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Geology of the Liverpool district — a brief explanation of the geological map Sheet 96 Liverpool

A S Howard, E Hough, R G Crofts, H J Reeves and D J Evans

Bibliographic reference: Howard, A S, Hough E, Crofts, R G, Reeves, H J, and Evans, D J. 2007. Geology of the Liverpool district—a brief explanation of the geological map. . Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 96 Liverpool (England and Wales).

© NERC 2007 All rights reserved. Copyright in materials derived from the British Geological Survey's work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining permission. Contact the BGS Intellectual Property Rights Section, British Geological Survey, Keyworth, e-mail ipr@bgs.ac.uk. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

Keyworth, Nottingham: British Geological Survey.

Printed in the UK for the British Geological Survey by B&B Press Ltd, Rotherham.

(Front cover) Front cover. Granite cladding disguises the concrete and steel structure of the Royal Liver Building on Liverpool waterfront. Built in 1908–11, this was one of the tallest buildings at that time which used this relatively new construction technique (Photograph: T Cullen; P535669).

(Rear cover)

(Geological succession) Geological succession at outcrop in the Liverpool district.

Notes

The word 'district' refers to the area of the geological 1:50 000 Series Sheet 96 Liverpool. National grid references are given in square brackets and all lie within 100 km square SJ. Symbols in round brackets after lithostratigraphical names are the same as those used on the geological map [symbols not shown in this digital version].

Acknowledgements

This Sheet Explanation incorporates contributions from E Hough (Permo-Triassic), R G Crofts (Quaternary), H J Reeves (Applied geology) and D J Evans (Structure). Series editor is A A Jackson: cartography was done by S Ward, and pagesetting by I Bolsover.

Maps and diagrams in this book use topography based on Ordnance Survey mapping. © Crown copyright. All rights reserved. Licence Number: 100017897/2007.

Geology of the Liverpool district (summary from rear cover)

An explanation of sheet 96 (England and Wales) 1:50 000 series map

(Rear cover)

(Geological succession) Geological succession at outcrop in the Liverpool district.

The City of Liverpool, Wirral and part of north Clwyd are included in this district, which is dissected by the estuaries of the Mersey and Dee (Afon Dyfrdwy). Both estuaries have been dredged to improve the shipping lanes and give access to the docks. Major coastal defences add to the artificial nature of parts of the coast but elsewhere sand-waves, dunes and tidal mudflats have built up around the natural coastline. Inland, superficial deposits of Quaternary age—mainly till and fluvioglacial sediments—conceal much of the bedrock, which consists of Carboniferous strata to the west of the Dee, in Wales, and largely Permian and Triassic strata to the east.

The oldest of the Carboniferous strata are the more erosion-resistant, Dinantian limestones of the Clwyd Limestone Group that forms a prominent escarpment in the west. Mudstone of the Bowland Shale Formation forms lower ground, and outcrops of the Pennine Coal Measures Group form the Flintshire and Wirral coalfields: the northernmost part of these formerly productive coalfields extend below the Dee estuary.

To the east of the Dee, Permian and Triassic strata underlie the City of Liverpool and Wirral and also extend offshore. These consist mainly of sandstone and mudstone that accumulated between the East Irish Sea and Cheshire basins, and are largely red beds owing to the arid conditions that prevailed at that time. Early Permian strata include aeolian and alluvial sandstones of the Appleby Group and coastal plain mudstones and evaporites of the Cumbrian Coast Group. The Triassic Sherwood Sandstone Group also originated largely as fluvial–alluvial sediments, and the Mercia Mudstone as coastal sabkha mudflats and evaporites. The more resistant formations, the Chester Pebble Beds and Helsby Sandstone, form escarpments and locally there are some spectacular exposures, both natural and artificial, of cross-bedded aeolian and fluvial sandstone.

A brief review of the mineral and groundwater resources is given here, as well as a summary of the engineering characteristics of the main lithological units, mining hazards, slope stability, dissolution and flooding.

Chapter 1 Introduction

This Sheet Explanation summarises the geology of the district covered by the geological 1:50 000 Series Sheet 96 Liverpool. The Bedrock and Superficial Deposits edition of the map shows the geology at the ground surface and adjacent sea bed. Thick superficial deposits conceal the underlying bedrock over much of the district, and the stratigraphical detail of bedrock strata has therefore been simplified for clarity. The Bedrock Geology edition provides full details of the stratigraphy and structure of the bedrock with the overlying superficial deposits omitted.

The district includes, on its north-eastern margin, the city centre and docklands areas of Liverpool. The Wirral peninsula, including the urbanised areas of Birkenhead and Wallasey, occupies much of the central part of the district, with the north Clwyd area of Wales occupying the south-west corner. The maps also display the geology of the adjacent offshore areas of the River Mersey and River Dee (Afon Dyfrdwy) estuaries and the south-east part of Liverpool Bay.

The bedrock geology of the district divides conveniently into two domains, with the Welsh part underlain by Carboniferous rocks and the English part underlain by Permo-Triassic rocks; the boundary runs approximately along the national border that bisects the Dee estuary (Figure 1). In north Clwyd, the topography climbs steadily south-westwards from the Dee estuary through the less erosion resistant mudstones of the Pennine Coal Measures Group and Bowland Shale Formation into the more resistant, Dinantian limestones of the Clwyd Limestone Group. Within these, the Loggerheads Limestone Formation is the most resistant to erosion, and forms a broken, south-facing escarpment that rises up to 250 m above OD and stands proud of the adjacent subdued terrain, which is mantled by clays, sands and gravels of glacial origin.

With the exception of a small outcrop of Upper Carboniferous strata around Neston, which includes the Wirral Coalfield, the Wirral peninsula and Liverpool city areas are underlain at rockhead by bedrock strata of Permo-Triassic age, consisting mainly of red sandstones of the Sherwood Sandstone Group. The more resistant sandstones of the Chester Pebble Beds and Helsby Sandstone formations locally form escarpments on the Wirral peninsula and in Liverpool, rising up to 80 m above OD. Adjacent lowerlying areas, generally at 30 m above OD or less, are underlain by less resistant, friable sandstones of the Wilmslow and Kinnerton Sandstone formations and red mudstones of the Mercia Mudstone Group, and are mantled by glacial till deposits up to 20 m thick.

The tidal estuaries of the rivers Mersey and Dee dominate the drainage system of the district. Both Wirral and north Clwyd are drained by a number of minor streams that flow mainly over low permeability glacial till deposits, but are locally deeply incised into the underlying bedrock. Recent coastal erosional and depositional processes have been modified considerably by the activities of man, including the construction of docks and coastal defences and the dredging of navigable channels in both the Mersey and Dee estuaries. Tidal sand flats are developed extensively in the Dee estuary, and sand wave 'fields' are well developed off the north-east tip of the Wirral peninsula at Hoylake. Coastal sand dunes are also present at Hoylake and to the west of Point of Ayr on the north Clwyd coast.

Although the oldest rocks preserved at surface in the district are of early Carboniferous age (340 Ma or million years before present), the geological history of the area was profoundly influenced by earlier events during the Palaeozoic. During the Silurian period (443 to 417 Ma), the district lay towards the northern margin of the Welsh Basin, a major depositional centre in which over 1000 m of mainly fine-grained marine sediments accumulated. In late Silurian to early Devonian times (417 to 400 Ma), these sediments were extensively folded, faulted and cleaved by compressional deformation associated with the late Caledonian (Acadian) orogeny, a period of major continental collision. Uplift associated with this orogeny was followed by a long period of erosion during the remainder of the Devonian period leaving, by early Carboniferous times (350 Ma), a low-lying landmass of deformed Silurian rocks (the Wales–Brabant Massif) with a connected series of subsiding, extensional marine basins (the Irish Sea and Craven basins) to the north. The northern margin of the Wales–Brabant Massif was flooded by a marine transgression in early Dinantian times (340 Ma), leading to the deposition of a thick sequence of mainly shallow marine, platform carbonates (Clwyd Limestone Group). To the north and north-east, coeval strata are represented by the deeper water, interbedded mudstones and limestones of the lower part of the Craven Group. By the late Dinantian (330 Ma), extension and rifting had effectively ceased, to be replaced by regional sag subsidence that persisted throughout the Namurian and Westphalian periods. Namurian (327 to 316 Ma) deposits were laid down over a residual Dinantian platform and basin topography. The Bowland Shale Formation records the gradual filling of the Irish Sea and Craven Basin by a southwards prograding delta system. The Millstone Grit Group includes both fluvial channel and sheet sandstones deposited in a delta plain environment. Delta plain conditions persisted during the Westphalian (316 to 306 Ma), characterised by cyclic deposition of lacustrine mud, fluvial sand, coal and marine bands (Pennine Coal Measures Group).

The red mudstones and sandstones of the Warwickshire Group reflect a transition to semi-arid alluvial plain conditions towards the end of the Westphalian (306 Ma) and the gradual uplift of the region associated with the onset of the Variscan Orogeny. This major period of continental collision extended through the late Carboniferous and early Permian (306 to 256Ma) periods, and resulted in the assembly of the world's continents into the single supercontinent of Pangaea. The district lay deep within this huge landmass and experienced a long period of uplift, associated with erosion and deep weathering of the pre-existing Carboniferous rocks in a hot and arid, continental climate. The Variscan Orogeny ended with a return to extensional tectonics, associated with the opening of the North and Central Atlantic, and leading to formation of rift basins in north-west Europe during the later part of the Permian and early Triassic periods (256 to 248 Ma). These basins accumulated thick successions of mainly terrestrial sediments derived from rapid denudation of adjacent elevated areas. The district lay on a low saddle (the Ellesmere Saddle; Chadwick et al., 1999) between two of these depocentres, the East Irish Sea and Cheshire basins. Early Permian sediments (Appleby Group) represent aeolian dune and alluvial fan environments, followed by a brief marine transgression in the late Permian (256 Ma), which resulted in the deposition of the red, coastal plain mudstones and evaporites of the Cumbrian Coast Group. Following regression of the sea, terrestrial deposition returned and persisted throughout the remainder of the Permian and the early part of the Triassic, filling the Cheshire and East Irish Sea basins with a thick sequence of fluvial and aeolian sediments, now preserved as the red sandstones of the Sherwood Sandstone Group. These sands were transported northwards across southern Britain by a major river system with a source in the Armorican mountain chain, which was created by the Variscan Orogeny and now has remnants in south Devon, Cornwall and Brittany. By Mid Triassic times (242 to 2227 Ma), a combination of rising sea level and denudation of the Armorican mountains resulted in a gradual transition to deposition now represented by the red, gypsiferous mudstones and siltstones of the Mercia Mudstone Group, the youngest bedrock strata preserved in the district.

