CHAPTER XXXIII,

SOUTH WALES,

By AUB RE Y STRA HA N, M.A., S c.D., F.R.S., F.G. S.

H E Geologists' Association has paid two visits to South T Wales---one to Cardi ff in 1888, and another to Gower in 1 9 0 2 . * T he regions then examined include a pa rt of the coalfield, but are situa ted for the most part on a p lain which intervenes between the Coal Measure Uplands and the Bristol Channel. Much of the pl ain is overspread by Secondary rocks, but marine erosion along the coast and sub aerial denudation elsewhere are freeing it from this covering, and revealing a platform carved out of Palseozoic rocks. The platform, though diversified by scar ps and hollows of ancient date, generally lies at a low level with 3. tendency to rise westwards as well as northw ards towards the coa lfield . Thus, while it lies buried at a depth of 4 0 0 feet und er Cardiff Docks, .it rises to a level of 2 0 0 to 300 feet above the sea in Gower. The Secondary covering accordingly tapers away westward s, so th at in Gower it is repr esented by an occasiona l small outlier, and in Pembrokeshire bv mere. traces. Geographicall y the two regions are known as the " Vale of Glamorgan " and the peninsula of Gower. The " Vale " extends from Cardiff pa st Bridgend, and is dominated on the north by bol d scarps of Old Red Sandstone and Carboniferous Limestone, backed by the still highe r moorl and of Pennant Grit. It is not in any sense a river-vall ey, but, on the contrar y, is crossed at right ang les by the rivers j nor is it a structural valley, ina smuch as the underlying platform has been cut out of rocks of varying hardness and dip . Its origin, therefore, forms one of th e sub­ jects of investigation which will be dea lt with after examination of the Secondary rocks which filled it. T he ravines by which the rivers escape from the coalfield into the " Vale " are deep and narrow, and wind upwards into a wild moorland region which was sparsely inhabited before the develop­ ment of the steam-coal industry. Such habitations as there,were, and the pa rish-churches, were placed on the higher slopes, clear of the roadless swamps and jungles of the vall ey-bottoms. The modern colliery-settlements, on the contrary, are crowded along the bottoms, while roads and rail wavs have rendered the whole region easy of access. The contrast-between the "hills," as the • T h is chapter was recei ved by the Editors In 19o8. The Association pa id a visi t to T enb v, at Easter, 1909. when rocks ranging from S ilurian to Coa l Measures wer e examined see Pr oc, Gmt. Assoc..vol. xxi, p . 177). SOUTH WALES.

coalfield is called, and the " Vale" is striking from every point of view. The Palseozoic rocks of the platform range in age from Wen­ lock Shale to Coal Measures, while the Secondary rocks include Keuper, Rhsetic, and Lower Lias. Between the two groups there exists an unconformity, the importance of which in the geological history of the British Isles can hardly be exaggerated. The grand exposures of the discordance between the Palseozoic and Secondary strata provided in the coast, and the relations of the two as observable in innumerable inland sections, enable us to realise that the earth-movements which came into action at the close of the Carboniferousperiod displayed an energy which has not been approached in all subsequent time. For it is plainly to be seen that the Paleeozoic strata were not only intensely folded and overthrust, but that they were uplifted to such a height that a thickness of upwards of a mile of material was denuded from some of the anticlinal folds before the Secondary rocks were laid upon them. These Secondary rocks, on the other hand, have been but little disturbed from their original position, and the platform on which they rest, though slightly tilted, preserves much of its original character as a plain. It follows, of course, that the Secondary rocks rest upon strata of a variety of ages, ranging from Silurian near Cardiff and in Gower to Coal Measures near Llantrisant. In describing the formations in stratigraphical order, it becomes necessary to oscillate between Cardiff and Gower more freely than would be advisable in an excursion programme. This disadvantage, how­ ever, is counterbalanced by the greater ease with which we can compare the developments of each formation in the different regions.

SILURIAN ROCKS.

