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[PLATE XXIV. ] CONTENTS, Page I Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016 370 MISS G. L. ELLES ON THE WENLOCK [May x9oo , 93. T~e ZONAL CLASSIFICATION of the WENLOCK SHALES Of th~ WELSH BORVERLA~ID. By Miss GERTRUDE L. ErLES, of Newnham College. (Communicated by J. E. MARR, Esq., M.A., F.R.S. Read December 20th, 1899.) [PLATE XXIV. ] CONTENTS, Page I. Introduction ......................................................... 370 II. Literature ............................................................ 372 III. The Builth District. (Map on p. 385.) ....................... 373 IV. The Long Mountain District. (Map on p. 395.) ............ 386 V. The Dee Valley ..................................................... 397 VI. General Conclusions and Correlation ........................... 40I VII. Pal~eontology ..................................................... 401 I. INTRODVC~I0~. In Southern Sweden occurs a series of shales of Wenloek age, which Tullberg divided into zones by means of their predominant graptolites? During a visit to that country in the early part of 1896, I was enabled, through Dr. TSrnquist's kindness, to spend some time in studying the fauna of these zones, and I determined that when I returned to England I would ascertain whether some such zonal classification did not also hold good for the beds of Wenlock age in our own country. During the past four years I have studied in detail the st~rata and the graptolites of the Wenlock Beds of the Welsh Borderland, and I find that such a classification can certainly be made. Further, as the existence of Tullberg's zones ~" has recently been called in question, and doubt has been thrown upon the validity of several of his species, it seems only fair to the memory of that geologist to bring forward the results that I have obtained, which appear to me to confirm his work completely. A glance at the Geological Survey maps of the Welsh Borderland would seem to indicate that the Wenloek Shales cover a consider- able extent of country. This, however, is not really the case : far from being of wide extent, the WeDlock Shales are generally present as a mere fringe below the Ludlow rocks. It is the Ludlows, and particularly the Lower Ludlows, which occupy the greater part of the area. Frequently the so-called ' Ludlow' of the Geological Survey maps of the Welsh Borderland is the equivalent of the Upper Ludlow of the maps of the typical area of Shropshire. The reason for this seems to be, that the Wenlock Shales and Lower Ludlow Beds of the Welsh Borderland are often identical in x , Sk~nes Graptoliter' pt. i (1882) Sver. Geol. UndersSkn. ser. C, no. 50, p. 15. 2 Frech & Rcemer, ' Leth0ea Palzcozoica ' vol. i, pt. iii (1897) pp. 650-653. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016 •ol. 56.] s~A~.Es 0F ~H~ W~LSH ~ORD~RLA~D. 371 lit~hological character, and were naturally mapped as one formation, while the Upper Ludlow is decidedly different in this respect, so the main line of separation was drawn at its base. Nevertheless, the Lower Ludlows may be at the present day readily distinguished from the Wenlock Shales palmontologically, for they contain a perfectly distinct graptolitic fauna. My own researches have led me to the conclusion that the best palmontolo~cal division of these two rock-groups is one which separates (a) A group characterized by graptolites of the priodor~.Flemingii-type (that is, Wenlock Shales) from (b) /t group characterized by graptolites of the co/onus-type (=Lower Ludlow). In this opinion I am supported by Miss E. ~I. It. Wood, who has been working for some time on the fauna of" the Lower Ludlow rocks. This classification is certainly of a practical working value; for, as a result of our field-work in many localities, we have both of us always found that the Flemingii-fauna dies away where the colonus- forms become abundant. I would therefore propose to restrict the term WenloekShale to the rocks containing the first of these faunal groups, and the term Lower Ludlow to the rocks con- taining the second. The present paper is concerned only with the Wenlock Shales and their graptolitie fauna. My work on these Wenlock Shales has extended over a considerable part of the Welsh Borderland, but I have chosen three special areas for detailed work. The most important district appears to be that which lies round the town of Builth. There the Wenlock Shales can be studied from base to summit ; this area therefore is taken as the typical one. There is no other district along the Welsh Borderland where the rock-succession is so complete and so fossiliferous throughout, though excellent confirmatory sections occur in the two other areas that I have studied, namely, in the Long