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Geology of the Rochdale district — a brief explanation of the geological map sheet 76 Rochdale

R G Crofts, E Hough, and K J Northmore

Bibliographic reference: Crofts, R G, Hough, E, and Northmore, K J. 2010. Geology of the Rochdale district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 sheet 76 Rochdale (England and Wales).

Keyworth, Nottingham: British Geological Survey © NERC 2010. All rights reserved

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Your use of any information provided by the British Geological Survey (BGS) is at your own risk. Neither BGS nor the Natural Environment Research Council gives any warranty, condition or representation as to the quality, accuracy or completeness of the information or its suitability for any use or purpose. All implied conditions relating to the quality or suitability of the information, and all liabilities arising from the supply of the information (including any liability arising in negligence) are excluded to the fullest extent permitted by law.

Maps and diagrams in this book use topography based on Ordnance Survey mapping. © Crown copyright. All rights reserved. Licence number 100017897/2010.

(Front cover) Cliviger Gorge (Photographer Tim Cullen, (P535961)).

(Rear cover)

Notes

The word 'district' is used in this sheet explanation to denote the area included in the geological 1:50 000 series sheet 76 Rochdale. National grid references are given in square brackets throughout this explanation. Unless otherwise stated, all lie within the 100 km square SD.

Acknowledgements

The series editors were A A Jackson and J E Thomas. Figures were drawn by P Lappage and page setting was by A Minks.

Geology of the Rochdale district (summary from rear cover)

(Rear cover)

An explanation of sheet 76 (England and Wales) 1:50 000 series map.

This sheet explanation provides an account of the district covered by geological gheet 76, Rochdale. The district extends from the northern suburbs of Bolton, Bury and Rochdale, across the Rossendale Moors to Blackburn, Accrington and Burnley. The solid rocks of Namurian and Westphalian age are well exposed across this central area within the Rossendale Anticline, a south-westerly trending open fold. The oldest strata in the area, the Craven Group, crop out in the extreme north-west of the district on the northern limb of the Pendle Monocline. The eastern edge of the sheet lies along the axis of the Pennine Monocline.

The Namurian and Westphalian strata range from about 306 to 316 million years old. They include the Millstone Grit Group with its massive sandstone units separated by mudstones, siltstone and subordinate coals. The sandstones form elevated ridges often with steep scarp faces and extensive dip slopes. These rocks are overlain by the Pennine Lower Coal Measures, which comprise mudstone, siltstone and sandstone with coal and seatearth horizons.

These rocks once provided a huge source of employment: the sandstones, particularly the Lower and Upper Haslingden Flags, provided sandstone flags and building stone, the coal fired the boilers of the hundreds of local cotton mills and the clays provided the raw material for bricks. A legacy of abandoned workings and made ground remains from these industries.

An extensive Quaternary succession dating from the Devensian is also described. A number of drift-filled valleys, some including laminated clay, have been identified and delimited. The late Quaternary period also saw an unprecedented period of landslide activity over much of the area, some of which is still active today.

The new geological map and this sheet explanation provide valuable information on a wide range of earth science issues. These include traditional aspects such as sedimentology and stratigraphy, but also cover applied aspects such as mineral and energy resources, waste disposal and foundation conditions. Both provide information that can assist planning and land-use conflict issues.

Chapter 1 Introduction

This sheet explanation provides a summary of the geology and applied geology of the district covered by the geological 1:50 000 series sheet 76 Rochdale, published in 2009. Detailed information can be found in individual technical reports.

The Rochdale district lies predominantly within the county of Lancashire while the area around Todmorden lies within West Yorkshire. The main population centres are the mill towns of Rochdale, Todmorden, Bacup, Rawtenstall, Haslingden, Blackburn, Accrington, Burnley, Darwen and Ramsbottom. These main population centres are separated by areas of agricultural land, with scattered villages and large expanses of moorland.

The bedrock is composed entirely of rocks deposited during the Carboniferous, about 359–299 million years ago (Figure 1); (Figure 2). The oldest rocks proved at surface in the district are Namurian (upper Carboniferous), which are known from boreholes to lie conformably on Tournasian to Visean (lower Carboniferous) mudstones and limestones. The Namurian rocks are represented by the Millstone Grit Group, a thick succession of interbedded sandstone, siltstone and mudstone and subordinate thin coals and seatearths.

The Millstone Grit Group occurs at outcrop over most of the south central and eastern areas, and in the north-western corner of the district. Commonly it forms upland scenery with extensive moorland associated with poor acid soils. The thick sandstones form a series of dissected escarpments with thin peat cover. Much of this area provides a catchment for water supplied to the main urban centres of the district as well as Bury and Bolton in neighbouring districts; the sandstones have also been extensively exploited for building stone. The Millstone Grit Group is overlain by the Pennine Coal Measures Group of Langsettian (Westphalian A) age, a succession of mudstone, siltstone and sandstone with subordinate coal, seatearth and ironstone. The Coal Measures crop out extensively in the district. Most of the larger urban areas, including Blackburn, Accrington, Burnley and Rochdale, are sited on the Coal Measures. The abundant water helped to stimulate early urban development and the rapid expansion in the 19th century owes much to the mineral resources, including coal, brick clay and sandstone. Today, mineral extraction is much reduced and coal mining has virtually ceased with coal extracted only from opencast sites.

Unconsolidated Quaternary deposits are present over much of the district and date from the last Devensian glaciation. The deposits either occur as a thin veneer on the higher ground or as thicker deposits on the lower ground, particularly within narrow channels carved into the bedrock. During the subsequent Flandrian, there was a significant development of landslips, which was caused by glacial overdeepening, stream rejuvenation in drift-filled valleys or faulting. Some of the glacial material has also been reworked to form river terraces and alluvium, and extensive peat development has occurred on the moorland.

History of research

The district covered by sheet 76 Rochdale was originally surveyed a scale of 1:10 560 and published as [Old Series] sheets 88NW and 89NE in 1874 and 1870, respectively, together with an account of the geology (Hull et al., 1875). The district was resurveyed at 1:10 560 scale in 1921–1923 and published at 1:63 360 scale as sheet 76 solid and drift editions in 1927. The maps were reprinted in 1947 (drift) and 1958 (solid) and reconstituted at 1:50 000 scale in 1974 and 1971, respectively. The accompanying memoir (Wright et al., 1927) provides a detailed account of the geology of the district with descriptions of localities and regional variations. The second resurvey was carried out on the 1:10 000 scale by R G Crofts, E Hough, C N Waters, R S Lawley and A S Howard assisted by G J Ager and K J Northmore between 1999 and 2002.