Younger Triassic and Mesozoic strata were probably deposited in the district but were subsequently removed by erosion during the Palaeogene (65 to 24 Ma). During the Pleistocene (1.8 Ma to 10 000 years ago), several glaciations affected the region, but evidence of only the most recent (Late Devensian) has been preserved. Two ice sheets covered the area: the Welsh Ice originated in the mountains of north Wales and the Irish Sea Ice encroached from the north, coalescing along a line crossing the extreme south-west corner of the district.

Both ice sheets laid down large volumes of glacial till and glaciofluvial deposits. Erosion by subglacial meltwater locally accentuated pre-existing river valleys that coincided broadly with the present Dee and Mersey estuaries. After retreat of the ice, which was probably complete by around 15 000 years ago, the unconsolidated glacial deposits were eroded and redeposited as head and alluvium deposits. The postglacial rise of sea level during the Holocene (10 000 years ago to present day) drowned the valleys of the Dee and Mersey, creating the modern estuaries in which extensive intertidal deposits continue to accumulate (Plate 1).

Chapter 2 Geological description

Carboniferous

Within the district, Carboniferous rocks crop out in north Clwyd and below adjacent parts of the Dee estuary. A small part of the outcrop extends into the Neston area of Wirral to form the Wirral Coalfield. Elsewhere,Carboniferousrocksareconcealed beneath the Permo-Triassic strata, and their stratigraphy and structure is interpreted mainly from seismic reflection data.

The Carboniferous rocks in the district (Figure 2) range from early Dinantian (Arundian) to late Silesian (?Westphalian D) in age. The lithostratigraphical nomenclature applied to these rocks follows the scheme proposed by Waters et al. (2007). This UK-wide scheme has replaced many of the local, often informal stratigraphical names used within geographically isolated outcrop areas with a rationalised, basin-wide nomenclature.

Sedimentation during the Carboniferous was profoundly influenced by local palaeogeography, syndepositional tectonism and glacio-eustatic fluctuations in sea level. The latter are manifested in sedimentary cylicity at various scales throughout much of the succession. The most significant palaeogeographic feature was the Wales– Brabant Massif, a landmass of deformed Lower Palaeozoic rocks that included the area of the present outcrop of the Clwyd Limestone Group in the south-west of the district. To the north-east lay the conjoined Irish Sea and Craven basins, regions of more rapid subsidence associated with Dinantian extension and rifting. The margin between the massif and adjacent basins was marked by the Nercwys–Nant-figillt–Alyn Valley Fault system (Davies et al., 2004), which experienced episodic movement throughout much of the Dinantian.

In late Chadian times, the northern margin of the Wales–Brabant Massif was flooded by a marine transgression, leading to the deposition of a thick, Arundian–Asbian sequence of mainly shallow marine, platform carbonates (Llanarmon Limestone and Loggerheads Limestone formations). Knoll-reef limestone build-ups developed on the platform margin (Figure 3). Basinal equivalents of Arundian to Asbian strata are not seen at outcrop in the district, but probably include turbiditic limestones similar to the Prestatyn Limestone Formation of the Rhyl and Denbigh district (Warren et al., 1984). By Brigantian times, rifting had effectively ceased, and thermal sag subsidence persisted throughout the Namurian and Westphalian (Leeder, 1982). The knoll-reefs were drowned as sea level rose. Sedimentation on the Wales–Brabant Massif took on a deeper water aspect: the shallow marine argillaceous limestones of the Cefn Mawr Limestone Formation were deposited on the platform, passing basinwards into the turbiditic limestones of the Teilia Formation. Carbonate deposition was brought to an end in the late Brigantian by a combination of regional uplift and eustatic sea level fall associated with the Gondwanan glaciation (Leeder, 1988). Sedimentation resumed in the Namurian with siliciclastic basin-fill deposits laid down over the residual Dinantian platform and basin topography, with thick spicular cherts (Pentre Chert Formation) deposited on the intervening slope. The overlying Bowland Shale Formation records the filling of the Irish Sea–Craven Basin by a southwards prograding delta system. The Millstone Grit Group is represented by the Gwespyr Sandstone Formation, which includes both fluvial channel and sheet sandstones deposited in a delta plain environment. Delta plain conditions, including episodes of coal formation and occasional marine transgressions, continued to prevail during the Westphalian (Pennine Coal Measures Group). The primary red beds of the overlying Warwickshire Group reflect a transition to a semi-arid alluvial plain setting towards the end of the Westphalian. Rocks of Stephanian age are absent, with Permo-Triassic rocks lying unconformably on strata of Westphalian age.

Clwyd Limestone Group (CwL)

The base of this group is not proved in the district, but is known to lie unconformably on Silurian rocks a short distance to the south in the adjacent Flint district (Davies et al., 2004). Terrestrial red beds ('Basement Beds') of early Dinantian age locally intervene. The outcrop is largely masked by glacial till, but locally the limestone stands proud of the till forming hills. The limestone has been quarried for stone, aggregate and cement making. Lead and zinc ores are common along veins and faults, and have been extensively mined in the district.

The Llanarmon Limestone Formation (LmL) outcrop is almost entirely masked by till deposits and exposure is limited to a few, long-disused small quarries in the extreme south-west of the district. The formation comprises pale grey, structureless or cross-bedded, medium to thick-bedded, peloidal and skeletal grainstones, with subordinate dark, thinner bedded packstones. Corals and brachiopods are common, the latter commonly preserved in discrete shell beds. The lower beds (early Arundian) were deposited on a north-eastwards facing carbonate ramp (Somerville et al., 1989), and the upper beds (late Arundian) on a shallow marine carbonate platform. The overlying Loggerheads Limestone Formation (LoL) consists mainly of pale grey, thick to very thick-bedded, skeletal and peloidal packstone, which is commonly pseudo-brecciated with a rubbly, mottled appearance. The formation is characterised by laterally persistent cycles ranging from 0.5 to 21 m thick, each capped by pitted palaeokarstic surfaces, volcanogenic palaeosols and well-developed calcrete profiles. The cycles each represent periodic flooding of the carbonate platform, followed by gradual shoaling and long periods of emergence. The formation contains a sparse but diverse assemblage of corals and brachiopods. Knoll-reef limestones (K) at Axton and Gelli are lateral equivalents of the Loggerheads Limestone and represent biogenic carbonate mud mounds that accumulated on the platform margin. They consist of unbedded, fine-grained limestone with a rich fauna of brachiopods, corals, bryozoans and crinoids. Cycles of flooding and emergence continued during deposition of the Cefn Mawr Limestone Formation (CML), which comprises highly fossiliferous, dark grey wackestones, packstones and grainstones occurring in a series of cyclic successions each up to 75 m thick, again capped by palaeokarstic surfaces and dark mudstone palaeosols. An abundant and diverse fauna includes crinoids, brachiopods, calcareous algae, fenestellid bryozoa, corals, molluscs, trilobites and foraminifera.

Craven Group (CRAV)

The Craven Group consists of mudstone, siltstone, argillaceous limestone and chert, deposited in the predominantly deep water, environment within the Irish Sea Basin. In north Wales, the group ranges from early Dinantian (?Chadian–Arundian) to late Namurian (Yeadonian) in age. Strata of Dinantian age within the group are lateral, basinal facies equivalents of the shallow water carbonates of the Clwyd Limestone Group. The oldest Craven Group strata occurring at outcrop in the district are of Brigantian age (Teilia Formation), but it is likely that deposition of the basinal facies mudstones and argillaceous limestones commenced as early as the late Chadian over much of the district (Davies et al., 2004).

Stratigraphical relationships at the base of the Craven Group in the district are complex and difficult to resolve with certainty due to poor exposure and locally thick Quaternary cover. Between Axton and Pentre-ffynnon Hall, the Teilia Formation forms the basal unit of the group and overlies the Loggerheads Limestone Formation, which is locally represented by knoll-reef facies. Farther south-eastwards, the Pentre Chert Formation forms the base of the group, and unconformably overlies the Cefn Mawr Limestone Formation.

The Teilia Formation (TlL) typically consists of dark grey, fine-grained, medium-bedded, argillaceous limestones interbedded with dark grey calcareous mudstone. The formation was deposited as an apron of limestone turbidites and hemipelagic mud on the marginal slope of the Dinantian carbonate platform. There are currently no exposures in the district, but the formation was formerly well exposed in a quarry at Trelogan (Hind and Stobbs, 1906), which has been backfilled. In the adjacent Rhyl district (Warren et al., 1984) the formation has yielded the bivalves Dunbarella persimilis, Posidonia becheri, and Arnsbergites sphaericostriatus, indicating a Brigantian (P1c) age.

ThePentre Chert Formation (PCF) lies with minor disconformity on the Teilia Formation in the Trelogan area, and with more marked unconformity on the Cefn Mawr Limestone around Holywell. Thickness also declines rapidly from 110 m near Llanasa to around 20 m at Holywell. The unit comprises colour 'banded', glassy to impure, granular-textured and wavy bedded cherts (Plate 2), interbedded with cherty mudstones and siltstones. Silicified limestone beds are also present towards the base. Recumbent folds and low-angle discordances, Pentre Chert Formation at Trelogan Quarry [SJ 1187 8027], indicate slumping and gravity slides down the platform margin during deposition. The chert may be partly primary, originating from hemipelagic deposition of sponge spicules and radiaolaria, but secondary re-mobilisation of silica and chertification of primary mudstone, sandstone and limestone has also taken place. Faunas in adjacent formations constrain the Pentre Chert to a late Brigantian (P1) to early Pendleian (E1b) age; given the unconformity at the base, the formation may be wholly of early Pendleian age.

