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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. There is the same iron-oxide and weathering of the limestone to a brown clay. Stalagmitic deposits are quite good. There is a remark• able breccia made up of fine-grained pieces of limestone and crinoidal fragments mixed together. This breccia is inter- bedded with the ordinary crinoidal and shell limestone. (c) Worston Hill shows more of the knoll character ; it rises somewhat sharply from the plain of Pendleside shales, and has the familiar rounded appearance. There is the same crinoidal debris, with the same rolled shells weathering out perfectly on long exposure. There is the same breccia, this time in greater abundance ; at least I have seen more of it, and it is again clearly interbedded with the irregularly stratified crinoidal and shell debris. (d) Worsaw is the highest and by far the most conspicuous of the series. It rises to a height of 725 feet above Ordnance datum, or more than 300 feet above the neighbouring plain. There is again the crinoidal ?^nd shell debris, sometimes quite well bedded, sometimes very obscurely bedded. Occasionally the limestone may be described as flaggy. The dips in this and in the knolls of Salt Hill, Bellman, and Worston are fairly consonant. They are almost uniformly towards the plain, Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015

WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES. 163

and the grit hills of the Pendle escarpment. In the case of Worsaw there are plenty of exposures, and, excepting in one case, the dips are fairly easy to determine and are as uniform in direction as can be expected in a folded country. The slope facing the low plain is a true dip-slope. The slope on the other side is an escarpment slope, and the beds may be seen dipping into the hill in many places. The following diagram shows these most important features. Fig. 2.

1 Fio-. 2. o SECTION THROUGH WORSAW KNOLL ACROSS THE STRIKE.

b = Pendleside Shales. c -• Upper Limestones, very "massive " as a rule^ Carboniferous Lime• c = Limestones, more "flaggy" ...... - 1 stone Series. c, = Limestones with shales ... ,...... J

There are swallow-holes on the top of this mass, and stalag- mitic deposits may be found in the wide joints, and I have also obtained fluor and pyrites there. (e) The next " knoll" is Gerna, though there are some apparently semi-detached masses or much reduced knolls between Worsaw and Downham village. Gerna appears as an island, and is so mapped. The shales all round it are mapped as the same, but those south of the " island," striking with the beds in front of the knolls previously described, contain the Pendle• side fauna—Pterinopecten papyraceus, Posidoniella Icevis and P. minor, &c, whilst the shales north of the Gerna mass are lithologically different, and contain a different fauna, the fossils being those of the limestone massif, as found at Thornton and Rain Hall, for instance. If the beds were coloured according to the fauna, the deep blue of Gerna would then be connected directly with the blue of the Chatburn anticlinal. Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015

164 WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES.

The structure of Gerna is not so manifest as in the other cases. There is one quarry, and a few rather poor exposures. In the quarry the dips are in accord with those of Worsaw and the other knolls to the west. The knoll is, however, covered with grass for the most part, and the small exposures seem to be mainly in the obscurely bedded limestone, and the dips are not easy to make out. On the survey map they are given as quaquaversal, and this would indicate the probability of some roll in the beds. The limestone at Gerna is exactly like that in the knolls already noticed. The hard unpromising limestone becomes easy to work for fossils when it has weathered and the matrix of calcareous mud is more or less dissolved away. I have got excellent fossils from the weathered and decomposed material. The same tufa and stalagmite are noticeable, and I have got perfect cubic crystals which have a core of pyrites, but are converted into limonite on the outside. Here, as always, there is evidence of the effects of long-continued percolation of water containing oxygen, carbon dioxide, &c. In one joint I found a well-worn quartz pebble, just such as occurs in the grits. (/) Sykes knoll stands up as a long narrow peninsula. It consists in part of crinoidal debris, and in part of fine-grained, apparently almost unfossiliferous, limestone. Exposures are not very numerous, but the dips on the whole seem to agree with the general direction in the other knolls. A fault runs alongside, and it is separated from Twiston knoll by a river valley. (g) Twiston knoll is the last of the series. It contains very fine crinoidal debris, with occasional shells ; this is some• times obscurely bedded, but in places it is most clearly well- bedded. There is again a fine-textured, apparently unfossiliferous, limestone at the base of the strata forming the knoll. There is another fine-grained deposit higher in the series, and the more obscurely stratified crinoidal and shell deposits seem to be interbedded. The dips are nearly all southwards. To sum up, the structures and structural relations common to this series of seven knolls are as follows :—(1) The occurrence Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015

