Geology of the Keswick district. A brief explanation of the geological map sheet 29 Keswick

D G Woodhall

Bibliographic reference: Woodhall, D G. 2000. Geology of the Keswick district. A brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 sheet 29 Keswick (England and Wales).

Keyworth, Nottingham: British Geological Survey, 2000.

© NERC copyright 2000.

The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright reserved Ordnance Survey licence number GD272191/2000.

(Front cover) Thirlmere reservoir viewed from Steel Fell [NY 322 121]. Screes form the low forested slopes on the right. Blencathra is in the background (north). The flooded valley lies within the Borrowdale Volcanic Group and is aligned along the main trace and splays of the Coniston Fault (MNS 06391).

(Rear cover)

(Index map)

Notes

The word 'district' refers to the area of the geological 1:50 000 series sheet 29 Keswick. National grid references are given in square brackets and all lie within 100 km square NY. Symbols in round brackets after lithostratigraphical names are the same as those used on the geological map.

Acknowledgements

This sheet explanation was compiled by D G Woodhall, Regional Editor for Scotland and Northern England, and is based on the approved version of the sheet description for the Keswick district authored by D G Woodhall. Full acknowledgements are to be found within the sheet description.

Geology of the Keswick district

The Keswick district is dominated by rugged mountainous topography of the Lake District National Park, and includes some of the highest fells, notably Skiddaw (928 m) and Helvellyn (930 m). The lakes of Loweswater, Crummock Water, Buttermere, Derwent Water, and Thirlmere are also situated within the district, along with part of Ennerdale Water, Bassenthwaite Lake and Ullswater. After a long history, mining and quarrying have now all but ceased, but past activities remain evident as mine entrances, waste tips and derelict buildings. Tourism now dominates the local economy and many public footpaths and bridleways traverse the district providing easy access to many geological features. Hill farming is the principal land use.

The solid geology is dominated by 495 to 450 million years old sedimentary and volcanic rocks of Ordovician age, but in the extreme north-east of the district there are small areas of younger rocks (about 380 to 330 million years old) of Devonian and early Carboniferous age. The resurvey of the Keswick district has significantly improved our understanding of the lithostratigraphy of the Ordovician rocks, particularly of the sedimentary Skiddaw Group. New formations and members in the overlying Borrowdale Volcanic Group are introduced, with those at the top of the group identified as the youngest known anywhere in the Lake District. These newly defined strata greatly extend our knowledge and understanding of the nature of volcanism, depositional processes and environment operating during deposition of the Borrowdale Volcanic Group. Mineralisation is evident throughout the district and includes deposits exploited by the last operating metal mines in the Lake district, namely Force Crag and Greenside.

There is a major unconformity between early Carboniferous rocks that formed about 330 million years ago and unconsolidated Quaternary deposits (drift) most of which accumulated within the last 30 000 years when the district was glaciated. The effects of this are displayed in the present landscape of glacially carved valleys, separated by rugged mountains, and by the widespread cover of glacial deposits.

The new geological maps ('solid' and 'solid and drift') and this sheet explanation provide valuable information on a wide range of earth science issues. These include traditional aspects such as sedimentation, volcanism, structure, metamorphism and mineralisation, but also cover applied aspect such as mineral, energy and water resources, waste disposal, foundation conditions, and conservation.

Chapter 1 Introduction

This sheet explanation provides a summary of the geology of the district covered by the geological 1:50 000 series sheet 29 Keswick, published in solid and solid and drift editions in 1999. A fuller description of the geology is provided by the sheet description (Woodhall, 2000), and detailed information can be found in technical reports accompanying some of the 1:10 000 scale geological maps.

The district lies within the northern part of the Lake District National Park, in the county of Cumbria. The main population centre is the town of Keswick. This and a number of villages are located in glacially eroded valleys separated by extensive rugged mountains, known locally as fells. Lakes now occupy parts of some of these valleys; the largest are Ennerdale Water (eastern part), Loweswater, Crummock Water, Buttermere, Bassenthwaite Lake (southern part), Derwent Water, Thirlmere and Ullswater (southern part). The economy of the district is based on tourism, but a legacy of past industrial activity occurs in the form of numerous disused mines and quarries (Plate 1). At present, hill farming is the principal land use.

The bedrock consists mostly of sedimentary and volcanic rocks deposited about 495 to 450 million years ago during the Ordovician (Figure 1). The oldest rocks are marine mudstones, siltstones and sandstones of the Skiddaw Group, some of which may be as old as late Cambrian. These strata were uplifted and eroded prior to subaerial volcanism that formed the Eycott and Borrowdale volcanic groups. The bedrock also includes areas of granite (e.g. the Ennerdale intrusion) emplaced as high-level components of the Lake District batholith. There was an early phase of intrusion during the late Ordovician, about 450 million years ago, and a later phase during the Early Devonian, about 400 million years ago (Figure 2). Contact metamorphism and mineralisation (mainly copper with some antimony) accompanied batholith intrusion. The Early Devonian batholith intrusion took place during the Acadian tectonic phase of the Caledonian Orogeny, which involved faulting, folding, cleavage development and regional metamorphism of the sedimentary and volcanic rocks. Uplift and erosion brought about by the orogeny is marked by the major unconformity that separates these rocks from overlying conglomerates, deposited during the Mid to Late Devonian, about 390 to 360 million years ago. Further erosion preceded limestone deposition during the early Carboniferous, about 340 million years ago. Lead-zinc and baryte minerals were deposited during the Carboniferous, and are probably related to the development of sedimentary basins around the Lake District and Pennine blocks.

The major unconformity between the Lower Carboniferous rocks and Quaternary deposits represents the whole of the Mesozoic and most of the Cainozoic eras, a period of about 340 million years. Any deposits of these eras have been removed during periods of uplift and erosion.

During the Quaternary, the district was subjected to successive glaciations (Plate 2) but most glacigenic deposits are those left by the Dimlington Stadial glaciation (about 26 000 to 13 000 years ago) during the Late Devensian. This was at its maximum about 22 000 years ago. Deglaciation commenced about 18 000 years ago, but small glaciers briefly returned to the heads of some valleys during the Loch Lomond Stadial (11 000 to 10 000 years ago) before renewed climatic amelioration at around 10 000 years ago brought about their retreat at the onset of the Holocene.

Survey history

The Keswick district was originally surveyed on a scale of six inches to one mile, and published as Quarter sheet 101 SE (one inch to one mile) with an accompanying memoir (Ward, 1876). An economic geology memoir concerned with lead-zinc ores also covers the district (Eastwood, 1921). Subsequently, Hartley (1941) published details of the geology of the area around Thirlmere and Helvellyn. The latest survey, on which this sheet explanation is based, commenced in 1981, and involved collaboration (on the Borrowdale Volcanic Group) with Liverpool University during the period 1990–1995. A 1:25 000 scale geological map of the Lorton and Loweswater area, in the north-west of the district, was published by the British Geological Survey in 1990.

Chapter 2 Geological description

Cambrian and Ordovician

The Skiddaw Group crops out in central and northern parts of the district (Figure 2) and consists of wacke sandstone, siltstone and mudstone, mostly of Ordovician (Tremadoc to Llanvirn) age, but with some possibly as old as Cambrian. The lithostratigraphy and biostratigraphy is summarised in (Figure 3) and only briefly described below. The lithostratigraphy is based on Cooper et al. (1995), but the biostratigraphy is based on a revised scheme proposed in a special BGS publication on the Skiddaw Group, which is currently in press. Cooper et al. (1995) and a more recent review by Stone et al. (1999) are important sources of additional references. The sedimentary strata of the Skiddaw Group accumulated as siliciclastic turbidites in deep water on the passive, northern margin of Gondwana, prior to or during the separation of the Eastern Avalonian micro-continent. The sediment was derived from ancient continental-arc volcanic rocks of Gondwana farther south. The presence of olistostromes and other evidence of soft-sediment deformation (Plate 3) indicate slope instability, particularly during the late Arenig, which was possibly related to the rifting of Avalonia from Gondwana.

