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Geology of the Coalville district — a brief explanation of the geological map Sheet 155 Coalville

P J Strange, J N Carney and K Ambrose

Bibliographic reference: Strange, P J, Carney, J N, and Ambrose, K. 2010. Geology of the Coalville district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 155 Coalville (England and Wales).

Keyworth, Nottingham: British Geological Survey

© NERC 2010 All rights reserved. Printed in the UK for the British Geological Survey by B&B Press Ltd, Rotherham.

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(Front cover) Snibston Colliery, Coalville, opened and developed by the famous engineers Robert and George Stephenson in 1832, in the concealed part of the Leicestershire Coalfield. Snibston produced coal continuously until December 1983; the site was subsequently preserved and converted into the Snibston Discovery Park Museum. (Photograph J Rayner; P618381).

(Rear cover)

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

Notes

The word 'district' refers to the area covered by the geological 1:50 000 Series Sheet 155 Coalville. Ordnance Survey National Grid references are given in square brackets; they lie within the 100 km squares SK or SP. Symbols in round brackets after lithostratigraphical names are the same as those used on the geological map. The serial number given with the plate captions is the registration number in the National Archive of Geological Photographs, held at the BGS.

Acknowledgements

The manuscript was edited by M A Woods and J E Thomas; cartography by Paul Lappage; pagesetting by A J Hill and A R Minks.

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

© Crown copyright. All rights reserved. Licence Number: 100017897/2010.

Geology of the Coalville district (summary from rear cover)

(Rear cover)

An explanation Sheet 155 Coalville (England and Wales).

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

The Coalville district lies to the west of Leicester and is mainly occupied by undulating countryside interspersed with small villages. In the north, coalmining settlements developed around Coalville and Measham, and in the north-east lies the higher ground of Charnwood Forest where the oldest rocks in the district, the Charnian Supergroup, crop out. Here, volcaniclastic strata and volcanic complexes indicate formation within an active volcanic arc of late Neoproterozoic age. In early Cambrian times, a marine transgression of the Iapetus Ocean deposited a thick mudstone sequence, the Swithland Formation and muddy sediments of the Stockingford Shale Group. In Late Ordovician times, crustal subduction to the east led to the intrusion of small dioritic batholiths. Late Caledonian deformation occurred at the end of the Silurian. No Silurian or Devonian strata are present and the geological record resumes in the early Carboniferous, when shallow marine carbonates were deposited on an eroded surface of much older rocks. These are overlain by deltaic sediments, formed during a period of widespread sedimentation which culminated in the deposition of the Pennine Coal Measures Group in the late Carboniferous. During the Permian Period, widespread erosion created a rugged topography which was largely buried as sedimentation resumed in the Triassic. Basal breccias and sandstones were succeeded by the Mercia Mudstone Group 'red beds.' No sediments of younger Mesozoic strata are preserved within the district.

By the earliest Quaternary times, the district lay within the catchment of the north-east-flowing Bytham River, which deposited sand and gravel along the major palaeovalley. During the Anglian cold stage, ice sheets advanced southwards and westwards depositing glacial till, together with glaciofluvial outwash sands and glaciolacustrine clays in a large proglacial or subglacial lake environment. Extensive postglacial deposits include river terrace deposits and alluvium associated with the present drainage pattern. Accumulations of soliflucted material, head, are extensive in Charnwood Forest. The district is a major producer of hard rock aggregates and has an important brick manufacturing industry using local clay and mudstone. Deep coal mining has ceased but surface mining continues. The geological, mineral extraction, and industrial legacy, have left many planning-related earth science issues that need consideration prior to development.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the geological 1:50 000 Series Sheet 155 Coalville, published as a Bedrock and Superficial Deposits edition in 2009. A more comprehensive account can be found in the Memoir, Geology of the country around Coalville (Worssam and Old, 1988).

In the north-east of the district, 'basement' rocks of Neoproterozoic, Cambrian and Ordovician age crop out. A thick sequence of Cambrian strata also occurs in the southwest of the district. The topography of the basement rocks in Charnwood Forest is hilly with numerous rocky knolls, whereas the younger strata of Carboniferous, Triassic and Quaternary age are characterised by an undulating, locally dissected landscape. The district ranges in elevation from about 60 m along the western margin to 279 m at Bardon Hill in the north-east, the highest point in Leicestershire.

Geological history

The oldest rocks to crop out are in Charnwood Forest and belong to the Charnian Supergroup, of late Neoproterozoic age. This sequence includes well-bedded to laminated volcaniclastic strata, which represent marine accumulations within an active volcanic arc. The Bardon Hill and Whitwick Volcanic complexes indicate that dacitic and andesitic volcanoes were present in the north-western part of the Charnian outcrop. In the Charnwood Lodge Volcanic Formation, boulder-rich volcanic breccias may represent proximal pyroclastic block flows erupted from these centres. Magmatic cessation was preceded by intrusion of the North and South Charnwood diorites. The waning stage of volcanism was marked by uplift and an influx of pebbly detritus, forming the Hanging Rocks Formation at the top of the Charnian Supergroup. In early Cambrian times, a marine transgression of the Iapetus Ocean deposited a thick, mudstone-dominated sequence that includes the Swithland Formation of Charnwood Forest. Elsewhere, muddy sediments of the Stockingford Shale Group accumulated over a period extending from the early Cambrian to Early Ordovician (Tremadoc) age. A recurrence of magmatism in Late Ordovician (Caradoc to Ashgill) times was related to the subduction of crust to the east of the district. Its expression was the series of small batholiths which constitute the South Leicestershire Diorite Suite and numerous lamprophyre and diorite sills of the Midland Minor Intrusive Suite. Folding, cleavage and metamorphism of the Neoproterozoic to Ordovician sequences is a late Caledonian event, attributed to end Silurian deformation.

No strata of Silurian or Devonian age are preserved in the district and it is not until Asbian or Brigantian times, the later stages of the early Carboniferous, that the geological record is resumed. These strata, which are wholly concealed, are dominated by shallow marine carbonates formed in warm equatorial seas fringing an eroded landmass of Neoproterozoic to Ordovician rocks. The overlying Namurian (Millstone Grit) sequence represents deltaic mudstones and sandstones formed during a period of widespread sedimentation that culminated in deposition of the Pennine Coal Measures Group in late Carboniferous (Westphalian) times.

The Coalville District includes three coalfields — the north-eastern part of the Warwickshire Coalfield, the southern half of the South Derbyshire Coalfield, and the southern part of the North-west Leicestershire Coalfield. Much of the sequence in all three coalfields comprises cyclic alternations of coal, mudstone, sandstone, siltstone and fireclay, laid down mainly in lower delta plain environments. Towards the end of Carboniferous times, earth movements involving local block uplifts initiated a change from swampy environments to better-drained, alluvial conditions in which the Warwickshire Group was deposited.

The Permian Period was dominated by erosion, which sculpted upstanding elements of the Neoproterozoic, Cambrian and Ordovician basement rocks into rugged hills and tors, separated by gullies and larger valleys ('wadis'). When sedimentation resumed in earliest Triassic times, this landscape underwent progressive burial, producing a highly irregular unconformity that is now seen in many quarries. The earliest Triassic 'cover' sequences to be deposited were basal breccias succeeded by the Sherwood Sandstone Group, which reflects a major fluvial episode. A change to more arid environments led to deposition of the Mercia Mudstone Group 'red beds', in which aeolian and lacustrine influences were interspersed with fluvial episodes that deposited numerous, thin beds of green, dolomitic siltstone and sandstone ('skerries') as well as two distinctive arenaceous marker beds, the Cotgrave Sandstone Member and the Arden Sandstone Formation (formerly Hollygate Sandstone Member). For the remainder of the Mesozoic (Jurassic and Cretaceous periods) the district probably experienced both marine and terrestrial environments (Cope et al., 1992), but no sediments are preserved.

Cenozoic erosion produced a subdued topography of dissected scarps and dip slopes and by earliest Quaternary times, the district lay within the catchment of the north-east-flowing Bytham River, which deposited preglacial sands and gravels in its various channels and floodplains. Ice sheets of the mid Pleistocene Anglian glaciation subsequently overwhelmed this drainage system and deposited widespread sheets of till, together with glaciofluvial outwash sands and glaciolacustrine clays. The postglacial history of the district dates from the retreat of the Anglian ice sheets, about 430 000 years ago. It was characterised by cycles of drainage development, involving incision and aggradation to produce river terrace deposits in addition to the superimposed effects of periglacial processes during the cold conditions that prevailed, for example, in Late Devensian times.

There is a thriving aggregate industry based on the quarrying of Neoproterozoic dacite, andesite, granodiorite and diorite, Ordovician granodiorite and associated metamorphosed country rock. Underground coal mining has now ceased in the district but there are still opencast coal reserves, either actively being exploited or subject to planning permission. Major brick clay quarries exist in the Mercia Mudstone. There is also extraction of Quaternary sand and gravel in the Cadeby and Huncote areas. Much of the Coalville urban area, and the former mining district between Moira and Measham are undergoing regeneration and here 'applied' geology has an important role to play in evaluating natural resource potential and predicting the constraints, and opportunities, that the rocks and superficial deposits of the district present to further development. The newly created National Forest provides the regeneration focus to the former South Derbyshire and Leicestershire coalfields, and significant tree planting and associated landscaping have transformed large areas of previously derelict land.

Chapter 2 Geological description

Neoproterozoic (Late Precambrian)

The district includes the southern part of Charnwood Forest, a classic area of British geology (Ambrose et al., 2007) exposing rocks belonging to the pre-Carboniferous 'basement' of the English Midlands. The Neoproterozoic rocks form numerous craggy inliers that protrude through an unconformable covering of Triassic strata and Quaternary deposits. Their outcrops are dominated by the Charnian Supergroup, which is cut by two generations of Neoproterozoic diorite intrusions. The lithostratigraphy for the Charnian Supergroup established by Moseley and Ford (1985), and modified for the Loughborough district by Carney et al., (2001), is followed here (Figure 1). Note is also taken of the revisions suggested by McIlroy et al. (1998), who maintained that the stratigraphically youngest strata, of the Brand Group, are probably of Cambrian age. Thus the nomenclature of this account differs in some respects from that used in the Coalville Memoir (Worssam and Old, 1988).

Charnian Supergroup rocks have an aggregate thickness of at least 3000 m in this district, with no base proved. Their horseshoe-shaped outcrop pattern (Figure 1) is controlled by the south-east-plunging Charnwood Anticline. All Charnian rocks are cleaved and recrystallised at epizonal (high greenschist-facies) metamorphic grades, but their volcanic origin is apparent upon microscopic investigation. Chemical studies of the igneous lithologies indicate that they are calc-alkaline in composition, comparable to the magmas of modern volcanic arcs founded upon oceanic or attenuated continental crust (Pharaoh et al., 1987a). Sedimentary structures that include normal grading, load-casting and slumped bedding are developed throughout the Charnian and reflect rapid deposition from debris flows and turbidity currents at depths below storm wave base. All of this activity took place within the 'Avalonian' volcanic arc, which lay off the margin of the Gondwana Supercontinent late in Neoproterozoic times (Pharaoh and Carney, 2000).

A latest Neoproterozoic age for the Charnian Supergroup is indicated by fossils (Plate 1) typical of the newly designated Ediacaran Period of time, which are principally found in the Maplewell Group (Boynton and Ford, 1995). Confirmation is provided by a U-Pb age of 559 Ma (Compston et al., 2002), and one of 562 Ma (BGS, work in progress), both determined from magmatic zircons in fossiliferous Maplewell Group volcaniclastic strata close to the horizon of the Sliding Stone Slump Breccia Member. These dates are consistently younger than the U-Pb zircon age of 603 Ma obtained for a diorite intrusion cutting Charnian Supergroup equivalents at Nuneaton (Tucker and Pharaoh, 1991), just to the south of the district.

