A Tectonic History of Northwest England

Home , Black Combe, Dent (fell), Northern Fells

A tectonic history of northwest England

FRANK MOSELEY

CONTENTS Introduction 56x Caledonian earth movements 562 (A) Skiddaw Slate structures . 562 (B) Borrowdale Volcanic structures . 570 (C) Deformation of the Coniston Limestone and Silurian rocks 574 (D) Comment on Ingleton-Austwick inlier 580 Variscan earth movements. 580 (A) General . 580 (B) Folds 584 (C) Fractures. 587 4 Post Triassic (Alpine) earth movements 589 5 References 59 °

SUMMARY Northwest England has been affected by the generally northerly and could be posthumous Caledonian, Variscan and Alpine orogenies upon a pre,Cambrian basement. The end- no one of which is entirely unrelated to the Silurian structures include early N--S and later others. Each successive phase is partially NE to ~NE folding. dependent on earlier ones, whilst structures The Variscan structures are in part deter- in older rocks became modified by succeeding mined by locations of the older massifs and in events. There is thus an evolutionary structural part they are likely to be posthumous upon sequence, probably originating in a pre- older structures with important N-S and N~. Cambrian basement and extending to the elements. Caledonian wrench faults were present. reactivated, largely with dip slip movement. The Caledonian episodes are subdivided into The more gentle Alpine structures also pre-Borrowdale Volcanic, pre-Caradoc and follow the older trends with a N-s axis of warp end-Silurian phases. The recent suggestions of or tilt and substantial block faulting. The latter a severe pre-Borrowdale volcanic orogeny are was a reactivation of older fault lines and rejected but there is a recognizable angular resulted in uplift of the old north Pennine unconformity at the base of the volcanic rocks. massifs relative to the downwarped Irish Sea The 'pre-Borrowdales' trends and those of the Basin and the Vale of Eden. pre-Caradoc movements are variable but are

i. Introduction

D tl R I N G T H E L A ST T E N Y E A R S there have been substantial and controversial additions to the vast geological literature on northwest England, and the tim£ would seem appropriate for a review of the nature, origin and time sequence of the major structures of this well documented region. It is not intended to deal with every aspect and it will be noticed in particular that the igneous intrusions and mineral veins receive only passing mention. The subject is most conveniently treated chronologically, starting with the Caledonian structures and continuing

J. geol. Soc. Lond. vol. x28, 1972, pp. 561-598, I2 figs., 2 pls. Printed in Northern Ireland.

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with less complex, but by no means simple, Variscan and post-Triassic structures. Whilst separating the structural episodes in this way, it is important to remember the strong influence of each episode on succeeding episodes, and that strong movements of the latter reactivated earlier structures.

2. Caledonian earth movements It is within the area of the Lake District and Howgill Fells that the Caledonian structures are best studied, but the Cross Fell, Ingleton and Austwick Inliers also provide direct evidence critical to interpretation. It is essential to emphasize at the outset that structural styles, particularly in the Lake District are closely related to rock facies and of fundamental importance to any understanding is the 3 ° ooo foot Lake District sandwich of highly competent Borrowdale volcanic rocks between incompetent Skiddaw Slates below and relatively incompetent upper Ordovician and Silurian slates and greywackes above. Each of these three divisions has a tectonic style peculiar to itself, although in each case a wide variety of structures is exhibited. It is not the purpose of this paper to detail historical developments of geological thought in northwest England. This has been effectively reviewed elsewhere at different times, most recently by Hollingworth et al. (z954) and Mitchell (I956A). It may be said in summary that after initial controversy about structural relationships between these three groups of rocks, and some misreadings of general structure by Green (i915), opinion seemed to be progressively hardening towards acceptance of views initially proposed by the pioneer surveyers a century ago. There was thus general belief in a conformable passage from Skiddaw Slates to Borrowdale Volcanics with complex folds in the former a result of their incompetence whilst the unconformity at the base of the Coniston Limestone first recorded by Aveline (I872) became an accepted fact, and there seemed little doubt that the major deformation was a result of post- Silurian orogenesis. Some of these opinions have now been questioned particularly by Simpson (z967, I968 ) and by Helm (x97 o) who believe that the Skiddaw Slates were severely folded and cleaved during a pre-Borrowdale Volcanic orogeny and that the volcanic rocks now rest upon them with major unconformity, thus explaining the differences in structural style in an apparently straightforward way. The interesting problem of the relationship between these two divisions is thus re-opened and Simpson and Helm's evidence has to be weighed against other conflicting evidence. Although structurally no single division of the Lake District rocks can be prop- erly assessed in isolation from the others it is nevertheless necessary to take them separately for descriptive purposes and in the pages following reference will be made in turn to structures within Skiddaw Slates, the Borrowdale Volcanics and the (essentially) Silurian slates and greywackes, and to the junctions between these major divisions.

(A) SKIDDAW SLATE STRUCTURES Simpson's (i967) structural account of the Skiddaw Slates revived interest, and has been followed by some ten articles and discussions already published, and

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another half dozen either in the press or in preparation. None would dispute a complex polyphase structural history, the principal bone of contention, already stated, being the time of the principle orogenic event, with two important alternatives. The first states that the Skiddaw Slates suffered a major orogeny with several complex phases of folding and cleavage prior to the formation of the Borrowdale Volcanics and the second, that the pre-volcanic folding represented a minor event and the major orogeny post-dated the volcanics. These alternatives represent an outstanding issue in Lake District geology at the present time, but before attempting to resolve it a summary of the visible structures is desirable.

I. Major Structures Study of small scale maps can give a rather misleading impression that the Skiddaw Slates are brought to outcrop by several major anticlines. It is quite clear however that such major structures are to be seen clearly only in the overlying Borrowdale Volcanic rocks and major structures within the Skiddaw Slates do not appear to conform to the same pattern, although they are not known with certainty over much of the outcrop. For example, the Ullswater anticline is a distinct major anticline as far as the volcanic rocks are concerned, but the Skiddaw Slates of the same area form a fault-bounded inlier with complex minor structures but unknown major structure (Moseley 1964). It is always the case that major structures within the Skiddaw Slates are not immediately obvious and in those areas so far examined there has been no consensus of opinion. For example, axial traces of major anticlines were placed by Rose (I 954, see also Mitchell 1956A, and Eastwood et al. 1968 ) along the outcrops of the major sandstones and were supposed to be upfolds of the Loweswater Flags in a two fold stratigraphical sequence of Loweswater Flags followed by Mosser-Kirkstile Slates. These folds do not coincide at all with those shown by Simpson (1967) who returned partially to an earlier stratigraphical interpretation of Dixon (1925) in which each of the sandstone outcrops was a different formation in a much thicker sequence. In assessing the published information now available, Simpson's more detailed structural analysis must be weighed against Rose's apparently more subjective summary (Rose I955) , although Rose's unpublished field maps and notes do contain a wealth of objective information, which formed the basis of his conclusions, and knowing of this it is impossible to say which is the more reasonable interpretation at the time of writing. It is to be hoped that Rose will publish his important data before long, but in any event the structures are so complex and the stratigraphy so difficult that much further work is necessary. More detailed structural studies are in fact now in progress, and together with studies of sedimentology, micropalaeontology and geochemistry should help towards a fuller understanding. For the time being Simpson's structural lines have been plotted on figure 12 for the ~rw fells (Bassenthwaite to Ennerdale). To the northeast is the large area of the Northern Fells much of which is covered by the recent Geological Survey Memoir (Eastwood et al. 1968 ). The fold axes plotted in this memoir are based on the hypothesis of upfolds in the Loweswater Flags (pp. II, 35 and 38), and could be suspect. Indeed D. E. Roberts (1971) in remapping part of this ground has failed to con- firm the locations of some of these fold axes.

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2. Minor Structures (i) Folds Whereas there is strong element of subjective interpretation in determination of the major folds most of the minor folds can be readily observed and measured in the field, and it is understandable that they could be of similar orientation and style to the major folds. The range in fold types (Figs. I, 2 and 3) is necessarily considered together with their orientation. Both are extremely variable and complex, there are at least three phases of movement recognisable in most areas, and up to now it has not been easy to correlate structural patterns from area to area, As more detailed analysis becomes available however, it appears that early northerly trending structures are everywhere to be found. They have been described from Black Combe (Helm & B. Roberts I968 and Helm x97o), the Northern Fells (D. E. Roberts 197 i), the Cross Fell Inlier (Burgess 1966 and figure 3), and they occur sporadically in the Buttermere-Crummock area, where they are dis- tinctly earlier than the EN~. F I of Simpson. In some localities (e.g. Black Combe, Helm i97o ) there are at least two sets of northerly (to NW) folds. The first folds are tight to open upright structures occasionally with an associated cleavage, and a plunge varying from zero to 9 o°" They are refolded by open recumbent folds with strain slip cleavage and the same trend and plunge as the first folds. The varying plunge of the northerly folds can be seen as a result of still later folding along ENE trends. These ENE structures are generally the more complex with tight upright folds and related high angle cleavage, refolded by more open recumbent folds with low angle strain slip cleavage (Fig. 3), the latter interpreted as gravity structures. Nor does this represent the end of the complexity as will be seen from the discussion on the Skiddaw Slate- Borrowdale Volcanic junction and from the suggested tectonic sequence in Table I. (ii) Cleavage Cleavage is associated with most of the fold types and ranges from horizontal to verticle and from slaty cleavage to fracture cleavage. Cleavage may or may not be parallel to the axial planes of the folds and generally there is more than one cleavage to be found, the inter-sections resulting in the well-known pencil slates of various parts of the Lake District. It is important to note, however, that the Skiddaw Slates are not invariably strongly cleaved, and this applies particularly to more competent formations such as the Loweswater Flags (Fig. I), whilst shales were long ago referred to by Ward (I876) in the Keswick area and are locally found around Ullswater (Moseley i964) and in the Cross Fell Inlier (Shotton 1935). At the same time reports of shale must be accepted with caution since cleavage parallel to bedding is common and can easily be mistaken for bedding alone in broken and weathered exposures. Figure 2 shows such an exposure with cleavage only immediately apparent on the short limb of the fold where intersections with bedding result in pencil slate. There are other exposures where cleavage is not apparent, for example only bedding is visible on some of the folds of Figs. I and 2 but cleavage shows up elsewhere under slightly different categories of weathering (C and D) and is also revealed by thin sections.

