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Geology of the Leicester district— a brief explanation of the geological map Sheet 156 Leicester

J N Carney and K Ambrose

Bibliographic reference: Carney, J N, and Ambrose, K. 2007. Geology of the Leicester district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50?000 Sheet 156 Leicester (England and Wales). Keyworth, Nottingham: British Geological Survey.

Keyworth, Nottingham: British Geological Survey, 2007.

(Front cover) Front cover. Exposure of strongly cleaved Precambrian volcaniclastic rocks of the Bradgate Form-ation, Charnian Supergroup, in Bradgate Park [SK?5422?1119]. Viewed towards the south-east, with Cropston Reservoir dam in middle distance (Photographer T?Cullen; P530782).

(Rear cover)

(Geological succession)

Notes

The word 'district' refers to the area covered by the geological 1:50?000 Series Sheet?156 Leicester. 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.

Acknowledgements

This Sheet Explanation was abridged by J?N?Carney from the Sheet Description, which contains full acknowledgements and references. The manuscript was edited by A?A?Jackson: cartography by R?Demaine: pagesetting by A?Hill.

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

© Crown copyright. All rights reserved. Licence Number: 100017897/2007.

Geology of the Leicester district (summary from rear cover)

An explanation of Sheet 156 (England and Wales) 1:50 000 series map.

The City of Leicester and ground to the east is included in this district. In the west is the wide floodplain of the River Soar, which skirts the hilly areas of 'basement' rocks that form Charnwood Forest and Mountsorrel. In the east, scarps and gentle dissected dip slopes form the wolds that are typical of the Lower Jurassic landscape of the Midlands, although the bedrock features are somewhat masked by glacial deposits.

Precambrian rocks exposed in the north-west comprise volcaniclastic strata of the Maplewell Group (Charnian Supergroup), overlain by conglomeratic beds of the Hanging Rocks Formation. Mudrocks of the Swithland Formation are thought to rest unconformably on the Precambrian rocks. They represent the basal part of a thick early Palaeozoic 'basement' sequence that ranges from Cambrian to early Ordovician age. Plutonic rocks of the Mountsorrel Complex and South Leicestershire Diorites intrude the mudrocks, and have been dated as Late Ordovician (Caradoc). The basement rocks were folded, fractured and cleaved during a deformational event thought to be of end-Silurian age in this region.

Triassic strata rest unconformably on the basement rocks, and are mainly of the Mercia Mudstone Group overlain by the thinner Penarth Group. The youngest bedrock comprises the Lower Jurassic Lias Group up to the basal part of the Middle Jurassic Inferior Oolite Group (Northampton Sand Formation). An extensive Quaternary sequence is described, dating from pre-Anglian times. It includes various glacigenic deposits, a sequence of river terrace deposits and alluvium of the Soar floodplain. Mass-wasting deposits, loosely classified as 'head', are widespread and landslides occur locally.

Granodiorite is extracted from the 'super-quarry' at Buddon Wood, Mountsorrel, and in the past ironstone was mined from the Lower Jurassic strata.

Earth science-related issues that should be considered when planning decisions have to be made are reviewed. Some of the principal factors include the physical properties of the rocks, slope stability, and the water and mineral resources of the district.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the geological 1:50?000 Series Sheet 156 Leicester, published as a Bedrock and Superficial Deposits edition in 2007. A more comprehensive account can be found in the Sheet Description (Carney et al., in press); the memoir from the original geological survey (Fox-Strangways, 1903) is available in facsimile form.

The oldest rocks crop out in the north-west of the district and are of Precambrian and Ordovician age. Overlying this 'basement' is an eastwards-younging sequence of Triassic and Jurassic strata, and there is a locally thick covering of Quaternary deposits. The topography of the basement rocks is hilly with numerous rocky knolls, whereas the Mesozoic strata are characterised by an undulating, locally dissected landscape, which ranges in elevation from about 44?m OD in the River Soar valley to a maximum of 230?m on Whatborough Hill [SK?7673?0591].

Geological history

The oldest rocks belong to the Charnian Supergroup of Late Precambrian (Neoproterozoic III) age. They consist of volcaniclastic strata of the Bradgate Formation, which represent marine accumulations within an active volcanic arc. The waning stage of volcanism was marked by uplift and an influx of pebbly detritus, forming the overlying Hanging Rocks Formation.

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 and possibly equivalent, strongly hornfelsed metasediments in contact with Mountsorrel Complex granodiorite. Relatively less metamorphosed mudstones, proved beneath the Triassic in deep boreholes around Leicester, contain fossils indicating an early Ordovician (Tremadoc) age, and are correlated with the Stockingford Shale Group. A recurrence of magmatism in late Ordovician (Caradoc) 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 Mountsorrel Complex, and South Leicestershire Diorites. Folding, cleavage and metamorphism of the Precambrian to Ordovician sequences is attributed to end-Silurian deformation.

In early Carboniferous times, thin Namurian to early Westphalian sedimentary strata were deposited along the northern fringe of the district. A tholeiitic basalt dyke, intruded into the Mountsorrel Complex, records a late Carboniferous igneous event.

The Permian period was dominated by erosion, which sculpted upstanding elements of the Precambrian and Ordovician basement into tors, gullies and larger valleys ('wadis'). When sedimentation re-commenced in earliest Triassic times this landscape underwent progressive burial, producing a highly irregular unconformity that is now seen in quarries. The earliest Triassic 'cover' sequence to be deposited was the Sherwood Sandstone Group, which is not exposed; it reflects a major fluvial episode, but is here only thinly developed. 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, very thin beds of green, dolomitic siltstone and sandstone ('skerries') as well as two arenaceous beds, the Cotgrave and Hollygate sandstone members. Near the top of the group, locally thick seams of gypsum occur within the Cropwell Bishop Formation. This is then sharply succeeded by the Blue Anchor Formation, with its highly distinctive grey to green dolomitic mudstones.

Continental sedimentation was brought to a close in latest Triassic (Rhaetian) times by a marine transgression, the first deposits of which were the grey mudstones and siltstones of the Penarth Group, laid down under brackish or lagoonal conditions. Throughout the Lower Jurassic, generally warm, shallow, subtropical seas in which the Lias Group accumulated occupied the district. The Blue Lias Formation consists of thin limestones interbedded with mudstone, the latter occurring in greater proportion within the overlying Charmouth Mudstone Formation. The group as a whole accumulated under progressively deepening waters, but a major regressive episode is represented by relatively shallow-water sequences such as the Dyrham Formation, and in particular the ferruginous Marlstone Rock Formation. The latter was followed by a reversion to deeper water, more quiescent conditions in the Whitby Mudstone Formation. The base of the Middle Jurassic Inferior Oolite Group is an unconformity, and in this district the group is represented by the Northampton Sand Formation, which is the youngest bedrock unit. It was deposited in a near-shore high-energy environment during a further regressive episode.

Cainozoic 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 sands and gravels in its various channels and floodplains. Ice sheets of the mid-Pleistocene, Anglian glaciation subsequently overwhelmed this drainage system and in the process deposited widespread sheets of till, together with glaciofluvial outwash sands and glaciolacustrine clays. These deposits are particularly thick where infilling preglacial or synglacial valley systems. 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 such as prevailed, for example, in late Devensian times. Modern erosion is dominated by stream incision, with land slippage common on oversteepend slopes developed on the Jurassic mudrocks.

The district is not particularly rich in minerals, but there is a thriving aggregate industry based on the quarrying of Ordovician granodiorite. There is further potential for the extraction of Quaternary sand and gravel. Triassic sandstones constitute important aquifers, and there are other, minor aquifers from which groundwater has been abstracted. Much of the Leicester urban area is 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.

Chapter 2 Geological description

Precambrian

Precambrian rocks in this district are restricted to Bradgate Park, which lies along the south-eastern fringe of Charnwood Forest. They belong to the Charnian Supergroup, the full exposed sequence of which is revealed within an anticlinal structure in the adjacent Coalville and Loughborough districts (Worssam and Old, 1988; Carney et al., 2001).

The Charnian Supergroup is at least 3800?m thick in the adjacent districts, where it consists mostly of volcaniclastic strata (Moseley and Ford, 1985). A late Precambrian (Neoproterozoic III) age is indicated by the presence of Ediacaran fossils in strata of the Maplewell Group (Boynton and Ford, 1995), and is in keeping with a SHRIMP U/Pb zircon age of 559?Ma obtained from that part of the sequence (Compston et al., 2002). In this district all of the Charnian strata belong to the Maplewell Group, the rocks of which have a calc-alkaline geochemical signature and were generated above a subduction zone bordering one sector of a Precambrian ('Avalonian') volcanic arc system (Pharaoh et al., 1987a). They are predominantly fine-grained, and consist of volcaniclastic turbidites deposited in moderate to deep waters. The cleavage fabric is a distinctive feature of these rocks that was imposed much later (see below). The Hallgate Member of the Bradgate Formation (BT) crops out within a small area of Bradgate Park adjacent to Coppice Plantation, just to the west of Cropston Reservoir. The exposures typically form small craggy knolls, as at the northern end of the outcrop [SK?5422?1119] where about 0.4?m of grey, volcaniclastic siltstone is overlain by 0.2?m of fine-grained volcaniclastic sandstone in thin, parallel-sided beds (Front cover). Southwards along the footpath skirting the plantation wall, other exposures e.g. [SK?5410?1092] show undulation, contortion and rafting of certain beds or laminae. A small quarry near the southern margin of Coppice Plantation [SK?5406?1088] exposes 3?m of massive to laminated volcaniclastic mudstone and siltstone. Individual beds commonly coarsen down to sandstone (Plate?1) and some beds show convoluted lamination. Under the microscope, siltstone laminae are crammed with angular fragments of quartz and plagioclase feldspar, whereas the very fine-grained, 'mud' layers contain unresolvable grainy material that may represent highly comminuted, and partly degraded or recrystallised volcanic ash.