The Blackbrook Group (BBG) is represented only by its uppermost unit, the Blackbrook Reservoir Formation (Blk), which crops out within the core of the Charnwood Anticline (Figure 1). Poorly exposed, it consists of medium grey, thinly bedded to laminated volcaniclastic siltstones and fine-grained sandstones (Blk(sa)) of medial to distal turbidite facies. In a small quarry at Charley Knoll [SK 4902 1577] some of the coarser beds show normal grading and have erosional bases; at Green Hill [SK 508 131], slump structures are seen in these strata, just below the Benscliffe Breccia.

Within the Maplewell Group (MG), the predominance of well-stratified volcaniclastic lithologies and occurrence of Ediacaran fossils together indicate marine deposition marginal to the volcanic arc. Maplewell Group strata are mainly represented by the Beacon Hill Formation which is rich in ashy, pyroclastic material; that formation coarsens progressively northwards and westwards into bouldery volcanic breccias of the Charnwood Lodge Volcanic Formation. Associated with those lithologies are volcanic complexes, containing andesitic to dacitic rocks of igneous aspect, representing the root-zones of Neoproterozoic volcanoes (Carney, 1999; 2000). The eastern and southern outcrops of the Beacon Hill Formation (BH) comprise a well-bedded and predominantly fine-grained sequence the base of which is marked by the Benscliffe Breccia Member (Ben). Termed 'Benscliffe Agglomerate' in the Coalville Memoir, this unit probably represents a particularly far-travelled subaqueous pyroclastic flow. It is a grey, massive to thickly bedded lithic lapilli tuff or volcanic breccia (Plate 2), with a coarse, crystal-lithic matrix. The breccia is typically 15 to 30 m thick, but when traced north-westwards it coarsens and thickens, eventually merging with the base of the Charnwood Lodge Volcanic Formation. The overlying, undivided beds of the Beacon Hill Formation, equivalent to the Buck Hills and Beacon Tuff members of the adjacent Loughborough district, are about 1100 to 1500 m thick. In the east and south they mainly consist of blue-grey, siliceous ('flinty'), laminated and graded tuffs, tuffaceous mudstones and tuffaceous siltstones, exemplified by exposures at the type locality on Beacon Hill (Plate 3). Recrystallised volcanic ash shards were noted in thin sections taken from here (Moseley and Ford, 1985). The Sandhills Lodge Member (ShL), exposed only in the north of the district, comprises about 20 m of pale grey, coarse-grained, crystal-rich tuffaceous sandstone enclosing fine-grained sedimentary rafts showing plastic deformation; vitric (y-shaped) shards are common to both matrix and volcanic clasts. Towards the top of the Beacon Hill Formation, the Park Breccia Member (PB), a few metres thick, is intermittently exposed; in Bradgate Park [SK 5309 1148] it consists of grey, coarse-grained sandstone crammed with fragments of laminated volcaniclastic mudstone. In the north-east of the district, a laterally equivalent sandstone-breccia sequence, about 120 m thick, is termed the Outwoods Breccia Member (OtB); it is best developed in the Loughborough district where it was placed at the top of the Beacon Hill Formation (Carney et al., 2001).

The Charnwood Lodge Volcanic Formation (CLV), about 1000 m thick, is considered a lateral equivalent to the Beacon Hill Formation. Its type area in the Charnwood Lodge Nature Reserve includes outcrops of spectacular volcanic breccias (Plate 4), which at the famous 'Bomb Rocks' locality (Watts, 1947) contain fragments of greygreen andesite up to 1.7 m in size. These rocks do not contain volcanic bombs, and are interpreted as the deposits of subaqueous pyroclastic block flows that were initially generated from collapsing volcanic domes or edifices (Carney, 2000), analogous to the recent activity on the Caribbean island of Montserrat (Ambrose et al., 2007). The roots of these volcanoes now occur as the Whitwick and Bardon Hill volcanic complexes.

The Whitwick Volcanic Complex encompasses massive to autobrecciated subvolcanic andesites and dacites occurring in close spatial association with the Charnwood Lodge Volcanic Formation. In this district it includes the Peldar Dacite Breccia (PDB), which is up to 520 m thick in Whitwick Quarry, where it consists of a fine-grained, hyaloclastite matrix enclosing ovoid masses of a distinctive, dark grey porphyritic dacite containing quartz and plagioclase feldspar phenocrysts up to a centimetre in size. Specimens of it can be viewed in the walls of Mount St Bernard Abbey [SK 4580 1620], 150 m north of the district. The breccia contains a sedimentary raft and is interpreted as a cryptodome (Carney, 2000). The Grimley Andesite (GyA), forms a body approximately 70 m thick south of High Tor Farm [SK 4598 1545], and is a green to greyish green, locally plagioclase-phyric andesite or dacite, thought to have been emplaced during the same magmatic event that formed the breccias of the surrounding Charnwood Lodge Volcanic Formation. The Sharpley Porphyrtic Dacite (SyP), a pale grey to lavender grey, fine-grained rock with prominent phenocrysts of plagroclase and quartz, is exposed within and immediately to the north of Whitwick Quarry [SK 4518 1602]. The Bardon Hill Volcanic Complex is probably entirely fault-bounded; for example, its southern contact with the Bradgate Formation is a 20 m-thick shear zone in Bardon Hill Quarry (Carney et al., 2000). The northern part of that quarry exposes the Peldar Porphyritic Dacite (PD), which is identical in lithology to the dacite masses in the Peldar Dacite Breccia. The rest of the quarry is occupied by green-grey, epidotised rocks of the Bardon Breccia (BBr), characterised by abundant angular to subrounded autoliths of andesite. The Bardon Breccia develops glassy, spherulitic textures near to the fault in the south of the quarry. Outside of these workings, it is exposed in crags around the summit of Bardon Hill, and as quarried blocks exhibited in the nearby conservation area (Ambrose et al., 2007). Rafts with contorted sedimentary strata enclosed within the Bardon Hill

Complex probably represent sea-floor accumulations into which the Peldar Dacite and Bardon Breccia were emplaced as semisolid domes and cryptodomes (Carney, 1999). The Complex also includes a large mass of Grimley Andesite, intermittently exposed at Birch Hill [SK 4782 1363].

The Bradgate Formation (BT) commences with the Sliding Stone Slump Breccia Member (SS), up to 10 m thick. This represents a major event during which strata were disrupted within submarine landslides and/or debris flows (Moseley and Ford, 1989). At its type locality in Bradgate Park [SK 5308 1131], the member fines progressively upwards, from basal coarse-grained volcaniclastic sandstone with abundant contorted and folded rafts of grey mudstone (McGrath, 2004). The northwesternmost occurrence of the member is a 3 to 4 m-thick bed overlying the Charnwood Lodge Volcanic Formation at Warren Hills [SK 4579 1516]. The stratigraphically higher beds of the Bradgate Formation, exposed in several places in the type area of Bradgate Park, typically consist of sharp-based and normally graded repetitions of volcaniclastic sandstone, siltstone and mudstone e.g. [SK 5321 1115]. At the Hanging Stone [SK 5223 1528], sandstone is thickly developed and particularly coarse-grained but above this, the uppermost beds of the formation fine down to predominantly mudor silt-rich, distal turbidites with sporadic, indistinct ash shards. In western outcrops the succession contains increased proportions of coarse-grained beds, as at Billa Barra Hill [SK 4668 1142]. In Bardon Hill Quarry, decimetre-scale, graded turbiditic sandstones to the south of the main fault contain andesitic or dacitic clasts thought to have been derived from the Bardon Hill Complex (Carney et al., 2000).

The Hanging Rocks Formation (HR) is about 40 m thick at its type section on Charnwood Forest Golf Course [SK 5247 1501]; a further exposure [SK 5417 1095] in Bradgate Park, 200 m east of the district, is described by Carney et al. (2009). At the golf course, beds of small-pebble conglomerate (Plate 5) up to 5 m thick contain subordinate intercalations of coarse-grained sandstone. Conglomerate is either massive or shows normal grading, and some beds have diffuse, planar stratification. The larger pebbles are principally composed of aphyric and microporphyritic andesite and dacite; some are flow-banded and contain volcanic ash shards, indicative of a welded tuff origin. They are unlike any Charnian lithology, as are the granitic pebbles containing microcline and perthitic feldspar (Carney, 1994). Such compositions suggest that the pebbles were derived from the erosion of a volcanic arc separate to that which gave rise to the underlying Charnian succession. They may have originated on a shoreline and been subsequently redeposited as flysch, by debris flows or turbidity currents in submarine fan or fan-delta environments, explaining the lack of shallow-water sedimentary structures. Delicate glass shards found in siltstones interleaved at the top of the succession indicate that arc volcanism had not entirely ceased at this stage (Carney, 1994), so the inclusion of the formation within the Brand Group, of probable Cambrian age, (McIlroy et al., 1998) is not followed here.

The two sets of Neoproterozoic intrusions found in the district (Figure 1), do not occur in mutual contact, but relationships between their equivalents in the Nuneaton inlier (Bridge et al., 1998) suggest that the North Charnwood Diorites (ND) are the oldest. They form subvertical to inclined sheets, up to 60 m wide and with north-westerly orientations. Along the ridge north-west from Hammercliffe Lodge [SK 491 126], coarsely crystalline hornblende diorite is cut by rhomboidal veins of pink aplite. In thin sections, the coarsely mottled texture of these diorites is seen to result from the enclosure of white, subhedral plagioclase crystals within a quartzo-feldspathic granular base containing several per cent of granophyre. The South Charnwood Diorites (SD) are the most silica and alkali-rich of the Neoproterozoic intrusions (Worssam and Old, 1988), as well as being the thickest and most areally extensive (Figure 1), and may have been emplaced when the Charnian arc had reached its most mature stage of development (Pharaoh et al., 1987b). Previously they were called 'Markfieldite', after their type locality in Hill Hole (Markfield) Quarry [SK 4853 1037]. There and in Bradgate Park e.g. [SK 5339 1013] a distinctive, coarse mottling is imparted by the enclosure of green-grey plagioclase and dark grey mafic aggregates within an abundant, pink to grey, granophyric base. Green-grey aplite dykes are locally present, as in the former quarry west of Groby Church [SK 5202 0763]. Thermally-induced spotting of Neoproterozoic metasediments adjacent to the intrusion was noted in Old Cliffe Hill Quarry (Carney et al., 2000).

