Excursion to the Northern Lake District

Excursion to the Northern Lake District 30 August-S September 1970

Report by the Directors: G. H. MITCHELL, F. MOSELEY, R. J. FIRMAN, N. J. SOPER, D. E. ROBERTS, M. J. NUIT & A. J. WADGE Received 4 June 1971

FOREWORD THE PARTY ASSEMBLED at the Cumberland and Westmorland College of Agriculture and Forestry, Newton Rigg, Penrith, on the evening of Sunday, 30 August 1970. Thanks are due to the Principal, Mr. W. Steele, B.SC., the Bursar, Mr. Jenkins, and their staff for their care and hospitality during our stay. A general discussion of Lake District geology and the daily plans took place on the first evening and a similar meeting was held every evening of the excursion. The accounts of the itineraries given below were supplied by the individual Directors, with additional discussion contributed by D. Helm, P. J. F. Jeans and B. Roberts, and edited by the Excursion Secretary, Dr. F. Moseley.At the final evening meeting Dr. Mitchell proposed a hearty vote of thanks to Dr. Moseley, to the individual Directors and to the Newton Rigg College.

a.H.M.

INlRODUCTION The geology of the Lake District has become increasingly well known since the classical pioneer surveys of the last century, and even in the relatively short time since the last Geologists' Association excursion in 1954 there has been a great volume of publication. Constant development of new ideas and techniques, and more detailed surveys create as many problems as they solve, and there is much work still to be done on almost every aspect of the geology. During this excursion a wide range of geology was examined but the strongest emphasis was on the structures of the Skiddaw Slates and their junction with the Borrowdale Volcanic rocks. 443

G. H. MITCHELL AND OTHERS

444

Geological Outline of the Northern Lake District

There are several fairly recent accounts of the general geology of the Lake District (Holling worth, 1955; Mitchell, 1956), and this short review is only intended to bring the reader up to date since the publication of those papers. The succession and outline geological map of Lakeland are shown in Figs. 1 and 2. The Lower Palaeozo ic rocks assume the greatest importance, and are readily subdivided into three units, each giving rise to a characteristic topography. The lowest unit , the Skiddaw Slates, gives the relatively smooth mountains of the north-west, the middle unit of the Borrowdale Volcanic rocks is associated with the craggy mountains of Central Lakeland, and the higher Silurian slates and greywackes with the lowerlying ground of the south. All these rocks were considerably deformed by the Caledonian Orogeny, which generally resulted in north-east trends of cleavage and folding, although there are numerous variations not all of which are properly understood. Towards the end of this orogeny important igneous intrusions varying from granitic to gabbroic in composition were

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EXCURSION TO THE NORTHERN LAKE DISTRICT 445

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30

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emplaced, but with the former predominant. Mineralisation was also important, but often remains of uncertain age; it is not always easy to disentangle Caledonian, Hercynian and Tertiary phases of mineralisation. The Devonian was notable mostly as a period of prolonged erosion, although the Mell Fell Conglomerate, 5000 ft. (1600 m.) thick, laid down locally in what must have been a deep valley, is thought to belong to this period. More important are rocks of Carboniferous age; the Carboniferous Limestone in particular, found round the margins of Lakeland, generally dipping outwards to give the geometry of the well-known Hercynian (and later) dome. The Carboniferous Limestone rests either on the Mell Fell Conglomerate or with strong unconformity on the Lower Palaeozoic rocks. The domed Carboniferous rocks were in their turn deeply eroded during the early part of the Permian, and red beds of the New Red Sandstones were deposited with moderate unconformity. During the last phase of folding and faulting, presumed to have been of Tertiary age, the New Red Sandstones were domed in their turn, modern drainage systems were initiated, and the central core of Lower Palaeozoic rocks once again re-excavated. The final events have been the establishment of well-marked erosional platforms, now at various heights above sea-level, and, perhaps with the strongest influence of all on today's scenery, the erosional and depositional effects of the Pleistocene glaciers. Skiddaw Slates This group of geosynclinal sediments, of which the base is not to be seen, has been variously estimated at from 7000 to 30,000 ft. (2300m-IO,OOO m.) in thickness according to which stratigraphical interpretation is preferred (Fig. 2). The disputed order of succession is a consequence of the sparsity of the fauna and the difficulty in understanding the complex structures (also disputed), which together make the Skiddaw Slates one of the most difficult rock groups to interpret in Britain. The older 'conventional' view of Rose (1955) and Jackson (1961), is that of fairly simple stratigraphy with a thick greywacke group at the base (the Loweswater Flags), passing by gradation into a predominantly pelitic facies of slates with occasional thin greywacke ribs. The repeated appearance of greywacke beds across the Skiddaw Slate outcrop is interpreted as evidence for anticlinal upfolds of Loweswater Flags, and the highest slate beds are considered to be conformably, or near conformably, overlain by the Borrowdale Volcanics. The major part of the folding and cleavage to be seen in the slates is therefore regarded as end Silurian, and the complexity of this folding, compared to the simple structures in the overlying volcanics, is believedto be due to the much greater competence of the latter.

EXCURSION TO THE NORTHERN LAKE DISTRICT 447

The alternative interpretation favoured by Simpson (1967) is that the outcrops of each major unit of greywacke and slate represent separate formations in a much thicker sequence, with the oldest rocks adjacent to the Borrowdale Volcanics, and separated from them by a major unconformity. This unconformity, he believes, marks a period of erosion following an intense orogenic deformation, during which the Skiddaw Slates were subjected to two phases of folding, each with its associated cleavage. During the last few years discussion on these two issues have been intense (Soper, 1970; Helm, 1970; Roberts, 1971; Jeans, 1972; Wadge, 1972; Moseley, 1972), and as yet there is no consensus of opinion. This is evident from the comments in the individual field excursion reports below. It is apparent that the problems of the Skiddaw Slates are so complex that much more detailed investigations into sedimentary and tectonic structures and into micropalaeontology and geochemistry are required. Such work is already in progress. The sedimentology is certainly of interest, with practically every outcrop revealing a variety of turbidite and other structures, on both a macro- and micro-scale. In many of the more highly cleaved slates these structures are only revealed where streams have smoothed outcrops, alternatively orientated specimens have to be collected and either sectioned or polished in order to see the structures. Their importance in both 'way up' determinations and in palaeogeographical studies will be apparent. Micropalaeontology has only recently been applied to the Skiddaw Group, but with extremely promising results (Lister, Burgess & Wadge, 1969)and it may be that ultimately such studies will give the most reliable estimates of the succession. It is clear, however, that a great deal of work will be necessary before fossil sequences can be established. Geochemistry also should be enlisted and could be of considerable assistance in the determination of stratigraphical successions and in deciding the true nature of the contact with the Borrowdale Volcanics, whilst the importance of even more detailed structural analyses hardly needs to be stressed. Borrowdale Volcanic Rocks

The Borrowdale Volcanic rocks, locally exceeding 15,000 ft. (5000 m.) in thickness, are massive, competent lavas and tuffs, which contrast strongly with the Skiddaw Slates in almost every respect. At a very early stage Ward (1876) noted the differing fold styles of incompetent Skiddaw Slates and competent Borrowdale Volcanics, with the latter distributed in much simpler open folds. The contrast in tectonic facies has resulted in planes of detachment or 'decollement thrusts' which separate the two rock groups locally. Other Skiddaw Slate-Borrowdale Volcanicjunctions are high angle

