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Geology of the Wellingborough district — a brief explanation of the geological map sheet 186 Wellingborough

A J M Barron, A N Morigi and H J Reeves

Bibliographic reference: Barron, A J M, Morigi, A N, and Reeves, H J. 2006. Geology of the Wellingborough district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 sheet 186 Wellingborough (England and Wales).

Keyworth, Nottingham: British Geological Survey, 2006. © NERC 2006. 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.

The grid, where used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright. All rights reserved. Licence Number: 100017897/2006.

(Front cover) Higham Ferrers churchyard, Cross and Bede House: the shaft of the cross is of Blisworth Limestone; Bede House is Northamptonshire 'polychrome' style with alternate courses of Northampton Sand and Blisworth Limestone [SP 961 685] (Photograph T P Cullen; (P535131)).

(Rear cover)

(Geological succession) Summary of the geological succession in the Wellingborough district.

Notes

The area covered by geological sheet 186 Wellingborough is referred to as 'the district'. National Grid references are given in square brackets [SP 3954 7264]. Boreholes are identified by the BGS Registration Number in the form (SP57SW/9), where the prefix indicates the 1:10 000 scale National Grid sheet. The registration number of BGS photographs is shown in the plate captions.

Acknowledgements

This sheet explanation was compiled by A J M Barron, A N Morigi and H J Reeves. N J P Smith contributed to the account of the concealed geology and the structure, I P Wilkinson and B M Cox to the Jurassic, M A Lewis to the hydrogeology and C Herbert to the Quaternary. C D Royles provided geophysics figures. The provision of mineral workings information by Northamptonshire County Council is acknowledged. The manuscript was edited by A A Jackson: cartography by R J Demaine and page-setting by C L Chetwyn.

Geology of the Wellingborough district (summary from rear cover)

An explanation of sheet 186 1:50 000 series map (England and Wales)

(Rear cover)

This sheet explanation describes the bedrock and superficial deposits of the Wellingborough district that lies in the English Midlands. Jurassic strata underlie the entire district but older rocks have been proved at depth and include Palaeozoic and Triassic strata. The lowest part of the Jurassic is also concealed so that the Whitby Mudstone Formation, the upper part of the Lias Group, is the oldest bedrock that crops out at the surface.

Middle Jurassic strata underlie most of the district, and the lower part forms a complex sequence of shallow marine and estuarine sediments with some coastal swamp and lacustrine deposits, the Inferior Oolite and Great Oolite groups. The southern limit of the iron-rich Northampton Sand Formation lies across the district, and the overlying strata includes the Blisworth Limestone Formation. The upper part, the Ancholme Group is of Mid to Late Jurassic age; it was deposited in deeper water and includes organic-rich bituminous mudstone.

Superficial deposits cover the higher ground and the valley floors concealing much of the bedrock, which crops out mainly on the valley sides. Gravelly sand of the Milton Formation predates the glacial till and was probably deposited by a south-easterly flowing river, the proto-Soar. The Bozeat and Oadby tills may represent two periods of ice advance and are associated with glacial outwash deposits.

Ironstone of the Northampton Sand Formation has been worked extensively here, and a brief description of this and other mineral resources is given, together with a summary of the engineering ground conditions that may be encountered within the bedrock and superficial deposits. Cambering of the valley sides and valley bulging occur, initiated in periglacial conditions that prevailed during the Pleistocene.

Chapter 1 Introduction

This sheet explanation provides a summary of the geology and applied geology of the district covered by the geological 1:50 000 series sheet 186 Wellingborough, published as a bedrock and superficial deposits edition in 2006. The district lies in the south-east of the English Midlands, and includes parts of Northamptonshire, Bedfordshire and Cambridgeshire.

The topography of the district is a plateau dissected by the valleys of the rivers Nene and Great Ouse and their tributaries, which are floored by late Quaternary deposits. Pleistocene superficial deposits (mostly of glacial origin) form the higher ground, and the underlying bedrock formations outcrop along the valley sides. Larger towns include Wellingborough, Burton Latimer, Rushden, Raunds and Higham Ferrers, which have expanded since World War II, initially as a result of steelmaking — the major industry up until the 1960s — but more recently through diversification and improved transport links. The district includes many pleasant villages, and arable farming is the main land use.

Ironstone and sand and gravel extraction have had a major impact on the district, leaving behind extensive areas of former workings. Originally agricultural land, new uses include leisure, light industry and landfill, but some areas are only partially restored or are flooded, with implications for engineering ground conditions and the hydrogeological regime.

Geological history

The Lower Palaeozoic marine rocks of much of southern Britain were deformed and uplifted during the final phase of the Caledonian orogeny, the Acadian deformation phase. Subaerial conditions were established in the early Devonian. In the late Devonian, a marine transgression from the south extended across the district.

Throughout the Carboniferous, the district formed part of the generally emergent Wales–Brabant High, and much of the Devonian succession may have been eroded. However, sea level rose at times in the early Carboniferous, the southern margin was submerged and marine carbonate was deposited across the south of the district. Subsequently, extensive uplift and erosion occurred, producing the widespread Variscan unconformity.

Much of Britain remained emergent through the Permian and early Triassic, and in the mid Triassic, a thick sequence of aeolian red-brown silicate mud (Mercia Mudstone Group) began to accumulate around the northern and western margins of the Anglo-Brabant Landmass (London Platform), which extended from eastern England into Belgium. The north-west-trending Orton Ridge (Figure 1) formed an extension of this 'high', and a marginal facies of the Mercia Mudstone Group was deposited around it. The Orton Ridge continued to influence deposition during the late Triassic trangression so that the Penarth Group thins across it and may be absent in the south. Subsequently, a regression resulted in the erosion of the uppermost Penarth Group strata in this district.

During the latest Triassic (Rhaetian), interbedded silicate mud and lime mud (Blue Lias Formation) was deposited in warm, shallow shelf seas lying north-west of the Wellingborough district (Figure 3). In the early Jurassic, the sea gradually transgressed the region, but marine deposition probably did not recommence in the Wellingborough district until the early Sinemurian, when a mud-dominated sequence (Charmouth Mudstone Formation) was deposited across the East Midlands Shelf, which stretched northwards from the Anglo-Brabant Landmass towards the Humber. In Pliensbachian times, a regression initially generated terrigenous mud and sand (Dyrham Formation), and a shallow marine environment was established, followed by postdepositional reducing conditions, resulting in ferruginous limestone and mudstone (Marlstone Rock Formation). A major sea-level rise re-established marine mud deposition (Whitby Mudstone Formation) across England in the early Toarcian, but later widespread uplift or regression resulted in erosion of much of this in eastern England.

In the early Mid Jurassic (Aalenian), a narrow seaway was established separating the low-lying land of the Anglo-Brabant Landmass to the south-east from the Pennine and Welsh landmass to the north-west. Iron, released by intense weathering on land was incorporated in ooids and carbonate cement in this shallow sea, forming the ironstone lithologies of the Northampton Sand Formation. The district lies at the south-eastern limit of its preservation, and possibly also at the original depositional limit. The complex sequence shows evidence of temporary emergence and erosion; lateral and vertical changes indicate areas of protected sedimentation where mud was deposited, areas that were rich in shelly faunas and where there was a periodic input of sand.

With further regression in the late Aalenian, the shoreline of the Anglo-Brabant Landmass advanced across the district; sand, silt and mud, preserved as the Grantham Formation farther north, may have been deposited here in a paralic environment. A global sea-level rise in the early Bajocian re-established the shallow open-marine conditions of the East Midlands Shelf in this district, which lay close to the shore. The Lincolnshire Limestone accumulated to the north, but any local correlatives of the Grantham or Lincolnshire Limestone formations were probably completely eroded following an episode of uplift in central England and the North Sea area in the mid to late Bajocian. Even the ironstones of the Northampton Sand may have been locally eroded during this event.

The Bathonian was characterised by widespread regression, and although the facies developed in much of Britain and this region indicate that crustal subsidence matched or exceeded sea-level fall, there is evidence locally of thinning and nondeposition in much of the sequence in the Wellingborough district that betrays the influence of the stable margin of the Anglo-Brabant Landmass. On the East Midlands Shelf, sand and mud of the Stamford Member were deposited in coastal swamps and lakes that developed during the early Bathonian (possibly in latest Bajocian). Above this, at least six cycles are recognised in the Rutland Formation of the region, consisting of marine transgression and deltaic progradation leading into salt-marsh conditions, followed by renewed inundation.

A marine transgression in the mid-Bathonian established a shallow lagoon, which may have extended farther south-east across the Anglo-Brabant Landmass than any of the previous Mid Jurassic incursions. Prolonged carbonate deposition produced the Blisworth Limestone Formation. Brief emergence was succeeded by deposition in a protected basin or lagoon with fine-grained terrigenous sediment from rivers (Blisworth Clay Formation).

A global sea-level rise near the end of the Bathonian submerged southern England and re-established shallow carbonate deposition (Cornbrash Formation), although a non-sequence within the Cornbrash indicates a relative sea-level fall at the Bathonian–Callovian boundary (Figure 3). The water deepened and extended farther south-east, and marine mud (Kellaways Clay Member) was deposited, followed by an influx of fine sand and silt (Kellaways Sand Member) possibly reworked from coastal or alluvial sources. As the marine basin became enlarged, the bottom water was less disturbed by currents, and organic-rich mud was deposited over a prolonged period (Oxford Clay Formation, Peterborough Member). However, in later Callovian and Oxfordian times, circulation had improved and more calcareous, less bituminous mud was deposited (Stewartby and Weymouth members).

Marine deposition continued through the late Jurassic and Cretaceous on the East Midlands Shelf. In the late Cretaceous to early Palaeogene, the Mesozoic strata were uplifted, tilted and gently deformed by a major orogenesis that affected the entire area of Britain. A period of deep erosion was initiated, which in central England continued into the Quaternary, removing the late Jurassic and younger beds from the Wellingborough district.