Cambrian and Tremadoc (Lower Ordovician)

These strata represent sediments laid down during a transgression of the Iapetus Ocean across the landmass of 'Avalonia'. They unconformably overlie Neoproterozoic strata and intrusions (Bridge et al., 1998), and are exposed in two geographically separate parts of this district. In Charnwood Forest, these strata are included within the Brand Group (BG), the stratigraphically lowermost unit of which is the Brand Hills Formation (BL). The main representative consists of the Stable Pit Member (SP), which has a type section at the Stable Pit in Bradgate Park (Figure 1). There, it consists of highly distinctive, pale grey, cross-bedded quartz-arenites identical to strata in the basal lower Cambrian Hartshill Sandstone Formation of Nuneaton (Bridge et al., 1998), and probably similarly deposited in a nearshore marine setting at the onset of the Cambrian marine transgression. Farther east, the member is best exposed at The Brand [SK 5348 1323], as a sequence at least 9 m thick in which turbidite-facies sandstones show normal grading, parallel stratification and poorly developed crossbedding. Pebbles of 'Markfieldite'-type granophyre-textured diorite occur in the basal, coarse-grained parts of these beds (Carney et al., 2000), which may have been deposited within subaqueous fans during erosion of the Charnian landmass. Although these strata are included within the Stable Pit Member by Worssam and Old (1988), they differ radically from the lithologies at the Bradgate Park type section; the two are never seen in contact. The other component, found only in the western outcrop of the Brand Hills Formation, consists of the Swithland Camp Member (SC), which at its type section [SK 537 122] is represented by purple-grey turbiditic siltstones with abundant intraformational clasts. Pebbles of 'Markfieldite' were found by McIlroy et al. (1998) in this unit, which may be a basal facies and part-lateral equivalent of the pebbly beds described above at The Brand. The Swithland Formation (SG) overlies the Hanging Rocks and Brand Hills formations, with a major unconformity implied at the unexposed contact with the former. In exposures [SK 5389 1212] close to the 'Great Pit', in the Swithland Wood type area, it consists of grey to purplish-grey, well-cleaved mudstones and siltstones with faintly developed bedding. Burrow-mottling is locally extensive at The Brand, and includes Teichichnus. This Phanerozoic trace fossil is better seen where the formation has been used for headstones, for example at Ratby churchyard [SK 5129 0593]; it is a major pointer to the Cambrian age of the unit (Bland and Goldring, 1995) and hence its probable correlation with the Stockingford Shale Group. The Stockingford Shale Group is exposed only in the south-west of the district, where it forms a westerly dipping succession unconformably overlain by Millstone Grit and Coal Measures and faulted against Triassic strata. It is a continuation of the more complete sequence in the Coventry district farther south which, with its abundant fauna of trilobites, brachiopods, acritarchs and protoconodonts, is an important reference section for lower Cambrian stratigraphy in England and Wales. Bridge et al. (1998) provide detailed biostratigraphical information for these beds, a summary of which is given in (Figure 2). Lower Cambrian to Lower Ordovician (Tremadoc) strata are also known from several deep provings in the central and western parts of the district (Worssam and Old, 1988).

Late Ordovician

Three assemblages of 'Caledonian' igneous intrusive rocks occur in the district, all of which have a subduction zone component revealed in their geochemistry (Pharaoh et al., 1987b). Their parental magmas are thought to have arisen within a continentaltype arc, their ages (Bridge et al., 1998) suggesting that this was contemporaneous with the Late Ordovician (Caradoc to Ashgill) igneous activity in the Lake District and Snowdonia. The Midlands Minor Intrusive Suite comprises a number of sills intruded into the Stockingford Shale Group between Atherstone and Merevale, in the south-west of the district. Exposures are poor and confined to small quarries and cuttings. These rocks lack essential quartz; they were termed 'Lamprophyres' by Worssam and Old (1988), and while olivine-bearing spessartite lamprophyre does occupy the thinner sills in the adjacent Coventry district, the suite also includes bodies, up to 70 m thick, of coarsegrained, pyritous hornblende diorite, typically with a lowermost, mafic-enriched layer (Bridge et al., 1998). The South Leicestershire Diorite Suite (SLD) is exposed within small inliers beneath Triassic strata, close to the south-eastern corner of the district. It mainly comprises quartz-diorites (Worssam and Old, 1988), and Le Bas (1972) suggested that the various occurrences are parts of a larger, zoned pluton ranging from diorite to tonalite in composition. Natural exposures occur on Croft Hill [SP 5101 9669], 60 m south of the district and overlooking Huncote Quarry [SP 5121 9692], which is now merged within the much-enlarged Croft Quarry. The rocks there consist of pale grey to pink, coarsegrained, inequigranular verging to porphyritic quartz-diorite; they are locally xenolithic and in the quarry are seen to be intruded by medium grey quartz diorite sheets of synplutonic origin, showing disruption and backveining by their coarse-grained host (Carney and Pharaoh, 1999). The inequigranular variety is found farther west at Barrow Hill Quarry [SP 487 972] and Yennards Quarry [SP 489 970], whereas farther east partially flooded quarries at Enderby Hill [SP 533 996] and Froane's Hill [SK 534 000] expose coarsegrained, equigranular diorite showing hydrothermal alteration along joints. In Charnwood Forest there are sporadic, narrow, subvertical quartz microdiorite dykes (pD). Since they intrude Cambrian strata they are possibly offshoots of the Mountsorrel Complex, a granodiorite pluton of Ordovician age cropping out about 2 km to the east of the district (Carney and Pharaoh, 1999). At the Stable Pit, Bradgate Park [SK 5337 0997], one of these dykes was intruded along a westsouth-westerly orientated fault; the latter's strike-slip sense of movement suggested by subhorizontal slickensides along a fracture in adjacent sandstones of the Stable Pit Member.

Carboniferous

Early Carboniferous (Visean) strata of the Peak Limestone Group are concealed, but have been proved in three boreholes. The best section is seen in the Rotherwood Borehole [SK 3458 1559], where the basal beds are plant-bearing fluviatile sandstones and siltstones which are succeeded by red mudstones and grey marine mudstones and siltstones. The bulk of the sequence, however, comprises shallow marine dolomitic carbonates. Fossil corals suggest a Brigantian age, whereas similar strata in the Stocks House Borehole [SK 1614 1052] contain specimens of the brachiopod Gigantoproductus maximus (McCoy) group and may thus be slightly older, probably late Asbian in age. The lithology and faunas support a correlation with the Ticknall Limestone Formation (TL) of the Loughborough district (Carney et al., 2001).

Namurian strata of the Millstone Grit Group (MG) crop out near Valley Farm [SK 345 155], where sandstones belonging to the Rough Rock (RR) and, farther north, a feature that may correspond to the underlying Chatsworth Grit (ChG) are identified. Millstone Grit strata, including sandstone of the Rough Rock (RR), also crop out around Waste Hill [SP 285 982], in the southwestern part of the district. In Leicestershire, the Rotherwood and Ellistown Colliery [SK 4390 1056] boreholes between them encountered marine bands appropriate to the Kinderscoutian, Marsdenian and Yeadonian substages of Namurian time, suggesting an unconformity is developed between these strata and the underlying Ticknall Limestone Formation. In addition to the typical beds of mediumto coarse-grained, feldspathic sandstone, Millstone Grit strata also include dark grey mudstones and siltstones, with a few thin coals and seatearths.

Strata of Westphalian age, comprising the Pennine Coal Measures Group, are distributed within parts of three coalfields. To the south-west between Shuttington and Atherstone, strata forming the north-eastern part of the Warwickshire Coalfield are about 500 m in thickness; the southern half of the South Derbyshire Coalfield, containing about 650 m of measures, lies farther to the north, whilst the area between Coalville and Desford is underlain by the concealed part of the North-west Leicestershire Coalfield, with about 340 m of Coal Measures that terminate to the east against the Thringstone Fault. The oldest Westphalian strata, of the Pennine Lower Coal Measures Formation (PLCM) are continuous across the north-west–southeast Ashby Anticline (the dividing line between the two latter coalfields) and many seams can be correlated between the two coalfields. The main marine bands (Figure 3) have been identified in all three coalfields. Coal Measures sedimentation accompanied progressive regional subsidence, and periodically the district was subjected to flooded, swampy conditions in which organic material accumulated to form coal seams. The Coal Measures represent cyclic sedimentary sequences deposited mainly in lowlying, delta plain environments. Typically these commence with dark grey mudstones, containing mussel bands or marine bands respectively denoting lacustrine or marine environments. These are succeeded by mudstones interbedded with laminated and micaceous siltstones indicating an overbank or lacustrine depositional environment, which in turn pass upwards into feldspathic sandstones of proximal delta or channel facies. Pale grey ganisteroid sandstones or mudstones represent the seatearths of some coal seams. The principal seams of the three coalfields, and their extent of working, are identified in (Figure 4), (Figure 5) and (Figure 6). The highest beds in the Leicestershire Coalfield are the Pennine Middle Coal Measure Formation (PMCM) of late Duckmantian age. They are intruded by the Whitwick Dolerite, a late Carboniferous or Permian sill, which was encountered in mine workings to the east and south-east of Coalville. In the South Derbyshire Coalfield, the main workable seams lie in the Lower and Middle Coal Measures, between the Kilburn and the Upper Kilburn coals. Above the Upper Kilburn Coal, and throughout the Pennine Upper Coal Measures Formation (PUCM), seams of the 'P' series (Figure 5) are thin but are separated by thick seatearths (up to 4 m), many of which are high quality refractory fireclays: in recognition of its industrial importance, the informal term 'Pottery Clays' has been applied to this part of the sequence.

The youngest Carboniferous strata were formerly referred to the Upper Coal Measures, but are now known as the Warwickshire Group. This distinctive sequence unconformably overlies weathered and red-stained Coal Measures strata and names such as 'red measures' or 'barren measures' have formerly been applied to it. In the South Derbyshire Coalfield the group is wholly concealed and commences, approximately 25 m above the Cambriense Marine Band, with the Etruria Formation (Et). These strata consist of variegated but mainly red-brown mudstones, which attain a thickness of 35 m in the Church Flats Borehole [SK 2605 1452]. In the Warwickshire Coalfield, the Etruria Formation occupies a small, fault-bounded outcrop [SK 251 039] south of Alvecote Priory close to the south-western margin of the district. Here it rests unconformably on the Aegiranum Marine Band, as also seen in the Wood End Shaft [SK 2510 9829], where 68 m of the formation are recorded. Elsewhere in the Warwickshire Coalfield the Coal Measures are unconformably succeeded by the outcropping Halesowen Formation (Ha). This formation overlies the Etruria Formation in the South Derbyshire Coalfield. It comprises green-grey mudstones and fine-grained, micaceous sandstones as exposed at Bramcote Hall [SK 2720 0428], with thin limestone beds containing common remains of the annelid Spirorbis. It is in turn overlain by beds formerly assigned to the Keele Formation, but which are now known as the Whitacre Member (Wit), of the Salop Formation. This comprises variegated mudstones (grey, green, red) with intercalations of green-grey to red, fine-grained sandstone, as can be seen in a quarry [SK 2729 0388] south of Bramcote Hall.

Triassic

Triassic strata occur at rockhead across much of the district. By earliest Triassic times (about 250 Ma) regional subsidence had commenced, resulting in the deposition of mainly continental facies across a landscape that had been moulded by erosion throughout the Permian Period. The Sherwood Sandstone Group represents the initial products of this sedimentation. It commences with the Moira Formation (Mo), occurring in the northern part of the district, and the equivalent Hopwas Breccia (HpBr) in the south-west. These comprise beds of breccia, sandstone and mudstone deposited in valleys on an uneven, eroded land surface. The Hopwas Breccia is dominated by clasts of Palaeozoic quartzite and Carboniferous Limestone, whereas the Moira Formation also contains easterly-derived fragments of Charnian rocks. The Moira Formation appears to pass laterally into the Kidderminster Formation near Measham [SK 336 119]; however, such lithologies are typically diachronous and a Permian age for some of these basal breccias is possible. Subsequently a major river system flowed northwards across the district depositing mainly sandstones, several hundred metres of which accumulated within the actively subsiding asymmetric graben of the Hinckley Basin and, farther north, the Needwood Basin (see inset at foot of Coalville Sheet 155 map). These strata, together with the overlying Mercia Mudstone Group, eventually covered the whole region, burying rugged palaeo-hill ranges that had developed on the Neoproterozoic rocks of Charnwood Forest. The Kidderminster Formation (KdM), formerly identified as the 'Bunter Sandstone and Bunter Pebble Beds' and 'Polesworth Formation' is largely confined to outcrops on the northern and southern margins of the Hinckley Basin, where it comprises yellowish brown to brownish red, friable medium-grained sandstones, locally pebbly and conglomeratic, with sporadic, lenticular beds of red mudstone. Exposures still occur (Plate 6) in the disused Acresford sand pit [SK 3032 1321]. Strata belonging to the Bromsgrove Sandstone Formation (BmS) form the upper part of the Sherwood Sandstone Group. They consist of fine- to medium-grained, commonly cross-bedded sandstones alternating with similarly thick (3–6 m) packages of red mudstone and siltstone. The sandstones locally form good scarp and dip-slope features and are sporadically exposed, as at the crossroads in Netherseal Village [SK 2876 1289] and in the small quarry nearby [SK 2819 1343].