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3. Skiddaw Slate-Borrowdale Volcanic Junction and discussion of the tectonic sequence It is indisputable that a considerable part of the contact is faulted, partly by low angle and partly by high angle faults. An overall thrust relation has been suggested for a number of areas, with the thrust or thrusts at or close to the junction, probably arising as a result of disharmony between incompetent slates and competent volcanics. They are therefore considered to be decollement structures (Rose 1954, Moseley i964). Where it is possible to see exposures of those faulted contacts mapped as thrusts the inclinations are always steeper than expected and an im- bricate type of structure has been suggested with occasional slices of Borrowdale Volcanics within the Skiddaw Slates (Moseley op. cit.) It is important however not to carry the thrust hypothesis too far, for example the junction sE of Ullswater has been revealed in the Tarn Moor tunnel to be a high angle fault (Firman I969) rather than a thrust (Moseley I96o), and the original x9th century survey has been vindicated once again. Interesting as these faulted contacts are, of more significance to the overall tectonic history is the current controversy whether or not the junction represents a major orogenic unconformity. In a variety of papers and discussions Simpson, Helm, B. Roberts, Burgess and Siddans express the belief that it does, whereas Rose, Gough, Eastwood et al., Soper, Brown, Nutt, Moseley & D. E. Roberts suggest otherwise. A likely region to yield solutions to the problem is the well exposed ground between Buttermere and Borrowdale, which is at present being reinvestigated in detail, as a follow-up to the work of Simpson (1967) and Soper (197oA). Simpson claims two phases of deformation (F 1 and F~) for the Skiddaw Slates prior to the deposition of the volcanics, each with ENZ trend, strong folding and cleavage. The volcanics near Buttermere, with no complex folding, are claimed to rest on low horizons of the Skiddaw Slates, with the alternatives of a large thrust, for which he notes there is no evidence, or a major unconformity. He did not comment on the N-S folds in the Skiddaw Slates of Newlands which are earlier than the EN~ cleavage. The frequently reported interbeds of slates and volcanics in some areas, and the conformity of the Latterbarrow Sandstone with the volcanics are explained by including them in the volcanic sequence with the unconformity at a lower stratigraphical level. Examples of early Ordovician movements from the Isle of Man and Southern Scotland are cited as evidence that orogeny at this time was widespread. A very different point of view is expressed by Soper (i97oA), who whilst recognising the possibility of a minor unconformity between the Slates and the Volcanics, makes the critical observation that at all localities where the junction is exposed there is one prominent ~NE cleavage in the slates (SI of Simpson) which passes upwards into basal tufts of the volcanics, whilst slate frag- ments within the ' tuff have the same cleavage as the tuff matrix, so that this S I cleavage must be post-volcanic. Soper's interpretation was challenged immediately on a number of counts. Helm (I97oB), and Helm & B. Roberts (in press) claim that of several cleavages in the slates only one, not necessarily the strongest, passes into the volcanics.

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Helm (op. dt.) also reported a previously cleaved slate fragment within basal Borrowdale tuff and described an angular unconformity between the Slates and volcanics of Black Combe, whilst Helm & Siddans (I97 I) after a detailed strain analysis of deformed 'birds eye' tuff from the volcanics, concluded that these rocks had not been affected by the movements which produced strong northerly folds and cleavage in the Skiddaw Slates and that the latter structures must therefore have pre-dated deposition of the volcanic rocks. However Soper & D. E. Roberts (~ 97 I) were able to reinforce Soper's former argument when they recorded andalu- site crystals from the Skiddaw Granite aureole, deformed by F2 and $2 of Simpson, and since the granite is known to be end-Silurian (Brown et al. ~964) they were able to reiterate that F2 and the closely associated F~ could not conceivably be pre-Borrowdale Volcanic (mid-Ordovician). It should be noted that D. E. Roberts (~97 I) has recorded earlier northerly folds, which he labels F~, and in his revised terminology Simpson F~ & F2 then becomes F2 & F 3 (Table ~). Finally both Jeans and Wadge (in press) have described localities (Newlands and Matterdale respectively) where there is an indisputable angular unconformity between the slates and the volcanics, although neither claims a pre-volcanic cleavage. It is important to note that the intensity and style of deformation of the Slates is to a large extent related to lithology (Fig. ~). The Skiddaw Slates adjacent to the Borrowdale Volcanics are mostly highly incompetent pelites and strongly deformed, but there are also more competent formations within the Slates which are little deformed. For example outcrops of the Loweswater Flags near Lorton (Scawgill Quarries, ~78~58) have a uniform, ~8 ° dip with no minor folding, poorly developed cleavage only seen in pelite bands, and an overall structural comparability with the

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Silurian Coniston Grit. Within the volcanic rocks cleavage is strong in the well known Honister Slates, largely a result of fineness of grain and presence of chlorite (Strens, unpublished thesis) but these rocks are still more competent than adjacent Skiddaw Slates and do not generally crumple into minor folds. The lavas, being massive and competent, like the Loweswater Flags, do not develop high grade cleavage, so that in general terms it will be seen that varying structural styles can be synchronous within the one small area. It is also worth noting that elsewhere in the Lake District both fight folding and 'F2 type' recumbent folds can be observed in Silurian strata. Returning to the problem of the northerly folds it is apparent that their strength varies considerably from one part of the Slate outcrop to another. On Black Combe (Helm I97oB), and in parts of the Cross Fell Inlier (Burgess I966 and Fig. 3) they represent an important deformation, with well defined cleavage, whereas in the Northern Fells and in the Buttermere-Borrowdale region they are less well developed and are without a related cleavage. Helm in describing the structures of Black Combe follows Simpson in his belief of a major orogeny between Skiddaw Slates and Borrowdale Volcanics, and he has recorded an angular bedding discordance between the two groups in Crookley Beck. However the time of cleavage formation remained in doubt and Soper contends (I97oA) that the cleavages in the Skiddaw Slates are also present in the Borrowdale Volcanics. Other similar angular unconformities already referred to have been observed in the Newlands Valley by Jeans (in press) and near Matterdale (Wadge in press). The former shows vertical north trending slates overlain by volcanics dipping 25 ° sE, and the latter vertical ENv. trending slates overlain by gently dipping conglomerate and volcanic rocks. At both these localities there is one strong v.Np cleavage (Simpson's S I) which affects both slates and volcanics. Before considering the significance of these observations reference must be made to the well known pre-Caradoc earth movements, which it is suggested, were more important than has been previously supposed. The resulting unconformity is best seen s.w. of Coniston, where the whole of the volcanic succession is cut out by the Coniston limestone overstep (Mitchell 1956B). Elsewhere evidence for these earth movements has been recorded from near Kentmere (Mitchell I929 and I934) , in the west of the Lake District (Firman I957 and Clark I964) and they also occur north of Ullswater (Moseley I964) where removal of the regional dip reveals a N-S asymmetrical anticline with an easterly limb inclined at 4o °. It is similarly noticed that the Eycott Hill lavas (Eastwood et al. I968 ) have a 4o ° dip and this is best explained not as the steeply plunging nose of an end-Silurian fold, but as the

FIG. 3" Some Skiddaw Slate structures in the Cross Fell Inlier. I, Brownber, (NY 7o6275), open recumbent folds with E.S.E. trends, folding an earlier cleavage. 2, Ellergill Beds in Ellergill (NY 67831 I) upright folds with S.W. plunge. An E.S.E. cleavage is poorly developed and apparently later than the folds. 3, Swindale Beck (NY 694285), upright folds with northerly trends, and a poorly developed related cleavage. Equal area projections, lower hemisphere.