The bedrock surface underlying the older Quaternary deposits is essentially planar and dips gently east-south-east. Braided rivers with headwaters in the West Midlands, flowed east-south-east and dissected the bedrock surface, depositing gravelly sand (Milton Formation). Deposition of this sand probably occurred mainly during the Early Pleistocene (but possibly as early as the Pliocene), because by the end of that time there is evidence that the north-eastward-flowing Proto-Soar had captured the headwaters of these rivers (Belshaw et al. 2005). Cooling of the climate during the Early Pleistocene culminated in the onset of full glacial conditions some 430 000 years ago. There is evidence for possibly two Quaternary ice advances into the district: the first deposited a Triassic- and Jurassic-rich till of uncertain affinity (Bozeat Till), and the second a chalk clast-rich till (Oadby Till). Emplacement of the tills was accompanied by deposition of glacial outwash (Glaciofluvial deposits). Superficial structures (cambering and valley bulging), which affect mainly the solid strata, are thought to have formed largely under the periglacial conditions of the Pleistocene.

After the retreat of the Oadby Till ice-sheet, episodes of down-cutting and aggradation during successive cold and temperate stages resulted in the formation of river terrace deposits (Nene Valley and Ouse Valley formations) along the river valleys. Late Pleistocene periglacial processes have affected some of the earlier deposits resulting in the development of head.

Following postglacial sea-level rise during the Holocene, the fine-grained alluvium of the modern river floodplains was deposited.

Chapter 2 Geological description

Concealed strata

Only the Lower Jurassic Whitby Mudstone Formation and younger rocks outcrop in the Wellingborough district; older strata are known from deep boreholes within or adjacent to the district (Figure 1).

Two major unconformities are present at depth within the district, separating three major tectono-stratigraphical units. The Variscan unconformity separates Triassic and younger strata above from Devonian and Lower Carboniferous strata beneath it (Figure 1) and the Acadian unconformity (late Caledonian) separates these rocks from Precambrian, Cambrian and Tremadoc strata below.

Contours on the Acadian and Variscan unconformities reveal the presence of a north-west-trending (Charnian) ridge, the Orton Ridge, which has had a prolonged influence on deposition and preservation of strata. On the ridge, late Precambrian (Neoproterozoic) felsic ash-flow tuffs (Pe) have been proved beneath Triassic rocks in boreholes at Orton (SP77NE/170), Great Oxendon (SP78SW/1) and Hollowell (SP67SE/24), to the west of the district. At the southern end of the Orton Ridge, an intrusion of unknown age is postulated to account for a gravity low (A) that extends into the north-west of the district (Figure 2a). This intrusion is possibly granite and underlies an estimated 200 km2; it is thought that the top of the intrusion does not reach the unconformity. Magnetic rocks, possibly up-faulted along the southern margin of this intrusion (B on (Figure 2b)), have not been proved in boreholes.

Cambrian to Ordovician (C–O) strata unconformably overlie the Precambrian rocks, surrounding their subcrop, but have not been proved in boreholes in the district. By comparison with the Kettering district (Herbert et al. 2005), they are likely to consist of deformed siltstone and sandstone.

Probable Upper Devonian (ORS) strata unconformably overlie Precambrian and early Palaeozoic rocks. Reddish and greenish grey sandy mudstone (Gas Council GH13 Borehole; (TL07NW/34)) are thought to be of Devonian age by analogy with similar marine fossil-bearing strata in the Wyboston Borehole to the south-east (Sheet 204 Biggleswade; Moorlock et al., 2003).

Over 4 m of Lower Carboniferous Dinantian (L Carb) strata was proved beneath the Penarth Group just above terminal depth in the Gas Council GNe No.1 Borehole (SP97SE/66). These strata comprise very hard, massive dolostone with thin greenish grey mudstone beds; they probably extend westwards to Northampton and possibly eastwards to Cambridge, but die out in the north-east of the Wellingborough district (Figure 1).

Pre-Penarth Group Triassic strata are probably present only in the north-east and extreme south-west of the district. The strata attain 24 m in thickness in the north, and comprise reddish brown and greenish grey mudstone and sandstone, possibly representing a marginal facies of the Mercia Mudstone Group (MMG). The Penarth Group (PnG) is up to 9 m thick in the Wellingborough district, and comprises sandy and pebbly limestone, sandstone and conglomerate.

The Lower Jurassic Lias Group is largely concealed beneath younger strata. The group thins from about 250 m in the north-west of the district to about 150 m in the centre to perhaps as little as 75 m in the south-east. This reflects the thinning of the overall sequence and progressive overlap of the component formations across the East Midlands Shelf and London Platform.

The Blue Lias Formation, present in part of the Kettering district to the north, is thought to be absent from this district, having been overlapped south-eastwards by the Charmouth Mudstone (Figure 3). The Charmouth Mudstone Formation (ChM) comprises bluish grey mudstone with some thin beds of limestone, possibly both as beds and concretions. It is probably about 150 m thick in the north-west, and deep boreholes in the centre of the district show that it thins to around 130 m, and it may be about 50 m in the south-east. The fauna includes bivalves, brachiopods and ammonites.

There is an upward transition into the overlying Dyrham Formation (DyS), which comprises grey micaceous mudstone and sandy mudstone, and is of Pliensbachian age. Impersistent beds and nodules of limestone and ironstone may be present. The estimated thickness of the formation in the district ranges from 30 m in the north to 3 to 5 m in the south-east.

The overlying Marlstone Rock Formation (MRB) ranges from 0.5 to 3.1 m in thickness where proved in boreholes. It may be absent locally. It comprises a condensed sequence of shell-fragmental, ferru-ooidal limestone with beds of ferruginous mudstone. It may subcrop beneath river terrace deposits in the Nene valley, south of Earls Barton [SP 855 620].

Exposed strata

Jurassic

Lias Group

The oldest formation at outcrop is the Lower Jurassic Whitby Mudstone Formation (WhM), the uppermost part of the Lias Group, which crops out in the valleys of the Nene and Ise, but is partly concealed beneath superficial deposits. It is dominated by bluish grey mudstone, and includes thin shelly limestones at the base. Thickness generally ranges from 35 to 56 m, but the formation thins to 20 m in the south-east. Boreholes near Wellingborough and south of Rushden indicate a local thickness of 65 m. Ammonites from the Whitby Mudstone in other boreholes indicate that the strata are no younger than the early to mid Toarcian Falciferum and Bifrons zones, which, together with data from outside the district, suggests that strata representing the upper three zones of the Lias Group are widely absent here (Figure 3).

Inferior Oolite Group

The outcrop of the Northampton Sand Formation (NS; formerly the Northampton Sand Ironstone) stretches from Lincoln to Chipping Norton in Oxfordshire: the Wellingborough district lies in the southern part of the area of its maximum development. The formation was formerly an important source of iron ore, supporting a major steel industry (see pp.23–24) around Wellingborough and Irthlingborough.

Hollingworth and Taylor (1951) and Taylor (1949) provide details of the lithological succession, stratigraphy, structure and petrology. They divided the succession into five lithological subdivisions, all of which are ferruginous. However, it is probable that only the two lower divisions occur in the Wellingborough district. The 'Lower Siderite Mudstone-Limestone Group' consists of sideritic limestone and mudstone, sandy towards the base, with sparse berthieroid ooids and a basal phosphatic nodule layer. Around 2 to 3 m thick, it corresponds with the 'bastard stone' normally left in the floor of ironstone quarries as a hard base. The overlying 'Main Oolitic Ironstone Group' consists of berthieroid ooid-ironstones with a cement that is predominantly of siderite. Shells are common in some beds, and there are local lenses of siderite mudstone. This is the principal 'workable stone' in the district, and ranges from 2 to 3 m thick, with a local maximum of about 4 m.

These thicknesses together give a general range for the Northampton Sand Formation of 5 to 5.5 m across much of the district, but it is absent throughout the south and east, and locally elsewhere (e.g. around Great Doddington [SP 88 65] and Orlingbury [SP 85 73]). In the extreme north-west it is 7 m thick, and may include thin representatives of the higher, typically more sandy divisions. In addition, it is possible that in places, in both boreholes and on maps, the arenaceous upper beds may have been included in the overlying Stamford Member. Otherwise, the apparent absence of the upper subdivisions and the variation in thickness may be attributed in part to erosion prior to the deposition of the Rutland Formation, but the widespread absence of the Northampton Sand across the south and east of the district may reflect its original depositional limit. The formation is no longer well exposed in the district: up to 2.5 m of bedded dark reddish brown ironstone are visible in disused 'gullets' (ironstone quarries) at Finedon [SP 9108 7224] to [SP 9132 7227] and Woodford [SP 946 767]. At the latter site, well-developed boxstone weathering is visible (Plate 1). This develops by alteration or leaching of iron from the relatively insoluble primary siderite and berthieroid minerals forming more soluble iron oxides. Once in solution in the formation water, these become mobile and may be re-precipitated in layers, possibly under alternate saturation and drying conditions, forming concentric iron-rich and iron-poor zones within the rock. This may be well seen in much of the stone used in masonry (Plate 6).

Bivalves are common in parts of the formation, and pectinids have been recorded from the west of the district. In addition, the brachiopod Rhynchonelloidea aff. subangulata and terebratulids have been reported. However, no biostratigraphically useful species have been recorded.

The Wellingborough district lies to the south-east of the area of preservation of the Grantham and Lincolnshire Limestone formations (Herbert et al. 2005), and thus there is a major stratigraphical break on top of the Northampton Sand Formation (Figure 3).

Great Oolite Group

The Great Oolite Group rests unconformably on the eroded surface of the Inferior Oolite or Whitby Mudstone, and was deposited after a period of uplift and erosion. It comprises, in ascending order, the Rutland, Blisworth Limestone, Blisworth Clay and Cornbrash formations (Figure 3). Local thinning in all these formations and their components in the centre of the district may be related to periodic uplift in the Bathonian on the Orton Ridge, but is more likely due to attenuation onto the London Platform.

The outcrop of the Rutland Formation (Rld; formerly the Upper Estuarine Series) extends from near Oxford, through the East Midlands to the Market Weighton High in east Yorkshire. In this district, the formation varies from 0 to 15 m thick; in the north and north-west it is generally over 10 m thick. It is fully exposed at Cranford St John [SP 923 764] (Plate 2), where it is over 8 m thick, and here at the top it passes locally into limestone facies similar to the Blisworth Limestone (Cox et al., 2002, pp. 260–263).