The Mercia Mudstone Group, originally known as the 'Keuper Marl', represents a marked lithological change to mudstones, deposited in mainly arid, aeolian environments with local development of playa mudflat and coastal sabkhas. Variations in clay mineralogy through the sequence (Carney et al., 2004) influence the engineering behaviour of these mudstones and siltstones. The sequence thickens dramatically southwards into the Hinckley Basin, where boreholes show a maximum thickness of about 300 m but gravity data suggest that, in the centre of the basin near Fenny Drayton [SP 350 970], as much as 600 m is preserved. The highly irregular nature of the unconformity with basement rocks is best illustrated at Bardon Hill (Plate 7) and Cliffe Hill quarries. Here, differential compaction causes mudstones to 'sag' downwards into narrow pre-Triassic valleys, or 'wadis'.

The group commences in the Tarporley Siltstone Formation (TpS), formerly the Sneinton Formation and 'Waterstones', the latter name originally used owing to the silvery appearance of its highly micaceous bedding planes. The formation comprises thinly interbedded and interlaminated micaceous sandstones, siltstones and mudstone showing red, grey and green variegation. It is overlain by the Sidmouth Mudstone Formation, which commences with generally red to pale brown laminated mudstones and siltstones of the Radcliffe Member (Rdc), formerly the Radcliffe Formation. The remainder of the formation largely comprising the Gunthorpe Member (Gun) and overlying Edwalton Member (Edw), is lithologically very similar, most outcrops being recognised by sticky, red or rarely green clays ploughed up in fields. Gypsum is also present, as disseminations, veinlets and nodules; for example, in the Leicester Forest East Borehole [SK 5245 0283]. The deposition of these members was punctuated by frequent sheet flood episodes, represented by thin (generally less than 1 m) beds of hard, grey-green dolomitic siltstone and sandstone ('skerries'), commonly showing a number of sedimentary structures including halite (salt) pseudomorphs, ripple marks, parallel lamination, cross lamination, mud cracks and load casts. Two particularly prominent sandstone beds are important marker horizons. The stratigraphically lowest of these is the Cotgrave Sandstone Member (Cot) (formerly the 'Thornton Skerry'), which separates the Gunthorpe and Edwalton members. This grey-green, argillaceous sandstone locally forms a marked feature e.g. [SK 5395 0610]. The stratigraphically higher marker bed, overlying the Edwalton Member, is the Arden Sandstone Formation (AS), which crops out only in the southeast of the district. In the adjacent Leicester district the equivalent unit, known as the Hollygate Sandstone Member (formerly the 'Dane Hills Sandstone'), consists of mediumgrained sandstone interbedded with redbrown siltstone and mudstone. It contains the Crustacean Euestheria minuta von Zieten together with finspines of the shark Palaeobates [Acrodus] keuperinus (Murchison and Strickland) and teeth of Acrodus indicating a marine influence during deposition.

The Branscombe Mudstone Formation (BcM) (formerly Cropwell Bishop Formation) is the uppermost part of the Mercia Mudstone Group preserved in the district. It is lithologically similar to the Sidmouth Mudstone Formation but with the presence of abundant gypsum as disseminations, veinlets, nodules and thin beds.

Quaternary

The Quaternary Period, covering about the last 2.6 million years or so, was marked in Britain by climates that oscillated between temperate and periglacial periods. During the past 900 000 years, those climate oscillations have been extreme, often ranging from Mediterranean-style temperate climates to periglacial/glacial climates during cold stages. These oscillations are reflected in the scheme of marine oxygen isotope (MIS) stages to which the deposits are tentatively referred on (Figure 7). The oldest Quaternary sediments are the preglacial deposits which are believed to be of 'Cromerian Complex' age. They comprise the Bytham Formation (Bth) (Figure 8), representing the main channel and river terrace deposits of the ancient 'Bytham River'. This flowed north-eastwards within a palaeovalley envisaged to have been about 10 to 15 m deep and 600 to 700 m wide where cut into Mercia Mudstone bedrock between Croft and Enderby (Rice, 1981). Where it crops out, the Bytham Formation gives rise to sandy ground strewn with pebbles of mainly Triassic, Carboniferous or recycled Lower Palaeozoic derivation. Although it is seldom exposed, instructive sections (Plate 8) are currently seen at the Huncote sand pit [SP 5181 9829], described in Bridge et al. (1998).

The continental ice sheets that commenced their advance across the district during the Anglian (MIS 12) cold stage laid down glacial deposits. There is currently a debate as to whether the youngest of these deposits, the Oadby Till, may in fact represent a later (MIS 10) and separate ice advance. Following a recent review of Quaternary nomenclature by Bowen (1999), the units named by Rice (1968) are now incorporated as members within the Wolston Formation (Figure 8). Although seldom exposed naturally, this stratified succession is of widespread distribution and elements of it have been described from a number of workings or disused pits (Worssam and Old, 1988). The sequence is most thickly developed within broad depressions on the bedrock surface, termed the Hinckley and Unthank palaeovalleys (Worssam and Old, 1988), which probably corresponded to north bank tributary valleys of the Bytham preglacial river system. The first ice advance originated in the Pennines region and travelled towards the south-south-east, depositing diamicton (boulder clay) of the Thrussington Till Member (T) (Figure 8); the sharp contact between till and the underlying Bytham Formation is exposed in the Huncote sand pit (Plate 8). The Oadby Till Member (Od) represents the lodgement deposits of a later ice advance, its dark grey clay matrix and erratics of flint and chalk indicating that the parent ice sheet had travelled across Cretaceous rocks that today crop out to the north-east of the district, in Lincolnshire and East Yorkshire. That these ice advances were closely synchronous is suggested by apparent interdigitation between the two till types, for example as seen in the various former sand pits around Huncote. The Bosworth Clay Member (B) (Figure 8), known as 'Wolston Clay' in certain adjacent districts, is a tabular body of fine-grade glaciolacustrine sediment mainly laid down after the Thrussington Till in a large proglacial or subglacial water body, part of 'Lake Harrison' named by Shotton (1953) after its discoverer, W J Harrison. Overlying this, glaciofluvial deposits of the Wigston Member (Wi) (Figure 8), previously called the 'Cadeby Sand and Gravel' in this district, are believed to have formed a glacial outwash plain, or sandur. Their generally high content of flint clasts suggests derivation from meltwaters emanating from the front of the encroaching ice sheet that deposited the overlying Oadby Till. Undifferentiated glaciolacustrine deposits were laid down in more restricted bodies of standing water, and commonly occur in association with glaciofluvial deposits.

Undifferentiated glaciofluvial deposits have a patchy distribution, many occurring as lenses or discontinuous sheets within the Thrussington and Oadby tills, or as patches overlying the latter. Glaciotectonic disturbance of the Anglian sequence was demonstrated by Rice (1981) in former exposures at the Huncote sand pit and in various cuttings along the M69 just outside of this district.

River terrace deposits are the remnants of earlier floodplain aggradations that now occur as flights of terraces along trunk valleys of rivers such as the Mease, Sence and Anker. They were formed under a wide range of conditions and record episodic climate change, coupled with continuing regional isostatic uplift that produced successive lateral and vertical incision from Anglian through to Holocene times. The terrace deposits mainly consist of sand-rich, matrix-supported, cross-bedded gravels in which quartzose pebbles and shattered flints are always prevalent. In the southwest of the district, terrace-like features of the Anker Sand and Gravel (A) (Bridge et al., 1998) stand up to 10 m above the surrounding alluvium, suggesting contemporaneity with relatively old terraces, such as the Birstall Member in the adjacent Leicester district (Figure 7). The other named terrace, the Wanlip Member (Wan), has been identified in the south-eastern corner of the map sheet; however, over much of the district only a First and Second River Terrace can generally be distinguished. Of these, deposits of the Second Terrace, which average less than 2 m in thickness, lie at various elevations (5–10 m) above alluvium, and may thus include elements of relatively old deposits such as the Anker or Birstall members. The First Terrace is probably late Devensian to early Holocene in age (Figure 7). It generally crops out within or marginal to floodplains and its deposits are found both underlying the alluvium and projecting 1 to 2 m above it.

Periglacial slope-wasting processes operated mainly during the various postAnglian cold periods, particularly during the Late Devensian and early Holocene stages. In this district they gave rise to head deposits, which are mostly accumulations of soliflucted material and surface hill wash. In Charnwood Forest, the head forms smooth aprons on the hill slopes and is typically marked by large numbers of Charnian blocks in a yellow-brown, silty clay matrix. Thicknesses of between 5 and 8 m have been recorded where head is banked against its source outcrop, as around Bardon Hill [SK 464 130]. Holocene (or Recent) alluvium (Figure 7), a complex of mud, silt, sand and gravel deposits, underlies the flood-prone tracts, water meadows and meander belts of the main rivers and tributary streams. Lacustrine alluvium forms isolated patches, for example to the west of Sheepy Magna [SK 316 013], which may represent the former locations of small, temporary lakes. Small spreads of peat such as occur at Osbaston Hollow [SK 418 062] are typically found close to springs emanating from glaciofluvial deposits. The distribution of calcareous tufa is associated with springs issuing from fine-grained sandstone beds within Mercia Mudstone, and is concentrated along nearby alluvial tracts. It commonly contains gastropod shells, and may be interbedded with peat deposits.

Landslide deposits formed mostly under the wetter freeze-thaw periglacial conditions of the Devensian. They occur mainly along steeper slopes of the district underlain by argillaceous bedrock or clayey superficial deposits, but are minor and have only been mapped at one locality near Beacon Hill [SK 503 156]. Other minor landslides have been noted on slopes underlain by Bosworth Clay, for example around Osbaston Hollow [SK 4176 0620].

Artificially modified ground reflects areas where human influences have changed the natural topography and physical character of bedrock and superficial deposits. Material that has been deposited on a natural or modified ground surface is shown as made ground, which includes industrial sites, road and railway embankments, colliery and quarry spoil tips. In many places, as for example at Bagworth New Wood [SK 442 086] and at the former site of Desford Colliery [SK 458 069], the old colliery spoil tips have been modified by recent landscaping. Landfill sites contain building and demolition rubble, waste from heavy industries and domestic and other waste. Made ground is most extensive, if not ubiquitous, in the main urban centres. Infilled ground comprises areas where excavations have been partly or wholly backfilled, such as the extensive opencast coal operations, and workings for sand and gravel, hard-rock aggregate and brick clay. In places where no surface indication of the original void has remained, the delineation of infilled ground relies on archival sources and may thus be imprecise. The infilling materials may include excavation and overburden waste, construction and demolition materials, industrial waste and domestic refuse. Worked ground represents those voids from which natural material has been extracted, for example open quarries and clay pits, road and railway cuttings and general landscaping. Disturbed ground is not shown on Sheet 155, for reasons of clarity, but is shown on the 1:10 000 series maps. It includes diffuse areas of former outcrop or shallow underground mineral extraction. Landscaped ground represents areas where the original surface has been extensively remodelled, but where it is impractical or impossible to delineate areas of cut or made ground. Constructional developments such as housing estates, playing fields or golf courses, and most urban areas, are associated with landscaped ground, and well illustrated in the extensive developments at Bardon Interlink Business Park [SK 450 115].