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easterly limb of a pre-Caradoc fold, possibly an east facing monocline. This is supported by asymmetric minor structures of similar orientation reported by D. E. Roberts (i97i) from the adjacent Skiddaw Slates. On different grounds Strens (unpublished thesis) has suggested N-S pre-Caradoc deformation for the Borrowdale region, supported by radiometric dates, to account for early fault and mineral vein orientation. Northerly folds also occur in the volcanic and slate interbeds of the Milburn Group in the Cross Fell Inlier, and there are similar but tighter folds in adjacent slates (Fig. 3). If the former is to be regarded as part of the Borrow- dale Volcanic episode, then post volcanic northerly-folding is implicit here also. It is thus possible to assign much of the northerly folding in the Skiddaw Slates to the pre-Caradoc episode, but the debate remains open, since the northerly cleavage common to parts of the slate outcrop is not to be seen in the volcanics, whilst at those critical localities where an angular unconformity has been recorded, there is no northerly cleavage in the slates. A series of folds not yet referred to are those described by Simpson as F3, which he believes to be the only representatives in the Skiddaw Slates of the end-Silurian movements. These folds, only locally developed, mostly have ~ trends and steepish plunges, and it is not easy to reconcile them with the main end-Silurian structures of the volcanic and Silurian rocks with which they are far from co- axial. Indeed this interpretation has already been questioned by Helm & Roberts (x968) and Soper & Brown (i968), and it is possible that, if F3 are truly later than the EN~. folds, they belong to the phases of end-Silurian and later northerly folding indicated on Table i. There is indeed no shortage of complexity in the structural history of the Silurian and later rocks, and it seems unlikely that Simp- son's comparatively simple F 3 can represent the whole of the end-Silurian movements. The author's conclusions on the problems just discussed are summarized in Table I. It seems certain now that the Borrowdale Volcanics are not conformable upon the Skiddaw Slates, nor were the principal tectonic movements affecting the Skiddaw Slates (F I, S I, F2, $2 of Simpson) pre-volcanic. Simpson may therefore be correct in his assertion that volcanic-slate interbeds are an essential part of the volcanic sequence, but wrong in his interpretation of the nature of the unconformity, and in his claim that the EN~. cleavage (with the related folding) was pre- volcanic. The unconformity truncates mainly N-S structures in the slates and the issue has narrowed down to the scale of the event, whether it was orogenic with associated cleavage, or local folding as a prelude to vulcanicity. The interpretation offered here is that pre-volcanic folding affected sediments which were still largely unconsolidated, and that there was later pre-Caradoc folding along the same lines, at which time the slates, now deeply buried beneath the volcanic pile, locally developed a northerly cleavage. The remaining phases can be regarded as end- Silurian and later.

(B) BORRO*qVDALE VOLCANIC STRUCTURES I. Folding The effect of the pre-Caradoc folds on the volcanic rocks has already been sufficiently discussed and need not be considered further. The end-Silurian structures

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superimposed on pre-Caradoc folds, are much the stronger, but with notable simplicity when compared with those in the Skiddaw and Silurian Slates. They are dominated by the main Lake District Anticline, an asymmetrical structure with a near vertical northern limb (Eastwood et al. 1968) and an undulating more gently inclined sE limb. The northern limb, with an E-w strike, only outcrops over an area of 12 × 2 miles and the bulk of the volcanic outcrop consists of the open folds of the sE limb. These folds are themselves large structures with half wave lengths of I to 2 miles, and dips ranging from I o to 6o degrees. Structures typical of much of this latter region are shown on figures 4 and 5 and on plates I & 2. Minor folds are uncommon and the concept of steeply dipping sharp isoclinal folds but with overall low 'sheet dip' advanced by Green (I915, I92O) has long been discarded. Other authors have mentioned belts of sharp folds in restricted localities (Hartley i942 , Mitchell I94o ) and whilst some such folds undoubtedly exist, many would now be identified as terminations of lavas and other phenomena related to the vulcanicity (Mitchell I956A).

2. Cleavage Over most of the volcanic outcrop only one cleavage is visible, although two cleavages at an acute agle to each other have been reported locally near Honister (Daniels, unpublished thesis). The cleavage intensity is variable and there are some regions where the rock is virtually uncleaved, others with weak fracture cleavage and yet others with strong slaty cleavage the latter generally being restricted to relatively narrow zones extending along the Caledonian strike. Intensity also varies according to lithology and availability of chlorite (Strens, unpublished thesis) with the best slates usually in the fine grained tufts. Cleavage planes are inclined from 5 °0 to vertical and some cleavage fans were noted long ago by Ward (i876), and recorded in more detail by Hartley (I942) who thought there may have been continuation of folding after the cleavage. It is not generally possible or advisable to use terms such as axial plane cleavage since in the open folds axial planes are not precisely defined.

3. Fractures (i) Low angle faults In general terms the highly competent volcanic rocks may be said to be partly separated from the underlying and overlying rocks by decollement thrusts, although high angle faults also form much of the lower boundary. The lower contact has already been described (p. 565) and no more need be said. The upper thrusts are rather different and largely confined to the highly incompetent Stockdale Shales which are completely sheared out in places (Mitchell 1956A, 1956B, see also p. 574). (ii) High anglefaults The rigid but brittle volcanic mass is cut by numerous high angle fractures of which mat dextral and N sinistral wrench faults are the most important. Most of these faults have had complex histories but at the time of formation many ended downwards and upwards at the bounding thrusts just referred to, and it is necessary

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to visualise not only the whole of the volcanic pile sliding along the planes of decollement, but slices of volcanic rock, including the Coniston Limestone, moving at different rates along the wrench faults (Moseley i96o, Fig. 9). The displacements of the wrench faults are up to a mile with the ww and N trends predominant in different areas, whilst the fault planes frequently form the wide hematite stained shatter belts noted by Mart (I916). In one case at least (the Kydal-Coniston complex) the whole of the volcanic outcrop is traversed, with the fault passing downwards into the Skiddaw Slates of St. John's vale, whilst upwards this fault and several others are strong enough to cut through the Stockdale Shales, but they are absorbed by the bedding thrusts near the base of the Coniston Grit (Fig. I2). In considering the total movement sequence of the faults one must take account of the fact that many of the wrench faults suffered subsequent movement as normal faults during the late Caledonian, the Variscan and succeeding episodes and this resulted in the block faulting with sudden changes in amount of dip slip, characteristic of the volcanic outcrop, and in the displacements of the Borrowdale Volcanic-Skiddaw Slate junction. (iii) Joints Firman, in his study of the Eskdale-Duddon area (I 96o), clearly showed that knowledge of joint orientation can help a great deal in structural interpretation. In particular he was able to demonstrate the presence of high angle shear joints in the Eskdale Granite parallel to the mew dextral and m~E sinistral wrench fault directions. At an acute angle to each set there is a second set which he interpreted as due to rheidal shear. Other ~¢w and N~. high angle joints particularly in the Borrowdale Volcanic rocks, are considered to be cross and longitudinal tension joints and probably formed late in the tectonic episode in the manner envisaged by Price (1966), whilst other relatively low angle joints were thought to be related to normal faults. Detailed studies of this kind have not been attempted in other parts of the Borrowdale Volcanic outcrop but from those observations so far published it seems likely that similar patterns exist.

C) DEFORMATION OF THE CONISTON LIMESTONE AND SILURIAN ROCKS

I. General The arcuate strike swing from NNE in the sw of the Lake District to E-W near Cautley and finally to ESE in the Ingleton and Austwick inliers of the Askrigg Block has frequently been referred to, most recently by Kent (I967)and Nutt (I967). There are many details to be added to this overall pattern and it is con- venient to refer to them first in stratigraphical order. It is apparent that the Conis- ton Limestone although unconformable on the Borrowdale Volcanic rocks is for the most part a competent formation and is deformed in much the same way. Upwards in the sequence, however, changes in facies result in differences in tectonic style, the changes usually taking place at fairly well defined stratigraphical horizons. The first important tectonic break occurs at the highly incompetent

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Stockdale Shales, which are cut through by numerous shears sub-parallel to the bedding. These shears are in essence bedding thrusts and this mechanism for relief of stress has largely inhibited development of other structures such as folds or high grade cleavage. The majority of the small wrench faults which displace the Coniston Limestone bend into these thrusts and the strike slip movement is thus turned into thrust movement in the manner depicted in De Sitter (1956). This is in fact similar to structures along part of the Skiddaw Slates-Borrowdale Vol- canicsjunction, where the strike slip movement of the wrench faults in the volcanics is transferred into movement along the Ullswater thrust (Moseley i96o , Fig. 9). A higher tectonic break occurs near to the junction of Coldwell Beds and the Con- iston Grits where there is a comparable zone of disharmonic thrusting. The larger wrench faults (Coniston, Rydal and Troutbeck) end in this zone and the displace- ment along them is translated into movement along the thrust complex in the manner just described. At a still higher level there is disharmony between the Coniston Grits and Bannisdale Slates, the former a competent group often with open folding reminiscent of the Borrowdale Volcanic structures and with poorly developed cleavage, and the latter with close packed folds and strong cleavage.

