THE GEOLOGICAL STRUCTURE of the COUNTRY ROUND COMBE MARTIN, NORTH DEVON.• by J
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201 THE GEOLOGICAL STRUCTURE OF THE COUNTRY ROUND COMBE MARTIN, NORTH DEVON.• By J. W. EVANS, D.Se., F.R.S. Read March 3rd, 19". y primary object in conducting an excursion of the M Geologists' Association to this neighbourhood is to make known the result of my investigation ofits geological structures, as I believe that they are of interest and importance not only for their own sake but because foldings and dislocations of a similar character are extensively developed in the south-west of England. Much still remains to be done with regard to the correlation of the strata represented both in the special area in question and elsewhere in the south-west, but I thought that it might be useful if at the same time I stated what progress I have up to the present been able to make in this direction and what problems still remain to be solved. The foundations of our knowledge of the area were laid by Sir Henry de la Beche in his official report on the " Geology of Cornwall, Devon and West Somerset," the first publication of the Geological Survey, which was issued as early as 1839, and by the classical work of Sedgwick and Murchison, published in the succeeding year. These can only be regarded as pre liminary surveys, but subsequent observations have served to confirm the substantial accuracy of their general conclusions. In 1866, however, J. B. Jukes attempted to show that the existence of fundamental strike faults had been overlooked, and that in consequence the accepted succession included a triple recurrence of the same strata. In the following year R. Etheridge, who made a rapid traverse of the area and had the assistance of the fine collections and local knowledge of R. H. Valpy of Ilfracombe, conclusively showed on palreontological grounds and by comparison with the Continental Devonian that jukes's contentions were erroneous and re-established the sue cession originally laid down. Unfortunately, Valpy's collec tions have been lost and there is now no means of confirming Etheridge's and his determinations. It is at the same time apparent that in some instances Etheridge has, in consequence of insufficient familiarity with the country, misunderstood Valpy's statements as to the localities from which fossils were reported.] • It is useful to remember that at the present time (1g••) the local magnetic deviationis about 16- W. All directions are, however, given in this paper by reference to the, true astronomical meridian. t A brief account by Valpy of the geology of Ilfracombe was issued anonymously. The 4th edition which I possess contains numerous obvious misprints, most of which were not in the original edition. 202 J. W. EVANS, Ln , Hicks, who visited the area in the nineties, contributed some interesting observations on the geological structures, but, as will be seen, I am unable to accept his views on the strati graphical position of the Morte Slates. I cannot close this brief historical review without reference to the excellent work of Mr. J. G. Hamling, of Barnstaple. It is a matter for congratulation that his valuable collections, which relate chiefly to the Baggy and, Marwood and Pilton Beds and provided the materials for Mr. Whidborne's description of their fauna, are safely housed in the Natural History Museum. BIBLIOGRAPHY. Geol. Surv. Map: Sheet 27 (Old Series). Geol. Surv. Index Map: Old Series, Sheet 10 ; New Series, Sheet 18. DE LA BECHE, SIR HENRY.-" Report on the Geology of Corn wall, Devon and West Somerset." Geol, Suruev, SEDGWICK, A., and MURCHISON, SIR R. 1. :_.i On the Physical Structure of Devonshire." Trans, Geol, Soc., Ser. 2, vol. 5, pp. 633'-652. 1866. JUKES, J. B.-" On the Carboniferous Slate (or Devonian Rocks) and the Old Red Sandstone of South Ireland and North Devon." Quart. JOU1'n. Geol, Soc., vol. 22, p. 320. JUKES, ]. B.-" Additional Notes on the Grouping of the Rocks of N. Devon and West Somerset," Dublin. HALL, T. M.-" On the relative distribution of Fossils throughout the N. Devon Series." Quart. [ourn, Geol, Soc., vol. 23, p. 371. ETHERIDGE, R-:-" On the Physical Structure of West Somerset and N. Devon." Quart. Journ. Geol, Soc.,vol. 23, p. 568. 186). VALPY, R H.-" Notes on the Geology of Ilfracombe and the Neighbourhood." Ilfracombe. 1878. BONNEY, T. G.-" Note on the Felsite of Bittadon, N. Devon." Ceol. Mag., pp. 207-209. 1881. USSHER. W. A. E.-" On the Palseozoic Rocks of N. Devon and W. Somerset." Ceol. Mag., pp. 441-448. 1888. MARR, J. E.-" On some effects of pressure on the Devonian Sedimentary Rocks of N. Devon." Geol, Mag., PI" 218-221. HICKS, H.