Upper Carboniferous

During Namurian times, when the oldest rocks (c. 315 Ma) that crop out in the Wigan area were deposited, the Lancashire area lay within a large, actively subsiding basin inundated by the sea. Extensive delta systems, fed with sediments eroded from the surrounding land surfaces, built out into the basin. As may be seen in modern delta systems, coarser-grained sediments (up to coarse sandstones) were deposited at the active fronts of the delta systems, whereas fine-grained background sedimentation, usually of mud-sized sediments, occurred in more quiescent areas of the basin. Toward the end of the Namurian, sedimentation periodically outstripped subsidence, and peaty swamps covered by dense forests of horsetails, club mosses, tree ferns and primitive conifers were established. Variations in sea level were reflected by distinct cycles of sedimentation, or cyclothems, each beginning at a sea-level highstand with the deposition of marine or near-marine shales (including fossiliferous ‘Marine Bands’), which passed up through mudstones into deltaic siltstones and sandstones. Through Westphalian times, from 315 to c. 300 Ma, the same pattern continued but with increasing dominance of shallow-water deposits. The cycles increased in number but became, generally, thinner. Periods when the area was above sea level and land flora were abundant became more frequent and, in many cases, more prolonged, resulting in complicated patterns of coal seam development. Although the general range of lithologies deposited was the same as during Namurian times, the lateral development of, and interrelationships between, the lithologies was far more complex, reflecting the different environments of deposition found in a large delta complex. The rocks deposited prior to the Westphalian have commonly been known as the Millstone Grit Group, whereas those deposited during the Westphalian are collectively known as the Coal Measures.

A period of structural upheaval after the deposition of the Coal Measures was followed by regional uplift and erosion prior to deposition of the Permo-Triassic strata. The effects of this period of earth movement, the Variscan Orogeny, commenced in the late Carboniferous, at c. 290 Ma. In the north of the Wigan area, pre-Permian uplift and erosion were such that Permo-Triassic strata rest directly on Westphalian A–C strata. In the south of the area, below the ground surface, the apparently conformable Westphalian D, Ardwick Group, is preserved, which suggests that continuous deposition took place until this time. It is estimated that between 500 and 1500 m of Variscan uplift and erosion occurred in the Wigan area. The resulting land surface was subjected to tropical weathering and the denuded Carboniferous rocks were superficially reddened by surface weathering.

Permo-Triassic

Following the Variscan Orogeny sedimentation recommenced in the early Permian, at c. 280 Ma, with a change to a new tectonic regime marked by widespread regional subsidence. Semi-arid, desert or near-desert, conditions prevailed and the Collyhurst Sandstone Formation of deep red, weak, medium- to fine-grained, well-sorted, non-pebbly sandstones was laid down and comprises fossil sand-dunes of wind-blown origin. The beds are generally lenticular and cross-bedding is common. Cementation generally is weak and, in extreme cases, the sediments remain as very dense sand. The lack of cementation gives the formation a high primary porosity.

Subsidence, briefly, exceeded sedimentation during the late Early Permian, with the deposition of the Manchester Marl Formation, which comprises dark, brick red or chocolate-brown mudstone, with minor, interbedded sandstone and limestone. It becomes coarser upwards, passing into interbedded mudstones, siltstones and sandstones that contain well-rounded, windblown sand grains. Periodic emergence is indicated by sand-infilled desiccation cracks. The area emerged during the later Permian and Triassic with the deposition of sediments in an arid, continental environment, the Sherwood Sandstone Group, which comprises weak, red–brown, fine- to medium-grained sandstone with beds of coarser well-rounded sand grains. A distinct facies, the Chester Pebble Beds, with well-rounded pebbles of quartz and quartzite, either randomly scattered or in crude lamination, can be distinguished from the otherwise uniform group.

Post-Triassic

The Wigan area is devoid of deposits laid down during the Jurassic, Cretaceous and Palaeogene, although apatite fission analyses of rocks in northern and central England suggest regional burial by over 1.5 km of deposits (Aitkenhead et al. 2002). Subsequent regional Palaeogene and early Neogene (Alpine) uplift and accompanying erosion removed this overburden and produced the Tertiary land surface, which was subsequently glaciated in the Quaternary.