Bradshaw (1978) described the Rutland Formation succession of eastern England in terms of seven named rhythms of sedimentary rocks, but not all are present in this district. The lowest of these, distinguished as the Stamford Member (St), comprises non-marine pale to dark grey and black sandy mudstone with a basal nodular ironstone bed, overlain by white and pale grey fine- to medium-grained sandstone, totalling up to 6 m. Fossil plant fragments and root traces are common. The Stamford Member is absent in much of the south and locally in the centre of the district. As the Grantham Formation (formerly the Lower Estuarine Series), of Aalenian age, is now thought to be absent in the district, strata and mapped outcrops formerly attributed to it here are now designated as Stamford Member.

The sedimentary rhythms comprising the Rutland Formation above the Stamford Member consist typically of a lower marine to brackish shelly mudstone and sandstone unit with trace fossils such as Rhizocorallium and Planolites, passing up into barren mudstone deposited in delta-top channels, and overlain by greenish grey mudstone with plant debris and rootlets; this last unit is interpreted as a saltmarsh deposit. However, much variation is seen, which suggests that the rhythmic depositional pattern was interrupted by transient marine incursions and periods of erosion (see below). Within one rhythm, a sequence of marine fossil-bearing strata is widely developed; it comprises varying proportions of interbedded calcareous mudstone, sandstone and bioclastic, sandy and ooidal limestone. This is the Wellingborough Limestone Member (W), and is distinguished throughout the western part of the district (1:50 000 sheet 186 Wellingborough). It is 2.4 m thick in the west, dying out eastwards, probably at its original depositional limit. The type section of the member (Cox et al, 2002, pp.258–260) is at Finedon Gullet [SP 926 699], where the uppermost 4 m of the Rutland Formation are exposed.

The Rutland Formation is or was formerly well exposed (Torrens, 1967) (Cox et al., 2002, fig. 4.14, pp. 255–263) at two other locations in the district: Irchester Old Lodge Pit [SP 914 647], and Cranford St John [SP 923 764] (Plate 2). The three rhythms immediately overlying the Stamford Member are absent. At two of these sites the uppermost (seventh) rhythm is also missing. Elsewhere, the Wellingborough Limestone rests directly on the Stamford Member or on the Northampton Sand Formation. Therefore, it is likely that generally throughout the district, no more than three or four of Bradshaw's rhythms are present, and commonly only one or two. This variability and the thinning and absence of the formation in much of the eastern third of the district may be due both to nondeposition and to intra-formational erosion, and accounts for the variation in thickness.

The macrofauna of the Rutland Formation is dominated by bivalves, including forms present in the 'deltaic' beds that were tolerant of low salinity. A more varied assemblage is found in the more marine beds, which includes brachiopods (Plate 3) such as Burmirhynchia sp. and ostracods. No age-diagnostic fossils have been recorded, but the formation is attributed a Bathonian age (Zigzag to Subcontractus or Morrisi zones) (Cox et al., 2002, fig. 4.21, p. 263), although the Stamford Member may extend down into the latest Bajocian (Parkinsoni Zone).

Soil developed on the Stamford Member is loamy; on the remainder of the Rutland Formation it is generally clayey and variably stony, and fossil oysters are common.

The Blisworth Limestone Formation (BwL; formerly the Great Oolite Limestone) was deposited in shallow marine lagoons following a transgression. A prolific benthonic fauna indicates deposition in protected conditions with some current activity, although compared with its approximate correlative, the White Limestone Formation of Oxfordshire, the higher clay and sand content suggests a greater terrigenous influence. The formation is typically 6 to 7 m thick, about 9 m in the north, and thins to 4 m in the east. It can be separated into two members — the Roade Member and the Irchester Member.

Limestone lithologies dominate, but lime-mudstone (marl) and mudstone beds occur, especially in the lower part. Bioclasts dominate the limestone allochems (largely angular and rounded shell fragments), with lesser amounts of coated grains (ooids and peloids) and quartz sand, set in lime mud or silt that is recrystallised in places, or in spar cement. There are three main rock types:

At the base of the Blisworth Limestone Formation, interbedded argillaceous, bioclastic packstone/wackestone and lime-mudstone contain a rich shelly fauna mainly of bivalves, and including the brachiopod Kallirhynchia sharpi Muir-Wood (Plate 3). Formerly the Sharpi Beds (Sylvester-Bradley and Ford, 1968; Cox et al., 2002), these strata are now formalised as the Roade Member (Cox and Sumbler, in press), which can be recognised at a number of localities in the district. At Cranford St John [SP 924 764], abundant Kallirhynchia sharpi is accompanied by bivalves including Anisocardia, Eocallista, Modiolus (Plate 3), Pholadomya, Praeexogyra and Rollierella. The Roade Member is placed within the Morrisi Zone on stratigraphical evidence (correlation with the Excavata Bed of the White Limestone Formation of Oxfordshire). Erosion at the base probably cuts out much of the Subcontractus and early Morrisi zones at Finedon Gullet and Cranford St John, where the member is 1.8 m thick (Cox et al., 2002, pp. 260–264).

The Irchester Member (Cox and Sumbler, in press) is dominated by shelly and shell-detrital limestones, which are ooidal or peloidal, with interbedded lime-mudstone beds. Within the member, an ooidal limestone bed yielding the brachiopod Digonella digonoides S S Buckman is inferred to represent the Digonoides Beds, recognised farther south. The bed commonly shows cross-bedding, indicating periodic higher energy conditions. Rootlet traces in the top bed of the member indicate emergence prior to renewed inundation (Cox et al., 2002, p.265).

The formation is moderately well exposed in the district: several metres of the lower beds are seen at the top of the face at Irchester Country Park [SP 915 657] (Sutherland and Hudson, 1982) and at Finedon Gullet [SP 926 699], and probably the entire thickness of the formation (9 m) is displayed at Cranford St John [SP 922 765 to 927 761] (Cox et al., 2002, pp. 260–264; (Plate 2)).

The Blisworth Clay Formation (BwC) comprises smooth plastic mudstone, characteristically variegated in colour, showing bluish grey, green, magenta and purple mottling, and root traces suggest deposition in a saltmarsh environment. Evidence of marine influence is lacking in this district, and common plant material and a basal ferruginous nodule bed demonstrate a terrigenous origin, with fluvial input of dissolved iron. The formation rests with a sharp conformable base on the Blisworth Limestone Formation. Thickness ranges between 3 and 5 m, thinning locally to less than 1 m in the centre of the district. It is not well exposed in the district, but up to 2 m of mudstone with the basal nodule bed may still be visible at Irchester Old Lodge Pit (Cox et al. 2002, p.256). The formation is largely barren of fossils — oysters seen in adjoining districts are not recorded here, and an Upper Bathonian age is inferred from its stratigraphical position. It develops a heavy, dark greyish brown soil, and colour mottling may be seen where the clay is deeply ploughed or dug.

The Cornbrash Formation (Cb) comprises thin, irregularly bedded bioclastic packstones and grainstones with rare superficial ooids and quartz sand. Thickness is generally between 1.5 and 2.7 m, thinning locally in the centre of the district to less than 1 m. The formation is very fossiliferous and includes brachiopods, bivalves, gastropods, echinoids and relatively common zonally indicative ammonites (Plate 4). It includes an important non-sequence between the Lower Cornbrash (Discus Zone) and Upper Cornbrash (Herveyi Zone), and thus the formation spans the Bathonian–Callovian boundary (Figure 3). When seen fresh, the limestones are usually grey, but in soil the rubbly or flaggy fragments weather pale brown due to disseminated iron in the matrix. The formation is probably no longer exposed in the district. About 0.8 m of shelly and sandy limestone were formerly exposed in a brick-pit at Bletsoe [TL 020 601], including fauna indicating the presence of both the Lower and Upper Cornbrash (Arkell, 1933 p.334).

Ancholme Group

The later Jurassic strata of the East Midlands Shelf are dominated by marine mudrocks of the Ancholme Group. Only the lower part is present in the Wellingborough district, comprising the Kellaways and Oxford Clay formations. The macrofauna of the formations is dominated by bivalves and ammonites; the latter are used for biostratigraphical zonation. However, weathering of the upper few metres below the soil usually destroys these, and generally only belemnites and the thick-shelled oyster Gryphaea are seen at the surface.

The Kellaways Formation (Kys) is between 5 and 7 m thick. It is subdivided into the Kellaways Clay and the Kellaways Sand members, a distinction recognised from Dorset to the Market Weighton Axis. The Kellaways Clay Member (KlC) comprises between 1.5 and 2.5 m of dark grey fissile mudstone. A lenticular sandy bed is developed locally at the base, near Little Addington [SP 953 743], but otherwise the silt and sand content increases upwards through the member. The Kellaways Sand Member (KlS) comprises 3 to 4.5 m of pale grey fine-grained sand and calcareous sandstone, interbedded with siltstone and mudstone. A lens of mudstone has been mapped near Denford [SP 996 763]. The formation is generally poorly fossiliferous, but at Riseley brick pit [TL 047 631], the uppermost part of the Kellaways Sand was exposed beneath the Oxford Clay, and here it is well cemented ('Kellaways Rock') and rich in bivalves, including Protocardia striatula, and Entolium corneolum, but these are of little biostratigraphical significance. Where the Kellaways Sand is best developed or cemented it may form a slight positive feature and broader outcrops. Soils on the Kellaways Clay are heavy and clayey; those on the Kellaways Sand are lighter and loamy.

The Oxford Clay Formation (OxC) is the youngest solid unit present and forms extensive outcrops in the eastern half of the district, but it is not exposed. Where complete it is divided into three: in ascending order, the Peterborough, Stewartby and Weymouth members (formerly the Lower, Middle and Upper Oxford Clay, respectively) (Cox et al., 1992). The Peterborough Member comprises dark brownish grey, fissile mudstone with a high organic (bitumen) content, which has fostered its use for brick making in the nearby Peterborough and Bedford districts. Aragonitic ammonites (Plate 4) and bivalves dominate its fauna, and are concentrated in shell beds. Layers of calcareous nodules or concretions are developed at a number of levels and beds of paler grey, blocky mudstone increase in abundance upwards. The Stewartby Member comprises pale to mid grey, calcareous, blocky mudstone. It is poorly fossiliferous, but the upper part may include thin, argillaceous, silty limestone beds with Gryphaea shells. The top of the uppermost of these, the Lamberti Limestone Bed, marks the top of the member, the boundary between the Callovian and Oxfordian stages, and hence that between the Middle and Upper Jurassic (Figure 3). The overlying Weymouth Member is similar in lithology to the Stewartby Member, but less silty and calcareous.

