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A stratigraphical framework for the upper Ordovician and Lower Devonian volcanic and intrusive rocks in the English Lake District and adjacent areas

By D Millward

Bibliographical reference: Millward, D. 2004. A stratigraphical framework for the upper Ordovician and Lower Devonian volcanic and intrusive rocks in the English Lake District and adjacent areas. British Geological Survey Research Report RR/01/07 54pp.

Integrated Geoscience Survey

British Geological Survey Research Report RR/01/07

Keyworth, Nottingham British Geological Survey 2004. © NERC 2004. All rights reserved. ISBN 0 85272 400 4

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Maps and diagrams in this book use topography based on Ordnance Survey mapping.

Keywords: Lake District, Lower Palaeozoic, Ordovician, Devonian, volcanic geology, intrusive rocks

(Front cover) View over the Scafell Caldera. BGS Photo D4011.

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Acknowledgements

The form of the lithostratigraphical framework for the Lake District volcanic rocks has developed during the life of the Lake District Regional Geological Survey project (1981–1999), led at various times by P M Allen, B Webb, A Reedman and P Stone, under the project management of D Land and D Fettes. All members of the team have made valuable contributions to the debate and are acknowledged here: P M Allen, M J Branney, B Beddoe-Stephens, E W Johnson, P Kokelaar, B Kneller, S Loughlin, N Mathieson, B McConnell, M G Petterson and D G Woodhall. Of these individuals who have contributed to the project, I would particularly like to thank the following for many hours of discussion and for their comments on the text and its content:

D G Woodhall B Beddoe-Stephens Development of the lithodemic scheme for the intrusive rocks has resulted directly from the huge effort put into the preparation of the Geological Conservation Review volume ‘Caledonian Igneous Rocks of Great Britain’, led by D Stephenson. I am grateful to D Stephenson for many hours of helpful discussion on the application of a formal lithodemic scheme to the Lake District rocks.

The report has been edited by P Stone and has benefitted from the comments from members of the BGS Stratigraphy Committee and from external reviews by P Brenchley and A J Reedman on behalf of the Geological Society of London’s Stratigraphy Commission.

Summary

This Research Report discusses the lithostratigraphical framework of all the Lower Palaeozoic volcanic and intrusive rocks in the English Lake District, and their correlatives in the nearby Cross Fell and Teesdale inliers. The volcanic rocks belong to three of the major groups recognised in the Lake District: Eycott and Borrowdale Volcanic groups, and the Windermere Supergroup. Summary definitions of more than 40 constituent formations and the many more members are given. The BGS map and Stratigraphical Lexicon codes for all the units are given in appendices.

The large number of formations present within a subaerial volcanic unit such as the Borrowdale Volcanic Group is difficult to summarise within a formal lithostratigraphical framework. Thus, an attempt is made to group formations into informal groupings with recognised associations through their eruptive or depositional histories. These are referred to in this report as ‘successions’ and ten are proposed.

A lithodemic scheme for the intrusive igneous rocks of the area is presented, based on work carried out in preparation for the Geological Conservation Review series publication on the Caledonian Igneous Rocks of Great Britain. The plutonic rocks and minor intrusions in northern England are grouped within a number of suites under the umbrella of the Caledonian Igneous Supersuite, which includes all intrusive igneous rocks associated with events that led to the Caledonian Orogeny.

The schemes presented are a contribution to the strati-graphical framework report for the Ordovician and Silurian rocks of northern England and are regarded as an essential precursor to any process-orientated account of the Lower Palaeozoic magmatism within the Lake District. This overview was undertaken as one of the concluding products of the Regional Geological Survey of the Lake District which lasted from 1981 until 1999.

Chapter 1 Introduction

This report summarises the upper Ordovician volcanic stratigraphy in the English Lake District, and its probable correlatives in the nearby Cross Fell, Cautley, Dent, Craven and Teesdale inliers. It also introduces a lithodemic scheme for the Lower Palaeozoic intrusive igneous rocks of the region. These igneous rocks were emplaced at the margin of Eastern Avalonia during closure of the Iapetus Ocean and the subsequent collision with Laurentia–Baltica which resulted in the Acadian Orogeny. The schemes presented herein and summarised in Fortey et al. (2000) and by Millward (2002), are intended as a contribution to the stratigraphical framework report for the Ordovician and Silurian rocks of northern England and are regarded as an essential precursor to any process-orientated account of the Lower Palaeozoic magmatism within the Lake District. The overview is based on the modern 1:10 000-scale geological mapping undertaken by the British Geological Survey as part of the Lake District Regional Geological Survey from 1981 until 1999.

The primary survey, from 1860 until 1890, produced the first complete geological maps of the Lake District.

However, it was some time before Marr (1900) and Green (1919), among others, applied a formal lithostratigraphy. During the ensuing 50 years almost all of the Borrowdale Volcanic Group was reinvestigated and major advances in the understanding of the lithostratigraphy were made by such notable researchers as J J Hartley, G H Mitchell and R L Oliver. In most of their works, a local lithostratigraphy was established for each area, each with its own terminology. In many cases precise correlation with adjacent areas was not made. Further, the complex, isoclinal fold model for the volcanic rocks proposed by Green (1920) strongly influenced lithostratigraphical interpretation. For example, Mitchell’s (1940) interpretation of the Coniston area is difficult to reconcile with his later contributions on the Dunnerdale and Seathwaite fells (1956a, 1963) (see Millward and Soper, 2000). Attempts at resolving correlation problems between areas have been made by Mitchell (1956b), Millward et al. (1978) and Moseley and Millward (1982), though the plethora of local names was rationalised in the recent publication of the Ambleside memoir (Millward et al., 2000a). The Borrowdale Volcanic Group-wide lithostratigraphical models produced by Millward et al. (1978), and Moseley and Millward (1982) were not based on new field surveys and contained much uncertainty caused mainly by the lack of fossiliferous strata, the lateral impersistence of many units and the complex faulting. The presence of many sills, previously identified as lavas and ascribed to stratigraphical formations poses further limits on these models (Branney and Suthren, 1988).

It is clear from previous work that abrupt variations in facies and thickness present a severe test in setting up a usable, unified stratigraphy across the inlier. Thus, the British Geological Survey mapping programme has provided a unique opportunity to establish a coherent stratigraphy of the volcanic successions and to provide a sound basis for interpretation of their facies. As a result of progress in the western part of the Lake District, there have been considerable advances in the understanding of Caradoc magmatism on the northern flanks of Avalonia in late Ordovician times (e.g. Petterson et al., 1992; Branney and Kokelaar, 1994; Beddoe-Stephens et al., 1995). Moreover, the work has a wider significance than the Lake District, because preservation of such subaerial volcanic successions in the geological record is unusual. The contribution made, for example by Branney and Kokelaar (1994), to the understanding of caldera development was fundamentally based on a stratigraphical approach.

1.1 Scope

1.1.1 Geographical coverage

The area considered in this report is the English Lake District and Cross Fell, in Cumbria. The resurvey of the major outcrops of igneous rocks in the Lake District by the British Geological Survey and its collaborators in several British universities started in 1981 and was completed in 1999; the area is covered by eight 1:50 000 geological maps (Figure 1). Resurvey of the Cross Fell area was carried out from 1963 to 1967; it has not been re-examined and recommendations in this report are taken from published works. Lower Palaeozoic volcanic rocks are also known to occur in the Teesdale inlier of the northern Pennines and in the Cautley, Dent and Craven inliers.

1.1.2 Stratigraphical scope

Volcanic rocks in the Lake District and Cross Fell form two major, middle Ordovician volcanic successions, fomalised as the Eycott and Borrowdale volcanic groups. Lower Palaeozoic volcanic rocks are also known to occur in the Teesdale inlier of the northern Pennines and these are taken to be part of the Borrowdale Volcanic Group (Dunham, 1990). Relatively thin volcanic formations representing the waning stages of Ordovician volcanism in northern England are also present within the overlying Dent Group, which forms the lower part of the Windermere Supergroup (Figure 2).

A lithodemic scheme for the intrusive igneous rocks is proposed in Chapter 5 of this report. This is part of a framework for all igneous rocks within the British Caledonides and is based on contributions made to the Geological Conservation Series Volume 17 which covers these rocks (Stephenson et al., 1999; Millward, 2002).

Chapter 2 Lithostratigraphical framework

Lower Palaeozoic volcanic rocks in the Lake District are represented principally by two lithostratigraphical units, the Eycott and Borrowdale volcanic groups. They form a major part of the sequence and contrast starkly with the marine sedimentary successions above and below, and from which they are separated by major, regional, unconformities (Wadge, 1978; Millward and Molyneux, 1992). In the northern part of the inlier the Eycott Volcanic Group has an outcrop of about 50 km² and a thickness of 3200 m (Millward et al., 1999, 2000b). The much thicker Borrowdale Volcanic Group (with a composite thickness of probably more than 6000 m) has an outcrop area of about 750 km². These figures compare with about 530 km² and at least 5000 m for the underlying Skiddaw Group, and 1050 km² and more than 8000 m for the overlying Windermere Supergroup respectively. The Skiddaw Group and Windermere Supergroup were each deposited over about 40 Ma, but the volcanism may have lasted for less than 5 Ma (Piper et al., 1997; Millward and Evans, 2003). A number of minor volcanic units are included within the Windermere Supergroup (Kneller et al., 1994).

2.1 Group terminology of the volcanic divisions

Until relatively recently, the volcanic rocks of the northern and central Lake District were considered to be part of a single unit which was originally referred to by Sedgwick (1832) and Aveline and Hughes (1872) as the ‘Green Slates and Porphyries’, and then successively as the ‘Lower Silurian Volcanic Series’ of Borrowdale by the Geological Survey (Ward, 1876; Aveline and Hughes, 1888), ‘Borrowdale Series’ by Marr (1916) and ‘Borrowdale Volcanic Series’ by Dakyns et al. (1897) and Green (1920). More than 50 years later the name was changed from ‘Series’ to ‘Group’ as required by modern lithostratigraphical usage (Wadge, 1978; Moseley, 1984). Nomenclature of the small number of volcanic units within the younger Windermere Supergroup herein generally follows the recommendations of Kneller et al. (1994).

Fitton and Hughes (1970) first recognised that the rocks forming the Eycott Volcanic Group are geochemically distinct from the rest of the Borrowdale Volcanic Group, but it was Downie and Soper (1972) who first used the term ‘Eycott Volcanic Group’. Wadge (1978) shortened the name to ‘Eycott Group’, incorporating parts of the Skiddaw Group considered by him to be the same age as the volcanic rocks; this included the Tarn Moor Formation and mudstones in the Ullswater, Bampton, Furness and Millom inliers. Moseley (1984) did not support such a chronostratigraphical approach, preferring the lithostratigraphical definition set out by Downie and Soper. The original definition of the Eycott Volcanic Group is recommended in this report, as argued by Millward et al. (1999, 2000b), though new subdivisions replacing the Binsey and High Ireby formations of Downie and Soper (1972) are preferred.

Moseley (1984) considered it important to maintain a uniform approach to the overall lithostratigraphy within the Lake District Lower Palaeozoic inlier. He thus resisted the temptation to consider the ‘Borrowdale Volcanic Group’ as a supergroup, despite previous use of the term ‘group’ for parts of the succession (e.g. Ullswater Group, Marr, 1900; Airy’s Bridge Group, Oliver, 1961; Place Fell Group, Moseley, 1960). Recently, Kneller et al. (1994) established the mudstones and sandstone that overlie the volcanic rocks as the Windermere Supergroup. Although of similar thickness to the Windermere Supergroup, the volcanic rocks are restricted in distribution to the Lake District, Cross Fell and Teesdale, and do not have direct correlatives elsewhere in the Lower Palaeozoic of Great Britain. Thus, it is recommended herein that group status is retained for the volcanic rocks.

2.2 Group definitions

2.2.1 Eycott Volcanic Group (EVG)

Distribution: Main outcrop in the northern Lake District from Bothel to the Caldbeck Fells; small inliers at Eycott Hill, Greystoke Park and Melmerby Fell (Cross Fell inlier).

Stratotype: Type area on Binsey [NY 2200 3500] to [NY 2400 3600]. Reference area on Eycott Hill [NY 3800 2900] to [NY 3950 3100].

Lithology

Dominantly sheets of andesite with some basalt and dacite; mostly considered to be lava flows, but with interbedded penecontemporaneous sills. Dacitic pyroclastic rocks of the Potts Ghyll Formation occur at the top of the group. Geochemically the volcanic rocks are characterised as medium K, and transitional between continental margin tholeiitic and calc-alkaline. The sequence also contains thin interbeds of volcaniclastic rocks. Basal tuffaceous sandstones constitute the Overwater Formation.

Lower boundary: Angular unconformity, with a profound change from deep marine mud-dominated sedimentary rocks of the Skiddaw Group, to volcanic rocks.

Upper boundary: Unconformably overlain by Carboniferous strata.

Thickness: Up to 3200 m.

Remarks: Originally included within the Borrowdale Volcanic ‘Series’ (e.g. Eastwood et al., 1968), but shown to be geochemically distinct from those rocks. Also included with Llanvirn mudstones within the Eycott Group of Wadge (1978). Lithologically very similar to the Birker Fell Formation of the Borrowdale Volcanic Group (Millward et al., 1999, 2000b). The Eycott Volcanic Group rocks have geochemical characteristics that are transitional between calc-alkaline and tholeiitic.

References: Downie and Soper (1972); Wadge (1978); Millward and Molyneux (1992); Millward et al. (1999, 2000b).

Maps: Cockermouth (23), Penrith (24), Keswick (29).

2.2.2 Borrowdale Volcanic Group (BVG)

Distribution: Extensive outcrop forming the craggy fells of the central Lake District, extending south-west to Millom Park; also crops out within the Furness inliers and the faulted inliers in Cross Fell and Teesdale. A small exposure of purple andesite in a stream north-north-east of Foggygill Farm [SD 722 988] in the Murthwaite Inlier at Cautley also probably belongs to the Borrowdale Volcanic Group. A concealed succession has been proved in west Cumbria, beneath Carboniferous and Permo-Triassic cover rocks.

Stratotype: No stratotype specifically defined. Except for some borehole successions in west Cumbria (Millward et al., 1994), no major parts of the group are undesignated and thus definitions of component formations apply.

Lithology: Medium to high K, continental-margin calc-alkaline basalt, andesite, dacite and rhyolite volcanic association of lavas, sills and pyroclastic rocks. Includes abundant intercalations of volcaniclastic sedimentary rocks. Many of the units of intermediate and silicic lavas and pyroclastic rocks contain phenocrysts of almandine–pyrope garnet (Fitton, 1972).

Lower boundary: Taken at the regional unconformity where there is a profound change from the marine mudrock succession of the Skiddaw Group to volcanic strata. Over most of the region this coincides with the base of the Birker Fell Formation. However, in the west of its outcrop the base of the group is defined by the base of the Latterbarrow Sandstone Formation.

Upper boundary: Unconformity, defined by the base of the Windermere Supergroup.

Thickness: Probably in excess of 6000 m, though much less locally.

Reference: Moseley (1984); Millward et al. (1994, 2000a).

Maps: Penrith (24), Alston (25), Whitehaven (28), Keswick (29), Appleby (30), Brough-under-Stainmore (31), Gosforth (37), Ambleside (38), Kendal (39), Ulverston (48).

2.3 Some problems affecting the nature of the lithostratigraphical framework

Subaerial volcanic sequences present a severe test of lithostratigraphical methodology (e.g. Fisher and Schminke, 1984). Interdigitation of units, the restricted nature of some units, lateral thickness and facies changes, and the presence of penecontemporaneous intrusions all contribute to complexity. Furthermore, correlation between areas is not always possible because of contemporaneous erosion, synvolcanic faulting and the interdigitation of products from contemporaneous adjacent volcanic centres.

The proposed lithostratigraphy of the Lake District volcanic rocks is centred upon widespread lithologically distinctive mappable units (Figure 5)." data-name="images/P991781.jpg">(Figure 3). Of particular value in this respect are the thick ignimbrite sheets which blanket the underlying formations: for example, the Airy’s Bridge, Whelter Knotts, Froswick and Lincomb Tarns formations. However, only the last named unit is sufficiently widespread to be of value throughout the Lake District. Relative to other units within the sequences the ignimbrite units were formed during major eruptions and can be regarded geologically as almost instantaneous events. By contrast, the sheeted andesite successions that comprise most of the Eycott Volcanic Group and Birker Fell Formation (Borrowdale Volcanic Group) contain few units that are widespread enough to provide a coherent internal stratigraphy. These units are generally undivided on the geological maps, despite their very thick successions, for example, 2400 m for the undivided part of the Eycott Volcanic Group (see Cockermouth, Sheet 23) and 1800 to 2700 m for the Birker Fell Formation (Ambleside, Sheet 38). These units are discussed further below, along with two other significant aspects of the lithostratigraphy.

2.3.1 Andesite sheets

Tabular andesite sheets form a significant proportion of the Borrowdale and Eycott volcanic groups. Interpretation of these bodies as lava or intrusion is of considerable stratigraphical significance. Stemming originally from Green’s (1919) work comparing brecciated andesite in the group with modern, autobrecciated lava, most researchers have assumed that most of the sheets are lava (e.g. Firman, 1957; Oliver, 1961; Mitchell, 1963; Clark, 1964; Millward et al., 1978). However, Green was also in no doubt that many sills are present. The lava only interpretation was challenged by Branney and Suthren (1988) who documented a number of andesite sheets that showed evidence of near surface intrusion and proposed that sills might constitute a significant proportion of the succession.

