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Geology of the Kettering district — a brief explanation of the geological map Sheet 171 Kettering

C Herbert, A J M Barron, H J Reeves and N J P Smith

Bibliographic reference: Herbert, C, Barron, A J M, Reeves, H J, and Smith, N J P. 2005. Geology of the Kettering district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 sheet 171 Kettering (England and Wales).

Keyworth, Nottingham: British Geological Survey, 2005.. © NERC 2005. 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.

Maps and diagrams in this book use topography based on Ordnance Survey mapping. © Crown copyright. All rights reserved. Licence Number: 100017897/2005.

(Front cover) Cottages in Rockingham built from Northampton Sand and Lincolnshire Limestone [SP 867 916]. (Photograph A J M Barron; GS1183)

(Rear cover)

(Geological succession) Geological succession at outcrop in the Kettering district.

Notes

The word 'district' refers to the area of sheet 171 Kettering. National Grid references are given in square brackets. Borehole records referred to in the text are prefixed by the code of the National Grid 25 km2 area on which the site falls, for example SP99SW, followed by the registration number in the BGS National Geosciences Records Centre.

Acknowledgements

C Herbert and A J M Barron wrote most of this geological account; N J P Smith provided details on the concealed geology and structure and H J Reeves provided information on the engineering geology. R J Demaine, P Lappage and G Tuggey produced the figures; C L Chetwyn was responsible for page setting; this sheet explanation was edited by A A Jackson.

The authors thank Northamptonshire County Council for information on mineral workings (ironstone, and sand and gravel).

Geology of the Kettering district.

An explanation of sheet 171 (England and Wales) 1:50 000 series map

(Rear cover)

This sheet explanation describes the geological 1:50 000 sheet 171 Kettering. The district lies at the northern end of an important development belt in the English Midlands, extending from Corby to Luton. The district includes the rapidly growing towns of Kettering and Corby, the smaller towns of Oundle and Thrapston, and many picturesque villages. The steel-making industry that was formerly centred on Corby and closed down in 1980 has had a big impact on the landscape, with large areas of opencast workings in the ironstone of the Northampton Sand Formation. There are also extensive sand and gravel workings in the river terrace deposits of the Nene valley; many are now flooded.

The concealed pre-Mesozoic basement rocks include Precambrian volcanic rocks in the south-west, Cambrian to Ordovician sandstone and siltstone with intrusions, and mudstone of probable Devonian age in the east. These strata are unconformably overlain, in upward succession, by the Triassic Mercia Mudstone and Penarth groups, and the Lower Jurassic Blue Lias Formation, all known only from boreholes.

The oldest rocks at surface belong to the Lower Jurassic Lias Group, which is dominated by mudstone and siltstone; the group crops out in the north-west and in valley bottoms elsewhere. The centre of the district is largely underlain by a gently dipping sequence of ironstone, sandstone, mudstone and limestone of the Middle Jurassic Inferior Oolite and Great Oolite groups, which is the source of a range of local building stones that contribute much to the character of the area. Mudstone of the Middle to Upper Jurassic Ancholme Group is the youngest solid rock of the district, and crops out chiefly in the east.

The Jurassic strata are partly concealed by Quaternary deposits, including a thick blanket of glacial till. In the north-west, the River Welland has a deep, broad and fairly straight valley, bounded to the south-east by a prominent escarpment. The valley carved by the River Nene is shallower and notably meandering, with more extensive postglacial fluvial deposits.

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 171 Kettering, published as a solid and drift edition in 2002.

The district lies mainly within Northamptonshire, with small areas in Leicestershire and Cambridgeshire. Topographically it forms a plateau dissected by the rivers Welland and Nene, with Quaternary (drift) deposits covering the higher ground and the underlying solid formations outcropping on the valley sides. The most conspicuous topographic feature is the Inferior Oolite escarpment, along the south-east side of the Welland Valley. The main population centres are Kettering and Corby, both of which underwent rapid expansion after the Second World War, coincident with the expansion of ironstone extraction and processing, which was the main industry in the area. Since the closure of the last ironstone quarry in the early 1980s, these areas have seen considerable light industrial redevelopment. The remainder of the district is predominantly rural with scattered villages: arable farming is the main land use.

The ironstone extraction and steel industry had a major impact on the character of the district, leaving extensive areas of former workings only partially restored. These have a range of new uses, including recreational areas, industrial estates and landfill. Ironstone working has had a profound and ongoing effect on the geology of the district, affecting the outcrop pattern and having implications for the hydrogeological regime and engineering ground conditions.

Geological history

Late Precambrian calc-alkaline volcanism generated a complex succession across Wales and central England, which was subsequently folded and cleaved. These rocks are thought to extend beneath at least part of the Kettering district. Throughout most of the early Palaeozoic in this region, thick sequences of marine clastic sediments accumulated. In the late Ordovician, these strata were intruded by basic sills and granodiorite plutons. Later, the Lower Palaeozoic rocks of much of southern Britain were deformed and uplifted. By the early Devonian, the region was land with a sea to the south, which gradually encroached on this district, and mud was deposited in a near-shore environment during the late Devonian.

The region formed part of a long-lived 'high' throughout the Carboniferous and Permian, and remained an area of net erosion during much of the Triassic, part of the Anglo-Brabant High (or London Platform). A thick sequence of red-brown aeolian silicate mud (Mercia Mudstone Group) was deposited around the margins of this landmass (East Midlands Shelf and Worcester Basin), extending into the Kettering district. In latest Triassic times, the area was flooded and marine mud and lime mud (Penarth Group) was deposited.

In the earliest Jurassic, silicate mud and lime mud (Blue Lias Formation) were deposited in a warm, shallow shelf sea that extended into the north-west of the district. Deeper water conditions were established (Charmouth Mudstone Formation), and extended right across the district. A lowering of sea level allowed silt and sand into the depositional basin (Dyrham Formation), but later anoxic (low oxygen) conditions were established, resulting in ferruginous mudstone and ooidal ironstone (Marlstone Rock Formation). A rise in sea level re-established marine mud deposition (Whitby Mudstone Formation), but later regression or uplift caused much of this to be eroded.

In the early Mid Jurassic, a narrow seaway formed between a low-lying landmass to the south-east, the London Platform, and the Wales–Pennine landmass to the north-west. Here, iron from subaerial weathering was incorporated in ooids and carbonate cement, forming ooidal ironstone (Northampton Sand Formation). Sand, silt and mud (Grantham Formation) were deposited on low-lying coastal land. Global sea-level rise in the early Bajocian re-established shallow open marine conditions on the East Midlands Shelf that lay to the north-west of the Anglo-Brabant Landmass. A protected carbonate shelf developed and deposition of shells, ooids and pellets dominated (Lincolnshire Limestone Formation). Major uplift caused the limestone surface to be eroded through the mid to late Bajocian and mud was deposited in hollows. In the south of the district, erosion reduced the preserved area of Lincolnshire Limestone (Figure 3), and beyond the limits of the limestone, the Grantham Formation was probably removed completely, and even the Northampton Sand may have been eroded locally.

Although the Bathonian is characterised by widespread regression, the facies developed in Britain indicate that subsidence kept pace with or exceeded sea-level fall. On the East Midlands Shelf, coastal swamps developed, and sand and silt was deposited (Stamford Member). There followed a number of marine transgressions and deltaic progradation. Six cycles have been identified; typically each cycle consists of marine mudstone, nonmarine mudstone and siltstone, overlain by mudstone with rootlets that indicates salt-marsh conditions (Rutland Formation).

Lagoonal conditions were established across the East Midlands Shelf, and calcareous grains and mud were deposited (Blisworth Limestone Formation). Rootlet traces in the topmost bed of this sequence indicate emergence in the late Bathonian. Clay deposition continued (Blisworth Clay Formation) in a basin or lagoon with some evidence of fluvial deposition. In the latest Bathonian, much of southern Britain was submerged during a global rise in sea level. Limestone of the Cornbrash Formation accumulated at this time, but a non-sequence separates the lower and upper Cornbrash, which indicates a relative fall in sea level at the Bathonian–Callovian boundary. As the water deepened and extended farther across the Anglo-Brabant Landmass, marine clay deposition (Kellaways Clay Member) was widespread, and was succeeded by a sheet of fine sand and silt (Kellaways Sand Member). Organic-rich mud (Peterborough Member) was deposited as the depositional basin deepened, but by late Callovian times the organic content of the mud was lower and the calcareous content was higher (Stewartby Member).

Through most of the late Jurassic and Cretaceous, the East Midlands Shelf remained an area of marine deposition — mudrock, chalk, sandstone and limestone. Uplift in the early Palaeogene initiated a period of erosion, which continued through the remainder of the Palaeogene and Neogene into the Quaternary, and the younger strata were removed from the Kettering district. The bedrock surface underlying the older Quaternary deposits is essentially planar, dipping gently east-south-east, but it is dissected in places by pre-Pleistocene channels, locally infilled with fluvial deposits. There is evidence in the district of two ice advances, the earlier depositing a till of uncertain affinity and the later a chalky flinty till. Glaciofluvial sand and gravel deposits are present, which may be associated with either till. Superficial structures, which mainly affect the bedrock, are thought to have formed largely under periglacial conditions.

Chapter 2 Geological description

Concealed geology

Only Lower Jurassic and younger rocks outcrop in the Kettering district, but older strata have been proved in a number of deep boreholes within or surrounding the district (Figure 1). These include Thorpe by Water Borehole, in the Stamford district to the north (sheet 157), and two series of Gas Council boreholes (prefixed GST and GH), which were drilled to find subsurface gas storage sites. A number of others in surrounding districts (e.g. Orton Borehole, sheet 170 Market Harborough District,) yield useful information. Part of a seismic reflection profile (Shell SE88-01) has been acquired within the district, with the rest of the survey covering an area to the north-east (sheet 158).

