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Geology of the Leicester district. Sheet description of the British Geological Survey 1:50 000 Series Sheet 156 Leicester (England and Wales)

By J N Carney, K Ambrose, C S Cheney, P R N Hobbs

Bibliographical reference: Carney, J N, Ambrose, K, Cheney, C S, and Hobbs, P R N. 2009. Geology of the Leicester district. Sheet description of the British Geological Survey, 1:50 000 Series Sheet 156 (England and Wales).

Authors: J N Carney, K Ambrose, C S Cheney, P R N Hobbs.

Contributors:

• Stratigraphy, structure, Quaternary geology: A Brandon, E Hough, R J Thomas, R K Westhead, P R Wilby, R J Rice^‡1 
• Biostratigraphy: J B Riding, I P Wilkinson, M Williams
• Geophysics: C P Royles
• Seismic interpretation and basin analysis: T C Pharaoh, N J P Smith
• Baseline geochemistry: L E Ander

Keyworth, Nottingham: British Geological Survey 2009. © NERC copyright 2009. ISBN 978 0 85272 573 3

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

(Back cover)

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Acknowledgements

In this report, J N Carney was responsible for overall compilation; K Ambrose contributed to sections on the Triassic, Jurassic and Quaternary, C S Cheney wrote the section on the Hydrogeology of the district and P R N Hobbs wrote much of the Engineering Geology section. M Williams checked the Jurassic faunal nomenclature used in this report. C P Royles evaluated gravity and aeromagnetic data bearing on the deep geophysical structure of the district. The manuscript was edited by A A Jackson: Cartography by R J Demaine and P Lappage; and page setting by A Hill.

The cooperation of landowners, tenants and quarry owners in permitting access to their lands is gratefully appreciated. Natural England is thanked for allowing us study access to the Tilton [on the Hill] cutting SSSI. We acknowledge all those who permitted the transfer of their data records to the National Geological Records Centre, BGS Keyworth. We are especially grateful to Dr R J Rice, who for the duration of this project made available all of his 1:10 000 annotated field slips compiled during 1961–63 for a PhD Thesis on the Quaternary geology of central Leicestershire.

The codes for geological units shown on the map face of Sheet 156 Leicester are indicated in parenthesis where first introduced in the text. A shorter account with some details of the stratigraphy, mineral resources and applied geology given herein, is provided with the published Sheet Explanation for Sheet 156. Much new information is introduced in this Sheet Description but it does not contain the level of lithological and palaeontological detail for the Jurassic rocks that is found in the original memoir (Fox-Strangways, 1903).

Notes

Throughout this report ‘the district’ refers to the area covered by the geological 1:50 000 Sheet 156 Leicester. This map is compiled from fieldwork conducted in thirty-five 1:10 000 series maps, or part of maps, identified on the inset at the foot of Sheet 156 Leicester. The 10 000 scale maps are available in digital or hard copy format. Annotated manuscript fieldslips at 1:10 000 are archived at BGS, both as hard copy and as raster scans, and are the primary source of the field-based information. Some geological linework may have been modified by geologists following completion and scanning of these fieldslips, however, and such modifications will appear only on the updated primary digital files for the relevant 1:10 000 sheets. Some cartographical simplification of the geological linework may have been undertaken during the process of compiling the 1:50 000 scale map from the primary 1:10 000 scale linework.

All National Grid references are given in square brackets; all lie within the 100 km squares SK or SP.

Borehole records referred to in the text are given a geographical name where possible. Locations, terminal depths and BGS registered numbers are given in (Table 10).

Numbers preceded by the letters P or A refer to the BGS collection of photographs; those preceded by the letter E refer to the BGS sliced rock collection.

Geology of the Leicester district—summary

This Sheet Description provides a brief account of the geology of the district covered by Geological Sheet 156 Leicester. The district encompasses the whole of the Leicester conurbation and extends eastwards to Whissendine and Horninghold, northwards to Mountsorrel and southwards to Whetstone. It includes the eastern fringe of Charnwood Forest, a large tract of rural Leicestershire and, in the east, part of Rutland. The oldest rocks crop out close to the western edge of the sheet, as small inliers of predominantly fine-grained volcaniclastic strata belonging to the Charnian Supergroup, of Precambrian (Neoproterozoic III) age. They are overlain by Cambrian mudrocks, which are only patchily exposed, but which evidently include Ordovician (Tremadoc) representatives encountered in deep boreholes. In late Ordovician (Caradoc) times, a series of small granodiorite batholiths was emplaced. Some of these are exposed whereas others, concealed by younger strata, have been proved by drilling or geophysical investigations. A phase of folding and mild metamorphism occurred in latest Silurian times, towards the end of the Caledonian orogeny, imposing a highly penetrative cleavage on the more argillaceous basement rocks.

The Mesozoic rocks comprise a strongly unconformable ‘cover’ to the basement, and both dip and ‘young’ eastwards. Arid conditions prevailed throughout much of the Triassic, when the distinctive red-beds of the Mercia Mudstone Group were deposited. The base-Triassic unconformity is particularly irregular around Mountsorrel and Charnwood Forest, which formed dissected hill ranges at the end of the Permian Period, but by latest Triassic times sedimentation had largely buried these upstanding areas. A shallow sea subsequently covered most of the region and strata of the Penarth Group accumulated, although at this early stage of the transgression the highest parts of Charnwood Forest may have remained emergent. Quiescent, warm tropical marine conditions became established during deposition of the overlying Lias Group. Such environments persisted throughout the Early Jurassic, although there was an episode in which water depths had shallowed significantly enough to usher in the high-energy, nearshore conditions that resulted in deposition of ooidal ironstones of the Marlstone Rock Formation. The uppermost part of the outcropping sequence occurs mainly as erosional outliers capped by Middle Jurassic strata of the Northampton Sand Formation, part of the Inferior Oolite Group and representing a second shallowing cycle of the Jurassic sea.

Quaternary geological processes chiefly moulded the landforms and drainage systems seen today. About two million years ago, in earliest Quaternary times, the landscape was dominated by the major north-east flowing Bytham River, which deposited sands and gravels. This drainage system was subsequently over-run by the ice sheets of the Middle Pleistocene glaciation, which left behind superficial deposits consisting of glaciofluvial outwash, glaciolacustrine clay, and till. These deposits now form a veneer covering the bedrock or hill-top capping, but in places are more thickly developed within palaeovalleys. Development of the modern landscape followed the retreat of the ice sheets, about 430 000 years ago. The various cycles of drainage development involved valley incision and aggradation, with further slope modification resulting from periglacial processes, particularly during Devensian times. A major economic legacy of these geomorphological cycles was the formation of five separate generations of sandy and gravelly river terrace deposits in the trunk valleys of the Soar and Wreake rivers.

Industrial development has been centred on Leicester, which was well served by canals and railways developed during the 19th century. The district was never particularly rich in minerals and although brick-clay, limestone, sandstone and ironstone were quarried in the past, these resources are no longer exploited. Current economic value comes from a thriving aggregate industry based on the quarrying of ‘hard rock’ (Ordovician granodiorite) and sand and gravel (Quaternary alluvium and river terrace deposits). Basal Triassic sandstones constitute a major aquifer in the general region, and there are other, minor aquifers from which groundwater has been abstracted. However, the district relies mostly on water supplied from elsewhere. Much of the Leicester city area is undergoing regeneration and here 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.

(Table 2)." data-name="images/P946735.jpg">(Table 1) Geological succession of the Leicester district. Thickness range shown in brackets in metres. ka age in thousand years. Ma age in million years. *A new national lithostratigraphical scheme of nomenclature for the Mercia Mudstone is indicated in (Table 2).

Chapter 1 Introduction

This Sheet Description provides an account of the geology of the district covered by the 1:50 000 Geological Series Sheet 156 Leicester, published as a Bedrock and Superficial Deposits edition in 2007. A simplified map of the bedrock geology is shown in (Figure 1), and the geological succession is summarised in (Table 2)." data-name="images/P946735.jpg">(Table 1). A brief summary of the geology is also provided in the Sheet Explanation that accompanies the published map.

The district lies within the administrative areas of Leicestershire, Rutland and Leicester City. The city of Leicester is a major conurbation and the main centre of population, as well as being the commercial and industrial hub of this part of the East Midlands. There are other, much smaller population centres in the rural area surrounding the city, where the land is given over mainly to arable and livestock farming.

A hilly and wooded landscape characterises the Ordovician outcrops of Mountsorrel and the Charnwood Forest massif of Precambrian rocks, in the north-west of the district. A more gently undulating topography is developed on the surrounding younger rocks, with a maximum elevation of 230 m above OD on Whatborough Hill in the east, and a minimum of about 44 m on the River Soar floodplain, in the north-west (Figure 2). The Soar is the main river in the district; it drains northwards to the Trent and occupies a wide valley. Tributary streams include the Wreake, Rothley Brook and Sence, each with a large floodplain. The main watershed separates streams flowing westwards into the Soar from those flowing to the River Witham, beyond the eastern margin of the district.

Bedrock geology is dominated by Mesozoic rocks of Triassic and Jurassic age, which either crop out or subcrop beneath Quaternary deposits over about 90 per cent of the district. These strata are tilted gently to the east or east-south-east and give rise to a subdued topography, the main element being a west-facing scarp slope developed on the Jurassic rocks, which is capped by the Marlstone Rock Formation. The escarpment and associated Marlstone dip-slope are embayed and deeply dissected, respectively by west- and east-draining stream systems. In places these features are extensively masked by Quaternary deposits, and there are significant areas of landsliding on some oversteepened slopes. East of the dip slope, the Whitby Mudstone crops out, surmounted by small outliers of Northampton Sand Formation that give rise to prominent flat-topped hills such as those of Whatborough and Robin-a-Tiptoe.

The district has limited natural resources; however, it currently supports an important aggregates industry, which is based on the quarrying of ‘hard rock’ around Mountsorrel, in the north-west, and the extraction of sand and gravel from Quaternary deposits within or bordering the floodplains of the Soar and Wreake rivers.

Outline of geological history

About 600 million years of geological history are represented by the rocks and superficial deposits of the district summarised in (Table 2)." data-name="images/P946735.jpg">(Table 1). The distribution of the outcropping Solid formations is indicated in (Figure 2), which also highlights those units of particular economic or environmental significance. These include, for example, the Cropwell Bishop Formation, which locally contains thick gypsum beds, and the Mountsorrel Complex, which hosts a ‘superquarry’ from which hard-rock aggregate is currently being extracted.

The north-western margin of the district lies on the fringe of Charnwood Forest, a hilly terrain of rough pasture and heathland, with craggy knolls in Bradgate Park exposing volcaniclastic rocks of the Charnian Supergroup, formed in the seas surrounding a volcanic arc that was active in Late Precambrian times. Following deposition of conglomerates represented by the Hanging Rocks Formation, and perhaps a subsequent interlude of erosion or non-deposition, a marine transgression in early Cambrian times laid down mudstones of the Swithland Formation. The Stockingford Shale Group contains closely similar lithologies, although at a lower metamorphic grade, and material from deep boreholes indicates an early Ordovician (Tremadoc) age for part of the group. Later in the Ordovician, in Caradoc times, subduction-related magmatism resulted in the emplacement of granodiorites, quartz-diorites and diorites of the Mountsorrel Complex and South Leicestershire Diorites, representing the exposed tops of small batholiths. Of similar age is the wholly concealed Melton Mowbray Grandiorite, penetrated by drilling but also indicated by the coincident aeromagnetic anomaly ‘high’. The earliest recorded deformation in the district dates from the end-Silurian, when the Charnian and Cambro-Ordovician sequences were cleaved and folded.

Following this tectonism, the district formed part of the northern margin of a rigid basement block, the Wales-London-Brabant Massif. The crust of this basement domain largely escaped the Variscan tectonic episodes that led to the accumulation of thick, fault-controlled, Carboniferous sedimentary sequences farther north. A thin veneer of Namurian to early Westphalian sedimentary strata is nevertheless recorded at depth, overlying basement rocks in the north of the district and forming the southernmost part of the Vale of Belvoir Coalfield. These strata accumulated during a post-rifting episode, when Carboniferous sedimentation expanded outside the margins of the former graben. The end-Carboniferous (Variscan) uplift reactivated many of the rift-bounding faults and inverted the Westphalian basins farther north, ushering in a new cycle of erosion that spanned most of the Permian period. In the north-west, the basement rocks resisted peneplanation to survive as a deeply dissected hill range, sculpted into bare-rock tors, gullies and larger valleys (‘wadis’) by a combination of fluvial activity and wind-action. This landscape is currently being exhumed from beneath a covering of Triassic strata, and is demonstrated by features on the highly irregular base-Triassic unconformity seen in the Buddon Wood (Mountsorrel) Quarry.

Throughout the Permian, when the district lay just to the north of the equator, the landscape was that of an arid peneplain dominated by the rocky massifs of Charnwood Forest and Mountsorrel. In earliest Triassic times, however, regional-scale subsidence had set in and progressive aggradation started to bury the old topography. The first strata to be preserved in this setting reflect a major fluvial episode, with river systems depositing arenaceous beds of the Sherwood Sandstone Group. These sandstones are not found at outcrop, but have been the target of deep drilling since they constitute a major aquifer in the East Midlands. There was a subsequent change to arid, desert conditions and a thick red-bed sequence, the Mercia Mudstone Group, was deposited. The principal outcropping mudrock units comprise the Gunthorpe, Edwalton and Cropwell Bishop formations, the last containing locally thick seams of gypsum. At the very top of the Mercia Mudstone, the distinctive grey to green dolomitic mudstone of the Blue Anchor Formation represents a marked environmental change prior to the latest Triassic marine transgression. Fluvial episodes are distinguished by numerous beds of green siltstone and sandstone (‘skerries’) within the Mercia Mudstone; the chief skerries occur at the base and top of the Edwalton Formation are known, respectively as the Cotgrave and Hollygate (Dane Hills) sandstone members. The Hollygate Sandstone, being particularly thick, has been exploited as a groundwater resource in parts of the city of Leicester.

The Penarth Group is of Rhaetian (latest Triassic) age, and is represented by the Westbury Formation and overlying Cotham Member (Lilstock Formation). The Westbury Formation was deposited in shallow-water marine conditions and the Cotham Member in a brackish, or lagoonal setting at the start of a marine transgression. Marine sedimentation continued to dominate in the district until at least the Mid Jurassic. The landscape in the west is subdued where the Mercia Mudstone Group crops out, but the ground rises to a low, sinuous, dissected escarpment where the main outcrop of the Blue Anchor Formation and Penarth Group occurs, capped by the Lias Group.

The Jurassic strata of the Lias Group were deposited in fully marine conditions, following the Rhaetian transgression. The group accumulated in generally warm, shallow, subtropical seas under well-oxygenated conditions leading to the accumulation of carbonates and argillaceous sediments with a diverse marine fauna in the Blue Lias and Charmouth Mudstone formations. The overlying Dyrham Formation and in particular the ferruginous Marlstone Rock Formation, locally a resource of ironstone, represent a regressive, high-energy episode. There was then a reversion to deeper water, more quiescent conditions, in which the Whitby Mudstone Formation was deposited. Overlying this, the Inferior Oolite Group represents a change to nearshore, high-energy conditions resulting in deposition of the uncomformable Northampton Sand Formation, the stratigraphically youngest bedrock unit of the district. Lias Group strata have a gentle regional dip to the south-east and significantly influence the topography of the district. Their variable resistance to erosion has produced a series of dissected cuestas, exemplified by the Northampton Sand outlier of Whatborough Hill [SK 7674 0592].

Quaternary Superficial Deposits (‘drift’) are widely distributed and locally very thickly developed in the district. Pre-glacial deposits of the Bytham Formation represent a ‘palaeo-floodplain’, and are restricted to the former course of the trunk stream and tributaries of the preglacial Bytham River, in the north and west of the district. The overlying glacigenic deposits of the Wolston Formation were laid down during the Middle Pleistocene glaciation of about 480 000 - 430 000 years ago. The lodgement tills of this event comprise the Thrussington Till Member, of northerly (‘Pennine’) derivation, a Lias-rich till of probable north-north-easterly derivation, and the very extensive Oadby Till Member with its characteristic suite of chalk and flint debris brought from the east or north-east by the youngest ice sheet. Outwash or subglacial melt-water sands and gravels of these tills are signified by the associated glaciofluvial deposits, whereas clays and silts of glaciolacustrine origin represent the ponding of subglacial or proglacial drainage. In the relict palaeovalley of the Bytham River, superficial deposits form a local stratigraphy in which Thrussington Till is overlain by, and in places interleaved with, glaciolacustrine deposits of the Rotherby Clay and Glen Parva Clay. A subsequent glaciofluvial phase is represented in part by the Wigston Member, which was deposited before the ice advanced once more and the Oadby Till was deposited.

The present landscape of the district was moulded by a complex series of post-glacial denudation cycles. Each of these has produced its own characteristic suite of deposits, which are generally the most thickly developed along and within the floodplains of the Soar, Wreake and Sence rivers. Those tracts contain the main outcrops of modern alluvium, as well as remnants of earlier floodplain levels represented by the River Terrace Deposits of the Soar Valley Formation, of which there are five separate generations. These are locally thick and constitute a major resource of sand and gravel in the district.

Quaternary slope deposits, of largely periglacial origin, are developed in all parts of the area, but form particularly well differentiated sequences in the Stapleford ‘clay vale’. The Little Dalby and Burton Lazars head deposits date back to the ‘Wolstonian’ stage. Younger head of Holocene-age is widespread at the foot of slopes and as ‘valley deposits’; landslide deposits are common on the steeper escarpment slopes of the district.

History of survey and research

The district was originally surveyed on the 1:63 360 scale by H H Howell, J W Judd and W H Holloway, and published on the old series sheets 63 NE, SE and 64 in 1855–72. The primary survey at the 1:10 560 scale, by C Fox-Strangways, was published as a Solid and Drift edition in 1903. D A Wray carried out minor revisions during remapping of the Marlstone Rock Formation in 1940, and in 1941 F B A Welch revised parts of the Northampton Sand Formation. The sheet was reprinted in 1954 without any further amendments and again, in 1975, when it was reconstituted on to a 1:50 000 scale topographical base. A geological memoir was published (Fox-Strangways, 1903) and there is a subsequent memoir detailing the geology of the iron-bearing Marlstone Rock and Northampton Sand formations (Whitehead et al., 1952).

The new resurvey of the district at 1:10 000 scale was completed during 2000–2004, a period that included a major interruption to rural fieldwork caused by the foot-and-mouth outbreak of 2001. Although the survey was principally field-based, it has also involved the collation and compilation of archival data sources; the chief of these are borehole records, the field-slips of Fox-Strangways and the early 1960s field-slips of J R Rice, who surveyed the Quaternary geology of the Soar valley at a time when significant parts of the Leicester conurbation had not yet been developed. This Sheet Description also includes the findings of researchers from various geological disciplines within BGS, as detailed in the list of contributors (see Acknowledgements).

Chapter 2 Precambrian

The Precambrian rocks of the district form the south-eastern fringe of Charnwood Forest, which represents an uplifted block of the cratonic basement that otherwise deeply underlies this part of the English Midlands (Pharaoh et al., 1987a). They consist of volcaniclastic strata belonging to the Charnian Supergroup, which is most extensively exposed in the Coalville and Loughborough districts, to the west and north-west of the present sheet.

Charnian Supergroup

The lithostratigraphical scheme for this supergroup is based on the terminology of Watts (1947) that was formalised by Moseley and Ford (1985) and modified further by Carney (1994). The rocks of the Leicester district belong to the upper division, the Maplewell Group, the volcanic provenance of which has long been recognised (see Watts, 1947 and references in Ford, 1979). The group is typical of the Charnian sequence as a whole, in that it is mainly composed of well-bedded or laminated volcaniclastic strata. These are admixtures of grains representing material that was either eroded or ejected from the volcanic axis and preserved in depositional basins surrounding it. The component grains consist of fine-grained andesitic to dacitic volcanic rock, together with pyroclastic material, such as ash shards, and angular crystal fragments. The general absence of cross-bedding or wave-generated structures indicates that deposition of the Charnian volcaniclastic sequence primarily took place below storm wave base. Structures such as normal grading suggest transport of material by turbidity currents and debris flows, most probably involving pyroclastic flows in areas proximal to the volcanic source regions (Carney, 2000a). Chemical analyses of the more primary components, exposed as volcanic or subvolcanic complexes farther to the north-west (Carney, 2000a; Carney et al., 2001), show that the parental magmas were calc-alkaline, similar to those of modern evolved volcanic arcs founded upon oceanic or attenuated continental crust (Pharaoh et al., 1987b).

A latest Precambrian (Neoproterozoic III) age for the extrusive phase of the Charnian arc is indicated by Ediacaran fossils. These are mainly found in strata of the Bradgate and Beacon Hill formations, in the adjacent Coalville district (Boynton and Ford, 1995), and their presence indicates that probably the whole of the Charnian Supergoup belongs to the newly designated Ediacaran Period of the Neoproterozoic erathem (Knoll et al., 2004). Direct age dating of a volcaniclastic bed (in the adjacent Coalville district) within the Maplewell Group by the SHRIMP U/Pb method yielded zircon age groupings of 573 ± 1 Ma, 559 ± 2 Ma and 549 ± 2 Ma. (Compston et al., 2002). The 559 Ma date was preferred for the eruptive event leading to the deposition of this bed. It is, however, in conflict with a previous estimate, based on a correlation between intrusive rocks of the South Charnwood Diorites (‘Markfieldite’), exposed in the Coalville district (Worssam and Old, 1988), and a chemically and petrographically identical Precambrian intrusion at Nuneaton, which has yielded a U/Pb age of 603 ± 2 Ma (Tucker and Pharaoh, 1991). Further U/Pb determinations by the TIMS method are currently in progress by BGS/NIGL. It should be noted that Precambrian volcanic rocks proved in the Orton and Glinton boreholes, respectively located 40 km to the south-east and 75 km east of Charnwood Forest, have also yielded relatively older U-Pb isotope ages of 616 ± 6 Ma and 612 ± 21 Ma respectively (Noble et al., 1993). These rocks are not the precise chemical equivalents of those in Charnwood Forest, however, and their relevance to the age of Charnian volcanicity is therefore questionable.

In terms of their geochemistry, the Charnian Supergroup and South Charnwood Diorites are directly comparable with Precambrian rocks exposed within the Nuneaton inlier, 23 km to the south-west (Bridge et al., 1998). All of these Precambrian occurrences were originally part of the Avalonian volcanic arc system, of latest Neoproterozoic age, and are collectively referred to as the ‘Charnwood Terrane’ of basement rocks (Pharaoh et al., 1987b). The Charnian sequence underwent epizonal (high greenschist facies) metamorphism and was folded into a south-east plunging anticline in end-Silurian times (Chapters 9 and 10), and the strata in the Leicester district represent part of the southern and eastern rim of that structure.

Bradgate Formation (BT)

In the north-western part of the Leicester district, strata belonging to the Hallgate Member of the Bradgate Formation crop out within a small area of Bradgate Park on the eastern margin of Coppice Plantation. The unit belongs to the uppermost (Maplewell Group) division of the Charnian Supergroup (Moseley and Ford, 1985), which has an estimated thickness of about 200 to 500 m in the adjacent Coalville district (Worssam and Old, 1988). The sporadic, craggy exposures in the Leicester district, however, show only several metres of these strata. At the northern end of the outcrop [SK 5422 1119] an exposure (see Front cover) overlooking Cropston Reservoir consists of about 0.4 m of grey- to cream-weathering volcaniclastic siltstone overlain by about 0.2 m of very fine-grained volcaniclastic sandstones in parallel-sided beds 10 to 20 mm thick. In similar lithologies intermittently exposed along the footpath to the south-west, the beds and laminae are locally rafted and contorted [SK 5410 1092]. Near the southern margin of Coppice Plantation [SK 5406 1088], the exposures include a small quarry. There, the lower part of the succession consists of at least 1.8 m of grey volcaniclastic siltstones stacked as internally massive beds of 0.15 m average thickness; the boundaries to these beds are commonly defined by sharp-based sandstone laminae about 3 mm thick. The overlying strata consist of about 1 m of grey volcaniclastic siltstone in thick beds that grade downwards to sandstone bases, the latter from 10 to 40 mm thick (Plate 1). The structural geology of these exposures is discussed in Chapter 10.

In a thin section of fine-grained volcaniclastic rock from the southern margin of Coppice Plantation (E74302), the siltstone laminae are crammed with angular crystal fragments of quartz and plagioclase feldspar. The very fine-grained, ‘mud’ layers are probably tuffaceous; they contain abundant, grainy, comminuted material, which is mainly unresolvable but shows faint shardic outlines in parts, suggestive of a high volcanic ash content. There is a strong preferred orientation of very fine, felted white mica, of metamorphic origin, superimposed upon the original mineral assemblage.

Key Localities

Bradgate Park: [SK 5422 1119] to [SK 5406 1088]

Hanging Rocks Formation (HR)

The uniqueness of this unit, setting it apart from the rest of the Charnian Supergroup, lies in its content of well-rounded pebbles. Its stratigraphical relationship with the Maplewell and Brand groups is in some doubt, but here it is placed at the top of the former, because there is evidence that it may have been deposited during a late continuation of Charnian volcanism. In the past, however, it has been included in the overlying Brand Group (Moseley and Ford, 1985), though bounded above and below by unconformities (McIlroy et al., 1998). The type section, and only other known locality for the unit, occurs farther north in the Coalville district (Sheet 155), at the Outwoods-Hangingstone Hills SSSI (Worssam and Old, 1988; Carney, 2000b).

In Bradgate Park^‡2 , 7.4 m of these strata are exposed at the foot of the slope east of Coppice Plantation. No top or base to the unit is seen, but a dip of about 70° to the east is suggested by the attitude of coarse partings interpreted as bedding planes. A preferred orientation of spindle-shaped conglomerate pebbles defines a fabric dipping at about 75° to the NNW; this is not depositional, but is due to later stretching in the plane of the Charnwood cleavage. The lowermost (westernmost) bed consists of more than 3.4 m of grey, medium- to coarse-grained volcaniclastic sandstone. In the topmost 0.5 m of the sandstone there are sporadic, well-rounded granules and pebbles, the latter up to 20 mm across, demonstrating that this bed is part of the Hanging Rocks Formation, rather than belonging to the underlying Bradgate Formation. Conglomeratic lithologies comprise the upper (easternmost) three beds. They are very poorly sorted, with no internal structure, and although the pebbles are locally close-packed, there are areas, particularly in the lowermost bed, where the medium- to coarse-grained sand matrix is dominant (Plate 2). Most pebbles fall in the size range of 5 to 15 mm, but granules are also common. Larger pebbles, up to 100 mm across, are rather more sporadic in occurrence and in the thickest (1.3 m) conglomerate bed are concentrated in the upper 0.5 m. The pebbles are roughly spherical to strongly oblate in shape, and have rounded to subangular or, more rarely, sharply angular outlines. Their lithologies were analysed by J Moseley (see Worssam and Old, 1988, p.18), who identified 29 per cent as being of volcanic origin (rhyolite/dacite and trachyte), with the remainder consisting mainly of quartz, pelite and quartzite. Sampling is no longer allowed here, but a superficial examination suggests an abundance of pink to cream pebbles with sporadic quartz phenocrysts, which are probably of a similar type to those found at Hangingstone Hills, and thus may be composed of fine-grained dacitic tuff and flow-banded welded tuff (Carney, 2000b). Large sedimentary clasts (about 0.4 m across) of pale to dark grey mudstone are concentrated at the base of the topmost conglomerate bed.

A notable feature of this exposure is the low topographical position of the Hanging Rocks Formation, relative to older strata of the Hallgate Member (Bradgate Formation), occurring on the hillside several metres above. Alternative explanations for these field relationships, discussed in Sutherland et al. (1994), are that:

• the beds lie within the Hallgate Member, but as they contain well-rounded pebbles, they would be unique amongst the lithologies of the Bradgate Formation
• or here the Hanging Rocks Formation is occupying a channel cut into the Hallgate Member.

The explanation preferred by Carney and Pharaoh (2000) is that this exposure forms part of a downfaulted inlier of the Hanging Rocks Formation; the steep dip observed for these beds certainly suggests structural complexity and would in any case be at odds with a channel-fill interpretation.

Pebble composition and roundness indicate deposition of the Hanging Rocks Formation on an eroded surface developed on a volcanic terrain. The likely source of the sediment is from fluvial or shoreline environments. Nevertheless, the poor sorting and matrix-supported nature of the conglomerates, their general lack of organisation and, at the Hangingstone Hills locality (Carney, 2000b), the presence of parallel stratification and grading indicate a final episode of transport by subaqueous sediment gravity flow, perhaps as turbidity currents with eventual deposition in a submarine fan or fan-delta environment. Shards of volcanic glass have been identified in thin sections of the unit from the Hangingstone Hills locality (Worssam and Old, 1988; McIlroy et al., 1998; Carney, 2000b), indicating contemporary volcanism. If this volcanism is found to be associated with Charnian (i.e. Precambrian) magmatism then the Hanging Rocks Formation should logically be included at the top of the Charnian Supergroup rather than as an unconformity-bounded unit at the base of a Brand Group that also contains the Swithland Formation of probable Lower Cambrian age (e.g. McIlroy et al., 1998).

Key Locality

Bradgate Park, east of Coppice Plantation [SK 5417 1095].

Chapter 3 Cambrian and Ordovician

The Cambrian-Ordovician strata that are exposed along the eastern edge of Charnwood Forest are included within the Brand Group, whereas the rocks exposed along the western contact of the Mountsorrel Complex and proved in certain deep boreholes in the Leicester city area, are equated with the Stockingford Shale Group. These strata form much of the ‘basement’ to the Carboniferous and Mesozoic strata of the district, (Figure 12).

Brand Group

Moseley and Ford (1985) formally named this group for the outcrops around The Brand, which adjoins the western margin of the district. McIlroy et al. (1998) suggested a tripartite division of the group into the Hanging Rocks Formation, described above, overlain by the Brand Hills Formation (not represented in this district), which is in turn succeeded by the Swithland Formation. Originally the Brand Group was included as the third, and youngest, division of the Charnian Supergroup. In this account, however, it is regarded as a stand-alone unit, separate from the Charnian Supergroup, because as discussed below there is now evidence that the Swithland Formation may be of Cambrian age rather than Precambrian as previously thought.

Swithland Formation (SG)

The Swithland Formation, or ‘Swithland and Groby Slates’ of Fox-Strangways (1903), crops out along the eastern and southern fringes of Charnwood Forest. In the adjacent Coalville district (Worssam and Old, 1988) it is more than 375 m thick, with no top seen. It constitutes the uppermost unit of the Brand Group, which crops out around the south-western and south-eastern flanks of the Charnwood anticline (Worssam and Old, 1988; fig. 2). The name was formalised by Moseley and Ford (1985), who designated type sections on ‘The Brand’ estate (Sheet 155 Coalville) and in Swithland Wood; both localities extend into the north-western corner of the Leicester district.

The Cambrian age for the unit is based on the discovery of the trace fossil Teichichnus in local gravestones that had been cut from the Swithland Formation (Bland and Goldring, 1995; McIlroy et al., 1998). The contact with the underlying Hanging Rocks Formation is nowhere exposed, although in the Hangingstone Hills section the two units appear to be gradational and structurally conformable (Carney, 2000b). An intervening unconformity was nevertheless favoured by McIlroy et al. (1998) and would provide a convenient stratigraphical datum for the Precambrian to Cambrian transition in Charnwood Forest.

In the Coalville district, the equivalent unit, named as the ‘Swithland Greywacke Formation’; crops out more widely and consequently has been better studied. There, it consists largely of cleaved silty mudrock, with detrital constituents of quartz, feldspar and fine sand grade lithic fragments in a matrix of white mica and chlorite (Worssam and Old, 1988). Bedding and lamination are expressed by variations in these constituents, and white mica is preferentially developed along a closely spaced (less than 1 mm) cleavage (see Structure).

In the Leicester district, the formation is exposed along the footpath skirting the eastern edge of Swithland Wood. It consists of grey to maroon slaty mudrock, showing rare bedding planes, cut by a millimetre spaced cleavage that has been accentuated by weathering. In thin section (E74299) the lithology is a metasiltstone consisting of abundant angular grains of quartz and plagioclase feldspar, which may be tightly packed or matrix-supported. Detrital Fe-Ti oxide grains constitute about 5 per cent of the rock. White mica and oxides show a strong preferred orientation locally, reflecting the slaty cleavage seen at outcrop. Similar rocks farther north [SK 5404 1284] form the southernmost exposures of ‘The Brand’ locality.

Key localities

Swithland Wood footpath [SK 5425 1200]; [SK 5420 1224]

Stockingford Shale Group (SSH)

These strata were laid down during a widespread transgression of the Iapetus Ocean across an eroded Precambrian landmass. Their type section is at Nuneaton, just over 20 kms west of the Leicester district (Taylor and Rushton, 1971; Bridge et al., 1998). In the Leicester district, possible hornfelsed equivalents of these strata are sporadically exposed around the western edge of the Mountsorrel Complex. More obvious correlatives of the Stockingford Shale Group are proved in deep boreholes, one of which, the Crown Hills Borehole, has yielded important biostratigraphical information.

Provings in deep boreholes^‡3 

A ‘basement’ consisting of fractured and cleaved mudrock, equated with the Stockingford Shale Group, has been proved beneath Triassic strata of the Sherwood Sandstone Group in four boreholes (see (Figure 4) for locations of some of these boreholes). The provings (with depth ranges) are at: Crown Hills (255–305 m); Lodge Farm or Spinney Hills (226–249 m); Knighton Fields (183–251 m) and Nansen Road (248–252 m). The last was completed in 1997 but is very poorly documented. Descriptions for the first three are given in Fox-Strangways (1903) and, although they are generally poor, the overall impression is of rocks that are hard, dark grey and pyritous, locally with quartz veins and a well-jointed or ‘slaty’ structure.

Samples of the sub-Triassic rocks from the Crown Hills Borehole are available for examination at Leicester Museum, and in the BGS collections (see Information Sources). Some of the BGS samples show silty laminae dipping at about 20°; this dip suggests a minimum true thickness of about 47 m for the sequence (Molyneux, 1991). The typical lithology consists of hard, dark or medium grey, micaceous mudrock and silty mudrock with paler grey siltstone laminae. Parting typically occurs along a weakly developed, bedding parallel fissility that is interpreted as a diagenetic fabric by Pharaoh et al. (1987a, fig. 7a). These authors describe a thin section (E50334) with a mineralogy consisting of finely intergrown white mica and chlorite that are locally aligned where defining the fissility; coarser mica and silty quartz grains, together with very sporadic Fe-Ti oxide grains, are of detrital origin. Other thin sections held by BGS indicate a regular interbedding of siltstone and muddy siltstone (E50335); (Plate 3a), with slight normal grading developed in the former; in this sample stringers of Fe-Ti oxides define a closely spaced, secondary cleavage fabric that cuts the bedding-parallel fissility at a high angle. In a further thin section (E50333); (Plate 3b) siltstone layers show disruption into asymmetric boudins due to overprinting by a secondary fabric developed oblique to the bedding fissility.

The age of the Crown Hills basement sequence is constrained biostratigraphically by its shelly faunas, including Eurytreta sabrinae (Callaway), ‘Hyolithes’ aff. magnificus calvus (Lake), Asaphellus homfrayi (Salter) and Micragnostus calvus (Lake), indicating a horizon above the Clonograptus tenellus Biozone of the Tremadoc (Bulman and Rushton, 1973; Molyneux, 1991). A full faunal listing is available at the BGS (Wilkinson, 2003a; see also Information Sources).

Similar mudrocks, also with Tremadoc faunas, dipping at 40 to 60°, were proved at the base of the Leicester Forest East Borehole, 1.5 km west of the Leicester district (Worssam and Old, 1988). They were also intersected in drifts at Merry Lees Colliery, about 7 km west of the district and very close to the Thringstone Fault. There they are tightly folded and steeply dipping, with an east-west strike (Butterley and Mitchell, 1945; Worssam and Old, 1988).

In the north-west of the district, the Hall Farm Borehole (SK51SE/391) demonstrated ‘hard grey shale’ between 60 and 98 m, below the Mercia Mudstone Group. This proving (Figure 4) could be in either the Swithland Formation or Stockingford Shale Group, both of which may, as discussed below, form a continuous sequence.

About 5.6 km beyond the south-eastern corner of the district (Figure 4), the Thorpe by Water Borehole passed through strata of the Mercia Mudstone Group and cored 81.26 m into the underlying basement. The borehole log records interbedded siltstone and sandstone, which is medium grey to pale grey and locally pyritous. Bedding and lamination indicate a tectonised sequence characterised by variable dip that is near-vertical in places. The strata are fractured, faulted and cleaved and are cut by narrow (up to 0.6 m) sheets of pale grey feldsparphyric igneous rock. Sedimentary structures include load-casting, wispy and slumped bedding and rare normal grading, with burrowing and various other features indicative of bioturbation. The few fossils recovered include a horny brachiopod and a possible graptolite, and suggest an Ordovician or early Silurian age (Molyneux, 1991). Borehole samples yielded a Rb/Sr date of 468 ± 36 Ma (mid-Ordovician; Bath, 1974), but the epizonal metamorphic grade of these rocks (Pharaoh et al., 1987a, fig. 6) suggests that this value very probably reflects isotopic resetting caused by an episode(s) of heating and recrystallisation, subsequent to deposition of these strata. The wide error range renders the determination uninterpretable in terms of a precise age for the metamorphic event.

Exposures adjacent to the Mountsorrel Complex

Hornfelsed metasedimentary rocks are exposed at three places along the western contact of the Mountsorrel Complex (Figure 3). They are locally affected by a weak cleavage or fracture cleavage, and by folding, and have a metamorphic mineral assemblage (see Chapter 9) attributed to recrystallisation within the contact aureole of the Mountsorrel Complex. The occurrences are barren of fossils, and so their correlation with the Stockingford Shale Group is highly tentative.

In the grounds of Quorn House, a small disused quarry known as the ‘old gravel pits’ reveals a few metres of grey or purple-brown rock. When slabbed and polished, samples of these rocks show sedimentary laminae 5 to 10 mm thick composed of dark green-grey to pink mudstone alternating with siltstone that coarsens downwards to fine-grained sandstone. Most beds appear to be highly lenticular and disrupted, structures that resemble soft-sediment deformation. A polished slab (Plate 4a), however, shows an unusual nodular structure that is more likely to have been produced by the growth of contact- metamorphic minerals. (Plate 4a) also shows a 2 mm-wide veinlet of pink, fine-grained granite or granodiorite, e.g. (E72434) and similar granitic veins, up to several centimetres wide, were seen in other parts of the exposure, which is located within about 10 m of Mountsorrel Complex granodiorite. No penetrative cleavage is seen, but the exposure shows a well-spaced fracture system orientated at 100° (Figure 3) and thus related to the regional Charnwood cleavage (see Chapter 10).

The second occurrence of possible Stockingford Shale strata consists of poorly accessible exposures (Plate 4b) around the top edge of a partially flooded quarry on the south-western shore of Swithland Reservoir, opposite the island of Brazil Wood (Figure 3). The tightly folded and hornfelsed fine- to medium-grained metasedimentary rocks exposed here are grey to pinkish grey with dark grey, commonly coarsely micaceous, sedimentary laminae or thin beds. A pink dioritic intrusive sheet is just accessible, and may be the thick sheet (about 1 m) noted by Lowe (1926). Metasedimentary rocks exposed a few metres from this sheet show a weak, penetrative cleavage (see Chapter 10), whereas this structure is missing from rocks immediately adjacent to the diorite. The continuation of the hornfelsed rocks farther east was formerly demonstrated in a trench dug between this locality and Brazil Wood, prior to the filling of Swithland Reservoir (Fox-Strangways, 1903). In this trench, Watts (1947) noted that numerous dykes and veins of ‘somewhat basic granite and microgranite’ cut ‘altered slates’.

The third occurrence, on the eastern (Kinchley) shore of Swithland Reservoir, by Kinchley Hill [SK 5597 1389], was originally documented by Lowe (1926, p.19) who described it as consisting of ‘indurated slate some 8 feet long by 2 feet 6 inches high’. Metasedimentary rocks were found near high water mark along the Kinchley shore [SK 5599 1397] during the present survey, but at a location (Figure 3) some 70 m due north of where Lowe (1926; locality ‘D’ on the sketch map) originally showed them. The metasediments are grey to brown-grey and laminated to thinly bedded, with an average bed thickness of about 10 to 20 mm; the dip is about 35° due south. Some of the beds show normal grading, from a fine sandstone base upwards to siltstone and mudstone. The exposure is cut by a coarse, near-vertical fracture cleavage orientated east-north-east. A thin section of metasediment (E74301) consists of a structureless, micaceous hornfels with possible pseudomorphs of cordierite and pyroxene (Plate 7b), and is described in more detail in Chapter 9. The metasediment occurs within several metres of Mountsorrel granodiorite, the latter evidently of marginal facies as it is ‘basified’ with 30 to 40 per cent of mafic mineral aggregates. It could represent a xenolithic inclusion within the main body of the Mountsorrel Complex, but it may also be interpreted as a septa or embayment of country rock along an irregular intrusive contact with the Mountsorrel Complex (Figure 3). The latter interpretation is to some extent borne out by the geophysical survey of Davies and Matthews (1966, fig. 2), which produced a contoured magnetic gradient map showing a prominent ‘col’ between Kinchley Hill and Brazil Wood, broadly corresponding to the zone where the metasedimentary rocks occur. Farther south, and close to locality ‘D’ where hornfelsed metasediments were noted by Lowe (1926), a further exposure [SK 5597 1388] showed dark grey, medium-grained, even-textured rocks, but these are not metasedimentary and as discussed later, are composed of hornblende-bearing quartz microdiorite (Chapter 4).

Lowe (1926, p.18) considered that these hornfelsed strata were different from the Swithland Formation of the Brand Group, with which they had been correlated by Hill and Bonney (1878). Le Bas (1968, p.48) suggested that they are correlatives of the Cambrian to Tremadoc-age Stockingford Shale Group of Nuneaton (Bridge et al., 1998), which is a reasonable supposition given that mudrocks of this age have been found at depth elsewhere in the East Midlands region; for example, in the Crown Hills Borehole in Leicester (see above). It should be noted, however, that the Swithland Formation, which was formerly thought to be Precambrian, is now believed to be of Cambrian age because it contains the Phanerozoic trace fossil, Teichichnus (Bland and Goldring, 1995). It thus seems possible that the strata here tentatively correlated with the Stockingford Shale Group could be part of a continuous sequence with the Swithland Formation, in which case the differences between them could be related to changes in the depositional environments, source regions, or even the degree of alteration and metamorphism (Chapter 9).

Key localities

‘Old Gravel Pit’ in Quorn House Park [SK 5583 1556]; Swithland Reservoir, west of Brazil Wood [SK 5567 1355].

Chapter 4 Ordovician intrusive rocks

These rocks crop out sporadically in the west of the district. They form the exposed parts of small batholiths which, although largely concealed by younger strata, can be traced in the subsurface of the East Midlands through borehole or seismic data, and in particular by their aeromagnetic expression (Evans and Allsop, 1987; Lee et al., 1990). Their subsurface distribution in this district is illustrated in (Figure 12), and their morphology and likely structure at depth is discussed in Chapter 11. The plutons are of calc-alkaline chemistry and they, together with volcanic rocks encountered in boreholes to the east of the Leicester district, mark the axis of a continental-type magmatic arc that extended eastwards into Belgium in Ordovician times (Le Bas, 1972, 1981; Pharaoh et al., 1991, 1993; Millward, 1999). This arc system bears the imprint of Early Palaeozoic deformations, one of which is represented by the Charnwood Forest structures described in Chapter 10. It is incorporated into a complex and variable, largely concealed basement domain termed the ‘Eastern Caledonides’ by Pharaoh et al. (1987a). The two intrusive associations represented in the Leicester district are the Mountsorrel Complex and South Leicestershire Diorites. A third intrusion, the Melton Mowbray Granodiorite, is a concealed body proved by drilling just to the north of the district. A basic dyke that cuts the Mountsorrel Complex may be of Carboniferous age, and is described in Chapter 5.

Mountsorrel Complex (Msr)

The Mountsorrel Complex gives rise to hilly country in the north-western part of the district. It consists mainly of granodiorite, with subordinate developments of diorite and gabbro exposed near the southern margin of the outcrop. The main outcrop (Figure 3) is centred on what was formerly Buddon Wood [SK 5620 1501], most of which is now Buddon Wood Quarry. The satellitic granitoid knolls of Cocklow Wood [SK 569 151] and Hawcliff Hill [SK 5730 1515] have been largely removed, and the eponymously named quarries replacing them are now assimilated into aggregate treatment areas. Farther east the original ‘Mountsorrel Quarry’ [SK 577 150], more recently known as ‘Castle Hill Quarry’, was being worked at the turn of the 19th century but in the last decade of the 20th century it was a landfill site, and it is now largely restored and landscaped. Surrounding the quarried areas are numerous natural or partly quarried exposures that form craggy knolls separated from each other by Mercia Mudstone and/or Quaternary deposits that occupy the lower lying ground. The Mountsorrel Complex is considered to be the expression of a partially exhumed Triassic mountainland (Bosworth, 1912) and thus the Triassic unconformity is extremely irregular; it is seen in the faces of Buddon Wood Quarry and described in the section below on the Mercia Mudstone Group. On geophysical grounds the Mountsorrel Complex was thought to extend beneath the cover rocks for several kilometres eastwards, as far the village of Thrussington (Hallimond, 1930; McLintock and Phemister, 1931); however, aeromagnetic maps for the district (Figure 14); (Figure 16) indicate that the anomaly beneath Thrussington belongs to a separate Ordovician intrusion, the Melton Mowbray Granodiorite (Carney et al., 2004).

Early workers (e.g. Hill and Bonney, 1878) believed that the Mountsorrel rocks were a continuation of the Precambrian basement seen in Charnwood Forest, a few kilometres to the west. Lowe (1926) drew attention to their predominantly granodioritic composition, however, and Jones (1927) suggested on the basis of jointing that they represented an intrusion of ‘post-Charnian’ age. Watts (1947) concluded that the petrological affinities of the Mountsorrel rocks are with the igneous ‘Caledonian group’ as represented in the Lake District. Meneisy and Miller (1963) broadly confirmed this by obtaining a K-Ar age of 379 ± 17 Ma on quartz diorite from Brazil Wood (Figure 3). The age of emplacement is, to a certain degree, better constrained by U-Pb isotopic analysis of zircons extracted from the exposed rocks of the Mountsorrel Complex. This method yielded an Ordovician (Caradoc) age of 451 to 452 Ma, and was derived by Noble et al. (1993), who recalculated the Mountsorrel data obtained by Pidgeon and Aftalion (1978) by merging it with values obtained from the nearby intrusive rocks of the South Leicestershire Diorites Suite. The former authors urge caution, however, in that the Mountsorrel age was obtained on zircons that were abraded and are significantly discordant. The age nevertheless falls within the spectrum of values between 449 ± 13 and 457 ± 20 Ma obtained from tuffs and plutonic rocks proved by drilling elsewhere in the basement of eastern England (Noble et al., 1993). This activity was thus broadly contemporaneous with the Ordovician volcanism of the Lake District and Snowdonia.