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faults and some are regarded as near conformable or unconformable as indicated above. The volcanic rocks show considerable lateral and vertical variation. The lower part of the succession is dominated by andesitic lava, often quite basic, as on Eycott Hill. Higher up there are numerous alternations of bedded and coarse andesitic tuffs, ignimbrites, rhyolites and andesites, and there is perhaps a tendency for acidic elements to be more pronounced in this upper part of the sequence (see Mitchell, 1956). The field characteristics of most of the rock types have frequently been described. The andesites are often flow-banded, flow-jointed or flow brecciated, the latter on occasions looking very much like explosion breccia, with tuffisite-like veins penetrating the rock. Porphyritic, aphanitic and amygdaloidal varieties are common. Andesitic tuffs are extremely widespread and some bedded horizons showing sedimentary structures of great variety are in demand as ornamental stone. During recent years worldwide study of ignimbrites has received much impetus. Many of the varieties described from other regions are to be found in the Lake District, but apart from Oliver's work (1954 and 1961) and summaries by Nutt (see the Haweswater Excursion, this report) they have received little attention. Further details of the volcanic rocks will be found in the daily itineraries of the excursions. Intrusions The larger intrusions of the Lake District post-date the Caledonian tectonic events, and several have been dated at slightly less than 400 Ma. (Brown and others, 1964). Of those visited the Carrock Fell Complex, Skiddaw Granite and Shap Granite occupy classical positions in geological literature, but the Threlkeld Microgranite is smaller and less well known. The Carrock Fell Complex and Skiddaw Granite have adjacent outcrops and have been described recently in some detail (Eastwood and others, 1968). The former is the older and consists of highly inclined east-west sheets of gabbro, granophyre and diabase, whilst the latter is notable for the associated greisen and the intense metamorphism produced in the Skiddaw Slates. The Shap Granite is equally well known, with the most recent work by Firman (1957), who studied the associated metasomatism, thermal metamorphism, epidotisation and hydrothermal mineralisation. The Threlkeld Microgranite is generally regarded as a laccolith intruded along the Skiddaw Slate-Borrowdale Volcanic junction. The Haweswater Complex, recently mapped by Nutt, includes dolerite, gabbro and diorite and probably represents one of the major eruptive centres for the Borrowdale Volcanic rocks (Nutt, 1970). It is thus in a different category to the other intrusions.

EXCURSION TO THE NORTHERN LAKE DISTRICT 449

Coniston Limestone, Silurian and Later Rocks These rocks were not studied to any extent on the excursion. A short time was spent examining the Coniston Grits exposed in the new M6 motorway cuttings, and it is worth noting the contrasts between folds in these rocks, the more open folds of the Borrowdale Volcanic rocks and the tighter folds of the Bannisdale Slates. The Mell Fell Conglomerate (Capewell, 1955)was also examined briefly. It is composed largely of greywacke boulders of Silurian type, and, on Little Mell Fell, is penetrated by small basic intrusives. Background information to these and other rocks and to glacial and post-glacial events is to be found in existing publications (Hollingworth, 1955; Mitchell, 1956). F.M.

Monday, 31 August (Excursion 1a) The Shap Area

Director: R. J. FIRMAN The party assembled in the Wasdale Granite Quarries to examine the classic exposures of the Shap Granite. The Director commented that very little research work had been done on the Granite since Grantham published his account in 1928. The quarry face was now much farther from the granite contact and it appeared that mineral veins and xenoliths were less abundant than formerly. This was particularly true of bismuthinite and Coniston Limestone xenoliths both of which were now quite rare. All the stages of the granite from Grantham's 'Early Basic' to 'Stage III' could still be found although Grantham's interpretations might be questioned in the light of recent intensive research on granites all over the world. Late stage alterations of the granite were of considerable importance. Kaolinised zones parallel to master joints were worthless, both for use as an aggregate and as a building stone, yet it is closely adjacent to these zones of alteration that the highly prized 'dark' Shap Granite occurs. This means that the Shap 'Dark' is both difficult and expensive to obtain. The Director drew attention to two such alteration zones in the west end of the quarry. Here the rock is so decomposed that it can be removed by hand. In trying to fulfil a large order for Shap 'Dark', charges were put in chambers between these two alteration zones. The effects of the disastrous blast can still be seen in the quarry and surrounding countryside. Press photographs (Daily Mail, 29 May 1952) prove that boulders weighing at least 2 tons were propelled high into the air; smaller boulders went through the 1 ft. reinforced concrete wall of the crushing plant; one boulder tragically killed a quarryman who was sheltering behind a railway wagon

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350 yards away and chips of Shap Granite flew as far as the A6 road. This illustrates the importance of considering the zones of weakness in the quarry when planning large-scale blasting operations. In this example it was planned to drop 90,000 tons but only about 60,000 tons were displaced. The party then examined the contact in the stream north of Wasdale Head Farm, and outcrops of metamorphosed Coniston Limestone, Stockdale Rhyolite and Borrowdale Volcanics exposed west of the farm. The Lower Coniston Limestone described by Harker & Marr (1891) and said by them to contain large grossular garnets and vesuvianite was not located. Although the Director had examined samples in the Harker collection at Cambridge he had never found it in the field-probably the exposures are now covered by peat. After lunch in Shap the party went to the Shap Blue Quarry where the Director outlined the sequence of metamorphism and mineralisation. When he had published a detailed account of this quarry he had wondered whether all the metasomatism should be attributed to the infiltration of fluids from the granite (Firman, 1957, particularly the discussion on pp. 221-2). Recent excavations for the Wet Sleddale Dam had revealed a series of quartz-calcite-chlorite veins, with some haematite, dipping in the same general direction as the garnet-bearing veins in the Blue Quarry about a mile away. Thus the weight of evidence now seemed to support metamorphism of pre-existing veins. The discovery that afternoon of a sample of wollastonite by Mr. Thompson of Carlisle further strengthens the case for metamorphism of pre-existing quartz-calcite veins. This material, the first record of wollastonite from this quarry, came from a dump of material probably extracted from the deeper flooded level of the quarry. After collecting samples of garnet, epidote, actinolite, laumontite, etc., the party went by coach to Tebay.

(Excursion Ib) Folded Silurian Strataof the Lone Gorge Director: F. MOSELEY After leaving Shap the remaining part of the afternoon was spent examining the section alongside A685, one mile south of Tebay. This section and several others adjacent to it were excavated mostly in 1969 during the construction of the M6 motorway. The motorway route through the Lune Gorge had to compete with the existing main-line railway and main road. A considerable amount of blasting and realignment of the latter was necessary and the overall result was a series of steps along the hillside with the highest occupied by the A685, the middle one occupied by

EXCURSION TO THE NORTHERN LAKE DISTRICT 451 the motorway, in places at two levels, and the lowest by the railway. This has given a total vertical section of over 100 ft. (30 m.). The A685 road provides the most accessible of the sections, and the party split up to examine that part which is north of Borrow Beck (about a quarter of a mile (0'5 km.) long). The exposure shown on Plate 21 is entirely within the Coniston Grit Group and is on the south-east limb of the Bannisdale Syncline, a major fold with a half wave length of more than five miles (8 km.), which can be traced across most of the southern Lake District (Moseley, 1968). The Coniston Grits were seen to consist of massive greywacke beds ranging from 6 in. (15 em.) to 10 ft. (3'3 m.) in thickness, with subsidiary laminated greywacke siltstone and mudstone. 'Minor' folds were particularly obvious and were large concentric, but with complications where thicker mudstone units were involved. It was pointed out that the folding observed here was more severe than that to be observed in the Borrowdale Volcanic rocks, but less complex than that in the Bannisdale Slates of nearby areas. This was attributed largely to differences in competence of these various rock groups. In detail many other structural complexities were apparent, and, additional to the obvious upright folds, there were open recumbent folds (comparable to those which have been referred to as F2 (Simpson) in the Skiddaw Slates), low angle shears, kink bands which deform cleavage and high angle wrench faults. On another theme there were comments on the engineering problems encountered in the road cuts. It was noticed that several factors such as the strike perpendicular to the road line, the near-horizontal fold-axes, and the dominance of greywacke, combined to make fairly stable sections. Some instability results from oblique intersections of wrench faults with 1 to 2 ft. (30 cm.-60 em.) shatter belts, and from some of the close-spaced northerly trending joints inclined at high angles into the section with a tendency for heavy slabs to fall away. These difficultieshad been countered by extensive use of rock bolts, and by grouting the worst of the faults which were also effectivewater channels, the water being piped behind the section and under the road.

