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Geology of the Selby district. A brief explanation of the geological map sheet 71 Selby

J R Ford, A H Cooper, S J Price, A D Gibson, T C Pharaoh and H Kessler

Bibliographic reference: J R Ford, A H Cooper, S J Price, A D Gibson, T C Pharaoh and H Kessler. 2008. Geology of the Selby district. A brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 71 Selby (England and Wales).

© NERC 2008 All rights reserved

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(Front cover) View looking south-west from Hagg Bridge [SE 7170 4514] along the Pocklington Canal and The Beck across the Vale of York to Drax Power Station 25 km away. Flat Vale of York glacial lake deposits are incised by the alluvial tract of the river (P969308).

(Rear cover)

(Geological succession) Summary of the geological succession in the district.

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

Notes

The word 'district' refers to the area Sheet 71 Selby. National grid references are given in square brackets: [0862 2333]. Unless otherwise indicated, all should be preceded by 'SE' (or the digits 4 and 4: [40862 42333]) to indicate the 100 km grid square. Symbols in round brackets after lithostratigraphical names are the same as those used on the geological map. The serial number in plate captions is the National Archive of Geological Photographs registration number, held at the BGS.

Acknowledgements

The authors acknowledge the contributions from: C P Royles — geophysical images, and D Appleton and C Scheib — Radon information. Sheet Explanation edited by D T Aldiss and J P Stevenson; figures drafted by R J Demaine and page setting by C Chetwyn.

Additional assistance and data provided by York City Council, North Yorkshire County Council, the Coal Authority, the Environment Agency, Yorkshire Water, British Waterways, Railtrack, Mineral Valuers Office, the National Soil Resources Institute, Cranfield University and the UK Onshore Geophysical Library.

Thanks are due to the many landowners, farmers and householders for property access during the survey.

The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright. All rights reserved. Licence No. 100037272/2008.

Geology of the Selby district (summary from the (Rear cover))

An explanation of sheet 71 (England and Wales) 1:50 000 series map

The low-lying landscape of the Vale of York includes some of the best preserved lowland glacial topographical features in the UK. The story of the landscape dates mainly from the last ice age and is one of glaciers advancing into proglacial lakes, leaving moraines on their retreat. Lobate terminal moraines extend across the Vale of York through Escrick and, to the north, through York itself. Extensive glacial lake deposits with laminate clays and coverings of sand occur both to the south of the Escrick Moraine and in the areas between and behind the York and Escrick moraines. The district is traversed from north to south by the rivers Ouse and Derwent with the River Wharfe joining the Ouse in the west of the district, each with a flood plain of varying width. Beneath the superficial deposits of the glacial landscape the bedrock geology records a history of deserts, warm seas and coal swamps. In the extreme north-east of the district Cretaceous chalk and Jurassic mudstones form rising ground on the flanks of the Yorkshire Wolds. The sequence here preserves evidence of subtropical seas in the

Cretaceous and shallow marine environments in the Jurassic. Beneath these the Triassic Mercia Mudstone Group contains mudstone with a little gypsum deposited in a desert environment with ephemeral lakes. The western half of the district is underlain by the Triassic Sherwood Sandstone Group of reddish-brown desert sandstones. These form one of the main aquifers in the district.

At greater depths Permian rocks (including dolostone, mudstone, gypsum and salt of the Zechstein Group) are present. Underneath these, Carboniferous coal-bearing rocks are present at depths of between 300 metres in the west and 1000 metres in the north-east. These strata have been exploited beneath the western half of the district in the Selby coalfield, which ceased production in 2004.

The low-lying nature of the alluvial and glacial lake deposits means the district is prone to flooding, a situation locally aggravated by mining subsidence. The glacial lake clay deposits constitute an enormous resource of brick clay, but the sequence includes running sand units at shallow depths constituting a geological hazard for development.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district covered by the geological 1:50 000 Series Sheet 71 Selby, published as a combined bedrock and superficial edition in 2008.

The district extends from Selby in the south to include approximately half of the city of York in the north-west corner. Pocklington is located in the north-east corner of the district and Holme-on-Spalding-Moor towards the south-east corner. The remainder of the area is moderately populated with a mixture of small to medium-sized villages, many forming part of the York commuter belt. Administration of the district is split between three authorities; York Unitary Authority is responsible for approximately the north-west quarter, Selby District Council and North Yorkshire County Council administer the south-west quarter while the East Riding of Yorkshire Unitary Authority is the local government in the eastern half of the district.

The bedrock of the district divides into three belts. In the west, occupying a little more than half the district, the strata are part of the Triassic Sherwood Sandstone Group. The eastern belt, covering a little more than one third of the district, is underlain by the Triassic Mercia Mudstone Group while a small area of Jurassic and Cretaceous rocks including the Chalk is present in the north-east corner of the district. The majority of the area is heavily blanketed by superficial deposits dating mainly from the last (Devensian) ice-age; consequently there are no exposures of the Sherwood Sandstone Group and only limited exposures of the Mercia Mudstone Group. Bedrock is present at the surface mainly in the north-east of the district where it forms ground that rises to an elevation of 204 m on the Chalk of the Yorkshire Wolds. Most of the district, where the superficial deposits are thick, is of very low elevation, around 5 to 8 m above sealevel. The alluvial areas of the River Ouse downstream of Naburn Lock are lower and the river is tidal. The lower parts of the River Derwent were once also tidal, but are now protected by a barrage where the Derwent joins the River Ouse.

At depth beneath the area Permian rocks of the Zechstein Group, including dolostone, mudstone, gypsum and salt, are present. Underneath these, Carboniferous coal-bearing rocks are present at depths of between 250 m in the west and 1000 m in the north-east. These strata have been exploited beneath the western half of the district in the Selby coalfield, which ceased production in 2004.

The thick glacial deposits lend their character to the scenery and agriculture. The land in the north of the district is hummocky reflecting the presence of moraines and eskers. The land in the south of the district is largely flat being formed of clay and sand deposited in a former glacial lake. The majority of the land covered by superficial deposits is arable, but the steeper slopes leading onto the Wolds are pastoral.

History of research

The Selby district was originally surveyed at six inches to one mile and published in 1885 as the one inch to one mile (1:63 360) 'Old Series' sheet 93SE. A brief general description of the district was given by Dakyns et al. (1886). In 1967–1968 a partial resurvey was done by G D Gaunt around Selby town and along the southern margin of the district to join up with the resurvey of the Goole district. The recent resurvey of the district was undertaken mainly between 2001 and 2004 and has employed digital data manipulation including the use of digital terrain modelling, 3D borehole interpretation and National Soil Resources Institute data.

Chapter 2 Geological description

Early Palaeozoic

The pre-Carboniferous basement of the district has not been proved by boreholes, but seismic data in the east suggests that it lies at a depth of 2600 m near Market Weighton and around 3600 m in the north of the district. By comparison with surrounding areas, it is likely that this basement comprises deformed metasedimentary rocks of early Palaeozoic age, locally intruded by granite. It forms the continuation towards the Lake District of the concealed Caledonide belt of eastern England (Pharaoh et al., 1987), part of the Anglo-Brabant Deformation Belt (Winchester et al., 2002) deformed during the Acadian Orogenic Phase in early–mid Devonian time.

Carboniferous

Dinantian

Seismic reflection data suggest that the Dinantian sequence comprises up to 900 m of strata thinning eastwards to around 600 m over the Market Weighton Axis (Figure 1). By comparison with surrounding areas, and from evidence from the Spaldington 1 Borehole [SE 7927 3245], the strata are inferred to be mainly limestones with some sandstones towards the top of the sequence.

Namurian

Evidence from seismic data and two boreholes suggest that the Namurian strata are about 1100 m thick in the west of the district, thinning dramatically to about 500 m in the east. Spaldington 1 Borehole proved Pendleian to Yeadonian strata, including most of the coarse-grained sandstones (grits) recognised to the west in the Leeds district. As in the exposed area to the west, the sequence between the sandstones comprises mainly mudstones and siltstones with common fine-grained sandstone beds.

Westphalian

The Westphalian strata are truncated by the unconformable Permian sequence. Seismic data indicate that about 400 m of Westphalian Pennine Coal Measures Group strata remain in the west with 972 m of beds proved by the Wheldrake 1 Borehole [SE 6766 4608] in the east. The thickest Westphalian sequence is preserved in the core of a broad syncline in the north and central parts of the district. The boreholes suggest that the Pennine Lower Coal Measures Formation (Westphalian A–Langsettian) is around 250 m thick. The Pennine Middle Coal Measures Formation is also proved and comprises the Westphalian B (Duckmantian) sequence, about 313 m thick, and the Westphalian C (Bolsovian) sequence, about 215 m thick. The upper part of the sequence is formed by part of the Pennine Upper Coal Measures Formation, about 150 m thick and included in the Westphalian C (Bolsovian).

Lithologically the strata are very similar to those seen in the exposed coalfield to the west; they consist mainly of interbedded mudstone, siltstone and sandstone with lesser amounts of coal, seatearth and ironstone, all deposited in cyclic sequences. The mudstones are grey to black, mottled pale grey, commonly micaceous with planar lamination, or massive bedding; ironstone nodules are also common. They are commonly overlain by an upward gradation to siltstones, which are typically medium grey with flaser and lenticular bedding, ripple cross-lamination and parallel lamination; they commonly contain plant debris. The siltstones grade both vertically and laterally into sandstones that are mainly fine to medium-grained. In the Middle Coal Measures Formation, the main sandstone present is the Woolley Edge Rock between the Newhill Coal and the underlying Meltonfield Coal. The Woolley Edge Rock comprises channelled sandstones locally forming a sequence more than 25 m thick.

There are 41 coal seams proved in the area by the exploration for the Selby coalfield. The sequence and details of the individual coals are shown on the 1:50 000 scale Selby geological map. The Barnsley Seam was the main seam worked in the Selby coalfield. It ranges from less than 2 m thick at over 1000 m depth in the north-east to about 3 m thick at 300 m depth in the south-west, the general dip of the sequence being to the north-east (International Mining Consultants, 2002). Two other seams are greater than 1.5 m thick over a widespread area, the Stanley Main (locally united with the Kents Thin) and the Dunsil; the Stanley Main was locally worked to the east of Riccall. Some other seams including the Haigh Moor, Swallow Wood, High Hazels, Kents Thick and Parkgate are locally more than 1.5 m thick, but the majority of the other seams in the area are thin.