The east-north-east-trending Causey Pike Fault divides the outcrop of the Skiddaw Group into two distinct stratigraphical belts, the Northern Fells Belt and the Central Fells Belt (Figure 1), (Figure 2), (Figure 3). In the Northern Fells Belt the group consists of the Bitter Beck, Watch Hill, Hope Beck, Loweswater and Kirkstile formations, but it is locally undivided. The Bitter Beck Formation (BBF), of late Tremadoc age (Araneograptus murrayi Biozone), crops out in the extreme north-west of the district and consists of at least 500 m of strata. These have been thrust southwards, possibly by as much as 5 km, over the Kirkstile Formation along the Watch Hill Thrust. The Watch Hill Formation (WHg), of late Tremadoc to early Arenig age, has no outcrop within the district, but is shown at depth in section 1 on the geological map. The Hope Beck Formation (HBe), of Arenig age (Tetragraptus phyllograptoides to Didymograptus varicosus biozones), is present as partly fault-bounded inliers in the north-western part of the district, at Swinside, between Crummock Water and Bassenthwaite Lake. Only the upper 400 m of the formation are present. The Loweswater Formation (LWF), also of Arenig age (Didymograptus varicosus and Didymograptus simulans biozones), mostly crops out in the north-western part of the district. Palaeocurrent flow was broadly from the northern quadrant, and variations in direction probably reflect the influence of basin-floor topography. Small-scale slump folds up to a few metres across occur locally, and there is evidence of bioturbation. The Kirk Stile Formation (KSt), of Arenig to early Llanvirn age (Didymograptus simulans to Didymograptus artus biozones), dominates the Skiddaw Group in the Northern Fells Belt, and the Causey Pike Fault marks the southern edge of the outcrop (Figure 2). The formation either rests on, or is faulted against, the Loweswater Formation. Large parts of the Kirk Stile Formation are hornfels within the thermal metamorphic aureoles of the Crummock Water and Skiddaw intrusions. Beds with slump folding (Plate 3) are bounded by less disturbed and undisturbed beds, indicating that the deformation was either syndepositional or early postdepositional. These structures are products of late Arenig or early Llanvirn soft-sediment deformation, possibly caused by tectonic activity.

In the Central Fells Belt, the Skiddaw Group is dominated by the Buttermere Formation (BUF), of early Tremadoc to late Arenig age, which crops out south of the Causey Pike Fault between Buttermere, Derwent Water and Threlkeld Common (Figure 2). The formation is an olistostrome deposit, at least 1500 m thick (Figure 3). Some angular olistoliths were at least partly lithified before being incorporated in the deposit, whereas others are ragged with injection structures from the matrix suggesting they were in a plastic state when redeposited. The olistoliths and matrix are intensely deformed by minor folds and shears, many of which formed during emplacement of the olistostrome. A characteristic feature of the formation is the juxtaposition of beds of widely different ages, for example, lower Tremadoc acritarch assemblages within a few metres of those of late Arenig age, as in Swinside Gill [NY 190 177]. There is no evidence that the assemblages are mixed. The Robinson Member (RNM) consists of several large sandstone-rich olistoliths, up to 1 km across and 250 m thick. The age of olistostrome emplacement is constrained by the Tremadoc to late Arenig age of the olistoliths and matrix in the Buttermere Formation and the latest Arenig to Llanvirn age of the overlying Tarn Moor Formation. Emplacement is inferred to be in the late Arenig, possibly at about the gibberulus–cucullus biozone boundary, and probably took place in one massive slumping episode involving downslope movement towards the north-west and/or west. The Tarn Moor Formation (TMF), of late Arenig to Llanvirn age, constitutes the highest part of the Skiddaw Group in the Central Fells Belt. It crops out only in the eastern part of the district (Figure 2). The base has not been proved, but its age indicates that it probably overlies the Buttermere Formation. Its lower boundary is presumed to be unconformable on the highly disrupted Buttermere Formation (Figure 3). The lower part of the Tarn Moor Formation contains uppermost Arenig to lowest Llanvirn acritarchs, graptolites (Aulograptus cucullus [formerly Didymograptus hirundo] and Didymograptus artus biozones) and trilobites. In the neighbouring Appleby district there is evidence of the Didymograptus murchisoni Biozone.

The Eycott Volcanic Group (EVG) is restricted to the southern part of Eycott Hill in the extreme north-east of the Keswick district (Figure 2), where about 800 m of strata unconformably overlie the Skiddaw Group of the Northern Fells Belt. The group consists of subaerially erupted basaltic andesite and andesite lavas up to 90 m thick, with a few interbedded volcaniclastic rocks. The latter are mostly sandstone units up to 5 m thick, but in places there are units of pyroclastic rock up to about 20 m thick, mostly in the form of lapilli-tuff emplaced as fall and/or flow deposits, but in places as welded dacitic ignimbrite. Geochemically the rocks have continental margin, medium-K characteristics that are transitional between calc-alkaline and tholeiitic (Millward et al., 1999).

The Borrowdale Volcanic Group crops out south of the Causey Pike Fault where it overlies the Skiddaw Group of the Central Fells Belt. The contact is an unconformity. Palaeomagnetic studies suggest that the Eycott and Borrowdale Volcanic Groups are penecontemporaneous (Piper et al., 1997). The Borrowdale Volcanic Group consists of continental margin type, medium to high-K, calc-alkaline magmas (Beddoe-Stephens et al., 1995) in the form of subaerially erupted basaltic, andesitic, dacitic and rhyolitic lavas, pyroclastic fall, surge and flow deposits, and numerous high-level sills (Branney, 1988; Petterson et al., 1992). The group also includes significant thicknesses of volcaniclastic sedimentary rocks deposited in fluviolacustrine environments. The Birker Fell, Whorneyside, Airy's Bridge, Lingmell, Seathwaite Fell, Lincomb Tarns, Esk Pike and Middle Dodd Dacite formations all occur in the adjacent Ambleside district, and consequently there is more information available in the BGS memoir for this district (Millward et al., 2000). The Helvellyn and Deepdale formations are confined to the Keswick district where they were recognised for the first time as a result of this resurvey. There is more information on these formations in the sheet description (Woodhall, 2000).

The Birker Fell Formation (BFA) crops out between the Ennerdale intrusion and the eastern edge of the district (Figure 2). The formation consists mostly of undivided andesite lavas and sills (Figure 4), which locally form prominent trap topography (Plate 4). Basal, lenticular volcaniclastic sedimentary and pyroclastic rocks crop out in Ennerdale, Borrowdale, Low Rigg, Threlkeld and Matter-dale. In Ennerdale and Borrowdale the lower part of the formation includes a number of lenticular members (Figure 4). The Grange Crags Member (GCg) and the overlying Ashness Member (AhT) occur in the lowest part of the formation around Derwent Water, and in many places the former rests directly on the Skiddaw Group. The more persistent Eagle Crag Member (Eag) lies about 350 m above the base of the formation; it rests on and is overlain by an undivided andesite-dominated succession. This member has been intruded by numerous andesite sills, but incorporates the slates quarried at Honister. There are about 1700 m of undivided strata above this member, but additional members occur south of the Burtness Comb Fault. Here, up to 60 m of undivided basaltic andesite above the Eagle Crag Member is locally overlain by the Seatallan Dacite (StaD) and more extensively by the Craghouse Member (ChT). Undivided strata above the latter locally incorporates the Haystacks Member (Hys) and the overlying Round How Member (Rnd).