The Oxford Clay weathers to heavy greyish brown clay soils, generally lacking fossils. It has not been subdivided on sheet 186 Wellingborough owing to an extensive cover of superficial deposits. However, data from adjoining areas indicates that the Peterborough, Stewartby and Weymouth members are about 19 m, 25 m and 22 m thick, respectively, within the district, and the Oxford Clay is probably 50 or even 60 m thick in the east. Thus, the full thickness of the Peterborough and Stewartby members is present here, and probably also the lowest few metres (6 to perhaps 16 m) of the Weymouth Member.

Quaternary

Milton Formation

The Milton Formation underlies till in the south-west of the district, outcropping around Denton, Yardley Hastings and Bozeat (Plate 5), and in the north east around Bythorn. In each area the deposits are thought to have been contiguous with similar deposits disposed in narrow belts stretching away to the west-north-west (Figure 4). The deposits consist mainly of planar- and trough-bedded sand and pebbly sand up to 13 m thick with minor clay and silt lenses. Pebbles, consisting largely of Jurassic limestone and ironstone, are entirely of local derivation. In contrast, the sand fraction, which consists mainly of medium-grained quartz, is probably derived from Triassic sandstones, the nearest outcrop of which lies some 50 km to the north-west of the district. The sedimentary structures, provenance of the sand and the location of the deposits in west-north-west-trending channels points to deposition in braided rivers that drained catchments as far away as the West Midlands (Belshaw et al., 2005).

The existence of these distinctive deposits to the south of Northampton and north of the Wellingborough district (e.g. at Brigstock; Sheet 171) has long been known (see Belshaw et al., 2005). The fact that the Milton Formation underlies all unequivocal glacial deposits and contains only clasts of locally derived material indicates a preglacial age for the formation: Marine Isotope Stages (MIS) 13 to 16 according to Bowen (1999)(Figure 5). Belshaw et al. (2005) have suggested a Pliocene minimum age for the Milton Formation, arguing that the rivers depositing it must have been truncated by the Bytham River/Proto-Soar (Figure 4), thought likely to have become well established in the Early Pleistocene.

Sand and gravel of unknown age and origin

Deposits of clayey sand up to 2 m thick outcrop locally near Higham Ferrers where they underlie the Bozeat Till. It has not been possible to establish either their genesis or relationships to other superficial deposits in the district.

Glacial deposits

Two tills have been mapped in this district: an upper till that is correlated with the Oadby Till, and a distinctive lower till that is here named the Bozeat Till. The relationship of the two tills is uncertain, as no intervening organic deposits have yet been recorded. They may be separate tills representative of discrete glacial events or merely different facies of the Oadby Till.

Bozeat Till

The Bozeat Till is a dark bluish grey diamicton consisting of sandy, silty clay with clasts mainly of Jurassic limestone and ironstone, some quartz and quartzite, derived Jurassic fossils, rare flint and, very rarely, chalk. It is present only in the western part of the district where it is up to 5 m thick and always underlies the Oadby Till. Evidence for the exact nature of the relationship with the latter is sparse and inconclusive. The junction with the Oadby Till is always sharp but without any weathering of the surface of the Bozeat Till. In the adjacent Kettering district an exposure near Brigstock was reported by Taylor (1963) to show a till very similar to the Bozeat Till preserved in 'dip-and-fault gulls' in underlying gravels, overlain unconformably by the Oadby Till. This suggests a hiatus between the emplacement of the two tills.

Oadby Till

The Oadby Till is typical of what was formerly known as the 'Chalky Boulder Clay' of central and eastern England. Bowen (1999) has assigned the chalky till of this region to the Lowestoft Till, but this and surveys of adjacent districts have shown it to be contiguous with the Oadby Till (Oadby Member of the Wolston Formation of Bowen, 1999). It is present throughout the district as a dissected plateau and is commonly up to 15 m thick, but locally up to 30 m. It is an olive-grey to grey diamicton that weathers to yellowish brown, and comprises silty clay with abundant clasts of chalk, flint, Jurassic limestone, sandstone and ironstone, quartz, quartzite, and Carboniferous limestone and sandstone. Other more exotic clasts have also been recorded including dolerite, tuff, schist, gneiss and granulites. The silt and clay content of the till is derived almost wholly from Jurassic mudstone formations. Traditionally, the Oadby Till has been referred to the Anglian Stage, MIS 12 (Figure 5) as in Bowen (1999), but other workers (e.g. Sumbler, 1995) have suggested a younger MIS 10 age. It may also be significant that in this district no deposits older than MIS 9 are known to overlie the till.

Glaciolacustrine deposits

Boreholes along the Nene valley in the adjacent Northampton district encountered thick deposits of glaciolacustrine laminated clay and silt (locally exceeding 20 m) interleaved with thin tongues of diamicts containing chalk and flint clasts and infilling a subglacial valley partially buried beneath more recent fluvial deposits (Horton, 1970). Similar deposits crop out where the Nene valley enters the Wellingborough district from the west [SP 824 626] but these are the only evidence for the eastward continuation of this buried valley as boreholes drilled downstream proved bedrock at shallow depths.

Glaciofluvial deposits

Glaciofluvial deposits comprise sand and gravel with clasts of Jurassic limestone, sandstone and ironstone, flint, quartz and quartzite and locally chalk. They occur as sparsely distributed discontinuous bodies resting on, within and beneath both tills. They are outwash deposits of the ice sheet that deposited the tills.

River terrace deposits

Sand and gravel deposits occurring along the valleys of the River Nene and its tributary the River Ise, and the River Great Ouse and its left bank tributaries the River Kym, River Till, and the Pertenhall Brook are regarded as river terrace deposits. These were mapped on the principle that they underlie flattish 'benches' or terraces in the valley sides that could be grouped on the basis of their elevation above the present-day floodplain, and it was assumed that the highest of these terraces was the oldest and the lowest the youngest. Unfortunately, this apparent simplicity belies the complexities revealed by detailed examination of the deposits of the Nene and Great Ouse. This has shown that deposition was almost certainly multiphase and probably multistage through a range of climatic conditions. This account adopts the lithostratigraphy and chronostratigraphy of Bowen (1999; (Figure 5)), but the reader should note the apparent differences in chronostratigraphy between the river terrace deposits of the Ouse and Nene — two adjacent rivers with, presumably, a similar history. Moreover, Bowen appears to assign some of the terrace deposits to temperate (marine isotope) stages contrary to the model of Bridgland (2000) in which terrace deposition occurs across at least three stages with the main aggradations in the cold stages.

The Nene Valley Formation encompasses three river terrace deposits of which only two have been mapped within this district. The correlation, largely notional, with the numbered terrace deposits is summarised in (Figure 5). The surface of the lowest and most extensive 'First Terrace', the Ecton Member, is about 2 m above the floodplain, but the deposits are continuous beneath the alluvium, which lies in a channel cut into them. The composition of the gravel fraction comprises about 32% flint, 28% quartzite, 38.5% local ironstone, sandstone and limestone and 1.5% chalk (Castleden, 1976). The maximum thickness varies between 4 and 4.5 m. Bones of horse, mammoth, musk ox, woolly rhinoceros, bison and reindeer have been found in the deposits, an assemblage indicative of a cold climate (Castleden, 1976). The Grendon Member ('Second Terrace') is much more restricted in extent. Its surface lies between 5 and 9 m above the floodplain. The member is up to 7.6 m thick and has a similar composition to the Ecton Member.

The Ouse Valley Formation also includes three terrace deposits and all are represented in the short stretch of the river within the district. The tributary valleys contain only the lowest (First) terrace. The surface of the lowest terrace, the Felmersham Member, is at about 2 m above the floodplain ('Second Terrace'). It comprises sand and gravel about 3 m thick with a composition similar to that of the Nene terrace deposits. A fossiliferous sandy silt infilling a channel cut into the Felmersham Member, the Radwell Member, contains plant macrofossils, mollusca, coleoptera and ostracoda, indicative of cool temperate conditions (Rogerson et al., 1992). The surface of the terrace underlain by the Stoke Goldington Member lies between 5 and 7 m above the floodplain ('Second Terrace'). The thickness of the member here is not known but elsewhere it is up to 8 m thick (Bowen, 1999), and the composition is similar to that of the Felmersham Member. The type locality, which lies outside this district, exhibits a similar complexity of deposition as that above. There the basal unit of the deposit comprises sand and gravel, of presumed pre-MIS 7 age, cut out by a channel filled with deposits referred to MIS 7. This is overlain by a further sand and gravel unit, which is assumed to be MIS 6 as the contents of a channel cutting into it have been assigned to MIS 5e (Green et al., 1996). This complex of deposits underlies one terrace surface. The highest terrace, the Biddenham Member, some 11 to 13 m above the floodplain, is underlain by sand and gravel with a similar composition to the younger terraces and is up to 4 m thick. At the type site (south of the district) fossiliferous clay layers within the deposits contain a temperate molluscan fauna including Belgrandia marginata (Bowen, 1999). Acheulian human artefacts have also been found (Harding et al., 1991).

Alluvium

The present-day floodplains of the rivers are underlain by Alluvium comprising silt and clay up to 4 m thick, with peat locally. In the main river valleys, the alluvium occupies channels cut into the lowest river terrace deposit. Bowen (1999) has classified the alluvium as a member of either the Nene Valley or Ouse Valley formations (Figure 5) but here the term alluvium has been retained.

Alluvial fan deposits

Spreads of stony, sandy clay up to 3 m thick have been recorded where small tributary valleys debouch into the main valleys, principally along the River Ise.

Head

Head has been mapped along small valleys in the south-east of the district. However, it may be presumed that Head occurs much more widely as thin deposits on the lower flanks of the valleys. Head is a diamicton and its composition reflects that of the upslope materials from which it has been derived by solifluction. Typically it consists of stony, sandy clay, and it may be up to 3 m in thickness.