Chemical compositions for the Mountsorrel rocks are provided in a number of publications (Taylor, 1934; Le Bas, 1972; 1981; Worssam and Old, 1988; Webb and Brown, 1989; Pharaoh et al., 1993). The suite as a whole is comparable to certain intrusions of the Brabant Massif of Belgium and the English Lake District (Le Bas, 1972; Pharaoh et al., 1993). It shows no iron enrichment, moderate enrichment of large ion lithophile (LIL) elements (K, Rb and Ba), Th and Ce, and relative depletion of Nb (and Ta), which are patterns typical of calc-alkaline volcanic arc magmatism. These rocks therefore provide evidence for Ordovician (Caledonian) calc-alkaline magmatism in eastern England, which probably was associated with subduction of oceanic lithosphere from an eastern ‘Tornquist’ branch of the Iapetus Ocean (Pharaoh et al., 1993).

General accounts of these rocks can be found in Fox-Strangways (1903), Lowe (1926), Le Bas (1968) and Carney and Pharaoh (1999). The coarse-grained varieties that form the bulk of the Mountsorrel Complex can be classified as granodiorite on the basis of their modal analyses, geochemistry and normative mineralogical composition. Accessible examples of typical pink, coarse-grained, granodiorite can be seen around Castle Hill, by the playing field north of Halstead Road, and at the Castle Hill Quarry SSSI. The texture is markedly inequigranular, verging to porphyritic. The granodiorite consists of pink, idiomorphic plagioclase crystals, up to 10 mm in length, set in a grey, coarse-grained base showing prominent quartz aggregates. In a thin section from the exposure north of Halstead Road (E73857), plagioclase forms large euhedral crystals with oscillatory zoning (Plate 5a); they are set in a hypidiomorphic-granular base featuring small euhedra of turbidly altered plagioclase crystals and granular intergrowths of quartz and perthitic orthoclase. Mafic minerals form less than 10 per cent of the rock and consist of clusters of coarse green-brown biotite. A further exposure at the Halstead Road Field wildlife area shows inequigranular granodiorite with common rounded diorite xenoliths up to 0.1 m across; one xenolith, 35 mm wide, is foliated and has a partially preserved chilled margin, suggesting that it may represent a disrupted synplutonic basic sheet. Sporadic, discontinuous aplite veins are also seen at this locality. A dyke was formerly described at the small disused granodiorite quarries by Wood Lane [SK 5665 1552], but was not confirmed during this survey. Prominent shatter zones and a fault are present, however, some of which could superficially resemble well-jointed dyke rock.

Carney and Pharaoh (1999) provided descriptions of granodiorite within the main pit of the Buddon Wood Quarry. The fresh granodiorite in the eastern part of the quarry is a grey, coarse-grained, inequigranular rock speckled with biotite. Later alteration has caused reddening of feldspars and replacement of mafic minerals within decimetres-wide zones of closely spaced joints that trend east-north-east (about 300°); the joint surfaces show slickenfibre development. In this part of the quarry, the granodiorite is homogeneous, with only sporadic, small, rounded xenoliths of dark grey, medium-grained diorite. Farther west, however, diorite xenoliths become more numerous, and two phases of aplite intrusion can be recognised in fallen blocks; the second phase is characterised by pronounced chilling of the sheet margin.

In addition to the dioritic xenoliths noted above, possible metasedimentary xenoliths, with tabular shapes and fine grain size, also occur and are illustrated by a photomicrograph in Lowe (1926, fig. 29). That particular example consists of a mosaic of feldspar, some of which show triple-point boundaries suggestive of annealing recrystallisation in sedimentary inclusions incorporated into the main mass of the granodiorite.

Aplite sheets, many with a northerly trend, have been noted in granodiorite at several exposures (Figure 3). Two phases of aplite intrusion are commonly present at the western and eastern margins of the Mountsorrel Complex. In the west, at the summit of the prominent knoll (‘Craig Buddon’) overlooking the Swithland Reservoir spillway, a vertical aplite sheet, 0.45 m thick, striking north-north-east, is cut by a second sheet dipping at 40° to the north-east (Plate 5b). At Castle Hill, just below the War Memorial [SK 5824 1494], the second phase of aplite intrusion forms a narrow sheet with chilled margins that crosscuts an earlier north-west-trending aplite sheet. At this exposure the host granodiorite shows a diffuse, near-vertical possibly magmatic foliation, trending west-north-west. Lowe (1926) describes thin sections of aplite, consisting of a fine-grained quartzo-feldspathic ‘mosaic’, in which K-feldspar is prominent, with sporadic scraps of muscovite, biotite, Fe-Ti oxides and chlorite.

Granitoid dykes cutting the complex were described by Lowe (1926). One of these, with an east-south-east trend and thickness of just over 3 m was observed in a disused quarry [SK 5659 1553] by Wood Lane. It consists of phenocrysts of largely epiditozed plagioclase, and chloritized amphibole, set in a fine-grained, intergranular, plagioclase-rich matrix. Grey to pink, fine-grained lithologies were noted at this overgrown locality during the present survey, but the margins of the dyke could not be located. Across Wood Lane from here Lowe illustrated a further dyke, with a northerly trend, in a former quarry [SK 5685 1551] that is now filled in. The lithology is non-porphyritic, consisting of medium-textured prisms of plagioclase interspersed with chlorite, the latter possibly altered biotite. Termed ‘mica-orthophyre’ by Lowe, this dyke is clearly more basic than many other Mountsorrel lithologies, with a silica content of 51.23 per cent (Timins, 1867; cited in Lowe, 1926). A dyke on the western side of Brazil Wood, intruded into quartz-diorite, has an east-west trend and was classified as ‘augite-andesite’ by Lowe (1926, fig. 27). It was not sought during the present survey but was described by Lowe as a very fine-grained, almost glassy, dark grey lithology with small phenocrysts of plagioclase in a cryptocrystalline base packed with Fe-Ti oxide granules and very small plagioclase laths.

‘Basified’ granodiorite has been observed both peripherally and within the main body of granodoirite, although its distribution shown in (Figure 3) is tentative due to the discontinuous nature of the exposures (see also Le Bas, 1968, fig. 11). Near the northern edge of the Mountsorrel Complex, north of Buddon Wood Quarry, a small quarry on the western side of Rowhele Wood [SK 5633 1565] exposes inequigranular granodiorite basified with 15 to 20 per cent of mafic mineral clumps. In a thin section (E72433) the clumps mainly consist of hornblende, with pale brown to olive-green pleochroism, forming small crystals and hypidiomorphic aggregates; green-brown biotite is present in subordinate amount. Fe-Ti oxides are common, particularly in the hornblende clumps, and there is a further mafic phase that is now pseudomorphed but possibly represents original pyroxene. Along the western pluton margin a more extreme basified variant of granodiorite occurs 10 m south of the exposure of hornfelsed Stockingford Shale country rocks at the ‘old gravel pits’ [SK 5583 1550]. It contains 30 to 40 per cent of mafic minerals, mainly distributed as irregular clumps (E72435) of hornblende that has been completely pseudomorphed by chlorite. At the western base of the ‘Craig Buddon’ locality mentioned above, a small quarry exposes a pale grey, mafic-rich ‘basic granodiorite’ (Le Bas, 1968), suggesting proximity to the margin of the pluton.

The exposure on the knoll by the Kinchley shore of Swithland Reservoir [SK 5607 1404] was suggested by Lowe (1926) and Le Bas (1968) to be of diorite, but is more similar to coarse-grained, xenolithic, basified granodiorite. This exposure adjoins xenolith-rich granodiorite, all occurrences being within an area of strong magnetic contrast (Davies and Matthews, 1966, fig. 2), which may signify proximity to basic igneous bodies within the granodiorite. South of here, basified granodiorite crops out regularly along the Kinchley shore, where it apparently encloses screens, or forms an irregular intrusive margin with hornfelsed metasedimentary rock equated with the Stockingford Shale (see above). The possibility of an embayed contact between metasediment and the basified granodiorite margin in the Kinchley-Brazil Wood area, shown in (Figure 3), is supported by the intricate pattern of strong magnetic gradients revealed by the survey of Davies and Matthews (1966, fig. 2).

Within the main body of the Mountsorrel Complex, basified granodiorite is exposed at Cocklow Wood Quarry [SK 5683 1498], where the lithology consists of pink, coarsely inequigranular granodiorite with 30 to 40 per cent mafic minerals. The mafic minerals typically form large (up to 5 mm) and small laths, a habit indicative of amphibole, but also occur in clumps up to a centimentre across. At the large disused quarry at Nunckley Hill Spinney [SK 5695 1426], near to the southern exposed edge of the Mountsorrel Complex, basified granodiorite (also noted by Lowe, 1926) consists mainly of a coarsely inequigranular rock with common dioritic xenoliths and up to 30 per cent mafic clumps. This locally grades into a finer grained, grey, equigranular variety with abundant (30 per cent) small mafic minerals scattered throughout. In a thin section (E74300) the finer grained rock is remarkably unaltered, consisting of abundant small, zoned plagioclase euhedra in aggregates with quartz, all enclosed within large poikilitic plates of perthitic K-feldspar. Mafic minerals include: green hornblende, rarely with clinopyroxene cores; aggregates of hornblende, with green to brown pleochroism; very common small laths of pale brown to red-brown biotite and sporadic Fe-Ti oxides.

Xenolith-rich granodiorite is exposed at the small quarry by Kinchley Hill, which is the most southerly occurrence of the main granodiorite. Here, the granodiorite is a pink, coarse-grained, highly inequigranular lithology containing common pale grey feldspar xenocrysts and dark grey mafic clumps. The xenoliths are up to a metre across, and consist of grey, fine- to medium-grained diorite. Some are highly angular but others have rounded, somewhat cuspate, margins (Plate 6a). Many are invaded by apophyses, schlieren and pegmatitic segregations of the enclosing granodiorite. The xenoliths may have sharp margins (Plate 6b), or have gradational contacts with the host granodiorite. In a thin section across a xenolith-host contact (E73865) the host granodiorite is a fresh, coarse-grained lithology with common euhedra of brown biotite partly altered to chlorite. The proportion of biotite in the host granodiorite increases within several millimetres of the xenolith contact, which is a sharp but irregular boundary across which feldspar crystals in the granodiorite project into the xenolith (Plate 7a). The quartz-diorite xenolith is fine- to medium-grained with a poikilitic texture, also noted by Lowe (1926, fig. 16), in which aggregates of large, untwinned ?K-feldspar and quartz enclose abundant small laths and prisms of green hornblende, brown biotite, plagioclase and apatite. At the granodiorite contact, the lath-shaped minerals in the quartz-diorite xenolith locally show fluxional alignment around projecting crystals of the granodiorite host. In turn, the quartz-diorite xenolith is traversed by a 4 mm thick granodiorite veinlet that originates from the host rock. A spectacularly granophyric-textured quartzo-feldspathic vein, up to 0.3 m thick, cutting both xenoliths and host granodiorite, was illustrated in Lowe (1926, fig. 18). In nearby Kinchley Wood [SK 5607 1404] exposures are of a considerably less xenolithic, basic granodiorite lithology with about 35 per cent of mafic clumps.

During the present survey all of these occurrences were extensively covered with lichen, but Lowe (1926) has described them in some detail. Drawing analogies between Kinchley Hill and a locality at Sorel Point, Jersey, where acid and basic magmas interacted, he concluded that the granodiorite was emplaced into the diorite when the latter was still hot. This explains the lack of chilled contacts between the two phases and the apparent recrystallisation and acidification of the dioritic xenoliths. It also accords with the description of the xenolith contact given above, which suggests that crystals in the dioritic xenoliths were capable of local flowage-alignment along the host contact. Le Bas (1968) supported incorporation of the xenoliths when both they and the host rock were still hot. An acid-basic magma interaction at Kinchley Hill is also favoured by J Cobbing (written communication, 1999), but he found no evidence for prominent quartz ocelli or white-rimmed K-feldspar megacrysts of the type seen, for example, at Tregastel in Brittany.

Quartz-diorite is uniquely exposed on the hill at Brazil Wood, which is now an island in Swithland Reservoir. On the lower, southern slopes of the hill it is markedly heterogeneous, with veinlets and schlieren of pink, coarse-grained granodiorite in a grey, medium-grained host. At the summit of the hill the lithology is a more homogeneous, medium-grained, grey, quartz-diorite although with patchy areas of more coarsely crystalline, leucocratic rock showing pink feldspar megacrysts (Plate 8a). In a thin section (E73866) the summit rock is medium-grained, with a hypidiomorpic-granular texture consisting of laths and prisms of zoned plagioclase and 40 to 50 per cent of green-brown hornblende, some of the latter intergrown with, or fringing, relicts of pale green clinopyroxene. In places the plagioclase and quartz coarsens into poikilitic plates enclosing hornblende and smaller plagioclase laths (Plate 8b), a texture reminiscent of the diorite xenoliths at Kinchley Hill. Brown biotite is less abundant than hornblende and has been largely replaced by chlorite. The other principal constituents are Fe-Ti oxides and quartz, the latter forming interstitial pools amounting to about 10 to 15 per cent of the rock. The feldspar megacrysts are of plagioclase, either singly or in aggregates of 3 to 4 crystals. The magnetic survey of Davies and Matthews (1966, fig. 2) shows a slightly arcuate, ridge-like positive magnetic anomaly projecting to the east of Brazil Wood, indicating that the diorite may extend farther in that direction (Figure 3). The magnetic data also show that this anomaly is separated from the Kinchley Hill xenolithic granodiorite by a pronounced negative ‘col’, which as discussed above may represent an embayment of the metasedimentary country rock.

Quartz-microdiorite forms small exposures [SK 5597 1388] by the waterline on the Kinchley shore of the Swithland reservoir, close to locality ‘D’ featured in Lowe (1926) as an occurrence of hornfelsed metasediment. The microdiorite is a grey, fine-grained rock, similar in appearance to the xenoliths in the granodiorite described above from Kinchley Hill, but of a different petrography, in particular lacking the poikilitic textures of the diorite xenoliths and the diorite in Brazil Wood. In a thin section (E74303), the microdiorite consists of fluxionally orientated plagioclase laths, zoned outwards to untwinned feldspar, with abundant interstitial granular quartz aggregates (Plate 8c). The mafic minerals include common yellow-brown biotite, in places as large, poikilitic plates, together with disseminated Fe-Ti oxides and euhedral microphenocrysts of green hornblende that are extensively altered to chloritic minerals. Due to the isolated nature of the exposures, it is uncertain whether these microdiorites represent part of a large xenolith within the granodiorite, or are extensions of the Brazil Wood diorite outcrop. The latter alternative, suggesting a locally embayed margin to the Mountsorrel Complex, is shown in (Figure 3).

Gabbro forms the largely submerged and now inaccessible knoll in Swithland Reservoir to the north-west of Brazil Wood [SK 5566 1373]. Le Bas (1968), noting the accounts of Lowe (1926) and Taylor (1934), described the rock as a crushed and altered hornblende gabbro consisting of labradorite and ophitic brown hornblende, the latter commonly enclosing relict green clinopyroxene. Lowe (1926) described the hornblende as having pleochroism ranging from brown to yellow ‘uralite’, to a pale green ‘actinolitic’ fibrous variety. The other minerals included quartz, Fe-Ti oxides and secondary green hornblende. Corrected norms from chemical analyses published by Taylor (1934) suggested to Le Bas (1968) that the gabbro was originally olivine-normative, though its final assemblage reflects the introduction of acidic igneous material.

Magmatic evolution

The relationships seen in the Mountsorrel Complex suggest the up-rise of basic magma, represented by the gabbro, into the main mass of granodiorite before the latter had cooled. The quartz-diorite at Brazil Wood represents a large mass of hybridised magma formed by this process, whereas at Kinchley Hill hybridisation was more piecemeal, with the basic phase broken up and incorporated as xenoliths showing varying degrees of plastic deformation and acidification (Le Bas, 1968). The main-phase biotite granodiorite was subsequently veined by at least two phases of aplite sheets. These were interpreted by Taylor (1934) to be the relatives of a sodic ‘aplogranite magma’ which, after hybridising at depth with gabbro or diorite, had given rise to the main body of granodiorite. Alternatively, however, the aplites could simply be differentiates of the granodiorite injected back into the partially cooled pluton. The late magmatic environment featured the extensive retrogression of primary minerals, particularly in the hornfelsed metasedimentary rocks of the contact aureole, as described in Chapter 9.

Key localities

Main granodiorite: Playing field by Halstead Road [SK 5774 1443]; Halstead Road Field Wildlife Area [SK 5737 1417]; Buddon Wood Quarry [SK 564 152]; Castle Hill Quarry SSSI [SK 5759 1496].

Aplite sheets: ‘Craig Buddon’ summit, by Swithland Reservoir Spillway [SK 5575 1501]; Castle Hill [SK 5812 1492]. Basified granodiorite: ‘Old gravel pits’ [SK 5583 1550]. Xenolith-rich granodiorite: small quarry by Kinchley Hill [SK 5618 1399]. Quartz-diorite: Brazil Wood [SK 5580 1364].

South Leicestershire Diorites (SLeD)

Small, quarried outcrops of these rocks are located around Enderby, near to the south-western edge of the Leicester district (Figure 12). Le Bas (1972) suggested that all of these occurrences represent the exposed part of a large, if somewhat irregularly shaped, composite pluton (see Chapter 11). Representative chemical analyses are given in Worssam and Old (1988).

Quartz-diorite was formerly exposed in Coal Pit Lane Quarry at Enderby [SP 5414 9912], which is now partly restored and overgrown. It is a pink, medium-grained rock studded with small (about 2 mm), white plagioclase phenocrysts. In a thin section (E25490) the rock is composed of fine-grained, hypidiomorphic-granular aggregates of plagioclase, colourless clinopyroxene and quartz, with accessory amounts of Fe-Ti oxides. Clinopyroxene and plagioclase are both heavily altered, the latter in part to a brown epidote mineral. This rock was originally classified as porphyritic microtonalite by Le Bas (1972), but the thin section suggests that there is probably less than the 20 per cent of quartz required for tonalite, and that quartz-diorite is therefore a more accurate term.

Enderby Warren Quarry [SK 5413 0007], which is now a restored landfill site, formerly exposed rocks which, according to modal analyses carried out on four samples by Worssam and Old (1988), have a mineralogy appropriate to quartz-diorite. The lithology described from here by Worssam and Old (1988) is dark purplish grey and mottled with pink feldspars, with a coarse, even-grained texture. Late-stage hydrothermal alteration was visible along south-east dipping joints. Samples held at BGS show a predominantly pink rock with dark green-grey mafic clusters; textures vary between medium- and coarse-grained (Plate 9). Three thin sections show that the rock is pervasively altered; in one (E62188), highly sericitised plagioclase forms large hypidiomorphic crystals and these, together with colourless clinopyroxene euhedra, are enclosed by granular quartz aggregates. The hornblende forms small, completely altered laths. The palygorskite mineralisation at Enderby Warren Quarry is described in Chapter 13.

Porphyritic microtonalite belonging to the larger, Countesthorpe pluton (Figure 12); (Chapter 11) was proved at 193 m depth beneath Triassic strata in the Countesthorpe Borehole (Le Bas, 1972), 1 km south of the district.

Melton Mowbray Granodiorite

This intrusion is inferred to be of the same (Ordovician) age and tectonic setting as the Mountsorrel Complex and South Leicestershire Diorites. It is wholly concealed, but was intersected over a 12 m interval at the base of the Kirby Lane Borehole (Figure 12), about 350 m north of the Leicester district. Core samples show coarsely inequigranular, xenolithic granodiorite with prominent pink feldspar prisms up to 5 mm across (Carney et al., 2004). In its upper several metres the intrusion shows clay mineral alteration, probably due to weathering, and it is then capped by strata of the Millstone Grit Group. The aeromagnetic anomaly gradient map (Figure 16) shows that the Melton Mowbray intrusion is flat-topped, consistent with the interpretation that it underwent erosional truncation prior to deposition of the Carboniferous strata; its inferred subcrop is shown on (Figure 12).

Chapter 5 Carboniferous

During this time the district formed part of the northern edge of the Wales-London-Brabant Massif, which is delimited by the Sileby Fault (Carney et al., 2004). In this relatively uplifted structural domain, the Leicester district would have received a thin covering of strata compared with the Melton Mowbray district farther north in which Dinantian, Namuriuan and Westphalian sequences were thickly deposited within fault-bounded basins (Ebdon et al., 1990). Carboniferous rocks are absent from the deep borehole provings around Leicester (Chapter 3), but (Figure 12) shows that Dinantian strata, tentatively correlated with the Widmerpool Formation, are inferred to occupy a small subcrop on the northern, downthrown side of the Sileby Fault. Furthermore, seismic interpretations suggest that Namurian and Westphalian strata wedge in beneath Triassic cover and may extend up to 4 km south of the fault.

The nearest proving of Carboniferous strata is in the Kirby Lane Borehole (Figure 12), which shows a 110 m-thick sequence overlying the clay-weathered surface of the Melton Mowbray Granodiorite (Carney et al., 2004). The succession commences in 25 m of dark grey mudstone with seatearth layers, a thin coal seam and thin sandstone beds. On the borehole log, the Superbilinguis and Cancellatum marine bands were tentatively identified, indicating the Marsdenian Stage of the Namurian and demonstrating that these strata are attenuated representatives of the Millstone Grit Group. A 12.7 m-thick bed of sandstone surmounting this sequence may either be the Rough Rock (Yeadonian Stage), or a channellised part of the Crawshaw Sandstone of the Pennine Lower Coal Measures Formation. The last alternative implies that the Subcrenatum Marine Band has been omitted, indicating a disconformity between the Lower Coal Measures and Millstone Grit. The overlying Lower Coal Measures sequence, including the possible Crawshaw Sandstone interval, is 85 m thick beneath the erosional base of the Sherwood Sandstone Group. It consists of dark grey mudrocks, seatearths, siltstones and sandstones; there are sporadic thin coals and marine bands, one of which is identified with the Listeri (Alton) Marine Band.

Basalt dykes, thought to be of Carboniferous age, cut the Mountsorrel Complex in Buddon Wood Quarry [SK 5620 1501]. The main dyke, 2 to 3 m thick, consists of grey-green, fine- to medium-grained basalt. A smaller dyke, about 0.6 m wide, interpreted as an offshoot, is emplaced into the granodiorite about 100 m to the south. The main dyke (Figure 3) is emplaced along the southern margin of a fault zone (about 20m wide) in which the host granodiorite is extensively fractured and brecciated; the dyke itself was more mildly fractured subsequent to emplacement. Both the dyke and fault are parallel to one of the dominant master joint sets in the Mountsorrel granodiorite (Chapter 4), and to major Carboniferous structures such as the Sileby and Normanton Hills faults farther north (Carney et al., 2004). Fractures offset the dyke, one of which, in the western quarry face, has a shallow north-easterly dip and a reverse sense of throw (top to south-west). To the east of Mountsorrel Quarry, Lowe (1926) located ‘traces’ of the apparent continuation of this dyke in Cocklow Quarry where, however, it was ‘much decomposed’. He did not give its precise location, but extrapolation of the dyke’s trend in Buddon Wood Quarry would place it (Figure 3) along the northern face of Cocklow Quarry, around [SK 5702 1522], much of which is now obscured by part of the aggregate processing plant. The dyke may re-appear farther east at the ‘old Mountsorrel Quarry’, now known as Castle Hill Quarry, but for this to be the same body it must have been offset (Figure 3), as discussed in Chapter 10. The dyke at Castle Hill Quarry was said to be 6.7 m wide at its western end (Fox-Strangways, 1903) and a one metre thickness of microgabbro is still exposed at the SSSI in the west of the quarry [SK 5759 1496]. It chills towards its locally sheared margin. Both Fox-Strangways and Lowe (1926) show that the dyke has an east-south-easterly trend across Castle Hill Quarry. Eastwards from there, Fox-Strangways (1903) found it in a small quarry on the outskirts of Mountsorrel [SK 5819 1482], 250 m east of Castle Hill Quarry, where it was said to be only four inches (10 cm) wide.

A thin section (E71133) of the main basalt dyke in Buddon Wood Quarry shows a fine- to medium-grained, intergranular texture of heavily altered plagioclase laths and about 10 per cent Fe-Ti oxides, many with acicular habit suggestive of ilmenite. Between the plagioclase aggregates are interstitial areas of chlorite studded with euhedral apatite; quartz pools also occur interstitially, forming a few per cent of the rock. Original mafic anhedra are totally replaced by carbonate and Fe-Ti oxides; none have typical olivine outlines, however. The microgabbro dyke at the SSSI in Castle Hill Quarry (E74298) has a similar mineralogy but is medium-grained, with clinopyroxene visible, through secondary chloritic alteration, as relict laths and plates, some of which subophitically enclose plagioclase terminations.

Although a Carboniferous age is still the most likely for these dyke rocks, the absence of olivine, also noted by Lowe (1926), and presence of interstitial quartz, suggests an oversaturated, tholeiitic composition. This is not typical of the olivine-bearing Whitwick Dolerite, which was proved at depth in Coal Measures strata of the Coalville district farther west (Worssam and Old, 1988). Nor is it typical of the Lower Coal Measures lavas, breccias and intrusions encountered in the concealed Vale of Belvoir coalfield about 20 km to the north-east (Carney et al., 2004); they are mostly alkali olivine basalts, and although some quartz-normative types are represented in the analytical data of Kirton (1984), modal quartz has not been reported.

Key locality

Dolerite sheet: Castle Hill Quarry SSSI [SK 5759 1496].

Chapter 6 Triassic

Fox-Strangways (1903) assigned all of the outcropping Triassic strata in this district to the ‘Keuper’ division, although noting the presence of the underlying ‘Bunter’ strata in the region. Since that account was published lithostratigraphical nomenclature has been modified a number of times (Table 2), particularly by Elliott (1961) for Nottinghamshire, and nationally by Warrington et al. (1980). Charsley et al. (1990) subsequently revised Triassic nomenclature in the Nottingham district, and a new national scheme is now in place (Howard et al., in press a). In order to achieve local consistency within the East Midlands region, the subdivisions used for the Leicester Sheet follow Charsley et al. (1990); they are compared with the other schemes in (Table 2).

The three principal Triassic subdivisions are: the Sherwood Sandstone Group, which is wholly concealed by younger strata, and the outcropping Mercia Mudstone Group and Penarth Group. Triassic (Rhaetian) strata also form the lowermost beds of the Lias Group and are described in Chapter 7.

No strata of unequivocal Permian age are recorded in the district. ‘Breccia and sandstones’, 4.7 m thick, resting on Tremadoc basement was reported in the Lodge Farm Borehole (Fox-Strangways, 1903). This may be a representative of the Permian Basal Breccia seen in the Melton Mowbray district (Carney et al., 2004), but it may also represent a pocket deposit of Triassic age, preserved on an irregular basement landsurface. The principal factor effecting the erosion of Carboniferous strata from all but the northern part of the district, and preventing sedimentation during the Permian, was the sustained elevation of this area throughout the 50 million-year period that followed the culminating, end-Carboniferous phase of the Variscan Orogeny. By early Triassic times, however, regional subsidence resulted in progressive burial of this landscape by the Sherwood Sandstone and subsequently the Mercia Mudstone Group. Descriptions of the highly irregular unconformity that separates the Mountsorrel Complex from its mantle of Triassic strata are given in the section on the Mercia Mudstone Group.

Sherwood Sandstone Group

These strata have been proved in boreholes and probably occur at depth throughout the central and northern parts of the district. In the Evington and Knighton Fields areas of central and eastern Leicester, the four deep boreholes penetrating basement rocks (Chapter 3), indicate an approximate thickness range of between 12 and 32 m. Because of the paucity of lithological detail in the borehole records, however, it is not always clear which parts of the sequence are in the Sherwood Sandstone and which form the lowermost, sandstone-rich unit (Sneinton Formation) of the overlying Mercia Mudstone Group. The Sherwood Sandstone beneath Leicester seems lithologically similar to the Bromsgrove Sandstone Formation, which is well documented farther west in the adjacent Coalville district (Worssam and Old, 1988); however, the conglomeratic Nottingham Castle Sandstone Formation is the main Sherwood Sandstone unit farther north in the Melton Mowbray district (Carney et al., 2004). It was encountered in the Kirby Lane Borehole (Table 10) for further details." data-name="images/P946714.jpg">(Figure 4a), and so may occur beneath many northern parts of the Leicester district. The Sherwood Sandstone Group attenuates to the north-west, around the Mountsorrel and Charnwood Forest basement massifs, as well as southwards across the district, as shown in (Table 10) for further details." data-name="images/P946714.jpg">(Figure 4a). It is absent in the Thorpe By Water Borehole, sited about 6 km to the east of the district, and probably also in the Countesthorpe Borehole (Poole et al., 1968), about 1 km south of the district. Nor is it present in the Great Oxendon Borehole [SP 7343 8275], about 17 km to the south.

Bromsgrove Sandstone Formation (BmS)

This unit consists of alternating beds of mudstone, siltstone and sandstone. It was known to Fox-Strangways (1903) as Bunter ‘Basement Beds’ (Table 2). The formation is generally considered to be Anisian in age, but is not fossiliferous in the East Midlands region, apart from a few reptilian footprints (Sarjeant, 1974). It passes upwards, within a few metres of vertical section, into the more thinly interbedded mudstone-siltstone-sandstone sequence of the Sneinton Formation, basal to the Mercia Mudstone Group. Such is the gradational nature of this transition, however, that its precise position cannot be accurately determined from the poorly documented borehole logs of the Leicester district.

The alternations between mud- and sand-dominated lithologies that characterise this formation are interpreted as upward-fining alluvial cycles (e.g. Worssam and Old, 1988), an association suggestive of deposition within a meandering fluvial system. Heavy mineral studies of these sandstones in the Loughborough district indicate a local provenance, which is particularly recognisable in the stratigraphically lower beds of the formation due to the abundance of lithic grains rich in chlorite that were probably derived from greenschist-facies rocks of the Charnian Supergroup (Knox, 1996).

The most detailed lithological record of the formation is that of the Leicester Forest East Borehole, 1.5 km west of the district (Table 10) for further details." data-name="images/P946714.jpg">(Figure 4a), which proved 25.8 m of Bromsgrove Sandstone, between the ‘Waterstones Division’ (Sneinton Formation) and basement (Worssam and Old, 1988, fig. 18). The succession includes beds of red-brown, fine to medium-grained, micaceous sandstone between 0.1 and 5 m thick. They are interbedded with red-brown, micaceous, silty to sandy mudstone beds with a similar thickness range to the sandstones. Mudstone strata are thinly bedded and have layers, lenses and wisps of sandstone; gypsum nodules are common. To the south of the district, about 9 m of sandstone and ‘marl’ overlying basement rock in the Cottage Homes (Countesthorpe) Borehole (Poole et al., 1968) is probably the Sneinton Formation.

In the Leicester city area, the Crown Hills Borehole proved about 30 m of these strata, between 255 and 223 m depth above Tremadoc basement. The log (Fox-Strangways, 1903) suggests regular alternations of sandstone, up to 5.8 m thick, and ‘red and blue marl’, the latter up to 4.4 m thick. In the same publication, similar alternations of red sandstone and marl are more sparsely documented over a thickness of about 27 m (194–220 m depth) in the Lodge Farm Borehole. To the south, however, the Knighton Fields Borehole proved only about 12 m of fine- and coarse-grained sandstone above basement rocks.

The Bromsgrove Sandstone laps against the upstanding basement massifs of Charnwood Forest and Mountsorrel (Table 10) for further details." data-name="images/P946714.jpg">(Figure 4a); it is therefore not seen in any of the Mountsorrel quarry sections which, being at relatively high topographical elevations, only expose the overstepping Mercia Mudstone sequence. The succession occupying the lower lying ground between these two hilly areas, west of Swithland Reservoir, was investigated by Davies and Matthews (1966) using sesmic refraction profiling. This work indicated the presence of a palaeovalley about 50 m deep on the basement rocks. Although the Gunthorpe Formation (Mercia Mudstone) is mapped at surface hereabouts, the low seismic velocities suggest that the Triassic valley fill consists mainly of sandstone. If this is proved to be Bromsgrove Sandstone then the Gunthorpe Formation is very much reduced in thickness hereabouts, estimated at 30 to 40 m. The closest borehole, at Rushey Fields Farm [SK 5437 1428], was drilled to 37 m through the palaeovalley fill but unfortunately the log is vague. The log cites ‘marl’ but also ‘very hard rock’ in part, which is a common driller’s term for sandstone.

Mercia Mudstone Group

The Mercia Mudstone Group (Warrington et al., 1980) encompasses strata formerly referred to by Fox-Strangways (1903) as the ‘Keuper Marl’. It crops out in the western part of the district and elsewhere is ubiquitous at depth. Little detail is known from the lower part of the sequence and the reader is referred to accounts of the adjacent Coalville (Worssam and Old, 1988) and Melton Mowbray districts. In the latter, geophysical logs, calibrated against fully cored lithological sections, were the principal means by which the different formations in the concealed parts of the group could be correlated (Carney et al., 2004). Boreholes in the Leicester city area indicate that the group is between 175 and 210 m thick (Table 10) for further details." data-name="images/P946715.jpg">(Figure 4b), although lithological descriptions are poor. The record of the Countesthorpe Borehole (Poole et al., 1968), about 1 km south of the sheet, is incomplete, but suggests that the group is about 165 m thick and rests directly upon basement at 192 m depth. This thickness estimate assumes that the top of the Mercia Mudstone, which was not recorded in the borehole, lies about 20 m below the surface. The group thins progressively south-eastwards, across the shelving surface of the London Platform basement (London-Brabant Massif). It is probably less than 70 m thick in the south-eastern corner of the district (Table 10) for further details." data-name="images/P946715.jpg">(Figure 4b), with only 45 m being proved in the Thorpe by Water Borehole. Farther south the group continues to attenuate, with 19.5 and 3 m, respectively, proved in the Oxendon Hall and Orton boreholes of the Market Harborough district (Poole et al., 1968).

The nomenclature for the Mercia Mudstone Group, (Table 2), follows that adopted for the Nottingham district (Charsley et al., 1990), rather than the scheme of Elliott (1961) that has been used in the adjacent Coalville district. Not all of Elliott’s formations are readily mappable at surface and consequently some of them, including those modified by Warrington et al. (1980), were downgraded to member status and combined into the six formations named by Charsley et al. (1990). In ascending order, these units are the Sneinton, Radcliffe, Gunthorpe, Edwalton, Cropwell Bishop and Blue Anchor formations. In the near future, new names are likely to be introduced for certain members and formations (Table 2), owing to a nationwide rationalisation of Triassic nomenclature that is currently being undertaken (Howard et al., in press. a).

The Mercia Mudstone Group rests conformably upon the Sherwood Sandstone Group in the western part of the district and is overlain, disconformably by the Westbury Formation of the Penarth Group. Its age is expressed in terms of international stages based on successions outside of the UK. In this district it is dated as early Mid-Triassic (Anisian) at the base to latest Triassic (Rhaetian) at the top of the group (Warrington et al., 1980). No faunal determinations have been made for the Leicester district; however, in the Coalville district palynological data is presented for the Radcliffe, Gunthorpe and Edwalton formations (Worssam and Old, 1988 table 4). Farther north, in the Melton Mowbray district, determinations from the Asfordby Hydro Borehole gave poorly constrained ages for the Cotgrave Sandstone Member and Hollygate Sandstone Member, respectively forming the base and top of the Edwalton Formation (Warrington, 1997; Carney et al., 2004).

A profile through the Mercia Mudstone Group, showing variations in clay mineralogy and maturity, was produced for the Melton Mowbray district, based on determinations made in the Asfordby Hydro Borehole about 9 km north of the Leicester district (Kemp, 1999; Carney et al., 2004, fig. 18). The principal finding is that the middle Edwalton to lower Cropwell Bishop interval contains high concentrations of smectite ‘swelling’ clay minerals, the geotechnical implications of which are discussed in Chapter 13.

Conditions of deposition

Bosworth (1912) suggested that the mudrocks constituting the bulk of the Mercia Mudstone Group around Leicester and Charnwood (the Gunthorpe, Edwalton and Cropwell Bishop formations) represented aeolian accumulations. This was basically endorsed by Jefferson et al. (2002), although they questioned Bosworth’s use of the term ‘loess’ and suggested that a more appropriate analogue for the structureless mudstones found in parts of the group, particularly the Cropwell Bishop Formation, would be with the modern parna of south-eastern Australia. In that environment, aeolian sheets and dunes occur, consisting of clay aggregates or pellets formed by material winnowed from soils, alluvium or dried-up lake floors. Those authors cited recent work on the clay mineral aggregate structure of the Mercia Mudstone by Hobbs et al. (1998 - updated to Hobbs et al., 2002) to support this analogy. Arthurton (1980) had earlier suggested a complex of different environments within an arid, coastal flat setting comparable to the modern Ranns of Kutch. There is a consensus among recent workers that the laminated and ‘deformed’ mudstone beds are more definitely waterlain, probably in ephemeral, shallow-water bodies such as playa lakes, and the intercalated siltstone and sandstone beds were deposited during short-lived sheet flood episodes. Sandstones of the Cotgrave and Hollygate members reflect more prolonged fluvial events and for the latter member a marine connection is additionally indicated by the occurrence of shark remains. The presence of gypsum may indicate a high water table, charged with sulphate-rich water, or a greater degree of marine influence with precipitation due to evaporation in shallow water (Mader, 1992) particularly during deposition of the Cropwell Bishop Formation. Both marine and continental water sources were possibly involved in the formation of the Blue Anchor Formation (Taylor, 1983). This is the highest division of the Mercia Mudstone Group and is transitional to the Penarth Group.

The highly irregular nature of the unconformity between Triassic strata and basement rocks is demonstrated by the mapped boundaries and, in particular, by the frequent exposure of deep, Trias-filled palaeovalleys in the various quarries of Charnwood Forest and the Mountsorrel Complex (Bosworth, 1912). Many of Bosworth’s original observations on this unconformity are confirmed by the present exposures around the margins of the modern Buddon Wood Quarry [SK 5634 1506]. There, the Triassic strata commonly exhibit catenary dips, and rarely synsedimentary faults, which are features attributed to the differential compaction of sediment in the deeper parts of the palaeovalleys, relative to their margins (Plate 19a). The unconformable contact with underlying pale grey Mountsorrel granodiorite is seen to the right of the photograph (P609283)." data-name="images/P609283.jpg">(Plate 10a). Between the palaeovalleys, the Mountsorrel granodiorite was sculpted into smooth-surfaced, rocky tors. The rounded and markedly fluted surfaces of the latter (Plate 10b), also illustrated by Bosworth (1912, fig. 21), were evidently moulded by the abrasive action of sand-laden winds. Present topographical considerations suggest that the Mountsorrel Complex would have been completely buried beneath sediment by Upper Triassic (Edwalton/Cropwell Bishop formations) times. The occurrence of pebbles of possible Charnian derivation in the uppermost Triassic (Rhaetian) bone bed at Spinney Hills, 10 km to the south-east (Wignall et al., 1989; see also Penarth Group) nevertheless indicates that parts of Charnwood Forest remained emergent throughout the Triassic.

Sneinton Formation (Snt) (including the Radcliffe Formation)

The Sneinton Formation, previously part of the ‘Lower Keuper Sandstone’ of Fox-Strangways (1903) is known only from certain of the deep boreholes in the Leicester district. In the general region it consists of alternating beds of siltstone and sandstone, and is characterised by an abundance of mica which, when concentrated along bedding planes, gives rise to a watery appearance, hence the former alternative name of ‘Waterstones’ for this unit (Table 2).

In the adjacent Coalville district, 11.5 m of this formation were encountered in the Leicester Forest East Borehole. The description by Worssam and Old (1988) is of red-brown to green-grey variegated, interlaminated micaceous mudstones and siltstones alternating with beds of green-grey, fine- to medium-grained sandstone up to 1 m thick. The sequence is overlain by about 6 m of finely laminated, variegated mudstones and siltstones, which were equated with the Radcliffe Formation. In the Leicester district, boreholes through strata appropriate to this part of the sequence are invariably poorly documented, but general thickness considerations suggest that around 15 to 20 m of the Sneinton and Radcliffe formations may be present. In the Crown Hills Borehole. A sample in the BGS collection of red-brown, laminated, highly micaceous siltstone from 680 ft (207.2 m) depth suggests the presence of the Sneinton Formation at that level, although the log indicates only ‘red and blue marl and gypsum’. In the Lodge Farm Borehole, a description of ‘soft sandstones and water’ and ‘red sandstone and marls’ between 173 and 193 m depth is suggestive of the Sneinton and Radcliffe formations. More convincing descriptions of the formation in the Willow Brook Borehole note the interbedding of ‘sandstone with mica’, sandy marl with mica’ and ‘sandy marl with skerry’ between 169 and 190 m depth, just above a predominantly ‘sandy marl’ and sandstone sequence inferred to be the Bromsgrove Sandstone.

The Sneinton Formation represents deposition on a broad alluvial plain crossed by ephemeral streams and sheet floods, with bodies of standing water and accumulations of wind blown sediment. The Radcliffe Formation is of lacustrine origin (e.g. Elliott, 1961).

Gunthorpe formation (gun)

The Gunthorpe Formation, of red-brown mudstone and silty mudstone, represents part of the ‘Lower Gypseous Series’ of former Triassic terminology (Fox-Strangways, 1903). It encompasses the ‘Carlton’ and ‘Harlequin’ formations of Elliott (1961), who placed the boundary between those units immediately above a thin, dolomitic sandstone which he named the ‘Plains Skerry’. Mapping in districts farther north has, however, shown that several resistant dolomitic siltstone and sandstone beds occur in this part of the succession, and that there is no single mappable marker horizon between the two formations. Those units were therefore amalgamated (Table 2), as the Gunthorpe Formation (Charsley et al., 1990; Howard et al., in press a).

The formation may be 70 to 80 m thick in the Leicester district, but this estimate is based on the records of six deep boreholes, which are poorly documented. It passes gradationally downwards into the Radcliffe (or Sneinton) Formation. Its top is taken at the base of the Cotgrave Sandstone Member of the overlying Edwalton Formation, which otherwise is of similar lithology to the Gunthorpe Formation. A thickness of 66 m was recorded in the Leicester Forest East Borehole 1.5 km farther west in the Coalville district, and in the Kirby Lane Borehole 350 m north of the district. In the former proving, the Cotgrave Sandstone is shown as a grouping of several sandstone beds within about 15 m of vertical section (Worssam and Old, 1988, fig. 18). Restriction of the Cotgrave Sandstone sensu stricto to the thickest of these sandstone beds would increase the thickness of the Gunthorpe Formation to just over 70 m in the Leicester Forest East Borehole.

Only the stratigraphically youngest part of the Gunthorpe Formation crops out in the Leicester district. It occurs near the western margin, in the valley of the Rothley Brook [SK 543 070], where the outcrop of the Cotgrave Sandstone Member has been traced from mapping in the adjacent Coalville district, and, tentatively, west of Rothley Plain [SK 5630 1374] (see below). The latter correlation suggests that the Gunthorpe-Edwalton formational contact north of Rothley Plain might lap around the southern margin of the Mountsorrel granodiorite, continuing north-eastwards into the Soar valley before being cut out at the Sileby Fault in the adjacent Melton Mowbray district. The thickness of the Gunthorpe Formation therefore attenuates around the flanks of the Mountsorrel Complex, where it may additionally be underlain by a thick basal Triassic sandstone-dominated sequence (see Sherwood Sandstone Group).

The narrow outcrop in the Rothley Brook valley is largely unexposed; however, a 23 m thickness of Gunthorpe Formation was penetrated below the inferred Cotgrave Sandstone Member in a borehole there (SK50NW/99). The log described strata consisting of red-brown to grey-green mudstone and silty mudstone with sporadic beds of grey-green dolomitic siltstone up to 0.3 m thick; near-horizontal veins of fibrous gypsum occurred at some levels.

In the north-western part of the district, strata tentatively correlated with the Gunthorpe Formation are well exposed within palaeovalleys carved into Mountsorrel granodiorite on the northern face of Buddon Wood Quarry [SK 5636 1544]. Such sequences invariably show catenary dip (Plate 19a). The unconformable contact with underlying pale grey Mountsorrel granodiorite is seen to the right of the photograph (P609283)." data-name="images/P609283.jpg">(Plate 10a), indicative of syn- to post-depositional compaction within the palaeovalleys. They consist mainly of red-brown, structureless or faintly laminated mudstone and silty mudstone, with scattered, rounded, sand- or granule-size quartz and feldspar crystal grains of probable aeolian derivation. Prominent tabular beds of grey-green, dolomitic siltstone, generally less than 0.5 m thick, are spaced at regular intervals of 1.5 to 2 m vertically within the succession. The stratigraphically lowest dolomitic siltstone beds include layers and lenses of coarse-grained to granule-size, poorly sorted sandstone, either structureless or cross-bedded, with abundant subangular or angular fragments of granodiorite and pink or white feldspar crystals. A measured 9.7 m-thick succession above the axis of one particularly deep palaeovalley [SK 5622 1555] includes 11 beds of poorly micaceous, dolomitic siltstone or sandstone up to 0.65 m thick. The dolomitic siltstone beds commonly have structureless basal parts but become laminated and more mud-rich towards their tops; many show crude normal grading, upwards from a basal lag of poorly sorted, coarse-grained mud-matrix supported sandstone. Sandstone beds are less common in that sequence; they are up to 0.35 m thick and of multi-storey type, consisting of 10 to 20 mm-thick beds with structureless, coarse-grained, commonly channellised bases fining up to laminated siltstone tops. The intervening red-brown or grey-green mudstones are either structureless or, particularly in their upper parts, laminated and micaceous with small-wavelength (10 mm), straight-crested ripples. The latter were possibly formed in shallow water or ephemeral lakes under the influence of a prevailing north-westerly wind direction. The basal sandstone, 0.4 m thick, is particularly coarse-grained and packed with feldspar grains in an abundant silty to muddy matrix.

Little information on the stratigraphically lower parts of the Gunthorpe Formation is forthcoming from the deep boreholes of the district, owing to their poor documentation. The log of the Willow Brook Borehole indicates a succession of ‘red marl’, ‘sandy marl’, ‘skerry’ and gypsum (Fox-Strangways, 1903). In that proving, strata that include ‘grey skerry’ and ‘sandy marl and skerry’, between 24 and 33 m from the top of the formation, are at an equivalent stratigraphical position to the ‘Plains Skerry’ identified by Worssam and Old (1988) in the Leicester Forest East Borehole, west of the district.

Edwalton Formation (Edw)

The Edwalton Formation, defined by Elliott (1961), consists mainly of red-brown mudstone and silty mudstone. It includes a basal unit, the Cotgrave Sandstone Member, and its top is defined at the upper surface of the Hollygate Sandstone Member (Table 2). These are feature-forming units, and hence are usually mappable, although the Cotgrave Sandstone is less easy to distinguish in the field, nor can it be readily identified from the poorly documented borehole records of the Leicester district.

The formation with its two sandstone members is between 40 and 50 m thick. Its outcrop is confined to the western part of the district, and is in part controlled by north-east- and north-west-trending fault systems. The paucity of natural exposures is due mainly to an extensive covering of Quaternary superficial deposits.

Mudrocks of the Edwalton Formation, between the sandstone members, seldom form natural exposures. They were formerly extracted from small pits around Mountsorrel [SK 5940 1396]; [SK 5854 1404] and at Sileby, but these are now either restored or much overgrown. The pits on the southern outskirts of Sileby [SK 6062 1498] revealed sections in red-brown, silty mudstone with numerous thin, tabular beds of grey-green dolomitic siltstone or sandstone (Plate 11a). South of Mountsorrel there are sporadic exposures of a dolomitic siltstone bed, which is laterally variable, and a ditch section [SK 5893 1352] shows 0.2 m of yellow-brown, coarse-grained sandstone. The 22 m of Edwalton Formation strata above the Cotgrave Sandstone in the Fox’s Glacier Mints Borehole consists of brown mudstone with common sandy layers and laminae, and with sporadic thicker beds of sandstone one of which is about 9 m above the Cotgrave Sandstone. A similar sequence is described for the formation in the Leicester Forest East Borehole; the log shows red-brown, less commonly grey-green, silty to very sandy mudstone, which is structureless, crudely bedded or laminated. Geotechnical boreholes at Farnham Bridge, east of Rothley, indicate red-brown or grey-green, silty mudstones with layers of gypsum appearing below 10 m depth, e.g. Borehole (SK51SE/165).

