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Geology of the Woodbridge and Felixstowe district — brief explanation of the geological map Sheets 208 and 225 Woodbridge and Felixstowe

S J Mathers and N J P Smith

Bibliographic reference: Mathers, S J, and Smith N J P. 2002. Geology of the Woodbridge and Felixstowe district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 Sheets 208 and 225 Woodbridge and Felixstowe (England and Wales).

Keyworth, Nottingham: British Geological Survey.

© NERC 2003 All rights reserved. Printed in the UK for the British Geological Survey by B&B Press Ltd. Rotherham

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 NERC permission. Contact the BGS Intellectual Property Rights Manager, British Geological Survey, Keyworth. 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 mouth of the River Ore looking north-east towards Orford Ness in the far distance in 1995. (Aerofilms 647602).

(Rear cover)

Notes

The word 'district' refers to the combined areas of Sheet 208 and 225 Woodbridge and Felixstowe. 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 TM34SE, followed by its registration number in the BGS National Geological Records Centre. Selected water wells from the BGS National Well Record Archive are prefixed by the code of the 100 km2 area upon which the site falls, for example TM24 followed by its registration number. Lithostratigraphical symbols shown in brackets in the text, for example (Tms) are those shown on the published map.

Acknowledgements

This Sheet Explanation was compiled by S J Mathers. D S Brew contributed information on the Holocene and offshore geology and N J P Smith on the concealed strata. The manuscript was edited by R D Lake. 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.

The grid where it is used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright reserved Ordnance Survey licence number GD 272191/2002.

Geology of the Woodbridge and Felixstowe district (summary from rear cover)

(Rear cover)

(Figure 1) Geological succession in the Woodbridge and Felixstowe district.

This Sheet Explanation provides a summary of the geology of the district covered by the combined geological 1:50 000 Series Sheets 208 Woodbridge and 225 Felixstowe including the adjacent offshore area. It is written for the guidance of those who use the geological maps, and may wish to be directed to further geological information about the area.

The Woodbridge and Felixstowe district comprises a low-lying coastal part of Suffolk bordering the North Sea. The highest ground lies in the north-west at around 30 m above OD and is underlain by loamy soils; farther south and east, extensive heathlands are developed on broad plateaux underlain by predominantly sandy soils. Most of the area is rural, given over to arable farming and forestry. The principal towns are the market town of Woodbridge and the seaside resort and the port of Felixstowe. The coastline includes the gravel spits of Orford Ness and Landguard Point, and is backed by extensive drained salt-marshes and mudflats.

Throughout the last two millenia the strategic position of the district with respect to continental Europe has resulted in a long military association since the construction of a Roman fort at Felixstowe and later the 12th century castle at Orford. Martello towers were constructed during the Napoleonic wars to strengthen defences and in more recent times several key airbases were sited in the district.

The oldest rocks seen at the surface comprise the Palaeogene marine clays of the Thames Group. The area is dominated by geologically young deposits laid down over the last three million years. These include the shallow-water, Pliocene, marine Crags overlain by the Quaternary, proto-Thames, river deposits of the Kesgrave Formation. During the Anglian glaciation, about half a million years ago, 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. Extensive swathes of glacial deposits were laid down at the fringe of this ice mass. During the succeeding cold stages, after the ice had decayed, the area experienced prolonged harsh periglacial conditions. The recent climatic amelioration that began about 16 000 years ago, resulted in the gradual rise in sea level leading to deposition of the postglacial (Holocene) fluvial, estuarine and coastal sediments of the district during the last 7000 to 8000 years.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the combined geological 1:50 000 Series Sheets 208 Woodbridge and 225 Felixstowe including the adjacent offshore area. 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 Woodbridge and Felixstowe district comprises a low-lying coastal part of Suffolk bordering the North Sea. The highest ground lies in the north-west at around 30 m above OD and is underlain by loamy soils developed on glacial deposits. Farther south and east, extensive heathlands are developed on broad plateaux underlain by the predominantly sandy fluvial deposits of the ancestral River Thames and the marine, shelly sand 'crag' deposits for which this area is famous.

The main rivers comprise the Deben, the Alde-Ore and its right-bank tributary the Butley River. Along the coast and within the Deben estuary, the coastal belt contains extensive drained salt-marshes and mudflats developed landward of the protective storm beach gravel deposits that comprise Orford Ness [TM 455 495] and Landguard Point [TM 283 313]. These marshes form good arable land and are protected by extensive bunds constructed following the devastation of the 1953 storm surge event. Orford Ness is one of the longest spits in Britain and over the last 1500 years it has grown and diverted the River Alde–Ore some 15 km south-west from its historic river mouth at Slaughden [TM 464 554].

Much of the area is rural, and is mainly given over to arable farming, forestry, and pig rearing. The principal settlements are the market town of Woodbridge [TM 275 495] and the seaside resort of Felixstowe [TM 299 349], together with its port [TM 280 335] on the northern bank, near the mouth of the Orwell–Stour estuaries. Felixstowe has developed as Britain's principal east coast container port for trade with continental Europe, owing to its proximity to the continent and the presence of protected estuaries.

Historically the strategic position of the district with respect to Continental Europe has resulted in a long military association. The remains of a Roman fort occur just offshore at Felixstowe and, as early as the 12th century, Henry II constructed the castle at Orford to guard against foreign invasion. The King's Fleet, the forerunner of the Royal Navy, was stationed in the artificial channel of the same name [TM 310 380] just north of Felixstowe on the Deben estuary (Plate 1) as early as 1346, when, during the reign of Edward III, the fleet carried troops to invade northern France and win the Battle of Crecy. Fortifications have existed at Landguard Point since the 16th century and during the Napoleonic war the fort was strengthened and Martello towers were constructed along the coastline to counter the threat of invasion.

In early geological time, during the Lower 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 late Devonian or Carboniferous rocks have been proved 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, which extends into the southern North Sea in a south-easterly direction. This massif was a positive feature throughout the Upper Palaeozoic and much of the Mesozoic. It was submerged again in the Lower Cretaceous and then continued to slowly subside during the accumulation of Chalk sediments in the Upper Cretaceous. Uplift, tilting and erosion at the end of the Cretaceous was followed by renewed sedimentation in the Palaeogene, initially marginal 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 downtilting of the region in response to subsidence in the adjacent Southern North Sea Basin. The succeeding Quaternary, proto-Thames, fluviatile sediments of the Kesgrave Formation were deposited predominantly 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 offshore base levels well below OD, and aggradation led to the deposition of river terrace deposits. During the ultimate glacial stage, the 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, estuarine and coastal sediments of the district during the last 7000 to 8000 years.

Chapter 2 Geological description

The geological succession present at outcrop and at shallow depth in the district is shown in (Figure 1). The deepest unit encountered in boreholes within the district is the Chalk Group. Much of the evidence of the concealed strata is drawn from regional geophysical interpretation and deep boreholes sited adjacent to the district.

Palaeozoic

Silurian and Devonian

The deep boreholes at Harwich [TM 2593 3278], Stutton [TM 1500 3340] and Weeley [TM 1474 2183] to the south-west of the district and the Lake Lothing borehole at Lowestoft [TM 5380 9260] — all reached Palaeozoic basement beneath the sub-Mesozoic unconformity (Figure 2). These basement rocks comprise grey, sandy, fissile mudstones 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). 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.