The Hanging Rocks Formation (HR) is exposed at the foot of a south-facing slope [SK?5417?1095], one of only two such occurrences in Charnwood Forest: volcaniclastic pebbly sandstone (about 3.4?m) is overlain by conglomerate (4?m). The latter comprises three beds, all mainly composed of small pebbles (5–15?mm) and granules either close-packed within, or separated by, a coarse to very coarse sand matrix (Plate?2); there are sporadic large pebbles, up to 100?mm across. The pebbles are mainly of cream, fine-grained, quartz-phyric tuff and welded tuff, but quartz and quartzite clasts were previously recorded. The north-north-west dipping pebble elongation of this exposure is an artefact of the local Charnian cleavage. In relation to older Bradgate Formation strata on the hillside several metres to the north, the formation has an apparent low topographic expression, which here is attributed to local faulting that has also imparted the steep, north-easterly dip of these beds. Fluvial or shoreline reworking of material eroded from a volcanic landsurface is most probably the source of the pebbly detritus so unique to the Hanging Rocks Formation; however, in the adjacent Coalville district, these strata contain microscopic shards of volcanic glass indicating contemporary volcanism, possibly during the waning stages of Precambrian magmatism (Worssam and Old, 1988). Other workers place this formation as an unconformity-bounded unit at the base of the Brand Group, which also includes Cambrian strata.

Cambrian and Ordovician

Two major groupings of these rocks are recognised; those cropping out along the eastern edge of Charnwood Forest form part of the Swithland Formation (of the Brand Group), whereas the rocks exposed along the western contact of the Mountsorrel Complex, and proved in certain deep boreholes in the Leicester urban area, are referred to the Stockingford Shale Group. The Swithland Formation (SG), commonly referred to as the 'Swithland Slates', is at least 200?m thick in Charnwood Forest (Figure?1). It was formerly included within the Charnian Precambrian sequence, but a Lower Cambrian age is now thought likely, following the discovery of the trace fossil Teichichnus in local gravestones fashioned from these rocks (Bland and Goldring, 1995). The contact with the underlying Hanging Rocks Formation is nowhere exposed, but a gradational passage, or an intervening unconformity, have both been suggested. In this district the exposures occur along the footpath skirting the eastern edge of Swithland Wood [SK?5425?1200]; [SK 5420?1224], wherein lies part of the formation's type section (Moseley and Ford, 1985). The grey to maroon metasiltstones show bedding, but exposures are dominated by weathering along a millimetre-spaced slaty cleavage. Under the microscope the rocks contain abundant fine-grained mica locally showing a strong preferred orientation, reflecting the cleavage seen at exposure.

Probable correlatives of the Stockingford Shale Group (SSh) occur as borehole provings at between 180–250?m depth, beneath the Sherwood Sandstone Group, in the Evington area of Leicester. The Crown Hills Borehole [SK?6245?0384] penetrated a 47?m (true) thickness of slightly metamorphosed, hard, dark or medium grey, micaceous mudrock and silty mudrock with paler grey siltstone laminae, some of which are graded or show disruption into raft-like masses. Parting typically occurs along a weakly developed, bedding-parallel shaly fissility that is interpreted as a diagenetic fabric. Borehole samples have yielded shelly faunas demonstrating an early Ordovician age, above the Clonograptus tenellus Biozone of the Tremadoc (Molyneux, 1991).

Three localities expose hornfelsed metasedimentary rocks along the western contact of the Mountsorrel Complex intrusion. These rocks are barren of fossils. They are variously folded, cleaved and fractured, and have been extensively recrystallized (see below). In the grounds of Quorn House, a small disused quarry known as the 'old gravel pits' [SK?5583?1556] reveals a few metres of grey or purple-brown, thinly bedded, granodiorite-veined metasedimentary rocks. Farther south, grey, thinly bedded and locally folded, hornfelses (Plate?3) are exposed around the top edge of a partially flooded quarry [SK?5567?1355]. Numerous granodiorite dykes and veins were formerly seen, cutting these rocks. The third occurrence is near the water line on the eastern shore of Swithland Reservoir by Kinchley Hill [SK?5597?1389], only a few metres from an exposure of basified granodiorite. It consists of grey to brown-grey, thinly bedded, fine-grained metasedimentary rock with a mineral assemblage indicative of the pyroxene-hornfels facies of contact metamorphism.

Ordovician intrusive rocks

The Mountsorrel Complex and South Leicestershire Diorites represent the exposed tips of small batholiths, which although largely concealed by younger strata can be traced in the subsurface by their magnetic expression (Figure?10). A third intrusion, the Melton Mowbray Granodiorite, has been confirmed beneath Carboniferous strata in the Kirby Lane Borehole [SK?7324?3176], 350?m north of the district. Previous disputes concerning the age of these intrusions were resolved by a U-Pb determination of 451 to 452?Ma on zircon fractions from the Mountsorrel Complex and South Leicestershire Diorites at Enderby (Pidgeon and Aftalion, 1978), indicating a late Ordovician (Caradoc) age. The plutons are all of calc-alkaline composition, indicative of their generation above a subduction zone, and the parental magmas arose within a continental-type arc. Their emplacement partly overlapped with Ordovician magmatism in the Lake District and Snowdonia. The Ordovician plutons are part of a largely concealed basement domain, termed the 'Eastern Caledonides' by Pharaoh et al. (1987b).

The Mountsorrel Complex (Msr) forms craggy outcrops in the Buddon Wood area (Figure?2), which is now mostly occupied by the eponymously named quarry [SK?562?150]. This hilly landscape is the expression of a pre-Triassic mountainland that is currently being exhumed from beneath its unconformable covering of Mercia Mudstone strata (Plate?5).

The granodiorite is grey to pink and inequigranular, verging to porphyritic, with small, dark grey dioritic xenoliths; it is accessible at localities such as Castle Hill Quarry Site of Special Scientific Interest (SSSI) [SK?5759?1496], around Castle Hill itself [SK?5812?1492] and by Halstead Road [SK?5737?1417].

Pink aplite sheets commonly show northerly trends (Figure?2), with two generations indicated by cross-cutting relationships (Plate?4). Basified granodiorite typically forms a peripheral envelope around the main granodiorite but occupies a more central position at Cocklow Quarry. At the knoll by the Kinchley shore of Swithland Reservoir [SK?5607?1404], it appears similar to the normal inequigranular granodiorite, with the addition of between 20 to 40 per cent of mafic minerals in irregular clumps that include hornblende, biotite and secondary chlorite.

Xenolith-rich granodiorite is exposed on the quarried knoll by Kinchley Hill [SK?5618?1399]. It contains abundant dark grey xenoliths of quartz microdiorite up to a metre across; some are highly angular whereas others have rounded, somewhat cuspate margins reminiscent of pseudo-pillows. This latter xenolith shape prompted Le Bas (1968) to suggest that the quartz-diorite parent was not completely solidified at the time of its incorporation by the granodiorite magma.

Quartz-diorite forms the island of Brazil Wood [SK?5580?1364] in Swithland Reservoir. It is a grey, markedly heterogeneous rock with veinlets and schlieren of pink granodiorite. Under the microscope the felsic minerals consist of quartz and zoned plagioclase, the latter in part forming poikilitic crystals; green-brown hornblende associated with relict clinopyroxene forms up to 50 per cent of the rock. Magnetometer surveys previously conducted suggest that the diorite extends a little way to the east of Brazil Wood, as shown in (Figure?2). Quartz-microdiorite forms small exposures of pale grey, medium-grained rock by the waterline on the Kinchley shore of the Swithland Reservoir [SK?5597?1388].

Gabbro forms the submerged knoll in Swithland Reservoir, to the north-west of Brazil Wood [SK?5566?1373]. Previous accounts describe it as a crushed and altered hornblende gabbro consisting of labradorite and ophitic brown hornblende, the latter commonly enclosing relict green clinopyroxene.

The South Leicestershire Diorites (SLeD) occur as small, isolated outcrops that have been extensively quarried out. In Coal Pit Lane Quarry at Enderby [SP?5414?9912], the pink, medium-grained quartz-diorite is studded with small (about 2?mm), white plagioclase phenocrysts set in a quartz-plagioclase base; under the microscope clinopyroxene and plagioclase are both heavily altered. Quartz-diorite also occurs at Enderby Warren Quarry [SK?5413?0007], which is now a restored landfill site. It is a dark, purplish grey to pink, medium- to coarse-grained rock composed mainly of pink plagioclase, quartz and green-grey clusters of clinopyroxene and chloritised hornblende. Triassic mineralisation has resulted in coatings of palygorskite along joint planes.

Carboniferous

Seismic reflection profiles indicate that concealed Carboniferous strata wedge in along the north-eastern margin of the Leicester district. They were proved in the Kirby Lane Borehole, 350?m north of the district, which shows 110?m of Namurian and lowermost Westphalian strata resting on the weathered surface of the Melton Mowbray Granodiorite and overlain by Triassic strata (Carney et al., 2004).

A basic dyke, 2 to 3?m thick, cuts the Mountsorrel Complex in Buddon Wood Quarry [SK?5620?1501]. The dyke seen at Castle Hill Quarry SSSI [SK?5759?1496] is probably the same body displaced by strike-slip faulting (Figure?2), but only about 1?m remains out of the 6 to 7?m thickness originally exposed here. The dyke rocks are of fine- to medium-grained basalt or microgabbro, with intergranular or subophitic textures. Although a Carboniferous age is most likely, these rocks contain interstitial quartz and are thus dissimilar to the alkali olivine basalts that are interleaved with Westphalian strata in the Vale of Belvoir coalfield farther north (Carney et al., 2004).