Siliceous sandstones proved below the Bowland Shale Formation in the Heswall Borehole probably correlate with the early Namurian Cefn-y-fedw Sandstone Formation (CfS) of the Flint district to the south (Davies et al., 2004). This formation is not represented at outcrop in the south-west of the Liverpool district, where the Bowland Shale Formation lies with gradational contact on the Pentre Chert.

The Bowland Shale Formation (BSh) (formerly Holywell Shale Formation) comprises mudstones with subordinate thin quartzitic sandstones, argillaceous limestones, sideritic nodule beds and impersistent coal seams with underlying seatearths. The mudstones are typically dark grey, fissile or blocky, becoming olive-brown, silty and finely micaceous towards the gradational contact with the overlying Gwespyr Sandstone. The boundary with the underlying Pentre Chert Formation is also gradational, marked by a progressive upward decrease in chertification over a stratigraphical interval of about 5 m. Several ammonoidbearing 'marine bands', each comprising highly fossiliferous, black, fissile mudstone several metres thick, were proved in the Abbey Mills No. 1 and No. 4 boreholes near Holywell (Ramsbottom, 1974). The Cravenoceras malhamense Marine Band (Pendleian, E1c) is the lowest recognised ammonoid horizon, with the highest (Cancelloceras cancellatum Marine Band Gla1) indicating that the formation ranges up to early Yeadonian in age. The formation was deposited in a marine, prodelta environment.

Millstone Grit Group (MG)

As in the central Pennines, the Millstone Grit Group is characterised by feldspathic sandstones deposited during cyclic progradation of a major delta system. In the Liverpool district, the entire group is mostly composed of a single unit, the Gwespyr Sandstone Formation, and is Yeadonian in age.

The Gwespyr Sandstone Formation (Gwp) ranges from 150 to 170 m thick in the district. Its outcrop extends from Gwespyr south-eastwards towards Holywell and Bagillt, locally forming a prominent north-eastwards dipping cuesta standing proud of the Quaternary deposits. The unit is well exposed in former sandstone quarries around Gwespyr and Holywell, and in stream sections to the west of Gwespyr and between Gyrn Castle and Mostyn. The boundary with the underlying Bowland Shale Formation is gradational and diachronous, with interdigitation of mudstone and sandstone units. The Gwespyr Sandstone is split into two leaves by a mudstone unit containing the Cancelloceras cumbriense Marine Band (G_1b1) (Ramsbottom, 1974), though this split cannot be mapped consistently at surface. Up to 20 m of mudstone beds intervene between the top of the Gwespyr Sandstone and the base of the Subcrenatum Marine Band, which marks the boundary with the overlying Pennine Lower Coal Measures. In the Flint district to the south, the Subcrenatum Marine Band is mostly absent or unrecognisable and the Gwespyr Sandstone persists upwards to within 10 m of the Llwyneinion Half Yard Coal (Davies et al., 2004). The upper part of the Millstone Grit in that district is therefore of early Langsettian (Westphalian A) age.

The lower beds of the Gwespyr Sandstone, below the Cancelloceras cumbriense Marine Band, consist of a distinctive, greenish grey, fine-grained, planar and ripple laminated sandstone, resembling the Haslingden Flags of Lancashire (Aitkenhead et al., 2002), and possibly with a western provenance. The upper beds consist of medium to coarse-grained micaceous and feldspathic sandstone, which is typically medium to thick bedded, but large scale cross-stratification is not as prevalent as in the equivalent Rough Rock of the Pennines. Like the Rough Rock, the upper beds were deposited by delta progradation with a northerly sediment source, but the persistence of sandstone deposition well into the Westphalian in the adjacent Flint district implies a contemporaneous high to the south that may also have provided a local source of detritus (Davies et al., 2004).

Pennine Coal Measures Group (PCM)

The Pennine Coal Measures Group includes the productive coal-bearing strata between (and including) the Subcrenatum Marine Band and the base of the overlying Warwickshire Group. In the Liverpool district, the Coal Measures crop out along the flanks of the Dee estuary between Flint and Point of Ayr, forming the North-west Flintshire Coalfield, and in a smaller area around Neston, forming the Wirral Coalfield. Both coalfields and their associated underground coal workings extend below the Dee estuary.

In the English coalfields, the group has beendividedintothe Pennine Lower, Middle and Upper Coal Measures formations using the most persistent ammonoidbearing marine bands as boundary markers. In the Liverpool district, the Cambriense (Top) Marine Band, which marks the base of the Pennine Upper Coal Measures, is absent, and there is a highly diachronous and locally unconformable passage from the Pennine Middle Coal Measures Formation into the overlying Etruria Marl Formation of the Warwickshire Group.

The group consists of numerous upward-coarsening units or cyclothems, each a few tens of metres thick and consisting of mudstone, siltstone and sandstone capped by a coal seam. Each cyclothem represents the gradual progradation of a delta lobe into a freshwater lake environment, eventually leading to emergence, swamp formation and ultimately colonisation by bog and forest floras. Mudstones deposited in the freshwater lakes contain abundant freshwater mollusc and fish faunas. Sandstones deposited on the delta top are typically fine grained, well laminated and micaceous with plant fragments, and were deposited in a range of environments that included distributary river channel, levee, crevasse splay and mouth bar. Colonisation by bog and forest plants led to the formation of thick peat deposits overlying clay (seatearth) or silica sand (ganister) soils with abundant rootlets. Coal seams resulted from compression and thermal maturation of the peat beds following deep burial. Less common lithologies include nodular sideritic ironstones and 'marine bands', the latter containing abundant brackish water faunas such as Lingula, Estheria and fish, and more rarely fully marine bivalves and ammonoids.

There are few gross differences in lithology between the Pennine Lower and Middle Coal Measures formations. The most significant change occurs approximately midway within the Pennine Lower Coal Measures, above which the cyclothems are generally thinner and coal seams are thicker and more persistent; most of the extensively worked seams in the North-west Flintshire Coalfield (Figure 4) lie above this level. With the exception of the Vanderbeckei (Llay) Marine Band, marine bands are restricted to the lower part of the Pennine Lower Coal Measures and the upper part of the Pennine Middle Coal Measures.

The coal seam nomenclature formerly used by the National Coal Board (latterly British Coal) in the North Wales Coalfield as a whole is based largely on that of the Denbighshire Coalfield (Figure 4). Correlation with the locally named seams in the North-west Flintshire Coalfield is mostly firmly established, with the exception of seams between the Nant and Red coals, where uncertainty remains owing to the complex pattern of seam splitting. Correlation of seam names between the Flintshire and Wirral coalfields is more speculative, and is unlikely to be reliably resolved now that coal exploration and mining activities have ceased in the region. Only the lower part of the Pennine Lower Coal Measures Formation (PCML) crops out extensively onshore. The outcrop in the Wirral Coalfield is largely masked by till and no surface exposures occur. The Pennine Lower Coal Measures Formation in the Mostyn area is intermittently exposed in several stream sections, notably Nant Felin-blym west of Mostyn Hall, where Shanklin (1956) recorded exposures of both the Subcrenatum and Listeri Marine Bands. The 30 m of strata between these marine bands include a 15 m-thick unit of flaggy, fine-grained micaceous sandstone that has been mapped extensively between Mostyn and Gwespyr. This has been termed the Upper Gwespyr Sandstone (Calver and Smith, 1974) but is differentiated from the Gwespyr Sandstone as a separate, unnamed unit in this account.

The upper part of the Pennine Lower Coal Measures and the overlying Pennine Middle Coal Measures Formation (PMCM) have only small areas of outcrop onshore in the district, and are obscured below thick superficial deposits. Information on the stratigraphy of this part of the succession (Figure 4), which includes most of the extensively worked coals in the North-west Flintshire Coalfield, is derived entirely from mine plan, shaft and borehole data, the last originating mainly from exploration for the proposed offshore extension of the Point of Ayr Colliery in the late 20th century. The Hollin Rock Member, a feldspathic sandstone occurring in several leaves and totalling up to 150 m thick, lies above the Hollin Coal and is proved only in boreholes and shafts in the south of the district. It represents a local but persistent influx of sand from the Wales–Brabant Massif, and may be the earliest manifestation in the district of uplift associated with the Variscan Orogeny.

Warwickshire Group (Wk)

The term Warwickshire Group was introduced by Powell et al. (2000) to encompass the primary red-bed strata intervening between the Pennine Coal Measures Group and the unconformable base of the Permo-Triassic in the coalfields of England and Wales. In the Liverpool district, the group includes strata formerly assigned to the Upper Coal Measures (Wedd et al., 1923) and more recently the Red Measures (Calver and Smith, 1974) or Red Measures Group (Davies et al., 2004). Only the lower part of the group crops out onshore, the remainder occurring mainly below the Dee estuary, where up to 425 m of strata are preserved locally within a series of tilted half-graben, and are markedly overstepped by the basal Permo-Trias unconformity.

The Warwickshire Group is not exposed in the district and all information is derived from boreholes and geophysical seismic profile data. It consists predominantly of red, purple and grey mottled mudstone and sandstone with common seatearths, rare coal seams and limestones that contain Spirorbis and ostracods. Only the lowest subdivision, the Etruria Marl Formation (Et) is differentiated on the map; broad equivalents of the Halesowen and Salop formations of the English Midlands are recognisable in some borehole descriptions in the Dee estuary and offshore, but the logs are insufficiently detailed and the boreholes too scattered to delineate boundaries. The onshore outcrop of the Etruria Marl Formation consists of grey, red and purple mottled mudstones and sandstones with common fireclays, formerly included in the Middle Coal Measures as the Buckley Fireclay (Strahan, 1890, Wedd and King, 1924) or Buckley Fireclay Group (Calver and Smith, 1974). Davies et al. (2004) include these strata in the Ruabon Marl, together with the overlying red and purple mudstones with thin beds of Spirorbis limestone more typical of that formation. As in the Flint district to the south, the base of the group is markedly diachronous and locally unconformable, analogous to the Symon Unconformity at the base of the group in parts of central England (Powell et al., 2000). The stratigraphical position of the unconformity varies considerably across faults (Calver and Smith, 1974; Davies et al., 2004), indicating the local onset by Bolsovian (Wesphalian C) times of syndepositional tectonism associated with the Variscan Orogeny. The Etruria Marl Formation is predominantly an alluvial plain deposit, laid down in drier, more oxidising conditions than the underlying Coal Measures (Besly, 1988).