The formations above the Birker Fell Formation are predominantly volcaniclastic as a result of a fundamental change in the nature of the volcanism and overall composition of its products, from predominantly effusive to explosive, and from mainly andesitic to dacitic. The Whorneyside Formation (Wny) consists of andesitic pyroclastic rocks produced during the initial stages of development of a caldera; the Scafell caldera of Branney and Kokelaar (1994). This was tightened into a syncline, the Scafell Syncline (Figure 2), during the Acadian phase of the Caledonian Orogeny. The Whorneyside Formation is confined to a narrow outcrop west of the Coniston Fault, along the northern limb of the Scafell Syncline, where it rests unconformably on the Birker Fell Formation (Figure 2). It has been intruded by numerous andesite sills, and thins eastwards from a maximum of about 140 m in Borrowdale before wedging out in the vicinity of the Coniston Fault. Ignimbrite in the lower part of the formation constitutes the Wet Side Edge Member (WSE) (Figure 5). In Sour Milk Gill [NY 230 122], the upper part of the Whorneyside Formation includes strata deposited in shallow ephemeral lakes (Branney, 1991), and it is from these strata that nonmarine trace fossils have been recorded (Johnson et al., 1994).

The Airy's Bridge Formation consists of dacitic and rhyolitic pyroclastic rocks produced during the main phase of development of the Scafell caldera (Branney and Kokelaar, 1994) (Figure 5). It rests conformably on the Whorneyside Formation, and likewise crops out along the northern limb of the Scafell Syncline west of the Coniston Fault (Figure 2). The outcrop is widest where the maximum thickness (650 m) is attained, but north-west of Ullscarf the outcrop narrows significantly as the thickness decreases eastwards towards the Coniston Fault. Adjacent to this fault, on the western side of Thirlmere the formation is no more than 40 m thick. It is not apparent east of the Coniston Fault. The lower part of the Airy's Bridge Formation comprises the Long Top Member (LTT), whereas the upper part consists of the Crinkle Member (Crk) with the impersistent Bad Step Tuff (Bdp) at the base (Figure 5). In places, the stratigraphical position of the Bad Step Tuff is occupied by up to 15 m of eutaxitic lapilli-tuff (ZR), which is considered to be a distal equivalent. The parataxitic foliation, a distinctive feature of the Crinkle Member, is defined by abundant high-aspect-ratio (up to 200:1) fiamme, and is commonly steeply dipping or vertical due to rheomorphism, particularly near faults. The rheomorphism, along with intercalated mesobreccias, probably formed in response to syndepositional fault movement associated with the piecemeal subsidence of the Scafell caldera.

The Lingmell Formation (Lme) consists of partly reworked pyroclastic rocks and locally occurring lavas associated with the waning phase of caldera development (Figure 5). It rests conformably on the Airy's Bridge Formation and thins from about 75 m in Borrowdale, eastwards towards the Coniston Fault where it wedges out. The Rosthwaite Rhyolite (RsR), and its associated intrusion, is confined to Rosthwaite Fell [NY 256 118], and the Scafell Dacite (ScD) crops out in the extreme south of the district between Kirk Fell [NY 195 105] and Green Gable [NY 215 107].

The Seathwaite Fell Formation (Set) consists of subaqueously deposited volcaniclastic sandstone, similar to that shown in (Plate 3), with intercalations of pebbly sandstone, breccia and tuff (Figure 6). There are a number of high-level basaltic andesite and andesite sills. In Borrowdale, eastwards as far as Thirlmere, the formation rests conformably on the Lingmell Formation, but east of the Coniston Fault, it rests directly on the Birker Fell Formation. West of the Coniston Fault, the Seathwaite Fell Formation is preserved within the Scafell caldera, but the presence of strata farther east is evidence that deposition was more widespread. The volcaniclastic sandstones include many graded beds, interpreted as wave-reworked turbidite and/or pyroclastic fall deposits. There are scattered occurrences of ripple cross-lamination that indicate south-south-east to north-east-directed palaeocurrents. Some of the pebbly sandstone and breccia intercalations are designated as members (Figure 6). The Cam Crags Member (CCr) occurs in the lowest part of the formation across Borrowdale and it persists as far east as Ullscarf where it wedges out. Farther north, the more impersistent Bell Crags Member (BCr) lies at a similar stratigraphical level. The formation is mostly undivided above these members, but a persistent unit of pebbly sandstone and breccia in the middle part of the formation constitutes the Pavey Ark Member (Pav). This crops out in Borrowdale and north-eastwards as far as Bell Crags, adjacent to which it wedges out. It also crops out south of Wythburn. Between Borrowdale and Bell Crags, some of the breccia in the lower part of the member contains highly irregular vesicular andesitic pyroclasts, which are considered to have been fluid when incorporated in the deposit. The presence of these pyroclasts has formed the basis of the overall interpretation of the Pavey Ark Member as the deposits of eruption-related subaqueous 'gravity-flows' (Kneller and McConnell, 1993). The uppermost part of the formation includes the Glaramara Tuff (Gmt). This crops out extensively, and attains its maximum thickness, on Glaramara [NY 246 105].

The Lincomb Tarns Formation (LTa) consists mostly of dacitic to rhyodacitic ignimbrite, the eruption of which was probably accompanied by renewed caldera development, but in this case mainly to the east of the Coniston Fault. The formation crops out around southern and northern parts of Thirlmere, either side of the Coniston Fault. South-west of Thirlmere, in outcrops that extend from the Wythburn Fells south-westwards to Ullscarf and Glaramara, up to 400 m of mostly undivided ignimbrite is preserved within the Scafell caldera and rests on the Seathwaite Fell Formation. The contact is unconformable due to ignimbrite emplacement on an irregular low-relief erosion surface. In the extensive outcrop on the Helvellyn range, immediately east of the Coniston Fault, the Lincomb Tarns Formation is in places at least 580 m thick; the base is either faulted or complicated by andesite sills. A number of members are newly recognised as a result of this resurvey (Figure 6). The Tarn Crags Member (TCr) is impersistent at the base. The Thirlmere Member (Thl) dominates the lower part of the formation and in many places closely resembles the Crinkle Member of the Airy's Bridge Formation. It displays evidence of rheomorphism and includes intercalations of mesobreccia. The member is also present in small outcrops immediately west of the Coniston Fault, at Wythburn, where it rests either on the Tarn Crags Member or directly on the Seathwaite Fell Formation. The Raise Beck Member (RBk) overlies the Thirlmere Member immediately south and east of Thirlmere, but is not apparent west of the Coniston Fault. The volcaniclastic sandstone of this member was probably deposited during a hiatus in the volcanism. The Lincomb Tarns Formation above the Raise Beck Member consists of up to 275 m of undivided, dacitic lapilli-tuff. Around the northern part of Thirlmere, the formation is dominated by the Thirlmere Member, which mostly rests on the Birker Fell Formation. The Tarn Crags Member crops on and adjacent to Great How [NY 313 187] where it is exceptionally thick (280 m).

The Esk Pike Formation (EsP) consists of volcaniclastic sedimentary and pyroclastic rocks (Figure 6) that rest on the Lincomb Tarns Formation. The contact is most distinct where it is an erosion surface on the under- lying ignimbrite. The sedimentary rocks are mostly sandstones that closely resemble those of the Seathwaite Fell Formation. The formation crops out to the south-west and east of the southern part of Thirlmere, on either side of the Coniston Fault (Figure 2). Those to the south-west are outliers preserved in the Scafell Syncline, but the more extensive outcrops to the east are mostly preserved in a depression of probable volcanotectonic origin and separated from the syncline by the Coniston Fault. The Birkhouse Moor and Hogget Gill faults define the northern and eastern margins of the volcanotectonic depression (Figure 2). This structure is interpreted as a caldera that formed during eruption of the Lincomb Tarns Formation, but which experienced further subsidence during deposition of the Esk Pike and later formations. Lenticular units of mass-flow deposited pebbly sandstone and breccia occur within the Esk Pike Formation; these attain maximum thickness and grain size adjacent to major faults, notably the Browncove Fault, suggesting fault controlled sedimentation. Contempora-neous volcanism is indicated by intercalations of tuff and lapilli-tuff, some of which was emplaced as ignimbrite, and basaltic andesite and andesite lavas (as well as sills) up to 150 m thick.