Recent advances in Silesian geology have necessitated some amendments to the previous surveys. Following the work of Ramsbottom et al. (1978), various publications have addressed regional and local aspects of the cyclicity (Holdsworth and Collinson, 1988), sedimentology (Collinson and Banks, 1975; Broadhurst and Simpson, 1983; Bristow, 1988; Collinson, 1988; Guion and Fielding, 1988; Guion et al., 1995; Chisholm et al., 1996; Waters, et al., 1996; Brettle, 2001) and palaeontology (Trueman and Weir, 1946; Calver, 1968; Eager et al., 1985) of the Namurian and Westphalian. Kirby et al. (2000) provide an insight into the regional structural history of the Carboniferous.

In addition to work of the Geological Survey, the Quaternary deposits have been studied by Baldwin (1911), Jowett (1914) and Johnson (1985).

Chapter 2 Geological description

Namurian

Namurian rocks crop out over much of the north-west, east and south of the district and occur at depth beneath Westphalian rocks in the north. They comprise a dominantly argillaceous succession, the upper part of the Craven Group, conformably overlain by the Millstone Grit Group — a thick succession of interbedded mudstone and siltstone (generally poorly exposed) and sandstone (traditionally referred to as 'grit'). A maximum thickness of 1800 m of Namurian strata has been estimated at outcrop, with seismic data suggesting that the total thickness diminishes to about 1300 m towards the south of the district.

During the Namurian period, approximately 320 m years ago, northern England lay within a large, actively subsiding basin, the Pennine Basin. Extensive delta systems built out into the basin, supplied with sediment derived from the land surface to the north, but also from the west in the early Yeadonian (G1) stage (see below). Variations in sea level are reflected in the Millstone Grit Group by distinct cycles of sedimentation, each beginning at a high sea-level stand with the deposition of marine or near-marine shales, including fossiliferous marine bands. The marine bands are generally a few centimetres thick, but may attain a thickness of several metres. They can be recognised across large areas, and as each marine band generally contains a distinctive and diagnostic marine faunal assemblage, they are important marker horizons. About 50 marine bands are recognised in the Millstone Grit Group of the Pennines (Holdsworth and Collinson, 1988). The marine bands commonly pass up through mudstones into siltstones and sandstones representing a transition from sedimentation in the delta slope to deposition within distributary channels on the delta top. During late Namurian times, the top of each cycle (when sea level is at its lowest) was often marked by the formation of soils and the development of widespread vegetation. Once subjected to compaction and lithification the soil horizons become seatearths, and organic deposits coal. This pattern of deposition is repeated with each rise in sea level, and the deposits of an individual cycle are known as a cyclothem. The ideal cyclothem described above may be interrupted or modified.

The Namurian is divided into seven substages, which in order of decreasing age are: Pendleian (E1), Arnsbergian (E2), Chokierian (H1), Alportian (H2), Kinderscoutian (R1), Marsdenian (R2) and Yeadonian (G1); these in turn are subdivided into chronzones (Ramsbottom et al., 1978). The boundaries of these chronostratigraphical subdivisions are recognised biostratigraphically by the presence of diagnostic ammonoid (goniatite) faunas. Formerly, the Millstone Grit Group was defined as a chronostratigraphical unit, the Millstone Grit Series (Wright et al., 1927), which included all strata of Namurian age, including the dominantly argillaceous Upper Bowland Shales. The 'series' was subdivided into seven, arbitrary defined, divisions, namely Pendleside Series, Pendle Grit Group, Sabden Shales, Kinderscout Grits, Middle Grits, Haslingden Flag Series and Rough Rock. To conform to current usage the Millstone Grit Group has been mapped as a lithostratigraphical unit that includes all strata of Namurian age above the Craven Group.

The Craven Group (Crav) comprises four formations. Only the uppermost, the Bowland Shale Formation (BSh), is present at outcrop, and is Pendleian in age. The base of the formation is taken as the base of Cravenoceras leion Marine band. The dominant lithology is dark grey, fissile mudstone. The group crops out in the extreme north-west of the district near Wilpshire.

The Millstone Grit Group (MG) comprises about 1800 m thickness of interbedded mudstone, siltstone and sandstone. Within the group six formations are recognised and defined by Waters, et al. (2007). In ascending order they are, Pendleton, Silsden, Samlesbury, Hebden, Marsden and Rossendale formations (Figure 1).

The Pendleton Formation (Pen), of Pendleian age, includes the Pendle Grit (PG) and the Warley Wise Grit (WWG). The former is a coarse-grained, feldspathic sandstone interbedded with subordinate silty mudstone and siltstone, deposited on a prodelta slope and interpreted as of turbiditic facies. The base of the formation is defined as the base of the lowest coarse-grained turbiditic sandstone present above the Bowland Shale Formation. The Warley Wise Grit is a medium-grained, cross-bedded and cross-stratified sandstone with sporadic pebbles and some siltstone interbeds. The top of the formation is identified by the base of the Cravenoceras cowlingense Marine Band. The marine band is not known at outcrop in the area but is known from the Garstang district (Aitkenhead et al., 1992).

The Silsden Formation (Sil) is Arnsbergian in age. It comprises dark grey mudstone with marine fossils with paler mudstones and siltstones; sandstones are very rare. In this district the succession is best known from the Fletcher Bank and Holme Chapel boreholes.

The Samlesbury Formation (Sam) is Chokierian to Alportian in age and comprises grey mudstones and siltstones. The top of the formation is defined by the base of the Hodsonites magistrorus Marine Band.

The Hebden Formation (Heb) is Kinderscoutian in age and includes the Todmorden Grit (TG) and the Lower (LKG) and Upper (UKG) Kinderscout Grits. The former is a coarse-grained, feldspathic sandstone interbedded with subordinate silty mudstone and siltstone, deposited on a prodelta slope and interpreted as of turbiditic facies, while the Kinderscout Grits are delta-top sandstones. The top of the formation is defined by the base of the Bilinguites gracilis Marine Band.

The Marsden Formation (Mar) is Marsdenian in age and comprises mudstone and siltstone with a number of delta-top sandstones. Many of the sandstones show similar petrographical and sedimentological features and can only be distinguished from one another by their position relative to known marine bands. The sandstone nomenclature is broadly similar to that used in the previous survey but with the rationalisation to a single name for the Revidge Grit, Gorpley Grit and Fletcher Bank Grit to the Fletcher Bank Grit. In addition, the Brooksbottoms Grit (BBS) is now known to be coeval with the Huddersfield White Rock (WHR) (Waters, et al., 2008), and the Holcombe Brook Grit (HB) to be later in age. The top of the formation is defined by the base of the Cancelloceras cancellatum Marine Band.