The Upper Coal Measures Formation lies above the Cambriense Marine Band. It comprises mainly siltstone and mudstone, with sandstones and poorly developed coals. It includes the Ackworth Rock (25–59 m) and the Brierley Rock (0–27 m), both well-developed sequences of fine-grained sandstone with siltstone.

Permian

Within the Selby district, Permian rocks (Figure 2) are not exposed, but are proved at depth. As a result of the Variscan earth movements, they rest unconformably on the Carboniferous rocks. In early Permian times, the land surface took the form of a major land-locked basin in tropical palaeo latitudes extending from eastern England across to Germany and Poland. The Selby district lay at the western margin of this basin (Smith, 1989).

Rotliegendes Group

After the Carboniferous, newly uplifted areas were subject to intense, mainly subaerial erosion in a desert environment resulting in the breccias and the overlying desert sand dunes that make up the Yellow Sands Formation of the Rotliegendes Group. In boreholes the formation comprises fineto medium-grained sandstones with rounded wind-blown grains and a light bluish grey colour due to the oxidation state of the ferruginous pellicles that coat the sand (Smith, 1992; Ruffell et al., 2006); at outcrop they weather to an orange colour. Over most of the area the sandstones are from 0 to 10 m thick, but they locally form dune structures and these thicken to 25 m.

Zechstein Group

In the late Permian, the basin flooded rapidly and became a major enclosed evaporitic sea in which the cyclic sequence of the Zechstein Group was deposited. The sequence in the area is known only from boreholes and mine workings; it is summarised in (Figure 2) and more details are given by Smith (1989). Its cyclicity was caused by repeated flooding and evaporation. Smith (1970, 1974) proposed five English Zechstein cycles, but Tucker (1991) proposed a seven-cycle sequence stratigraphical model of highstands (mainly carbonate) and lowstands (mainly evaporitic). This model has now been widely accepted (Ruffell et al., 2006).

Within the district, it can be observed that, generally, where the Cadeby Formation is thick, the evaporitic formations are relatively thin. The Cadeby Formation thickens from around 50 or 60 m in the west to about 108 m at a reef edge in the middle of the district, thinning to 27 m in the north-east. The halite and anhydrite deposits are thickest in the middle and east of the district ((Figure 2); Smith, 1989). The anhydrite formations contain anhydrite at depth, but at shallow levels and in contact with the dolostone formations the rock is commonly hydrated to gypsum. The salt deposits are confined to the deeper parts of the basin and wedge out westwards towards outcrop where they have been removed by groundwater dissolution as well as by stratigraphical thinning. Many of the Zechstein Group formations are differentiated only at depth in the basin. Towards outcrop only the undivided Roxby and Edlington formations can be recognised, separated by the Brotherton Formation (Figure 2).

Triassic

Sherwood Sandstone Group

During the Triassic Period, lithospheric extension and rifting continued on the sites of the Permian basins. The basins experienced continental desert aeolian and fluvial red-bed sedimentation with the deposition of the dominantly arenaceous Sherwood Sandstone Group. In the Selby district the Sherwood Sandstone Group ranges between 300 m and 435 m thick. It is fairly homogeneous and it has not proved possible to subdivide it in this area. The lower boundary of the group is an interbedded transition over a few metres from the underlying calcareous mudstones of the Permian Roxby Formation. The group comprises mainly fine to coarse-grained, reddish brown, aeolian and fluvial sandstones. These are commonly cross-stratified and contain channel structures, sporadic layers of mudstone clasts, and a few pebbly beds. The Sherwood Sandstone Group is locally important as the major aquifer. The upper boundary of the group is an interbedded transition over a few metres into the calcareous mudstones of the overlying Mercia Mudstone Group.

Mercia Mudstone Group

The Mercia Mudstone Group is very poorly exposed throughout the district and is mainly seen in a weathered state as heavy reddish brown clay soil. A few small sections are visible in streams and ditches. Where it is exposed it is mainly weathered red-brown calcareous mudstone (formerly called marl), but in a few places fibrous gypsum veins up to 20 cm thick are present, for example near Holme-on-Spalding-Moor [SE 8250 3619]. The Mercia Mudstone Group in the district ranges from 180 to 240 m thick. The basal part of the group includes grey, pyritic and micaceous mudstones with limestone and sandstone beds. The lower part of the sequence includes abundant gypsum beds and numerous secondary gypsum veins; here the sequence is also micaceous with sandstone and dolostone beds. However, the remainder of the group is dominantly composed of red-brown calcareous and gypsiferous mudstone. A semi-terrigenous evaporitic playa lake environment is suggested by these deposits. The upper part of the group is affected by a local marine influence, probably associated with deposition in coastal lagoons (Powell et al., 1992).

Penarth Group

The Penarth Group typically crops out in lowlying ground, obscured by superficial deposits or Jurassic landslide deposits. The group represents a transition from a terrestrial to a shallow marine environment. Outside of the district, the sequence has an erosional base and rests unconformably on the mudstones of the Mercia Mudstone Group. It represents the Rhaetian, a relatively short interval of about 4 million years, and is a precursor to the Jurassic sequence that overlies it. The Penarth Group is relatively thin comprising approximately 10 m of pale bluish grey, grey and greenish grey calcareous mudstone.

Jurassic

Strata of Jurassic age are represented in the district by the 'calcareous shales' of the Redcar Mudstone Formation (Powell, 1984; Rawson and Wright, 1995). This unit belongs to the lower part of the Lias Group. Succeeding formations of Jurassic age are not represented due to reduced sedimentation and increased erosion associated with the Market Weighton Axis. Regionally, the Jurassic sequence dips to the north-east, at approximately 2° to 3°.

Redcar Mudstone Formation

The Redcar Mudstone Formation spans the age range from Hettangian to Early Sinemurian, a period of approximately 5 million years commencing around 205 million years ago. The formation rests with a sharp and irregular contact on mudstones of the Penarth Group.

The formation comprises dark grey siltstone and mudstone, with common belemnites. Subordinate lithologies include shelly limestone and ferruginous sandstone, which both occur in beds less than 0.5 m thick. These were originally sediments laid down in a shallow marine environment. Sediment accumulation and preservation during the Jurassic was significantly affected by the structural control of the Market Weighton Axis (Figure 1). Consequently, the Redcar Mudstone Formation varies in thickness across the district, increasing from an estimated minimum of 40 m in the south to approximately 54 m in the north.

This formation typically gives rise to a slight steepening of the land surface, characterised by poorly drained clay soil. The mudstones and siltstones are commonly deeply weathered and poorly consolidated near to the ground surface. The incompetent nature of these rocks frequently results in extensive and complex landslides that also affect the Cretaceous strata.

Cretaceous

An extensive outcrop of Cretaceous strata to the east of the Vale of York forms the prominent Yorkshire Wolds. The succession comprises approximately 100 m of predominantly chalk lithology, subdivided into the Early Cretaceous Hunstanton Formation and the Late Cretaceous Chalk Group. In this district, the Chalk ranges through the Ferriby and Welton formations to the Burnham Formation (Wood and Smith, 1977; Whitham, 1991; Sumbler, 1999). The Cretaceous strata dip gently to the north-east (at approximately 1°) on the northern limb of a broad anticline. However, steeper dips are observed adjacent to faulting, and in areas where large blocks of strata are affected by landslides.

The sediments that formed the Chalk were deposited in a shelf sea environment that extended over much of present-day Europe. Sedimentation started around 100 million years ago and lasted approximately 40 million years. Cretaceous strata in the district represent the earliest 25 million years of this period.

Hunstanton Formation

The Hunstanton Formation represents the Early Cretaceous on the published Selby map. However, at its base there is a thin discontinuous (up to 1 m thick) ferruginous sandstone, believed to be the equivalent of the Carstone Formation, which is too thin to be shown on the map.

The Hunstanton Formation is of Albian age. It is commonly known as the 'Red Chalk'. This formation, along with the Carstone Formation, where it is present, typically rests with a slight angular unconformity on the underlying Jurassic mudstones. The Hunstanton Formation comprises rubbly to massive chalks, typically becoming sandy towards the base. Belemnite fragments and rounded ironstone nodules are common, although both may be derived. The formation is typically pink to medium brownish red in colour due to the presence of finely disseminated iron oxides. However, weathering of these minerals can result in it taking a grey to ochreous colouration. The formation thickness is estimated to be between approximately 1 and 2 m. Local thickness variation may occur as result of irregularities in the pre-Hunstanton topography, and syn-sedimentary effects of the Market Weighton Axis.

Because of its hardness compared with the underlying Jurassic rocks, this formation typically gives rise to a steepening of the land surface, and distinctive red-coloured brash in freshly ploughed fields. No significant pits or quarries are recorded locally in the Hunstanton Formation. Its basal contact with the less permeable Jurassic lithologies is commonly marked by springs, several of which are exploited for domestic water supply. The overall distribution and morphology of the Hunstanton Formation is significantly disrupted by down-slope movements associated with rotational landslides and earth flows originating in the underlying mudstones.

Chalk Group

The Ferriby Formation is the lowermost formation of the Chalk Group, resting conformably on the Hunstanton Formation. The Ferriby Formation is of Cenomanian age, and is approximately equivalent to the Grey Chalk Subgroup of the Southern Province (Mortimore et al., 2001). This formation is commonly referred to in older literature as part of the 'chalk without flints'. The Ferriby Formation typically weathers to a pale yellow colour, and is largely composed of pale grey to pale yellow-coloured chalk, with occasional pale pink horizons in the upper part. Texturally, the chalk ranges from fine-grained massive chalk to medium-grained limestone with abundant shell fragments. The Ferriby Formation is characteristically argillaceous, including many laterally persistent marl seams (beds of clay-rich chalk) that can reach several centimetres in thickness. Field observations in the district suggest that the Ferriby Formation attains a maximum thickness of approximately 15 m. This represents a significant thinning of the sequence with respect to the surrounding area, probably due to the close proximity of the Market Weighton Axis and its effect on Cretaceous sedimentation.