Calcareous tufa

Deposits of pale calcareous silt were recorded in places. These are interpreted as calcareous tufa deposits, chemically precipitated from calcium-carbonate-saturated water percolating through the limestone formations or more rarely the chalk clast-rich till, and deposited at springs, commonly at the base of the Northampton Sand Formation, adjoining the alluvium. The deposits are probably less than 2 m thick and may be very soft and loose or partially cemented but are likely to be unstable if loaded.

Artificial ground

Artificial ground is exceptionally widespread in the district and is principally the legacy of large-scale extraction of ironstone from the Northampton Sand Formation, which is now discontinued, and sand and gravel largely from the Nene Valley and Ouse Valley formations. The old ironstone quarries, commonly infilled, provide the district's main sites for brownfield development, while some of the former sand and gravel workings form lakes that are now used for recreation and nature conservation.

Made ground is material that has been deposited by man on the pre-existing ground surface. It includes 'engineered fill' such as road and rail embankments, spoil heaps and landfill. Worked ground is indicated on the map where natural material has been removed by man and includes mineral workings (quarries and pits). Some areas of Worked round may have been partially backfilled. Road and rail cuttings are not shown on geological sheet 186. Infilled ground is worked ground that has been largely or completely backfilled with spoil or landfill to, or above, the level of the natural land surface, and may be restored to other uses (Plate 2). Some ill-defined areas of sand and gravel workings along the Nene and Great Ouse valleys, which may be restored, landscaped or flooded, are shown as disturbed Ground.

Structure

The major Acadian (Caledonian) and Variscan unconformities, caused by repeated phases of earth movements and erosion, lie at depth across the district. The depth of the Acadian unconformity increases from 200 m below OD in the north to over 500 m in the south (Figure 1), indicated by an extensive aeromagnetic low (C) in the southern half of the district (Figure 2b). Above it, thickening of Upper Palaeozoic strata to the south-east of Rushden and Raunds (Figure 1) demonstrates that the Orton Ridge does not join eastwards with a shallow concealed basement ridge north of Wyboston (Biggleswade district).

The structure of the Jurassic strata at outcrop in the district generally conforms to the regional gentle east-south-easterly dip of less than half a degree. In places this is modified by flexuring or the effects of faulting or superficial structures (see p.29). As a result, locally steeper dips may be observed or deduced. A number of minor faults cut the bedrock outcrops, and in the north and west an east–west or east-south-east trend is predominant; displacement is generally a few metres, but is locally up to 10 m and possibly 15 m at Raunds [SP 999 733]. At Riseley [TL 04 63], a block of Great Oolite Group strata has been faulted upwards possibly as much as 10 m, and tilted to dip south-east. It is bounded by two north-east–south-west faults. The cause of this structure is unknown, but it overlies the area where the Upper Palaeozoic is thickest (Figure 1).

Chapter 3 Applied geology

Hydrogeology

In the Wellingborough district, groundwater is encountered in both Jurassic bedrock formations and in the superficial deposits. However, nearly 99 per cent of all groundwater is abstracted from the river terrace deposits (Figure 6) and two large abstractions for mineral washing account for 97 per cent of this. No sources in the district are currently utilised for public supply, and as long ago as 1909 Woodward and Thompson (1909, p.17) deemed the terrace deposits polluted along the Nene valley. The hydrogeology of the Jurassic aquifers is described by Allen et al. (1997) and Jones et al. (2000). The limestone-, ironstone- or sandstone-dominated units form a sequence of thin aquifer horizons (each typically between 3 and 9 m thick) separated by mudstone-dominated formations with similar thickness ranges, which act as aquitards. The water-bearing units are the Dyrham, Marlstone Rock, Northampton Sand, Rutland (Stamford and Wellingborough members), Blisworth Limestone, Cornbrash and Kellaways (Kellaways Sand Member) formations. Apart from the Dyrham and Marlstone Rock formations, which do not crop out, all of these permeable units may feed spring lines. The hydraulic relationships between aquifers may be complicated by the local thinning or absence of an aquitard or by faults with small displacements (less than 5 m) which can juxtapose them.

The Dyrham and Marlstone Rock formations are present only at depth in the district. Water was obtained from them in the past but was often slightly brackish and there are few current abstractions. However, water from a well at Finedon [SP 9109 7006] was used for public supply up until the 1950s.

The ironstone-dominated Northampton Sand Formation was formerly widely utilised for water resources (Woodward and Thompson, 1909). Groundwater flow is via both matrix and fracture flow. The highest yields are obtained at relatively shallow depths (typically less than 10 m), where weathering and oxidation have enhanced the porosity of the formation. The natural water quality is generally good, but hard and ferruginous.

The overlying Stamford Member sandstones and sands are normally in hydraulic continuity with the Northampton Sand. However, locally the two are separated by a mudstone bed and form two discrete aquifers. The mudstone beds of the Rutland Formation act as aquitards, but the Wellingborough Limestone Member has yielded water that is hard but generally of good quality.

The limestone-dominated Blisworth Limestone and Cornbrash formations have low intergranular permeabilities and fracture flow predominates. Solution hollows occur on the outcrop of the Blisworth Limestone Formation close to the base of the till near Orlingbury [SP 854 724]: the aquifers are therefore particularly vulnerable to surface pollution. Where at high elevations they are generally unsaturated. Elsewhere, they are only capable of providing limited local supplies (Woodland, 1942). Boreholes at Chelveston [SP 99 67] struck water in the glaciofluvial gravels at the base of the superficial deposits, in the Blisworth Limestone and in the beds beneath. However, the main industrial supply came from the Blisworth Limestone. The water quality is generally good but hard. Few boreholes have been sunk in the east of the district where the overburden is thicker and salinity levels may be elevated.

The Kellaways Sand Member is characterised by very low hydraulic conductivities, partly due to the high fines content. The porosity is highly variable, depending on the degree of cementation. The water may be of poor quality.

Glaciofluvial sand and gravel deposits historically provided small supplies for villages and farms, but these are not generally still in use. The river terrace deposits along the River Kym and other streams in the east, and higher terrace gravels in the south in the Great Ouse valley at Sharnbrook formerly provided local supplies. However, the Ecton Member of the Nene Valley Formation is the most important groundwater resource, with large public supplies of hard water having been obtained in the past at Earls Barton, Denford, Ringstead, Wollaston and Irthlingborough.

Mineral resources

Ironstone

Within the district the most extensively exploited mineral resource (Figure 7) is the Northampton Sand Formation, with records of iron ore workings in the wider region going back to the 11th century. In more recent times, between the mid 19th century and the closure of the steelworks in 1962 and last quarries in the district in 1969, the formation was worked for ironstone to support the steel industry largely in the west and north, around Wellingborough, Irthlingborough and Burton Latimer. Both opencast and underground working was undertaken, accounts of which are given (up till the 1940s) by Hollingworth and Taylor (1951), and (until closure) by Tonks (1988–1992). In the early days the ore lying near the surface was worked by hand, but following progressive mechanisation from the late 19th century, huge areas were opencast by steam- or electric-powered shovels, capable of removing thick overburden, commonly of up to 20 m. The ore bed, some 2 to 4 m thick, was then drilled and blasted, and loaded into trains of wagons. The Northampton Sand was typically worked to within 1 to 2 m of the base of the formation, the lowest beds, the 'bastard stone' forming a hard base for site traffic. During operation the opencast pits were progressively backfilled with spoil and overburden generated by the mineral extraction process, which may consist of compacted sand, sandstone, mudstone, limestone, till (boulder clay), unusable ironstone and clinker derived from steel making. Mining of ironstone on a small scale was undertaken in the late 19th century at Cogenhoe [SP 841 603], but in the early 20th century took place on a much more extensive scale around Irthlingborough and Finedon with access gained by adits (tunnels). A considerable volume of unworked ironstone remains, largely under thick overburden, but as a result of technological and economic changes in the steel industry this is not regarded as a resource.

Building stone

Sutherland (2003) gives a thorough and well-illustrated description of the building stones of Northamptonshire. The ironstones and limestones of the Northampton Sand Formation have found favour as a local building stone, both on its own, in random arrangements (Plate 6) and in the characteristic Northamptonshire 'polychrome' style, where it is alternated in courses with a pale limestone, usually the Blisworth Limestone (Front cover). This may also be utilised on its own, both dressed and as rubblestone, and even in the shafts of churchyard crosses (Front cover). The Wellingborough Limestone was also employed locally for rough stonework. However, for cornerstones (Plate 6) and fine mouldings such as door and window frames, stone from the Lincolnshire Limestone Formation (often Weldon Stone) was brought in from the north.

Industrial materials

The pure white silica sandstone of the Stamford Member, which overlies the ironstone through much of the district, proved of value as a refractory (ganister) for furnace linings, and was worked at several (ironstone) quarries, including Earls Barton [SP 862 639] and Burton Latimer [SP 892 758]. The Blisworth Limestone Formation has been worked in the past for limestone for metallurgical flux around Irthlingborough and Finedon.

Sand and gravel

Sand and gravel has been extracted from river terrace and glaciofluvial deposits and from the Milton Formation, and active and disused pits are found throughout the district. The majority of workings are in the deposits of the Ecton Member, which are the most widely distributed, and have been exploited along the entire length of the Nene valley in the district, from Cogenhoe to Ringstead, usually from beneath an overburden of alluvium, although some reserves remain. Typically they comprise gravel and sandy gravel, with a mean fines content of about 7 per cent and gravel clasts dominated by ironstone, limestone and flint (Harrisson, 1983). Other river terrace deposits of the Nene are not extensive, and typically comprise very clayey sandy gravel. These and thin flint gravel river terrace deposits along the streams in the east of the district are unlikely to provide a workable resource. A newly opened pit at Bozeat [SP 898 606] extracts sand from the Milton Formation (Plate 5). There are also recently disused sand and gravel pits in the Felmersham and Stoke Goldington members in the Great Ouse valley south of Sharnbrook [TL 00 58]. Some of the former sand and gravel pits in the district now have leisure and wildlife conservation uses e.g. [SP 873 623] and [SP 886 636]. The main historical and current mineral resources of the district are summarised in (Figure 7).