Permian and Triassic

Within the district, bedrock of Permo-Triassic age underlies much of Wirral, the Mersey estuary and the City of Liverpool. The outcrop continues offshore, where these rocks underlie much of the Dee estuary, and farther north, much of the eastern Irish Sea. The Permo-Triassic succession comprises four groups (Figure 5), but only the upper two, the Sherwood Sandstone and Mercia Mudstone groups, are known to crop out onshore. The boundary between the Permian and the Triassic has not been proved chronostratigraphically in the UK but, conventionally, is drawn at the base of the Sherwood Sandstone Group (Warrington et al., 1980). There are practical difficulties applying this definition in the Liverpool district, because the Upper Permian Cumbrian Coast Group pinches out towards the south of the district. Beyond its limit, the Appleby Group (Lower Permian) is indistinguishable from, and has therefore been included within, the lowermost part of the Sherwood Sandstone Group.

During the Permo-Trias, the Liverpool district was located in the northern foreland of the Armorican mountain belt, approximately 20ºN of the contemporary Equator. Structurally, the district lay on the Ellesmere Saddle, which separated the adjacent, more rapidly subsiding East Irish Sea and Cheshire extensional basins (Chadwick et al., 1999), where thicker Permo-Triassic successions accumulated.

The nomenclature used for the Sherwood Sandstone Group in the district is taken from the Cheshire Basin and is based on the scheme of Warrington et al. (1980). Correlation with the alternative nomenclature used in the East Irish Sea Basin, and its onshore extension into Lancashire, is shown in (Figure 5).

Appleby Group (Apy)

The Appleby Group is represented in the district by the Collyhurst Sandstone Formation (CS), and is of probable Early Permian age. Onshore, the Collyhurst Sandstone is inferred to be present at depth below much of the area, but does not crop out at surface. Coal exploration drilling in the Dee estuary encountered up to 16 m of red-brown, coarse-grained sandstone with rounded 'millet seed' grains, comparable to the Collyhurst Sandstone in its type area around Manchester, and suggesting an aeolian depositional environment. In the south of the district, beyond the southernmost depositional limit of the overlying Manchester Marls Formation (see below) the Collyhurst Sandstone is inseparable stratigraphically from the Triassic Kinnerton Sandstone Formation.

Cumbrian Coast Group (CCO)

The Cumbrian Coast Group is represented by the Manchester Marls Formation (MM) of probable Late Permian age. The formation is proved by coal exploration drilling in the Dee estuary, which encountered up to 27 m of dark reddish brown mudstone. The Heswall Borehole proved a thin bed of rock salt at 625 m depth which, together with beds of red mudstone noted within the adjacent sandstone units, may represent a marginal facies of the Manchester Marls near its southernmost limit of deposition.

Sherwood Sandstone Group (SSG)

The Sherwood Sandstone Group encompasses sandstones formerly assigned to the 'Bunter' and lower 'Keuper' series (Figure 5). The group is predominantly of Early Triassic (Scythian) age but, in the south of the district, the lowermost strata may be equivalent to the Collyhurst Sandstone of Permian age. The highest formation, the Helsby Sandstone, is early Mid Triassic (Anisian) in age. The four constituent formations (Figure 6) broadly reflect an alternation between aeolian and fluvial sedimentation. The Chester Pebble Beds and Helsby Sandstone formations are generally more strongly lithified and more resistant to erosion; their outcrops form the higher ground on Wirral and in Liverpool and are locally free of superficial deposits. In contrast, the Kinnerton Sandstone and Wilmslow Sandstone formations are weaker and more friable, and generally form lower ground mantled by thick till deposits.

The Kinnerton Sandstone Formation (KnS) underlies much of the district at depth, but the onshore outcrop is restricted to a small area near Neston. No exposures were recorded in the district during this resurvey, but the upper part of the formation, and the boundary with the overlying Chester Pebble Beds, is well exposed at Burton Point [SJ 3026 7358], nearby in the adjacent Flint District (Davies et al., 2004) The formation has been penetrated by several boreholes both onshore and offshore, in which the sandstone is typically described as slightly cemented, red-brown and pale yellowish grey mottled, and fine to medium grained. At Burton Point and in other exposures elsewhere in the Cheshire Basin, the sandstone is markedly cross-stratified, and consists dominantly of subrounded to well rounded quartz grains. Cross-stratification within the formation was formed by migration of transverse aeolian dunes, with planar laminated sandstones deposited by flash floods in interdune areas (Macchi, 1991).

The Chester Pebble Beds Formation (CPB) underlies much of the southern and eastern parts of Wirral, and much of the residential areas of Liverpool to the east of the city centre. The base of the formation is taken at the lowest occurrence of pebbly sandstone overlying the Kinnerton Sandstone. The formation is poorly exposed in the district, with exposures limited mainly to quarry sections and on parts of the foreshore along the Mersey estuary, notably at Wallasey. The formation is composed of red-brown or pinkish red, fine to medium-grained sandstone, with subordinate dark reddish brown mudstone interbeds. Extraformational quartz and quartzite pebbles occur within the lower 80 m of the formation, but are less common in higher beds.

The sandstone is characterised by erosive-based, trough cross-stratified sets deposited by low- to medium-sinuosity braided rivers. Cross-bedding azimuth measurements ranging from 275º to 015º are consistent with the regional north-westward to northward palaeocurrent flow suggested by Earp and Taylor (1986). Pebbles generally become rarer to the north, consistent with a sediment source in the contemporary Armorican mountains, which lay to the south of Britain at this time (Wills, 1956).

The Wilmslow Sandstone Formation (WlS) gives rise to a gently undulating, low-lying topography, mostly mantled by till deposits. The base of the formation is taken where sandstone above the Chester Pebble Beds becomes entirely pebble free, which coincides with a gradual transition to generally weaker cementation, finer and more well-rounded grains and more vivid, deep red-brown and yellowish grey mottling. In practice, because of the general scarcity of pebbles in the upper part of the Chester Pebble Beds, the boundary can be difficult to place.

The formation comprises red-brown, yellowish grey and white, slightly cemented fine- to medium-grained sandstone with rare silty sandstone interbeds. The formation is cross- and planar-stratified (Plate 3), with bimodal grain-size lamination a common feature. To the west of the Frankby Fault, the upper part of the formation locally displays distinctive soft-sediment deformation and is differentiated as the Thurstaston Sandstone Member (TsS). The base of this member is marked by the Thurstaston Hard Sandstone Bed (ThHB), a laterally impersistent, well-cemented sandstone up to 2 m in thickness. The Thurstaston Sandstone Member thickens northwards from 23 m at Heswall to 62 m at West Kirby.

The Wilmslow Sandstone Formation is dominantly aeolian in origin. Azimuths of cross-bedding foresets range from 275º to 350º, indicating deposition by prevailing palaeowinds blowing towards the north-west. The Thurstaston Hard Sandstone Bed is interpreted as a fluvial channel deposit cutting into underlying damp, cohesive sediment, and further fluvial sedimentation episodes occur within the overlying Thurstaston Sandstone Member (Macchi, 1991). The soft-sediment deformation displayed in the latter, especially at Thurstaston cutting [SJ 245 846] (Rice, 1939) is some of the finest exposed in the UK. It is possible that dewatering and slumping of sand beds was initiated by seismic events (Macchi, 1991).

The Helsby Sandstone Formation (Hey) comprises both pebbly and pebble-free sandstone with subordinate mudstone beds. Pebbles include both intraformational mudstone and sandstone clasts, and more exotic quartz and quartzite pebbles. The base of the formation is taken where pebbly sandstone becomes dominant over the largely pebble-free Wilmslow Sandstone Formation, associated with a transition to brown, micaceous and well-cemented sandstone. This junction may correlate with the Hardegsen Unconformity (Evans et al., 1993), formed by an episode of uplift and erosion in late Early Triassic (Scythian) times.

The formation was extensively exploited for building stone in the district, and is locally well exposed in quarries and road cuttings. Excellent natural exposures occur in the cliffs bounding Hilbre Island in the Dee estuary, where a prominent and persistent breccia bed up to 0.4 m thick can be mapped around the western side of the island. Reptilian footprints, including Chirotherium and Rhynchosaurids have been described from sections at Storeton and Hilbre Island (e.g. Cunningham, 1838; Tresise, 1991, King and Thompson, 2000).

To the east of the Woodchurch Fault, the formation is undivided, and forms the prominent high ground at Irby, Storeton and Bidston hills, and at Wallasey. Two members are mapped to the west of the Woodchurch Fault.The Delamere Sandstone Member (DmS) comprises brown, pebbly, medium- to coarse-grained sandstone and subordinate mudstone beds, and is up to 90 m thick. The overlying Frodsham Sandstone Member (FsS), consists of up to 20 m of pebble-free, red-brown medium-grained sandstone. In Cheshire, the Delamere Sandstone is interpreted mainly as a fluvial sandstone with subordinate aeolian units (Thompson, 1970; Mountney and Thompson, 2000). Fluvial sandstone dominates in the Liverpool district, with cross-stratified bedforms indicating deposition mainly as sandbars under waning flood conditions within a seasonal river system in a semi-arid climatic setting (Thompson, 1970). At Frodsham, Cheshire, the Frodsham Sandstone Member shows cross-stratification structures indicative of dome-shaped and transverse aeolian dunes (Thompson, 1969).

Mercia Mudstone Group (MMG)

The lower part of the Mercia Mudstone Group underlies part of the central and northern part of Wirral, and occurs within a series of small graben to the west of the Woodchurch Fault. Due to extensive cover of superficial deposits, exposures within the group are not common, and are restricted to a few small exposures in streams. The stratigraphical nomenclature adopted for the district follows the national scheme of Howard et al. (2007). The lowest two formations of the group are present within the district and are differentiated primarily on grain size; the Tarporley Siltstone Formation contains a substantial proportion of sandstone beds compared with the mudstone-dominated strata of the overlying Sidmouth Mudstone Formation.