Cotgrave Sandstone Member (Cot)

The Cotgrove Sandstone Member is a useful marker horizon, although rather diffuse and hence confusing in places. In the adjacent Coalville district the equivalent ‘Cotgrave Skerries’ unit was identified in the Leicester Forest East Borehole, as a sequence, about 15 m thick, of mudstone, containing nine beds of green-grey, fine- to medium-grained sandstone generally less than 1 m thick. The equivalent sandstone grouping was mapped across the eastern part of the Coalville district and its most prominent bed, averaging 2 m in thickness, was named the ‘Thornton Skerry’ by Worssam and Old (1988). The outcrop of this bed passes into the Leicester district along the south side of the Rothley Brook valley at Glenfield [SK 5395 0610], and for convenience it is here equated with the Cotgrave Sandstone Member. Locally, it belongs to a significant grouping of sandstone and siltstone beds, which are proved in boreholes about 1 km north-eastwards along the floor of the Rothley Brook valley; for example, the 3.15 m thickness of green-grey siltstone and sandstone intersected below glacial deposits (SK50NW/100), and a proving of sandstone, at least 1.3 m thick, below those deposits in Borehole number (SK50NW/97) close by. Farther south, a sandstone bed about 3 m thick at 36 m depth in the Fox’s Glacier Mints Borehole is equated here with the Cotgrave Sandstone.

The outcrop of the Cotgrave Sandstone is tentatively identified farther north, as a prominent feature [SK 5630 1374] strewn with brash of brown, fine-grained sandstone north-west of Rothley Plain, and as a similar feature on red, sandy mudstone south of that village [SK 5665 1325]. Between these localities, a ‘skerry’ was formerly seen in a brick pit [SK 5690 1364] west of Rothley Plain.

In the east of the district, the presence of the Cotgrave Sandstone is suggested by a few of the better-documented boreholes; for example, in the Lodge Farm Borehole about 2.6 m of ‘marl, grey rock and sandy marl’ at 87 m depth may represent the member.

Hollygate Sandstone Member (Hly)

The Hollygate Sandstone Member consists of thick sandstones interbedded with gypsiferous mudstones and siltstones. It is 10 to 15 m thick, on average, and thus forms a marker bed for the top of the Edwalton Formation that is both recognisable in the field and identifiable on most borehole logs. It equates with the ‘Upper Keuper Sandstone’ (Fox-Strangways, 1903); (Table 2), and the more locally named ‘Dane Hills Sandstone Group’, for example, as coined by Horwood (1913). Whereas Fox-Strangways (1903) thought it to be somewhat discontinuous and lenticular, the present survey indicates that it is persistent, although displaced by faulting. The formal name Hollygate Skerry Member was applied by Warrington et al. (1980), and subsequently modified to ‘Hollygate Sandstone Member’ by Charsley et al. (1990). The member is of similar (Carnian-Norian) age to and thus correlated with the Arden Sandstone Formation of the West Midlands (Warrington et al., 1980); (Table 2).

The member consists of several thick beds of pale greenish grey to brownish grey sandstone, with minor interbedded red-brown and grey-green mudstone. In other parts of the East Midlands it typically forms a cuesta with a long dip slope that is undulatory in places; however, in the Leicester district the dip slope is generally capped by Quaternary superficial deposits.

A disused quarry in Western Park [SK 558 043] exposes up to 1 m of buff to greenish grey, fine- to medium-grained, cross-bedded sandstone. In the nearby railway cutting [SK 5565 0420] to [SK 5538 0415] there are excellent, near-continuous exposures through part of the formation, the best occurring immediately beneath the overbridge [SK 5565 0420], where around 3.2 m of buff to greenish grey, fine- to medium-grained, moderately well sorted sandstone is exposed (Plate 12a). It is cross-bedded in sets that are between 0.3 to 0.4 m and locally up to 0.75 m thick. These sets show a unidirectional easterly current direction. They have truncated top surfaces, with undulating contacts and dewatering structures locally developed at that interface. Easterly current directions also characterise the contemporaneous Arden Sandstone at its type area in Warwickshire (Old et al., 1991), perhaps suggesting that the arenaceous beds of the latter and Hollygate Sandstone are the deposits of a single river system that formerly traversed the English Midlands.

Petrographical examinations of Western Park samples (G K Lott written communication, 2002) have shown that the Hollygate Sandstone consists of fine- to medium size, monocrystalline and polycrystalline, ragged quartz grains, with subordinate grains of feldspar and sparse grains of carbonate, mudstone and spheroidal dolomite. The feldspar and mudstone grains commonly show evidence of corrosion and partial leaching. All of these grains form an open, porous framework (Plate 12b); however, brown, ferruginous clay coats some grains and forms meniscus-shaped bridges between them. The ragged framework grains all have a thin coating of carbonate cement. Irregular patches of fine, zoned, rhomb-shaped dolomite sporadically fill the interstices, and discrete dolomite rhombs occur throughout the framework. Some samples also show cemented laminae comprising fine, rhomb-shaped, dolomite crystals. Cementation is generally weak, with quartz and K-feldspar grains all showing narrow, discontinuous euhedral overgrowth development; ferroan carbonate patches occur rarely.

Fossils from mudstone beds lying both above and within the sandstones of the member at the former Ashleigh House Quarry [SK 5683 0466] included Estheria minuta together with plant remains and crustacean or annelid tracks; the sandstones yielded fin-spines of the shark Acrodus keuperinus and teeth of Acrodus (Horwood, 1913).

In the north of the district, the Hollygate Sandstone is up-faulted along a displacement inferred to coincide with the Wreake valley. Several exposures of grey-green, fine- to medium-grained sandstone occur along the length of a drainage ditch [SK 6302 1438] south of Ratcliffe on the Wreake, it was noted by Fox-Strangways (1903) on the Fosse Way ‘above Lewin Bridge’, around [SK 6241 1360], and during the present survey it was augered in fields just to the west of the latter locality.

The Hollygate Sandstone is an important local aquifer (Chapter 13) and has consequently been proved in many wells and borings, particularly in the Soar valley area of central and north-western Leicester. From these provings it is suggested that the northwards disappearance of the Hollygate Sandstone outcrop is due mainly to faults that throw down to the north-east, thus progressively lowering the top-surface of the member by a total of about 20 m and allowing preservation of the mudstones of the overlying Cropwell Bishop Formation. The borehole descriptions are unfortunately poor and in many it is probable that the sandstone component, which is weathered and friable at these shallow depths (generally 10 to 20 m below surface), was ‘running’ and consequently lost during drilling. In the Charnwood Street Borehole, around [SK 600 048 but the precise location is not known] the Hollygate Sandstone interval commences at about 39 m depth and is about 12 m thick; the log shows a predominance of ‘sandstone’ in beds between 1 and 6.8 m thick (Fox-Strangways, 1903, p.69). Gypsum and ‘marl’ are also described from this datum in many other boreholes. A borehole (SP59NE/15) at Whetstone, 30 m south of the district, indicates that the Hollygate Sandstone occurs at only 15 m depth, below about 9 m of Cropwell Bishop Formation mudstone. The log of this borehole records at least 10 m of ‘grey sandstone and shale’, which was equated with the Arden Sandstone of the West Midlands.

Key localities

Western Park: Disused quarry [SK 558 043]; Railway cutting [SK 5565 0420] to [SK 5538 0415].

Cropwell Bishop Formation (CBp)

This formation is similar in lithology to the Edwalton Formation, but is characterised by a much higher content of gypsum, which can form beds of appreciable thickness. It ranges between about 40 and 50 m thick although in the Kirby Lane Borehole, just to the north of the district, geophysical logs suggest it is only about 34 m. The formation encompasses the ‘Upper Gypseous Series’ of Fox-Strangways (1903), and the strata that Elliott (1961) had subdivided into the ‘Trent’ and ‘Parva’ formations. The latter, in its upper part, includes the distinctive ‘Tea-green Marls’ unit (Table 2), for which Warrington et al. (1980) introduced the term Blue Anchor Formation, renaming the lower part the ‘Glen Parva Formation’. The Glen Parva and Trent units were amalgamated by Charsley et al. (1990) into the Cropwell Bishop Formation because their mutual boundary was defined on characters recognised only in cored boreholes (Elliott, 1961), and could not be delineated between the sporadic surface sections that are normally available

The formation crops out along the lower slopes of the Penarth Group escarpment, occupying much of the low-lying ground occupied by the Soar and Wreake valleys, and central Leicester city area. It is now virtually unexposed, save for the occasional temporary excavations, but was formerly seen in several brickpits. Mostly these mention red mudstone with sporadic thin ‘skerry’ beds of dolomitic siltstone, and beds of gypsum. In the former brickpit at Gipsy Lane [SK 6172 0694] a thick, composite gypsum bed several metres below the junction with the Blue Anchor Formation showed an irregular top-surface indicative of partial dissolution (Plate 11b).

The section currently exposed at this conservation site is as follows:

At another part of this quarry the uppermost beds of the formation, immediately below the Blue Anchor Formation, consist mainly of 0.1 to 0.8 m thick alternations between red-brown and greenish grey structureless mudstone.

Contrasting styles of mineralisation were seen in this pit, and are reviewed in Chapter 13. Gypsum dissolution features in many of the former brick pits are mentioned in the accounts of Horwood (1913). For example, in the former Humberstone pit [SK 6120 0594] he noted that gypsum formed partly decomposed masses containing cavities filled with the enclosing red mudstone. Similarly, within the former Thurmaston pit [SK 6188 0933] a gypsum bed was noted to occur as isolated masses. There, a further bed, of ‘anastomosing fibrous’ gypsum, is the apparent lateral equivalent of the thick bed of gypsum in the nearby railway cutting [SK 620 095].

The gypsum bed in the above railway cutting has a thickness of between 1.8 and 3 m according to Fox-Strangways (1903). It may equate with the gypsum bed 3.2 m thick recorded at 78 m depth in the Crown Hills Borehole, stratigraphically positioned about 14 m below the top of the Cropwell Bishop Formation. At roughly the same datum a ‘15 ft’ (4.6 m) thick gypsum bed was reported from a former brick pit [SK 639 113] near Syston (verbal communication by a local resident to JNC in 2003). Gypsum in ‘considerable quantities and large masses’, evidently a particularly thick bed, was encountered (Browne, 1893, p.196) in the footings for the Knighton railway tunnel [SK 5898 0263] to [SK 5901 0261]. All of these occurrences suggest the presence of a single, continuous, thick gypsum bed that may correlate with the Tutbury Gypsum of the adjacent Melton Mowbray district (Carney et al., 2004).

The 37 m of Triassic beds that occur below the Blue Anchor Formation in the Thorpe by Water borehole, to the east of the district, mostly consist of structureless red-brown mudstone with common reduction spots and with gypsum veins and nodules. Locally, however, the beds are sandy, with some crude lamination sporadically present and traces of mica occuring throughout. There are also green, commonly dolomitic mudstones, fine- to coarse-grained sandstones (particularly below 263 m depth), and the lowermost 5 m has a well-developed siltstone lamination. Lithological similarities to the Sneinton Formation are suggested by these features; however, the sequence is more likely to represent a marginal facies of the Mercia Mudstone equivalent to its upper part elsewhere, the lower part having been cut out by the progressive onlap of the strata across the London Platform. The base of the Blue Anchor Formation in this borehole is probably an unconformity, as observed elsewhere in the Midlands (Old et al., 1987; Barclay et al., 1991).

Blue Anchor Formation (Ban)

The Blue Anchor Formation (Warrington et al., 1980) was formerly known as the ‘Tea-green Marl’ of the Rhaetic Beds (Fox-Strangways, 1903), but was placed by Elliott (1961) within the upper part of the ‘Parva Formation’ of the Mercia Mudstone Group (Table 2). It is generally considered to be Norian to Rhaetian in age (Warrington et al., 1980), although at a location in the Nottingham district, about 25 km north of the Leicester district, microfossils recovered are predominantly Rhaetian (Howard et al., in press b). The formation averages about 5 to 6 m thick in the central and southern parts of the district, possibly increasing to around 8 m in the north. In the Kirby Lane Borehole, just north of the district, it is about 5 m thick and in the Thorpe by Water Borehole to the east, it is 6.99 m thick. As noted by Kent (1968), however, it is seldom exposed except in quarries, now mostly backfilled, exploiting the higher, gypsiferous part of the underlying Cropwell Bishop Formation. At outcrop, the formation generally gives rise to dark grey, clayey soils tinged green or pale grey where mudstone fragments have been ploughed up. The outcrop is narrow in the west of the district, where it forms much of a low escarpment capped by the Penarth and Lias groups; in many areas, that feature is more subdued due to a mantle of Quaternary deposits.

Throughout much of central England the formation rests unconformably on red-brown mudstone of the underlying Cropwell Bishop Formation. This relationship was recognised in the Worcester (Barclay et al., 1997) and Warwick areas (Old et al., 1987) and has been traced more widely using geophysical logs into the Nottingham area (Howard et al., in press b). In the Leicester district this junction was commonly seen to be abrupt, as at Sherrif’s brickpit, Gipsy Lane. In the former Moore’s brick pit at Spinney Hills, around [SK 6019 0454] the Blue Anchor Formation was seen resting on a surface (of the Cropwell Bishop Formation) that was ‘hollowed out in long curves’ (Horwood, 1913, p.209). Augering on the outcrop to locate this boundary suggests that locally, red-brown mudstone of the Cropwell Bishop Formation apparently interdigitates with grey-green ‘Blue Anchor’-type mudstone, although this may have been produced by periglacial disturbance of the boundary. The upper boundary of the formation, with the Westbury Formation of the Penarth Group, is invariably sharp (Fox-Strangways, 1903), may be burrowed as seen in the Thorpe by Water Borehole, and commonly is defined by the base of a bone bed (e.g. Wignall et al., 1989). This important datum marks the onset of a widespread latest Triassic (Rhaetian) marine transgression across the region (Warrington and Ivimey-Cook, 1992).

In the Leicester district, only a few metres of the Blue Anchor Formation is currently exposed, at Gipsy Lane Quarry, above the junction with the underlying Cropwell Bishop Formation (Plate 13). The Gipsy Lane section, and also the many descriptions of the formation in former quarries, indicates a sequence of grey to green, dolomitic siltstone and mudstone with an irregular blocky fracture. The former brick pits around Spinney Hills [SK 6019 0454] were in green mudstone with ‘blue nodules in the lower part’ (Horwood, 1913, p.209). A blocky structure is suggested by the conchoidal fractures mentioned by Horwood, who also noted the presence of halite pseudomorphs, ripple marks and fossils that include abundant fish scales of the Semionotid type, teeth, and Estheria minuta. In the south of the district, the entire formation, 6.1 m thick, was formerly exposed at the South Wigston (Glen Parva) brickpit [SP 5847 9851]. There, it was said to contain well-defined layers of greyish yellow sandstone with shark’s teeth (Fox-Strangways and Browne, 1901). In the Thorpe by Water Borehole, the formation consists mostly of pale green, structureless mudstones and siltstones with paler, hard dolomitic beds. Some local internal bedding was noted, formed by sandy laminae, and mudcracks were recorded in places. There are also two prominent beds of medium-grained sandstone and these, together with those seen at South Wigston, suggest the development of a marginal marine facies.

Key locality

RIGS at Gipsy Lane [SK 615 068].

Penarth Group

The mudstones and siltstones of the Penarth Group form a narrow but continuous outcrop in the west of the district. The group occupies the middle and upper slopes of an embayed escarpment capped by more resistant strata of the Lias Group, although in many places Quaternary deposits extensively mantle that feature. Descriptions of the group are based mainly on several former quarry sections, the larger ones showing (Wignall et al., 1989) that its thickness increases southwards, from about 8 m at Gipsy Lane [SK 6172 0694] to 12 m at the South Wigston (Glen Parva or Wigston Junction) brickpit [SP 5847 9851]. South of the district the group is 3.3 m and 5.5 m thick, respectively, in the Orton and Oxendon Hall boreholes. The Penarth Group is only 2 to 4 m thick farther north, around Melton Mowbray (Carney et al., 2004), but it is 11.16 m thick in the Thorpe by Water Borehole 5.6 km to the east of the district.

The Penarth group comprises the Westbury Formation and the overlying Lilstock Formation. In the latter only the Cotham Member has been mapped, even though the overlying Langport Member (Warrington et al., 1980) is probably present in many parts (Swift, 1995a); it was proved in the Thorpe by Water Borehole, where it is 1.3 m thick.

Warrington et al. (1980) introduced the term ‘Penarth Group’ as a substitute for ‘Rhaetic Beds’ (Table 2), which was the name used for these strata in the original memoir for this district (Fox-Strangways, 1903). The boundaries of the group do not, however, coincide with those of the Triassic Rhaetian Stage of time, which encompasses strata between the middle of the underlying Blue Anchor Formation (Mercia Mudstone Group; information from G Warrington, BGS, 1996) and the lowest part of the overlying Wilmcote Member, basal to the Lias Group.

Penarth Group strata have yielded conodonts in the Melton Mowbray and adjacent districts, which were first discovered by Swift (1989; see also, Swift and Martill, 1999). The succeeding ‘Pre-planorbis Beds’ of the Wilmcote Limestone Member (Lias Group, see below) additionally contain the conodont Misikella posthernsteini, confirming an uppermost Triassic, Rhaetian age for those beds and hence for the Penarth Group.

Wignall et al. (1989) showed that in the Leicester area the Penarth Group is bounded by unconformities. Its lower junction with the Blue Anchor Formation is in places irregular (see previous section), and the basal component of the Westbury Formation is commonly a bone bed. The upper junction is an erosion surface in the north, as attested by the former exposure at Gipsy Lane (Wignall et al., 1989), which also shows that the planorbis Subzone is apparently missing from the basal part of the overlying Blue Lias Formation.

In the Leicester district these strata were always very poorly exposed, but there are many detailed accounts of it in the various brickpits and other temporary sections cut into the Rhaetic escarpment (Wignall et al., 1989; Browne, 1893; Fox-Strangways, 1903; Horwood, 1913; and references therein; Swift and Martill, 1999).

Conditions of deposition

The sedimentology of the Penarth Group (Swift and Martill, 1999) indicates a marked environmental change from the continental-style deposition that characterised most of the Triassic Period. Marine microplankton occur throughout the unit in the general region (e.g. Fisher, 1972; Orbell, 1973; Morbey, 1975), suggesting deposition in shallow seas on the gently subsiding East Midlands Shelf immediately after a widespread Rhaetian (Late Triassic) transgression (Warrington and Ivimey-Cook, 1992). Wignall et al. (1989) suggested that the faunas of the Westbury Formation were indicative of fluctuating conditions of salinity and temperature, which inhibited burrowing organisms thus preserving sedimentary lamination. They noted that the formation coarsened upwards, a trend also seen in southern England and which has been attributed to a slowing down of relative sea-level rise, and eventual relative sea-level fall (Hesselbo et al., 2004). The overlying Lilstock Formation (Cotham Member) has lesser proportions of organic matter, is paler coloured, and contains micritic and calcareous beds, some with hardgrounds (Swift and Martill, 1999). The occurrence of the bivalve crustacean Euestheria suggests deposition in fluctuating, but generally brackish salinity (Wignall et al., 1989). Evidence of shallower water, and the development of an emergent surface has been reported within the Cotham Member in southern Britain (Hesselbo et al., 2004). In the Leicester district, the occurrence of wave-rippled sandstones may suggest deposition in a shallow, nearshore and intermittently stormy sea environment (Wignall et al., 1989). Later a relative rise in sea-level is reflected in the Cotham Member, and culminated with the deposition of restricted lagoonal (Swift, 1995a) or carbonate ramp beds of the Langport Member, at the top of the Lilstock Formation. Drowning of this ramp occurred at the onset of the Lias Group deposition (Hesselbo et al., 2004), and is suggested by the relationships seen at South Wigston (Wignall et al., 1989) and possibly at Spinney Hills. Farther north, however, uplift and erosion probably accounts for the absence of the upper part of the Cotham Member and whole of the Langport Member at Gipsy Lane (Wignall et al,. 1989). It is possible that this erosional surface cuts further down-section since the Langport Member and entire Cotham Member are absent from the Penarth Group in boreholes around Asfordby, in the adjacent Melton Mowbray district, about 16 km north-east of Gipsy Lane (Carney et al., 2004).

Westbury Formation (Wby)

Very little of this lower formation of the Penarth Group is exposed, but former exposures have shown that it consists of dark grey, laminated mudstone. The strata correspond to the ‘Lower Rhaetic’, ‘Black Shales’ or ‘Avicula contorta shales’ of the earlier survey (Fox-Strangways, 1903). At outcrop the formation generally forms dark grey, clay-rich soils and in auger holes the weathered strata are rather nondescript, yellow or silvery grey, sticky clays.

The thickest development of the Westbury Formation (3.7 m) was formerly seen above the Blue Anchor Formation at the South Wigston (Glen Parva) brickpit [SP 5847 9851]. The base of the unit features only a poorly developed bone bed, seen as irregular patches of bone-rich sand in erosional hollows on the Blue Anchor Formation (Richardson, 1909; Wignall et al., 1989). Wignall et al. note that the sequence coarsens upwards, from grey-black laminated mudstone into sandy and silty mudstone; the fossil fauna includes the species detailed below at Spinney Hills, plus Chlamys yalonensis. Horwood (1913, p.213) mentions sandy mudstones and sporadic gritty sandstone laminae appearing at 1 m above the base. The comprehensive fossil listing of this section provided by Richardson (1909) emphasises the faunal diversity of the Westbury Formation compared with overlying strata of the Cotham Member. The junction between the two units (according to the thicknesses given by Wignall et al.) occurs within Richardson’s 13 foot thick (3.96 m) ‘bed 10’.

Farther north Horwood (1913), and other authors reviewed in Wignall et al. (1989), describe a similar sequence with a minimum thickness of 3 m, consisting of dark grey, locally pyritic mudstones coarsening up to sandy mudstones and very thin sandstones in the former Moore’s brickpit at Spinney Hills [SK 602 042]. Wignall et al. noted that the basal bone bed here and in nearby workings was particularly well developed, although always less than 0.1 m thick. From it, Horwood (1913) recovered Gyrolepis, Saurichthys apicalis, Hybodus minor, Ceratodus, Nemacanthus monilifer, Ichthyosaurus, Plesiosaurus and Sargodon tomicus. This bone bed contained sporadic pebbles, which Wignall et al. suggested were derived from Charnwood Forest, 10 km to the north-west. Those authors also noted the occurrence of a bed of complete ophiuroids (brittle-stars) approximately 1.3 m above this bone bed, with Aplocoma damesii illustrated in Swift and Martill (1999). The bivalve species listed here included Rhaetavicula contorta (Porlock), Protocardia rhaetica (Merian) and Eotrapezium ewaldi (Bornemann). The last species crowds a one-inch (2.5 cm) thick sandstone that was named the ‘Axinus bed’ by Harrison (1876). At Spinney Hills, Wignall et al. (1989) take the boundary with the overlying Cotham Member to coincide with a colour change from grey-black to green-blue mudstone; the interface corresponds with the disappearance of Euestheria and most of the other Westbury fauna with the exception of E. ewaldi.

The northernmost exposure of the Westbury Formation at the time of survey was the Gipsy Lane brick pit, where only the lowermost 1 to 2 m was visible; however, a complete 3 m-thick section was available for study to Wignall et al. (1989). They noted a sequence of grey-black laminated mudstones and sandy mudstones, the latter with the sand occurring as millimetre-thick, burrow-mottled laminae. The fauna is dominated by Eotrapezium ewaldi and Rhaetavicula contorta, which are abundant on some bedding planes; Modiolus and ‘Gervillia’ are less common species, and a rare find of Aplocoma suggested the presence of the ophiuroid bed also seen at Spinney Hills and South Wigston. Here, the transition to the overlying Cotham Member is marked by the loss even of the impoverished E. ewaldi fauna.

The Westbury Formation is poorly documented in the borehole provings of the district, with the notable exception of the Billesdon Brook Borehole, near Frisby, which records ‘Avicula contorta?’ at 218 m depth. The fossil was evidently recovered near to the base of a ‘dark shaly’ succession 13 m in thickness, which arguably represents the whole of the Penarth Group. In outlying areas, the formation is 2 to 4.4 m thick in the Asfordby area of the Melton Mowbray district and may be as thin as 1.2 m in the Kirby Lane Borehole. In that proving the formation is a dark grey to black, fissile, fossiliferous mudstone with sandy and pyritic laminae and a few thin beds of sandstone (Ambrose, 1999). In the Thorpe by Water Borehole only the basal 0.78 m of the Penarth Group has been assigned to the Westbury Formation; it comprises medium grey, fissile, locally sandy mudstone with fish debris and bivalves.

Key locality

RIGS at Gipsy Lane [SK 615 068].

Lilstock Formation

In central and southern England, the Lilstock Formation includes the Cotham Member and the overlying Langport Member (Warrington et al., 1980).

Cotham Member (Ctm)

This member consists of grey-green, somewhat sandy and locally pyritous laminated mudstones, with thin, discontinuous beds of nodular limestone; it was informally referred to as the ‘Upper Rhaetic’ (e.g. Fox-Strangways, 1903). The outcrop typically forms the upper part of a scarp slope that is capped by more erosionally resistant limestone beds forming the basal unit of the Blue Lias Formation. The Cotham Member is no longer exposed, but augering of the weathered outcrop generally produces silty, yellow-grey or green-brown mottled clay with common small, irregular, white to buff calcareous nodules (‘race’). Fragments of nodular, porcellanous limestone with a conchoidal fracture form a field brash on parts of the outcrop, but some of this may include material from the overlying Langport Member.

As noted by Wignall et al. (1989), the southwards thickening of the Penarth Group is controlled mainly by Cotham Member, which thickens from about 5 m at Gipsy Lane brickpit [SK 615 068] to about 8.3 m at South Wigston brickpit [SP 5847 9851]. The base of the Cotham Member was formerly seen in several quarries and is a sharp junction with the underlying Westbury Formation, (see above). The upper boundary with the Langport Member is generally sharp, and in the north of the district there is evidence for an erosional break with some Cotham Member strata having been removed prior to deposition of the Lias Group. Although the Cotham Member is absent from much of the south-eastern corner of the adjacent Melton Mowbray district (Carney et al., 2004), it re-appears southwards across the Leicester district, and 10.36 m are proved in the Thorpe by Water Borehole, just to the south-east of the district. In that borehole it comprises alternating pale green and chocolate brown, variably laminated fissile or blocky mudstone with local burrowing, slumping and cross-lamination. These beds become grey in colour down-section.

In the South Wigston brickpit [SP 5847 9851], Horwood (1913, p.213) recorded a sequence dominated by grey-blue or blue laminated mudstone. He recognised the upper 3.7 m of this sequence as the ‘Upper Rhaetic’ and referred to the underlying beds as ‘Lower Rhaetic’. In a later re-appraisal, Wignall et al. (1989) evidently included much of what Horwood referred to as ‘Lower Rhaetic’ within their Cotham Member, which was estimated as 8 m in thickness. These later authors showed the Cotham Member to consist mainly of sandy and silty mudstone with subordinate beds of ‘pyritic blue shales’. Horwood (1913) described packages of pyritic, blue or grey-blue ‘shales’ with common sandy partings, lamination and sporadic nodular limestone beds with septaria. Richardson (1909) provided further descriptions of the section.

Disc-shaped septarian nodules up to 0.6 m across were described (Wignall et al., 1989) from road cuttings in the Cotham Member at Knighton [SK 599 006]; internal cavities contained crystals of celestine partially replaced by baryte. Only very sporadic fossils of E. ewaldi were recovered.

At Gipsy Lane brickpit, Wignall et al. (1989) noted that the Cotham Member was devoid of fossils. Septaria were present, as were thin beds of wave-rippled sandstone, which increased in abundance up the section. Although the top of the member was not exposed, Wignall et al. recovered a block of porcellanous limestone, believed to represent the upper bed of the Cotham Member. This block featured an irregular erosion surface blanketed by shelly Lias Group limestone containing the subzonal ammonite Caloceras johnstoni, in addition to bone fragments.

Langport Member

In this district the Langport Member has not been mapped as it is represented by only a single bed of nodular, yellow-weathering but blue-hearted porcellanous limestone. This bed occurs at the very top of the Cotham Member at the South Wigston and Spinney Hills brick pits described above, and has also been described from a number of other temporary sections in the Leicester district, as reviewed by Swift (1995a). In the northern part of the district, and south-eastern corner of the adjacent Melton Mowbray district, this member is probably missing due to erosional truncation at the base of the overlying Lias Group (see above). Like the Cotham Member, however, it probably re-appears in the south-eastern part of the Leicester district since the Thorpe by Water Borehole shows it to be 1.3 m thick. There it consists of pale grey, fine-grained limestone with thin mudstone beds; burrow structures, shell debris and a few pebbles occur near the base. The Langport Member contains a sparse marine fauna of mussels and oysters, and there is also a low-diversity conodont fauna (Swift, 1995b).

The upper boundary of the Langport Member is a major flooding surface in the south of the district. Thus at the South Wigston brickpit, the member is abruptly succeeded by argillaceous limestone yielding Psiloceras planorbis, which thus belongs to the Jurassic beds of the overlying Blue Lias Formation (Wilson and Quilter, 1884; Richardson, 1909). In other parts of central England this junction commonly shows abundant animal burrows, in keeping with a hardground surface.

Chapter 7 Latest Triassic and Jurassic

These strata comprise the Lias Group and lower part of the Middle Jurassic Inferior Oolite Group. The sequence dips at about 1° towards the south-east, but is locally affected by faulting and flexuring. There are few exposures and therefore few detailed descriptions, other than for sections formerly recorded in quarries and temporary exposures that are now mostly overgrown. Moreover, Quaternary superficial deposits mantle about 60 per cent of the outcrop.

Lias Group

The Lias Group (Powell, 1984) has an aggregate thickness of about 300 m and occupies a large proportion of the Jurassic outcrop and subcrop in the Leicester district. It comprises in upward sequence: the Blue Lias, Charmouth Mudstone, Dyrham, Marlstone Rock and Whitby Mudstone formations. This nomenclature follows the recommendations of Cox et al. (1999), pertaining to Jurassic strata deposited across the East Midlands Shelf and southern England. It partially replaces the former scheme of Brandon et al. (1990), which was used farther north, including the equivalent strata in the adjacent Melton Mowbray district.

Biostratigraphical control in the early Jurassic of England and Wales is based on ammonite biozones defined with considerable precision (Dean et al., 1961). The stages in the Lower Jurassic of the East Midlands have been well documented for the age-equivalent strata in the nearby Grantham district (Brandon et al., 1990; Berridge et al., 1999), and Melton Mowbray district (Carney et al., 2004). Only a few new fossil finds were made during the present survey. Most of the sections described by earlier workers are no longer exposed but good accounts are given by Fox-Strangways (1903), Hallam (1955) and Howarth (1980), among others. For the adjacent Melton Mowbray district, Carney et al. (2004) included illustrations of some of the more common fossils, which may for example be encountered as brash in fields or in ditch diggings. The Thorpe by Water Borehole, sited just outside the south-eastern corner of the district, has provided a fully cored sequence through the Lias Group, downwards from the basal beds of the Whitby Mudstone Formation. Geophysical logs have enabled correlations to be made with these beds in the south of the Melton Mowbray district (Figure 5), and they give a good estimate of the likely thickness of the various ammonite biozones that constitute the Lias Group in the Leicester district.

In this account, fossil names have been updated to the modern terminology (written communication, M Woods, BGS, 2004) where it has been possible to do so unambiguously; however, where no data could be found to decide the status of the fossil, the old name has been retained.

The lowermost 2 to 3 m of the Lias Group is devoid of ammonites and would therefore, by definition, be of latest Triassic age if the base of the Jurassic is to be drawn at the lowest occurrence of ammonites of the genus Psiloceras (see discussion in Hesselbo et al., 2004). A Rhaetian (latest Triassic) age for these strata is indeed confirmed by their conodont assemblage, which will be discussed below. For descriptive purposes, however, these pre-planorbis strata are included with the remainder of the Lias Group, although they may be largely missing in the southern part of the district.

Conditions of deposition

The Lias Group accumulated on the East Midlands Shelf in generally warm, shallow, subtropical seas that covered the London-Brabant massif following the latest Triassic (Rhaetian) marine transgression. The north-sloping margin of the London-Brabant massif controlled the onlapping nature of Liassic sedimentation (Donovan et al., 1979). Thus in the south-eastern corner of the district, the Thorpe by Water Borehole shows that the lowermost two members, Wilmcote Limestone and Saltford Shale, are absent; these are equivalent to the Barnstone and Barnby members, farther north. By contrast, for the Rugby Limestone Member, Charmouth Mudstone and Dyrham formations, the London-Brabant massif had little influence on sedimentation apart from a slight southward thinning of the formations (Figure 5).

At first, poorly oxygenated bottom waters resulted in deposition of a dysaerobic facies of faunally restricted, laminated, organic-rich sediments (Wignall and Hallam, 1991), recognised at the base of the Blue Lias Formation. With rising sea level, and a consequent transition to hemipelagic shelf environments (Weedon, 1986) well-oxygenated conditions became established leading to the accumulation of carbonates and argillaceous sediments with a diverse marine fauna in the Blue Lias and Charmouth Mudstone formations. Cyclic deposition, involving beds of laminated mudstone passing up into calcareous mudstone and thence to bioturbated limestone, has long been recognised in strata equivalent to the Blue Lias Formation, and such variations involve bottom water oxygen conditions varying from anoxic, with a limited benthonic fauna, to better oxygenated, coarser-grained substrates with a more diverse fauna, the underlying causes being complex (Waterhouse, 1999). The Dyrham Formation and in particular the ferruginous Marlstone Rock Formation, with its locally prominent cross-bedding, represents a shallow water, high-energy, regressive episode on the East Midlands Shelf. The subsequent reversion to deeper water, more quiescent conditions, in which was deposited the overlying Whitby Mudstone Formation, represents one of the most important transgressive events in the Jurassic according to Hallam (2001). A new cycle then commences at the Toarcian-Aalenian boundary, when a shallowing led to the deposition of the Northampton Sand Formation (Inferior Oolite Group).

Blue Lias Formation (BLi)

The Blue Lias Formation, as defined by Cox et al. (1999), encompasses strata between the top of the Penarth Group and base of the Charmouth Mudstone Formation. It broadly equates with the lower half of the ‘Lower Lias Clays and Limestones’, or ‘Limestone Series’ of the previous survey and is the lateral equivalent of part of the Scunthorpe Mudstone Formation (Figure 5); (Table 2), which was the name given to these strata by Brandon et al. (1990) and used farther north in the Melton Mowbray and Grantham districts (Berridge et al., 1999; Carney et al., 2004). The changeover in nomenclature as it affects the Scunthorpe Mudstone and Blue Lias formations currently takes place at the Leicester-Melton Mowbray sheet boundary, and is discussed later (see Charmouth Mudstone Formation).

The Blue Lias Formation has an estimated thickness of 30 to 115 m, and has three subdivisions, which are, in ascending order, the Wilmcote Limestone, Saltford Shale and Rugby Limestone members. These units can be traced southwards across Britain as far as the Bristol area (Ambrose, 2001); however, the Saltford Shale and Rugby Limestone members are difficult to recognise within the Leicester district and consequently they have been included as undifferentiated ‘Blue Lias Formation’ on the map.

Although the base of the Blue Lias Formation is sharply defined at the junction with the underlying Penarth Group, its upper boundary with the Charmouth Mudstone Formation is not easily traced in the field. Farther north, in the Melton Mowbray district, the Vale of Belvoir outcrop of the Scunthorpe Mudstone Formation, which includes equivalents of the Blue Lias, contains several mappable limestone beds. Its upper boundary in the Melton Mowbray district, marking the change to the relatively limestone-free sequence of the Charmouth Mudstone, occurs at a stratigraphical level (top of the Foston Member); (Figure 5) that is well above the top of the Blue Lias Formation in the Thorpe by Water Borehole. In this borehole, the highest two members of the Scunthorpe Mudstone, the Beckingham and Foston members, which have no equivalent in the Blue Lias Formation as defined by Ambrose (2001), can be recognised in the overlying Charmouth Formation (Figure 5); (see below). In the Leicester district, limestone marker beds are seldom traceable in the field, being mostly mantled by Quaternary deposits, but there are sporadic identifications of zonal ammonites, reviewed below. Such occurrences, in conjunction with biostratigraphy and thickness comparisons with the adjacent Melton Mowbray district, suggest that in the south the upper boundary probably occurs within the bucklandi Zone, although farther east in the Thorpe by Water Borehole, just to the east of the district, the Rugby Limestone Member, uppermost Blue Lias Formation, extends up into the base of the semicostatum Zone, a diachronicity which is also apparent on a national scale (Ambrose, 2001). Using lithology and the fossil occurrences as a guide, the junction with the Charmouth Mudstone has been traced across the Leicester district mainly on the basis of topography and assuming a uniformly gentle easterly to south-easterly regional dip. Undetected stratal thickness variations, faulting and/or flexuring, and fluctuations in rockhead (base of Quaternary) elevations are all factors that introduce inaccuracies to the placing of this boundary on the map, which should therefore be regarded only as a rough guide.

The Blue Lias Formation is at least 50 m thick in the Crown Hills area of Leicester, and could be as much as 115 m in total farther north, where the top lies at higher stratigraphical levels (Figure 5). This compares with around 60 m in the Rugby area of the Warwick district (Old et al., 2001).

Wilmcote Limestone Member

This member consists of laminated mudstones with numerous thin interbeds of argillaceous limestone. Previous names for this part of the sequence (Table 2) include ‘Strensham Series’ (Quilter, 1886), and ‘Limestone Series’ (Fox-Strangways, 1903), but they later came to be known as the ‘Hydraulic Limestones’ in this region (Lamplugh et al., 1909). The latter term persisted late into the 20th century (e.g. Hallam, 1968) and reflects the exploitation of the limestone beds for hydraulic cement manufacture (Chapter 13). The current name of ‘Wilmcote Limestone Member’ was first adopted by Old et al. (1991) and formalised by Ambrose (2001); it is the terminology recommended by Cox et al. (1999). The member cannot easily be differentiated in the field from the rest of the Blue Lias Formation because it is widely covered by Quaternary deposits or, as in the Leicester city area, by urbanisation. The only known complete sequence is in the Crown Hills Borehole, which proved a thickness of about 8 m (Figure 5). This compares with 11 to 12 m for the equivalent beds that constitute the Barnstone Member (Scunthorpe Mudstone Formation) at Barrow on Soar, 2 km to the north of the district (Carney et al., 2004).

In the Melton Mowbray district, macrofaunas from these beds include a rich assemblage of vertebrates, and are reviewed in Carney et al. (2004). Throughout the East Midlands, the member and its equivalents have yielded faunas indicating an age between the latest Triassic (Rhaetian) to early Jurassic (Hettangian-Lower Sinemurian). The lowermost strata are devoid of ammonites and the informal name ‘Pre-Planorbis Beds’ was assigned to them (Trueman, 1915; Kent, 1937). At that stratigraphical level, conodont faunas from north of the Leicester district include Misikella posthernsteini (Swift, 1989, 1995b), confirming a latest Triassic, Rhaetian age for the ‘Pre-Planorbis Beds’. This is in keeping with current practice, reviewed above, in which the base of the Hettangian — and hence of the Jurassic — in Britain occurs within the Lias Group and is taken at the lowest occurrence of ammonites of the genus Psiloceras.

The absence of the lower part of the planorbis Zone, and hence of the Rhaetian ‘Pre-Planorbis Beds’, in parts of the Leicester city area is suggested by the few metres of basal Wilmcote Member strata that were seen above the Langport Member of the Penarth Group at the former Spinney Hills brick pit [SK 602 042]. This section, described by Wignall et al. (1989), commences with just over a metre of paper shale with indeterminate ammonites, overlain by a 2 m-thick ‘hard nodular limestone’ with a diverse fauna that includes the subzonal ammonite Caloceras johnstoni. A trench close to this location recorded a very similar sequence of basal Wilmcote Limestone Member strata above the Penarth Group (Bates and Hodges, 1886). At Gipsy Lane pit [SK 621 071], a further apparent omission of early planorbis Zone strata was deduced by Wignall et al. from the occurrence of a block showing an erosion surface on the Cotham Member (Penarth Group) blanketed by shelly Lias Group limestone containing Caloceras johnstoni.

Excavations for road works at Knighton [SK 6025 0075] exposed about 4 m of the lower Wilmcote Limestone Member, which consisted of 0.1 m thick beds of poorly fossiliferous, laminated limestones alternating with thinner beds of echinoderm-rich mudstones (Wignall et al., 1989). The presence of the entire planorbis Zone was suggested by the occurrence of the ammonites Psiloceras planorbis and Caloceras johnstoni, although the planorbis Subzone was less than 4 m thick. In the vicinity, Browne (1893, p.183) had remarked that excavations in the Knighton Church Road area [SK 606 014] had recovered fissile limestones identical to those quarried for lime (from the Barnstone Member) at Barrow on Soar.

Farther to the south, the lowermost Wilmcote Limestone Member was revealed in the South Wigston brickpit [SP 5847 9851]. This section was still open when visited by Wignall et al. (1989), and showed 2.75 m of mainly ‘paper shales’, with a bed of laminated limestone at the base. The fossils recovered included: Modiolus minimus, Plagiostoma sp., Liostrea hisingeri, Gryphaea arcuata, echinoderms and the ammonite Psiloceras planorbis. The last occurred within the basal limestone (see also Richardson, 1909), suggesting the presence of the planorbis Subzone, but absence of the Pre-Planorbis beds.

The lowermost beds of the Wilmcote Limestone, in particular the ‘Pre-Planorbis Beds’, are absent at the Spinney Hills, Gipsy Lane and South Wigston localities, but not in the Knighton area. Such variations suggest that the southwards onlap of the Lias Group onto the London Platform, with consequent omission of the lower Lias Group beds (Donovan et al., 1979), may have occurred either across an uneven surface or was complicated by localised synsedimentary tectonic activity.

Saltford Shale Member

The Saltford Shale Member forms the middle part of the Blue Lias Formation, but may attenuate and thin out in the east of the district (Figure 5). It is identified by the predominance of mudstone and relative paucity of limestone interbeds. The strata are generally fossiliferous, but there are very few finds of zonal ammonites in the Leicester district. The name ‘Angulata Clays’ was applied to this part of the sequence by Swinnerton and Kent (1949, 1976), Hallam (1968) and Cope et al. (1980). The unit is part-equivalent to the Barnby Member of north Leicestershire, Nottinghamshire and Lincolnshire (Brandon, 1990), but unlike that unit, which is of liasicus zonal age, is considered to range from the planorbis to the liasicus zones. Only a few fossils were collected from these strata during the present survey, but comprehensive faunal listings of previous workers’ findings are provided by Fox-Strangways (1903).

The Saltford Shale Member can be clearly identified in the Crown Hills Borehole (Figure 5) as a 17 m sequence of ‘blue clay’, with one thin limestone bed. Its base occurs at 40.9 m and ?Psiloceras was identified at 35 m (BGS collection) indicating the planorbis Zone and giving a zonal thickness of at least 14 m. This is consistent with the Warwick area where the lower part of the Saltford Shale is of planorbis Zone age (Old et al., 1987; Ambrose, 2001). It should be noted, however, that locally in the Leicester city area the johnstoni Subzone (of the planorbis Zone) has been proved in the Wilmcote Limestone Member of the underlying Penarth Group, implying that the Psiloceras planorbis Subzone is absent. Away from Crown Hills, the member can only be been traced with difficulty through the Leicester urban area and in the outlying rural areas, where most of the outcrop is covered by glacial deposits. Consequently it has been placed together with the Rugby Limestone Member within the undifferentiated Blue Lias Formation outcrop on the Leicester Sheet 156.

Rugby Limestone Member

The Rugby Limestone Member, about 30 m thick, forms the uppermost subdivision of the Blue Lias Formation, and is probably the only one that is present in the south-east of the district (Figure 5). This part of the sequence was not named either formally or informally by past workers, although some applied the name ‘Hydraulic Limestones’ (e.g. Fox Strangways, 1903), due to the local exploitation of these strata for lime making. The unit is the lateral age-equivalent of the upper part of the Scunthorpe Mudstone Formation (Granby and possibly the Beckingham and Foston members) of north Leicestershire, Nottinghamshire and Lincolnshire (Brandon, 1990); (Figure 5). The geophysical log profiles of the two sequences are quite different, however, and indicate a considerably higher concentration of limestone interbeds in the Rugby Limestone as compared to the Scunthorpe Mudstone sequences.

The outcrop of the member cannot easily be traced in the Leicester district, owing to urbanisation and a thick Quaternary cover. Therefore, together with the underlying Saltford Shale Member, it is merged into an otherwise undifferentiated ‘Blue Lias Formation’ outcrop on the map. The Rugby Limestone consists of a sequence of interbedded, fossiliferous argillaceous limestones (‘cementstones’) and mudstones. The mudstones are generally blocky, but in detail the member can show a cyclic development of fissile anaerobic mudstone (‘paper shale’) passing up into blocky mudstones that become more calcareous upwards, the cycle terminating with a bed of argillaceous limestone. Bioturbation and burrows are commonly found, and some limestone beds are nodular. The equivalent stratigraphical unit has been described in detail on the Dorset and Glamorgan coasts (Hallam, 1960). The limestones are thought to have formed through a combination of primary and secondary processes (e.g. Hallam, 1964).

Details of the age range for the member in the Leicester district are very limited. The few ammonite findings prove the angulata and bucklandi zones; however, in common with the neighbouring Warwick district, the likely age range is high liasicus (laqueous Subzone) to mid bucklandi (rotiforme Subzone) zones (Old et al., 1987). Ambrose (2001) demonstrated a diachronous top and base to the member across the Midlands. In the Thorpe by Water Borehole, just to the east of the district, the member ranges from the liasicus Zone up to the lower semicostatum Zone in age. The Crown Hills Borehole proved the lowermost 23.9 m of the member, yielding ?Schlotheimia (BGS collection) at 7.3 m and an indeterminate schlotheimid at 18.7 m, probably indicative of the angulata Zone.

In the Evington area, the Rugby Limestone was formerly exposed in two lime pits. In Crown Hills No. 2 quarry [SK 6150 0393], Fox-Strangways (1903, p.74) recorded around 4 m of interbedded limestones and ‘shales’. Each bed was generally less than 0.15 m thick, with the exception of the uppermost ‘shale’ bed, which exceeds 1.08 m thickness. The second quarry, Crown Hills No. 1 [SK 6204 0374], formerly exposed 6 m of interbedded limestones and ‘shales’, each generally less than 0.15 m thick with the exception of a single 1.4 m-thick limestone bed (Fox-Strangways, 1903, p.74). Fox-Strangways (p.25) reported ‘Agassiceras scipionianum’?, among other fauna, from this quarry. This suggests an improbably young, semicostatum Zone age for this exposure, which is close to the inferred base of the member and thus more likely to be of angulata Zone age. In his original description of this quarry, Quilter (1883) noted ‘yellowish beds of limestone and clay in the upper part, and blue beds and clay in the lower part’. He found a varied fauna of bivalves, crinoids, brachiopods and echinoids together with the ammonites A. charmassei and A. scipionianus (duly reported by Fox-Strangways, see above). Despite the latter ammonite identification, which may as discussed be erroneous, he concluded that the sequence was appropriate to the ‘upper part of the lower Bucklandi bed, near to the junction of the middle and lower Bucklandi beds’.