~. Fo/ds It is notable that over much of the southern Lake District the sequence from the Coniston Limestone to the base of the Coniston Grit dips steeply and uniformly to the s~. and the greater part of the folding, starting in the Coniston Grits, is to be found in the Bannisdale Slates. There are exceptions of course, such as the minor folds in Ashgill and Stockdale Shales between Torver and Coniston and complex structures in the Cautley area and Cross Fell Inlier where there are Variscan and other complications. The strong folding of the Coniston Limestone of Ireleth in the southwest, is probably less intense than the Old Series maps seem to indicate, and W. C. C. Rose who is remapping the area believes that the outcrop pattern results from faulting (personal communication). Turning to other features of a general nature, Norman (unpublished thesis) has observed that near Coniston the steeply dipping strata below the Coniston Grits are cut by numerous upthrusts, additional to those thrusts already referred to. These upthrusts dip more steeply than the bedding, and he suggests that they represent overturned limbs of the folds that partially replace them to the south. Another notable characteristic is the tendency especially within the Bannisdale Slates for sharply folded belts to alter- nate with uniformly dipping strata (Moseley i968 ). Referring next to specific details of the folds it may be noted that there is folding on a variety of scales, which can be classified into major folds with half wave lengths of about 2 miles and minor folds parasitic upon them with half wave lengths of from 5 to I OO yards. Majorfolds have been located with certainty only in the regions south of Coniston and near to Shap, re-survey being necessary elsewhere. Within the Coniston Grit these folds are generally more open in character and minor folds are less common than in the succeeding Bannisdale Slates (compare Fig. 7 with Fig. 6 and with Moseley 1968, Fig. I). It is generally the case that minor folds obscure the locations of major fold axes, which only emerge after detailed mapping.

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Minor folds exhibit a variety of styles depending on lithology and location. It has already been noted that they are better developed in the argillaceous formations and particularly in the Bannisdale Slates, this clearly being attributable to the relative incompetence of these rocks, a recurring theme in interpretation of structural styles of the rocks of Northern England. The majority are asymmetrical, with the asymmetry noticeably related to the limb of the major fold upon which they occur. For example the northern (ss~. dipping) limb of the Bannisdale Syn- cline is corrugated by asymmetric minor folds with short moderately inclined NNW dipping limbs, and long near vertical ssF. dipping limbs. The opposite is the case on the southern (NNW dipping) limb of this syncline where it is the ~NW dipping minor fold limbs which approach the vertical (Moseley I968). This can also be seen south of Coniston (Norman op. tit.) and to a lesser degree near Underbarrow, where it is also possible to demonstrate upthrusts and high angle axial plane faults (Fig. 6). Foldplunge is variable in both inclination and direction. In the east a five degree ~.N~ plunge has been attributed to post-Carboniferous tilt (Moseley 1968) and in the west where plunges are both to NE and sw or where the N~ plunge varies in amount there have been suggestions of N to Nw cross folds made independently by Norman, Stabler and O'Connor in unpublished theses. South of Coniston, Nor- man has recorded consistent mE plunges of 3 o°, rather steeper than usual and, since most of the folds hereabouts are strongly asymmetrical with steeper ~v dipping limbs, the axial trends (07 °0 ) and axial traces (0600-065 °) show considerable divergence in azimuth. One further and rather interesting feature to be found within the outcrops of the Bannisdale Slates and Kirby Moor Flags is a localised swing to a ~-s strike with concomitant steepish dips to the east. Such regions are near Whitbarrow (SD 4584), Underbarrow (Fig. 7) and Cartmel (SD 3580) and they surprisingly exhibit little folding. For example the strong ENd. folds of Fig. 6 die out completely a short distance further east and are not to be seen near Underbarrow (Fig.7) , although the near vertical ~N~ cleavage continues unchanged, the situation is exactly the same near Whitbarrow. It is therefore clear that initially these zones must have pre-dated the EN~. folds, with the steep easterly dipping strata resisting the later folding, presumably because they were already in a relatively stable condition, although it will be seen (p. 584) that there was also substantial Variscan

FIG. 6. Map of the Bannisdale Slates of Wcst-Undcrbarrow, Wcstmorland. (SD 4592). A-B-line of section (profile). Equal area projections, lower hemisphere :-- I. 24o bedding poles. P-mean plunge. 2. 23o cleavage poles. X-maximum of 63 cleavage-bedding intersections on SE dipping fold limbs. Y-maximum of 57 cleavage-bedding intersections on NW dipping fold limbs, b-mean bedding planes. Map redrawn partly from unpublished field maps and data by J. R. Chatten, and partly from the authors' field maps. The original field maps of W.T. Avcllne provided a rcllablc basis which rcqulred almost no modification.

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movement along the N-s lines. It is possible that they represent a reactivation of the pre-Caradoc folds (p. 569) and it has already been suggested that the latter may be equated with phases of northerly folds in the Skiddaw Slates, and that these may be related to a N-S trending pre-Caledonian basement, possible ofpre- Cambrian age.

3. Cleavage Two cleavages affect the Silurian rocks although the second cleavage is restricted to a few localities only. The first and dominant cleavage is an invariable characteristic of the argillaceous rocks but is poorly seen in the massive greywackes. It is usually a fracture cleavage which varies in intensity and spacing according to lithology and in some of the finer grained mudstones passes gradationally into slaty cleavage. Orientation is also affected by lithology and there are many examples of cleavage refraction in sympathy with graded bedding, whilst near Shap cleavage in mudstone rotates into a sinistraljoint set in the interbedded greywackes, suggesting either that the joints were already present or that deformation which gave a fracture cleavage in the mudstones resulted instead, in 'wrench joints' in the greywackes (Moseley 1968). In most of the areas for which detailed analysis is available it is apparent that the principal cleavage is not parallel to axial planes of folds. It often fans in the manner characteristic of fracture cleavage (op. cit. I968, Fig. 2) and more significantly frequently has a strike about 5 ° oblique to that of the axial planes of folds, suggesting an overall clockwise rotation of the stress field during the final stages of folding (Moseley I968 ). This characteristic is clearly revealed in the west Underbarrow area by unpublished data recorded by J. R. Chatten (Fig. 6). In particular the inset stereogram to this map shows the different orientation of cleavage-bedding intersections on opposing fold limbs, which would be expected from such a structural relationship. The second cleavage indicated above has been locally observed south of Coniston and west of Kendal (Norman and Ahmed in unpublished theses). It is a high angle cleavage and is generally about 2o ° oblique to the principal cleavage with an ~--w trend, probably representing a late Caledonian pulse with continued clockwise rotation of the stress field.

4. Faults The fault pattern of dextral and sinistral wrench faults and N~ trending upthrusts is similar to that in the underlying rocks and has already been sufticiently described.

FIo. 7" North-south structures in the Silurian rocks of Underbarrow (SD 4790). Geological map and cleavage and bedding data partly from unpublished work by J. R. Chatten, and partly from work by the author, with W. T. Aveline's six inch field maps as a reliable basis. X cleavage-bedding intersections. Note that the upper Bannisdale Slates of this map area (B) have recently been reclassified as Lower Underbarrow Flags (Shaw, I97x ).

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There are, however, a few items of special interest worth mention. It is notable for example that the arrangement of minor fold axes is different on opposite sides of faults suggesting that folding and faulting were to some extent synchronous (Moseley 1968). Secondly there is the interesting development of composite wrench and thrust faults mapped by Norman (op. cit.) south of Coniston. These faults are basically north trending sinistral wrench faults which bend into NE trending thrusts inclined at about 45 ° to the south-east, and then bend back into wrench faults to form dog-leg outcrops (Fig. i2). A third element requiring comment is the reactivation of the faults during the Variscan and Tertiary episodes, easily demonstrated by observing faults such as the Trias boundary fault (SD Io.9o , SD 5o.5 o) and the Cartmel-Artle Beck fault (SD 5o.7 o) which pass from the Lower Palaeozoic to younger rocks. It is certain that many of the faults crossing the Silurian and other Lower Palaeozoic outcrops suffered such renewed movement in Variscan and again in later time. This later movement was, however, different in kind to the Caledonian faulting being normal rather than wrench and was responsible for the dip slip block faulting, a common feature of most of the Lower Palaeozoic outcrop. 5. Joints Study of joints is rather a specialism in an article such as this and the problems can be considered in outline only. The most important aspects are strong development of high angle joints with NW and N trends, parallel to the principal faults and probably with similar wrench type origins. There is local occurrence of low angle thrust joints inclined both to NW and sE (Fig. 7), and there is an abundance of near horizontal joints best interpreted as late stage tensional structures. It is also of importance to note that joint orientation and patterns on adjacent fold limbs can differ markedly. It is thus necessary to exercise care in selecting field methods for recording the joint data (Moseley I968 ).

(D) COMMENT ON THE INGLETON-AUSTWICK INLIER No reference has been made to the Ingleton-Austwick inlier where the situation is different to that of the main outcrop, although the same general principles apply. The fold trends swing to ESE, parallel to the line of the Craven Faults and to the zone of magnetic anomaly in the basement (Bott 1967), which suggests that all are related. The two major stratigraphical divisions, the Ingletonian (regarded by many as Pre-Cambrian) and the upper Ordovician and Silurian show considerable differences in structure with the former isoclinally folded and strongly cleaved (Leedal & Walker i95 o) and the latter deformed in much the same way as the Silurian of the main outcrop (King & Wilcockson I934). All these structures are co- axial however and the question of conformity or unconformity has not been settled.