The argillaceous nature of the Ferriby Formation typically gives rise to fairly heavy 'marly' soils. Deep ploughing commonly unearths cobble-size fragments of rock, resulting in a characteristic cover of pale yellow to pale grey flint-free chalk brash. In common with the underlying Hunstanton Formation, the apparent extent and distribution of the Ferriby Formation is strongly affected by complex large-scale mass movement originating in the less competent Jurassic strata. The Ferriby Formation supports several pits, of which a very few remain active as a local source for road base materials.

The Welton Formation ranges from the Late Cenomanian to mid Turonian in age. This formation correlates closely with the Holywell Nodular Chalk and New Pit Chalk formations of the Southern Province. The first flint horizon of the Chalk Group occurs in the upper part of the Welton Formation; hence this unit was commonly described as the lower part of the 'chalk with flints'. In common with the Ferriby Formation that conformably underlies this unit, the lower part of the Welton Formation includes persistent marl seams, and beds containing abundant shell fragments. The upper part of the formation is dominated by hard white flint-bearing chalk that typically forms massive to thickly bedded units. The flints occur along discrete horizons as characteristically irregular or rounded nodules.

Fieldmappingandmicropalaeontological evidence indicate that the Welton Formation reaches a thickness of approximately 30 m. It is known to increase in thickness to both the north and south of the district suggesting that the influence of the Market Weighton Axis continued during the sedimentation of this formation. Common flint horizons and the relatively hard-weathering chalk of the Welton Formation typically result in a steep, moderately well-featured slope. Freshly ploughed soils show abundant chalk and flint brash. Many small pits occur in the Welton Formation, although all are currently disused.

Spanning the age range from the Late Turonian to Early Santonian, the Burnham Formation correlates approximately with the Lewes Nodular Chalk and Seaford Chalk formations of the Southern Province. This formation together with the conformably underlying Welton Formation has traditionally been called the 'chalk with flints'. The formation comprises thinly-bedded hard white chalk with abundant flints and sporadic marl seams. In contrast to the Welton Formation, the Burnham Formation is characterised by bands of tabular and semi-tabular flint. Regionally, the Burnham Formation is known to reach a thickness of approximately 140 m. However, only the lowest 50 m of the formation is represented in the Selby district, typically capping high ground on the Wolds. Here, the landscape forms gentle slopes with distinct features made by fairly continuous bands of tabular flint. Soils developed over the Burnham Formation contain abundant chalk and flint brash including cobble-size blocks of tabular flint. Several large pits have exploited the chalks and flints of the Burnham Formation, but current extraction is restricted to local farm use.

Quaternary

The Quaternary era has lasted from about 2 million years ago to the present. In Great Britain it is characterised by climatic fluctuation that has resulted in long periods of extreme cold (Ice Ages or glaciations) separated by much warmer periods. The Vale of York area was directly affected by several glaciations, each leaving its mark on the landscape. Most of the superficial deposits seen today in the Vale of York relate to the last glaciation, which occurred during the Devensian Stage between 115 000 and 13 000 years ago. Older, pre-Devensian deposits exist on the higher ground of the Wolds in the east of the district and possibly at depth beneath younger deposits in the Vale of York. These older deposits are included within the Albion Glacigenic Group, which includes all the Anglian Stage glacial deposits in southern Britain and the buried deposits in northern Britain. The Devensian glacial and proglacial deposits are included in the Caledonia Glacigenic Group. The head deposits, cover sands, fluvial and alluvial deposits are related to river catchments outside of the glacial limits. They are included in the Yorkshire Catchments Subgroup and form part of the Britannia Catchments Group. As such, some of the deposits span an age range from the early Devensian to the Flandrian.

Pre-Devensian

Highly dissected remnants of till and glaciofluvial outwash deposits demonstrate that ice once covered the area, reaching elevations in excess of 100 m. Correlations with equivalent deposits to the south suggest that these remnants relate to the Anglian glaciation (approximately 0.5 million years ago) and belong to the Albion Glacigenic Group. Relict Anglian deposits may also exist beneath Devensian cover in the Vale of York.

Older Till

Remnants of the Older Till are preserved on the eastern margin of the Vale of York where the ground rises onto the Wolds. The till occupies characteristically flat-lying benches in the landscape, where it is typically up to 15 m thick. Lithologically the till is predominantly composed of unsorted gravelly clay varying to clay with occasional gravel and some cobbles and boulders. The clast composition includes locally-derived lithologies such as chalk, flint and Jurassic limestone, mixed with far-travelled lithologies including Carboniferous sandstone and rare igneous material thought to be from northern England.

Older Glaciofluvial Deposits

Isolated patches of high-level gravel occur in the east of the area and as a capping to Church Hill at Holme-on-Spalding-Moor [SE 8209 3895]. These Older Glaciofluvial Deposits are characterised by moderately well-sorted gravel to cobble-size clasts of sandstone, flint and rare ironstone. These clasts are commonly wind-eroded (ventifacts), forming facetted driekanter, or pitted with a desert varnish on the surface. Up to 3 m of gravel have been recorded, typically forming a low-relief cover to the Older Till or to bedrock. Some of the gravel is interpreted as a remnant of a previously extensive outwash fan extending from the Chalk uplands of the Wolds. The deposits capping Church Hill may have been deposited by outwash when the low hill was surrounded by ice at a greater elevation.

Devensian Stage

The Devensian Stage brought the last glacial events to affect the Vale of York. During this time, ice-flows down the North Sea blocked the Humber estuary and impounded the drainage from the Vale of York, creating a vast lake known as Lake Humber (Figure 3). Fine-grained proglacial sediment deposited in this lake is preserved as the Hemingbrough Glaciolacustrine Formation. Ice also advanced southwards down the Vale of York, ploughing into the northern part of Lake Humber. Deposits of till represented by the Vale of York Formation were laid down by the ice as it rode over the lake sediments, forming morainic ridges at the front of the ice-flow. The southern limit of the Devensian ice in the Vale of York is marked by the Escrick Moraine (Figure 3), (Figure 4) and (Figure 5).

As the ice began to melt more rapidly than its flow was advancing, its southern limit retreated northwards. When the ice melted, sand and gravel-filled channels on, within or below the ice were left to form esker deposits including those of the Crockey Hill Esker Member. Subsequent minor readvances of the ice formed successively younger moraines in the north of the district (Figure 3), (Figure 4) and (Figure 5). These occur as elongate ridges, which were formed at the limit of ice advance by marginal melting and the pushing action of the ice. Meltwater from the retreating ice became ponded behind these moraines, forming a series of lakes from which the fine-grained sediment of the Alne and Elvington glaciolacustrine formations was deposited as extensive areas of laminated clay. All these Devensian deposits belong to the North Pennine Glacigenic Subgroup, which is a component of the Caledonia Glacigenic Group (Figure 4).

As ice in the North Sea retreated, the Humber estuary became unblocked and Lake Humber was able to drain. The exposed lakebed supported the localised development of peat deposits, which were subsequently buried beneath sands of the Breighton Sand Formation carried into the area by the surrounding rivers and proglacial drainage.

Basal Glaciofluvial Deposits

Borehole evidence from across the district proves the local occurrence of sand and gravel deposits at the base of the glacial succession, in direct contact with the underlying bedrock. These concealed deposits reach a maximum thickness of 2 m, showing an irregular distribution that may relate to preglacial fluvial systems and early fluvioglacial events. In the west of the district, this unit is characterised by sand derived from local weathering of the Sherwood Sandstone Group, and gravel lithologies indicative of a Pennine source including light-coloured sandstone, limestone and grey mudstone. Borehole records from the east of the area indicate a generally greater proportion of chalk and flint material derived from the Wolds.

Hemingbrough Glaciolacustrine Formation

The Hemingbrough Glaciolacustrine Formation (formerly known as 'Silt and Clay of the 25-Foot Drift of the Vale of York'; Edwards et al., 1950; Gaunt, 1976, 1994) forms a characteristic low-relief landscape, typically occupying an elevation range between 5 and 8 m. This laterally persistent unit is characterised by a sequence of up to 24 m of laminated clay and silt with occasional sand beds, resting directly on bedrock or underlain by Basal Glaciofluvial Deposits. The Hemingbrough Glaciolacustrine Formation was deposited in the low-energy, glacial lake environment that developed ahead of the Devensian ice-sheet. Over a large part of the district a tripartite division of the Hemingbrough Glaciolacustrine Formation is possible comprising the lower Park Farm Clay Member, the middle Lawns House Farm Sand Member, and the upper Thorganby Clay Member. The area where these members are recognised is shown as 'Domain B' in (Figure 6). The type section was designated by Thomas (1999). Undivided areas of the formation are shown as 'Domain A' in (Figure 6).

The Park Farm Clay Member is defined by a type section at Park Farm, 2 km north of Riccall [SE 6220 4055]. Here up to 7 m of greyish brown, laminated clay and silt are exposed in a disused clay pit. Borehole evidence indicates that very silty fine-grained sand beds occur in the lower part of the unit. Borehole intersections prove a sharp contact with the underlying Basal Glaciofluvial Deposits. The unit was overridden by ice in the north of the district, where it is locally preserved beneath the Vale of York Formation. Adjacent to the south and east of the Escrick Moraine, just beyond the southern limit of the Devensian ice-sheet, this unit is locally affected by glaciotectonism and contains drop-stones. A gradual thinning from over 16 m in the north of the district to approximately 10 m in the south is shown by boreholes. This part of the Hemingbrough Formation is interpreted to continue as the laminated clays that extend northwards under the Vale of York Formation beneath York and into the Harrogate district (Cooper and Burgess, 1993).

The Lawns House Farm Sand Member is defined by a type section at Lawns House Farm, 1.5 km west of Ellerton [SE 6862 3991], where up to 1 m of reddish yellow, very silty fine-grained sand is exposed. Local auger hole intersections prove a sharp to gradational contact with the underlying Park Farm Clay Member. The unit forms a lobe extending south from the Escrick Moraine for up to 15 km, and thinning gradually from a thickness of approximately 2.5 m. This unit is frequently water-saturated, supporting a weak hydrostatic head, and so is prone to 'running'.