The younger part of the sequence was formerly exposed in the quarries east of Kilby Bridge [SP 6138 9692], where 21 limestone beds were recorded (Fox-Strangways, 1903). When this quarry was studied by Kent (1937) only the top seven limestone beds were still visible, but they yielded fossils that included: Gryphaea arcuata incurva, Vermiceras and Schlothiemia suggesting a position within the angulata and bucklandi zones, of Hettangian-Lower Sinemurian age. Less than a metre of calcareous mudstone from this sequence remains exposed in a flooded quarry [SP 6125 9714] north of the railway line.

During the present survey, limestones typical of the member were noted in fields at Thurnby [SP 64 04] on the east side of Leicester. Fox-Strangways (1903) recorded exposures of ‘limestones and shales’, clearly equating with the Rugby Limestone, at Ashby Folville, north-east of Twyford, at South Croxton, south-west of Beeby and at Keyham. From the stream bed at Ashby Folville [SK 7058 1196] specimens of ?Angulaticeras sp. and Coroniceras sp. were recovered during the present survey, suggestive of the conybeari Subzone of the bucklandi Zone (M Howarth, National History Museum, written communication to M Woods, BGS, 2004). A further small exposure of calcareous mudstone found during the present survey in a ditch half way up the valley side west of South Croxton [SK 6814 1029], yielded a large specimen of Coroniceras, also suggestive of the conybeari Subzone (M Howarth, written communication to M Woods, BGS, 2004). Farther south, Fox-Strangways encountered strata of the bucklandi Zone, finding Arietites bucklandi, together with Gryphaea arcuata and many other fossils, in the stream bed by the viaduct near Keyham [SK 6816 0550]. Other good sections were reported by Fox-Strangways (1903, p.24) along the then newly completed railway cutting west of the Ingarsby tunnel to Thurnby [SK 6679 0482] to [SK 6462 0450]. These yielded abundant fauna from the greater part of the bucklandi Zone, including the upwards transition to the semicostatum Zone, Farther west, where the railway crosses the Uppingham Road [SK 6283 0484], Fox-Strangways noted brown and grey mudstones with limestone beds containing Lima gigantea, Gryphaea arcuata, Modiola minima and Ammonites catenatus, suggesting that the transition into the underlying angulata Zone occurred at that point. Fossil faunas indicative of the higher beds of the Blue Lias Formation were identified in a few former exposures, documented by Fox-Strangways (1903).

Charmouth Mudstone Formation (ChM)

This formation has an estimated thickness range of 100 to 180 m; the Thorpe by Water Borehole, sited just beyond the south-eastern corner of the district, records a total thickness of 159.9 m. Farther north in the adjacent Melton Mowbray district, about 155 m of the equivalent beds are recorded of which, however, the lower 45 m was placed within the Scunthorpe Mudstone Formation, (Figure 5) and see below. The formation corresponds broadly with the upper part of the ‘Lower Lias Clays and Sands’ of the previous survey, and to the poorly defined ‘Obtusum-Oxynotum Clays’, ‘Sandrock’ and ‘Upper Clays’ of Swinnerton and Kent (1949, 1976). Its current name follows the scheme for Jurassic lithostratigraphy proposed by Cox et al. (1999). The formation thus encompasses all of the former ‘Brant Mudstone Formation’ and the upper part of the Scunthorpe Mudstone Formation (Beckingham and Foston members) of Brandon et al. (1990), as indicated in (Figure 5).

Correlations between these strata, and the changes of nomenclature within the Lias Group between this and the adjoining Melton Mowbray district, are summarised in (Figure 5). The sequence prevailing over most of the Leicester district is not well known in detail, but is probably similar to that described from the nearby Thorpe by Water Borehole, about 6 km beyond the south-eastern sheet margin. In that borehole, geophysical logs of the Charmouth Mudstone show highly distinctive profiles, between 142 and 170 m depth, corresponding to the Foston Member that forms the upper part of the Scunthorpe Mudstone Formation in the Melton Mowbray and Grantham districts (Brandon et al., 1990). The Thorpe by Water sequence, however, comprises mudstones with a carbonate component dominated by beds of hard calcareous mudstone and thin argillaceous limestones, whereas the Foston Member of Brandon et al. (1990) typically contains beds of bioclastic, commonly ferruginous limestone. In such respects, the beds of the Thorpe by Water Borehole are more similar to those of the Charmouth Mudstone as defined across southern Britain by Ambrose (2001) and in consequence, the Beckingham and Foston age-equivalents are placed within it. In reality, a gradual lateral transition southwards across the Leicester district probably occurs between the two sequences, with some elements of the typical ‘Scunthorpe’ stratigraphy persisting, as perhaps seen in the Lowesby brickyard locality described below. Due to the lack of exposure, it has not proved possible to map this transition and so, for convenience, the changeover in nomenclature between the Scunthorpe Mudstone and Blue Lias formations currently takes place at the Leicester-Melton Mowbray sheet boundary. The Sileby Fault, being a major geological discontinuity close to that sheet boundary, would be a better, if still somewhat arbitrary, location for this change to occur.

The Thorpe by Water Borehole also proves the presence of the ‘70’ and ‘85’ Marker Members of Horton and Poole (1978) on the geophysical logs. The ‘70’ Marker Member is equivalent to the Loveden Gryphaea Bed of the Melton Mowbray district (Figure 5), which also occurs in the extreme north of the Leicester district; in the borehole it is represented by siltstone and very silty, calcareous mudstone. The ‘85’ Marker Member is equivalent to the Jericho Gryphaea Bed of the Melton Mowbray district, but in the borehole it comprises mudstones and calcareous mudstones with thin limestones. No horizons particularly rich in Gryphaea were found at this stratigraphical level in the Leicester outcrops, despite the fact that one good exposure formerly existed (see below). In similar fashion, the Brandon Sandstone, which is a prominent bed in the lower part of the Charmouth Mudstone in the Melton Mowbray district, was not encountered in the Leicester district and has no obvious lithological representative in the Thorpe by Water Borehole, although this shows a small geophysical feature at 117 m depth where the sandstone would be expected to occur (Figure 5).

The Charmouth Mudstone Formation contains fossils including bivalves, ammonites, belemnites, crinoids, gastropods, brachiopods, foraminifera and ostracods. The ammonite zonations, based on findings in the Thorpe by Water Borehole, are summarised in (Figure 5). In that proving the formation commences in the semicostatum Zone, but in the Leicester district it probably dates back to the Lower Sinemurian, high bucklandi Zone, ranging upwards to an undefined biostratigraphical level approximately around the davoei-margaritatus zonal boundary (Lower-Upper Pliensbachian boundary). At that level the beds grade upwards into the Dyrham Formation, which is generally more silty and micaceous. In the field this junction commonly approximates to a concave break in slope, with the Dyrham Formation occupying the higher ground. There are a few minor stream sections in the district but most of the exposures reported in Fox-Strangways (1903) consisted of temporary sections, ditch diggings and cuttings that are now overgrown.

In a former brickpit north of Ingarsby [SK 6887 0617] ‘ironstone’ fragments were formerly dug from beds thought to be equivalent to the semicostatum Zone (Fox-Strangways, 1903). This lithology may compare with the age-equivalent Foston Member (the former ‘Ferruginous Limestone Series’ or ‘Plungar Ironstone’) of the Melton Mowbray district (Carney et al., 2004), although Hallam (1968) noted that ‘no definite trace’ of such ferruginous oolitic ironstones at this stratigraphical level occur south of the Vale of Belvoir, in the Melton Mowbray district. In a further brickyard [SK 7166 0808] north-west of Lowesby, the 6 m section described in Fox-Strangways (1903, p.27) contained, near the top, at least 1.8 m of ‘shales with ferruginous nodules’. This lithology, combined with an identification of Oxynoticeras conyarti, and other ammonites, may suggest a stratigraphical level comparable with the Sand Beck Nodule Bed, which in the Grantham and Melton Mowbray districts occurs close to the base of the Charmouth Mudstone, in the middle to lower part of the oxynotum Zone (Brandon et al., 1990). The many other fossils listed by Fox-Strangways included Gryphaea cymbium, Pholadomya, Hipppopodium ponderosum, Cardinia listeri, Modiolus scalprum, ?Pseudolimea pectinoides, Ostrea and Pentacrinus. Farther north, strata belonging to the middle part of the formation (jamesoni Zone) were reported (Fox-Strangways, 1903) from a locality west of Little Dalby [SK 767 145]. The fauna there included Uptonia jamesoni, Hippopodium ponderosum, Gryphaea obliquata, G. cymbium, ?Oxytoma sp. and Pentacrinus. Fox-Strangways (1903) cited a report by Quilter of Oxynoticeras oxynotum from a succession of ‘shale…with a band of small ironstone nodules’ in a former brick pit [SK 6944 0354] by the Uppingham Road east of Houghton on the Hill, but this was not found during the present survey.

The Loveden Gryphaea Bed, indicative of a level at the top of the raricostatum Zone (Figure 5), has been traced into the northernmost part of the district, on the basis of common Gryphaea fossils in the soil (Ambrose, 2001). It evidently persists as a geophysical ‘marker’ (see above), although as its fossil content disappears its outcrop cannot be extended beyond Burton Brook [SK 762 131]. It does have local topographical expression [SK 750 102], however, as a feature-forming bed rich in siderite mudstone nodules to the west of Burrough on the Hill.

Temporary exposures of beds and their faunas appropriate to the middle part of the Charmouth Mudstone Formation were opened during the construction of the Billesdon Bypass, and are fully described by Blake (1986). The most comprehensive section was about 10 m of strata revealed to the east of Coplow Farm [SK 7084 0294]. It consisted mainly of laminated and poorly laminated grey mudstone, locally silty and sporadically micaceous, with thin beds of ironstone and commonly ferruginous limestone. In the middle part is a thicker concentration of ironstone and ferruginous limestone intercalations, and the upper part of the section consists of more than 5 m of mudstone with ironstone nodules. The abundant fauna described by Blake is exemplified by the assemblage from the ferruginous, shelly limestone of ‘bed 12’, which includes Cardinia sp., Gryphaea sp., Hippopodium ponderosum, Mactromya sp. Oxytoma. inequivalvis, Plicatula spinosa, Pseudolimea sp., Pseudopecten sp., Tertarhynchia dunrobinensis, Isocrinus and other species. The occurrence of the ammonite Acanthopleuroceras valdani (d’Orbigny) in strata from beds 1 to 12 indicated to Blake a stratigraphical position within the valdani Subzone of the ibex Zone; this means that the brachiopod Gibbirhynchia curviceps (Quenstatd), which was found in bed 5 and was the first known in Britain, comes from that zone. The part of this exposure with the ferruginous, shelly limestone interclations (beds 6–14) may correlate with the ‘85’ Marker Member of Horton and Poole (1977), which is also the Jericho Gryphaea Bed in the sequence farther north (Berridge et al., 1999; Carney et al., 2004) and is of ibex Zone age (Figure 5).

In cuttings for the mineral railway east of Cold Newton, around [SK 711 074], Fox-Strangways (1903) noted several sections with fossil faunas that include Uptonia jamesoni and Androgynoceras capricornus, appropriate to the middle and uppermost parts of the Charmouth Mudstone Formation. At the deep railway cutting south of Cold Newton [SK 7187 0599], for example, he noted Arietites sauzeanus? and various bivalves including Avicula, ?Pseudolimea pectinoides, Pleuromya costata, abundant Plicatula spinosa, Terebratula and Pentacrinus. From a ditch near Hammer’s Lodge [SK 7272 0620] Androgynoceras capricornus was recovered, indicating a location either on or just beneath the Dyrham Formation. A further finding of Androgynoceras capricornus, above a succession of mudstones containing ironstone nodules, was reported by Fox-Strangways from the stream due east of Rolleston [SK 745 004], but the present mapping has placed this within the Dyrham Formation.

In the Thorpe by Water Borehole (Figure 5), the Charmouth Mudstone comprises pale to dark grey, fissile and blocky, locally calcareous, fossiliferous mudstones with common nodules of siderite-mudstone and calcite-mudstone. Parts of the sequence show fine mica or siltstone wisps and laminae. Phosphatic pebbles occur at a number of levels between 102.8 to 107.75 m and 125.78 to 138.4 m, and are commonly associated with burrowed surfaces below, signifying the presence of minor non-sequences. There are locally abundant pyritic trails and burrow mottling, with U-shaped burrows and Chondrites present.

Dyrham Formation (DyS)

The Dyrham Formation is estimated at 10 to 25 m thick where it crops out in the central and eastern parts of the district. In the Thorpe by Water Borehole, about 6 km south-east of the district, it is 26.5 m thick. The unit corresponds to the ‘Clays and Sands’ and the lower part of the ‘Middle Lias’ of the earlier mapping, and to the ‘Clays above the Semicostatus Beds’ of Lamplugh et al. (1909). The topmost bed, which is of local occurrence and is commonly in sharp contact with the overlying Marlstone Rock Formation, is informally known as the ‘Sandrock’. Previously the Sandrock was regarded as part of the Marlstone unit (e.g. Lamplugh et al., 1909; Berridge et al., 1999), but here as well as in the adjacent Melton Mowbray district it is now included within the Dyrham Formation in accordance with the scheme of Cox et al. (1999).

The formation commences within the daveoi Zone in the Thorpe by Water Borehole (Figure 5). In the Leicester district, most of the faunal identifications come from near the top of the formation. Thus Woodward (1893) reported Amaltheus margaritatus from the upper Dyrham Formation at the Tilton on the Hill railway cutting [761 055] and Blake (1987) described the same exposed sequence in detail. There, he and earlier workers found Amaltheus margaritatus and A. subnodosus, indicating the presence of the subnodosus Subzone of the margaritatus Zone. Microfossils recovered from the top of the formation at the Tilton cutting (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6) included the forams Lingulina tenera subsp. A (Copestake and Johnson, 1989) and Dentalina matutina, indicative of the margaritatus Zone, while higher up the occurrence of Lenticulina muensteri acutiangulata suggests the presence of foram zone JF11 and the subnodosus-gibbosus subzones of the margaritatus Zone (Wilkinson, 2001). The gibbosus subzone could thus be restricted to a level immediately below the Marlstone Rock (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6) and is either very thin or absent, suggesting a non-sequence at the junction between the Dyrham and Marlstone Rock formations. Dinoflagellate cysts recovered from the uppermost bed of the Dyrham Formation at Tilton (Bed 1 of Hallam, 1968) further suggest a margaritatus Zone age (Riding, 2001); the freshwater/brackish alga Botryococcus is prominent at this level although other palynomorphs indicated a marine influence also. It should be noted that dinoflagellate cysts indicative of the spinatum Zone (Riding, 2001) were recovered from the overlying beds 3 and 4 of Hallam (1968), which are included within the basal part of the Marlstone Rock Formation and referred to below.

The outcrop of the formation is located generally in the middle and upper parts of the escarpment capped by the Marlstone Rock Formation and which is commonly landslipped. Natural exposures are not common, but cuttings and small sections in landslip back-scarps reveal that the lower part of the formation consists of micaceous siltstone, very fine- to fine-grained sandstone and mudstone. The siltstone is pale to medium grey in colour, ochreous to yellow-weathering, and generally poorly cemented. Some beds contain siderite-mudstone nodules, and there are impersistent beds of well-cemented, fossiliferous sandstone and ferruginous limestone.

Judd (1875) described brickyards between Whissendine and Pickwell exposing ‘blue, highly micaceous clay with septaria crowded with fossils’ and yielding the fossils A. margaritatus, Belemnites elongates, Helicina expansa, Avicula inaequivalvis, Mytilus hippocampus, Modiola scalprum, Cardium truncatum, Pleuromya unioides and Pentracrinus subangularis. A further brick yard [SK 781 090] between Somerby and Ouston exposed the following (Judd, 1875):

Judd listed a number of fossils collected from this pit, including A. margaritatus, belemnites, bivalves and crinoids.

At the Tilton railway cutting SSSI (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6), the following partial section was visible at the time of the present survey:

It should be noted that these strata were erroneously described as ‘Sandrock’ or ‘calcareous sandstone’ of beds a-c by Howarth (1980). Similarly, the sandstone beds 2a-b and 3b described by Blake (1987) are now reclassified (above) as the sandy and ferruginous, shell-detrital limestone.

Fox-Strangways (1903) observed a thick sequence in excess of 13 m at Tilton on the Hill; however, some of this was recorded ‘in the stream’, which is presumably the stream flowing parallel to the cutting north of the road.

Farther south, in the brick-pit at Billesdon [SK 7224 0296], Fox-Strangways (1903, p.32) described 9 m of the Dyrham Formation, which was the most complete section of the unit at that time. It consisted mainly of grey mudstone with several ‘nodular’ bands, becoming more ferruginous and sandy upwards. The highest part of the section consisted of at least 3 m of strata including ‘yellowish sand beds’ and, at the top, ‘dark red, ferruginous, sandy limestone’. North of Billesdon Lodge South, ironstone exploration boreholes, e.g. (SK70SW/33) showed that the upper part of the Dyrham Formation comprised a sequence about 5 m thick consisting of yellow mudstone with intercalated beds of ‘soft’ sandstone.

Elsewhere, exposures are confined mainly to ditch sections and a few undercut stream banks. Of these the most prominent occurs in the River Chater at Withcote [SK 7927 0508] where 2.6 m of dark grey, micaceous, fissile siltstone interbedded with very silty mudstone and containing clay ironstone nodules is exposed beneath 2.0 m of head. Other exposures of the formation are visible nearby. About 800 m north of Horninghold [SP 8072 9804] 0.8 m of grey, fissile, micaceous mudstone with siltstone laminae and clay ironstone nodules is exposed beneath 1.5 m of head in a stream bank. At Loddington [SK 7930 0225] 0.3 m of pale grey and ochreous mottled micaceous siltstone is exposed in a river bank, beneath the Marlstone Rock Formation.

The upper ‘Sandrock’ component of the formation is typically a yellow-brown and friable lithology up to 2 m thick. It is, however, impersistent and has been noted only in the north of the district in the Pickwell–Somerby–Burrough on the Hill area. There, it is revealed by its field brash that includes calcareous sandstone, ferruginous sandstone and shell-detrital wackestones with a siderite mudstone matrix. The ‘Sandrock’ was observed only at one locality [SK 7615 1178] north of Burrough on the Hill, where 0.3 m of fine-grained sandstone is exposed, dipping at about 10º to the south-west: the high dip results from cambering.

In the Thorpe by Water Borehole, the formation comprises very silty and micaceous, fossiliferous mudstones with common siltstone laminae or wisps, together with siltstones containing a few thin beds of limestone and fine-grained sandstone; siderite-mudstone, calcite-mudstone and phosphate nodules also sporadically occur. The beds are generally fissile but locally hard and blocky. They contain a rich fauna dominated by bivalves but also including ammonites, gastropods, belemnites, echinoid debris and foraminifera; shell pavements are common as is bioturbation and the trace fossil Chondrites. A full faunal list is available in unpublished BGS manuscripts.

Key localities

Tilton railway cutting SSSI [SK 7635 0530] to [SK 7615 0557]

Marlstone Rock Formation (MRB)

The Marlstone Rock Formation comprises sandy, shell-fragmental, ooidal limestone, which weathers to limonitic ironstone. It also includes shell-detrital, ferruginous limestones and is particularly noted for its content of brachiopods. The ‘Sandrock’, was included by some past workers in the Marlstone Rock Formation but is now placed at the top of the underlying Dyrham Formation (see above). Judd (1875) referred to the whole of the Middle Lias, including the Marlstone Rock Formation, as the ‘Marlstone’, using the name ‘Marlstone Rock Bed’ for the ironstone. Wilson (1885) used the term ‘Lias Marlstone’, whereas the ‘Rock-bed’, constituting the upper part of the ‘Middle Lias’, was defined by Fox-Strangways (1903). It subsequently became known as the ‘Marlstone’ (Lamplugh et al., 1909), and the term ‘Marlstone Rock Bed’ was re-introduced later (e.g. Whitehead et al., 1952; Poole et al., 1968) in adjacent districts. Cox et al. (1999) recommended the formal name ‘Marlstone Rock Formation’ and designated the national type section as the SSSI at the Tilton railway cutting of this district (Wilson, 1885; Wilson and Crick, 1889; Hallam, 1955, 1968; Howarth, 1980, 1992).

The Marlstone Rock Formation has been described in detail by Judd (1875), Fox-Strangways (1903) and Hallam (1955), and some of the lateral variations in the Leicester district are summarised in (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6). It is between 2 and 4 m thick over most of the central and northern parts, achieving its greatest development of 6 to 9 m in the area just north of Billesdon. Southwards and eastwards from the Tilton on the Hill area the formation thins to 1 to 2 m; it is only 0.17 m thick in the Thorpe by Water Borehole, just beyond the south-eastern corner the district. Hallam (1955) summarised the possible reasons for the thinning, noting that it might be due to pre-Toarcian erosion, to lateral passage, to a condensed sequence or to any combination of these. He favoured the first of these, following Arkell (1933), who stated that ‘Here, the greater part [of the Rock-Bed] seems to have been removed by erosion prior to deposition of the Upper Lias….’ Whitehead et al. (1952, p.103) similarly attributed the southerly attenuation to erosion of the upper part of the bed but Howarth (1980) noted that there was no evidence of non-sequence in the upper part (tenuicostatum Zone) at the Tilton on the Hill section. The concomitant southward facies change, from ooidal grainstone and ferruginous limestone to a dominantly mudstone sequence, would favour interpretation as a condensed sequence (Hallam, 1968) and lateral passage resulting in the greater dominance of mudstone that is more prone to thinning as a result of compaction during burial.

The formation weathers to a rusty brown colour and produces soils of a characteristic deep orange-brown, strewn with field brash. It shows a progressive facies change from the neighbouring Melton Mowbray district, southwards across the sheet. The nearest outcrops in the Melton Mowbray district, at Holwell [SK 741 234], are composed entirely of ooidal iron grainstone (Carney et al., 2004), whereas in the northern outcrops of the Leicester district, around Pickwell (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6), Somerby, Burrough on the Hill and Whissendine an interbedded sequence of ooidal iron grainstone and ferruginous fossiliferous limestone is present. Southwards, this stratigraphy changes into one featuring an upper ooidal iron grainstone and a lower ferruginous limestone. In the extreme south, between Allexton and Horninghold, the formation is represented by ferruginous mudstone with thin beds of ferruginous limestone. Eastwards from here, the ooidal iron grainstone facies appears and in the Thorpe by Water Borehole, a little farther to the east in the neighbouring Stamford district, only 0.17 m of the formation is present, consisting of a fossiliferous limestone conglomerate with intraformational and phosphatic clasts, set in an ooidal matrix. The probable equivalent of this is the basal conglomeratic bed that is locally well developed in the Melton Mowbray district, and has been noted in parts of the Leicester district (see below). Westwards into the Tilton on the Hill area, another facies change occurs whereby the ferruginous limestone is laterally replaced by a very sandy, ferruginous limestone that is locally a calcareous sandstone (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6). A similar but less sandy facies occurs north of Tilton on the Hill but the westward extent of this facies is not known.

Petrographically, the formation is a shell-detrital limestone with a sideritic cement. Ooids are of a dull green colour when fresh and are composed of the iron silicate mineral berthierine (Young and Taylor, 1989), rather than chamosite as was formerly thought (e.g. Whitehead et al., 1952). Other minerals include siderite (iron carbonate), both as a cement and as a replacement to some berthierine ooids, and calcite as a cement and in the shell fragments. The supergene alteration of berthierine and siderite produces the amorphous, yellow-brown iron oxide, limonite (Hallam, 1968), which accounts for the rusty weathering colours as well as enhancing the iron content of the rock.

Marlstone Rock is richly fossiliferous (Wilson and Crick, 1889; Fox-Strangways, 1903, p.30–37; Hallam, 1955; Howarth, 1980). Particularly common are brachiopods, such as the terebratulid Lobothyris punctata and the rhynconellid Tetrarhynchia tetrahedra. Their concentrations in nests and beds suggest the presence of life assemblages (Hallam, 1961). Comprehensive details of the fauna of the Marlstone at the Tilton cutting (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6) were given by Hallam (1955). In the present account, however, the unit encompasses the strata between (and including) beds 2–8 of Hallam (1955), whereas the latter author restricted it to beds 6–8. From the top of the formation (‘Transition bed’ or bed 8) Hallam (1955) recovered Tiltoniceras antiquum and Dactylioceras semicelatum; the latter was also noted by Howarth (1980) and is indicative of the semicelatum Subzone of the tenuicostatum Zone. To these are added Harpoceras? serpinatum, identified by Wilkinson (2001). Hallam (1955) noted that the occurrence of Toarcian ammonoids indicated that the boundary between the spinatum Zone and the younger tenuicostatum Zone may occur near the top of the unit, a situation also found in the Grantham district (Berridge et al., 1999). Calcareous microfossils from the uppermost ‘Transition Bed’ of the Marlstone Rock Formation at the Tilton railway cutting provided further age definition, indicating the Kinkinella sermiosensei/Kinkinella intrepida ostracod Zone, ranging from the tenuicostatum to bifrons ammonite zones (Wilkinson, 2001). Although Hallam (1955, 1968) described the ‘Transition Bed’ as resting disconformably on the ironstone, Howarth (1980) concluded that it did not exist at Tilton as a separate bed and that instead it formed a weathered top to the Marlstone.

Ammonites are generally rare in middle and lower parts of the sequence and the zonal ammonite Pleuroceras spinatum, which was identified in the Grantham district (Berridge et al., 1999), does not appear in any of the faunal listings so far published for this district (Fox-Strangways, 1903; Hallam, 1955; Howarth, 1980) nor for the Melton Mowbray district farther north (Lamplugh et al., 1909; Carney et al., 2004). Some evidence for the age of the very base of the formation is, however, forthcoming from calcareous microfossils recovered by Wilkinson (2001). Those faunas include the foram Rheinholdella macfadyeni, which is common and abundant in the tenuicostatum Zone, suggesting that much of the formation is of Toarcian age despite the earlier suggestion of Howarth (1980) that the basal part is of a spinatum Zone age. The findings of Wilkinson (2001) are, however, complemented by dinoflagellate cyst associations from the Tilton cutting (Riding, 2001). The two stratigraphically lowermost samples, from just above the base of the formation, represented by beds 3 and 4 of Hallam (1968), together with palynomorphs from slightly higher up, indicate a level within the spinatum Zone (Riding, 2001). Higher up in this section, the basal part of bed 7 of Hallam (1968) contained dinoflagellate cysts again indicative of the uppermost Pliensbachian, spinatum Zone, but the younger samples (upper bed 7 and lower bed 8) contained assemblages referable to the lowermost Toarcian, tenuicostatum Zone (Riding, 2001). Taken together, the dinoflagellate cyst faunas suggest that the lower part of the formation lies within the spinatum Zone, and that the Pliensbachian-Toarcian boundary occurs about 4 to 5 m above the base of the Marlstone Rock Formation, which is about 5.7 m thick at the Tilton section (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6).

The Marlstone lithology, being relatively resistant to erosion, forms the crest of a prominent dissected scarp which, although locally drift-covered, dominates the central and north-eastern part of the district. Structure contours (Figure 7) indicate that the formation has a south-easterly regional dip although where sufficient subsurface data is present, as in ironstone exploration areas, it is shown to be locally flexured and faulted. In areas where the formation was valued as a source of iron ore or building stone there are numerous small surface workings on the broad dip slope, but it is only in the area immediately north of the former Tilton on the Hill railway station [SK 75 06] that there were significant open casts. The Marlstone was extensively investigated by drilling for the purposes of ironstone resource assessment in the Somerby, Tilton on the Hill and Billesdon areas and some details of this activity are given in Chapter 13. Useful accounts of lithological and chemical variations in the Marlstone Rock Formation can be found in Whitehead et al. (1952) and Wheeler (1967). Nowadays most of the shallow pits have been restored to agriculture, but a few clean sections remain.

The formation is best seen at the SSSI and national type section in the Tilton railway cutting, where a complete sequence is exposed (Plate 14a), together with the uppermost beds of the underlying Dyrham Formation and lower part of the overlying Whitby Mudstone Formation (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6). The section was first described in detail by Wilson and Crick (1889) who gave a comprehensive faunal listing. Woodward (1893) briefly described the cutting, as did Fox-Strangways (1903). Hallam (1955) divided the Tilton sequence into nine beds, of which Bed 1 is now taken to be the Dyrham Formation and Bed nine the Whitby Mudstone Formation. He recognised four distinct facies, but these were based largely on fossil assemblages and are not easily correlated with the lithologies. He also described the lower part as being composed of sandstone, but noted that this ‘Sandrock’ at Tilton differed from that in the Melton Mowbray district in two respects. It showed a more gradual downward passage into the ‘shales’ (of the Dyrham Formation), and it contained a large number of bivalves. In their comprehensive account of the Tilton cutting and surrounding Marlstone outcrops, Whitehead et al. (1952), noted that this basal part of the Marlstone was calcareous sandstone comparable to the ‘Sandrock’ of the Melton Mowbray district. The current work, summarised in (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6), has demonstrated that at Tilton these lower beds (2–5 of Hallam, 1968) consist of limestone and sandy iron grainstone, and are lithologically more akin to the Marlstone Rock Formation than the underlying Dyrham Formation. Recent petrographic work on Tilton by BGS (G K Lott, written communication, 2002) also suggests that the lower ‘hard bed’ of the Tilton sequence is a very sandy, ferruginous limestone (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6), although it may locally be a calcareous sandstone. The most comprehensive exposures occur immediately by the entrance into the cutting [SK 7616 0555], where the following section was measured:

On the east side of the Tilton railway cutting [SK 7615 0563] 0.4 m of grey, coarsely crystalline, ooidal, shell-detrital limestone is exposed. Locally abundant crinoid debris define a cross-lamination structure, with current directions recorded to the north, south, east, south-west, and south-east.

A partial exposure in the stream [SK 8100 1095] near Cold Overton Grange shows 1.1 m of deeply weathered, ochreous, ferruginous limestone with shells, shell debris and scattered ooids. At the base the unweathered rock is seen, blue-grey in colour.

A former quarry on Pickwell Main Street [SK 7895 1135] exposes the following section, which is also summarised in (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6):

From the same quarry, Judd (1875) recorded Tetrarhynchia tetrahedra and Lobothyris punctata, reporting a thickness of around 4.9 m, which may, however, have included the ‘Sandrock’, resting on ‘clays’. Immediately south of Somerby [SK 7820 0977] there are several exposures of up to 1 m of ferruginous limestone containing brachiopod nests, with ooidal iron grainstone visible near top of escarpment.

At Loddington, a river cliff exposure [SK 7930 0225], illustrated in (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6), showed:

Hallam (1955) noted just over 3 m of marlstone at this exposure. He collected Tetrarhynchia tetrahedra, Lobothyris punctata, Gibbithyris northamptonensis, Pseudopecten dentatus and Entolium cf. lunularis.

A disused quarry north of Tilton on the Hill [SK 7370 0652] exposes two beds (0.7 and 0.4 m thick) of ochreous, iron grainstone with an ooidal matrix. The upper bed (0.7m thick) contains about 40 per cent of coarse- to very coarse-grained bioclastic debris, mainly of crinoids with some belemnites. These beds are underlain by 0.3 m of ochreous, fine-grained, ooidal and shell-detrital sandy limestone, which is of a similar facies (though less sandy) to the lower beds in the Tilton railway cutting. About 2 km to the east, a disused quarry near Cold Newton [SK 7186 0519] exposed 2.5 m of cross-laminated, ooidal iron grainstone with scattered shells and belemnites, which would appear to be the uppermost bed of the formation.

In the valley of the River Chater, a disused quarry [SK 7866 0537] described by Hallam (1955) exposed about 3.7 m of strata, the lower part of which is no longer visible. Two main brachiopod beds, A and B, were identified there and can be correlated across the district as shown in (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6). Nowadays the quarry exposes 1.3 m of ochreous to buff, fine-grained, shelly and shell-detrital limestone with scattered ooids; only 0.2 m of the overlying bed is exposed, consisting of ochreous, fine-grained, ooidal and shell-detrital iron grainstone. In stream banks nearby [SK 7860 0532] to [SK 7869 0532] a number of other Marlstone exposures can be seen. Many other sections were recorded by Hallam (1955), to which the reader is referred for further details.

Judd (1875) described sections in several Marlstone pits around Robin-a-Tiptoe Hill [SK 775 045], but unfortunately their precise locations are now uncertain. In one pit, he noted a 0.15 m-thick bed, which he called ‘First Jack’, made up almost wholly of shells of Terebratula punctata and Rhynchonella tetrahedra. This was separated, by 1.8 m of ‘Building Stone’, from a similarly fossiliferous lower bed, the ‘Second Jack’, which also contained flattened nodules or concretions and had a central parting of clay. These two beds are most probably equivalents of beds B and A respectively of Hallam (1955), as shown in (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6). In addition to the usual ammonites, brachiopods, bivalves, belemnites and crinoids, Judd also mentioned abundant fossilised wood fragments from this quarry.

Horwood (1907a and b, 1908, 1910) described a former quarry, probably [SK 718 049] in the Marlstone between Tilton on the Hill and Billesdon Coplow. He noted (1907a) that ‘the very characteristic gastropod and cephalopod zone of the Transition Bed is well developed’ (at the top of the Marlstone). He further noted that in the upper part of the formation, there was a ‘very well marked encrinital limestone band, varying from a foot [0.3 m] to eighteen inches [0.45 m]’ occurring 0.9 to 1.2 m from the top of the formation. He also recorded the echinoid Eodoadema granulata. He found the crinoid-rich bed to be present on Tilton Hill (1908) and at Burrough on the Hill (1910). However, this may not be a particularly meaningful correlation as crinoid-rich beds have been noted at several levels in the Marlstone at different localities.

Whitehead et al. (1952) recorded a section from Fryers Quarry, just south-east of Somerby, probably [SK 785 100], which showed 2.1 m of pale brown, ooidal and ferruginous limestone with bluish grey bands towards the base. In an old quarry [SK 767 044], west of Robin-a-Tiptoe Hill, they record 1.8 m of brown and bluish grey ooidal ferruginous limestone with shelly bands, some crowded with Lobothyris punctata and Tetrarhynchia tetrahedra, overlain in places by the Whitby Mudstone Formation. About 75 m east of the quarry, a full thickness (about 4.5 m) of the formation was visible in crags alongside the stream, which included a hard siliceous bed about 0.6 m above the base.

A ditch section [SK 817 060] about 1 km south-west of Braunston in Rutland exposed grey mudstones of the Whitby Mudstone Formation overlying the Marlstone Rock Formation. The latter comprises an upper ferruginous, shell-detrital limestone with scattered ooids, a middle bed of soft, chocolate-brown mudstone and a lower ooidal iron grainstone bed that rests on micaceous siltstone and sandstone of the Dyrham Formation. The visible section was disturbed and exact thicknesses could not be determined, but the Marlstone is probably no more than about 2 m thick. The middle mudstone was the only occurrence of its type noted this far north, the main facies change to mudstone occurring about 5 km farther south.

On the till-capped plateau north of Billesdon Lodge South, ironstone exploration boreholes, e.g. (SK70SW/33) recorded an above average 8 to 9 m thickness for the Marlstone, although the lithological descriptions given are not particularly useful. East and south of here Quaternary deposits extensively cover the Marlstone, but the few provings that are available indicate an average thickness for the area of 3 to 4 m, for example around Tilton Grange [SK 7560 0445] and Somerby, although it is uncertain whether this included the very base of the unit and, if present, any ‘Sandrock’. East of Keythorpe Hall Farm [SP 7709 9930], ploughed fields are strewn with large tabular brash of ironstone, some of which show ripple marks and abundant shell remains. The thickness here is nevertheless probably no greater than 1 to 2 m, and the outcrop of the Marlstone is now demonstrated to be considerably narrower than that shown on the former maps. Just to the south-east of here, Fox-Strangways (1903, p.35) illustrates a previous observation by J W Judd on a former exposure near the valley floor [SP 776 986]. This shows only a one-foot thickness (30 cm) of Marlstone, occurring as a single bed with convexities of its upper surface suggestive of large-scale ripple forms. The fossils included Belemnites, Lobothyris punctata and others not listed. Although the Marlstone is inferred to continue southwards from here, its outcrop is largely covered by Quaternary deposits and it remains thin. For example, the former exposure at Hallaton brickyard [SP 7885 9751] showed (Fox-Strangways, 1903, p.36) two beds of ‘rubbly ironstone’, each about 0.5 m thick, within a 3.6 m sequence that consisted mainly of yellow and grey nodular mudstone. Fossils recovered from here included Amaltheus margaritatus, Oxytoma inequivalvis, ?Entolium liasiantum and ?Protocardia truncata. Farther south of the district, descriptions by Poole et al. (1968) indicate that in the Hallaton-Slawston area the Marlstone Rock Formation is unlikely to be much more than 30 cm thick (see also, Fox-Strangways 1903, p.34).

Key localities:

Tilton railway cutting (type section) [SK7635 0530] to [SK 7615 0557]; Pickwell quarries [SK 7841 1157]; [SK 7895 1130]; Loddington river cliffs [SK 7930 0225]; Disused quarry in the Chater valley near Sauvey Castle [SK 7866 0537].

Whitby Mudstone Formation (WhM)

The term ‘Whitby Mudstone Formation’ was defined by Powell (1984) for the Yorkshire basin, and its usage was extended throughout the East Midlands by Cox et al. (1999). It replaces the former ‘Upper Lias Clay’ division of Fox-Strangways (1903). The formation consists mainly of grey mudstone with a few beds of nodular limestone, and has calcite- and siderite–mudstone nodules scattered throughout. It is 40 to 50 m thick on average, and where it is not covered by Quaternary deposits it gives rise to undulating, dissected country, in places forming the lower part of the steep escarpment capped by outliers of the Northampton Sand Formation.

At the Tilton railway cutting [SK 764 054], Howarth (1980) placed the lowermost 2.8 m of Whitby Mudstone in the exaratum Subzone and the overlying 6.2 m in the falciferum Subzone, both of the Harpoceras falciferum Zone (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6). In the lower subzone, he recorded the ammonites Harpoceras serpentinum, H. elegans and Dactylioceras sp. In the upper Subzone, he recorded H. falciferum and Phylloceras heterophyllum.

A similar age is suggested by the rich, low-diversity calcareous microfauna recovered from these strata just above the Marlstone Rock Formation at Tilton, which indicate (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6) the JF13 and JF14 foram zones (Wilkinson, 2001). Marine microplankton assemblages from the same locality further suggest that the lower part of the Whitby Mudstone Formation is of falciferum Zone age, and thus represents the peak of the early Toarcian anoxic event (Riding, 2001). Dinoflagellate cysts and miospores from the stratigraphically higher Whitby Mudstone strata in the railway cutting, however, include representatives of the lowermost bifrons Zone, during which there was a re-establishment of circulating oceanic waters. In the Grantham district, Berridge et al. (1999) suggested that the base of the formation was disconformable on the underlying Marlstone Rock, resulting in the sharpness of the contact; this is also seen at the Tilton railway cutting in the present district although Howarth (1980) argued that there was no evidence for non-sequences on biostratigraphical grounds. The lowermost strata of the formation in the Grantham district belong to the falciferum Zone, and the upper part is in the bifrons Zone (Berridge et al., 1999). The top of the formation is probably an unconformity surface because in this part of the Midlands region Upper Toarcian (Levesqui Zone) beds were either not deposited or have been removed by erosion prior to or during deposition of the overlying Northampton Sand Formation (Cox et al., 1999).

The presence of ooids and pisoids in the basal part of the formation are noteworthy features of the Tilton exposures. They are in a similar stratigraphical position to, and may thus be comparable with the phosphatic ooids described by Horton et al. (1980) from the ‘Cephalopod Limestone Member’, basal Upper Lias, at Empingham about 12 km east of the district. Geochemical analyses by Chatwin (1998) showed that the ooids and pisoids at Tilton are composed predominantly of calcium phosphate, together with variable proportions of goethite, calcite and aluminosilicates. Horton et al. (1980) argued that the presence of phosphate ooids suggested formation in sea water of a moderate depth, possibly in a zone where a current of colder, nutrient-rich water was moving southwards from the deeper Yorkshire basin. The phosphate was probably initially concentrated in the shallower waters, closer to the London Platform. There, current activity formed the ooids and pisoids, which were then transported by storms out into the basin and deposited in deeper water. Chatwin (1998) showed that the Whitby Mudstone at Tilton was largely composed of a mixture of illite, muscovite and quartz, with traces of chamosite and possibly authigenic kaolinite.

The principal Whitby Mudstone exposure in the district (Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6) is at Tilton railway cutting [SK 761 055] (Wilson and Crick, 1889; Woodward, 1893, Fox-Strangways, 1903; Whitehead et al., 1952; Hallam, 1955; 1968; Howarth, 1980). Here, the lower part (up to 9 m) of the formation consists of the grey, locally ooidal and pisoidal mudstones with thin limestones - composition and origin were discussed above. The following description of the Tilton section incorporates the work of Jones and Mathieson, Leicestershire Museums (1972, unpublished manuscripts), and Leicestershire Museums (1979 manuscripts) (Bed numbers in curly brackets.). There is much additional data from R G Clements (RGC; 1986 manuscript field data), Hallam (1955, 1968) and Howarth (1980).

One other exposure of note occurs by a barn at Withcote Lodge [SK 8080 0493], where 2.6 m of grey, ochreous-stained, fissile mudstone with scattered lime-mudstone nodules contains dactylioceratid ammonoids, bivalves and belemnites.

A quarry [SK 7885 1130] in Main Street, Pickwell, formerly exposed about 1.2 m of Whitby Mudstone, described as ferruginous with ‘thin ribs of fissile, blue limestone weathering white, and which contain abundant fish fragments, crustacean and other fossil impressions’ (Wray, 1941).

Fox-Strangways (1903, pp.37–40) noted that there were very few exposures of this formation, temporary or otherwise, and therefore drew upon the former geological memoir for Rutland (Judd, 1875), written at a time when agricultural developments had opened up a number of temporary sections. The most important of these occurred at the brickyard at Knob Hill [SP 8211 9796], east of Hallaton, where finely laminated mudstone with large selenite crystals was exposed. The lowermost beds north of here ‘in the left bank of the stream at Hallaton Ferns’ showed, above the Marlstone Rock, ‘paper shales with fish and insect limestones’ overlain by ferruginous beds containing ‘Ammonites serpentines’? and Hildoceras bifrons. The paper shales almost certainly equate with the early Toarcian anoxic event recognised in the succession at the Tilton railway cutting (see above); the facies represents a widespread horizon of anaerobic lithologies comparable with the ‘Dumbleton Beds’ of the Cotswolds (Hallam, 1968). Farther north, Judd (1875) noted about 18 m of the Whitby Mudstone exposed, probably at the Moor Hill brickyard [SP 7814 9882]. This succession of pale grey to blue-grey, laminated and locally ferruginous mudstones contained large selenite crystals and belemnites, but ammonoid remains were not identifiable. West of here, Whitby Mudstone dug from a pond at Keythorpe Park [SK 7695 0028] was richly fossiliferous, with Dactylioceras commune, ‘Ammonites annulatus’, Dactylioceras holandrei, ‘Ammonites radians’, Hildoceras bifrons, ‘Belemnites compressus’, Nuculana ovum and Inoceramus dubius. Across the road, west of Keythorpe Hall, a brickyard [SK 7636 0026] exposed ‘finely laminated blue-grey clays’ with the same fauna. Judd (1875) also described a 4.6 m section in Whitby Mudstone exposed just above the Marlstone Rock in a pit [SK 816 004]; (precise location uncertain) at Allexton. This consisted of blue-grey laminated mudstone with subordinate, thin beds of argillaceous limestone, the latter said to be comparable in quality to the limestones of Barrow on Soar and used for lime making at Tugby. The fauna of the limestone beds consisted of Belemnites sp., ‘Ammonites serpentines’,’ A. elegans’, ‘A. annulatus’, Ostrea, Inoceramus dubius, Astarte, Lima and Pteroperna. Large masses of wood converted to jet were noted in the mudstones. The former railway cutting at East Norton Station [SK 793 002] exposed 6 to 9 m of thin shales (Whitby Mudstone), overlain by 1.5 to 3 m of till (Browne and Fox-Strangways, 1901). The section is described as ‘somewhat obscured by slipping’.

Inferior Oolite Group

This sequence, formerly known as the ‘Inferior Oolite’ (Fox-Strangways, 1903), is the stratigraphically youngest Jurassic division exposed in the district. Its base is an unconformity (see below) that marks the commencement of the Middle Jurassic (Aalenian Stage), which coincided with a major regressive event (Hallam, 2001) and change to relatively shallow water, compared with the environment that prevailed during deposition of the underlying Whitby Mudstone Formation. The only representative of the group in this district is the stratigraphically lowest division, the Northampton Sand Formation, which represents deposition under nearshore, high-energy conditions.

Northampton Sand Formation (NS)

The Northampton Sand Formation is up to 10 m thick in the district. It was formerly known as the ‘Northampton Sand’ (Fox-Strangways, 1903) and ‘Northampton Sand Ironstone Formation’ (Taylor, 1949), and is the lateral equivalent of the ‘Northampton Ironstone’ of Hallam (1968). In its type development to the east and south, the unit has a varied lithology. In this district, however, Taylor (1949, plate II) suggested that only the ‘sandy marginal facies’ of the ‘Northampton Sand Ironstone Formation’ is represented. Field evidence nevertheless indicates that both the typical ferruginous sandstone and ooidal iron grainstone lithologies are present, together with some local occurrences of non-ferruginous sandstone and ferruginous limestone, both seen on Whatborough Hill [SK 769 059]. The formation is of Aalenian, opalinum Zone age in the Market Harborough district to the south (Poole et al., 1968), although no fossils have been recovered from it in this district. It rests unconformably on underlying strata of the Whitby Mudstone Formation, as discussed in the previous section.

The formation is relatively resistant to erosion and forms a series of six outliers surmounting steep slopes developed on the Whitby Mudstone in the south-eastern part of the district, the most prominent being Whatborough Hill [SK 77 06] and Robin-a-Tiptoe Hill [SK 774 044]. Cambering is a pronounced feature of many of these outcrops, (see Chapter 10), and dip-and-fault structures were also noted in the extreme east of the district [SK 827 023].

Soils on the outcrop are of dark reddish brown sandy loam with abundant brash of hard, limonite-cemented, quartzose sandstone and some ooidal ironstone. The lithology of field brash material on Whatborough Hill was correlated by Taylor (1949) with his ‘Upper Siderite Mudstone-Limestone Group’ division, which is of variable lithology and includes sideritic or calcareous sandstones and shelly ooidal ironstones. Fox-Strangways (1903) drew attention to the common occurrence of red-brown hematite veining, along joints or bedding planes, in many fragments found in the soil. The only exposure mentioned by Fox-Strangways was on the western edge of the outcrop north of Hallaton, where a sketch by Judd (1875, p.107) featured a small pit [SP 799 991] showing Northampton Sand overlain by till. The only exposure seen during the present survey was on the south-east side of Whatborough Hill [SK 7714 0585], where ooidal iron grainstone and ferruginous limestone are seen.