3- Variscan earth movements (A) OENERA- The Variscan tectonics of Nw England can be referred to successive phases of earth movement although it is probable that all except the main post-Carboniferous

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movements were mild and none were of orogenic dimensions. Local non- sequences at two levels (G1/G2 and S~[S~Da) within the Lower Carboniferous of the Craven Lowlands have been recorded, with attenuation and breccia beds (Hudson & Mitchell I937). Towards the end of the Lower Carboniferous (DI/P1) there appears to have been movement along the Mid-Craven Fault (Sudetic movements) resulting in a fault scarp against which the Bowland shales were banked (Hudson I933, I944; Black I958 ). Hudson & Mitchell (I937) also suggested that south of the stable block there was strong folding at this time, with the Bowland Shales of the Skipton Anticline resting with strong discordance on tightly folded Lower Carboniferous rocks. The evidence for this folding rests to a large extent on differences in tectonic style between beds in the core and in the envelope of the Skipton Anticline, and the arguments are to some extent parallel to those relating to the Skiddaw Slate-Borrowdale Volcanic relationship. It was suggested by the author (i962) that this greater complexity to be found in the cores of all the Ribblesdale Folds did not necessitate a major unconformity, but was better explained by a combination of two processes (i) the crumpling which is normal to the core of a concentric fold and (ii) disharmonic folding, whereby argillaceous beds crumpled more readily than competent sandstones and limestones. On these bases the suggested strong Sudetic unconformity was rejected, although the likelihood of minor earth movements was not denied. At a later stage the Malvernian is represented by local unconformities such as that of the Morganian red beds on the Westphalian of the Ingleton Coalfield (Ford I954) and is probably of similar intensity to the preceding movements. It is, however, the post-Carboniferous (presumably Saalian) movements which were responsible for the major tectonics and it is reasonably certain that the important Variscan folding and faulting belongs largely to this phase. It is necessary to remember however that the Caledonian basement exerted a strong influence both on Carboniferous sedimentation and on the orientation and style of the Variscan structures. Most important was the controlling influence of the Alston and Askrigg Blocks and the Lake District Massif, rendered stable and quite distinct from adjacent regions by the Caledonian orogeny, the late Caledonian granites of Weardale and Wensley- dale and by the lenticular pile of the Lake District volcano. These problems have been discussed by many authors, and most recently and fully by Bott (I967). The stable regions upon which relatively thin Carboniferous deposits were laid down remained comparatively unaffected by the Variscan earth movements but they are flanked by downwarped basins with thick Carboniferous sequences; which were much more strongly deformed. The general ~.-w Variscan stress over Northern England resulted in the buckling together of the Lake District and the stable North Pennine Blocks, with the complex structures of the Gross Fell Inlier and the Dent Line forming between them. Additional to this Caledonian control there is reason to think that some individual structural patterns could be post- humously related to Caledonian structures. Turner (I949) drew attention to 'Caledonian trends' over much of the Carboniferous outcrop, making particular reference to the N~ trends of the Ribblesdale folds, although Westoll (I967) suggested that the latter could be a variety of drag fold related to dextral movement of the Craven Faults. It has been known for a long time that many faults

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affecting the Carboniferous rocks are reactivated Caledonian faults, and recently Moseley and Ahmed (i967) attempted to show that Variscan joint orientation also was related to fractures in the underlying Caledonian basement.

B) FOLDS I. Massifs The Carboniferous strata on the Alston and Askrigg Blocks are generally near horizontal, but there are local zones of disturbance of considerable structural importance. These include shallow folds between the Askrigg and Alston Blocks of which the Cothersdale syncline (Reading I957) is the most notable whilst the Burtreeford disturbance (Dunham 1948) is a well known N-s monoclinal flexure on the Alston Block with features in common with the N-s monoclines to be described below. The Lake District Massif is rather different with the Carboniferous rocks tending to dip outwards in all directions in a dome like structure, although in detail and especially in the south of the Lake District there is more complexity which will be referred to in the paragraphs below.

2. Basin Much more interesting structures are to be found on the margins of the stable blocks and in the basins. They include the gentle N-s Pennine anticline, the N~- trending Ribblesdale folds, several N-s sharp monoclinal structures including the Dent Line and irregular ~.-w folds on the northern margin of the Alston Block. The Pennine anticline is largely outside the area of discussion but it is interesting to note the way it bends to a NE trend in the extreme north, to become parallel to the Ribblesdale folds.

3. Particulars of the majorfold groups (i) The northerlyfolds very largely but not entirely within the confines of the Central Basin, are mostly monoclinal with vertical to inverted east facing limbs. The Dent Line bordering the western margin of the Stable Blocks is the best known of these structures and has been described by Turner (i927, i935). In most cases there is complex faulting associated with the steep monoclinal limbs and usually these faults have westerly downthrows giving displacements in the opposite sense to those of the monoclines. This is well illustrated by the Augill Beck section across the Dent Line (Turner i935) and can also be seen in the Hutton monocline and the Knots anticline (Fig. Io). Another similar structure is that of the Silverdale disturbance (Fig. Io) which consists of a monocline with subsidiary sharp folds extending more than a mile across the strike. The probability that the Silverdale folds extend north into the Silurian tract of the southern Lake District has already been commented on (p. 577), and the N-s strike and the steep easterly dips of the Bannisdale Slates near Underbarrow (Fig. 7) would seem to be partially of Variscan age. Similarly, zones of N-s strike and easterly dip in the Silurian rocks west ofWhitbarrow (SD 4584) and near Cartmel (SD 358o) (also coincident

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with a prominent magnetic high) may be suspected as northward continuations of fold belts in the Arnside-Morecambe Bay region, whilst unknown structural complexities may occur at the approximate position of the Ribble estuary, where these N-S structures converge on the NF. trending Ribblesdale folds and where there is a marked gravity low. (D. H. Griffiths, private communication). In general terms the N-s folds must be attributed to z-w compression, the strata belonging to the central Pennine basin being deformed by a number of such fold belts, but these reduce in intensity as they are traced northwards onto the massif, the exception being the Dent Line which can be regarded as a deformed zone produced by the Lake District massif riding against the Askrigg Block. Further north where the Lake District pushed against the Alston Block there are analo- gous structures in the easterly directed thrusts, upthrusts and folds of the Cross Fell Inlier (Shotton 1935, Shotton et al. 1968). As a final comment it may not be a coincidence that the northward continuation of the Hutton monocline is to be found in the Kendal wrench fault, which traverses Silurian strata, and perhaps all is part of the story of Variscan inheritance of structures already existing in the Caledonian basement, and indeed as previously suggested in an even deeper pre- Cambrian basement beneath. (ii) The Ribblesdale folds are a marked contrast to the northerly folds, both in trend and in the detailed structures they exhibit (Fig. I o). They are largely concentric structures and with half wave lengths between I and 4 miles and they involve strata of diverse lithology including shales, sandstones and limestones, with resulting intense disharmonic relations. Complex major and minor structures have been established in the Skipton Anticline (Hudson & Mitchell I937) , the Sykes Anticline (Moseley x962 ) and the Clitheroe folds (Earp et al. I96I), in- eluding numerous small scale tight folds, with some associated thrusting which occurs both in the cores of major folds and in incompetent strata such as the Wors- ston and Bowland shales. These complex structures are to be compared with the much simpler folds in overlying competent formations such as the Pendle Grit, a contrast in fold style and tectonic facies which has already been referred to (p. 58I), and is reminiscent of the contrast between the Skiddaw Slates and the Borrowdale Volcanics. An unusual feature previously referred to (p. 58I), is the apparently anomalous NE trend of the Ribblesdale Folds. Wager (193 I) illustrated the relation between these structures and the Askrigg Block using a model, and Turner (i949) and Westoll (I967) have both put forward suggestions to explain this trend. Westoll's suggestion that the orientation of the Ribblesdale folds and the deflection of the Pennine axis were consistent with a dextral wrench movement for the Craven Faults is an interesting one, which solves the difficulty of anomalous trend but creates another, since the upthrusts of the Dent line and Cross Fell inlier, and the Nw orientation of the Craven Faults in relation to an ~.-w stress imply sinistral movement. Turner's hypothesis that the Ribblesdale folds are posthumous upon Caledonian structures escapes the difficulties just mentioned. It is not necessary to take this hypothesis too literally, since any type of NE Cale- donian structure at depth, whether fold or cleavage, could give a trend of weakness capable of being propagated upwards through the Carboniferous rocks during a period of renewed stress.

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A Silverdale Hutton Hutton Disturbance Man Roof Man

I I ! o ~ 2 ~miles

B Knots Ouernmore Anticline Disturbance Caton Shales

' 'l:.l '" "-- ~\~ .~" \\ ", ",. Grouo I ,;~\ ~ -.- ....--- -- ..z__ .-I /a o\ - ~ .... i

0 ,~mile ~ ~/ • ° • ~1, ~1' ~ ~ ~ ~ ~. I i o ~mile Marl Hill Tunnel f-- . - = ".- ":- .

....,-_-:--~....~.=-~,, ., --,,~- ,~.,~ ~////,~m~X~~,~,:.%::¥~.,~-~ I_.- • ~.-..--~. ~..:¢--/~.~:: ,,,.,

Bowland b Worston Shales

D Skipton Anticline

G~ i ~. \- ~zmile . .o~ ~ x~

[ C-D, Shales 5 Limestones

FIG. 9. Sections to show typical folds in the Carboniferous rocks. A & B--north trending monoclines (author), C & D---Pdbblcsdale folds. C---Clitheroe fold belt after Earp et al 196x, D after Hudson & Mitchell 1937.