Tuesday,l September (Excursion 2) Gowbarrow, Ullswater

Director: F. MOSELEY The principal objects of the excursion were to study the Borrowdale Volcanic rocks of the Ullswater Group which include basic and normal andesites and various andesitic tuffs, the junction between them and the Skiddaw Slates (high and low angle faults), and the Mell Fell Conglomerate

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with instrusions of basalt. The total distance walked was about seven miles (l I km.) across gently undulating fellsides rising to a maximum altitude of 1500 ft. (300 m.). The excursion was entirely within the limits of the geological map of Gowbarrow (Moseley, 1964, pl. 7). The party left the bus at Hallsteads (436213 1), and visited the localities listed below, rejoining the bus in the afternoon at Airy Force car-park. Locality 1. At point 434215 on the minor road there are excellent views both to the south-east and north-west. To the south-east Barton Fell can be seen, with its sequence of lavas and tuffs of the Borrowdale Volcanics making prominent features and crags along the fellside. These bands are truncated by a fault at the bottom of the crags marking the junction with the Skiddaw Slates (Plate 22, C). Immediately to the north-west is Birk Crag, where lavas make distinct features dipping at about 25° to the northeast. Locality 2. (432216). Several old quarries exhibit a variety of features, as follows. The first shows a junction between slates and volcanics (tuff). It is a faulted junction inclined about 35° south-west. The Skiddaw Slates below this fault have two high angle cleavages with west-north-west and north-north-east trends which compare with the east-north-east trends of cleavage in the adjacent volcanics . The second quarry is in strongly cleaved coarse tuffs (east-north-east trend) in which the bedding can only be determined with difficulty. The third quarry is similar to the second , but bedding, revealed by alternation of coarser and finer beds, is more distinct, and makes a small angle with a well-developed set of low angle joints (the western and highest part of the quarry). Drs. D. G. Helm and B. Roberts noted that in the first quarry, in addition to the obvious cleavages, there were several minor structures that bear on the nature of the junction. In the slates, the earliest tectonic structure is a slaty cleavage (S1, this exposure), sub-parallel to the bedding, and is demonstrably refolded about sub-vertical axial surfaces. An axial-planar fracture cleavage (S2, this exposure) causes pencilling of the slates. These second-generation folds (F2, this exposure) which, in the slates, plunge at 50° to 257°, are similar in their open style to the single set of minor folds (plunging at 30° to 254°) in the overlying tuffs. Both sets of folds possess rounded hinges and curved limbs. Drs. Helm and B. Roberts correlate the F2 folds in the slates with the only folds seen to deform the tuffs. Furthermore, they suggest the fracture cleavage in the slates is equivalent to the principal slaty cleavage (strike 260°, dip 70° north) in the tuffs, i.e. these are representatives of the main end-Silurian slaty cleavage in the Lake District. This cleavage, with slight refraction, apparently passes uninterruptedly through the junction. 1

All National Grid references given in this account lie in 100 km , sq uare NY.

EXCURSION TO THE NORTHERN LAKE DISTRICT 453

Drs. Helm and B. Roberts dispute Dr. Moseley's contention that the junction is here a fault. In the first instance, the junction appears to be undisturbed, and, secondly, it occurs at different heights in the exposed face, suggesting that the volcanics rest on an erosional surface cut in the underlying slates. In conclusion, they find the evidence from this outcrop supports a pre-volcanic phase of deformation which produced tight minor folds and a slaty cleavage. Dr. Soper was inclined to reject the interpretation of pre-volcanic cleavages in the Skiddaw Slates. The small-scale irregularities of the junction did not preclude its tectonic origin. The gross discordance between the junction and bedding of the overlying volcanic rocks suggested a thrust. The absence of either a basal conglomerate of the 'tuffaceous slate'

Fig. 3. Map to show the route followed and the localities visited on the Gowbarrow excursion (Excursion 2)

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facies, both of which are widely developed in the north-eastem Lake District, may also be of relevance. Dr. Moseley said that there were a number of features not taken into account by Drs. Helm and B. Roberts, and in consequence their argument could not be sustained. First, the structures in the upper 1·5 m. of the slates, beneath the junction with the volcanics, are different to those lower down, and include lineations plunging 25° to 35° south-west, sub-parallel to the junction. Second, this junction is strongly transgressive to the bedding in the overlying tuffs, which dip 30° north-east, and, third, mapping of the junction in the immediate vicinity shows that it cuts across more than 300 m. of volcanic rocks with the volcanics of Birk Crag 250 m. above the lowest exposed volcanics, and that its outcrop can only be interpreted in terms of a low angle plane (Moseley, 1964, pl. 7). These observations indicate a low angle fault as the only reasonable conclusion, and the difference in tectonic style and orientation of structures above and below this fault (the first observation) can be no complete surprise in rocks of such contrasting competence. Locality 3. Ascending Birk Crag (430217) a variety of andesitic lavas above the tuffs of Locality 2 were crossed. 'Flow jointing' revealed the dip 0 (25 north-east). This is an excellent locality for a panoramic view (Plate 22, C). The features of Barton Fell were seen again (Locality 1), and further south-west the effect of the Howtown Fault is revealed by the kink between the two lower reaches ofUllswater (Moseley, 1960). Continuing the view to the south-west, the Skiddaw Slate outcrops of Sandwick and Swinbums Park were clearly distinguishable, terminated on all sides by the abrupt escarpments of volcanic rocks. Locality 4. The route continued through Hagg Wood to 426223, where bedded tuffs dipping 40° to the east-north-east were well seen. The constant north-north-west strike of Localities 2, 3, 4 and 5 is at right-angles to the regional Lake District strike, and is attributed to the pre-Caradoc phase of folding. Locality 5. A well-defined path was followed across Priests Crag. At 427228 there are basic andesites with well-developed flow jointing. The junction with the Skiddaw Slates can be seen at the base of the crag and clearly transgresses a number of lava flows in a sinous outcrop which suggest a relatively low angle fault. The volcanic outlier of Knots, visible to the south, is regarded as a klippe, whilst it is probable that the Mell Fell Conglomerate seen to the north, rests unconformably upon a hilly surface of volcanic rocks. Locality 6. A track leads to the Folly (429240), where there are basic intrusives in the Mell Fell Conglomerate (Capewell, 1954). An overgrown quarry west of the track is particularly interesting and shows high angle contacts between amygdaloidal basalt and conglomerate.