The Rossendale Formation (Ros) is Yeadonian in age and comprises mudstone and siltstone with a number of finger-bar sandstones which include the Lower (LH) and Upper Haslingden Flags (UH) (Collinson and Banks, 1975) (Plate 1). These sandstones have a uniquely westerly provenance as proved by sedimentological studies. The youngest sandstone in the formation is the Rough Rock (RR), a northerly sourced, coarse-grained and pebbly deltaic sandstone. The top of the formation is defined by the base of the Subcrenatum Marine Band.

Further information and descriptions of the sandstones, coals and marine bands within the Millstone Grit Group is given in (Figure 3) and (Figure 4).

Westphalian

Rocks of the Pennine Coal Measures Group, of Westphalian age, outcrop over much of the north, west and east central parts of the district including most of the main urban areas. During Westphalian times, about 316 to 306 million years ago, the pattern of sedimentation described in the Namurian continued, but as sedimentation kept pace with subsidence of the Pennine Basin shallow-water conditions were eventually established. In Langsettian times deposition occurred in a delta-plain environment that was above sea level for much of the time and land floras were abundant.

The Pennine Coal Measures Group is divided into Lower, Middle and Upper formations, of which only the Pennine Lower Coal Measures Formation (PLCM) of Langsettian (Westphalian A) age is present in the district. The Coal Measures rest conformably on the Millstone Grit Group, the base being taken as the base of the Subcrenatum (six inch) Marine Band. The Coal Measures consist of interbedded mudstone, siltstone and sandstone with subordinate coal and seatearth deposited in cyclic sequences. The mudstones are dark grey to black, weathering to orange-brown, planar laminated and micaceous, or massive. Commonly they contain non-marine bivalves, for example the Daubhill Mussel Band which may be used as a chronostratigraphical indicator. The mudstone of the Old Lawrence cycle is distinctly greenish indicating a western provenance (Chisholm et al., 1996). The mudstones are commonly overlain gradationally by siltstones, which are typically medium-grey with ripple cross-lamination and parallel lamination and commonly containing plant debris. The siltstones grade both vertically and laterally up into sandstones. The sandstones (Figure 5) and (Plate 2) commonly form positive, mappable, topographical features, and are thus distinguished on the map from the mudstones and siltstones, which are shown as Lower Coal Measures (undivided). The sandstones can be thin and laterally impersistent but many are extensive and basin-wide. The sandstones (Figure 6) are mainly medium grained, but varying from fine to coarse grained, and comprise subangular to subrounded quartz and feldspar grains with a variable mica content. They are grey where fresh but weather to yellowish brown. Again, some sandstones are greenish, notably the Old Lawrence Rock, indicating a western sediment source. Sedimentary structures include planar lamination, cross-bedding and ripple cross-lamination together with lenticular bedding. Coalified plant fragments are common, as are trace fossils locally. Seatearths are palaeosols which developed during plant colonisation and are characterised by the presence of rootlets. They occur in all lithologies being referred to as ganister where developed on sandstone and fireclay where they are formed on mudstone. In general, pedification destroys the primary sedimentary structures.

Coal seams are extensive, and many are developed on a regional scale, but they vary laterally in thickness and composition, particularly by the variation in the number of dirt partings present within a seam. The coals generally cap upward-coarsening sedimentary cycles, and are underlain by seatearths. Thirty-two named seams have been identified in this area (Figure 7). Marine bands are thin beds of black mudstone with marine fauna; commonly they overlie coals or seatearths. They are generally only a few centimetres thick but may rarely attain thicknesses of up to one metre. Marine bands can be recognised across large areas; they represent eustatically controlled flooding events and are important as marker horizons. Marine band faunas are discussed fully by Calver (1968). The Subcrenatum, Listeri and Amaliae marine bands are recognised across the district. Additionally, a number of Lingula bands have been proved which may correlate with the Langley and Burton Joyce marine bands found elsewhere in the Pennine Basin. However, due to the lack of modern correlative data for the coal measures within the district, correlation with better researched areas, such as Wigan, is still rather tentative. Until proved otherwise, the entire Westphalian succession within the Rochdale district is thought to lie within the Lower Coal Measures (Aitkenhead et al., 2002).

Quaternary

About 40 per cent of the district is covered by superficial deposits (drift), which comprise glacial, periglacial and postglacial deposits. The nature of these deposits is shown in (Figure 8). Despite much of the district being urbanised, changes have been made where new data, such as field observations, temporary open sections, topographical features, auger holes and borehole data, have revealed changes from the previous survey. The superficial deposits and bedrock are locally covered by artificial (man-made) deposits, the product of human modification of the natural environment, most notably in areas of urban and industrial development. The man-made deposits shown on the map represent those that were identifiable at the date of the survey. They are delineated by recognition in the field and by the examination of documentary sources, in particular topographical maps, aerial photographs and site investigation data. Only the more obvious man-made deposits can be mapped by these methods and boundaries shown may be imprecise.

The present-day topography is largely the result of glacial processes active during the Pleistocene, but has been modified by alluvial processes and mass movement, mainly landslips. The district was probably affected by at least three glaciations, although evidence for earlier phases has been obliterated by the final Devensian phase. The ice sheets would have covered the whole area and deposited till.

During the Devensian glaciation the district was subject to an ice mass moving southwards from Lancashire and Cumbria. Results of pebble counts show that Lake District pebbles may exceed 30 per cent of the erratic suite, demonstrating that the till in the area was deposited by southerly flowing ice.

The till generally has a grey clay matrix, and the erratic suite consists mainly of Carboniferous sandstones, but in the extreme north-east of the district it contains many cobbles of Carboniferous limestone derived from the Clitheroe area to the north. Striations also indicate north–south ice movement. The till sheet is a complex of lodgement till and ablation till. The lodgement till was deposited beneath moving ice, and forms a blanket that mimics the bedrock surface. The ablation till formed by the slow release of debris during glacial melting, and can be identified by kettle-kame topography, for example around Smithy Bridge. However, thicker successions occupy channels cut into the bedrock (Figure 9). These channels may have been cut during early glacial events or eroded by proglacial meltwaters as the late Devensian ice advanced.

The ice sheets, or water moving beneath or at the edge of them, also excavated glacial drainage channels in the bedrock (Figure 9); (Plate 3). Many glacial drainage channels have been identified during the surveys and are identified on the map.