Chapter 8 Quaternary

Uplift of the East Midlands region from the Palaeogene through into Quaternary times (Chapter 10) and the concomitant erosion led to the complete removal of all but the lowest part of the thick marine Jurassic and Cretaceous sequences that once covered the district (Green et al., 2001). The Quaternary Period, covering the last two million years, is marked in Britain by extreme oscillations of climate ranging from severely cold glacial or periglacial to mild temperate conditions. These oscillations are reflected in the scheme of marine oxygen isotope stages (MIS), to which the deposits of this district are tentatively referred (Table 3).

The Quaternary sequence of the northern and western parts of the Leicester district commences with Preglacial Deposits, of fluvial sands and gravels, which probably date back to at least Neogene times. They accumulated within the Bytham river basin (Rose, 1989), one of a series of major east-draining systems that transported quartzite-rich, sandy and gravelly sediments from central and southern England into the North Sea region (Gibbard and Lewin, 2003). When ice sheets subsequently advanced across the English Midlands during the severely cold Anglian stage (MIS 12) Glacial Deposits were laid down, thickening into, and completely infilling, the Bytham river basin and mantling the early Quaternary topography elsewhere. There are three principal varieties of till, together with their related glaciofluvial sands and gravels. These deposits commonly interleave with glaciolacustrine silts and clays, particularly where they are thickly accumulated in palaeovalleys.

The present-day drainage was initiated during the waning phases of the Anglian glaciation and subsequent downcutting has dissected the glacigenic deposits, partly revealing the Bedrock. A rockhead contour map shows (Figure 8) that the Quaternary deposits mantle an undulating topography, largely inherited from preglacial times but with a tributary drainage pattern remarkably coincident with today’s actively eroding valleys. Major steep-sided features, such as the uplands of Mountsorrel and Burrough Hill [SK 7639 1185] were evidently only thinly mantled, and due to subsequent erosion are largely free of glacial deposits today. The River Terrace Deposits are the post-Anglian fluvial sediments, which now occur as flights of terraces along the trunk valleys and larger tributaries. They record climate change coupled with continuing uplift and lateral and vertical incision. It is this regime of erosion that is responsible for much of the present-day undulating topography of the district, characterised by valleys separated by interfluves mantled by glacigenic deposits dominated by till.

Widespread deposits of head, which include deposits related to terrace cryoplanation, are evidence for periglacial conditions occurring at various times following the Anglian glaciation. The clay vales, such as the Stapleford Vale in the north-east of the district, and the much larger Vale of Belvoir farther north in the Melton Mowbray district, developed through repeated episodes of periglacial slope mass movement, these being partly reflected by the head that forms the Slope Terrace Deposits (Brandon, 1999; Carney et al., 2004). Evidence of landsliding is found along the steeper escarpment slopes. Holocene (MIS 1) events are marked by further fluvial incision and terracing of Late Devensian and early Holocene sediments along the main valleys and larger tributaries of the Soar and Wreake, together with the formation of the alluvium of the modern floodplains. Human influence has led to accelerated colluviation linked to deforestation and modern farming practises and a change of hydrodynamic styles brought about by the introduction of flood prevention schemes.

Permanent sections in the Quaternary deposits are rare and are only mentioned in the text if significant.

Preglacial deposits

Preglacial deposits represent the fluvial sands and gravels laid down within the Bytham River catchment, a north-east flowing drainage system that dominated the preglacial landscape of central England and the East Midlands (e.g. Rose, 1989; 1994; Graf, 2002). The valley occupied by the trunk stream can be traced by mapping, and by calculations of rockhead elevation contours on the base of the deposits, from the Coventry district to the south-west (Bridge et al., 1998, fig. 42). It enters the Leicester district via a rather narrow and poorly defined palaeovalley to the north of Enderby [SK 540 011] and continues north-eastwards along the modern Soar valley into the adjacent Melton Mowbray district. There, its graded floor, locally preserved from erosion, declines eastwards, in the inferred direction of flow, from about 60 m to 50 m above OD over a distance of 25 km (Wyatt, 1971; Brandon, 1999; Carney et al., 2004, fig. 23). A major tributary valley in the north-west, shown on (Figure 8), may represent the south-flowing ‘Derby River’ (Brandon, 1999), which possibly followed a bifurcating course around the Mountsorrel massif. The deposits of the Derby River were, however, largely removed following synglacial and postglacial downcutting, the latter causing a major diversion of this part of the Soar, which now flows northwards into the Trent.

The Bytham River deposits are poorly exposed today, but were formerly extracted from several sand and gravel quarries. They continue to represent an important potential local aggregate resource and are consequently known from recent subsurface exploration programmes around Rearsby [SK 655 145], in the north of the district (e.g. Rice, 1991; Brandon, 1999).

Bytham Formation

This unit consists of the largely unconsolidated deposits formerly called the Bytham Sands and Gravels (Brandon, 1999). The formation is part of the Dunwich Group, in the recently formalised nomenclature of McMillan (2005), and it includes all of the pre-Anglian fluvial deposits of the Bytham River and tributaries occurring in this district. It was referred to as the ‘Older Sand and Gravel’ by Fox-Strangways (1903), who also hinted at its preglacial age. Parts of the ‘Quartzose Sand’ unit recognised by Deeley (1886), and subsequently adopted by Fox-Strangways, also seem certain to belong to the Bytham Sands and Gravels. The deposits were named the ‘Thurmaston sand and gravel’ by Rice (1968), but this was later changed to ‘Baginton Sand and Gravel’ (Rice, 1991), the term also used in the adjacent Coalville district (Worssam and Old, 1988). Those deposits were considered by Brandon (1999) to represent only the last cycle of aggradation of the Bytham Sands and Gravels, on a graded floor of the Bytham valley, some details of which are shown in (Figure 9a) and (Figure 9b). The ‘Bytham Terrace Deposits’ of Brandon (1999) form raised outcrops and relate to earlier periods of sediment aggradation along the valley. They are included within the Bytham Sands and Gravels, however, and are mainly described in the account for the adjacent Melton Mowbray district (Carney et al., 2004). A further component of this unit, the ‘Brooksby sand and gravel’, was encountered in shell-and-auger borings to the south of Brooksby village (Figure 9b), within a bedrock channel cut below the more characteristic preglacial fluvial sequence (Rice, 1991), and will be discussed below.

A precise age for the Bytham Formation has yet to be determined for the Leicester district, but correlations with its equivalents elsewhere in central and eastern England, summarised by Graf (2002, p.2), suggest that the deposits are of Middle Pleistocene, Cromerian Complex age, probably MIS 15 or 14 through to very latest MIS 13. Organic remains from the ‘Brooksby sand and gravel’, described below, suggest cool or cold climatic conditions early in the history of the local Bytham valley, and it is probable that the deposits constituting the main phase of Bytham river development were laid down under increasingly colder climates as the Anglian ice sheets approached the region.

Details of the stratigraphy, clast composition, provenance and sedimentology of the formation are given by Engineering Geology Ltd (1985a, b) and Rice (1968, 1991). The gravel deposits, which contain virtually no detritus younger in age than Triassic, consist of two compositional suites, the most important constituents (Rice, 1968) being quartzite and quartz pebbles (‘Bunter’ pebbles) derived from the Sherwood Sandstone Group. Of slightly lesser importance are fragments of Upper Carboniferous sandstone and of Lower Carboniferous chert and limestone. The content of chert fragments becomes noticeably raised in the deposits between Thurmaston and Rearsby, suggesting an increased input of material from the southern Pennines (Rice, 1991), which perhaps originated from a southwards-flowing ‘Derby River’ that entered the preglacial drainage system at about the location of the present Soar-Wreake confluence (Brandon, 1999).

It is mainly through the fieldwork and borehole studies of Rice (1968, 1991) that the outcrops of the Bytham Formation were identified in this district. From these data, supplemented by the recent resurvey, the course of the Bytham River has been reconstructed, based partly on the calculation of rockhead elevations at the base of the deposits (Figure 8). The main deposits represented are those laid down at the time of maximum development of the river system, ‘final stage’ deposits of (Figure 9b). The outcrops bordering the modern Soar valley, however, lie at elevations in excess of 60 m and locally are over 70 m, and at least some of these may represent the remnants of terraces that formerly bordered the Bytham River floodplain, the main channel of the river having been obliterated during downcutting of the modern Soar River. A bipartite division is commonly demonstrated where the Bytham River deposits are fully developed. This consists of fine-grained red sand overlying coarser sand and gravel (Figure 9a), and is typical of the Baginton Sand and Gravel equivalents in adjacent districts as seen, for example, in the pit at Huncote (Bridge et al., 1998; fig. 43), about 3 km south-west of the district margin [SP 582 982].

In outcrops to the east of Rearsby, around [SK 66 14], the Bytham Formation overlies bedrock and is in turn overlain by glacial or river terrace deposits (Figure 9a). Borehole investigations hereabouts are summarised by Rice (1991) and Brandon (1999), the former noting that there is generally an upper layer of sand, between 1.6 and 5.4 m thickness, and a lower layer of ‘fine to medium gravel’, between 0.5 and 3.4 m thick (Figure 9a). The gravel showed variable sorting, but the upper sand layer is consistently well sorted, consisting of fine to medium grain size, and of a reddish brown colour, with common coal-rich layers or laminae.

Farther to the south-west, in the now restored pit [SK 615 101) at Thurmaston, Rice (1968) measured at least 20 ft (6 m) of the upper sand component. This consisted of red sands with coaly laminae, exhibiting foreset bedding inclined to the north-north-east, below which was about 2 m of gravel, in which virtually all of the clasts were of northerly or westerly derivation. North-west of here, the deposits were again thickly developed in the former pit at Rothley [SK 565 123], where Rice (1968) noted 6.7 m of cross-bedded sand underlain by at least 4.2 m of gravel. The sand component here is less well sorted than at Thurmaston, and includes lenses of chocolate-coloured clay or silt, as well as gravel. The foreset beds indicate deposition from currents flowing ‘north of east’. The Rothley gravels include Mountsorrel and Charnwood basement clasts (Rice, 1968, table 1), suggesting that they represent deposits of the left-bank tributary river shown in (Figure 8). The present resurvey has further extended the distribution of these deposits, via disconnected outcrops around Swithland, to the north-west, e.g. [SK 551 135] where fragments of Mountsorrel Complex granodiorite occur as field brash, in addition to the typical quartzose pebbles and minor chert. Rice (1991) also assigned the outcrops on Wanlip Hill [SK 593 113] to the Bytham Formation noting that, if local Triassic material is discounted, the dominant clasts are quartzose but are accompanied by significantly raised proportions of Carboniferous sandstone and chert. The north-easterly current direction observed at Rothley by Rice (1968) is broadly in keeping with the east to north-east measurements taken at other exposures of these deposits in the Wreake valley area (Carney et al., 2004, fig. 23). It should be noted, however, that in the pit at Huncote mentioned above one of us (KA) found that the upper sands show more variable current directions (including N, E, S, NE) than the underlying gravel (almost exclusively to the north-east), perhaps indicative of a change in regime from a braided river to a meandering one.

South of Wanlip, the Bytham Formation is largely obscured by urban development, but on the evidence of boreholes and former surveys the deposit can be traced as a near-continuous sheet, several metres thick, overlying Triassic bedrock. Elevations on the base of the deposit increase southwards from about 65 m OD. Its top surface, where temporarily exposed, shows evidence of disturbance caused by emplacement of the overlying Thrussington Till (Rice, 1968). The deposits were formerly quarried near Stocking Farm [SK 5775 0707], where Browne (1902) recorded about 6 m of red sand. Farther south, at the former Leicester Abbey sand pit [SK 5772 0622], previous work summarised by Rice (1968, p.465) suggests the presence of at least 11 ft (3.3 m) of pink, coal-bearing sand showing foreset bedding indicative of currents flowing towards the north-north-east. The description by Deeley (1886), probably at this locality, indicates that over a depth of about 1 m immediately beneath the Thrussington Till the sand has lost its cross-bedded character; this perhaps reflects disturbance associated with cryoturbation. The base of the Bytham Sand and Gravels hereabouts occurs just below 70 m OD.

On the right bank of the Soar, south of Leicester city centre, outcrops equated with the Bytham Formation include that at Aylestone. In this former pit [SK 5750 0055] a basal gravel with abundant quartzite pebbles and Lower Jurassic shell fragments was overlain by at least 4.3 m of red, cross-bedded sand with coaly laminae (Rice, 1968 and references therein). At just over 70 m OD, the base of these deposits is slightly higher than those encountered farther north, at the Abbey sand pit (see above). Farther south, at the former Blaby brick-pit [SP 563 987], Rice (1968) described a complex sequence in which typically red, coaly, cross-bedded sand is interleaved in complex fashion with Thrussington Till and rafts of Mercia Mudstone. This exposure may represent inclusions of Bytham deposits, or meltwater deposits of the Thrussington ice sheet, which have been subjected to glaciotectonic deformation and transport; they are shown as Glaciofluvial Deposits on the map of the district.

Brooksby sand and gravel

In Borehole (SK61NE/21), located just to the south of the modern Wreake valley, 7.8 m of deposits representing the Brooksby sand and gravel was penetrated below 60 m OD. The Brooksby sand and gravel, which lies beneath the basal part of the Bytham Formation (Figure 9a), contains two organic-rich layers in that borehole. The material investigated from these layers initially suggested relatively mild climatic conditions (Rice, 1991, table 1), and subsequent studies have confirmed this (Coope, 2006; Stephens et al., 2008). A survey of the deposits in several exploration boreholes in the vicinity of the Rearsby Brook valley suggested to Brandon (1999) that the Brooksby sand and gravel represents deposits within a palaeochannel of the Bytham river system that is about 6 m deep and 150 to 300 m wide, and has the morphology of an overdeepened meander scour (Figure 9a).

Glacial deposits (Wolston Formation)

The nomenclature of the glacial deposits has been formalised (McMillan et al., 2005), and the various components are now members of the Wolston Formation, which in turn belongs to the Albion Glacigenic Group of pre-Ipswichian age. The deposits were accumulated during the Middle Pliestocene glaciation, which, as recognised by Deeley (1886), involved two principal ice sheets. A trans-Pennine ice sheet initially covered the district from the north-west and was followed by an ice sheet originating to the east (locally north-east) of the district. These distinct but probably closely contemporaneous ice advances correspond to deposition of the Thrussington Till Member (trans-Pennine sheet) and, from the eastern ice sheet, the ‘Lias-rich’ Till Member and the Oadby Till Member. The tills all represent a complex association of lodgement facies and associated meltout deposits. There is ample evidence for this sequence from both within and outside the district in the form of superimposition of the glacigenic suites, but there are also instances where two or more till varieties are interleaved, or are thoroughly mixed. Ice striae and erratic orientations from this and adjacent districts confirm that the Thrussington ice originated in the Pennines farther north and advanced in a south-south-easterly direction, whereas the Oadby ice originated in the east and travelled towards the west-south-west. The ‘Lias-rich till’, intercalated between the Thrussington and Oadby tills, was first recognised in the Melton Mowbray district (Carney et al., 2004), and from its erratic content appears to be largely of north-easterly derivation. Of the three tills, the Oadby Till has the widest distribution, occurring throughout the Leicester district, whereas the Thrussington Till is limited to central and western parts of the district by the rising Jurassic bedrock surface (Figure 8).

The absence of any intervening interglacial sediments supports the current consensus that the Thrussington and Oadby glaciations both date from the same severely cold stage (MIS 12; Bowen, 1999) and their deposits are thus thought to be of Anglian age (Sumbler, 1983; Bowen, 1999). Previously, Shotton (1953) had ascribed them to a considerably later (post-Hoxnian-pre-Ipswichian) glacial event-the ‘Wolstonian glaciation’. This is now considered unlikely, although Sumbler (1995, 2001) has suggested the possibility that the Oadby Till (and its equivalent in the Thames Valley, the Moreton Drift) could be of MIS 10 age, based on the chronology of certain terrace aggradations in the Thames and Severn catchments. As discussed by Keen (1999), this would effectively represent a second, ‘Wolstonian’ glaciation, post-dating that of the Anglian Stage as currently defined.

The present outcrop distribution suggests that a cover of glacigenic deposits was once ubiquitous, veiling the earlier landscape and, as the rockhead contour map (Figure 8) shows, forming a partial mantle to the pre-existing topography that is now much dissected by the development of the post-glacial drainage system. Within the preglacial Bytham palaeovalley, which is broadly coincident with the present Wreake valley in the north of the district (Figure 8) and (Figure 9), there is a well-differentiated stratiform glacigenic sequence (Rice, 1968), comparable with the ‘Wolston’ succession that was deposited in the continuation of the Bytham valley farther west in Warwickshire (Bridge et al., 1998). The following named components of the Wolston Formation are present, in order of superposition (McMillan , 2005):

• Oadby Till Member, locally with a basal
• Lias-rich Till (Oadby Till Member)
• Wigston Member (sand and gravel)
• Rotherby Clay Member
• Glen Parva Clay Member
• Thrussington Till Member

Elsewhere in the district the thickness of glacial deposits varies considerably from place to place. In part, this is due to downcutting at the base of the various till units, a process that has locally resulted in overdeepening to form networks of synglacial palaeovalleys. These palaeovalleys are particularly well defined in the lower lying ground of the western part of the district (Figure 8). There, they form sinuous, north-north-easterly or northerly trending features, the floors of which have locally been cut to below 50 m OD. The Narborough Palaeovalley (‘Narborough Furrow’ of Rice, 1981) is mainly composed of Chalky (Oadby) till according to Rice (1981), although red Thrussington Till is also present. Brown sand or clay, the latter possibly of glaciolacustrine type, are the other main constituents. This is shown by Borehole (SP59NW/19), in which almost 24 m of red, brown and grey sand with subordinate intercalations of silt and clay were recorded, resting on Mercia Mudstone at just over 44 m OD. Farther north, near Narborough, flinty sands and gravels form extensive spreads. On this outcrop, a borehole at Barton’s Nursery [SP 5535 9936] noted by Rice (ms fieldslip, 1962) proved 8 m of sand and gravel underlain by at least 4 m of sand. Only 50 m to the south-east, a further borehole proved at least 15 m of glaciofluvial deposits in which was intercalated a 10 m-thick layer of chalky (Oadby) till. In the axis of the sinuous Rothley Palaeovalley, at least 32 m of till was penetrated in Borehole (SK51SE/349), with no base reached at the level of final drilling (48.3 m OD); the sequence consisted of 21 m of Oadby Till underlain by at least 11 m of Thrussington Till. In a further borehole (SK50NE/35), located within the present valley of the Rothley Brook, 17 m of Thrussington Till was proved. The Cropston Palaeovalley detours round the western side of the Mountsorrel uplands and may join with the Rothley Palaeovalley farther north. The borehole at the Cropston sewage works (SK51SE/12) shows at least 20 m of palaeovalley fill, with no base reached. These deposits consisted of 10.2 m of Thrussington Till underlain by glaciolacustrine deposits of red-brown, laminated clay and silt with a 2.6 m-thick intercalation of red-brown glaciofluvial sand.

Thrussington Till Member (T)

The Thrussington Till corresponds to the ‘Early Pennine Boulder Clay’ of Deeley (1886), and was named as the ‘Older Boulder-clay’ by Fox-Strangways (1903). Its present name was given by Rice (1968), for the characteristically red-brown till that is of widespread occurrence at the base of the glacial sequence in the central and western parts of the district. In areas where Thrussington Till overlies deposits of the Bytham Formation the contact is sharp, although in detail somewhat irregular as a result of cryoturbation and/or glacitectonic deformation (Figure 9a). Towards the east of the district the elevation of the rockhead surface progressively rises and the Thrussington Till eventually pinches out, commonly being overstepped by younger glacial deposits such as the Oadby Till and Lias-rich till. (Figure 8) shows that the eastern limit of the till is sinuous where it either infills pre-existing valleys on the rockhead surface or was removed by erosion prior to, or during, emplacement of the Oadby Till.

Thrussington Till is seldom exposed, but the few sections available during this resurvey, and that by Rice (1968), show that it is a diamicton characterised by a matrix of brown to reddish brown, silty or sandy clay. Stones within the till include abundant ‘Bunter’ quartzite pebbles and grey-green dolomitic siltstone (‘skerry’) fragments, both of Triassic derivation, along with Carboniferous sandstone, chert, coal and limestone carried in from the Pennines region. Field brash on till outcrops extending southwards from Mountsorrel across Leicester commonly include pink or grey granodiorite, derived from the Mountsorrel Complex, as well as lithologies from Charnwood Forest. One notable occurrence, a block of Mountsorrel granodiorite weighing about 20 tons, was given the name ‘Humber Stone’ [SK 6242 0704] and is one of the many large boulders in the district listed by Fox-Strangways (1903, p.53) as glacial erratics. It was presumably carried south-eastwards from Mountsorrel during the Pennine ice advance, and is located on an outcrop of Thrussington Till.

Auger samples of typical Thrussington Till commonly show an upper, weathered layer, about 0.4 m thick, of yellow-brown, silty or sandy clay. This darkens progressively downwards into brown and red-brown clay with smears of red sand. The erratics include quartzose pebbles and fragments of green-grey Triassic siltstone.

In the central parts of the district, the junction between the Thrussington and overlying Oadby or Lias-rich tills is generally sharp and readily mappable, either by augering or by observations of sudden changes in brash type and soil colouring in ploughed fields, e.g. [SP 630 982]. Farther west, however, the glacigenic sequence is locally complex, as evidenced by the temporary sections formerly visible in the grounds of Leicester University [SK 595 030]. Here, West and Donner (1956) described, beneath Oadby Till, 3 m of red-brown Thrussington Till. In the latter, a preferred erratic orientation of 325° is consistent with a north-north-westerly derivation. Subsequent excavations here, (Rice, 1968), revealed that, at least locally, the Thrussington Till was of a hybrid type, with patches of blue clay and Liassic limestone fragments. Moreover, it included a thin (1 ft) [30 cm] layer of typical blue-grey, chalk and flint-rich Oadby Till. The nearby railway cutting [SK 5903 0254] exposed further sections in hybrid till with a red-brown and blue-grey mottled matrix and some Liassic clasts (Browne, 1893), prompting Rice (1968) to conclude that in some places there were no satisfactory lithological criteria for distinguishing between Thrussington and Oadby tills.

In a more recent borehole from the Leicester University campus (SK50SE/763) a varied sequence was revealed (samples taken below 6.7 m depth are held in the BGS borehole core collection). The log of this borehole, showing the stratigraphical position of the samples illustrated in (Plate 17a), (Plate 17b), (Plate 18a) and (Plate 18b), is as follows:

Features that suggest an ice-contact origin for this sequence are the repetition and the thinness of the till beds, and the development of stratification in till, which could be a result of deformation due to flowage. The tills are intercalated with sands of possible meltout origin, and also with laminated silts and clays that may represent glaciolacustrine deposits or their deformed equivalents. The whole sequence was possibly deposited close to the front of the Thrussington ice sheet, which subsequently overrode and tectonised it. Farther south, in the former Blaby brick-pit, complex relationships between Thrussington Till and glaciofluvial deposits are described below.

Across the Soar valley to the south-west, around Abbey Farm [SK 544 010], successions proved both by augering and by M1 Motorway site investigation boreholes show that the Thrussington Till contains large masses of grey, chalk- and flint-rich Oadby Till; the latter commonly changes over several metres into a hybrid, red-brown to grey till with Triassic as well as Cretaceous fragments. Nearby, in former excavations along the route of the M1 Motorway, Poole (1968) noted that there were patches of Oadby Till within Thrussington Till, and also described lateral and vertical gradations between both types of till. A section (Figure 10) exposed during the construction of the M1 Motorway near Enderby Grange (Poole, 1968), shows that at least some of the local complexities to the Quaternary stratigraphy are caused by glaciotectonic disturbance of the drift sequence. This evidently occurred along the western margin of the Narborough Palaeovalley (Figure 8), and it involved folding and probable interthrusting between and within the glacial deposits, and also possibly the Mercia Mudstone bedrock; this was followed by re-folding of the imbricated sequence (Figure 10). Poole (1968) suggested that this deformation indicated compression directed towards the south-west, probably at a time when a locally thickened glacial sequence was being overridden by the Oadby Till ice sheet.

The Thrussington Till thickens considerably west of the Soar valley, from a thin (less than 5 m), impersistent layer in the Stocking Farm area [SK 577 064] into the Rothley Palaeovalley (see above).

Biostratigraphy

Preliminary palynological investigations have been undertaken on three samples of the Thrussington Till Member in order to determine provenance trends. The results demonstrate that in the Leicester city area, at least, the proportion of ‘local’ Triassic, Rhaetian and Jurassic floras as opposed to far-travelled, Carboniferous forms decreases westwards. The southernmost sample of Thrussington Till, from a deposit overlying Lias Group bedrock east of Canal Street in Wigston [SP 5902 9811], contained a reworked palynoflora with a majority (83%) of palynomorphs of Late Triassic, Rhaetian age, together with about 5 per cent of Carboniferous (Namurian or Westphalian) spores, the remainder being of Quaternary derivation (Riding, 2004). The dominance of Rhaetian spores suggests at least in part derivation from either the Blue Anchor Formation, Penarth Group or ‘Pre-Planorbis’ beds of the lowermost Lias Group, all of which crop out close by. The absence of earlier Triassic forms is not surprising since palynomorphs are extremely rare in the continental-facies strata of the Mercia Mudstone Group. About 5 km farther north, a further till sample from the diverse sequence encountered in the borehole at Leicester University (see above) contained a markedly increased (40.9%) content of Carboniferous spores, 26.4 per cent of Jurassic (Toarcian and mid-late Jurassic) miospores and 6 per cent Triassic miospores and dinoflagellate cysts, the remainder not being diagnostic of age (Riding, 2005). The third sample, from Braunstone, west of the River Soar [SK 5478 0129], overlying Triassic bedrock, was remarkable in containing a palynoflora entirely comprised of Carboniferous (Westphalian) miospores, which could suggest that a major component of the (presumed) Mercia Mudstone source rocks represents oxidised, reworked Coal Measures mudrock.

Lias-rich Till Member (O(L))

A Lias-rich till was recognised by Fox-Strangways (1903), who termed it the ‘Older Boulder-clay (upper part)’, implying close correspondence with the Thrussington Till, which it commonly overlies. On the other hand, Rice (1968) called it the ‘Lower Oadby Till’, in recognition of the fact that, unlike the Thrussington Till, it is apparently devoid of Triassic or Carboniferous material. The till lithology is indicated mainly by field brash or augering as there are few exposures. Typically, it consists of a brown to grey, somewhat silty matrix enclosing erratics dominated by Jurassic limestone, Marlstone and ironstone, together with quartzite pebbles derived from the Triassic Sherwood Sandstone Group. Chalk is typically absent, as is flint, but in places it may occur in trace amounts as very small granules.

The nature of the erratic assemblage, dominated as it is by Jurassic rock fragments and Rhaetian palynomorphs (see below), suggests that this till was derived from ice originating in the north-east, which travelled along the outcrop of the Penarth Group, Lower Lias and Inferior Oolite strata.

Lias-rich till is particularly thick in the north, between Gaddesby and Queniborough, as shown by the borehole (SK61SE/3) at Barrowcliffe Farm, in which 19 m of ‘brown silty clay with stones’ underlies about 3 m of chalky (Oadby) till and, in turn, rests on 3.7 m of sand and gravel. Near Gaddesby, mapping suggests that the Lias Till locally penetrates downwards into the Thrussington Till [SK 684 118] and includes masses of the latter [SK 5850 1335]. Southwards the till is only patchily represented beyond Barkby Thorpe [SK 646 096]; it was noted by Fox-Strangways (1903, fig. 7) in cryoturbated contact with bedrock in a former section at Spinney Hills [SK 602 045], and Rice (1968) saw it at the former Aylestone sandpits [SK 5750 0055]; (see also below) and as far south as Blaby brickpit [SP 563 987]. During the present survey a lens of Lias-rich till was augered near Oadby [SP 6195 9980] and, outside the district, at Countesthorpe [SP 570 956].

Biostratigraphy

A palynological study was carried out on an auger sample south of Seagrave Road [SK 6390 1635], about 500 m north of the district margin. This sample contained an abundant residue of wood fragments and palynomorphs. The latter (Riding, 2004) included about 3 per cent of Carboniferous (Namurian or Westphalian) spores but the great majority (82%) consisted of a ‘Penarth Group’ assemblage that included spores, pollen and dinoflagellate cysts diagnostic of a Late Triassic, Rhaetian age, together with taxa such as Classopollis meyeriana that range throughout the Triassic and Early Jurassic.

Oadby Till Member (OD)

Rice (1968) named this unit for the deposits formerly referred to as ‘Chalky Boulder-clay’ by Fox-Strangways (1903) and ‘Great Chalky Boulder-clay’ by Deeley (1886). It overlies bedrock or earlier Quaternary glacigenic deposits, and is particularly extensive on interfluve ridges. The Oadby Till gives rise to yellowish brown, clayey soils strewn with abundant angular, granule to boulder-size fragments of flint, Jurassic limestone, ironstone and rounded quartz pebbles; Gryphaea shells are particularly conspicuous. Chalk, being prone to dissolution, is not common as fragments in soil except where the till has been excavated or deeply ploughed. Where Oadby Till is augered, the upper leached part is a yellow or yellowish brown clay, somewhat sandy and with small flint fragments but no chalk. Chalk fragments do not occur in abundance until about 0.4 to 0.6 m lower down, their appearance coinciding with a change to pale grey or medium grey, rather less decalcified clay. In some areas a flinty remanié deposit, accompanied by very thin veneers or pockets of yellow clayey sand, is indicative of a former Oadby Till cover. The matrix of fresh Oadby Till is grey to blue-grey, although red or red-brown patches and mottles may occur where mixing has taken place with the Thrussington Till, giving rise to the hybrid till varieties described above as well as a Triassic and Lias-rich Oadby Till variant north of Twyford [SK 727 112]; similar mixed tills were noted farther north-west by Carney et al. (2001).

Oadby Till is more widespread in the district than was previously thought. For example, near Great Glen [SP 657 984] it is exposed at stream level in a meander bank of the River Sence, where Lias mudstone was previously mapped. It is commonly the only till present over much of the central and eastern parts, where rockhead elevations are highest. There it mantles an irregular bedrock topography and oversteps earlier Quaternary deposits preserved in the lower lying parts. Such relationships are also seen west of Mountsorrel, where Thrussington Till is intermittently developed in topographical ‘lows’, for example west of Swithland Reservoir [SK 554 144] and at Rothley [SK 569 129], and is overstepped by Oadby Till that mantles a rising bedrock surface on the Mountsorrel Complex. The culmination of this trend is seen on the southern face of Buddon Wood Quarry [SK 565 147], where about 15 m of Oadby Till is exposed within a palaeovalley (Plate 19a). There, it rests with sharp contact upon Mercia Mudstone, which itself occupies a Triassic palaeovalley that was partially re-excavated by ice before or during infill by the till. In this exposure, the till (Plate 19b) has a dark blue-grey, clay-rich matrix enclosing abundant boulders, cobbles and granule-size fragments of Chalk, flint, Jurassic limestone and calcareous mudstone; a calcareous microfossil assemblage was also recovered from a further occurrence in the eastern part of the quarry and is described below. Quartzose pebbles form a minor proportion of the till clast suite. They have an ultimate Triassic provenance, from the Sherwood Sandstone Group, but were probably recycled during earlier phases of the glaciation and may have been finally incorporated as the Oadby ice sheet overrode pre-existing glaciofluvial deposits.

Typical Oadby Till thicknesses average about 10 to 15 m. In the east of the district, many till outliers have a thickness in excess of 20 m, and a maximum-recorded thickness of around 35 m is attained to the north [SK 82 08] of Braunstone in Rutland. In that area, till mantles the lower slopes, but commonly has not been preserved on the steep upper slopes and tops of the more upstanding features: for example at Whatborough Hill [SK 767 059], which has an elevation of 230 m above OD. A minimum elevation of around 90 m for the till base occurs in the Eye Brook [SP 825 998]. At least 12 m of Oadby Till is present near the western margin of the district, proved in Borehole (SK50NE/358) on the plateau around Beaumont Leys.

In the south-west of the district, the base of the Oadby Till declines into the Soar valley, but is then caught up in large-scale disturbances of the glacigenic sequence. At the former Aylestone sand pit [SK 5750 0055], for example, descriptions by Deeley (1886) suggest that the Oadby Till was apparently ‘forced’ over the Thrussington Till, with both deposits being ‘contorted and ploughed up’. Around Braunstone [SK 554 014], elevations on the base of the Oadby Till decline to less than 60 m and the deposit is cut through by the floodplain of the River Soar. Between there and Enderby Warren [SK 563 000], masses of Oadby Till occur interleaved with Thrussington Till and glaciofluvial deposits [SK 545 008]. These complex relationships, described above (Figure 10), are at least in part due to the glaciotectonic disturbance of a thick Quaternary sequence that was perhaps confined within a palaeovalley. A Motorway borehole (SK50SW/52) indicates at least 15 m of brown-grey till with chalk, although other boreholes close by also record masses of red-brown till within the grey variety. Farther north, on the floor of the Rothley Brook, boreholes e.g. (SK50NW/33) commonly show ‘dark grey boulder clay’ (?Oadby Till) stratigraphically below Thrussington Till mapped along the valley sides.

Biostratigraphy

Calcareous microfossils were recovered from the Oadby Till Member in an exposure near the top of the eastern face of Buddon Wood Quarry [SK 5656 1524]. The microfauna (Wilkinson and Riding, 2006) comprised frequent early Jurassic foraminifera and common early Jurassic ostracods, the latter indicative of Hettangian (late P. planorbis to S. angulata) macrofaunal zones. The rare late Cretaceous foraminifera comprise generally long-ranging species, but no older than ‘mid’ Coniacian Foraminiferal Zone BGS14 (M. coranguinum macrofaunal Zone) and no younger than the Santonian part of BGS18 (U. socialis macrofaunal zone). The provenance of the chalk clasts is considered to be the Upper Burnham to lower Flamborough chalk formations of eastern and north-eastern England (Lincolnshire and Yorkshire) or upper Lamplugh-lower Jukes formations (of the southern North Sea Basin) and coeval with the Seaford Chalk of southern England. The palynoflora includes Jurassic and Late Cretaceous palynomorphs in keeping with the calcareous microfossil fauna, but also a significant proportion of Rhaetian and Carboniferous forms, the latter two inferred to have been worked in during passage of the Oadby ice across Penarth Group and Thrussington Till outcrops respectively (Wilkinson and Riding, 2005).

Rotherby Clay Member (Rot)

The Rotherby Clay, as first named by Rice (1968), corresponds to the ‘Loam (Laminated Clay and sand)’ division on the 1976 edition of the geological Sheet 156 Leicester. It was considered part of the ‘Older Boulder-clay’ category of Lamplugh et al. (1909), in the adjacent Melton Mowbray district. The deposit forms narrow, discontinuous outcrops in the north, along the various tributary valleys of the River Wreake, and is probably 3 to 7 m thick on average. It consists of clay and silt that accumulated when glacial or subglacial water was ponded within the Bytham palaeovalley. Rotherby Clay occupies a consistent stratigraphical position, between Thrussington Till and the Wigston Member, and in this respect at least it is analogous to the Wolston Clay, which was deposited within the former ‘Lake Harrison’ farther west in Warwickshire (Bridge et al., 1998). Both these clay sequences may therefore belong to the same phase of glacial or subglacial ponding of meltwaters in this region.

The Rotherby Clay Member was formerly worked at the Rotherby brick-pit [SK 679 163], 350 m north of the Leicester district margin. It was described by Lamplugh et al. (1909), partly from drilling investigations, as comprising 6.4 m of ‘laminated brick-clay or sandy loam of a reddish colour with a pebble here and there’.

Augering of the deposit near Spinney Farm [SK 6357 1577] showed blue-grey, stoneless silty clay with pale grey carbonate nodules, or ‘race’, becoming sandy close to the junction with the overlying Wigston Member. On the south side of the Wreake valley, a borehole (SK61NE/153) on the Rotherby Clay outcrop showed 4 m of yellow-brown, sandy silt becoming pebbly 0.7 m lower down and underlain by Thrussington Till. Other boreholes in the vicinity indicate that the deposit is lithologically complex, including grey silty clay with sporadic pebbles (probably dropstones), but in places dominated by sand and pebbly sand. The westernmost outcrop of Rotherby Clay [SK 6170 1455] rests directly on Mercia Mudstone Group.

Glen Parva Clay Member (GP)

South of the main Rotherby Clay occurrence there are further outcrops of grey to red-brown clay or silty clay, which correspond to the ‘Glen Parva clays’ of Rice (1968). These deposits are typically interleaved with glaciofluvial deposits and/or till and, unlike the Rotherby Clay, they do not maintain a definite stratigraphical horizon, appearing instead to represent sporadic developments of the glaciolacustrine facies within the more complex glacial sequence that typifies the south-western part of the district.

Farther south, Glen Parva Clay crops out near Eyres Monsall as a lens interleaved within Thrussington Till. Observations in deep trenches [SP 5735 9925] by Rice (ms fieldslip, 1962) indicated mainly mottled red and blue clay with only sporadic clasts (‘Bunter’ pebbles, Triassic siltstone and Lias Group limestone) and intercalated, discontinuous layers of red sand up to 1.8 m thick. A borehole (SP59NE/8) through this outcrop demonstrated at least 6.1 m of clay, mainly consisting of ‘hard red marl’ but with ‘pockets’ of sand and, near the top, a layer of sandy clay capped with sand that is 1.5 m in aggregate thickness. There is some lithological variability, however, with a further borehole (SP59NE/6) showing the same sandy layer underlain by ‘mainly blue’ clay. Farther east [SP 5880 9915], trenches available to Rice (ms fieldslip, 1962) showed red, purple and blue clay with very few pebbles, those present being of Triassic derivation. These clays, admixed with lenses of Thrussington Till, were again seen by Rice following landsliding in a railway cutting farther north [SP 5928 9994]. Former trenches in Wigston Magna [SP 6070 9820], examined by Rice (ms fieldslip, 1962) showed laminated blue-brown clay which was, however, associated with the chalky, Oadby Till rather than with Thrussington Till.

A continuation of the Glen Parva Clay has been mapped southwards across the River Sence valley; it lies in a similar stratigraphical position to the Wigston sequence, although at a slightly lower topographical elevation. Red, grey or brown stoneless clay, inferred to be of glaciolacustrine origin, was formerly seen or augered by J Rice (ms fieldslip) and was confirmed by augering during the present survey. In the outcrop [SP 585 969] east of Blaby this clay is interleaved with flint-rich glaciofluvial deposits which, occurring between the Thrussington and Oadby tills, are tentatively equated with the Wigston Member. A borehole (SP59NE/17) showed 6.1 m of ‘brick clay’, which is a common driller’s term for what is now mapped as glaciolacustrine material.

Glaciolacustrine deposits (undifferentiated)

Small developments of stoneless clay of similar appearance to the Rotherby or Glen Parva clays are known from temporary excavations and boreholes. Along the floor of the Rothley Brook, west of Beaumont Leys, boreholes encountered a particularly thick glacial sequence in the Rothley Palaeovalley (Figure 8), below Thrussington Till mapped along the valley sides. Many boreholes through this sequence showed till intercalated with glaciolacustrine clay and sand. For example, beneath the floor of the brook, borehole (SK50NW/33) more than 8 m of dark grey (?Oadby) till rests on 2.1 m of ‘stiff grey brown laminated silty clay’, which is in turn underlain by sand to the bottom of the hole. This thickened Quaternary sequence can be traced as far north as Cropston sewage works, where a borehole (SK51SE/12) shows, below Thrussington Till, at least 10 m of laminated red to grey glaciolacustrine clay interbedded with red-brown glaciofluvial sand and silt.

Around Leicester University, glaciolacustrine clay is mapped as a discontinuous layer, interleaved with Thrussington Till, which was formerly exposed in the railway cutting [SK 5895 4245] farther south. The cutting showed a lens (up to 2.4 m thick) of laminated clay with a few stones, sandwiched between till (Fox-Strangways, 1903, fig. 8). Nearby in temporary excavations at the University, Rice (1968, p.472) described at least 2 ft [60 cm] of purplish red clay and silt, underlying the Thrussington Till. This sequence has been proved in more recent boreholes (e.g. (SK50SE/763), see above for log), which showed a more complex succession in which four thin beds of laminated clay and silt are intercalated with Thrussington Till and glaciofluvial sands above Mercia Mudstone bedrock. Westwards, across the Soar valley, small developments of glaciolacustrine deposits are mapped in the Braunstone Park area [SK 560 032], either below Thrussington Till or between the latter and the Oadby Till.

Farther south, in the former M1 Motorway cutting at Enderby Grange, Poole (1968) described up to 2 ft [60 cm] of laminated clay, with sandy layers, between Oadby Till above and Thrussington Till below. A microfossil assemblage was recovered from here, as described below.

In the east of the district, five outcrops of undifferentiated glaciolacustrine deposits were mapped south and north-east of Owston. All are thin (maximum 5 m); they underlie glaciofluvial deposits and rest on Oadby Till. They consist of laminated chocolate-brown clay with minor silt and sand, and are interpreted as forming in small, localised proglacial ponds in front of the retreating Oadby Till ice sheet.

Biostratigraphy

A sample of clay (EGP8955), (Figure 10) collected by Poole from a glaciotectonised zone in the Enderby Grange cutting was checked for calcareous microfossils by Wilkinson (2003b), who found reworked forams and ostracods of Rhaetic to early Jurassic and late Cretaceous (Lower and Middle Campanian) age. In addition, however, there occurred Quaternary foraminifera indicative of cold, brackish water to brackish marine environments; no freshwater species were encountered. The reworked Quaternary fauna could suggest derivation from the matrix of Oadby Till that had incorporated sediment from the North Sea littoral zone, prior to transportation across eastern England.

Wigston Member (Wi)

The Wigston Member (Rice, 1968), comprises a semi-continuous sequence of glaciofluvial sand and gravel that is stratigraphically positioned above Thrussington Till or Rotherby Clay, and below the Oadby Till. Elsewhere, other sand and gravel deposits found at this datum, but which lack significant stratigraphical continuity, are considered within the ‘undifferentiated’ category of glaciofluvial deposits (see below). The Wigston Member is invariably flint-rich and may represent proglacial or subglacial outwash of the Oadby ice sheet into what remained of the Bytham valley following the Thrussington Till glaciation and subsequent Rotherby Clay lacustrine environment.

In the Wreake valley area as far west as Ratcliffe on the Wreake, the unit maps out as a near-continuous sheet up to about 7 m thick (Brandon, 1999; Carney, 2000c). The few former exposures described by Deeley (1886) and Lamplugh et al. (1909) indicated that its base is gradational with the Rotherby Clay but its upper contact is a sharp junction with the Oadby Till. There are few remaining exposures; however, to the north of the Wreake valley the deposit can locally be traced in the field, where it gives rise to a small scarp and ledge feature. Samples obtained by auger consist of red, grey or yellow sand with common to abundant quartz and quartzite pebbles of Triassic derivation, and numerous flints. An exposure by the side of a former sand pit at Spinney Farm [SK 6349 1570] showed red, medium-grained, cross-bedded sand with foreset inclinations indicative of a current direction towards the north-east. Close by [SK 635 155] a further pit is now completely restored but originally showed 4.5 m of red, coaly, cross-bedded sand, locally contorted and with lenses of Oadby and Thrussington Till (Rice, 1968).

Farther south, the Wigston Member re-appears as discontinuous sheets or lenses, up to several metres thick around and to the south of Barkby Thorpe [SK 622 087]. Just to the north of the latter point, the new mapping indicates a complex sequence with intercalations of Thrussington Till. Around Evington, e.g. [SK 625 032] the unit becomes more discontinuous and rather than being a sheet-like body, may represent sand-filled channels cut into the Thrussington Till. Farther east, around Thurnby [SK 657 036] a body of the Wigston Member appears from mapping to transgress the Oadby-Thrussington junction and becomes wholly intercalated within the Oadby Till, a possible example of localised glacitectonic disturbance. In the Oadby and Wigston area, e.g. [SP 6055 9900], the Wigston Member maps out as part of a complex sequence associated with till and glaciolacustrine deposits. In an excavation near the Memorial Park [SP 6054 9888], Rice (ms fieldslip, 1962) noted coarse, yellow, cross-bedded coaly sand resting on flint- and chalk-rich gravel; intercalated with the deposit was a lens of chalk-rich till.

South of the River Sence extensive sandy spreads have been mapped. These are part of a deposit that occurs mainly between the Thrussington and Oadby tills, and is interleaved with glaciolacustrine deposits (see above); they are tentatively equated with the Wigston Member. Augering and debris from animal burrows, e.g. [SP 5865 9725] indicate that it consists of yellow, grey and red sand and gravel with abundant quartzose pebbles and flint.

Glaciofluvial deposits (undifferentiated)

Patchy but locally very thick occurrences of sand and gravel have been mapped throughout the district. Their field relations and constituent clasts indicate that they represent glaciofluvial detritus washed out during both the Thrussington Till (generally red, non-flinty sands and gravels) and the Oadby Till (flint- and locally chalk-rich sands and gravels) ice-sheet regimes. Flint-rich glaciofluvial deposits include those identified as the ‘Chalky gravel’ by Deeley (1886) and Fox-Strangways (1903). They commonly occur along the junction between the Thrussington and Oadby tills, or the Lias-rich Till, e.g. west of Beaumont Leys [SK 5525 0775], in a similar stratigraphical position to that occupied by the Wigston Member. Sheet-like deposits of non-flinty, red or orange sand, up to several metres thick, can be traced along the till contact for some distance east of Queniborough, e.g. [SK 660 115]; [SK 690 123]. Impersistent, lensoid bodies of sand and gravel can occur in any part of a till deposit, as indicated by an exposure at Great Dalby [SK 7411 1424] showing a lens of fine-grained, orange sand intercalated within till that is typically of ‘Oadby’ type, but which also contains a raft of red-brown ‘hybrid’ till with coal in addition to chalk and flint fragments.

Deposits of yellow or orange flinty sand and gravel have been mapped around Kings Norton [SK 690 003] and Skeffington [SK 735 025], the latter forming a series of disconnected outcrops as far as Rolleston [SP 720 000] and northwards at least to Tilton on the Hill [SK 740 060], where 14 m of sand and gravel was recorded above bedrock in a borehole near Tilton Church. In a disused quarry [SK 7415 0607] about 200 m north of Tilton, 3.5 m of this deposit is exposed. It consists (Plate 20) of pale brown, fine- to coarse-grained gravel with a very poorly sorted sand matrix. The clast suite, measuring up to boulder size (0.6 m), consists of Jurassic limestones, ironstones, various fossils, and chalk. There is a seam of locally cross-laminated sand, and discontinuous layers of silt occur at the west end of the quarry. The beds in the deposit dip to the north-west, at up to 50°, and were described by Deeley (1886) as being vertical to locally overturned. Such structure suggests glaciotectonic disturbance, either during or soon after deposition of the sequence. In broad outline, the Tilton-Skeffington deposit represents a north-south-trending body, which locally rests on Oadby Till but is also channellised through it down to the bedrock, as seen to the south-west of Skeffington [SK 734 020]. Farther south, a complex of smaller bodies of flinty sand, with channel-like forms, and with bases cutting down southwards through the Oadby Till, is centred on Shangton Grange [SP 722 973].