Triassic

Triassic strata (Figure?3) crop out only in the western part of the district. They commence with the Sherwood Sandstone Group (SSG), which is wholly concealed but shown in boreholes to be 15 to 35?m thick beneath Leicester. To the east and south the group thins to less than 10?m, and is probably absent in some parts. Strata proved in the deep Leicester boreholes belong to the Bromsgrove Sandstone Formation (BmS), made up of fine-grained sandstone beds alternating with similarly thick (3–6?m) packages of red mudstone and siltstone (Figure?3). In northern parts of the district, pebbly sandstone of the Nottingham Castle Sandstone Formation (NtC), formerly known as the 'Bunter Pebble Beds', may be the sole Sherwood Sandstone representative.

The Mercia Mudstone Group (MMG; for new nomenclature see (Figure?4)), originally the 'Keuper Marl', represents a marked lithological change to mainly arid, aeolian environments (Figure?3). Variations in clay mineralogy through the sequence (Carney et al., 2004) influence the engineering behaviour of these rocks (Figure?12). Boreholes show maximum thicknesses of about 200?m beneath Leicester, but less than 70?m of these strata are present in the south-east of the district.

The highly irregular nature of the unconformity with basement rocks is demonstrated at Buddon Wood Quarry [SK?5634?1506]. Here the mudstone strata 'sag' downwards into pre-Triassic valleys, a result of differential compaction. On the intervening ridges, what were formerly bare-rock tors of Mountsorrel Complex granodiorite have been exhumed by quarrying, their surfaces smoothed and sculpted into fluted shapes by the abrasive action of sand-laden wind (Plate?5).

The largely concealed part of the group commences in the Sneinton Formation (Snt) formerly known as the 'Waterstones' owing to the silvery appearance of its highly micaceous bedding planes. It is normally overlain by laminated siltstones of the Radcliffe Formation, but this has not been identified in the Leicester district. The overlying Mercia Mudstone Group formations are lithologically very similar (Figure?3), most outcrops being recognised by sticky, red or rarely green clays ploughed up in fields. Their deposition was punctuated by numerous fluvial episodes, represented by thin (less than 1?m) beds of hard, grey-green dolomitic siltstone ('skerries').

Two particularly thick sandstone beds are formational markers (Figure?4). The stratigraphically lowest of these is the Cotgrave Sandstone Member (Cot) at the base of the Edwalton Formation. This grey-green argillaceous sandstone is probably equivalent to the 'Thornton Skerry' mapped in the Coalville district to the west. Its outcrop forms a feature south of the Rothley Brook, at Glenfield [SK?5395?0610]. The Hollygate Sandstone Member (Hly), at the top of the Edwalton Formation, is 10–15?m thick, and includes red mudstone interbeds. It was originally known as the 'Dane Hills' or 'Upper Keuper' sandstone, and is correlated with the Arden Sandstone of the West Midlands (Figure?4). Water exploration boreholes show that it is persistent beneath Leicester, although displaced by faulting. Exposures in the railway cutting adjoining Western Park show just over 3?m of buff to greenish grey, fine- to medium-grained, moderately well sorted sandstone (Plate?6). Fossils previously found in the associated mudstone beds included the arthropod Euestheria minuta together with plant remains and crustacean or annelid tracks; the sandstones yielded fin-spines of the shark Acrodus keuperinus and teeth of Acrodus indicating a marine influence.

In the Cropwell Bishop Formation (CBp) gypsum forms abundant disseminations, veinlets, nodules and thin beds, as formerly seen at the Gipsy Lane brickpit (Figure?3). Here, a sequence of several thin gypsum beds occurring below the junction with the Blue Anchor Formation possibly correlate with the Newark Gypsum farther north (Carney et al., 2004). Down-section, a single gypsum bed 2 to 3?m thick in the railway cutting [SK?620?095] near Thurmaston, and at 78?m depth in the Crown Hills Borehole [SK?6245?0384], is possibly the local continuation of the Tutbury Gypsum.

About 3?m of grey-green mudstones belonging to the Blue Anchor Formation (BAn) is exposed at Gipsy Lane (Figure?3), where previous exposures showed halite pseudomorphs, ripple marks and fossils that include abundant fish scales, teeth and Euestheria minuta. Farther south, in the former South Wigston (Glen Parva) brickpit [SP?5847?9851], beds of greyish yellow sandstone with shark's teeth were seen in the formation. Many of these earlier sections also revealed an irregular and disconformable contact with the underlying Cropwell Bishop Formation.

The Penarth Group crops out along the middle and upper slopes of an escarpment surmounted by the Lias Group, although that feature is in many parts masked by Superficial Deposits. The unit is poorly exposed, but former sections (Wignall et al., 1989) showed that it thickens southwards, from about 8?m at Gipsy Lane [SK?617?069] to 12?m at the former South Wigston brickpit [SP?5847?9851]. It corresponds exactly with the former 'Rhaetic Series' beds; however, the Triassic Rhaetian Stage encompasses strata between the middle of the underlying Blue Anchor Formation and the lowest part of the overlying Wilmcote Limestone Member of the Lias Group. Wignall et al. (1989) showed that the Penarth Group rests disconformably on the Blue Anchor Formation.

The Westbury Formation (Wby) corresponds to the 'Lower Rhaetic', 'Black Shales' or 'Avicula contorta shales' of the earlier surveys. Its deposition was a consequence of the widespread Rhaetian transgression, which is also confirmed by the occurrence of microfossils (acritarchs) indicative of a significant marine influence. The unit generally coarsens upwards, from fissile, dark grey, laminated mudstone to thin, pyritous sandstone. Dark grey, clay-rich soils are typical on the outcrop. The 'Rhaetic Bone Bed', at the disconformable base of the formation, is generally less than 0.1?m thick, and in the former brickpit at Spinney Hills [SK?602?042] it yielded fossils that included: Gyrolepis, Saurichthys apicalis, Hybodus minor, Ceratodus, Nemacanthus monilifer, Ichthyosaurus, Plesiosaurus and Sargodon tomicus, as well as sporadic pebbles thought to have been derived from Charnwood Forest, 10?km to the north-west. Here, Wignall et al. (1989) found a bed of fossil brittle-stars, including Aplocoma damesii, approximately 1.3?m above this bone bed, together with the bivalves: Rhaetavicula contorta (Porlock), Protocardia rhaetica (Merian) and Eotrapezium ewaldi (Bornemann). The boundary with the overlying Cotham Member coincided with the disappearance of most fauna, with the exception of E. ewaldi. The Cotham Member (Ctm) represents the Lilstock Formation at map-scale. It consists of green-blue or grey-green, somewhat sandy and locally pyritous laminated mudstones, with thin beds of nodular limestone. The latter commonly have hardground surfaces, indicative of semi-emergent conditions. Nearshore, intermittently stormy sea environments are further suggested by the occurrence of wave-rippled sandstones. Later in the Cotham Member, a relative sea-level rise culminated in restricted lagoonal or carbonate ramp conditions recorded by deposition of the Langport Member; an impersistent, thin, nodular, porcellanous limestone at the top of the Lilstock Formation. The top of the Member is at least locally an erosion surface, as demonstrated at Gipsy Lane where the bed is blanketed by shelly Lias Group limestone containing bone fragments and the subzonal ammonite Caloceras johnstoni, the planorbis Subzone apparently being missing (Wignall et al., 1989).

Latest Triassic and Jurassic

These strata were deposited in shallow-water to semi-emergent marine environments (Figure?5). As they are seldom exposed there are few detailed descriptions; past workers recorded sections in quarries and temporary exposures, but these are mostly restored or overgrown.

The Lias Group (Figure?5), of latest Triassic and Early Jurassic age, either crops out or forms the rockhead beneath Superficial Deposits over approximately 70?per cent of the district. The strata dip between 1° and 2° to the south-east, and this structure, combined with differential erosion of the harder beds, produces scarp and dip-slope features of varying magnitude that are locally displaced by faulting. The group is subdivided according to the recommendations of Cox et al. (1999). Biostratigraphical control of the succession is based on ammonite biozones defined with considerable precision in Britain; however, only a few new fossil finds were made during the present survey, and this Sheet Explanation therefore relies on former accounts, notably by Fox-Strangways (1903), Hallam (1955) and Howarth (1980).

The Blue Lias Formation (BLi) (Figure?5) is the lateral equivalent of the Barnstone, Barnby and Granby members of the Scunthorpe Mudstone Formation north of the Sileby Fault (Carney et al., 2004). Its upper boundary with the Charmouth Mudstone Formation is not easily traced in the field, but in the south of the district approximates to the junction between the bucklandi and semicostatum ammonite zones. The Wilmcote Limestone Member (Wct) (Figure?5) was formerly called the 'Limestone Series' and 'Hydraulic Limestones', and it equates with the Barnstone Member of the Melton Mowbray district (Carney et al., 2004). The lowermost 2 to 3?m of strata are devoid of ammonites and the informal name 'Pre-Planorbis Beds' was assigned to them. In the Melton Mowbray district, conodont faunas indicate that these lower beds are of latest Triassic, Rhaetian age. The top of the 'Pre Planorbis Beds' therefore marks the commencement of the Jurassic Period, and is taken at the first occurrence of ammonites of the genus Psiloceras. A complete 9?m-thick sequence through the Wilmcote Member was proved in the Crown Hills Borehole [SK?6245?0384], which showed beds of hard limestone, generally less than 0.5?m thick, alternating with dark grey mudstone. The 'Pre-Planorbis beds' were possibly encountered in the Knighton area of Leicester, but were absent from the Spinney Hills and Gipsy Lane brickpits, and from the former pit at South Wigston [SP?5847?9851] when visited by Wignall et al. (1989). Fossils from there included: Psiloceras planorbis, Modiolus minimus, Plagiostoma sp., Liostrea hisingeri, Gryphaea arcuata and echinoderms. The Saltford Shale Member (Figure?5) was formerly known as the 'Angulata Clays' and equates to the Barnby Member of the Scunthorpe Mudstone Formation as defined farther north. It has not been differentiated on the map, but can be identified in the log of the Crown Hills Borehole as 17?m of 'blue clay', with one thin limestone bed. The member forms the middle part of the Blue Lias Formation in the Leicester urban area, but thins and disappears towards the eastern margin of the district. The Rugby Limestone Member (Figure?5) is not differentiated as such on the map, but it probably comprises the whole of the Blue Lias Formation in the east of the district. It is the lateral age-equivalent of the Granby and Beckingham Members of the Scunthorpe Mudstone Formation farther north, but is considerably more calcareous, consisting of fossiliferous argillaceous limestone beds ('cementstones'), in regular alternation with mudstone. In the Evington area of Leicester, the Rugby Limestone was formerly exposed in two lime pits: Crown Hills No.?1 [SK?6204?0374] and No.?2 [SK?6150?0393]. In a further pit near Kilby Bridge [SP?6138?9692], 21?limestone beds were formerly recorded.