Engineering ground conditions

Three of the most important ground conditions relevant to construction and development are the suitability of the ground to support structural foundations, the ease of excavation and the use in engineering works. These issues are summarised for the main engineering geological units in the district in (Figure 8). Important geohazards which may occur within the district include ground heave and subsidence, cambering and valley bulging, instability over old mine workings, slope stability and mass movement, gas emission related both to the landfill and the underlying geology (natural radon), and the flooding of low-lying ground.

Ground heave and subsidence

The Oxford Clay, Blisworth Clay and Whitby Mudstone formations are dominated by clay minerals that can attract and absorb water. These undergo significant volume changes in response to variations in moisture content. During the winter months, the clay absorbs large quantities of water, which is lost during dry periods, leading to extensive shrinkage and cracking. The alternating processes of expansion and contraction may cause structural damage to buildings and roads. Glacial till deposits may also contain clay minerals that are prone to shrinking and swelling.

Cambering and valley bulging

Cambering, the extension and lowering of near-surface strata, is an effect induced by gravity and occurs on valley sides where strong beds (such as limestone or ironstone) overlie weaker clay or mudstone. This is attributed (Parks, 1991) to a process that includes stress relief, caused by erosion of the valley floor, and frost induced heave, coupled with creep processes during periglacial conditions in the Pleistocene. As a result, the thickness of the underlying clay or mudstone beds is reduced, and the overlying strong, cap rock is lowered as a 'camber'. The lowered strata may then undergo brittle fracture forming blocks separated by vertical joints normal to the direction of movement, on which minor vertical displacements may take place ('dip-and-fault' structures, Horswill and Horton, 1976). As the blocks move laterally down slope any joints may open causing wide fissures ('gulls'), which may be empty but are likely to be filled with loose rock and soil. However, gulls may also result from the formation of cavities in limestone, formed by the enlargement of joints by dissolution. This may be evident from a regular pattern or orientation of gulls parallel to joint sets or not at right angles to the inferred direction of extension. Many may develop by a combination of these causes. Cambering is probably not an ongoing process here, but may merge into landslide movements downslope.

Instability related to mine workings

There are extensive underground workings in the ironstones of the Northampton Sand Formation, notably in the Irthlingborough to Finedon area. Although these lie at up to 25 m depth, they may present subsidence hazards. Their presence and condition should be investigated prior to major development.

Slope stability and mass movement

Very few landslides have been identified in this district. However, slopes over 10° that are cut into the Whitby Mudstone, Rutland, Blisworth Clay or Oxford Clay formations or the Kellaways Clay Member are prone to failure, particularly where adjoining permeable layers give rise to springs. Slopes exceeding 3° should also be considered as potentially unstable due to the effects of periglacial processes (Culshaw and Crummy, 1991). Head and till may also contain thinly interbedded sequences of sands and clays, which are prone to landslip due to the presence of springs and high confined pore pressures that lead to loss of strength.

Natural radon emissions

Radon is a natural radioactive gas produced by the radioactive decay of radium and uranium. Radon is found in small quantities in all rocks and soils, although the amount varies from place to place. Geology is the most important factor controlling the source and distribution of radon (Appleton and Ball, 1995) and relatively high levels of radon emissions are associated with particular types of bedrock and unconsolidated deposits. The Northampton Sand Formation has the highest radon potential in the Wellingborough district due to the presence of phosphate nodules, heavy minerals and metal oxides, which contain traces of radiogenic elements.

Radon that enters poorly ventilated enclosed spaces such as basements, certain buildings, mines and tunnels may reach high concentrations in some circumstances. Inhalation of the radioactive decay products of radon gas increases the chance of developing lung cancer.

Radon Affected Areas have been declared by the National Radiological Protection Board (NRPB) where it is estimated that the radon concentration exceeds the Action level (200 Bq m-3) in 1 per cent or more of homes (Green et al., 2002). About half of the Wellingborough sheet has been identified by the NRPB as Radon Affected.

Radon protective measures may need to be installed in new dwellings (and extensions to existing ones) in areas where it is estimated that the radon concentration exceeds the Action Level in 3 per cent or more of homes (BRE, 1999).

Information sources

Further geological information held by the British Geological Survey relevant to the Wellingborough district is listed below. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth.

Searches of indexes to some of the collections can be made on the Geoscience Index System in BGS libraries and on the BGS web site (www.bgs.ac.uk), which also includes access to part of the photograph collection and stratigraphical lexicon. A copy of the BGS catalogue of maps, books, data and services is available on request.

Maps

SP85NW SP87NE SP96SW TL05NE TL07SW
SP85NE SP87SW SP96SE TL06NW TL07SE
SP86NW SP87SE SP97NW TL06NE TL15NW
SP86NE SP95NW SP97NE TL06SW TL16SW
SP86SW SP95NE SP97SW TL06SE TL16NW
SP86SE SP96NW SP97SE TL07NW TL17SW
SP87NW SP96NE TL05NW TL07NE TL17NW*
*1:10 000 scale map unavailable due to incomplete 1:10 560 scale survey.

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 the Wellingborough district there are sites and logs for about 2100 boreholes, for which index information has been digitised. For further information contact: The Manager, National Geological Records Centre, BGS, Keyworth.

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 186 Wellingborough are held in the Lexicon database. This is available on the web site at www.bgs.ac.uk. Further information on the database can be obtained from the Lexicon Manager at BGS, Keyworth.

BGS photographs

Copies of photographs of the district are deposited for reference in the BGS Library, Keyworth.

Material collections

Enquiries concerning the collections listed below should be directed in the first instance to the Chief Curator, BGS Keyworth. Charges and conditions of access to the collections are available on request from BGS Keyworth.

Palaeontological collection

Macrofossils and micropalaeontological samples collected from the district are held at BGS Keyworth.

Petrological collections

Hand specimens and thin sections are held in England and Wales Sliced Rocks Collection at BGS Keyworth.

Bore core collection

The National Geosciences Records Centre, BGS Keyworth, holds samples and entire core from a small number of boreholes in the Wellingborough district.

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.

Earth science conservation sites

Information on the Sites of Special Scientific Interest and other conservation sites in the Wellingborough district is held by English Nature, Headquarters and Eastern Region, Northminster House, Peterborough, PE1 1UA. Tel: 01733 455000.

Addresses for data sources

BGS Hydrogeology enquiry service; wells, springs and water borehole records.

British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX0 8BB. Telephone 01491 838800. Fax 01491 692345.

References

British Geological Survey holds most of the references listed below, and copies may be obtained via the library service subject to copyright legislation (contact libuser@bgs.ac.uk for details). The library catalogue is available at: http://geolib.bgs.ac.uk

Allen, D J, Brewerton, L J, Coleby, L M, Gibbs, B R, Lewis, M A, MacDonald, A M, Wagstaff, S J, and Williams, A T. 1997. The physical properties of major aquifers in England and Wales. British Geological Survey Technical Report, WD/97/34. Environment Agency R&D Publication 8.

Appleton, J D, and Ball, T K. 1995. Radon and background radioactivity from natural sources: characteristics, extent and relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/2.

Arkell, W J. 1933. The Jurassic System in Great Britain. (Oxford: Clarendon Press.)

Bate, R H. 1967. The Bathonian Upper Estuarine Series of eastern England, Part 1 Ostracoda. Bulletin of the British Museum (Natural History), Geology, Vol. 14, 23–66.

Belshaw, R K, Hackney, G D, and Smith, K A. 2005. The evolution of the drainage pattern of the English Midlands from the late Tertiary to the Early Pleistocene: the significance of the Milton Formation. Quaternary Newsletter, Vol 105, 16–35.

Bowen, D Q. 1999. A revised correlation of Quaternary deposits in the British Isles. Geological Society of London Special Report, No. 23.

Bradshaw, M J. 1978. A facies analysis of the Bathonian of eastern England. Unpublished PhD thesis, Oxford.

BRE. 1999. Radon: guidance on protective measures for new dwellings. Building Research Establishment, BR211.

Bridgland, D R. 2000. River terrace systems in north-west Europe: an archive of environmental change, uplift and early human occupation. Quaternary Science Reviews, Vol. 19, 1293–1303.

Castleden, R. 1976. The floodplain gravels of the River Nene. Mercian Geologist, Vol. 6, 33–47.

Cox, B M, Hudson, J D, and Martill, D M. 1992. Lithostratigraphic nomenclature of the Oxford Clay (Jurassic). Proceedings of the Geologists'Association, Vol. 103, 343–345.

Cox, B M, and Sumbler, M G. in press. Bathonian–Callovian correlation chart. in A correlation of Jurassic rocks in the British Isles. Cope, J C W (editor). Special report of the Geological Society of London.

Cox, B M, Sumbler, M G, Wyatt, R J, and Page, K N. 2002. The Middle Jurassic stratigraphy of the East Midlands. 227–312 in British Middle Jurassic Stratigraphy. Geological Conservation Review Series. Cox, B M, and Sumbler, M G (editors). No. 26. (Peterborough: Joint Nature Conservation Committee/Chapman and Hall.)

Culshaw, M G, and Crummy, J A. 1991. SW Essex–M25 Corridor: Engineering geology. British Geological Survey Technical Report, WN 90/2.

Green, B M R, Miles, J C H, Bradley, E J, and Rees, D M. 2002. Radon atlas of England and Wales. National Radiological Protection Board, NRPB-W26.

Green, C P, Coope, G R, Currant, A P, Holyoak, D T, Ivanovitch, M, Robinson, J E, Rogerson, R J, and Young, R C. 1996. Pleistocene deposits at Stoke Goldington, in the valley of the Great Ouse, UK. Journal of Quaternary Science, Vol. 11, 59–87.

Harding, R, Keen, D H, Bridgland, D R, and Rogerson, R J. 1991. A Palaeolithic site rediscovered at Biddenham, Bedfordshire. Bedfordshire Archaeology, Vol. 19, 87–90.

Harrisson, A M. 1983. The sand and gravel resources of the country around Kettering and Wellingborough, Northamptonshire: description of 1:25 000 sheets SP97 and parts of SP87 and TL07; and SP86 and 96. Mineral Assessment Report of the Institute of Geological Sciences, No. 114.