EXCURSION TO THE NORTHERN LAKE DISTRICT 455

Locality 7. From Locality 6 the party walked across the side of Little Mell Fell, across the Hause and thence to Great Meldrum (415223), from where the faulted Skiddaw Slates and Borrowdale Volcanics to the southeast of Ullswater, already seen from Locality 3, were again well seen. Locality 8. The walk continued to the crag above Ulcat Row (342224), where there are gently dipping andesites belonging to the upper part of the Ullswater group, with characteristic flow brecciation (fragments recessed by weathering) and irregular flow banding and flow folding. Locality 9. Higher up Gowbarrow Fell, at 344219, more flow banding was suggested, but Dr. B. Roberts pointed to the eutaxitic texture and suggested an andesitic ignimbrite. After some discussion most of the party agreed with this interpretation. Locality 10. Traversing Airy Crag to 410217, a succession of th in andesite flows was seen, which results in well-defined 'trap topography'. To the east are rhyolites and ignimbrites, and separating them a well-marked northsouth fault depression, but time was now closing in and the party could not afford to stop long. The route back to the bus followed the fault-line to Locality 11 (411208), where there is a good view of the Ullswater region, and then proceeded to Airy Force (400206). The stream here is partly controlled by joints in andesite and the waterfall may have receded from the volcanic slate junction. Farther downstream the junction of the Skiddaw Slates and Borrowdale Volcanics can be seen at 400205 (a high angle fault), and andesitic intrusions in the Skiddaw Slates occur at 402201, but there was insufficient time to visit these localities, and the coach was joined at the car-park (400200).

Wednesday, 2 September (Excursion 3) Borrowdale Director: N. J. SOPER The object of this excursion was to examine the structural and stratigraphic relationships between the Borrowdale Volcanic rocks and the Skiddaw Slates in Borrowdale. The Director had previously described localities near Hollows Farm, Grange-in-Borrowdale (249171), on which he had based the following conclusions (Soper, 1970): (a) The junction is essentially a passage upwards from pelite with minor tuffaceous bands to andesitic tuff, with, however, evidence of considerable reworking and a small erosional unconformity. (b) The Skiddaw Slates have a steep east-north-east-trending slaty cleavage which is deformed by a gently inclined strain-slip cleavage

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G. H. MITCHELL AND OTHERS

(correlated with S1 and S2 of Simpson, 1967). S1 passes across the junction to become the dominant cleavage in the volcanic rocks and is thus of post-volcanic rather than pre-volcanic age. The intention had been to visit Localities A-E and g-h (Soper, 1970, fig. 5) around Hollows Farm and then cross into Newlands, but discussion was so lively and protracted that the party reached only Locality E before time and weather ran out. Locality 1. At the roadside below Shepherd's Crag (263187) striped pelitic Skiddaw Slates are exposed a few metres below the junction, which is scree-covered. Bedding (So) is steeply inclined as is the east-north-easttrending slaty cleavage (S1). There are small sub-horizontal 'second' folds, a congruous microfold lineation (F2 L2) and steeply plunging microfold lineation on So and S1 (possibly La). Mr. D. E. Roberts remarked that the variable trend of the F2 L2 microfold lineation, visible at this locality, could be attributed to their being refolded about Fa fold-axes of north-south trend. Locality 2. Quayfoot Quarry (252167) is an old slate quarry in andesitic tuff, illustrating the apparent structural simplicity of the Borrowdale Volcanic rocks compared to the Skiddaw Slates, as exemplified by Locality 1. Bedding (So) and steep, east-north-east-trending slaty cleavage (S1) are present. The Director suggested that S1 at the two localities were the same structure. He also pointed out Goat Crag on the opposite side of the valley, raising the possibility that cleavage features in the volcanic rocks here, and elsewhere in the Honister slate belt, suggested that S1 was weakly deformed by flat 'second' style structures. The alternative suggestion was made that the variation in attitude of the cleavage was due entirely to contact strain. Locality 3. Roche moutonnee of Skiddaw Slates at Grange-in-Borrowdale (253175). Here the bedding, which is tightly folded on a small scale, strikes north-south, and there is notable absence of the regional eastnorth-east cleavage. Locality 4. At the roadside near Scarbrow Wood (249171) (Locality A, Soper, 1970, fig. 5) Skiddaw Slates are seen, close to the junction. The outcrop contains a thin inverted psammitic band, rarely found so high in the slate succession. The Director hoped that the party would not conclude that the whole of the slates beneath the junction were inverted, as this would be fatal to his interpretation. Further excavation revealed that the inverted portion of the band formed the short common limb of a tight fold pair. The Director recalled exposures in Wamscale Beck, Buttermere (200135), where tight folding in the slates could be seen very close to the gently inclined junction, and said, as the S1 cleavage crossed the junction, one must invoke either great ductility contrast or pre-volcanic, pre-cleavage folding. In view ofthe broadly gradational nature of the junction in Borrow-

EXCURSION TO THE NORTHERN LAKE DISTRICT 457 dale, he favoured the former. Mr. I. Burgess interpreted the fold pair as a downward-facing structure and invoked pre-volcanic folding. Locality 5. (Locality B, op. cit.). This exposure in Scarbrow Wood contains the principal lithological break associated with the slate-volcanic junction. Dr. Helm noted that in his view this statement required qualification, namely, that the break referred to by Dr. Soper is probably intra-Borrowdales rather than inter-Borrowdales/Skiddaw Group. If Dr. Helm is correct then the latter junction would have to be present at a lower topographic level. Dr. Soper commented that to draw the slate-volcanic junction within the dominantly pelitic slates seemed reasonable at this locality, but this militated against the junction being interpreted as a major orogenic unconformity, particularly as all observers seemed to agree that the slates immediately below the tuff had suffered a similar sequence of deformations to the Skiddaw Slates proper. He then observed that andesitic tuff, with dark blue slate fragments, rested on an eroded surface of pelitic material, the pelite being locally conglomeratic, with mudstone fragments in a mudstone matrix. Dr. Helm collected samples from here and has since confirmed that the underlying pelites are tuffaceous. In view of the presence of tuffaceous material below this junction (e.g. at Locality g, op. cit.), the Director suggested that the unconformity was trivial, and of the type to be expected in association with the emergence of volcanic islands. Mr. P. J. F. Jeans observed that the difficulty with this exposure (Locality 5) was the lack of bedding in the pelites immediately below the junction. This sections which he had made revealed that immediately below the junction there is either an unbedded tuffaceous pelite, or pelite with bedding so fragmented that its original orientation was impossible to discern (laharic?). When bedding is seen in the pelites below the junction it usually lies parallel to the SI cleavage. Locality 6 (Locality C, op, cit.). Another exposure of the erosional junction. The Director demonstrated a steep slaty cleavage in the underlying pelitic slate and a similarly orientated but weaker cleavage in the tuff, suggesting that the only reasonable interpretation was that they were the same structure, expressed in different lithologies. Dr. B. Roberts showed that the cleavage in the pelite was cut by a weak fracture cleavage and suggested that it was the latter which passed through. The Director regarded it as illogical to correlate a weak cleavage in a weak rock with a powerful cleavage in a strong rock. Dr. Roberts regarded this as feasible if the weak rock had been previously cleaved. The Director replied that this was a preconception, and he would prefer to correlate the weak cleavage in the pelite with joints in the tuff. Dr. Roberts cheerfully admitted that he was biased and the party moved on to the next locality.

