Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015 158 THE STRUCTURE OF SOME CRAVEN LIMESTONES. PART II. ON THE STRUCTURE AND ORIGIN OF THE CRAVEN LIMESTONE KNOLLS. BY A. WILMORE, B.SC, F.G.S. Read at the Meeting held at Colne, March 29th, 1907.) There are in certain parts of Craven a number of knolls of limestone which are somewhat conical, or dome-shaped, and which are mainly or wholly composed of greyish-white or bluish- grey limestone, and are often, but not always, very fossiliferous. These knolls have been the source of some contention, and a number of papers have been written dealing, more or less directly, with observations on the nature of, and theories concerning the origin of these structures (see bibliography, p. 169). The knolls occur in certain well-defined areas, more or less linear sets, of apparently a common type, being seen in the following districts :—(a) Downham, Chatburn, and Clitheroe ; (6) Cracoe, Thorpe, and Burnsall, close to the grit fells ; (c) near Rylstone and Grassington; (d) Malham; (e) Stockdale and Scaleber, east of Settle; (/) Slaidburn district. There are isolated or detached masses, which have some of the character• istics of the knolls, (g) on the Greenhow extension of the Cracoe fold, (h) at Fogga, near Hellifield, (i), at Xewsholme, near Gis- burn. It is obvious that the distribution of these knolls is in• timately associated with the well-known directions of disturb• ance in the district. The key to the structure of the district is furnished by the Craven Faults and the associated folds to the south and south-west of them (see Plate XIX, to which the above letters refer). Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015 X X X X Knolls. ^™•••• Faults. Permian. Coal Measures. Millstone Grit. Pendleside and Carboniferous Limestone. Pre-Carboniferous Rocks. A.R.D. Bust. Proc. Yorks. Geol. Soc, Vol. XVI Plate XIX. Scale: J inch = 1 mile. Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015 WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES. 159 It will be necessary to recall the chief features of the physical geography of the district. First we have the Craven Highlands, to the north of the inner fault, the country of the dales proper and the land of the famous scars. Here the strata are almost hori• zontal, the Carboniferous Limestone series being succeeded by the Shales with Limestone and then by the Millstone Grit which forms the summits of Penyghent, Ingleborough, and Fountains Fell. Dr. Wheelton Hind informs me that he has recently proved the existence of the Pendleside fauna in the southern part of the Highland area to the north of Malham, but I do not know of anything corresponding to the knolls of Lower Craven in this area. The limestones are sometimes quite as irregularly bedded, and there is the same variation in fossil- iferous character, but the rounded detached hills do not occur. The Northern Branch of the Craven Faults runs from Ingleton, north of Malham, to the neighbourhood of Pateley Bridge. The middle branch runs from Ingleton to Malham and on to Winterburn. The southern " branch " is a complex system branching off near Settle, and running south to the neighbourhood of Skipton. The faults at Barnoldswick, at Twiston (near Downham), and the limestone boundary fault at Clitheroe, as well as the north-eastern boundary fault of the Burnley Coalfield, are somewhat similar in direction to the Craven Faults (Plate XIX). It has often been pointed out that the throw of the Craven Faults decreases as they are traced eastwards, and becomes comparatively small in the knoll district (see especially Hind and Howe, Q.J.G.S., Ivii.). Is it not difficult to conceive of a very great difference in the succession of the beds or a radical change in the fauna north and south of the faults, when the throw is so small '? Are not any observed differences rather to be attributed to a gradual change in the succession and in the fauna as the beds are traced from south to north independ• ently of the faults themselves ? The succession in the country Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015 160 WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES. between the northern and middle branches of the Craven Faults is not very different from the succession further south. Secondly, we have the rolling country of Lowland Craven, where the rocks are folded. The strata are mainly shales and limestones, with apparently a very variable lithological sequence in different parts of the district. There are well-marked anti- clinals, the one which strikes S.W. to N.E. from Clitheroe to Grassington being the most important. There are