The strike of the Precambrian and Lower Palaeozoic rocks trends north-west (Charnian) and the rocks young north-eastwards across the district (Figure 1). Upper Palaeozoic strata are probably aligned north-east, if the borehole provings are representative of their distribution. Mesozoic strata thin south-eastwards onto the London Platform, also with a north-east regional strike.

Precambrian

Precambrian rocks proved in the Orton (dated at 616 ± 6 Ma) and Oxendon Hall (615 Ma; Noble et al., 1993) boreholes, in the adjacent Market Harborough district, probably extend into the south-west part of the district (Smith, 1985a, b; (Figure 1)). These rocks are felsic ash-flow tuffs, cleaved and dipping between 45º and 50º.

Cambrian–Ordovician

Interbedded cleaved and faulted siltstone-sandstone strata, containing small igneous intrusions, were encountered in the Thorpe by Water Borehole, just to the north-west of the district. Fossil fragments and bioturbation were recorded and a Rb/Sr date of 468 ± 36 Ma gives a minimum age of deposition (mid-Ordovician). Along strike, Gas Council Borehole GH10 [TL 0597 8740], encountered very hard grey-green and pink quartzite, which falls within the area considered to contain Ordovician rocks (Figure 1).

A prominent magnetic anomaly (Figure 7a) extends north-west, to Mountsorrel and beyond, and is concordant with the strike of the Lower Palaeozoic rocks. Boreholes on the anomaly (e.g. Rempstone and Kirby Lane boreholes) have proved granodiorite, similar to that of Mountsorrel, which is Ordovician in age. The shape of the anomaly (Figure 7a) has been used to indicate the limits of the intrusion on (Figure 1).

Devonian

No Lower to Middle Devonian rocks are present within the district. Probable Upper Devonian strata, comprising red and green sandy and silty mudstones, have been found in some of the Gas Council GH and GST-series boreholes, where they rest unconformably upon Lower Palaeozoic rocks. They trend north-east, and may occupy much of the subsurface south-east of the MH5 aeromagnetic anomaly, which terminates against the Oundle Fault (Figure 1). Borehole data and an interpretation of the Shell seismic profile suggest they also occur as a thin cover overlying Ordovician granodiorite north-west of the Oundle Fault. The nearest dated equivalent strata were cored in the Wyboston Borehole to the south-east.

Triassic

The cross-section on the map shows up to 17 m of the Sherwood Sandstone Group underlying the Mercia Mudstone Group, but its occurrence may differ from that indicated. A sequence of grey, green and red mudstone and argillaceous siltstone with thin porous sandstones, thickens from approximately 15 m in the south-east to about 50 m in the north-west of the district, and in the Thorpe by Water Borehole the basal bed comprises a breccia. These strata may be interpreted as a littoral facies of the Mercia Mudstone.

The Penarth Group thins from about 12 m in the north of the district to approximately 4 m in the south and comprises grey mudstone and limestone. Significantly, the Penarth Group is thinner in the Orton Borehole than in surrounding boreholes, because it was deposited on a basement ridge (Orton Ridge). Here it includes two conglomerates, the lower of which contains quartz porphyry clasts, indicating derivation from exposed basement.

Jurassic

Lias Group

The Lower Jurassic Lias Group is over 260 m thick in the north-west of the district, thinning to about 200 to 220 m in the centre and to perhaps as little as 150 m in the south-east. The Blue Lias Formation was proved in Thorpe by Water Borehole [SP 8857 9648] (Ambrose, 2001), some 100 m beyond the district to the north. Only the uppermost member, the Rugby Limestone Member, is present here. It comprises 32 m of interbedded mudstone and limestone, which extend into the north-west of the Kettering district, thinning out towards the south-east.

The oldest rocks seen at surface in the district crop out in the Welland valley, where the Charmouth Mudstone Formation is succeeded by the Dyrham and Marlstone Rock formations (Figure 2). The overlying Whitby Mudstone Formation crops out along the valleys of the Ise Brook, River Nene, and some of the tributaries, in places brought to the surface by valley bulging.

The Charmouth Mudstone Formation (ChM) typically comprises bluish grey mudstone, with some thin beds of tabular limestone and discontinuous layers of limestone concretions. The uppermost beds of the formation also contain scattered ironstone (sideritic mudstone) nodules. The total thickness of the formation in the Thorpe by Water Borehole is possibly as much as 178 m, but it thins to about 150 m in the centre of the district, and may be as little as 106 m in the south-east. The mudstone weathers to greyish brown clay, giving rise to very heavy soils. The fauna includes bivalves, brachiopods and ammonites, although only belemnites and the most robust varieties of bivalves (e.g. Gryphaea) are likely to be found in the soil. The formation ranges from Sinemurian (Semicostatum Zone) to Pliensbachian.

There is a gradual upwards transition into the coarser siliciclastic deposits of the overlying Dyrham Formation (DyS), which comprise pale to dark and greenish grey micaceous silty and sandy mudstone. The formation is of Pliensbachian age. These coarser deposits give rise to characteristic light micaceous soils, and are more permeable than the underlying clays, hence the base of the Dyrham Formation is often marked by a seepage line corresponding to the first occurrence of sandy beds. Impersistent beds or lenses of limestone and calcareous sandstone, as well as nodules of cementstone and ironstone are present, but fossils appear to be absent. The sedimentation pattern is cyclical, with limestones and sandstones tending to occur at the top of cycles. The formation thins south-east from possibly 23 m at the Thorpe by Water Borehole towards the East Midlands Shelf, where it is absent.

In this district, the overlying Marlstone Rock Formation (MRB) is a highly condensed sequence of shell-fragmental, ooidal ferruginous (berthierine and siderite) limestone with abundant detrital fine quartz sand and subordinate beds of sandy and shelly ferruginous mudstone. Bivalves dominate the fauna, with amaltheid ammonites indicating a Pliensbachian to Toarcian age. The formation reaches a maximum thickness of about 1 m and can be traced only locally near Bringhurst [SP 830 923]; [SP 842 915] and Gretton [SP 891 935]. Taylor (1963) concluded that along most of the Welland valley, the Dyrham Formation is directly overlain by the Whitby Mudstone Formation, although recent studies in the north-west indicate that the Marlstone Rock may be more extensively developed than shown on the map.

The Whitby Mudstone Formation (WhM) is dominated by bluish grey mudstone, with scattered pyritic and cementstone nodules. It ranges from 49 to 58 m, but thins to less than 40 m in the south-east. At the base is a variable unit, about 3 m thick, which includes the characteristic 'paper shales', bounded at both base and top by distinctive limestones. The basal limestone is discontinuous, and comprises highly fissile, smooth flattened nodules up to 15 cm thick. The upper limestone is shelly, rich in foraminifera, and has an irregular rubbly or nodular surface. These beds are correlated with the Fish Beds and Cephalopod Limestones members of Horton et al. (1974) and yield a rich fauna of early Toarcian age. Locally, for example at Gretton [SP 890 940], the nodular limestone is well developed, but the 'paper shales' and basal limestone are apparently absent (Taylor, 1963).

Thick mudstone beds overlie the basal unit, and give rise to a very heavy greyish brown soil. In the upper 7 m of the formation, Taylor (1963) noted nodules of clay ironstone. Beds immediately below the Northampton Sand Formation have yielded an ammonite fauna indicative of the lower to mid Toarcian Bifrons Zone, indicating that the upper three zones of the Lias are absent hereabouts (see below).

Inferior Oolite Group

The Inferior Oolite Group rests unconformably on the Lias Group, and marks a change in the depositional environment. It is divided into three formations, the Northampton Sand Formation, Grantham Formation and Lincolnshire Limestone Formation (Figure 2) and (Figure 3).

Northampton Sand Formation

The Northampton Sand Formation (NS; formerly the Northampton Sand Ironstone) crops out from Lincoln to Chipping Norton, Oxfordshire, and reaches its maximum development in this district, where it was formerly an important source of iron ore, and supported a major steel industry (see p.23) around Corby and Kettering. The stratigraphy, structure and petrology of the Northampton Sand was detailed by Taylor (1949) and Hollingworth and Taylor (1946, 1951), who subdivided it into five lithological units (numbered I to V). The lower two units (I and II) are well represented in the Kettering district, and show little variation in distribution and thickness across the western half of the district (Figure 4). Thin representatives of units III and V are found locally, and up to 0.9 m of siderite mudstone of the 'Upper Siderite Mudstone-Limestone Group' (IV) is present widely at the top of the formation.

  1. the 'Lower Siderite Mudstone-Limestone Group' — 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 (see p.23).
  2. Main Oolitic Ironstone Group — sideritic berthieroid ooid-ironstone with a predominantly siderite cement and local lenses of siderite mudstone. Shells are common in some beds. This is the principal 'workable stone' in the district, and ranges between 2 and 4 m thick.

Where the ironstone has been worked, the formation ranges in thickness from 5 to 8 m. Overall it thins eastwards, but the variation within the majority of the outcrop is not systematic and may depend both on original depositional thickness and on erosion before deposition of the Grantham Formation. It is absent east of a line approximately coinciding with the Nene valley (Figure 3), which may be close to the original depositional limit, although it may also have been locally eroded away hereabouts.