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of crinoidal and shell debris, with occasional corals and polyzoa ; (2) the presence of rolled and broken shells ; (3) the occurrence of mineral deposits—stalagmite, fluor, pyrites, and iron oxides ; {4) the presence of interbedded breccias made of fine-grained material and fragments, principally crinoidal; (5) the variation in the character of the bedding, well-bedded material alternating with more obscurely bedded deposits in every knoll; (6) the general agreement of the dip of the beds, with only such irregu• larities as are likely to occur in a folded country, and which can be matched in every part of the Craven Lowlands. It is now necessary to consider the strata on each side of this knoll line. To the south there are the fine black shales, with characteristic thin limestones passing upwards into the shales with grits. These are characterised, as Dr. Hind and Mr. Howe have shown, by a well-defined fauna (see also the Survey memoir on the Burnley Coalfield). On the north side, that is behind the knolls and towards the middle of the Chat burn - Clitheroe anticlinal, there are more flaggy limestones, with mud- stones and soft shales. The beds have still the same general dip. They may be well studied in the immediate neighbourhood of Downham and in the brook sections between Worsaw Hill and Chatburn. These beds contain the same general fauna which occurs at Thornton-in-Craven, at Rain Hall, at News- holme, and other places in the area coloured dark blue on the survey map. (I hope in a future paper to give further particu• lars and detailed lists of the fauna of Thornton and Downham.) These shales and " mudstones " are not on the same horizon as the shales south of the knolls, the faunas being quite different. The recognition of the Pendlesides, as constituting a distinct stratigraphical series, is, in my opinion, most important. Whether these beds are eventually known as the Pendleside series or as the upper shales with limestones matters less than the insistence on the fact that they are quite distinct lithologically and pahe- ontologicaDy from the various beds of the series below. The greyish limestones, the blue limestones, and the variable shales of the Carboniferous Limestone series contain, on the whole, the same well-defined fauna. This fauna is quite different from that of the Pendleside shales and limestones immediately Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015

166 WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES.

succeeding them. Grey or " white " limestones occur not only in the knolls at the top of the " massif," but also nearer the base of the series exposed in the Chatburn district. The Carboniferous Limestone is at least 2,500 feet thick unless there is repetition of the beds, and whether there is much repetition can only be determined by careful collecting from every horizon. I hope it will be possible to work out the whole 2.500 feet or so of the limestone massif in zones, as has been so ably done by Dr. A. Vaughan for the Bristol area, and I commend a share in the work to any members of our society who can find time to assist. The general succession of the beds from the grit escarp• ment of Pendle through the most conspicuous of the knoll hills, Worsaw, is as follows :— Millstone Grits. Pendleside shales with limestones and grit bands. Greyish-white crinoidal limestone. More flaggy limestone. Flaggy limestones with shale bands. Blue and grey massive limestones, with shales. Thin bedded dark limestones and shales. The beds from the base of the Pendleside Shales down• wards contain the same general fauna. The succession would then be as in Pig. 3.

& ®

Fig. 3.

DIAGRAMMATIC SECTION THROUGH THE HILLS BETWEEN THE CRAVEN LOWLANDS (A) AND THE LOTHERSDALE ANTICLINAL (B). Vertical scale exaggerated.

The partial isolation of the Worsaw mass and its conse• quent knoll-like appearance is partially, if not wholly, accounted for by the trend of the old river valleys. There is a manifest Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015

WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES. 167

longitudinal river course which has produced the rather wide plain between the Worsaw mass and the Chatburn ridge. The river seems to have run behind the knoll mass, and to have flowed out, between it and the Worston mass, into the valley of Pendleside beds, and the course is marked at the outer end by a tiny stream now. The river has been captured by a trans• verse stream, and it now flows off across the strike through the lower part of Chatburn village. Worston knoll and the low hills of Bellman and Salt Hill are similarly accounted for. The more flaggy limestones and the limestones with shales have been more readily worn down than the massive obscurely bedded crinoidal and shell debris higher in the series, and hence these latter beds now stand up as the knolls, the southern faces of these knolls being the well marked dip-slopes. The soft Pendleside shales immediately succeeding these limestone beds with their breccias have been cut down by the longitudinal stream drainages, and have thus intensified the knoll-like appearance. I do not deny that there may have been lenticular thicken• ings of these deposits, but it seems to me that Sir A. Geikie's explanation completely expresses the facts as regards deposition. He says, " The original reef-knolls described by Mr. Tiddeman from the Clitheroe district appeared to me to be due to the irregular aggregation of submarine organic debris in situ, though I could not detect any true reef structure."* The present appearances are due then, partly to the nature of the deposits, and partly to the ordinary agents of denudation acting differentially on the beds of varying texture. It is noteworthy that the organic debris and breccia deposits with their rolled shells and other peculiarities are absent from the Barnoldswick-Thornton area. There the beds which lie immediately below the Pendleside shales and limestones are blue limestones with abundant shale bands. The fauna of these blue limestones and shales is, however, as nearly that of the knoll deposits of Chatburn as could be expected. I hope to show in a subsequent paper that the crinoids, echinoids,