Rocks of Silurian age appear at the surface in two tracts within the region under description, though in both cases their existence remained long unsuspected. The one lies about two miles north of Cardiff, and was first recognised by the Rev. Norman Glass in 1861,* while the other occupies a narrow strip in the main anticlinal axis of Gower, and was first detected by Mr. R. H. Tiddeman in 19°1. t Cardiff or Rumney lnlier.-This inlier forms low rounded hills on the two sides of the Rhymney, where that river enters the maritime marsh, and is usually known as the Rumney Inlier, after the village of that name j . The outcrop belongs to * Geologist, vol. iv, p. 168. t " Summary of Progress of the Geological Survey for 1901," Mem. Geol. Survey. p. 38. ::: This spelling appears to have -been adopted to distinguish the village from the town of Rhyrnney, which. is situated several miles farther up the river. 828 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. the northern limb of the Cardiff-Cowbridge anticline, and the strata dip northwards, passing in that direction under Old Red Sandstone. Southwards they are overspread, quite uncon­ formably, by Keuper Marl, by which they have been deeply stained. The Keuper Marl dips and thickens towards the south, so that its base attained a depth of 400 feet below sea-level in a boring near the Cardiff Docks. Except for this boring, the southward extension of the Silurian rocks, under the Trias, is unknown. The inlier was mapped and described by Professor Sollas in 1879,* and much information concerning it was added by the late John Storrie in later years. According to Professor Sollas, all the strata exposed belong to the Ludlow and Wenlock groups. Though they differ from those of Woolhope, Dudley and Malvern in the poor development of limestones and in the prevalence of sandstones, they resemble in these respects the Silurian rocks of the Usk inlier, which had originally been supposed to be of Caradoc and Llandilo age on account of their lithological char­ acters. The occurrence of some obscure vegetable organisms called Nematophycus, and of the still more doubtful Pachytheca, together with Pentamerus oblongus, var. laois, was suggestive of a Llandovery age for the Rumney rocks, but on the evidence of the other fossils Professor Sollas was led to assign them, as stated, to the Ludlow and Wenlock, and to extend the range of the fossils named into the Wenlock. The most continuous and accessible section in the Silurian inlier is that of the road-cutting made in Roath Park, in 1893, along the eastern side of N ant Mawr (Roath Brook). At the southern end of the park, near Fairoak, the Keuper Marl tapers away upon a gently rising slope cut in mudstone with a high nor­ therly dip. The first cutting shows these beds, deeply stained, but containing Heliolites, Strophomena, Atrypa, Phaco-ps, Encri­ nurus, Trinucleus concentricus, etc. These are followed, in ascending order, by a massive coarse quartz-grit which crops out at the eastern end of the embankment of the lake, and to which the name of Rhymney Grit was given by Professor Sallas. Else­ where the grit is usually less coarse, and in a quarry on the eastern side of the River Rhymney, 300 yards south of Rumney Church, Professor Soli as found it to consist of 2376 ft. of sand­ stone, flaggy in places and ripple-marked, in others passing into a fine-grained conglomerate. Above it lay yellow sandstone, containing partings of green and blue shales and yielding Ctenodonta subcequalis and Pachytheca, while below it a grey sandstone yielded Grammysia cingulata and Rhynchonella stricklandii. These details have not been recognised, however, in Roath Park.

* Quart. [aurn, Geol. Soc., vol. xxxv, p. 475. This and other papers are summarised in "The Country around Cardiff;' llfem. Geol. Survey, 1902, chap. tt. SOUTH WALES.

The next cutting, 250 yards north of the embankment of the lake, shows deep-red and grey shale with thin bands of ferruginous quartz-grit. These were thought to be Old Red Sand­ stone, until the making of the cutting disclosed encrinital mud­ stones among them, and red or grey mudstones with Ludlow fossils above them. The junction actually occurs at Cefn-coed­ fach, where coarse red quartz-grit, clay, and deep-red micaceous sandstone, undoubtedly of Old Red Sandstone age, are poorly exposed. The Rhymney Railway traverses the Silurian inlier in a cutting which is now much obscured. A coralline limestone, possibly representing the Wenlock Limestone, crops out 70 yards south of the bridge, and an anticline in red and mottled mudstones is cut through north of the bridge. The red staining is conspicuous, and in a cutting on the new Cardiff Railway, to the east of the Heath, some Triassic red marl and conglomerate were found by Mr. G. H. Dutton, showing how close overhead the Trias was in all this part of the Silurian inlier. At Pen-y-Ian a quarry exhibits some of the oldest rocks of the inlier. It shows fossiliferous mudstones with about 12 ft. of sandstone, which weathers into large spheroids. The fossils were considered by Professor Sollas to prove a Wenlock age, though they had somewhat of a Llandovery facies. They include Petraia bina, Encrinurus punctatus, Lepta:na transversalis, L. sericea, and many others. The Rhymney grit crops out in a road-cutting north of Llwyn-y-grant-isaf', and in several quarries near the road running north-west from Pen-y-Ian. The river-section at Rumney is now in great part inaccessible; it contains a thin limestone which was assigned by Prof. Sollas to the horizon of the Wenlock Limestone. Above this there followed about 360 ft. of Ludlow Beds, and above these again deep-red micaceous sandstone of Old Red Sandstone age. At the junction a brownish grit full of fragments of fishbones (Unchus tcnuistriatus and others) was observed in loose blocks by Prof. Sollas, and subsequently in situ by Mr. Storrie. The last exposure of the Rhymney Grit occurs in the quarry already mentioned. A red limestone, which may represent part of the Wenlock Lime­ stone, crops out in a lane 200 yards east of Ty-rnawr. It is to be noticed that though the Ludlow Beds are represented, and though there 1S no visible discordance between them and the Old Red Sandstone, yet there is a great mass of strata developed on the northern side of the coalfield, near Llandilo, which has not been recognised at Rumney. These include the Tilestones, the Trichrug Beds, the Grammysia Beds, and others. * Gower Inlier.-The sections in the Silurian rocks in Gower are extremely poor, and their interest lies chiefly in the light * "The Country around Ammanford," Mem. Geol, Survey, 1907. 830 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. they throw upon the relation of those rocks to the Old Red Sandstone. But for Mr. T'iddernan's discovery it would scarcely have been anticipated that the 3,500 ft. of strata comprised in the Old Red Sandstone near Cardiff, had dwindled to about 300 ft. in parts of Gower. Strictly, there are two inliers on the crest of the anticline. The smaller lies north-west of Penmaen Workhouse, while the larger occupies the top of Cefn-y-Bryn farabout two miles. To­ wards the south the Silurian rocks are overlain naturally by the upper beds of the Old Red Sandstone, and though the junction has not been seen, it is known to be sharp; presumably it is unconformable, but no basal conglomerate is known to occur. The Silurian rocks, so far as they are visible, consist of soft olive-coloured shales, and have yielded Rlzynclzonella, Beller­ ophon, Modiolopsis (?), Chonetes, crinoid-ossicles, and entomostraca.