In the south-western part of the district, glaciofluvial deposits form part of a lithologically variable drift stratigraphy, some of which accumulated within the Narborough Palaeovalley (see above). In the former Blaby brick-pit [SP 563 987], a highly contorted sequence was described (Rice, 1969), in which red, cross-bedded sands with a high content of coal, and gravel lenses containing Triassic and Liassic detritus, showed chaotic folding and were intricately interbedded with Thrussington Till. Rice considered these features to indicate the contemporaneous deposition of till and glaciofluvial material, perhaps in a proglacial (ice-contact) environment, followed by the wholesale slumping of beds around bodies of melting stagnant ice. Glaciotectonic deformation, recorded farther west in M1 Motorway cuttings (Figure 10), is described above. To the east of the Blaby brick-pit, in the former Blaby Wharf sand pit [SP 5700 9869], Deeley (1886) described 21 feet [6.4 m] of sand, which contains abundant chalk and flint; cross-bedding indicated currents from the east and west.

At Leicester University, up to 5.75 m of non-pebbly sand was proved below Thrussington Till and above glaciolacustrine clay in a recently drilled borehole, (SK50SE/763), see above.

In the east of the district, there are a number of generally small outcrops of undifferentiated glaciofluvial deposits. Most overlie the Oadby Till and are interpreted as outwash sheet deposits. Some smaller outcrops occur within the Oadby Till and may represent englacial sediments or sandy and gravelly pockets of till. The most extensive, at Cold Overton [SK 810 102], is 5 to 10 m thick and composed of orange-brown, fine- to coarse-grained, poorly sorted sand with some flint and quartzite pebbles. Two outcrops east of Belton-in-Rutland show a pronounced south-westerly dipping base, possibly suggesting channellisation; one deposit [SK 835 004] underlies the Oadby Till. Sands and gravels with flint, chalk and Jurassic limestone fragments are typical of the eastern parts of the district, where they are locally thick and commonly have steep dips, reflecting glaciotectonic disturbances. One such sequence, from a sand and gravel bed within Oadby Till in a pit south of Owston [SK 7761 0697] was illustrated by Fox-Strangways (1903, fig. 11) and is 5 to 10 m thick. An exposure of glaciofluvial sand and gravel observed during the present resurvey at Gaulby Lodge Farm [SK 694 998] consisted of 1.5 m of cross-bedded, pale brown sand and gravel with pebble beds. The foreset beds are apparently oversteepened, being inclined at about 40° due east. An unusual feature of the south-eastern part of the district is the occurrence of small outcrops of coarse, poorly sorted gravels with abundant cobbles of chalk as well as flint. Some of these bodies are elongated, with bases cutting downwards into the Oadby Till, e.g. [SP 7135 9705]. They may represent the fills of deeply incised, ephemeral meltwater channels in which the chalk erratic component was concentrated and preserved from subsequent dissolution.

River Terrace deposits (Soar Valley Formation)

The post-glacial deposits generally comprise sand-rich, matrix-supported, trough cross-bedded gravels rich in quartzose pebbles and flint. It has been proposed (McMillan, 2005) that these deposits should be formalised as members within the Soar Valley Formation of the ‘Trent Catchment Subgroup’. River terrace deposits reflect late- to post-glacial river basin initiation and later incision associated with the development of the Soar, Wreake and Sence catchment systems. They form parallel, sheet-like spreads of sand and gravel, rarely more than 5 m thick, and are mapped not only on lithological grounds but also by recognition of their terrace morphology. Although these terraces appear to be composed of flat-topped landforms, in detail they are extremely low relief, planar surfaces that slope gently inwards towards the axis of the main valley. Terrace formation coincided with periods when deposition outpaced erosion, and because of the repeated cycles of fluvial incision and periglaciation the preservation potential of a given terrace deposit is inversely proportional to its age. In order of decreasing age (and topographic elevation), the named terrace deposits shown on the map comprise: the Knighton, Birstall, Wanlip, Syston and Hemington members. Terrace nomenclature and chronology is summarised in (Table 3) and is largely based on the names proposed by Rice (1968), with minor modifications by Brandon (1999). It should be noted that the Quorndon Terrace of Rice (1968) is equivalent to the Syston Member of this account. The correlation and tentative assignment of the terraces to various marine oxygen isotope stages (Table 3) suggests that most were formed under periglacial climatic regimes and are probably the deposits of braided floodplains that filled most of the valley floor.

Altimetric information and constructed terrace long profiles (Figure 11) have greatly assisted correlations of the more isolated terrace deposits within the Soar valley. These data indicate between 3 and 7 m of incision between the various terrace aggradations, with a greater height differential between the Knighton-Birstall and Birstall-Wanlip terraces than between the latter and the Syston Terrace, as was also demonstrated by Rice (1968, fig. 13). For the Wreake terraces, the average thalweg separation calculated by Brandon (1999) was about 7 m. Stone clasts in terrace deposits derive from reworking of the older Superficial Deposits and bedrock within the river catchment and the proportions of the main constituents vary between the valleys, although ‘Bunter’ pebbles and shattered flints are always prevalent, generally in similar proportions. Later head accumulation and cryoplanation typically modify the terrace form, the degree of cryogenic involution generally increasing with the age of the deposit.

The Knighton Member (Kn) represents remnants of the oldest and highest terrace. The unit was identified in the Melton Mowbray district, to the north (Carney et al., 2004), where it was named in anticipation that it would correspond with the sandy deposits that Rice (1968, fig. 12) found distributed across the ‘Knighton Surface’, a landform identified near Barkby Thorpe, around Victoria Park, Knighton, and south of Mountsorrel. These areas are primarily till-covered, and no significant sand and gravel deposits have been identified during this re-survey. However, better examples of the Knighton terrace were found closer to the Soar valley, to the north-east and south-east of Rothley [SK 5915 1400]; [SK 5945 1130]. Flinty gravels at the former location are at an elevation of about 60 m as compared with the adjacent Soar floodplain that lies at just over 45 m OD (Figure 11). The Knighton terrace was identified farther north at Barrow on Soar (Melton Mowbray district), and its gravelly deposits were possibly the source of a complete skeleton of the straight-tusked elephant ‘Elephas antiques Falconer’ (Plant, 1859; Carney et al., 2004).

Outcrops of the Birstall Member (Bir) have been mapped at various points along the Soar valley, as far south as Braunstone [SK 572 020]. The latter outcrop lies on the reconstructed long profile of the Birstall terrace (Figure 11), and was noted by Rice (ms fieldslip) as featuring extensive spreads of flinty sand and gravel. On the opposite (east) side of the Soar, the small outcrop centred around Friar Lane [SK 5855 0425] was mainly inferred from topography and the occurrence there of sand and gravel. Terraces formed by the deposit are extensively developed along the north side of the Queniborough Brook [SK 647 135], which is a tributary of the Wreake.

The Wanlip Member (Wan) is particularly widespread, with numerous terrace remnants occurring in the district. These are situated on rocksteps, and the associated terrace surface is about 4 to 5 m above the alluvium of the Soar and Wreake floodplains (Figure 11). The Wanlip terrace is most extensively developed at the confluence of the Soar and Wreake south of Cossington. Here, a borehole (SK61SW/46) proved 4 m of this deposit, consisting mainly of pale brown sand with a layer 0.7 m thick of brown to grey-green sandy clay. A borehole (SK50SE/289) at St Peter’s Lane in central Leicester, proved the base at 5.4 m depth; this is possibly not a true thickness, however, as it is measured from a ground level that may have been raised by artificial deposits.

The Syston Member (Sys) is the ‘floodplain terrace’, confined as it is within the Soar, Wreake and Sence valley floors. In this situation, it forms degraded terraces with surfaces protruding 1 to 2 m above the surrounding alluvium, which has thus aggraded onto the terrace remnants, the base-levels of which may in places lie up to 4 m below the modern floodplain surface (Figure 11). The associated terrace features commonly back on to bedrock or older Quaternary deposits forming the edge of the floodplain, but as noted by Rice (1968), the Syston terrace deposits are never separated from the floodplain alluvium by a bedrock step. The member probably comprises much of the lower part of the coarse deposits that are typically found beneath the clayey and silty alluvium of the Soar and Wreake valleys. Subsurface provings in the Soar valley suggest that a thickness of between 2.5 and 4 m is typical (Figure 11), but locally where the deposit is channellised, the latter value may be exceeded.

Radiocarbon dating and faunal evidence, reviewed below, suggest that at least in part, the Syston Member was deposited either just before or at the onset of the Late Devensian (Weichselian) Last Glacial Maximum, an intensely cold period which is considered to have lasted for some time between 35 000 and 15 000 years BP in eastern England (Briant et al., 2004). Subsequently, Early Holocene incision by anastomosing and chute channels would have dissected this deposit into low terraces and isolated eyots above the alluvium, as is the case in the adjacent Melton Mowbray district (Carney et al., 2004). On the Soar floodplain north of the Leicester city area, this topography has been largely destroyed by the extensive sand and gravel workings on the floodplain.

In the former Pontylue gravel pit at Syston [SK 610 110], Bell et al. (1972) described a 3.5 m section of the Syston Member resting on bedrock. Gravel showing a concentration of ice-wedge casts passes down into braid plain deposits that contain organic layers in the lowest metre. Material recovered from the organic layers yielded a radiocarbon date of 37 420 (±1670) years BP, suggesting deposition of the overlying gravels during the mid-Weichselian cold climate. From the same location, insect, bivalve and plant remains were listed, together with mammalian remains, collected earlier in 1967. The latter were of cold-stage aspect and included Mammuthus primigenius Blum (Woolly mammoth) and Rangifer tarandus Linn (Reindeer). Just to the north, Bell et al. (1972) additionally reported Coelodonta antiquitatus Blum (Woolly rhinoceros), probably from below a cover of Soar floodplain alluvium in the Wanlip gravel pits [SK 601 116], and Megaloceras giganteus Blum (Giant deer) from a similar level at the sewage-disposal works [SK 600 113]. Further finds of Mammuthus primigenius were made at the former ‘Sydney Street’ pit, around [SK 5976 0561] off Belgrave Road, and at Wood Street, around [SK 5891 0512] (Browne, 1889). It is noteworthy that the Mammuthus remains documented from Abbey Meadow, around [SK 589 061] by Browne (1889) come from a depth of 2 to 3 m beneath the surface of the Soar floodplain, again providing evidence that the Syston Member extensively underlies that feature.

Syston gravels with syndepositional ice wedge casts (noted by A Brandon, BGS, 1993) were formerly exposed in a gravel pit [SK 555 182] 2 km north of the Leicester district margin. This working revealed lenses of organic silt with pollen remains indicating a glacial or early Lateglacial environment of deposition (Brown et al., 1994). A ¹⁴C age determination on material from a further organic layer, just above bedrock, gave an older limiting value of about 28 000 years BP for deposition of these sands and gravels, within the Late Glacial Maximum period discussed above.

In the northern part of the Soar floodplain [SK 597 147] and bordering the Sence floodplain [SP 577 878], the Holocene (or Recent) ‘floodplain alluvium’ has been subdivided into Hemington Member (He) and Alluvium (see below). The Hemington Member typically forms broadly convex, subdued features rising to about 0.5 m above the alluvium. Ridge and furrow cultivation commonly marks such ground, indicating that it was beyond the range of low-magnitude, seasonal flooding in historical times. Both deposits are similar, however, and it is probable that the member simply represents positive topographical elements of the alluvium, such as levees and point-bars.

Hemington terrace deposits and alluvium both comprise a complex of deposits, consisting of grey and brown mottled clayey silt, 2 to 3 m thick on average, interspersed with sand and gravel point-bar deposits. This is underlain by up to 5 m of sand and gravel, probably representing buried continuations of the Syston Member (see above). The silty to clayey soils capping the Hemington deposits give rise to marshy, poorly drained conditions in contrast to the light, sandy soils of the older terraces. The Hemington Member probably occurs along the Wreake valley but, since the distinction with the modern alluvium is not clear in many places, on the published map they are depicted as ‘Alluvium, undivided’. The same is true along the upper reaches of many smaller tributary valleys where the narrow tracts of alluvium commonly interdigitate with, or pass beneath, valley-fill mass wasting deposits. One valley in which the distinction between the Hemington deposits and alluvium is particularly well marked (though not shown on the published map) is that of Whissendine Brook, in the extreme north-west of the district, around [SK 830 158].

River Terrace Deposits, undifferentiated constitute the minor occurrences of terraced sands and gravels along various tributary streams. They are generally too isolated to be classified with the named river terrace deposits, although they are probably all of Devensian age. Some minor patches of terrace deposits are numbered in order to differentiate aggradations of different ages, although the numbering implies no correlation between different valleys.

In the east of the district, at Whissendine [SK 829 148], a series of First Terrace outcrops occur along on Whissendine Brook. Since all are composed of clay, and lie about 1 m above the modern alluvium, they may correlate with Hemington Terrace Deposits (above). Much of the valley floor occupied by the Queniborough Brook, e.g. [SK 676 126] consists of clays and silts belonging to the First Terrace. These deposits are locally sharply incised by the modern river channel, which occupies a relatively narrow, meandering floodplain. Farther west, in the Rothley Brook valley [SK 0820 5500], First Terrace deposits may correlate with the Syston Member.

Peat

Peat is a minor component of the Holocene floodplain alluvium of the River Soar (see below). It is mappable only in the south-western corner of the sheet, where the main outcrop is centred on the Narborough Bog SSSI [SP 549 979]. According to website information from the Leicestershire and Rutland Wildlife Trust (www.lrwt.org.uk), the peat formed about 6000 years ago and is 1.8 m deep; its pollen assemblage is described by Brown (1999). The bog is the remnant of what originally was a larger area of peat marshland, much of which has since been drained for agricultural and flood defence purposes. Its original extent southwards, to beyond the M1 Motorway, is suggested by augering and boreholes, one of which (SP59NW/4) shows an upper 2.8 m thick alluvial layer consisting of 0.3 m of ‘peaty topsoil’ underlain by 2.8 m of interbedded brown, grey and black organic-rich clay and silt.

Alluvium

The Holocene (or Recent) ‘floodplain alluvium’ comprises the deposits formed by flooding and meander migration of the main channel in the various streams and major river systems of the district. Topography on the alluvium is generally of the order of 1 to 1.5 m vertical height and is expressed as gentle slopes or undulations, many of which are developed on fluvial features such as levees, backswamps and point bars. The Soar floodplain alluvium typically consists of a basal point bar or channel gravel overlain by overbank-facies deposits of grey and brown mottled clayey silt. For example, on the floodplain west of Syston a borehole (SK61SW/19) recorded a sequence consisting of yellow-brown silty clay (1 m) underlain by a further metre thickness of yellow-brown sandy and silt clay with gravel lenses, this in turn overlying a basal 2 m-thick layer of medium to coarse sand passing down to flinty gravel. The basal sands and gravels commonly rest directly on bedrock. They may be part of the alluvial sequence, but in the major trunk rivers, where they are up to 5 m thick and contain cold-stage mammalian faunas (see above), they are clearly suballuvial deposits representing an earlier valley fill of Syston Member.

Two quarry sites on the Soar floodplain, showing profiles through predominantly fine-grade alluvial sequences above suballuvial gravels, were fully described and geochronologically dated by Brown et al. (1994). At the first site, near Cossington [SK 596 137], organic material in clayey silt alluvium occupying a palaeochannel indicates that the present floodplain regime dates back to at least 10 200 years ago. At a second site, farther south near Birstall [SK 598 084], organic material from the alluvium layer ranged in age from about 5160 to about 3500 years BP.

A partial exposure of floodplain alluvium open at the time of the resurvey was in a working gravel pit [SK 5965 1420] on the Soar floodplain opposite Mountsorrel; it showed that the upper layer of dark brown, silty, blocky to laminated clay was about 3 m thick, and was separated from the underlying gravel by 0.3 m of dark grey to black, organic-rich clay. Organic-rich lenses in floodplain alluvium could reflect the courses of former abandoned channels or widespread marshlands similar to the Narborough Bog (above). For example, many boreholes through the Soar floodplain in central Leicester, opposite Leicester University, record an upper ‘black clay’ layer less than 1 m thick. Borehole (SK50SE/518) from here showed an upper alluvial layer, 3.2 m thick, composed of blue-grey to brown silty clay with shells underlain by similar sediment containing peaty inclusions. This layer becomes increasingly sandy downwards and eventually passes into gravel at least 0.8 m thick at the base of the borehole.

Borehole (SK61SW/66) indicates the typical alluvial sequence of the Wreake floodplain. It shows an upper component of green-brown to grey silty clay 2.5 m thick, becoming gravelly in the lower 0.5 m and underlain by 1.5 m of gravel.

Alluvium of the minor tributary valleys is seldom more than 3 m thick and is generally composed of yellow, brown or grey, clayey silt or organic-rich silt. Thin lenticles or more continuous bodies of coarse, flint-rich gravel are common at or near to the base of such deposits, but are generally only thinly developed.

Slope terrace deposits

Slope terrace deposits were first distinguished during the survey of the Melton Mowbray district, to the north (Brandon, 1999; Carney et al., 2004). All such deposits consist of head, the origin of which is discussed by Brandon (1999). Slope terrace deposits comprise locally extensive deposits of head occurring in the Stapleford clay vale, in the north-east of the district. This mudstone vale has developed since Middle Pleistocene times, through successive episodes of periglacial slope wasting during the cold stages, and the formation of planoconcave solifluction terraces. These slope terrace deposits were subsequently incised by fluvial action during the warm interglacial stages and this incision, induced by regional uplift, has led to continual base level lowering throughout the Soar–Wreake catchment and hinterland. Consequently, there is a close correspondence between the solifluction terrace staircase and the fluvial terrace staircase of the adjacent river valley (Table 4).

Subsurface investigations carried out on these deposits in the Vale of Belvoir are summarised by Carney et al. (2004). The Slope Terrace Deposits there comprise two or three layers, each about 1 m thick. The lower layer consists of brecciated grey mudstone and grey clay, which overlies undisturbed, grey, Lower Jurassic mudstone. The middle layer rests on a basal shear plane and comprises slickensided and sheared grey clay. It is overlain by a layer of sand and gravel containing abundant oxidised siderite mudstone fragments; this upper layer is commonly cryoturbated with, and thus appears part of, the middle slickensided layer.

Two phases of the Slope Terrace Deposits have been recognised and mapped in the Stapleford clay vale in the north-east of the district. The oldest of these is the Little Dalby Head (LD), which is a terraced deposit about 2 m thick that locally forms a near-continuous flat [SK 78 14] at 100 to 105 above OD, its surface inclined gently to the north (Ambrose, 2001). This deposit is probably equivalent to the Pen Hill head of the Vale of Belvoir (Carney et al., 2004) and is thus of ‘Wolstonian’ age. It is not exposed, but surface debris and augering show a brown to grey, sandy and pebbly clay with common flint, quartzite, ironstone and limestone pebbles. The occurrence of flints sets this deposit aside from the Lias-rich Till, which it otherwise resembles. The colluvial deposits of adjacent valleys mostly merge down-slope with the Little Dalby Head, rather than being incised into it.

The Burton Lazars Head (BL) of the Stapleford vale is equivalent to the Harby Head of the Vale of Belvoir, and is of ‘Wolstonian’ age. It lies at 95 to 100 m above OD, forming gently north-sloping terraces with surfaces about 2 m lower than the older Little Dalby Head (Ambrose, 2001). It is of similar thickness and composition to the latter deposit, and probably incorporates at least some material eroded from it.

Head, undifferentiated

Head sensu stricto is a periglacial deposit formed during the various cold stages that have affected the district (Table 3). It is of extremely widespread occurrence, can be as much as 3 m thick, and is important because of its commonly hazardous geotechnical properties (Chapter 13), although it can be difficult to differentiate from other diamictons such as till. The undifferentiated head mapped in the Leicester district accumulated as a result of several closely related processes and it underlies extensive ‘solifluction terrace’ aprons on the flanks of many valleys. Although most of the surviving head is Late Devensian in age, it was a product of all stadial periods and older head (not generally mapped separately) can be identified by the cumulative effects of periglacial processes during later stadials. Related periglacial mass-wasting features observed in the district include cambering (Chapter 10).

Head deposits similar to those of the Leicester district have sometimes been referred to as ‘colluvium’, the definition of which, however, differs between authors. Bates and Jackson (1980) used ‘colluvium’ as a general term applied to any loose heterogeneous and incoherent mass of soil material and/or rock fragments deposited by rain-wash, sheet wash, or slow continuous down slope creep, usually collecting at the base of gentle slopes or hillsides. Turner (1996) added that the term ‘colluvium’ is generally used to refer to weakly stratified, diamicton-like deposits that have been transported by gravity and which can give rise to lobate topographic patterns reflecting slope instability and solifluction.

The composition of head mainly depends on the nature of the source bedrock and/or superficial deposits up slope. It generally consists of a grey to brown silty clay diamicton with a variable content of dispersed stones. Sandy head is widespread below outcrops of Glaciofluvial Deposits or Bytham Formation, giving rise to sloping ground veneered by sandy, flinty and pebbly soils, an example of this occurring on the outskirts of Birstall [SK 593 101]. Such material may represent hill-wash rather than soliflucted material (see below). Sandy and silty head partially filling wide topographic depressions is typified by the outcrop on either side of Barkby Lane [SK 624 103]. A borehole (SK61SW/62) through this deposit indicates 1.1 m of brown sandy clay underlain by 0.5 m of sand.

Valley deposits

Valley deposits form a partial infill to many small valleys of the district, giving rise to the flat floors, and are consequently mapped as predominantly narrow outcrops. They are mostly a Holocene accumulation of surface hill wash into valleys, but may also contain layers of soliflucted material and alluvium, the latter representing periods of fluvial activity on the valley floor. Such material can locally be up to a few metres thick but is commonly less than this. It is generally sandier and lighter in texture than head, and may also contain layers of fluvial sand or peat where it occurs in the valley-floor; mostly it consists of brown, silty or sandy clay with rock fragments.

Landslide deposits

Slope instability is inherent in the district along many of the steeper valley sides and larger cuesta escarpments formed by argillaceous bedrock or superficial deposits. The resulting landslides mainly originate in mudstone or clay strata containing thin permeable beds and are marked by hummocky landforms. They are readily identified both on the ground and on aerial photographs, and in many respects are similar to the landslides of the adjacent Melton Mowbray district, (Carney et al., 2004). The geohazard potential of such landslipped ground is discussed in Chapter 13.

Landsliding is a characteristic feature of oversteepened slopes that form the embayed Marlstone Rock escarpment overlooking the west of the district, although some parts of it appear to be unaffected. Large-scale complex landsliding involving rotational movement in the bedrock has usually occurred in association with spring lines in the Dyrham and Marlstone Rock formations, but has also affected the upper part of the Charmouth Mudstone and certain outcrops of Superficial Deposits. The toes of the landslides are commonly difficult to delineate where there is a downslope gradation into mudflows. The latter can be detected by the development of 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 0621]. Smaller-scale rotational landsliding can occur along the steep sides of minor valleys, where it commonly affects Dyrham Formation strata immediately below the junction with the Marlstone Rock Formation [SP 7778 9765].

On slopes formed by the Charmouth Mudstone Formation, south-westwards from Debdale Spinney [SK 720 126], hummocky ground and pronounced backscars, some of very recent origin, are well developed in many places (Ambrose, 2001). Many of the landslides are associated with springs issuing either from the base of the Marlstone or within and at the base of the Dyrham Formation. The angle of the affected escarpment slopes ranges up to about 21° at the steepest, the slips at Debdale Spinney being probably caused by oversteepening of the mudstone slopes, which here are inclined at 14 to 16°. In places, the toe of the landslide is poorly defined, and it may grade into mudflows that extended well beyond the natural break of slope at the base of the escarpment, which has been taken for convenience as the limit of the slipped deposits. Other landslides on oversteepend clay/mudstone slopes capped by till occur east of Horninghold [SP 826 961] and south of Allexton [SP 811 996].

The landslide deposits are generally an admixture of rocks derived from whichever part of the local bedrock or Quaternary succession has been affected. Debris can include ironstone from the Marlstone Rock Formation, sandstone and siltstone from the Dyrham Formation and mudstone from the Charmouth Mudstone, and debris- or mud-flow deposits (breccias and diamictons) are commonly seen. A ditch exposure [SK 7587 1213] to [SK 7576 1193] on the north-west flank of Burrough Hill, sited 100 to 130 m below the top of the escarpment, showed up to 1 m of grey and ochreous clay with common fragments of iron-oolite derived from the Marlstone Rock Formation.

Landsliding is common on oversteepened slopes of the Whitby Mudstone Formation immediately below the Northampton Sand outcrop, as at Whatborough Hill [SK 77 06] where the natural slope angle is around 10º. Similar slope angles are present in the extreme east of the district, on west-facing slopes below the Northampton Sand outcrop, east of Belton-in-Rutland [SK 828 021], where pronounced roational slip features are developed. The north facing escarpment of this outcrop unusually appears to not be slipped; however, much of it is under arable cultivation and the slip features may have been smoothed over by this process.

South-west of Whatborough, Colborough Hill [SK 760 050] is entirely landslipped, apart from a very small area at the summit. The landsliding has occurred within the Whitby Mudstone Formation, producing unstable slopes which maintain an angle of around 10º. Near the eastern sheet margin, close to Ridlington, slopes [SK 8266 0228] developed on the Whitby Mudstone show a hummocky topography with masses of the Northampton Sand Formation representing material introduced from higher up, possibly by a combination of landsliding and cambering under periglacial conditions.

Landslides are locally present along the ‘Rhaetic’ (Penarth Group) escarpment in the adjacent Melton Mowbray district (Carney et al., 2004) but, perhaps owing to the extensive mantle of Quaternary deposits, have not been found in this situation in the Leicester district.

Chapter 9 Metamorphism

Two metamorphic events are recognised in the ‘basement’ (pre-Carboniferous) rocks of the district. The second of these, involving the imprint of a penetrative cleavage, is also of considerable relevance to later structural events, as discussed in Chapter 10.

Ordovician thermal metamorphism

Thermal metamorphism of mudrocks presumed equivalent to the Stockingford Shale Group (Chapter 3) is the earliest recognisable metamorphic event in the district. It dates to Caradoc times, and is seen at three localities within the contact aureole on the western flank of the Mountsorrel Complex.

Hornfels from the ‘old gravel pit’ [SK 5583 1556] consists of metasedimentary rock with nodular structure (Plate 4a); a thin section (E72434) shows triple-point intersections of the boundaries between quartz and feldspar detrital grains, indicative of annealing recrystallisation. The original mud component is recrystallised to interlocking aggregates of colourless mica, less than 1 mm across, and larger sieve-textured biotite plates have grown across these in places. Tentative identification suggest that garnet in granular masses, locally stained red and altered to white mica, forms part of the nodular aggregates, and also occurs as small equant crystals. Quartz aggregates, euhedral Fe-Ti oxides, white mica of probable secondary origin, coarse laths of biotite and sporadic chlorite are the other minerals. Cordierite (see also below) was noted at this locality by J Phemister (quoted in Watts, 1947, p.76), but could not be confirmed in this thin section. Prominent pseudomorphs, some with lath-like or hexagonal to rectangular outlines, are infilled with a felt of mica and may represent an original aluminosilicate.

A further exposure of hornfels (Chapter 3), along the top of the flooded quarry on the western side of Swithland Reservoir [SK 5567 1355], is in grey, recrystallised mudrock. A thin section (E71073) shows it consists mainly of an equigranular, fine- to medium-grained aggregate of quartz, white mica, biotite, carbonate and pale yellow-green epidote. The abundance of epidote and Fe-Ti oxides in this sample is in keeping with the chemical analysis that showed 11.62% combined FeO and Fe₂O₃ (Hill and Bonney, 1878). A thin section (E73867) of hornfels with a millimetre-scale foliation at this locality shows oriented laths of coarse, pale pink-yellow mica and a pale brown, epidote-like mineral.

A small, natural exposure of grey, thinly bedded hornfelsed metasedimentary rock occurs near the reservoir waterline by the Kinchley shore [SK 5599 1397]. In a thin section (E74301) the rock is devoid of foliation, and has an assemblage dominated by interlocking plates of pale brown biotite, larger laths of white mica, Fe-Ti oxides and granular quartz, the last occasionally aggregated and showing triple-point grain boundaries. Studded throughout are small pinnite pseudomorphs, many with pseudohexagonal outlines, interpreted as the sites of original cordierite. An equally small, relict high-relief mineral with parallel extinction and grey interference colours, altering to a pale yellow serpentinous mineral along fractures (Plate 7b), may be the orthopyroxene, enstatite (oral communication, R J Merriman, BGS, 2002). K-feldspar was not identified, but may have been pseudomorphed by secondary retrogressive minerals (see below).

These hornfels uniquely demonstrate a prograde metamorphic mineral assemblage formed at the time of emplacement of the Mountsorrel Complex (Chapter 4). Further analytical and microprobe work would be required to confirm the mineral assemblages; however, this preliminary petrographical study suggests that the rocks at the ‘old gravel pit’ and western Swithland shore probably belong to the hornblende-hornfels facies of contact metamorphism discussed in Winkler (1979, p.58), and indicate temperatures of about 600°C (R J Merriman, written communication, 2004). On the other hand, the hornfels on the Kinchley shore, with cordierite and possible enstatite is suggestive of a higher grade - the ‘pyroxene hornfels’ or K feldspar-cordierite hornfels facies, the development of which is normally restricted to the innermost zone of a contact aureole (Winkler, 1979, p.58), and would imply burial depths in excess of 6 km (Winkler, 1979, p.101).

A second metamorphic assemblage in these rocks is of retrograde origin, dominated by the pseudomorphing of some earlier prograde minerals, and the formation of mica, epidote, chlorite and serpentinous minerals. It evidently post-dated the emplacement of the granite, since feldspars in a granitic veinlet in the ‘old gravel pit’ exposure (E72434) are similarly retrogressed. R J Merriman (written communication, 2004) notes that much of this mica replacement may be due to rehydration of the hornfels relatively soon after the dehydration reactions caused by contact metamorphism. These retrogressive minerals may in part have been modified by subsequent end-Silurian deformation (see below), causing the crystallisation of aligned mica and epidote parallel to the weak cleavage developed in the hornfels seen on the western shore of the Swithland Reservoir. There is, however, no obvious penetrative cleavage fabric at the ‘old gravel Pit’ and Kinchley shore localities (Chapter 3).

Latest Silurian regional metamorphism

Regional metamorphism of latest Silurian age appropriate to low, high and very high epizonal (metapelitic) white mica crystallinity grades is widely distributed in mudrocks of the Charnian Supergroup and Swithland Formation (Merriman and Kemp, 1997) and is considered to be co-eval with the formation of a cleavage in those rocks (see next section). By contrast, in the Leicester Forest East Borehole, just beyond the eastern margin of this district, mudrocks of the Stockingford Shale Group showed much lower, diagenetic white mica crystallinity values (Pharaoh et al., 1987a). Diagenetic to anchizonal grades of metamorphism were also predicted by Pharaoh et al. (1987a) for the equivalents of these strata proved in deep boreholes beneath Leicester (Chapter 3), and this is borne out by the very weak tectonic imprint seen in these samples, relative to the highly penetrative cleavage of the Charnian Supergroup and Swithland Formation. Thus, within the context of a single tectonic event, it would appear that the latter two rock masses represent crust that was cleaved and metamorphosed at deeper levels than the surrounding Stockingford Shale Group. Such a large metamorphic discordance suggests significant tectonic displacements within the basement in the west of the district. Preliminary argon-series isotope measurements (Chapter 10) suggest that in the Charnian Supergroup and Swithland Formation this metamorphism, and the accompanying cleavage fabric, was imprinted during latest Silurian times.

Chapter 10 Structure

The structures of the district can be divided into those related to Palaeozoic deformation, and the relatively minor faults, flexures or folds that have been mapped in areas of Triassic or Jurassic outcrop. Major Palaeozoic structures commonly either coincide with or are defined by geophysical lineaments and boundaries of regional importance (Lee et al., 1990), as shown by the aeromagnetic and gravity maps discussed in Chapter 11. Some of these also displace Carboniferous and Mesozoic strata, indicating that they have been ‘posthumously’ rejuvenated at various times since their formation (Turner, 1949).

End-Silurian deformation

A penetrative cleavage is widespread throughout the Precambrian and Lower Cambrian rocks of Charnwood Forest, including those exposed in the extreme west of this district. In mud- or silt-grade rocks it is a pervasive, closely spaced (less than 1 mm), east-south-easterly trending fabric. The cleavage cuts obliquely (transects) the axis of the south-east-plunging anticline (Figure 12) that affects these rocks (e.g. Worssam and Old, 1988, fig. 2), although both types of structure are considered to have formed at the same time. The cleavage transection geometry is demonstrated in 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 has long been controversial, because cleaved Charnian rocks have shown a confusing spread of radiometric (K-Ar) dates, from Precambrian to Carboniferous times (Meneisy and Miller, 1963). More recent work, however, has constrained the age of the cleavage by utilising the more precise Ar40 /Ar39 techniques on fabric-forming mica from a variety of Charnwood rocks that include the Swithland Formation. The results (Carney et al., 2008) suggest a very strong late Caledonian imprint, at between 425 and 416 Ma, or Late Silurian on the timescale of Gradstein et al. (2004). The causative plate tectonic event may therefore predate the Acadian Phase of the Caledonian orogeny, which elsewhere in southern Britain is regarded as mainly Devonian (Emsian) in age. Cleavage fabrics in the Stockingford Shale Group proved in deep boreholes around Leicester are by implication of the same age, although the metamorphic grade is lower (see below).

The main fracture and cleavage fabrics affecting hornfelsed equivalents of the Stockingford Shale Group at the three exposures close to the western margin of the Mountsorrel Complex have east-south-east orientations (Figure 3). These structures are therefore part of the same Silurian event described above. At the flooded quarry by the Swithland Reservoir shore west of Brazil Wood [SK 5567 1355], the millimetre-spaced, easterly trending penetrative cleavage (Chapter 3) is in part defined by the alignment of epidote and mica parallel to the axis of a small-amplitude isoclinal fold (Plate 4b) with a subhorizontal axis. Stockingford Shale strata proved in deep boreholes in Leicester show a much weaker fracture-cleavage type of fabric developed obliquely to an earlier bedding-parallel fabric that may have been imposed during burial (Chapter 3).

Igneous rocks of the Mountsorrel Complex resisted the penetrative effects of the Silurian deformation, but are widely affected by metre-scale, spaced fracture systems. In the Buddon Wood Quarry, prominent fracture belts, many showing slickenfibre development, have easterly orientations (Figure 3) and may thus be related to the deformation. Such zones can be up to tens of metres in thickness, and within them the normally grey granodiorite becomes pink in colour, a feature similar to that described by King (1968) from Castle Hill Quarry. Other fractures have north-east and south-east orientations (Figure 3). Their age is uncertain, but one major fracture zone, with north-east orientation, predates the ?Carboniferous dolerite sheet exposed at the Castle Hill Quarry SSSI [SK 5759 1495].

Carboniferous deformation

Carboniferous faulting is well documented farther north, where it commenced in earliest Dinantian times with the formation of major asymmetric grabens. Their predominantly east-south-east orientation possibly reflects tectonic control related to the Silurian cleavage direction discussed above (Carney et al., 2004). The Sileby Fault (Figure 12) had an aggregate northerly downthrow of about 1750 m during syn-Dinantian crustal extension, confining sedimentation to the Hathern Shelf, an asymmetric half-graben located on the hanging-wall side of the fault (Ebdon et al., 1990). The fault, which was reactivated with reversed throw during the end-Carboniferous Variscan inversion event (Fraser and Gawthorpe, 2003), delineates the northern edge of the Wales-London-Brabant Massif. The massif remained as a relatively stable, uplifted tectonic domain throughout Carboniferous times.

The possibility that the basalt dykes in Buddon Wood and Castle Hill quarries are Carboniferous in age (Chapter 5) is based in part on their petrography. Fractures related to the main dyke are host to an unusual style of bitumen mineralisation (Chapter 13), which could have resulted from basin fluids circulating along dilational structures, either before or after dyke emplacement, that opened during the various Carboniferous (Variscan) tectonic movements. The dyke is offset by later, cross-cutting fractures (Figure 3), one of which, seen in the western face of Buddon Wood Quarry, has a shallow north-easterly dip and a reverse sense of throw (top to south-west), perhaps suggestive of Variscan compression.

The basic dyke exposed farther east, at Castle Hill Quarry [SK 5759 1496], shows a very similar trend and thus may be a continuation of the main dyke seen in Buddon Wood Quarry. Lowe (1926) noted the (presumably same) dyke in Cocklow Wood Quarry, as discussed in Chapter 4, but it was not recorded in Hawcliff Quarry, which was open at the time of his survey. Thus the dyke could have been displaced by a fault between Cocklow and Hawcliff quarries. Assuming that the smaller dyke in Buddon Wood Quarry is a minor offshoot, one possible solution, shown in (Figure 3), is that the main dyke has been displaced by a north-easterly trending strike-slip fault with a dextral throw of about 200 m. Such a fault, if extrapolated south-westwards, would displace the inferred western margin of the Mountsorrel Complex. Farther north, another dextral fault, with north-westerly trend, is inferred to displace the dyke across the central part of Buddon wood Quarry, and to cause a further displacement to the western intrusive margin of the host granodiorite.

The age of this faulting is uncertain. Aeromagnetic evidence, discussed below, suggests that the inferred north-east-trending dextral fault may be geometrically related to the margins of the anomalies defining the Kirby Lane and Rempstone Ordovician intrusions. This suggests an early Palaeozoic inheritance, but the fact that a possible Carboniferous dyke is displaced may indicate a later, Variscan rejuvenation along the same line.

Post-Jurassic deformation

End-Triassic earth movements, possibly at the time of the Rhaetian marine transgression, are suggested by the northwards thinning of the Penarth Group and corresponding erosional truncation of the upper part of the Cotham Member, prior to Lias Group deposition (Chapter 6). For convenience, however, all of the surface bedrock structures of the district are regarded as having last been active in post-Jurassic times, when the district to the south of the Sileby Fault occupied the Midland Platform tectonic province of Green et al. (2001). The principal post-Jurassic structure is the regional east to south-easterly dip of about 0.5° seen in the Mesozoic strata, and demonstrated by the structure contour map for the Marlstone Rock Formation (Figure 7). There are, however, local modifications to this structure caused by flexuring associated with faulting.

The Sileby Fault has an estimated post-Jurassic, northerly downthrow of about 130 m in the north of the district (Carney et al., 2004). Although mapped at the surface as a broadly arcuate structure, geophysical evidence discussed in Chapter 11 suggests that at depth it may be rooted within intersecting west-north-west and west-south-west linear basement structures.

Faults in the west of the district are principally distinguished on the basis of field and borehole evidence, which taken together suggest displacements of the Hollygate Sandstone Member. Three north-west-trending faults, each with a throw of about 10 to 15 m to the north, are responsible for the preservation of the Hollygate Sandstone within narrow, fault-bounded outcrops. The northernmost of these faults brings in the Cropwell Bishop Formation, approximately 10 to 15 m of which lie above the Hollygate Sandstone in the Abbey Park area [SK 586 057]. A basement control over this faulting is suggested by geophysical compilations (e.g. Worssam and Old, 1988, fig. 32) showing that these three faults lie along the southerly extrapolation of a major linear north-west-trending aeromagnetic anomaly coincident with the outcrop of a late Precambrian intrusion, the South Charnwood Diorites (see also, Chapter 11). A further north-west fault, throwing southwards, in the Evington-Spinney Hills area [SK 610 040] is inferred on more speculative grounds, from an apparent displacement of the top of the Sherwood Sandstone in deep boreholes; it may be associated with a southwards down flexure suggested by south-easterly dips of 3 to 5° recorded in the two former limestone quarries at Evington. The persistence of a north-westerly structural ‘grain’ north of Evington is suggested by the markedly linear trend of many east-bank tributaries of the River Soar. Exposures were generally too poor to confirm that these valleys follow faults, although a possible south-throwing fault is mapped along the valley to the north of Barkby [SK 645 105].

The outcrop of the Hollygate Sandstone to the north of the Wreake, and its absence at surface farther south, is attributed to the action of a southwards-throwing downfault trending north-eastwards along the course of the Wreake valley. This fault may continue south-westwards, confining the Hollygate Sandstone and overlying Cropwell Bishop Formation to the east in the Glenfields area [SK 5519 0600]. A north-north-east-trending fault, coincident with the Soar valley and extending northwards into central Leicester, is also required to account for the apparent lowering of strata on the east bank of the Soar, compared with the west bank where older strata such as the Hollygate Sandstone crop out, e.g. [SK 572 0040]. This fault is depicted on the map as being displaced by the north-westerly faults referred to above, although the reverse case is equally possible. The geophysical summary map (Figure 15) supports the presence of this fault, which may be rooted within a north-north-easterly basement lineament dividing the magnetic anomalies produced by the Croft and Countesthorpe intrusions, M4 and M3 on (Figure 15).

In the central and eastern parts of the district, the effects of faulting and flexuring are largely masked by poor exposure. Little evidence could be found for the course of the ‘Loddington Fault’ as proposed by Fox-Strangways (1903). A north-throwing fault may, however, occupy the course of the Eye Brook, which has a linear north-westerly orientation. Evidence for a structure in that vicinity is suggested by ironstone exploration boreholes around Tilton Grange [SK 755 043], which show a steepening of dip of the Marlstone Rock Formation from 0.5° to the south-east to 4° south as the valley is approached, although cambering may have played a part in causing this. Farther west, the north-easterly fault trend is again seen between Rolleston [SK 740 004] and Billesdon [SK 727 033], and is in part supported by ironstone exploration drilling at the latter locality and around Green Lane [SK 730 044]. East-north-easterly faulting north of Hallaton, in the east of the district [SP 794 979], has been detected by apparent displacements of the Marlstone Rock outcrop.

There is no direct evidence for the actual age of this post-Jurassic deformation. Data based on apatite fission track analysis and vitrinite reflectance summarised by Green et al. (2001) nevertheless suggest that to the north of the Sileby Fault (i.e. within the East Midlands Shelf tectonic province), there were two well-defined uplift and erosion events dated at Early and Late Cainozoic. South of the fault these events were considerably less marked, producing only minor displacements such as those seen in the Leicester district.

Superficial structures caused by periglacial activity

Cambering typically occurs in association with resistant Jurassic strata that cap steeply sloping escarpments or valley sides. It is a pronounced feature of many Northampton Sand and some Marlstone outcrops, producing pseudo-dip slopes that either accentuate the regional easterly direction, or reverse it; for example around the outliers to the north [SK 789 030] and north-east [SK 799 025] of Loddington. The anomalous dip of the formation at the latter locality was remarked upon by Fox-Strangways (1903, p.42), but thought to have been of tectonic origin, caused by an east-south-east-trending fault. In the extreme east of the district [SK 827 023], dip-and-fault structures were also noted affecting the Northampton Sand. Complex fold structures in mudstones occupying valley bottoms are another manifestation of periglacial activity in the Melton Mowbray district (Carney et al., 2004); such ‘valley bulge’ features are also likely to be also present in the Leicester district.

Chapter 11 Geophysical information on the concealed geology

A map (Figure 12) showing the likely elements of the ‘concealed’ (i.e. pre-Carboniferous) basement rocks that underlie this district has been compiled from interpretations of gravity and aeromagnetic data, together with limited borehole information and seismic reflection profiling. The patterns of Bouguer gravity and aeromagnetic anomalies within and immediately adjacent to the district (Figure 13) and (Figure 14) result from the contrasts in physical properties (Table 4) between the various basement rock masses. The principal geophysical features are summarised and labelled on the composite anomaly interpretation map (Figure 15).

The Bouguer gravity anomaly map (Figure 13) features a deep gravity low, G1 on (Figure 15), which reaches a minimum of -13 mGal in the adjacent Melton Mowbray district. This Bouguer gravity low is coincident with the thick accumulations (up to 4.5 km) of Carboniferous strata within the Widmerpool Half-graben north of the district (e.g. Ebdon et al., 1990). The north-dipping plane of the main graben-bounding structure, the Normanton Hills Fault, is revealed by the steep Bouguer anomaly gradient (GL1). To the south, the Sileby Fault (Figure 12) has a lesser Carboniferous throw, of about 1.75 km down to the north. This fault is also part of GL1, and probably merges at depth with the Normanton Hills Fault. To the south-west it continues as lineament GL2, forming one of a series of lineaments defining the northern edge of the Charnwood Massif in the Loughborough district (Carney et al., 2001).

To the west an extensive gravity high (G2) coincides with outcropping Precambrian rocks of the Charnian Supergroup in the adjacent Coalville district. The pronounced, north-west-trending linear gravity feature (GL3) that flanks G2 forms part of a col dividing it from the ridge-like anomaly (G3) to the north-east. GL3 coincides with the Newtown Linford Fault in the Coalville district (Worssam and Old, 1988, fig. 2) and, together with the coincident linear aeromagnetic anomaly (M8), discussed below, forms a major basement feature that evidently controls the north-west-trending faults mapped in central Leicester (Figure 1); (Chapter 10). The gravity ridge (G3) also has Precambrian source rocks; it may be delimited by an inferred structure, the North East Charnwood Boundary Fault (Pharaoh and Carney, 2000), which represents part of the north-eastern margin of a rigid tectonic block, the Midlands Microcraton.

The marked Bouguer gravity gradient (GL4) extending south-eastwards beneath the city of Leicester could represent a progressive deepening of Precambrian basement below weakly metamorphosed Ordovician (Tremadoc) mudrocks encountered, for example, in the Crown Hills Borehole at 235 m depth (Chapter 3). The north-east linearity defined by this gradient may be the expression of a basement structure that controlled the locations of the north-easterly fault systems parallel to the Soar and Wreake valleys (Figure 1). To the south and east, this Bouguer gravity gradient drops off rapidly to a low (G4) that reaches a minimum some 20 to 30 km south of the district. This gradient is a feature of regional significance seen clearly, for example, on the 1:1.5M scale gravity anomaly map of Great Britain (Smith and Edwards, 1997). To the north-east of the main gravity low is an area of generally higher Bouguer gravity values (G5), perhaps representing a shallowing or up-folding of Charnian-type basement; the transition with G4 is in part occupied by linear gradient zones GL5 and GL6. The north-west fault postulated along the course of the Eye Brook may be controlled by a basement fracture coincident with GL5. To the west, a further north-westerly trending lineament, GL7 (Figure 13) and (Figure 15), is coincident with the Thringstone Fault, a major Variscan reverse fault, with a probable earlier inheritance, that terminates the outcrop of the Precambrian Charnian rocks in the adjacent Coalville district (Worssam and Old, 1988, fig. 2).

Magnetic anomalies (Figure 14) across the northern part of the Leicester district form part of a prominent geophysical feature (Smith and Royles, 1998), 10 to 15 km wide on average, that extends for 125 km from Derby to St Ives (in Cambridgeshire). They are interpreted as the expressions of small batholiths lying at relatively shallow crustal depths (Busby et al., 1993). Their Ordovician age, confirmed by isotopic determinations on the Mountsorrel Complex (Chapter 3), indicates that these bodies constitute part of the Eastern Caledonides basement domain (see Chapter 4). Individual anomaly shapes suggest discrete plutons separated by cols of presumed early Palaeozoic basement strata (Figure 12). Alternatively, the cols may represent major crustal fault zones occupied by sheared and demagnetised intrusive rocks.

