594 F. Mosd~ WESTOLL,T. S. 1967 . In discussion: Carboniferous joints in the north of England and their relation to earlier and later structures, by F. Moseley and S. M. Ahmed, Proe. Yorks. geol. So¢. 36, 82-84 . WH~TTON, J. T., MYERS, J. O. & WATSON, I.J. i956. A gravimeter survey in the Craven district of north-west Yorkshire. Pro¢. Y~rks. geol. So¢. 30, 259-287. 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 un- conformity 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 Several unpublished theses are referred to by the author.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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/6/594/4884602/gsjgs.128.6.0594.pdf by guest on 30 September 2021 A tectonic history of northwest England 595 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 con- clusion 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 partic- ularly 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 inten- sity 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 con- cealed 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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/128/6/594/4884602/gsjgs.128.6.0594.pdf by guest on 30 September 2021 596 F. Moseley 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 develop- ment 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 deforma- tion 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.