Results from the Lake District Regional Geological Survey show that within volcaniclastic sedimentary units in the upper part of the Borrowdale Volcanic Group there are many andesite, basaltic andesite and dacite sills. Evidence for this is normally clear cut and this radically alters previous interpretations of the stratigraphy. One such example is the ‘Wrengill Andesites’, a unit that featured in many early accounts, which must now be an inappropriate lithostratigraphical division because most of its components are intrusions. The abundant andesite sills are not considered within this stratigraphical framework, though they contribute significantly to the development of the volcanic succession and are included within the lithodemic scheme for intrusive igneous rocks of the region.

Particular difficulty is presented by the sequence of tabular andesite sheets that constitutes the Birker Fell Formation. Petterson et al. (1992) and Beddoe-Stephens et al. (1995) concluded that, although sills are present in the formation, most of the sheets are lava. However, field evidence does not always provide a clear origin for each sheet, and opinions vary among workers as to the relative proportions of lava and sills. It is nevertheless important to recognise that a part of the formation consists of sills. Beddoe-Stephens et al. considered the sills within the Birker Fell Formation succession to be comagmatic and probably contemporaneous with the lavas.

2.3.2 Concealed volcanic rocks in west Cumbria

The existence of buried, pre-Carboniferous volcanic rocks in west Cumbria has been known for many years from borings, mainly for hematite exploration (Smith, 1924; Eastwood et al., 1931). However, the recent investigations near Gosforth, undertaken by Nirex as part of an assessment of that area as a possible site for the construction of an underground disposal facility for nuclear waste, have shown the presence of a complex volcanic lithostratigraphy, mostly concealed beneath Permo-Triassic and Carboniferous cover rocks (Bowden et al., 1998). More than 9800 m of volcanic rock cores were obtained from 22 of the 29 deep boreholes drilled; individual volcanic sequences varied from 67 m to 1143 m (uncorrected for dip) ((Table 1); borehole locations are given in Akhurst et al., 1997, fig. 3). The cleaved volcanic rocks are considered to be part of the Borrowdale Volcanic Group (Millward et al., 1994; Akhurst et al., 1997). The stratigraphical sequences from many of these boreholes can be matched, at least in part, with the succession at outcrop east of Gosforth and in the Bleng valley (Akhurst et al., 1997). However, several boreholes contain volcaniclastic rocks that cannot be correlated with the successions proved at outcrop or even within other boreholes. Such sequences are seen for example in Sellafield 3, 11A and 13A boreholes and their stratigraphy remains undesignated at present (Table 1; Millward et al., 1994). It has been argued that the west Cumbria succession contains some of the youngest units within the Borrowdale Volcanic Group (Akhurst et al., 1997).

2.3.3 Side Pike volcanic complex (SIPI)

Branney and Kokelaar (1994) described an exceptionally coarse and chaotic volcanotectonic megabreccia (sensu Lipman, 1984), covering an area of more than 5 km² between Great Langdale and Wrynose Pass [NY 280 057] to [NY 300 030], as the Side Pike complex (see 1:50 000 Sheet 38, Ambleside, for location). This is bounded to the north and west by faults, and passes southward and eastward into structurally more coherent blocks cut by volcanotectonic faults. The megabreccia comprises blocks up to more than 500 m across, composed of a variety of pyroclastic and volcaniclastic sedimentary lithofacies associations. Many of the larger blocks have a coherent stratigraphy that can be matched with parts of the succession elsewhere. However, though the internal stratigraphy of some other large blocks can be correlated from one block to another, they cannot be matched with any part of the regional stratigraphy. These ‘exotic’ blocks were derived either from formations not otherwise preserved in the district, or from known formations with unrecorded local facies variations. The name ‘Side Pike complex’ is used on Sheet 38 (Ambleside) and is described in Millward et al. (2000a). Side Pike volcanic complex is a useful informal stratigraphical division and is therefore retained.

Chapter 3 Biostratigraphical and palaeomagnetic correlation

In the Lake District, the age and time span of the Borrowdale Volcanic Group is poorly constrained between the Llanvirn Tarn Moor Formation (Skiddaw Group; Cooper et al., 1995) and the Ashgill (Cautleyan) Dent Group, and has been assigned customarily to the Llandeilian and early Caradoc interval (chronostratigraphy sensu Fortey et al., 2000). However, in the Cross Fell Inlier, silicic tuffs of the uppermost Borrowdale Volcanic Group are overlain by shelly siltstones of the ‘corona facies’ at the base of the Dufton Shale Formation (Dent Group) (Burgess and Holliday, 1979). The faunas in the siltstones are Longvillian (upper Caradoc) and indicate an upper limit on the age of the volcanic sequence in that area.

Dark grey mudstone (Holehouse Gill Formation) from Holehouse Gill in the Ulpha valley (Figure 5)." data-name="images/P991781.jpg">(Figure 3), first reported by Numan (1974), contains an acritarch microflora that was interpreted as probably Caradoc (Harnagian–Soudleyan) in age by Molyneux (1988). Re-examination of the microflora in the light of recent work elsewhere now casts doubt on this interpretation (S G Molyneux, pers. comm. 2001). Thus, there is no biostratigraphically significant unit within the Borrowdale Volcanic Group.

The absolute age of the Borrowdale Volcanic Group is constrained by radiometric dates on the volcanic rocks of 457 ± 4 Ma (Sm-Nd on garnet-whole-rock pairs; Thirlwall and Fitton, 1983), and 451.0 ± 1.4 and 452.8 ± 0.73 Ma (U-Pb zircon; Millward and Evans, 2003). These are Caradoc according to the most recent comparative timescales (Compston, 2000; Fortey et al., 2000).

Biostratigraphical evidence for the age of the Eycott Volcanic Group comes from palynological assemblages recorded from the Overwater Formation at the base of the group by Downie and Soper (1972), and Millward and Molyneux (1992). The earliest Llanvirn (Abereiddian) age deduced by Downie and Soper (1972) could not be substantiated by Millward and Molyneux (1992), who concluded that the volcanic rocks could not be older than Llanvirn and might be Caradoc. At the eastern extent of the largest outcrop of the Eycott Volcanic Group is a small outlier of the Caradoc Dent Group; contacts of this unit, the Drygill Formation (Dean, 1963), are partly faulted and partly intrusive, but it is clearly younger than the volcanic rocks (Millward et al., 1999). Rundle (1979) obtained a K-Ar age of 468 ± 10 Ma for the Mosedale Gabbros (Carrock Fell Complex), which is considered by Hunter (1980) to be contemporaneous with the volcanic rocks. This date is within the error range of the Sm-Nd garnet age obtained from the Borrowdale Volcanic Group.

Palaeomagnetic data from both the Eycott and Borrowdale volcanic groups reported by Piper (1997) and Piper et al. (1997) show that the two groups exhibit uniform normal polarity and have a relatively strong and stable remanent magnetisation dominated by a single characteristic component acquired during cooling on emplacement. Together with the radiometric ages on the Borrowdale Volcanic Group, this strongly supports the hypothesis that the two groups were emplaced contemporaneously in Caradoc times during a period lasting no more than 5 Ma.

Chapter 4 Stratigraphical divisions

The preferred stratigraphical divisions of the Eycott and Borrowdale volcanic groups and the volcanic units in the Windermere Supergroup in Cumbria are arranged below by group, in roughly ascending stratigraphical order. The stratigraphy of the Eycott Volcanic Group has been described in detail recently by Millward et al. (1999). The stratigraphical relationships within the Borrowdale Volcanic Group are illustrated in (Figure 5)." data-name="images/P991781.jpg">(Figure 3). The volcanic units within the Dent Group of the Windermere Supergroup are shown in (Figure 2).

The stratigraphy of the Borrowdale Volcanic Group is described in detail in the Ambleside and Ulverston memoirs with additions included in the Keswick sheet description (Millward et al., 2000a; Johnson et al., 2001; Woodhall, 2000a). Where possible, previously well established unit names in common usage are retained. Approximate comparison between the new and old units within the Borrowdale Volcanic Group is summarised in (Table 2) and (Table 3). Superficially, these charts suggest that much of the previous lithostratigraphy remains little changed. However, considerable differences between old and new units are revealed. Old names are commonly redefined; for example, in part of the Seathwaite Fells, Mitchell’s (1963) Kidson How Tuffs belong to the newly defined Whorneyside and Airy’s Bridge tuff formations but, elsewhere, to the Low Water Tuff Formation. Furthermore, many of the definitions of the base of formations have changed. As a result of this revision it has been necessary to abandon some terms used previously.

More than 40 formations and a large number of members have been erected within the Borrowdale Volcanic Group. Informal groupings of formations into larger units have proved to be useful in the interpretation of the development of the volcanic succession. In their attempts at producing a unified lithostratigraphy for the Borrowdale Volcanic Group, Millward et al. (1978), and Moseley and Millward (1982) noted that the succession comprised two distinctive parts; a lower one comprising dominantly andesite lavas and an upper one of intermediate and silicic pyroclastic rocks, and volcaniclastic sedimentary rocks. This division is recognised throughout the entire outcrop and is referred to informally as the ‘lower’ and ‘upper’ Borrowdale Volcanic Group respectively.

In attempting to provide a framework for the interpretation of the development of the volcanic field Millward (2002) proposed ten informal groupings that he referred to informally as ‘successions’. The definitions of these are set out in (Table 4), which shows the constituent formations, the main lithofacies associations present, approximate thickness and the definitions of the base and top; a summary of the main styles of volcanic and/or sedimentary processes that occurred is also given. The relationships between the successions present in the central Lake District are shown diagrammatically in (Figure 4) and the approximate distributions of all but the succession in Cross Fell in (Figure 5).

A summary of the formal definitions of the volcanic lithostratigraphical units in the Lake District is given below. British Geological Survey Lexicon codes are given after the formal names. Appendices 1 and 2 list the lexicon and map codes for this succession. Appendix 3 gives a summary of unit names that are no longer required.

4.1 Eycott Volcanic Group

4.1.1 Overwater Siltstone Formation (OV)

Stratotype: Type section in the Overwater spillway, between the confluence of the River Ellen and the southern end of Chapel-house Reservoir [NY 2582 3551].

Lithology: Grey, laminated siltstone and tuffaceous fine-grained sandstone with channel fills of sedimentary breccia. The formation is intruded by sills. Composed mostly of siliciclastic material; with minor reworked contemporaneous tephra.

Ranking: Locally, the basal division of the Eycott Volcanic Group.

Lower boundary: Defined by the base of the lowest bed of breccia or siltstone overlying the angular unconformity on the Skiddaw Group.

Upper boundary: Overlain locally by conglomerate or by basaltic andesite and andesite within the undivided part of the group.

Thickness: About 8 m in the type section, but up to 10 m elsewhere.

Remarks: Previously considered to be a passage sequence between the Skiddaw Group and overlying volcanic rocks; included previously within the now obsolete Binsey Formation. Occupies a similar position to the Latterbarrow Sandstone Formation at the base of the Borrowdale Volcanic Group.

References: Millward and Molyneux (1992); Millward et al. (1999).

Map: Cockermouth (23).

4.1.2 Potts Ghyll Tuff Formation (POG)

Stratotype: Type section in Gill Beck, 500 m upstream from Potts Gill (2.5 km south-south-west of Caldbeck) [NY 3189 3688] to [NY 3190 3669].

Lithology: Massive vitroclastic and lithic-rich acid andesitic to dacitic lapilli-tuff (interpreted as non-welded ignimbrite).

Ranking: Locally the uppermost exposed division of the Eycott Volcanic Group.

Lower boundary: Not exposed in detail; taken at upward change from dominantly andesite to volcaniclastic deposits within the Eycott Volcanic Group.

Upper boundary: Overlain unconformably by Carboniferous strata.

Thickness: The maximum thickness is at least 800 m, though the upper boundary is concealed beneath Carboniferous rocks.

Remarks: Lithology depicted on older versions of Cockermouth sheet but previously included as part of the now obsolete ‘High Ireby Formation’.

Reference: Millward et al. (1999).

Map: Cockermouth (23).

4.2 ‘Lower’ Borrowdale Volcanic Group

4.2.1 Latterbarrow Sandstone Formation (LAS)

Stratotype: None given by Allen and Cooper (1986); recommend type area between the River Calder and Latterbarrow Beck [NY 066 117] to [NY 077 109].

Lithology: Bedded quartz-wacke sandstone with intercalated mudstone in the upper part.

Ranking: Locally significant, non-volcanic basal formation of the Borrowdale Volcanic Group in west Cumbria.

Lower boundary: Taken at the marked change to sandstone from silty mud-stone containing an acritarch assemblage of Tremadoc to Arenig age. The junction is nowhere exposed, but has been interpreted as an unconformity on mapping evidence.

Upper boundary: Defined by the base of Devoke Water Member of the Birker Fell Formation.

Thickness: 0 to c. 400 m.

Remarks: Previous authors (e.g. Eastwood et al., 1931; Jackson, 1978; Wadge, 1978; Moseley, 1984) included these rocks in the Skiddaw Group. Following Simpson (1967), and Allen and Cooper (1986) this should be regarded as the basal formation of the Borrowdale Volcanic Group.

References: Allen and Cooper (1986); Akhurst et al. (1997).

Maps: Whitehaven (28), Gosforth (37).

4.2.2 Greenscoe Tuff Formation (GCOE)

Stratotype: Type area at Greenscoe Quarry [SD 222 760] and at nearby Greenscoe Cragg.

Lithology: Crudely stratified units of silty mudstone, andesitic lapillituff and tuff-breccia; cut by andesite intrusions.

Ranking: Basal formation of the Borrowdale Volcanic Group in the Furness Inlier, south-west Cumbria.

Lower boundary: Unconformably overlies the Skiddaw Group.

Upper boundary: Unconformably overlain by the Dent Group (Windermere Supergroup).

Thickness: 0 to 300 m.

Remarks: Interpreted as a vent filling by Soper (1970) and as a maar deposit by Johnson (1997) and Johnson et al. (2001), but alternatively could be a valley-fill sequence of debris-flow deposits.

Reference: Johnson et al. (2001).

Map: Ulverston (48).

4.2.3 Whinny Bank Tuff Formation (WNB)

Stratotype: Type area on Whinny Bank, 1 km west of The Green, Millom [SD 169 845].

Lithology: Bedded units of andesitic tuff and lapilli-tuff with large amounts of grey silty mudstone pyroclasts, particularly at the base, but with an increasing juvenile pyroclast component upwards.

Ranking: Basal formation of the Borrowdale Volcanic Group in Millom Park, south-west Cumbria.

Lower boundary: Taken at the base of the lowest volcanic unit above the unconformity with the Skiddaw Group.

Upper boundary: Defined by the base of either the Po House or Waberthwaite formations.

Thickness: 100 to 170 m.

Remarks: Previously known as the Mottled Shale (Green, 1913) and Whinnybank Tuffite (Mathieson, 1986). Interpreted as phreatomagmatic fall-out and surge deposits.

Reference: Johnson et al. (2001).

Map: Ulverston (48).

4.2.4 Po House Tuff Formation (PHT)

Stratotype: Type area at Po House Bank, Whicham Valley [SD 151 823].

Lithology: Parallel, thin- and medium-bedded andesitic tuff and lapilli-tuff. Interpreted as pyroclastic fall-out deposits, with a named ignimbrite at the base.

Ranking: Formation near the base of the Borrowdale Volcanic Group in Millom Park; overlies the Whinny Bank Formation.

Lower boundary: Taken at the base of a lapilli-tuff unit (pyroclastic flow deposit) that marks the change to tuff and lapilli-tuff from strata of the Whinney Bank Formation. The Po House Tuff lacks the grey silty mudstone component of the unit beneath.

Upper boundary: Defined by the base of Waberthwaite Formation.

Thickness: Up to 400 m.

References: Mathieson (1986); Johnson et al. (2001).

Map: Ulverston (48).

4.2.4.1 Dog Crag Tuff Member (DCT)

Stratotype: Type area on Dog Crag, Po House Bank, Whicham Valley [SD 149 823].

Lithology: Massive, eutaxitic dacitic lapilli-tuff (ignimbrite).

Ranking: Basal member of the Po House Formation.

Remarks: Not used on the Ulverston sheet (48) where the formation remains undivided, but the member is described by Johnson et al. (2001).

4.2.5 Birker Fell Andesite Formation (BFA)

Stratotype: Type area at Devoke Water to Yoadcastle and Stainton Pike [SD 145 967] to [SD 154 940]. Reference sections/areas at: High Rigg [NY 300 225] to [NY 315 200]; Haycock to Red Pike [NY 140 100] to [NY 160 100]; Cockley Beck to Grey Friar [NY 247 017] to [NY 262 004]; Hallin Fell [NY 426 190] to [NY 440 205].

Lithology: Predominantly a tabular, sheeted succession of feldsparphyric andesites, usually with blocky autobreccia at the base and top and massive to flow-banded and flow-jointed centres. Includes locally restricted sequences of basalt and dacite lavas, including for example the Throstle Garth Basalt and Great Whinscale Dacite members respectively. Interbedded units of pyroclastic and volcaniclastic sedimentary rocks are designated as members where they are distinctive, laterally extensive and recognised as stratigraphical markers. Examples of pyroclastic units include the Cockley Beck, Craghouse and Little Stand tuffs; sedimentary units include the Eagle Crag and Blisco sandstone members. Some units of andesite, dacite and pyroclastic rocks are garnetiferous, notably the Cockley Beck Tuff.

Ranking: Major formation comprising most of the lower part of the Borrowdale Volcanic Group.

Lower boundary: Oversteps the Latterbarrow Formation eastwards to overlie the Skiddaw Group.

Upper boundary: Defined by the base of the overlying pyroclastic and sedimentary formations.

Thickness: From about 980 m in the north-eastern part of the outcrop to 2700 m in Eskdale.