Structure and metamorphism

The main structures reflect the location of the district at a boundary between two major basement provinces (Lee et al., 1990), the Midlands Microcraton and Eastern Caledonides. Their mutual junction is thought to be defined by the Thringstone Fault. Surface faults and folds in the district mainly have north-westerly orientations, and are related to the structural grain of the 'Eastern Caledonide' basement. Subsidiary faults trending east to north-east are also attributed to a Caledonide inheritance. Northerly 'Malvernian' basement trends, characteristic of Midlands Microcraton crust, are mainly found in the extreme west of the district, bordering the Needwood Basin.

Two phases of pre-Carboniferous deformation are recognised. The first, in latest Neoproterozoic times, was non-penetrative and is seen in the 'Old' Cliffe Hill Quarry [SK 475 105], where the South Charnwood Diorites were intruded through previously flexured strata of the Bradgate Formation (Carney et al., 2000). A much younger event folded the Charnian Supergroup and Brand Group together within a south-east-plunging anticline, at the same time imposing epizonal (greenschist facies) metamorphism and a pervasive west-north-westerly penetrative cleavage which cuts across (transects) the anticlinal axis (Figure 1). The Charnian cleavage fabric was almost certainly imposed at the same time as the folding and Ar-Ar isotope dating suggests that it was formed in latest Silurian to earliest Devonian times, around 425 to 416 Ma (Carney et al., 2008), thus pre-dating the mid Devonian Acadian Orogeny. In contrast, Cambrian strata in the west of the district are uncleaved and at considerably milder (lower anchizonal) metamorphic grades (Bridge et al., 1998). Many of the major faults in Charnwood Forest have north-easterly orientations, notably the Abbot's Oak Fault (Figure 1), for which a dextral strike-slip throw of about 1300 to 1500 m is suggested (Carney, 1994). Namurian to Westphalian deformation involved limited faulting in response to the onset of thermally induced subsidence across the region e.g. (Fraser and Gawthorpe, 2003). The consequences of this were the observed westward thickening of the Lower Coal Measures across the Boothorpe Fault and Ashby Anticline, and the interseam variations noted between the Warwickshire, South Derbyshire and Leicestershire coalfields. Compression during the end Carboniferous Variscan inversion event reactivated basement structures, reversing the throws of many faults. 'Coalfield synclines' were formed, separated by inversion anticlines, the most important being the basement-cored structure intervening between the Warwickshire and South Derbyshire coalfields, shown by the cross-section on Sheet 155 Coalville. The Thringstone Fault was activated, its reverse movement demonstrated by the hade of 40° north-east where intersected by the Merry Lees Drift mine [SK 4680 0591]. To the east of that fault, Precambrian, Cambrian and Namurian rocks were uplifted by at least 550 m, with the Coal Measures preserved in a strongly asymmetrical syncline developed on the fault's south-western side (Worssam and Old, 1988). Other major coalfield inversion structures include the Warton, Polesworth, Boothorpe and Netherseal faults. Syn-Triassic deformation was most pronounced in Early Triassic times, when several hundred metres of Sherwood Sandstone strata were deposited within the actively subsiding Hinckley and Needwood basins, the margins of which were controlled by the reactivation of Variscan structures as growth faults (Carney, 2007). During a subsequent phase of more uniform subsidence and sedimentation, lasting throughout the later part of the Triassic, former topographical features such as the Charnwood uplands were progressively buried beneath Mercia Mudstone Strata (Plate 7). Post-Triassic deformation is demonstrated by displacements along, for example, continuations of the Warton and Polesworth faults. This may be a manifestation of Cenozoic reactivations generated by far-field stresses associated with the Alpine Orogeny and North Atlantic opening. Palaeogene to Quaternary deformation is principally manifested by the flights of late Quaternary river terraces developed along the major drainage systems. They are in part attributed to drainage renewal within a regime of regional uplift that can be traced at least as far back as the early Neogene.

It should be noted that faults commonly occur as zones up to tens of metres wide, and that the portrayal of such faults as a single line on the map is therefore a generalisation.

Geophysical evidence of the concealed geology

The patterns of gravity and aeromagnetic anomalies shown by (Figure 9a) and (Figure 9b) result from the contrasts in physical properties between various rock masses, and can be used to assess the influence of 'basement' structure on the geological development of the district. The Bouguer anomaly map (Figure 9a) shows ridge-like, north-westerly gravity 'highs' corresponding to the uplifted basement blocks represented by Charnwood Forest and the Nuneaton–Merevale area; these are separated by a gravity 'low' centred on the Hinckley Basin with its thick Triassic deposits. The gravity anomaly ridge at Charnwood Forest shows offsets corresponding to the traces of the Abbot's Oak and Newtown Linford Faults; its western boundary is, however, located some distance from Charnwood Forest and coincides with the Boothorpe Fault and associated structures. The aeromagnetic map (Figure 9b) features a linear, ridge-like anomaly which, with an east-south-east trend, coincides with the Neoproterozoic South Charnwood Diorite intrusion. Shallow basement coinciding with the Stockingford Shale outcrop is indicated by the north-westerly aeromagnetic anomaly in the south-western corner of the sheet. The Ordovician pluton of the South Leicestershire Diorite Suite in Croft Quarry is revealed by the small circular anomaly just outside the south-eastern corner of the sheet.

Chapter 3 Applied geology

Geological factors should be considered at an early stage in planning urban, industrial or rural development because these may give rise to variations in ground conditions over small areas. This natural diversity in geotechnical properties may be exacerbated within a single rock unit by considering the superimposed effects of weathering and/ or periglacial processes. Where mineral resources have been quarried the legacy is areas of derelict land, which have their own unique and highly variable geotechnical and chemical characteristics. By considering the interplay between natural geological and artificial, man-made factors at an early stage in the planning process, appropriate remediation or mitigation measures can be taken prior to development. Geological and geotechnical information may also be used to identify opportunities for development, particularly in respect of leisure, recreation and protection of sites of nature conservation interest including geodiversity.

Mineral resources

Minerals of current interest are those that can be won at the surface; their distribution and outline information is given in the Mineral Resources Map for Leicestershire and Rutland (Harrison et al., 2002) and detailed information is provided in the Leicestershire Minerals Plan Local Review. In the southwest part of the district, within the county of Warwickshire, mineral resource information is covered by a separate document (Bloodworth et al., 1999). Factors that may hinder surface mineral extraction are: significant thicknesses of overburden including man-made deposits, adverse geological conditions rendering the resource economically inaccessible, sterilisation of resources by urban development, conflicts with other forms of land use, possible detrimental effects on the landscape and possible interference with flow paths in floodplain situations. Quarrying activities are important for their impact on ground conditions, both currently and post-restoration, and their visible surface effects are portrayed on Sheet 155 Coalville by areas of worked ground, infilled ground and made ground. The areas around the major quarries, for example at Bardon Hill [SK 457 131] and New Cliffe Hill [SK 458 108], have extensive screening banks of made ground and landscaping to minimise the visual impact of the quarries on the local landscape. The increasing use of quarries and pits for waste disposal has the potential for producing a widely developed, but localised, hazard from toxic leachates and dangerous gases (see below).

Hard rock resources are known to have been worked in the district from Neolithic times but it was during the 19th century that mineral extraction came to dominate this part of Leicestershire (McGrath, 2007). In 1832, George and Robert Stephenson opened the Leicester and Swannington railway to transport coal and hard rock, and it was from this time that the major quarries developed. By 1900, Leicestershire was producing more than one million tonnes of 'Charnwood Forest granite'. Early uses for the stone included paving setts and kerbing, which were widely used on the London streets. Today, the two main quarrying sites at Bardon Hill (Aggregate Industries) [SK 458 129] and Cliffe Hill (Midland Quarry Products) [SK 475 105] are together producing more than eight million tonnes of high quality crushed rock aggregate annually. Besides extensive use in asphalt and roadstone, there is considerable demand for the aggregate as railway ballast and in concrete. Much of the aggregate is destined for the Midlands, East Anglia and south-east England.

The Coalville district is well known throughout the country as a major brick clay producer, with large clay pits excavated into the Mercia Mudstone at Measham [SK 336 108], Heather [SK 394 104] , Ibstock [SK 417 107], Ellistown [SK 438 108], and Desford [SK 462 068]. The colour of the bricks produced varies from pale buff to red, depending on the mineralogy within the horizons worked in the Mercia Mudstone. At Ibstock, Leicester South Pit [SK 417 107], for example, three horizons produce differing fired brick colours. Given the wide extent of the Mercia Mudstone outcrop, the brick clay resources of the district are vast.

Sand and gravel is worked over an extensive area to the east of Cadeby, where the Wigston Member (previously the Cadeby sand and gravel) has been extracted in wide shallow excavations between Cadeby, Brascote and Kirkby Mallory.

Coal has been the long-lived industry on which the major urban developments such as Coalville were founded. Following nationalisation, the newly formed National Coal Board invested in major development work at the 11 remaining collieries in the district and the area became the 'test-bed' for new mining equipment and techniques. In the North-west Leicestershire Coalfield, the shallow dipping strata and the relative absence of faulting helped to pioneer the modern form of longwall retreat mining, resulting in the highest productivity figures for any British coalfield. In 1969, the district employed 9640 in the mining industry and produced 7.14 million tonnes of coal. In the south-western part of the district, the last deep mine, at Baddesley [SP 278 971], closed in 1989. In the Northwest Leicestershire Coalfield, deep mining ceased with the closure of Bagworth Colliery [SK 444 087] in 1991, but opencast mining has continued. Extensive opencast workings extracted more than eight million tonnes of coal from the Sence Valley northwest of Ibstock over a sixteen-year period, ending in 1996, with excavations to depths of 130 m below surface [SK 400 125]. The site has now been reinstated and carefully landscaped as the Sence Valley Forest Park (Plate 9). In the south-west of the district, extensive opencast workings have exploited the steeply dipping Lower Coal Measures of the Warwickshire Coalfield, with surface workings and shallow drift (inclined adit) mining continuing up to 1999 (at Colliery Farm [SP 2843 9767]) in the vicinity of Baxterley and Merevale (Plate 10). Opencast coal workings at Long Moor Surface Mine, to the south-west of Ravenstone [SK 392 130], commenced early in 2008 and will extract 725 000 tonnes of coal for power station generation over a three-year period.

Within the South Derbyshire Coalfield, economically important fireclays occur within the 'Pottery Clays' succession of the Middle Coal Measures, which encompasses seams at and above P39. The fireclays occur as a number of particularly thick seatearth horizons, and assay up to 35.6 per cent alumina in this district according to Worssam and Old (1988). They were formerly mined and quarried for pottery, sanitary and refractory use in the Moira area e.g. around [SK 314 161], but the main focus of this industry has now moved farther north, into the Loughborough district.

In Charnwood Forest, Neoproterozoic and basal Triassic rocks are host to numerous, small-scale base metal mineral occurrences, which include traces of gold at Bardon Hill and Whitwick quarries (Ince, 2007 and references therein).