The Thorganby Clay Member is defined by a type section at Thorganby [SE 6797 4133], where 0.5 m of stiff, greyish brown, laminated clay and silt are exposed. Auger hole intersections confirm a gradational to sharp contact with the underlying Lawns House Farm Sand Member (see also (Plate 1)). A gradual thinning from approximately 4 m in the north of the district to 1 m in the south is recorded in boreholes. This unit is inferred to postdate the formation of the Escrick Moraine; it is not affected by glaciotectonism, nor does it contain drop-stones.

Vale of York Formation

The Vale of York Formation is an extensive glaciogenic sequence present to the north of the Escrick Moraine. It extends from west of Stillingfleet [SE 5949 4078], past Escrick [SE 6327 4230] to Wilberfoss [SE 7240 5078] in the north of the district. The Vale of York Formation is mainly till, but it also includes glaciofluvial sand and gravel deposits. It commonly overlies silt and clay beds of the Hemingbrough Glaciolacustrine Formation (Park Farm Clay Member), but elsewhere bedrock. The deposits display a variable composition ranging from sandy gravelly clay with common cobbles and boulders to slightly clayey sand and gravel (Plate 2). Clast composition, which includes Carboniferous limestone and sandstone with sporadic volcanic material, supports a northern-England provenance for the till.

The till of the Vale of York Formation commonly forms topographic features that are related to the action of moving ice. Two distinctive morainic ridges occur in the area. The first, referred to as the Escrick Moraine, occurs along a line between Stillingfleet [SE 5949 4078] and Wilberfoss [SE 7240 5078]. The deposits of gravelly sandy clay till that form this ridge constitute the Escrick Moraine Member of the Vale of York Formation. Evidence of the Escrick Moraine being pushed into the lacustrine deposits was visible at Newton upon Derwent Clay Pit [SE 7270 5030]. The second ridge, referred to as the York Moraine, occurs in a series of lobate arcs extending from Copmanthorpe [SE 5629 4734] north-eastwards to near Heslington [SE 6200 510]. A splay from this main trend also occurs extending from Askham Bryan [SE 5539 4846] to West Field [SE 5670 5070]. It includes gravelly sandy clay, clayey sand and a little sand and gravel that make up the York Moraine Member. In other areas, for example a broad area between York and Wheldrake, the till of the Vale of York Formation forms a gently undulating topography disposed in gentle mounds or ridges; these features reflect a series of re-advances of the icefront during its general retreat from the principal morainic ridges. The Vale of York Formation is generally around 10 to 14 m thick, but reaches a maximum thickness of 38 m in the moraines.

Two sand and gravel members of the Vale of York Formation are recognised in the Selby district. The Poppleton Glaciofluvial Member is present to the west of York around 'The Grange' [SE 5578 5098] where it commonly overlies sandy gravelly clay of the Vale of York Formation, but in some place, bedrock. The deposit comprises bedded sand and gravel, gravelly sand, or sandy gravel with rare clay horizons. It typically ranges in thickness between 2 and 9 m with a maximum thickness of approximately 20 m. This deposit forms a terrace-like feature and may represent a kame formed between the ice margin and the York Moraine Member.

Much of the sand and gravel in the Vale of York Formation was probably deposited by glaciofluvial systems under, within and on top and on top of the ice-sheet. Now the ice has melted these form features on the land surface, eskers. The most notable of these in the Selby district form gentle north – south trending ridges including the Crockey Hill Esker, the deposits of which form the Crockey Hill Esker Member. This is seen between Crockey Hill [SE 6255 4647] and Escrick [SE 6284 4313], but it also forms broad mounds between Crockey Hill and Fulford [SE 6086 4979].

Elvington Glaciolacustrine Formation

The Elvington Glaciolacustrine Formation is present to the north and west of Elvington [SE 7000 4783] in an area between the Escrick and York moraines shown as 'Domain C' in (Figure 6). Here it mainly overlies the sandy gravelly clay of the Vale of York Formation. The deposit comprises thinly to thickly laminated silt and clay with common sand beds, which in the upper part of the sequence are often saturated with water so that they form 'running sand' horizons. Gravel-sized calcareous nodules are common at approximately 1.5 m below ground level and gypsum crystals have been observed at approximately 5 m below ground level. The thickness of the deposits typically ranges between 3 and 6 m. The elevation of the upper surface of this unit is between 10 and 12 m above OD, at a higher level than that of the Hemingbrough Formation, to the south of the Escrick Moraine. The sediments of the Elvington Glaciolacustrine Formation are interpreted to have been deposited in a low energy proglacial lake that developed between the York and Escrick moraines as the Vale of York ice retreated northward to York.

Alne Glaciolacustrine Formation

The Alne Glaciolacustrine Formation is present in a small area of the Selby district around Woodthorpe [SE 5754 4969], where it comprises laminated silt and clay with occasional sand beds and is typically around 2.5 m in thickness. It overlies the sandy, gravelly clay of the Vale of York Formation. The elevation of the top of the Alne Glaciolacustrine Formation is between 14 and 17 m above OD. Its distribution is indicated as 'Domain D' in (Figure 6). The sediments of the Alne Glaciolacustrine Formation were probably deposited following further retreat of the Vale of York ice to the north of York and represent the distal outwash material deposited in the proglacial lake. They are impounded at their southern margin by the York moraine and generally differ from the deposits to the south in that they have much less sand associated with them or covering them. It is possible that the Alne glacial lake existed after Glacial Lake Humber had drained, dammed by the York Moraine.

Pocklington Gravel Formation

The Pocklington Gravel Formation (formerly known as 'Older Littoral Sand and Gravel'; Gaunt, 1981) is present in the north-east of the district, resting directly on bedrock or interdigitating with the Hemingbrough Glaciolacustrine and Breighton Sand formations. The formation is defined by a type section at Pocklington [SE 8042 4801], where temporary exposures showed more than 1 m of light-coloured, matrix-supported, sandy gravel, dominated by sub-rounded chalk and subangular flint fragments, with minor rounded ironstone clasts (Plate 3). The gradational lower contact with weathered Mercia Mudstone is characterised by red-brown clayey gravel or gravelly clay. The Pocklington Gravel Formation represents a glaciofluvial fan deposit that extends westwards from deeply incised valleys in the Wolds into the Vale of York. It attains a maximum thickness of 5 m in the valley bottoms, spreading and thinning over approximately 8 km from the Wolds. A significant component of this deposit may represent reworked material derived from the Older Glaciofluvial Deposits. The chalk-and flint-dominated facies of the Basal Glaciofluvial Deposits may represent a lateral equivalent of the Pocklington Gravel Formation, suggesting a continuity of sediment input from the Wolds throughout the Devensian.

Breighton Sand Formation

The Breighton Sand Formation is included in the Yorkshire Catchments Subgroup of the Britannia Catchments Group. The formation was formerly known as 'Sand of the 25-Foot Drift of the Vale of York' (Edwards et al., 1950; Gaunt, 1976, 1994; Cooper and Gibson, 2003). It forms a blanket of sandy sediment that overlies either the till of the Vale of York Formation, the Hemingbrough Glaciolacustrine Formation, the Elvington Glaciolacustrine Formation, the Alne Glaciolacustrine Formation or the Pocklington Gravel Formation, or rests directly on bedrock. The formation is defined by a type section in the vicinity of Breighton Airfield [SE 7100 3530], where auger holes prove 1.2 m of weakly laminated, grey-brown, fineto medium-grained clayey sand at surface, underlain by silty clay of the Hemingbrough Glaciolacustrine Formation. The formation has a typical thickness of 1 to 1.5 m, reaching a maximum thickness of 4 m, and is characterized by an erosive base. The Breighton Sand Formation comprises three geographically discrete members: the Bielby Sand Member in the east of the district, the Naburn Sand Member to the north of the Escrick Moraine, and the Skipwith Sand Member to the south of the Escrick Moraine (Figure 7).

The Bielby Sand Member is defined by a type section at Bielby [SE 7925 4444], where 1 to 2 m of reddish yellow, slightly clayey to slightly silty sand with occasional fine-grained gravel is exposed beneath recent overbank alluvial deposits. The unit rests erosively on the Bielby Peat Bed and the Pocklington Gravel Formation. The Naburn Sand Member is defined by a type section at Naburn [SE 5984 4528], where a 2.5 m thickness of upward-fining, yellowish brown, silty fine-grained sand is exposed at surface. Auger hole intersections prove an erosive lower contact with the Vale of York Formation. The Skipwith Sand Member is defined by a type section at Little Skipwith [SE 6518 3937], where 1 m of weakly bedded, yellowish grey, slightly gravelly clayey medium-grained sand with thin and discontinuous clayey peat laminae is exposed at surface (Plate 4). Auger hole intersections prove an erosive lower contact with the Hemingbrough Glaciolacustrine Formation.

Thin peat beds of up to 0.3 m thickness occur within the Breighton Sand Formation, including the basal Bielby Peat Bed and Skipwith Peat Bed. The former unit is defined by a type section at Bielby [SE 7925 4444], where 0.3 m of dark greyish brown reed roots are exposed, overlain by the Bielby Sand Member and underlain by the Hemingbrough Glaciolacustrine Formation. The latter unit is defined by a type section at Skipwith Common [SE 6682 3769], where auger hole intersections prove 0.2 m of dark reddish brown compressible peat, overlain by the Skipwith Sand Member and underlain by the Hemingbrough Glaciolacustrine Formation.

The Breighton Sand Formation is interpreted as having been deposited by a complex interaction of fluvial and aeolian processes. The sand may have initially been deposited in a periglacial environment by fluvial systems that were initiated at the end of the glacial period in the Vale of York. Subsequently, the sediments were probably modified and partially redistributed by aeolian processes that continue to the present day. Geomorphological evi-dence suggests that a separate fluvial system was associated with the deposition of each sand member (Figure 7). The Bielby Sand Member is inferred to have been deposited by a system flowing south between the Escrick Moraine and the Wolds; the Naburn Sand Member was deposited by a system flowing south behind the Escrick Moraine; and the Skipwith Sand Member was deposited by a system that broke through the Escrick Moraine, flowing south to meet the east-flowing fluvial system responsible for the extensive spread of Breighton Sand Formation in the south of the district.