Mesozoic

Cretaceous

The sub-Mesozoic unconformity deepens from about 300 m below OD, just west of the district, to about 450 m below OD off the coast (Figure 2). The overlying rocks in the Harwich, Stutton, Weeley and Four Ashes [TM 0223 7186] boreholes together with those at Stowlangtoft [TM TL 9475 6882] are of the Gault Formation. However, 12 m of Lower Greensand was encountered in the Lake Lothing borehole but no older Mesozoic rocks are known from the surrounding parts of East Anglia. The Gault Formation, of Lower Cretaceous age, is probably present throughout the district and is about 15 to 20 m thick. In the Weeley Borehole the lower part is a smooth grey mudstone, overlain by a very thin green sand with pebbles and black phosphatic nodules, and capped by grey laminated mudstone. Mudstones and subordinate sandstones were encountered in the Harwich (Whitaker and Thresh, 1916) and Stutton boreholes (Whitaker, 1906). The overlying Cambridge Greensand is present in the Stutton Borehole comprising 2 m of green sandstone and phosphatic nodules overlying glauconitic, calcareous mudstone (Whitaker, 1906).

The Chalk Group (Ck) thickens gradually north-eastwards across the district from 275 m to nearly 300 m. The uppermost beds probably belong to the Newhaven Chalk (sensu Bristow et al., 1997) whereas younger stages are preserved beneath the basal Cainozoic (Palaeogene) unconformity farther north in Norfolk (Whittaker, 1985). The Lower Chalk is typically grey and marly; it is possible to identify the Belemnite Marl and Chalk Marl in the Weeley Borehole whilst the Totternhoe Stone is recognised in the Stutton Borehole (Whitaker, 1906). The Middle Chalk, in the Weeley Borehole, is grey with nine flint layers, cream or white coloured, becoming rough, nodular and shelly near the base (Melbourn Rock); the Upper Chalk is grey or white with layers of flint (Whitaker and Thresh, 1916).

Cainozoic

Palaeogene

Palaeogene strata belonging to the Lambeth Group and Thanet Sand Formation (LT) occur at depth beneath the district. There are no surface exposures, but the strata are believed to be present at rockhead beneath the Holocene deposits of the Deben valley between Woodbridge [TM 275 495] and Waldringfield [TM 284 447]. Around Woodbridge about 10 m are present whereas farther south around Felixstowe and to the east around Orford [TM 423 500] about 15 m are preserved (Boswell, 1928). Boswell (1915, 1916a) presented data to suggest that the thickness of this unit and the overlying Thames Group were attenuated along a south-east-plunging anticlinal 'axis of instability'. The contoured top of Chalk Group surface (shown in an inset on the published map) lends some support for folding along parts of Boswell's axis in the Woodbridge–Waldingfield area. A separate broad doming of the strata results in Chalk being exposed at sea bed south-east of Felixstowe.

At the base of the sequence there is a bed of glauconite-coated flints overlain by glauconitic sands. These may belong to the Thanet Sand Formation with the basal unit being the Bullhead Bed (Morris, 1876) but the classification is uncertain. 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 interpreted as the Reading Formation of the Lambeth Group. The basal, green, glauconite-bearing sediments are of marine origin, and 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 sediments of the Thames Group (Tms) are the lowest stratigraphical unit exposed onshore and they are present at crop or beneath younger deposits in all but two small parts of the onshore area. These are firstly in the extreme north-west of the district around Wickham Market [TM 302 558], where the Thames Group feathers out and the overlying Red Crag oversteps north-westwards on to Palaeogene deposits, and secondly beneath the alluvial deposits of the Deben valley between Waldringfield and Woodbridge where a buried channel cuts through the Thames Group into the underlying strata. The strata dip very gently towards the south-east with a maximum preserved thickness onshore of about 30 to 35 m in the coastal parts. They crop out along the lower valley flanks of the Deben valley and its tributaries as far upstream as Eyke [TM 317 517] and on the western side of parts of the lower reaches of the Butley River. Offshore, the deposits are present extensively at the sea bed except in the south-west. Farther north-east in the offshore area up to about 90 m of Thames Group sediments are preserved. The top of the Thames Group corresponds approximately to the form of the base of the Crag deposits as depicted on the inset on the published map.

The term Thames Group, here used in the sense of King (1981) and Ellison et al. (1994), corresponds to the London Clay including the basement bed of the older literature, but is now defined as comprising a lower Harwich Formation and an upper London Clay Formation. It was not practicable to separate these two formations onshore, although a boundary between the two was constructed for the offshore area based on seismic evidence (British Geological Survey, 1985, 1989).

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 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.

The Harwich Formation (HAR) 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 part of the sequence but are more sporadic in the lower part. 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 smectite (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 comprising a concretionary cementstone layer with a substantial ash content (Elliott, 1971). The type sections for the formation (Ellison et al., 1994) lie close to the south-western corner of the district and 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).

Along the Deben estuary, foreshore and river cliff exposures at Ramsholt [TM 297 427] reveal multiple weathered ash beds in a subhorizontal attitude; a further now largely overgrown river cliff section [TM 294 454] below Nettle Hill also contains several horizontal ash beds and blocks from a stone band. Opposite Woodbridge, the foreshore at the base of the largely overgrown Ferry Cliff [TM 280 485] was excavated to reveal the basal 'Suffolk Pebble Bed' (George and Vincent, 1976); they also noted the Harwich Stone Band, 6.7 m higher in the cliff face.

Much of the mapped distribution of the Thames Group in the Deben catchment, to the north and west of Ramsholt, is probably of the Harwich Formation. Down-dip (to the south-east) thicker sequences are present in the coastal areas around Felixstowe, Bawdsey [TM 348 398] and Capel St Andrew [TM 375 480] where the overlying London Clay crops out.

Detailed studies of the coastal outcrops south-west 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 are 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 (LC) as defined by Ellison et al. (1994) consists mainly of blue-grey, silty clay and clayey silts that weather to a characteristic chocolate-brown colour. Subordinate coarser sand beds and pebble seams, some glauconitic, are also present marking the base of trangressive events. Onshore within the district, up to about 15 m of strata are present overlying the Harwich Formation around Felixstowe, Bawdsey and Capel St Andrew. These deposits probably belong to the Walton Member (A2) of King (1981). Old cliff sections around Felixstowe detailed by Whitaker (1885) were almost all covered over by the end of the 19th century by coastal defences and stabilisation structures. Samples from a few extant low cliff exposures north-east of the town were examined by Jolley and Spinner (1991) and assigned to the Walton Member. Well records around Orford suggest a maximum onshore thickness of 20 to 25 m for the London Clay. Offshore, a greater thickness is preserved probably reaching about 65 m towards the eastern district margin.

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 Coralline Crag and Red Crag formations 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 Bawdsey–Ramsholt–Felixstowe area and the informal term 'Trimley Sands', after the twin villages [TM 277 368] north-west of Felixstowe, was proposed (Balson, 1990a).

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 Balson (1990a). 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, and based on a thorough recent review a Syltian (latest Miocene) age is probable for most of the concretions (Balson, 1990a, and references therein). However, some of the phosphatised material associated with the concretions points to 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

The main onshore body of the Coralline Crag Formation (CCg) comprises a north-north-east-aligned buried ridge extending from the Aldeburgh area, just north of the district to the south-west of Orford (Figure 3)a. South-west of this occurrence, thin patches of Coralline Crag are also present beneath alluvium and younger Crag deposits immediately south-west of the Butley River and small outcrops of Coralline Crag are also present at Rockhall Wood ('Sutton' of many authors) [TM 305 440] and Ramsholt Cliff. The Coralline Crag is composed mainly of shelly calcarenites and shelly sands; that are cemented and yellow-brown at surface. At depth however, the unoxidised deposits are unlithified and greenish reflecting a glauconite content.

