The Caledonian History of the North-Eastern Irish Sea Region

Home , Binsey, Black Combe, Cross Fell, Skiddaw, Skiddaw Group, Watch Hill (Cockermouth)

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LETTERS 375

JOHNSON, M. R. W. and DALZIEL, I. W. D. 1966. Metamorphosed lamprophyres and the late thermal history of the Moines. Geol. Mag. 103, 240-249. MILLER, J. A. and BROWN, P. E. 1965. Potassium-argon age studies in Scotland. Geol. Mag. 102,106-134. NICHOLLS, G. D. 1951. The Glenelg-Rattagan igneous complex. Q. Jl geol. Soc. Lond. 106, 309-344. MS received 18th July 1968

THE CALEDONIAN HISTORY OF THE NORTH-EASTERN IRISH SEA REGION

Sms,—Simpson (1967; 1968) has described three phases of folding, Fx, F2 and F3, from the Skiddaw Group in the core of the Lake District anticline. In some ways these episodes of deformation are comparable to those which produced the ¥lt F2 and F3 structures in the Skiddaw Group in the Black Combe inlier (Helm, in Helm, Roberts and Simpson 1963). Fx and F2 in both areas belong to an intra- Lower Ordovician orogenic episode which pre-dates the deposition of the Borrowdale Volcanic Group. Fi (Black Combe) folds, however, have a northerly trend while Fx (Lake District) folds are Caledonoid; F2 (Black Combe) folds are more variable in attitude and tightness than F2 (Lake District) folds. In general, in both areas, F3 minor folds may plunge towards the north-westerly quarter and the associated axial-plane cleavage, S3, usually has a north-westerly trend (Simpson 1967, fig. 4f and g, fig. 6, and fig. 7; Helm 1968, p. 67). In the Black Combe area (Helm 1968) the major, primary end-Silurian phase of folding (F4) produced the north-easterly trending Borrowdale anticline with an associated axial-plane cleavage in the Borrowdale Volcanic Group and brought the Skiddaw Group of the inlier to its present structural level. The response to the F4 movements in the slaty inlier was the development of a suite of minor F4 folds congruous with the Borrowdale anticline and an associated axial-plane cleavage, S4. The latter cuts and displaces S3, and is itself modified by two later end-Silurian cleavages, S6 and S6. The Skiddaw Group of the inlier displays the effects of six phases of deformation while the Borrowdale Volcanic Group displays only three. Hence the tectonic history of the Black Combe area apparently differed from that of the main tract of the Lake District as described by Simpson (1968). In the structural chronology suggested for the Black Combe area (Helm, op. cit.) the F3 structures pre-date the Borrowdale Volcanic Group whereas in the Skiddaw Group of the main tract the F8 structures are equated by Simpson (1968, table 2) with the main-phase Fx (end-Silurian) folding which formed the Lake District anticline. Thus the N.-S. compressive force (Simpson 1967, p. 415) which was responsible for the flexing of the volcanic sequence into the east- northeasterly trending Lake District anticline and which produced the Caledonoid fold-pair around Crummock Water in the Skiddaw Group is also stated to have Downloaded from http://sjg.lyellcollection.org/ by guest on October 2, 2021

376 LETTERS produced a suite of north-westerly trending minor structures and the Lorton Vale fold-pair, also in the Skiddaw Group (op. cit., p. 415 and Plate 19). This implies that the N.—S. compression simultaneously produced both north-westerly and east-northeasterly trending major and minor structures. The writers find this interpretation difficult to accept, especially since in Fig. 4 (Simpson 1967, p. 401) the plot of plunges of F3 structures shows two maxima, one in the N.W. and the other in the S.E. quadrant, and the poles form a girdle about an axis trending N.E. It is suggested that the N.W.-S.E. trending structures may pre-date and have been re-folded by the movements which produced the Lake District anticline. If this is so, the Skiddaw Group in the Lake District has been subjected to three intra-Lower Ordovician movements, as has the Skiddaw Group in Black Combe. It may well be, therefore, that the F3 structures in the Manx Group in the Isle of Man are also intra-Lower Ordovician. The demonstration of the possibility of such an alternative explanation serves to illustrate the need to exercise extreme caution in correlating fold-phases over long distances. In particular Simpson's correlation of the F3 structures in the Manx Group of the Isle of Man (of end-Silurian age, according to Simpson 1968, pp. 144-145,160) with the end-Silurian F3, structures in N. Wales (Roberts 1967, pp. 380, 388) must be regarded with circumspection until more evidence is produced.