Sutton Sand Formation

The Sutton Sand Formation is also included in the Yorkshire Catchments Subgroup of the Britannia Catchments Group. It ranges in age from Late Devensian to Holocene and includes blown sand formed by modern aeolian processes. The unit has been correlated with deposits to the north of York (Matthews, 1970) and to the east of Doncaster in the Isle of Axholme area (Gaunt, 1994). The blown sand commonly forms dunes reaching 2 m or more in thickness, and overlies either alluvium, the Hemingbrough Glaciolacustrine Formation, or the bedrock. In many places the sand is very difficult to distinguish from the underlying Breighton Sand Formation or its constituent members; consequently unless it is very extensive and thick it has not always been separated from them on the map. The Sutton Sand and Breighton Sand are notable for turf farming and root vegetable agriculture (Plate 5); they are highly susceptible to wind erosion, being notable for dust storms during tilling that often deposit modern blown sand deposits along fence lines and the edges of woods.

Flandrian Stage

After the draining of Glacial Lake Humber and the deposition of spreads of outwash and blown sand, the local rivers cut down through the soft glacial and glaciolacustrine deposits to form the present day drainage channels. The modern major drainage uses paths established during the Devensian glaciation. The River Ouse flows through York in a narrow gorge through the York Moraine, along a route that almost coincides with the ice-sheet drainage marked by the route of the Crockey Hill Esker. The confinement of the river where it cuts through the moraine causes severe flood problems in York itself. The course of the River Derwent is that established by drainage from Glacial Lake Pickering southwards between the Devensian ice-sheet and the rising ground of the Yorkshire Wolds (Figure 3). In the west, the course of the drainage round the south of the Escrick Moraine was controlled by the Devensian ice-margin (Cooper and Gibson, 2003) and the River Wharfe still follows that course.

Alluvium

Alluvium comprises postglacial river deposits associated with the main drainage systems of the district, including the rivers Ouse and Derwent. These deposits are assigned to the Yorkshire Catchments Subgroup of the Britannia Catchments Group. They comprise soft interbedded silt and clay with occasional sand beds and common organic horizons, including thin peat beds. Thicknesses range from 0.5 to 10 m. A basal sand and gravel lag deposit commonly underlies the silt and clay. The alluvial deposits are confined to low-lying areas, which are highly susceptible to flooding.

Peat

Recent peat in the area is commonly associated with alluvium. The peat is variably preserved but commonly forms persistent beds of less than 1 m thickness. An extensive area of peat occurs along the course of the Pocklington Canal north of Melbourne [SE 7500 4450]. Other, smaller areas of peat are developed either on top of, or to the north of the York Moraine in places such as Askham Bogs [SE 5700 4800].

Landslides

The gentle topography of the region means that few slopes are steep enough to promote landslide activity. The largest landslides in the region occur on the gentle slopes north-east of Pocklington, where the Chalk lies on top of the Redcar Mudstone Formation over the Penarth Group. Despite their degraded appearance it is possible to distinguish discrete blocks of displaced material, indicating that these landslides are of a rotational nature, although they pass downslope into mudslides and flows. The subdued features of these landslides indicate that they are very old, possibly related to the cold, wet periglacial environment after the last glaciation or even of the, Little Ice Ages, of the seventeenth and nineteenth centuries. However, fresh cracks and disturbed vegetation visible today indicate that these landslides can be subject to local re-activation if ground conditions are altered, for instance by excavation or a change of drainage. Minor landslides may also occur adjacent to actively eroding river courses in the low-lying Quaternary deposits.

Artificial Deposits

Artificial deposits represent areas of the ground that have been extensively modified by the activities of man. In the Selby district the main types of artificial deposits are made ground, worked ground, infilled ground and landscaped ground. These areas were delineated using documentary evidence including historic topographic maps, aerial photographs and site investigation data, and observations recorded during field survey. Only the more obvious man-made deposits can be mapped by these methods and the boundaries shown may not be precise, especially in areas that were active at the time of resurvey — such as landfill sites. Generally, only deposits more than about 1.5 m thick are shown. On the 1:50 000 Series map the artificial deposits have been generalised and the component 1:10 000 Series maps show more detail and include smaller deposits.

Made Ground is present in areas where material has been placed on the former natural ground surface. In the Selby district made ground most commonly comprises: colliery spoil associated with mining; former and current road and railway embankments; flood and screening embankments; abandoned military infrastructure (former airfields); landraising and fill in worked ground associated with inert waste (dominantly soil) and waste disposal (dominantly domestic and industrial refuse). The thickness and composition of made ground is extremely variable. It may be made up of natural material or of artificial material such as plastic and metal. The majority of engineering fill for embankments is inert, but waste disposal fill may include contaminants.

Worked Ground has been excavated. In the Selby district this category includes numerous former and current brick pits, extensive fishing ponds and chalk pits. It also includes road and railway cuttings.

Infilled Ground represents areas that have been excavated and then partially or wholly backfilled. In the Selby district, former clay pits and sand and gravel pits subsequently used for landfill of municipal non-hazardous and inert waste dominate this category.

Landscaped Ground is mapped where the ground surface has been extensively remodelled but where it is difficult to delineate areas of worked ground and of made ground. In this district landscaped ground is most commonly associated with golf courses.

Structure and concealed geology

Lithological and stratigraphical details of the deep geology are given in the sections about the Early Palaeozoic, Carboniferous and Permian rocks. The depositional patterns within the district for all these rocks and to a lesser degree the overlying sequences have been controlled by major basement structures (Figure 1). The pre-Carboniferous basement of the district forms part of the concealed Anglo-Brabant Deformation Belt that extends through eastern England from the Lake District to Belgium (Pharaoh et al., 1987; Verniers et al., 2002; Winchester et al., 2002). This was deformed during the Acadian orogenic phase in early to mid Devonian times.

Geophysical imaging of the area shows a magnetic high crossing the district from north-west to south-east (Figure 8). This is interpreted as Cambrian magnetic basement or regionally metamorphosed basement rocks (Lee et al., 1990). Superimposed on this trend there is another structure visible on both the magnetic and Bouguer anomaly maps (Figure 8) and (Figure 9). This is named the Market Weighton Axis, which is thought to be caused by a buried granite (Bott et al., 1978; Donato and Megson, 1990); this controlled Mesozoic sedimentation and much of the structure in the district. The Bouguer anomaly map for the Selby district also shows a general decrease from west to east, coincident with the easterly thickening of Permo-Trias strata into the North Sea basin. Local variations in this anomaly map are caused by depositional thickness changes and reactivation faulting and folding in the rocks that overlie the pre-Carboniferous basement. These were produced by the interaction between block and basin structures developed around the northern margin of the Midlands microcraton that abuts this tectonic belt.

An area to the north of the district (Figure 1) comprises the eastern corner of the Askrigg Block, which dips eastwards beneath the Jurassic sequence. South of this there is a zone of intense faulting forming the Craven Fault Zone (including the North Craven Fault), which continues eastwards into the Howardian Hills – Flamborough Head Fault Zone. The latter has strongly affected all the bedrock strata including the Jurassic and Cretaceous rocks. South of these fault zones the Carboniferous rocks are thick within the Harrogate Basin and probably also the area to the east. The southern margin of this basin is marked by the Leeds Monocline, a structure seen at outcrop to the west of the district. Its continuation eastward just cuts the north-west corner of the Selby district and marks the approximate northern limit of the workable coal seams. Coal is present further north, but the amount of faulting and folding coupled with the presence of the city of York make extraction largely uneco nomical.

At depth beneath the Selby district the Dinantian strata dip towards the north-east at 2° to 3°, ranging from a depth of 2600 m below OD in the west to 3700 m in the east (Figure 10). Above them the Namurian rocks thin rapidly to the north-east so that their dip is steeper in the west (2.5°) than in the east (1.8°). As a consequence of this rapid thinning, the overlying Westphalian strata are preserved in a broad synclinal structure with its deepest point and thickest sequence of about 1000 m in the north-east of the district. In the west the Westphalian strata dip at about 4° to the north-east, but in the central part of the district this reduces to about 2° to the north. As a consequence, the depth of the Barnsley Coal, which is the principal worked seam, increases from about 250 m in the west to about 1000 m in the east, which is the limit to which it can be worked. In the north-central part of the district its depth increases to 1600 m.

The Selby district is cut by numerous en échelon west-south-west – east-north-east to south-west – north-east trending normal faults delineating grabens and horsts connected by transfer ramps. The faults generally have throws of 10 to 30 m in the southern part of the district, but in the middle and north the throws increase to between 10 and 100 m. In general, the magnitude of throw is similar at all stratigraphic levels, so these faults are presumed to be mainly of postlate Triassic age. Movement on the faults mainly predates the deposition of the Cretaceous rocks in the east of the district. These structures controlled the way that the Selby coalfield was exploited by determining the orientation and layout of the panels that were extracted. The structures also strongly influence groundwater abstraction, groundwater quality and the likelihood of penetrating the Sherwood Sandstone aquifer. Two significant graben structures crossing the district are recognised. The larger is the Osgodby Fault Trough running into the Seaton Ross Graben, together up to 1 km wide and 23 km long (Hawkins and Aldrick, 1994). This structure is controlled by a north-north-west-dipping master normal fault on its southern side, with a south-south-east-dipping antithetic fault on its north side. By contrast, in the eastern part of the district, this polarity is reversed. The Pocklington Fault Trough (and other structures) have a master fault on their southern side, and a minor antithetic fault on the north. The cause of this change in polarity from west to east is unknown. In general, the magnitude of the throw, up to 100 m, increases towards the northern part of the district, approaching the hinterland of the Howardian Hills–Flamborough Head Fault Zone.