Despite widespread exploitation from opencast pits prior to 1980, the Northampton Sand Formation and the overlying strata are currently very poorly exposed in the district. It is currently visible along 500 m of the lower levels of a 'gullet' (ironstone quarry) at Twywell [SP 940 772] to [SP 944 775] (now a country park). The formation is highly fossiliferous, and bivalves and brachiopods are locally abundant. Ammonites are rare, but those found here and in neighbouring areas indicate the Scissum Zone of the Aalenian. At outcrop, the formation is typically deeply weathered to brown or reddish brown limonitic sandstone, and abundant debris occurs in the distinctive reddish brown soil. Redistribution of the ferric oxide during the weathering process gives rise to a characteristic 'boxstone' structure (Taylor, 1949; (Plate 1)).

Grantham Formation

The Grantham Formation (GrF; formerly Lower Estuarine Series) rests on the Northampton Sand Formation in the north-west of the district, but is inferred to overstep it towards the north-east where it rests directly on the Whitby Mudstone Formation. It is absent in the south (Cox and Sumbler, 2002; (Figure 3)), where it has been removed by erosion (together with the Lincolnshire Limestone) before the deposition of the Rutland Formation. The boundary with the Northampton Sand Formation is generally sharp and erosional, but it may appear transitional where both formations are ferruginous and sandy. Where the Grantham Formation rests on the Whitby Mudstone Formation, it may include a bed of reworked Northampton Sand pebbles at the base. The soil developed on this formation is generally stoneless, loamy and pale brown.

The Grantham Formation consists of a complex sequence of pale to dark grey fine silicate-sandstone and siltstone, sandy mudstone and mudstone containing common plant debris. It includes both marine and nonmarine facies and typically comprises a tripartite succession (Taylor, 1963, fig. 10 sections 1–14):

In general, the Grantham Formation ranges from 5 to 8 m thick in this district, but isopachytes show an east-north-east-trending belt, 2.5 to 5 km wide (Figure 3), where the formation is thin or absent, having been removed by channelling before the deposition of the Upper Lincolnshire Limestone. The upper boundary may be difficult to locate where the basal beds of the Lincolnshire Limestone are very sandy, but it may be distinguished where shell debris and peloids are present.

The formation is very poorly exposed in the district: up 6 m of fine-grained sand, mudstone and sandy mudstone are visible beneath the Upper Lincolnshire Limestone near Weldon [SP 9145 8852], and about 3 m of sandstone and mudstone may be seen in a quarry face at Geddington [SP 884 821]. The formation is barren of macrofossils, and is attributed a mid Aalenian age from its stratigraphical position.

Lincolnshire Limestone Formation

The Kettering district lies at the southern margin of the outcrop of the Lincolnshire Limestone Formation (Figure 2) and (Figure 3), which extends north to the Humber. On the 1:50 000 series map, the formation is generally subdivided into lower and upper units, and consists of bedded limestone; composition varies laterally and vertically through changes in the proportions of various types of carbonate grains (allochems), sand (mainly quartz) and cement. The grains include shell and skeletal fragments (bioclasts), aggregate grains and intraformational rock fragments (intraclasts) and coated grains (ooids and peloids/pellets). A more detailed description is given by Taylor (1963).

The Lower Lincolnshire Limestone (LLL) includes four principal lithologies, listed below, which approximates to their predominance in ascending order:

  1. Fissile silici-sandy limestone (Collyweston Slate type) occurs at the base and locally may contain skeletal fragments.
  2. Very fine to fine ooidal, peloidal limestone, locally rich in bioclasts and skeletal fragments.
  3. Bimodal (very fine to fine and medium to coarse) ooidal, peloidal limestone, locally rich in bioclasts and skeletal fragments.
  4. Medium to coarse ooid-limestone.

The cement of these rocks may be composed predominantly of spar deposited from solution (grainstone texture), mechanically or chemically derived lime mud (packstone/wackestone) or a combination. Bivalves and gastropods dominate the macrofauna of the Lower Lincolnshire Limestone and brachiopods are rare.

The Upper Lincolnshire Limestone (ULL) is more homogeneous in character than the lower unit: it includes the same grain and cement types, but it is characterised by a greater abundance of bioclasts, a generally coarser allochem grain size, dominance of grainstone texture (spar cement) and the presence of cross-bedding especially in the lower part (Taylor, 1963). In addition to molluscs, brachiopods and coral are also present. Near Corby, the 'Weldon Stone/Freestone' occurs in the upper part of the unit. This is a coarse shelly ooid-grainstone that was quarried and mined south-west of Weldon village [SP 92 89] as a high-quality building stone (p.24).

An undulating erosion surface marks the base of the Upper Lincolnshire Limestone and in places it rests in channels cut in, and locally through, the underlying Lower Lincolnshire Limestone and even the Grantham Formation (Figure 3), for example near Weldon [SP 923 917]; [SP 925 897] and Stanion [SP 920 870]; [SP 935 862]. From sections and interpretation of the isopachytes of the Grantham Formation (Lower Estuarine Series) Taylor (1946) inferred substantial subparallel channels, up to 5 km long, within a belt up to 5 km wide (Figure 3). These run parallel to the southern boundary of the formation. The surface of Lower Lincolnshire Limestone underlying the Upper unit is capped by a widespread bored hardground.

The thickness of the Lincolnshire Limestone Formation is highly variable, with a general maximum between 10 and 12 m reaching 14 m locally. The variability is due to original depositional variation, channelling and erosion prior to deposition of the Rutland Formation. The maximum thickness of both Upper and Lower Lincolnshire Limestones is about 12 m.

An ammonite from the Lower Lincolnshire Limestone near Geddington [SP 863 824] indicates either the uppermost Aalenian Concavum Zone or lowermost Bajocian Discites Zone (Barker and Torrens, 1971). No ammonites have been recorded from the Upper Lincolnshire Limestone in the district, but farther north the Lower Bajocian Laeviuscula Zone is present.

The Lincolnshire Limestone Formation is no longer well exposed in the district. About 5 m of the upper unit is visible in a quarry face near Weldon [SP 9196 8881]. Up to 3.5 m of shelly ooidal limestone and calcareous sandstone of the lower unit is exposed in a gullet at Geddington [SP 884 821].

The predominance of grainstone texture and the presence of cross-bedding indicate that the upper unit was deposited in a higher energy environment than the lower.

Great Oolite Group

The Great Oolite Group rests unconformably on the Inferior Oolite, and consists predominantly of mudstone and limestone. The group is divided, in ascending order, into the Rutland, Blisworth Limestone, Blisworth Clay and Cornbrash formations.

Rutland Formation

The Rutland Formation (Rld; formerly the Upper Estuarine Series) is a succession of shallowing-upwards cycles of marine to nonmarine sedimentary rocks extending through the East Midlands. Bradshaw (1978) recognised and named seven rhythms. The lowest of these is the Stamford Member (St), which is entirely nonmarine and comprises up to 5 m of white and pale grey sandstone and siltstone, with minor darker mudstone and an impersistent basal nodular ironstone bed. Following the work of Bradshaw and others, it is now thought that the Grantham Formation (see above) is generally absent beyond the southern limit of the Lincolnshire Limestone Formation. Thus south of this line, strata formerly attributed to the Aalenian Grantham Formation (Lower Estuarine Series) are now assigned to the late Bajocian–Bathonian Stamford Member or its correlatives. These include some of the 'Lower Estuarine Series' sections, which Taylor had noted as lithologically different from the others figured (Taylor, 1963, fig. 10, sections 15–21, p.40). As a result, within the Kettering district, the Stamford Member is distinguished on the map only in the south. However, it is thought to be present within the undifferentiated Rutland Formation in the north, where it overlies the Lincolnshire Limestone, and is shown in one location at this level near Weekley [SP 885 810].

Above the Stamford Member, each rhythm typically comprises a basal unit of grey sandstone and mudstone with marine to brackish shells, which passes up into poorly fossiliferous mudstone (delta-top distributary channel deposits) and greenish grey mudstone with rootlets and plant debris (saltmarsh deposits). Some of the rhythms may be partly or wholly absent in the Kettering district, through erosion or non-deposition. This may account for the largely unsystematic changes in thickness of the formation across the district, which ranges from 5 to 14 m. Many of the outcrops of undifferentiated Rutland Formation include all the above lithologies. However, the Wellingborough Limestone Member (W; formally Upper Estuarine Limestone) has been mapped here and in the Wellingborough district to the south. This member is a marine sequence of shelly calcareous mudstone, and argillaceous sandy and bioclastic limestone, developed within the lower part of one rhythm. It attains a thickness of 3 m, and outcrops can be traced widely in the south and locally in the north near Fotheringhay [TL 05 94].

The Rutland Formation has yielded a rich shelly fauna dominated by bivalves that include those tolerant of reduced salinity in the 'deltaic' beds, and a more varied assemblage with brachiopods such as Burmirhynchia sp. in the more marine beds. No age-diagnostic fossils are known and by inference the formation is attributed an age ranging from the Zigzag to Subcontractus or Morrisi zones (Bathonian; Cox and Sumbler, 2002, p.263), although the Stamford Member may extend down into the latest Bajocian (Parkinsoni Zone). The formation gives rise to a clayey or loamy, variably stony soil, and oysters are commonly found. Exposures in the formation degrade rapidly, but about 10 m of predominantly argillaceous beds are visible in a quarry at Geddington [SP 884 821].