Glaciofluvial deposits form extensive spreads around Rochdale and Rishton and in the Darwen valley; they are related to the outwash phase in the late Devensian. Other glaciofuvial deposits lie buried in the drift-filled channels (see (Figure 9)). The deposits comprise grey-brown fine- and medium-grained sand and are likely to be derived from local Carboniferous sandstone bedrock.

Periglacial weathering occurred once the ice had receded. The intense cold caused the development of permafrost conditions in the subsoil with shattering and weathering of rock due to freeze-thaw processes. These processes helped promote the formation of head and landslide deposits. Head is generally confined to the steep slopes below sandstones that form prominent escarpments. It is difficult to delineate in urban settings and head may therefore be more extensive than indicated on the map. Most of the landslides are associated with one or more of the following:

• glacially oversteepened valleys
• faulting
• stream rejuvenation in drift-filled valleys

The classes of landslide range from tumbled blocks through slides and flows to rotational failure (Plate 4). Following the Devensian glaciation, when the modern drainage pattern became established, rivers were re-established in the drift-filled valleys, reworking the glacial deposits by alluvial processes to form alluvium and locally river terrace deposits. In many cases, subsequent landslides kept pace with downcutting to maintain only narrow alluvial developments. River terrace deposits are developed mainly in the Irwell valley. Alluvial fan deposits and debris cones occupy the point where narrow tributary valleys join the main valley.

Lacustrine deposits form generally thin deposits that have accumulated in hollows within the till surface or are more extensive in kettle holes. Glaciolacustrine deposits are thicker where ice dammed lakes developed. Both deposits comprise laminated silt and clay with peat.

Peat may also occur in abandoned glacial drainage channels, and on the high moorland throughout the district. These once-extensive spreads of moorland peat are now much degraded and dissected due to erosion by rain and wind, which continues to remould the outcrops.

Made ground is shown on the map where the natural ground surface is concealed by deposits that result from human activity. The main categories include civil engineering works, spoil from sandstone quarrying, opencast spoil, building and demolition rubble, waste from heavy industries and domestic and other waste in raised landfill sites. The most extensive areas of made ground are found in urban areas where the topographical features associated with specific deposits may have been smoothed over prior to development. In such areas, the extent of made ground is often based on site investigation data.

Infilled ground comprises areas where the natural ground has been removed and the void partly or wholly restored with man-made deposits. Sandstone quarries, clay pits, opencast coal workings and disused railway cuttings have been used for the disposal of waste materials. For example, the former Scout Moor sandstone quarries are filled with domestic and other inert waste. Where quarries have been restored and either landscaped or built on, there may be no surface indication of the extent of the backfilled void. In such cases, the location of these sites is taken from archival sources, in particular old topographical and geological maps.

Worked ground is where material is known to have been removed, for example in unfilled quarries and excavations for roads and railways. Disturbed ground is associated with ill-defined surface workings such as shallow sandstone quarries and areas of bell pits. Landscaped ground comprises areas where the original surface has been extensively remodelled, but where it is not feasible to delineate areas of cut or made ground. Most urban areas are associated with landscaped ground. Landscaped and disturbed ground is not shown on the 1:50 000 scale geological map, but where possible they are delineated on the constituent 1:10 000 scale geological maps.

Structure and concealed geology

In earliest Carboniferous times, a major rift basin system developed in northern England in response to regional back-arc extension, caused by northward subduction of the Rheic Ocean. Extension was much diminished in Namurian and Westphalian times and a regional 'post-rift' or 'sag' basin developed (Leeder, 1982). In latest Carboniferous times (about 300 million years ago), final closure of the Rheic Ocean caused large-scale thrust and nappe emplacement in central Europe and southern Britain. This period of tectonic activity is known as the Variscan Orogeny. On the foreland to the north of the foldbelt, deformation was much less pervasive so that northern England tectonic disturbance was largely restricted to basin inversion, with partial reversal of some of the earlier Carboniferous basin-controlling normal faults, associated folding and regional uplift.

Much of the district lies within a large, south-west-trending, open fold, the Rossendale Antciline, which passes into the Burnley Syncline and Pendle Monocline on the northern edge of the district (Figure 10). The eastern edge of the district lies across the north-trending Pennine Anticline. Across the Rossendale Anticline, strata have a broadly northerly or southerly dip of 2° to 5°. In the north-west of the district, steep southerly dips (up to 40°) define the southern limb of the Pendle Monocline. The Pennine Anticline has an assymetrical form with dips of between of 2° to 5° on the eastern limb and up to 30° on the western limb. The structure of the Pennine Anticline has been described in detail by Evans et al. (2002), who concluded that the fold represents a positive inversion structure developed over a north–south basement fault, reactivated and subjected to oblique slip during north-west–south-east-directed Variscan compression.

In the Rochdale district, the Dinantian extensional basin system is concealed by Namurian and Westphalian post-rift strata. Seismic reflection data show a mosaic of tilt blocks bounded by large east-north-east-trending syn-rift normal faults, with throws commonly of several hundred metres. Basement depths are less than 2000 m on fault-bounded horsts such as the Central Lancashire High (Figure 10). The Holme Chapel Borehole encountered basement, comprising purple slate of early Palaeozoic age, at a depth of 1968 m (Kirby et al., 2000) thought to represent the Ingleton Group or Windermere Group. These positive syn-rift structures are characterised by relatively thin overlying Dinantian successions (about 800 to 1200 m thick). Carbonate platforms developed around and over the Central Lancashire High (Evans and Kirby, 1999) while thicker, more basinal Dinantian sequences in excess of 2000 m are present in the Rossendale basin in the southern half of the district. Biostatigraphical correlation with more extensive Dinantian carbonate platform deposits in the Askrigg Block has been largely unsuccessful (Riley and McNestry, 1988).

The post-rift Namurian and Westphalian successions are generally of more uniform thickness than the underlying Dinantian strata, although the lower part of the Namurian succession shows a significant southward thinning. The thinning probably relates to the basinward thinning and fining of late Namurian deltaic deposits overlying a relatively deep basin fill deposited during early Namurian times. The post-rift strata are dominated by north-north-west faults, as mapped at surface. Some of these may have been active as Dinantian transfer faults, while others may have Variscan or even younger origin. Surface displacements are generally smaller than the subsurface displacements, but some are in excess of 100 m.

The Pennine Anticline shows a sinistral offset of up to 4 km along the Todmorden Smash Belt. South of the Todmorden Smash Belt, new evidence (Evans et al., 2002) suggests that the Pennine Anticline is underlain by a complex of high-angle, down-to-the-west reverse faults, which connect to an east-dipping, low-angle, down-to-the-west reverse fault named the Central Pennine Reverse Fault.