Chapter 3 Applied geology

Water Resources

The Sherwood Sandstone Group is the main aquifer in the Selby district. There is a small area of the Chalk Group aquifer in the north-east. The Sherwood Sandstone is characterised by both fracture and intergranular porosity, resulting in a high storage capacity. Fault planes may act to enhance flow, forming recharge boundaries, or may form impermeable barriers to flow (Shand et al., 2002). A cover comprising superficial deposits and the Mercia Mudstone Group limits recharge from surface infiltration across much of the district. Recharge is predominantly from the major river courses in the west and north-west of the district. In many places, the cover may also prevent upward percolation of groundwater from the aquifer, which under some conditions can lead to the buildup of artesian pressures. This cover and the general disposition of the geology in the district generate a general variation in hydrologic head across the district, highest in the west-north-west, decreasing towards to the east-south-east.

Water extracted from the Sherwood Sandstone is of poor quality towards the east of the district where the aquifer is overlain by a thick cover. High levels of salinity here will in most cases preclude the extraction of potable water, but the supplies are generally suitable for agricultural and industrial purposes.

The high hydraulic conductivities of all of the sandstone and chalk units within the district can make them susceptible to contamination from the ground surface, but the extensive thick superficial cover on the low ground helps to protect the Sherwood Sandstone aquifer.

In the past shallow wells have been sunk into the Mercia Mudstone and superficial deposits to provide local supplies, but their poor quality, low yields and irregular supply has led in most cases to abandonment in favour of overland supplies.

Mineral resources

Coal is the most significant mineral resource in the Selby district and over 121 million tonnes were extracted from the Selby coalfield. The depth of the deposits and complexity of the deep geology in the region meant that they could not be exploited until the 1970s when new mining technologies and the rising price of oil made such deep coal extraction an economically viable source of energy. Plans for the Selby Mine were approved in 1976, when it was the largest deep mine coal project undertaken in the world. It started producing in 1983. The mine had two parallel coal conveyor drifts running for 10 and 12 km in a north-easterly direction from their entry point at Gascoigne Wood Mine [SE 5280 3190], lying to the south-west of the Selby district. The mine included 740 km of underground roadways that linked 285 km2 of exploitable coal seams. Collier and maintenance access was via 10 shafts at 5 satellite mines situated at North Selby [SE 6480 4430], Whitemoor [SE 6650 3580], Stillingfleet [SE SE 60053 40477] (Plate 5), Riccall [SE 6380 3700] and Wistow [SE 5740 3560]. Over most of the worked area only the Barnsley Seam was exploited, but east of Riccall a small area of the Stanley Main Coal was also worked. Pillars were left under sensitive towns and villages, but many low-lying areas were affected by subsidence requiring substantial raising of flood embankments and regrading of land drainage.

The mining strategies employed at Selby allowed it to become one of the most productive collieries in the world. In 1993/4 its peak output was over 12 million tonnes and accounted for 45 per cent of the entire UK deep-mined coal production. After that time the production declined with the coalfield finally being closed on 26th October 2004 due to 'geological complications'. Considerable reserves remain, but because of the low-lying nature of the area it is unlikely that the remaining deep reserves of coal in this mining complex will be exploited in the near future.

The potentially short life cycle of the Selby complex was taken into account when extraction licenses were originally granted in the 1970s. Stringent planning conditions mean that the colliery sites should be environmentally restored and the surface structures have either been demolished or adapted for other industries.

The thick cover of superficial deposits over much of the district means that few minerals from the bedrock formations have been worked. Disused quarries of various minerals for local use are found across the district. Mineral deposits of historical importance in the region are listed in (Figure 11).

Engineering ground conditions and hazards

A broad impression of how a particular unit can be expected to behave under normal conditions can be determined from its lithological and geotechnical characteristics. The major geological considerations for development are outlined in (Figure 12). This provides a general guide to each of the units. Ground conditions are also influenced by geological structure, topography, weathering and human activity. The effects of any undermining, faults and variable depths and degrees of weathering should also be considered on a site-by-site basis.

Flooding

The low topographic relief and drainage pattern of the district mean that much land lies close to the meandering routes of the rivers Ouse, Derwent, and Foulness, and Pocklington Beck, making large areas prone to flooding. Flooding can be especially severe in the centre of York where the River Ouse is constrained to a narrow flood plain as it passes through the York Moraine. The extensive flat-lying terrain underlain by glaciolacustrine silt and clay deposits is also prone to flooding when severe rainfall is allied with high river levels.

Subsidence due to undermining

Mining in the Selby coalfield was done primarily by the longwall method at depths of between 390 and 1044 m. As a consequence of the depth, much of the foundering within overlying strata will have been accommodated within the rock mass, but significant subsidence at the surface has occurred and future settlement may be possible. The approximate extent of underground workings is shown in (Figure 13). It should be borne in mind that even minor levels of subsidence could affect buildings, flood defences and agricultural activity. A standing agreement exists between the Environment Agency, the Internal Drainage Board and British Coal to remediate the effects of any subsidence in sensitive areas, including wetlands that are particularly susceptible to changes in local drainage patterns.

Made ground

Made ground in the district is of varying composition and quality, ranging from engineered structures such as embankments to non-engineered landfill. The materials and the method of their emplacement will greatly influence the quality and engineering behaviour of the fill. Other major areas of made ground are associated with the former coal mines, which have extensive screening banks surrounding them. Detailed assessments are required prior to any development on made ground, especially where a change of use is being considered: design criteria of the existing fill may not satisfy the requirements of the new use.

Slope stability

The low topographic relief of the district means that problems associated with slope stability are limited to a few areas. As previously described, there are significant landslides associated with the Jurassic escarpment north of Pocklington, where degraded landforms suggest the presence of a series of shallow rotational landslides. Generally slopes of more than 7° in the Penarth Group, Redcar Mudstone Formation and the Chalk should be considered prone to slope instability. It is likely that such slopes would have been extensively affected by landsliding during and after the last glaciation and may contain zones of weakness and relict shear surfaces. Although stable under current climatic conditions, these slopes may be susceptible to re-activation if the ground is significantly changed, for instance by excavation, changes in drainage or future climate change. Glacial and periglacial deposits within the area should also be assumed to contain relic shear surfaces, which represent potential planes of weakness.

A large part of the Selby district is underlain by the clay and silt of the Hemingbrough Formation. These deposits pose a potential compressible ground problem (Taylor et al., 1976) and may cause instability if encountered during excavation. Over part of the district the Hemingbrough Formation contains the Lawns House Farm Sand Member. This lithology is at shallow depth and prone to 'running sand' behaviour. This occurs when the water pressure within a body of sand or silt is released, for example during excavation. The weak material then flows to release the pressure and fill available space. This can be especially dangerous when creating a new excavation, but can also cause problems by infiltration of service pipes (Plate 1).

Land gas

Made ground, including landfill, may contain hazardous substances or organic material that will produce methane and other gases upon decomposition. Such gases may be prone to migration and possibly explosion if they are confined. Disused mine shafts and adits may act as pathways for the migration and collection of gases, but modern mineshaft capping techniques should prevent such problems. There are only 10 coal mine shafts in the district and their locations are precisely known.

Radon emissions

Radon is a radioactive gas produced naturally by the radioactive decay of radium and uranium. Radon is found in small quantities in all rocks and soils, although the amount varies from place to place. Geology is the most important factor controlling the source and distribution of radon (Appleton and Ball, 1995). Less than one per cent of the Selby district is classified as a radon affected area (Green et al., 2002). The government recommends that houses in radon affected areas should be tested for radon. A study of geological radon potential (BRE, 1999) indicates that the only bedrock unit in the district that may have slightly elevated radon level is the Mercia Mudstone Group, where it is not covered by superficial deposits.

Information sources

Sources of further geological information held by the British Geological Survey relevant to the Selby district and adjacent areas are listed here.

Information on BGS publications is given in the current BGS Catalogue of Geological Maps and Books, available on request and at the BGS website (www.bgs.ac.uk). BGS maps, memoirs, books, and reports relevant to the district may be consulted at BGS and some other libraries. They may be purchased from the BGS Sales Desk, or via the bookshop on the BGS website. This website also provides details of BGS activities and services, and information on a wide range of environmental, resource and hazard issues.

Searches of indexes to some of the materials and documentary records collections can be made on the BGS website.

Geological enquiries, including requests for geological reports on specific sites, should be addressed to the BGS Enquiry Service at Keyworth. The addresses of the BGS offices are given on the back cover and at the end of this section.

Maps

Copies of maps from these and earlier large-scale surveys are available for reference in the BGS Libraries at Keyworth and Edinburgh, and at the BGS London Information Office in the Natural History Museum Earth Galleries, South Kensington. Copies for purchase are produced on a print-on-demand basis.

Digital geological map data

In addition to the printed publications, many BGS geological maps are available in digital form. Details are given on the BGS website. National coverage of digital geological map data (DiGMapGB) is derived from geological maps at scales of 1:625 000, 1:250 000 and 1:50 000. Selected areas also have digital geological data derived from 1:10 000 scale geological maps. Digital geological data for offshore areas is derived from 1:250 000 scale geological maps.

Books

Documentary records collections

Detailed geological survey information, including large scale geological field maps, is archived at the BGS. Enquiries concerning unpublished geological data for the district should be addressed to the Manager, National Geoscience Data Centre (NGDC), BGS Keyworth.

Borehole and trial pit records

Borehole records for the district are catalogued in the NGDC at BGS Keyworth. Index information, which includes site references, names and depths for these boreholes, is available through the BGS website. Copies of records in the public domain can be ordered through the same website, or can be consulted at BGS Keyworth.

Hydrogeological data

Records of water wells, springs, and aquifer properties held at BGS Wallingford can be consulted through the BGS Hydrogeology Enquiry Service.

Geophysical data

These data are held digitally in the National Gravity Databank and the National Aeromagnetic Databank at BGS Keyworth. Seismic reflection data from coal and hydrocarbon exploration programmes is available for the north of the district. Indexes can be consulted on the BGS website.

BGS Lexicon of named rock units Definitions of the stratigraphic units shown on BGS maps, including those named on Sheet 71 (Selby), are held in the BGS Stratigraphic Lexicon database, which can be consulted on the BGS website. Further information on this database can be obtained from the Lexicon Manager at BGS Keyworth.

