of silt and fine sand several metres thick also occur, associated with tidal channel sedimentation.

Peat is mapped at surface along the poorly drained margins of the alluvium along the Gipping floodplain and its tributaries at several locations northwards from Claydon [TM 132 498]. In the Orwell estuary, coarse clastic sand and pebbly sand deposits are found fringing the intertidal Beach Deposits, especially at the base of gravel-capped cliffs where an abundant supply of sand and gravel can be derived by cliff erosion.

Landslide, artificial deposits and worked ground

Several areas of Landslide Deposits have been delineated, relating to the collapse of cliffs along the Orwell estuary (see below). The more extensive areas of artificial deposits and worked ground are shown on the map but many minor occurrences have been omitted for clarity. The 1:10 000 scale maps of the district, listed in the Information sources show a more detailed distribution. Made Ground is shown in areas where natural, artificial or waste material has been deposited by man upon the natural ground surface. Major areas within the district occur flanking the lower reaches of the River Gipping in Ipswich and downstream along the flanks of the Orwell estuary where intertidal flats and marshes have been built over in order to extend the port facilities. Other spreads of Made Ground include major road and rail embankments, reservoirs and seawall defences, these linear spreads are generally marked on topographic maps and so are not included on the geological map sheet. Worked Ground is shown where natural materials are known to have been removed, for example in quarries and pits, the larger ones are shown on the map sheet. Most of these relate to the extraction of aggregate, chalk, brick clays and phosphate nodules. Many of the former sand and gravel quarries along the Gipping valley are flooded and are managed as wetland habitats and recreational facilities. Infilled Ground comprises areas where the natural ground has been removed and the void so created has been wholly or partially backfilled with natural or waste materials or a combination of both as commonly found in large landfills. Some of the largest landfills are shown on Sheet 207, including Foxhall Heath [TM 235 438] and [TM 242 437], Little Blakenham [TM 112 486], Valley Farm [TM 116 432], Broom Hill [TM 129 482] and parts of the pits at Waldringfield Heath [TM 260 448], Layham [TM 010 400], Shrubland Park [TM 120 537], Kesgrave [TM 227 466] and Great Blakenham [TM 113 502]. Landscaped Ground consists of areas that have been extensively remodelled or landscaped with complex patterns of cut and fill, too small to be identified separately. These mainly comprise areas of landscaped parkland within Ipswich and sewage works.

Structure

The district covers an area that lay on the culmination of the London Platform or Anglo-Brabant Massif, an area that stood above sea level for much of the Mesozoic and was finally submerged in the Cretaceous with the deposition of the Gault. The Chalk Group and underlying Gault rest unconformably on Silurian rocks (Smith, 1985a, b). Older magnetic Palaeozoic rocks form the core of the massif.

The Caledonian rocks are part of the East Anglian–Brabant Caledonide chain (Pharaoh et al., 1995), deformed by thrusting and uplift in response to closure of the Tornquist Ocean that lay between the continents of Avalonia and Baltica in the late Ordovician. The bounding thrust to the Caledonian chain, bordering the Midlands Microcraton, lies farther south-west, in a line joining the Thringstone Fault and north Kent (Smith, 1985a,b). Late Caledonian deformation (Acadian Orogeny) produced cleavage in Silurian and early Devonian rocks (Pharaoh et al., 1987) and probably renewed thrusting on north-east-dipping faults. The aeromagnetic anomaly map of the region shows the north-west-trending tectonic strike of this chain. Belts of magnetic rocks reach into East Anglia and appear to be a continuation of the Caradoc calc-alkaline rocks of the East Midlands (Noble et al., 1993). These magnetic rocks are overlain by Silurian rocks to the west and south of the district.

Surrounding this district, the Gault Formation and (offshore) Chalk Group rest unconformably on early Palaeozoic rocks, and older Mesozoic strata appear to be absent. If there has been fault reactivation along either a north-west or north-east trend, then the possibility remains that small basins containing Mesozoic strata are present, but have not been drilled After the initial extensional faulting and subsidence in early Mesozoic times, affecting basin development in the Southern North Sea and Weald, the entire region subsided in response to thermal relaxation.

The regional distribution of the late Pliocene–Pleistocene, shallow marine Red Crag and Norwich Crag formations in southern East Anglia provides evidence for the easterly down-tilting of the region during the last two to three million years. This is probably in response to subsidence in the Southern North Sea Basin (Moffat and Catt, 1986; Mathers and Zalasiewicz, 1988). The tilt is estimated at about 1 m per km. The longitudinal gradients of the terraces of the easterly flowing river that deposited the Kesgrave Formation (estimated age between 1.8 and 0.46 Ma) show a gradual decline in the younger deposits (Whiteman and Rose, 1992, fig. 3).