OLD RED SANDSTONE.

The fossils leave us in no doubt that both Upper and Lower Old Red Sandstone are represented in South 'Vales, and it was anticipated, in view of the uncon­ formable relations which have been stated to exist between the two elsewhere, that there would be no difficulty in separating them here, though it was certain that there was no obvious dis­ cordance. So far from this being the case, we are still in doubt where exactly to draw the line; a great mass of red sandstones (Brownstones) had long to be left as of doubtful age, and, even now, it is little more than a suggestion that they should be included in the Upper Old Red Sandstone. The fossils on which we rely for proving the presence of Upper Old Red Sandstone are Arclzanodon (Anodonta) jukesi,* found at Talgarth, 7Yz miles east of Newport, and a scale of Holoptychius observed by Muchison in a crag two miles north-: west of Crickhowel. t More recently fish-teeth allied to Carboni­ ferous forms, t and plants § which cannot be older than Upper Old Red, have been obtained by Mr. Dixon from Caldy and Kidwelly respectively. The Lower Old Red Sandstone, which consists chiefly of red marls with impersistent cornstones, and occasional conglom­ erates, has yielded a fairly rich list of characteristic fishes. The collections made by J. E. Lee and others near Newport and Abergavenny are in the Geological Department of the British Museum, and for further information concerning them reference

• Described bv Mr. R. B. Newton, Geot. Maf(.• dec. 4. vol, vi (,899), p. 245. + "Silurian System," ]839. pp. ]72. 588. 601. ~ "Summary of Progress for 190$." Mem. Geol. S.urver, p.~ 45. § «West Gower." Mem. Geol. Survey, 1907, p. 6. SOUTH WALES . should be made to the " Catalogue of Fossil Fishes," by Dr. A. Smith Woodward. Cardiff. *-In the neighbourhood of Cardiff three sub-divi­ sions of the Old Red Sandstone may be di stinguished: . (I) The uppermost strata, which pass up into the Ca rboni­ f erous sha les, consist of yellow and red sandstones with thin red marls, and,below these, of impersistent grey conglomerates .packed with pebbles of vein-quartz. · The upper par t of this sub-division corresponds to the strata which yielded Archanodon, and the whole of it is assig nable to the Upper Old Red. Thick­ ness about 2 0 0 ft. (2) A series cf dull-red grits with subordinate marls, const i­ tuting the Brownstones, No fossils excep t indeterminable plant­ remains have been obta ined, and the group, comprising a thick­ ness of about 500 It. , is assigned to the Upper Old R ed Sand­ stone with' some doubt.' (3) A mass of red marls with deep-red micaceous sandstones, all of Lower Old Red Sandstone age, and about 2,800 feet thi ck. The marls often contain calcareous concretionary nodules, which at times become so abundant as to coalesce and form a compact limestone or "cornston e." In other ca:ses a marl with nodul es appears to hav e suffered cont emporaneous erosion, the result of which has been to wash away nhe matrix and leave a residue of partly- rolled nodules forming a "conglomera tic cornstone." In the marls there sometimes occur true conglomerates contai ning well-rounded pebbl es of older rocks. Beds of this character are well exhibited north of Cardiff. These three groups are crossed in ascending order as we proceed from the Silurian inlier northwards towards the coalfield. The ba se has been recognised in the bank s of the Rhymney above Cae Castell, and aga in in the northern end of Roath P ark, as already men­ tioned . Near Coed-y-gores also there are fragments of con­ glomerati c sandstone and grit speckled with decomposed felspar, which not improbably belong to the actua l basement- bed. Above them comes a mass of red marl s, which may be examined in many old pi ts and road -sections on the way to St. Mellons. They con­ tain green bands and spots, and often show empty cavities, due probably to the weath erin g-out of calcareous matter. One thick ba nd contains large isolated grains of quartz, the dissemination of whi ch through a homogeneous mass of fine sediment is 110 ! easy to explain. The resemblance of Old Red and New Red marls is note­ worthy. Near Newport, where the one rests on the other with little or no conglomerate intervening, their distinction is far from obvious. The newer marl , however, is more homogeneous, and

* II Th e Country around Cardiff." M em. Geol . Survey, ] 9 0 2 , chap. iIi. l b. Il N ewp ort , U ]899, chap. 3. 832 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. does not contain the irregular sandstones and cornstones which characterise the older deposit. Its tint, moreover, is somewhat brighter. On the other hand, the older marl is devoid of the deposits of gypsum, celestine, and rock-salt which are familiar features ,in the New Red Marl. The resemblance, notwithstand­ ing, is sufficient to point to a general community in origin. The characters of both indicate land-locked water of restricted cir­ culation; in no respect do they suggest the products of an open sea. Comparison of the two marls may be made in several places on the north and south sides of the Silurian inlier, but more readily in the Cardiff Brick Co.'s clay-pit at Maendy, where the following remarkable section was laid open to view in 1901 :

ft. GLACIAL. •. Gravel, 6 ft. to. ••.• 20 Red marl with some green bands . 30 Breccia in red marl, in irregular beds; TRIAS (Keuper Marl) the" dolomitic conglomerate" of old authors, and the" granite" of 1the workmen •..•. 3° OLD RED SANDSTONE Purple marl with irregular sandstones, seen to •