* Text Book of Geology, Fourth Edition, vol. ii., p. 1041. Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015

168 WILMORE : THE STRUCTURE OF SOME CRAVEN LIMESTONES.

corals, and polyzoa are identical in the Thornton and Rain Hall beds of blue limestone and muds and in the limestone knolls. The brachiopods are only different in that muddy water forms predominate in the one, whilst Spiriferid and Rhyn- chonnelid forms are plentiful in the other. The difference is quantitative and not qualitative. An apparent peculiarity of the knolls to which attention has been very often drawn is their extremely fossiliferous character. This is partly due to secondary causes. It is well to remember that some of the knoll limestones are apparently unfossiliferous until they are examined in thin section or until their fossils are " weathered out." There are some recent exposures in the Twiston knoll which are apparently quite unfossiliferous, whereas all the weathered surfaces and all the old walls on and near that knoll show plenty of fossils. The shells and crinoids and other remains of organisms 3,ve held together.by a fine calcareous cement. Long-continued weathering attacks this cementing matter, and reveals the megascopic fossils, and renders them quite easy to extract. Everyone who has worked on the limestones knows how beauti• fully fossiliferous are the old walls, and how much better their western faces are than their eastern faces. It is the same in the joints of the knoll limestones. Masses which appear quite hopeless on newly fractured surfaces show excellent fossils of various types where the bedding planes have been long ex• posed or where the joints are accessible. Now the dip of the knoll limestones is evidently very favour• able to the action of percolating water, with its dissolved gases and organic acids. In the more thinly bedded masses the action reveals fossils along the frequent joints and along the parting planes of the bedding, as has been already pointed out. In the more irregularly bedded limestones the meteoric waters have occasionally percolated throughout the whole mass, the cementing material has been partly dissolved away, the iron compounds converted into hydrates, the remainder of the cement converted into a yellow or brown clay, and the shells and other fossils left so that they can be lifted out with the fingers. Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015

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The progressive development of this phenomenon may be well seen in many new cuttings, the upper part is fossiliferous. the lower part apparently very much less so. Similar appear• ances may often be seen in the regularly bedded limestones of the districts where there are no knolls in the restricted sense. It may be well to recall the evidence for long continued drainage in the interior of these knolls, which are so admirably situated for the manifestation of its full action—frequent stalag• mite deposits, the fluor along the joints (this is not common),

the pseudomorphs of Limonite (2 Fe2033H20) after Pyrites, and the occasional grit pebbles found in the underground channels. It remains to be said that these observations and con• clusions refer to the Down ham and Clitheroe knolls in particular, and whilst it is quite likely that other knolls in other areas may allow of a different explanation, yet I think that at least some of these knolls may be thus accounted for. I wish in conclusion to express my indebtedness to the writings of Prof. Marr and Messrs. Hind and Howe on this sub• ject, and especially would I express my obligation to the work of Mr. Tiddeman. Whilst venturing to differ from some of his conclusions, I am continually feeling in my work in Craven how much I owe to his pioneer work in this part of the country.

BIBLIOGRAPHY.

1. R. H. TIDDEMAN. Report of the British Association, 1889, page 600, et. seq. 2. R. H. TIDDEMAN. Transactions Leeds Geological Association, Pt. VI., 1891, p. 112. 3. R. H. TIDDEMAN. Excursion Guide (to Aire Valley) for the Leeds meeting of British Association, 1890. 4. R. H. TIDDEMAN. " On the Formation of Reef-Knolls," Geol. Mag., January, 1901, pp. 20-23. c Downloaded from http://pygs.lyellcollection.org/ at University of St Andrews on March 16, 2015