The Coralline Crag rests on the London Clay throughout the district. The form of the basal surface is shown as a component of the base of Crag contour map on the published map. This shows that in the southern part of the main outcrop. The base falls eastwards, though this simple pattern is complicated by a north-trending buried channel beginning just north of Orford. Such features have been ascribed to sea-bed scouring of the Thames Group sediments by tidal currents (Funnell, 1972) or fluvial action (Carr, 1967). Farther north, towards Aldeburgh, the base of the Coralline Crag lies at a more constant level between 10 and 15 m below OD. The straight north-western edge of the deposit (Figure 3)a results from the later incision of a deep, sharply bounded north-east aligned trough infilled with Red Crag sediments (Zalasiewicz et al., 1988). Over much of the area the preserved thickness of the deposit is 15 to 20 m and this thins markedly towards its margins. Sequences over 20 m thick are locally preserved south of the Alde valley. The form of the Coralline Crag as an elongate buried ridge was noted by Harmer (1898, 1910) who suggested that this might reflect the original depositional structure of a sand bank.

The Coralline Crag was first recognised as a distinct formation by Charlesworth (1835) who divided the 'crag-formation' of eastern England. Subsequent subdivision by Prestwich (1871) and Wood and Harmer (1872) was superseded by the work of Balson (1981a, b) who showed that a vertical sequence of primary, laterally continuous sedimentary units is present in the Coralline Crag. It was also suggested (Balson, 1983) that the primary sedimentary fabric in part controls the extent and nature of aragonite leaching and lithification of these carbonate-rich sediments. Many of the important localities were described by Daley and Balson (1999). However, a vertical sequence of facies could only be demonstrated at two localities, Rockhall Wood, Sutton and 'The Cliff', Gedgrave [TM 3972 4863]. Subsequently seven BGS boreholes enabled three members to be recognised (Figure 3)a, b; Balson et al., 1993), corresponding in part with the facies defined by Balson (1981a).

The Ramsholt Member, the lowest part of the formation, is up to 7.5 m thick and present throughout the extent of the Coralline Crag within the district. It corresponds to Facies A2 of Balson (1981a, b) and Facies A of Balson and Taylor (1982). The section at Ramsholt Cliff [TM 298 428] is the stratotype. The Ramsholt Member is divided into two informal lithological units.

  1. The basal transgressive deposits, 0.6 to 1.8 m thick, typically comprise a basal lag of gravel containing abundant phosphorite pebbles overlain by fine- to medium-grained calcareous sands. This unit is characterised by a very low mud content (generally less than 7 per cent) and a low calcium carbonate content, which increases upwards but is generally less than 50 per cent and locally as low as 35 per cent — the lowest values of any of the Coralline Crag sediments. Aragonite is the dominant carbonate mineral.
  2. The upper informal unit of calcareous, medium-grained sand is between 0.9 and 6.75 m thick. In boreholes (Balson et al., 1993), a basal phosphorite pebble layer indicates a slight unconformity (Figure 3)b. The mud content is variable and it has a CaCO3 content averaging 63 per cent, but a lower proportion of aragonite. Sections occur at Ramsholt Cliff and Rockhall Wood (Balson 1990b; Balson et al., 1991).

Borehole cores reveal a general lack of sedimentary structure within the Ramsholt Member due to extensive bioturbation. It is generally rich in mollusc shells when compared with the other two members. Much of this difference can be attributed to the selective dissolution of aragonite from the latter as the majority of mollusc species have aragonitic shells. A comparatively restricted bryozoan fauna (with calcite skeletons) is present. The material is generally well preserved with Metrarabdotos monilifera and species of Cellaria being predominant. Various microfaunal studies indicate an Early Pliocene age.

The Aldeburgh Member is present only in the north of the district where it overlies the Ramsholt Member (Figure 3)b; it includes Facies C of Balson (1981a, b). Around Aldeburgh, just north of the district it attains a maximum thickness of about 15 m. An old quarry at Aldeburgh Hall [TM 453 566] (Balson et al., 1991) serves as a reference section for the upper part of the member. The variably sorted sand fraction is medium grained and generally shows a coarsening-upward trend. In exposures (Balson, 1981a) the stratification is horizontal to low angle; abundant burrows and silt drapes are present. The mud content is generally low with an average of 6.6 per cent. The calcium carbonate content averages 65 per cent. Aragonite is usually absent due to postdepositional dissolution.

Balson (1981a, b, 1983) noted that surface exposures are characterised by an abundant and diverse bryozoan fauna. However, in the borehole cores of this member, bryozoan fragments are rare, the skeletal material being dominated by calcitic mollusc debris. Only a few bryozoan-rich levels were found, generally towards the top of the unit. Thus Facies C of Balson represents only a part of the Aldeburgh Member. The exposed part of this unit has been interpreted as winnowed material that formed upcurrent of the probable sand ridge deposits of the Sudbourne Member (Balson, 1981b).

The Sudbourne Member is equivalent to Facies B of Balson (1981a, b) and it overlies the Ramsholt Member in the Orford–Gedgrave area. The contact is clearly unconformable: the basal beds locally contain small muddy clasts derived from the underlying member and local small phosphorite pebbles. At the contact, the upper surface of the Ramsholt Member is locally burrowed. North-east of Sudbourne, the Sudbourne Member overlies and probably interdigitates with the Aldeburgh Member (Figure 3)b. The pit exposure near Crag Farm was extended by the Crag Farm Borehole [TM 4283 5230] (Balson et al., 1993) and the combined sequence forms the composite stratotype (Plate 2). This sequence includes about 12 m of Sudbourne Member sediments overlying the Ramsholt Member. The well-sorted sand fraction is medium to fine grained and shows a general fining trend to the south-west. The mud content of this unit varies between 3 and 24 per cent (Balson, 1981a). The CaCO3 content averages 65 per cent and aragonite is generally absent due to postdepositional dissolution. Exposures show well-developed large-scale cross-stratification suggesting deposition in a relatively high-energy environment by migrating sandwaves. The cross-stratification shows almost unidirectional migration of the bedforms to the south-west. Only certain bryozoans with rapid growth rates and/or rooted bases were also able to colonise the mobile sands, as did epifaunal molluscs such as pectinids (Balson, 1981b). Much of the skeletal debris in the member is abraded and probably derived.

The Red Crag Formation (RCg) is present across much of the onshore part of the district and at the sea bed in the offshore area north of Orford Ness. The only areas where the Red Crag does not appear to have been originally deposited are on the buried ridge of Coralline Crag from south west of Orford to Aldeburgh (Figure 3)a and on the isolated outlier of Coralline Crag at Rockhall Wood, Sutton. In these locations, the Red Crag appears to be banked up against 'highs' of Coralline Crag. This relationship was seen at Rockhall Wood where sections showed cemented blocks of Coralline Crag reworked from an ancient cliffline into the Red Crag sediments (Prestwich, 1871; Boswell, 1928). This relationship also demonstrates that the lithification of the upper parts of the Coralline Crag took place prior to the onset of Red Crag deposition.