Herbert, C, Barron, A J M, Reeves, H J, and Morigi, A N. 2005. Geology of the Kettering district. British Geological Survey Sheet Explanation, Sheet 171 (England and Wales).

Hollingworth, S E, and Taylor, J H. 1951. The Northamptonshire Sand Ironstone: stratigraphy, structure and reserves. The Mesozoic Ironstones of England. Memoir of the British Geological Survey.

Horswill, P, and Horton, A. 1976. Cambering and valley bulging in the Gwash valley at Empingham, Rutland. Philosophical Transactions of the Royal Society of London, Vol. A283, 427–462.

Horton, A. 1970. The drift sequence and subglacial topography in parts of the Ouse and Nene basin. Report of the Institute of Geological Sciences, No. 70/9.

Jones, H K, Morris, B L, Cheney, C S, Brewerton, L J, Merrin, P D, Lewis, M A, MacDonald, A M, Coleby, L M, Talbot, J C, McKenzie, A A, Bird, M J, Cunningham, J, and Robinson, V K. 2000. The physical properties of minor aquifers in England and Wales. British Geological Survey Technical Report, WD/00/4. Environment Agency R&D Publication 68.

Moorlock, B S P, Sumbler, M G, Woods, M A, and Boreham, S. 2003. Geology of the Biggleswade district — a brief explanation of the geological map. British Geological Survey Sheet Explanation, Sheet 204 (England and Wales).

Parks, C D. 1991. A review of the mechanisms of cambering and valley bulging. 373–380 in Quaternary Engineering Geology. Forster, A, Culshaw, M G, Cripps, J C, Little, J A, and Moon, C F (editors). Geological Society Engineering Geology Special Publication, No. 7.

Rogerson, R J, Keen, D H, Coope, G R, Robinson, E, Dickson, J, and Dickson, C A. 1992. The fauna, flora and palaeoenvironmental significance of deposits beneath the low terrace of the River Great Ouse at Radwell, Bedfordshire, England. Proceedings of the Geologists' Association, Vol. 103, 1–13.

Sumbler, M G. 1995. The terraces of the rivers Thame and Thames and their bearing on the chronology of glaciation in central and eastern England. Proceedings of the Geologists' Association, Vol. 106, 93–106.

Sutherland, D S. 2003. Northamptonshire Stone. (Wimborne, Dorset: The Dovecote Press.)

Sutherland, D S, and Hudson, J D. 1982. Irchester Country Park: the quarry face: a geological guide. (Northampton: Leisure and Libraries Department, Northamptonshire County Council.)

Sylvester-Bradley, P C, and Ford, T D (editors). 1968. The geology of the East Midlands. (Leicester: Leicester University Press.)

Taylor, J H. 1949. Petrology of the Northampton Sand Ironstone Formation. Memoir of the Geological Survey of Great Britain.

Taylor, J H. 1963. Geology of the country around Kettering, Corby and Oundle. Memoir of the Geological Survey of Great Britain, Sheet 171 (England and Wales).

Tonks, E S. 1988–1992. The ironstone quarries of the Midlands: history, operation and railways. Parts I to IX. (Cheltenham: Runpast Publishing.)

Torrens, H S. 1967. The Great Oolite Limestone of the Midlands. Transactions of the Leicester Literary and Philosophical Society, Vol. 61, 65–90.

Woodland, A W. 1942. Water supply from underground sources of the Oxford–Northampton district: Part 1 - General Discussion. Wartime Pamphlet of the Geological Survey of Great Britain, No.4.

Woodward, H B, and Thompson, B. 1909. The water supply of Bedfordshire and Northamptonshire. Memoir of the Geological Survey of Great Britain.

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

(Index map)

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.

(Index map)

Almost all BGS maps are available flat or folded and cased. 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. Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

Figures and plates

Figures

(Figure 1) Variscan subcrop map showing contours on the Variscan and Acadian unconformities below OD, and the location of deep boreholes.

(Figure 2a) Bouguer gravity anomalies shown as a colour shaded relief illuminated from the north. Contour interval 1 mGal (1 mGal = 1x10−5 m/s2).

(Figure 2b) Total field magnetic anomalies shown as a colour shaded relief illuminated from the north. Contour interval 10 nT.

(Figure 3) Main subdivisions of the Mesozoic succession of the district and adjacent area. Vertical ruling indicates non-sequences. Not to scale.

(Figure 4) Preglacial drainage of the district and adjoining areas (adapted from Belshaw et al. 2005, fig 2). Conjectural courses of the river that deposited the Milton Formation and its tributaries are shown in pink. Occurrence of possible Milton Formation sands in red hatching. Present-day drainage is in blue.

(Figure 5) Quaternary stratigraphy of the Wellingborough district (after (Bowen, 1999))

(Figure 6) Licensed groundwater abstraction in the district, for industrial and agricultural use (data supplied by Anglian Region, Environment Agency, December 2004).

(Figure 7) Mineral resources of the district.

(Figure 8) Engineering constraints on selected bedrock and superficial deposits.

Plates

(Plate 1) Boxstone weathering in the Northampton Sand Formation, Woodford gullet [SP 946 767]. Hammer is 0.3 m long (GS 1303).

(Plate 2) Blisworth Limestone and Rutland formations exposed at Cranford St John gullet [SP 922 765] to [SP 927 761], a partially backfilled and restored ironstone quarry (GS1306).

(Plate 3) Fossils of the Blisworth Limestone and Rutland formations; (i) Inoperna plicata from the Blisworth Limestone Formation of the Great Addington Borehole (depth 9.8 m) (BGS specimen SEH455). Maximum fossil dimension: 76 mm; (ii) Modiolus imbricatus from the Blisworth Limestone at Little Irchester (BGS specimen SEH474). Maximum dimension: 70 mm; (iii) and (iv) Kallirhynchia sharpi with encrusting serpulid from the Blisworth Limestone, Old Quarry, Finedon Hill Farm. (BGS specimen SEH509). Maximum dimension: 14 mm; (v) Camptonectes (Camptochlamys) obscura on a slab of Blisworth Limestone from the Great Addington Borehole (depth: 9.0–9.4 m). (BGS specimen SEH443) Maximum fossil dimension: 41 mm; (vi) Rhynchonellid from the Rutland Formation at Earls Barton (BGS specimen SEH458) Maximum fossil dimension: 18 mm.

(Plate 4) Fossils of the Oxford Clay and Cornbrash formations; (i) Kosmoceras (Gulielmites) jason from the Oxford Clay of Denford (BGS specimen 28312). Maximum dimension: 47 mm; (ii) Nucleolites orbicularis from the Cornbrash of Bozeat Hill (BGS specimen h388). Maximum fossil dimension: 31 mm; (iii) Obovothyris obovata from the Cornbrash of Bozeat Hill (BGS specimen h384). Dimension: height 22 mm; (iv) Macrocephalites fragment, labelled 'M (Kamptocephalites) aff subtumidus' from the Cornbrash of Bourne End brickyard (north of Sharnbrook) (BGS specimen JR1446). Maximum dimension: 74 mm; (v) Goniomya literata from the Cornbrash of Bozeat Hill (BGS specimen h389). Maximum dimension: 47 mm; (vi) Trigonia elongata from the Cornbrash of Bozeat Hill (BGS specimen h381). Maximum dimension: 55 mm.

(Plate 5) Milton Formation exposed at Bozeat sand pit showing well developed micro-faulting [SP 898 606] (GS1307).

(Plate 6) Woodford church, close-up of the masonry showing the Northampton Sand (orange, brown), Blisworth Limestone (flaky, pale grey) and Lincolnshire Limestone (pale cream) [SP 969 767] (GS1308).

(Front cover) Higham Ferrers churchyard, Cross and Bede House: the shaft of the cross is of Blisworth Limestone; Bede House is Northamptonshire 'polychrome' style with alternate courses of Northampton Sand and Blisworth Limestone [SP 961 685] (Photograph T P Cullen; (P535131)).

(Rear cover)

(Geological succession) Summary of the geological succession in the Wellingborough district.

Figures

(Figure 5) Quaternary stratigraphy of the Wellingborough district (after Bowen, 1999)

Marine Isotope Stage Nene Valley Formation Ouse Valley Formation
1 Nene Member (Alluvium) Ouse Member (Alluvium)
2
3 Ecton Member ('First Terrace')
4
5a-5d Radwell Member Felmersham Member ('First Terrace')
5e
6 Stoke Goldington Member ('Second Terrace')
7 Grendon Member ('Second Terrace')
8
9 Orton Longueville Member ('Third Terrace') Biddenham Member ('Third Terrace')
10
11
12 Wolston Formation
13 to ? Milton Formation

(Figure 6) Licensed groundwater abstraction in the district, for industrial and agricultural use

(Data supplied by Anglian Region, Environment Agency, December 2004)

Washing and dust suppression Irrigation Fanning and domestic Cooling Other Totals
Fluvial sands and gravels 4 140 000 (2) 91 597 (4) 31 573.8 (12) 4 263 170.8 (18)
Till 114 (1) 114 (1)
Jurassic 3765 (3) 3765 (3)
Great Oolite 5000 (1) 2123 (2) 17 359 (10) 24 482 (13)
Northampton Sands 29 913 (18) 29 913 (18)
TOTAL GROUNDWATER 4 145 000 (3) 93 720 (6) 82 724.8 (44) 4 321 444.8 (53)
SURFACE WATER 2 700 000 (2) 196 639.54 (18) 3 062 935 (2) 13 410 (2) 5 972 984.54 (24)
  • Quantities of water in m3/a
  • The number in brackets refers to the number of licences.