At outcrop, faults may occur as a single discrete plane, or as a zone up to several tens of metres wide containing several fractures, each of which accommodates some displacement. The portrayal of such faults as a single line on a map is therefore a generalisation. The position of a fault may be based on the interpretation of topographical features, surface outcrops, site investigation data and underground mining, but the evidence is rarely sufficient to locate a fault precisely. Only rarely are faults exposed in the district. In an area of thick and extensive superficial deposits, the positioning of faults relies almost entirely on projection from underground mining information. Geological faults in this district are of ancient origin and are currently inactive. However, the Rochdale district, in common with other parts of northern and central Britain has been affected historically by minor earthquakes. A number of earthquakes occurred in October–November 2002 with a macroseismic epicentre beneath east Manchester, a depth of focus of 5 km or less and an inferred maximum magnitude 3.2. Minor earthquakes may arise through reactivation of faults by undermining, when general subsidence effects may be concentrated along them. Underground mining has ceased in the district, and although minor residual subsidence may still occur, it is increasingly unlikely that this will result in significant fault reactivation.

Chapter 3 Applied geology

Geological factors have had a significant role in the industrial and social history of the Rochdale district as well as its landscape (Figure 11). It lies within the Lancashire and Burnley coalfields, and has a history of coal mining, sandstone quarrying and brick making that has left a legacy of derelict and despoiled land. Coal mining has now ceased and opencast mines are only of limited extent. Similarly, sandstone quarrying and brick making are also in major decline from production levels reached in the early 20th century. The area has suffered significantly from landslides, particularly in the incised valleys found throughout the area. By considering the nature and location of earth science issues at an early stage in the planning process appropriate action can be taken to ensure that the site and development are compatible, and that appropriate mitigation measures can be taken prior to development. The information may also be used to identify opportunities for leisure and recreation as well as protecting sites of nature conservation interest.

Energy and mineral resources

By far the greatest coal production from the district was obtained from the Westphalian Coal Measures. In addition, coal seams were also worked from within the Namurian rocks, particularly the Sand Rock Coal. Although up to 25 coal seams have been mined, the economics of underground mining are unlikely to be favourable in the future and opencast workings are likely to be limited. The main factors hindering further opencast extraction are the thickness of overburden that includes Quaternary and artificial deposits, sterilisation by urban development, conflicts for land use and possible further detrimental effects on the landscape. The potential for coalbed methane is likely to be low due to the extent of former workings and lack of thick cover (Glover et al., 1993; Kirby et al., 2000).

The search for oil resources has taken place by seismic survey and borehole drilling. Despite favourable source rocks being located beneath the district (Kirby et al., 2000), no oil or gas has been proved.

Sandstone was formerly quarried on a large scale and provides stone for local, national and international use. The most notable units worked are the Upper and Lower Haslingden Flags. They have been extensively quarried and mined, providing paving stones for Trafalgar Square and the streets of New York City.

Mudstone has been worked for brick making and dam construction. Brick making is still a major industry in the Accrington area. The favoured horizons are the mudstones lying between the Dyneley Knoll Flags and the Old Lawrence Rock, locally known as the 'Accrington Mudstones', and below the Rough Rock were the Upper and Lower Haslingden Flags are thin or absent.

Limestone cobbles and boulders are common in the till and have been worked in the north-east of the district, although there is no carbonate bedrock in the area. These workings are known as hushings ('hush' meaning to scour boulders from till) and occur over considerable areas of Worsthorne Moor (Plate 5) and Hapton Park. The limestone boulders were quarried, cleaned by water, burnt and transported as lime for industrial use.

Surface mineral workings

Quarries and pits that remain open represent an important resource as they provide a suitable repository for waste, may be reopened for further mineral extraction or, as in the case of Thurns Quarry, may provide educational, recreational and wildlife value. However, they can also be a constraint to development as steep rock faces may be unstable. Quarries and pits occur widely within the district having been worked mostly for sandstone and mudstone.

Engineering ground conditions

The key parameters relevant to construction and development are the suitability of the ground to support structural foundations, the ease of excavation of the material and its value in engineering earthworks and fills. Some of these issues are summarised for the main engineering geological units in the district (Figure 12). Foundation conditions are not only affected by the engineering properties of the substrate, but also by factors such as geological structure, slope stability, the presence of undermining and degree of weathering. The seismic history of this district is also relevant, as it shows the potential for large-magnitude earthquakes to occur (see above, page 18). Variable man-made conditions, notably landfill sites and areas of colliery spoil, are a potential problem with respect to severe differential settlement. Colliery spoil may also contain iron pyrites that is prone to oxidise and produce sulphate-rich, acidic leachates, which could be harmful to concrete present in foundations or buried services, thus requiring the use of sulphate-resisting cement. This oxidation process may also result in expansion and differential heaving of foundations constructed on such deposits. Large volumes of quarry spoil, both ancient and modern, are common in the district, and areas affected may present poor foundation conditions if large cavities are present, or if deposited on steep slopes. Mudrocks, in particular the Coal Measures seatearths, may weather to show an increase in natural moisture content, plasticity and swelling potential.

Subsidence risk due to undermining may be a potential constraint in areas of former underground mining of coal and sandstone. Coal mining was particularly important in much of the district (Figure 6). In such areas, the principal concerns relate to ground stability caused by the collapse of unsupported shallow workings, such as bell pits and areas of pillar and stall workings. Structures straddling a fault may be susceptible to uneven settlement in areas prone to mining subsidence. Arup Geotechnic (1991) provided a review of mining subsidence in the UK. Information on recorded shafts and abandoned mines is lodged with the Coal Authority, who should be consulted prior to development in a coalfield area. Additional information on the distribution of ancient near-surface workings, including hitherto unrecorded shafts, that could be located in the field or on aerial photographs, is given on the relevant 1:10 000 scale geological map.

Mine-drainage water may be a problem in areas of disused collieries, which are common throughout the district. Such water has high acidity, in addition to iron and commonly elevated levels of manganese, aluminium and sulphates; where it reaches the surface it can devastate the flora and fauna.

Slope stability is an issue particularly where building development has extended on to steep valley sides. The majority of landslip features may be currently considered inactive. However, renewed instability may occur if the slope is adversely disturbed by undercutting or loading, or if increased volumes of water are introduced, such as may occur during development.