The Keuper Marl dips at 9°, as compared with a dip of 20° in the Old Red Sandstone, and the unconformable junction of the two is visible all round the pit. On the east side of the pit there are two small north-and-south faults in the Old Red'Sandstone which do not affect the New Red Marl, while on the north side there is an east-and-west fault, which is obviously of post­ Triassic age, for it cuts through both formations. Another fine view of Old Red marls may be obtained in some clay-pits one mile north-east of Whitchurch. Resuming our northward- traverse from Roath Park, after passing over the marls we reach a red gravelly conglomerate, which crops out at Llanishen Station, but is better developed a mile and a half farther north, at the entrance to the tunnel. It contains well-rounded pebbles of reddish quartzitic grit of un­ known source, averaging about half an inch, but ranging up to five inches in length. Though so conspicuous here, this con­ glomerate is not known to occur elsewhere in Glamorganshire. It lies about 700 or 800 ft. below the top of the red marl sub­ division, and is, of course, quite distinct from the quartz-con­ glomerates of the Upper Old Red Sandstone. From the tunnel-mouth the ground rises rapidly to the escarp­ ment of the Brownstones, which is followed by the quartz-con­ glomerates and yellow or red sandstones already mentioned. The total thickness of the Old Red Sandstone is estimated at 3,500 ft., of which 2,800 ft. belong to the red marl sub-division, while SOUTH WALES.

200 ft. certainly, and 500 ft. more with some doubt, are referred to the Upper Old Red Sandstone. Gower.*-The development of Old Red Sandstone in Gower differs from that in any other part of South Wales. The com­ plete sequence can be ascertained along the southern sides of the Silurian inliers. In the first place, the marls, which are 2,800 ft. thick at Cardiff, have disappeared on the flanks of Cefn-y-Bryn. A little farther west they come in again, and reach a thickness of 700 ft. in Rhossili Down; but locally they are overlapped by the Brown­ stones, which in their absence rest directly upon the Silurian rocks. The Brownstones, while presenting their usual char­ acters, scarcely exceed 100 ft. in thickness, as compared with 500 ft. at Cardiff; but, on the other hand, the conglomerates above them have expanded to 200 ft., and are crowded with pebbles of vein-quartz and quartzite, with many angular jaspers. The lower part of the conglomerate is red, but the upper part is conspicuously white, and passes up into Lower Limestone Shales. These conglomerates and the Brownstones form a continuous andconforrnrable series, and though they have yielded no fossils in Gower, are assumed to be all of Upper Old Red Sandstone age on the strength of their passage up into the Carboniferous shales. The sudden overlap of the red marls seemed therefore at first sight to give the long-sought-for evidence of an unconformity between Upper and Lower Old Red Sandstone. It must be remembered, however, that the Brownstones themselves have lost four-fifths of their thickness, and the same argument would apply to them, in only a slightly less degree. The conglomerates alone have suffered no attenuation; in fact, they show a development which cannot be matched elsewhere in South Wales. The over­ laps can be best explained on the supposition that the Old Red marls were deposited on a very uneven platform, and that each member in succession overlapped its predecessor on the flanks of eminences in that platform. The red jaspers were found by Mr. Dixon to represent rocks of widely different origin. One was an igneous rock, probably a perlitic pitchstone; another had been highly calcareous, and was probably of sedimentary origin; while there were many grains of jasper representing several kinds of igneous rocks. All had been reduced to their present condition by silicification and reddening with hrematite, before their incorporation in the Old Red Sandstone. This form of alteration, as well as a rounding of the sandgrains in the Brownstones and their siliceous cement, recalls the phenomena observable in desert regions.r The passage up into the Carboniferous shales, to which reference has been made, can be seen in a dingle 300 yds, north­

• If The Country around Swansea," Mem. Geol. Survey, I!;07J chap. it. lb. ,I West Gowr-r," 1007, chap. if. t E. Dixon. jf West Gower," Mem. Geol. Survey, 19°'1, pp. 6, 7. 55 834 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. east of Fairy Hill House, at the western end of Cefn-y-Bryn, The section shows in descending order:

Gre-y shales and ostracods, and a band of sandstone with Iamellibranchs, Grey sandy limestone with fragments of grey and red limestone and quartz grains...... • about 4 ft. Red marl .. about 4 ft. Pebbly grey quartzite.

CARBONIFEROUS LIMESTONE SERIES. This series includes three main sub-divisions, namely: The Upper Limestone Shales, which are present only in that part of South Wales lying west of the Tawe Valley (Swansea Vale). In Gower they are not less than IS0 ft. thick. The Main Limestone, in which shales are thin or absent. The thickness of this sub-division amounts to about 700 ft. in the main escarpment north of Cardiff, but is probably double that amount in the Vale of Glamorgan, while in Gower it reaches 3,000 ft. The Lower Limestone Shales, which are about ISO to 200 ft. thick. These include in the upper part shales and slabby lime­ stones,and in the lower part a fairly persistent band of richly encrinital limestone, 50 to 100 ft. thick, which has been traced upon the maps through Monmouthshire and much of Glamorgan­ shire. In Gower they are about 450 ft. thick. The palreontological equivalents" of this lithological group­ ing are expressed in the following table:

UPPER LIMESTONE SHALES Dibunophyllum-zone, upper part. DibunOPhYllum.zone, lower part. Seminula-zotv»: Dark crinoidal lime. MAIN LIMESTONE 'lSyringothyris-zoneJ stone. . { Camma-oohte. Laminosa-dolomite. Zaphrentis-zone. LOWER LIMESTONE SHALES Clsistopora-zone. The southward or south-westward thickening of the limestone is a noticeable feature throughout South Wales. The series dwindles to its smallest dimensions at the north-eastern outcrop, near Abergavenny, where it is only 100 ft. thick, and in the out­ lier of Pen-cerig-caleh, near Crickhowel. Traced westwards along the northern outcrop it expands to 500 or 600 ft., which thickness it maintains for several miles, but in Pembrokeshire it dwindles again, and is finally overlapped by the Millstone Grit. Along the southern margin of the coalfield, on the other hand, it ranges in thickness from between 800 and 900 ft. near Cardiff • Using the classification established by Dr. Vaughan. Quart. Journ, Geoi. Soc., vol. \xi ('905), p. 181. SOUTH WALES. to about 2,400 ft. in Gower, with a still further increase in the scatte red outcrops to the south of that margin. The exact man­ ner in which the attenuation in the north-east is brou ght abo ut can only be ascertained by tracing the changes in each at the pal eeon­ tological zones enumera ted above , a work likely to yield results of great inte rest . T here is certainly some overlapping by the Mill­ stone Grit towards the north-east, for not only are the Upper Limestone Shales absen t, but there are obvious signs of erosion of th e limestone und er the basa l conglomerate of the Millstone Grit. The greater part of the decrease, however, is pro bably du e to attenua tion in each of the zones, as has been shown -by Mr. Dixon to be the ca se in Gower. T he original margin of th e limestone­ sea passed through Pembrokeshire, and doubtless was not far distant from Pen-cerig-calch and Abergavenn y. In fact, th e existing limits of th e formation, as defined by denudation, are a reflection of the origin al margin of deposition. Cardift.*-The limestone is characteristically exhibited in th e gorge at Walnut Tree by which the Taff escapes from th e coal­ field. Some of it is brown and bituminous, but it conta ins one or more bands of light-coloured oolite, while much of it shows a close approach to dolomite in containing 30 per cent. of car­ bonate of magnesia. The upper part of th e main limestone as develop ed on Cefn-on, four miles farther east, consists of pur ­ plish and mottled dolomite with partings of shale or smooth­ textured ar gillaceous dolomite. The outcrop of the Lower Limestone Shales is marked by a depression at the foot of the limestone-scarp, and that of the encrinital band by a su bsidiary scarp in the dep ression, features which are recognisable for many miles. Ca stell-coch is built upon thi s subsidiary scarp. The alteration of parts of the limestone into hsematit e is to be noticed both in Garth Wood and Fforest-fawr, on the west and east sides respectivel y of the T aff gorge. The ore occupie d cha m­ bers hollowed out along joints, bedding-planes, or any other pa rt ing which had given passage to underground water. The de­ posits were consequently most irregular in shape and size. The largest were found in Garth Wood, and were worked by a sha ft sunk at the top of the hill and by a level driven in from the foot. One of the ch ambers measured 60 [1. by 2 4 ft . There are excavations also, on a smaller scale, in Fforest-fawr, and in a gash-vein which cuts through the hill 200 yds. south of Walnut Tree Bridge. In other cases the ore has replaced beds of limestone, so as to take the form known t o miners as "flats." The lowest bands of the Main Limestone and the band' included in the Lower Lime­ stone Shales are th e most genera lly subject to the alteration, th e shal e probably having guided the water by which the iron was

'" H Th e Country aro und N ewport;" b-fem. Geol. Survey, 1899, pp . ~ : ·27 lu. ff Cardiff," I ~ 02 , pp. 2 2 -2 8 . 836 GEOLOGISTS ASSOCIATION JUBILEE VOLUME. introduced. In a thin band of encrinital limestone, near Rhubina, the crinoidal and polyzoonal structures have been re­ placed by opaque iron-oxide, but are imbedded in a clear crystalline matrix, so as to give microscopic slides of extraordin­ ary beauty. (See" The Country around Newport," Frontispiece.) Gower. *-The whole of the Carboniferous Limestone Series is represented in Gower, and all the zones established by Dr. Vaughan in the Bristol district have been recognised by Mr. Dixon, working partly in company with Dr. Vaughan. The Lower Limestone Shales (Cleistopora-zone) are seldom visible. Southgate and Reynoldston are both built upon them, but there are only a few road-side sections in dark shales and limestones. The shales are better seen at the Rhossili Rocket House and at Pitton Mill. The exposure of their base at Fairy Hill has already been noted. Their thickness has been estimated to be between 450 and 500 ft. at Cheriton. The Zap hrentis-zcue, according to Mr. Dixon, forms the cliffs between Rhossili, Worm's Head, and Tears Point, while the rugged coast from Mewslade Bay to Port Eynon exhibits a great development of crinoidal and oolitic limestone referable to the upper part of the Syringothyris-zone and the Seminula-zone. The Dibunophyllum-zone forms a ridge running from Overton to the sea at Sedger's Bank, north of Port Eynon,and the upper part of it, corresponding to the Upper Limestone Shales, is some­ times visible at low tide in the south side of Port Eynon Bay. The thickness of the zones composing the Main Limestone (p. 834) were estimated by Mr. Dixon in the southern coast, and along Barry Pill 011 the northern side of Gower. The net result of a southward expansion in every zone was an increase from 2,000 to 3,000 ft., but the greatest expansion was observed in zones above the Caninia-oolite. Including the Lower Lime­ stone Shales (450 ft.), and the Upper Limestone Shales (not less than IS0 ft.), the thickness of the Carboniferous Limestone Series in the south coast must be something over 3,600 ft. The Upper Limestone Shales and their relations to the strata above and below them can be best studied near Bishopston. There a road leading to Clyne Common occupies a.cutting in the topmost strata of the limestone-series, which are so rarely visible. The beds are somewhat crumpled, but consist of at least 30 ft. of white and yellow clays, and over them about IS ft. of more distinctly bedded clays, hard and soft, with thin irregular seams of limestone and chert. They contain wavellite (a hydrous phosphate of alumina) in discs of pale-green radiating crystals, and yield trilobites, brachiopods, etc., which are characteristic of the highest part of the Dibunophyllum-zone. Below them at a few yards distance the more massive lower parts of that zone crop out .