Blisworth Limestone Formation

The transgression that submerged the Rutland Formation deltas, eroding the upper beds in places, established a shallow, protected carbonate lagoon in which the Blisworth Limestone Formation (BwL; formerly the Great Oolite Limestone) was deposited. A prolific bottom-dwelling fauna indicates moderate current activity, although the more muddy and sandy character compared with its approximate correlative, the White Limestone Formation of Oxfordshire, suggests that deposition occurred closer to land.

The formation thickens from about 4 m in the north of the district to about 8 m in the south, and is typically 5 to 6 m thick. It is dominated by limestone lithologies, but beds of lime-mudstone ('marl') occur at several levels (Taylor, 1963). The limestone allochems are dominated by bioclasts (mainly shell fragments, both angular and rounded), with lesser amounts of coated grains (ooids and peloids) and quartz sand, in lime mud, silt (in places recrystallised) or spar cement. The three main rock types are:

Bivalves, including many oyster species, dominate the abundant fauna, which also includes echinoids, corals and brachiopods, the last concentrated at two discrete levels.

The basal beds of the Blisworth Limestone Formation comprise interbedded argillaceous, bioclastic limestone with lime mud cement, and lime-mudstone rich in bivalves. It also includes the brachiopod Kallirhynchia sharpi Muir-Wood in abundance, making these strata regionally recognisable; they were formerly known as the Sharpi Beds, now formalised as the Roade Member (Cox and Sumbler, in press). Above the Roade Member, bioclastic limestones continue to dominate, locally becoming ooidal or peloidal, and with subordinate interbedded lime-mudstone beds, all rich in oysters, and now termed the Longthorpe Member (Cox and Sumbler, in press). A persistent bed containing ooids and brachiopods, including Digonella digonoides S S Buckman, gives an indication of periodic higher energy conditions; it is inferred to represent the Digonoides Beds, seen farther south. Rootlets are reported in the uppermost bed at Thrapston (Cox and Sumbler, 2002, p.265), suggesting emergence prior to deposition of the Blisworth Clay.

Probably the entire thickness of the formation (here almost 9 m) is displayed at Cranford St John [SP 924 764] (Cox and Sumbler, 2002, pp.260–264), just to the south of the Kettering district. About 1.5 m of limestone are visible at Twywell Country Park [SP 9397 7748]. The uppermost 4 to 5 m of the formation were formerly exposed beneath the Blisworth Clay at Thrapston [TL 000 776] (Cox and Sumbler, 2002, pp.264–266).

Blisworth Clay Formation

The base of the Blisworth Clay Formation (BwC; formally Great Oolite Clay) rests with sharp conformity on the Blisworth Limestone. Beds of oyster limestone present locally close to the base and higher in the formation indicate some marine influence. However, most of the formation comprises smooth plastic mudstone, characteristically variegated in colour, showing bluish grey, green, magenta and purple mottling, with rootlet traces and lignite fragments. These, and the presence of ferruginous nodule beds (a persistent bed occurs at the base) indicate a considerable terrigenous influence. The beds were possibly deposited in a saltmarsh environment with fluvial input of clay and dissolved iron. The formation ranges between about 3 and 6 m in thickness, thinning to the south-east (Taylor, 1963, fig. 15). The type section is within the Kettering district, in a quarry at Thrapston [TL 000 776] [Cox and Sumbler, 2002, pp.264–266] where 2.5 m of clay (mudstone) are exposed, the thickness has probably been reduced by cambering. It is not well exposed elsewhere in the district. The formation is largely barren apart from oysters that include Praeexogyra hebridica (Forbes) in the limestone beds. An Upper Bathonian age is inferred from its stratigraphical position. It develops a heavy, dark greyish brown soil, and the colour mottling may be seen where deeply ploughed or dug.

Cornbrash Formation

Bioclastic limestone of the Cornbrash Formation (Cb) indicates a return to marine shelf conditions and carbonate deposition in late Bathonian times. This formation is generally less than 3 m thick, thinning north-westwards to less than 0.3 m in places (Taylor, 1963, p.106, fig. 17). The formation comprises irregularly bedded, shell-detrital packstone and grainstone with rare coated peloids and quartz sand; it is interbedded with thin shelly lime-mudstone beds. On a freshly broken surface the limestone is commonly blue-grey, but weathers brown owing to a significant iron content in the matrix, and produces a distinctive reddish brown stony soil. A rich fauna includes brachiopods, bivalves, gastropods and echinoids, and ammonites that are zonally indicative are relatively common. Both the Lower and Upper Cornbrash are present in this district. The Lower Cornbrash is characterised by rather rubbly weathering; the Upper Cornbrash is more flaggy. An important nonsequence occurs between the Lower Cornbrash (Discus Zone) and Upper Cornbrash (Herveyi Zone), thus the formation spans the Bathonian–Callovian boundary (Taylor, 1963, fig. 17; (Figure 2)). The formation is well exposed at Thrapston [TL 000 776] where 1.7 m of Upper Cornbrash overlie up to 0.13 m of Lower Cornbrash (Cox and Sumbler, 2002). Douglas and Arkell (1932) reported that the Cornbrash was fully exposed at Barnwell [TL 049 838] where the upper division is 1.5 m thick and the lower 1.0 m.

Ancholme Group

The expansion and deepening of the basin in early Callovian times established open marine conditions on the East Midlands Shelf, and the mudrock-dominated Ancholme Group was deposited. Only the lower part is present in the district, the Kellaways and Oxford Clay formations. Ammonites are used for biostratigraphical zonation, but weathering of the upper few metres below the soil usually destroys these fragile fossils, and generally only belemnites and the thick-shelled oyster Gryphaea are seen at the surface.

Kellaways Formation

A rapid passage up from limestone into argillaceous strata marks the base of the Kellaways Formation (Kys), which is between 5 and 8 m thick and is subdivided into two members. The Kellaways Clay Member (KlC) comprises between 1.5 and 3 m of dark grey, smooth or slightly silty, fissile claystone; the base is locally sandy. The member passes up through increasing silt and fine sand content into the Kellaways Sand Member (KlS). This comprises 3 to 5 m of light grey fine-grained sand and calcareous sandstone, interbedded with siltstone and mudstone. Even when seen unweathered, the formation is poorly-fossiliferous and it is no longer exposed in the district.

The Kellaways Formation is thought to be present overlying the Cornbrash throughout the district, but it could not be recognised consistently during the earlier survey (Taylor, 1963). Thus, it is not shown on the 1:50 000 series map along much of the Harper's Brook valley or the west side of the Nene valley, and hereabouts the outcrops shown as Oxford Clay Formation include the Kellaways Formation at the base. 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.

Oxford Clay Formation

The Oxford Clay Formation (OxC) is the youngest solid unit present in the district and forms extensive outcrops and subcrops beneath glacial deposits in the centre and east of the district. It comprises grey mudstone with minor thin calcareous beds, and where complete is divided into three: in ascending order the Peterborough, Stewartby and Weymouth members (formerly the Lower, Middle and Upper Oxford Clay, respectively). These members have not been shown on the 1:50 000 series map because of the difficulty in distinguishing them at outcrop and the cover of superficial deposits. The Peterborough Member comprises dark brownish grey, fissile mudstone with a high organic (bitumen) content; this characteristic has fostered its use for brick making in the nearby Peterborough and Bedford districts. Crushed aragonitic ammonites and bivalves, in parts pyritised, dominate the shelly fauna, and are concentrated in shell beds. At a number of levels, bands of calcareous nodules or concretions (cementstone) are developed. Beds of paler grey, blocky mudstone increase in abundance in the upper part. The Stewartby Member comprises light to mid grey, smooth or silty, calcareous, blocky claystone. It is poorly fossiliferous, although there are some beds with numerous bivalve shells, and the macrofauna is generally preserved in pyrite. To the south, at Stewartby near Bedford (sheet 203), it includes in the upper part a number of thin muddy limestone beds with common Gryphaea shells, and these may persist into this district. The overlying Weymouth Member is generally similar in lithology to the Stewartby Member, but is less silty and calcareous. All three members weather to olive-brown clay near the surface, producing heavy greyish brown clay soils, generally lacking any indicative fossils.

From recent BGS studies, the Peterborough and Stewartby members are estimated to be about 18 and 25 m thick, respectively, and there may be as much as 50 m of Oxford Clay present on the downthrow side of a fault around Fayway [TL 06 78]. Thus, it is possible that not only is the full thickness of the Peterborough and Stewartby members present here, but also the lowest few metres of the Upper Jurassic Weymouth Member. Where fully developed the Weymouth Member is about 22 m thick.

Quaternary

Superficial deposits cover approximately 50 per cent of the district. This area is essentially a dissected till plateau, with glacial till deposits (up to 30 m thick) and associated sand and gravel blanketing the higher ground of the interfluves. The district lay well to the south of the Devensian glaciers, but was subjected to periglacial conditions during the last glacial maximum. The glacial deposits of this district were deposited during earlier glaciations and are generally thought to be Anglian in age. River terrace deposits and Alluvium are found along the major rivers. (Figure 5) summarises the nomenclature of Quaternary deposits in the district.

Glacial and preglacial deposits

Till

Two distinct glacial tills have been recognised in the district. The upper till, formerly widely known as 'Chalky Boulder Clay', is a structureless diamicton, comprising stiff pale bluish grey to dark grey clay, weathering to brown near the surface, with abundant chalk fragments, ranging from silt grade (flour) to coarse well rounded gravel. In addition to the chalk, there is a variable erratic content that includes flint, quartzite ('Bunter' pebbles derived from the Sherwood Sandstone Group), ironstone and limestone of local origin, and fossils reworked from the underlying deposits, including Gryphaea. Far-travelled igneous and metamorphic rocks are very minor constituents.