Pollution potential

Artificial (man-made) deposits may contain toxic residues, either as primary components or generated by chemical or biological reactions, and are thus potential sources of pollution. Significant sites of potential pollution include areas of landfill and former gasworks, chemical works, textile mills, iron and steel works, railway sidings and sewage works. Leachate migration may be a problem where groundwater percolates through waste and becomes enriched in potentially harmful soluble components. The resultant leachate may migrate laterally in permeable superficial deposits or bedrock adjacent to the site, according to the depth of the unsaturated zone. This is potentially a most serious hazard at landfill sites situated on deposits in hydraulic continuity with Coal Measures or Millstone Grit aquifers, and may be exacerbated within areas that are highly faulted. Similar problems may also be encountered with river terrace deposits and alluvium.

Gas emission

Gas emission may represent a hazard in areas associated with the accumulation of 'greenhouse gases' (methane and carbon dioxide) and carbon monoxide. These gases can be generated naturally in Coal Measures and Millstone Grit strata, or by the decomposition of materials in landfill sites. They may migrate considerable distances through permeable strata and accumulate in poorly ventilated enclosed spaces such as basements, foundations or excavations. Methane is potentially explosive, may act as an asphyxiant and may cause vegetation die back. Carbon monoxide is potentially explosive and is toxic at low concentrations.

Water resources

Reservoirs in upland areas and incised valleys throughout the area provide the principal source of domestic and industrial water supply for the district. Springs most commonly discharge to slopes where groundwater flow in sandstones or superficial deposits is interrupted by impermeable mudstone or clay. Groundwater provides public water supplies for isolated villages and farms and licensed water abstraction for industrial purposes throughout the district. The major bedrock aquifers are in the Millstone Grit Group and to a lesser extent in the Coal Measures. Sands and gravels within alluvium and river terrace deposits form minor aquifers in this district.

Flooding is only a potential problem within the main watercourses. Information about flood risk limits can be provided by the Environment Agency. Sheet 76 Rochdale shows that the major river floodplains contain limited tracts of river terrace deposits, which are slightly higher than the surrounding alluvium. The distribution of these deposits and the alluvium, should therefore form the basis for any plan to manage flood risk. However, exceptional weather conditions feeding into confined valleys may result in flash floods and localised extreme flooding events as experienced at Todmorden in June 2000.

Conservation sites

Several sites have been identified in the district as important to earth science research and teaching and for recreational purposes. Some are in natural sections while others are located in active as well as disused quarries and pits such as Thurns Head Quarry [SD 874 186] (Plate 1).

Information sources

Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. Geological advice for this area should be sought from the Head of Geology and Landscape, England, BGS Keyworth. Further information held by the British Geological Survey relevant to the Rochdale district is listed below. Searches of indexes to these and many other data collections can be made on the Geoscience Index System in BGS libraries or by accessing the Geoscience Data Index (GDI) via the BGS Websitewww.bgs.ac.uk.

Maps

• Geological maps
• 1:625 000
• Solid Geology map, south sheet, 2007; Quaternary Geology, 1977
• 1:250 000
• Liverpool Bay, solid geology, 1978; Sea-bed Sediment and Quaternary,1984
• 1:50 000 and 1:63 360
• Sheet 67 Garstang (solid with drift), 1990
• Sheet 68 Clitheroe (solid and drift), 1975
• Sheet 69 Bradford (solid and drift), 2000
• Sheet 75 Preston (solid and drift), 1982
• Sheet 77 Huddersfield (solid and drift), 2003
• Sheet 84 Wigan (solid), 1977
• Sheet 84 Wigan (solid and drift), 1935 (1:63 360 scale)
• Sheet 85 Manchester (solid), 1975
• Sheet 85 Manchester (solid and drift), 1970†
• Sheet 86 Glossop (solid with drift), 1981
• 1:10 000 and 1:10 560
• Details of the original geological surveys are listed on editions of the 1:10 000 or 1:10 560 geological sheets. Copies of the fair-drawn maps of these earlier surveys may be consulted at the BGS Library, Keyworth. The maps covering the 1:50 000 series sheet 76 Rochdale are listed below, together with surveyors' initials and dates of the survey. The surveyors were R G Crofts, E Hough, C N Waters, R S Lawley and A S Howard. The maps are published and are also available for public reference in the libraries at BGS, Keyworth and Edinburgh and the BGS London Information Office in the Natural History Museum, South Kensington, London. All sheets coloured and in digital format are available for purchase from the BGS Sales Desk. Those with accompanying technical reports are shown thus*.

• Digital geological map data
• In addition to the printed publications noted above, many BGS maps are available in digital form, which allows the geological information to be used in GIS applications. These data must be licensed for use. Details are available from the Intellectual Property Rights Manager at BGS Keyworth. The main datasets are:
• DiGMapGB-625 (1:625 000 scale)
• DiGMapGB-250 (1:250 000 scale)
• DiGMapGB-50 (1:50 000 scale)
• DiGMapGB-10 (1:10 000 scale)
• The current availability of these can be checked on the BGS web site at: www.bgs.ac.uk/products/digitalmaps/digmapgb.html
• Geophysical maps
• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1997
• Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1998.
• High resolution geophysical and radiometric surveys
• Plot-on-demand contoured images synthesising data from recent airbourne gravity and magnetic surveys and including distributions for a range of radioactive elements are available.
• Geochemical atlas
• Baseline geochemical survey data for the district will be available in the near future.
• Hydrogeological maps
• 1:625 000
• England and Wales (1977)
• 1:100 000
• Groundwater vulnerability map, central Lancashire area (Sheet 10)

Books

• British Regional Geology
• Pennines and adjacent areas, 2002
• Memoirs
• Sheet 67 Garstang, 1992
• Sheet 68 Clitheroe and Nelson, 1961
• Sheet 69 Bradford and Skipton, 1953
• Sheet 69 Bradford district, 2000
• Sheet 75 Preston, 1978†
• Sheet 76 Rossendale Anticline, 1927†
• Sheet 77 Huddersfield and Halifax, 1930†
• Sheet 84 Wigan, 1938†
• Sheet 85 Manchester, 1931†
• Sheet 86 Glossop, 1953†
• † out of print; facsimile copies may be purchased from BGS libraries at a tariff set to cover the cost of copying
• The structure and evolution of the Craven Basin and adjacent areas, 2000
• Sheet explanations
• Sheet 69 Bradford district, 2000
• Sheet 77 Huddersfield, 2005
• Technical reports
• Technical reports relevant to the district (see above) may be purchased from BGS or consulted at BGS and other libraries.
• Geology
• Reference numbers for the technical reports covering the geology of individual or combined 1:10 000 scale geological sheets is in the list of 10 000 scale maps.
• Mineral resources
• Information on mineral resources is available from BGS. Mineral Resources maps at 1:100 000 scale are available from BGS with accompanying reports or single sheet print-on-demand versions. They include details of mineral resource statistics and planning permissions. There are none available for this area.
• Biostratigraphy
• There is a collection of internal BGS biostratigraphical reports; details are available on request.