.. If The Country around Swansea," Afem. Geol. SU7veYt I~07, chap. iii. I b, OJ'Vest Gower," 1907. chap. iii. SOUTH WALES.

Upon these clays there rest black shales with impersistent seams of radiolarian chert, graduating up into a great mass of dark shales, the whole yielding a completely different fauna. We therefore select the base of the shales with radiolarian cherts as the division between Upper and Lower Carboniferous rocks. The relations of the cherts and shales to the Millstone Grit of other parts of South Wales will be discussed later on. The uppermost bands of the limestone-series usually occur in the form of clays and rottenstones, as a result of decalcification along the outcrop, but in a quarry at Oystermouth they show themselves as thin-bedded black limestones, commercially known as "black lias," which, indeed, they much resemble. The lower part of the Dibunophyllum-zone includes some remarkable strata, which are admirably exhibited near Mumbles and in Oxwich Point. One variety which attracts immediate attention was described by Mr. Tiddeman as "pseudo-brecci­ ated." It has the appearance of having been broken into small fragments and recemented. He noticed, however, that it characterised one horizon of the limestone for several miles, and on closer examination saw that the appearance was due to a form of dolomitisation. The subject was further investigated by Mr. Dixon, who found that a surface when polished shows the sup­ posed fragments to be dark patches of foraminiferal limestone with a calcite-matrix, while the intervening lighter parts consist chiefly of large dolomite-crystals, but include some foramini­ feral limestone also, which is continuous with that of the darker patches. (See Plate I in" The Country Around Swansea.") Some of the bedding-planes of this same group of strata show pitted surfaces. They are dotted over with shallow depressions, which appear to have been originally filled with clay. With these there are commonly associated coaly beds, possibly formed of fucoids and containing minute gasteropoda. For this group of strata, which is upwards of 200 ft. thick, Mr. Tiddeman pro­ posed the name Mumbles Head Beds. In continuing his researches on dolomitisation, Mr. Dixon found means to distinguish contemporaneous dolomites from vein-dolomites. * As an example of the former he quotes the Mumbles Head Beds and the Laminosa-dolomite, while speci­ mens of subsequent dolomitisation abound in the faults and fissures between Mumbles Head and Pwll-du, and again in some light-coloured limestones on the west side of Port Eynon.

MILLSTONE GRIT. Along the northern and eastern margins of the coalfield the Millstone Grit has been sub-divided into a sandstone at the top, known as the Farewell Rock, a mass of shales with subordinate