REFERENCES HELM, D. G. 1968. Univ. London Ph.D. thesis (unpubl.). , ROBERTS, B. and SIMPSON, A. 1963. Polyphase folding in the Caledonides south of the Scottish Highlands. Nature, Lond. 200,1060-1062. ROBERTS, B. 1967. Succession and structure in the Llwyd Mawr syncline, Caernarvonshire, North Wales. Geo/. J. 5, 369-390. SIMPSON, A. 1967. The stratigraphy and tectonics of the Skiddaw Slates and the relationship of the Borrowdale Volcanic Series in part of the Lake District. Geol.J. 5, 391-418. 1968. The Caledonian history of the north-eastern Irish Sea region and its relation to surround ing areas. Scott. J. Geol. 4,135-163.

D. G. HELM Department of Geology, Goldsmiths' College, University of London.

B.' ROBERTS Department of Geology, Birkbeck College, University of London. MS received 5th June 1968 Downloaded from http://sjg.lyellcollection.org/ by guest on October 2, 2021

LETTERS 377 SIRS,—In a recent paper, Dr A. Simpson (1968) postulated a major orogeny within the Lower Ordovician, on the basis of his work on the Skiddaw Slates of the western Lake District. He made no reference to the Cross Fell inlier, however, where we have been working recently. The particular interest of this area lies in the fact that the change in tectonic style upon which Dr Simpson sets such value here occurs within beds hitherto assigned to the Skiddaw Slates. In the northern part of the inlier (north of Flagdaw), and in a limited area below Roman Fell, a thick sequence of mudstones, spilitic lavas and tuffs, including both the Milburn and Ellergill Beds of Shotton (1935), is almost uncleaved and disposed in broad open folds of Caledonoid trend. Graptolites of the D. bifidus Zone and associated microfossils (Wadge, Owens and Downie 1967) occur in this sequence. Throughout much of the southern part of the inlier however the Skiddaw Slates are, in contrast, affected by polyphase folding and associated cleavages (A. Rep. Inst. Geol. Sci. 1966—for 1965, pp. 55-56). These beds have not yet yielded fossils of zonal significance and no unfaulted contacts have been found between beds showing the two tectonic styles. If however the major change in style is to be interpreted as implying an inter vening major orogenic event, then it must have occurred at about the D. hirundoj D. bifidus zonal boundary, for D. extensus Zone and D. hirundo Zone graptolites have been recorded from polyphase-folded beds in the Lake District (Simpson 1967). The orogeny is thus late-Arenig rather than late-Llanvirn. It is clear that the structural condition of the mudstones of D. bijidus age, recorded from several localities in the Skiddaw Slates of the eastern Lake District is critical, for if the structural break proves to transgress the approximate time-planes as marked by the graptolite zones, it may well be necessary to reassess the conventional inter pretation of polyphase folding, and possibly to conclude that the structural contrast is due solely to the relative incompetence of the Skiddaw Slates under the stresses of an end-Silurian orogeny.