The Middle Dodd Dacite (MdD) consists of a number of separate lava flows that rest conformably on the Esk Pike Formation (Figure 7). These crop out in the extreme south-east of the district (Figure 2). South of the Hogget Gill Fault, lava is preserved as outliers, whereas farther north other flows are preserved beneath younger strata.

The Helvellyn Formation (Hlv) consists of dacitic ignimbrite preserved within the volcanotectonic depression bounded by the Coniston, Birkhouse Moor and Hogget Gill faults (Figure 2),(Figure 7), and rests on the Esk Pike Formation. The contact is sharp and in places is either a low-relief erosion surface or is highly irregular due to loading. The maximum thickness of 400 m is attained between Helvellyn and Grisedale, but the formation thins southwards to 200 m along the eastern side of Grisedale, 130 m at Fairfield, and to 80 m in Rydal Head.

The Deepdale Formation (Dpd) closely resembles the Seathwaite Fell and Esk Pike formations in that it consists of subaqueously deposited volcaniclastic sandstone (Plate 5), with intercalations of pebbly sandstone, breccia, and pyroclastic rocks (Figure 7). There are a number of andesite sheets mostly emplaced as high-level sills, but a few are possibly lavas. The formation is preserved in the volcanotectonic depression bounded by the Coniston, Birkhouse Moor and Hogget Gill faults. It is present only as small outliers west of the Deepdale Hause Fault but crops out extensively to the east (Figure 2). It rests on the Helvellyn Formation, except in the extreme east where there is overstep onto the Middle Dodd Dacite and the Esk Pike Formation. The basal contact is most distinct where it rests on a fissured, high-relief erosion surface on the Helvellyn Formation. The most prominent intercalations of pebbly sandstone, breccia and pyroclastic rocks form members (Figure 7). The basal Cawk Cove Member (Cwk) crops out along Deepdale and adjacent to Patterdale, and is made up of detritus derived from the Helvellyn Formation. The Blind Cove Member (Bld) lies in the middle part of the formation. It crops out extensively north of Deepdale where it is directly overlain by dacitic ignimbrite of the St Sunday Crag Member (SSC). South of Deepdale, this ignimbrite is absent and the Blake Brow Member (Blk) overlies the Blind Cove Member. They are usually separated by an andesite sheet, which is possibly in part extrusive. The Cockley How Member (Cky) is a dacitic ignimbrite that it is geochemically distinct from that of the St Sunday Crag Member. It crops out only to the south and east of Deepdale, where its stratigraphical relationship with the Blake Brow Member is uncertain. The Dove Crag Member (Dvd) forms outliers on Hart and Dove crags [NY 370 111] and [NY 374 106], where it rests on an erosion surface on bedded sandstone. This member is the highest preserved part of the Deepdale Formation.

Contemporaneous intrusions (A) in the Borrowdale Volcanic Group consist of high-level sills of basaltic andesite, andesite and dacite. Many were emplaced into unconsolidated volcaniclastic rocks, with the formation of peperitic margins (Branney and Suthren, 1988), but the distinction between lavas and sills remains uncertain in many parts of the Borrowdale Volcanic Group succession. This is particularly the case with the Birker Fell Formation. The numerous andesite sheets within the volcaniclastic Eagle Crag Member of that formation, together with similar sheets in the overlying Whorneyside Formation are interpreted as sills. Similar sheets, some with peperitic margins occur sporadically throughout the upper part of the Borrowdale Volcanic Group, particularly within the Seathwaite Fells, Esk Pike and Deepdale formations.

Devonian and Carboniferous

Devonian rocks are represented by the Mell Fell Conglomerate (MFC) (Wadge in Moseley, 1978), which crops out in the extreme north-east of the district (Figure 2), mostly on Great Mell Fell [NY 400 255]. The conglomerate rests unconformably on the Skiddaw and Borrowdale Volcanic groups. No fossils have been recorded, but a Middle to Late Devonian age is preferred because the conglomerate is lithologically distinct from the basal Carboniferous strata, and because it rests on strata deformed during the Early Devonian Acadian phase of the Caledonian Orogeny. The conglomerate is polygenetic; clasts of greywacke sandstone derived from the Windermere Supergroup predominate, but there are some of limestone, calcite, volcanic rock and feldspar.

Carboniferous rocks consist of the Chief Limestone Group (Seventh Limestone: LM7), which unconformably overlies the Eycott Volcanic Group, and locally the Mell Fell Conglomerate, in the extreme north-eastern part of the district (Figure 2). The limestone is about 75 m thick, pale grey and bedded, with interbedded calcareous mudstone, siltstone and cross-bedded sandstone. It is a continuation of basal marine Lower Carboniferous (Dinantian) rocks present in the adjacent Cockermouth district (Eastwood et al., 1968).

Intrusive igneous rocks

Granitic intrusions of late Ordovician and Early Devonian age at Ennerdale, Threlkeld and Skiddaw are high-level components of the Lake District batholith, which is otherwise present at shallow depths throughout the district (Figure 8). The Ennerdale intrusion is the most extensively exposed, and has intruded the Skiddaw Group and the lowest part of the Borrowdale Volcanic Group around Ennerdale Water (Figure 2). The contact with the Skiddaw Group appears to be steep sided, but seismic reflection data reveal that the overall form of the intrusion is a sheet-like mass about 1100 m thick (Evans et al., 1993). It consists of granophyric microgranite or granite (gG), for which isotopic ages indicate a late Ordovician age (Hughes et al., 1996). The laccolithic Threlkeld microgranite (FG) is also of late Ordovician age (Rundle, 1981). It crops out on Low Rigg [NY 305 230], Threlkeld Knotts [NY 328 238], Bramcrag [NY 320 220] and around White Pike [NY 339 229], and has likewise intruded the lowest part of the Borrowdale Volcanic Group as well as the Skiddaw Group. A small outcrop of biotite granite (GG), with phenocrysts of coarse perthite up to 5 cm long, at Sinen Gill [NY 229 281], is the only exposed part of the Skiddaw intrusion within the district. Isotopic ages indicate an Early Devonian age (Rundle 1992). A high-level component of the Lake District batholith was intruded along the Causey Pike Fault during the Early Devonian but remains concealed. An elongate zone of contact metamorphism, known as the Crummock Water Aureole indicates the presence of this intrusion. There are numerous minor intrusions within the Skiddaw Group, in the form of dykes, sills and small plugs of strongly altered lamprophyre (L), basalt (B), dolerite (D), gabbro (E), andesite (A), diorite (H), rhyolite (R), microgranite (FG) and hornblendite (UA). Basalt and andesite dykes are widely scattered throughout the Borrowdale Volcanic Group and many probably acted as feeders to the various lavas and high-level sills in the group. Most of these minor intrusions are believed to be of Ordovician age, but a few are known or suspected to be of Early Devonian age. The latter include altered lamprophyre dykes in the Skiddaw Group on Sale Fell [NY 193 297], from which biotite has yielded an Early Devonian isotopic age (Rundle, 1979). In the Borrowdale Volcanic Group around Thirlmere, there are a few microgranite dykes, including the well-known Armboth dyke, that are not cleaved, unlike the country rocks, and therefore an Early Devonian or younger age is suspected.