(Figure 7) Mineral resources of the district

Mineral resource Source Activity Principle use
Ironstone Northampton Sand Formation Formerly worked extensively in quarries and underground mines Steel-making, building stone
Sand and gravel River terrace deposits, glaciofluvial deposits, Milton Formation Working quarries and pits, many former workings Concrete aggregate building and asphalt sand
Limestone Blisworth Limestone Formation Formerly worked extensively in quarries Building stone, locally used as aggregate and agricultural lime
Clay Oxford Clay Formation No activity, may have been worked locally in the past Brickmaking

(Figure 8) Engineering constraints on selected bedrock and superficial deposits

Engineering Geological units Description/characteristics Engineering considerations
geological units Foundations Excavation Engineering fill Site investigation
Highly variable artificial deposits (includes landfill and non-engineered) Worked ground

Made ground

Infilled ground

 Highly variable composition, depth and geotechnical properties Highly variable. May be highly compressible. Hazardous waste may be present causing leachate and methane production Usually diggable Highly variable. Some material may be suitable Essential todetermine depth, extent, condition and type of fill and chemistryof groundwater. Care needs to be taken as presence of pollution and contaminated ground likely
Non-cohesive River terrace deposits

Glaciofluvial deposits

Milton Formation

 Medium dense to dense yellow grey (weathers reddish brown/ yellow) sand and gravel. May contain buried channels and lenses of clay, silt and peat. Variable particle grading and composition. Variable thickness Generally good. Thick deposits in buried channels may be significant in foundation design Diggable. Trench support may be required. May be water bearing Suitable as granular fill Important to identify the presence and dimension of buried channels and characteristic of infilling deposits. Geophysical methods may be advisable
Kellaways Sand Member Light grey/yellow, medium dense sand occasionally silty sand. Locally cemented. 3–4.5 m thick Generally good, but may have problems with artesian boiling Easily diggable unless cemented. Trench support will be required. Running sand a hazard. Difficulty dewatering Suitable for general fill, but has low remoulded strength Essential to determine depth and extent of strata and the groundwater conditions
Calcareous tufa White to light grey or brown loose silt. Locally poorly cemented. Variable thickness Poor foundation. Soft and highly compressible; risk of differential settlement Easily diggable. Trench support will be required. May be water bearing Unsuitable Essential to determine depth and extent of deposit and the groundwater conditions
Mixed (cohesive/ non-cohesive) Alluvium Soft mid brown to pale grey (weathers dark brown/yellow) clay/sandy clay, with sand, gravel and peat lenses. High moisture content and variable composition. Variable thickness Poor foundation. Soft highly compressible zones may be present; risk of differential settlement Diggable. Moderate stability, decreasing with increasing moisture content. Running sand possible below water table and in pockets at perched water tables. Risk of flooding Generally unsuitable Determine the presence, depth and extent of soft compressible zones and depth to sound strata
Head Soft to firm mid brown-grey (weathers orange) sandy stony clay. Variable thickness Poor foundation. Soft highly compressible zones may be present; risk of differential settlement. Shallow shear zones may also be present Diggable. Moderate stability, decreasing with increasing moisture content Generally unsuitable Determine the presence, depth and extent of deposit and depth to sound strata. Investigate whether any shear zones are present
Mixed (cohesive/ non-cohesive) Till Firm blue-grey to olive-grey (weathers mid brown) clay with sand and some gravel, cobbles of chalk and flint, rare boulders. Lenses of sand may be present. Variable thickness Generally good foundation, although sand lenses may cause differential settlement. Possibility of pre-existing slips can also cause strength reduction Easily diggable. Trench support may be required if sand lenses or pre- existing slips encountered. May encounter running sand conditions in sand lenses Generally suitable Determine the depth and extent of deposit, especially the frequency and extent of sand lenses. Investigate whether any pre-existing slips and shear planes are present
Uniform Clay Oxford Clay Formation Kellaways Clay Member Stiff dark greenish grey to dark grey (weathers dark brown) clay with some shell fragments and pyrite. Occasional fissuring. Kellaways Clay 1.5-2.5 m thick, Oxford Clay up to 60 m thick Generally good provided suitable design is adopted as strata liable to ground heave and subsidence (shrink swell). Possible strength variability due to fissuring Easily diggable. Difficulty dewatering. Stress relief fissuring is a problem in excavations Suitable for general fill (mixing or screening required) Essential to determine depth and extent of strata, along with the extent of fissuring as stress relief fissuring is a problem
Plastic Clay Blisworth Clay Formation Dark grey to blue grey (weathers dark reddish brown), stiff occasionally very stiff clay. Occasionally fissuring and gravel of mudstone. 0.7-5 m thick Generally fair foundation, although strata liable to ground heave and subsidence. Possibility of pre-existing slips can also cause a strength reduction Easily diggable. Trench support may be required depending on the presence of slips and shears. Difficulty dewatering Suitable for general fill (mixing or screening required) Essential to determine depth and extent of strata, especially the frequency and extent of any pre-existing slips and shear planes
Ironstone Northampton Sand Formation Moderately strong to strong grey-green (weathers dark reddish brown) interbedded ferruginous sandstone and ferru-ooidal limestone with some mudstone beds and bioclasts. 0-7m thick Generally good provided suitable design is adopted Difficult to dig. Blasting generally required Suitable for general fill (mixing or screening required) Important to identify the presence of cambering and locally weaker zones (gulls)
Massive Hard Limestone Blisworth Limestone Formation Moderately strong to strong mid grey to greenish grey (weathers pale yellow) limestone with some mudstone bands. Locally ooidal. 4-9 m thick Generally good provided suitable design is adopted. Bed thickness and presence of highly weathered zones or solution features needs to be assessed Dependent on discontinuity spacing and clay interbeds. Ripping or pneumatic tools or blasting generally required Suitable for high grade fill, if care taken in selection and extraction Important to identify the presence of locally highly weathered zones or solution features
Well-jointed shelly limestone Cornbrash Formation Strong dark blue-grey (weathers grey speckled pinkish white) coarse-grained bioclastic slightly ferruginous limestone. 0.9-2.7 m thick Generally good provided suitable design is adopted. Bed thickness and presence of highly weathered zones needs to be assessed Dependent on discontinuity spacing and clay interbeds. Ripping or pneumatic tools or blasting generally required Suitable for high grade fill, if cam taken in selection and extraction Important to identify the presence of locally highly weathered zones
Undivided mudstone, siltstone, sandstone Rutland Formation, including Stamford Member Weak to moderately strong interbedded greenish grey (weathers brown-grey) mudstone, siltstone and sandstone with sand and thin limestone beds. 0-15 m thick Generally fair foundation provided suitable design is adopted, although sand beds may cause differential settlement. Possibility of pre-existing slips can also cause a strength reduction Easily diggable where rocks are weathered. Trench support may be required if sand beds or pre-existing slips encountered. May encounter running sand conditions in sand beds Suitable for general fill under controlled compaction conditions (mixing or screening required) Need to determine depth and properties of weathered zone. In situ loading tests advisable to assess bearing strengths at selected sites
Mudstone Whitby Mudstone
Formation		 Weak to moderately weak medium dark grey (weathers dark brown) mudstone. 21-65 m thick Generally good provided suitable design is adopted, as strata liable to ground heave and subsidence (shrink-swell). Possible strength variability due to fissuring, pre-existing slips and weathering Easily diggable where rocks are weathered. Ripping or pneumatic tools may be required at depth. Trench support may be required if pre-existing slips encountered Suitable for general fill under controlled compaction conditions Important to determine the extent of fissuring

(Geological succession) Summary of the geological succession in the Wellingborough district.

QUATERNARY HOLOCENE Artificial deposits (infilled ground, worked ground and made ground etc)
Alluvial fan deposits Stony, sandy clay up to 3 m
Alluvium Mainly silt and clay, locally peat up to 4 m
Tufa Calcareous tufa up to 2 m
PLEISTOCENE TO HOLOCENE
Head Stony, sandy clay up to 3 m
NENE VALLEY FORMATION River terrace deposits: sand and gravel, with silt and clay locally 4 to 7.6 m
OUSE VALLEY FORMATION River terrace deposits: sand and gravel, with silt and clay locally 3 to 8 m
PLEISTOCENE Glaciolacustrine deposits Laminated clay and silt up to 10 m
Glacial deposits Mainly stony sandy clay (Bozeat Till and Oadby Till) with sand and gravel locally up to 30 m
MILTON FORMATION Gravelly sand up to 13 m
UPPER JURASSIC ANCHOLME GROUP OXFORD CLAY FORMATION Mudstone, pale grey (Weymouth Member) up to 6 m up to 50 m
MIDDLE JURASSIC Mudstone, pale to medium grey (Stewartby Member) up to 25 m
Mudstone, dark brownish fissile grey, (Peterborough Member) up to 19 m
KELLAWAYS FORMATION Sand and sandstone, fine, with siltstone and mudstone (Kellaways Sand Member) 3–4.5 m 5–7 m
Mudstone, dark grey, with sandy lenses (Kellaways Clay Member) 1.5–2.5 m
GREAT OOLITE GROUP CORNBRASH FORMATION Limestone, bioclastic and shelly 0.9–2.7 m
BLISWORTH CLAY FORMATION Mudstone, colour-mottled 0.7–5 m
BLISWORTH LIMESTONE FORMATION Limestone, bioclastic, ooidal, peloidal, and lime-mudstone 4–9 m
RUTLAND FORMATION Mudstone with subordinate limestone and sandstone, including: Mudstone, sandstone and limestone, shelly (Wellingborough Limestone Member) 0–2.4 m 0–15 m
Sandstone and sandy mudstone (Stamford Member) 0–6 m
INFERIOR OOLITE GROUP NORTHAMPTON SANDFORMATION Ironstone, ooidal, and ferruginous and sandy limestone and mudstone 0–7 m
LOWER JURASSIC LIAS GROUP WHITBY MUDSTONE FORMATION Mudstone, grey, with shelly limestone at base 20–65 m
MARLSTONE ROCK FORMATION Ferruginous limestone and mudstone 0–3 m
DYRHAM FORMATION Mudstone and sandy mudstone 3–30 m
CHARMOUTH MUDSTONE FORMATION Mudstone with minor tabular and nodular beds of limestone 50–150 m
TRIASSIC PENARTH GROUP Limestone, sandstone and conglomerate Up to 9 m
MERCIA MUDSTONE GROUP Mudstone and sandstone Up to 24 m
CARBONIFEROUS Dolostone and mudstone unknown
DEVONIAN OLD RED SANDSTONE SUPERGROUP Sandy mudstone unknown
CAMBRO- ORDOVICIAN Cleaved siltstone and sandstone unknown
PRECAMBRIAN Cleaved felsic tuff and granite intrusion unknown