REFERENCES SHOTTON, F. W. 1935. The stratigraphy and tectonics of the Cross Fell Inlier. Q. Jl geol Soc. Lond. 91, 639-704. 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. 1968. The Caledonian history of the north-eastern Irish Sea region and its relation to surround ing areas. Scott. J. Geol. 4,135-163. WADGE, A. J., OWENS, B. and DOWNIE, C. 1967. Microfossils from the Skiddaw Group. Geol. Mag. 104, 506-507. A. J. WADGE I. C. BURGESS Institute of Geological Sciences, Leeds 15. MS received 23rd May 1968 Downloaded from http://sjg.lyellcollection.org/ by guest on October 2, 2021

378 LETTERS SIRS,—Dr Simpson's (1968) hypothesis of an important intra-Lower Ordovician orogenic episode in the north-eastern Irish Sea region rests entirely on his inter pretation of the relationship between the Borrowdale Volcanic and Skiddaw Slate Groups in the Lake District. This interpretation demands that the first and second phases of deformation in the Slates were followed by a period of profound erosion prior to the deposition of the Volcanic Group. It is clearly desirable to test this hypothesis by examining exposed contacts between the two groups and although the junction in many areas is tectonic, sufficient evidence from unmoved contacts is available to raise serious objections to the hypothesis. One of the writers (N.J.S.) has recently reinvestigated the small outcrop of Borrowdale Volcanics within the Skiddaw Slates at Greenscoe near Dalton in Furness. A variety of volcanic types are present, including lavas, imbedded agglomerates and tuffs, tuffisitic breccias and veins of intrusive tuff, which may be interpreted as the filling of a volcanic vent which is partially obscured by faulting and the Coniston Limestone unconformity. A single steeply inclined strain-slip cleavage is present, striking between 000° and 060°. This is very closely spaced in the slates, more open in the tuffaceous rocks, crudely developed in the limestone and absent in the lavas. It is the sole cleavage present in the slates and is almost certainly the first cleavage (Sx) of Dr Simpson. The fragmental rocks of the vent contain much material derived from the Skiddaw Slates. Evidence from numerous thin sections shows that the cleavage passes through slate fragments and volcanic matrix alike; bedding is visible in some fragments but none shows evidence of an earlier cleavage. A contact between the vent and country rock slates is well exposed; a marginal intrusion breccia consists of disoriented mudstone and silty fragments with a single cleavage traversing both fragments and matrix. The sum total of evidence from this locality suggests that the Sx cleavage in the Skiddaw Slates is post-Coniston Limestone and hence end-Silurian. The evidence will be presented in detail elsewhere. Currently, the writers are investigating the junction between the Skiddaw Slate and Borrowdale Volcanic Groups between Buttermere and Borrowdale, which is exposed at a number of localities. There is evidence to indicate that the steep Si cleavage in the slates, which is axial planar to isoclinal folds, passes through into the volcanics wherever a thin tuff is present below the massive Lower Andesites which are affected by very open folding. This is perhaps a surprising circumstance, but less difficult to accept than Dr Simpson's correlation of the powerful slaty cleavage in the tuffaceous horizons of the Borrowdale Volcanics with the weakly developed S3 cleavage in the Skiddaw Slates (1967). Further evidence is accumulating which tends to confirm the opinion of Rose (1954) that there is a local passage between the two groups, in an area where Dr Simpson's reading of the Skiddaw Slate stratigraphy demands a profound unconformity at the base of the Borrowdale Volcanics. At a number of localities close to the Skiddaw Slate—Borrowdale Volcanic Downloaded from http://sjg.lyellcollection.org/ by guest on October 2, 2021

LETTERS 379 junction in the north-eastern part of the Lake District interbedded argillaceous slate and endesitic tuff, or slate of intermediate composition (' mottled tuff') are exposed, suggesting a passage between the two groups. These may be seen near Binsey (Trotter in King 1948), around Ullswater (Moseley 1964) and between Swindale and Bampton (Mitchell 1956, p. 413). Currendy, officers of the Institute of Geological Sciences are reinvestigating an extensive development of similar rocks in the Cross Fell inlier (Mr A. J. Wadge, personal communication). In the west of the region the Latterbarrow Sandstone is interposed between the two groups. Simpson (1967) interprets this as the local basal member of the Borrowdale Volcanic succession and in view of its lithological disparity with the Skiddaw Slates, suggests an unconformity at its base. We accept this possibility, although the hiatus may be of less significance than the pre-Coniston Limestone unconformity. Dr Simpson's argument that the main cleavage in the Skiddaw Slate predates the Latterbarrow Sandstone is unconvincing: massive horizons in the Watch Hill Grits are uncleaved. In the Gosforth Memoir (Trotter et at. 1937) a case is made for a thrust at the base of the Latterbarrow Sandstone, which Dr Simpson does not accept. On the basis of our own observations we tentatively interpret the sandstone-slate junction as a tectonic dislocation inclined less steeply than the bedding in the overlying sandstone. In such poorly exposed ground no firm conclusions can be reached. Thus, while recognising Dr Simpson's contribution towards the understanding of the polyphase deformation of the Skiddaw Slates, we regard his main thesis, that of an intra-Lower Ordovician orogeny, as unproven.