Metamorphism

Contact metamorphism by the Ennerdale intrusion has produced a distinct aureole within the Skiddaw Group between Ennerdale Water and Buttermere (Figure 2). The rocks contain biotite, together with secondary carbonate, and incipient spots of cordierite and/or andalusite. In the Borrowdale Volcanic Group a less distinct aureole is defined by the presence of biotite and/or horn-blende, with or without horn-felsing. A spaced fracture cleavage cutting the hornfels indicates that the metamorphism, and therefore the intrusion, predated the Acadian phase. The aureole of the Skiddaw intrusion spans the Keswick and adjoining Cockermouth districts (Figure 2). The rocks in this case contain andalusite, cordierite and biotite, with chiastolite and spotting in the outermost part of the aureole. The Crummock Water aureole is a highly elongate zone of metasomatised mudstone and siltstone (Cooper et al., 1988) (Figure 2), (Figure 8). The rocks are bleached as a result of carbon loss, but the overall mineralogy is little affected by this and other geochemical changes. However, small aggregates and porphyroblasts of white mica and chlorite have formed, and there has been tourmaline veining. The aureole postdates the regional (Acadian) cleavage, and isotopic ages indicate that it is associated with Early Devonian magmatism.

Regional and burial metamorphism of the Skiddaw Group, as indicated by white-mica (illite) crystallinity, has produced grades ranging from diagenetic, through the anchizone, into the lower epizone (Fortey, 1989). Metamorphism of the Borrowdale Volcanic Group (Meller, 1998) has overprinted an earlier hydrothermal event that resulted in the deposition of chalcedony in vesicles and veins. Burial metamorphism in the volcanic rocks is defined by calc-silicate and calcite-hosted secondary mineral assemblages. A maximum grade of prehnite-actinolite to actinolite-pumpellyite facies was reached during the Devonian immediately prior to the Acadian phase. Regional metamorphism of the volcanic rocks associated with this orogeny produced tectonically aligned white mica. Pervasive carbonate in the rocks and as veins is either coeval with the regional metamorphism, or due to a later postmetamorphic event.

Structure

The structure of the Keswick district consists of extensive folding, a regional cleavage, and many faults (Figure 2). Most structures formed in response to Acadian phase deformation during the Early Devonian (Soper et al., 1987), but pre-Acadian structures are apparent in the Skiddaw and Borrowdale Volcanic groups, and some post-Acadian deformation is suspected.

Pre-Acadian deformation is marked by syndepositional, soft-sediment folding, and the development of an olistostrome, in parts of the Skiddaw Group (Stone et al., 1999, and references therein), and regional basin extension and volcanotectonic faulting in the Borrowdale Volcanic Group (Branney and Soper, 1988; Branney and Kokelaar, 1994). Many of the faults in the Borrowdale Volcanic Group have been interpreted as volcanotectonic structures on the basis of closely associated thickness and facies changes, local angular unconformities, gravity collapse structures, and ductile deformation structures in ignimbrites. The north–south Coniston Fault was an important basin extension structure during deposition of the Borrowdale Volcanic Group. It appears to mark the eastern limit of the products, and area affected by collapse, of the Scafell caldera. An increase in thickness of the Seathwaite Fell Formation westwards across the fault is consistent with syndepositional dip-slip movement with downthrow of the western side. However, the juxtaposition of the Seathwaite Fell and Lincomb Tarns formations, with the latter on the east side, indicates that there was at least some downthrow to the east. The youngest formations of Borrowdale Volcanic Group east of the fault are preserved in a broad fault-bounded basin. This is probably at least partly volcanotectonic, and may be the remnants of a caldera that formed during the eruption of the Lincomb Tarns Formation ignimbrite. The Birkhouse Moor and Hogget Gill faults along the basin margin (Figure 2) have experienced hundreds of metres of downthrow to the south and north-west respectively. Within the basin, a number of major north–south to north-east–south-west-orientated faults are associated with thickness and facies variations within the Esk Pike, Helvellyn and Deepdale formations, which indicate that these faults are also syndepositional structures.

Acadian deformation of the Skiddaw Group is marked by north-east-trending folds (Figure 2) with a regional, axial planar cleavage. The folds have amplitudes up to hundreds of metres, are gently plunging, steeply inclined, and range from open to isoclinal. The cleavage fabrics vary from penetrative, slaty and pressure solution types, to spaced fractures. Crenulation cleavages and related folds were produced during later stages of the Acadian event, and some are related to a set of south-directed thrust faults; the Watch Hill, Loweswater, Gasgale and Causey Pike thrusts (Figure 2). The Early Devonian Skiddaw intrusion postdates the regional cleavage, but predates the later crenulation cleavages. The largest of the south-directed thrust faults forms part of the Causey Pike Fault, which is an important crustal-scale feature. However, bedding and cleavage are sigmoidally deformed across the Causey Pike Fault suggesting sinistral movement of the fault during the later stages of the Acadian deformation, possibly synchronous with development of the Crummock Water Aureole. Reversed movement on the Causey Pike Fault postdates the aureole. Acadian deformation of the Borrowdale Volcanic Group developed a north-east-trending regional cleavage, and probably reactivated at least some pre-existing faults. The Coniston Fault was reactivated during this period of deformation and experienced strike-slip movement (Moseley, 1993). Acadian deformation tightened the caldera depressions and sedimentary basins into open synclines, for example the Scafell Syncline (Figure 2).

Post-Acadian deformation, referred to as Hercynian by Soper and Moseley (in Moseley, 1978), was mostly end-Carboniferous (Saalian), and involved reactivation of Acadian structures, notably the north-west 'wrench' faults. It is suspected that post-Carboniferous and post-Triassic dip-slip movement took place along the Coniston Fault.

Mineralisation

The Lower Palaeozoic rocks of the Keswick district host a wide variety of epigenetic mineral veins, and genetic relationship between the Lake District batholith and the distribution of vein mineralisation has been advocated (Firman in Moseley, 1978). There is no uniformity of vein orientation, but many copper veins are orientated east–west, whereas the lead-zinc veins are commonly nearly north–south. The nature and extent of mineralisation is summarised in (Figure 9). The age is based mainly on a genetic classification of Lake District mineralisation by Stanley and Vaughan (1982). However, more recent field data strongly suggest that some copper mineralisation is of late Ordovician age in that it predates the regional cleavage-forming event in the Early Devonian (Acadian phase) (Millward et al., 1999). There is a possible genetic link to the final phases of the Caradoc magmatism, on the basis of mineralisation style and its relationship with the volcanic rocks.

At Force Crag there is evidence for vertical zonation of the mineralisation; abundant baryte with some manganese oxides and relatively scarce lead-zinc sulphides gives way downwards to more sulphide-rich lead-zinc mineralisation with very little baryte. At Greenside there is similar zonation, along with an increase in chalcopyrite in the sulphide assemblage at depth. The metals in these veins may have been derived from Lower Palaeozoic sedimentary rocks as well as from basement granites, possibly in part due to convective leaching by Carboniferous sea water.

The graphite deposit near Borrowdale is associated with a basalt intrusion in the Borrowdale Volcanic Group. The origin of the graphite has been explained in terms of the catalytic reduction of large volumes of carbon monoxide or by derivation from deeply buried carbonaceous rocks, but no wholly satisfactory mechanism for these processes has been identified. The age of the graphite mineralisation is uncertain, but an Early Devonian age is suspected.