458

G. H. MITCHELL AND OTHERS

Locality 7 (Locality D, op. cit.). Gently inclined Fa folds and Sa kinkbands are to be seen in the slates below the junction. Locality 8. Immediately to the west of Scarbrow Wood (247171) (Locality E, op. cit.), a locality popularised by Shackleton (1967), who described it as a transitional slate-volcanic junction. With the exception of Drs. Helm and Roberts the party agreed that the contact was faulted, but Dr. M. J. C. Nutt questioned the Director's description of the volcanics as flow-banded andesite, suggesting that they were bedded tuffs. The Director diverted attention to the possible pre-cleavage age of the fault. Excavation of the lower part of the outcrop had revealed a new exposure of the junction, on the south side of the fault. Here an apparently perfect transition from pelite to tuff was uncovered, with a single cleavage passing through both rock types. The Director hailed this as vindication of his interpretation. Mr. Jeans noted that he had first cleared this exposure during the summer of 1969, when specimens of all lithologies were collected across the junction. Thin sections revealed an apparently complete lithological gradation from pelite to cleaved tuff and massive tuff. Bedding in the pelite at the lowest part of the exposure is parallel to the Sr cleavage (56 south-east), but bedding and cleavage diverge as the junction is approached, the cleavage remaining fairly constant with a dip of 50°, but the bedding becomes progressively shallower. In the cleaved tuff it is 40°, and in the massive tuff 25°. The Si cleavage is continuous from pelite to tuff, and one instance of a microscopic Fa (?) kink-band deforming Si was found in the tuffs. Referring to all the exposures seen at the junction Mr. Jeans also observed that perhaps the most important part of this whole problem was the lack of clear bedding in the pelites below the junction, which means that one was trying to define a junction without knowing the nature of the junction itself. For example, if an unconformity separates the two Groups then we may assume it to lie at the lithological junction between slates and volcanics. But it is reasonable to assume that the initiation of the Borrowdale Volcanic episode could be marked by a pelitic phase before the advent of volcanic material, in which case the stratigraphic and structural unconformities would lie wholly in pelite. In view of the intense deformation suffered by the pelites, such an unconformity would be very difficult to discover. Dr. Moseley, adding to the last comments, said that one could contemplate erosion and disturbance of the upper few feet of the Skiddaw Slates at the beginning of vulcanicity. It was not difficult to imagine that this would have destroyed bedding just as it is destroyed on any present-day eroding surface (especially if clay or shale), and this could explain why bedding becomes indistinct as the junction is approached. 0

EXCURSION TO THE NORTHERN LAKE DISTRICT 459 Thursday, 3 September (Excursion 4) Mungrisdale-Carrock Fell Director: D. E. ROBERTS The purpose of the excursion was to examine folded Skiddaw Slates of the Loweswater Flags on Raven Crags, near Mungrisdale, and folded arenaceous units in the Skiddaw Slates, exposed in the River Cal dew, two miles west of Mosedale. In the latter case the 'slates' have been hornfelsed by the Skiddaw Granite and the resulting hard rock displays to perfection a whole series of complex folds, which include polyphase deformation. A brief examination was also made of the Skiddaw Granite, the Grainsgill Greisen, the pneumatolytic suite at Carrock Mine, and the Carrock Fell Gabbro. The unconformity between the Carboniferous Limestone and the Borrowdale Volcanic Rocks was examined at Berrier, and a short visit to Flusco Quarries demonstrated the highly fossiliferous nature of the Carboniferous Limestone in this area. Locality 1. At Mungrisdale Quarry (363306) black southerly dipping slates were seen with an east-west cleavage subparallel to the bedding. The bedding and cleavage are folded by open folds with sub-horizontal axial planes (similar in style and orientation to F2 of Simpson). Two prominent thrusts are sub-parallel to the axial planes and are probably the result of slip caused by the stresses acting at the time of formation of the folds (Plate 23, A). At the eastern end of the quarry two sandstone bands occur within the slates. These sandstones have probably been confused by many visitors to this locality with the two quartz dolerite dykes which are marked on the 1 in. Geological Survey Map, Sheet 23 (Cockermouth), near this position. These dykes are no longer visible but two hollows behind the quarry, one containing fragments of dyke rock, mark positions where they may have outcropped, now completely covered by soil and vegetation. Where the sandstone bands are visible on the quarry floor they are seen to cut and be displaced by several north-south minor faults. In the adjoining slates these faults are represented by tight minor north-south folds with a steep southerly plunge (Plate 23, B). It was noted that folds of this type are present over much of the eastern half of the Northern Fells and are demonstrably later than the sub-horizontal folds. Two weak cleavages associated with these two sets of folds are visible in the slates of the quarry wall here, and can be seen to be later than the bedding-cleavage. The earlier of these cleavages is a faint east-west cleavage associated with the sub-horizontal folds, and is cut by a weak fracture cleavage associated with the steep north-south folds. Locality 2. On Raven Crags (362307) interbedded sandstones, silts and shales were examined which had been classified by Jackson (1961), on the

s

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36

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CJ SlATES }SKJDDAW CII SLATES &SST SLATES

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o

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32

1

I 31

Fig. 4. Map of the Carrock Fell-Mungrisdale area (Excursion 4), showing some of the more generalised structures in the Skiddaw Slates and part of the aureole of the Skiddaw Granite

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PROC. GEOL. ASS., VOL. 83 (1972)

PLATE 21

B

A

c

G

AI

5

GI

G

100 YARDS

E

F

EI

N

B

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EXPLANATION OF PLATES PLATE 21 M6-A685 sections in the Coniston Grits one mile south of Tebay (Excursion l b), Folds A, B, E and F. The southern limb of A(AI) is cut by a small thrust and other similar thrusts or 'thrust joints' can be seen at D and EI. High angle, north-trending wrench-faults, oblique to the road-section, occur at G and C, and another fault at F. G in the A685 section is continued by GI in the M6 section. A rock-bolt has been placed in cleaved mudstone, parallel to the fold-axis at AI, and other rock-bolts can be seen more usefully placed across joints and the fault below C PLATE 22 A and B. M6 (A685) structures in the Coniston Grits (Excursion 1b). A: upright fold with fracture cleavage in the hinge and recumbent minor fold on the limb. This fold is immediately to the right of G 1 on Plate I. B: open recumbent fold in near vertical strata. C and D: Excursion 2. Views to the east (C) and south-east (D) from near Birk Crag (locality 3). SS: Skiddaw Slates; BV: Borrowdale Volcanics; F: fault; UK: Ullswater fault; HF: Howtown fault; dotted lines: outcrops of volcanic features PLATE 23 Skiddaw Slate structures in the northern fells, Excursion 4. A: Mungrisdale Quarry showing two thrusts and minor F3 folds. B: Skiddaw Slates with steeply plunging north-south F. folds. C: Syncline in Loweswater Flags, Raven Crags. D: Hook refolded fold in hornfelsed Skiddaw Slates, River Cal dew, at the mouth of Grainsgill Beck

PROC. G EOL. ASS., VOL. 83 (1972)