REFERENCES KING, W. B. R. 1948. Guide to excursion A2. North-west of England. Int. geol. Congr. XVIII (Gt. Britain). MITCHELL, G. H. 1965. The geological history of the Lake District. Proc. Yorks. geol. Soc. 30, 407-463. MOSELEY, F. 1964. The succession and structure of the Borrowdale Volcanic rocks northwest of Ullswater. Geol. J. 4,127-142. ROSE.W. C. C. 1954. The sequence and structure of the Skiddaw Slates in the Keswick-Buttermere area. Proc. Geol. Ass. 65, 403-406. 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. 1968. The Caledonian history of the north-eastern Irish Sea region and its relation to surround ing areas. Scott. J. Geol. 4,135-163. TROTTER, F. M. et al. 1937. The geology of the Gosforth District. Mem. Geol. Surv. U.K. N. J. SOPER Department of Geology, P. E. BROWN University of Sheffield, St. George's Square, Sheffield, 1. MS received 17th May 1968 Downloaded from http://sjg.lyellcollection.org/ by guest on October 2, 2021

380 LETTERS

Dr A. SIMPSON (University of London) replies to Helm and Roberts: It is unfor tunate that Dr Helm's recently completed examination of the Black Combe area has not yet been published. The detailed results of this work which are contained in his thesis, and upon which the above comments are based, are not therefore easily available for assessment by the general reader. I am, however, fully aware of Dr Helm's investigation since I first suggested the area to him. I followed the progress of Dr Helm's work closely and am fully conversant not only with his resultant thesis but also with the field evidence upon which his conclusions are based. The basic Caledonian tectonic history of the Black Combe area is comparable with that of the core of the Lake District anticline some twenty miles to the north. Indeed the roughly triangular and north-easterly closing area of the Skiddaw Group in the core of the Borrowdales anticline-at Black Combe furnishes a relatively small replica of the much larger belt of the Skiddaw Group in the core of the parent easterly-plunging Lake District anticline to the north. The Skiddaw Group at Black Combe, as Dr Helm's work has shown, was affected by two strong phases of deformation (^ and F2), each accompanied by severe folding and cleavage-formation, prior to the accumulation of the Borrowdale Volcanic Group on these truncated intra-Lower Ordovician structures. In various parts of the inlier the base of the Borrowdale Volcanic Group is adjacent to each of the formations into which Dr Helm has subdivided the Skiddaw Group of this area. In one particularly convincing series of outcrops on the north-western fringe of the inlier, strongly deformed and cleaved Skiddaw Group, steeply dipping and striking north-south, can be observed in knife-edge contact with undeformed basal Borrowdale rocks dipping northward at c. 30°. The intervening surface of erosion is somewhat irregular and displays no sign of detachment. As Dr Helm and Roberts have outlined, the simple anticline which flexes the Borrowdale Volcanic Group was produced by the end-Silurian folding and has brought the complex Skiddaw basement to its present structural level in the core of the fold. The sporadic Caledonoid axial-plane cleavage in the volcanic rock continues to lower structural levels as a Caledonoid crenulation or strain- slip cleavage superimposed upon the pre-Borrowdale Sx and S2 of the more intense intra-Lower Ordovician phases of deformation. These then are the three strongly developed episodes of deformation in the Black Combe area. The two severe phases pre-dating the Borrowdale Volcanic Group can be correlated with the Fx and F2 phases of the main Skiddaw belt while the formation of the rather open anticline in the superstructure of the Borrowdale Volcanic Group is clearly synchronous with the parent Lake District anticline. In their letter Drs Helm and Roberts state that ' the Skiddaw Group of the inlier displays the effects of six phases of deformation while the Borrowdale Volcanic Group displays only three. Hence the tectonic history of the Black Combe area apparendy differed from that of the main tract of the Lake District Downloaded from http://sjg.lyellcollection.org/ by guest on October 2, 2021