The Charmouth Mudstone Formation (ChM) (Figure?5) includes the upper part of the Scunthorpe Mudstone Formation recognised farther north (e.g. Carney et al., 2004) and corresponds to the upper part of the 'Lower Lias Clays and Sands' of the previous survey. The sequence is not well known, but is probably comparable to strata in the Thorpe by Water Borehole, about 6?km beyond the south-eastern corner of the district, which consist of pale to dark grey, fissile and blocky, locally calcareous, fossiliferous mudstones with common siderite-mudstone and calcite-mudstone nodules, and phosphatic pebbles. Geophysical logs of this borehole show the presence of the '70' Marker Member, which is equivalent to the Loveden Gryphaea Bed farther north, but in the Leicester district this cannot be traced south of Burrough on the Hill [SK?750?102].

In a former brickpit north of Ingarsby ?[SK?6887?0617], 'ironstone' fragments were dug from beds possibly equivalent to the semicostatum Zone, suggesting a datum equivalent to that of the Foston Member ('Ferruginous Limestone Series' or 'Plungar Ironstone') of the Melton Mowbray district, although Hallam (1968) noted that 'no definite trace' of those ferruginous oolitic ironstones have been found south of the Vale of Belvoir. A further brickyard [SK?7166?0808] north-west of Lowesby, formerly exposed a 6?m section (Fox-Strangways, 1903) featuring 'shales with ferruginous nodules'. Oxynoticeras conyarti, and other ammonites, suggest a stratigraphical level comparable with the Sand Beck Nodule Bed, near the base of the Charmouth Mudstone as defined farther north.

Temporary exposures of mudstone with ferruginous limestone beds in the middle part of the Charmouth Mudstone Formation, opened during the construction of the Billesdon Bypass [SK?7084?0294], yielded an abundant fauna (Blake, 1986), which included the ammonite Acanthopleuroceras valdani (d'Orbigny), proving a stratigraphical position within the valdani Subzone of the ibex Zone.

The Dyrham Formation (DyS) (Figure?5) corresponds to the 'clays and Sands' and the lower part of the 'Middle Lias' of the earlier mapping. It crops out along the escarpment capped by the Marlstone Rock Formation, where cuttings and small landslide back-scarps reveal pale to medium grey, yellow-weathering, poorly cemented micaceous siltstone with beds of mudstone and fine-grained sandstone. Beds with siderite-mudstone nodules, and less persistent beds of well-cemented, fossiliferous sandstone and ferruginous limestone also occur. The topmost bed is an impersistent unit informally known as the 'Sandrock', which was previously regarded as part of the overlying Marlstone Rock Formation. It has been noted in the Pickwell– Somerby–Burrough on the Hill area, where field brash includes calcareous sandstone, ferruginous sandstone and shell-detrital wackestone with a siderite mudstone matrix. Macro and microfossils from the upper part of the formation at the Tilton railway cutting SSSI [SK?761?055] indicate the presence of the subnodosus–gibbosus sub-zones of the margaritatus Zone; the gibbosus Subzone is either very thin or absent, suggesting a non-sequence at the base of the overlying Marlstone Rock Formation.

The Marlstone Rock Formation (MRB), previously known as the 'Marlstone Rock Bed', is relatively resistant to erosion and forms the crest of a prominent dissected scarp dominating the central and north-eastern parts of the district. It produces characteristic deep orange-brown soils, strewn with field brash. The formation reaches its greatest thickness, of 6–9?m, just north of Billesdon Lodge [SK?7321?0452], but farther south it thins to less than a metre. Where well developed it consists mainly of sandy, shell-fragmental berthierine-ooidal limestone, shell detrital, ferruginous limestone and iron grainstone (Figure?5). The ochreous weathering colours, and locally high iron content, are due to the alteration of berthierine and siderite to the iron oxide, limonite. At the national type section represented by the SSSI in the Tilton railway cutting [SK?7635?0530] to [SK 7615?0557], the formation is 5.7?m thick (Plate?7). Microfossils (dinoflagellate cysts) collected here suggest that the lower part of the formation lies within the spinatum Zone, with the Pliensbachian–Toarcian boundary occurring about 4 to 5?m above the base. Near the top of the formation, the ammonite fauna is indicative of the semicelatum Subzone (tenuicostatum Zone). Calcareous microfossils from the uppermost, 'Transition Bed' of the Marlstone Rock Formation at the type section additionally indicate the tenuicostatum to bifrons ammonite zones.

The Whitby Mudstone Formation (WhM) (Figure?5) is equivalent to the 'Upper Lias Clay' division of Fox-Strangways (1903). It crops out along the lower part of the steep escarpment capped by outliers of the Northampton Sand, where rare exposures indicate grey mudstones with a few beds of nodular limestone; calcite- and siderite-mudstone nodules are scattered throughout. At the Tilton SSSI [SK?764?054] the lower several metres of the formation consists of grey mudstone containing phosphatic ooids, and thin limestone beds. The ammonites Harpoceras falciferum, H. serpentinum, H. elegans, Dactylioceras sp. and Phylloceras heterophyllum have all been found in these beds, which record the peak of an early Toarcian anoxic event.

The Inferior Oolite Group commences in the Mid Jurassic (Aalenian Stage). This coincided with a major regressive event and an abrupt change to relatively shallow water, compared with the environments prevailing during deposition of the Whitby Mudstone Formation. In this district, the sole representative of the group is the Northampton Sand Formation (NS), which reflects deposition under nearshore, high-energy conditions. This unconformable unit is the lateral equivalent of the 'Northampton Ironstone' farther south, and as it is relatively resistant to erosion it forms a series of six outliers, the most prominent being Whatborough Hill [SK?77?06] and Robin-a-Tiptoe Hill [SK?774?044]. On the outcrop, soils of dark reddish brown, sandy loam are strewn with fragments of ferruginous sandstone and ooidal iron grainstone; nonferruginous sandstone and ferruginous limestone also occur on Whatborough Hill.

Quaternary

The Quaternary Period, covering approximately the last two million years, is marked in Britain by extreme oscillations of climate, ranging from severely cold glacial or periglacial periods to mildly temperate conditions. These oscillations are reflected in the scheme of marine oxygen isotope stages (MIS), with which the Superficial Deposits ('Drift') are tentatively correlated on (Figure?6). Rockhead contours show that the Quaternary deposits mantle an undulating topography that was largely inherited from preglacial times, and which has been exploited by some modern drainage systems. The oldest Quaternary sediments are the Preglacial Deposits. They comprise the Bytham Sand and Gravel Formation (Figure?7) that represents the main channel and river terrace deposits of the north-east-flowing Bytham palaeo river, the valley of which was a major feature of the early Quaternary landscape of central England. These deposits are seldom exposed but their outcrops, revealed by sandy soils, suggest that the Bytham River floodplain was between 3 and 5?km wide. The Bytham Sand and Gravel contains abundant quartz pebbles; it was formerly extracted from quarries such as that near Rothley [SK?565?123] and has been prospected around Rearsby [SK?655?145]. The continental ice sheets that advanced across the district during the Anglian cold stage destroyed the Bytham river system and laid down the Glacial Deposits of the Wolston Formation. In the north of the district, these deposits are well stratified and up to 50?m thick where infilling the Bytham River valley. The first ice sheet advanced from the Pennines region and travelled towards the south-south-east, depositing the Thrussington Member till (Figure?7). A further ice lobe encroached slightly later, depositing the Oadby Member till (Plate?8). Its dark grey clay matrix and clasts of flint and chalk, indicate that the parent ice sheet had incorporated material from Cretaceous outcrops to the east. The Lias-Rich Oadby till, which locally intervenes between the Oadby and Thrussington tills, probably represents material brought by ice travelling along the Jurassic outcrop, from the north-north-east. The Rotherby Member (Figure?7) is a tabular body of clay-rich sediment laid down after the Thrussington till, probably in a large proglacial or subglacial lake within the remnants of the Bytham palaeovalley. Farther south its equivalent, the Glen Parva Member, shows more complex interbedding with glaciofluvial sands and gravels. Undifferentiated Glaciolacustrine Deposits represent deposition in more restricted bodies of standing water, and commonly occur in association with glaciofluvial deposits. The Wigston Member (Figure?7) is a semicontinuous sheet of glaciofluvial sands and gravels intercalated between the Rotherby Member and the base of the Oadby Member till. The abundant flint fragments suggests that this unit represents proglacial outwash of the 'Oadby till' ice sheet as it advanced into the Bytham palaeovalley. Undifferentiated Glaciofluvial Deposits (Figure?7) have a patchy distribution and many of the narrower outcrops may represent the fills of palaeochannels. Extensive outcrops of yellow or orange flinty sand and gravel have been mapped near Kings Norton [SK?690?003] and Skeffington [SK?735?025]. This deposit can be traced northwards at least to Tilton on the Hill [SK?740?060], where a borehole near the church proved 14?m of sand and gravel above bedrock. A few metres of this deposit exposed in a disused quarry [SK?7415?0607] north of Tilton includes chalk clasts, and showed vertical to locally overturned bedding suggestive of glaciotectonic disturbance.