Documentary collections

Basic geological survey information, which includes 1:10 000 or 1:10 560 scale field slips and accompanying field notebooks, are archived at BGS, Keyworth.

Borehole and shaft data

Borehole and shaft data for the district are catalogued in the BGS archives at Keyworth. For further information contact: The Manager, National Geosciences Record Centre, BGS, Keyworth.

Mine plans

BGS maintains a partially complete collection of plans of underground mines for coal and other minerals.

Gravity and aeromagnetic data

Gravity and aeromagnetic data are held digitally in the national Gravity Databank and the National Aeromagnetic Databank at BGS, Keyworth. A limited amount of seismic reflection data is also available for the district.

Hydrogeological data

Hydrogeological data, water boreholes, wells and springs and aquifer properties are held in the BGS database at Wallingford.

BGS lexicon

BGS Lexicon of named rock unit definitions can be accessed on the BGS website. Definitions of the named rock units shown on BGS maps, including those shown on the 1:50 000 series Rochdale sheet 76 are held in the BGS Lexicon database.

Material collections

Palaeontological collection

Macrofossils and micropalaeontological samples collected from the district are held at BGS Keyworth.

Petrological collections

Petrological samples collected from the district are held at BGS Keyworth.

Geochemical samples

A database of stream sediment and water samples collected from the district are held at BGS Keyworth.

Borehole core collection

Samples and entire core from a small number of boreholes in the Rochdale district are held by the National Geosciences Record Centre, BGS, Keyworth.

BGS photographs

Copies of the photographs used in this report are deposited for reference in the BGS Library, Keyworth; prints and transparencies can be supplied at a fixed tariff. Part of the photographic archive can be accessed via the BGS website.

Other relevant collections

Mine abandonment plans

Coal abandonment plans are held by the Coal Authority, Mining Reports, 200 Lichfield Lane, Mansfield, Nottinghamshire, NG18 4RG.

Groundwater licensed abstractions, catchment management plans and landfill sites

Information on licensed water abstraction sites, for groundwater, springs and reservoirs, Catchment Management Plans with surface water quality maps, details of aquifer protection policy and extent of washlands and licensed landfill sites are held by the Environment Agency.

Geological conservation sites

Information on the Sites of Special Scientific interest present within the Rochdale district is held by English Nature, Pier House, Wallgate, Wigan, Lancashire, WN3 4AL.

References

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

Aitkenhead, N, Bridge, D McC, Riley, N J, and Kimbell, S F. 1992. Geology of the country around Garstang. Memoir of the British Geological Survey, sheet 67 (England and Wales).

Aitkenhead, N, Barclay, W J, Brandon, A, Chadwick, R A, Chisholm, J I, Cooper, A H, and Johnson, E W. 2002. British regional geology: the Pennines and adjacent areas (Fourth Edition). (Keyworth, Nottingham: British Geological Survey.)

Arup Geotechnics. 1991. Review of mining instability in Great Britain. Report to the Department of the Environment, Arup Geotechnics, Ove Arup and Partners. (London: HMSO.)

Baldwin W. 1911. The Pleistocene lakes of Rochdale. Transactions of the Rochdale Literary and Scientific Society, Vol. 10, 17–21.

Brettle, M J. 2001. Sedimentology and high-resolution sequence stratigraphy of shallow water delta systems in the early Marsdenian (Namurian) Pennine basin, northern England. Unpublished PhD thesis, University of Liverpool.

Bristow, C S. 1988. Controls on sedimentation of the Rough Rock Group (Namurian) from the Pennine Basin of northern England. 114–131 in Sedimentation in a synorogenic basin complex: the Upper Carboniferous of north west Europe. Besley, B M, and Kelling, G (editors). (Glasgow and London: Blackie.)

Broadhurst, F M, and Simpson, I M. 1983. Syntectonic sedimentation, rigs, and fault reactivation in the Coal Measures of Britain. Journal of Geology, Vol. 91, 330–337.

Calver, M A. 1968. Distribution of Westphalian marine faunas in northern England and adjoining areas. Proceedings of the Yorkshire Geological Society, Vol. 37, l–72.

Chisholm, J I, Waters, C N, Hallsworth, C R, Turner, N, Strong, G E, and Jones, N S. 1996. Provenance of Lower Coal Measures around Bradford, West Yorkshire. Proceedings of the Yorkshire Geological Society, Vol. 51, l53–166.

Collinson, J D. 1988. Controls on Namurian sedimentation in the Central Province basins of northern England. 85–101 in Sedimentation in a synorogenic basin complex: the Upper Carboniferous of north west Europe. Besley, B M, and Kelling, G (editors). (Glasgow and London: Blackie.)

Collinson, J D, and Banks, N L. 1975. The Haslingden Flags (Namurian, G1) of south-east Lancashire: bar finger sands in the Pennine basin. Proceedings of the Yorkshire Geological Society, Vol. 40, 431–458.

Eagar, R M C, Baines, J G, Collinson, J D, Hardy, P G, Okolo, S A, and Pollard, J E. 1985. Trace fossil assemblages and their occurrence in Silesian (mid-Carboniferous) deltaic sediments of the Central Pennine Basin, England. 99–149 in Biogenic structures: their use in interpreting depositional environments. Curran, H A (editor). Society of Economic Palaeontologists and Mineralogists, Special Publication, No. 35.

Evans, D J, and Kirby, G A. 1999. The architecture of concealed Dinantian carbonate sequences over the Central Lancashire and Holme highs, northern England. Proceedings of the Yorkshire Geological Society, Vol. 52, 297–312.

Evans, D J, Walker, A S D, and Chadwick, R A. 2002. The Pennine Anticline, northern England — a continuing enigma? Proceedings of the Yorkshire Geological Society, Vol. 54, 17–34.

Glover, B W, Holloway, S, and Young, S R. 1993. An evaluation of coalbed methane potential in Great Britain. British Geological Survey Technical Report, WA/93/24.

Guion, P D, and Fielding, C R. 1988. Westphalian A and B sedimentation in the Pennine Basin, UK. 153–177 in Sedimentation in a synorogenic basin complex: the Upper Carboniferous of north west Europe. Besley, B M, and Kelling, G (editors). (Glasgow and London: Blackie.)

Guion, P D, Fulton, I M, and Jones, N S. 1995. Sedimentary facies of the coal-bearing Westphalian A and B north of the Wales–Brabant High. 45–78 in European coal geology. Whateley, M K G, and Spears, D A (editors). Geological Society of London Special Publication, No. 82.