Remarks: Interpreted mainly as a succession of lavas forming a major, low-profile, plateau-andesite volcanic field (Petterson et al., 1992). Includes contemporaneous sills (see section 2.3.1.)

References: Petterson et al. (1992); Millward et al. (2000a); Beddoe-Stephens (2000a, b); Campbell (1995); Loughlin (2002), Millward (2003); Woodhall (2000a, b).

Maps: Whitehaven (28), Keswick (29), Appleby (30), Gosforth (37), Ambleside (38), Ulverston (48).

4.2.5.1 Grange Crags Andesite Member (GCG)

Stratotype: Reference section on Grange Crags near and around the village of Grange [NY 253 173] to [NY 260 178].

Lithology: White to pink-weathered, very fine-grained, aphyric andesite lava flow with some flow foliation and autobreccia.

Ranking: Local member near the base of the Birker Fell Formation west of Keswick.

Reference: Beddoe-Stephens (1997); Woodhall (2000a).

Map: Keswick (29).

4.2.5.2 Ashness Tuff Member (AHT)

Stratotype: Reference section on crags next to the campsite at Ashness Farm [NY 272 193] and adjacent to the car park at the top of Lodore Wood Crags [NY 269 194].

Lithology: Massive to weakly eutaxitic, lithic-rich lapilli-tuff (ignimbrite).

Ranking: Local member near the base of the Birker Fell Formation west of Keswick.

Reference: Beddoe-Stephens (1997); Woodhall (2000a).

Map: Keswick (29).

4.2.5.3 Devoke Water Tuff Member (DKT)

Stratotype: Type area on Birkby Fell and Stord’s Hill, south and southwest of Devoke Water [SD 146 965] to [SD 155 960]; reference area on Capel Crag [NY 071 102].

Lithology: Poorly bedded, basic lapilli-tuff with substantial content locally of pyroclasts of Skiddaw Group rocks.

Ranking: Local member at the base of the Birker Fell Formation in Eskdale and the Bleng valley.

Remarks: Interpreted as phreatomagmatic fall-out and surge deposits, possibly reworked.

References: Petterson et al. (1992); Akhurst et al. (1997); Millward et al. (2000a).

Maps: Whitehaven (28), Gosforth (37), Ambleside (38).

4.2.5.4 Little Meldrum Tuff Member (LMEL)

Stratotype: Type area on Little Meldrum [NY 422 227], Ullswater.

Lithology: Coarse tuff, lapilli-tuff and lapillistone with abundant pyroclasts from the Skiddaw Group.

Remarks: Interpreted as phreatomagmatic deposits, reworked locally.

Ranking: Local member at the base of the Birker Fell Formation north-west of Ullswater.

Reference: Beddoe-Stephens (2000a).

Map: Appleby (30).

4.2.5.5 Lanty Crag Breccia Member (LTY)

Stratotype: Type area on Lanty Crag [NY 500 202], 2 km south-southwest of Helton, Shap.

Lithology: Heterogeneous succession of volcaniclastic breccia and conglomerate with some bedded volcaniclastic sandstone. Associated with aphyric andesite sheets.

Ranking: Local member at the base of the Birker Fell Formation in the north-eastern Lake District.

Remarks: Previously known as the Bampton Conglomerate (Nutt, 1970); the name ‘Bampton’ has been used elsewhere in British stratigraphy and is unavailable. Interpreted as phreatomagmatic deposits, possibly with substantial sedimentary reworking.

Reference: Beddoe-Stephens (2000b).

Map: Appleby (30).

4.2.5.6 Birkby Fell Andesite Member (BKB)

Stratotype: Type area on Birkby Fell and Woodend Height, south-west and south of Devoke Water [SD 1475 9600] to [SD 1550 9535].

Lithology: Sheets of dark grey to greenish grey, porphyritic basaltic andesite and subordinate andesite with clinkery autobreccia (aa-lava flows). Interbedded volcaniclastic sedimentary and pyroclastic rocks.

Ranking: Local member in the lower part of the Birker Fell Formation south of Devoke Water.

References: Petterson et al. (1992); Millward et al. (2000a).

Maps: Gosforth (37), Ambleside (38).

4.2.5.7 Cockley Beck Tuff Member (CKT)

Stratotype: Type area between Cockley Beck and Dale Head Close [NY 240 010] to [NY 240 000].

Lithology: Massive, garnetiferous, eutaxitic lapilli-tuff (ignimbrite), with some interbedded units of low-angle cross-laminated tuff (pyroclastic surge deposits).

Ranking: Local tuff marker member within the lower part of the Birker Fell Formation; widespread in Eskdale and the upper Duddon Valley.

Remarks: Usually referred to as Cockley Beck Tuff.

References: Petterson et al. (1992); Millward et al. (2000a).

Maps: Ambleside (38).

4.2.5.8 Dale Head Breccia Member (DHD)

Stratotype: Dale Head to Rowantree How [NY 241 006] to [NY 249 006].

Lithology: Massive units of heterolithic volcaniclastic breccia and sandstone, interpreted as debris-flow deposits.

Ranking: Local member of the Birker Fell Formation in the upper Duddon valley.

Reference: Millward et al. (2000a).

Map: Ambleside (38).

4.2.5.9 Nettle Crags Tuff Member (NCT)

Stratotype: Type area at Nettle Crags, 1 km east-north-east of Bootle [SD 1160 8864] to [SD 1166 8872].

Lithology: Massive, eutaxitic to parataxitic dacitic lapilli-tuff (ignimbrite).

Ranking: Local member of the Birker Fell Formation in the area to the north of Black Combe.

Reference: Johnson et al. (2001).

Map: Ulverston (48).

4.2.5.10 Eagle Crag Sandstone Member (EAGC)

Stratotype: Type area on High Stile [NY 167 148] to Comb Crags [NY 175 145]; reference area on Fleetwith Pike and Honister Quarries [NY 210 140].

Lithology: Bedded succession of volcaniclastic siltstone, sandstone, conglomerate and breccia; subordinate tuff, eutaxitic lapillituff and accretionary lapilli-tuff.

Ranking: Local member within the Birker Fell Formation in the west of the outcrop.

Remarks: Subaqueous sequence of reworked volcaniclastic detritus and subaerially erupted pyroclastic deposits. Intruded by many shallow-level sills.

References: Petterson et al. (1992); McConnell and Kneller (1993); Woodhall (2000a).

Map: Keswick (29).

4.2.5.11 Seatallan Dacite Member (STAD)

Stratotype: Type area on Seatallan, 4.5 km west of Wasdale Head [NY 140 084]; reference area between summit of Haycock and Scoat Tarn [NY 145 104] to [NY 154 102].

Lithology: White to pink-weathered flow-foliated feldspar-phyric dacite lava.

Ranking: Local member within the Birker Fell Formation of the Wasdale area.

References: Petterson et al. (1992); Millward et al. (2000a); Woodhall (2000a).

Maps: Keswick (29), Gosforth (37), Ambleside (38).

4.2.5.12 Craghouse Tuff Member (CHT)

Stratotype: Type area in Crag House Park, north of Santon Bridge, west Cumbria [NY 112 028].

Lithology: Massive, eutaxitic, andesitic to dacitic lapilli-tuff (ignimbrite).

Ranking: Local member of substantial thickness within the Birker Fell Formation of the Wasdale and western fells area.

References: Petterson et al. (1992); Akhurst et al. (1997); Millward et al. (2000a).

Maps: Keswick (29), Gosforth (37), Ambleside (38).

4.2.5.13 Little Stand Tuff Member (LIT)

Stratotype: Type area on Little Stand [NY 2464 0334].

Lithology: White-weathered, massive, crystal (K-feldspar)-rich, vitroclastic rhyolitic tuff (ignimbrite), locally with abundant lithophysae.

Ranking: Widespread member within the south-western part of the Birker Fell Formation outcrop.

Remarks: Usually referred to as the Little Stand Tuff. U-Pb zircon age of 451.6 ± 1.4 Ma (Millward and Evans, 2003).

References: Petterson et al. (1992); Millward et al. (2000a).

Maps: Ambleside (38).

4.2.5.14 Great Whinscale Dacite Member (GWD)

Stratotype: Type area on Crook Crag and Great Whinscale, south of Eskdale [SD 197 990].

Lithology: Single unit of white-weathered, platy flow-foliated and autobrecciated aphyric dacite lava.

Ranking: Widespread member within the south-western part of the Birker Fell Formation outcrop; overlies the Little Stand Tuff.

Remarks: Usually referred to as Great Whinscale Dacite.

References: Kanaris-Sotiriou et al. (1991); Millward et al. (2000a).

Map: Ambleside (38).

4.2.5.15 Grey Friar Tuff Member (GY)

Stratotype: Type area from Grey Friar to Seathwaite Tarn [NY 250 000] to [SD 240 980].

Lithology: Massive, very lithic-rich dacitic lapilli-tuff (ignimbrite).

Ranking: Local member within the south-western part of the Birker Fell Formation outcrop.

Remarks: Usually referred to as the Grey Friar Tuff. Locally overlies Great Whinscale Dacite, but provides marker unit in areas where Little Stand Tuff and Great Whinscale Dacite are missing.

Reference: Millward et al. (2000a).

Map: Ambleside (38).

4.2.5.16 Blisco Sandstone Member (BCO)

Stratotype: Type area on Pike of Blisco [NY 271 042].

Lithology: Bedded succession of fine to very coarse-grained volcaniclastic sandstone with interbeds of tuff and lapilli-tuff.

Ranking: Member within the Birker Fell Formation; overlies the Little Stand Tuff.

Remarks: Blisco Formation of Branney and Soper (1988). Sequence of reworked and re-sedimented tephra with intercalations of pyroclastic rocks.

Reference: Millward et al. (2000a).

Map: Ambleside (38).

4.2.5.17 Haystacks Andesite Member (HAYS)

Stratotype: Reference sections on crags at Seat [NY 186 134] and on Haystacks [NY 195 133].

Lithology: Columnar and flow-jointed porphyritic andesite lava flows with intercalations of volcaniclastic rocks.

Ranking: Local member of the Birker Fell Formation south of Buttermere.

Reference: McConnell and Kneller (1993); Woodhall (2000a).

Map: Keswick (29).

4.2.5.18 Round How Breccia Member (RND)

Stratotype: Reference section on crags on Little Round How [NY 208 132].

Lithology: Poorly- to well-bedded, volcaniclastic sandstone and breccia, locally slumped.

Ranking: Local member of the Birker Fell Formation south-east of Buttermere.

Remarks: Overlies the Haystacks Member.

Reference: McConnell and Kneller (1993); Woodhall (2000a).

Map: Keswick (29).

4.2.5.19 Wrighthow Basalt Member (WGTB)

Stratotype: Type area at Wrighthow Crags, north of Hollins Farm, west Cumbria [NY 107 037].

Lithology: Clinopyroxene and plagioclase-phyric basalt lava flows. Lenses of massive basalt are intercalated with scoriaceous autobreccia.

Ranking: Local member within the upper part of the Birker Fell Formation in the Wasdale area, west Cumbria.

References: Petterson et al. (1992); Akhurst et al. (1997); Millward et al. (2000a).

Maps: Gosforth (37), Ambleside (38).

4.2.5.20 Throstle Garth Basalt Member (TGH)

Stratotype: Type area at Throstle Garth [NY 226 040] and the river Esk nearby.

Lithology: Scoriaceous aphyric to sparsely olivine-phyric basalt lava flows.

Ranking: Local member of the Birker Fell Formation in Eskdale.

References: Petterson et al. (1992); Beddoe-Stephens et al. (1995); Millward et al. (2000a).

Map: Ambleside (38).

4.2.5.21 Frith Crag Breccia Member (FRCR)

Stratotype: Type area on Swindale Foot Crag [NY 5190 1383].

Lithology: Unit of massive, matrix- to clast-supported tuff-breccia.

Ranking: Local member within the Birker Fell Formation east of Haweswater.

Remarks: Previously Frith Crag Tuffs of Nutt (1970).

Reference: Millward (2003).

Map: Appleby (30).

4.3 ‘Upper’ Borrowdale Volcanic Group: Scafell Caldera Succession

4.3.1 Whorneyside Tuff Formation (WNY)

Stratotype: Type section at Whorney Side waterfall, Crinkle Crags [NY 261 054].

Lithology: Unit of brown-weathered, massive, eutaxitic andesitic lapilli-tuff (Wet Side Edge Member; ignimbrite) overlain by laminated and thinly bedded tuff containing many units of accretionary lapilli (phreatoplinian fall-out deposit).

Ranking: Widespread pyroclastic formation in the western Lake District, forming the basal unit of the Scafell Caldera succession.

Lower boundary: Taken at the base of lapilli-tuff and tuff resting markedly unconformably on andesite sheets of the Birker Fell Formation.

Upper boundary: Defined by the base of the overlying Airy’s Bridge Formation.

Thickness: 0 to 450 m.

Remarks: Marks the base of the ‘upper’ Borrowdale Volcanic Group in the western part of the Borrowdale Volcanic Group outcrop and records a fundamental change in eruption style from dominantly lava to explosive, pyroclastic.

References: Branney (1991); Branney and Kokelaar (1993); Millward et al. (2000a).

Maps: Keswick (29), Ambleside (38).

4.3.1.1 Wet Side Edge Tuff Member (WSE)

Stratotype: Type area on Wet Side Edge [SD 270 020].

Lithology: Brown-weathered, massive, eutaxitic andesitic lapilli-tuff (ignimbrite). The upper part is abruptly transitional into the overlying bedded tuff.

Ranking: Basal member of the Whorneyside Formation.

Remarks: Whorneyside Ignimbrite of Branney (1991).

Reference: Millward et al. (2000a).

4.3.2 Airy’s Bridge Tuff Formation (AIB)

Stratotype: Type area from Long Top to Crinkle Crags [NY 240 040] to [NY 250 050].

Lithology: White to pink-weathered, massive, eutaxitic and foliated, garnetiferous rhyodacitic tuff and lapilli-tuff (ignimbrites). The sequence is stratified and divided into two members; the lower one, the Long Top Tuff, dominantly of eutaxitic ignimbrites and the higher one, the Crinkle Tuff comprising very densely welded ignimbrites. The boundary between the members is also a marked geochemical break (Beddoe-Stephens and Mason, 1991).

Ranking: Pyroclastic formation in the western Lake District, and forming the major part of the Scafell Caldera succession.

Lower boundary: Taken at the base of the lowest dacitic lapilli-tuff unit; abruptly transitional from the Whorneyside Formation locally, elsewhere unconformable on the Whorneyside and Birker Fell formations.

Upper boundary: Defined by the base of various pyroclastic and sedimentary formations that overstep onto it.

Thickness: 0 to 1010 m.

Remarks: The first major silicic pyroclastic unit in the western Lake District. U-Pb zircon age of 452.8 ± 0.7 Ma determined on the Crinkle Tuff Member (Millward and Evans, 2003).

References: Branney et al. (1993); Branney and Kokelaar (1994); Millward et al. (2000a).

Maps: Keswick (29), Ambleside (38).

4.3.2.1 Long Top Tuff Member (LTT)

Stratotype: Type area as for formation.

Lithology: Bedded units of massive, eutaxitic tuff and lapilli-tuff (ignimbrite). Includes, locally, a basal unit of densely welded ignimbrite (Oxendale Tuff, see below). Intercalated thin units of bedded tuff and lapilli-tuff, interpreted as phreatomagmatic surge and fall-out deposits (Stonesty, Cam Spout and Hanging Stone tuffs, see below).

Ranking: Basal member of the Airy’s Bridge Formation.

Stonesty Tuff (STF)

Stratotype: Representative section at Stonesty Pike [NY 2489 0401].

Lithology: White-weathered, flinty, bedded and cross-bedded accretionary lapilli-tuff-bearing unit.

Ranking: Widespread marker bed, locally seen at the base of the Long Top Member.

Remarks: Not seen where the Oxendale Tuff is present.

Oxendale Tuff (OXT)

Stratotype: Representative sections are in Hell Gill [NY 2590 0545], on the south side of The Band [NY 2670 0546], and southeast of Seathwaite Tarn [SD 256 976] to [SD 259 980].

Lithology: Pink-weathered, lava-like rhyodacitic welded tuff.

Ranking: Bed, locally seen at the base of the Long Top Member in Langdale, Wrynose and the central fells.

Cam Spout Tuff (CPT)

Stratotype: Representative section at Cam Spout [NY 2145 0478].

Lithology: Cream- and brown-weathered, bedded fine tuff with accretionary lapilli.

Ranking: Widespread thin marker bed within the Long Top Member.

Hanging Stone Tuff (HNG)

Stratotype: Representative section on Base Brown [NY 2295 1195].

Lithology: Cream- and brown-weathered, bedded fine tuff with accretionary lapilli.

Ranking: Widespread marker bed at the top of the Long Top Member.

4.3.2.2 Crinkle Tuff Member (CRK)

Stratotype: Type area on the summit of Crinkle Crags between Long Top and Shelter Crags [NY 2485 0475] to [NY 2495 0580].

Lithology: White-weathered, eutaxitic, parataxitic and foliated lapilli-tuff with interbeds of mesobreccia (interpreted as densely welded ignimbrites), and thin units of bedded tuff (interpreted as pyroclastic fall-out and surge deposits).

Ranking: Upper member of the Airy’s Bridge Formation. Includes a distinctive named ignimbrite unit (Bad Step Tuff) and a named marker bed (Rest Gill Tuff).

Bad Step Tuff (BDP)

Stratotype: Representative section on Bad Step, near the summit of Crinkle Crags [NY 2487 0485].

Lithology: Pink to white-weathered, lava-like rhyolitic tuff, locally with heterolithic pyroclastic breccia and autobreccia.

