177 EXCURSION TO TENBY, EASTER, I9c9. APRIL 8TH TO 15TH. Director: ARTHUR L. LEACH. Excursio» Secrctary: RAY~[OND H. CK\NDLER. (Rt/ort by THE DIRECTOIL) THE official party left Paddington at I I.30 a.m., on Thursday, April Sth, and arrived at Tenby at 6.30 p.m.; but many members had assembled at the headquarters, the Coburg Hotel, during the ~ay, and upwards of sixty sat down to dinner on Thursday evemng. Fridav, April 9th. UPPER A:\D Lowrr, CARI30NIFEROUS ROCKS. Favoured by delightful weather the party walked to the south side of the harbour, where the Director drew attention to the chief features of the coast. A heat-haze overhung Carmarthen Bay, veiling 'Worm's Head, in Gower, which, like Tenby, is on the south crop of the Carboniferous Limestone, but Ragwen, ~lJ."fort· . -'-. ..~I.", Casl kH~ 1. f .." + - - - - ..2.- -- FIG. 12.-DIAGRA~IMATIC SECTION OF THE CARI30NIFEROUS LIMESTONE (Avonian), TENllY.-rf. L. leach. 5 "U pper subb·zone fl Seminu!,I-zO:lC. a. Lamiflosa-dolomite. S t : L ower su -zcne b. Caninia-oolite. Cs' Upper sub-zone l S.l'ringoth.vns- c. Shales and impure limestones. C r- Lower sub-zone j zone. d. Massive Limestones (with Michelinia Zz' Upper sub-zone-Zaphrmtis-zone. megastoma). x. Prominent band of Productus e. Thin-bedded reddish limestones. " giganteus." ./. Massive Limestones. Length of Section, 2,000 feet. on the north crop, could be discerned. Between Ragwen and Tenby extends a remarkable coast section through the greatly­ disturbed heds of the western extension of the South Wales coal-field. The north side of Tenby stands on the edges of the PROC. GEOL. Assoc., VOL. XXI, PART 4, 19°9] 14 E XCURSION TO T E N RY, EAST ER, 19° 9. upturned and locally overfolded beds composing the southern border of the coal-field. In the immediate foreground a set of brownish grits dipped steeply south into the cliff, and 100 yatds to the north-west a group of black Upper Carboniferous shales was seen also dipp ing south, apparently under the Carboniferous Lim estone. These shales, extending nearly ISO yards along the foreshore, are separated from still higher Carbon iferous rocks by a great fault,'*' recently mapped and described by NIl. E. L. Dixon, who names it the " Ritec " fault, since it runs westward several miles along the valley of that stream. On closer examination the shales were seen to include large " bullions " (ellipsoidal conc retions), thin limestones, and carbonaceous layers. De la Beche (l lfolt. Ceol. Survey, vol. i, p. 134), after describing a thick er series (1,600 ft.) of black cherty shales overlying the" Rottenstones " (Carboniferous Limestone) at Bishopston in Gower, remarked: "At T enby we recognise the continuation of these beds, though but a few feet thick, in the carbonaceous shales, with nodules and irregul ar seams of dark argillaceous limestones (containing Goniatites) and the hard sandstones which there really succeed the Carboniferous Limestone in ascending order, th ough the contortions have been such, that beneath the town where this part of the series is well-exposed, these beds have the deceptive appearance of dipping und er the limestone." In De la Beche's opinion these shales were part of a lenti cular mass (thinning off east and west) interposed between the Carboniferous Limestone and the true Coal Measures, not definitely corresponding to the Millstone Grit of the North of England and of the north crop of he South Wales coal-field, but appearing to be a continuation of the black limeston es above the Pilton and Petherwin groups in Devonshire. In Gower, and at T enb y, " it is," he remark s, "difficult to draw fine lines of distinction between the Car­ boniferous Limestone and the Coal Measures." T his difficulty probably explains the discrepancy between Horizontal Section No. I, Sheet :2 (1844), in which only Coal Measures are indicated, and the r-inch geological map ( [845), which shows a broad out­ crop of Millstone Grit between Tenby and Waterwynch. After the Director's state ment of these views the shales were examin ed. From the "bullions " Dr. Wheelton H ind obtained C lyphioceras diadema, Pterinopecten papy rai:ea, Posidonie!la lcevis, P osidonomya cr. memoranacea, Orthoceras sp. (?), .Jfachrocheilina e!egans, several gasteropods, and ostracods in abundance. The peculiar form of Glyphioceras diadema found in the bullions possessed coarse ribs, an acutely angular periphery and a very wide umbilicus, exposing all th e whorls, corresponding thus to the variety which is characteristic of the Chokier beds of Belgium, and of beds of the same horizon at Lisdoonvarna, Co. Clare. • S um . Prog; CeDI. S urve)' for 1904 and subsequent years . EXCURSION TO TENBY, EASTER, 19°9. 