* .e The Country around Swansea," Mem. Geol. SU1'vey, I9Q7, pp. 13-20. 838 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. sandstones in the middle, and a purely siliceous grit at the bot­ tom, often packed with pebbles of vein-quartz. This Basal Grit is sharply marked off from the underlying limestone, and not infre­ quently fills hollows eroded in the surface of that rock. Upwards the three sub-divisions of the Millstone Grit shade one into the other, and the Farewell Rock graduates into the Coal Measures. The divisional lines are arbitrary. The series expands southwards and westwards, and in so doing changes type; the grits cease to be conglomeratic, and finally, in the most southerly exposures, disappear, leaving shale, with subordinate sandstones only, to represent the formation. As an example of the first type we may take the Millstone Grit of the moorlands near the head of the Vale of Neath, and as an example of the second, the shales of the Bishopston neighbour­ hood. An intermediate stage is shown near Bridgend. In the outcrop which lies nearest to Cardiff, various stages in the south-westerly expansion of the Millstone Grit can be deter­ mined. North of Risca the Basal Grit is absent in some places, and represented by lenticular patches of quartz-conglomerate in others; the Farewell Rock, on the other hand, is persistent and often conglomeratic. At Risen the Basal Grit develops, together with the shales above it (which here contain a small seam of coal) j but the total thickness of Millstone Grit is only 462 ft. South of the Rhymney the lower grit disappears again, leaving shales next above the limestone. Twelve miles farther on again the lower part of the Millstone Grit consists of alternations of grit and conglomerate with shales, and the tota.l thickness of the formation amounts to 1,4°0 or 1,500 ft. In Gower the grits have disappeared, and in their place we find the shales shown near Bishopston. These are exposed, above the radiolarian cherts referred to previously, to a thickness of 1,500 ft., and are followed by a considerable further mass; above these there is a sandstone which possibly corresponds to the Farewell Rock. It must be admitted that at first sight the Bishopston sequence seemed to have little in common with that of any other part of South Wales. But it was clear that in the topmost part of the Dibunophyllum-zone we had a reliable foundation from which to build upwards. The strata of this sub-zone had long been known on the north side of the coalfield, where they had been worked for" rottenstone," the porous and impalpably fine residue of decalcified limestones. These were followed by about 50 ft. of clays with some radiolarian chert, and then by grit and quartzite of the usual character of the Basal Grit. It appeared, therefore, that the grit and quartzite corresponded in stratigraphical position with the dark shales of Bishopston. The palreontological evidence confirmed this view. Posi­ doniella laois, Pterinopecten papyraceus, Glyphioceras bilingue, G. spirale, G. reticulatum, G. diadema, Lingula, and others SOU)'H WALES. were obtained from Old Castle and elsewhere on Barland Com­ mon to the north of the Bishopston road-cutting. They may be compared with lists given in "The Country Around Aberga­ venny," pp. 47, 48, and" The Country Around Merthyr Tydvil," pp. 49, 50, in which Pterinopecten papyraceus, Posidoniella lavis, Glyphioceras bilingue, and G. reticulatum are recorded from the Millstone Grit, together with many others. We there­ fore class the radiolarian cherts and the shales above them as Millstone Grit. The radiolarian cherts crop out in the north side of the road­ cutting. They show thin white lenticular streaks, and were found bX Dr. G. J. Hinde to contain some species identical with those from the Culm chert. No other fossils occur. Above them come black cherts without white streaks, and black cherty shale followed by at least 1,400 ft. of the shale which underlies Bar­ land Common, and yielded the fossils enumerated above. The Pendleside Series of Dr. W. Hind must be represented by a part of these shales. The fauna, in his opinion, though limited is definite. Moreover, a single fragment, identified by him as Posidonomya becheri, was obtained at Port Eynon by Dr. Vaughan and Mr. Dixon. It is quite impossible, however, to distinguish Pendleside Series from Millstone Grit, either at Bishopston or to the north of the coalfield. A marine fauna not differing essentially from that of the shales extends up into the productive Coal Measures, and no line, stratigraphical or palseontological, save of the most arbitrary description, can be drawn for the top of the Pendleside Series. The only other section worth mention occurs at Port Eynon, where the shales, overlain by Triassic conglomerate and stained purple, have been dug for the manufacture of paint. The chert­ beds are faulted out, but that they exist in the neighbourhood is proved by an abundance of fragments in the drift on the coast south of Overton.

COAL MEASURES. The Coal Measures of South Wales have been sub-divided into an Upper Coal Series, a Pennant Series, and a Lower Coat Series. There is perfect conformity from top to bottom of the three series, and they are distinguished chiefly as a matter of convenience. No correlation with the Upper, Middle, and Lower Coal Measures of other coalfields. is implied. As regards thickness, the Coal Measures follow the same general rule as the other Carboniferous formations. They are thinnest in the north and east, thickest in the south and west. The Lower Coal Series, for example, expands from 625 ft. at the eastern end of the coalfield to 1,400 ft. in the central part, and to upwards of 4,000 ft. at the south-western margin. ThE' 8 40 GEOLOGI ST S ' ASSOCIATION JUBILEE VOLUME.

Pennant Series expands no less rapi dly, and as it does so the massive sandstones which characterise it in the east become split up by shales with coal-sea ms, for which reason the name Pennant Series was substituted for the old name Penn ant Sandstone. The greatest thickness of Coal Measures believed to exist anvwhere in South Wales amounts to 8,237 ft. This is founded onrecent measurements in th e Loughor Va lley. A shaft sta rting in the U pper Coal Series reached its base at a depth of 1 ,037 ft . At this spot the Pennant Series may be estimated to be 3,200 ft. t hick , without serious error. The Lower Coal Series is more d iffic ult to estimate, but is not likely to be less than 4,0 0 0 ft. In the part of the coalfield nearest to Cardiff these three sub­ d ivisions present their cha racteristic features. The outcrop of the Lower Coal Series, with its numerous coal -seams and shales with argillaceous ironstone (" mine-ground "), is marked by a swampy depression, which is almost continuous round the coal­ field. From it the ground rises sharply to the Pennant esca rp­ ment. Surmounting this we gain a moorland plateau, from which we descend again into low ground underlain by the Upper Coal Series of the Caerphi lly and Llantwit syncline. T1)e synclin e is followed immed iately by th e P ontypri dd anti­ cli ne, which is one of the most import ant structural features in the coa lfield. It sta rts betwee n the Sirhowy and Rhymney valleys, and, passing under Mynydd Egl wysilan and Pontypridd, develops into a great monocline, from which the beds plunge down southwards at angles of 40° to 50°. F art her west again it passes into a fau lt, the effect of which is to throw up the beds on its north side about 3,0 0 0 ft. As a result of this fold and fractur e, coals which might have been inaccessible are bro ught tip well within the limits of deep mining . In former days " mine " or ironstone was more energetically worked than coal. No " mine " is now raised, and th e coal­ field owes its reputat ion chiefly to its smokeless steam-coal. Th is is worked from seams in the Lower Coal Series on the north side of the anticline, and in this part of the coal-field is reached only by deep shafts, for the same seams where they come to the surface in the South Crop are bituminous. It was the discovery of the existence of thi s steam-coal which led to the development of th e Rhondda Vall eys about 70 years ago, and which is the cause of most of the enterprise exhibited in other parts of the coalfield. Levels driven into the sides of the hills are now comparatively little used. The seams most in requisition are not accessible by this method of working, and those which can be reached are either worked out or of inferior quality . One example, however, deserves mention. At the Cwrn Bran Colliery, north of New­ port, a level, sta rting in the Old Red Sandstone, has been dr iven under the hill -ranges which enclose the coalfield . It passed SOUTH WALES.