A second till, distinguished on the basis of colour and composition, occurs below the chalky till, in places separated from it by glaciofluvial sand and gravel. Formerly referred to as the 'Lower Boulder Clay' (Hollingworth and Taylor, 1946), the lower till comprises a stiff, dark brownish grey to greenish grey clay with abundant gritty quartzose material, fragments of Jurassic limestone and ironstone, white-coated sandstone, friable green sandstone, and quartzite pebbles (probably derived from the Sherwood Sandstone Group). Calcareous 'race' concretions may be found as a minor constituent, while towards the base of the deposit, overlying the Oxford Clay, an increasing proportion of pale greenish grey mudstone fragments may be included. Harrisson (1981) states that the lower till can be distinguished from the upper, chalky till by the absence of chalk and flint. However, Taylor (1963) noted that in some areas the deposit contained rare weathered flint and chalk fragments. The lower till is not extensive in outcrop, occurring as isolated pockets infilling irregularities in the pre-glacial land surface. Presumably it was once more extensive but was removed by erosion as the ice sheet that carried the chalky till advanced. Because of the restricted outcrop of the lower till, the two tills have not been distinguished on the map and are grouped together as till.

The upper, chalky till is believed to be equivalent to the Oadby (Till) Member of the Wolston Formation as classified by Bowen (1999) (although he attributes most glacial deposits of this district to the Lowestoft Formation). As such it is the most widespread till of the English Midlands, generally accepted to have been deposited by the main advance of the Anglian ice sheet. The current consensus is that this ice advance occurred during oxygen isotope stage 12, although it is considered by some authors (Sumbler, 1995; Bowen, 1999) that the Oadby Till was deposited subsequent to the main Anglian advance (oxygen isotope stage 10).

The affinity of the lower till is more problematic. The lithology is similar to that recorded in the Towcester, Milton Keynes (Horton et al., 1974), Melton Mowbray (Carney et al., 2003) and Wellingborough districts, where deposits of this type are more extensive. It clearly predates the upper till but it is uncertain whether it is the result of an earlier glaciation, or was deposited as part of the same glaciation event that produced the more extensive chalky till. Other workers have included it as a distinct lithology of the Oadby (Sumbler, 1983) or Thrussington (Bridge et al., 1998) tills, both are now included as members of the Wolston Formation of Bowen (1999). The presence of a laminated deposit containing organic debris, and of possible interglacial or interstadial lacustrine origin, separating the upper, chalky till from the lower till in a borehole 2 km north-west of Oundle [TL 0210 8907] was originally reported by Merritt (1982). Further investigations have found possible evidence of a climatic amelioration from this material (Riding, 2004). The full significance of this finding is under investigation.

Glaciofluvial sand and gravel

Glaciofluvial sand and gravel deposits are closely associated with till deposition, occurring within and below the Oadby Till. Typically, they comprise medium-grained, subangular to rounded sand, composed mainly of quartz, flint and ironstone, and subangular to rounded gravel, composed of ironstone, limestone, flint and quartzite, in varying proportions (Harrisson, 1981).

The mode of deposition of these deposits is uncertain, as their original morphology is not preserved. They have been interpreted as outwash gravels deposited during the various phases of ice advance and retreat (Harrisson, 1981, 1983). Those below the till, which form the most extensive deposits, may have been deposited as proglacial outwash deposits, or sandurs, formed in front of the advancing ice sheet (but see below). Those occurring as lenses within the till are probably best regarded as glaciofluvial ice contact deposits. Isolated patches of glaciofluvial sand and gravel are found below the level of the base of the till at Southwick [TL 033 927] and in the Nene valley [TL 062 874]; [TL 064 864]. These deposits do not have the morphological form of terraces (Taylor, 1963), and represent the only possible ice-retreat deposits in the district.

Evidence of an earlier phase of sand and gravel deposition, predating the deposition of the lower till, was formerly exposed in sand and gravel pits at Brigstock and Stanion. In contrast to the later glaciofluvial sand and gravel, these earlier deposits are composed entirely of locally derived material, and may correlate with the Milton Formation of Bowen (1999) for which Clarke and Moczarski (1982) infer an early Pleistocene fluvial origin. They are poorly recorded and of uncertain extent. The section at Brigstock, which is no longer visible, exposed a total thickness of 4.9 m of interbedded brown sand and beds of (water-worn) ironstone and limestone pebbles (Taylor, 1963), resting on bedrock and overlain by the lower till. Boreholes prove that similar material extends farther to the south beneath the lower till.

The different types of sand and gravel deposit described above have not been distinguished on the map due to the limited extent of outcrop and lack of exposure, and are grouped together as glaciofluvial sand and gravel.

Fluvial deposits

River terrace deposits are found in the Nene and Welland valleys and the lower reaches of Willow Brook, a major tributary of the Nene. The spatial relationship of the drift in the Nene valley is shown schematically in (Figure 6). The Third Terrace Deposits (1.0 to 6.3 m thick) comprise clean, well-sorted gravels with seams of clay, silt and fine- to coarse-grained sand, and occur 10 to 15 m above the present floodplain. They are of limited extent, preserved only at Tansor, Aldwincle and Barnwell. The surface of the more extensive Second Terrace Deposits (1.3 to 8.5 m thick) is typically 5.5 to 7 m above the present floodplain. They are of a similar composition to the Third Terrace Deposits, but generally less silty and clayey. The interbedded medium to coarse sands and fine to medium gravel of the First Terrace Deposits (1.5 to 11 m thick) form a gently sloping shelf rising up to 3 m above the present floodplain and continuing beneath the more recent alluvium, infilling a broad, shallow valley cut below the present base level.

The clast composition of the terrace gravels typically includes limestone, ironstone, flint, chalk and sandstone, with minor igneous and metamorphic erratics. Jurassic limestone is the most abundant clast type. The occurrence of chalk, flint and other erratics indicates that the river terrace gravels are derived in part from reworked till. Fossil evidence and radiocarbon dating indicate a mid-Devensian age for the First Terrace Deposits (Castleden, 1976). They were deposited from a high bedload periglacial braided stream system, and represent a period of mid-Devensian aggradation. There is no absolute age data for the second and third terrace deposits, but their similar morphology and composition, together with evidence of frost heave and solifluction (Harrisson, 1981), suggests a periglacial environment of deposition.

The fluvial deposits of the Nene valley have been classified by Bowen (1999; (Figure 5)). Only two terraces are preserved along the Welland valley; their correlation with those of the Nene valley is not known.

Alluvium (0.5 to 4.5 m thick) is the most recent fluvial deposit, found along the valleys of the rivers Nene, Welland, and Ise, and their tributaries. Typically, it comprises silt and clay with gravel lenses, occupying broad meander belts, usually cut into the underlying First Terrace Deposits.

Along minor tributary valleys, the composition of the alluvium is likely to be more variable. Further details of distribution, thickness, and composition of the Alluvium and River Terrace Deposits of the Nene is given by Harrisson (1981, 1983).

Other Quaternary deposits

Limited deposits of stony, silty clay and gravel occur in some valleys and have been mapped as head. They accumulated by processes of weathering and solifluction, primarily under cold climatic conditions.

Landslip has occurred along the Inferior Oolite escarpment to the south-east of the River Welland, where steep slopes in the Whitby Mudstone Formation became unstable. The slips are believed to have occurred under colder, wetter climatic conditions than at the present day, possibly during the Devensian.

Unmapped tufa deposits may also be present, where they have been precipitated from calcium carbonate-rich spring waters issuing at or near the base of outcrops of the Northampton Sand or Blisworth Limestone formations. They comprise pale, loose, friable calcareous silt, probably less than 3 m thick, and are highly compressible.

Artificially modified ground

The more extensive areas of artificial deposits and worked ground are shown on the map but minor occurrences have been omitted for clarity, and are shown on the 1:10 560 and 1:10 000 scale maps of the district (see Information sources).

Made ground occurs in areas where material was deposited by man upon the natural ground surface, mainly on road and rail embankments and in urban areas, and typically comprises engineered fill.

Worked ground is shown where natural materials are known to have been removed in quarries and pits. In this district, there are a small number of active and disused pits associated with the extraction of brick clay and limestone. The ironstone extraction methods (see Infilled ground) mean that former ironstone workings are almost invariably wholly or partially backfilled.

Infilled ground comprises areas where the natural ground has been removed, and the void wholly or partially backfilled with man-made deposits, which may be either natural or waste materials, or a combination of both. Extensive areas of former ironstone workings, centred around Corby and to the north and east of Kettering, have been mapped as infilled ground. These can be divided into two categories.

  1. Infill of natural material: During the extraction process the pits have been partially or wholly backfilled with overburden. The principal method of ironstone extraction, in which a working 'gullet' was opened, then the overburden stripped and deposited in the space behind as the working face moved forwards, has resulted in the majority of former workings being restored to a partially backfilled state (see Mineral resources).
  2. Waste material: Some of the former pits are known to have been re-used and infilled with inert domestic [SP 966 784] and industrial waste [SP 905 913]. Other formerly worked areas may contain refuse materials.

Limestone quarrying in the area was often closely associated with ironstone extraction. Where limestone formed part of the overburden it was utilized as a resource in its own right. Such workings, now infilled, are found in the Lincolnshire Limestone for instance between Stanion and Weldon [SP 92 88] area and west of Islip [SP 97 78].