Over much of the district the Red Crag Formation rests directly on the Thames Group. This basal Red Crag surface is shown in the composite inset on Sheets 208 and 225 and reveals an overall fall of the contact eastwards. Closed scoured hollows and small basins are evident in both the onshore and offshore areas. Elsewhere in East Anglia, faulting has been invoked to explain abrupt thickness variations and changes in level of the basal contact of the Red Crag (Bristow, 1983). Faulting appears to be absent in this district and the changes in elevation and thickness are here thought to result from sea-bed scouring of the Thames Group sediments by tidal currents (Funnell, 1972) or fluvial action (Carr, 1967). The thickness of the Red Crag sediments is largely a reflection of the morphology of its basal contact. In the hollows and basins offshore, the sediments locally reach 30 to 40 m in thickness whereas onshore 25 m is probably the maximum and 10 to 15 m a more typical thickness. The Red Crag is overlain without marked discontinuity by the Chillesford Sand in the north-eastern part of the district.

The Red Crag comprises medium- to coarse-grained, poorly sorted, shelly sands, which exhibit an overall fining-upward trend, and are commonly decalcified in their upper parts. The type section is from 6.82 to 26.00 m in the Wantisden Hall Borehole [TM 3601 5215]. Near the surface the sands have a distinct red colouration but at depth, they are green, commonly with a glauconite 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 (mainly derived from the London Clay) and 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 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). This unit has been described at Neutral Farm, Butley (Plate 3) [TM 372 511] and Bawdsey Cliff [TM 345 385] to [TM 350 390] by Dixon (1979) and 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), but Balson et al. (1991) suggested deposition under the influence of rectilinear tidal currents in a constricted estuary. Pit sections include Orford Lodge, Chillesford [TM 390 508] and Vale Farm, Sutton [TM 317 456] (Balson et al., 1991). Many of the classic localities were described by Balson (in Daley and Balson, 1999).

An exhaustive faunal list was compiled (Boswell, 1928 and references therein). Study of the molluscan faunas led Harmer (1900) to suggest that, when traced northwards from Walton on the Naze towards Aldeburgh, progressively younger and cooler climate faunal assemblages could be recognised. He termed these, in decreasing age, Waltonian, Newbournian and Butleyan: the Newbournian Red Crag is found to the south-west of the Deben and the Butleyan Red Crag to the north-east.

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, on the basis of 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). Subsequent detailed studies investigated the Red Crag in its steeply bounded basin north of Aldeburgh (Zalasiewicz et al., 1988). Here the basal Pre-Ludhamian Sizewell Member comprises shelly coarse-grained sands with thin clays. It is overlain by Ludhamian (Thorpeness Member) which exhibits two coarsening-upward cycles of shelly fine- to medium-grained sands. This deep basin extends south-westwards into the district around Iken [TM 410 560] and Tunstall [TM 358 551]. It is probable that the upper part of thick Red Crag preserved in that area contains some Ludhamian sediments.

The Norwich Crag Formation (NCg) is represented by a conformable sequence of two distinct mappable lithological members: the Chillesford Sand Member, and the more restricted, overlying Chillesford Clay Member (Figure 4). These deposits are only preserved in the north-eastern part of the onshore area where they rest without marked disconformity on the Red Crag and overstep it to lie unconformably on the buried ridge of Coralline Crag in the Orford–High Street [TM 433 553] area. The formation locally attains a thickness of 15 m. This main crop extends westwards as far as Hollesley [TM 354 450] in the south and Campsey Ash [TM 331 559] in the north. Three small outliers of Norwich Crag have been identified, from surface and borehole evidence, to the east of the Deben valley. The Chillesford Sand Member (CfS) is present almost throughout the distribution of the Norwich Crag. It is only locally cut out, or laterally replaced, by the Chillesford Clay east of Butley [TM 368 512]. The Chillesford Sand comprises up to 13 m of well-sorted, fine- to medium-grained, micaceous, quartz sand. The type section (Zalasiewicz and Mathers, 1985) is from 1.42 to 6.82 m depth in the Wantisden Hall borehole [TM 3601 5215]. Throughout much of the area, the Chillesford Sand is decalcified, but local patches of shelly material are preserved, notably in the Chillesford Church Pit [TM 3829 5231] where the deposit was formerly termed the 'Scrobicularia Crag' due to the abundance of this bivalve. This shelly development of the deposit also contains abundant Spisula and paired valves of Mya in life position. Much of the shell material is fine grained, abraded, and appears, at least in part, to be derived from the older Red and Coralline Crag formations. The Chillesford Sand passes upwards through a gradual interbedded transition into the Chillesford Clay.

The Chillesford Sand 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.

The Chillesford Clay Member (CfC) has a more restricted elliptical distribution stretching from around Butley, north-eastwards through Sudbourne [TM 413 530] towards Aldeburgh, just north of the district (Figure 4)a. The base of the deposit falls from about 15 m above OD in the south-west to about 10 m in the north-east of the district (Figure 4)b. The type section is the Chillesford Brickyard Pit [TM 3880 5258] ((Plate 4); Zalasiewicz and Mathers, 1985).

The deposits comprise up to 6 m of unfossiliferous, buff or pale grey, silty clay with poorly defined sand laminae and intercalations. The deposit is bioturbated and burrowed; shelly debris including the bivalves Nucula and Yoldia were recovered from the Captain's Wood Borehole [TM 4237 5455] (Zalasiewicz et al., 1991). The deposit is thought to have formed as high intertidal mudflats. Studies of the palaeomagnetism, pollen and foraminifera of the deposit tentatively suggest a Baventian–Pre-Pastonian age.

Quaternary

Cromerian–Beestonian

The Kesgrave Formation (Kes) occurs extensively in southern East Anglia as a series of cold-phase braided stream terrace deposits. These comprise sand and gravel deposits laid down in the proto-Thames drainage system between the deposition of the 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 (Figure 5). 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 (Kemp, 1985). This clay-rich layer, typically 1 to 2 m thick, contains clear evidence of reddening and other processes normally associated with subtropical soils. Also lenses of temperate organic material have been found within some of the terraces 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. In the south and east they cap much of the high ground at about 20 to 25 m above OD. In the north-west the deposits are overlain, and cut through, by the deposits of the subsequent Anglian glaciation. The Kesgrave Formation comprises interbedded, pale yellow sand, pebbly sand and gravel, 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 material into younger glaciofluvial and fluvial deposits has also occurred. There are few exposures of the deposits within the district, but a small disused pit [TM 3525 4475] at Hollesley reveals up to 2.0 m of horizontally stratified quartzose gravels resting on the Chillesford Sand Member. No rubified palaeosol unit has been recorded within the district.

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 literature therein). They were subdivided into an upper 'Sudbury Formation' found north-west of this district, and a lower younger 'Colchester Formation' that includes all the deposits within the district (Figure 5). A series of four distinct levels or terrace remnants have been identified within the 'Colchester Formation'. Most of those within the district fall within the highest Waldringfield Member and crop out at elevations of 20 to 25 m above OD. The deposits in the Felixstowe and Bawdsey area may belong to the Ardleigh Member that lies at a slightly lower altitude (Figure 5), and was distinguished by Whiteman and Rose (1992). The 'Colchester Formation' is believed to have been deposited between about 0.9 and 0.45 Ma. (Oxygen Isotope stages 21–12), an interval of time broadly corresponding to the Cromerian and Beestonian Stages of the Pleistocene (Rose et al., 1999, and references therein).