Colliery spoil heaps

The intensive mining activity in the Wigan area has caused a huge volume of waste material to be brought to the surface. During early mining operations the amount of waste produced was comparatively low but once mechanization became commonplace the volumes of waste increased quickly, partly as a result of the growth of the industry and partly because of the less selective methods of mining. Usually the waste was carried by tramway or railway, conveyor belt or aerial ropeway away from the minehead and tipped on adjacent land. To minimize the distance of transportation and the area of land required, tips were very high and steep-sided, with slopes at the angle of repose of the waste material.

The survey of derelict land carried out by the Wigan Metropolitan Borough Council in 1993 classified 623 ha of spoil tips as derelict land. Older colliery spoil heaps may contain a relatively high proportion of coal and carbonaceous material that may catch fire and burn (‘spontaneous combustion’). In some instances the coal content of spoil heaps has been sufficiently high to make extraction of the coal economically viable. Coal-washing operations have been carried out at Bickershaw Colliery tip and Astley Green tip. Such operations are beneficial in that, as well as having economic benefit, they reduce the potential for spontaneous combustion.

The Mines and Quarries Inspectorate administers the Mines and Quarries (Tips) Act 1969 and has responsibility under that Act for the inspection of working tips. Responsibility for the inspection of disused tips rests, under part II of that Act, with local authorities. Unreclaimed colliery spoil heaps are inspected annually and short summary reports on the condition of tips are prepared with site locations plotted on 1:10 000 scale maps. The reports are principally concerned with safety aspects and potential hazards such as condition of shafts (and relevant safety measures), erosion, signs of combustion, leachate and standing water bodies.

Backfilled opencast coal sites

Modern opencast coal mining started in the Wigan area during the Second World War and a large number of small sites were worked until the late 1950s. There followed a break in opencast mining activity until the early 1970s, when the working of larger, deeper sites began. Mining operations were not always systematically recorded; sometimes only a rough outline of the worked site is available and details of the nature of backfill are lacking.

Most sites were worked and restored quickly, usually within 2 years of commencing. It is unlikely that any significant volumes of foreign waste were imported to early opencast sites because of the pressure to work and restore sites quickly, and the relatively high costs of road transport at that time. Subsequent site investigations on these sites have not encountered any materials other than overburden.

Later sites (post-1970s) tend to be larger and deeper, with higher overburden-to-coal ratios. They tended to work multiple seams, including thin seams not previously economically viable. It was not uncommon for sites to produce an excess of material because the volume of excavated material is greater as a result of the increase in voids when broken up (bulking). In general, the infill of opencast sites is low in coal and carbonaceous material and comprises mostly mudstone–siltstone and sandstone of the site overburden.

Clay

The clay resources of the area comprise superficial deposits of till and Carboniferous (Namurian and Westphalian) mudstones and fireclays. Both types have been used in brickmaking in the past but there are no active brick clay workings at present. The only clay extraction within the district was at Crankwood [SJ 624 998], near Abram, where very small amounts of till were worked in a site adjacent to the Leeds and Liverpool Canal for use as a ‘puddling clay’ by the British Waterways Board.

Glass sand

The Shirdley Hill Sand is characteristically uniform in both composition and grain-size distribution, reflecting its wind-blown origin. Most of the sand particles fall in the range 500–125 μm, which is ideal for glassmaking but unsuitable for construction use. The sand was formerly of considerable importance for the manufacture of flat glass in St Helens. Typically the top 0.5–1 m of purer sand beneath the topsoil was removed. Here the action of peaty groundwater had leached some of the iron with the effect of upgrading the upper part of the deposit. Because of the thin nature of the deposit large areas were rapidly worked and restored by installing drainage and spreading the topsoil behind the advancing face. However, extraction ceased in the mid-1970s.

Sand (aggregate)

Deposits of sand associated with till and Late Glacial drainage are fairly widespread in the Wigan area and have been of particular importance as an aggregate resource in the area to the north, near Standish, where most of the more recent operations have been located. For the most part, the deposits are fine grained and comprise sands, rather than gravel, which have been derived from local Carboniferous sandstones. They are usually suitable for use in mortar and asphalt and, after washing, for use as the fine aggregate in concrete, but are at the fine end of the specified range.

To the south of Wigan, Triassic sandstones of the Sherwood Sandstone Group are worked in conventional quarries and the sandstone is sold as crushed material for construction fill. The crushed rock is friable and easily disaggregates. It is too weak for use as concrete aggregate or as roadstone and does not conform to any of the British Standard specifications (BS 882, BSI 1992; BS 4987, BSI 2003). The upper part of the sandstone, immediately below the drift cover, is sometimes weathered to the extent that the cementing material has disintegrated leaving the original quartz grains as sand, which is sometimes worked.