Where quarries and pits have been filled, the ground restored and landscaped, built on or returned to agricultural use, there may be no obvious surface indication of the extent of the infilled area. In such cases, the boundary of the site can sometimes be distinguished on aerial photographs, or is taken from archival sources, including local authority records, and old topographical and geological maps.

Disturbed ground has been mapped where sand and gravel has been worked mainly from beneath the alluvium along the Nene valley; ill-defined excavations and spoil are in complex association and many pits are flooded or backfilled.

Structure

Surface strata dip gently towards the east or south at less than 1º (see cross-section on sheet 171) and a number of east-south-east-trending faults cross the district. The subsurface structure has been interpreted from a number of deep boreholes in and adjacent to the district and from a seismic profile in the north-east. Two major unconformities divide the rocks into three tectonostratigraphical units; these mark the base of the Mesozoic sequence and the base of the Upper Palaeozoic sequence. Silurian rocks are absent in this area so that Upper Palaeozoic rocks rest on the oldest tectonostratigraphical unit comprising Precambrian, Cambrian and Ordovician rocks (Figure 1).

The base of the Mesozoic sequence is marked by a reflector that is interpreted as the Variscan unconformity. Contours on this surface (Smith, 1985b) show a prominent ridge (Orton Ridge) trending south-east through the Oxendon Hall and Orton boreholes (Figure 1). This ridge appears to be the continuation of Charnwood Precambrian strata and intrusions, which are exposed to the north-west, and it probably extends into the Wellingborough district. In this district, the depth to the unconformity increases to the north-east of the ridge reaching about 215 m.

A gravity anomaly located just south of Orton (Figure 7b), suggests the presence of a low density granite, which may also extend into the district (Allsop et al., 1987). The top of this granite does not reach the unconformity.

Chapter 3 Applied geology

Hydrogeology

The limestones and sandstones of the Jurassic form a sequence of aquifers separated by mudstone-dominated formations which act as aquicludes. The Environment Agency classifies the Lincolnshire Limestone Formation as a major aquifer, and the Kellaways Sand Member, Cornbrash, Blisworth Limestone, Northampton Sand, Marlstone Rock and Dyrham formations as minor aquifers (Allen et al., 1997; Jones et al., 2000, table 6.5). Faulting complicates hydraulic relationships: faults with relatively small displacements can have a major impact on the hydrogeological regime because the aquifer units are thin.

Fracture flow dominates in the limestones, which typically have low intergranular permeability. The Lincolnshire Limestone Formation is a major water resource farther to the north. However, in this district close to the southern limit, the largest yields have been obtained where the formation is in hydraulic continuity with the underlying Northampton Sand Formation (Taylor, 1963), which occurs where the Grantham Formation is incomplete or absent. For example, the pumping station at Little Oakley had a yield of 820 m3 per day.

The transmissivities and storage coefficients of the Cornbrash and Blisworth Limestone are relatively low, because the formations are thin (Jones et al., 2000). Consequently, they are generally capable of yielding only limited local supplies (Woodland, 1942). Water quality is generally good but hard. However, elevated salinity levels are found in the confined portions of the aquifers.

The Kellaways Sand Member is characterised by very low hydraulic conductivities with poor water quality.

The Northampton Sand Formation is the only minor aquifer of regional importance as a water resource. Groundwater flow is via both matrix and fracture flow: primary porosity is thought to be more significant for water storage and transport than in the limestones, except where extensive fracture systems are developed (Jones et al., 2000). The highest yields are obtained at relatively shallow depths, where weathering and oxidation have enhanced the porosity of the formation (Taylor, 1963). Water is generally of good quality, but hard. Historically, the formation was heavily utilized as a water resource. However, large-scale ironstone extraction resulted in artificially lowered water levels (Taylor, 1963). Water levels are now generally higher than prior to ironstone extraction, due to increased infiltration. Nevertheless, the formation is no longer utilized for major supplies in the district.

The Marlstone Rock Formation is of insufficient thickness and continuity to be a useful resource in the district. The underlying Dyrham Formation is also unlikely to provide reliable water supplies.

Further details of the aquifer physical properties are given by Allen et al. (1997) and Jones et al. (2000).

Quaternary deposits are also of importance as a groundwater resource. The river terrace gravels of the Welland and Nene have historically provided numerous small domestic supplies in the villages (Taylor, 1963). Glaciofluvial deposits have also been exploited locally. The First Terrace Deposits underlying the alluvial plain of the Nene and Welland are the most important Quaternary aquifer, from which large public water supplies have been obtained at Gretton, Oundle, and Thrapston (Taylor, 1963).

Mineral resources

Within the district the most extensively exploited mineral resource is the Northampton Sand Formation, with records of iron ore workings going back to the 11th century. In more recent times, between the mid 19th century and the closure of the steelworks and the last quarries in 1980, the formation was worked for ironstone to support the steel industry largely in the west and south, around Corby and Kettering. Both opencast and underground working was undertaken, accounts of which are given by Hollingworth and Taylor (1951) and Tonks (1991, 1992). Huge areas were opencast by mechanical shovels, later by draglines, the larger models of which (weighing up to 1800 tons) removed the overburden where present, commonly up to 20 m and exceptionally up to 35 m thick (Plate 2). The ore bed, some 2 to 4 m thick, was then drilled and blasted, and smaller machines loaded it 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 trafficable base. 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 even clinker derived from the smelting process of the ironstone. Mining of ironstone took place around Slipton and Islip [SP 94 80] to [SP 97 79] and at Cranford St Andrew [SP 91 77] with access gained by both shafts and adits. There remain considerable unworked areas of ironstone within the district, largely in the west. However, for a number of economic and other reasons they are not regarded as a mineral resource (Bloodworth et al., 2000).

The Lincolnshire Limestone and Blisworth Limestone formations have both been worked in the past to extract limestone for building stone, roofing slates, metallurgical flux and burning to produce agricultural lime. The Upper Lincolnshire Limestone includes the renowned Weldon Stone or Freestone which was worked in quarries and underground workings between Weldon and Corby, although at least part of these were worked away during ironstone extraction. Weldon Stone has been used in college buildings locally in Oundle and Uppingham, in Oxford and Cambridge, and in a number of cathedrals. Taylor (1963, pp. 64 and 136) reported that fissile sandy limestone for roofing slates (Collyweston Slate) was formerly extracted from the base of the Lincolnshire Limestone near Kirby Lodge [SP 9122 9226]. A fissile layer that the quarry men called 'pendle' extracted from the Blisworth Limestone in a quarry west of Oundle [TL 021 882] may have been used for the same purpose. Currently there remain two quarries working the Lincolnshire Limestone for aggregate, and one extracting building stone from the Blisworth Limestone. There are considerable resources of both (Bloodworth et al., 2000).

Sand and gravel has been extracted from river terrace and glaciofluvial deposits throughout the district. The majority of workings are in the river terrace deposits of the River Nene valley, but it is likely that considerable resources remain (Harrisson, 1981, 1983; Bloodworth et al., 2000). The most widely distributed and consistently graded deposits are those of the first terrace, which have been extensively worked around Thrapston and Oundle, usually from beneath an overburden of alluvium. Typically, they comprise medium- to coarse-grained sand and fine gravel, with a mean fines content of less than 10 per cent. Third Terrace Deposits in the district are generally considered to be thin and silty. Second Terrace deposits, although less silty than those of the third terrace, are inconsistently graded (fines 4 to 27%, sand 46 to 56%). They may locally provide a workable resource, and have been worked in the past, south-east of Oundle [TL 047 876] and near Fotheringhay Lodge [TL 080 946]. Many of the former sand and gravel pits in the district now have educational, recreational and wildlife conservation uses e.g. [TL 035 874].

The main historical and current mineral resources of the district are summarised in (Figure 8).

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 of extracted material in engineering works. These issues are summarised for the main engineering geological units in the district (Figure 9). Some important geohazards within the district include ground heave and subsidence, cambering and valley bulges, slope stability and mass-movement, gas emissions related to both the landfills and the underlying geology (natural radon), and the flooding of alluvial ground.

Ground heave and subsidence

The Oxford Clay, Blisworth Clay, Whitby Mudstone and Charmouth Mudstone formations are dominated by clay minerals that can attract and absorb water. These undergo significant volume changes in response to variation 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, services and roads. Glacial till deposits may also contain clay minerals that are prone to shrinking and swelling.

Cambering and valley bulge

Cambering — the extension and lowering of near-surface strata — is a gravitational effect seen on valley sides where strong beds (such as limestone) 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 move down slope and are extended by brittle fracture or the opening of existing joints. Continued downhill (lateral) movement of the blocks may result in wide fissures (gulls), which may be open but are likely to be filled with loose rock and soil. However, gulls may also be formed in limestone by the dissolution along joints and enlargement of cavities. 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 thought to have taken place largely during the Pleistocene, but may be ongoing, and cambers may merge into landslide movements downslope. Another consequence of the stress relief in narrow valleys with clay strata is valley bulging; weaker material below the valley floor is forced upwards above its normal stratigraphical position, becoming folded and possibly faulted. This process may also disrupt the downslope ends of cambers on the valley side.

Slope stability

A number of landslides have been identified in this district. They are found mainly on the Whitby Mudstone Formation and outcrop in the north-west of the district, along the Welland valley between Cottingham and Gretton.

Slopes over 10° in the Whitby Mudstone, Blisworth Clay, Kellaways Clay or Oxford Clay are prone to failure. Slopes exceeding 3° should also be considered as potentially unstable due to periglacial processes (Culshaw and Crummy, 1991). Head, Till and the Blisworth Clay Formation can also contain thinly interbedded sequences of sand and clay. These are prone to landslide due to the presence of springs and high confined pore pressure, which 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). Relatively high levels of radon emissions are associated with particular types of bedrock and unconsolidated deposits. The Northampton Sand Formation and the Lincolnshire Limestone Formation have the highest radon potential in the Kettering district.