Fault-control of parts of the Crag basin was postulated by Bristow (1983) but abrupt changes in elevation of the basal Crag surface have also been interpreted as the infill of palaeovalleys and or tidal scour hollows (Funnell, 1972; Mathers and Zalasiewicz, 1988).

Chapter 3 Applied geology

Geological factors have an influence on ground conditions and hence the planning and nature of future urban and industrial developments. By giving consideration to geological conditions at an early stage in the planning process, it may be possible to mitigate some of the problems commonly encountered during construction work. The diverse local geology gives rise to variable ground conditions and some significant aspects of this are discussed below. Important factors are water resources, mineral workings, gas emissions and the risk of flooding.

Water resources

Two principal aquifers are present within the district: the shallow Crag (Norwich, Red and Coralline) aquifer, floored throughout almost all the area by the impermeable London Clay, and the deeper and more important Chalk aquifer. The Chalk aquifer is confined in parts of the area by the overlying clay-dominated Palaeogene strata.

In the Crag aquifer, which is up to about 30 m thick, the groundwater chemistry is variable. At outcrop or beneath younger sands and gravels, the total hardness is commonly 100 to 300 mg/l and the chloride ion content is fairly low at 30 to 70 mg/l. Decalcification of shelly material results in a high proportion of carbonate hardness at outcrop. Where the Crag aquifer is overlain by Lowestoft Till, the hardness, mainly non-carbonate increases to over 400 mg/l and the chloride ion content rises to around 150 mg/l. Chlorides also increase towards the coast and along the tidal reaches of rivers due to saline infiltration.

The Chalk water table within the region is generally a modified reflection of the surface topography, so the groundwater and surface water divides are usually related. At outcrop groundwater levels show marked annual and seasonal variations of up to 10m but where confined down dip beneath Palaeogene strata the aquifer is characterised by small annual fluctuations in the piezometric surface of 1.5 to 1.8 m.

The total hardness of the water from the Chalk outcrop in the main valleys is usually 250 to 350 mg/l of which 65 to 80 per cent is usually carbonate. The chloride ion content is generally below 100 mg/l. Beneath the Red and Norwich crags and superficial sands and gravels the groundwater is usually of similar quality but beneath the crags the water is ferruginous. Beneath the Lowestoft Till hardness increases dramatically with total hardness in the range 550 to 750 mg/l with subequal carbonate and non-carbonate components. The chloride ion content is generally in excess of 150 mg/l and higher values of calcium and magnesium sulphates are recorded. Beneath the confining Palaeogene clays the non-carbonate hardness increases and connate water causes a sharp increase in salinity to over 500 mg/l in the south-east of the district. A well [TM 110 470] near Bramford encountered saline water at 48 m below OD overlain by acceptable freshwater. In the area around the Orwell estuary saline intrusion has resulted from the depression of groundwater levels, to below sea level, due to heavy pumping.

Surface mineral workings

The geological units that have been dug within the area are summarised in (Figure 5). Few of these deposits are of current economic interest.

Historically Chalk has been worked in the district as an important source of agricultural lime and as carbonate rock for cement manufacture at many sites along the Gipping valley and tributaries between Needham Market and Bramford, and also at Offton. Today lime products are still produced by Needham Chalks from their quarry south of Needham Market [TM 095 543]. The Palaeogene clays were formerly dug as a local brick clay at many locations in and around Ipswich and accounts of many old brickworks are contained in Boswell (1927). The Red Crag was worked in the south-east of the district in numerous small pits for coprolites (phosphate nodules) used in the manufacture of fertilisers.

The Kesgrave Formation is a regionally important source of sand and gravel. Active quarries in the district are at Waldringfield Heath, Layham and Shrubland Park . The glaciofluvial deposits have been dug locally for sand and gravel and hoggin (clayey gravel for road bases. The Lowestoft Till was dug in the west [TM 100 503] of the pit at Great Blakenham and blended with Chalk to produce cement. The River Terrace deposits have been extensively worked for sand and gravel along the Gipping valley from both the exposed River Terrace deposits and those buried beneath the Alluvium. Assessments of the sand and gravel resources of parts of the district are contained in Allender and Hollyer (1972, 1973, 1981), Hollyer (1974) and Hopson (1981).

Foundation conditions

Potential problems related to ground stability are shown in (Figure 6).

Ground heave and subsidence

The Thames and Lambeth Groups are predominantly composed of clays, some of which have a relatively high smectite content (Burnett and Fookes, 1974). These undergo significant volume changes in response to variations in moisture content. Seasonal effects, in which vegetation, especially trees, play a dominant role, were identified by Driscoll (1983) as the most important influences on the amount of shrinking and swelling. During winter months, the clays absorb large quantities of water that is lost during dry periods when it leads to extensive cracking. The alternating processes of expansion and contraction may cause structural damage to buildings and roads.