Ranking: Distinctive bed within the Crinkle Member.

Reference: Branney et al. (1992).

Rest Gill Tuff (REG)

Stratotype: Representative section in Rest Gill, Crinkle Crags [NY 2446 0558] to [NY 2500 0536].

Lithology: Brown and turquoise-weathered, stratified tuff with common scour and fill structures.

Ranking: Widespread marker bed overlying the Bad Step Tuff.

4.3.3 Lingmell Tuff Formation (LME)

Stratotype: Type area from Lingmell to Scafell and Great Gable [NY 200 080] to [NY 200 060] to [NY 210 100].

Lithology: Pale-weathered, stratified sequence of garnetiferous tuff, lapilli-tuff and pyroclastic breccia (intercalated thin ignimbrites and pyroclastic surge deposits); subordinate reworked volcaniclastic sedimentary rocks. Includes named dacite and rhyolite lavas.

Ranking: Uppermost, dominantly pyroclastic, formation in the Scafell Caldera succession.

Lower boundary: Taken at the change in lithology from massive, very densely welded lapilli-tuff of the upper part of the Airy’s Bridge Formation, to a diverse sequence of dominantly thin units of less densely welded lapilli-tuff and tuff.

Upper boundary: Defined by the base of the sedimentary Seathwaite Fell Formation.

Thickness: 0 to 450 m.

Remarks: Represents the waning phases of activity within the Scafell Caldera succession.

References: Kneller et al. (1993); Millward et al. (2000a).

Maps: Keswick (29), Ambleside (38).

4.3.3.1 Scafell Dacite Member (SCD)

Stratotype: Type area from Little Narrow Cove to Aaron Crags [NY 2250 0675] to [NY 215 085].

Lithology: Pale-weathered, flow-foliated, garnetiferous dacite lava, locally with autobreccia.

Ranking: Member within the Lingmell Formation.

4.3.3.2 Rosthwaite Rhyolite Member (RSR)

Stratotype: Type area on Rosthwaite Fell [NY 255 116].

Lithology: Pinkish weathered, flow-foliated crystal-poor rhyolite lava with autobreccia.

Ranking: Member within Lingmell Formation, associated with an intrusion.

4.4 ‘Upper’ Borrowdale Volcanic Group: Duddon Basin Succession

4.4.1 Waberthwaite Tuff Formation (WB)

Stratotype: Type area on Corney Fell, west Cumbria [SD 151 902] to [SD 161 917].

Lithology: Massive, poorly sorted, generally lithic-rich andesitic to dacitic lapilli-tuff and tuff with intercalations of pyroclastic breccia (ignimbrite).

Ranking: Major pyroclastic formation confined to the south-west Lake District.

Lower boundary: Taken at the marked change in lithology at the base of lithic-rich lapilli-tuff; in part oversteps the Airy’s Bridge Formation to rest on andesites of the Birker Fell Formation.

Upper boundary: Defined by the base of the Duddon Hall Formation.

Thickness: Up to 1900 m.

Remarks: Included within the Airy’s Bridge Formation on the 1:25 000 map Devoke Water and Ulpha [SD 1 9], but shown to be distinct lithologically and geochemically; the Waberthwaite Tuff is generally more lithic rich. Laterally equivalent to the Millom Park Formation (name now obsolete) as used on the Ulverston sheet.

Reference: Millward et al. (2000a); Johnson et al. (2001).

Maps: Ambleside (38), Ulverston (48).

4.4.1.1 Wrayslack Tuff Member (WRT)

Stratotype: Type area on Wrayslack, c. 1 km west of Duddon Bridge [SD 186 882].

Lithology: Massive, andesitic lapilli-tuff (ignimbrite).

Ranking: Local member within the Waberthwaite Formation.

4.4.1.2 Barrow Tuff Member (BART)

Stratotype: Type area on Barrow, 1.2 km north-west of Duddon Bridge [SD 186 887].

Lithology: Pink-weathered, foliated, lava-like dacitic lapilli-tuff with autobreccia at top (ignimbrite).

Ranking: Local member within the Waberthwaite Formation.

4.4.1.3 Harrath Tuff Member (HHT)

Stratotype: Type area between Harrath and Corner Croft, near Millom [SD 1570 8105] to [SD 1783 8387].

Lithology: White-weathered, densely welded, foliated rhyolitic tuff (ignimbrite).

Ranking: Local member within the Waberthwaite Formation.

4.4.1.4 Beck Wood Tuff Member (BWT)

Stratotype: Type area in Beck Wood, 500 m west of Beck Farm, near Millom [SD 1600 8100] to [SD 1560 8057].

Lithology: Densely welded rhyolitic tuff (ignimbrite).

Ranking: Local member within the Waberthwaite Formation.

4.4.1.5 Duddon Bridge Tuff Member (DBT)

Stratotype: Type area between Barrow and Duddon Hall [SD 186 887] to [SD 193 996].

Lithology: Massive, andesitic lapilli-tuff (ignimbrite).

Ranking: Locally, the uppermost member of the Waberthwaite Formation.

4.4.2 Duddon Hall Tuff Formation (DH)

Stratotype: Type area from Wallowbarrow Heald to Hollin House Tongue in the Duddon Valley [SD 210 970] to [SD 220 960]; reference section near Goat’s Hause, Coniston [SD 266 982].

Lithology: Planar-bedded andesitic tuff, massive units of lapilli-tuff and some of pyroclastic breccia.

Ranking: Pyroclastic formation within the Duddon Basin succession.

Lower boundary: Taken at the base of the succession of bedded tuffs, overlying massive dacitic or rhyodacitic lapilli-tuff of the Airy’s Bridge and Waberthwaite formations.

Upper boundary: Defined by the base of various overlying formations.

Thickness: 200 to 450 m.

Remarks: Resembles the Whorneyside Formation; the bedded part of the sequence is interpreted as a phreatoplinian fall-out deposit and the lapilli-tuff and breccia units as pyroclastic flow deposits. Intruded by andesite sills locally.

Reference: Millward et al. (2000a); Johnson et al. (2001).

Maps: Ambleside (38), Ulverston (48).

4.4.3 Holehouse Gill Mudstone Formation (HLH)

Stratotype: Type section in Holehouse Gill [SD 190 926] to [SD 181 932].

Lithology: Dark grey, silty mudstone with interbeds of sandstone and granule conglomerate.

Ranking: Non-volcanic formation confined to the Duddon Basin.

Lower boundary: Taken at the base of dark grey silty mudstones resting unconformably on bedded andesitic tuffs of the Duddon Hall Formation.

Upper boundary: Defined by the base of the overlying Ulpha, Wallowbarrow and Dunnerdale formations.

Thickness: 0 to 450 m.

Remarks: Described as the only biostratigraphically significant formation within the Borrowdale Volcanic Group by Molyneux (1988), though this is now in doubt (S G Molyneux, pers. comm., 2001).

Reference: Millward et al. (2000a).

Maps: Ambleside (38).

4.4.4 Ulpha Andesite Formation (ULP)

Stratotype: Type area south-east of Ulpha [SD 190 920] to [SD 20 920].

Lithology: Plagioclase-phyric andesite lavas with autobreccia.

Ranking: Lava formation confined to the Duddon Basin.

Lower boundary: Taken at the base of the lowest lava overlying bedded andesitic tuff of the Duddon Hall Formation, or mudstones of the Holehouse Gill Formation.

Upper boundary: Defined by the base of the overlying Wallowbarrow and Dunnerdale formations.

Thickness: 0 to 800 m.

Reference: Millward et al. (2000a); Johnson et al. (2001).

Maps: Ambleside (38), Ulverston (48).

4.4.5 Wallowbarrow Tuff Formation (WAL)

Stratotype: Type area near Wallowbarrow Crag [SD 222 967].

Lithology: Massive, andesitic lapilli-tuff and pyroclastic breccia (ignimbrite).

Ranking: Pyroclastic formation confined to the Duddon Basin.

Lower boundary: Taken at the base of the distinctive lithology overlying the Duddon Hall and Holehouse Gill formations.

Upper boundary: Defined by the base of the overlying Dunnerdale and Lickle formations.

Thickness: 0 to 70 m.

Reference: Millward et al. (2000a).

Maps: Ambleside (38), Ulverston (48).

4.4.6 Dunnerdale Sandstone Formation (DNF)

Stratotype: Type area on Dunnerdale Fells between Yew Pike and Croglinhurst [SD 203 925] to [SD 212 898].

Lithology: Coarsening upward unit of bedded volcaniclastic sandstone, local intercalations of sedimentary breccia. Bed thickness changes from thin at base to thick and very thick towards top. Synsedimentary deformation and water-escape structures are common, particularly in uppermost part. Contains ichnofossils (Johnson et al., 1994).

Ranking: Volcaniclastic formation confined to the Duddon Basin.

Lower boundary: Taken at the base of the sandstone resting unconformably on the Wallowbarrow, Ulpha and Holehouse Gill formations.

Upper boundary: Defined by the base of the overlying Lickle Formation.

Thickness: 0 to 400 m.

References: Millward et al. (2000a); Johnson et al. (2001).

Maps: Ambleside (38); Ulverston (48).

4.4.7 Lickle Tuff Formation (LICK)

Stratotype: Type area on the Dunnerdale Fells to the south-west of Caw [SD 230 944]; reference sections between Yew Pike and Stickle Tarn [SD 204 925] to [SD 213 928] and from Tarn Hill to Great Stickle [SD 2080 9197] to [SD 2117 9159].

Lithology: Three units of massive dacitic and rhyolitic tuff and lapillituff (ignimbrite) with subordinate intercalated volcaniclastic sandstone that has been intruded locally by andesite and basaltic andesite sills.

Ranking: Pyroclastic formation confined to the Duddon Basin and Coniston Fells.

Lower boundary: Defined by the base of the pyroclastic Kiln Bank Member overlying the Dunnerdale Formation in the Duddon Basin and by the base of the Paddy End Member overlying the Duddon Hall Formation in the Coniston Fells.

Upper boundary: Defined by the base of the overlying Caw, Lag Bank and Low Water formations.

Thickness: 200 to 500 m.

Reference: Millward et al. (2000a); Johnson et al. (2001).

Maps: Ambleside (38), Ulverston (48).

4.4.7.1 Kiln Bank Tuff Member (KLN)

Stratotype: Type area as for the formation.

Lithology: Massive, crystal-rich eutaxitic dacitic lapilli-tuff (ignimbrite).

Ranking: Basal, pyroclastic member of the Lickle Formation confined to the Duddon Basin.

4.4.7.2 Paddy End Tuff Member (PER)

Stratotype: Type area Old Man of Coniston between Goats Water and Levers Water [SD 266 976] to [SD 280 993]; reference section as for formation.

Lithology: Pink-weathered, densely welded, foliated rhyolitic tuff locally with very coarse pyroclastic breccia at the base (densely welded ignimbrite, in part lava-like).

Ranking: Pyroclastic member of the Lickle Formation in the Duddon Basin and Coniston Fells.

Remarks: Generally referred to as the Paddy End Rhyolite.

4.4.7.3 Stickle Pike Tuff Member (STPI)

Stratotype: As for the formation.

Lithology: Pale-weathered, eutaxitic to parataxitic, rhyolitic lapilli-tuff (densely welded ignimbrite). Geochemically very similar to the Paddy End Tuff.

Ranking: Pyroclastic member of the Lickle Formation overlying the Paddy End Tuff, but confined to the Duddon Basin.

4.4.8 Caw Sandstone Formation (CAW)

Stratotype: Type area on Caw [SD 230 944].

Lithology: Coarsening upwards unit of bedded volcaniclastic sandstone and breccia. A basal unit comprises laminated and thinly bedded medium to coarse-grained sandstone that includes several beds of accretionary lapilli-tuff. Passes gradationally into massive to impersistently bedded heterolithic breccia in the rest of the formation.

Ranking: Volcaniclastic formation confined to the Duddon Basin.

Lower boundary: Taken at the base of the sandstone or breccia sequence that overlies rhyolitic lapilli-tuff of the Lickle Formation.

Upper boundary: Defined by the base of the overlying Lag Bank Formation.

Thickness: 0 to 700 m.

Remarks: Sedimentary formation entirely within the Duddon Basin.

Reference: Millward et al. (2000a).

Maps: Ambleside (38).

4.4.9 Lag Bank Tuff Formation (LBT)

Stratotype: Type area near Lag Bank [SD 240 930] to [SD 237 947].

Lithology: Poorly sorted, massive dacitic lapilli-tuff containing abundant white-weathered plagioclase-phyric dacite lapilli (ignimbrite).

Ranking: Pyroclastic formation within the Duddon Basin and Coniston Fells.

Lower boundary: At the base of the lapilli-tuff unconformably overlying sandstone of the Caw Formation or densely welded tuff of the Paddy End Member (Lickle Formation).

Upper boundary: Defined by the base of the overlying Seathwaite Fell Formation.

Thickness: 0 to 600 m.

Reference: Millward et al. (2000a).

Maps: Ambleside (38).

4.4.10 Low Water Tuff Formation (LWW)

Stratotype: Lad Stones, above Beck Fells, Coniston [NY 2953 0010] to [SD 2940 9930].

Lithology: Sheets of uniform, massive, welded dacitic lapilli-tuff (ignimbrite) separated by units of bedded volcaniclastic sandstone and tuff.

Ranking: Pyroclastic formation in the Coniston Fells.

Lower boundary: Taken at the lowest extent of dacitic lapilli-tuff overlying rhyolitic tuff of the Paddy End Member or at the base of sandstones underlying the lapilli-tuff where present.

Upper boundary: Unconformably overlain by the Seathwaite Fell Formation.

Thickness: 0 to 400 m.

Remarks: Includes the Lingmoor Fell Formation (now obsolete) as used on the Ambleside map.

Reference: Millward et al. (2000a).

Maps: Ambleside (38).

4.5 ‘Upper’ Borrowdale Volcanic Group: Haweswater Caldera Succession

4.5.1 Scalehow Tuff Formation (SCW)

Stratotype: Type area on Birk Fell, Ullswater, from Hare Shaw to Scalehow Beck [NY 4015 1775] to [NY 4110 1840].

Lithology: Stratified sequence of andesitic and dacitic lapilli-tuff and intercalated tuff, including some accretionary lapilli-tuff (interpreted as a mixed sequence of pyroclastic density current deposits and pyroclastic fall-out deposits. The upper part of the formation contains some bedded volcaniclastic sandstone produced as a result of reworking by tractional currents and mass-flow processes.

Ranking: Volcaniclastic formation in the Borrowdale Volcanic Group of the Ullswater to Haweswater area.

Lower boundary: Taken at the sharp change from andesitic and dacitic sheets of the Birker Fell Formation to a dominantly pyroclastic succession.

Upper boundary: Defined by the base of the Whelter Knotts Formation.

Thickness: 0 to 340 m.

Remarks: Previously included in the Birk Fell Group of Moseley (1960). Not distinguished on the current printed edition of Sheet 29.

Reference: Woodhall (2000b).

Maps: Keswick (29), Appleby (30).

4.5.2 Whelter Knotts Tuff Formation (WLT)

Stratotype: Type area from Kidsty Pike to Whelter Knotts, west of the Haweswater reservoir [NY 227 125] to [NY 473 137].

Lithology: White- to pink-weathered, massive, garnetiferous silicic tuff and eutaxitic lapilli-tuff (ignimbrite), with subordinate foliated vitrophyric tuff (densely welded ignimbrite) and bedded units of lithic-rich tuff and lapilli-tuff (phreatomagmatic fall-out and surge deposits).

Ranking: Major stratified pyroclastic succession east of Ullswater.

Lower boundary: Taken at the abrupt change to silicic pyroclastic rocks from the andesite of the Birker Fell Formation or volcaniclastic rocks of the Scalehow Formation.

Upper boundary: Defined by the base of the Mardale Formation.

Thickness: Up to 300 m.

Remarks: Previously part of the Place Fell Group (Moseley, 1960); Atkinson and Fordingdale, Stanegarth and Whelter Knotts, Hugh’s Laithes Pike and Haweswater ignimbrite groups (Nutt, 1970). Not distinguished on the current printed edition of Sheet 29.

Reference: Beddoe-Stephens (2000a); Woodhall (2000b); Millward (2003).

Maps: Keswick (29), Appleby (30).

4.5.2.1 Powley’s Hill Tuff Member (PLH)

Stratotype: Type area west of Powley’s Hill, Naddle Forest, Haweswater [NY 502 136].

Lithology: Heterogeneous succession of massive and bedded units of crystal, pumice and lithic-rich silicic tuff and lapilli-tuff; significantly garnetiferous in places. Interpreted as a mixed sequence of pyroclastic density current deposits and pyroclastic fall-out deposits.

Ranking: Pyroclastic member at the base of the Whelter Knotts Formation.

4.5.2.2 Pinnacle Howe Tuff Member (PNH)

Stratotype: Type area on Pinnacle Howe, 2.2 km south-west of Bampton [NY 497 167].

Lithology: Pink-weathered, foliated, crystal poor, vitrophyric tuff (densely welded ignimbrite).

Ranking: Pyroclastic member of the Whelter Knotts Formation.

4.6 ‘Upper’ Borrowdale Volcanic Group: Kentmere Basin Succession

4.6.1 Mardale Sandstone Formation (MRL)

Stratotype: Type area on the fellside on the south-east side of Swindale, near Shap [NY 510 122] to [NY 521 133].

Lithology: Mixed succession of generally bedded volcaniclastic sandstone comprising reworked and re-sedimented detritus mainly of andesitic composition; localised soft-sediment deformation. Intercalated units of volcaniclastic breccia, tuff and lapilli-tuff, the most prominent of which have been designated as members.