minor folds striking nearly at right angles to these, seen in the Marton and Gargrave district. Almost every exposure—and they are very numerous—shows some signs of intense folding or faulting. Naturally the thin-bedded muddy limestones show the folding best, but it is also seen in the more massive and unevenly bedded grey limestone. It is almost exclusively on the borders of this region of folding that the grey fossiliferous knolls occur, and there are knolls in the most perfectly bedded blue Lime• stones with Shales, which occupy exactly corresponding positions, and which are as nearly like them as their different lithology will admit. The faults and folds are obviously closely connected, be• cause the latter end off sharply against the former, and it is difficult to resist the conclusion that they are due to the same series of movements. An illustration of the folding is given in Plate XX. Thirdly, there is the region of grit moorlands of Todmorden and Hebden Bridge, to the south-east of the coalfield of Burnley. Here we have a foreland of the Craven folded system. The grits lie nearly horizontally, and the hills become tabular masses, thus reproducing some of the features of the country north of the Craven Faults. Just as these faults separate the tabular Craven Highlands from the folded Craven Lowlands, so there is a series of faults—not nearly so complete or so well seen—separating the lowlands from the foreland of grit hills. The hills of the eastern part of the Pendle Range are of the nature of escarpments, on the flanks of shallow synclinals at the edge of the folded country, while those in the western Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015 Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015 WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES. 161 part consist of the middle limb of an anticlinal fold. The relation ox the eastern hills to the folded country is shown in the diagrammatic section, Fig. 1. Fig 1. SKOTIOF FROM FENDM2 TO CHATBCRN THROUGH WQKSAW KXOIJ,. a. Base of Millstone Grit Series. Pendle (? Farey's) Grit. b. Pendleside Series, Shales with variable limestones and grits. c. Carboniferous Limestone Series, "with coarse, massive "Knoll" Limestone at the top. Referring again to the diagram map. Plate XIX, it will be seen that the distribution of the knolls is intimately related to the structure of the country, as briefly outlined above. It seams clear that the knolls depend more upon the " tectonics " of the district than upon any peculiarities of de• position, though I do not deny that such peculiarities may have existed, and may have had something to do with the development of the knolls. I propose in this paper to study rather more intimately the remarkable series of knolls in the Downham-Clitheroe dis• trict. These are marked (A) on Plate XIX. There are five at least of these knolls forming a well-marked series, namely, the hills of Twiston, Sykes, Gerna, Worsaw, and Worston, though I do not see why the low hill of Bellman Quarry, Salt Hill, and Clitheroe Castle Hill should not be included. They do not form the same " ovoid eminencies," but the limestone itself, the bedding, the fossils, the secondary deposits of stalagmite, fraor, pyrites, and limonite pseudomorphs after pyrites, are precisely similar. Omitting the Castle Hill, which I have not been able to study very fully, and beginning at the Clitheroe end of the series, the following is a brief description of the knolls. (See one-inch Geol. Map, 92, S.W., new series, sheet 68.) Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015 162 WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES. (a) Salt Hill. This is a low rounded hill, now partly re• moved by quarrying. The limestone is for the most part a well-bedded greyish-white crinoidal debris, containing broken and rolled shells. The stems and plates of crinoids and the shells weather out from the massive limestone on long exposure. There is abundant iron oxide in parts, and the limestone some• times weathers to a clay of a brownish-yellow colour. There are layers of light blue very fine-grained limestone, with occasional megascopic fossils in excellent preservation. This quarry has been one of the favourite collecting grounds for Cephalpods, Euomphalus, Spirifers, and other Brachipods. (b) Bellman Quarry Hill is of inconsiderable height, but forms a low rounded knoll. It consists largely of crinoidal debris, sometimes quite well bedded, sometimes obscurely bedded. The same plates and stems weather out as on Salt Hill, and cups or heads are quite plentiful. Shells sometimes come out on weathered masses in great perfection.