Radon that enters poorly ventilated enclosed spaces such as some basements, buildings, caves, 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). Approximately two-thirds of the Kettering district has been identified by the NRPB as Radon Affected Areas.

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).

Flooding

Low-lying alluvial ground adjacent to active stream and river courses may be prone to flooding during periods of exceptional rainfall. Flood protection measures may be required in these areas. It is important that all drainage courses are regularly maintained.

Information sources

Further geological information held by the British Geological Survey relevant to the Kettering district is listed below. It includes published maps, memoirs and reports.

Enquiries concerning geological data and advice for the district should be addressed to the BGS Enquiry Service. BGS hydrogeology enquiry service (wells, springs and water borehole records) can be contacted via the BGS web site or at Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB. Telephone 01491 838800. Fax 01491 692345.

Other information sources include borehole records, fossils, rock samples, thin sections, hydrogeological data and photographs. Searches of indexes to some of the collections can be made on the GeoIndex system in BGS libraries and on the BGS web site.

Maps

A range of small-scale geological, hydrogeological, geophysical and geochemical maps that include the Kettering district are available. These are listed in the current BGS catalogue, available on request or accessible online.

Books and reports

Documentary collections

Boreholes

Borehole data for the district are catalogued in the BGS archives (National Geological Records Centre) at Keyworth. For the Kettering district there are sites and logs for about 7500 boreholes, for which index information has been digitised. For further information contact: BGS Records Enquiries (South), 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 171 Kettering are held in the Lexicon database. This is available on the BGS web site. Further information on the database can be obtained from the Lexicon Manager at BGS, Keyworth.

BGS Photographs

Photographs of the district are deposited for reference in the National Archive of Geological Photographs, BGS, Keyworth. Part of the collection can be viewed online (see back cover for addresses). Those prefixed with BAAS derive from the British Association for the Advancement of Science collection lodged with BGS.

Material collections

All enquiries regarding the National Geological Materials Collections should be directed to BGS Collections Enquiries, 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 the England and Wales Sliced Rock Collection at BGS Keyworth.

Bore core collection

The National Geological Materials Collections, BGS Keyworth, hold samples and entire core from a small number of boreholes in the Kettering district.

Other relevant collections

Groundwater

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 present within the Kettering district is held by English Nature, Headquarters and Eastern Region, Northminster House, Peterborough, PE1 1UA.

See back cover for BGS addresses and web site.

References

Most of the references listed below are held in the libraries of the British Geological Survey at Keyworth (Nottingham) and Edinburgh. Copies of the references can be purchased subject to the current copyright legislation. BGS Library catalogue can be searched online at http://geolib.bgs.ac.uk

Allen, D J and seven others. 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.

Allsop, J M, Ambrose, K, and Elson, R J. 1987. New data on the stratigraphy and geophysics in the area around Hollowell, Northamptonshire, provided by a coal exploration borehole. Proceedings of the Geologists' Association, Vol. 98, 157–170.

Ambrose, K. 2001. The lithostratigraphy of the Blue Lias Formation (Late Rhaetian-Early Sinemurian) in the southern part of the English Midlands. Proceedings of the Geologists' Association, Vol. 112, 97–110.

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, No. WP/95/2.

Barker, M J, and Torrens, H S. 1971. A new ammonite from the southernmost outcrop of the lower Lincolnshire Limestone (Middle Jurassic). Transactions of the Leicester Literary and Philosophical Society, Vol. 65, 49–56.

Bloodworth, A J, Cameron, D G, Morigi, A N, Highley, D E, and Holloway, S. 2000. Mineral resource information for development plans. Phase 1 Northamptonshire: Resources and Constraints. British Geological Survey Technical Report. No. WF/00/4.

Bowen, D Q (editor). 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.

Bridge, D M, Carney, J N, Lawley, R S, and Rushton, A W A. 1998. Geology of the country around Coventry and Nuneaton. Memoir of the British Geological Survey, sheet 169 (England and Wales).

British Standards Institution. 1999. Code of practice for site investigations. BS 5930. (London: British Standards Institution.)

Carney, J N, Ambrose, K, Brandon, A, and Royles, C P. 2003. Geology of the Melton Mowbray district. Sheet Explanation of the British Geological Survey, 1:50 000 series sheet 142 Melton Mowbray (England and Wales).

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

Clarke, M R, and Moczarski, E R. 1982. The sand and gravel resources of the country between Rugby and Northampton, Warwickshire and Northamptonshire: description of 1:25 000 sheet SP66 and parts of SP56, 57, 65, 67, 75 and 76. Mineral Assessment Report of the Institute of Geological Sciences, No. 107

Cox, B M, and Sumbler, M G. 2002. British Middle Jurassic Stratigraphy. Geological Conservation Review Series, No. 26. (Peterborough: Joint Nature Conservation Committee/Chapman and Hall.)

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). Geological Society of London Special Report.

Culshaw, M G, and Crummy, J A. 1991. SW Essex — M25 Corridor: engineering geology. British Geological Survey Technical Report, No. WN/90/2.

Douglas, J A, and Arkell, W J. 1932. The stratigraphical distribution of the Cornbrash: II. The north-eastern area. Quarterly Journal of the Geological Society of London, Vol. 88, 112–170.

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.

Harrisson, A M. 1981. Sand and gravel resources of the country south-west of Peterborough, in Cambridgeshire and east Northamptonshire: description of 1:25 000 resource sheets TL 09, 19 and SP 98, TL 08. Mineral Assessment Report of the Institute of Geological Sciences, No. 60.

Harrisson, A M. 1983. The sand and gravel resources of the country around Kettering and Wellingborough, Northamptonshire: description of 1:25000 sheets SP97 and parts of SP87 and TL07; and SP86 and 96. Mineral Assessment Report of the Institute of Geological Sciences, No. 114.

Hollingworth, S E, and Taylor, J H. 1946. An Outline of the Geology of the Kettering District. Proceedings of the Geologists' Association, Vol. LVII, 204–233.

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

Horton, A, Shephard-Thorn, E R, and Thurrell, R G. 1974. The geology of the new town of Milton Keynes. Report of the Institute of Geological Sciences, No. 74/16.

Jackson, I. 1978. Sand and gravel deposits of parts of south Nottinghamshire, east Leicestershire and north Northamptonshire — a collation of existing information. British Geological Survey Technical Report, WF/MN/78/1.

Jones, H K, and 12 others. 2000. Physical properties of minor aquifers in England and Wales. British Geological Survey Technical Report, WD/00/4. Environment Agency R&D Publication 68.

Merritt, J W. 1982. A possible interglacial or interstadial deposit near Oundle, Northamptonshire. Quaternary Newsletter, Vol. 37, 10–11.

Noble, S R, Tucker, R D, and Pharaoh, T C. 1993. Lower Palaeozoic and Precambrian igneous rocks from eastern England and their bearing on late Ordovician closure of the Tornquist Sea: constraints from U-Pb and Nd isotopes. Geological Magazine, Vol. 130, 835–846.

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 of London Engineering Geology Special Publication, No. 7.

Richardson, L, and Kent, P E. 1938. Report of weekend field meeting in the Kettering district. Proceedings of the Geologists' Association, Vol. 49, 59–76.

Riding, J B. 2004. A palynological investigation of the Oxford Clay Formation and the Quaternary succession of Northamptonshire (sheets 171 and 186). British Geological Survey Internal Report. No. IR/04/046.

Smith, N J P. 1985a. Map 1: Pre-Permian geology of the United Kingdom (south). 1:1 000 000. (British Geological Survey.)

Smith, N J P. 1985b. Map 2: Contours on the top of the pre-Permian surface of the United Kingdom (south). 1:1 000 000. (British Geological Survey.)

Sumbler, M G. 1983. A new look at the type Wolstonian Glacial deposits. Proceedings of the Geologists' Association, Vol. 94, 23–31.

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.

Taylor, J H. 1946. Evidence of submarine erosion in the Lincolnshire Limestone of Northamptonshire. Proceedings of the Geologists' Association, Vol. 57, 246–262.

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. 1991. Ironstone quarries of the Midlands: history, operation and railways. Part V: The Kettering Area. (Cheltenham: Runpast Publishing.)

Tonks, E S. 1992. Ironstone quarries of the Midlands: history, operation and railways. Part VI: The Corby area. (Cheltenham: Runpast Publishing.)

Woodland, A W. 1942. Water supply from underground sources of the the Oxford – Northampton district. Geological Survey of Great Britain.

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

The map below shows the sheet boundaries and numbers of the 1:50 000 series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland. The west and east halves of most Scottish 1:50 000 maps are published separately.

(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) Subcrop of strata beneath the Mesozoic unconformity; contours are shown for the Caledonian (Acadian) unconformity (in metres below OD), and location of deep boreholes.

(Figure 2) Lithostratigraphical classification of the Jurassic strata of the district: stippling indicates nonsequences and lateral absence.

(Figure 3) Limits of the Northampton Sand, Grantham and Lincolnshire Limestone formations with isopachytes of the Grantham Formation (after Taylor, 1946, plate 22; 1963, fig. 10).

(Figure 4) Comparative sections of the Northampton Sand Formation (after Taylor, 1963, fig.9). See (Figure 3) for location of numbered sections. Roman numerals (I to V) indicate stratigraphical divisions of Hollingworth and Taylor (1951).