Holdsworth, B K, and Collinson, J D. 1988. Millstone Grit cyclicity revisited. 132–152 in Sedimentation in a synorogenic basin complex: the Upper Carboniferous of north west Europe. Besley, B M, and Kelling, G (editors). (Glasgow and London: Blackie.)

Hull, E, Dakyns, J R, Tiddeman, R H, Ward, J C, Gunn, W, and De Rance, C E. 1875. Geology of the Burnley Coalfield. Memoir of the Geological Survey of Great Britain. Quarter Sheets 88NW, 89NW and NE and 92SW. (London: HMSO.)

Johnson, R H. 1985. The imprint of glaciation on the west Pennine uplands. 237–262 in The geomorphology of north-west England. Johnson, R H (editor). (Manchester: Manchester University press.)

Jowett, A. 1914. The glacial geology of east Lancashire. Quarterly Journal of the Geological Society of London, Vol. 70, 199–231.

Kirby, G A, Baily, H E, Chadwick, R A, Evans, D J, Holliday, D W, Holloway, S, Hulbert, A G, Pharaoh, T C, Smith, N J, Aitkenhead, N, and Birch, B. 2000. The structure and evolution of the Craven Basin and adjacent areas. Subsurface memoir of the British Geological Survey.

Leeder, M R. 1982. Upper Palaeozoic basins of the British Isles–Caledonide inheritance verses plate margin processes. Journal of the Geological Society of London, Vol. 139, 479–491.

Ramsbottom, W H C, Calver, M A, Eager, R M C, Hodson, F, Holliday, D W, Stubblefield, C J, and Wilson, R B. 1978. A correlation of Silesian rocks in the British Isles. Geological Society of London Special Report, No. l0.

Riley, N J, and McNestry, A. 1988. Biostrati-graphy and correlation of Boulsworth and Holmes Chapel boreholes. British Geological Survey Technical Report, WH/PD/88/375.

Trueman, A E, and Weir, J. 1946. A monograph of British Carboniferous non-marine lamellibranchia. Palaeontographical Society Monograph, Vol. 32, Part 1, pp. 18.

Waters, C N, Aitkenhead, N, Jones, N S, and Chisholm, I J. 1996. Late Carboniferous stratigraphy and sedimentology of the Bradford area, and its implications for the regional geology of northern England. Proceedings of the Yorkshire Geological Society, Vol. 51, 87–101.

Waters, C N, Browne, M A E, Dean, M T, and Powell, J H. 2007. Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore). British Geological Survey Research Report, RR/07/01.

Waters, C N, Chisholm, J I, Benfield, A C, and O'Beirne, A M. 2008. Regional evolution of a fluviodeltaic cyclic succession in the Marsdenian (late Namurian Stage, Pennsylvanian) of the Central Pennine Basin, UK. Proceedings of the Yorkshire Geological Society, Vol. 57, 1–28

Wright, W B, Sherlock, R L, Wray, D A, Lloyd, W, and Tonks, L H. 1927. The geology of the Rossendale anticline. Memoir of the Geological Survey of Great Britain, Sheet 76 (England and Wales).

Figures and plates

Figures

(Figure 1) Summary of the geological succession in the district.

(Figure 2) Geology of the Rochdale district.

(Figure 3) Principal units of the Millstone Grit Group.

(Figure 4) Stratigraphy of the Millstone Grit Group. SI Six Inch Coal; SR Sand Rock Coal; HBC Holcombe Brook Coal; C Coal.

(Figure 5) Stratigraphy of the Lower Coal Measures. Coal seams: DH Doghole; BF Burnley Four-Foot; Mn Maiden; LY Low Yard; LB Lower Bottom Stream (Clifton Blindstone); C Inferior Cannel; FT Fulledge Thin (Habergham Blindstone); K King; L Lady; Ch China (Cliviger Two-Foot); Cr Crackers; D Dandy; A Arley (Cliviger Four-Foot); DH DibHole; C Coal; P Pasture; CM Cemetry; Ca Cannel; UM Upper Mountain (Hogards); IS Inch; U Upper Foot; LM Lower Mountain; LF Lower Foot (Little); B Bassy; M Margery.

(Figure 6) Westphalian sandstones of the Rochdale district.

(Figure 7) Coal seams of the Burnley and Lancashire coalfields.

(Figure 8) Glacial, periglacial and postglacial deposits.

(Figure 9) Meltwater channels and drift filled valleys in the Rochdale district.

(Figure 10) Main structures of the district.

(Figure 11) Principal mineral resources of historical importance in the district.

(Figure 12) Engineering geological characteristics of the rocks and soils in the district. A general guide only to the engineering characteristics of the deposits in the district is indicated below. In practice, ground conditions from site to site are further influenced by such factors as geological structure, topography, the presence of undermining, the presence of faults, the variable depths and degrees of weathering and human activity; such factors should also be considered prior to undertaking engineering development.

Plates

(Plate 1) Upper Haslingden Flags in Thurns Quarry showing soft-sediment deformation (arrowed), of sandstone into underlying silty mudstone [SD 874 186] (P710972).

(Plate 2) Cox Green Quarry [SD 718 146] showing the Ousel Nest Grit in the quarry face and the Woodhead Hill Grit forming the skyline feature (P710973).

(Plate 3) Rushy Hill meltwater channel looking east: the meltwater has cut through the dip slope of the Helpet Edge Rock (left) and into the underlying mudstones causing the sandstone on the north side of the channel to fail [SD 8900 1615] (P710974).

(Plate 4) Landslide deposits in the Rossendale Valley above and below the Lower Haslingden Flags [SD 827 213] (P710975).

(Plate 5) Limestone hushings at Sheddon Plantation showing mounds of sandstone cobbles left after removal of limestone cobbles and clay matrix [SD 895 295] (P710976).

(Front cover) Cliviger Gorge (Photographer Tim Cullen, (P535961)).

(Rear cover)

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

Figures

(Figure 3) Principal units of the Millstone Grit Group

(Figure 6) Westphalian sandstones of the Rochdale district

(Figure 7) Coal seams of the Burnley and Lancashire coalfields

(Figure 8) Glacial, periglacial and postglacial deposits

(Figure 11) Principal mineral resources of historical importance in the district.

(Figure 12) Engineering geological characteristics of the rocks and soils in the district

A general guide only to the engineering characteristics of the deposits in the district is indicated below. In practice, ground conditions from site to site are further influenced by such factors as geological structure, topography, the presence of undermining, the presence of faults, the variable depths and degrees of weathering and human activity; such factors should also be considered prior to undertaking engineering development.