Anglian–Hoxnian

Glacigenic deposits of the Anglian Stage are extensively distributed throughout East Anglia and although they have been partly eroded, especially along drainage lines, their overall distribution still broadly reflects the area of original ice-cover. Two principal lobes of ice were present within the region during the Anglian Stage. One was derived from the north-east and deposited the Cromer Formation (North Sea Drift) throughout much of east Norfolk with a characteristic suite of Scandinavian-sourced erratic pebbles. Overlying and locally interdigitating with these deposits are the deposits of the Lowestoft Formation, derived from the north-west and dominated by the widespread 'chalky boulder clay' till, which occurs at surface over much of East Anglia. The Lowestoft Formation extends farther south, covering much of Suffolk including parts of this district and north Essex.

There are three categories of deposits associated with the Lowestoft Formation within this district: Lowestoft Till, Lacustrine Deposits and Glaciofluvial Deposits. The formation occurs as a semi-continuous spread on the plateau in the north-west. On the margins and sides of the Deben valley upstream of Melton [TM 283 506], glacial deposits are also found in steeply downcutting channels, some of which appear to be broadly aligned with the present valley (Figure 6). These indicate substantial Anglian downcutting along the line of parts of the valley. However, the presence of true tunnel valleys with undulating thalwegs (Woodland, 1970) has not been established within the district. It is possible that glaciotectonic deformation may have affected sediments in the Deben valley, as seen in the adjacent Gipping valley to the west (Slater, 1927).

Farther to the south and east, local developments of glacigenic deposits are also preserved, principally in intricate channel systems. Detailed geological surveying has revealed the presence of an extensive well-preserved network of ice-marginal drainage channels incised into the pre-Anglian deposits (Mathers and Zalasiewicz, 1986; Mathers et al., 1991). The infill of many of the channel segments is dominated by clay-rich lithologies whereas the deposits into which they are cut (Crags, Kesgrave Formation) are predominantly arenaceous. This results in a marked conductivity contrast that can be mapped. The Aldeburgh–Snape system immediately north of the district (Mathers and Zalasiewicz, 1986) contains a well-ordered infill sequence interpreted as the result of proglacial drainage that was overridden by the advancing ice sheet. Farther south, the Hollesley system (Mathers et al., 1991) contains a less ordered infill produced at a stationary ice-margin without significant further ice-advance. In these channels, thin Lowestoft Till deposits are commonly interstratified with glaciofluvial deposits comprising sands, gravels and silts. The final infill of some of these channel elements comprises lacustrine deposits indicating blockage and later deposition in isolated hollows of thinly stratified silts and clays, some of which appear glacigenic. However, in some locations such as Rookery Farm, Eyke [TM 330 514] and east of Shottisham [TM 320 447], silts and clays pass up into organic-bearing material indicating that localised depressions within the channel systems persisted during climatic amelioration, related to the succeeding Hoxnian Interglacial ((Figure 7)a, b; Mathers et al., 1993).

Rose et al. (1985) showed that early Anglian periglacial soil structures (Barham Arctic soil) overprint the temperate rubified palaeosols found resting on the Kesgrave Formation. Some of their localities lie close to this district.

The Lowestoft 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 weathered to an oxidised yellowish brown clay with flint pebbles; decalcification is common. In the north-west of the district, the till is commonly up to 5 to 8 m thick on the plateau. At depth, the deposit is over-consolidated and massive with local subordinate interbeds of water-sorted sands and gravels. The diamicton is regarded as a lodgement till sheet composed of the unsorted detritus carried by the ice-sheet. Locally at the base of the sequence, or in channels beyond the main crops in the north-west of the district, thinner beds (1 to 30 cm) of poorly consolidated till are found complexly interstratified with waterlaid silts, sands and gravels with some evidence of soft sediment deformation. Such soft beds while clearly of similar petrological character are commonly more silty than the massive variety and are variously interpreted as flow, water-laid or basal melt-out tills.

The Glaciofluvial Deposits comprise water-laid sands, gravels and silts of very variable character. In the north-west of the district, these 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 compositionally distinct from the Kesgrave sands and gravels and contain more nodular and freshly broken flint, and proportionally less quartz and quartzite. Locally chalk is also preserved within the deposits where decalcification is incomplete. The glaciofluvial deposits occur as low elongate mounds capping the Lowestoft Till plateau, and also on the shoulders and in deeply incised channels on the valley flanks of the Deben catchment (Figure 6). They are also present as lenses up to 3 m thick within the Lowestoft Till.

Farther south however, the distinction between the gravelly deposits is difficult to establish. This is probably due to the increased incorporation of Kesgrave material into the ice sheet and possible reworking of Kesgrave deposits by relatively clean meltwater issuing from the ice margin. Patches of suspected glaciofluvial deposits of this type have been mapped east of Sutton and the occurrence of isolated glacial channel deposits in such areas supports this particular interpretation. Reworking of the deposits found on the extensive plateau tops may also have occurred. Similar uncertainty exists with the classification of small spreads of gravel and sand within the lower reaches of the Deben and Butley River valleys. Possibly many of the small patches of sand and gravel at 4 to 10 m above OD are quite old in terms of the progressive valley incision. Their clast composition contains 'Kesgrave' and 'Anglian' elements and they are tentatively regarded as Anglian outwash.

The Lacustrine Deposits are mainly restricted to isolated channels and thin plateau-top sheets beyond the main till sheet (Dalton, 1886; Boswell 1914, 1916b). They comprise laminated silts and clays with thin sand partings. Detailed studies of these deposits established that they were deposited in isolated blocked-off remnants of channel systems, in the late Anglian and locally into the succeeding Hoxnian Interglacial (Mathers et al., 1991, 1993).

Post–Hoxnian–Devensian

Patches of sand and gravel fringing the estuarine tidal flats and salt-marsh deposits at elevations of 1 to 2 m above OD are classified as Second Terrace Deposits. Their age 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. Their base probably lies 1 to 3 m below OD.

At lower elevations, sand and gravel deposits were recorded in several boreholes drilled through the Holocene deposits in the Deben, Orwell and Alde–Ore valleys. These deposits are classified as First Terrace Deposits and typically comprise a thin lag gravel 1 to 2 m thick, resting on bedrock. They occur within buried channels cut to levels of about 15 m below OD near the present coastline and are part of systems that have been traced offshore to much lower levels. The buried channel of the Alde–Ore system flowed eastwards through Slaughden (Figure 8) during the Devensian. These deposits are depicted in the schematic cross-section, inset on 1:50 000 Series Sheet 208 and 225.

Head deposits represent mass-movement deposits that have accumulated at or near the base of slopes within the district. While most slopes contain some material of this kind, deposits have been mapped only where they exceed 1.0 m in thickness. The deposits are not thought to exceed 2.5 m. Lithologically the material is of varied character reflecting the materials upslope from which it derives. For example, those located on the outcrop of the Thames Group 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.

Head deposits are thought to be mainly developed in periglacial areas during cold phases when limited vegetation cover and alternating freeze and thaw conditions aid mobility of near surface material and its movement downslope. The head deposits in the area are likely to be mainly associated with the last (Devensian) glaciation when the district remained beyond the extent of 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 7–8 000 years BP. Within the district, the rising sea levels were accompanied by sedimentation in fluvial, coastal and marine environments producing the suite of Flandrian deposits that continue to form at the present day.