Ranking: Volcaniclastic formation in the area south-east of Ullswater.

Lower boundary: Taken at the abrupt change from silicic tuff and lapilli-tuff of the Whelter Knotts Formation to generally well bedded volcaniclastic rocks.

Upper boundary: Defined by the base of the overlying Froswick Formation.

Thickness: Probably up to 980 m; much faulted and further inflated by andesite sills.

Remarks: Includes the Keasgill Tuffs of Nutt (1970).

References: Millward (2003); Beddoe-Stephens (2000a); Woodhall (2000b).

Maps: Appleby (30), Kendal (39).

4.6.1.1 Brown Howe Tuff Member (BNH)

Stratotype: Type area on the fellside of Mardale Banks, south-east of Haweswater reservoir [NY 479 122] to [NY 485 130].

Lithology: Pale grey, parallel-bedded acid andesitic/dacitic coarse tuff and fine lapilli-tuff; locally intercalated thin units of pyroclastic breccia containing ragged bombs of scoriaceous andesite/dacite. Interpreted as pyroclastic fallout and density current deposits.

Ranking: Pyroclastic member at the base of the Mardale Formation.

4.6.1.2 Cawdale Tuff Member (CAWD)

Stratotype: Type area on Four Stones Hill, Bampton [NY 492 163].

Lithology: Massive coarse tuff and tuff-breccia, locally with pyroclastic breccia and pebbly volcaniclastic sandstone. Interpreted dominantly as pyroclastic density current deposits, but with some reworking by tractional currents.

Ranking: Pyroclastic member of the Mardale Formation.

4.6.1.3 Rowantreethwaite Tuff Member (RWT)

Stratotype: Type section on Ritchie Crag, 1.7 km south-south-east of the Haweswater Hotel, Haweswater [NY 490 123].

Lithology: Well bedded (striped) dacitic lapilli-tuff and tuff, with some pyroclastic breccia and accretionary lapilli-tuff. Interpreted as pyroclastic fall-out and surge deposits.

Ranking: Pyroclastic member of the Mardale Formation.

4.6.1.4 Gouther Crag Breccia Member (GRCG)

Stratotype: Type area on Gouther Crag, 500 m south of Truss Gap, Swindale [NY 514 126].

Lithology: Massive poorly sorted volcaniclastic breccia containing abundant pink-weathered rhyolite clasts. Interpreted as mass flow deposit, probably derived from the collapse of a lava.

Ranking: Volcaniclastic member of the Mardale Formation.

4.6.1.5 Great Ladstones Breccia Member (GTLA)

Stratotype: Type area on Great Ladstones, Rafland Forest, near Shap [NY 532 122] to [NY 536 127].

Lithology: Massive to weakly bedded volcaniclastic breccia and pebbly coarse-grained sandstone. Interpreted as mass-flow deposits.

Ranking: Volcaniclastic member of the Mardale Formation.

4.6.2 Froswick Tuff Formation (FSW)

Stratotype: Type section on Swindale Common, from Fewling Stones to High Wether Howe [NY 5130 1175] to [NY 5145 1080]; reference section on eastern slopes of Froswick [NY 437 085], west of the river Kent, Kentmere.

Lithology: Pale-weathered, eutaxitic to parataxitic andesitic to dacitic lapilli-tuff (ignimbrite), locally passes into tuff-breccia and includes thin units of bedded tuff and lapilli-tuff.

Ranking: Major pyroclastic formation extending from the eastern part of the Ambleside area through Troutbeck and Long Sleddale to Wet Sleddale.

Lower boundary: Marked by the abrupt change from bedded volcaniclastic rocks of the Mardale Formation to massive pyroclastic rocks.

Upper boundary: Defined by the base of the overlying Woundale or Seathwaite Fell formations.

Thickness: Up to 640 m.

Remarks: Previously Froswick Tuffs (Mitchell, 1929).

References: Beddoe-Stephens (2000a); Woodhall (2000b); Millward (2003).

Maps: Appleby (30), Kendal (39).

4.6.3 Woundale Tuff Formation (WOU)

Stratotype: Type area in Woundale, north of Troutbeck [NY 410 070 to 425 100].

Lithology: Andesitic tuff and lapilli-tuff, heterogeneous with interbedded volcaniclastic sandstone in the upper part of the formation.

Ranking: Local pyroclastic formation extending from the eastern part of the Ambleside area through Troutbeck, Kentmere and Longsleddale.

Lower boundary: Marked by the abrupt change from uniform massive dacitic tuff of the Froswick Formation to a variable succession of pyroclastic rocks.

Upper boundary: Defined by the base of the overlying Seathwaite Fell Formation.

Thickness: Up to 800 m.

Remarks: Previously the upper part of the Froswick Tuffs of Mitchell (1929). Considered to represent the product of phreatomagmatic volcanism that formed a tuff-ring that was contemporaneously reworked.

Reference: Woodhall (2000b).

Maps: Appleby (30), Kendal (39).

4.6.3.1 Doup Crag Tuff Member (DPT)

Stratotype: Type area from Doup Crag to Hart Crag on the west side of the Troutbeck valley, Windermere [NY 419 092] to [NY 412 083].

Lithology: Bedded and cross-bedded, andesitic lapilli-tuff.

Ranking: Member of the Woundale Formation.

4.6.4 Kentmere Pike Tuff Formation (KMR)

Stratotype: Type area on Kentmere Pike and Hallow Bank Quarter, Kentmere [NY 468 081] to [NY 453 074].

Lithology: Dacitic lapilli-tuff, massive (ignimbrite); some interbedded tuff, lapilli-tuff and volcaniclastic sandstone.

Ranking: Densely welded ignimbrite formation within the Borrowdale Volcanic Group.

Lower boundary: Taken at the base of the dacitic lapilli-tuff overlying volcaniclastic sedimentary rocks; boundary usually obscured by penecontemporaneous andesite intrusions.

Upper boundary: Defined by the base of the overlying Lincomb Tarns Formation, but relationships commonly obscured by intrusions.

Thickness: Up to 350m.

Remarks: Kentmere Pike Rhyolites of Mitchell (1929); Kentmere Formation (in part) of Soper and Numan (1974).

Map: Kendal (39).

4.6.5 Wet Sleddale Andesite Formation (WETS)

Stratotype: Type area on the fellside from Tod Crags south to Little Saddle Crag, Shap Fells [NY 520 103] to [NY 528 083].

Lithology: Dark-grey and blue-grey, porphyritic andesite and basaltic andesite sheets with abundant scoriaceous autobreccia and thin intercalations of bedded volcaniclastic sandstone and tuff.

Ranking: Lava formation or sill complex within the Borrowdale Volcanic Group.

Lower boundary: Taken at the base of the lowest andesite sheet in a thick sequence of andesite sheets.

Upper boundary: Defined by the base of the overlying Lincomb Tarns Formation.

Thickness: Possibly up to 1200 m; uncertainty due to faulting.

Remarks: Mechanism of emplacement not certain because of poor exposure. Part of the Wrengill Andesites of Mitchell (1929).

Reference: Millward (2003).

Maps: Appleby (30), Kendal (39).

4.7 ‘Upper’ Borrowdale Volcanic Group: Rydal Succession

4.7.1 Seathwaite Fell Sandstone Formation (SET)

Stratotype: Type area on Seathwaite Fell, Borrowdale [NY 220 090] to [NY 240 110]; important reference sections at Walna Scar Quarries and Torver High Common [SD 248 958] to [SD 266 953], and Great Intake to Holme Fell (including the Tilberthwaite Quarries) [NY 304 020] to [NY 318 012].

Lithology: Bedded to massive units of fine to coarse-grained volcaniclastic sandstone with intercalations of tuff, lapilli-tuff, pyroclastic breccia, pebbly sandstone and conglomerate. In the Central Fells area, the formation is divided into members, only some of which have been depicted on the 1:50 000-scale maps. Few of these units have been extended beyond the Scafell Syncline.

Ranking: A major volcaniclastic formation within the upper Borrow-dale Volcanic Group.

Lower boundary: In the type area, a facies transition or a sharp contact with Lingmell Formation pyroclastic rocks; overlaps the Scafell Dacite and the Rosthwaite Rhyolite. In the Coniston Fells the formation overlies the pyroclastic Low Water and Lag Bank formations unconformably. Around High Street the Seathwaite Fell Formation rests on the Woundale Formation.

Upper boundary: Defined by the base of the Lincomb Tarns Formation, or in the Kentmere area by the Kentmere Pike Formation.

Thickness: About 300 to 540 m in the central fells; 30 m on Lingmoor Fell; possibly greater than 1100 m in the Tilberthwaite area.

Remarks: A formation that occurs throughout much of the Lake District. The base is diachronous and facies controlled. The sequence in the central fells seems to be condensed following eruption of the pyroclastic rocks of the Scafell caldera succession. By contrast, in the Duddon Basin and Coniston Fells much of the Duddon Basin succession is equivalent to the lower members of the Seathwaite Fell Formation and the upper members are greatly thickened.

The formation was emplaced by a mixture of tractional and mass-flow processes in a lacustrine environment. The volcaniclastic material was derived from active fault scarps, topographical highs and contemporaneous eruptions.

References: Kneller and McConnell (1993); Millward et al. (2000a); Woodhall (1998, 2000b).

Maps: Keswick (29), Appleby (30), Ambleside (38), Kendal (39).

4.7.1.1 Three Tarns Sandstone Member (TTA)

Stratotype: Not defined by Kneller and McConnell (1993). Recommend type area from Pike o’ Stickle to Harrison Stickle [NY 274 074] to [NY 284 073].

Lithology: Well bedded, predominantly white-weathered fine-grained sandstones and siltstones.

Ranking: Basal member of the Seathwaite Fell Formation in the south and east of the Scafell Syncline.

4.7.1.2 Bell Crags Breccia Member (BCR)

Stratotype: Type area around Bell Crags [NY 298 148], near Thirlmere.

Lithology: Matrix-supported, pebble to cobble-grade breccia and pebbly to coarse-grained sandstone.

Ranking: Member within the Seathwaite Fell Formation. Represents deposition from mass flows

4.7.1.3 Cam Crags Sandstone Member (CCR)

Stratotype: Type area from Thorneythwaite Fell to Glaramara [NY 240 120] to [NY 247 105].

Lithology: Dark brown weathered, coarse-grained volcaniclastic sandstone to breccia.

Ranking: Member within the Seathwaite Fell Formation confined to the northern side of the Scafell Syncline.

4.7.1.4 Harrison Stickle Breccia Member (HTK)

Stratotype: Type area on Harrison Stickle [NY 282 073].

Lithology: Pebble-grade breccia to pebbly sandstone; some lapillituff.

Ranking: Member within the Seathwaite Fell Formation in the Scafell Syncline.

4.7.1.5 Dungeon Ghyll Sandstone Member (DGY)

Stratotype: Type area around Langdale Pikes [NY 275 070] to [NY 281 075].

Lithology: Bedded siltstone, sandstone and breccia with widespread slumping and disruption.

Ranking: Member within the Seathwaite Fell Formation in the Scafell Syncline.

4.7.1.6 Pavey Ark Breccia Member (PAV)

Stratotype: Type area on Pavey Ark [NY 285 078].

Lithology: Brown-weathered, massive to poorly bedded, pebbly volcaniclastic sandstone to breccia, in the type area containing ragged vesicular andesite spatter. Interpreted as the product of an eruption-generated subaqueous gravity flow.

Ranking: Member of the Seathwaite Fell Formation widespread between Scafell and Grasmere

4.7.1.7 Bowfell Links Sandstone Member (BFL)

Stratotype: Not defined by Kneller and McConnell (1993). Recommend type area on Bowfell Links [NY 247 063].

Lithology: Thin-bedded, parallel and ripple-cross-laminated siltstone and fine-grained sandstone.

Ranking: Member within the Seathwaite Fell Formation.

4.7.1.8 Cockley Pike Sandstone Member (CPK)

Stratotype: Not defined by Kneller and McConnell (1993). Recommend type area Stake Pass to High Raise [NY 265 088] to [NY 281 095].

Lithology: Coarse to very coarse-grained sandstones, commonly massive or weakly stratified over tens of metres.

Ranking: Member within the Seathwaite Fell Formation.

4.7.1.9 Sprinkling Tarn Sandstone Member (SPR)

Stratotype: Not defined by Kneller and McConnell (1993). Recommend type area near Sprinkling Tarn [NY 228 091].

Lithology: Bedded, coarse-grained and pebbly sandstones interbedded with various pyroclastic rocks and their reworked equivalents.

Ranking: Member within the Seathwaite Fell Formation; includes the Glaramara Tuff, a widespread marker unit.

Glaramara Tuff (GMT)

Stratotype: Type area on the summit of Glaramara [NY 2460 1050].

Lithology: Bedded sequence of white to pale buff weathered, silicic tuff, eutaxitic lapilli-tuff and accretionary lapilli-tuff.

Ranking: Kneller and McConnell (1993) and Millward et al. (2000a) designate this as having bed status within the Sprinkling Tarn Member.

4.7.1.10 St Raven’s Edge Tuff Member (SRE)

Stratotype: St Raven’s Edge [NY 405 085], east side of Kirkstone Pass, Cumbria.

Lithology: Massive andesitic lapilli-tuff, in part eutaxitic (ignimbrite).

Ranking: Member within the Seathwaite Fell Formation in the eastern Lake District.

4.7.2 Lincomb Tarns Tuff Formation (LTA)

Stratotype: Type area from Glaramara to Allen Crags [NY 246 103] to [NY 238 088]; reference sections in Torver Beck [SD 2717 9644] to [SD 2753 9622], Wythburn Fells [NY 3060 1350] to [NY 3015 1270], Pass of Dunmail Raise to Seat Sandal [NY 3340 1045] to [NY 3440 1150] and Threshthwaite Cove [NY 415 103].

Lithology: Eutaxitic, dacitic lapilli-tuff (ignimbrite), typically lithic rich with both silicic and mafic lithic clasts. In part stratified with distinctive units designated as members; elsewhere massive.

Ranking: The most widespread ignimbrite formation of the Borrowdale Volcanic Group.

Lower boundary: Rests sharply on sandstones of the Seathwaite Fell Formation and taken at a sharp change to pyroclastic rocks. In parts the contact is conformable, but in other areas there is an angular discordance.

Upper boundary: Defined by base of the Esk Pike or Tarn Hows formation.

Thickness: Varies form about 170 m near Scafell in the west of the outcrop to more than 800 m north-east of Tarn Hows. There are significant changes across some faults.

References: McConnell (1993); Woodhall (1998, 2000b); Millward et al. (2000a); Millward (2003).

Maps: Keswick (29), Appleby (30), Ambleside (38), Kendal (39).

4.7.2.1 Tarn Crags Tuff Member (TNCR)

Stratotype: Type section along Tarn Crags [NY 3032 1351] to [NY 3085 1348].

Lithology: Bedded coarse dacitic tuff and lapilli-tuff, sparsely garnetiferous. Interpreted as pyroclastic density current and fall-out deposits.

Ranking: Locally, basal member of the Lincomb Tarns Formation.

4.7.2.2 Thirlmere Tuff Member (TMR)

Stratotype: Type area between Whelpside and Dunmail Raise [NY 3250 1450] to [NY 3345 1110]; reference sections on the east side of Steel Fell [NY 320 115] and Thirlmere to Dunmail Raise [NY 3340 1045] to [NY 3300 1585].

Lithology: White- to pink-weathered, parataxitic garnetiferous tuff and lapilli-tuff, locally with mesobreccia (densely welded, rheomorphic ignimbrite).

Ranking: High-grade ignimbrite within the Lincomb Tarns Formation, but locally occurs at its base.

Remarks: Referred to as the Thirlmere Rhyolite by Hartley (1941).

4.7.2.3 Raisebeck Volcaniclastic Member (RABK)

Stratotype: Type section in Raise Beck [NY 3351 1195] to [NY 3406 1208].

Lithology: Massve to bedded tuff and bedded volcaniclastic sandstone. Interpreted as mass-flow and tractional sediments deposited during hiatus in explosive silicic eruption.

Ranking: Member within the Lincomb Tarns Formation.

4.7.2.4 Goat Scar Dacite Member (GSD)

Stratotype: Type area on Goat Scar [NY 476 068] on the west side of Longsleddale.

Lithology: Dacite, in part interpreted as lava dome and coulee-type flow; partly intrusive and part extrusive with thin interbeds of lapilli-tuff.

Ranking: Member in the Lincomb Tarns Formation.

4.8 ‘Upper’ Borrowdale Volcanic Group: Helvellyn Basin Succession

4.8.1 Esk Pike Sandstone Formation (ESP)

Stratotype: Type section on Esk Pike [NY 236 075]; reference sections on Wythburn Fells [NY 300 117] to [NY 308 128], Calf Cove (Rydal Head) [NY 3575 1075] and Black Crag (Rydal Head) [NY 3610 1075] to [NY 3615 1105].

Lithology: Volcaniclastic sandstone and mudstone in the type area. Bedded and massive volcaniclastic sandstone with intercalations of breccia, tuff and lapilli-tuff in areas to the east.

Ranking: Volcaniclastic sedimentary formation within the upper Borrowdale Volcanic Group.

Lower boundary: Taken at a change in lithology at an erosion surface on lapilli-tuff of the underlying Lincomb Tarns Formation to dominantly sandstone.

Upper boundary: Not seen in the type area. In the area around Helvellyn, it is defined by the base of various overlying formations, including the Middle Dodd Dacite, Deepdale and Helvellyn formations.

Thickness: At least 170 m in the central fells and up to 200 m in the Helvellyn area.