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When individual folds are examined in detail other complexities become apparent which must have some bearing on ultimate interpretation. For example the Sykes anticline is a concentric fold which becomes tighter into the core and is likely to end downwards at a type of decollment above the Clitheroe limestone (Moseley 1962 , Fig. 9). Larger folds such as the Skipton, Slaidburn and Clitheroe anticlines are comparable, but extend to deeper stratigraphical levels. One more aspect of some importance is the en-echelon arrangements of folds (Hudson 1933, Hudson & Mitchell 1937, Moseley I962). The Sykes anticline in particular is made up of numerous NN~. en-echelon periclines within the main NE structure, and this can be explained in terms of an v.-w regional stress, in conjunction with some factor producing NE structures (e.g. the Caledonian grain), with a resulting sinistral rotational shear pattern.

There has been reference to Variscan faults and joints in the previous discussion, and they have been the subject of recent papers (Spears I96I, Moseley & Ahmed I967, Doughty 1968), so that only a summary is required here. The faults can be divided into two groups. First, the northerly faults associated with the northerly monoclines, already sufficiently discussed, and secondly the m~r and N~. faults which form a conjugate system. These latter faults maintain remarkably uniform trends over the whole region (Fig. 12) regardless of the orientation of local folds, and considering the evidence of all structures it seems likely that they had origins as wrench type structures in response to a regional ~-w stress, although they are now seen as normal faults. The parallel trends of the Nw (to m~w), N~. (to ~.NF.) and N Variscan faults and Caledonian structures (NW dextral wrench, NE cleavage and N sinistral wrench), with an implicit posthumous relation has already received consideration and need not be expanded further. Structure contour maps, with different folding patterns on opposite sides of faults (e.g. Moseley 1954, I956; Reading i957) suggest that normal faulting and folding were to some extent contemporaneous. The joints have the same consistent trends OfNW, N and NE as the faults, typified by the Hutton Roof region (Fig. lO). They differ in inclination however, and it is certain that they are not small or incipient faults, but are different structures. It is conceivable that the ~-w stresses responsible for initiation of the faults also built up strain energy in the rock, to be released during a later tensional phase as joint fractures (Price 1966 ) . It is not essential however to regard this as positively the last event of the Variscan earth movements of this region, since there is some suggestion that normal faulting and some folding continued after joint formation (Moseley & Ahmed 1967), and it is possible to envisage alternating compressional and tensional pulses during the final phases of movement. The principal trends, like those of the faults, bear such a close relation to the fracture planes in the Lower Palaeozoic rocks (Caledonian), that a posthumous relation is likely, and Moseley & Ahmed (0p. tit.) suggested that this was the case, and that the Variscan pattern was likewise transmitted upwards into the Permo-Triassic rocks during the later

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CARBONIFEROUS " LIMESTONE STRUCTURES of

; ,--~-- ...... HUTTON ROOF

x'~ ',: :: .". ". 7 ',',, "':il!~ii!: . I I ~i,iii Half Mile i i l I "~ ::" i:"~,~Newbiooin One Kilometr¢ iii:: N tII i ",'} ~../i!i I/L !,~,.~i::ii!2.~a t79/_I

...... :;':::':'::',.

__ ...::" ...... !i !'.,,r ÷ %% s /// "," : Y/.-. . / /f'ta ,t :: : : ::.;f~ /-: :i;!i!'~.. /Jf '11,,, v ~i~',~ ...... ,., ", ? i!!ii!iii!i!--!; /" ":.7 ...... ::/ (,

, \~ ../i~/ /¢! i'i!~i

ul / Burton l :

--//--/\mAx Faults & masterjoints Outcrops of successive beds of 'D' limestone (scars) HM-KF Line of the Hutton ~line-Kendal-fault HRM Line of the HuttonRoof monocline 1-15 Rose diagrams of joint localities(ground measunnents) shown on map

A - Total of IO51 ground measurments of joints

B - Total of 520 oerial photograph lineaments of faults-masterjoints Glacial drift

FIG. IO. Structural map of the Carboniferous Limestone outcrop of Hutton Roof (SD 5478) showing bedding, faults and joints.

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Alpine movements. This should not be taken as implying passage of individual joints across an unconformity, and Doughty (1968) has in fact shown that this does not happen in the Craven district. Attending to some details ofjoint formation it can be seen that both primary bedding and lithology exert strong influence, with joints invariably nearly perpendicular to bedding, and joint density related to lithology (Doughty 0p. dt.).

4. Post Triassic (Alpine) earth movements

The principal effects of this phase of earth movements were reactivation of existing faults, uplift and tilting of the Askrigg and Alston Blocks, gentle doming of the Lake District and warping along the Pennine axis. Renewed post-Triassic movement of many of the faults as normal faults has been referred to in numerous publications. The Nw faults were particularly active, in some cases with displacements of several thousand feet. Movement along the Outer Pennine fault was largely instrumental in the uplift of the Alston Block, and combined with the rise of the Lake District dome resulted in the Vale of Eden trough. Movement along the South-Craven fault resulted in uplift of the Askrigg Block and must be visualised in the broader context of the downwarp of the Irish Sea basin. Between the Craven faults and Morecambe Bay there are numerous row faults traversing the Coal Measures and Millstone Grit, most of which have sw downthrows (Figs. i and i2). Since both the South-Craven and the faults around Morecambe Bay displace Permo Triassic rocks it is reasonable to suppose that most of the inter- vening faults also had a post-Trias phase of movement with the general picture of the uplifted Askrigg Block separated from the Irish Sea basin by a series of step faults, features of a Tertiary landscape still partially reflected by present day topog- raphy. One final observation which has been a theme throughout this paper is that all the Tertiary structures, not only the faults owe their orientation and development to earlier structures.

N.E. ASKRIGG Scales (approximate) BLOCK Horizontal I J 5 miles UP-FAULTED S.W. vertical : : : I I 4000 feet J

IRISH SEA BASIN [FYLDE| O ~

,:.'EL.,:.'::'..":.': __ -:--'..:'~ '" ''.'~'~'~,~ - ~'--c. ~.~ "-.'~-- 71~CA~BO.iF~.OUS ,l~,l Lk',~,,c',/X~,v,k%~s .

FIe. I x. Section from the Irish Sea to the Askrigg Block to show Tertiary uplift by step faulting.

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ACKNOWLEDGEMENTS. This paper has benefited from numerous discussions with others working on similar problems and in addition to the publications consulted frequent reference has been made to unpublished theses and reports, particularly those of T. N. Norman, R. G. J. Strens, S. M. Ahmed, B. O'Connor, C. S. Stabler, J. R. Chatten, E. B. Daniels, P. J. F. Jeans.

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MATCHELL G.H. 1929. The succession and structure of the Borrowdale Volcanic Series of Trout- beck. Kentmere and the western part of Long Sleddale. Q. Jl geol. Soc. lord. 85, 9-44. I934. The Borrowdale Volcanic Series of the country between Long Sleddalc and Shap. Q. Jl geol. So¢. lord. 9 o, 418-444 . 194o. The Borrowdale Volcanic Series of Coniston, Lancashire. Q. Jl geol. So¢. lord. 96, 3o x- 319 • I956A. The geological history of the Lake District. Proc. Yorks. geol. Soc. 30, 4o7-463 • I956B. The Borrowdale Volcanic Series of the Dunnerdale Fells, Lancashire. Lpool Manchr geol. J. x, 428-449. - I967. The Caledonian orogeny in Northern England. Proc. Yorks. geol. Soc. 36, x35-x38. MOS~LEY, F. x954. The Namurian of the Lancaster Fells. Q. J1 geol. Soc. lord. xo9, 423-454. 1956. The geology of the Keasden area, west of Settle, Yorkshire. Proc. Yorks. geol. Soc. 30, 331-352. 1960. The succession and structure of the Borrowdale Volcanic rocks south-east of Ullswater. Q. Jl geol. So¢. load. x x6, 55-84. , I962. The structure of the south-western part of the Sykes Anticline, Bowland, West York- shire. Proc. Yorks. geol. So¢. 33, 287-314. 1964 . The succession and structure of the Borrowdale Volcanic rocks northwest of Ullswater. Geol. J. 4, I27-x42- I968. Joints and other structures in the Silurian rocks of the southern Shap Fells, Westmor- land. Geol. 2'. 6, 79--96. I97oA. In discussion: Three critical localities on the junction of the Borrowdale Volcanic rocks with the Skiddaw Slates in the Lake District, by N.J. Soper, Pro¢. Yorkx. geol. So¢. 37) 484-486. I97oB. In discussion: Stratigraphy and structure in the Black Combe Inlier, English Lake District, by D. G. Helm, Pro¢. Yorks. geol. 8o¢. 38, x34-138. & AHM~.D, S. M. I967. Carboniferous joints in the north of England and their relation to earlier and later structures. Pro¢. Yorkx. geol. Soc. 36, 6x--9o. I~YERS,J. O. & WARDELL,J. 1967. Gravity anomalies of the Askrigg Block south of Wensleydale. Pro¢. Yorks. geol. Soc. 36, I69-I73. Ntrrr, M. J. C. x967. The sub-Carboniferous basement in Northern England. Discussion, Pro¢. Yorks. geol. Soc. 36, 228. x968. Borrowdale Volcanic Series and associated rocks around Haweswater, Westmorland. Proc. geol. Soc. x649, xx2-II 3. x97o. In discussion: Stratigraphy and Structure in the Black Combe Inlier, English Lake District by D. G. Helm, Proc. Yorks. geol. Soc. 38, t43-x44. OERT~L, G. I970. Deformation of a slaty, lapillar tuffin the Lake District, England. Bull. geol. soc. Am. x8, xx73-xx87. OLrWR, R. L. x96x. The Borrowdale volcanic and associated rocks of the Scafell area, English Lake District. Q. 31 geol. Soc. lord. xx7, 377-417 . PAR~SON, D. x936. The Carboniferous succession in the Slaidburn district. Q. Jl geol. Soc. Lord. 92, 294-33 z. Pssc~, N.J. x966. Fault and joint development in brittle and semi-brittle rock. x + x76 pp. Pergamon Press, Oxford. I~ADXNO, H. G. I957. The stratigraphy and structure of the Cotherstone Syncline. Q. Jl geol. Soc. Lord. xx3, 27-56. ROBERTS, B. I97o. In discussion: Three critical localities on the junction of the Borrowdale Volcanic rocks with the Skiddaw Slates in the Lake District by N. J. Soper, Proc. Yorks. geol. Soc. 37, 483-484 • ROBERTS, D. E. x97o. In discussion: Three critical localities on the junction of the Borrowdale Volcanic rocks with the Skiddaw Slates in the Lake District, by N. J. Soper, Proc. Yorks. geol. Soc. 37, 486. x97x. Structures oftbe Skiddaw Slates in the Caldew Valley, Cumberland. Geol. aT. 7, 225- 238.