The Tarporley Siltstone Formation (TpS) comprises interbedded brown, very fine-grained sandstone, reddish brown and grey-green siltstone, and red-brown mudstone in approximately equal proportions. Siltstone and sandstone beds are characteristically highly micaceous. Preserved bedding structures include desiccation polygons and flute casts; parallel, wavy and climbing ripple lamination are common. Mudstone clasts commonly form lags at the base of the sandstone units. The formation was deposited in an estuarine or lacustrine setting, with silty mudflats prone to periodic desiccation and sheet floods. Thicker sandstone units were deposited in broad, fluvial channels (Warrington and Ivimey-Cook, 1992).

Good exposures of the Tarporley Siltstone Formation occur in a stream section west of Heswall known as the Dungeon [SJ 2510 8310], described by Benton et al. (2002). Samples from the formation in the Saughall Massie Borehole yielded spores and pollen broadly indicative of an early Mid Triassic (Anisian) age (Warrington, 1999).

The base of the overlying Sidmouth Mudstone Formation (SiM) is gradational, taken above the level where mudstone becomes the dominant lithology. The formation comprises red-brown mudstone with thin beds of dolomitic grey-green siltstone and very fine-grained sandstone. The mudstone is characteristically blocky with ill-defined primary structures, although traces of lamination and desiccation features are preserved. Primary halite beds have not been proved within the district, but thin beds, veins and nodules of gypsum have been reported in some borehole logs. The mud accumulated in shallow lakes or as aeolian dust adhering to damp mudflats in a marginal sabkha environment, and brecciation occurred syndepositionally by repeated episodes of desiccation, and the growth and solution of evaporitic minerals (Arthurton, 1980). It is probable that thin beds of sandstone were deposited by flash floods on subaerial mudflats or by ephemeral sediment pulses in shallow, temporary lakes (Warrington and Ivimey-Cook, 1992).

Quaternary

More than 90 per cent of the district, including the Dee and Mersey estuaries and Liverpool Bay, is covered by natural superficial (drift) deposits. These are shown on the Bedrock and Superficial Deposits edition of the map, and their principal characteristics and stratigraphical relationships are summarised in (Figure 7) and (Figure 8).

Onshore the present-day topography was largely shaped by glacial processes during the Pleistocene, while offshore the bathymetry continues to be modified by marine and estuarine processes. The district has probably been subjected to at least three major glacial advances, although evidence for earlier events has been obliterated by the most recent Late Devensian glaciation. The rockhead topography of the district suggests that prior to the Late Devensian glaciation major drainage was coincident with the present-day courses of the Mersey and Dee estuaries, with a former coastal cliffline forming an embayment across the northern part of Wirral, several kilometres inland of the present coastline. During glaciation, subglacial meltwater excavated new channels and locally overdeepened pre-existing valleys, in some cases to several tens of metres below present-day sea level (Howell, 1973). These channels are commonly filled with glacial till or, locally, by laminated clay or stratified sand and gravel deposits. Several of these 'buried' channels have been identified during the resurvey and are shown on the Bedrock and Superficial Deposits edition of the map. A buried valley beneath the current Mersey channel, first postulated by Reade (1873), was later confirmed in the Liverpool area during the excavation of the Mersey Railway Tunnel (Reade, 1885).

The channel beneath the current course of the River Mersey is partly filled with till and is excavated to a depth of 28 m below OD (Boswell, 1925, 1937). The channel has also been confirmed at Dingle (Renewable Energy Enquiries Bureau, 1992, fig. 2) where the channel is excavated to about 50 m below OD. Borehole evidence indicates that the Mersey buried channel has an undulating base formed from a line of interconnected enclosed hollows, typical of a subglacial origin. Further buried channels have also been confirmed by boreholes at Dingle (Owen, 1947), Birkenhead and Bromborough. The channel at the first locality is excavated to at least 15 m below OD and filled with till, while at the last, the channel is excavated to about 25 m below OD. However, their hydrological relationship to the main Mersey buried channel is unclear. A buried channel has also been proved at Hooton and was first noted by Jones (1937). It is excavated to at least 41 m below OD, is orientated north– south and there are indications that its floor falls to the south. The infilling sediment is mainly till, but sand and laminated clay occur locally.

During the Late Devensian glaciation, two distinct ice sheets invaded the district. The first to arrive was the Welsh ice (Thomas, 1985, Jansson and Glasser, 2005), fed by glaciers originating from the Snowdonia and Arenig mountains of north Wales, followed by the Irish Sea ice, which entered the district from the north. The two ice sheets converged and interacted to the south-west of a line between Walwen and Pen-ffordd-llan (coinciding almost exactly with the route of Offa's Dyke). Two distinct Till deposits resulted, each with a distinctive erratic suite, although it has not been possible to map them out separately across the district. Till deposits in Liverpool and Wirral include a diverse suite of erratics derived from the Lake District and Scotland (listed by Wedd et al., 1923), together with marine shell fragments (Reade, 1874). In contrast, till in the extreme south-west of the district has a grey matrix and contains many pebbles of Lower Palaeozoic siltstone and sandstone indicating derivation from Welsh ice. The two tills also contrast strongly in their surface morphology; the Irish Sea till generally has a smooth, gently undulating profile, whereas the Welsh till is moulded into drumlins, best seen to the south-west of Tre Abbott [SJ 105 785].

The Irish Sea till is well exposed at Dawpool cliffs, Thurstaston [SJ 240 830], first described in detail by Slater (1929) and more recently by Brenchley (1968), Lee (1979), and Glasser et al. (2001). Here, the till consists of a lower, clast-rich, well consolidated red-brown diamicton, overlain by a complex interbedded succession of sand, gravel and laminated silty clays, which in turn is overlain by poorly consolidated, clast-poor red-brown diamicton. This broadly tripartite succession of lower till, middle sand and upper till is typical of the Cheshire region, and has been considered as the product of two, separate ice advances, with the intervening middle sand representing an interglacial deposit (Poole and Whiteman, 1966; Lee 1979). More recently, the tripartite succession has been interpreted as the product of a single glaciation, with the upper till interpreted variously as a lacustrine, glaciomarine or ablation till deposit laid down during deglaciation (Brenchley, 1968; Eyles and McCabe, 1989, Davies et al., 2004). Glasser et al. (2001) argue that the entire deposit was laid down as a deformation till beneath the advancing Irish Sea ice, with the middle sand and gravel units representing hiatuses in that advance. Glasser and Hambrey (1998) suggest that, during deglaciation at the end of the Devensian period, the Irish Sea ice remained thin and active, without a long period of ice stagnation and wasting. This is supported by the general lack of ice stagnation and retreat features on Wirral, such as morainic ridges, kettle and kame topography and eskers.

Glaciofluvial deposits consist of very poorly sorted gravelly, cobbly, clayey sands and form extensive spreads to the west of the River Dee, where they are related to the complex interaction between the Welsh and Irish Sea ice. North-westwards from Holywell a series of moundy glaciofluvial deposits were probably deposited in a kame belt lying close to the suture of these two ice sheets. At Pantasaph, glaciofluvial deposits form an extensive flat spread of sand and gravel that was formed by outwash from decaying ice at the end of the Devensian period. Glaciofluvial deltaic deposits ,consistingofwellsorted,cross-stratified sandy gravels, occur in the extreme south-west of the district, where they are closely associated with Glaciolacustrine deposits that consist of grey, largely pebble-free silty clays. This association forms part of a complex belt of proglacial fluvial and lacustrine deposits extending south-eastwards towards the Wrexham area, deposited along the suture zone of the Irish Sea and Welsh ice sheets during uncoupling and melting at the end of the Devensian (Thomas, 1985).

Periglacial weathering occurred once the ice had receded. The intense cold caused the development of permafrost conditions in the subsoil and the shattering and weathering of rock due to freeze-thaw processes. These processes promoted the formation of head deposits and landslides. Head is generally confined to the upper parts of valleys or occurs at the base of steep slopes where weathered Triassic sandstones or poorly consolidated glaciofluvial deposits occur upslope. Head is difficult to delineate in urban settings and therefore may be more extensive than indicated on the map. Landslides occur on steeper slopes, usually in excess of 10°, underlain by till deposits or by mudstones of the Pennine Coal Measures (see below). Where streams cut through thick till into water-saturated and weathered sandstone, the valley sides are often prone to landslides.

In the immediate postglacial period, the Shirdley Hill Sand (SH), an aeolian cover sand, was deposited prior to extensive re-vegetation of the landscape. Regionally, the deposit consists typically of up to 2 m of yellowish-grey, unconsolidated, fine-grained sand. Sedimentological and mineralogical studies (Wilson et al., 1981) have established the provenance to be glaciofluvial deposits in the Irish Sea Basin rather than locally degraded Triassic sandstones.

Following the Devensian glaciation, rising sea level flooded the lower courses of the Mersey and Dee valleys to form the present-day estuaries. On land, the modern drainage pattern was established, though most of the smaller streams in the district are misfit, and follow deeply incised courses initiated by glacial meltwater. Alluvium , typically consisting of sandy gravel overlain by brown, silty clay, is the deposit of the modern floodplains. Accumulations of Peat may occur locally within the alluvium, usually formed within abandoned river courses and enclosed hollows on the floodplain. Alluvial fan deposits occur at the mouths of deeply incised streams draining the Pennine Coal Measures to the south-west of the Dee estuary. These consist of poorly sorted gravelly sand with common cobbles and were probably deposited rapidly by meltwater during the late stages of deglaciation.

Coastal zone deposits preserved above or inland of Mean High Water represent:

• either tidal flat deposits that have emerged through natural processes such as saltmarsh colonisation or silting up of lagoons behind spits, coastal barriers or dunes
• land that has been reclaimed through artificial construction of coastal defences or docks.

They include Intertidal Deposits, Tidal Flat deposits, Saltmarsh deposits and Tidal River or Creek deposits. All consist of well-laminated sand, silt and clay. The grain size within these deposits tends to fine upwards gradually as continued deposition led to emergence, which was associated with an increase in organic material that originated as transported debris, in situ accumulation or roots.

South of the Warren, near Point of Ayr, a series of low ridges composed of gravelly coarse sand are interpreted as abandoned Storm Beach deposits.