Glaciotectonic disturbances of the Anglian sequence were formerly demonstrated in excavations for the M1 Motorway near Enderby e.g. [SP?5440?9850]. There, various glacial deposits are thrust-imbricated with Mercia Mudstone bedrock. A structurally complex multicomponent glacial sequence was also formerly observed in the now restored Blaby brick-pit [SP?563?987]. Synglacial palaeovalleys have been identified, from mapping (Plate?8) and borehole evidence, in the west of the district. They have sinuous, northerly trending courses and are filled with up to 30?m of glacial deposits that include Thrussington and Oadby tills, glaciofluvial sands and gravels and glaciolacustrine clay.

The modern drainage of the district was initiated along meltwater routes during the waning phases of the Anglian glaciation. The River Terrace Deposits of the Soar Valley Formation (Figure?8) are the remnants of earlier floodplain aggradations that now occur as flights of terraces along trunk valleys such as the Soar and Wreake. 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 (Figure?6). The terrace deposits consist mainly of sand-rich, matrix-supported, cross-bedded gravels in which quartzose pebbles and shattered flints are always prevalent. The Hemington terrace and to a lesser extent the Syston terrace are only slightly raised above the alluvial tracts, and form a major suballuvial component of the Soar and Wreake valley fills. At the former Pontylue gravel pit at Syston [SK?610?110], the Syston Member contained ice-wedge casts and mammalian fossils that include Mammuthus primigenius (woolly mammoth), indicative of cold-stage conditions during the final (Late Devensian) glaciation of Britain (Figure?6), although the Devensian ice sheets stopped well short of the Leicester district. Periglacial slope-wasting processes, involving solifluction, occurred mainly during the various cold periods, such as the Late Devensian, that followed Anglian ice withdrawal. They promoted the development of the Slope Terrace Deposits (Figure?8), comprising the Little Dalby Head and Burton Lazars Head, each of which can be correlated (Figure?6) with named fluvial terraces of the adjacent river valleys. Head (Figure?8) where undifferentiated on the map is mostly a Holocene accumulation of soliflucted material and surface hill wash along the lower valley sides. It is thus a variable deposit, although it is generally sandier and lighter in texture than the periglacial head. Valley deposits (Figure?8) form the flat floors to many small valleys, which are not occupied by featured alluvial tracts. They are mainly accumulations of surface hill wash, but may also contain layers of soliflucted material and alluvium. These deposits are generally sandier than head, and may also contain layers of peat.

Holocene (or Recent) deposits of Alluvium (Figure?8) underlie the flood-prone tracts, water meadows and meander belts of the main rivers and tributary streams. Peat is a minor component of the Holocene floodplain alluvium of the River Soar, the main outcrop being at Narborough Bog SSSI [SP?549?979] where peat formation has been dated to about 6000 years ago.

Landslides 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. They include large-scale complex slippages, involving rotational movement in the bedrock, developed along spring lines in the Dyrham and Marlstone Rock formations. The toes of the landslides commonly show a subtle downslope gradation into mudflows, which give rise to lobate landforms on the more gently sloping ground below the main landslide, as in the valley north-west of Tilton on the Hill [SK?7362?9621]. On the Charmouth Mudstone outcrop south-west of Debdale Spinney [SK?720?126], landslippage is revealed by hummocky ground and pronounced backscars. Similar features occur on the Whitby Mudstone Formation around Whatborough Hill [SK?77?06] and Colborough Hill [SK?760?050] where landslippage occurred on ground with a natural slope angle of about 10°.

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 and quarry spoil tips. 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 residential centres where it is not shown on Sheet?156 Leicester. Infilled ground comprises areas where excavations have been partly or wholly backfilled, such as workings for sand and gravel, ironstone, hard-rock aggregate and brick clay. The infilling materials may include excavation and overburden waste, construction and demolition materials, industrial waste and domestic refuse, the latter partially filling Castle Hill Quarry [SK?5759?1496]. 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. Worked ground represents those voids from which natural material has been extracted, for example open quarries (Plate?9) and pits, road and railway cuttings and general landscaping. Disturbed ground is not shown on Sheet?156, for reasons of clarity, but is shown on the 1:10?000 series maps. It includes diffuse areas of former mineral extraction. Landscaped Ground is a further category shown only on the 1:10?000 series maps. It 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.

Metamorphism

Thermal metamorphism occurred within the contact aureole of the Mountsorrel Complex, which was intruded during late Ordovician (Caradoc) time. It was imposed on mudstone country rocks that are tentatively correlated with the Stockingford Shale Group. Hornfelsed metasediments at the 'old gravel pit' [SK?5583?1556] are recrystallised to fine-grained aggregates of white mica, biotite, garnet, quartz and feldspar, together with a pseudomorphed aluminosilicate mineral. Hornfelses exposed at the flooded quarry by Swithland Reservoir [SK?5567?1355] have a similar mineralogy, but possess a local foliation caused by preferred mineral orientations. The metasediments exposed along the waterline by the Kinchley shore [SK?5599?1397] include pinnite pseudomorphs after original cordierite and small euhedra of the orthopyroxene enstatite, a mineralogy suggesting the pyroxene hornfels grade of contact metamorphism. Regional metamorphism appropriate to low, high and very high epizonal (metapelitic) white mica crystallinity grades is widely distributed in the Charnian Supergroup and Swithland Formation. By contrast, mudrocks of the Stockingford Shale Group in the deep boreholes around Leicester show much lower, diagenetic to anchizonal grades of metamorphism. This regional metamorphism and the accompanying cleavage are attributed to the end-Silurian deformational event discussed below. However, the marked increase in metamorphic grade at Charnwood Forest suggests that significant basement discontinuities, possibly major crustal structures, may delimit that area.

Structure

End-Silurian deformation produced a pervasive, near-vertical, east-south-easterly trending cleavage fabric that cuts obliquely (transects) the axis of the south-east-plunging anticline affecting the Charnian Supergroup and Brand Group rocks in Charnwood Forest. This cleavage transection geometry is demonstrated at Bradgate Park, where measurements [SK?5405?1085] indicate that the main cleavage pole is offset, in an anticlockwise sense, by 8° from the local fold axis (Carney and Pharaoh, 2000). The timing of this deformation is constrained by argon isotope techniques (BGS work in progress) to around 425–416?Ma, about 20?Ma before the Acadian deformation recorded, for example, in Wales and the Lake District. A weak bedding-plane cleavage fabric characterises the Stockingford Shale Group in the Leicester boreholes, suggesting the presence of a major structural interface between there and Charnwood Forest. The Mountsorrel Complex was widely affected by metre-scale, spaced fracture systems, many with easterly orientation showing slickenfibre development. Carboniferous extension is indicated on seismic reflection profiles to the north of the Sileby Fault, which had an aggregate synsedimentary downthrow of 1750?m during the Dinantian. The profiles further demonstrate this fault was reactivated, with reverse throw, during the end-Carboniferous Variscan inversion event. The (?)Carboniferous basalt dyke cutting the Mountsorrel Complex shows an apparent offset along a north-east-trending strike-slip fault with a dextral throw of about 200?m (Figure?2). End-Triassic deformation, at about the time of the Rhaetian marine transgression, may have caused the northwards thinning of the Penarth Group and corresponding erosional truncation of the upper part of the Cotham Member, prior to Lias Group deposition. Post-Jurassic deformation may reflect Cainozoic uplift event(s). The principal structure is the regional dip of the Mesozoic strata, which is about 1° towards the east to south-east, but with local modifications due to flexuring and faulting. The Sileby Fault was rejuvenated, with a northward downthrow of about 130?m. In the Leicester urban area, the Hollygate Sandstone is stepped down by north-west-trending faults (Figure?1) averaging about 10 to 15?m throw. Along the Wreake valley, a south-east throwing downfault possibly continues south-westwards, confining the Cropwell Bishop Formation to the east in the Glenfields area [SK?5519?0600]. A south-west trending fault is also postulated along the southern Soar valley (Figure?1). It throws down to the east, accounting for the appearance of the Hollygate Sandstone in the Dane Hills area e.g. [SK?565?046]. In the central and eastern parts of the district, the effects of faulting and flexuring are largely masked by poor exposure, but they have been detected by ironstone exploration drilling; for example, around Green Lane [SK?730?044]. Superficial structures caused by periglacial mass wasting are expressed by the cambering of outcrops bordering valley sides, mainly in the Northampton Sand with some in Marlstone Rock. The pseudo-dip slopes either accentuate the regional easterly dip, or reverse it, for example, around the outliers to the north [SK?789?030] and north-east [SK?799?25] of Loddington.

Geophysical evidence on the concealed geology

The geophysical anomaly maps principally reflect the contrasting physical properties of rock masses partly or wholly concealed beneath the Mesozoic strata of the district. On the Bouguer gravity anomaly map (Figure?9), the deep gravity low to the north of the district coincides with thick accumulations of Carboniferous strata within grabens; the gravity gradient that forms the southern edge of the anomaly, in part, corresponds with the Sileby Fault. A second major feature, the oblong-shaped gravity 'high' to the west, coincides with outcropping Precambrian rocks; it shows a north-west-orientated 'grain' of lineaments, their continuations possibly controlling the faults described above in the Leicester urban area, and shown in (Figure?1). The magnetic anomaly map (Figure?10) includes discrete, high amplitude anomalies, one of which, in the north-west corner of the district, is the magnetic expression of the Mountsorrel Complex. The causative body for the larger anomaly to the east is the concealed Melton Mowbray Granodiorite, which was proved beneath Carboniferous strata 350?m north of the district in the Kirby Lane Borehole (Carney et al., 2004). Magnetic gradients show that these plutons have flat tops, indicating their erosional truncation at the unconformity with overlying Carboniferous or Triassic strata. The two smaller magnetic anomalies just outside the south-western corner of the district coincide with surface or borehole provings of the South Leicestershire Diorites. Limited seismic reflection surveys associated with oil and coal exploration show that Carboniferous strata (Millstone Grit and Lower Coal Measures) form an unconformity-bounded sequence that pinches out southwards, about 4?km into the district.

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.