-" Some Examples of Folds and Faults m the Devonian Rocks at and near Ilfracombe." Geol, Mag., pp. 2-9. HICKS. H.-" On the Morte Slates and Associated Rocks in N. Devon and W. Somerset." Quart. Jour". Geol, Soc., vol, 52, p. 254. 1896. HICKS H.-" The Paleeozoic Rocks of West Somerset and North Devon." Proc, Geol, Assoc., vol. 14, pp. 357-370. 1908. HAMLING, J. G.-" Recently discovered fossils from the Lower and Upper Devonian Beds of North Devon." T1'"tls. Devon. Assoc., vol. 40, pp. 276-280. 19 10. HAMLING, ]. G. and ROGERS, 1.-" Excursion to North Devon." Proc, Geol, Assoc., vol. 21, p. 457 (with map on a scale of lin. to the mile). EVANS, ]. W. and POCOCK, R W.-" The Age of the Morte Slates." Ceol.Mag., pp. 113-115. EVANS, J. W.-" Excursion to West Somerset," Proc, Geol. Assoc., vol. 25, pp. 100, 102. EVANS, ]. W.-" Correlation of the Devonian Rocks of North Devon," Ceol. Mag., pp. 147-9. EVANS. J. W.-" Devonian of Great Britain (Sedimentary Rocks)," Handb, del' regionale1f Geologie, vol. 3, part I, The British Isles, pp. 104- 137. STRUCTURE OF COUNTRY ROUND CO~IBE MARTIN. 203 The dominant feature of the Palreozoic rocks which emerge in the south-west of England from beneath the Mesozoic strata is the great synclinorium that traverses the County of Devon from east to west. The centre is occupied mainly by Carbon iferous rocks and the northern and southern margins by tracts of Devonian strata. These form the coast line of West Somerset and North Devon from Minehead to Barnstaple and extend some fifteen miles to the southward from the north coast. They also appear further to the east in the Quantock Hills as an inlier which is separated from the main mass by a trough of Permian and Triassic rocks. The succession of the rocks in North Devon, as I under stand it, and their correlation with rocks of other areas, is given on pages 204 and 205, and .in the course of my description of the rocks I shall indicate how far my views differ from those put forward by other authors. I have provisionally called the minor divisions of the rocks near Combe Martin by local names, as their correlation with similar divisions elsewhere is still uncer tain. Except in the case of these minor divisions the estimates of the thickness of the strata are intended merely to give some idea of the probable thickness. The actual amount is quite uncertain. On the one hand the faulting, as will be seen, tends to omit portions of the succession, and on the other hand local folding increases the apparent thickness. Any calcula tion based on the dip of the cleavage, or even the average apparent dip of the strata, would be far too high. In the accompanying sketch map (Fig. 54, p. 209) I have confined myself to indicating the positions of the principal faults and the lithological characters, as the information available at present is not sufficient to justify the insertion of definite boundaries between the different divisions. Except for local variations and folds the rocks have a dip -directed approximately due south and averaging about 35°, but they bend over- in an anticline in the extreme north at the Foreland east of Lynmouth. From that point southward there is an upward succession, and a similar, but more gradual, suc cession can be followed along the coast to the west-south-west, oblique to the- dip, as far as Morte Point, where the coast line changes direction and continues nearly due south, approximately parallel to the dip, to Bideford Bay. The general distribution -of the beds is shown in the accompanying diagrammatic map (Fig. 53, p. 206) taken from :\Ir. Etheridge's paper.* It must be noted, however, that the trend of the strata, over the greater part of the area, of about 13c north of west, is not the actual • 0..11" /014",. Geol. Soe., vol. niii., 1867. p. s80. A more detailed map. e:ltendiog fr_ B irnstaple or Bideford Bay to a point east of the Foreland on a scale of three quart.... of an iRCII 10 the mile. was prepared by Mr. Hamling, and published in P'OI. Gtol. A.soc.• nl. xxi., 1910. 1'. 472 , 204- J. W. EVANS, STRATIGRAPHICAL SUCCESSION IN NORTH DEVON Recent Alluvium Quaternary { Pleistocene Head Stl'tlJigl'aphical lweak. Upper Culm [C] LC I {Pendleside Series with Coddon Cherts Carboniferous ewer u m Cyathaxonia Zone. 1M] Stl'atigl'aphical Brea», 1Upper Pilton Beds (M] Feet Middle Pilton Beds } Lower Pilton Beds (M] 2000 Baggy and Marwood Beds (L] 1200 Pickwell Down Sandstones [e] 30 0 0 Marte Slates [M] 15°0 Upper Woolscott Cleave Slates. Coarse Devonian arenaceous slates [M] 250 Slates with subordinate grits and lime stones [M] 1000 David's Hole Beds. Striped calcareous slates with intercalated limestones [M] (thickness uncertain on account of folding and faulting) 25° Red Limestone Series.