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RosE, W. C. C. 1955. The sequence and structure of the Skiddaw Slates in the Keswick-Buttermere area. Pro¢. Geol. Ass. 65, 403-406. x96o. In discussion: The succession and structure of the Borrowdale volcanic rocks south- east of Ullswater by F. Moseley, Q. dl geol. Soc. Lond. Ix6, 55. ROWELL, A.J. & SCA~LAN,J. E. 1957. The relation between the Yoredale series and the Millstone Grit on the Askrigg Block. Proc. Yorks. geol. Soc. 3x, 79-9 o. SHACKLETON, R. M. x967. The sub-Carboniferous basement in Northern England. Discussion. Pro¢. Yorks. geol. Soc. 36, 227. SHAW, R. W. L. x97x. The faunal stratigraphy of the Kirkby Moor Flags of the type area near Kendal, Westmorland, Geol. J. 7, 359-38o. SHOTTON, F. W. x935- The stratigraphy and tectonics of the Cross Fell Inlier. Q. Jl geol. So¢. Lond. 9 x, 639-7o4. , WADGE,A. J. & BURGESS, I. C. x968. The Cross Fell area. Report of a field meeting. Pro¢. Yorks. geol. 8o¢. 36, 34o-344 • SmDANS, W. B. x97o. In discussion: Stratigraphy & Structure in the Black Combe Irdier, English Lake District by D. G. Helm, Proc. Yorks. geol. Soc. 38, x44-I47. SIMPSON, A. 1963 . The stratigraphy and tectonics of the Manx Slates Series, Isle of Man. Q. dl geol. So¢. Lond. xx9, 367-4oo. 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, 39I-4 x8" I968A. The Caledonian history of the north-eastern Irish Sea region and its relation to surrounding areas. Scott. 3'. Geol. 4, 135-x 63. x968B. The Caledonian history of the north-eastern Irish Sea region. Scott. J. Geol. 4, 38o-385 . I97O. In discussion: Three critical localities on the junction of the Borrowdale Volcanic Rocks with the Skiddaw Slates in the Lake District, by N. J. Soper, Pro¢. Yorks. geol. Soc. 37, 496-488. SOPER, N.J. x97oA. Three critical localities on the junction of the Borrowdale Volcanic rocks with the Sklddaw Slates in the Lake District. Proc. Yorks. geol. So¢. 37, 461-493 • x97oB. In discussion: Stratigraphy and structure in the Black Combe Inlier, English Lake District, by D. G. Helm, Pro¢. Yorks. geol. So¢. 38, x32-x34. -- & BROWN, P. E. x968. The Caledonian history of the north-eastern Irish Sea region. Scott. 3.. Geol. 4, 376-377. -- & ROBERTS, D. E. x97x. Age of cleavage in the Skiddaw Slates, in relation to the Skiddaw aureole. Geol. Mag. xoS, ~,93-3o2. SPEARS, D. A. x96 x. Joints in the Whin Sill & associated sediments in Upper Teesdale, Northern Pennines, Pro¢. Yorks. geol. Soc. 33, 2 I-3O. TROTTER, F. M. & HOLLINGWORTH, S. E. 1928. The Alston Block. Geol. Mag. 65, 433-448. ~, HOLLINGWORTH, S. E., EASTWOOD,T., & ROSE, W. C. C. 1937. The geology of the Gosforth district. Mem. geol. Surv., vii + I36 pp. H.M.S.O. London. TURNER, J. S. x927. The Lower Carboniferous succession in the Westmorland Pennines and the relations of the Pennine and Dent faults. Pro¢. Geol. Ass. 38, 339-374. 1935. Structural geology of Stainmore, Westmorland, and notes on the late-Palaeozoic (late-Variscan) tectonics of the north of England. Pro¢. Geol. Ass. 46, x2 I-I51. I949. The deeper structure of central and northern England. Pro¢. Yorks. geol. So¢. 27, 28o- 297. WADG~, A. J. x97o. In discussion: Stratigraphy and structure in the Black Combe inlier, English Lake District, by D. G. Helm, Proc. Yorks. geol. Soe. 38, 137-i 38. & BURGESS, I. C. x968. The Caledonian history of the north-eastern Irish Sea region. Scott. d. Geol. 4, 377. WAGER, L. R. x93 I. Jointing in the Great Scar Limestone of Craven and its relation to the tee- tonics of the area. Q. Jl geol. Soc. Lond. 87, 392-424. WARD, J. C. i876. The geology of the northern part of the English Lake District. Mem. geol. Surv., x2 + I32 pp. H.M.S.O. London.

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Received 22 November I97o; revised manuscript received I3 July x97x ; read 24 November x97 x.

F. Moseley, Department of Geology, University of Birmingham, Edgbaston, Birmingham.

DISCUSSION In a written contribution Dr. G. H. MITCHELL said: Of the many problems of Lake District tectonics two in particular are outstanding. One is the relationship of the Borrowdale Volcanic Group to the Skiddaw Slates. The other concerns the nature of the junction between the Coniston Limestone Group and the Borrow- dale Volcanic Group. Over the past too years these two junctions have provided geologists with the problem of tectonic relationships. The upper junction is now generally held to be an unconformity. The lower junction has been described as conformable; at other places an unconformity has been invoked while elsewhere a normal fault, a thrust, or a lag fault have been postulated. Unfortunately exposures of the actual junction between the Borrowdale Volcanic Group and the Skiddaw Slates are for long distances poor and obscured by drift deposits, or disturbed by the passage of ice. There is often ample excuse for controversy regarding the structural relationships. Furthermore it is clear that the incidence of cleavage varies greatly from place to place and from bed to bed. Nevertheless during the last few years much new mapping has been done and it is good to attempt to correlate present knowledge. This Dr. Moseley has done, including in his account the Caledonian, end-Silurian, Variscan and Alpine movements and so providing information for discussion of the tectonics of the north-west of England as a whole. The road to the solution of these problems lies in detailed field surveys covering the whole district. Meanwhile Dr. Moseley's present summary is a very useful statement of geological opinion on Lake District tectonics. Several unpublished theses are referred to by the author. It is to be hoped that speedy publication of these works will be possible.

Mr. W. C. C. Rose congratulated Dr. Moseley on a stimulating and timely paper concerning a subject on which a general appraisal of our knowledge was much needed. His own particular interest dated from as long ago as 193o when he joined the Cumberland Unit of the Geological Survey. Arising out of official work on the Cockermouth Sheet he decided to continue six-inch mapping of the Skiddaw Slates southwards towards Keswick and Buttermere. War in I939 put an end to

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the project before the junction with the Borrowdale Volcanics was reached and only a short summary has been published. However, this includes a section illus- trating conclusions about the sequence and showing the general position of the main anticlinal and synclinal axes. Unfortunately it does not include the map giving the evidence on which the conclusions are based. The conclusions do not agree with those of Dr. A. Simpson, derived from a more recent structural analysis of the Skiddaw Slates of approximately the same area. The difference is important because Dr. Simpson relies on his interpretation of the structure and sequence in the Skiddaw Slates as providing critical evidence in support of his general conclusion that a major orogenic episode preceded the Borrowdale volcanicity. The interpretation of the sequence and structure which emerged from the speakers mapping was based mainly on stratigraphical evidence--chiefly the identification of lithological units which could be mapped, supplemented by a detailed examination of 'way up' evidence. In parts of the ground, particularly in the extreme south near the outcrop of the Borrowdale Volcanics, the story is com- plicated by severe faulting, shearing and compressional effects, and the evidence less secure. Even so, the general interpretation of the structure and sequence in this ground is reasonably reliable and it certainly stands on firmer ground than that put forward by Dr. Simpson. Further stratigraphical work is more likely to resolve present controversies than structural studies; in particular detailed palaeontological evidence from those areas where there is none at present. If some of the effort at present being put into detailed structural analysis could be directed towards stratigraphy and palaeon- tology the combined results would be more rewarding and less controversial. Dr. Moseley's reference to micropalaeontology, which offers the possibility of supplying diagnostic evidence of age where graptolites cannot be found, is particularly welcome. The outstanding question concerns the nature of the junction between the Skiddaw Slates and the Borrowdale Volcanics. Mr. Rose supported Dr. Moseley's general assessment and particularly his view that the principal tectonic movements affecting the Skiddaw Slates were not pre-Borrowdale Volcanics in age; that the main structures may include some pre-Caradoc elements but that they were largely end-Silurian. He asked if Dr. Moseley had any views on the possible local effect on the intensity of folding and cleavage, and on their orientations, of known intrusive masses which were either being emplaced or were already emplaced during compression. Could not some of the complex folding seen in the Skiddaw Slates be due to intrusions, known or concealed, acting as rigid blocks causing differential com- pressive effects on relatively incompetent strata ? There are many minor intrusions in the Skiddaw Slates and some of these are quite large. There is also some evidence in the form of apparently thermally metamorphosed slates suggesting large concealed intrusions. With the exception of the Skiddaw Granite we do not yet know the age of the Skiddaw Slate intrusions but if some are Ordovician their presence must have affected the pattern of some of the structures produced by end-Silurian movements and may even have done so in the case of intrusions of end-Silurian age.