Blown sand fringes the north and west shores of north Wirral and Point of Ayr. It generally builds into dunes, although the morphology has locally been destroyed where the dunes have been artifically stabilised and coastal defences have been constructed — notably to the north of Wallesey. The area inland of these dunes includes reclaimed tidal flat and lagoonal deposits of Holocene age, locally lying below Mean Sea Level. These deposits include up to three separate peat beds (Kenna, 1986), each deposited following emergence and colonisation of back-barrier lagoons driven by fluctuations in groundwater levels (Innes et al., 1990).

Coastal Zone deposits (Sea bed sediments) below Mean High Water are classified compositionally according to the ratio of mud, sand and gravel. These deposits are in constant movement due to the action of wave and tidal currents and artificial dredging operations. Their grain size generally reflects the intensity of current activity, with sand deposits dominating the deeper, tidal-current prone parts of the Mersey and Dee estuaries and the storm wave-prone coasts along the north of Wirral and west of Point of Ayr (Plate 1). Finer grained muddy sand and sandy mud deposits predominate in the south-eastern part of the Dee estuary offshore of Neston, where they merge gradually with Saltmarsh deposits across the Mean High Water Mark.

Artificially modified ground is ubiquitous in areas of human development. It has been delineated by interpretation of surface morphology in the field and by examination of documentary sources, in particular historical topographic maps, aerial photographs and site investigation data. Only the thicker and more extensive areas are shown on the Bedrock and Superficial Deposits edition of the 1:50 000 map. More detailed information is available on the 1:10 000 scale geological maps of the district, including major areas of landscaped and disturbed ground, which is not shown at 1:50 000 scale.

Made Ground represents areas where artificial deposits lie on the original ground surface. It includes civil engineering earthworks such as road, rail and coastal defence embankments, spoil from sandstone quarrying, building and demolition rubble, waste from heavy industries, and domestic and other waste in raised landfill sites. Docks are indicated as areas of Made Ground, but include areas of Worked Ground and Infilled Ground that are impractical to delineate separately. Worked Ground represents artificial excavations that have not been backfilled, including sandstone and limestone quarries and engineered cuttings for roads and railways. Infilled Ground comprises partly or completely filled excavations, including landfill sites and quarries partly backfilled with spoil. Where quarries have been fully restored and developed subsequently, 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 historical, topographical and geological maps.

Concealed geology and structure

The subsurface structure and evolution of the region is described in detail by Kirby et al. (2000) and Smith et al. (2005), and summarised with additional local details below.

Geophysical evidence suggests that Silurian rocks form the Pre-Carboniferous basement below the entire district (Smith et al., 2005). Basement is not penetrated by boreholes in the district, but rocks of Silurian age underlie Dinantian strata at outcrop in neighbouring parts of the Flint district to the south (Davies et al., 2004). Structurally, the district is dominated by faults with a dominant north–south or north-north-west trend. The style of faulting is well displayed by cross-sections on the Bedrock edition of Sheet 96, which were compiled with extensive use of interpreted seismic reflection data. The cross-sections illustrate a broad structural symmetry, with eastward downthrow on major faults in the western half of the district mirrored by westward downthrow on faults in the east. Splays of both antithetic and synthetic faults are associated with the main Caldy, Woodchurch and Shaw Street Faults.

Faults in the district have a long history of activity. The main north–south-trending faults were active during episodes of Dinantian rifting, but are likely been have reactivated, or at least have been influenced by structures within the basement. The Gwespyr and Axton Faults probably represents plays of the major Nercwys–Nant– figillt–Alyn Valley Fault system (Davies et al., 2004), which marked the local boundary between the Wales–Brabant Massif and Irish Sea–Craven basins, and was associated with substantial syndepositional thickness and facies change throughout much of the Dinantian. Seismic reflection data also indicate substantial westward thinning of Dinantian strata across the Woodchurch and Shaw Street faults. Syndepositional fault movement declined during the Namurian and early Westphalian, but basin inversion associated with the onset of the Variscan Orogeny initiated further movement in the late Westphalian, indicated by differential erosion across faults at the base of the Warwickshire Group (see above). Extension and rifting was renewed in the Permian and early Triassic, associated with opening of the North and Central Atlantic rift systems. Although there is no direct evidence for Triassic syndepositional fault movement in the district, evidence abounds in adjacent parts of the Cheshire and East Irish Sea Basins, and it is likely that much of the antithetic and synthetic faults displacing the Triassic strata were also initiated during this period. Further post-Triassic fault displacement has undoubtedly taken place but cannot be quantified in the absence of younger strata.

Dinantian strata in the south-west of the district typically dip gently north-eastwards. Major faults, including the Axton and Gwespyr faults, generally have a north-north-west trend with downthrow up to 300 m towards the east. Dips locally increase to 20° or more towards the north-east of the Dinantian outcrop, coinciding with the inferred, subsurface position of the Nercwys–Nant-figillt–Alyn Valley Fault system. Minor faults and non-displacive lead and zinc mineral veins with a predominant east–west trend are common on the outcrop of the Loggerheads and Cefn Mawr Limestone formations. Mineralisation was probably associated with Triassic extension, which expelled hot brines from basinal Craven Group strata with precipitation in dilated faults and fractures in juxtaposed early Carboniferous limestones (Davies et al., 2004).

Upper Carboniferous strata within and adjacent to the north-west Flintshire coalfield are folded into low-amplitude dome and basin structures confined within faulted blocks. Steeper north-easterly dips occur below the Dee estuary and seismic reflection data indicate a series of north-eastwards tilted half-graben to the north-east of Point of Ayr. Triassic rocks on Wirral and in the Liverpool area are also preserved within a series of very gently eastwards-tilted half-graben, each bounded to the east by a major fault. Of these, the Woodchurch and Shaw Street faults are the most significant, with throws in Triassic strata exceeding 400 m down towards the west. Both faults have well developed splays of antithetic and synthetic faults, including the Thurstaston and Castle Street faults, antithetic to the Woodchurch and Shaw Street faults respectively. Exposure of the major faults is very rare in the district; where seen the faults are normally filled with fine-grained gouge up to 2 m wide. Granulation seams (cataclastic fractures) are well developed in parts of the Wilmslow and Helsby sandstone formations. These are zones (up to 5 m wide) where grain size is reduced associated with faulting, and are best exposed at Thurstaston Hill (Plate 4) and Bilsden Court Gardens [SJ 285 888].

Chapter 3 Applied geology

The resources and constraints posed by the geological environment are essential considerations for planning and development processes. As with most urban centres and their hinterlands, the availability of local mineral and water resources have played a major role in the industrial and economic development of the Liverpool region. Exploitation of these resources, together with other human modification of the geological environment through activities such as construction works and waste disposal, have left a legacy of variable and locally difficult ground conditions that require appropriate remediation or management for safe and effective development. Geological and geotechnical information may assist with bringing marginal land into productive use and to identify opportunities for development, particularly in respect of leisure, recreation and protection of conservation sites.

Detailed guidance on geological constraints for planning and development are published by the former Department of the Environment, Transport and the Regions (DETR) in the form of Planning Policy Guidance notes (PPGs) and Mineral Planning Guidance notes (MPGs.) Ellison et al. (1997) and Ellison and Smith (1997) have reviewed the use and availability of geological information in support of planning and development. The key issues for the Liverpool district are described below.

Groundwater

The permeable Permo-Triassic sandstones of the Sherwood Sandstone Group form the principal aquifer in the district. Abstraction for both industrial and public supply grew dramatically during the Victorian period on both Wirral and around Liverpool, but in recent years abstractions have declined, leading locally to engineering and flooding problems associated with rising groundwater (see below). The region obtains most of its water supplies from surface sources in north Wales, principally the River Dee and Lake Vrynwy, and from groundwater boreholes in Cheshire, but local boreholes in the district continue to contribute significantly to supply.

The four formations within the Sherwood Sandstone Group are in hydraulic continuity and behave as a single aquifer unit, though with variations in aquifer properties. Though all the sandstones have substantial intergranular porosity, most groundwater flow takes place along fractures and bedding planes within the rock. Increasing closure of fractures with depth limits the aquifer to an effective depth of around 200 m. The Chester Pebble Beds Formation is normally the best aquifer with the highest yields (typically 1000 to 2000 cubic metres per day), and is sufficiently competent for fractures to remain open and for boreholes to remain stable without casing. The Kinnerton and Wilmslow Sandstone formations are less competent and boreholes tend to pump sand. The Helsby Sandstone is generally the most strongly cemented sandstone, so that intergranular porosities are reduced and fractures tend to be more regularly distributed with lower connectivity, leading to comparatively low formation permeabilities (Allen et al., 1997).

The Permo-Triassic sandstone aquifer is confined below the Mercia Mudstone Group in the northern part of Wirral, and is partly confined below glacial till elsewhere. Although the permeability of the till is variable due to variations in thickness and the presence of fractures and porous lenses, it is generally sufficiently thick to inhibit recharge and to offer some protection to the aquifer by attenuating pollutants.

Recent research (Mohamed and Worden, 2006) on the effect of faulting in the Permo-Triassic aquifers in the Liverpool area indicates that transmissivity across major faults can be substantially reduced, effectively dividing the aquifer into hydraulically separate, fault-bounded compartments. The presence of fine-grained impermeable fault-gouge along fault planes may limit permeability (Mohamed and Worden, 2006), but cementation of fractures by silica or barytes, as seen at outcrop on Thurstaston Hill (see above), may also be a factor locally. Recharge of aquifer 'compartments' may therefore depend considerably on the presence of surface outcrops of unconfined aquifer within faulted blocks.

Saline intrusion due to over-abstraction has occurred adjacent to the Mersey estuary, although faults (notably the Castle Street Fault in Liverpool) can present effective barriers to intrusion (Mohamed and Worden, 2006), especially if they are oriented parallel to the coastline. Nitrate pollution from agricultural fertilisers may present a problem in areas where the sandstones crop out at surface without a protective cover of till.