LETTERS 381 as described by Simpson (1968)'. This statement, perhaps because of its necessary brevity, is somewhat misleading. The only phases of deformation which are persistently and strongly developed in the Skiddaw Group of Black Combe are what Helm has termed Fx, F2 and F4 (the latter represents the downward continua tion of the cleavage associated with the Borrowdales anticline). As described above these are recognisable in the region to the north. The structures referred to by Helm and Roberts as a closing intra-Lower Ordovician F3 phase of open N.W.—S.E. folds and a steep to vertical axial-plane cleavage, are rare and weakly developed. In his thesis Helm completes his description (p. 67) of these structures in precisely eight lines and on p. 96 states that * these are too poorly represented to be considered further in the present context'. The same is true of Helm's F5 and F6 ' phases' in the Skiddaw Group. Neither are associated with any folds but are reflected merely by rare and very weak strain-slips. Their counterparts in the Borrowdale Volcanic Group, while superimposed on the dominant Caledonoid end-Silurian structures, are similarly extremely weak and sporadic. Dr Helm's F3, F6 and F6 structures are so weak and sporadic that I hesitate at present to attach any regional significance to them. Until further evidence is forthcoming it seems doubtful whether they should even be considered as distinct deformational episodes. In particular there are no real grounds for endeavouring to extend Dr Helm's extremely weak ' F3' further north, and to suggest the even more distant correlation with the very strong F3 structures in the Isle of Man is extending the argument to an even more unfounded extreme. The query raised by Drs Helm and Roberts in the last part of their third para graph is discussed in my 1967 paper (pp. 406-8 and 415). I find it difficult to under stand the reasoning behind their fourth paragraph. The fact that the poles of a set of linear structures form a girdle does not necessarily mean that such structures have been refolded. They must surely appreciate that the linear structures associated with the intersection of an earlier fold or folds by a later crosscutting cleavage will He in a girdle or partial girdle. In Simpson (1968) I have suggested a correlation between the F3 structures in the Isle of Man and the end-Silurian F8 structures in North Wales. This possible correlation was, as stated, based on grounds of similarity in style and orientation. Drs Helm and Roberts seem hesitant to accept this correlation but do not produce any evidence or argument to invalidate my suggestion.

Dr A. SIMPSON replies to Wadge and Burgess: My conclusion that the Borrowdale Volcanic Group rests unconformably on the previously deformed Skiddaw Group was presented in my 1967 paper dealing with the north-western part of the Lake District. The considerable amount of evidence and discussion upon which this Downloaded from http://sjg.lyellcollection.org/ by guest on October 2, 2021