Water resources and flooding

Strata of the Sherwood Sandstone Group, where concealed beneath the thick Mercia Mudstone cover, have been utilised for groundwater in at least six public water supply boreholes in the district. Limited quantities of groundwater suitable for domestic or small-scale agricultural use were obtained from the Mercia Mudstone Group, but boreholes penetrating the Hollygate Sandstone Member farther east, in the Leicester urban area, show yields of between 1.5 to 12.5 l/s; virtually all is used for industrial purposes. Prior to the demise of deep coal mining in the North-west Leicestershire Coalfield in 1991, an estimated 7.9 Ml/d were pumped from the eight active collieries. Nowadays, most of the district's water is supplied from reservoirs in Derbyshire but there is one significant reservoir at Thornton [SK 474 075] taking water from local streams draining from the foothills of Charnwood Forest. Local farms have obtained limited water supplies from lenses of sand and gravel occurring within till, and from glaciofluvial deposits. Flooding is a potential hazard in low-lying parts of the Sence Valley and its tributaries, and may be exacerbated if certain climate change predictions are fulfilled. Prolonged rain can result in widespread floodplain inundation, whereas shorter, much heavier bursts of rainfall can result in flash flooding in the narrower valleys. A broad relationship exists between geology and the potential extent of flooding. In catchments, the permeability of bedrock can affect the rate of run-off, whereas on floodplains the edge of the modern (Holocene) alluvium approximates to the maximum flooding limit. In the vicinity of Atherstone, there are significant areas of elevated ground occupied by river terraces, and their distribution allows zones of generally lower flood frequency to be recognised within the floodplain.

Geology and planning

Geological assessments are necessary when planning for land-use development, the control of natural resources and the prediction of areas with potential geohazards. There is also a valuable geological heritage in the rock outcrops, quarry exposures and natural landscape. These considerations are set in the context of the continuing need to provide land for housing, commercial, industrial, waste disposal and other developments. In this district, the maximisation of mineral resources such as hard rock aggregate and sand and gravel are further issues that continually arise. The key parameters relevant to construction and development of a site are related to the physical characteristics of the geology, which determines the foundation conditions — the suitability of the ground to support structural foundations, the ease of its excavation and its worth in engineered earthworks and fills. The various issues are summarised for the main engineering geological units in the district in (Figure 10). Factors such as geological structure, slope stability, natural weathering and potential for flooding are also locally important.

Bedrock solution may be anticipated in the parts of the district that are underlain by the Mercia Mudstone Group. However, the unit most prone to this, the Branscombe Mudstone Formation with its sporadic gypsum beds, is only thinly developed in the south-east and no surface features attributable to gypsum dissolution have been recognised in the Coalville district.

Slope stability is a potential issue particularly where building development is extended on to steep valley sides. Undisturbed natural slopes of the district are generally stable in our present climate; unstable conditions would be favoured by increased ingress of water from natural or artificial sources, and at times of exceptionally heavy and sustained rainfall.

Mining subsidence caused by underground coal extraction has been observed in many parts of the district underlain by workings in the South Derbyshire and Leicestershire coalfields. Many rows of miner's cottages and even substantial buildings such as Bagworth Church [SK 449 079] had to be demolished following severe structural damage, especially in the period post-1950. Subsidence also created significant damage to local roads between Desford and Bagworth, and subsidence flashes (water-filled hollows) developed close to the former site of Desford Colliery [SK 455 069]. Similar water-filled flashes were formerly observed along the line of the Coalville (A511) bypass, 600 m south west of Coalville Community Hospital [SK 437 138]. To the south of Broomleys Primary School [SK 442 139], marked linear subsidence features probably indicate the extent of mined panels close to the Thringstone Fault. Similar linear subsidence features were noted in the vicinity of Battleflat [SK 445 108] which may mark approximately the eastern extent of workings from Ellistown Colliery [SK 438 104]. The last deep mine (Bagworth) closed in 1991 and no instances of mining subsidence have been recorded since.

Artificial (man-made) deposits may contain toxic residues, and are thus potential sources of pollution. Significant sites in this district would include areas of landfill and quarry spoil, industrial complexes, railway sidings and sewage works. In old or modern landfill sites with inadequate containment structures, leachate migration into surface watercourses and groundwater could occur if developed on permeable superficial deposits (e.g. glaciofluvial deposits, river terrace deposits, alluvium). Minor aquifers, or impermeable lithologies of the Mercia Mudstone Group may also be rendered susceptible to fluid flow as a result of groundwater movement along faults, joints or zones of gypsum dissolution. Gas emissions may also represent a local hazard. Landfill gas is a type of bacteriological methane, formed by the biodegradation of organic matter in landfill sites under anaerobic conditions; it can migrate from the landfill site through permeable substrates, or along faults and joints, both vertically and laterally. As a general rule none of the bedrocks of the district should be considered for the disposal of degradeable waste material without suitable arrangement for its safe containment. Radon (Rn-222) is a naturally occurring gas, which is derived from rocks, soils and groundwater containing uranium (U) and thorium (Th).

In the Coalville district natural radon levels are generally low (Sutherland and Sharman, 1996). Advice on potential radon hazard and measures for the alleviation of radon buildup in properties can be obtained on application to the Enquiries Desk at the British Geological Survey, Keyworth.

Earthquakes, although of generally small magnitude, nonetheless pose a potential problem in the East Midlands region, which is one of the more seismically active parts of the UK. The historical database shows that damage would be caused by a repeat of the Derby earthquake of 11 February 1957, with an epicentre near Diseworth [SK 450 250], about 10 km north of the district. With a magnitude of 5.3 ML (aftershock: 4.2 ML) and maximum intensity of 6 to 7 EMS, it caused widespread damage to chimneys and roofs in the general area. The BGS earthquake seismology team can provide a detailed analysis of these events and assessments of local seismic risk to major constructions.

Baseline geochemistry

Systematic sampling to establish geochemical baselines for stream waters, stream sediments and soils was undertaken throughout this district as part of the Geochemical Baseline Survey of the Environment (G-BASE) project within the BGS (Scheib et al. 2008). All samples (stream sediment, water and soil) were analysed for at least 50 inorganic substances, all of which are naturally occurring and many of which may also be enhanced, or depleted, in concentration by anthropogenic activity. Further information on the project rationale, protocols and methods may be found in Johnson et al. (2005) and Fordyce et al. (2005). Analysis of 315 regional soils and 160 urban soils samples show that lead (Pb) concentrations are highest in the urban survey area of Leicester, but are also relatively high around the villages of Measham and Donisthorpe to the north and Atherstone and Polesworth to the south, where concentrations may be related to the local coal-extraction industry as well as general urban sources. The regional data are currently being described for later release in a publication and are available on application or can be licensed (see Information sources below).

Conservation sites

Exposures of rocks and superficial deposits, which demonstrate the geology of the area can either occur naturally, or are revealed in quarries and cuttings. The main way such sites can be preserved is by designation as Sites of Special Scientific Interest (SSSI), Regionally Important Geological and Geomorphological Sites (RIGS) or Local Nature Reserves. Listings of SSSIs and RIGS for this district are held by the Leicestershire and Rutland Wildlife Trust. A full audit of geological and landscape features and sites relevant to local authority issues has been compiled by the BGS in partnership with several other locally based organisations, as part of the Leicestershire and Rutland Local Geodiversity Action Plan financed by the Aggregates Levy Sustainability Fund (Ambrose, 2004). The Aggregates Levy Sustainability Fund also supported the publication of a walkers map at 1:25 000 scale showing the rocks and landscape of Charnwood Forest, an accompanying book and a DVD detailing many important geological features of the area (Ambrose et al., 2007).

Information sources

Sources of further geological information held by the British Geological Survey relevant to the Coalville district and adjacent areas are listed here.

Information on BGS publications is given in the current BGS Catalogue of Geological Maps and Books, available on request and at the BGS website (www.bgs.ac.uk). BGS maps, memoirs, books, and reports relevant to the district may be consulted at BGS and some other libraries. They may be purchased from the BGS Sales Desk, or via the bookshop on the BGS website. This website also provides details of BGS activities and services, and information on a wide range of environmental, resource and hazard issues.

Searches of indexes to some of the materials and documentary records collections can be made on the BGS website.

Geological enquiries, including requests for geological reports on specific sites, should be addressed to the BGS Enquiry Service at Keyworth. The addresses of the BGS offices are given on the back cover and at the end of this section.

Maps

Books

Documentary records collections

Detailed geological survey information, which includes 1:10 000 or 1:10 560 scale field slips and accompanying field notebooks is archived at BGS, either as hard copy or digital (scans). Enquiries concerning unpublished geological data for the district should be addressed to the Manager, National Geological Records Centre (NGRC), BGS Keyworth.

Boreholes and trial pit records

The BGS holds collections of boreholes and trial pit records, which can be consulted at BGS Keyworth, where copies of records in the public domain may be purchased. Index information, which includes site references, names and depths for these boreholes can be accessed through the BGS website, where copies can be ordered.

Hydrogeological data

Records of water boreholes, wells and springs and aquifer properties held at BGS Wallingford can be consulted through the BGS Hydrogeology enquiry service.

Geophysical data

These data are held digitally in the National Gravity Databank and the National Aeromagnetic Databank at BGS Keyworth. Seismic reflection data from coal and hydrocarbon exploration programmes is available for parts of the district.

BGS Lexicon of named rock units

Definitions of the stratigraphical units shown on Sheet 155 Coalville are held in the Lexicon database, which can be consulted on the BGS website. Further information on this database can be obtained from the Lexicon Manager at BGS Keyworth.

BGS photographs

The photographs used in this Sheet Explanation are part of the National Archive of Geological Photographs, held at BGS in Keyworth and Edinburgh. Part of the collections can be viewed at BGS libraries at Keyworth and Edinburgh, and on the BGS website. Copies of the photographs can be purchased from BGS.

Materials collections

Information on the collections of rock samples, thin sections, borehole samples (including core) and fossil material can be obtained from the Chief Curator, BGS Keyworth. Indexes can be consulted on the BGS website.

External collections

Plans of underground and opencast coal workings are held by the Coal Authority, 200 Lichfield Lane, Mansfield, Nottinghamshire, NG18 4RG.

BGS Hydrogeology Enquiry Service

British Geological Survey, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB. Telephone: 01491 838800; Fax: 01491 692345.

Conservation sites

Sites of Special Scientific Interest are the responsibility of the Joint Nature Conservation Committee, Monkstone House, City Road, Peterborough, PE1 1JY.

Leicestershire and Rutland Local Geodiversity Action Plan, which includes a full audit of geological and landscape features, sites relevant to local authority geodiversity issues, geological trails and education packs, are detailed in Ambrose, 2004.

References

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

Ambrose, K. 2004. Leicestershire and Rutland Geodiversity Action Plan. British Geological Survey Commissioned Report, C R/04/063N.

Ambrose, K, Carney, J N, Lott, G K, Weightman, G, and McGrath, A. 2007. Exploring the landscape of Charnwood Forest and Mountsorrel. A walkers' guide to the rocks and landscape of Charnwood Forest and Mountsorrel. (Keyworth, Nottingham: British Geological Survey.)

Bland, B H, and Goldring, R. 1995. Teichichnus Seilacher 1955 and other trace fossils (Cambrian?)from the Charnian of Central England. Neues Jahrbuch für Geologie, Abhandlungen, Vol. 195, 5–23.

Bloodworth, A J, Cameron, D G, Harrison, D J, Highley, D E, Holloway, S, and Hough, E. 1999. Mineral Resource Information for Development Plans: Phase One Warwickshire: Resources and constraints. British Geological Survey Technical Report, WF/99/2.

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

Boynton, H E, and Ford, T D. 1995. Ediacaran fossils from the Precambrian (Charnian Supergroup) of Charnwood Forest, Leicestershire. Mercian Geologist, Vol. 13, 165–183.

Bridge, D, 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).

Carney, J N. 1994. Geology of the Thringstone, Shepshed and Loughborough districts 1:10 000 sheets S K 41 N W, S K 41 N E and S K 51 N W. British Geological Survey Technical Report, WA/94/08.