Sandstone

Carboniferous sandstones from the Millstone Grit and Lower Coal Measures have been a traditional source of building stone and flagstone in Lancashire. The Cannel Rock and the sandstone above the Ince 7 foot coal were worked for local use on the Haigh Estate. Near Haigh village both the Trencherbone Rock and the Toddington Delf were also worked (Anderson & France 1994). During the last 50 years the stone markets have declined and sandstone is now used mainly for the production of crushed rock. Although sandstones within the Coal Measures occur extensively in the Wigan district, significant areas are covered with superficial deposits and resources are small.

In general, the Carboniferous sandstones in Lancashire are too weak and susceptible to frost damage for them to be used for roadstone or concrete aggregate. They may be used in road construction below the level of possible frost damage and for some of the less demanding concrete applications but, for the most part, crushed Carboniferous sandstone is used as a construction fill. Sandstones within the Triassic, Sherwood Sandstone Group are usually substantially weaker than Carboniferous sandstones. They are not worked in the Wigan area but where unweathered might find application as construction fill.

Peat

An extensive area of lowland peat occurs on Chat Moss in the extreme southeastern part of the area, and has been extensively worked for horticultural purposes, for use as either a growing medium or a soil conditioner.

Coal

Methane

Though coal mining is in decline there has been an increasing interest in the methane potential of the coalfields of Britain; methane could be extracted from abandoned coal mines and the network of dilated fractures in the rock surrounding them (abandoned mine methane) as well as from unmined coal (virgin coalbed methane) from surface boreholes. The mines in the Wigan Coalfield were recognized as producing large amounts of methane during their use and in abandonment have the potential for the extraction of significant amounts of methane, although potential quantities have not been calculated. The production of methane in this manner offers relatively easily won, clean fuel that although resulting in carbon dioxide on combustion, will also prevent methane, a much more potent greenhouse gas, escaping to the atmosphere.

Classification and material characteristics

The geological materials in the Wigan Metropolitan area have been classified on the basis of their engineering geological behaviour with regard to land use and construction (Table 1). The primary division is into ‘engineering rocks’ and ‘engineering soils’. In broad terms, engineering soils comprise the superficial geological deposits and some weathered, weak bedrock formations. Engineering rocks comprise the strong and unweathered ‘bedrock’ formations.

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Table 1

Engineering geological classification of gelogical formations in the Wigan Metropolitan Borough.

The primary divisions are subdivided on lithological grounds into ‘sandstone’ and ‘mudstone’ rocks, and the soils are divided into the equivalent ‘non-cohesive’ (sand) and ‘cohesive’ (clay) units. No mappable units of entirely cohesive soil are present in the area and a category of ‘mixed cohesive–noncohesive’ has been used to indicate units that comprise combinations of discrete bodies of cohesive and noncohesive material. ‘Organic’ soil, in the form of peat, is present in significant, mappable but geographically restricted amounts, and is found mainly in the moss lands in the SE of the area. A fourth category of ‘highly variable mixed’ material has been designated to describe the extensive areas of artificial deposits that cover much of the area.

The secondary divisions are further divided on the basis of strength in the case of the sandstones, mudstones and mixed cohesive–noncohesive materials, and on grain size for the noncohesive soils. The accuracy of the classification is constrained by the fact that it is based on mapping that was carried out for geological, not engineering geological or geotechnical, purposes.

The use of a conventional 2D engineering geological map is limited by its inability to convey information on materials below the surface. To give some indication of the materials and their engineering behaviour below the geological material exposed at the surface, Forster et al. (1995) produced a map using a variation of the ‘stripe’ method of Pasek & Rybar (1961) used by Dearman (1991) (Fig. 6). Where bedrock formations crop out at the surface, or where superficial deposits are greater than 5 m thick, the engineering geological unit is shown as a solid block of colour. Where the superficial material is less than 5 m thick the colour of the superficial engineering geological unit is shown in stripes, interspersed with stripes of the colour of the underlying bedrock colour. However, there is insufficient detail available to indicate where there are other superficial materials present between that exposed at the surface and the bedrock interface. Other superficial materials may be anticipated below thin alluvium or within till, where sand bodies are known to be present within the predominantly clay-rich material.