CO

u

PLATE 22

PROC. GEOL. ASS., VOL. 83 (1972)

o

PLATE23

EXCURSION TO THE NORTHERN LAKE DISTRICT 461

basis of their graptolites, as belonging to the Loweswater Flags. These rocks have been folded into gently plunging north-north-west-south-southeast folds, and the folds have Z profiles at the southern end of the crag above Undercrag Farm. However, approximately 100 yds. (30 m.) northwest of Undercrag Farm (363308), a gentle syncline occurs (Plate 23, C) with the above mentioned Z folds to the south-west of it and with northsouth folds to the north-east of its projected axis. That these folds are an early structure in the Skiddaw Slates is evidenced by their being cut by an east-west fracture cleavage which can be related to the early east-west cleavage in Mungrisdale Quarry, and they are regarded as being of preCaradoc but post-Volcanic age (FI). Drs. Helm and B. Roberts disagreed with the Director to the extent that, while accepting these folds as being the earliest structures, they regarded their time of formation as more likely to be pre-Borrowdale Volcanic. No definite F2 folds are visible on Raven Crags but they are well displayed nearby on Bowscale Fell. These structures the Director regarded as being the earliest of the end-Silurian structures, and the sub-horizontal folds with the associated crenulation cleavage (Mungrisdale Quarry) he regarded as being later end-Silurian structures (Fs and Sa), The steeply plunging folds with the weak fracture cleavage (Mungrisdale Quarry) he classed as a fourth set of structures (F4 and S4) and considered them possibly as late as Amorican. Locality 3. Here in the River Caldew (330326) a magnificent section of complex folds in hornfelsed Skiddaw 'Slate' was examined. The alteration from an arenaceous rock to a biotite-cordierite hornfels is caused by the metamorphic effectsofthe Skiddaw Granite. This section has recently been described (Roberts, 1971) and the conclusions arrived at were that the structures like some of those already referred to can be classified as being the earliest in the Lake District (pre-Caradoc) and have undergone refolding during the end-Silurian movements (Plate 23,0). This has resulted in steep plunges and variable orientation of fold-axes. Many fold styles and several re-folded folds were examined. Some members of the party considered some of the folds in this section to be slump structures, but the Director, whilst agreeing that there may be some small slump structures present, noted that the style of the folds was unlike that of slump folds, and elsewhere, outside the metamorphic aureole folds of the same type, could be seen with associated cleavage. Locality 4. The granitic part of the Grainsgill greisen outcrop of the Skiddaw Granite was next examined in the bed of the River Caldew, upstream of the mouth of Grainsgill Beck (326325). The petrology of this rock is well documented, and the party was able to note its characteristic features. In the northern part of the Grainsgill outcrop the granite has been altered to greisen by late stage boron-fluorine metasomatism. Also in PROC. GEOL. ASS., VOL. 83, PART <4, 1972

31

G. H. MITCHELL AND OTHERS

462

Grainsgill Beck (327327), in addition to the greisenisation of the granite, the hornfels had been partly converted to greisen for a distance of up to 3 ft. (l m.) from the margin of the main body. The transition was seen to be a gradual one, butting across the structural lines of the hornfels and evidenced by the appearance of white mica in the hornfels as the greisen is approached. The Director informed the party that in thin section the rock

++++++++ ++++++++ +~ IF~Er 2~O 1

++

+ METRES 80 + + ++-r-rTT++ ++:+ + -!:~"i­ 1§Q!L I :(+ + +

+++++++

Fig. 5. Large-scale map of the Carrock Mine area (Excursion 4), showing the major mineral veins and some of the workings (after Hitchen, 1934)

EXCURSION TO THE NORTHERN LAKE DISTRICT 463

along the margin often displayed a sharp boundary between biotite and white mica, whilst on both sides of this boundary the rock had many of the characteristics of the hornfels. Locality 5. At Carrock Mine (322329), mineralisation of the area was discussed. The wolfram- and scheelite-bearing pneumatolytic veins, like the Skiddaw Granite, have been well documented, and numerous mineral species were demonstrated from small specimens found on the spoil-heaps. Locality 6. Below Snailshell Crag, on Carrock Fell, near Mosedale (356326), some of the gabbros of the Carrock Fell complex were examined. At this locality, the post-Borrowdale Volcanic age of the Carrock Fell complex can be demonstrated by the xenol iths of Borrowdale Volcanic rock present in the Gabbro. Some are as much as 40 ft. (13 m.) in diameter. Locality 7. Near Berrier (388306), the unconformity between Borrowdale Volcanics, dipping at approximately 40° east, and the gentle easterly dip of the Carboniferous Limestone was examined. There was a discussion of whether the Borrowdale Volcanics of this area were on the nose of the main Lake District Anticline, or whether their dip could be due to their being on the limb of a north-south pre-Caradoc fold. Locality 8. The Carboniferous Limestone was further examined at Flusco Quarries, near Stainton (478282), and here the party were able to examine var ious sedimentary structures within the limestone, as well as large colonies of Lithostrotionjunceum, some up to 4 ft. (1'3 m.) in diameter, and colonies of Lonsdaleia floriformis, together with the other less-frequent faunal elements.

Friday, 4 September (Excursion 5) Haweswater Director : M. J. C. NUTT The purpose of this visit was to examine recent published (Nutt, 1968) and unpublished work on the Borrowdale Volcanic Rocks of the Haweswater area, continuing from work already demonstrated to the Yorkshire Geological Society (Nutt, 1966). Particular attention was paid to the presence of a volcanic cycle which is repeated many times in the eastern Lake District. This cycle in upward sequence consists of (a) coarse andesitic tuffs, (b) andesite lavas and (c) acid volcanic rocks, mainly ignimbrites (Fig. 7). Direct evidence for a local unconformity at the base of the Borrowdale Volcanic rocks (Nutt, 1966, 432) could not be examined for lack of time, but further evidence in the form of a major conglomerate was seen. A major east-north-east-trending syncline-the Haweswater Syncline-occurring to the north-east of the

G. H. MITCHELL AND OTHERS

464

lake, and the evidence for a large intrusive complex of pre-cleavage 'dolerites'-the Haweswater Complex-were also demonstrated. Travelling by coach from Newton Rigg to Bampton, via Penrith and Askham, the party alighted at the Bampton Quarries (51401824), ready to spend the rest of the day on foot. Within the quarries an upward sequence of tuffaceous sandstone, conglomerate and andesite, forming the basal members of the Bampton Howes Andesites, was examined. The Director explained how conglomerates and tuffaceous sandstones, both derived from volcanic rocks, occurred locally at the base of the Borrowdale Volcanic rocks throughout the eastern Lake District, although usually

Non-welded vitric and pumice tuff with or without possible sillars towards the base. This zone may not be present.

Generally between 200-400 feet

Welded to sl i qh tl y welded vitric (felsiticl tuffs with a eutaxitic structure.

thic«

:::.----- --~

-- --- - -

-- -- ---

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-

Strongly welded to welded vitric tuffs
Welded vitric (felsltic) tuffs with a eutcxitic structure. Cavities and quartz veining may be present.

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Non-welded to' poorly welded vltr-ic. pumice, and lithic tUffs, grading to lithic tuffs at the base. Between ignimbrite units this zone is usually absent.

Fig. 6. Idealised vertical section of the ignimbrites occurring around Haweswater (Excursion 5)

EXCURSION TO THE NORTHERN LAKE DISTRICT 465

faulted against the Skiddaw Slates. These conglomeratic rocks are taken to be intra-Borrowdale in age (Nutt, 1968, 113), and the presence of an interbedded andesite, in the area north-west of Bampton (50152015), supports this view. Mudstone phenoclasts within the conglomerate are believed to have been derived from the Skiddaw Slates (Nutt, 1966,432), and it was pointed out that no evidence of a pre-conglomerate cleavage has been found within the mudstone phenoclasts. West of the quarries (51281828) an andesite flow-breccia was noted, although the Director pointed out the absence of flow-breccia at the base of the andesite overlying the conglomerate. Continuing westwards across The Howes the party crossed numerous scarp features representing in-

t

I

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I 1000

feet

+

470170

Atkinson Ignimbrite Ignimbrite

-= -::-:...:

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0000000 o 000

Medium Unbedded Tuffs

........