Carney, J N. 1999. Revisiting the Charnian Supergroup: new advances in understanding old rocks. Geology Today, Vol. 15, 221–229.

Carney, J N. 2000. Igneous processes within late Proterozoic volcanic centres near Whitwick, north-western Charnwood Forest. Mercian Geologist, Vol. 15, 7–28.

Carney, J N. 2007. Geological evolution of Central England with reference to the Trent Basin and its landscapes. Mercian Geologist,Vol. 16, pp. 231–240.

Carney, J N, and Pharaoh, T C. 1999. Croft Hill. 221–223 in Caledonian Igneous Rocks of Great Britain. Stephenson, D, Bevins, R E, Millward, D, Highton, A J, Parsons, I, Stone, P, and Wadsworth, W J (editors). Geological Conservation Review Series, No. 17. (Peterborough: Joint Nature Conservation Committee.)

Carney, J N, Horak, J M, Pharaoh, T C, Gibbons, W, Wilson, D, Barclay, W J, and Bevins, R E. 2000. Precambrian Rocks of England and Wales. Geological Conservation Review Series, No. 20. (Peterborough: Joint Nature Conservation Committee.)

Carney, J N, Ambrose, K, Brandon, A, Cornwell, J D, Hobbs, P R N, Lewis, M A, Merriman, R J, Ritchie, M A, and Royles, C P. 2001. Geology of the country between Loughborough, Burton and Derby. Sheet Descripton of the British Geological Survey, Sheet 141 (England and Wales).

Carney, J N, Ambrose, K, Brandon, A, Royles, C P,Lewis, M A, and Sheppard, H. 2004. Geology of the country around Melton Mowbray. Sheet Description of the British Geological Survey, Sheet 142 (England and Wales).

Carney, J N, Alexander, P, Pringle, M S, Pharaoh, T C, Merriman, R J, and Kemp, S J. 2008. 40Ar-39Ar isotope constraints on the age of deformation in Charnwood Forest, U K. Geological Magazine, Vol. 145, 702–713.

Carney, J N, Ambrose, K, Cheney, C S, and Hobbs, P R N. 2009. Geology of the country around Leicester. Sheet Description of the British Geological Survey Sheet Description, Sheet 156 (England and Wales).

Compston, W, Wright, A E, and Toghill, P. 2002. Dating the late Precambrian volcanicity of England and Wales. Journal of the Geological Society of London, Vol. 159, 323–339.

Cope, J C W, Ingham, J K, and Rawson, P F (editors).1992. Atlas of Palaeogeography and Lithofacies. Geological Society of London Memoir, No. 13.

Fordyce, F M, Brown, S E, Ander, E L, Rawlins,B G, O'Donnell, K E, Lister, T R, Breward, N, and Johnson, C C. 2005. G SU E: urban geochemical mapping in Great Britain. Geochemistry: Exploration, Environment, Analysis, Vol. 5, 325–336.

Fraser, A J, and Gawthorpe, R L. 2003. An Atlas of Carboniferous Basin evolution in Northern England. Geological Society of London Memoir, No. 28.

Harrison, D J, Henney, P J, Cameron, D G, Spencer, N A, Evans, D J, Lott, G K, Linley, K A, and Highley, D E. 2002. Mineral Resource Information in Support of National, Regional and Local Planning: Leicestershire and Rutland (comprising City of Leicester, Leicestershire and Rutland). British Geological Survey Commissioned Report, C R/02/24N.

Ince, F. 2007. The mineralogy of Bardon Hill Quarry, Coalville, Leicestershire. Journal of the Russell Society, Vol.10, 27–39

Johnson, C C, Breward, N, Ander, E L, and Ault, L. 2005. G-B AS E: Baseline geochemical mapping of Great Britain and Northern Ireland. Geochemistry: Exploration, Environment, Analysis, Vol. 5, 347–357.

Le Bas, M J. 1972. Caledonian igneous rocks beneath Central and Eastern England. Proceedings of the Yorkshire Geological Society, Vol. 39, 71–86.

Lee, M K, Pharaoh, T C, and Soper, N J. 1990. Structural trends in central Britain from images of gravity and aeromagnetic fields. Journal of the Geological Society of London, Vol. 147, 241–258.

McGrath, A. 2004. A geological walk around Bradgate Park and Swithland Wood. (Keyworth, Nottingham: British Geological Survey.)

McGrath, A. 2007. The rock quarries of Charn-wood Forest. Mercian Geologist, Vol. 16, 241–262.

McIlroy, D, Brasier, M D, and Moseley, J M. 1998. The Proterozoic–Cambrian transition within the 'Charnian Supergroup' of central England and the antiquity of the Ediacara fauna. Journal of the Geological Society of London, Vol. 155, 401–413.

Moseley, J, and Ford, T D. 1985. A stratigraphical revision of the late Precambrian rocks of the Charnwood Forest, Leicestershire. Mercian Geologist, Vol. 10, 1–18.

Moseley, J, and Ford, T D. 1989. The sediment-ology of the Charnian Supergroup. Mercian Geologist, Vol. 11, 251–274.

Pharaoh, T C, and Carney, J N. 2000. Introduction to the Precambrian rocks of England and Wales. 3–17 in Precambrian Rocks of England and Wales. Carney, J N, Horák, J M, Pharaoh, T C, Gibbons, W, Wilson, D, Barclay, W J, Bevins, R E, Cope, J C W, and Ford, T D (editors). Geological Conservations Review Series, No. 20. (Peterborough: Joint Nature Conversation Committee.)

Pharaoh, T C, Webb, P C, Thorpe, R S, and Beckinsale, R D. 1987a. Geochemical evidence for the tectonic setting of late Proterozoic volcanic suites in central England. 541–552 in Geochemistry and mineralization of proterozoic volcanic suites. Pharaoh, T C, Beckinsale, R Dand Rickard, D (editors). Geological Society of London Special Publication, No. 33.

Pharaoh, T C, Merriman, R J, Webb, P C, and Beckinsale, R D. 1987b. The concealed Caledonides of eastern England: preliminary results of a multidisciplinary study. Proceedings of the Yorkshire Geological Society, Vol. 46, 355–369.

Rice, R J. 1968. The Quaternary deposits of Central Leicestershire. Philosophical Transactions of the Royal Society of London, Vol. 262(A), 459–508.

Rice, R J. 1981. The Pleistocene deposits of the area around Croft in South Leicestershire. Philosophical Transactions of the Royal Society of London, Series B, Vol. 293, 385–418.

Scheib, A J, Nice, S E, Fordyce, F M, Johnson, C C, Morigi, A N, Carney, J N, and Schofield, D I. 2008. Soil geochemical baseline data for the urban areas of Corby, Coventry, Derby, Leicester, Northampton, Nottingham and Peterborough in the East Midlands. (Keyworth, Nottingham: British Geological Survey.)

Shotton, F W. 1953. Pleistocene deposits of the area between Coventry, Rugby, and Leamington, and their bearing on the topographic development of the Midlands. Philosophical Transactions of the Royal Society of London, Series B, Vol. 237, 209–260.

Sutherland, D S, and Sharman, G. 1996. Radon— in Northamptonshire? Geology Today, Vol. 12, 63–67.

Tucker, R D, and Pharaoh, T C. 1991. U-Pb zircon ages for late Precambrian igneous rocks in southern Britain. Journal of the Geological Society of London, Vol. 148, 435–443.

Watts, W W. 1947. Geology of the ancient rocks of Charnwood Forest, Leicestershire. (Leicester: Leicester Literary and Philosophical Society.)

Worssam, B C, and Old, R A. 1988. Geology of the country around Coalville. Memoir of the British Geological Survey, Sheet 155 (England and Wales).

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

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

(Index map)

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

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

Figures and plates

Figures

(Figure 1) Pre-Triassic map of Charnwood Forest.

(Figure 2) Summary details of the western outcrop of the Stockingford Shale Group.

(Figure 3) Westphalian marine bands in the Coalville district.

(Figure 4) Coal seams of the Warwickshire Coalfield.

(Figure 5) Coal seams of the South Derbyshire Coalfield.

(Figure 6) Coal seams of the North-west Leicestershire Coalfield.

(Figure 7) Quaternary chronology and correlation for Coalville and adjacent districts.

(Figure 8) Morphology and lithology of preglacial and glacial deposits.

(Figure 9a) Gravity map of the Coalville district.

(Figure 9b) Aeromagnetic map of the Coalville district.

(Figure 10) Geotechnical data for the main geological units in the Coalville district.

Plates

(Plate 1) Charnia masoni (Ford) (P550159). Specimen is 21 cm long.

(Plate 2) Benscliffe Breccia Member viewed at the Pillar Rock, Benscliffe Wood [SK 5149 1244]. Fragments of andesite and dacite occur in a coarse-grained volcaniclastic matrix (P708661).

(Plate 3) Tuffs at Beacon Hill (P708662).

(Plate 4) Volcanic breccia, 'Bomb Rocks' (P708663a).

(Plate 5) Sandstone overlain by conglomerate, Hanging Rocks Formation (P708664).

(Plate 6) Kidderminster Formation, Acresford (P708665).

(Plate 7) Bardon Hill Quarry: Neoproterozoic rocks and the Triassic unconformity (P549628).

(Plate 8) Bytham Formation overlain by Thrussington Till, Huncote sand pit (P549727).

(Plate 9) Sence Valley Forest Park, Ibstock (P626932). View east towards Bardon Quarry and the higher ground of Charnwood Forest. Extensive opencast coal workings ceased in 1996 and the area landscaped and reinstated for recreational and agricultural use.

(Plate 10) Opencast coal mining at Colliery Farm between Baxterley and Atherstone (P509620). Note the inclined drift under construction to work the Bench Coal below the opencast pit.

(Front cover) Snibston Colliery, Coalville, opened and developed by the famous engineers Robert and George Stephenson in 1832, in the concealed part of the Leicestershire Coalfield. Snibston produced coal continuously until December 1983; the site was subsequently preserved and converted into the Snibston Discovery Park Museum. (Photograph J Rayner; P618381).