Mineral resources

Current and past mineral extraction in the district is summarised in (Figure?11). Minerals of current interest are those that can be won at the surface and their distribution, together with outline information, is given in the Mineral Resources Map for Leicestershire and Rutland (British Geological Survey, 2002). Further detailed information is provided in the Leicestershire Minerals Plan Local Review. 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?156 Leicester by areas of Worked Ground and Infilled Ground. 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).

Water resources and flooding

Springs commonly issue from the base of the Marlstone Rock (Cheney, 2004). One of the largest, with a yield of 2.6 litres per second, recorded near Somerby [SK?7776?0996], formerly provided the supply for that village. Other small springs occur at the junction of the Dyrham and Charmouth Mudstone formations. Concealed strata of the Sherwood Sandstone Group have been prospected for groundwater; however, only two water supply boreholes use this source and neither contains any significant hydrogeological record. 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 in the Leicester urban area show yields of between 1.5 to 12.5?l/s; virtually all is used for industrial purposes. The Lias Group represents a minor, multilayered aquifer, but where interconnectivity is favourable relatively high yields can result (up to 1.9?l/s). Water supplies from the Dyrham and Marlstone Rock formations determined the location of many villages, and groundwater yields of up to 1.1?l/s have been recorded, the water quality being generally good, but hard. The main source of water for Leicester prior to the 19th century was from shallow wells and springs developed on the River Terrace Deposits of the Soar valley. These sources, being highly prone to contamination, were discontinued and today there are only two agricultural supplies: one from River Terrace Deposits, and the other from River Soar floodplain (alluvium and possibly underlying River Terrace Deposits). Nowadays, most of the city's water is supplied from reservoirs in Derbyshire. Limited supplies have been obtained where lenses of sand and gravel occur within till, and from glaciofluvial deposits, with a number of wells drawing water for individual villages in the district. Water levels and yields of many of these wells declined during prolonged dry periods and in some cases, the supply was not sufficient to meet demand.

Flooding is a potential hazard in low-lying parts of this district, 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. 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. A flood risk management strategy has been prepared by the Environment Agency for the Soar and its major tributary valleys.

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?12). Factors such as geological structure, slope stability, natural weathering and potential for flooding are also locally important.

Bedrock solution may be anticipated in the western parts of the district that are underlain by the Mercia Mudstone Group, in particular by the Cropwell Bishop Formation with its locally thick gypsum beds. Gypsum dissolution typically occurs within a 'solution zone' that is several metres thick below the base of the subsoil or superficial deposits, and is mainly accomplished by groundwater flow along bedding planes, joints and fissures. No surface features attributable to dissolution have been recognized in the Leicester district, but the 'karstic' modification of gypsum beds was formerly revealed in brick pits around Leicester.

Slope stability is a potential issue particularly where building development is extended on to steep valley sides. Many areas of landslide are shown on Sheet?156 Leicester and these, together with outcrops of Head and Slope Terrace Deposits that may contain relict shear surfaces, have the potential to be reactivated by loading, undercutting or excavating into slopes. Undisturbed natural slopes of the district are generally stable in our present climate; however, the steeper slopes, which are mostly developed on deeply weathered Liassic mudstone bedrock or head derived from it, may be susceptible to movement. Unstable conditions would be favoured by increased ingress of water from natural or artificial sources, and at times of exceptionally heavy and sustained rainfall.

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 and Lias groups, 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 Leicester district natural radon levels are generally low except over outcrops of the Lias Group, in particular the Marlstone Rock Formation. The Northampton Sand, Charmouth and Scunthorpe Mudstone formations rank slightly lower in this respect (Sutherland and Sharman, 1996). Advice on potential radon hazard and measures for the alleviation of radon build-up in properties can be obtained on application to the Enquiries Desk at the British Geological Survey, Keyworth.

Quarrying (Plate?9) has focussed on the extraction of brick clay around Leicester, sand and gravel on the Soar and Wreake floodplains, ironstone around Tilton on the Hill and hard-rock aggregate west of Mountsorrel (Figure?11). Numerous small ironstone, clay, and limestone quarries have been backfilled and some, particularly the ironstone workings of the Marlstone Rock Formation, are partially filled and degraded. On geological maps quarry boundaries are based on the best information available, and some are likely to be imprecise in detail. Constraints to the further development of worked-out excavations are related to geotechnical problems of variable ground conditions, including drainage, between the natural surface and the fill; for example, settlement problems may occur within some of the larger infilled quarry sites. The properties and nature of the backfilled material must also be considered, in particular its possible interaction with the wall-rocks of the repository, and any risks associated with the migration of gases or leachates from the fill into the surrounding geological deposit.

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 15?km north-west 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 Global Seismology Unit can provide a detailed analysis of these events and assessments of local seismic risk to major constructions.

Conservation sites ?Exposures of rocks and superficial deposits, which can 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 their being made into 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 BGS in partnership with several other locally based organizations, as part of the Leicestershire and Rutland Local Geodiversity Action Plan financed by the Aggregates Levy Sustainability Fund (Ambrose, 2004).

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

Information sources

Other information sources include borehole records, fossils, rock samples, thin sections, hydrogeological data and photographs. Searches of indexes to some of the collections can be made on the Geoscience Index System in BGS libraries and on the BGS website (www.bgs.ac.uk).

Maps

Books

BGS memoirs, books, reports and papers relevant to the district are arranged by topic. These may be consulted at BGS and other libraries, or may be purchased from BGS sales desk. Memoirs that are out of print are available on a print-on-demand basis.

Documentary collections

Basic geological survey information, which includes 1:10?000 or 1:10?560 scale field slips and accompanying field notebooks are archived at BGS, either as hard copy or digital (scans). Charges and conditions of access to these records are available on request from the Manager, National Geological Records Centre.

Boreholes and site investigation reports

BGS holds collections of borehole 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 web site, where copies can be ordered.

Hydrogeological data

Records of water boreholes, wells and springs and aquifer properties are held in the BGS (Hydrogeology Group) database at Wallingford.

Gravity and magnetic 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 the north of the district.

BGS Lexicon of named rock unit definitions

Definitions of the rocks and superficial deposits shown on Sheet 156 Leicester are held in the Lexicon database, available through the BGS web site. Further information on the database can be obtained from the Lexicon Manager at BGS Keyworth.

BGS (Geological Survey) photographs

Copies of the photographs used in this report, and of others taken during this and previous surveys are deposited for reference in the National Archive of Geological Photographs, BGS, Keyworth; part of the collection can be viewed online at BGS web site. Colour or black and white prints can be supplied at a fixed tariff.

Materials collections

Petrological collections

The petrological collections for the district include hand specimens and thin sections. Information on the databases of rock samples, thin sections and geochemical analyses can be obtained from the Manager, Mineralogy and Petrology Section, BGS, Keyworth.

Borehole core collections

Samples have been collected from core taken from some of the boreholes in this district. They are registered in the borehole collection at BGS Keyworth.

Palaeontological collections

The collections of biostratigraphical specimens are taken from surface and temporary exposures, and from boreholes throughout the district. The samples are held at BGS Keyworth. Enquiries concerning all the macrofossil material should be directed to the Curator, Biostratigraphy Collections, BGS Keyworth.

Geochemical samples

A database of silicate and trace element analyses of rock samples, including some from the Leicester district, is held by the Minerals and Geochemical Surveys Division of the BGS.

Relevant collections held outwith BGS

Plans of ironstone workings may be held at the relevant office of Leicestershire County Council.

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 includes a full audit of geological and landscape features, sites relevant to local authority geodiversity issues, geological trails and education packs, (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 Commisioned Research Report, CR/04/063N.

Blake, D M.?1986. A new Lower Lias exposure at Billesdon, Leicestershire. Transactions of the Leicester Literary and Philosophical Society, Vol. 80, 47–57.

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

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.

Brandon, A. 1999. Geology of the Wreake Valley (SK61NE, SK71NW, SK71NE and SK81NW). British Geological Survey Technical Report, WE99/17.

Carney, J N, and Pharaoh, T C. 2000. Bradgate Park. 33–37 in Precambrian rocks of England and Wales. Geological Conservation Review Series, No. 20. (Joint Nature Conservation Committee, Peterborough).

Carney, J N, Ambrose, K, Brandon, A, Royles, C P, Cornwell, J D, and Lewis, M. A.?2001. Geology of the country between Loughborough, Burton and Derby. Sheet Description of the British Geological Survey, 1:50?000 Series Sheet 141 Loughborough (England and Wales).

Carney, J N, Ambrose, K, Brandon, A, Royles, Lewis, M A, and Sheppard, H.?2004. Geology of the country around Melton Mowbray. Sheet Description of the British Geological Survey, 1:50?000 Series Sheet 142 Melton Mowbray (England and Wales).

Carney, J N, Ambrose, K, Cheney, C S, and Hobbs, P R N. in press. Geology of the country around Leicester. Sheet Description of the British Geological Survey, 1:50?000 Series Sheet 156 Leicester (England and Wales).

Cheney, C S. 2004. The hydrogeology of the Leicester district (1:50?000 scale geological sheet?156). British Geological Survey Internal Report, IR/04/085.

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.

Cox, B M, Sumbler, M G, and Ivimey-Cook, H C. 1999. A formational framework for the Lower Jurassic of England and Wales (onshore area). British Geological Survey Research Report, RR/99/01.

Fox-Strangways C.?1903. The geology of the country near Leicester. Memoir of the Geological Survey of Great Britain, Sheet 156 (England and Wales), (London: HMSO).

Hallam, A.?1955. The palaeontology and stratigraphy of the Marlstone Rock Bed in Leicestershire. Transactions of the Leicester Literary and Philosophical Society, Vol. 49, 17–35.

Hallam, A. 1968. The Lias. 188–210 in The geology of the East Midlands. Sylvester-Bradley, P C, and Ford, T D (editors). (Leicester: Leicester University Press.)