Coarse Unbedded TUffs

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--.--

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Faults (crossmark indicates downthrow side) . Dip and st'r ike of Bedd ing

Fii. 7. Geology of the area around Seal Hole (Excursion 5)

466

G. H. MITCHELL AND OTHERS

dividual lava flows or porphyritic andesite, but with some tuff horizons present. West of The Howes a major feature forming the base of the overlying Stanegarth Ignimbrites was seen. The party examined welded rhyolitic tuffs occurring near the base of the lowest ignimbrite (50241803). In arguments concerning the origin of the green streaks or fiamme in these rocks the Director upheld the nuee ardente load compaction theory for the origin of ignimbrites and fiamme in the Haweswater area. Farther west to the south of Moorahill (49401804), on the south-eastern limb of the Haweswater Syncline, coarse andesitic tuffs in the overlying Cawdale Tuffs were examined along with large terminal and medial moraines at the confluence of Cawdale and Willdale. The same tuffs were again examined on the north-western limb of the syncline around Carhullan (49001821). Hereabouts the tuffs are strongly cleaved, with a belt of intense cleavage striking along the Cawdale Valley, up which the party proceeded. Lunch was taken at Seal Hole (47201743) while members examined the local geology (Fig. 7). The area illustrates a facies and thickness change across a north-west-trending line (Nutt, 1968, 113), presently occupied by a minor fault. Proceeding up Sealhole Grain two ignimbite units (Fig. 6) within the Atkinson Ignimbrites were briefly examined. Hereabouts these ignimbrites, overlying the Cawdale Tuffs, form the north-western limb of the Haweswater Syncline. Crossing hill peat deposits the party came upon coarse andesitic tuffs in the overlying Keasgill Tuffs, at an exposure (46821631) in the axial region of the Haweswater Syncline. Then members proceeded eastwards along the old peat road, to an exposure (47831638) on the south-eastern limb of the syncline, showing a nodular horizon (Fig. 6) within the Atkinson Ignimbrites. In the col between Willdale and Fordingdale (48211651) members examined exposures of dolerite belonging to the Haweswater Complex. East of the col an exposure (48451630) was examined of silicified and tourmalinised breccia. The Director explained how a number of such breccias in this area occurred apparently in pipes cutting the ignimbrites. Southwards (48421570) the party crossed coarse andesitic and rhyolitic tuffs in the Cawdale Tuffs. The underlying Stanegarth Ignimbrites were examined in the excellent exposures north-west of The Force (48601551), whilst at The Force a vertical contact between cleaved slightly baked welded tuffs and a cleaved fine-grained dolerite was seen. This contact was originally described by Walker (1904, 99), who first recognised the intrusive nature of some of the rocks around Haweswater, although Green was the first to note that the intrusion pre-dated cleavage (1917, 27). Members agreed that this age assessment was correct. Continuing down to the lakeside, an exposure (48631542) of dolerite

EXCURSION TO THE NORTHERN LAKE DISTRICT 467

with aplite veins was examined. The Director explained that the aplite veins contained tourmaline identical with that in the silicifiedtourmalinised breccia, seen earlier in the afternoon. Noting evidence of copper-mining on the way (49721600), the party returned along the lakeside to Burnbanks, where the coach waited. Saturday, 5 September (Excursion 6) Matterdale-Threlkeld Director: A. J. WADGE As a contribution to the debate on the relationship between the Skiddaw and Borrowdale Volcanic Groups, three localities along their junction were visited between Matterdale and Keswick. The Director wished to emphasise how fruitful a stratigraphical and palaeontological approach to this problem could be, as much of the earlier excursions had tended to concentrate, perhaps to too great an extent, on the structural evidence, which was rarely unequivocal. Locality 1. In the south bank of Matterdale Beck (38842344), conglomerate was seen resting with striking unconformity upon steeply dipping mudstones, which have yielded graptolites from the Didymograptus bifidus Zone of the Llanvirn. The conglomerate, dipping gently to the south-west, contained mudstone fragments, particularly near the base, and thin bands of sandstone higher in the sequence. The graptolites indicated that the Skiddaw Group mudstones in this section were younger than any known from western Lakeland, where Arenig faunas only have been recorded. Thus, the unconformity between Skiddaws and Borrowdales at Matterdale might well be represented farther west by a break of even greater magnitude. The principal cleavage in this section, nearvertical and trending north-east, affected both Groups; there were clear signs of other cleavages in the Skiddaw mudstones, which did not appear to penetrate the overlying conglomerates. It was the view of the Director that this type of evidence was a whole order of magnitude weaker than the simple stratigraphic relationships shown in this section, since it was always possible to maintain that the cleavages in question failed to penetrate the younger rocks because the latter were much more massive. He concluded that this section seemed to demonstrate, more clearly than any other he knew in the Lake District, that the junction between the Skiddaws and Borrowdales was an angular unconformity, here approaching 90° in places, and that the faunal evidence, sparse though it was, tended to support the view that the unconformity was regional in extent. Dr. Moseley agreed that evidence of unconformity could not be denied at this exposure, but the question to be asked was whether this represented

468

G. H. MITCHELL AND OTHERS

an orogenic event or a minor phase of movement. The angular discordance between slates and conglomerate was considerable, but the dominant cleavage in the slates, locally parallel to bedding, went through into the conglomerate, indicating a post-volcanic orogenic episode. The alternative would be to claim renewed post-volcanic cleavage along already established pre-volcanic planes in the slates for which there seemed to be no evidence. Dr. D. G. Helm and Dr. B. Roberts took a different stand to that just mentioned. In the blue-black Skiddaw slates with thin psammitic laminae they demonstrated structures, only one of which, a slaty cleavage, could be matched in the overlying coarse basal conglomerate of the Borrowdale Volcanic Group. Helm and Roberts recorded the attitude of the bedding in each rock group as follows: Skiddaw Slates-striking between 018 and 036°, and dipping between 83° north-west and 66° south-east; in the overlying tuffaceous conglomerate the bedding has a strike of 142° and dips gently at 25° west. In short, the steeply dipping bedding in the Skiddaw slates was more or less at right-angles to that in the Borrowdales. In addition, the Skiddaw Group displayed a fold plunging at 45° to 207°, and a second cleavage striking at 009° and dipping at 37° east, neither of which was represented in the overlying Borrowdale Group. A single cleavage, with typical Caledonoid trend (striking between 037° and 052°), was seen cutting both slates and conglomerate. It was then suggested by Drs. Helm and B. Roberts that the evidence given above could only be interpreted in terms of a pre-Borrowdale Volcanic Group deformation or deformations of orogenic proportions. In answer, Dr. Soper said that he had no alternative but to accept an unconformable junction between the two Groups at this point and that he was coming round to the view that pre-volcanic deformation now seemedlikely. However, this was not the point at issue. A local sub-volcanic unconformity did not prove pre-Borrowdale orogenesis. Drs. Helm and B. Roberts had here, as elsewhere, been unable to demonstrate that the main slaty cleavage in the Skiddaw Slates was truncated by the overlying volcanic rocks. Locality 2. Farther west, near Threlkeld, a bed of conglomerate at least 4 m. thick was seen at the base of the Borrowdale sequence at Low Rigg (30142227). It contained mainly mudstone and andesite fragments and the crude false-bedding of the deposit seemed to suggest a northerly source. Although the junction with the Skiddaw Group was not exposed hereabouts, it was examined at Causeway Foot (29072200), a short distance to the west, where a thin conglomerate containing both mudstone and andesite fragments unconformably overlay Skiddaw Group mudstones. Within the limited exposure of this small stream section, the angular contrast between the bedding in the two Groups was not very great. A large mass of andesite in the stream-bed was thought to be a boulder within the Borrowdale clastic sequence. 0