(Figure 5) Nomenclature of Quaternary deposits in the district.

(Figure 6) Spatial relationship of river terrace deposits and alluvium of the River Nene between Cotterstock and Elton. Sections based on borehole and other data (after Harrisson, 1981).

(Figure 7a) Total field magnetic anomalies shown as a colour shaded relief illuminated from the north. Contour interval 10 nT.

(Figure 7b) Bouguer gravity anomalies shown as a colour shaded relief illuminated from the north. Contour interval 1 mGal (1 mGal = 1 x 10−5m/s2).

(Figure 8) Mineral resources of the district.

(Figure 9) Engineering geology characteristics of rocks and soils that outcrop in the district.

Plates

(Plate 1) Boxstone weathering in the Northampton Sand Formation, at Twywell [SP 9415 7726] (P593310).

(Plate 2) Opencast ironstone working in the 1950s, Earlstrees quarry [SP 885 905] (photograph provided by Dennis Taylor of Corby; GS1179).

(Front cover) Cottages in Rockingham built from Northampton Sand and Lincolnshire Limestone [SP 867 916]. (Photograph A J M Barron; GS1183)

(Rear cover)

(Geological succession) Geological succession at outcrop in the Kettering district.

(Index map) Index to the 1:50 000 series maps of the British Geological Survey. The map below shows the sheet boundaries and numbers of the 1:50 000 series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland. The west and east halves of most Scottish 1:50 000 maps are published separately. Almost all BGS maps are available flat or folded and cased. The area described in this sheet explanation is indicated by a solid block. 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

(Figure 5) Nomenclature of Quaternary deposits in the district

Taylor et al. (1963) This publication Bowen (1999)
Alluvium Nene Valley Alluvium Nene Valley Formation Nene Member Nene Valley Formation
First Terrace First Terrace Deposits Ecton Member
Second Terrace Second Terrace Deposits Grendon Member
Third Terrace Third Terrace Deposits Orton Longueville Member
Late glacial gravels Glaciofluvial sand and gravel Wolston Formation Lowestoft Formation
Chalky boulder clay gravels Glaciofluvial sand and gravel
Chalky boulder clay Till (Oadby Member)
Pre-chalky boulder clay gravels Glaciofluvial sand and gravel
Lower boulder clay Till (lower till) ?
Pre/ early glacial gravels Glaciofluvial sand and gravel Milton Formation

(Figure 8) Mineral resources of the district

Mineral resource Source Activity Principal use
ironstone Northampton Sand Formation formerly worked extensively in quarries and underground mines steel
sand and gravel River terrace deposits

Glaciofluvial sand and gravel deposits

Rutland Formation

Kellaways Sand Formation

 working quarries and pits; formerly worked extensively concrete aggregate; building and asphalt sand
limestone Blisworth Limestone Formation

Lincolnshire Limestone Formation

 working quarries and pits; formerly worked extensively in quarries and underground mines building stone, locally used as aggregate, agricultural lime and roofing slates
clay Oxford Clay Formation no activity; may have been worked locally in the past brickmaking

(Figure 9) Engineering geology characteristics of rocks and soils that outcrop in the district

Engineering geological unit Geological units Description/characteristics Engineering considerations
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 casing leachate and methane production Usually diggable Highly variable. Some material may be suitable Essential to determine depth, extent, condition and type of fill and chemistry of groundwater. Care needs to be taken as presence of pollution and contaminated ground likely
Non-cohesive River terrace deposits

Glaciofluvial sand and gravel deposits

 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. 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, fine sand occasionally silty sand. Occasional fine gravel of calcium carbonate. Generally good, but may have problems with artesian boiling Easily diggable. 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
Mixed (Cohesive/ Non-cohesive) Alluvium Soft mid brown to pale grey (weathers dark brown/yellow) clay, occasional sand, gravel and peat lenses. High moisture content and variable composition Poor foundation. Soft highly compressible zones may be present; risk of differential settlement Diggable. Moderate stability, decreasing with increasing moisture content. Running sand conditions possible below the 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) gravelly sandy clay. 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 the deposit and depth to sound strata. Investigate whether any shear zones are present.
Till Firm blue-grey to olive-grey (weathers mid brown) caly with some gravel, occasional cobbles of chalk and flint. Occasional lenses of sand and gravel. Generally good foundation, although sand lenses may cause differential settlement. Possibility of pre-existing shear plans can also cause strength reduction Easily diggable. Trench support may be required if sand lenses or pre-existing shear planes present. May encounter running sand conditions in sand lenses Generally unsuitable Determine the depth and extent of the deposit, especially the frequency and extent of sand lenses. Investigate whether any pre-existing 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. Generally good provided suitable design is adopted as strata liable to ground heave and subsidence. Possibility of strength variability due to fissuring Easily diggable. 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 stress relief and fissuring.
Plastic clay Blisworth Clay Formation Dark grey to blue grey (weathers dark reddish brown) stiff occasionally very stiff clay with some bioclastic fragments. Occasional fissuring. Generally fair foundation, although strata liable to ground heave and subsidence. Possibility of pre-existing shear planes can also cause a strength reduction. Easily diggable. Trench support may be required depending on the presence of shear surfaces Suitable for general fill (mixing or screening required Essential to determine depth and extent of strata, especially whether and pre-existing shear planes are present.
Ironstone Northampton Sand Formation Moderately strong to strong grey-green (weathers dark reddish brown) interbedded sandstone and oolitic limestone with some mudstone beds and bioclastic fragments 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 local highly weathered zones
Massive hard limestone Blisworth Limestone Formation

Lincolnshire Limestone Formation

 Moderately strong to strong mid grey to greenish grey (weathers pale yellow) limestone with some clay bands and bioclastic fragments. Locally ooidal. 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 care taken in selection and extraction Important to identify the presence of local highly weathered zones
Well-joined shelly limestone Cornbrash Formation

Marlstone Rock Formation

 Strong dark blue-grey to light grey (weathers grey speckled pinkish white) coarse-grained bioclastic or ferruginous limestone with some clay bands Generally good provided suitable design is adopted. Bed thickness and presence of highly weathered zones needs to be assessed Dependant 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 local highly weathered zones
Undivided mudstone, siltstone, sandstone Rutland Formation

Grantham Formation

Dyrham Formation

 Weak to moderately strong interbedded greenish grey (weathers brown-grey) siltstone, mudstone, and sandstone with some thin limestone sand and clay beds Generally fair foundation provided suitable design is adopted, although sand and clay beds may cause differential settlement. Possibility of pre-existing shear planes can also cause a strength reduction Easily diggable where rocks are weathered. Trench support may be required if sand beds or pre-existing shear planes 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 siu loading tests advisable to assess bearing strengths at selected sites
Mudstone Whitby Mudstone Formation

Charmouth Mudstone Formation

 Weak to moderately weak medium dark grey (weathers dark brown) micaceous mudstone with some bioclastic fragments Generally good provided suitable design is adopted. Possible strength variability due to fissuring and weathering. Presence of highly weathered zones need to be assessed Easily diggable where rocks are weathered. Ripping or pneumatic tools may be required at depth. Trench support may be required if pre-existing shear planes are present Suitable for general fill under controlled compaction conditions Essential to determine depth and extent of strata, along with the extent of fissuring

(Geological succession) Geological succession at outcrop in the Kettering district

Quaternary Holocene Artificial deposits (infilled ground, worked ground and made ground etc)
Landslip
Tufa up to 3 m
Nene Valley Formation Alluvium; silty clay with gravel lenses up to 4.5 m
Pleistocene River terrace deposits; mainly sand and gravel up to 11 m
Head; stony silty clay and gravel up to 3 m
Wolston Formation Glacial and glaciofluvial deposits; till (stony clay) up to c. 30 m
sand and gravel up to 5 m
Upper Jurassic Ancholme Group Oxford Clay Formation Weymouth Member; mudstone, light grey up to 7 m up to 55 m
Middle Jurassic
Stewartby Member; mudstone, light to medium grey c. 25 m
Peterborough Member; mudstone, dark brownish grey, shaly c. 18 m
Kellaways Formation Kellaways Sand Member; sand and sandstone, fine, with siltstone and mudstone 3–5 m 5–8 m
Kellaways Clay Member; mudstone, dark grey 1.5–3 m
Great Oolite Group Cornbrash Formation limestone; bioclastic and shelly 0.3–3 m
Blisworth Clay Formation mudstone, mottled, with beds of limestone 3–6 m
Blisworth Limestone Formation limestone, bioclastic, ooidal, peloidal, and lime-mudstone 4–8 m
Rutland Formation mudstone, limestone and sandstone, including: Wellingborough Limestone Member, mudstone and limestone, shelly 0–3 m 5–14 m
Stamford Member, sandstone and siltstone 0–5 m
Inferior Oolite Group Lincolnshire Limestone Formation limestone, bioclastic, ooidal, peloidal 0–12 m
Grantham Formation sandstone, siltstone, sandy mudstone and mudstone 0–8 m
Northampton Sand Formation ironstone, ooidal, sandy, and ferruginous mudstone and limestone 0–8 m
Lower Jurassic Lias Group Whitby Mudstone Formation mudstone, grey, with nodular limestone at base 40–58 m
Marlstone Rock Formation limestone, ferruginous, shelly, and ooidal, and ferruginous mudstone 0–1 m
Dyrham Formation mudstone and sandy mudstone 6–15 m
Charmouth Mudstone Formation mudstone with minor tabular and nodular beds of limestone 106–178 m
For a description of the concealed strata see chapter 2.