Concealed geology

The district is traversed by a number of east-north-east-trending geophysical lineaments, of which the Crummock (gravity) and Ullswater (gravity and magnetic) lineaments are the most prominent (Lee, 1989) (Figure 8). These probably mark fundamental basement fractures, which were initiated prior to the erosion of the Borrowdale Volcanic Group, and influenced both the structural development of the group and of the intrusive form of the Lake District batholith. The latter is a key component of the concealed geology of the central part of the Lake District. Detailed interpretations of regional gravity and aeromagnetic data (Lee, 1989 and references therein) form the basis for the configuration of the batholith as shown in (Figure 8) and in the two cross-sections on the geological map sheet 29 Keswick.

Quaternary

Pre-Late Devensian deposits consist mainly of till, the Thornsgill Formation of Boardman (1991), which crops out at several localities in the valleys of Mosedale and Thornsgill becks in the north-eastern part of the district (Figure 10). It is not distinguished from younger till on sheet 29 Keswick solid and drift map. The older till rests on the Skiddaw Group; it is up to 14 m thick, intensely weathered, and contains erratics derived from the west. Microfabrics suggest ice movement in an east-north-east direction (Boardman, 1991). The older till underlies a pre-Late Devensian peat bed, compressed beneath younger till.

Widespread deposits of younger till, although not dated, are probably products of the Dimlington Stadial. This till, which probably equates with the Threlkeld Till of Boardman (1991), is typically a relatively impermeable, unsorted deposit in which rock debris ranging in size from sand to boulders occurs in a matrix of clay, silty clay or silt. It includes lenticular deposits of glaciofluvial sand and gravel. Till commonly underlies featureless ground, but in places it forms low drumlins elongated parallel to the ice-flow direction. The major ice-flow directions are shown in (Figure 10). The overall direction of ice movement was from south to north, but east of Keswick it moved north-eastwards, constrained by the Skiddaw–Blencathra massif, before turning northwards. Some of the highest mountains in the district may have remained just above the maximum altitude of the Dimlington Stadial ice sheet at its maximum thickness (Lamb and Ballantyne, 1998).

Conspicuous landforms and glacigenic deposits of the Loch Lomond Stadial are confined to the heads of north- and east-facing valleys. The landforms consist of corries, partly bounded by steep slopes and cliffs. Moraines, made up of unsorted cobble and boulder gravel, and linear ridges of till were deposited from a series of valley-head glaciers (Plate 6), first identified throughout the Lake District by Sissons (1980).

Silt, sand and gravel deposited during the Flandrian (Holocene) forms alluvium beneath present-day floodplains, river terraces, numerous alluvial fans and debris cones, and lacustrine deposits. Peat covers many smooth upland areas, and occurs locally in some lowland areas. Alluvial fans occur downstream from abrupt changes in stream gradient, and are common where streams meet the alluvial plains of larger watercourses. Pollen assemblages have been recovered from organic deposits, which underlie an alluvial fan in the Seathwaite valley. They indicate that human-induced vegetation changes, possibly following settlement of the area between about 900 and 1000 AD, immediately preceded fan development.

Chapter 3 Applied geology

The Keswick district lies almost entirely within the boundaries of the Lake District National Park, designated as such in 1951, which altogether consists of 2280 km² of upland of outstanding natural beauty. Hill farming is the principal land use, but tourism now dominates the local economy. There has been a long history of mining (Figure 9) and slate quarrying, and the aftermath of these activities remains, in the form of mine entrances, waste tips and derelict buildings in many parts of the district.

Mineral resources

Metalliferous minerals exploited in the district consist of mainly of lead-zinc and copper ores (Figure 9). The earliest records of mining date back to the sixteenth century (Figure 9), but at Goldscope there is evidence of workings that may be several hundred years earlier (Adams, 1995). Copper mining declined in the seventeenth century as no new sources were discovered, and lead mining declined towards the close of the nineteenth century. The decline of lead mining was partly offset by the rising demand for industrial minerals such as baryte. Mining activity peaked during the late eighteenth and nineteenth centuries. Lead, zinc and baryte were extracted from the Force Crag Mine, but between 1984 and 1992, a combination of poor market conditions, problems with the processing plant and eventual collapse of the lowest mine level forced the operators to abandon the mine in March 1992. Greenside Mine operated almost continuously over a period of 200 years, and was the most successful of all the Lake District metal mines. According to Adams (1995) it yielded a total of about 2 400 000 tons (2 438 400 tonnes) of ore and 2 000 000 ounces (56.7 tonnes) of silver, all from a single vein. The last ore was extracted in April 1961, and the mine was abandoned in 1962. Waste products from some of the numerous abandoned mine workings throughout the district may cause pollution where water tables are high or flooding is likely. Many old workings may be unrecorded, and details of many others are probably incomplete.

Nonmetalliferous minerals exploited within the district consist of baryte and graphite. The former was not a saleable commodity until the late 1860s (Adams, 1995). Economic quantities were extracted from the Force Crag Mine, but although abundant in the upper workings at Greenside, it was not of commercial significance. The graphite mine at Seathwaite gave rise to the Keswick pencil industry in about 1790. However, this was late in the mine's history, because workings are known to date back to 1555 (Adams, 1995).

The main construction materials available in the district consist of slate, which has been extracted since at least as early as 1870 from an extensive system of surface and underground quarries on either side of the Honister Pass [NY 210 135] to [NY 224 143]. The slate consists of green, cleaved volcaniclastic rocks of the Eagle Crag Member (Birker Fell Formation). Only Honister quarries [NY 215 140] have been recently worked. At the beginning of the twentieth century most of the slate was used for roofing purposes, but by the Second World War the use of tiles predominated and slate extraction declined. Since about 1950 the best slate has been used either for monumental (decorative) or architectural purposes, but the latter has declined since about 1970. There is potential use of mine spoil as a crude aggregate, particular for sub-base and farm-track construction. Areas of usable spoil exist at a number of mines, for example Barrow and Yewthwaite. Microgranite was extracted from Threlkeld and Bramcrag quarries for use as aggregate. Quarrying ceased in 1980, and Threlkeld Quarry is now the site of a mining museum. Building stone has been worked locally from small quarries, and has been derived from glacial erratics. It has been used in some buildings, but mostly in the construction of many miles of dry stone walls. Sand and gravel occurs within the glacial and post-glacial deposits of the district, but no large extensive deposits are known, and none have been worked commercially.

Energy resources

Although there is a variable thickness of blanket peat across much of the district, none appears to have been exploited for fuel or horticultural use.

The Lake District batholith has been investigated as a potential hot dry rock geothermal resource, because of its size, and possible above-average content of the main heat-producing radioactive-elements, uranium, thorium and potassium (Lee, 1986). Measurements in boreholes on the Skiddaw and Shap granites, (Cockermouth and Kendal districts respectively) showed heat-flow values above the UK average. No heat-flow measurements were made in the Keswick district. The Skiddaw and Shap granites are limited in surface extent, and therefore the measurements are probably not representative of the rest of the batholith. Heat-flow over the central Lake District is possibly significantly above the national average, but more borehole measurements are needed to confirm this.

Water resources

The mountainous terrain of the district equates with a high annual rainfall (about 2500 mm) and consequently there is abundant surface water in the form of lakes and rivers. However, the bedrock generally has low permeability and intergranular porosity, with groundwater flow and storage restricted to joints and fractures in weathered rock and along fault planes (Patrick, in Moseley, 1978). Consequently there are no regionally significant aquifers in the district, most water supplies being taken from the surface sources. Recharge to bedrock aquifers is restricted by a combination of poorly permeable rock, clay-rich drift cover and steep mountain slopes. The annual infiltration rate is estimated at less than 40 mm (Patrick, in Moseley, 1978). Groundwater is also present in superficial deposits within the narrow glaciated valleys, in river gravels and other granular material. These sources often have a shallow water table and can transmit larger volumes of groundwater compared with bedrock. Hillside till is another source of groundwater from springs.