BGS Photographs

The photographs used in this Sheet Explanation are part of the National Archive of Geological Photographs, held at BGS in Keyworth and Edinburgh. Part of the collection can be viewed at BGS libraries at Keyworth and Edinburgh, and on the BGS website. Copies of the photographs can be purchased from BGS.

Materials collections

Information on the collections of rock samples, thin sections, borehole samples (including core) and fossil material can be obtained from the Chief Curator, BGS Keyworth. Indexes can be consulted on the BGS website.

External collections

Coal mine abandonment plans

Copies of all known abandonment plans are held by the Mining Records Office, Coal Authority, Bretby Business Park, Ashby Road, Burton upon Trent, Staffordshire DE15 0QD. These plans are held by the Coal Authority in the public domain, but are not available for reference at BGS.

Groundwater licensed abstractions, Catchment Management Plans and landfill sites

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

Earth science conservation sites Information on the Sites of Special Scientific Interest present within the Selby district is held by English Nature, Headquarters and Eastern Region, Northminster House, Peterborough, PE

References

Most of the references listed here can be consulted at the BGS Library, Keyworth. Copies of BGS publications can be obtained from the sources described in the previous section. The BGS Library may be able to provide copies of other material, subject to copyright legislation. Links to the BGS Library catalogue and other details are provided on the BGS website.

Appleton, J D, and Ball, T K. 1995. Radon and background radioactivity from natural sources: characteristics, extent and relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/2.

Bott, M H P, Robinson, J, and Kohnstamm, M A. 1978. Granite beneath Market Weighton, east Yorkshire. Journal of the Geological Society, London, Vol. 135, 535–543.

B R E. 1999. Radon: guidance on protective measures for new dwellings. Building Research Establishment Report, B R 211.

Cooper, A H, and Burgess, I C. 1993. Geology of the country around Harrogate. Memoir of the British Geological Survey, Sheet 62 (England and Wales).

Cooper, A H, and Gibson, A. 2003. Geology of the Leeds district—a brief explanation of the geological map. Sheet Explanation of the British Geological Survey, 1:50 000 Sheet 70 Leeds (England and Wales).

Dakyns, J R, Fox-Strangways, C, and Cameron, A G. 1886. The geology of the country between York and Hull. Memoir of the Geological Survey of England and Wales.

Donato, J A, and Megson, J B. 1990. A buried granite beneath the East Midland Shelf of the Southern North Sea Basin. Journal of the Geological Society, London, Vol. 147, 133–140.

Edwards, W, Mitchell, G H, and Whitehead, T H. 1950. Geology of the district north and east of Leeds. Memoir of the Geological Survey of Great Britain, Sheet 70 (England and Wales).

Gaunt, G D. 1976. The Devensian maximum ice limit in the Vale of York. Proceedings of the Yorkshire Geological Society, Vol. 40, 631–637.

Gaunt, G D. 1981. Quaternary history of the southern part of the Vale of York. 82–97 in The Quaternary in Britain. Neale, J, and Flenley, J (editors). (Oxford: Pergamon Press.)

Gaunt, G D. 1994. Geology of the country around Goole, Doncaster and the Isle of Axholme. Memoir of the British Geological Survey, Sheets 79 and 88 (England and Wales).

Green, B M R, Miles, J C H, Bradley, E J, and Rees, D M. 2002. Radon Atlas of England and Wales. National Radiological Protection Board Report, NRPB-W26.

Hawkins, T R W, and Aldrick, R J. 1994. The pattern of faulting across the western sector of the Market Weighton Block, Vale of York. Proceedings of the Yorkshire Geological Society, Vol. 50, 125–128.

International MiningConsultants. 2002. Review of the Selby Complex. Report for the Department of Trade and Industry.

Lee, M K, Pharaoh, T C, and Soper, N J. 1990. Structural trends in central Britain from images of gravity and aeromagnetic fields. Journal of the Geological Society, London, Vol. 147, 241–258.

Matthews, B. 1970. Age and origin of aeolian sand in the Vale of York. Nature, London, Vol. 227, 1234–1236.

Mortimore, R N, Wood, C J, and Gallois, R W. 2001. British Upper Cretaceous Stratigraphy. Geological Conservation Review Series. No. 23. (Peterborough: Joint Nature Conservation Committee.)

Pharaoh, T C, Merriman, R J, Webb, P C, and Beckinsale, R D. 1987. The concealed Caledonides of eastern England: preliminary results of a multidisciplinary study. Proceedings of the Yorkshire Geological Society, Vol. 46, 355–369.

Powell, J H. 1984. Lithostratigraphical nomenclature of the Lias Group in the Yorkshire Basin. Proceedings of the Yorkshire Geological Society, Vol. 45, 51–57.

Powell, J H, Cooper, A H, and Benfield, A C. 1992. Geology of the country around Thirsk. Memoir of the British Geological Survey, Sheet 52 (England and Wales).

Rawson, P F, and Wright, J K. 1995. Jurassic of the Cleveland Basin. 173–208 in Field Geology of the British Jurassic. Taylor, P D (editor). (The Geological Society, London.)

Ruffell, A H, Holliday, D W, and Smith, D B. 2006. Permian: arid basins and hypersaline seas. 269–293 in The Geology of England and Wales. Brenchley, P J, and Rawson, P F (editors). (The Geological Society, London.)

Shand, P, Tyler-Whittle, R, Morton, M, Simpson, E, Lawrence, A R, Pacey, J, and Hargreaves, R. 2002. Baseline Report Series 1: The Triassic Sandstones of the Vale of York. British Geological Survey Commissioned Report, C R/02/102N.

Smith, D B. 1970. The palaeogeography of the British Zechstein. 20–23 in Third Symposium on Salt. Rau, J L, and Dellwig, L F (editors). Vol. 1. (Cleveland, Ohio: Northern Ohio Geological Society.)

Smith, D B. 1974. Permian. 115–144 in The Geology and Mineral Resources of Yorkshire. Rayner, D H, and Hemingway, J E (editors). (Leeds: Yorkshire Geological Society.)

Smith, D B. 1989. The late Permian palaeogeography of north-east England. Proceedings of the Yorkshire Geological Society, Vol. 47, 285–312.

Smith, D B. 1992. Permian. 275–305 in Geology of England and Wales. Duff, P McL D, and Smith, A J (editors). (The Geological Society, London.)

Sumbler, M G. 1999. The stratigraphy of the Chalk Group in Yorkshire and Lincolnshire. British Geological Survey Technical Report, WA/99/02.

Taylor, R K, Barton, R, Mitchell, J E, and Cobb, A E. 1976. The engineering geology of Devensian deposits underlying P FA lagoons at Gale Common, Yorkshire. Quarterly Journal of Engineering Geology, Vol. 9, 195–216.

Thomas, G S P. 1999. Northern England. 91–98 in A revised correlation of Quaternary deposits in the British Isles. B OW E N, D Q (editor). Special Report of the Geological Society, London, No. 23.

Tucker, M E. 1991. Sequence stratigraphy of carbonate-evaporite basins: models and application to the Upper Permian (Zechstein) of northeast England and adjoining North Sea. Journal of the Geological Society, London, Vol. 148, 1019–1036.

Verniers, J, Pharaoh, T C, andre, L, Debacker, T N, De Vos, W, Everaerts, M, Herbosch, A, Samualsson, J, Sintubin, M, and Vecoli, M. 2002. The Cambrian to mid Devonian basin development and deformation history of eastern Avalonia, east of the Midlands Microcraton: new data and a review. 47–93 in Palaeozoic amalgamation of Central Europe. Winchester, J A, Pharaoh, T C, and Verniers, J (editors). Geological Society of London Special Publication, No. 201.

Whitham, F. 1991. The stratigraphy of the Upper Cretaceous Ferriby, Welton and Burnham formations north of the Humber, north-east England. Proceedings of the Yorkshire Geological Society, Vol. 48, 227–254.

Winchester, J A, and the P AC E T MR Network Team. 2002. Palaeozoic amalgamation of Central Europe: new results from recent geological and geophysical investigations. Tectonophysics, Vol. 360, 5–21.

Wood, C J, and Smith, E G. 1977. Lithostratigraphical classification of the Chalk in North Yorkshire, Humberside and Lincolnshire. Proceedings of the Yorkshire Geological Society, Vol. 42, 263–287.

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

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

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

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, and from BGS-approved stockists and agents.

Figures and plates

Figures

(Figure 1) The principal structural features in and around the Selby district.

(Figure 2) Generalised Permian and Triassic sequence in the Selby district.

(Figure 3) Outline of the glacial geology of the district.

(Figure 4) Schematic correlation diagram showing the spatial and temporal relationships of the subdivisions of the Quaternary sequence in the Selby district.

(Figure 5) Schematic cross-section through the glacial and post-glacial deposits of the Selby district.

(Figure 6) The Selby district showing distribution of glaciolacustrine domains: A = Hemingbrough Glaciolacustrine Formation (Undivided); B = Tripartite subdivision of Hemingbrough Glaciolacustrine Formation into the Park Farm Clay Member overlain by the Lawns House Farm Sand Member, in turn overlain by the Thorganby Clay Member; C = Elvington Glaciolacustrine Formation underlain by till of the Vale of York Formation; D = Alne Glaciolacustrine Formation underlain by till of the Vale of York Formation.

(Figure 7) The distribution of the Breighton Sand Formation and its component members in the Selby district.

(Figure 8) Magnetic anomaly map for the Selby district showing a broad regional anomaly trending north-west to south-east through the district. This magnetic high coincides approximately with the northern edge of the Selby coalfield. However, the main cause of the magnetic anomaly is deep-seated, and is interpreted to be related to either buried Cambrian magnetic basement, or regionally metamorphosed basement rocks (Lee et al. 1990). A strong magnetic high in the south-east of the district is coincident with the western edge of the Market Weighton Axis.

(Figure 9) Bouguer anomaly map for the Selby district showing a general gradient from west to east, coincident with the easterly thickening of Permo-Trias strata into the North Sea Basin. The east of the district is marked by a pronounced gravity low associated with the Market Weighton Axis. This structure interpreted as a large buried granite batholith (Donato and Megson, 1990), marks the southern limit of the Jurassic to Early Cretaceous Cleveland Basin.