179 The whole fauna appeared to Dr. Hind to link the "bullion beds" with his Pendleside Series, a group occurring between the Carboniferous Limestone and the Millstone Grit in the English Midlands, and containing a characteristic assemblage of fossils. This series is well shown at Chokier, and is represented in the West of Ireland; Dr. Hind claims also" a limited but definite Pendle­ side fauna, Posidonieiia iaois and Glypht'oceras btl£ngue,"* for the black shales of Bishopston in Gower, but the Bishopston shales are assigned by Dr. Strahan to the Millstone Grit, t the true grits of the north crop being held equivalent to the shales of the south 'Crop. In response to a request for an expression of opinion, Dr. Wheelton Hind writes: "The' bullion' beds exposed along the North Shore belong to the Pendleside series of the Midlands, the Namurian of Belgium, rather than to the true Millstone Grit series, to which they bear little resemblance in fauna or lithology." Leaving the bullion beds and noting "barrel-post rock," a breccia on the line of the Ritec fault, the folded and faulted shales and grits of the north cliff were examined. The structures shown in the cliffs afforded good illustrations of the powerful disturbances which have affected the coal-field. Near Middle Rock a sharp anticline showed in its almost vertical north limb and the more gently inclined south limb features which characterise most of the anticlines in this district, and indicate pressure from a centre of disturbance in the south. The exact horizon of these beds is doubtful. Mr. E. L. Dixon] says: "The section exposes shales, with various lamellibranchs but no Goniatites, which cannot be correlated at present with any part of the series [Millstone Grit, Coal Measures] outcropping to the north." Some fossils obtained during the excursion are therefore of interest. On the south side of First Point in a curious rubbly bed, with plant fragments and annelid tracks, Gastrioceras iiszer! was found, and a shale imme­ diately south of this bed yielded a species of Carbonico/a in abundance. Dr. Wheelton Hind considered these fossils to indicate Lower Coal Measures rather than Millstone Grit, since Gastrioceras l£stfr£ characterises the upper part of the Millstone Grit and the Lower Coal Measures of the Midlands. Thus the most important and interesting feature of the section ex­ posed along the North Shore from the" bullions" to First Point is its resemblance to the Belgian sequence where the Namurian (only about 150 feet thick) with Glypht'oceras diadema succeeds the Visean conformably. The Namurian in Belgium is classed with the Lower Coal Measures. * Geol, Mag. (Ig06), p. SOl. t "The Country around Swansea." Mem; Geol, Survey (Ig07). t Sum. Prog: GeJl. Suru., Ig05, P. 65· 180 EXCURSION TO TENBY, EASTER, 1909. The party now returned to the harbour and went thence to the Castle Hill. Near the Life-boat house thinly-bedded lime­ stones with abundant chert nodules were seen overlying a thick grey oolite. Above the chert beds come highly crinoidal lime­ stones with Zaphrentids and a brachiopod, probably Spirifer aff. clathratus. Some time was spent on the summit of the hill in studying the general view of the coast-line before descending to the South Sands. The limestone on the south side of the Castle Hill was seen to be much brecciated and unfossiliferous. From St. Catherine's Rock the section shown in Fig. 12 was first examined as a whole, and afterwards the party worked along the base of the cliffs to study the details of the sequence. FIG. I3·-SECTION SHOWING BRECCIA, PROBABI.Y OF TRIASSIC AGE, LYDSTEP CAVERNS, TENBY. (See page 185.) The lowest beds in the axis of the anticline are much jointed and weathered dolomitic limestones-the Laminosa-dolomites of C1-containing few fossils except occasional specimens of Zaphrentis omalusi, Syringopora d. reticulata, and small crinoid ossicles. Only a thin band of white oolite-Caninia-oolite­ intervenes between these dolomites and the thinly-bedded shaly C~ and gritty beds at the base of C2• These lowest beds of indicate probably a temporary upheaval of the floor of the Carboniferous sea, producing thus shallow-water conditions of deposition at this point. At Pendine, about 7 miles to the EXCURSION TO TENBY, EASTER, 1909. 181 north-east, where greater movement must have occurred, a con­ glomerate intervenes in this position between the Zaphrentis and Seminula zones. The sequence is continued by dark massive limestones still in C2, abounding in Michelinia cf. megastoma and Bellerophon, and con­ taining also Orthoaeras sp. Caninia cylindrica, Zaphrentis cornu­ copiee, Syringopora cf. reticulata and large crinoid stems; these are followed by redder limestones, apparently unfossiliferous, until Lithostrotion cf. martini appears a few yards above a little fold in the northern limb of the anticline. A mass of this coral was seen just at the beginning of the wall which ends the section north of Lexden.