Howarth, M K. 1980. The Toarcian age of the upper part of the Marlstone Rock Bed of England. Palaeontology, Vol. 23, 637–656.

Le Bas, M J.?1968. Caledonian igneous rocks. 41–58 in The geology of the East Midlands. Sylvester-Bradley, P C, and Ford, T D (editors). (Leicester: Leicester University Press.)

Molyneux, S G.?1991. The contribution of palaeontological data to an understanding of the early Palaeozoic framework of eastern England. Annales de la Societe de Belgique, Vol. 114, 93–105.

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

Pidgeon, R T, and Aftalion, M.?1978. Cogenetic and inherited U-Pb systems in granites: Palaeozoic granites of Scotland and England. 183–220 in Crustal evolution in northwestern Britain and adjacent regions. Bowes, D R, and Leake, B E (editors). (Liverpool: Liverpool: Seel House Press.)

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 D, and 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.

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

Wignall, P B, Clements, R G, and Simms, M J. 1989. The Triassic–Jurassic boundary beds of the City of Leicester. Transactions of the Leicester Literary and Philosophical Society, Vol. 83, 25–31.

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 theBGS London Information Office at the NaturalHistory Museum Earth Galleries, and from BGS-approved stockists and agents.

Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

Figures and plates

Figures

(Figure 1) Geological map showing the main bedrock units and structures.

(Figure 2) Geological sketch map of the Mountsorrel Complex.

(Figure 3) Sherwood Sandstone and Mercia Mudstone groups. The darker shadingdenotes the main outcropping units.

(Figure 4) New national scheme ofnomenclature for the Mercia Mudstone Group.

(Figure 5) Environment of deposition, lithology and stratigraphy of the Lias Group.

(Figure 6) Quaternary deposits correlation.

(Figure 7) Morphology and lithology of the Quaternary glacial andpreglacial deposits.

(Figure 8) Morphology and lithology of the Quaternary postglacial deposits.

(Figure 9) Regional Bouguer gravity anomaly map. Bouguer gravity anomalies in milligals (mGal) calculated against the Geodetic Reference System 1967, referred to the National Gravity Reference Net 1973.

(Figure 10) Regional magnetic anomaly map. Total field magnetic anomalies in nanotesla (nT) relative to a local variant of IGRF90.

(Figure 11) Mineral resources and minor occurrences (M = million).

(Figure 12) Geotechnical properties of the principal lithological units.

Plates

(Plate 1) Precambrian strata of the Bradgate Formation in a small quarry south of Coppice Plantation, Bradgate Park (P609270). The scale bar is one centimetre.

(Plate 2) Small-pebble conglomerate typical of the Hanging Rocks Formation (P609271). Field of view is about 15 cm wide.

(Plate 3) Hornfelsed mudrock exposed on western shore of Swithland Reservoir. Field of view is about 4 m high (P599668).

(Plate 4) Aplite sheets cutting Mountsorrel granodiorite at the summit of 'Craig Buddon' [SK 5575 1501], overlooking Swithland Reservoir spillway (P599686).

(Plate 6) Hollygate Sandstone Member in the railway cutting in Western Park. The cross-bedding indicates a dominant current direction towards the east (P549720).

(Plate 7) Marlstone Rock at Tilton railway cutting showing the principal divisionsincluding the three brachiopod beds of Hallam (1955) (P601558).

(Plate 8) Oadby till (dark grey) filling a palaeovalley on Triassic strata(red-brown, bedded) and Mountsorrel granodiorite (pale grey rocks at far upper right) on the south-east face of Buddon Wood (Mountsorrel)Quarry (P599688).

(Plate 9) Aerial view of Buddon Wood Quarry, in granodiorite of the Mountsorrel Complex (P540721).

(Front cover)

(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 3) Sherwood Sandstone and Mercia Mudstone groups. The darker shadingdenotes the main outcropping units

Group Formation Environment and lithology Key locality or borehole (B) Age
MERCIA MUDSTONE BLUE ANCHOR (BAn) (5-8) Arid playa mudflats with marine influences. Grey-green, blocky, dolomitic mudstone Gipsy Lane SSSI Norian to Rhaetian rUPPER TRIASSIC
CROPWELL BISHOP (CBp) (40-50) Environments and lithologies similar to Edwalton and Gunthorpe formations, but more gypsiferous; lower, thick (Tutbury) seam and higher, thin (? Newark) gypsum seams Gipsy Lane SSSI [SK 6172 0694] Mainly Norian
EDWALTON (Edw) (40-50) Wind-blown sediments on arid playa mudflats or coastal sabkha, with fluvial sand input. Similar to Gunthorpe Fm with, at base and top, green-grey sandstone-rich sequences, respectively of the Cotgrave and Hollygate sandstone members Western Park railway cutting [SK 5565 0420] (Hollygate M) Late Ladinian and Carman
GUNLHORPE (Gun) (70-80) Similar environment to Edwalton Formation Red-brown, rarely green, blocky mudstone with common beds of green-grey, hard, dolomitic siltstone or fine-grained sandstone. Common gypsum veinlets and disseminations Leicester Forest East (B) (see below) Ladinian MIDDLE TRIASSIC
SNEINION (Snt) (0-20) Fluvial sand, silt and mud deposited on a broad alluvial plain. Red to grey, fine- to medium- grained micaceous sandstone thinly interbedded with red, brown and green, laminated, micaceous mudstone and and siltstone - Leicester Forest East (B)

(see below)

-

 Anisian
SHERWOOD SANDSTONE BROMSGROVE SANDSTONE (BmS) (0-35) Fluvial sand and overbank mud and silt deposited in meandering rivers. Red to grey, fine- to medium-grained argillaceous sandstone; red mudstone or siltstone interbeds Leicester Forest East (B) (SK50SW/71) [SK 5245 0283] Scythian to Anisian

(Figure 5) Environment of deposition, lithology and stratigraphy of the Lias Group

Named units† Depositional environment, main lithology and landform Typical fossils Age (biozones)
WHITBY MUDSTONE FORMATION (WhM) (c.45) Relatively deep water, quiescent conditions following a major marine transgression; discomformable on unit below. Blue-grey, fissile, fossiliferous mudstone, locally pyritous and ochreous; beds of calcareous siltstone and nodular limestone. Exposed at Tilton railway cutting (see below) Pseudomonotis sp., Dactylioceras annulatum, Harpoceras, belemnites cf. elongatus Toarcian, Harpoceras falciferum to Hildoceras bifrons
MAIDSTONE ROCK FORMATION (MRB) (1-9) Condensed sequence; dunes and bars deposited during a regressive, high-energy episode. Sandy, shell-fragmental, ooidal limestone and ferruginous limestone, with abundant secondary limonite and rusty weathering. Either massive, with brachiopod 'nests', or cross-bedded. May be pebbly at base. Forms a strong cuesta overlooking west of district. Exposure: Tilton railway cutting [SK 761 055]; Chater Valley [SK 7886 0537]; Loddington [SK 7930 0225] Lobothyris punctata, Tetrarhynchia tetrahedra, Pseudopecten, Tiltoniceras, Dactylioceras semicelatum, Harpoceras Upper Pliensbachian to L Toarcian, ?Pleuroceras spinatum to tenuicostatum
DYRHAM FORMATION (DyS) (10-20) Shallow marine, with nearshore terrigenous influence indicated by silty detritus and sandstone beds. Pale to medium grey, micaceous, ochreous-weathering siltstone with beds of fine-grained, friable sandstone. Locally a prominent yellow sandstone ('sandrock') at top. Upper part seen at Tilton railway cutting (see above) Amaltheus marginatus, A. subnodosus, Belemultes elongatus, Avicula inequivalvis, Modiola, Cardium Upper Pliensbachian ?daveoi and margaritatus
CHARmoum MUDSTONE FORMATION (ChM) (150-180) Generally well-oxygenated, marine conditions and deposition of grey, fissile mudstone, locally fossiliferous. Abundant nodules at several levels comprising calcite-mudstone, phosphate or siderite-mudstone. Exposures are few and limited Ammonites, Gryphaea, Pseudopecten, Pinna, Plagiostoma, Pholadomya, Cardinia, Hippopodium Sinemurian—L/U Lower Pliensbachian, bucklandi to ?margaritatus
BLUE LIAS FORMATION (BLi) (55-120) Progressively deepening, hemipelagic to shelf tropical marine environments Rhaetian to Lower Sinemurian
Rugby Limestone Member (c.30) Offshore marine environment characterised by 'cyclic' depositional couplets consisting of blue-grey, laminated mudstone (anoxic conditions) grading up to calcareous mudstone and argillaceous, bioturbated and locally Shelly limestone. Overgrown exposures in IngarsbyThurnby railway cutting [SK 6679 0482 to 6283 0484] Ammonites, Gryphaea, Lima gigantea, Pholadomya, Pseudopecten, Pinna, Plagiostoma pseudomonotis, Modiolus Hettangian to Lower Sinemurian (Liasicus to mid bucklandi)
Saltford Shale Member (0-15) Warm oxygenated marine environment. Deposition of blue-grey, fossiliferous mudstone with a very few beds of argillaceous limestone reflecting shallower conditions Liostrea, Cardinia, Caloceras, Psiloceras planorbis Mainly Hettangian (Liasicus Zone)
Wilmcote Limestone Member (Wit) (0-10) Warm, poorly oxygenated (dysaerobic) marine conditions and deposition of faunally-restricted, organic-rich sediments. Blue-grey, commonly fissile mudstone with numerous thin beds of pale grey argillaceous limestone Liostrea, Gryphaea, Plagiostoma, Caloceras, Psiloceras planorbis Rhaetian to Hettangian (planorbis Zone)