Unweathered Thames Group deposits contain pyrite (iron sulphide) that on weathering is oxidised to yield sulphate ions in solution. Any calcium carbonate present may react with the sulphate to precipitate selenite crystals. This involves an eight-fold increase in volume compared with the original pyrite and can cause disruption and weakening of the strata. The weathering of selenite produces sulphuric acid, so that resultant high concentrations of sulphate in the ground or groundwater may weaken concrete foundations that are not designed to resist chemical attack.

Slope stability and mass-movement movement

The river cliffs along the Orwell estuary are relatively unstable because their lower parts are composed of London Clay. Areas of landslipped ground are shown on the Sheet 207. Periodically, rotational slips occur at the cliff faces followed by the erosion and redistribution of the slipped material by wave and tidal action. Elsewhere slopes on Thames Group sediments over 7° are prone to failure, and slopes exceeding 3° should be considered as potentially unstable due to weathering by periglacial processes (Culshaw and Crummy, 1991.

Chalk dissolution

Chalk is prone to dissolution because of the action of acidic rainwater and groundwater especially in the near-surface vadose zone. Such effects have been observed in the form of cavities and pipes within the Chalk adjacent to the district even where it is overlain by 10 m or more of other deposits (see for example Boswell, 1927, p. 53). Locally, large karstic features may lead to foundering of the overlying strata. A borehole (TM14SE/381) in the Orwell estuary encountered a water-filled void 8 m deep in Chalk immediately beneath the superficial deposits. Possible further evidence of solutional collapse is suggested by the presence of circular meres at the margin of the floodplains of the Gipping at Bosmere [TM 098 547] and the Brett at Semer [TL998 466]. An alternative possibility for these meres is that they represent foundering of strata deposited over ground ice as in pingo-like structures.

Other hazards

Natural Radon emissions

Radon is a naturally occurring radioactive gas that is produced by the progressive decay of uranium, which is found in small but variable quantities in all soils and rocks. Radon released from rocks and soils is normally quickly diluted in the atmosphere and does not present a hazard. However, radon that enters poorly ventilated spaces such as basements may be potentially hazardous and may lead to an increased risk of serious illness including lung cancer.

Within the district there is a risk from radon due to the concentration of phosphorite nodules, which have high uranium levels, in the basal part of the Red Crag Formation. These deposits are best developed in the south-east of the district where the potential risk from radon is perceived to be low-moderate but is greater than elsewhere in East Anglia. In this area basic radon protection should be provided in all new dwellings (Lomas et al., 1996; Miles et al., 1996; Department of the Environment, 1999; Appleton et al., 2000).

Flooding

Low-lying alluvial ground in the main valleys and the tidal flats and salt-marshes, fringing the Orwell estuary are prone to regular flooding. In many of the estuarine areas, bunds have been constructed along the river sides by excavating the adjacent mud deposits.

Information sources

Further geological information held by the British Geological Survey relevant to the Ipswich district is listed below. It includes published maps, memoirs and reports. Enquiries concerning geological data for the districts should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth.

Other information sources include borehole records, fossils, rock samples, thin sections, hydrogeological data and photographs. Searches of indexes to some of the collections can be made on the Geoscience Index system in BGS libraries and is also available at BGS web site (see back cover for address):

Books

Maps

Documentary collections

Boreholes

Borehole data for the district is catalogued in the BGS archives (National Geological Records Centre) at Keyworth on individual 1:10 000 scale sheets. For further information contact the Manager, National Geological Records Centre, BGS, Keyworth.

BGS hydrogeology enquiry service; wells, springs and water borehole records. British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, OXO 8BB. Telephone 01491 838800. Fax 01491 692345.

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on BGS maps, including those for this district are held in the Lexicon database. This is available on BGS web site. Further information can be obtained from the Lexicon Manager at BGS, Keyworth.

BGS photographs

The BGS Photographic Collection houses a number of photographs from the Ipswich district. Part of the collection can be viewed online.

Other relevant collections

Groundwater licensed abstractions, Catchment Management Plans and landfill sites

Information on licensed water abstraction sites, for groundwater, springs and reservoirs, Catchment Management Plans with surface water quality maps, details of aquifer protection policy and extent of washlands and licensed landfill sites are held by the Environment Agency.

Radon potential

For more detailed follow-up geological radon potential assessment based on BGS 1:10 000 scale geological maps contact: Land Survey South Enquiry Service, BGS Keyworth, Nottingham NG12 5GG or e-mail geohelp@bgs.ac.uk.

References

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Allender, R, and Hollyer, S E. 1972. The sand and gravel resources of the area south and west of Woodbridge, Suffolk. Report of the Institute of Geological Sciences, No. 72/9.