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Fig. 6

Engineering geology stripe map of part of the Wigan area around Leigh (for key see Table 1).

Shallow mining

The shallow mining map showed information regarding known workings, shafts and boreholes that had encountered old workings. However, other areas may contain unknown, unrecorded and so far undetected shallow workings. Three general categories that indicated the relative likelihood of unrecorded, old, abandoned, shallow mining being present were defined for planning guidance. These were defined using information such as coal seams that were known to have been worked in the area, the presence of these seams within 30 m of the surface and the geological structure defined by fault blocks.

A1 Abandoned shallow mine workings are unlikely. Areas where no coal seams are believed to occur within 30 m of the surface.

A2 Abandoned shallow mine workings are possible. Areas where coal is believed to occur within 30 m of the surface, but no evidence was found during the study of seams having been worked locally within the fault block, although they are known to have been mined elsewhere in the Borough.

A3 Abandoned shallow mine workings are possible and proven. Areas where coal is believed to occur within 30 m of the surface and evidence was found of seams having been worked within the fault block.

Potential for contaminated land

Many sites were identified that were being, or had been, used for industrial purposes but not all were potentially contaminative. Similarly, many areas of made ground were identified but only some were landfills, which were potentially significant to future land use. The judgement as to whether a land use was potentially contaminative or a significant land fill was based on categories of land quoted in the following documents:

The Department of the Environment and the Welsh Office joint circular 21/87. Development of Contaminated Land (Anonymous 1987);

Registers of land that may be contaminated: a Consultation Paper, the Department of the Environment, May 1991 (Anonymous 1991);

Draft Regulations contained in proposals for the Environmental Protection Act 1990 Section 143 Registers issued by the Department of the Environment, July 1992 (Anonymous 1992).

The planning guidance map showed only those areas where there was a known potential for contamination or landfill problems:

B1 Past and present potentially contaminative industrial use.

B2 Landfill sites potentially restrictive of future land use.

Potential for groundwater contamination

The Permo-Triassic rocks in the south of the area mainly comprise the Sherwood Sandstone, which is a major aquifer for public water supply. This aquifer had a high priority for groundwater protection and four hydrogeological divisions were identified based on the summary groundwater vulnerability map supplied to Wigan Metropolitan Borough Council by the National Rivers Authority—North West Region (now the Environment Agency) for use in their development plan (Anonymous 1993).

C1 Total catchment area.

C2 Principal recharge areas.

C3 Areas with significant water source protection zones, which, in turn, contain the most developmentally restrictive inner source protection zones.

C4 Public water supply boreholes.

The inner source protection zones were unlikely to extend more than 100 m from a source but the outer source protection zones were much larger. The zones could be subject to revision without notice in the event of changes in the abstraction regime or improved hydrogeological modelling. Protection zones were calculated only for major public water supply abstractions and the presence or absence of a designated zone was no indication of the presence or otherwise of other as yet unmodelled abstractions.

Mineral resources

Mineral resource potential in the area has been discussed briefly above and only three potentially significant mineral resources were identified. The extraction of coal by deep mining was not thought to be viable in the area in the foreseeable future. Three mineral potential zones were identified.

D1 Coal worked by opencast methods may be possible in areas within the exposed coalfield north of the Permo-Triassic outcrop.

D2 Sand and gravel from superficial deposits of glacial origin.

D3 Sand from bedrock deposits of weak sandstone where the cover of superficial deposits is not too great.

Major faults

Faults may be a potential hazard to development in several ways. In the past, during the period of intense underground deep and shallow mining of coal the subsidence was often concentrated as movement on existing faults. Since mining has ceased this is no longer a major problem. However, the faults may still be the focus for movement if future excavations on or below the ground or rising groundwater levels disturb their stability.

The main current, potential hazard associated with faults is their ability to act as a pathway for fluids and gases. Fluid transport might include the transmission of leachate from a leaking landfill site or the passage of groundwater following a rise in the water table after the halt in mine water pumping. Gases that may be transported include methane from landfill or natural sources.

The information on which the planning guidance map was based was not comprehensive and the information that could be shown on the map was limited by scale and space; its purpose was to advise the user of the possibility of encountering potential, significant geological constraints on development and to indicate where reference to the thematic maps and text was necessary, possibly followed by specialist advice and site investigation, if appropriate.