EXCURSION TO THE NORTHERN LAKE DISTRICT 469

During the journey between these localities, the geological work recently undertaken by Mr. R. S. Arthurton and the Director along the route of the proposed A66 trunk road between Penrith and Workington was summarised, and particular points of interest, such as pot-holes in the Carboniferous Limestone, and major landslips on the Skiddaw Group outcrop, were briefly demonstrated. Causeway Foot was the last exposure to be visited, and the party then returned to Newton Rigg, which had proved to be an excellent centre at a most reasonable cost, with the added attraction (for some) of table-tennis . The secretary made a (disputed!) claim to be the excursion champion.

REFERENCES BItOWN, P. E., J. A. MILLER & N. J. SOPER. 1964. Age of the Principal Intrusions of the Lake District. Proc. Yorks. geol. Soc., 34, 331--42. CAPEWELL, J . G. 1954. The Basic Intrusions and an Associated Vent near Little Mell Fell, Cumberland. Trans. Leeds. geol, Ass., 6, 243-8. - - - . 1955. The Post-Silurian Pre-Marine Carboniferous Sed imentary Rocks of the Eastern Side of the English Lake District. Q. JI geo l, Soc. Lond., HI, 23--46. EASTWOOD, T., S. E. HOLLINGWORTH, W. C. C. ROSE & F. M. TROTTER. 1968. Geology of the Country around Cockermouth and Caldbeck. Mem. geol, Sun'. U.K. H.M.S.O., London. EWAItT, A. 1962. Hydrothermal Alteration in the Carrock Fell Area , Cumberl and . o eot. Mag., 99,1-8. FIRMAN, R. J. 1957. Fissure Metasomatism in the Volcanic Rocks Adj acent to the Shap Granite, Westmorland. Q. JI geol. Soc. Lond., H3, 205-11. GRANTHAM, D. R. 1928. The Petrology oftbe Sbap Granite. Proc. Geol. A ss., 39, 299331. GItEEN, J. F. N. 1917. The Age of the ChiefIntrusions of the Lake District. Proc, Geol. Ass. ,28,1-30. HARlCER, A. 1894. Carrock Fell. A Study in tbe Variation of Igneous Rocks Masses. Part I ; The Gabbro. Q. J/ geol. Soc. Lond. , 50, 311-17. - -- . 1895. Carrock Fell. Pans II and Hl : The Granophyre and Grainsgill Greisen. Q.J/geol. Soc. Lon d.,SI, 125--48. - - - and J. E. MARR. 1891. The Shap Granite and Associated Rocks. Q. J/ geol. Soc. Lond., 47, 266--328. HELM, D. G. 1970. Stratigraphy a nd Structure in the Black Combe Inli er, English Lake District. Proc. Yorks. geol, Soc. , 38, 105--48. HITCInN, C. S. 1934. The Skiddaw Granite and its Residual Products. Q. J/ geol. Soc . Lond., 90, 158-200. HOI.L1NGWORTH, S. E. 1955. The Geology of the Lake District. A Review. Proc. Geol. Ass., 65, 385--402. JACKSON, D. E. 1961. Stratigraphy of the Skiddaw Group between Buttermere and Mungrisda1e, Cumberland. Geol. Mag., 98, 515-28. JEANS, P. J. F. (in press). The Junction between the Skiddaw Slates and Borrowdale Volcanics in Newlands Beck, Cumberland. Geol. Mag. LISTER, T. R., I. C. BURGESS & A . J. WADGE. 1969. Microfossils from the Cleaved Skiddaw Slates of Murton Pike and Brownber (Cross Fell Inlier). Geol, Mag., 106,97-9. MITCHELL, G. H. 1956. The Geological History of the Lake District. Proc. Yorks . geol. Soc., 30, 407-63 .

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G. H. MITCHELL AND OTHERS

MOSELEY, F. 1960. The Succession and Structure of the Borrowdale Volcanic Rocks South-East of Ullswater. Q. II geol. Soc. Lond., 116, 55-84. - - - . 1964. The Succession and Structure of the Borrowdale Volcanic Rocks North-West of Ullswater. Geol. I., 4, 127-42. ---.1968. Joints and other Structures in the Silurian Rocks of the Southern Shap Fells, Westmorland. Geol. I., 6, 79-96. - - - (in press). A Tectonic History of N.W. England. /1 geol. Soc. NUTT, M. J. C. 1966. Field Meeting; Haweswater. Proc. Yorks. geol. Soc., 35, 429-33. - - - . 1968. Borrowdale Volcanic Series and Associated Rocks around Haweswater, Westmorland. Proc, geol, Soc., 1649, 112-13. - - - . 1970. The Borrowdale Volcanic Series and Associated Rocks around Haweswater, Westmorland. Unpublished Ph.D. Thesis, University of London. OLIVER, R. L. 1954. Welded tuffs in the Borrowdale Volcanic Series, English Lake District, with a Note on Similar Rocks in Wales. Geol. Mag., 91, 473-83. - - - . 1961. The Borrowdale Volcanic and Associated Rocks of the Scafell Area, Lake District. Q. II geol Soc. Lond., 117, 377-417, ROBERTS, D. E. 1971. Structures of the Skiddaw Slates in the Caldew Valley, Cumberland. Geol. I., 7, 225-38. ROSE, W. C. C. 1955.The Sequence and Structure of the Skiddaw Slates in the KeswickButtermere Area. Proc. Geol. Ass., 65,403-6. SHACKLETON, E. H. 1967. Lakeland Geology. Clapham, Yorks. SIMPSON, A. 1967. The Stratigraphy and Tectonics of the Skiddaw Slates and the Relationship of the Overlying Borrowdale Volcanic Series in Part of the Lake District. Geol.J., 5,391-418. SOPER, N. J. 1970. Three Critical Localities on the Junction of the Borrowdale Volcanic Rocks. Proc. Yorks. geo/. Soc., 37, 461-93. WADGE, A. J. (in press). Sections through the Skiddaw-Borrowdale Unconformity in Eastern Lakeland. Proc. Yorks. geo/. Soc. WARD, J. C. 1876. The Geology of the Northern Part of the English Lake District. Mem, Geol. SUTV. U.K. H.M.S.O., London, 12-132. WALKER, E. E. 1904. Notes on the Garnet-Bearing and Associated Rocks of the Borrowdale Volcanic Series. Q. /1 geol, Soc. Lond., 60, 70-105. G. H. Mitchell 57 Ladysmith Road Edinburgh EH9 3EY

N. J. Soper Department of Geology The University Sheffield Sl 3JD

F. Moseley Department of Geology The University Birmingham Bl5 2TT

D. E. Roberts Department of Geology The University

R. J. Firman Department of Geology The University Nottingham NG7 2RD

M. J. Nutt and A. J. Wadge Institute of Geological Sciences Ring Road Halton Leeds LSl5 8TQ

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