Minor bedrock aquifers in the district include the limestones of the Clwyd Limestone Group and sandstone beds within the Millstone Grit and Pennine Coal Measures groups. The limestones constitute aquifers of karstic type, in that groundwater flow takes place mainly though fissures, conduits and caves that have been enlarged by solution. Yields can therefore be highly unpredictable. Groundwater ingress was a major limiting factor for metalliferous mining in north Wales, requiring excavation of major drainage tunnels as workings proceeded to greater depths in the saturated zone. Although the limestone may yield locally viable supplies on a small scale, contamination due to local lead and zinc workings is a major consideration. Similarly, the presence of contaminated and acidic mine waters in abandoned coal workings limits the value of minor sandstone aquifers in the North-west Flintshire Coalfield. Groundwater quality within shallow, minor aquifers in the till and alluvium is highly variable and susceptible to pollution.

Mineral resources

The district has a long and diverse history of mineral extraction, reflecting its varied geology, though most quarrying and mining operations have now ceased. The main mineral resources of the district are summarised in (Figure 9).

Sand and gravel

Glaciofluvial Sand and Gravel deposits have been worked from small quarries in many parts of the district, notably in north Clwyd. The only quarries active at the time of survey were at Spring Hill [SJ 212 757], near Bagillt, and near Cae Rhys Farm [SJ 108 756] in the extreme south-west of the district. The latter worked a small outcrop of Glaciofluvial deltaic deposits for horticultural and ornamental purposes. At Raby Hall [SJ 334 806], weathered sandstones of the Wilmslow Sandstone Formation have also been worked for sand in the past.

Brick Clay

The Till and Tidal Flat deposits are the main clay resources of the district. In the past both deposits were extensively worked for brick making. The brickworks at Newton Carr [SJ 245 897] is the only clay pit still in operation in the district. Many former clay pits are backfilled, such as Morton Clay Pit, Morton [SJ 2560 9080] and Leasowe Road, Leasowe [SJ 2800 9190] (Hough, 1998).

Coal

Coal has been mined in north Wales since Roman times. The earliest workings were at outcrop, and as these surface workings were exhausted, mining gradually proceeded deeper down the dip of the seams, accessed via adits and pits. The industrial revolution not only brought greater demand, but also supplied the means (mechanical winding and pumping of groundwater) that enabled deeper coal to be exploited. By the 19th century, coal was being extracted in the North-west Flintshire coalfield from numerous private collieries, though normally at depths less than 100 m, leaving a legacy of abandoned shafts, adits and shallow underground workings. By the early 20th century, collieries became progressively deeper but declined in number, with workings starting to encroach below the Dee estuary. By the Second World War, only Point of Ayr Colliery remained in operation, finally closing in 1996. Coal was exploited in the smaller Wirral Coalfield from 1765 with the opening of Neston Colliery. Workings had extended over 3 km westwards under the Dee estuary by the time the colliery closed in 1928.

The principal worked coals are listed in (Figure 4). The Main ( Five Yard) Coal was by far the most productive seam, accounting for over 50 per cent of the total coalfield production. The coals were of the highly volatile, medium to strongly caking class, and utilised mainly for manufacturing (steam raising), domestic supply and coal gas-making.

Sandstone

The main locally derived building stones in the district were quarried from the Chester Pebble Beds and Helsby Sandstone formations. Many of the prominent buildings in Liverpool and Wirral, including Liverpool's Anglican Cathedral (Plate 5), Lime Street Station and Birkenhead Town Hall, were constructed from locally quarried stone from these formations, but the stone was also exported more widely. One of the most important quarries in the district was at Storeton Hill [SJ 314 850], (Plate 6), which was active from Roman times until the mid-20th century. Stone from the quarry was used to clad parts of the Empire State Building in New York, and the quarry itself was famed as a locality for fossil reptilian footprints, with many specimens now preserved in the Liverpool and Chester museums. The quarry was filled with spoil from the Birkenhead Tunnel.

The Gwespyr Sandstone was formerly quarried at Gwespyr for walling, building and paving stone. Flags of the sandstone were used for a Roman bathhouse and fort at Prestatyn.

Limestone

Limestone was formerly quarried at several locations in north Clwyd around Axton, Glol and Lloc. Smaller quarries, mainly in the Llanarmon Limestone, provided local sources of building and walling stone, with larger quarries in the more thickly bedded Loggerheads Limestone Formation supplying limestone for crushed aggregate and cement-making purposes. All mining has now ceased in the district.

Chert

Chert has been quarried in Clwyd for a variety of purposes, mainly as trimmed blocks for lining pan mills, for making refractory bricks, and as an aggregate (Campbell and Hains, 1988; Davies et al., 2004). A former quarry in the Pentre Chert Formation at Trelogan [SJ 1187 8021] is now a licenced landfill site.

Metalliferous minerals

The history of lead and zinc mining in north Clwyd is reviewed by Davies et al. (2004). These ores occur mainly in steeply dipping fissure veins or mineralised faults, normally on the outcrops of the Loggerheads Limestone and Cefn Mawr Limestone formations but locally extending into both underlying and overlying formations. Mining operations reached their peak in the mid 19th century, but all extraction in the region had ceased by 1978. Although significant resources remain, the requirement for drainage is a serious constraint to any future exploitation.

Engineering ground conditions and hazards

The three most important aspects of ground conditions relevant to construction and development in the district are the suitability of the ground to support structural foundations, the ease of excavation, and the use of geological materials as engineering fill. These issues are summarised in (Figure 10) and the geological context of the most significant constraints is described below. Further information on geological hazards and constraints associated with specific sites is available from the BGS Georeports Service (see Information sources)

Shallow mining and tunnelling

Abandoned mine workings are present mainly in the south-west of the district, where coal, lead and zinc were mined in the past. In the North-west Flintshire and Wirral coalfields, ground instability may be associated with former coal mine entries or collapse of shallow coal workings of pillar and stall type. The 1:10 000 scale geological maps of the district show the surface outcrops of individual coal seams and hence provide an indication of areas where unrecorded shallow coal workings may occur. Information about the local coal mining subsidence and shaft locations can be obtained from the Coal Authority (see Information sources).

Lead and zinc ores in the south-west of the district were mined via numerous vertical or steeply inclined shafts sited on or close to the surface outcrop of the mineral vein. These voids present not only a potential ground stability problem for building development but also a fall hazard for people and livestock. Shafts in the Pantasaph area to the south-west of Holywell were mapped extensively by Campbell and Hains (1988), but numerous additional uncharted shafts undoubtedly exist throughout the mined area. Mapped positions of known mineral veins are shown on the 1:10 000 scale geological maps of the district and provide an indication of the potential location of unrecorded shafts and shallow workings (see also Information sources).

In the Edge Hill area of Liverpool, a number of underground brick-lined tunnels and chambers were excavated in the Chester Pebble Beds Formation in the early 1800s. They were commissioned by Joseph Williamson for purposes unknown, but possibly as an act of philanthropy to provide work for the local unemployed. Some tunnels have been restored and are open to the public, while others are uncharted and undoubtedly some remain to be discovered.

During the 1800s, the Liverpool and Manchester Railway was constructed in underground tunnels and in large cuttings beneath the City of Liverpool. The Wapping Tunnel is over 2000 m long, and is cut through largely unlined sandstone bedrock between the docks and Edge Hill. Stables and boiler houses associated with Crown Street Station at Edge Hill (the world's first passenger railway station) were cut into the Chester Pebble Beds Formation.

Dissolution features

Dissolution features occur within the limestones of this district, mainly within the Loggerheads and Llanarmon Limestone formations. Limestone is prone to dissolution by mildly acidic rainwater or groundwater, resulting in enlargement of naturally occurring joints and bedding planes, and formation of cavities and pipes. This process was more active during the prolonged period of periglacial conditions in the Pleistocene, exacerbated by freezing and thawing. These voids may occur in the shallow surface with only thin limestone roofs, or may be concealed beneath till. Site investigations to detect potential voids are therefore recommended for proposed developments on the outcrop of the limestone formations.

Slope stability and mass movement

A number of landslides have been identified in the district. On Wirral, unprotected areas of the coastline, such as Dawpool Cliffs south-east of West Kirby [SJ 216 854] south-east wards and the New Ferry coast [SJ 340 859] to [SJ 343 853], may be subject to coastal erosion, leading to coastal landslides. Historically other parts of the coastline have been subjected to severe coastal erosion, but this has been greatly reduced by the construction of sea defences along the north Wallasey coast (Hough, 1998). In the south-west of the district most of the landslides occur along steep-sided valleys dissecting the outcrop of the Pennine Coal Measures Group, such as south-east of Holywell [SJ 198 756].

Slopes exceeding 3° should also be considered as potentially unstable due to relict shear surfaces left by periglacial processes (Culshaw and Crummy, 1991). Head and till deposits can also contain thinly interbedded sequences of sands and clays. These are prone to landslip due to the presence of springs and high confined pore pressures, which lead to loss of strength. This process gives rise to landslides along the flanks of the Rivacre valley [SJ 380 775] in the south-east of the district.

Compressible ground

Compressible ground may be associated with poorly compacted Made Ground or Infilled Ground, and with natural superficial deposits. Of the latter, alluvium and Coastal Zone deposits may contain significant beds or lenses of compressible peat and require thorough site investigation prior to constructional development.

Flooding

Low-lying ground adjacent to active stream and river courses may be prone to flooding during periods of exceptional rainfall. The outcrops of alluvium or Coastal Zone deposits on the geological map indicate areas subject to river and coastal flooding in the recent geological past and, in conjunction with information on ground elevation and historic flood records, can assist with prediction of flood-prone areas prompting the inclusion of protective measures at an early stage in the planning and design process.

With the decline in groundwater abstraction in parts of the district (see above), notably in Liverpool city itself, rising groundwater due to rebound may present flooding problems for underground installations and tunnels. A system of dewatering boreholes has recently been installed in Liverpool for the Mersey Loop Line rail tunnels to counter this problem.

Gas emissions

Methane, carbon dioxide and carbon monoxide are present naturally in concealed Coal Measures strata, or can be produced by decomposition of artificial material in landfill sites. These gases can migrate considerable distances through bedrock and superficial deposits, and may accumulate in enclosed, ill-ventilated spaces such as basements, buildings, excavations, caves, mines and tunnels. If accumulation occurs these gases can reach high concentrations and may become a hazard. Methane presents a fire or explosion hazard; carbon dioxide and carbon monoxide can cause asphyxiation or poisoning of humans and animals, and dieback of vegetation.