Remarks: Marked facies variation between the central fells and the area east of the Coniston Fault. Sediments emplaced in a fluviolacustrine environment by mass-flow and tractional processes with volcaniclastic material derived from active fault scarps and contemporaneous explosive volcanism.

References: McConnell (1993); Millward et al. (2000a); Woodhall (1998, 2000b).

Maps: Keswick (29), Appleby (30), Ambleside (38), Kendal (39).

4.8.2 Middle Dodd Dacite Formation (MDD)

Stratotype: Type area on the summit of Middle Dodd [NY 397 096] and a reference section on Little Hart Crag [NY 3885 1010], Kirkstone Pass.

Lithology: White-weathered, flow-foliated, feldspar-phyric dacite lava.

Ranking: Formation within the upper Borrowdale Volcanic Group.

Lower boundary: Taken at the base of the dacite resting on volcaniclastic sedimentary rocks of Esk Pike Formation.

Upper boundary: Top not seen in the type area or reference section. Probable equivalent lithologies to the north are overlain by the Helvellyn and Deepdale formations.

Thickness: 0 to 170 m.

References: Millward et al. (2000a); Woodhall (1998).

Maps: Keswick (29), Appleby (30), Ambleside (38), Kendal (39).

4.8.3 Garburn Dacite Formation (GAD)

Stratotype: Type area at Garburn, between Troutbeck and Kentmere [NY 432 043] to [NY 440 048].

Lithology: Plagioclase-phyric dacite lavas, in part autobrecciated.

Ranking: Local formation within the upper Borrowdale Volcanic Group of Kentmere.

Lower boundary: Taken at the base of the dacite on volcaniclastic sedimentary rocks of the Esk Pike Formation. In part separated by andesite intrusions.

Upper boundary: Unconformably overlain by the Windermere Supergroup.

Thickness: 0 to 100 m.

Remarks: Upper Rhyolites of Mitchell (1929) and, in part, the Garburn Formation of Soper and Numan (1974).

Map: Kendal (39).

4.8.4 Brock Crags Andesite Formation (BKCG)

Stratotype: Type area on Brock Crags about 1 km north-east of Hartsop, Ullswater [NY 418 136].

Lithology: Andesite lavas, in part autobrecciated, with thin intercalations of lapilli-tuff and volcaniclastic sandstone.

Ranking: Local formation within the upper Borrowdale Volcanic Group of the Ullswater area.

Lower boundary: Taken at the base of the lowest andesite on a few metres of volcaniclastic sedimentary rocks assigned to the Esk Pike Formation. In some poorly exposed parts it may overlie silicic tuff of the Lincomb Tarns Formation.

Upper boundary: Top not seen in the type area.

Thickness: 0 to 290 m.

Reference: Woodhall (2000b).

Map: Appleby (30).

4.8.5 Tarn Hows Tuff Formation (THW)

Stratotype: Type area from Tarn Hows to Iron Keld Plantation [NY 330 000] to [NY 338 008].

Lithology: Massive to very weakly stratified, andesitic coarse tuff and lapilli-tuff, with a very rough weathered surface exhibiting conspicuous surface holes. Emplacement dominantly from pyroclastic density currents.

Ranking: Highest formation in the Borrowdale Volcanic Group in the Coniston area, only occurring east of the Coniston Fault.

Lower boundary: Taken at the change from dacitic to andesitic pyroclastic rocks; overlies the Lincomb Tarns Formation.

Upper boundary: Not seen; overlain by the lowest formation of the Dent Group (Windermere Supergroup).

Thickness: 0 to 420 m.

Remarks: Similar stratigraphical position to the Esk Pike Formation, with which it may be laterally equivalent.

Reference: Millward et al. (2000a).

Map: Ambleside (38).

4.8.6 Helvellyn Tuff Formation (HLV)

Stratotype: Type area in the Helvellyn range includes Nethermost Cove [NY 350 145], Ruthwaite Cove [NY 350 136] and Cock Cove [NY 350 132].

Lithology: Massive, dacitic lapilli-tuff (ignimbrite), eutaxitic in its lower part.

Ranking: Major pyroclastic formation within the Helvellyn range.

Lower boundary: Rests sharply on sandstones of the Esk Pike Formation.

Upper boundary: Defined by basal beds of the Deepdale Formation, representing one of the highest units proved in the Borrowdale Volcanic Group.

Thickness: From about 80 m at Rydal Head to 400 m between Helvellyn and Grisedale; maximum unknown.

Remarks: Equates with the middle and upper parts of the Felsitic and Basic Tuffs of Hartley (1941).

Reference: Woodhall (1998).

Map: Keswick (29).

4.8.7 Deepdale Sandstone Formation (DPD)

Stratotype: Type section in the western part of Deepdale from Mart Crag [NY 372 124] to Greenhow End [NY 371 121], The Step [NY 369 119], Link Cove [NY 371 117] and Hart Crag [NY 368 113]. Important reference section on St Sunday Crag [NY 367 136], Grisedale.

Lithology: Bedded and massive volcaniclastic sandstone with intercalations of pebbly sandstone, breccia, tuff and lapilli-tuff.

Ranking: Volcaniclastic formation within the Helvellyn range, restricted between the Deepdale Hause and Hoggett Gill faults.

Lower boundary: In the west of the outcrop, taken at the erosion surface on top of the Helvellyn Formation where the lapilli-tuff is succeeded by pebbly sandstone; elsewhere, overstep on to the Middle Dodd Dacite and Esk Pike Formation where the base of the formation is defined by the base of the Cawk Cove Member.

Upper boundary: Not seen.

Thickness: Greater than 650 m.

Remarks: Stratigraphically the youngest unit seen at outcrop in the central Lake District. Lithologically similar to both the Seathwaite Fell and Esk Pike formations. Sediments emplaced in fluvial and lacustrine environment by mass-flow and tractional processes, with material derived from active fault scarps and contemporaneous volcanism. Also includes intercalations of ignimbrite.

Reference: Woodhall (1998).

Maps: Keswick (29), Appleby (30).

4.8.7.1 Cawk Cove Sandstone Member (CWK)

Stratotype: Type section between Mart Crag and the south side of Mossydale [NY 3766 1225].

Lithology: Stratified pebbly volcaniclastic sandstone and some bedded coarse-grained volcaniclastic sandstone.

Ranking: Basal member of the Deepdale Formation in the central and south-west part of its outcrop.

4.8.7.2 Blind Cove Sandstone Member (BLCV)

Stratotype: Reference section on prominent north to north-west-facing crags (St Sunday Crag and Blind Cove) [NY 3740 1410]

2.7 to 4 km south-west of Patterdale.

Lithology: Pebbly volcaniclastic sandstone with intercalations of fine to coarse-grained volcaniclastic sandstone in units 2 to 30 m thick.

Ranking: Member within the Deepdale Formation.

4.8.7.3 St Sunday Crag Tuff Member (SSC)

Stratotype: Reference section on crags on the south side of Lord’s Seat, Deepdale [NY 3793 1325] to [NY 3805 1335]; reference section on St Sunday Crag [NY 3694 1385] to [NY 3695 1382].

Lithology: Massive, rhyodacitic tuff and lapilli-tuff with abundant fiamme (ignimbrite).

Ranking: Pyroclastic member within the upper part of the Deepdale Formation, resting on the Blind Cove Member.

4.8.7.4 Cockley How Tuff Member (CKY)

Stratotype: Reference section on crags adjacent to Cockley How 1.8 km south-south-west of Deepdale [NY 3940 1290].

Lithology: Dacitic lapilli-tuff, parataxitic at its base (ignimbrite).

Ranking: Pyroclastic member within the upper part of the Deepdale Formation.

4.8.7.5 Blake Brow Breccia Member (BLBR)

Stratotype: Type section at Link Cove [NY 367 115] to [NY 372 115]; reference section at Blake Brow [NY 367 115] to [NY 372 115],

3.5 km south-west of Deepdale.

Lithology: Volcaniclastic breccia and pebbly sandstone in units up to 75 m thick.

Ranking: Member of the Deepdale Formation; interdigitates with the Greenhow End Member.

4.8.7.6 Dove Crag Tuff Member (DOCR)

Stratotype: Type section at Hart Crag [NY 370 111] and reference section at Dove Crag [NY 374 106].

Lithology: Eutaxitic, andesitic lapilli-tuff (ignimbrite).

Ranking: Pyroclastic member of the Deepdale Formation; the highest unit seen.

4.9 ‘Upper’ Borrowdale Volcanic Group: The Gosforth Succession

4.9.1 Moorside Farm Tuff Formation (MOF)

Stratotype: Partial type section in Sellafield No. 2 Borehole (NY00SE/28) 1495.5 to 1520.2 m below rotary table elevation.

Lithology: Welded lithic-rich lapilli-tuff and pyroclastic breccia (ignimbrite).

Ranking: Oldest formation seen in the volcanic succession west of the Thistleton Fault, in west Cumbria.

Lower boundary: Not seen.

Upper boundary: Overlain by volcaniclastic sandstones of the Broom Farm Formation.

Thickness: Greater than 114 m (core length).

Remarks: Only seen in cores from the base of Sellafield No. 2 Borehole.

References: Millward et al. (1994); Akhurst et al. (1997).

4.9.2 Broom Farm Sandstone Formation (BRFA)

Stratotype: Type section in Sellafield No. 2 Borehole (NY00SE/28) 1482.37 to 1495.5 m below rotary table elevation.

Lithology: Bedded coarse-grained and pebbly volcaniclastic sandstone, intercalated with laminated fine-grained volcaniclastic sandstone and siltstone. The coarser units are medium and thick bedded, commonly with convolutions.

Ranking: Formation within the volcanic succession west of the Thistleton Fault in west Cumbria.

Lower boundary: Placed at the sharp base of a siltstone bed marking a downward change from bedded volcaniclastic sedimentary rocks to lapilli-tuff.

Upper boundary: Overlain by welded lapilli-tuff of the Bleawath Formation.

Thickness: Borehole core length of 13.5 m.

Remarks: Only seen in cores from Sellafield No. 2 Borehole.

References: Millward et al. (1994); Akhurst et al. (1997).

4.9.3 Bleawath Tuff Formation (BLEA)

Stratotype: Type section in Sellafield No. 2 Borehole (NY00SE/28) 1066.5 to 1482.37 m below rotary table elevation.

Lithology: Massive, fiamme-rich, eutaxitic to parataxitic, andesitic to dacitic lapilli-tuff (densely welded ignimbrites).

Ranking: Formation within the volcanic succession west of the Thistleton Fault in west Cumbria.

Lower boundary: Taken at a sharp base of lithic-rich welded lapilli-tuff overlying volcaniclastic sandstone.

Upper boundary: Defined by the base of the Brown Bank Formation.

Thickness: A core length of 416 m recorded in the Sellafield No. 2 Borehole.

Remarks: Seen in cores from Sellafield No. 2, 4, 5 and 10A boreholes; not seen at outcrop. Type section represents the only complete section through the formation.

References: Millward et al. (1994); Akhurst et al. (1997).

4.9.4 Brown Bank Tuff Formation (BBA)

Stratotype: Type section in Sellafield 10A Borehole (NY00SW/38) 1053.0 to 1408.5 m below rotary table elevation; reference section in the Sellafield RCF1 Borehole (NY00SE/35)

908.7 to 1150 m (section incomplete) below rotary table elevation.

Lithology: Heterogeneous stratified succession of massive units of lithic-rich tuff, lapilli-tuff, tuff-breccia and pyroclastic breccia (ignimbrites). Most of the units are andesitic in composition, though there are also important dacitic/rhyolitic units of densely welded lapilli-tuff, including the important marker unit of the Seascale Hall Member. There are several thin units of bedded lapilli-tuff (pyroclastic fall-out deposits) and volcaniclastic sandstone.

Ranking: Formation within the volcanic succession west of the Thistleton Fault in west Cumbria.

Lower boundary: Taken at the lowest occurrence of lithic-rich lapilli-tuff overlying the pumice-rich lapilli-tuff of the Bleawath Formation.

Upper boundary: Defined by the base of the Fleming Hall Formation.

Thickness: From 45 to more than 250 m (uncorrected core lengths).

Remarks: Seen in Sellafield boreholes 2, 4, 5, 10A, RCF1, RCM1, RCM2, and possibly in boreholes 8A, 9 and RCF3.

References: Millward et al. (1994); Akhurst et al. (1997).

4.9.4.1 Seascale Hall Tuff Member (SEHA)

Stratotype: Type section in the Sellafield 10A Borehole (NY00SW/38); reference section in the Sellafield RCF1 Borehole (NY00SW/35) 908.71 to 994.09 m below rotary table elevation.

Lithology: Eutaxitic to parataxitic rhyolitic lapilli-tuff with pale coloured rhyolitic clasts (ignimbrite).

Ranking: Within the Gosforth sequence this unit has very distinctive wireline log characteristics that, along with its lithology, make it the most important unit for correlation (Millward et al. 2002).

4.9.5 Fleming Hall Tuff Formation (FLH)

Stratotype: Type locality at Table Rock [NY 0885 0360]; reference section in Sellafield No. 5 Borehole (NY00SE/30) 489.83 to 892.42 m below rotary table elevation.

Lithology: Eutaxitic and parataxitic andesitic tuff and lapilli-tuff (rheomorphic ignimbrite); has high magnetic susceptibility.

Ranking: Formation within the volcanic succession west of the Thistleton Fault in west Cumbria.

Lower boundary: Defined by the base of the Town End Farm Member.

Upper boundary: Not seen in the Sellafield boreholes; poorly exposed at outcrop, but defined by the base of the Lowcray Formation.

Thickness: 180 to more than 380 m.

Remarks: The only formation in this succession proved at outcrop and in the Sellafield boreholes 2, 4, 5, 10A, 11A, 12A, RCF1, RCF2, RCF3, RCM1, RCM2, RCM3.

References: Millward et al. (1994); Akhurst et al. (1997).

Map: Gosforth (37).

4.9.5.1 Town End Farm Tuff Member (TOEF)

Stratotype: Type section in Sellafield No. 5 Borehole (NY00SE/30) 736.95 to 892.42 m below rotary table elevation.

Lithology: Eutaxitic and parataxitic andesitic lapilli-tuff (rheomorphic ignimbrite).

Ranking: Basal member of the Fleming Hall Formation.

4.9.5.2 Sides Farm Breccia Member (SIFA)

Stratotype: Type section in the Sellafield Borehole 5 (NY00SE/30) 669.18 to 736.95 m below rotary table elevation; reference section in the Sellafield RCM3 Borehole (NY00SE/39) 811.08 to 887.00 m below rotary table elevation.

Lithology: Clast-supported blocky breccia in the Sellafield No. 5 Borehole; in the Sellafield RCF and RCM boreholes comprises thick beds of graded clast and matrix supported, pumice and/or lithic-rich lapilli-tuff, tuff-breccia and pyroclastic breccia.

Ranking: Member within the Fleming Hall Formation.

4.9.5.3 Longlands Farm Tuff Member (LOFA)

Stratotype: Partial type section in the Sellafield No. 4 Borehole [NY 00 SE 29] 411.75 to 795.90 m below rotary table elevation.

Lithology: Geochemically homogeneous, eutaxitic and parataxitic andesitic tuff and lapilli-tuff (rheomorphic ignimbrite); some coignimbrite breccia.

Ranking: Only member of the Fleming Hall Formation at outcrop; best seen in borehole cores.

Remarks: Lithologically very similar to the Town End Farm Member.

4.9.6 Lowcray Tuff Formation (LOCY)

Stratotype: Poorly exposed, mainly in lower reaches of the River Bleng [NY 088 063] to [NY 085 053], which is taken as the type section.

Lithology: Crystal-rich parataxitic tuff and lithic-rich coarse lapilli-tuff. Locally there is a weak eutaxitic fabric, suggesting that these rocks were emplaced as pyroclastic flows (ignimbrites).

Ranking: Formation within the volcanic succession west of the Thistleton Fault in west Cumbria.

Lower boundary: Not seen but mapped at the lowest lapilli-tuff above the densely welded tuffs of the Fleming Hall Formation.

Upper boundary: Defined by the base of the overlying Blengdale Formation.

Thickness: About 650 m.

Reference Akhurst et al. (1997).

Map: Gosforth (37).

4.9.7 Blengdale Tuff Formation (BGD)

Stratotype: Type area in Blengdale Forest with numerous small quarries north-west of forestry track south-west of Gait Kirk; reference section on Ponsonby Fell [NY 0803 0735], near Gosforth.

Lithology: Rhyolitic parataxitic tuff (rheomorphic ignimbrite), overlain by pyroclastic breccia locally.

Ranking: Uppermost formation of the volcanic succession west of the Thistleton Fault in west Cumbria.

Lower boundary: Taken at base of the distinctive lithology.

Upper boundary: Top not seen; unconformably overlain by Permo-Triassic strata.

Thickness: About 250 m.

Remarks: One of the most compositionally differentiated units within the Borrowdale Volcanic Group and possibly the youngest. May be equivalent to the Yottenfews Formation in the Sellafield No. 7 Borehole to the west.

Reference: Akhurst et al. (1997).

Map: Gosforth (37).

4.9.8 Newton Manor Sandstone Formation (NEM)

Stratotype: Partial type section in the Sellafield No. 7A Borehole (NY00SE/36) 771.5 to 1010.00 m below rotary table elevation.

Lithology: Dominantly bedded volcaniclastic sandstone, mainly coarse and very coarse grained, medium and thick beds; a variety of other facies are also present, including parallel laminated siltstone and well sorted fine to medium- grained sandstone with soft-sediment deformation, and units of cross-bedded sandstone. Some intercalated thin units in the upper part of tuff and lapilli-tuff interpreted as pyroclastic fall-out and flow deposits.

Ranking: Formation within the volcanic succession west of the Thistleton Fault in west Cumbria.