382 LETTERS conclusion was based need not be reiterated here. I must point out, however, that the existence of this sub-Borrowdale unconformity is not solely supported by a ' change in tectonic style' upon which, according to Mr Wadge and Mr Burgess, I am supposed to ' set such value '. The most obvious large-scale indi cation of this unconformity is of a purely stratigraphical character. Along the southern fringe of the Skiddaw belt the basal Borrowdale volcanic rocks overstep two Skiddaw formations (Kirkstile Slates and Blakefell Mudstones) which occupy a low stratigraphical position in the thick Skiddaw sequence. The junction is essentially unmoved. The supplementary evidence of strong deformation of the Skiddaw Group prior to the accumulation of the Borrowdale Volcanic Group further supports the occurrence of severe intra-Lower Ordovician earth-movement in this part of Great Britain. It seems to me that Mr Wadge and Mr Burgess attach undue significance to the fact that the Skiddaw Group in most of the southern part of the Cross Fell inlier shows strong polyphase folding and cleavage while in the north only open, and presumably later folds of Caledonoid trend are present. In any region display ing polyphase folding it is unusual for the various generations of folds, and their accompanying cleavages, to be equally developed in every outcrop. Localised tracts are not uncommon (e.g. on large-scale early fold limbs) where earlier, more severe small-scale structures become inconspicuous, leaving only more open, later folds to flex the strata. I cannot accept the suggestion that the intra-Lower Ordovician orogenic event took place between the D. hirundo (late Arenigian) and D. bifidus (early Llanvirnian) zones i.e. within the time of deposition of the Skiddaw Group. In the main Skiddaw belt this stratigraphical break occurs at the base of the Borrow dale volcanic sequence. In the Cross Fell inlier, however, this junction of the Borrowdale Volcanic Group and the underlying Skiddaw Group is not well exposed. Past literature on the subject (e.g. Shotton 1935) suggests that this important contact is mostly cut out by faulting and only survives at the surface in two isolated and localised localities. This circumstance would appear to pre clude any definite conclusion concerning the relationship between the two groups. In the last few lines of their communication Mr Wadge and Mr Burgess show signs of returning to the older view that the structural contrast between the Skiddaw Group and the overlying Borrowdale Volcanic Group is due solely to disharmonic folding during the end-Silurian orogeny. Although such a mechanism has been favoured in the past, no real evidence of any kind has ever been put forward to support it. The ' disharmonic theory * simply provided a convenient and not too rigorous way of accounting for the obvious contrast in complexity between the Skiddaw Group and the Borrowdale Volcanic Group. My views on the Caledonian history of the north-western part of the Lake District and the relationship between the Borrowdale Volcanic Group and the Skiddaw Group, are based on an area of 126 square miles. In this region the Downloaded from http://sjg.lyellcollection.org/ by guest on October 2, 2021

LETTERS 383 Skiddaw Group is for the most part well exposed and in the Buttermere region forms mountains approaching 3000 feet high. In the Cross Fell inlier the Skiddaw Group occupies a very restricted and not too well exposed area. I have no doubt that the field observations by Mr Wadge and Mr Burgess on the Skiddaw Group in the Cross Fell inlier will be of consider able interest and value. Perhaps, however, they are endeavouring to extract large-scale conclusions from an area which is simply not adequate to yield them.

Dr A. SIMPSON replies to Soper and Brown: I was interested to read that Dr Soper has been examining the volcanic rocks and associated slates forming the small inlier at Greenscoe. I accept his field observations concerning the constitution of the volcanic rocks and the presence of a single steep cleavage, striking towards the north-eastern quarter, which cuts both the volcanic rocks and the associated slates. I might add that in parts of the same area the slates to which he refers have the character of blue mudstone, sometimes rather ashy and dip in conformity with the overlying pyroclastic rocks. Lenticles of the same mudstone are occasionally intercalated within the volcanic sequence. Dr Soper has apparently assumed that the pelitic material associated with the Borrowdale volcanic rocks belongs to the Skiddaw Group. The same conclusion was reached by J. F. N. Green some fifty years ago and Dr Soper has evidently not questioned its validity. Comparable pelitic material occurs in association with the basal tuffs and lavas of the Borrowdale volcanic sequence in other parts of the Lake District. For example near Binsey, one of the localities mentioned by Drs Soper and Brown, intercalations of brownish shale occur between andesite flows and can be seen in a quarry at Whitefield Cottage where the sequence dips 32° at 351° on the northern limb of the Lake District anticline. Similar intercalations of tufFaceous mudstone occur in association with the basal Borrowdale volcanic rocks on the south-eastern flank of the Black Combe inlier of the Skiddaw Group. During the last fifty years such pelitic intercalations, at and near the base of the Borrowdale volcanic sequence have been repeatedly identified, without any apparent justification, as Skiddaw Slates. This assumed marginal intercalation of the two groups is the basis of the view expressed in the past by several workers that there is a conformable passage from the Skiddaw Group into the Borrowdale Volcanic Group. It seems to me that the rocks in question are simply pelitic intercalations in the basal part of the Borrowdale Volcanic Group. In the south-western part of the main Skiddaw belt these basal sediments assume an arenaceous lithology in the form of the localised Latterbarrow Sandstone which towards its top only is intercalated with the basal Borrowdale tuffs. Conversely, of course, the occurrence of thin lava and ash bands in the highest Lower Llanvirn beds of the Skiddaw Group, as for example at Cross Fell, does not Downloaded from http://sjg.lyellcollection.org/ by guest on October 2, 2021