Along the major river valleys freshwater Alluvium and tidal flat, channel and saltmarsh Mud deposits have accumulated infilling the channels of the rivers established by strong downcutting during the preceding 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. The limits of the two deposits at surface are drawn on the accompanying map sheet to reflect the approximate limit of the present tidal influence in the individual valleys. At depth the deposits are known to interdigitate in a complex fashion. These sequences locally exceed 10 m in thickness where they infill buried channels, such as the former channel of the River Alde (Figure 8). Limited borehole evidence in the Deben valley also confirms a buried channel located close to the alignment of the present river cutting to levels near the coast of about 15 m below OD. Beneath the Languard Point spit Flandrian deposits are also found to comparable depths. The alluvium and tidal flat, channel and saltmarsh deposits are composed of mud and silt with a variable organic content and are poorly consolidated near the surface. Locally a basal peat layer is present as well as thin interbeds of peat that rarely exceed 0.5 m in thickness. Peat is also mapped at surface at the poorly drained margin of the Deben floodplain west of Eyke [TM 317 517]. Throughout the area the alluvium deposits are a mixture of silt and mud, and locally contain shelly debris, pebbles and thin interbeds of sand. The tidal deposits are similarly varied and while organic mud is the dominant component, lenses of silt and fine sand several metres thick occur associated with tidal channels.

Specific investigations of the stratigraphy of the freshwater alluvium, peat and the tidal flat, channel and saltmarsh sequences of the district have been reported by Carr and Baker (1968), Carr (1971), Brew (1990) and Brew, Funnell and Kreiser (1992). As part of the recent survey of the district two cored boreholes have been sunk to further investigate these sequences. One was located on Aldeburgh Marshes [TM 455 560] near the northern edge of the district, the other near Chantry Point [TM 423 485] south of Orford (Figure 8).

Beneath Aldeburgh Marshes a four-part lithostratigraphy is present comprising a basal peat, overlain, in turn, by saltmarsh and intertidal silty clay, freshwater peat, and finally an upper saltmarsh and intertidal silty clay. Carr and Baker (1968) report a basal peat (core 182) dated at 8640 ± 145 14C years BP at 12.7 m below OD and 8460 ± 145 14C years BP at 13.7 m below OD. The intercalated peat horizon is reported by Carr and Baker (1968) from Core 183 at 3.4 m below OD yielding a date of 3460 ± 100 14C years BP. In the recent BGS borehole TM45NW9 [TM 4493 5600] a freshwater peat-rich sample from 3.32 to 3.35 m below OD gave a date of 3663 ± 44 14C years BP (AA-35807) while the transgressive top of the peat at 2.96 to 2.98 m below OD gave a date of 3382 ± 42 14C years BP (AA-35806).

Farther south at Chantry Point, a second BGS borehole (TM44NW/82) [TM 4228 4846] proved 11.85 m of Holocene sediment resting on Coralline Crag. A basal, black, laminated peat, 0.07 m thick, overlies the Crag surface. The base of the peat is barren of foraminifera and is interpreted as freshwater, whereas the top contains a few T. inflata and J. macrescens indicating saltmarsh conditions. A sample from the transgressive contact at the top of this peat at 11.80 to 11.82 m below OD gave a date of 7019 ± 82 14C BP (AA-35808).

Carr and Baker (1968) and Carr (1971) also dated several peat horizons in boreholes from nearby King's [TM 445 500] and Lantern Marshes [TM 450 515]. Core 136 contained a thin peat, which has been dated using pollen to about 7500–8000 years BP. A basal peat in Core 73 has a pollen-based date of about 7600–7700 years BP. Core 50 contained a basal peat which has been dated at 7010 ± 130 14C years BP at 9.5 m below OD. Carr (1971) describes a thin layer of silty clay (marsh clay) overlying the peat and beach gravel with some sand above the clay; he therefore suggested that the date sets a limit to the earliest deposition of spit sediment at the locality.

Coarser clastic deposits are found associated with the spits and beaches of the area. The prominent spit of Orford Ness and the smaller Landguard Point spit are composed of medium to coarse gravel predominantly of clasts of subrounded to well-rounded flint. Landwards the deposits of these spits interdigitate with tidal mud deposits. Elsewhere along the coast, in particular where extensive cliff erosion has occurred at Bawdsey and Felixstowe and around the mouths of the Ore and Deben, the beach and shoreface deposits comprise sand and gravel. Large stretches of cliffs have now been protected reducing the potential supply of material to the adjacent beaches. Collectively all these coarse clastic deposits are depicted as shoreface and beach Sand and Gravel on the map. These deposits locally reach 15 m thick beneath parts of Orford Ness. Along the Landguard Point spit, sequences of about 15 m of Flandrian deposits are characterised by an upper and lower bed (each 3–7 m thick) of coarse flint gravel separated by a layer of tidal Mud deposits of similar thickness. The historical evolution of Landguard Point from 1804–1925 is depicted in Boswell (1928).

The historical development of Orford Spit was traced by Steers (1926) and Carr (1969, 1970, 1972). It probably originated as an offshore bar formed at a lower sea-level stand, possibly from glacial outwash gravel. It subsequently moved landward with sea-level rise as a storm beach, constricting the various river channels as it did so. The dominance of drift processes over estuarine tidal flow has given rise to several such spits along this coast, of which Orford is much the largest (Carr, 1969, 1970). Longshore drift from Aldeburgh probably closed the exit of the River Alde at an early stage, and the river diversion has been maintained since that time. It has not had a simple development by progressive elongation but appears to have varied in length through time, controlled by the effects of storm breaching and subsequent closure by beach drifting at its south-western end. Periods of rapid accumulation of sediment have been followed by times of severe regression. Orford Castle was completed in 1173 near to the mouth of the river at that time. Between the 12th century and today the spit has doubled in length (that is a further 8 km to Shingle Street). From estimates of sea-level rise, Carr (1970) took the ages of the beach ridges to range from pre-Roman to the 12th or 13th century. Detailed geomorphological studies at Shingle Street, at the mouth of the River Alde–Ore, have been conducted by Cobb (1957) and Randall (1973).

Patches of Blown Sand occur on either bank at the mouth of the Deben. The deposits comprise up to 5 m of clean, well sorted, medium-fine sand distributed as dune forms derived by aeolian action eroding sand from adjacent dry beach surfaces. These dunes are well vegetated and so are stable.

Beyond the shoreface in the offshore area several north-north-east- aligned linear sand banks are present. These banks vary from 3 to over 10 km in length, and are generally less than 1.5 km wide and are commonly 5 to 10 m in height. These are depicted on the Sheet 208/225 Woodbridge and Felixstowe as bank Sand deposits. They form part of an extensive development of linear sand banks that lie off much of the coast of southern East Anglia and Essex.

Offshore from Slaughden tidal estuarine Mud deposits associated with the former more easterly outflow of the Alde–Ore river system are preserved on the sea bed extending out about 7 km from the coast. These deposits comprise up to 12 m of mud and silt with sand interbeds; they are overlain by the sand deposit of one of the linear banks.

Landslip, Artificial Deposits and Worked Ground

A small area of Landslip was recorded at the base of the cliffs north-east of Bawdsey Manor [TM 335 378]. These cliffs are relatively unstable (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. Within the district large areas of made ground occur at the port of Felixstowe, as road embankments and as reservoirs. Sea-wall defences are marked on the topographical base to the geological Sheets 208 and 225. Worked Ground is shown where natural materials are known to have been removed, for example in quarries and pits, road and rail cuttings. Many small pits and quarries are present within the district: the larger ones are shown on the map face. Most of these relate to the digging of local sources of aggregate, brick clays and phosphate nodules from the Red Crag. 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. These are of restricted extent in the district, consisting mainly of small pits backfilled with waste materials. 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. Such areas are of restricted extent in the district.