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Dr. A. SIMPSON said that his 1967 conclusions were apparently rejected by Dr. Moseley. Namely that the Skiddaw Group had been subjected to widespread intra-Lower Ordovician evorogenic deformation before the essentially subaerial Borrowdale Volcanic Group, with associated basal sediments, was laid down unconformably on top. Little reference appeared to be made to the tectonic and stratigraphical reasoning upon which this conclusion was based. In contrast, Dr. Moseley evidently accepted the presence of an angular unconformity between the two Groups in at least that portion of the junction around Newlands Beck. He found it difficult to accept Dr. Moseley's proposed structural sequence for the Skiddaw Group in the Lake District. In particular, the relegation of the bulk of Skiddaw deformation to pre-Caradoc and end-Silurian phases did not seem realis- tic. Style and geometry were not necessarily criteria for structural correlation in strata of different ages. The speaker (Simpson, Scott. J. geol. 1968 ) had inferred that the Lake District, during late Tremadocian to Lower Llanvirnian time, formed part of a rapidly subsiding Manx-Skiddaw trough in which the Skiddaw Group, underlain by the north-easterly extension of the older Manx Group from the Isle of Man, had a combined thickness in the order of 55 ooo feet. Since the depth of burial of any pre-Cambrian basement under the Lake District was certainly considerable, it seemed unlikely that such a basement could exert any control over the development and alignment of the structure in the Lake District Lower Palaeozoic cover. Such a control seemed to be an important theme in Dr. Moseley's paper.

Dr. HELM expressed his delight at Dr. Moseley's affirmation of his belief in an angular unconformity at the base of the Borrowdale Volcanic Group, but was surprised that Dr. Moseley was unable to accept that the unconformity resulted from any of the principal movements that affected the Sldddaw Group. He was not surprised, however, that Dr. Moseley was unable to place, with any certainty, the movement phases seen in Black Combe within his own structural sequence. For example D I (Helm) makes its appearance twice, once below and once above the unconformity, and later movements are likewise uncertainly placed. Perhaps this is because Dr. Moseley has not appreciated the severity of deformation in Black Combe. For instance he describes the first folds as 'tight to open,' they are in fact tight to isoclinal. He says they are upright but since the plunge is o-9 ° some must be vertical. The second folds he describes as 'open recumbent folds' but they too are often isoclinal. The third folds, by which Dr. Moseley presumably means those of D 4 age, are not ENE but NE trending and can in no sense be described as being 'generally the more complex'. D4 folds in the Skiddaw Group are open and congruous with the 'Borrowdales' anticline. Dr. Moseley suggests that in the lower Palaeozoic the earliest folds were N-S and that, because they possess no axial-plane cleavage, they were formed out of wet sedimenl. How then does he reconcile this argument with the fact that the D I folds in Black Combe, which are demonstrably the earliest, do possess a strong metamorphic fabric, occasionally axial-planar but more frequently sub-parallel with the bedding. The writer would be interested to know how Dr. Moseley is apparently able to

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differentiate N-s folds of pre-Borrowdale Volcanic Group age from N--s folds of post-Borrowdale Volcanic Group age in the Skiddaw Group. Black Combe continues to be an embarrassment to the opponents of a major tectonic break. Clearly, any explanation of the lower Palaeozoic geological history of the Lake District must take the evidence from Black Combe fully into account. In Dr. Helm's opinion this has not been satisfactorily done in Dr. Moseley's paper.

Dr. B. ROBERTS welcomed the fact that Dr. Moseley now recognises the angular unconformity at the base of the Borrowdale Volcanic Group. It was important to emphasise that the unconformity was recognised in 197 ° by Helm at Black Combe after a detailed structural analysis and in I967 by Simpson in the western part of the main outcrop of the Skiddaw Group on structural and stratigraphical grounds. However in rejecting the claim for severe polyphase deformation of the Skiddaw Group prior to the accumulation of the Borrowdale Volcanic Group Dr. Moseley should make it clear that he is accepting the unpublished work of Rose and others, together with the isolated pieces of work of Soper (i97o) but rejecting the essence of the published work of Helm and Simpson. It seemed odd that Dr. Moseley should at last recognise the angular unconformity but at the same time deny the validity of the original evidence. Was Dr. Moseley's opinion that Helm and Simpson were right about the existence of the unconformity, but for the wrong reasons ?

In replying Dr. MOSELEY regretted that the discussion had centred around one small part of the structure of north-west England, although it was perhaps understandable, with the spate of recent publications on the Skiddaw Slate- Borrowdale Volcanic junction, that it was this subject which excited most interest. Dr. Mitchell had referred to other matters, however, and is certainly correct in suggesting that detailed field surveys are necessary to fill obvious gaps on the map. Mr. Rose's remarks were welcome, and his suggestion that all aspects of geology should be used to attack the problem must be followed if ultimate agreement was to be reached. With regard to the effects of intrusions on structures he was unable to offer objective comment although surveys now in progress may soon answer some of the questions. Drs. Simpson, Helm and Roberts had all emphasized the thorny problem of the Skiddaw Slate-Borrowdale Volcanic junction. Interpretations were still polarised into those who thought the unconformity was a small one and those who believed it to reDresent a break of orogenic dimensions. It is significant that during the sum- mer of 197o protagonists from both camps met on some of the critical exposures, and even then, with well exposed evidence in clear view, the interpretations were just as divergent. One of the exposures visited was Hollows Farm, Borrowdale, (Soper i97o ) for which Helm and Roberts (I97 I) now suggest a major unconformity, an interpretation which has been rejected entirely by Soper and by Jeans (I97I) who have both completed detailed surveys of the outcrop independently of each other. Dr. Simpson had referred to the stratigraphy he had established as well as to the structure, but his is not the only work on this ground and Mr. Rose, who

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spent many years mapping a large area has a diametrically opposite interpretation, both in terms of sequence and thickness. Dr. Moseley noted that when he first wrote this paper he had no real knowledge of Rose's unpublished field maps and had been prepared to accept Simpson's maps as the more reliable. Since then, however, Rose has made his maps available to several research workers mapping the same areas, and without exception they have commented on the accuracy of the detailed observations. It is therefore a question of whose stratigraphy to accept. The other question of an unconformity of some kind has not been seriously doubted for some years and was referred to by the author in the discussion of Helm 197o (p. 135 ). It is the nature of the unconformity which is disputed. The unconformity in Newlands Beck is certainly not the structure which was referred to by Simpson in the same region, and this was made quite clear by Jeans (in press and in the discussion of Helm 197o ). It is a truncation by the volcanics of northerly folds with no associated cleavage, whilst the ~.I~ S I cleavage of Simpson (also supposedly truncated by the volcanics) passes through slates and volcanics alike. This is discussed in the paper. Dr. Moseley acknowledged that he had made reference to unpublished work. In assessing any problem it was reasonable to use all the evidence available. He did not reject the work of Simpson and Helm, indeed he had complimented both those authors on the value of their work, both in this paper and in past discussions. However, he believed their interpretations should be questioned. Also, he doubted whether Dr. Soper would be delighted by his work being called 'isolated,' but if this is the case, then he has other isolated pieces of work lying in wait. Dr. Helm had particularly referred to Black Combe and in this connection Dr. Moseley affirmed that he was not satisfied about the severity which had been read into the early movements there. Dr. Helm himself (x 970) had observed that D I folds were difficult to discern and axial plane cleavage was only sporadically developed in the hinges. This degree of deformation, bearing in mind the incompetence of the Skiddaw Slates, could well be accommodated in the author's first two phases. The evidence for the second of these depended on N-s folding in the volcanic rocks, and in volcanic-slate interbeds. This is explained in the text. 'Bedding cleavage' was a doubtful commodity which required further research, and in thin section it closely resembled 'bedding' in formations such as the Carbon- iferous Bowland Shales. Concluding Dr. Moseley reiterated that if this particularly intractable problem was to be solved it was essential to have a combined assault through structural geology, stratigraphy, sedimentology, geochemistry and micropalaeontology, in a more balanced rather than a more specialized approach. Such studies are now under way.

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