384 LETTERS necessarily indicate a conformable passage by intercalation to the younger Borrowdale Volcanic Group. Indeed, in view of the marked Lower Ordovician volcanicity elsewhere in the country, a surprising feature of the thick Skiddaw Group is that comparable volcanic bands do not occur more widely through the preponderant Arenig portion of the sequence. The single, broadly Caledonoid cleavage observed by Dr Soper at Greenscoe is well developed only five mile to the north-west in the basal Borrowdale volcanic rock (with intercalations of tuffaceous mudstone) forming the south eastern limb of the end-Silurian anticline at Black Combe. This same cleavage can be followed into the complex Skiddaw Group occupying the core of the anti cline where it is superimposed upon and cuts through earlier, severe generations of folds and cleavages. The latter, including Sx, are confined to the Skiddaw core and pre-date the accumulation of the Borrowdale Volcanic Group. Quite apart from my comparable findings further north in the Lake District, this additional evidence clearly nullifies the contention by Drs Soper and Brown that the end- Silurian cleavage which affects the Borrowdale Volcanic Group is equivalent to the first cleavage (Sx) of the Skiddaw Group. Drs Soper and Brown contend that between Buttermere and Borrowdale 4 there is evidence to indicate that the steep Sj cleavage in the slates, which is axial planar to isoclinal folds, passes through into the volcanics wherever a thin tuff is present below the massive Lower Andesites which are affected by very open folding'. One question which I feel the above writers should consider is that if this is one and the same cleavage, impressed on a conformable sequence, why does the thin basal tuff, presumably a layer of no great competence, not display isoclinal folding comparable to the striped slates below? In the paper by Rose (1954) to which Drs Soper and Brown refer, there is no evidence of any kind to support a conformable passage from the Skiddaw Group to the Borrowdale Volcanic Group. With regard to the Latterbarrow Sandstone, Drs Soper and Brown have presented my views in a somewhat partial way. My statement that this formation should be regarded as the local basal member of the Borrowdale Volcanic Group, primarily separated from the underlying Skiddaw Group by an unconformity, is not based solely on its' lithological disparity with the Skiddaw Slates' but more pointedly on the fact that it oversteps the Kirkstile Slates and Blakefell Mudstones, formations low in the thick Skiddaw sequence (Simpson 1967, p. 413). Further more I do not reject the possibility of subsequent, subsidiary end-Silurian thrusting in this localised area (Simpson 1967, p. 415). My findings concerning the unconformable relationship between the Borrow dale Volcanic Group and the underlying Skiddaw Group, resulting from intra- Lower Ordovician orogenesis, are based on the examination of the western two- thirds of the main Skiddaw belt, a substantial tract of country. A considerable amount of stratigraphical and structural evidence was presented and discussed Downloaded from http://sjg.lyellcollection.org/ by guest on October 2, 2021

LETTERS AND PUBLICATIONS 385 in my 1967 paper. Without reference to any supporting evidence and without producing any valid evidence of their own, Drs Soper and Brown conclude in a somewhat peremptory fashion that' we regard his main thesis, that of an intra- Lower Ordovician orogeny, as unproven '. I feel that I can leave judgement in this matter to the interested reader.

MS received 13*/» August 1968

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