Anomaly M7 (Figure 15), in the adjacent Melton Mowbray district, reaches over 200 nT magnitude and extends for 10 km north-south. Its source is the Rempstone Granodiorite, inferred to be of Ordovician age, which was proved in the footwall of the Normanton Hills Fault during oil exploration drilling (Carney et al., 2004). A prominent col, occupied by the westerly extension of the Sileby Fault, separates this anomaly from M2, which is coincident with proven Ordovician plutonic rocks of the Mountsorrel Complex (Chapter 4). Farther east, anomaly M5, with its subsidiary anomaly M6, exceeds 250 nT. The source of this anomaly is the Melton Mowbray Granodiorite, proved in the Kirby Lane Borehole (Figure 12) 350 m north of the district margin (Carney et al., 2004). The north-western margin of this granodiorite is markedly linear, and may be controlled by north-east fractures parallel with the gravity feature GL4. A mapped fault, coincident with the Wreake valley, may be the surface expression of this north-east-trending structure. The belt of magnetic anomalies continues east and south-eastward with significant highs (M9 and M10). These anomalies correlate in part with residual gravity lows (Figure 13) and (Figure 15), suggesting that the causative bodies are plutonic rocks with relatively low densities similar to samples from the Melton Mowbray Granodiorite, which has values around 2.57 Mg m-3 (Table 4).

Modelling by Arter (1982) suggests that the Ordovician intrusions have the form of cupola-like bodies possibly extending down to 7 km, although their vertical depths are generally poorly constrained. On the other hand, basement reflectors on the CHARM II seismic profiles (Maguire, 1987), and others examined during this resurvey, could be interpreted (written communication, N J P Smith, BGS, 2005) to indicate that the Mountsorrel and Melton Mowbray series of intrusions have relatively flat-lying bases and form north-dipping, sheet-like bodies overlying a more obviously reflective possible-early Palaeozoic basement succession, the expression of which is better developed beyond the district, to the north of the Sileby Fault. The interpretation of sheeted bodies is also consistent with the aeromagnetic anomalies that abruptly terminate south and west of the Mountsorrel outcrop. In this north-western area, the patterns of negative magnetic anomalies or low-gradient cols (Figure 14); (Figure 16) may be explained by varying widths of the footwalls of structures such as the Sileby Fault, where no rocks of the intrusions are present.

Magnetic anomalies M3, 4 and 4a (Figure 15) are all sourced from plutons of the South Leicestershire Diorites. To the south of the Leicester district, M3 is a large, circular anomaly representing an intrusion of porphyritic tonalite, proved below Triassic strata in the Cottage Homes Borehole at Countesthorpe (Chapter 4). The exposed Croft quartz-diorite body is represented by M4 and the quartz-diorite formerly quarried at Enderby (Chapter 4) by the very subdued anomaly M4a. These three bodies could be part of a single zoned intrusion, as suggested by Le Bas (1972), in which case the compartmentation into separate anomalies (Figure 14); (Figure 16) could reflect dissection by crustal shear zones. Alternatively they may be discrete, structurally controlled plutons. Thus M4 and 4a are separated from M3 by the important geophysical lineament, GL4 (Figure 15).

The magnetic gradient map (Figure 16) shows that many of the larger plutons (M8, M7, M5/6, M9, M10) have steep sides but ‘flat’ tops (i.e. the central areas are lacking a strong magnetic gradient). By analogy with the anomaly coincident with the Mountsorrel Complex, M2 (Figure 15), this ‘flatness’ may represent bevelling of the intrusion by erosion surfaces, an interpretation illustrated by the sub-Triassic and sub-Carboniferous continuations to these plutons inferred in (Figure 12). It should be noted, however, that the limit of magnetic expression of the plutons probably occurs at crustal depths rather than being related to their subcrop. This is demonstrated by the Hall Farm Borehole, which was sited over strongly magnetic basement but proved (probable) early Palaeozoic mudrocks (Figure 12). The erosional truncation of at least some concealed plutons is demonstrated in the Kirby Lane Borehole, which shows that the top of the Melton Mowbray Granodiorite is weathered and overlain by strata of the Millstone Grit at 352 m depth (Chapter 4).

Anomaly M8, in the adjacent Coalville district, has a pronounced north-west-south-east linearity, parallel to the GL3 and G3 gravity features (Figure 15). This anomaly coincides with the outcrop of Precambrian intrusions of the South Charnwood Diorites (‘Markfieldite’). As discussed above, this complex linear zone may control the locations of the north-west-trending faults affecting Triassic strata in central Leicester.

The broad, low amplitude magnetic high (M1) occurring centrally in the Leicester district may indicate either the presence of a deeply buried intrusion, or a belt of more highly magnetised strata within the early Palaeozoic metasedimentary basement.

Limited seismic reflection surveys conducted by oil and coal exploration companies in the north of the district show the extent of Carboniferous (Namurian and Westphalian) strata, which thin and pinch out southwards (Figure 12). The results of smaller scale seismic refraction experiments carried out by Davies and Matthews (1966) west of Buddon Wood and Swithland Reservoir are summarised in the section on the Bromsgrove Sandstone Formation (Chapter 6). They show that no Carboniferous rocks are present beneath Triassic cover in that area, as confirmed by the Hall Farm Borehole, which penetrated possible early Palaeozoic mudrocks (Figure 12).

A seismic refraction survey, crossing the northern part of the district, included the basement outcrops of Charnwood Forest and Mountsorrel, and was tied in the east to the Kirby Lane Borehole (Arter, 1982). The interpreted profile, which is also shown in Evans and Allsop (1987, fig. 8) suggested that the Mountsorrel Complex was emplaced through a north-east-dipping surface composed of Precambrian (Charnian) basement with a velocity of 5.6 to 5.7 kms-1. An alternative proposal, that this intrusion is a sheeted body with an essentially concordant base, is discussed above. The refraction line that was chosen is strongly oblique to the gravity gradient GL2 on (Figure 15), and so the fate of the Charnian basement farther east is not known; it may pass beneath early Palaeozoic (Stockingford Shale) metasediments, or be structurally terminated against the latter, or against Carboniferous cover, by the North East Charnwood Boundary Fault, discussed above. The uncertainty is largely due to the critical area of the section being occupied by the Melton Mowbray Granodiorite, anomaly M6 on (Figure 15), and by the structures associated with gravity lineament GL4 discussed earlier. Whitcombe and Maguire (1981) interpreted a further east-west seismic refraction profile (the Eastern Seismic Line), which traversed the Melton Mowbray district just to the north of the line described by Arter (1982). They noted that the Charnian basement refractor was interrupted eastwards by a buried intrusion, corresponding to the Rempstone Granodiorite, M7 of (Figure 15). This granodiorite has seismic velocities of about 4.9 kms-1, similar to those for the Melton Mowbray Granodiorite characterised by Arter (1982). East of it, they interpreted the refractor to be a continuation of Charnian Precambrian basement, rather than being composed of early Palaeozoic strata.

Chapter 12 Artificially modified ground

Only selected categories of man-made deposits, or ground otherwise modified by human activity, are shown on Sheet 156 Leicester. Descriptions of the other categories are nevertheless included in this account since they have a bearing on the local ground conditions; they are shown in their full complexity on the digitised 1:10 000 Series map sheets upon which Sheet 156 is based. Artificial deposits are the legacy of a long history of human modification of the natural environment. They were delineated by recognition in the field and by compilations from documents that include: modern and archival topographical maps, archival geological fieldslips, aerial photographs and site investigation reports. In places where these data are absent, the boundaries shown may be imprecise.

Made ground

Made ground represents areas where material has been deposited on the natural ground surface. The main categories in this district include: industrial sites, road, railway and canal embankments, quarry spoil, building and demolition rubble, waste from light industries, domestic and other waste in raised landfill sites. Made ground is most extensive and thickly developed around and in the city of Leicester, where the industrial infrastructure of the district is concentrated; in this area a number of derelict industrial ‘brownfield’ sites exist. Parts of the River Soar floodplain that were extensively remodelled during canal construction and river channel straightening are also portrayed as areas of made ground. For the residential areas of Leicester city and the rural villages, however, made ground is generally not indicated on the geological maps since it is considered to be ubiquitous, although relatively thinly developed. It should be noted that on 1:10 000 scale maps patches of made ground will also be present within areas of Landscaped or Disturbed ground.

Worked ground

This represents areas where material is known to have been removed, for example in hard-rock or sand and gravel quarries, brick pits, ironstone quarries on the Marlstone Rock outcrop, and road, railway or canal cuttings. Although many quarries are regarded as voids, they will nevertheless contain a certain amount of made ground on their floors or sides, which cannot be indicated on the maps. In the case of gravel quarries that are now landscaped as wetlands, it has generally not been possible to ascertain whether islands were created by leaving mounds of unworked gravel, or by the introduction of made ground. Due to the difficulty of obtaining completion plans for such quarries, their boundaries should in most cases be considered as estimates based on survey, aerial photography, borehole evidence and local hearsay.

Infilled ground

This comprises areas where the natural ground surface has been removed and the void partly or wholly backfilled with man-made deposits. Mineral excavations for sand and gravel, brick clay, ironstone, limestone and disused railway cuttings are the principal repositories for the disposal of waste materials. The latter may include excavation and overburden waste, construction and demolition waste, domestic refuse and industrial waste. Where excavations have been restored and either landscaped or built on, no surface indication of the original void may remain and their delineation relies on the availability of archival sources, some of which may be inaccurate. During this re-survey incomplete documentation existed for some of the large, restored or partially restored gravel pits. During the field survey, it was noted that most of the restored gravel pits on the Soar and Wreake floodplains had their surfaces built up 1 to 2 m above the level of the surrounding alluvium.

Disturbed ground

Areas of disturbed ground are shown on the 1:10 000 series geological maps of this district, but for reasons of clarity they have been left off the 1:50 000 series geological map. It commonly encompasses ground that has experienced the effects of mineral extraction, but where it is impossible to accurately delineate separate areas of made, worked or infilled ground.

Landscaped ground

This category is 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. Landscaped areas on the maps generally denote constructional developments such as housing estates and medieval villages, and recreational sites such as playing fields or golf courses. Most residential areas are built on ground that has been remodelled by landscaping, but it is not convenient to depict this on the maps.

Chapter 13 Applied geology

This chapter provides a brief overview of the natural resource and earth science factors that should be taken into account during or before urban, industrial or rural planning and development processes. In the Leicester district (Sheet 156) there are some natural variations in ground conditions over small areas and the resultant diversity in geotechnical properties may be further enhanced within a single rock unit by considering the superimposed effects of weathering and/or periglacial processes. The district is not rich in mineral resources, but where these have been exploited the legacy of quarrying is areas of derelict land, which have their own unique and highly variable geotechnical and chemical characteristics. However, by considering the interplay between natural geological and 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.

It should be noted that this section mainly collates the available earth science data of relevance to planning and development. It should be used in conjunction with, but not as a replacement for, the detailed and comprehensive development plans and specifications (e.g. for waste disposal, water supply) that are produced by the local councils.

Mineral resources and quarrying

Minerals in the district that are currently of interest are those that can be won at the surface. 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 (Leicestershire County Council, 1995), together with a review of environmental considerations, and planning obligations and conditions. Factors that may hinder surface mineral extraction are:

• significant thickness of overburden, including natural drift deposits and man-made deposits, disposal of which becomes uneconomic;
• adverse geological conditions rendering the resource economically inaccessible and dewatering problematic;
• 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 on floodplains; and
• the extraction of mineral resources in a way that may lead to problematical engineering ground conditions, depending on infill materials and methods of compaction, and limiting future development.

Past quarrying activities are important for their impact on current ground conditions, and their visible surface effects are portrayed on the accompanying 50 000 maps by areas of worked ground and infilled ground (Chapter 12). It should be noted that the locations of all workings, large or small, that were documented during the present survey can be established by interrogating the BGS digital databases from which the 50 000 map and component 10 000 maps have been assembled. 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, as reviewed further below.

Brick and agricultural clay

Brick clay was mainly obtained from weakly consolidated mudstone, which may require some crushing and mixing with water to provide workable ‘clays’. There is no current exploitation, but the largest of the former brick-pits were located on the Mercia Mudstone Group, particularly the outcrops in the Leicester city area. The most worked unit was the Cropwell Bishop Formation, which also contains locally thick gypsum beds as formerly seen at Gipsy Lane (Plate 11b). Examples of brick clay workings in other units include the pit in the Edwalton Formation south of Sileby (Plate 11a) and (?)Gunthorpe Formation at Rothley Plain [SK5690 1364].

A number of generally small brick-pits in the Lias Group are shown on the accompanying map Sheet156 Leicester. By the turn of the 19th century these had all been superseded by material from the Mercia Mudstone brick-clay workings (Fox-Strangways, 1903). They originally furnished strictly local supplies, and those that have survived appear to have been opened near to trunk roads or on the outskirts of rural villages, for example at Ashby Folville [SK 7055 1227].

Pitting has also occurred in Quaternary deposits of the Oadby Till; for example above Great Dalby [SK 7465 1492], and in such workings it is possible that flint and chalk were sought for, as well as brick or agricultural clay.

Limestone

Limestone for cement-making was formerly extracted from the lower part of the Blue Lias Formation in the small quarries at Crown Hills, around Evington [SK 6204 0374]; [SK 6150 0393], as well as farther south around Kilby Bridge [SP 6138 9692]. A detailed account of the Evington workings was given by M McIntosh (1986; unpublished, copied to the Evington Local History Society). They apparently already existed in 1860 and were further developed in the early 1880s, but had lapsed by the middle part of that decade. In the Melton Mowbray district farther north, similar limestones were quarried and mined from the base of the Lias Group (Barnstone Member) for cement and concrete products, building stone and agricultural purposes; however, the limestones of the Leicester district were said to have been of lesser quality (Fox-Strangways, 1903).

Parts of the Marlstone Rock outcrop were quarried for lime. The resource was of ‘tolerably good quality’ near Tilton on the Hill, according to Judd (1875), who also noted that the Marlstone was extracted for lime from small pits [SK 7670 0445] near Robin-a-Tiptoe Hill.

Ironstone

The Marlstone Rock Formation, as the principal potential ironstone resource, has been extensively quarried in many parts of the East Midlands outcrop (Whitehead et al., 1952; Tonks, 1992). In the Leicester district it has been worked since ancient times from many small pits, e.g. [SK 7215 0521] when it was, however, principally used for building, road metalling, and lime burning (see above). The Northampton Sand Formation is the lateral equivalent of the ‘Northampton Ironstone’ farther south, but in this district it has not been worked for ironstone.

The extraction of Marlstone for iron ore in the Leicester district probably commenced within a year or so of the opening of the GNR-LNWR railway line, in 1879, to the east of Tilton on the Hill (Tonks, 1992). Whitehead et al. (1952, p.108) reported that quarrying was continuing on the western side of the line at the Tilton Ironstone Quarry, north-east of Halstead [SK 757 061], although it is clear from the account of Fox-Strangways (1903) that by 1903 these workings had ceased. They were, however, reopened by the Stanton Ironworks in 1911 and were further extended in 1933; smaller quarries existed to the north-west of Tilton [SK 737 065]. By 1950 all quarrying to the west of the line had ceased (Tonks, 1992) and the focus of operations moved to the east of the line, around White Lodge [SK 762 058]. The last ironstone quarries in the district were located farther north, to the north and south of Stone Lodge [SK 762 068], and finally closed down in 1961 (Tonks, 1992). Iron ore exploration drilling has been targeted at the outcrops north-west of Skeffington [SK 733 043], east of Tilton on the Hill [SK 770 067] and around Somerby [SK 779 100], but none of these prospects was subsequently developed. The primary mineralogy of the ironstone was believed to consist of chamosite ooids, associated with a siderite-calcite matrix (Taylor, 1949; Whitehead et al., 1952); however, the ooids are now known to be principally composed of berthierine (Young and Taylor, 1989). The weathering of these primary minerals to limonite (Hallam, 1968, plate 9a) enriches the ore and also accounts for the rust-brown colour of the rocks.

An assessment of the iron ore potential of the district (Whitehead et al., 1952, p.131) showed that the workable ore, with grades reaching to 33 per cent iron, was mainly concentrated in the central part of the outcrop, around Tilton on the Hill, where the Marlstone is also thickest. At Tilton on the Hill the upper about 2.5 m of Marlstone Rock was worked for ore, the lower sandy beds being depleted in iron. Assays of Marlstone across the district provided in Whitehead et al. (1952) showed that workable ore, with iron contents of 25 to 30 per cent, existed in the Burrough on the Hill-Somerby area and between Tilton on the Hill and Billesdon. The formation varied considerably in iron content vertically through the sequence, and typically the topmost bed and lower part were of poor grade. An analysis from the ironstone outcrop near Springfield Hill, just to the north-west of Tilton on the Hill [SK 735 066] illustrates this variation as follows:

The very high silica content of the lower part demonstrates that locally this part of the formation is a sandstone.

Sand and gravel

Quaternary sands and gravels in the district that are considered to have mineral resource potential are:

• suballuvial deposits and river terrace deposits of the Soar and Wreake floodplains; and
• preglacial fluvial deposits of the Bytham Formation.

In the past, glaciofluvial deposits have provided a local resource from small workings located on estates or close to villages, but although they are locally thick, no major exploitation has taken place.

River terrace and suballuvial deposits had been intensively quarried on the Soar floodplain northwards of Leicester city centre to beyond the Wreake-Soar confluence. This phase of extraction largely commenced in the 1960s, but was drawing to a close at the time of this resurvey, with the final workings west of Cossington [SK 598 144] undergoing restoration. In 2007, a quarry was opened to extract sand and gravel of the Bytham Formation from an area just south of Brooksby [SK 671 153], which has an estimated reserve of 4.5 million tonnes (Leicestershire County Council, 1995). A background survey of sand and gravel potential, mainly relating to geological sheets adjacent to Sheet 156 Leicester, was carried out by Mathers and Colleran (1987) and further information is given in the BGS mineral resources map for Leicestershire and Rutland (2002).

Building stone

The Marlstone Rock Formation and underlying cemented sandstone (‘Sandrock’) at the top of the Dyrham Formation have both furnished building stone from numerous small quarries, seen in the distinctive, rust-brown coloured houses and village churches in the central and eastern parts of the district (e.g. Lott, 2001). Judd (1875) noted that around Somerby, Pickwell and Burrough on the Hill the lower beds of the Marlstone were worked for road metal and only the upper 10 to 12 feet [3 to 3.6 m] of the unit was favoured for building stone.

Slate was worked during the 19th century from quarries in Swithland Wood, just outside the north-western margin of the district. Fox-Strangways (1903) noted the coarseness of the Swithland Formation resource compared with Welsh slates, the importation of which had by then led to the demise of the local workings (Herbert, 1944), although the Swithland stone is once again in local demand for roof replacement and/or extension work. Pink and grey granodiorite from quarries in the Mountsorrel Complex around Buddon Wood (see below) has been used for buildings and walls, and examples can be seen in Mountsorrel, Cossington and Rothley. It was also exported to cities and towns throughout the country for use as kerbstones and setts (Lott, 2001).

Igneous rocks–hard-rock aggregate

Apart from their building stone qualities (see above), the Ordovician igneous rocks have been the primary source of hard-rock aggregate in the district for over two centuries (McGrath, 2007). Extraction at Mountsorrel was initially centred on the Broad Hill area and began in earnest in 1795 with the opening of the canal on the nearby River Soar floodplain. Expansion re-commenced in 1854, when the Mountsorrel Granite Company was formed, and by 1860 the workings had been linked to the Midlands County Railway. Archival maps dating from the 1880s show that the ‘Mountsorrel Quarries’ [SK 577 149] had already achieved a large size; they were eventually renamed ‘Castle Hill Quarry’ and continued working up till 1967. Before this, the focus of quarrying had shifted westwards, with Hawcliff Quarry opened by 1904 and Cocklow Wood by 1930. Buddon Wood Quarry [SK 562 151] was opened in 1973 and is now the largest working ‘granite’ quarry in Europe (Plate 21), with an output of 4.5 to 5 million tonnes in 2003 and reserves estimated at 180 million tonnes. The stone is crushed and processed on site and then transported by conveyor belt across the Soar valley to the railhead near Barrow on Soar [SK 5860 1685]. Its quality meets the specifications for hard roadstone with more durable characteristics, the most noteworthy of its recent uses being for the Channel Tunnel (Boucher, 1994).

Base metal and hydrocarbon mineralisation

No commercially exploitable quantities of these minerals occur in the district, but small showings have been found. Ordovician rocks of the Mountsorrel Complex are the hosts to at least three styles of mineralisation. The late magmatic environment favoured the early generation of a molybdenite-pyrite-topaz assemblage, which could formerly be seen, in association with aplite and pegmatite veins, at Castle Hill Quarry (King, 1959, 1968). A continuation of this mineralisation on joint surfaces also involved molybdenite, together with tourmaline, pyrite, pyrrhotine and topaz. This was, however, modified within the same zones by a lower temperature, second-generation mineral assemblage of: dolomite, epidote, chlorite, quartz, chalcopyrite and pyrite together with minor amounts of galena, calcite, hematite and baryte. King (1968) compared the latter style of mineralisation with that seen in the Ordovician granite at Shap in Cumbria.

The third style of mineralisation is spatially associated with the Carboniferous dyke seen at Castle Hill Quarry [SK 577 150]. King (1959, 1968) identified this as a low-grade hydrothermal mineralising event in joints parallel to the dyke. It consists of cavernous veinlets with dolomite, calcite, bitumen, a clay mineral and pyrite. Aucott and Clarke (1966) and King (1968), among others, considered the bitumen compound to have had an abiogenic origin. In a later review of the evidence, however, Parnell (1988) found that the Mountsorrel hydrocarbons had a carbon isotope ratio similar to that of modern petroleum samples. Moreover their alkane and pyrolysate fractions have yielded abundant biomarkers (Xuemin et al., 1987), supporting the earlier suggestion of King (1959), that the bitumen was derived by migration from pre-existing Carboniferous rocks. It is possible that the bitumen originated from the expulsion of hydrocarbon-rich fluids during the end-Carboniferous inversion of the Widmerpool Half-graben, a deep sedimentary basin located to the north of the Sileby Fault and containing an estimated 5000 m of strata (e.g. Ebdon et al., 1990, Carney et al., 2004). Such fluids could have migrated into the adjacent basement rocks along fault and joint systems undergoing dilation during this tectonic event.

The South Leicestershire Diorites intrusion exposed at Enderby Warren Quarry features joints coated with the white, fibrous mineral, palygorskite in association with calcite (Tien, 1973). Evans and King (1962) suggested that meteoric waters percolating downwards from overlying sediments in Triassic times deposited this mineral.

Two main styles of Triassic mineralisation were observed by Faithfull and Hubbard (1988) in former exposures of the Cropwell Bishop Formation at Gipsy Lane brickpit [SK 620 071]. Minerals occurring both within and on the surface of gypsum beds included malachite, and possible bornite and djurleite. Also associated with gypsum were small, subspherical masses of the uraniferous mineral coffinite, together with malachite, crythrite, azurite, a copper sulphide, a nickel arsenide and a Co-Ni-Cu-As sulphide complex. In the second style, green ‘reduction spots’ surrounding kernels of a black, organic-rich vanadium mineral were scattered throughout the sandy dolomitic beds of the formation.

Water resources

This section is summarised from the report on the hydrogeology of the Leicester district by Cheney (2004). The mean annual rainfall ranges from a minimum of about 630 mm in the Wreake valley, on the northern fringe of the district, to more than 700 mm over the higher ground of Mountsorrel in the north-west, and over the more elevated eastern areas around Cold Overton, Billesdon and Whatborough Hill (Meteorological Office, 1977). The mean (1971 to 2000) annual evapotranspiration for the central part of the district is about 515 mm, using data derived from MORECS (Thompson et al., 1981).

Prior to the 19th century, the city of Leicester and other urban centres in the district were dependent on shallow wells, springs and surface watercourses for their water supplies. Leicester developed on the gravelly River Terrace Deposits of the Soar valley, which provided well-drained sites from which most of the supply was obtained from public draw wells, one of which (the Plancke or Klanche well) was located near the top of the present Cank Street (Richardson, 1931). These sources were highly prone to contamination, however, and so in 1847 a private water company was formed to provide the city with a potable water supply. Surface water impoundments such as Thornton reservoir (located to the west of the district), together with the associated treatment plant and supply mains, had been constructed by 1854. This was followed by the Cropston and Swithland reservoirs of this district, in 1866 and 1896 respectively. Additional reservoirs were constructed farther north, under the auspices of the Derwent Valley Water Board, during the first half of the 1900s to provide additional water supplies to Leicester, Nottingham, Sheffield and Derby. Increasing demand, due to continuing industrial expansion and rising population, outstripped Leicester’s portion of the supply from these sources by the mid 1950s, prompting the formation of the River Dove Water Board to secure additional supplies. Currently over 92 per cent of the water supplied to Leicester and the surrounding area originates from the rivers Dove and Derwent; these rivers drain southwards into the Trent from the Derbyshire Peak District and have been the responsibility of Severn Trent Water since 1989.

Groundwater abstraction licence and water use data for the Mercia Mudstone Group, Lias Group and Superficial Deposits minor aquifers are shown in (Table 5). Surface water abstraction licence data (for the categories used for groundwater use), has been included in this table for comparative purposes. The vulnerability of the geological units in the district to contamination by groundwater infiltration is discussed by Palmer and Lewis (1998), and portrayed in an Aquifer Vulnerability Map (see Information Sources). Of the three aquifers, the greatest abstraction, (although not particularly large in terms of total quantity), is from the Mercia Mudstone Group. Groundwater from this source is of particular importance to the industrial sector, with abstractions concentrated in the vicinity of Leicester. The relatively small scale use of this aquifer for agricultural and private supply purposes is probably a reflection of the proximity of the Mercia Mudstone outcrop to urban areas that are well served by water mains supplies. The Lias Group aquifer is of particular importance to the agricultural sector with a large number of small licensed abstractions (average about 1100 m³/a), for general agricultural and domestic use. Little groundwater from the Superficial Deposits is abstracted in the district despite its widespread distribution along the valley of the River Soar. The two agricultural supplies obtained from the Superficial Deposits are in fact from River Terrace Deposits, whilst the abstraction used for mineral washing from River Soar floodplain (alluvium and possibly underlying River Terrace Deposits) may include some contribution from the underlying Mercia Mudstone Group aquifer.

Ordovician and Precambrian basement rocks

These are well-indurated rocks with little or no primary permeability or porosity. Limited quantities of groundwater may occur within and move through joints and fractures, and yields are dependent on wells and boreholes intersecting such water bearing fractures. Usable quantities of groundwater would not appear to be obtainable from these rocks in the Leicester district.

Sherwood Sandstone Group

These sandstones constitute the most important aquifer in the English Midlands, but are of variable, and generally limited, thickness (Table 10) for further details." data-name="images/P946714.jpg">(Figure 4a), and found only at considerable depth, in the Leicester district; the nearest outcrop is about 6 km to the north-west. Where present, the Sherwood Sandstone aquifer is likely to be confined by the overlying Mercia Mudstone Group. Sandstones also occur in the Sneinton Formation at the base of the Mercia Mudstone, where the groundwater is commonly in hydraulic continuity with the underlying Sherwood Sandstone aquifer, but can also form a localised minor aquifer in its own right. In practice, few (if any) boreholes solely utilise the Sneinton Formation, most continuing downwards into the Sherwood Sandstone aquifer.

Of the two water supply boreholes in the district that penetrated the Sneinton Formation and underlying Sherwood Sandstone Group aquifer, neither borehole record contains any significant hydrogeological information for these horizons. The first borehole was drilled in 1892 in Knighton Fields Lane [SK 5922 0197]; it penetrated the Mercia Mudstone Group (including the Hollygate Sandstone, Cotgrave Sandstone and Plains skerry) to a depth of 169 m below surface, then entered approximately 30 m of the combined Sneinton Formation and Sherwood Sandstone aquifer, before terminating in Ordovician mudrocks at 251 m depth. No casing details were provided, but it is probable that the sandstone beds within the Mercia Mudstone contributed to the total yield of the borehole. The borehole was tested at 27.3 m³/h (7.5 l/s) for a water level drawdown of only 1.7 m but a much larger production yield of almost 136 m³/h (38 l/s) was reported in 1949. The water quality was also reported to have been good. This combination of very high yield and good quality water may indicate that a substantial proportion of the total yield originated from the Sneinton/Sherwood Sandstone aquifer.

The second borehole, drilled in 1997 in Nansen Road, Leicester [SK 6104 0401], penetrated the whole of the Mercia Mudstone sequence above a 30 to 40 m thickness of Sneinton Formation/Sherwood Sandstone Group aquifer. Water strikes were reported at depths of 11.5 m (in the Mercia Mudstone) and at 243 m (in the Sherwood Sandstone). The second strike appears to have provided a yield of almost 2.5 m³/h (0.7 l/s) but water quality was not reported. This ‘test’ borehole was backfilled on completion.

Three other investigation boreholes were drilled to the north-east of Leicester, encountering over 30 m of sandstones beneath 160 m to almost 200 m of Mercia Mudstone strata. None of the available records provides hydrogeological information.

Mercia Mudstone Group

This unit has traditionally been regarded as weakly permeable and at best a poor aquifer, most commonly referred to in the context of forming a confining upper limit to the Sherwood Sandstone aquifer. Limited quantities of groundwater suitable for domestic or small-scale agricultural use are nevertheless commonly obtainable (Jones et al., 2000), and in the Leicester district the group provides higher yields than any of the other potential aquifers. The largest yields are from boreholes that penetrate the Hollygate Sandstone Member (formerly the Dane Hills Sandstone), at the top of Edwalton Formation.

Arenaceous or silty beds (‘skerries’) are present throughout most of the mudstone sequence (Jones et al., 2000), but with the exception of those in the Sneinton Formation (above), and Cotgrave and Hollygate Sandstone members, they are generally less than 1 m thick. They commonly have a strong dolomitic cementation, however, and thus can contain and transmit limited quantities of groundwater through fractures. Although such beds constitute only a small proportion of the nominal saturated depth of boreholes and wells, they confer a very large proportion of the yield, with the mudstones contributing little or nothing. In situ rising head and packer tests in investigation boreholes drilled along the line of a sewage tunnel between Wanlip and Leicester provided values of coefficient of permeability ranging from 3 × 10–10 m/s to 10⁻⁵ m/s. The very low values were thought to be associated with the mudstones and the higher ones with the ‘skerries’ (Atkinson et al., 2003). The ‘skerries’ commonly possess a very small outcrop area, limiting recharge to that which moves slowly through the mudstones. Storage is therefore rapidly depleted on pumping and in consequence, yields can decline dramatically with time. Failure to penetrate a dolomitic bed, or intersecting one that is only poorly fractured (not an uncommon event), results in a dry or very low yielding borehole.

The Cropwell Bishop Formation is probably the most prospected Mercia Mudstone unit of the district. Yields generally range up to 1.6 m³/h (0.45 l/s) but there are also a number of records indicating that ‘very little water’ was obtained. Exceptionally, a yield of 4.6 m³/h (1.3 l/s) was obtained from a borehole in Friday Street, Leicester [SK 5866 0530].

Yields obtained from the Hollygate Sandstone Member commonly range from less than 5 m³/h to 45 m³/h (about 1.5 to 12.5 l/s) for drawdowns (where recorded) that only rarely exceed 10 m. Virtually all of these relatively high yielding boreholes are located within the Leicester urban area and were drilled to supply groundwater for industrial purposes. Smaller yields were obtained from a number of other boreholes, but in many the member was not fully saturated. A test yield of over 50 m³/h for a drawdown of only 1.6 m was obtained from a borehole located at the Union Works, Leicester [SK 5902 0639], whilst pumping rates of over 90 m³/h were recorded for boreholes located at the Beaumanor Brewery [SK 5880 0713] and Ash Street [SK 6024 0539], although both were originally tested at considerably lower rates. This wide variation in yields and associated drawdowns in boreholes where the Hollygate Sandstone Member is fully saturated is likely to reflect differences in the degree of fracturing present. It may also reflect the extent to which the original cementation has been leached; for example, most samples collected at outcrop show a very high macroporosity due to the abundance of voids between the framework grains (Plate 12b). The Hollygate Sandstone Member subcrops beneath alluvium and river terrace deposits over a small part of the River Soar floodplain [SK 578 050], west of central Leicester and there, the aquifer is likely to be in hydraulic continuity with both the superficial deposits and the river channel. The sandstone aquifer therefore has the potential to be recharged as well as contaminated by river water, although its continuity is disrupted by north-west- and north-trending faulting. Where such recharge does occur, the aquifer could be capable of sustaining the relatively high yields documented from local boreholes in the Leicester urban area.

The few boreholes that only penetrate the Edwalton Formation or the Gunthorpe Formation mudrocks produced little or no water. The three boreholes known to have penetrated the intervening Cotgrave Sandstone Member, defining the base of the Edwalton Formation, produced yields of the order of 3.5 m³/h (1 l/s), although the resulting water level drawdowns are not recorded.

Groundwater quality in the Mercia Mudstone

Fox-Strangways (1903) considered that although considerable amounts of water could be obtained from the Mercia Mudstone Group, this water was too hard for domestic use due to the quantity of gypsum present in the mudstones. Few partial chemical analyses are available for boreholes or wells that penetrate the group in the district, but comments contained on drilling records often indicate that the water was hard, or very hard, was occasionally unfit for drinking and that groundwater mineralisation may increase with borehole depth. Total dissolved solids concentrations range from about 700 mg/l in a borehole of about 40 m depth, to 2600 mg/l in a 60 m borehole. Total hardness ranges from over 300 to 2600 mg/l (as CaCO₃), much of this being permanent, with associated sulphate concentrations of between 163 and 1260 mg/l. Recorded chloride concentrations range from about 40 to 120 mg/l, again showing some correlation with borehole depth.

Lias Group

The Lias Group is a minor, multilayered aquifer in which the subordinate limestones and sandstones, including ferruginous sandstones (ironstones), constitute aquifer horizons confined by the overlying and underlying impermeable mudstones. Aquifer horizons are relatively thin, rarely extend over large areas and may be laterally discontinuous over distances of kilometres or less. Faults with quite limited displacements may split an aquifer into separate, hydraulically isolated compartments, or may juxtapose different aquifer horizons, placing them in hydraulic continuity. Intergranular permeabilities are generally low in limestone beds, and water movement takes place through fractures that are commonly irregularly distributed in a vertical and horizontal sense; those that extend for significant distances, however, may result in relatively high yields where interconnected. Fracture development in the sandstones, friable sands, and ironstones is likely to be less pronounced than in the limestones and primary porosity may play a greater role in water storage and transport. Controls on permeability are likely to be more closely linked to the degree of induration and grain size distribution.

In the Blue Lias and Charmouth Mudstone formations intergranular permeabilities are generally low in the limestones and water movement takes place through fractures that have commonly been enlarged by solution. Yields are very variable, being dependent on whether or not a borehole intersects an interconnected system of water-filled openings. The limestone aquifer horizons generally only have small outcrop areas and in consequence recharge is limited. They are characterised by relatively high secondary permeabilities but since they are generally thin, transmissivities are relatively low, as are storage coefficients. Water resources are in consequence limited, with both yields and water levels commonly falling significantly after only a few hours pumping. Where formations are highly fissured pumping can affect sources some distance away. Recorded yields obtained from the Blue Lias and Charmouth mudstones generally range between 0.7 and 3 m³/h (0.2 to 0.8 l/s) for highly variable amounts of water level drawdown in response to pumping. Higher yields of about 4 m³/h (1.1 l/s) occur in the vicinity of Illston-on-the-Hill [SP 707 993] and range from about 4.4 to 6.8 m³/h (1.2 to 1.9 l/s) in the area to the south-west and south-east of Twyford [SK 730 100]. However there are also numerous records of very low yielding and effectively dry boreholes elsewhere in the district.

Dyrham Formation and Marlstone Rock Formation are generally penetrated in the same borehole and it is therefore only rarely possible to define the separate contributions from each formation, although the greater part of the total yield is likely to be from the Marlstone Rock. A number of villages (such as Pickwell, Somerby, East Norton, Goadby, Launde, Loddington and Withcote) have had their positions originally determined by the availability of usable water supplies derived from this formation (Richardson 1931). Recorded borehole yields range from about 1.1 to 3.6 m³/h (0.3 to 1 l/s), with lower yields being recorded where the Marlstone Rock is only partially saturated. In contrast to the Blue Lias and Charmouth Mudstone formations, records of very low yielding boreholes are comparatively rare and there are no effectively dry boreholes. Springs commonly issue from the base of the Marlstone Rock and a particularly large yield of 9.5 m³/h (2.6 l/s) is recorded from such a spring near Somersby [SK 7776 0996], which formerly provided the supply for that village. Similarly small springs commonly occur at the base of the Dyrham Formation at its boundary with the underlying mudstone. The structure contour map of (Figure 7) may be used to estimate the depth to the base of the Marlstone Rock Formation, and hence of a potential local water supply.

Whitby Mudstone Formation contains a few thin limestone beds that could act as aquifer horizons; however, most outcrops are in relatively elevated situations, on the crest of hills in the district, and consequently groundwater is likely to drain rapidly from the more permeable horizons that are present. Virtually all boreholes that penetrate this formation continue into the underlying Marlstone Rock and Dyrham formations, from which water supplies are obtained. Fox-Strangways (1903) considered that this unit provided the poorest supply of any formation in the district, and only very low yields (at best) have been obtained from boreholes solely penetrating it. Springs that formerly provided a supply to the village of Cold Overton [SK 8130 1187] yielded up to a maximum of 3.8 m³/h (1 l/s) (early in the year) but had been known to decline to as little as 0.5 m³/h (0.13 l/s), presumably following a period of particularly low rainfall.

Groundwater quality in the Lias Group

Even partial chemical analyses of groundwaters from these rocks are rare. The quality of water from Lias limestones is generally good but hard, with calcium and bicarbonate ions likely to predominate. Fox-Strangways (1903) and Richardson (1931) considered that the scant water supplies available from the limestones and shales of the Blue Lias and Charmouth mudstones (formerly the Lower Lias) at depth was likely to be hard, liable to be brackish or saline and to commonly contain hydrogen sulphide due to the break down of pyrites. The few adverse comments regarding water quality on the vast majority of boreholes and wells penetrating the Lias in the district would nevertheless imply that, in general, water of usable quality has been obtained. The presence of poor quality water was, however, recorded in two boreholes near Ashby Folville, with one [SK 7147 1177] simply noting ‘salty water’ and another [SK 7084 1468] noting an unpleasant odour. High salinity was indicated at New York Farm, South Croxton [SK 6627 1049] where a chloride ion concentration of 220 mg/l was recorded, whilst at Borrough Court near Marefield [SK 7425 0080] it was noted that the water was highly corrosive to the pipes. Water quality from the Marlstone Rock is generally good but hard and commonly ferruginous. Fox-Strangways (1903) indicated that such groundwater was liable to contain elevated concentrations of iron, sometimes rendering it unsuitable for domestic use. Numerous boreholes and wells have been drilled and used to provide small-scale supplies from the Dyrham Formation and Marlstone Rock Formation but adverse comments regarding water quality are rare, suggesting that, in general, water of usable quality has been obtained.

Inferior Oolite/Northampton Sand Formation

Within the Leicester district the Northampton Sand Formation occurs as small outliers located on the crest of hills. In consequence any recharge that enters these rocks rapidly drains through them and the formation does not constitute a viable aquifer within the district.

Superficial deposits

The original water supply for Leicester was provided from shallow wells sunk in the alluvium and river terrace deposits of the Soar floodplain. These wells would have been relatively low yielding and highly prone to contamination from surface pollution. In addition, groundwater contained in the superficial deposits was most probably in hydraulic continuity with the River Soar and its tributaries and many of the wells thus would have dried up during prolonged drought periods. These wells were abandoned as the city expanded and turned to more reliable potable sources of surface water for public supplies.

Few sources exist in the district that have obtained water supplies solely from the superficial deposits. There are, however, numerous records of shallow wells, presumably dug into them, that were later deepened by drilling through their floor into the underlying Mercia Mudstone Group or Lias Group. The superficial deposits were commonly cased out, with water supplies being obtained from the underlying strata, and consequently only small unusable quantities of groundwater were obtainable from them.

Till deposits are effectively impermeable, but limited supplies have been obtained where lenses of sand and gravel occur within them. Limited supplies of water have also been obtained from glaciofluvial deposits, with a number of wells drawing water for individual villages in the district. Archive records indicate that 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.

Little water quality information is available for groundwater from the superficial deposits. It is probable that it is generally of usable quality, but the records for few of the wells have confirmed that they are prone to contamination from surface pollution.

Flooding

Flooding is a potential geohazard in many low-lying parts of this district during heavy falls of rain, and may in the future 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. The most serious flood event in recent times occurred during the Easter period of 1998, in the floodplain of the Wreake valley. It caused flooding from Cossington northwards along the Soar valley to the Trent confluence, but as it was primarily caused by rain falling over the Wreake catchment, it did not greatly affect the city of Leicester. Flood defences in central Leicester were improved after the major flood of 1903, and many of the newest housing developments, such as that at Syston [SK 616 117], have flood defences incorporated into them. Groundwater flooding and urban drainage back-up, as opposed to overland flowage, may nevertheless present problems if water table levels on the floodplain are sufficiently high during extreme rainfall events.

In this district a broad relationship exists between geology, geomorphology and the potential extent of flooding. In catchments, the permeability of bedrock can affect the rate of runoff, whereas on floodplains Quaternary geological processes are largely responsible for width and topography (‘roughness’) and the volume of alluvial fill that is capable of absorbing floodwaters. The modern (i.e. Holocene) alluvium correlates with the deposits left behind by previous maximum-flood events, or by channel migrations across the floodplain, suggesting that the alluvium outcrop, as mapped, is a credible geological indicator of floodable ground. It should be stressed, however, that the alluvium represents only those preserved deposits of flooding events: the waters that deposited them may have extended further, beyond the alluvium boundary as mapped. The present re-survey has shown that in the major floodplains, such as that of the River Soar, there are also significant areas of ground within or marginal to the floodplain that are topographically only slightly higher (generally by up to 2 m) than the modern alluvium tracts. These more elevated areas correspond to outcrops of the ‘floodplain terrace’, named the Syston Member, and their distribution is a further indicator of floodable ground, albeit with a generally lower frequency of flood-occurrence compared with the alluvium.

A flood risk management strategy for the River Soar and major tributary valleys, incorporating maps with statistically calculated flooding limits, has recently been completed by the Environment Agency. For smaller tributary valleys, where such information may not be available, the distribution of alluvium, floodplain terrace and/or valley deposits (including colluvium), shown on Sheet 156 Leicester and derivative 1:10 000 scale maps, can be used as approximate geological indicators of flooding.

The Leicester district covers numerous valleys, many being moderately incised but some seen as little more than gentle depressions and revealed only during this survey by the mapping of narrow strips of alluvium or valley deposits. Such valley systems focus runoff, which can result in flash flooding at times of short-lived and localised, but exceptionally heavy, bursts of rain.

Engineering geology and geotechnical properties

The suitability of superficial and bedrock materials of the district for foundations and other aspects of construction work depends mainly on their geotechnical properties. A qualitative summary is provided in (Table 6) and (Table 7). This is based on regional assessments, and reviews of published and unpublished literature (Hobbs, 1998; Hobbs et al., 2004) and summaries of selected geotechnical reports for adjacent districts (Poole et al., 1968; Worssam and Old, 1988; Hobbs, 1998; Charsley et al., 1990; Carney et al., 2004). No new sampling or testing was undertaken for the Leicester district, but geotechnical coverage of geological units is generally good. Details of the coverage and quality of data, the methodology used in processing the data, the limitations of results and analysis of geotechnical properties are provided in the various references given in Hobbs (1998).

When considering geotechnical properties, particular attention should be paid to the possibility of rock fabrics being modified in the near surface zone. For example, mudstones from the Lias Group and Mercia Mudstone Group, which crop out over about 30 per cent of the district, have commonly been affected not only by weathering but by periglacial processes, such as solifluction and cryoturbation, which were active during previous cold climatic regimes. These processes have typically disrupted strata, converting them to lithologies composed of ‘clasts’ of mudstone in a matrix of softer clay. Such features, described in the relevant weathering schemes developed for these strata (Chandler, 1969, 1972), have an important effect on geotechnical properties and hence engineering behaviour. The process of engineering reworking during, for example, excavation, and fill handling and preparation, results in a reduction in the strength, and in some cases an increase in plasticity, of these mudstones.

Sandstones and siltstones, particularly those of the Lias Group, are variably cemented and hence have strength parameters that differ widely, even on a local scale. In some cases they are virtually unconsolidated and may be better described as sand and silt rather than sandstone and siltstone, particularly if referring to borehole cores where the drilling process has caused a certain amount of disturbance and/or flushing of the strata penetrated.

A list of selected geotechnical properties for the Mercia Mudstone Group and Lias Group in the East Midlands, taken from Hobbs et al. (2002 and 2004 respectively), is given in (Table 8).

Mercia Mudstone Group is the predominant bedrock unit in the west of the district, where it crops out along major valleys such as the Soar, and in parts of the Leicester city area. When fresh, these strata generally consist of stiff to hard, silty clays of low to intermediate plasticity. Analyses of fresh mudrocks from the Asfordby Borehole in the adjacent Melton Mowbray district has shown that parts of the Edwalton Formation and the lower half of the Cropwell Bishop Formation contain unusually high amounts (up to 40–60%) of smectite clay minerals (Kemp, 1999; see also Carney et al., 2004) with the consequent potential for shrink/swell clays to be present in this part of the succession. Moreover, studies have shown that Mercia Mudstone undergoes considerable strength loss on weathering or human re-working (Chandler, 1969; Hobbs et al., 2002). Boreholes with geotechnical information in the west of the district indicate that the subcropping parts of the Mercia Mudstone Group, commonly the Edwalton or Cropwell Bishop formations, can be affected by weathering down to at least 20 m, with weathering zones IV to II of Chandler and Davis (1973) identified. Weathering effects are further exacerbated where gypsum dissolution has occurred, as discussed below.

In the Leicester district, there has been a recent example of Mercia Mudstone becoming disaggregated as a result of engineering operations (Atkinson et al., 2003). Mineralogical analyses revealed that the affected rocks contained unexpectedly high levels of smectite clay minerals which, when allied to the aggregated nature of these mudstones (Hobbs et al., 2002), resulted in the clogging of a full-face tunnelling machine operating in closed mode. The problem occurred within the 2.4 m diameter Abbey Sewer tunnel, which was driven during 1993 from Wanlip [SK 601 109] southwards beneath the Soar floodplain to near Abbey Park in central Leicester. Although Atkinson et al. (2003) thought that these problems occurred in strata spanning the Edwalton and overlying ‘Trent’ (i.e. Cropwell Bishop) formations, the present mapping suggests that tunnelling was probably entirely confined to the lower part of the Cropwell Bishop Formation, which as noted above contains significant proportions of smectite ‘swelling’ clay minerals.

The net allowable bearing capacity of Mercia Mudstone is in the range of 100 to 600 kPa, with moderate, rarely high, settlement. The unit is locally abundant in gypsum, so sulphate attack on buried concrete is possible and class 2 or class 3, rarely class 4 or 5, concrete mixes may be required locally (BRE, 1996). Excavations on Mercia Mudstone can generally be carried out by digging, or by ripping in harder parts, and support is required in weathered or voided material. In some cases, the near-surface Mercia Mudstone may be disrupted by periglacial processes, as described above.

Penarth, Lias and Inferior Oolite groups: mudrocks in these groups can be of intermediate to very high plasticity, with strength generally increasing with depth as weathering and stress relief effects decline. Such clays may be subject to slope instability and high compressibility, as well as shrinkage and swelling problems. Excavations can generally be carried out by digging, but ripping may be necessary for sequences with strong limestone beds. Water may flow along joints in limestone beds, causing flooding to excavations.