Structure

The district covers an area near the culmination of the London–Brabant Massif, defined by the youngest supercrop formation (Gault), overlying Silurian rocks (Smith, 1985a, b), which are, in turn, probably unconformable on older magnetic Palaeozoic rocks, forming the core of the massif.

The district lies within the East Anglian–Brabant Caledonide chain (Pharaoh et al., 1995). This chain was deformed by thrusting and uplifted in response to closure of the Tornquist Ocean, situated between Avalonia and Baltica in late Ordovician times. 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, 1985). Another probable thrust separates Devonian and Silurian rocks, south of the district, and this fault was possibly responsible for the 1884 Colchester earthquake. Deformation during the Acadian orogeny also produced cleavage in Silurian and some Devonian rocks (Pharaoh et al., 1987) and probable renewed thrusting on north-east-dipping faults. The aeromagnetic anomaly map (inset on the 1:50 000 Series Sheet 208 and 225) 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 appear to be overlain unconformably by Silurian rocks.

The London–Brabant Massif was a long-lived (late Palaeozoic and early Mesozoic) positive feature, an amalgamation of the East Anglian–Brabant Caledonide chain and the Midlands Microcraton. Mesozoic strata older than Cretaceous are absent from the area. After initial extensional faulting and subsidence in early Mesozoic times, which also affected the surrounding basins in the southern North Sea and the Weald of Kent, the area subsided in response to thermal relaxation, and the Cretaceous strata, Gault Formation, Upper Greensand and Chalk Group, rest unconformably on early Palaeozoic rocks.

A narrow north-west-trending graben has been recorded on sparker profiles in Hollesley Bay cutting the Thames Group. The profiles indicate that the faults do not extend down into the Chalk Group but they do overlie a broad depression in the top of the Chalk. It seems possible that either warping or extensive dissolution collapse may be invoked to explain this structure. Onshore, small-scale near-vertical faults have been observed in the Thames Group, the largest with a throw of 3.5 m in the cliffs at Felixstowe (Whitaker, 1885).

The regional distribution of the shallow marine (late Pliocene-Pleistocene) Red and Norwich Crag formations in southern East Anglia provides evidence for the easterly downtilting of the region during the last two to three million years. This is probably in response to subsidence in the adjacent Southern North Sea Basin (Mathers and Zalasiewicz, 1986; Moffat and Catt, 1986).

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 show a gradual shallowing with decreasing age (Whiteman and Rose, 1992, fig. 3); these rivers were maintained over a long time period (between 1.8 and 0.46 Ma).

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). Furthermore, the systematic sparker profiles of the offshore Crag sequence show no evidence of faulting. An orthogonal joint system has however been detected within the lithified Coralline and Red Crag sediments and interpreted as the product of tectonic subsidence (Balson and Humphreys, 1986).

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. Other 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 that is confined throughout the onshore area by the clay-dominated Palaeogene strata.

In the Crag aquifer, which may be 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 noncarbonate increases to over 400 mg/l and the chloride ion content rises to around 150 mg/l. Chlorides also increase near the coast and the tidal reaches of rivers, due to saline infiltration.

The confined Chalk aquifer is about 300 m thick and characterised by small annual fluctuations in the piezometric surface of 1.5 to 1.8 m. Across the area, connate water causes a sharp increase in salinity from about 100 mg/l in the north-west to the coastal belt (extending 5 to 10 km inland as far a line between Hemley [TM 285 424] and Tunstall) where values are greater than 500 mg/l. Locally close to the coast, these reach several thousand mg/l. Immediately inland of this belt, salinities probably increase with depth within the aquifer where a shallow wedge of relatively fresh water overlies denser saline water.

Surface mineral workings

The geological units that have been dug within the area are summarised in (Figure 9). Few of these deposits are of current economic interest.

The Chillesford Sand was dug at Chillesford as a local building sand whilst nearby the deposits of the Chillesford Clay are dug and transported to Aldeburgh to make the characteristic red-firing Aldeburgh bricks. The Kesgrave Formation is an important source of sand and gravel within the region although the major sites of current extraction lie beyond the district. Locally the glaciofluvial deposits have been dug as a source of sand and gravel but they commonly have a higher clay content and are inferior to the other local alternatives. Assessments of the sand and gravel resources of parts of the district are contained in Allender and Hollyer (1972, 1973) and Hollyer and Allender (1982).

Foundation conditions

There are a range of potential problems related to ground stability in the district. (Figure 10) gives the principal ones.

Ground heave and subsidence

The Thames Group is 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, leading 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 to 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

Cliffs are present along the coast at Bawdsey and Felixstowe and along the Deben estuary. These cliffs are relatively unstable because their lower parts are composed of London Clay. The cliffs within the Felixstowe urban area and those immediately below Bawdsey Manor were landscaped, stabilised and protected in the 19th century. Periodically, small rotational slips and rock falls occur at the other locations followed by the erosion and redistribution of the slipped material by wave and tidal action. Regionally, 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

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. Although Chalk is not exposed at surface onshore within the district, it is present at a modest depth (10 to 20 m) along the Deben valley upstream of Waldringfield. Two closely spaced water wells [TM 2750 4900] drilled close to the floor of the Deben valley in Brook Street, Woodbridge encountered Chalk at anomalous depths (Whitaker, 1903; Boswell, 1913). The deeper borehole reached Chalk at 53.5 m below OD at a location where a depth of about 10 m below OD would be expected from nearby boreholes (Figure 11). Faulting is apparently absent and the presence of an apparently 'normal stratigraphy' discounts the possibilities of a buried channel cut into the chalk or glacial disturbance, despite the views expressed by Boswell (1913). A more plausible explanation is that gentle foundering has occurred into a tightly confined solution hollow perhaps no more than 100 to 200 m across with some 40 m of collapse. Other similar features may be present along the Deben and adjacent valleys.

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 Felixstowe–Bawdsey area where the potential risk from radon is perceived to be greater than that found elsewhere in East Anglia. This area is classified as one where basic radon protection should be provided in all new dwellings (Miles et al., 1996; Lomas et al., 1996; Department of the Environment, 1999; Appleton et al., 2000).

Flooding

Low-lying alluvial ground in valleys and the tidal flats and salt-marshes, fringing the Deben estuary and coast, have a long history of flooding. Major works of flood defence near Aldeburgh occurred in 1947 when a concrete sea wall (fronted by groynes) was constructed along the frontage south of Fort Green [TM 465 561] to the Martello tower south of Slaughden. The most severe episode occurred during the North Sea storm surge of 1953, when most of the pre-existing sea and estuarine defences were breached, causing widespread flooding and damage to agriculture. The sea defences have been considerably improved and heightened in recent times to afford a greater degree of

protection than before. In many of the estuarine areas, bunds have been constructed along the river sides by excavating the adjacent mud deposits. Nevertheless, many of these coastal areas remain at some risk of flooding when similar extreme storm events occur. Upstream of Woodbridge, the floodplain of the Deben is also prone to inundation during heavy precipitation.

Coastal erosion

The east-facing coast of Orford Ness and the beach continuing northwards have shown consistent landward movement. Since the 16th century, six Aldeburgh streets that ran parallel to the coast have been lost to the sea. Slaughden comprised a village of 20 houses in the late 16th century but now only a Martello tower remains. The foreshore in this area was recharged with over 200 000 cubic metres of shingle in 1965. The harsh coastal regime along this stretch of coast has necessitated frequent replacement of groynes and repairs to the sea wall around Slaughden; breaches of the sea wall have occurred, most notably in 1963.