Lower boundary: Not reached in boreholes.

Upper boundary: Defined by the base of the Yottenfews Formation in Sellafield No. 7A Borehole.

Thickness: At least 200 m of core length are seen, including andesite sills, in the Sellafield No. 14A Borehole.

Remarks: Only known from the Sellafield No. 7A and 14A boreholes.

References: Millward et al. (1994); Akhurst et al. (1997).

4.9.9 Yottenfews Tuff Formation (YOT)

Stratotype: Partial type section in the Sellafield No. 7A Borehole (NY00SE\4) 577.83 to 718.15 m below rotary table elevation.

Lithology: Eutaxitic to parataxitic rhyolitic tuff and lapilli-tuff. A stratified sequence of ignimbrites that includes the distinctive, spherulitic Sella Park Member.

Ranking: Formation within the volcanic succession west of the Thistleton Fault in west Cumbria.

Lower boundary: Defined by the base of the Sella Park Member resting on bedded volcaniclastic sandstones.

Upper boundary: Not seen; overlain by Carboniferous strata.

Thickness: At least 143 m in core length.

Remarks: Not seen at outcrop, but from the geochemistry it may be correlated with the Blengdale Formation.

References: Millward et al. (1994); Akhurst et al. (1997).

4.9.9.1 Sella Park Tuff Member (SEP)

Stratotype: Type section in the Sellafield No. 7A Borehole (NY00SE\4)

680.96 to 771.5 m below rotary table elevation.

Lithology: High-silica rhyolitic eutaxitic to parataxitic tuff (ignimbrite) with abundant spherulites.

Ranking: Member at the base of the Yottenfews Formation.

4.10 ‘Upper’ Borrowdale Volcanic Group: Succession In The Cross Fell Inlier

4.10.1 Studgill Tuff Formation (SGIL)

Stratotype: Not defined by Burgess and Holliday (1979). Crops out on the north-east face of Dufton Pike [NY 699 267] which is recommended as the type area.

Lithology: Lapilli-tuff and volcaniclastic sandstone.

Ranking: Local formation within the Borrowdale Volcanic Group in the Cross Fell inlier.

Boundaries

The lower boundary is not seen. The upper boundary is mapped at the base of the overlying Knock Pike Formation.

Thickness: About 300 m.

Remarks: Not designated on the published 1:50 000-scale maps, but described by Burgess and Holliday (1979). This unit is very poorly exposed and details of the lithofacies seen are not given.

Reference: Burgess and Holliday (1979).

Maps: Penrith (24), Appleby (30), Brough-under-Stainmore (31).

4.10.2 Knock Pike Tuff Formation (KNPK)

Stratotype: Not defined by Burgess and Holliday (1979). It is best exposed in the large roadstone quarry on the north-east side of Knock Pike [NY 6870 2850], which is recommended as a partial type area.

Lithology: Massive, pale-grey, eutaxitic and parataxitic, silicic lapillituff and tuff; most is interpreted as densely welded ignimbrite, though some may be lava.

Ranking: Local formation within the Borrowdale Volcanic Group in the Cross Fell inlier.

Boundaries

The poorly exposed base is mapped at the change from intercalated volcaniclastic rocks of the Studgill Formation to densely welded silicic tuff. The top of the formation is defined by the base of the overlying Harthwaite Formation.

Thickness: About 500 m.

Remarks: Not designated on the published 1:50 000-scale maps, but described by Burgess and Holliday (1979).

References: Burgess and Wadge (1974); Burgess and Holliday (1979).

Maps: Penrith (24), Appleby (30), Brough-under-Stainmore (31).

4.10.3 Harthwaite Tuff Formation (HAWT)

Stratotype: Not defined by Burgess and Holliday (1979). There is a good section in Harthwaite Sike [NY 7084 2484] to [NY 7079 2481] which is recommended as a partial type section; a section in the lowest part of the formation is seen in a small quarry [NY 7103 2524] on Dod Hill and is recommended as a reference section.

Lithology: Silicic lapilli-tuff, volcaniclastic sandstone and banded siltstone.

Ranking: Uppermost formation with the Borrowdale Volcanic Group locally in the Cross Fell inlier.

Boundaries

The base is mapped at the change from densely welded silicic tuff to a mixed sequence of volcaniclastic rocks. The formation is overlain by sedimentary rocks of the Dent Group.

Thickness: About 300 m.

Remarks: Not designated on the published 1:50 000-scale maps, but described by Burgess and Holliday (1979). Though not stated, descriptions of this unit imply that the various lithofacies are interbedded.

Reference: Burgess and Holliday (1979).

Maps: Penrith (24), Appleby (30), Brough-under-Stainmore (31).

4.11 Volcanic units in the Dent Group (Windermere Supergroup)

4.11.1 Yarlside Volcanic Formation (YRL)

Stratotype: Type section in Stockdale Beck, Longsleddale [NY 4920 0558].

Lithology: Pink to pale grey, flow-banded silicic vitrophyre, locally includes massive tuff and lapilli-tuff, particularly at the base.

Ranking: Volcanic formation within the Dent Group (Windermere Supergroup).

Lower boundary: Taken at the base of the volcanic rocks resting on siltstone and sandstone of the Stile End Formation. In places overlies andesite within the Borrowdale Volcanic Group.

Upper boundary: Defined by the base of the overlying Kirkley Bank Formation.

Thickness: Up to 325 m.

Age:Ashgill (Cautleyan).

Remarks: Previously known as the Stockdale or Yarlside Rhyolite, the latter being the older term (Marr, 1892). Other rocks are classified as Stockdale Group, therefore previous designation as Stockdale Rhyolite Formation/Member now must be changed (Kneller et al., 1994). Interpreted as a succession of lavas by many authors, though a pyroclastic origin (rheomorphic ignimbrite) has been proposed (see discussion in Stephenson et al., 1999).

References: Kneller et al. (1994); Lawrence et al. (1986); Millward and Lawrence (1985).

Map: Appleby (30), Kendal (39).

4.11.2 High Haume Tuff Formation (HHR)

Stratotype: Type area near High Haume Farm, north-east of Dalton in Furness [SD 227 762].

Lithology: Massive coarse, rhyolitic crystal-lithic tuff with a weak eutaxitic texture.

Ranking: Volcanic formation within the Dent Group (Windermere Supergroup).

Lower boundary: Base of rhyolitic tuff on calcareous silty mudstone of the Kirkley Bank Formation.

Upper boundary: Defined by the base of the overlying Ashgill Formation.

Thickness: Up to 25 m.

Age: Ashgill (Rawtheyan; Zone 6).

Remarks: Previously the High Haume Rhyolite (Rose and Dunham, 1977). Interpreted as an ignimbrite.

References: Johnson et al. (2001); Kneller et al. (1994); Rose and Dunham (1977).

Map: Ulverston (48).

4.11.3 Appletreeworth Volcanic Formation (AVF)

Stratotype: Type area near Appletreeworth [SD 2444 9254].

Lithology: Pervasively altered and resedimented rhyolitic tuff; crudely bedded with locally disrupted bedding.

Ranking: Volcanic formation within the Dent Group (Windermere Supergroup).

Lower boundary: Taken at the base of the tuff overlying calcareous siltstone and limestones of the Broughton Moor Formation.

Upper boundary: Defined by the base of the overlying Ashgill Formation.

Thickness: Up to 5 m.

Age: Ashgill (Rawtheyan; Zone 6).

Remarks: Equivalent to the ‘Rhyolitic Ash’ of Marr (1916) and McNamara (1979). Emplaced as sediment–gravity flows.

Reference: Kneller et al. (1994); Millward et al. (2000a)

Map: Ambleside (38), Ulverston (48).

4.11.4 Sowerthwaite Formation (SOW)

The Sowerthwaite Formation occurs within the Dent Group of the Austwick and Crummack anticlines (Craven inliers) (Arthurton et al., 1988). At the base of the formation, the sequence of siltstones and mudstones contains a unit dominated by volcaniclastic rocks.

4.11.4.1 Dam House Bridge Tuff Member (DHBT)

Stratotype: Type section in Austwick Beck at Dam House Bridge [NY 7798 6953] to [NY 7799 6957].

Lithology: Siltstone and mudstone with interbedded units of lithic crystal tuff, probably of silicic composition.

Ranking: Basal member of the Sowerthwaite Formation.

Lower boundary: Taken at the base of the lowest bed of tuff overlying calcareous siltstones and argillaceous limestones of the Norber Formation.

Upper boundary: Taken at the top of the last bed of tuff where it is overlain by siltstone and mudstone within the main part of the Sowerthwaite Formation.

Thickness: Up to 40 m.

Age: Ashgill (Rawtheyan; Zone 6).

Reference: Arthurton et al. (1988).

Maps: Settle (60).

4.11.5 Cautley Mudstone Formation (CMU)

The Cautley Mudstone Formation occurs within the Dent Group of the Cautley and Dent inliers. The formation contains a single unit of volcaniclastic rocks near to its top.

4.11.5.1 Cautley Volcanic Member (CTV)

Stratotype: Type area of Wandale Beck (Murthwaite Inlier) and its tributaries on its western bank between the Odd Gill and the confluence with the River Rawthey [SD 707 980 to SD 713 990]; reference section in Backside Beck (Westerdale Inlier) [SD 696 992].

Lithology: Silicic tuffs, buff to pink in colour; thickly bedded at base becoming finer grained and thinly bedded at the top.

Ranking: Member near the top of the Cautley Mudstone Formation.

Lower boundary: Taken at the base of the lowest bed of volcaniclastic rock overlying grey flaggy mudstones of the main part of the formation.

Upper boundary: The tuffs pass upwards into grey splintery mudstone.

Thickness: Up to 24 m; 12.2 m in type area.

Age: Ashgill (Rawtheyan; Zone 6).

Reference: Ingham (1966).

Map: Kirkby Stephen (40).

Chapter 5 Lithodemic scheme for Caledonian intrusive rocks in the Lake District and northern England

Formalised schemes in lithostratigraphy are now widely accepted and are used on all modern British Geological Survey maps. By contrast, intrusive igneous rocks have not been subjected to such rigorous treatment previously. However, development of the digital geological map of Great Britain (DigMapGB), and major reviews of the igneous rocks of Great Britain undertaken as part of the Geological Conservation Review have provided both the driving force and the opportunity to develop a formal lithodemic scheme.

At present, the British Geological Survey’s Stratigraphy Committee has not published guidelines for lithodemic nomenclature. Lithodemic schemes produced for DigMapGB follow the recommendations of the North American Stratigraphic Code, as far as is practicable. The hierarchical scheme used is contained in the corporate dictionary BGS_DIC_LITHOSTRAT_RANK. In this table, the most significant departure from the North American code concerns the rank of ‘lithodeme’, which under the North American Code is considered to be equivalent to ‘formation’. For pragmatic reasons, in our schemes ‘lithodeme’, regarded as the fundamental unit, is equivalent to the lithostratigraphical rank of ‘bed’.

An extensive programme of geochemical analysis and radiometric dating of Lake District intrusive rocks has provided a wealth of data on the timing of episodes of intrusion relative to the sedimentary and extrusive igneous rocks. Various aspects of this study have been reported in papers by, among others, Rundle (1979, 1981, 1987, 1992), Hughes et al. (1996) and Millward and Evans (2003).

Large volumes of magma were intruded into the Lake District crust during Early Palaeozoic times. The intrusions with the largest outcrops are shown in (Figure 1), but in addition there are concealed plutons and many minor intrusions. Many of the exposed plutonic rocks in this region form a batholith that underpins an area of at least 1500 km². Surface observations and interpretation of available potential field and seismic data indicate that a considerable number of granitic intrusions may be present, many of which are concealed (M K Lee, in Millward et al., 2000a). The radiometric dates on exposed components of the batholith indicate that it was built up during several magmatic episodes. Hence, ‘Lake District batholith’ is not given lithodemic significance.

Millward (2002) and Millward and Evans (2003) recognised three phases of plutonic and minor intrusion emplacement (Figure 2). These were associated with both Caradoc and Ashgill volcanism, and with the Acadian phase of deformation in Early Devonian times. Because these events can be considered to have occurred as part of the Caledonian Orogeny, the intrusive rocks are assigned as components of the Caledonian Igneous Supersuite. The Caledonian Orogeny was defined by Stephenson et al. (1999) to include all of the convergent tectonic and magmatic events arising from the closure of the Iapetus Ocean in which many of the Upper Proterozoic and Lower Palaeozoic rocks of Great Britain had been deposited.

A formal lithodemic framework for the Lower Palaeozoic intrusive rocks of northern England has evolved during preparation of the Geological Conservation Review volume on the Caledonian igneous rocks of Great Britain (Stephenson et al., 1999); this review placed the Lake District rocks within the context of the Caledonian igneous rocks of Great Britain. Millward (2002) proposed a formalised scheme which is adopted here with some modification.

Suites of intrusive rocks in northern England are defined using three factors, the type of intrusion (pluton, sill or minor intrusion), its age (Ordovician or Devonian) and, in some cases, the composition (mafic or felsic). All of the Ordovician granitic plutonic rocks are included within the Lake District Ordovician Felsic Plutonic Suite (5.2, below), and the Early Devonian granitic plutons in the Northern England Devonian Plutonic Suite (5.5). One major, Early Devonian granite, the Cheviot Pluton, is similar in age and composition to the Shap, Skiddaw and Weardale granites in the latter suite, but because of its geographical position within the Southern Uplands, it is included in the Galloway Suite and is not defined here.

The many dykes and other minor intrusive bodies in the Lake District are treated in a manner similar to the plutonic rocks (5.4, 5.6, 5.7). The small number of mafic Ordovician plutonic bodies are included within the Lake District Ordovician Mafic Plutonic Suite (5.3). The large number of basaltic, andesitic and rhyolitic sills associated with the Borrowdale Volcanic Group constitute the Borrowdale Sill Suite (5.1).

5.1 Borrowdale Sill Suite (BVGS)

Includes all contemporaneous sills within the Borrowdale Volcanic Group. These range in composition from basalt to rhyolite. At present no individual sills or groupings of sills have been formally recognised as subsuites or sill complexes, though there is scope for such a development.

Stratotype: Type section in Pets Quarry, Kirkstone Pass [NY 392 073].

Lithology: Basalt, basaltic andesite, andesite, dacite and rhyolite, commonly with peperitic breccia margins.

Age: No radiometric ages have been determined, but features such as peperitic margins imply emplacement into unlithified sediment during the Borrowdale Volcanic episode.

References: Millward (2002); Millward et al. (2000); Johnson et al. (2001); Branney and Suthren (1988).

Maps: Penrith (24), Keswick (29), Appleby (30), Gosforth (37), Ambleside (38), Kendal (39), Ulverston (49).

5.2 Lake District Ordovician Felsic Plutonic Suite (LDFP)

This suite includes all felsic plutons of known and inferred Ordovician age within the region. Most are components of the Lake District batholith, though the Threlkeld Intrusion is a laccolith intruded at levels above the batholith. Though it is clearly not in the Lake District, the concealed Wensleydale Intrusion within the Askrigg Block was included within this suite by Millward (2002), based on evidence cited by Kirby et al. (2000).

5.2.1 Ennerdale Microgranite Intrusion (ENIN)

Stratotype: Type area on Bowness Knott [NY 112 156].

Lithology: Micrographic microgranite, with some diorite, and doleritic and dioritic hybrid rocks.

Radiometric age: 452 ± 4 Ma (U-Pb, zircon).

Remarks: Commonly referred to as the Ennerdale Granophyre. Intruded into the Skiddaw Group and lower part of the Borrowdale Volcanic Group.

References: Hughes et al. (1996); D J Fettes, in Stephenson et al. (1999); Millward et al. (2000a); Akhurst et al. (1997).

Maps: Whitehaven (28), Keswick (29), Gosforth (37), Ambleside (38).

5.2.2 Eskdale Granite Pluton (ESK)

Stratotype: Type area Beckfoot Quarry, Eskdale [NY 164 003].

Lithology: Medium and coarse granite and various types of microgranite.

Radiometric age: 450 ± 3 Ma (U-Pb, zircon).

Remarks: The Eskdale Granite is intruded into the uppermost Skiddaw Group and lower part of the Borrowdale Volcanic Group. The granite is cleaved locally. The Rb-Sr isochron age for the granite (429 ± 4 Ma) is considered to have been reset. The relative emplacement ages of the Broad Oak Granodiorite and the Ennerdale Microgranite have not been resolved.

References: Hughes et al. (1996); B Young, in Stephenson et al. (1999); Millward et al. (2000a); Akhurst et al. (1997); Rundle (1979).

Maps: Gosforth (37), Ambleside (38).

5.2.3 Broad Oak Granodiorite Pluton

Stratotype: Type area Waberthwaite (Broad Oak) Quarry [SD 112 944].

Lithology: Granodiorite and microgranodiorite.

Radiometric age: The Rb-Sr isochron age of 428 ± 71 Ma (Rundle, 1979) is considered to have been reset, and that this pluton was emplaced during Ordovician times.

Remarks: Emplaced into the Lower BVG and Duddon Basin succession. Its emplacement age relative to the Eskdale Granite Pluton has not been resolved because of lack of exposure of the contact. Previously grouped with the Eskdale Granite Pluton as the Eskdale intrusions, but the Broad Oak Granodiorite is a separate pluton emplaced at a higher structural level.

References: B Young, in Stephenson et al. (1999); Millward et al. (2000a); Johnson et al. (2001); Rundle (1979).

Map: Gosforth (37), Ambleside (38), Bootle (49), Ulverston (48).

5.2.4 Threlkeld Microgranite Intrusion (THKD)

Stratotype: Type area in Bramcrag Quarry [NY 320 220].