The radioactive decay of uranium, which is found in small quantities in all soils and rocks, produces radon, a colourless, odourless radio-active gas. Radon quickly disperses once released into the atmosphere, but can accumulate in poorly ventilated enclosed spaces such as basements, buildings and mines. In these circumstances, radon poses a significant long-term health risk. Variation in the radon levels between different parts of the country is controlled mainly by the underlying geology with regard to the presence of radon-producing minerals and the ability of the gas to reach the surface through discontinuities in the rock mass. Radon susceptibility assessments are available via the BGS Georeports Service (see Information sources).

Information sources

Further geological information held by the British Geological Survey and relevant to this district 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 provides details of BGS activities, services and information, including summaries of projects, lists of publications and contact points for advice on a wide range of environmental resource and hazard issues. Information on the geology of specific sites is available via the BGS Enquiry and GeoReports Services. A GeoIndex (http://www.bgs.ac.uk/ geoindex/home.html) is available on the website, and includes details of borehole and seismic line locations, small-scale geological maps of the UK, and many other items of data. Most publications listed below can be purchased via the BGS Sales Desk at BGS Keyworth or from the online bookshop on the BGS website. A catalogue of current BGS publications is available on request.

Maps

• Geological maps
• Sheet 96 Liverpool, Bedrock, 2006
• Sheet 96 Liverpool, Bedrock and Superficial deposits, 2006
• Onshore geological maps at 1:50 000 or smaller scales for the surrounding region are listed in the BGS catalogue or can be searched via the BGS Online Bookshop (http://shop.bgs.ac.uk/Bookshop/).
• The 1:10 000 scale geological maps of the district date from surveys carried out between 1998 and 2000, except for the Holywell area, which was surveyed in 1987. Maps are available as full colour plots, and can be purchased online or through the Sales Desk at BGS Keyworth.
• Geochemical atlas
• Regional geochemical atlas: parts of north-west England and north Wales, 1997
• Offshore geological maps
• 1: 250 000 scale
• Sheet 53N 04W Liverpool Bay, available as Solid (bedrock geology) and Sea bed sediments and Quaternary editions
• East Irish Sea Special Sheet, Parts 54N 06W
• Isle of Man, 54N 04W
• Lake District, available as Solid (bedrock geology)
• Groundwater vulnerability maps
• 1:100 000
• Groundwater vulnerability map of Chester. Sheet 16. (Environment Agency)
• Digital geological map data
• National coverage (onshore) of digital geological map data (DiGMapGB) is available at 1:50 000, 1: 250 000 and 1:625 000 scales. Selected areas, including the Liverpool district, are also available at 1:10 000 scale. Digital geological maps of offshore areas are available at 1:250 000 scale. Information on availability of data and licensing conditions are available from Map Data Delivery, BGS Keyworth.

Books and reports

• British Regional Geology
• The Pennines and adjacent areas. Fourth edition, 2002
• Memoirs
• Geology of the country around Flint (Sheet 108), 2000
• Geology of the country around Rhyl and Denbigh (Sheets 95 and 107, and parts of sheets 94 and 106), 1984
• Geology of Southport and Formby (Sheets 74 and 83), 1948† † out of print; facsimile copy available
• Applied Earth Science Mapping Report Deeside (North Wales) thematic geological mapping. Technical Report, WA/88/2
• Subsurface memoirs
• Structure and evolution of the Craven Basin and adjacent areas, 2000
• Structure and evolution of the south-west Pennine Basin and adjacent area, 2005
• UK Offshore Regional Report
• The geology of the Irish Sea, 1995
• BGS Technical reports for 1:10 000 sheets
• These reports describe the geology of areas covered by 1:10 000 geological maps in the district. They can be consulted in the BGS Library or purchased through the BGS Central Enquiries Desk, Keyworth.
• Geology of the north Wirral district Technical Report, WA/98/66 (Sheets SJ28NW, 28NE, 28SW, 28SE, 38NW and 39SW)
• Geology of the Bebington, Eastham and Garston Dock area. Technical Report, WA/98/58 (Sheets SJ38SW and 38SE) Geology of the Ellesmere Port area. Technical Report, WA/99/59 (Sheet SJ37NE)

Documentary collections

Detailed manuscript geological survey information, which includes 1:10 000 or 1:10 560 scale field maps and accompanying field notebooks, are archived at BGS, Keyworth. Access to these records is available on request from the Manager, National Geosciences Data Centre, BGS Keyworth.

Boreholes and site investigation reports

BGS holds collections of borehole records, which can be consulted at BGS Keyworth, where copies of records in the public domain may be purchased. Index information, which includes site references, names, and depths for these boreholes can be accessed through the BGS website, where copies can also be ordered. The following boreholes (with terminal depth) are cited in this sheet explanation:

• Abbey Mills No. 1 Borehole, (SJ17NE/1) [SJ 1949 7757], 363.9 m
• Abbey Mills No. 4 Borehole, (SJ17NE/4) [SJ 1949 7747], 243.8 m
• Heswall Borehole, (SJ28SE/1), [SJ 2526 8161], 1024.7 m
• Saughall Massie Borehole, (SJ28NW/22), [SJ 24313 88446], 150.0 m

Hydrogeological data

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

Gravity and magnetic data

These data are held digitally in the National Gravity Databank and the National Aeromagnetic Databank at BGS Keyworth.

BGS Lexicon of named rock unit definition

Definitions of the rocks and superficial deposits shown on BGS maps, including those shown on the 1:50 000 Series Sheet 96 Liverpool, are held in the Lexicon database, available through the BGS website. Further information on the database can be obtained from the Lexicon Manager at BGS Keyworth.

BGS (Geological Survey) photographs

Photographs (except (Plate 1) and (Plate 6) used in this sheet explanation, and others taken during the present resurvey or previous surveys, are held in the National Archive of Geological Photographs, BGS Library, Keyworth. Colour or black and white prints, transparencies and digital images can be supplied at a fixed tariff.

Materials collections

Rock samples and thin sections, fossil specimens, borehole samples and core are available for this district

For information concerning availability and access to materials contact the Chief Curator, National Geosciences Data Centre, BGS Keyworth.

External data sources

Mining information

Information on coal mining activities, mine entries and abandonment plans is available from The Coal Authority, 200 Lichfield Lane, Mansfield, Nottinghamshire, NG18 4RG; Telephone 0845 762 6848 (Mining Reports), 01623 638 233 (Mining Records), Website: http://www.coal.gov.uk/

Non-coal mine abandonment plans are held at the Flintshire Record Office, The Old Rectory, Hawarden, Flintshire, CH5 3NR; Telephone 01244 532364

References

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

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

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

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

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

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

Figures and plates

Figures

(Figure 1) Summary geology map.

(Figure 2) Stratigraphy of Carboniferous strata in the district and adjacent areas. Thickness (in brackets) in metres.

(Figure 3) Schematic profile of stratigraphical relationships of Dinantian and early Namurian strata in north-west Clwyd (not to scale). For lithostratigraphical unit codes see text. NNAF = Nercwys–Nant-figillt–Alyn Valley Fault complex.

(Figure 4) Principal coals (seam names in bold type) of the north-west Flintshire and Wirral coalfields. Correlation of coals between these two areas is uncertain. (Figure 4) Principal coals (seam names in bold type) of the north-west Flintshire and Wirral coalfields. Correlation of coals between these two areas is uncertain.

(Figure 5) Lithostratigraphy of Permo-Triassic rocks and correlation with adjacent basins.

(Figure 6) Lithological characteristics and depositional environments of Permo-Triassic formations within the district.

(Figure 7) Schematic diagram illustrating the stratigraphical and topographical relationships of superficial deposits in the district (not to scale).

(Figure 8) Characteristics of natural superficial deposits in the district.

(Figure 9) Principal mineral resources of the district.

(Figure 10a) Engineering geological units: soil and superficial deposits (based on BS5930 British Standards Institution, 1999; excavatability based on Pettifer and Fookes, 1994).

(Figure 10b) Engineering geological units: bedrock.

Plates

(Plate 1) Sand waves, beach ridges and spits formed by wave and tidal current action, Point of Ayr, Dee Estuary (Liverpool Coastal Group).

(Plate 2) Pentre Chert Formation at Trelogan Quarry [SJ 1187 8027], north-west Clwyd, illustrating lenticular bedding of chert and cherty limestone (P625706).

(Plate 3) Red (left) and Yellow Noses [SJ 300 940], New Brighton, Wirral. These former coastal headlands are formed from Wilmslow Sandstone Formation, and display cross-stratification of probable aeolian origin (P625707 and P625708).

(Plate 4) Linear ridge formed by silicified fracture zones in the Wilmslow Sandstone Formation, Thurstaston Hill [SJ 2424 8489], Wirral (P625709).

(Plate 5) Liverpool Anglican Cathedral, constructed of local Triassic sandstone from quarries at Woolton, Liverpool (FreeFoto.com).

(Plate 6) Storeton Sandstone Quarry [SJ 3153 8440], Wirral, photographed at its peak of building stone production in 1914. This quarry, in the Helsby Sandstone Formation, yielded numerous examples of Triassic reptilian (Chirotherium) footprints (P201626).

(Front cover) Front cover. Granite cladding disguises the concrete and steel structure of the Royal Liver Building on Liverpool waterfront. Built in 1908–11, this was one of the tallest buildings at that time which used this relatively new construction technique (Photograph: T Cullen; P535669).

(Rear cover)

(Geological succession) Geological succession at outcrop in the Liverpool district.

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

Figures

(Figure 4) Principal coals (seam names in bold type) of the north-west Flintshire and Wirral coalfields

Correlation of coals between these two areas is uncertain

(Figure 5) Lithostratigraphy of Permo-Triassic rocks and correlation with adjacent basins

(Figure 6) Lithological characteristics and depositional environments of Permo-Triassic formations within the district

(Figure 8) Characteristics of natural superficial deposits in the district

(Figure 9) Principal mineral resources of the district

(Figure 10a) Engineering geological units: soil and superficial deposits

(based on BS5930 British Standards Institution, 1999; excavatability based on Pettifer and Fookes, 1994)

(Figure 10a) Continued

(Figure 10b) Engineering geological units: bedrock