Information sources

Further geological information held by the British Geological Survey relevant to the Woodbridge and Felixstowe district is listed below. It includes published maps, memoirs and reports. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Geological advice should be sought from the Programme Manager, Integrated Geoscience Surveys (South), BGS, Keyworth.

Other information sources include borehole records, fossils, rock samples, photographs, geophysical, geochemical and thin sections, hydrogeological data. Searches of indexes to some of the collections can be made on the Geoscience Index system in BGS libraries, and BGS is providing increased online access to its national data sets. Web address: http://www.bgs.ac.uk.

Maps

Books

Documentary collections

Boreholes

Borehole data for the district are 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 shown on the 1:50 000 Series Sheet 208/225 Woodbridge and Felixstowe Sheet are held in the Lexicon database. This is available on Web Site http://www.bgs.ac.uk. 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 Woodbridge and Felixstowe districts.

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 e-mail geohelp@bgs.ac.uk.

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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.

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

(Index map)

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 and plates

Figures

(Figure 1) Geological succession in the Woodbridge and Felixstowe district.

(Figure 2) Location of deep boreholes, contours and subcrop pattern of the sub-Mesozoic unconformity. Borehole comprising 2 m of green sandstone and phosphatic nodules overlying glauconitic, calcareous mudstone (Whitaker, 1906).

(Figure 3) Coralline Crag Formation: geometry and relationships between the Ramsholt, Sudbourne and Aldeburgh members, based on Balson et al. (1993). a Location of BGS research boreholes b Geometry of deposits.

(Figure 4) Correlation of sequences through the Coralline, Red and Norwich crag deposits between Butley and High Street showing their geometry, based on Zalasiewicz et al. (1991). a Location of BGS research boreholes and sections b Cross-section

(Figure 5) Kesgrave Formation terrace levels modified from Whiteman and Rose (1992).

(Figure 6) Section through marginal Anglian glacigenic sediments and Solid formations.

(Figure 7) The Rookery Farm channel filled with Anglian glacigenic deposits and Hoxnian organic deposits (after Mathers et al., 1993). a. Location map with ground conductivity b. Section showing borehole and augerhole logs

(Figure 8) Basal surface of the Flandrian–Holocene sequence in the Orford Ness–Aldeburgh area and location of BGS research boreholes.

(Figure 9) List of materials dug within the district.

(Figure 10) Potential ground constraints.

(Figure 11) Anomaly in the top Chalk surface at Brook Street, Woodbridge. a. Location map showing borehole data. b. Graphic logs of anomalous boreholes and relationships (Culshaw and Crummy, 1991).

Plates

(Plate 1) Mouth of the River Deben (1998) taken from Felixstowe Ferry looking north-east to Bawdsey Manor. Note the flint-rich gravel beaches on either side of the river mouth [TM 330 375] (GS1162).

(Plate 2) Cross-stratified beds of calcarenite formed by sand wave migration, Sudbourne Member, Coralline Crag Formation; dip is to the south-west. Crag Farm, Sudbourne, 1982 (Type Section) [TM 428 523]. Section is 2.5 m high. (GS1164).

(Plate 3) Red Crag Formation showing a thick basal set of south-west-dipping cross-stratification formed by sand wave migration overlain by planar cross-stratified beds, Neutral Farm Pit, Butley, 1982. [TM 3718 5105] (GS1161).

(Plate 4) Laminated silt and clay of the Chillesford Clay Member, Norwich Crag Formation. Chillesford Brickyard Pit, Chillesford, (Type Section), 1998. [TM 3880 5258] (GS1163). Spade for scale.

(Front cover) The mouth of the River Ore looking north-east towards Orford Ness in the far distance in 1995. (Aerofilms 647602).

(Rear cover)

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

Figures

(Figure 1) Geological succession in the Woodbridge and Felixstowe district

Quaternary Thickness
Flandrian (Holocene) Alluvium, Peat, Blown Sand Marine and Coastal Deposits
Devensian to Post Anglian River Terrace Deposits (First and Second) Head
0.3–0.45 Hoxnian to Anglian Lacustrine Deposits
Anglian Glaciofluvial Deposits
Lowestoft Till
0.45–Ma 0.9 Cromerian to Beestonian Kesgrave Formation
Unconformity
Neogene Pliocene to 3.6 Norwich Crag Formation Chillesford Clay Member Silty clay, buff and grey with sand laminae and shell debris
Chillesford Sand Member Sand, fine-grained, micaceous, yellow-brown, locally shelly 13 m
Red Crag Formation Sand, shelly, medium to coarse-grained, with thin phosphorite and flint pebble seams to base; upper parts decalcified, red at surface, green at depth 40 m
Coralline Crag Formation Calcarenite, fine- to medium-grained, with abundant bryozoa and mollusc fragments, thin phosphorite pebble seams to base; yellow-brown at surface, green at depth 25 m
Unconformity
Palaeogene Eocene 52.0–54.8 Thames Group London Clay Formation Clay and silty clay, with thin sand and pebble beds, and septarian concretions, blue-grey 65 m
Harwich Formation Clay and silty clay, with thin ash bands and cementstone layers, blue-grey 20 m
Palaeocene 54.8 65.0 Lambeth Group and Thanet Sand Formation Colour mottled clay, silt and sand, and yellow-brown fine-grained, sands (locally lithified to sarsen); green at base 15 m
Unconformity
Cretaceous Upper Cretaceous Chalk Group Chalk, soft and nodular, grey and white, with abundant flint seams in upper parts 290 m

(Figure 9) List of materials dug within the district

Geological unit Worked for
Glaciofluvial Deposits sand and gravel aggregates
Lacustrine Deposits brick clay

ceramic clay
Lowestoft Till marl for soil improvement
Kesgrave Formation gravel sand and aggregates
Chillesford Clay brick clay
Chillesford Sand building sand
Red Crag phosphate for fertilizer (coprolites) track making
Coralline Crag building stone

track making
Thames Group (London Clay Fm) brick clay
Thames Group (Harwich Fm) cementstones

building stone

(Figure 10) Potential ground constraints

Geological unit Potential ground constraints
Worked Ground variable foundation conditions; unstable sides of old workings
Made Ground variable foundation conditions
Infilled Ground variable foundation conditions; leachate and methane production from waste
Landscaped Ground variable foundation conditions
Landslip unstable ground
Blown sand unconsolidated deposit
Peat compressible deposit, subsidence
Alluvium unconsolidated and compressible deposit, subsidence
Mud (Tidal flat, channel, saltmarsh) unconsolidated and compressible deposit, subsidence
Sand (offshore banks only) unconsolidated deposit
Sand and Gravel (shoreface and beach) unconsolidated deposit
Head variable unconsolidated deposit
Second Terrace Deposits high water table, undocumented former workings
Glaciofluvial Deposits perched water tables and springs
Lacustrine Deposits undocumented former workings
Lowestoft Till soft, decalcified and weathered to about 2 m depth, undocumented former workings
Kesgrave Formation undocumented former workings
Chillesford Clay Member undocumented former workings
Chillesford Sand Member unconsolidated
Red Crag Formation

Coralline Crag Formation

 unconsolidated decalcified upper parts, undocumented former workings

undocumented former workings
Thames Group ground heave, landslip and subsidence in clays; high sulphate content, perched water table and springs in sand layers
Chalk Group not encountered at surface but prone to dissolution producing cavities and leading to collapse of overlying strata