Allender, R, and Hollyer, S E. 1973. The sand and gravel resources of the country around Shotley and Felixstowe, Suffolk. Report of the Institute of Geological Sciences, No. 73/13.

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Index to the 1:50 000 Series maps of the British Geological Survey

The map below shows the sheet boundaries and numbers of the 1:50 000 Series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland. The west and east halves of most Scottish 1:50 000 maps are published separately. Almost all BGS maps are available flat or folded and cased.

(Index map)

The area described in this sheet explanation is indicated by a solid block.

British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGS-approved stockists and agents.

Figures, plates and tables

Figures

(Figure 1) Location of deep boreholes, contours (in metres below OD) and subcrop pattern of the sub-Mesozoic unconformity.

(Figure 2) Exposed and probable concealed Chalk Group stratigraphy of the Ipswich district.

(Figure 3) Stratigraphical range of key Chalk Group outcrops in the Ipswich district (not to scale). Key Chalk Group outcrops in the Ipswich district. Coe's Pit,Bramford [TM 1290 4820]; Coddenham Pits [TM 1198 5486] & [TM 1197 5473]; Needham Quarry [TM 0940 5395]; Great Blakenham [TM 1161 4986]; Little Blakenham [TM 1108 4899] & [TM 1086 4910]; Nedging Mill[TL 9910 4750]; Offton[TM 0734 4935]; Claydon[TM 1363 4966]

(Figure 4) Kesgrave Formation terrace levels (modified from Rose et al., 1999).

(Figure 5) List of materials worked within the district.

(Figure 6) Potential ground constraints.

Plates

(Plate 1) Glacial Sand and Gravel : the base cuts down into the chalk on the west flank of the Gipping valley. Photograph of the south-eastern face [TM 096 540] of the Needham Chalks pit, south of Needham Market (P616159).

(Plate 2) Planar contact of the Chillesford Sand Formation resting on the Chalk at Great Blakenham pit, northern face around [TM 108 504], the landfill is in the foreground (P616160).

(Plate 3) Chillesford Sand in the western part of the old pit [TM 095 553] east of Needham Market. The pencil is 15 cm long (P616163).

(Plate 4) Close-up view of clasts and bedding structure within the Kesgrave Formation at Shrubland Park: photograph of southern quarry face [TM 122 536]. The pencil is 15 cm long (P616161).

(Plate 5) Close-up view of a layer of Lowestoft Till within the Glacial Sand and Gravel, photograph of the south-western face [TM 095 539] of the Needham Chalks pit, south of Needham Market (P616162).

(Front cover) Front cover. The Orwell bridge across the estuary south of Ipswich, looking north. (Photograph: P. Witney; (P626235)

(Rear cover) Rear cover

(Geological succession) Geological succession in the Ipswich district.

(Index map) Index to the 1:50 000 Series maps of the British Geological Survey

Figures

(Figure 5) List of materials worked within the district

Geological unit Worked for
River Terrace Deposits sand and gravel aggregate
Glaciofluvial Deposits sand and gravel aggregate
Glacial Silt and Clay Deposits brick clay
Lowestoft Till marl for soil improvement
clay for cement
Kesgrave Formation sand and gravel aggregates
Red Crag, Coralline Crag phosphate for fertilizer
(coprolites), track making
Thames Group (London Clay Fm) brick clay
Thames Group (Harwich Fm) cementstone, building stone
Lambeth Group (Reading Formation) brick clay
Chalk cement, lime

(Figure 6) Potential ground constraints

Geological unit
Worked Ground
Made Ground variable foundation conditions
Infilled Ground variable foundation conditions; leachate and methane production from waste
Landscaped Ground variable foundation conditions
Landslip unstable ground
Peat compressible deposit, subsidence
Alluvium unconsolidated and compressible deposit, subsidence
Intertidal deposits (Tidal flat, channel, saltmarsh) unconsolidated and compressible deposit, subsidence
Beach Deposits (sand and gravel) unconsolidated deposit
Head, Head Gravel variable unconsolidated deposit
River Terrace Deposits high water table, undocumented former workings
Glaciofluvial Deposits perched water tables and springs
Lowestoft Till soft, decalcified and weathered to about 2 m depth, undocumented former workings
Kesgrave Formation undocumented former workings, unconsolidated
Chillesford Sand Member unconsolidated
Red and Coralline Crag formations unconsolidated decalcified upper parts, undocumented former workings
Thames Group ground heave, landslip and subsidence in clays; high sulphate content, perched water table and springs in sand layers
Lambeth Group and Thanet Sand Formation Variable ground conditions, ground heave
Chalk Group Prone to dissolution producing cavities and leading to collapse of overlying strata, undocumented former workings