Lithology: Microgranite; quartz and garnet microphyric.

Radiometric age: 451.0 ± 1.1 Ma (U-Pb, zircon).

Remarks: Emplaced into the Skiddaw and Borrowdale Volcanic groups adjacent to the unconformity. The geochemistry and style of mineralogical alteration seen in the mass are similar to silicic rocks within the Borrowdale Volcanic Group. A Rb-Sr age of 438 ± 6 Ma is shown to have been reset.

References: Millward and Evans (2003); Rundle (1981); S C Loughlin, in Stephenson et al. (1999); Hughes et al. (1996).

Map: Keswick (29).

5.3 Lake District Ordovician Mafic Plutonic Suite (LDMP)

There are only two mafic plutonic bodies in the Lake District, the Carrock Fell Complex and the Haweswater intrusions. The former comprises a number of intrusions of distinct geochemical affinity and probable differing ages. Some aspects of the emplacement ages within the complex remain poorly understood. The age of the Haweswater intrusions is inferred because there is no radiometric-age determination.

5.3.1 Carrock Fell Intrusion-complex (CFCO)

Stratotype: Type area on Carrock Fell [NY 340 320] to [NY 360 342].

Lithology: Layered gabbro–microgranite complex.

Remarks: Includes a number of component intrusions, named below, with varying geochemical affinity. These have proved or inferred Ordovician ages.

Radiometric ages: See component units.

References: Millward and Evans (2003); Rundle (1979, 1992); Hunter (1980); D Millward, in Stephenson et al. (1999).

Map: Cockermouth (23).

5.3.1.1 Mosedale Gabbros (CFCOM)

Stratotype: As for the Carrock Fell Complex.

Lithology: Layered cumulate gabbro and hypersthene gabbro; quartz gabbro; xenoliths of hornfelsed basalt common and large adjacent to contact.

Radiometric age: 468 ± 9 Ma (K-Ar, whole rock).

Remarks: The oldest intrusion in the complex. Geochemically similar to the Eycott Volcanic Group and interpreted to have been emplaced as a near-horizontal sill at the base of the volcanic group. Within error, the radiometric age is comparable with the probable biostratigraphical age of the Eycott Volcanic Group.

5.3.1.2 Carrock Gabbro–Microgranite Intrusions (CFCOC)

Stratotype: As for the Carrock Fell Complex.

Lithology: Separate intrusions of granophyric microgranite, microgabbro, and a marginal zone comprising apatite-bearing iron-rich microgabbro, microdiorite, micromonzodiorite and microgranite.

Radiometric age: 452.4 ± 4.1 Ma (U-Pb, zircon) on the main microgranite mass.

Remarks: Main mass commonly referred to as the Carrock Granophyre, and the microgabbro as Carrock Diabase. Cross-cutting intrusive contact with Mosedale Gabbros. Believed to have been emplaced as a near-vertical sheet-like body in which crystal fractionation of an evolved, low-Mg tholeiitic basalt magma had occurred. Rb-Sr, whole rock isochron age of 435 ± 9 Ma reset.

5.3.1.3 Iron Crag Microgranite (CFCOI)

Stratotype: Type area Roughton Gill [NY 304 342] to [NY 307 344].

Lithology: Microgranite, forming an elongate, faulted mass.

Radiometric age: None determined.

Remarks: Cross-cutting, intrusive contacts with the Carrock intrusions.

Reference: Millward, in Stephenson et al. (1999).

5.3.1.4 Harestones Rhyolite (CFCOH)

Stratotype: Type area fell side north-east of Great Lingy Hill [NY 312 345].

Lithology: Quartz-phyric rhyolite.

Radiometric age: 419 ± 4 Ma (Rb-Sr, whole rock), believed to be reset.

Remarks: Intruded into mudstones of the Longvillian Drygill Shale Formation. The mudstones are bleached at the contact with the rhyolite and similar apophyses, giving a maximum age for emplacement. Faulted contact with other components of the complex. This is the only extant Silurian radiometric age for igneous rocks in the Lake District and is probably reset. A late Ordovician (Ashgill) age is thought more likely (Millward and Evans, 2003).

5.3.2 Haweswater Dolerite Intrusions (HAWS)

Stratotype: Type area on Wallow Crag to Kit Crag, south-east of Haweswater [NY 491 145] to [NY 496 151].

Lithology: Eight main plugs and dyke-like masses of dolerite with subordinate gabbro and microdiorite; minor intrusive breccia.

Radiometric age: None determined.

Remarks: Localised within an area of 19 km² around Haweswater and intruded into the Borrowdale Volcanic Group. An Ordovician age is considered probable because the rocks are cleaved and are geochemically similar to the Borrowdale Volcanic Group.

Reference: Millward and Beddoe-Stephens (1998).

Map: Appleby (30).

5.4 Lake District Ordovician Minor Intrusion Suite (LDOMI)

Minor intrusions of Ordovician age are widespread throughout the Lake District inlier. Included are basalt and dolerite dykes (Macdonald et al., 1988), garnetiferous rhyodacite dykes (Beddoe-Stephens and Mason, 1991), dacite, rhyolite, and andesite and hybridised andesite dykes, associated with intrusive breccia (McConnell et al., 2002; Millward et al., 2000a). There are few reliable radiometric dates, though many of these rocks are considered to be associated with the Borrowdale Volcanic Group or Eskdale and Ennerdale intrusions. At present, two groups of mafic intrusions are formally defined within the suite:

5.4.1 Embleton Microdiorite Intrusions (EMBL)

Stratotype: Type area in Close Quarry, Embleton [NY 175 309], near Cockermouth.

Lithology: Microdiorite.

Radiometric age: 444 ± 24 Ma (Rb-Sr, whole rock).

Remarks: Occurs within the Skiddaw Group, localised to an area of about 13 km² adjacent to the Watch Hill Thrust.

Reference: Rundle (1979).

Map: Cockermouth (23).

5.4.2 Bassenthwaite Diorite Intrusions (BADIN)

Stratotype: Type intrusion at Great Cockup [NY 271 324].

Lithology: Porphyritic hornblende diorite, microdiorite and hornblendite.

Radiometric age: 458 ± 9 Ma (K-Ar, whole rock).

Remarks: Widespread, within the Skiddaw Group of the main outcrop and in the Black Combe Inlier.

Reference: Rundle (1979).

Maps: Cockermouth (23), Keswick (29), Ulverston (48).

5.5 Northern England Devonian Plutonic Suite (NEDP)

In the Lake District, this suite comprises two exposed granitic bodies, the Skiddaw and Shap plutons. In addition, one concealed mass, the elongate Crummock Water Intrusion is inferred to have been emplaced at this time. Millward (2002) also postulated that the concealed Ulpha ‘Granite’ of Lee (in Millward et al., 2000b) could also have been emplaced at this time. The Weardale Granite, beneath the north Pennines, is also included in the suite but it is not defined here. This suite is equivalent in age to the Cheviot Volcanic Formation and the Galloway Suite of southern Scotland which includes the Cheviot Granite Pluton.

5.5.1 Shap Granite Pluton (SHAP)

Stratotype: Type area in Shap Pink Quarry, Shap [NY 557 084].

Lithology: K-feldspar megaphyric granite, forming a cylindrical pluton.

Radiometric age: 397 ± 7 Ma (K-Ar, biotite).

References: Rundle (1992); see also S C Loughlin in Stephenson et al. (1999).

Maps: Appleby (30), Kendal (39).

5.5.2 Skiddaw Granite Pluton (SKID)

Stratotype: Type area Grainsgill and River Caldew [NY 327 327].

Lithology: Granite, some quartz-muscovite rock (greisen).

Radiometric age: 399 ± 8 Ma (K-Ar, biotite).

References: Shepherd et al. (1976); S C Loughlin, in Stephenson et al. (1999).

Maps: Cockermouth (23), Keswick (29).

5.5.3 Crummock Water Felsic Intrusion

Lithology: Concealed, but probably granitic, as inferred from the major metasomatic aureole at the surface and from interpretation of geophysical data.

Radiometric age: 401 ± 3 Ma (Rb-Sr, whole rock), determined on aureole rocks.

Reference: Cooper et al. (1988).

Map: Whitehaven (28), Keswick (29).

5.6 Lake District Devonian Minor Intrusion Suite (LDDMI)

Comprises dykes and minor intrusions throughout the Lake District of two main rock types: quartz-feldsparphyric microgranite and porphyritic microgranodioritemicrogranite. All probably are related to the event which included emplacement of the Shap Pluton. The first rock type is distinctive, occurring for example in the Scafell area and one dyke there has a radiometric age of 393 ± 9 Ma (Rb-Sr, whole rock; Al Jawadi, 1987). Microgranodiorite-microgranite dykes from Lingmell on Scafell have given an age of 392 ± 4 Ma (Rb-Sr, whole rock; Al Jawadi, 1987) and a red microgranite from Holehouse Gill area (Ambleside sheet) has an age of 391 ± 19 Ma (Rb-Sr, whole rock; Rundle, 1987). Microgranite dykes cutting the Windermere Supergroup probably have a similar age (Soper and Kneller, 1990). A detailed study of the distribution and geochemistry of these dykes may enable recognition of swarms within the suite.

5.7 North Britain Siluro-Devonian Calc-Alkaline Dyke Suite (SDCAD)

Calc-alkaline dykes are widespread throughout the Caledonides of northern Britain. In the Lake District this suite is represented by lamprophyre dykes. These rocks are seen sporadically in the central Lake District, but are more widespread in the northern Pennine inliers (Macdonald et al., 1985). The K-Ar ages determined by Nixon et al. (1984), span the range 420–402 Ma, and are latest Silurian to Early Devonian. No formal division of the suite is proposed in this report.

Appendix 1 Alphabetical list of Ordovician lithostratigraphical and lithodemic units referred to in this report

Obsolete stratigraphical terms are given thus [ ] and are detailed in Appendix 3.

Appendix 2 Alphabetical list of abbreviations used for the Ordovician volcanic rocks (map codes) on British Geological Survey 1:50 000 and 1:25 000-scale maps of the Lake District.

Obsolete stratigraphical terms are given thus [ ] and are detailed in Appendix 3.

Appendix 3 Discontinued and obsolete stratigraphical terms

The usage of a small number of units included on maps, in the BGS Lexicon and in various accounts should be discontinued as a result of this overview. Such units are listed below along with reasons for this.

Binsey Formation

The lower of two divisions of the Eycott Volcanic Group following Eastwood et al. (1968) and formalised by Downie and Soper (1972), Wadge (1978) and Moseley (1984). The base of the formation was placed at the base of the ‘Lower Eycott Lavas’ of Eastwood et al. However, Millward et al. (1999) do not follow this stratigraphy. They showed that the Lower Eycott Lavas are sills and that most of the formation as originally defined differs little from the overlying High Ireby Formation.

Crosby Thwaite Formation

There is a Lexicon entry under this name for bedded pyroclastic rocks in the south-western Lake District. These rocks have since been assigned as part of the Whorneyside Formation and the name Crosby Thwaite Formation is regarded as obsolete.

Fell Green Formation

There is a Lexicon entry for dacitic and basaltic andesite sheets in lower part of Borrowdale Volcanic Group between Crookley Beck and Nettle Crags [SD 1160 8830] to [SD 1164 8865]. These rocks are lithologically similar to, and occur at the same stratigraphical level as, the Birker Fell Formation. They are shown as such on the Ulverston map and are described as such in the memoir (Johnson et al., 2001). The name is therefore obsolete.

Greenhow End Member

Woodhall (1998) described the main part of the Deepdale Formation as the Greenhow End Member. There is no requirement to name all parts of the formation and this member name is not shown on Keswick Sheet 29. The name is obsolete.

High Ireby Formation

The upper of two divisions of the Eycott Volcanic Group following Eastwood et al. (1968) and formalised by Downie and Soper (1972), Wadge (1978) and Moseley (1984). The base of the formation was placed at the base of the ‘Middle Eycott Lavas’ of Eastwood et al. (1968). However, Millward et al. (1999) do not follow these previous workers, arguing that the Middle Eycott Lavas may be sills and that most of the formation, as originally defined, differs little from the underlying Binsey Formation.

Lingcove Formation

Defined by Branney and Soper (1988), and Branney and Kokelaar (1994) for part of the Birker Fell Formation in the Scafell area. These rocks have been included within the Birker Fell Formation by the BGS and the name Lingcove Formation has not been used by the BGS. It is included here because of its previously published status.

Lingmoor Fell Formation

Designated for use on Sheet 38, Ambleside. Dacitic lapillituff, but the outcrop on Lingmoor Fell is probably equivalent to the Low Water Formation, though this has not been proved geochemically; the outcrop in the Ambleside area is possibly part of the Froswick Formation. The term Lingmoor Fell Formation is considered to be obsolete.

Millom Park Formation

Massive dacitic lapilli-tuff of the Millom Park area (Sheet 48 Ulverston). Considered to be laterally equivalent to Waberthwaite Tuff Formation and this name is preferred to Millom Park Formation, which now becomes obsolete.

Mosedale Andesite Formation

A partial Lexicon entry exists under this title for rocks subsequently included within the Birker Fell Formation. Mosedale Andesite Formation has not been published and this term should be considered to be obsolete.

Napes Needle Member

The Napes Needle Member was formally established by Kneller et al. (1993) to refer to the main part of the Lingmell Tuff Formation: i.e. all except the formally defined Scafell Dacite and Rosthwaite Rhyolite, which are members. Because the definition of the formation carries the main lithologies, this formally defined member is considered unnecessary.

Stockdale Rhyolite Formation

Renamed on precedence by Kneller et al. (1994) as the Yarlside Volcanic Formation and referred to as the Yarlside Formation by Fortey et al. (2000). It is recommended that use of the term Stockdale Rhyolite Formation is discontinued.

Stoneside Hill Formation

A formal Lexicon entry exists for the Stoneside Hill Formation to include dacite sheets with intercalated volcaniclastic sandstone and tuff in the lower part of the Borrowdale Volcanic Group in the area north of Black Combe. These rocks are included as part of the Birker Fell Formation on the Ulverston sheet (48) and in the memoir (Johnson et al., 2001). The term is obsolete.

Thornhow End Member

Used by Woodhall (1998) to refer to the basal part of the Deepdale Formation in northern part of its outcrop. Not shown on the Keswick map and regarded as the main part of the formation. There is no Lexicon entry.

Tongue House Member

The Tongue House Member was established for use on the British Geological Survey (1991) 1:25 000-scale map, Devoke Water and Ulpha, to encompass a widespread silicic volcanic event within the largely andesitic Birker Fell Formation. The member included the Little Stand Tuff, the Great Whinscale Dacite and thin intervening volcaniclastic sedimentary rocks. Outside this limited area this formal term was not found to be useful and each of the constituent units has been raised to member status. The term Tongue House Member is now obsolete

Worm Crag Formation

A partial Lexicon entry exists under this name for rocks subsequently included within the Airy’s Bridge and Whorneyside tuff formations. The term Worm Crag Formation is obsolete.

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references may be purchased from the Library subject to the current copyright legislation.

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Figures, tables

(Figure 1) The Lake District Lower Palaeozoic Inlier, showing the outcrops of the Skiddaw, Eycott Volcanic and Borrowdale Volcanic groups, the Windermere Supergroup, and major igneous intrusions. BGS 1:50 000 geological sheets are overlain.

(Figure 2) Summary chart showing the stratigraphical distribution and radiometric ages of igneous rocks in northern England. Radiometric dates are in Ma with errors quoted as 2 standard deviations: diamonds U-Pb, circles Rb-Sr, squares K-Ar. Open symbols represent radiometric data for igneous bodies, the ages of which are considered to have been reset; the inferred emplacement age is shown without errors. No radiometric ages have been determined for the Haweswater Intrusions and the tholeiitic dykes. See text for explanation of the inferred ages of Wensleydale Intrusion, Mosedale Gabbros and Harestones Rhyolite. The age of the Weardale Intrusion has been recalculated using more recent constants; the rhyolite minor intrusions are geochemically similar to the Ennderdale Intrusion (Millward et al. 2000a). CFC Carrock Fell Complex. Modified from Millward (2002).

(Figure 5)." data-name="images/P991781.jpg">(Figure 3) Generalised cross-sections through the BVG showing the relationships of the various lithostratigraphical units. Sills are omitted. All units shown are formations, except where stated otherwise. Approximate lines of cross-sections are shown in (Figure 5).

(Figure 4) Diagrammatic lithostratigraphical framework for the pyroclastic and sedimentary formations within the upper part of the Borrowdale Volcanic Group. Not to scale. Relative thickness is schematic only. Line of section approximately west to east. The successions in west Cumbria and Cross Fell are now shown. In the northern part of the central Lake District the Rydal succession rests on the Lower BVG. However, farther south in the Ambleside area, the base of the Rydal succession is nowhere seen.

(Figure 5) Lithostratigraphy of the Borrowdale Volcanic Group showing the distribution of informally designated successions within the predominantly volcaniclastic upper part of the group. Approximate lines of section shown in (Figure 5)." data-name="images/P991781.jpg">(Figure 3) are indicated.

Tables

(Table 1) Sellafield borehole data.

(Table 2) Comparison of the main lithostratigraphical classifications of the Borrowdale Volcanic Group in the western part of the Lake District (west of the Coniston Fault).

(Table 3) Comparison of previous lithostratigraphical classifications of the Borrowdale Volcanic Group in the eastern Lake District with the recommended scheme.

(Table 4) Summary of informally defined lithostratigraphical successions within the Borrowdale Volcanic Group of the English Lake District.

Tables

(Table 1) Sellafield borehole data.

(Table 4) Summary of informally defined lithostratigraphical successions within the Borrowdale Volcanic Group of the English Lake District.