(Figure 6) Quaternary deposits correlation

Stage Approximate age of commencement (in years BP) MIS Leicester district (nomenclature in part follows Rice, 1968; Brandon, 1999) Trent (above Nottingham) and Lower Dove
Holocene 0 Floodplain alluvium Head (undivided) & Valley deposits Floodplain alluvium warm, temperate
11 500 Hemington Member* Hemington Terrace Deposits cold, periglacial
15 000 Syston Member* Holme Pierrepont Sand and Gravel cold
26 000 2
Devensian 65 000 3 warm, temperate
Wanlip Member* Burton Lazars Head Beeston Sand and Gravel cold, periglacial
115 000 5d—a warm, temperate
Ipswichian 126 000 5e warm, temperate
195 000 Birstall Member* ?Little Dalby Head Egginton Common and and Gravel cold, periglacial
240 000 7 warm, temperate
297 000 8 Knighton Member* Etwall Sand and Gravel cold, periglacial
330 000 9 warm, temperate
367 000 10 Eagle Moor Gravel cold, periglacial
Hoxnian 400 000 11 warm, temperate
Anglian 450 000 12 Undifferentiated and Wolston Formation Oadby Member Wigston Member Rotherby Member Glen Parva Member Thrussington Mbr. Eagle Moor Sand and Gravel (?) Findern Clay Oadby Till

Thrussington Till

 cold
Cromeria Complex 500 000-450 000 ?12-14 Bytham Sand and Gravel Formation warm, temperate
* River terrace deposit members of the Soar Valley Formation

(Figure 7) Morphology and lithology of the Quaternary glacial andpreglacial deposits

Type Morphology and thickness Description
GLACIOLACUSTRINE DEPOSTS Thin, more persistent in west, where up to 7 m. Rotherby and Glen Parva members are semicontinuous, tabular sheet-like bodies Grey or red-brown clay or silt with interbedded sand; sporadic rock fragments (dropstones) and carbonate nodules ('race'); laminated to blocky. Rotherby Member and Glen Parva Member well developed in west
GLACIOFLUVIAL DEPOSITS Discontinuous outcrops; thickness variable but up to 14 m locally. Wigston Member forms a semicontinuous tabular sheet up to 7 m thick Brown, yellow or red, unconsolidated to loosely cemented fluvial sand, gravel or silt, commonly interleaved in till; clasts are of 'Oadby' (flint-rich) or 'Thrussington' (Triassic and Carboniferous) outwash derivation. The flint-rich, yellow or red Wigston Member is restricted to western parts
TILL (boulder clay) Wide featureless spreads, commonly on interfluves but locally preserved in palaeo- valleys, where up to 30 m recorded; 10-20 m elsewhere Oadby Member Lodgement till of brown to blue-grey clay, red in case of Triassic-rich variant; common scattered quartz pebbles and fragments of flint and chalk. Jurassic lime-stone and various Jurassic fossils. Lias-rich variant has Jurassic erratics but is largely devoid of flint or chalk
Thrussington Member Lodgement till of red to red-brown clay with admixed silt and sand. Fragments of green-grey Triassic siltstone or sandstone and quartz pebbles; also Carboniferous sandstone, coal and limestone, and fragments of Charnian rock and Mountsorrel granodiorite (e.g. 'Humber Stone' SK 6242 0709)
PREGLACIAL DEPOSITS Locally a continuous, tabular sheet, in places forms terrace-like outcrops; 7-10 m average, up to 15 m in Wreake valley near Brooksby [SK 669 157] Bytham Sand and Gravel Fluvial deposits of the north-easterly flowing Bytham River. Commonly comprises basal coarse sands and gravels overlain by red, fine- to medium-grained sand. Clasts are mainly of Triassic, Carboniferous or (strictly local) Jurassic derivation

(Figure 8) Morphology and lithology of the Quaternary postglacial deposits

Type Morphology and thickness Description
LANDSLIDE Hummocky, terraced or lobate features; variable thickness Slipped masses of bedrock or drift; rotational failures and associated mud/debris flows are common
PEAT Mainly as swampy flats in Soar floodplain Black to dark brown, commonly interbedded with organic-rich clay and silt
ALLUVIUM Underlies widespread flats along floodplains of trunk streams; narrower tracts in tributaries; up to 4m Heterogeneous complex of fluvial silt, sand and clay with flint-rich gravel lenses; organic-rich clay and peat in former meanders
HEAD (including valley deposits) In hollows, at bases of slopes or in narrow valleys; variable, 0-3 m Brown, unconsolidated sandy and clayey diamicton with local rock fragments. A complex of soliflucted and/or hill-wash material; may contain shear surfaces and lenses of silt or sand
SLOPE TERRACE DEPOSITS Underlie gently sloping, locally wide, planar surfaces in the Stapleford clay vale; up to 2-3 m Solifluction head-type deposit of poorly consolidated, brecciated and slickensided grey mudstone with basal shear plane; composition reflects that of upslope source material. Capped by or involuted with thin sand and gravel layer
RIVER TERRACE DEPOSITS (SOAR VALLEY FORMATION) Underlie wide, flat-topped terraces or more restricted spreads. Older and higher terraces are separated from alluvium by a 'bedrock step'; up to 5 m Grey, brown or yellow unconsolidated, flinty, fluvial silt, sand and gravel, locally with lenses of silt, clay or organic-rich 'peaty' clay. Hemington Terrace Deposits have a clay or silt capping up to 2 m thick

(Figure 11) Mineral resources and minor occurrences (M = million)

Mineral resource Mineral resources (main source in bold) Activity Use
Building stone 'Sandrock' of Dyrham Formation; Marlstone Rock Formation; Mountsorrel Complex (granodiorite) No significant current activity. Mountsorrel rocks mainly quarried for this during 19th century around Castle Hill/Buddon Wood Houses, walls, cladding. Stone sets (Mountsorrel granodiorite only)
Hard-rock aggregate Mountsorrel Complex, South Leicestershire Diorites Currently extracted at Buddon Wood Quarry [SK 564 152]; average 4-5 M tonnes/year reserves estimated at 180 Mt A hard, durable roadstone, used for constructional fill, rail ballast etc
Sand and gravel Soar and Wreake terrace deposits, glaciofluvial deposits, Bytham Formation, alluvium Numerous former large workings in Soar and Wreake river terraces or alluvium, and smaller workings in glaciofluvial deposits. 4-5 Mt estimated for prospect in Bytham Formation south of Brooksby [SK 671 153] Concrete aggregate, building sand
Limestone Blue Lias Formation; Rugby Limestone Member, Marlstone Rock Formation Small quarries in Leicester and at Kilby Bridge (Rugby Limestone) and Tilton (Marlstone); mainly during late 19th century. No current activity Cement, 'lime, building stone, agricultural use
Ironstone Marlstone Rock Formation Numerous small former workings. Larger quarries around Tilton on the Hill and Halstead [SK 757 061] during early 20th century to final closure in 1961 Buildings and roads, lime burning, iron smelting
Brickclay Mercia Mudstone Group, Lias Group Numerous small quarries and pits; larger workings in Mercia Mudstone around Leicester, e.g. Spinney Hills [SK 602 045]. No current activity Building bricks, agricultural clay
Gypsum Mercia Mudstone Group; Newark Gypsum Possibly worked as a by-product at brick pits around Leicester, including Spinney Hills where a shaft was said to exist
Base metals uranium Mountsorrel Complex, Mercia Mudstone Group Molybdenite, pyrite, chalcopyrite, topaz, galena at Castle Hill Quarry [SK 577 150]. Bornite and the uraniferous mineral, coffinite, found in Mercia Mudstone at the Spinney Hills pit
Hydrocarbon Generated from ?Carboniferous strata of Widmerpool Half-graben, to north of district Tar residues coating joints and fractures at Castle Hill Quarry. No commercial importance
Palygorskite South Leicestershire Diorites Joint coatings at Enderby Warren Quarry [SP 541 991]

(Figure 12) Geotechnical properties of the principal lithological units

Engineering geological units Geological units (see maps) Description/characteristics Foundations
SOILS, ARTIFICIAL DEPOSITS, LANDSLIDE DEPOSITS
MIXED

(COARSE/FINE)

 Till (boulder clay) Stiff—very stiff stony and sandy clay of intermediate to rarely high plasticity; silt and sand lenses Generally good. Possible uneven settlement or shrink/swell behaviour
Head and valley deposits Soft—firm clay, sandy silty, pebbly clay; heterogeneous Poor, may be relict shears. Strength close to residual values
Alluvium, Peat, Glacio- lacustrine Deposits Fine—coarse, clay, silt, sand, gravel. Organic-rich lenses or deposits (peat) Soft, compressible in case of peat and organic-rich clays. Thicker gravels good
COARSE River Terrace Deposits Glaciofluvial and Pre-glacial Deposits Medium dense sand and gravel with laminated silts Generally good. Greater thickness present in former river channels
ARTIFICIAL DEPOSITS Made Ground Infilled Ground Variable composition, thickness and density Poor, variable, uneven settlement, locally compressible. Gas, chemical and bio-hazards
LANDSLIDE DEPOSITS Landslide Variable, commonly weak; may have voids Generally unsuitable; may require stabilisation
ROCK
SANDSTONE Sandstone in Mercia Mudstone Group Northampton Sand Very weak to strong, thinly bedded sandstone; may weather to dense/very dense sand Generally good. Less so if cementation removed by weathering to sand
MUDROCKS Whitby Mudstone Dyrham Formation Charmouth Mudstone Blue Lias Formation Penarth Group Mudstone and siltstone weathering to soft/hard overconsolidated fissured, silty clay of intermediate to high plasticity Generally good. Design for shrink well behaviour in high plasticity clays. Sulphate attack on concrete in some strata
Mercia Mudstone Group Very overconsolidated mudstone and siltstone. Weathers to fissured, low to locally high plasticity clay. Gypsum nodules/veins/beds Generally good, but weaker, weathered strata may underlie stronger material. Sulphate attack on concrete. Shrinkable clays most abundant in Edwalton and Cropwell Bishop formations
LIMESTONES Marlstone Rock Blue Lias Formation Moderately strong to strong limestone, muddy limestone and ferruginous limestone Generally good. Remote possibility of hazard from dissolution voids, and karstic features