(Figure 10) Deep cross-section across the Selby district from the south-west [SE 5523 3364] towards [SE 6412 4192] then to the north-east [SE 8341 5087], vertical exaggeration x2; compiled from borehole, mine and seismic data. See summary table inside front cover for details of succession.

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

(Figure 12) Summary of engineering properties of principal geological units in the Selby district.

(Figure 13) Approximate area of coal mining in the Selby district.

Plates

(Plate 1) Thorganby Clay Member overlying the sands of the Lawns House Farm Sand Member (both of the Hemingbrough Formation) at ponds near Rossmore Lodge [SE 7300 4440]. Note the water table in the sand causing it to run and slump (P612468).

(Plate 2) Cut section from core showing the clasts and structure of the till of the Vale of York Formation at a depth of 3.8 m in the Crockey Hill Borehole [SE 6362 4550], field of view 6 cm wide. Note the fractured nature of the larger clasts indicating a significant degree of stress during the emplacement of the till (P696311).

(Plate 3) Pocklington Gravel Formation exposed in house foundation trenches at Pocklington [SE 8058 4823]. The gravel comprises mainly chalk and flint with abundant ironstone and Jurassic limestone. The dark brown soil is partly decalcified with mainly flint fragments (P696312).

(Plate 4) Skipwith Sand Member exposed at Skipwith [SE 6518 3937] (P612470).

(Plate 5) Stillingfleet Mine [SE 6036 4049], one of the satellite access mines for the Selby Coalfield. Viewed across flat glacial lake deposits with the Breighton Formation sand at the surface; the sand is ridged for growing potatoes (P696306).

(Front cover) View looking south-west from Hagg Bridge [SE 7170 4514] along the Pocklington Canal and The Beck across the Vale of York to Drax Power Station 25 km away. Flat Vale of York glacial lake deposits are incised by the alluvial tract of the river (P969308).

(Rear cover)

(Geological succession) Summary of the geological succession in the district.

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

Figures

(Figure 2) Generalised Permian and Triassic sequence in the Selby district.

GROUPS FORMATIONS DOMINANT LITHOLOGY THICKNESS
TRIASSIC Penarth Group Not divisible in area Pale bluish grey and greenish grey calcareous mudstone About 10 m
Mercia Mudstone Group Not divisible in area Red-brown calcareous mudstone with gypsum especially near the top. 180 m – 240 m
Sherwood Sandstone Group Not divisible in area Red-brown mainly finegrained sandstone with a little calcareous mudstone and mudflake conglomerate. Up to 435 m
PERMIAN Zechstein Group Roxby Formation (undivided) Roxby Formation (part) Red-brown calcareous and anhydritic or gypsiferous mudstone Up to 5 m
Littlebeck Anhydrite Formation Anhydrite W: 0 m E: 0 – 6 m
Roxby Formation (part) Red-brown calcareous and anhydritic or gypsiferous mudstone Up to 7.5 m
Sherburn Anhydrite Formation Grey and red-brown gypsum or anhydrite with mudstone SW: 3 m E: 7 m
Roxby Formation (part) Red-brown calcareous and anhydritic or gypsiferous mudstone 10 – (48 m where other fms are not recognised)
Boulby Halite Formation Salt W: 0 m E: 15 m
Billingham Anhydrite Formation Grey gypsum or anhydrite Up to 12 m
Brotherton Formation Light grey fine-grained dolomitic limestone W: 17m E: 34 m
Edlington Formation (undivided) Grauer Salzton Formation Salt and mudstone W: 0 m E: 7 m
Edlington Formation (part) Red-brown calcareous and gypsiferous mudstone 11 – (45 m where other fms are not recognised)
Fordon Evaporite Formation Salt with mudstone W – 0 m E – 11.5 m
Kirkham Abbey Formation Grey dolostone W: 0 m NE: 84 m
Hayton Anhydrite Formation Grey gypsum or anhydrite W: 4 m Central: 50 m NE: 79 m
Cadeby Formation Grey and light yellowish brown dolostone W: 50 m Central (reef): 108 m E: 27 m
Marl Slate Formation Dark grey laminated dolomitic pyritic mudstone W: 0 m E: 2 m
Rotliegendes Group Yellow Sands Formation (with local breccia) Light and medium bluish grey fine-grained aeolian sand 0 – 25 m as dunes

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

Mineral resource Source (main source in bold) Activity Use
Coal Coal Measures Significant extraction from the Selby area from 1983 to 2004 Electricity generation, industrial
Sand and gravel Alluvium, Breighton Sand Formation, Crockey Hill Esker Member, Vale of York Formation, Poppleton Formation, Pocklington Gravel Formation, Older Gravel Some small scale extraction Concrete aggregate, building
Chalk Ferriby, Welton and Burnham Formations Of historical local importance Concrete, agricultural lime
Brick clay Hemingbrough, Alne and Elvington Glaciolacustrine Formations silt and clay A few large pits and some small local workings Brick, tile and pipe making

(Figure 12) Summary of engineering properties of principal geological units in the Selby district

Engineering Considerations
Engineering Geological Unit Geological Units Description/Characteristics Excavation Foundation Conditions Engineering Fill Site Investigation
Engineering Soils Mixed Fine And Coarse Soils Stiff/Dense Till: Vale of York Formation, York Moraine and Escrick Moraine Members; Older Till Stiff to very stiff, sandy CLAY, with gravel and occasional cobbles with impersistent layers of peat and water bearing sands and gravels. Variable. Diggable, poor stability, flooding hazard. Presents few problems for shallow foundations. But locally variable. Suitable if selected carefully. Establish local variability.
Soft

Firm

 Head Soft to firm, sandy CLAY with variable gravel and silt content of low to high plasticity. Relict shear surfaces may be present. Diggable, poor stability, flooding hazard. Generally poor, may require piling. Locally high sulphate levels. Generally unsuitable. Determine presence and extent of shear surfaces and compressible zones Establish local variability.
Medium Dense Alluvium. Glaciolacustrine deposits: Alne and Elvington formations, Hemingbrough Formation including Thorganby and Park Farm clay members Alluvium: Very soft to firm, occasionally laminated or organic CLAYS, and sandy SILTS and loose to medium dense SANDS with impersistent layers or pockets of PEAT Glaciolacustrine deposits: stiff laminated and fissured CLAYS with medium dense, medium to fine SANDS. Diggable, poor stability, flooding hazard. Generally poor due to the presence of soft compressible zones. Raft or piled foundations may be required; locally high sulphate levels. Glaciolacustrine deposits may be suitable for shallow foundations and light structures, piled foundations may be required for heavier loads. Marked strength anisotropy in laminated deposits Generally unsuitable. Determine the presence, depth and extent of soft compressible zones and depth to bedrock. May require trial pitting as a matter of course. Need to establish local variability and presence of soft deposits.
Coarse Medium Dense Alluvial gravels River Terrace Deposits Breighton Sand Formation and constituent members, Lawns House Farm Sand Member, Poppleton Formation, sand and gravel in Vale of York Fm. Crockey Hill Esker Member Medium dense to dense, fine to coarse grained SANDS and angular to rounded, fine to coarse GRAVELS with variable clay content and local lenses of organic silt or brown pebbly sandy clay. Sometimes laminated. Diggable, immediate support/casing may be required, potential water ingress. Lawns House Farm Sand Member is a widespread running sand hazard. Generally suitable for light structures. Channel deposits may influence foundation design. Selected sands and gravel generally suitable as granular fill. Need to establish depth, lithological variation and presence of buried channels, geophysical methods may be required.
Organic Soils Very Soft Peat Fibrous/ amorphous peat. Diggable. Very poor, weak compressible, potentially acidic groundwater. Unsuitable. Determine extent and depth of soft compressible peat deposits.

Groundwater acidity should be ascertained before selection of buried concrete.
Highly

Variable Artificial Deposits

 Variable Made ground Worked ground In-filled ground Highly variable composition, thickness and geotechnical properties. Hazardous waste may be present in landfill areas. Usually diggable. Highly variable. May be highly compressible, giving rise to total and differential settlement, particularly in areas of non-engineered fill. Highly variable, some material may be suitable. Essential to follow current best practice. Including investigation of groundwater chemistry, presence of volatiles.
Landslides Variable Landslide Variable source material, presence of shear surfaces likely. Generally unsuitable for built development unless made suitable by appropriate engineered remedial works. Generally unsuitable. Determine stability conditions of landslide and adjacent slopes before development and design of remedial works.
Bedrock Chalk Weak To Strong Burnham, Welton, Ferriby and Hunstanton formations Moderately weak to moderately strong LIMESTONE. Properties depending upon weathering grade. Hard digging to easy ripping depending upon weathering grade. Usually good, dependant upon weathering grade and fracture spacing. Dissolution features may lead to an irregular rockhead and may cause problems with foundations if not detected and designed for. Suitable for general fill if carefully chosen and when dry. Important to assess weathering grade of rock. Geophysical methods and/or probing may be needed

to assess irregularity of rockhead. Groundwater protection status should be established.
Sandstone Weak To Moderately Strong (& Dense Sands) Sherwood Sandstone Group Weak to moderately strong (at depth) weakly cemented, friable, fine to medium grained SANDSTONE and loose to dense SAND; not exposed, present beneath superficial deposits. Digging/scraping, poor stability, flooding hazard. Piles required for heavier structures. Locally high sulphate concentrations. May be suitable. Determine depth of superficial layer, conditions highly variable. Possible artesian conditions.
Mudrocks Weak To Strong Mudstones of the Redcar Mudstone, Penarth Group and Mercia Mudstone Group*

*Calcareous with beds and veins of gypsum.

 Weak to moderately strong, moderately fissured silty, and sandy MUDSTONE and SILTSTONE. Hard digging or ripping depending on weathering state. Open excavations need protection from water to prevent deterioration. Generally good, heavier structures may require piling. Locally high sulphate conditions. May be suitable as general fill under controlled compaction, but may often be too wet to achieve optimum compaction. In-situ loading tests advisable. Any undermining by dissolution. Possible artesian water conditions locally.