The overall volume of groundwater present in the district is low, but it is still an important source of supply for many houses. British Geological Survey well records include 78 spring supplies in use, 35 shallow wells and 2 boreholes. Most sources in use are located on the lower slopes of hillsides in valleys close to dwellings, but there are many other springs present in the area that are too remote for exploitation. Spring supplies are in use particularly around Bassenthwaite Lake and to the east of Keswick.

Thirlmere reservoir was created from a pre-existing lake, the level of which was raised following the construction of a 16.5 m high masonry dam at the northern end. The 4.8 km long reservoir holds 41 000 million litres, when full, and is now owned by the North West Water Authority. The water is carried by gravity in a 'cut and cover' aqueduct from Thirlmere to Manchester, a distance of 155 km, and is treated south of Dunmail Raise. It is also used in conjunction with other water sources to supplement supplies to Cumbria and central Lancashire.

Waste disposal

There are at present no operational landfill sites, for waste disposal, in the Keswick district. A site at Town Cass, Keswick [NY 261 229] was operational until January 1987, and a wide range of domestic, commercial and industrial wastes was deposited. The site was reinstated in May of that year. Former landfill sites at Thornthwaite Mine [NY 224 259], Bog House [NY 238 245], Threlkeld Quarry [NY 329 246], Rannerdale Knott [NY 165 185], Holmcragg Wood [NY 251 172], Mill Moss [NY 396 157] and Rosthwaite Bridge [NY 256 149] were operational prior to 1976. However, the type and quantity of waste deposited on these sites is not known.

Foundation conditions

There is only a small amount of geotechnical data available for the district. The Skiddaw Group mudstones weather readily to form a weak material consisting of slabs and prisms. The weathering of these rocks may result in slope instability (see below) and maintenance problems, particularly in cuttings. The strength of the Borrowdale Volcanic Group rocks is reduced by the presence of small-scale veins filled with weaker haematite, calcite and dolomite. The porosity of the intact volcanic rocks is typically very low with values less than 1 per cent, but this may vary across formations.

Till throughout the district varies widely in its geotechnical properties, which reflect variations in the proportion of clay/silt to cobble and boulder-sized clasts. Site investigations for the Keswick northern by-pass revealed till which presented problems in construction. These included resilience against compaction and susceptibility to water content change when used as a fill, and unpredictable groundwater conditions in cuttings.

There have been a number of landslides in the district but in most cases their age and mechanisms of formation are unknown. In hard rock areas most are rock-falls. The largest landslide is on Buttermere Fell, at Gatesgarth [NY 204 156]. This covers an area of about 200 hectares and is a deep-seated landslide in the Buttermere Formation, possibly of interglacial origin, caused by the presence of down-slope dipping, east–west faults, and glacially over-steepened slopes. Smaller landslides occur east of Keswick, on the south- east side of Latrigg [NY 285 247] and [NY 282 242], along Glenderaterra Beck [NY 262 298], and at 15 other locations. About half of these involve till and other Quaternary deposits.

Seismicity

There were earthquakes within the district on 6 July 1787 and 9–14 July 1901(Musson et al., 1984). The former was felt over an area of about 500 km² including Threlkeld and Penrith, and an epicentre just north of Helvellyn is suspected. The 1901 events were possibly aftershocks of the Carlisle earthquake on 9 July of that year. Events in adjacent districts were felt widely throughout the central Lake District; for example the Whitehaven earthquake in 1786, and the Carlisle earthquakes of 1901, 1915, 1979 and 1980.

Conservation

Localities around the town of Keswick have been recognised, by English Nature, as being of national importance, providing valuable Earth science teaching sites. These have received new designations as SSSI (Site of Special Scientific Interest) and RIGS (Regionally Important Geological/ Geomorphological sites), and consequently have official recognition and some degree of protection from development or damage. There are nine SSSI and forty RIGS sites in the Keswick district. There are eleven sites within the district described in the Geological Conservation Review series volumes published by the Joint Nature Conservation Committee. Five are concerned with geological structures, and six with volcanic and intrusive igneous rocks.

Information sources

Further geological information held by BGS relevant to the Keswick district is listed below. This includes published maps, memoirs and reports, along with open-file maps and reports. Other sources include borehole records, mine plans, fossils, rock samples, thin sections, hydrogeological data and photographs.

Searches of indexes to some of the collections can be made on the Geoscience Index System in BGS libraries. This is a developing computer-based system, which carries out searches of indexes to collections and digital databases for specified geographical areas. It is based on a geographical information system linked to a relational database management system. Results of the searches are displayed on maps on the screen. At the present time (1999) the data sets are limited and not all complete. The indexes which are available are listed below:

• index of boreholes
• topographical backdrop based on 1:250 000 scale map
• outlines of BGS maps at 1:50 000, 1:10 000 and 1:10 560 scale and County series maps
• chronostratigraphical boundaries and areas from BGS 1:250 000 maps
• geochemical sample locations on land, aeromagnetic and gravity data recording stations and land survey records

Details of geological information available from the British Geological Survey can be accessed from the BGS Home Page at http://www.bgs.ac.uk.

Maps

• Geological maps
• 1:625 000
• United Kingdom (North sheet) solid geology, 1979; Quaternary geology, 1977.
• 1:250 000
• Sheet 54N 04W Lake District, solid geology, 1980
• 1:50 000
• Sheet 22 Maryport, solid geology, solid and drift geology, 1980, 1995E
• Sheet 23 Cockermouth, solid geology, 1977, solid and drift geology, 1975
• Sheet 24 Penrith, solid and drift geology, 1974
• Sheet 38 Ambleside, solid geology, 1996, solid and drift geology, 1998
• Sheet 37 Gosforth, solid geology, 1998
• Sheet 28 Whitehaven, solid geology, in press
• 1:25 000
• Sheet NY12 Lorton and Loweswater, solid and drift, 1990.
• 1:10 000 and 1:10 560
• Details of the original geological surveys are given on the 1:50 000 geological sheets. Copies of the fair-drawn maps of these early surveys are available for public reference at the BGS Library, Edinburgh. The maps at six-inch or 1:10 000 scale covering all or part of 1:50 000 geological sheet 29 Keswick are listed below, along with the surveyors' initials, dates of survey and Technical Report number. The surveyors were: P M Allen, R S Arthurton, R P Barnes, B Beddoe-Stephens, S D G Campbell, A H Cooper, N C Davis, D J Fettes, R A Hughes, B C Kneller, S C Loughlin, B J McConnell, J W Merrit, D Millward, M G Petterson, D E Roberts, G J Roycroft, P Stone, B C Webb, D G Woodhall and B Young. The maps are not published, but are available for public consultation in the BGS libraries in Edinburgh and Keyworth, and can also be consulted at the London Information Office in the Natural History Museum, South Kensington. Uncoloured dyeline copies are available for purchase from BGS sales desk.