Sulphur occurs in a variety of forms in Lias Group mudrocks, the best-known being gypsum and anhydrite. Sulphate, in particular pyrite, is formed in anaerobic conditions and is known from the Penarth and Lias groups. It may constitute up to 5 per cent of the whole rock. Thaumasite is a sulphate mineral that may be produced as a product of pyrite oxidation; it attacks buried concrete in a particularly aggressive manner, reducing it to a paste (BRE, 2005).

Igneous rocks: granodiorite of the Mountsorrel Complex tends to have a very high intact strength. Rock mass strength is variable, however, depending on the state of weathering, topography and bedrock profile. Rock fall is a hazard beneath exposed rock faces. Seasonal freeze/thaw tends to breakdown near-surface rock, enabling mobility of shattered blocks.

Till is an extremely important geological unit in areal terms, covering approximately 50 to 60 per cent of the district. The various tills that have been mapped are heterogeneous deposits, which may include large ‘erratics’, rafts of locally derived bedrock, and pipes, channels or incorporated slivers of sandy glaciofluvial and clay-rich glaciolacustrine material. The thickness variation of till may be considerable, and can occur within short distances due to irregularities in the underlying bedrock or drift surface. The geotechnical properties of till tend to match its lithological heterogeneity. Clay-rich till is usually described as over-consolidated and may be classified, in parts, as a stiff to very stiff cohesive soil (undrained shear strength, cu, ranges from 100 to 300 kPa). Over-consolidation of clay-rich facies may result in fissuring, softening, and weakening on exposure. Some tills also have reasonably high sand content, however, and these non-cohesive facies may cause problems of running conditions in excavation due to perched water tables. In the Coventry district, it was noted that the plastic limit of till (mainly Thrussington Till) varied widely, between 10 and 53 per cent (Bridge et al., 1998, table 18). Casagrande plots of relatively fresh tills analysed from borehole samples from the Leicester and Melton Mowbray districts (Figure 17) indicate that most are intermediate in terms of their plasticity, and hence shrink/swell characteristics. Weathering could, however, further enhance these properties.

Glaciolacustrine Deposits, which include the Rotherby and Glen Parva clays, most probably have similar geotechnical properties to the Wolston Clay of the Coventry district (Bridge et al., 1998; table 18). In the latter, moisture contents reach over 30 per cent and plasticity can be very high, with strengths generally lower than for till but still implying slight or pseudo-overconsolidation.

River Terrace Deposits typically consist of medium-dense to dense, fine-medium sand and medium gravel with lenses of laminated clay and silt. They normally provide good foundations, but are of irregular thickness and may become loosened and unstable in excavation, where running sand can also be a problem. Clayey lenses, where present, are probably of low to intermediate plasticity.

Peat is mapped in the south-west of the district [SP 549 979], and occurs in association with organic-rich silt and clay. Such deposits have also been proved in boreholes through the upper, silty component of typical floodplain alluvium sequences elsewhere in the district (Chapter 8). The properties of peat and its associated organic-rich deposits typically feature low to high plasticity, soft to firm consistency and medium to high compressibility with medium to very high consolidation. The most organic-rich material may have a moisture content ranging up to several hundred per cent.

Alluvium is a heterogeneous deposit typically consisting of clay, silt, sand, and gravel in varying proportions, but also including organic-rich silt and peat. Its geotechnical attributes reflect this lithological heterogeneity, with a very wide range for properties such as strength, moisture content, compressibility and plasticity; the last being typically low to intermediate. Most alluvial tracts have a 1 to 3 m capping of silt and clay, and it is in this layer that materials with the highest parameters of compressibility and plasticity (peat, organic silt, clay) are the most abundant. The gravel component ranges in density from loose to dense, and generally offers good bearing capacity. As the thickness of these deposits may vary considerably, marked lithological and geotechnical changes may also be expected laterally and vertically, linked with the formation of channels.

Head is a heterogeneous superficial deposit mantling bedrock and other superficial deposits. Its thickness may be highly variable, as may be its content, which reflects the source rocks that have formed it. Although the properties of head commonly resemble those of the source rock, it has features within it indicative of a reduced strength and increased plasticity. For example, limiting slope angles tend to reduce from 11° in a weathered Lias Group mudrock to 7° in a Lias-sourced head deposit, subject to the same environmental conditions (Chandler, 1972). Strength reduction is greatest within the relict shear zones (see below) that characterise head formed by solifluction in former periglacial climatic regimes. Mudslides that formed in areas of landsliding will contain shear zones with similar properties.

Ground conditions and geohazards

When considering the viability of engineering structures the geotechnical properties of the substrate must be examined (see above), as well as geological factors such as: local geological structures, slope stability, bedrock weathering and/or dissolution, potential for flooding and seismicity. Any of these may either enhance or give rise to problematical ground conditions and geohazards, which then act as a major constraint to development. Site-specific investigations should always be carried out prior to development.

Slope stability

Slope stability refers to the potential for a slope to undergo gravity-induced movement, principally due to landsliding. The stability of slopes is dependent on slope angle, on the nature, structure and strength of the underlying material (bedrock, superficial deposits or fill) and the influence of water.

For the Melton Mowbray district, Forster (in Carney et al., 2004) considered that undisturbed natural slopes have generally attained a considerable degree of stability in our present climate. Construction of any sort disturbs this equilibrium, however, and may present problems in certain circumstances. Natural slopes dominate the Leicester district, the steeper ones occurring along the escarpment capped by the Marlstone Rock Formation and surrounding the relatively small outliers capped by the Northampton Sand Formation. Steep slopes are also developed along certain deep valleys that dissect the Marlstone Rock dip-slope. This resurvey has found a large number of landslides in the central and eastern part of the district (Chapter 8). They are mainly of the shallow slump/mudslide type, and in places the related process of cambering has also occurred (Chapter 10).

The surface geotechnical properties of a given bedrock or superficial lithology will generally determine its susceptibility to mass movement. Mudstone and interbedded mudstone and sandstone are normally strong enough to sustain steep slopes without failure. Deeply weathered bedrock and material derived from it, such as head, is much weaker and has a reduced permeability. The weathered zone of slopes, and slopes mantled by Quaternary deposits are consequently susceptible to movement if there is increased ingress of water from natural or artificial sources. Under the present climate natural water input is not generally sufficient to promote movement, except near spring lines or following exceptionally heavy and sustained rainfall. However, under the wetter freeze thaw periglacial conditions during the Devensian, movement may have occurred, forming shallow landslips or solifluction aprons. The degraded products of such processes are today preserved as the numerous outcrops of Head and Slope Terrace Deposits. These include relict shear surfaces, indicating that Head has the potential to be reactivated by loading, undercutting or excavating into slopes, or by introducing water into the slope from drains or soakaways.

For bedrock lithologies without significant clay, such as the Marlstone Rock Formation, the main modes of slope failure are by cambering, rockfall, slab displacement or undercutting of steep faces. The last two types of failure are most likely to occur associated with quarrying operations, and the potential causes of failure are generally related to major planar structures such as joints, faults, bedding planes and cross bedding.

Bedrock dissolution

Bedrock solution may be anticipated in the western part 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 in the presence of freely flowing groundwater, within a ‘solution zone’ that is several metres thick below the base of the subsoil or superficial deposits (Elliott, 1961), and is mainly accomplished by groundwater flow along bedding planes, joints and fissures. No surface features attributable to such a dissolution process have been recognised in the Leicester district, but it has certainly accounted for the ‘karstic’ modification of gypsum beds formerly revealed in quarries around Leicester (Chapter 6). Although commonly attributed to glacial or periglacial conditions, such processes may be continuing, albeit much more slowly, today (Firman and Dickson, 1968). The slow, natural solution of gypsum within zones of enhanced groundwater flow could cause uneven settlement resulting in damage to heavy man-made structures, as has been suggested for ground adjacent to the Ratcliffe on Soar Power station in the Melton Mowbray district (Seedhouse and Sanders, 1993).

Mined ground and shafts

Underground mineral extraction has never been a significant activity in the Leicester district. There is, however, mention of a ‘gypsum shaft’ in the Spinney Hills-Evington area [SK 607 042] (M McIntosh, written communication, copied to the Evington Local History Society).

Surface quarrying

Until well into the 19th century, the surface extraction of minerals was almost entirely from a number of small pits, which are indicated on archival Ordnance Survey maps. Larger quarries for brick clay were subsequently opened around Leicester, for sand and gravel on the Soar floodplain, and for hard-rock aggregate at Mountsorrel, as reviewed above. 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. Only a few quarries now remain in their quarried state, with steep backwalls and limited fill in their bases. It is important to note that many former quarries, pits and ponds have been sited using BGS archives and old editions of Ordnance Survey maps and that other excavations may exist. The boundaries of those delineated are based on the best information available, and some are likely to be imprecise in detail. In areas where former workings are known or where a resource exists, site investigations should allow for the possible presence of backfilled excavations.

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. Problems of settlement may occur within some of the larger infilled quarry sites of the district. 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, including potentially explosive methane, or leachates from the fill into the surrounding geological deposit (see below).

Leachate movement and gas emissions

The increasing use of quarries and various types of pit for waste disposal has the potential for producing a widely developed, but localised, hazard from toxic leachates and dangerous gases. This is potentially a serious hazard at landfill sites situated on deposits in hydraulic continuity with an aquifer such as permeable bedrock or, in the case of the sand and gravel quarries, with the major rivers or their tributaries. Toxic and explosive gases, particularly methane, can be generated within waste tips and landfill sites. Such gases can migrate, sometimes through adjacent porous strata or along fissures, and accumulate within buildings or excavations either nearby or some distance away, as noted below.

Landfill gas is a type of bacteriological methane, formed by the biodegradation of organic matter in landfill sites under anaerobic conditions. It has a lower density than air and thus has the potential to migrate from the landfill site, both vertically and laterally. Many modern sites pack waste material densely and contain it within impermeable barriers, venting the gas through pipes to the surface. Other sites, if not contained or vented, may present a risk from gas or leachate migration. Gases can migrate, sometimes through adjacent porous strata or along faults and fissures, and accumulate within buildings or excavations either nearby or some distance away, as occurred at Loscoe in Derbyshire in 1986 when an explosion resulted (Williams and Aitkenhead, 1991). 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.

Carbon dioxide is a colourless, odourless, non-combustible gas that is very soluble, forming the potentially corrosive carbonic acid. The gas has a high density relative to air, and thus tends to accumulate in low areas; it can accumulate in depressions, such as trial pits, replacing air and resulting in the potential for asphyxiation. The oxidation or combustion of organic materials may result in higher than normal concentrations of carbon dioxide, which is typically a major constituent of landfill and sewage-derived gases. It commonly makes up 40 per cent of the volume of a typical landfill gas, but can vary between 16 to 57 per cent (Williams and Aitkenhead, 1991). Carbon dioxide to methane ratios from landfill sources are typically 30 per cent.

Toxic gases: the natural migration and build-up of toxic gases such as methane should also be considered if tunnelling or working in otherwise confined spaces. The bitumen mineralisation at Mountsorrel (see above), which was possibly introduced from Carboniferous sedimentary successions, indicates that there may be a potential for methane-generating compounds to exist at depth. Pathways provided by faults or fractures (e.g. Appleton et al., 1995) could allow such gases to migrate into Mesozoic strata that otherwise would not be considered hazardous in these respects.

Radon (Rn-222) is a naturally occurring gas, which is derived from rocks, soils and groundwater containing uranium (U) and thorium (Th). Nationally, the main areas of relatively high levels of radon are associated with ground underlain by rocks, or their weathering products, containing enhanced concentrations of uranium, and areas underlain by permeable rocks, superficial deposits and their weathering products. For the Leicester district it is anticipated that natural radon levels will be generally low except over the outcrops of Lias Group. Studies by Sutherland (1992) and Sutherland and Sharman (1996) show that elsewhere in the East Midlands the Marlstone Rock Formation is a significant radon generator, with the Northampton Sand, Charmouth and Blue Lias (Scunthorpe Mudstone) formations ranking slightly lower in this respect. The occurrence of uraniferous minerals in Mercia Mudstone strata (see above) is also noteworthy, however.

Radon commonly migrates from the source rocks by association with other gases, in particular methane and carbon dioxide. It may also be transported in groundwater, returning to a gas phase in areas of water turbulence or pressure decrease (e.g. waterfalls and springs). Radon may therefore occur in high permeability rocks present above a source rock. Major faults can act as conduits for radon migration while impermeable surface deposits, such as till, may form a surface capping, reducing levels of radon reaching the ground surface. Although concentrations of radon in open air normally do not present a hazard, in poorly vented confined spaces the gas can accumulate and may cause problems to individuals exposed to it for long periods of time. 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, British Geological Survey, Keyworth.

Baseline geochemistry

Systematic sampling to establish geochemical baselines for stream waters, stream sediments and soils was undertaken throughout this sheet area as part of the Geochemical Baseline Survey of the Environment (G-BASE) project within BGS. The objective of geochemical mapping is to establish both the natural chemical baseline, and to assess where this has been altered by anthropogenic activity.

Stream sediment and water samples were collected at an average density of one site per 1.5 km², where the natural drainage conditions permit, and soil samples were on a grid of one site per 2 km², from both surface (5–20 cm) and deep (35–50 cm) depths. Whilst urban areas were avoided in the regional sampling campaign, the built-up area of Leicester was sampled as part of a companion programme to establish the urban soil baseline in our cities. The collection of these soils was in all ways identical to that of the regional survey, with the exception that they were retrieved at a density of four per km². All the sample media were analysed for at least 50 inorganic substances, all of which are naturally occurring and many of which may also be enhanced, or depleted, in concentration by anthropogenic activity. Further information on the project rationale, protocols and methods may be found in Johnson et al. (2005) and Fordyce et al. (2005).

Illustrative examples of the resulting information are provided for this sheet area in (Figure 18) and (Figure 19), derived from 340 regional soils and 660 urban soils from Leicester. These two datasets demonstrate the influence of the geological environment, and the benefit of placing the urban data in context within the regional background. For example, the largest area of high arsenic concentrations in soils occur over parent materials consisting of the youngest Lias and the Inferior Oolite groups (Figure 18); compare with (Figure 1), and are far higher than those over other parent materials and over much of the urban area. Similarly high soil arsenic concentrations, due to entirely geological sources and natural processes, have been recognised previously over the same strata farther north as a result of an earlier phase of geochemical mapping (Breward, 2007; Palumbo-Roe et al., 2005), and the same interpretation can be extended to these data. Additionally, relatively high concentrations of arsenic can be seen in the centre of the urban area, largely over the alluvium of the river Soar. The urban nature and industrial history of Leicester may contribute contaminant arsenic, but there may also be an accumulation of naturally derived arsenic in the Quaternary riverine sediments. The lead map (Figure 19) shows that industrial and general urban activities have given rise to what is a clear anthropogenic anomaly, when compared with the regional baseline. Further details of the data acquired in the Leicester survey can be found in Scheib et al. (2008), and the regional data are currently being described for later release in a publication, the Central and Eastern England Geochemical Atlas. All data are, however, available to license by contacting the BGS Enquiries Desk, British Geological Survey, Keyworth.

Earthquakes

Earthquakes present an unpredictable and considerable hazard to development in certain parts of the world; however, in a country of low seismicity such as Britain, much historical research is required to assess seismic risk (e.g. Neilson et al., 1984). The largest seismic event to be recorded recently in this region was the Melton Mowbray earthquake of 28 October, 2001, the epicentre of which was located near Eastwell, 13 km north of the district. With a magnitude of 4.1 ML (intensity 5+ EMS), this was the fourth largest event to have been recorded by instruments in the region (BGS Global Seismology Dept, 2001). It was felt across much of the East Midlands, and as far afield as Warwickshire, and caused minor damage to chimneys and the walls of some houses. The cause of the earthquake is unknown, but the focus, at an estimated depth of 11.6 km, could reflect movement due to readjustments along structures such as the Sileby Fault (Carney et al., 2004).

Examination of the historical database shows that the main seismic risk in the district is a repeat of the Derby earthquake of 11 February 1957. With a magnitude of 5.3 ML (aftershock: 4.2 ML) and maximum intensity of 6 to 7 EMS, it was one of the most damaging UK earthquakes of the 20th century and was felt over the English Midlands, and as far as Hartlepool, Pwllheli, Norwich and Topsham (near Exeter). The epicentre was located about 12 km north-west of the district, near to Diseworth [SK 450 250] (see review in Carney et al., 2001).

Conservation

Geological sites of scientific and educational interest

Exposures of rocks and superficial deposits, which can demonstrate the geology of the area, are a considerable resource for educational and research purposes. In the Leicester district, such exposures 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 Sites (RIGS) or Local Nature Reserves (LNR). Listings of SSSIs and RIGS for this district are given in (Table 9).

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 organisations, as part of the Leicestershire and Rutland Local Geodiversity Action Plan financed by the Aggregates Levy Sustainability Fund. The project, described in Ambrose (2004), contains databases of geological, conservation and habitat information, and at the time of writing a number of new RIGS sites were under review. All of this information can be accessed either through the BGS and its project partners, or through the project’s website, which is hosted by the BGS website.

Information sources

Further geological information relevant to this district and held by the British Geological Survey is listed below. Searches of indexes to some collections can be made on computerised databases, either held at BGS or available on the BGS website (www.bgs.ac.uk). The latter contains details of BGS activities, services, data etc, including: summaries of BGS projects, BGS products and contact points for advice on a wide range of issues. A Geoscience Data Index (GDI) is available on the website, with details of borehole and seismic line locations, topographical backdrops based on various map scales, multi-scale geology of the UK and many other items of data. Geological information is also available by enquiry to the BGS Sales Desk at Keyworth. Digital geological coverage of the UK at 1:50 000 scale (DIGMAP) is now available, by enquiry to the BGS Sales Desk at Keyworth. Geological reports (Georeports) on specific sites can be requested by contacting the BGS Enquiries service.

Maps

Geology

• 1:625 000
• Solid geology map UK (south sheet), 1979; Quaternary Geology, 1977
• 1:500 000
• Tectonic map of Britain, Ireland and adjacent areas, 1996.
• 1:250 000
• Solid geology, East Midlands, 1983
• 1:100 000
• Mineral resources map for Leicestershire/Rutland, 2002
• 1:50 000 and 1:63 360
• Sheet 142 Melton Mowbray (Solid & Drift, with optional Sheet Explanation), 2002
• Sheet 141 Loughborough (Solid & Drift, with optional Sheet Explanation), 2001
• Sheet 155 Coalville (Solid & Drift), 1982 (a revised version is currently in preparation)
• Sheet 157 Bourne (Solid & Drift), 1964
• Sheet 170 Market Harborough (Solid & Drift), 1969
• 1:10 000 and 1:10 560
• Details of the original geological surveys are listed on editions of the 1:63360 geological sheets, or can be accessed from the BGS Geoscience Data Index. Copies of maps of these earlier surveys may be consulted at the BGS Library, Keyworth. The new maps covering the 1:50000 Series Sheet156 at 1:10000 scale are available in hard-copy (coloured) or digital formats and are indicated at the foot of Sheet 156 Leicester.

Geophysical maps

• 1:1 500000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas (1997). Smith, I F, and Edwards, J W F (compilers), British Geological Survey.
• Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas (1998). Smith, I F, and Royles, C P (compilers), British Geological Survey

Geochemical data and information

Regional baseline geochemical data are available for soils, stream waters and stream sediments in this area from the G-BASE survey. These data are currently being described for publication in the Central and Eastern England Geochemical Atlas (Scheib et al., 2008) and are available to licence through the BGS Enquiries Desk, Keyworth. General information on the project can be found at http://www.bgs.ac.uk/gbase

Hydrogeological map

• 1:625000
• England and Wales, 1977
• 1:100 000
• Groundwater vulnerability (Sheet 23)

Publications

Memoirs, books, reports and papers of the BGS relevant to the district arranged by topic. Most are either out of print or are not widely available, but may be consulted at BGS and other libraries. Some of these publications are cited in the References.

• British Regional Geology: Central England, 1969
• Memoirs and Sheet Descriptions (of this and fully adjacent map sheets)
• Sheet 142 Melton Mowbray. Memoir, 1909^‡4 
• Sheet 142 Geology of the country around Melton Mowbray. Sheet Description, 2004
• Sheet 156 Leicester. Memoir, 1903^‡5 
• Sheet 155 Coalville. Memoir, 1988
• Sheet 170 Market Harborough. Memoir, 1968^‡6 

Economic geology

• The Liassic ironstones (Mesozoic ironstones of England). Memoir, 1952^‡7 

Hydrogeology

• Wells and springs of Leicestershire. Memoir, 1931^‡8 .

Documentary collections

Basic geological survey information, which includes 1:10000 or 1:10560 scale field slips and accompanying field notebooks are archived at the 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, for these boreholes has been digitised. Summary details of boreholes mentioned in this report are given in Table If2.

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 whole of the district.

BGS Lexicon of named rock unit definition

Definitions of the rocks and superficial deposits on BGS maps, including those shown on the 1:50000 Series Sheet156 Leicester, are held in the Lexicon database, available through the BGS Website (see below). 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 the present resurvey or previous surveys are deposited for reference in the BGS library, Keyworth, and are indexed in the BGS Website. Colour or black and white prints and transparencies can be supplied at a fixed tariff. A comprehensive set of digital images is also available in the database of the Leicestershire and Rutland Geodiversity Action Plan (Ambrose, 2004), a copy of which resides at BGS.

Geochemical samples

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

A regional geochemical survey, involving soil samples and stream sediment analysis of the urban and rural areas has recently been completed and the data is now available for use, on application to the BGS Keyworth office.

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 group 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.

Collections held outside 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, which includes a full audit of geological and landscape features, sites relevant to local authority geodiversity issues, geological trails and education packs, as detailed in Ambrose (2004).

Addresses for data sources

• BGS Hydrogeology Enquiry Service: wells, springs and water borehole records.
• British Geological Survey, Hydrogeology Group, Maclean Building , Crowmarsh Gifford, Wallingford, Oxfordshire OXO 8BB. Telephone 01491 838800. Fax 01491 692345.
• London Information Office at the Natural History Museum, Cromwell Road, London, SW7 5BD. Telephone 0171 589 4090; Fax 0171 584 8270
• British Geological Survey (Headquarters), Keyworth, Nottingham, NG12 5GG. Telephone 0115 936 3100; Fax 0115 936 3200. Website http://www.bgs.ac.uk

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. 1999. Geology of the Ab Kettleby area (SK72SW). British Geological Survey Technical Report, WA/99/20.

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

Ambrose, K. 2001b. Geology of the Somerby area (SK71SE). British Geological Survey Technical Report, WA/01/09.

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

Ambrose, K. 2004b. Geology of the Whissendine area (SK81SW). British Geological Survey Internal Report, IR04/116.

Appleton, J D, Hooker, P J, and Smith, N J P. 1995. Methane, carbon dioxide, and oil seeps from natural sources and mining area: characteristics, extent, and relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/1.

Arkell, W J. 1933. The Jurassic System in Great Britain. No. 681. (Oxford: Oxford University Press.) (Leicestershire pp.110–1, 133, 157–8, 179, 207–8, 210–15, 305–11).

Arter, G. 1982. Geophysical investigations of the deep geology of the East Midlands. Unpublished PhD Thesis, University of Leicester.

Arthurton, R.S. 1980. Rhythmic sedimentary sequences in the Triassic Keuper Marl (Mercia Mudstone Group) of Cheshire, northwest England. Geological Journal, Vol. 126, 43–58.

Atkinson, J H, Fookes, P G, Miglio, B F, and Pettifer, G S. 2003. Destructuring and disaggregation of Mercia Mudstone during full-face tunnelling. Quarterly Journal of Engineering Geology and Hydrogeology, Vol. 36, 293–303.

Aucott, J W, and Clarke, R H. 1966. Amino-acids in the Mountsorrel bitumen, Leicestershire. Nature, Vol. 212 (5057), 61–63.

Bates, E F, and Hodges, S L. 1886. Notes on a recent exposure of the Lower Lias and Rhaetics in the Spinney Hills, Leicester. Transactions of the Leicester Literary and Philosophical Society, Vol. 1, 22–23.

Bates, R L, and Jackson, J A. 1980. Glossary of geology. Second edition. (Falls Church, Va: American Geological Institute.)

Bath, A H. 1974. Rb-Sr isotope data from unfossiliferous shales from drill-core from Lincolnshire (Thorpe by Water Borehole). British Geological Survey unpublished report.

Bell, F G, Coope, G R, Rice, R J, and Riley, T H. 1972. Mid-Weichselian fossil-bearing deposits at Syston, Leicestershire. Proceedings of the Geologists’ Association, Vol. 83, 197–211.

Berridge, N G, Pattison, J, Samuel, M D A, Brandon, A, Howard, A S, Pharaoh, T C, and Riley, N J. 1999. Geology of the country around Grantham. Memoir of the British Geological Survey, Sheet 127 (England and Wales).

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

Blake, D M. 1987. The Middle Lias silts and clays and Amaltheid ammonites from Tilton railway cutting, Leicestershire. Transactions of the Leicester Literary and Philosophical Society, Vol. 81, 29–34.

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

Bosworth, T O. 1912. The Keuper Marls Around Charnwood Forest. Leicester Literary and Philosophical Society (Leicester: Thornley and Son).

Boucher, K. 1994. Mineral Resources. 22–32 in Loughborough and its region. Boucher, K (editor). (Loughborough: Quorn Selective Repro.)

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

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

Brandon, A. 1999. Geology of the Wreake valley (SK61NE, SK71NW, SK71NE and SK81NW (western part). British Geological Survey Technical Report, WA/99/17.

Brandon, A, Sumbler, M G, and Ivimey-Cook, H C. 1990. A revised lithostratigraphy for the Lower and Middle Lias (Lower Jurassic) east of Nottingham, England. Proceedings of the Yorkshire Geological Society, Vol. 48, 121–141.

Breward, N. 2007. Arsenic and presumed resistate trace element geochemistry of the Lincolnshire (UK) sedimentary ironstones, as revealed by a regional geochemical survey using soil, water and stream sediment sampling. Applied Geochemistry 22, 1970–1993.

Building Research Establishment. 1996. Sulfate and acid resistance of concrete in the ground. Digest 363. (London: HMSO.)

Building Research Establishment. 2005. Special Digest 1: concrete in aggressive ground (SD1). (London: BRE Bookshop.)

Briant, R M, Coope, R G, Preece, R C, Keen, D H, Boreham, S, Griffiths, H I, Seddon, M B, and Gibbard, P L. 2004. Fluvial system response to Late Devensian (Weichselian) aridity, Baston, Lincolnshire, England. Journal of Quaternary Science, Vol. 19, 479–495.

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

British Geological Survey. 2001. Loughborough, England and Wales Sheet 14. Solid and Drift geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey.)

British Geological Survey. 2002. Leicestershire and Rutland: mineral resource information in support of national, regional and local planning. 1:100 000. (Keyworth, Nottingham: British Geological Survey.)

Brown, A G. 1999. Characterising prehistoric lowland environments using local pollen assemblages. Quaternary Proceedings, No. 7, 585–594.

Brown, A G, Keough, M K, and Rice, R J. 1994. Floodplain evolution in the East Midlands, United Kingdom: the Lateglacial and Flandrian alluvial record from the Soar and Nene valleys. Philosophical Transactions of the Royal Society of London, Series A. Vol. 348, 261–293.

Browne, M. 1889. The vertebrate animals of Leicestershire and Rutland. (Birmingham, England: Midland Educational Company, Ltd.)

Browne, M. 1893. Contributions to the history of the geology of the Borough of Leicester. Transactions of the Leicester Literary and Philosophical Society, Vol. 3, 123–239.

Browne, M. 1902. The geology of the Beaumont Leys Estate. Transactions of the Leicester Literary and Philosophical Society, Vol. 7, 5–41.

Browne, M, and Fox-Strangways, C 1901. Excursion to Barrow and Mount Sorrel. Transactions of the Leicester Literary and Philosophical Society, Vol. 6, 45–46.

BSI. 1990. British Standard methods of test for soils for civil engineering purposes. British Standards Institution, BS 1377.

Busby, J P, Kimbell, G S, and Pharaoh, T C. 1993. Integrated geophysical/geological modelling of the Caledonian and Precambrian basement of southern Britain. Geological Magazine, Vol. 130, 593–604.

Butterley, A D, and Mitchell, G H. 1945. Driving of two shafts by the Desford Coal Co. Ltd., at Merry Lees. Transactions of the Institute of Mining Engineers, Vol. 104, 703–713.

Carney, J N. 1994. Geology of the Thringstone, Shepshed and Loughborough districts: 1:10 000 sheets SK41NW, SK41NE and SK51NW. British Geological Survey Technical Report, WA/94/08.

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

Carney, J N. 2000a. Igneous processes within late Precambrian volcanic centres near Whitwick, northwestern Charnwood Forest. Mercian Geologist, Vol. 15, 7–29.

Carney, J N. 2000b. Outwoods-Hangingstone Hills. 43–48 in Precambrian rocks of England and Wales. Geological Conservation Review Series, No. 20. (Peterborough: Joint Nature Conservation Committee.)

Carney, J N. 2000c. Geology of the Barrow-upon-Soar and Seagrave areas: 1:10 000 sheets SK51NE and SK61NW. British Geological Survey Technical Report, WA/00/21.

Carney, J N, and Pharaoh, T C. 1999. Buddon Hill. 224–227 in Caledonian igneous rocks of England and Wales. Geological Conservation Review Series, No. 17. (Peterborough: Joint Nature Conservation Committee.)

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, (Peterborough: Joint Nature Conservation Committee.)

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Taylor, J H. 1949. The Mesozoic ironstones of England. Petrology of the Northampton Sand Ironstone Formation. Memoir of the Geological Survey of Great Britain. (London: HMSO.)

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Figures, plates and tables

Figures

(Figure 1) Simplified map of the main bedrock units and geological structures.

(Figure 2) Principal physical features and drainage of the district.

(Figure 3) Simplified geological sketch map of the Mountsorrel Complex. Triassic strata and Quaternary deposits are omitted for clarity.

(Table 10) for further details." data-name="images/P946714.jpg">(Figure 4a) Isopachyte maps of Triassic strata. (Compiled by N J P Smith, BGS.) Sherwood sandstone Group Boreholes: CO Countesthorpe (Cottage Homes); CH Crown Hills; HF Hall Farm; KF Knighton Fields; KL Kirby Lane; LF Lodge Farm (Spinney Hills); LFE Leicester Forest East; NR Nansen Road; TBW Thorpe by Water; WB Willow Brook. See (Table 10) for further details.

(Table 10) for further details." data-name="images/P946715.jpg">(Figure 4b) Isopachyte maps of Triassic strata. (Compiled by N J P Smith, BGS.) Mercia Mudstone.Boreholes: CO Countesthorpe (Cottage Homes); CH Crown Hills; HF Hall Farm; KF Knighton Fields; KL Kirby Lane; LF Lodge Farm (Spinney Hills); LFE Leicester Forest East; NR Nansen Road; TBW Thorpe by Water; WB Willow Brook. See (Table 10) for further details.

(Figure 5) Lias Group: comparative stratigraphical sections with boreholes drilled in adjacent districts. Faunal stages shown on the left of the diagram are those identified in the Melton Mowbray district (Carney et al., 2004).The red lines link units that can be correlated on grounds of lithology (Crown Hills Borehole) or geophysical log signatures (gamma ray and sonic). Depths to right of columns are in metres below ground level. See text for explanation of nomenclature.

(Figure 7) for locations." data-name="images/P946717.jpg">(Figure 6) Marlstone Rock Formation: comparative sections the across the Leicester district incorporating information from Hallam (1955, 1968), Howarth (1980), Blake (1987), Wilkinson (2001) and Riding (2001).See (Figure 7) for locations.

(Figure 7) Structure contour map for the Marlstone Rock Formation. Where the formation is concealed by Quaternary deposits, depth to the base can be estimated for any given location by subtracting the structure contour value from the topographic elevation.

(Figure 8) Rock head contour map for the western part of the district compiled from borehole and field survey data. The map shows the reconstructed distribution of the preglacial Bytham River deposits and the major palaeo valleys filled with glacial deposits, as they are inferred to have existed in Middle Pleistocene times.

(Figure 9a) Relationship between the Bytham Formation and other Quaternary deposits in the Wreake valley (modified from Brandon, 1999). Cross-section along the line shown in 9b. Selected boreholes are shown: to obtain the full BGS borehole reference number add the 10K sheet number (SK61SE or SK61NE) as a prefix. Other nearby boreholes, up to 100 m, have been projected into the line of the section.

(Figure 9b) Relationship between the Bytham Formation and other Quaternary deposits in the Wreake valley (modified from Brandon, 1999).The course of the Bytham valley around Brooksby, reconstructed from borehole data and field evidence. Quaternary glacigenic deposits are not shown, but see (Figure 9a).

(Figure 10) Section through tectonised Quaternary deposits formerly seen in the western bank of theM1 Motorway cutting near Enderby Grange (modified from Poole, 1968). The location of the cutting is shown in (Figure 8). The positions of the glacio-tectonic thrust faults are conjectural, and have been added by the present authors.

(Figure 11) River Soar terrace deposit and alluvium long profiles. Nomenclature follows that used on Sheet 156 Leicester. See Brandon (1999) for long profiles in the Wreake Valley.

(Figure 12) Pre-Triassic geology of the district, showing the subcrop of Carboniferous strata and of pre-Carboniferous ‘basement’ (modified from interpretations by N J P Smith and T C Pharaoh, BGS). Igneous intrusions: CP Countesthorpe; CR Croft; E Enderby; M Mountsorrel Complex; MM Melton Mowbray Granodiorite; R Rempstone Granodiorite. Boreholes: CH Crown Hills; CO Cottage Homes (Countesthorpe); HF Home Farm; KF Knighton Fields; KL Kirby Lane; LF Lodge Farm (Spinney Hills); LFE Leicester Forest East; TBW Thorpe By Water; WB Willow Brook

(Figure 13) Bouguer gravity anomaly map. The anomalies are shown as colour-shaded relief illuminated by an imaginary light source located to the north. A variable Bouguer reduction density has been used to calculate the anomalies. Contour interval 5 milligal (1 milligal=10–5m/s2).

(Figure 14) Aeromagnetic anomaly map. Total field magnetic anomalies are shown as colour shaded relief illuminated by an imaginary light source located to the north. Contour interval 50 nanotesla.

(Figure 15) Geophysical summary map showing Bouguer gravity anomalies (black contour lines), magnetic highs (red hatching), and principal lineaments (dashed blue lines GL1–7). Numbered featuresG1–5 and M1–10 are gravity and magnetic features referred to in the text.

(Figure 16) Horizontal derivative of magnetic anomalies depicting intensity of magnetic gradient: reds and browns indicate highs, greens and blues indicate lows. Shaded relief image with equal area colour-fill, illuminated from the north. Contour interval 25 nanotesla. The aeromagnetic anomalies can be identified by reference to (Figure 14) and (Figure 15).

(Figure 17) Casagrande plasticity plot for borehole samples of unweathered Oadby Till and Thrussington Till. The A-Line represents a boundary below which the behaviour of the deposit is considered to be abnormal.

(Figure 18) Arsenic concentrations in profile (35–50 cm depth) soils of the Leicester sheet area.

(Figure 19) Lead concentrations in profile (35–50 cm depth) soils of the Leicester sheet area.

Plates

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

(Plate 1) Graded volcani­clastic strata of the Bradgate Formation exposed in a small quarry south of Coppice Plantation at Bradgate Park. The upper bed fines upwards from medium-grained sandstone to siltstone and shows possible dune bedforms at the base. The scale bar is graduated in centimetres. (P609270).

(Plate 2) Small-pebble conglomerate of the Hanging Rocks Formation, exposed in Bradgate Park (P609271).

(Plate 3a) Stockingford Shale Group: close-up photographs of thin sections from the base of the Crown Hills Borehole. Thick laminae of siltstone/fine sandstone (pale grey) with possible normal grading (slight fining from right to left) and layer-confined soft sediment deformation. The section (E50335) from 291 m depth, measures 36 mm across (P609272).

(Plate 3b) Stockingford Shale Group: close-up photographs of thin sections from the base of the Crown Hills Borehole. The pale grey siltstone laminae show extensive soft-sediment disruption, in part accompanied by the development of an oblique fabric. The section (E50333) from 261.4 m depth, measures 45 mm across (P609273).

(Plate 4a) Mudrock lithologies baked by the Mountsorrel Complex, exposed at localities shown in (Figure 3). Polished slab of hornfelsed mudrock (JNC 850) at the ‘old gravel pits’, showing nodular development of metamorphic minerals and a pink granitic veinlet. Actual width of view is 55 mm (P609274).

(Plate 4b) Mudrock lithologies baked by the Mountsorrel Complex, exposed at localities shown in (Figure 3). Folded and cleaved mudrock in the south-eastern wall of a partly flooded quarry; western shore of Swithland Reservoir opposite Brazil Wood (P599668).

(Plate 5a) Mountsorrel Complex granodiorite. Photomicrograph (E73857): field of view is approximately 15 mm wide (P609275).

(Plate 5b) Mountsorrel Complex granodiorite. Summit of ‘Craig Buddon’, showing coarse-grained granodiorite of the Mountsorrel Complex cut by two aplite sheets (P599686).

(Plate 6a) Xenolithicgranodiorite at Kinchley Hill. Large diorite xenolith (to right of hammer head) in granodiorite. The hexagonal outline and concentric internal structure of the xenolith suggests cooling upon incorporation into the granodiorite. Reproduced as a scanned image from fig. 11 of Lowe (1926) (P609276).

(Plate 6b) Xenolithicgranodiorite at Kinchley Hill. Polished slab of junction between a diorite xenolith (dark area to right) and ‘basified’ Mountsorrel granodiorite. Field of view is 100 mm wide. (P609277).

(Plate 7a) Kinchley xenoliths andhornfelsed mudrock. Photomicrograph (E73865) of the diorite xenolith contact shown in (Plate 6b). A prominent rounded feldspar crystal juts out from the main body of the granodiorite, which occupies the lower right part of the image. Note the tendency for small plagioclase laths in the diorite xenolith to show a tangential orientation against the left-hand side of the prominent feldspar, suggesting flowage and/or compression of the diorite during incorporation into the granodiorite. Field of view is about 10 mm wide (P609278).

(Plate 7b) Kinchley xenoliths andhornfelsed mudrock. Photomicrograph (E74301) of mudrock from the Kinchley shore, metamorphosed at pyroxene hornfels facies. Note the development of coarse white mica plates (red and greeninterference colours) and, in centrefield, two interpenetrating laths of the orthopyroxene, enstatite (En), showing grey interference colours. Field of viewis 1.2 mm wide (P609279).

(Plate 8a) Dioritic facies of the Mountsorrel Complex. Inequigranular quartz-diorite showing pink plagioclase megacrysts, exposed near summit of Brazil Wood (P599687).

(Plate 8b) Dioritic facies of the Mountsorrel Complex. Photomicrograph (E73866) of Brazil Wood diorite showing common laths of partly altered hornblende (red and blue interference colours), zoned plagioclase and poikilitic quartz (white areas in lower part of slide). Field of view is 8 mm wide (P609280).

(Plate 8c) Dioritic facies of the Mountsorrel Complex. Photomicrograph (E74303) of fine-grained quartz-diorite at the Kinchley shore locality. Note the large poikilitic plates of biotite (red interference colours). Field of view is 1.7 mm wide (P609281).

(Plate 9) Polished slab of quartz-diorite (E62188), representing part of the South Leicestershire Diorites body formerly worked at Enderby Warren Quarry. The specimen is 42 mm wide (P609282).

(Plate 19a). The unconformable contact with underlying pale grey Mountsorrel granodiorite is seen to the right of the photograph (P609283)." data-name="images/P609283.jpg">(Plate 10a) Triassic strata at Buddon Wood Quarry. Catenary dip in ?Gunthorpe Formation of Mercia Mudstone Group, see also, (Plate 19a). The unconformable contact with underlying pale grey Mountsorrel granodiorite is seen to the right of the photograph (P609283).

(Plate 10b) Triassic strata at Buddon Wood Quarry. In the northern part of the Quarry, a newly exhumed exposure shows that the surface of the granodiorite is smooth, with parallel grooves or flutes (some containing adherences of Mercia Mudstone) caused by Permo-Triassic aeolian erosion and ‘sand-blasting’ of the bare rock surface prior to its covering by the Mercia Mudstone (P601557).

(Plate 11a) Mercia Mudstone exposed in former brick pits. Edwalton Formation at the Albion/Phoenix brickworks on the southern outskirts of Sileby. The pale grey beds represent intercalations of dolomitic siltstone (G. Warrington, 1966) (P609284).

(Plate 11b) Mercia Mudstone exposed in former brick pits. Upper part of the Cropwell Bishop Formation at the former Sherrif’s brickpit at Gipsy Lane, showing beds of white gypsum. Note the irregular top surface of the uppermost gypsum bed, indicative of dissolution (P549737).

(Plate 12a) Hollygate Sandstone Member at Western Park. Cross-bedded strata in the railway cutting (P549720)

(Plate 12b) Hollygate Sandstone Member at Western Park. Photomicrograph of the sandstone. It is fine- to medium-grained and composed mainly of subrounded quartz grains and subordinate feldspar (yellow-stained, with grainy texture). The specimen is weakly cemented with abundant voids (blue areas) and a macroporosity rating of very high. Photograph by G K Lott; width of photo is 3 mm (P609285).

(Plate 13) Blue Anchor Formation (green strata near top of face) exposed above red-brown mudstone of the Cropwell Bishop Formation at the Gipsy Lane SSSI (P549738).

(Plate 14a) Marlstone Rock exposures. Tilton railway cutting SSSI: annotated with the principal divisions including the brachiopod beds A and B of Hallam (1955) (P601558).

(Plate 14b) Marlstone Rock exposures. Loddington: showing base of the Marlstone resting on Dyrham Formation (P549904).

(Plate 15) Polished slab showing burrowed top surface of the Marlstone Rock Formation at the Tilton railway cutting (P609286).

(Plate 16) Polished slab from the base of the Marlstone Rock Formation at Tilton railway cutting, showing burrowing and shell-rich pockets (P609287).

(Plate 17a) Samples of Quaternary deposits cored from a borehole (SK50SE/763) at Leicester University campus. Glaciolacustrine clay and silt, between 9.7 to 10.0 m depth, showing disrupted laminae (P609288).

(Plate 17b) Samples of Quaternary deposits cored from a borehole (SK50SE/763) at Leicester University campus. Thrussington Tillwith tabular fragments ofdolomitic siltstone (pale green-grey), coal (black, lower right), quartzose pebbles and Mercia Mudstone (small, red, rounded fragments). Core is from 10.8 to 11.03 m depth. (P609298).

(Plate 18a) Further samples of Quaternary deposits from the Leicester University borehole. Thrussington Till with faint stratification, in part indicated by parallel orientation of tabular rock fragments. Core is from 13.22 to 13.47 m depth (P609290).

(Plate 18b) Further samples of Quaternary deposits from the Leicester University borehole. Glaciolacustrine silt and clay showing disrupted lamination. Core is from 15.34 to 15.62 m depth. (P609291).

(Plate 19a) Oadby Till on the south-eastern face of Buddon Wood Quarry. The till (grey) infills a palaeovalley cut into the Mercia Mudstone Group (red-brown). The latter in turn abuts the rising surface of the Mountsorrel Complex, which occurs as pale grey, rounded masses projecting through Mercia Mudstone and into the Oadby Till at far right of photograph (P599688).

(Plate 19b) Oadby Till on the south-eastern face of Buddon Wood Quarry. Close up of the Oadby Till showing white fragments of Chalk. Photo taken on the higher part of the quarry face, shown near the right-hand margin of (Plate 19a) (P599689).

(Plate 20) Steeply dipping glaciofluvial deposits in a disused quarry north of Tilton on the Hill. The upper, structureless gravel layer is probably composed of head (P549906).

(Plate 21) Aerial view of the south-western face of Buddon Wood Quarry. The Kinchley shore of Swithland Reservoir is in the background (P540721).

(Back cover)

Tables

(Table 2)." data-name="images/P946735.jpg">(Table 1) Geological succession of the Leicester district. Thickness range shown in brackets in metres. ka age in thousand years. Ma age in million years. *A new national lithostratigraphical scheme of nomenclature for the Mercia Mudstone is indicated in (Table 2).

(Table 2) Comparative lithostratigraphy for Triassic strata and the lower part of the Lias Group.

(Table 3) Correlation of the Quaternary deposits of the Leicester district. (correlations are modified from Brandon, 1999; Carney et al., 2004). *River terrace deposit members of the Soar Valley Formation.

(Table 4) Physical properties of rock units in the district and immediate environs (from BGS databases). n/d not determined.

(Table 5) Licensed water use in the district (derived from data provided by the Environment Agency in 1999). Note that the original listing does not differentiate between the wells and boreholes that penetrate each of these three minor aquifers. It has, therefore, been necessary to correlate the abstractions with specific borehole or well records held in the BGS National Water Well Archive in order to define from which aquifer the abstraction is occurring. Where it has not been possible to make such a correlation, the location of abstractions with regard to the underlying geology was used to designate the most proable aquifer.

(Table 6) Engineering geological summary - superficial deposits (‘soil’ units). *See (Figure 17).

(Table 7) Engineering geological summary - bedrock units. † Cone penetrometer test

(Table 8) Summary of selected geotechnical site investigation data for the East Midlands, taken fromHobbs et al. (2002) and Hobbs et al. (2004) respectively (median values only are shown). The table refers to any state encountered by the site investigation; a weathering state is not usually specified, although most site investigation data are from <15 m depth. WL (%) Liquid limit; Ip (%) Plasticity index; Cu (kPa) Undrained shear strength; SO4 (tot) Sulphate content (total); Intrm Intermediate.

(Table 9) Geological conservation sites in the district. Regionally Important Geological Sites (RIGS), and Sites of Special Scientific Interest (SSSI).

(Table 10) Brief details of boreholes mentioned in this report. *Borehole cited in text but located outside of the Leicester district.

Tables

(Table 2)." data-name="images/P946735.jpg">(Table 1) Geological succession of the Leicester district.

Thickness range shown in brackets in metres, ka - age in thousand years, Ma - age in million years. * A new national lithostratigraphical scheme of nomenclature for the Mercia Mudstone is indicated in (Table 2).

(Table 3) Correlation of the Quaternary deposits of the Leicester district

(correlations are modified from Brandon, 1999; Carney et al., 2004). *River terrace deposit members of the Soar Valley Formation.

(Table 4) Physical properties of rock units in the district and immediate environs (from BGS databases). n/d not determined

(Table 5) Licensed water use in the district (derived from data provided by the Environment Agency in 1999)

Note that the original listing does not differentiate between the wells and boreholes that penetrate each of these three minor aquifers. It has, therefore, been necessary to correlate the abstractions with specific borehole or well records held in the BGS National Water Well Archive in order to define from which aquifer the abstraction is occurring. Where it has not been possible to make such a correlation, the location of abstractions with regard to the underlying geology was used to designate the most proable aquifer.

(Table 7) Engineering geological summary - bedrock units. † Cone penetrometer test

(Table 8) Summary of selected geotechnical site investigation data for the East Midlands

Taken from Hobbs et al. (2002) and Hobbs et al. (2004) respectively (median values only are shown). The table refers to any state encountered by the site investigation; a weathering state is not usually specified, although most site investigation data are from <15 m depth. W_L (%) - Liquid limit, I_p (%) - Plasticity index, C_u (kPa) - Undrained shear strength, SO₄ (tot) - Sulphate content (total), Intrm - Intermediate.

(Table 9) Geological conservation sites in the district. Regionally Important Geological Sites (RIGS), and Sites of Special Scientific Interest (SSSI)

(Table 10) Brief details of boreholes mentioned in this report

* borehole cited in text but located outside of the Leicester district.