(Rear cover)

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

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

Figures

(Figure 2) Summary details of the western outcrop of the Stockingford Shale Group

Formation Description Age
Merevale Shale (MvSh) (>90 m) Mainly grey to green-grey mudstones with thin beds of siltstone and sandstone; marks change to more oxygenated conditions. Early Ordovician (Tremadoc)
Monks Park Shale (MPSh) (90 m) Dark grey, micaceous, pyritous mudstones with dolomitic limestone and phosphate nodules. Predominantly dysaerobic environments. Late Cambrian (Merioneth Series)
Moor Wood Sandstone(MWF)(20 m) Fine-grained sandstone with convoluted bedding; interleaved with micaceous mudstone. Late Cambrian (Merioneth Series)
Outwoods Shale (OwSh) (300 m) Dark grey and pale grey laminated to thinly bedded mudstone, burrowed and pyritous; sporadic thin sandstone with flute casts. Mid to late Cambrian (St David's to Merioneth Series)
Mancetter Shale (McSh) (50 m) Grey mudstone; tabular beds of load-casted, glauconitic, fine-grained sandstone. Mid Cambrian (St David's Series)
~disconformity~
Abbey Shale (ASh) (40 m) Green, blue-grey and black, laminated, pyritous mudstone, locally with calcareous and glauconitic sandstone and limestone beds. Anoxic in parts. Mid Cambrian (St David's Series)
Purley Shale (PSh) (200 m) Purple and grey interbedded blocky mudstone; sporadic thin siltstone and finegrained sandstone beds. Late early to early mid Cambrian (late Comley to early St David's Series)

(Figure 3) Westphalian marine bands in the Coalville district

Standard names Local names
Warwickshire S Derbyshire Leicestershire
BOLSOVIAN Cambriense Nuneaton Top
Shafton Shafton
Edmondia Edmondia
Aegiranum Overseal
DUCKMANTIAN Sutton Seven Feet Sutton
Haughton Haughton Not named
Clown Clown
Maltby Two Foot ?Two Foot
Vanderbeckei Molyneux Bagworth
LANGSETTIAN Burton Joyce
Langley Not named
Amaliae Not named
Meadowfarm
Parkhouse
Listeri West Hill Alton Alton
Honley
Springwood
Holbrook
Subcrenatum Fair Oak Not named Not named

(Figure 4) Coal seams of the Warwickshire Coalfield

Coal seam Map code Thickness (m) Former extent of workings
Half Yard Coal HY 0.6–1.3 Not worked
Four Feet Coal 4 FT 0.6–1.4
Thin Rider Coal TR 0.5–1.1
Two Yard Coal 2 YD 1.6–2.6 Worked underground and opencast
Bare Coal BA 0.3–1.7 No evidence of working
Ryder Coal R 0.4–2.1 Not been worked
Ell Coal E 0.3 Not mined underground but some opencast working
Threequarter Coal TQ 0.1–0.4
Nine Feet Coal 9 FT 0.7–2.0
High Main Coal HM 0.2–0.7 Worked underground at Birch Coppice
Smithy Coal SM 0.5–0.8 Worked underground and opencast
Seven Feet Coal 7 FT 1.1–2.5 Worked extensively underground and opencast
Yard Coal 0.9 Not worked
Deep Rider Coal DR 0.2–1.6 Worked underground and opencast
Double Coal D 0.9–1.4
Top Bench Coal 0.9–1.8
Bench Coal B 0.9–3.8
Stumpy Coal STU 0.3–0.5 Worked opencast at Holly Park [288 970]
Stanhope Coal 0.3–0.7 Not worked

(Figure 5) Coal seams of the South Derbyshire Coalfield

Coal seam (alternative name) Map code Thickness (m) Former extent of workings
P15–P29 Numerous minor seams 'Pottery Clays Formation': formerly worked opencast from numerous sites where clay stockpiled; coal mainly a by-product. Sites now in various stages of restoration — last workings ceased operation in August 2006
P31 (Derby) 0.3–0.6
P33/34 (Ell) 0.8
P39 0.6–0.8
Upper Kilburn (Dicky Gobler) UK 0.3–1.2 Worked opencast between Donisthorpe and Willesley
Block Coal (Stone Smut, Jack Dennis, Watson) BL 0.2–1.5
Yard Coal 0.2–0.8 Not worked
Upper Cannel Coal UC 0.3–1.0 Small-scale underground working at Oakthorpe Colliery
Little Coal (Five Feet, Four Foot) L 0.2–1.5 Worked underground and opencast
Little Kilburn Coal (Smith's Smuts, Cannel) LK 0–2.0.9
Rider Main 2.6–4.4
Over Main
Nether Main (Little Woodfield Rider)
Little Woodfield Coal LW 0.5–1.2
Lower Main Coal (Shale, Slate) LM 0.6–1.2
Woodfield Coal W 1.2–2.0 Worked underground between Measham and Rawdon
Stockings Coal (Two Yard, Low Main, Linton) ST 1.1–2.5
Eureka Coal (Bottom) E 0.3–1.2
Joyces Coal (Joicey) 0–0.1 Worked underground at Measham Colliery
Upper Stanhope Coal 0.3–1.2
Lower Stanhope Coal 0.2–0.4
Well Coal (Threequarter) 0.1–0.9
Twelve Inch Coal 0–0.6
Clod Coal C 0.3–1.7 Not worked
Kilburn Coal K 0.3–1.7 Worked opencast
Norton Coal N 0–1.7 Not worked
Belperlawn Coal 0–1.0 Not worked

(Figure 6) Coal seams of the North-west Leicestershire Coalfield

Coal seam (alternative name) Map code Thickness (m) Former extent of workings
Excelsior Coal 0.8–2.7 Worked underground (1965 to 1991) between Ellistown and Bagworth
Minge Coal (Stone Smut Rider, Forest) 1.1–2.4 Worked underground between Hugglescote and Bagworth
Five Feet Coal (Block, Stone Smut) 1.4–2.0 Worked underground between Whitwick and Bagworth
Splent Coal (Swannington) 1.1–1.6
Threequarters Coal (Jack Head, Two Foot, Soft) 0.1–0.6 Not worked
New Main Rider Coal (Slate Rider) 0.1–0.6 Not worked
New Main Coal (Slate, Swannington, Main, Ten Foot Mine) 0.6–2.0 Worked underground
Swannington Yard Coal 0.5–1.3 Not worked
Cannel 0.9–1.7 Not worked
High Main Coal (Stinking, Stinking Rider) HM 0.9–1.7 Worked underground and opencast
Upper Main Coal (Main, Main-Lower Leaf) UM 1.5–2.0
Smoile Coal (Ell) SM 0.5–1.2 Worked underground at Whitwick
Upper Lount Coal (Second Lount, Yard) UL 0.7–1.1 Poor quality but worked opencast
Middle Lount Coal ML 0.9–2.1 Worked extensively underground and opencast
Nether Lount Coal NL 1.1–3.2 Worked extensively underground and opencast
Yard Coal YD 0.8–1.2 Worked extensively underground and opencast
Lower Main (Roaster, Deep Main) LM, LMU, LML 1.4–3.1 Worked extensively underground and opencast
Clod Coal C 0.3–1.0 Worked opencast at Sence Valley
Kilburn Coal K 0.5–1.0

(Figure 7) Quaternary chronology and correlation for Coalville and adjacent districts

Stage (of the Quaternary) Approximate age (years BP) MIS Coventry district Coalville district Leicester district
Holocene 0

10 000**

 warm, temperate 1 Alluvium, peat, head etc Alluvium, lacustrine alluvium, peat, head Alluvium, peat, head etc
Devensian 15 000

26 000

 cold, glacial 2 First Terrace First Terrace Syston Member*
65 000 warm, temperate 3
80 000 cold, periglacial 4 Second Terrace (age uncertain) ?Second Terrace and Wanlip Member* Wanlip Member*
115 000 warm, temperate 5d–a
Ipswichian 126 000 warm, temperate 5e
195 000 cold, periglacial 6 Anker Sand and Gravel Member (age uncertain) Anker Sand and Gravel Member Birstall Member*
240 000 warm, temperate 7
297 000 cold, periglacial 8 Knighton Sand and Gravel*
330 000 warm, temperate 9
367 000 cold, periglacial 10
Hoxnian 400 000 warm, temperate 11
Anglian 450 000 cold, glacial 12 Undifferentiated and Oadby Till

Wolston Sand & Gravel

Wolston Clay

Thrussington Till

 Undifferentiated and Wolston Formation:

Oadby Till Member

Wigston Member

Bosworth Clay Member

Thrussington Till Member

 Undifferentiated and Wolston Formation:

Oadby Member

Wigston Member Rotherby & Glen Parva members

Thrussington Member
Cromerian Complex 850 000–

450 000

 warm, temperate ?12–

21

 Baginton Sand and Gravel Bytham Formation Bytham Sand and Gravel Formation
pale blue cold, glacial * River terrace deposit members of the Soar Valley Formation

dark blue cold, periglacial ** 14C age pink warm, temperate

(Figure 8) Morphology and lithology of preglacial and glacial deposits

Type Morphology and thickness Deposit description
Glaciolacustrine deposits Mainly developed and thus thickest in former 'Lake Harrison' basin in the vicinity of Market Bosworth, where they are up to 30 m thick.

Generally have broadly lensoid to sheet-like geometry.

 The Bosworth Clay Member is a bluegrey to brown, mottled clay or silt, locally laminated, with carbonate nodules and sporadic layers of sand, commonly red. Debris (?dropstones) of Triassic rocks and chalk. Locally interbedded with tills of both Thrussington and Oadby type.
Glaciofluvial deposits Laterally continuous tabular sheet up to 10 m thick. Other outcrops are discontinuous with variable thickness up to 9 m. Wigston Member: Generally brown and yellow, unconsolidated to loosely cemented fluvial sand and gravel with

clasts of flint, 'Bunter' quartz and locally chalk. Locally interbedded with red sand and gravel containing Carboniferous (e.g. coal) and Triassic clasts.
Till (boulder clay) Wide featureless spreads, commonly on interfluves but also preserved in palaeovalleys, where locally

deformed. The two named till members are each up to 15– 20 m thick locally. Undivided tills include complex mixtures

of both members, possibly due to glaciotectonic activity.

 Oadby Till Member: Lodgement till of brown to blue-grey clay with common scattered quartz pebbles and fragments of flint, chalk, Jurassic limestone and various Jurassic fossils; locally with Charnian fragments. Sandy to gravelly layers and lenses. A Lower Jurassic-rich variant ('Raunscliffe Till') seen in the A50 cutting [SK 487 107]. A Triassic-rich variant is locally present in the east.

Thrussington Till Member: Lodgement till of red to red-brown silty or sandy clay with sand or gravel lenses. Fragments of green-grey Triassic siltstone or sandstone and quartzose 'Bunter' pebbles; also Carboniferous sandstone, coal and limestone fragments. Charnian debris locally present.
Preglacial deposits Discontinuous sheets, in places forming terrace-like outcrops; at least 15 m seen in Huncote pits [SK 512 982]. Bytham Formation: Fluvial deposits of the north-easterly flowing Bytham River. Commonly comprises basal coarse sands and gravels overlain by red, fineto medium-grained sand. Clasts mainly

of Triassic, Carboniferous or Jurassic derivation.

(Figure 10) Geotechnical data for the main geological units in the Coalville district

Engineering geological units Geological units Characteristics Foundations
Soils
Mixed Loose– dense Till (boulder clay) Stiff–v.stiff stony sandy clay–variable Generally good but depends on extent of water-bearing sand/silt layers/lenses
Cohesive/noncohesive soils Soft– firm Head Soft–firm clay, sandy silty clay. Highly variable (head) Generally poor due to high variability,

presence of relict shears & plasticity
Alluvium Lacustrine alluvium Glaciolacustrine

deposits

 Soft–firm, loose–dense, fine-coarse, clay, silt, sand, gravel Soft, highly compressible (organic?) zones. Dense gravels are good
Non-cohesive soils Med dense River terrace deposits Glaciofluvial deposits Medium dense sand & gravel with laminated silts Generally good. Variable thickness in channels
Man-made deposits Made ground Infilled ground Highly variable in composition, depth, & density Very variable. May be very compressible. Pollution hazard
Landslide deposits Landslide Variable. Content as per origin but weaker; voided, possibly saturated Generally unsuitable unless special measures taken to stabilise ground
Rock
Weak sandstone Mercia Mudstone Group (parts)

Pennine Coal Measures Group

 Weak, thinly bedded, flaggy sandstone Generally good depending on thickness, cementation, & weathering state
Strong sandstone Pennine Coal Measures Group Millstone Grit Group

(parts)

 Strong to thickly bedded, blocky sandstone Good foundation. Presence of open joints, block movement possible on slopes?
Mudrocks Soft– hard Mercia Mudstone Group (parts)

Pennine Coal Measures Group (parts)

 Soft mudrock–hard clay. Fissured, jointed, shaly? Plasticity generally low but locally high Generally good, but variable. If high plasticity, subject to heave. Slaking on exposure. Sulphate attack on concrete
Very strong igneous, volcanic & metamorphic rocks Charnian Supergroup North and South Charnwood Diorites

South Leicestershire Diorites

 Strong to extremely strong, jointed & faulted Generally good. Removal of loose/ weathered material may be required