Mountain and in the Dee Valley. In the Long Mountain, the basal beds of the Wenlock Shales are not seen, while in the Dee Valley the upper part of the series contains but few fossils, and the succession is complicated by the coming in of the grits, which attain a great development farther north. I have purposely avoided, for the present, those districts where limestones or grits enter largely into the sequence, as my object has been to study the succession of lifo where the Wenlock Series was most typically developed. Now, the Wenloek is essentially a shale-formation, though locally, as is well known, some of the beds of shale are replaced at different horizons by beds of limestone or grits. Also, in the localities where limestones or grits form an important feature in the succession, the grapbolites are fewer in number both as regards genera and species, and a zonal classification will prove to be a matter of greater ditfieulty. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016 372 ~ms o. L. sI~ES os veB ~ [May t 900, II. I~T~.~TURB. The Builth District. Very little has been written about the Wenlock Shales of Buflth, though much attention has been paid to the Llandeilo Beds of that area. The Wenlock Beds merely receive a passing mention in Murchison's 'Silurian System' (1839) p. 315, and also in his ' Siluria' 5th ed. (1872) p. 59. In the last-named work, however, sections are drawn across the Builth country, and the Wenloek Beds are grouped together as a series of mudstones containing graptolites and Orthoceras. A summary account of the Silurian rocks of the Builth district is found in Mere. Geol. Surv. vol. i (1846) p. 22, and the chief localities for fossils are recorded in vol. fi, pt. i (1848) p. 327, etc. The only suggestion of their possible division into zones is. given by Lapworth in his ' Geological Distribu- tion of the Rhabdophora.' 1 He separates the zone of Cy,'tograptus Murchisoni from higher beds in which C. Linnarssoni is a character- istic fossil. At the end of the same paper he also states his opinion that the day will come when ' by the aid of the lowly graptolite the geologist of the future will be ab]e to read off the natural succession ' in the monotonous Silurian mudstones of Britain with ease and certainty. The Long Mountain. Prof. Watts has written two papers dealing with the Silurian rocks of the Long Mountain. In the first of these, ' On the Igneous & Associated Rocks of the Breidden Hills in East Montgomeryshire & West Shropshire,' ~ he divides the Wenlock Shales of the area into an upper and lower series. In a paper read before the British Association in 1890 and subsequently published in abstract, a he modifies his earlier views and considers that the beds represent the upper part of the Wenloek Shale together with the Wenloek Limestone. My own work confirms his later opinion. The Dee Valley. The Llangollen Basin has been studied by many geologists. The most important recent paper is one by Mr. Lake on ' The Denbighshiro Series of South Denbighshire.' 4 This paper contains a summary of the earlier work on the beds exposed in the area, and the author divides the Wenlock Beds of the district as follows :-- Moel Ferna Slates, with .V. TTemi~gii and ~/. im~don. Pen-y-glog Grit. Pen-y-glog Slates, with M. personatus, M. priodon, e~c. i Ann. & Mag. Nat. Hist. ser. 5, vol. vi (1880) p. 201. "-' Quart. Journ. Geol. Soc. vol. xli (1885) p. 532. 3 Rep. Brit. Assoc. 1890 (Leeds) p. 817. 4 Quart. Journ. Geol. See. vol. li (1895) p. 9. Downloaded from http://jgslegacy.lyellcollection.org/ at University of California-San Diego on January 17, 2016 Yolo 56,] SHALES OF THE WELSH BORDERLAND. 373 My work supplies some further details, and confirms his view ghat in part the Moel Ferna Slates are approximately on the same horizon as the beds with G~/rtogrc~tus Linnarssoni in the Long Mountain. III. THE B~rZLT~ ])ISTRICr. That part of the Builth district which is occupied by Wenloek Shales, and which I have mapped in detail, includes a roughly triangular area extending from Disserth on the north to Llanafan- fechan on the west and Aberedw Hill on the east. (See Map, fig. 4, p. 385.) The conspicuous features in the landscape are due to the igneous rocks associated with the Llandeflos, and the Ludlows also attain a considerable height south-west, south, and south-east of the town, forming Mynydd Aberedw and Mynydd Epynt. The Wenloek Shales only give rise to gentle undulations sloping down to the banks of the Wye and its tributaries, the higher ground being generally occupied by the harder and more flaggy beds of the series. Exposures are less common than in the older beds, and close to the town all are concealed beneath a tract of alluvium.
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