• Geophysical maps
• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1997
• Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1998
• 1:625 000
• Aeromagnetic map of Great Britain (and Northern Ireland), South sheet, 1965
• Bouguer anomaly map of the British Isles, Southern sheet, 1986
• 1:250 000
• 54N 04W Lake District; Aeromagnetic anomaly map, 1977, Bouguer gravity anomaly map, 1986
• 1:50 000
• Geophysical information maps (GIM) are available on a plot-on-demand basis for all 1:50 000 scale geological sheets in the United Kingdom. These summarise graphically the publicly available geophysical information held for each sheet in the BGS databases. Features include:
• regional gravity data: Bouguer anomaly contours and location of observations
• regional aeromagnetic data: total field anomaly contours and location of digitised data points along flight lines
• gravity and magnetic fields plotted on the same base map at 1:50 000 scale to show correlation between anomalies
• separate colour contour plots of gravity and magnetic fields at 1:125 000 for easy visualisation of important anomalies
• location of local geophysical surveys
• location of public domain seismic reflection and refraction surveys
• location of deep boreholes and those with geophysical logs
• Geochemical atlases
• 1:250 000
• The Lake District, 1992
• The geochemical Baseline Survey of the Environment (G-Base) is based on the collection of stream sediment and stream water samples at an average density of one sample per 1.5 km2. The fine (minus 150 µm) fractions of stream sediment samples are analysed for a wide range of elements, using automated instrumental methods.
• The samples from the Lake District were collected in 1978–80. The results are published in atlas form (British Geological Survey, 1992) and include Ag, As, Ba, Be, Bi, B, CaO, Cd, Co, Cr, Cu, Fe2O3, Ga, K2O, La, Li, MgO, Mn, Mo, Ni, Pb, Rb, Sb, Sn, Sr, TiO2, U, V, Y, Zn and Zr in stream sediments, pH, conductivity, fluoride, bicarbonate and U for stream waters. The geochemical data, with location and site information, are available as hard copy for sale or in digital form under licensing agreement. The coloured geochemical atlas is also available in digital form (on CD-ROM or floppy disc) under licensing agreement. BGS offers a client-based service for interactive GIS interrogation of G-BASE data.
• Hydrogeological maps
• 1:625 000
• England and Wales, 1977

Books and reports

The various memoirs, books, reports and papers relevant to the Keswick district are listed in the reference section. A more comprehensive reference list is included in the sheet description (Woodhall, 1999) and also in the current BGS Catalogue of geological maps and books. More details of the general geology can be found in BGS Technical Reports covering the geology of individual, part, or combined 1:10 000 scale geological sheets. BGS Technical and other reports may be purchased from BGS or consulted at the BGS and other libraries. More information on mineral resources can be found in the memoir by Eastwood (1921), now out of print but available for library consultation, and in a Technical Report WA/LD/84/1. Minerals present in the district are given in the glossary of Lake District minerals by Young (1987), which is available from the British Geological Survey and other bookshops. There is a collection of internal British Geological Survey Biostratigraphical Reports details which are available from the Biostratigraphy Group in the Keyworth office. An analysis of satellite imagery (Landsat) also appears in a Technical Report (WA/91/30). Popular publications consist of a 1:200 000 scale full colour satellite image poster for the Lake District and surrounds, a Holiday Geology Map Guide (Lake District) published in 1997, and a Holiday Geology Guide (The Lake District Story) which is in press. A multimedia CD-ROM, entitled Discovering geology: the Lake District, is available for Mac/PowerMac and IBM compatible computers; it includes geochemical and mineral distribution maps.

Documentary collections

Collections of records of borehole and site investigations, relevant to the Keswick district, are available for consultation at the BGS, Edinburgh, where copies of most records can be purchased. The collection consists of the sites and logs of about 110 boreholes. Index information, including site references, for these boreholes have been digitised. The logs are either hand-written or typed and many of the older records are drillerís logs. BGS maintains a mining and quarrying dataset; for the Keswick district there are notes and/or plans for 18 mines (including Force Crag and Greenside) and one quarry (Threlkeld).

Material collections

More than 100 Geological Survey photographs illustrating aspects of the geology of the Keswick district are deposited for reference in the BGS libraries at Edinburgh and Keyworth. Sheet albums of the more recent photographs are also held in the BGS Information Office in London. The photographs depict details of the various rocks exposed, both naturally or in excavations, and also some general views. Copies of the photographs can be purchased as black and white or colour prints, and 35 mm transparencies, at a fixed tariff, from the Photographic Department, BGS, Edinburgh.

There are petrological and palaeontological collections for the Keswick district. The former has more than 500 rock specimens and thin sections, mostly from the Borrowdale Volcanic Group and related intrusions. The latter is from surface and temporary exposures mostly within the Skiddaw Group. Registered samples are held by the British Geological Survey at Keyworth.

References

Most of the references listed below are held in the Libraries of the British Geological Survey at Keyworth (Nottingham) and Edinburgh. Copies of the references can be purchased subject to the current copyright conditions.

Adams, J. 1995. Mines of the Lake District Fells. (Skipton: Dalesman Publishing Company.)

Beddoe-Stephens, B, Petterson, M G, Millward, D, and Marriner, G F. 1995. Geochemical variation and magmatic cyclicity within an Ordovician continental-arc volcanic field: the lower Borrowdale Volcanic Group, English Lake District. Journal of Volcanology and Geothermal Research, Vol. 65, 81–110.

Boardman, J. 1991. Glacial deposits in the English Lake District. 175–183 in Glacial Deposits in Great Britain and Ireland. Ehlers, J, Gibbard, P L, and Rose, J (editors). (Rotterdam: Balkema.)

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Figures and plates

Figures

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

(Figure 2) Solid geology and structure of the district.

(Figure 3) Biostratigraphy and lithostratigraphy of the Skiddaw Group.

(Figure 4) Details of the Birker Fell Formation.

(Figure 5) Details of the Whorneyside, Airy's Bridge, and Lingmell formations.

(Figure 6) Details of the Seathwaite Fell, Lincomb Tarns and Esk Pike formations.

(Figure 7) Details of the Middle Dodd Dacite, Helvellyn and Deepdale formations.

(Figure 8) Three dimensional form of the Lake District batholith and the main geophysical lineaments.

(Figure 9) Mineralisation and summary of mining history in the district.

(Figure 10) Quaternary glaciations in the district.

Plates

(Plate 1) Force Crag Mine, Braithwaite [NY 2010 2166]: the treatment plant with spoil from the mines and, in the foreground, the settling ponds (D3856).

(Plate 2) View westwards from the summit of Place Fell [NY 406 169] along the lower part of the glacially eroded Glenridding valley with part of Ullswater in the foreground (MN506838).

(Plate 3) Slump folds in hornfelsed Skiddaw Group siltstones of the Kirk Stile Formation, within the Crummock Water aureole at Lad Hows [NY 1729 1925] (D3829).

(Plate 4) Prominent trap topography in the andesite-dominated Birker Fell Formation on High Rigg [NY 307 214], viewed from St John's in the Vale (Photograph D Millward).

(Plate 5) Bedded volcaniclastic sandstone of the Deepdale Formation, Link Cove [NY 3704 1188] near Fairfield (MNS06671).

(Plate 6) Hummocky glacial deposits (moraines) laid down by valley glaciers of the Lock Lomond Stadial along Grisedale (MNS06730).

(Front cover) Thirlmere reservoir viewed from Steel Fell [NY 322 121]. Screes form the low forested slopes on the right. Blencathra is in the background (north). The flooded valley lies within the Borrowdale Volcanic Group and is aligned along the main trace and splays of the Coniston Fault (MNS 06391).

(Rear cover)

(Index Map) Index to the 1:50 000 series maps of the British Geological Survey. The map below shows the sheet boundaries and numbers of the 1:50 000 series geological maps. The maps are numbered in three sequences, covering England and Wales, Northern Ireland, and Scotland. The west and east halves of most Scottish 1:50 000 maps are published separately. Almost all BGS maps are available flat or folded and cased. The area described in this sheet explanation is indicated by a solid block. British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGS-approved stockists and agents. Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

Figures

(Figure 4) Details of the Birker Fell Formation

(Figure 5) Details of the Whorneyside, Airy's Bridge, and Lingmell formations.

(Figure 6) Details of the Seathwaite Fell, Lincomb Tarns and Esk Pike formations.

(Figure 7) Details of the Middle Dodd Dacite, Helvellyn and Deepdale formations

(Figure 9) Mineralisation and summary of mining history in the district.