through all the Lower Carboniferous rocks in succession till it intersected the principal coal-seam at a distance of 2,070 yards from the mouth. In the part of the coalfield nearest to Cardiff the principal seams worked are those in the Lower Series, the two Rock Veins at the base of the Pennant Series and the Llantwit Veins at the base of the Upper Series, one of these being known also as the Bedwas Vein in the Caerphilly Basin. The Rock-fawr Seam, which, under a, variety of names, has been traced all round the coalfield, is overlain by a massive sandstone in which there occur conglomeratic bands filled with pebbles of quartz, ironstone, coal and shale. These attracted the attention of early observers in Garth Hill, but they are by no means confined to this locality or to this horizon. They may be attributed to the same sort of contemporaneous erosion as that which formed the conglomeratic cornstones in the Old Red Sandstone (p. 831). A sudden increase of strength in the current which distributed the sediment would account for them. The coal, however, appears to have had time to harden before it was broken up. Near Swansea, and along the north side of Gower a different set of seams has yielded the bulk of the coal. Several good seams occur in the upper part of the Pennant Series, and the lower part alone retains that massive character which charac­ terises the Pennant Sandstone in Monmouthshire. Several coals are worked also in the upper part of the Lower Coal Series, but not much has been done upon the lower coals. All along the northern side of Gower the lowest measures are sharply folded. The coals are bituminous notwithstanding, and thus furnish one example among several of disturbance of strata not being associated with anthracitisation of coal. The most prominent physical feature is the escarpment of the lower Pennant Sandstones and the Llynfi Rock (also known as the Cenrhos and Tormynydd Rock), while the sandstones in the upper part of the Pennant form parallel subsidiary ridges. The Llynfi Rock is the name given to the upper part of the Lower Coal Series by Mr. Tiddeman after the valley of that name. It exists in Monmouthshire, but it was not until the great expansion of the measures westwards and the increasing im­ portance of these sandstones was recognised, that a distinctive colour was adopted on the maps for them. The sandstones of this group show no essential difference to Pennant Sandstone. They crop out in the southern faces of Kilvey Hill and Town Hill, on the east and west sides respectively of the Tawe, and are quarried at Dunvant. The lower Pennant Sandstones form the crests and the northward dip-slopes of these hills, passing thence under the Hughes Vein, which introduces the less massive strata associated with the Two-foot, Three-foot, Six-foot, and Four-foot Veins. 842 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME.

This brings us to the close of the palseozoic age, so far as regards any formation existing in South Wales. The Carboni­ ferous Series is certainIy not complete now, but how much has been lost by denudation we have no means of ascertaining. When the stratigraphical record recommenced, the palreozoic strata had been subjected to that powerful folding referred to in the introductory remarks, and we may pause here to inquire how far the main physical features of the district were developed by the denudation which ensued upon that folding. This we can ascertain by determining the limits of the area which was sub­ sequently overspread by the earliest Secondary Rocks. So far as regards the Vale of Glamorgan, Gower, and the neighbouring parts of the Bristol Channel, the evidence is clear. Though almost the whole area is underlain by Old Red Sandstone and Lower Carboniferous Rocks, and though these massive rocks were thrown into sharp folds, yet Triassic and Liassic waters overspread the whole of it. Some prominent ridges, such as the limestone-escarpments. on the two sides of the Cowbridge-Cardiff anticline, or the Old Red Sandstone hills of Gower, kept their heads above water through Triassic and part of Liassic time, but there is no reason to doubt that they were all overwhelmed eventually, the region passing through a geographical stage such as that represented on the map, Fig. 133. On the other hand there is no suggestion that either Trias or Lias overspread the Pennant uplands. In one place only the Trias touches the foot of the Pennant scarp; it there ends off against the steep face, much as a recent lacustrine deposit ends against a lake-bank. It would appear, therefore, that the synclinal area of the coalfield had already asumed its character as an upland, while the anticlinal region to the south of it had already been planed down to a lowland, when the deposition of Keuper Marl commenced. Farther than this it is difficult to go. The abnor­ mality of the Rhsetic sediments around Bridgend suggests that there may have been a river entering the Rhsetic sea not far off, but it is not possible to indicate the site of its mouth.

TRIAS. The Keuper Marl is the earliest Triassic deposit represented in South Wales. Near Newport and Cardiff it presents a per­ fectly normal appearance as a bright-red marl, variegated with green spots or bands and associated with lenticular masses of gypsum, or more rarely of celestine. From Cardiff westwards, however, an increasing proportion of it passes into the condition of a breccia, derived from the shores of a number of islands which broke the surface of the Triassic lake. The breccia, it will be understood, is not the basement-bed of the Keuper formation, SOUTH WALES. but represents a condition which each bed of marl in turn assumed in the neighbourhood of a coast-line. Some of the islands survived through Keuper into Liassic times, and near these the Rhsetic and Lias in their turn also assume a littoral condition. The accumulations which gathered along the margins of the Keuper, Rhsetic and Liassic waters vary in a manner which throws much light on the physical conditions prevailing during those periods. The littoral Keuper consists of rearranged Old Red Sandstone conglomeratic material, or more commonly of fragments of Carboniferous Limestone, which are little worn, though they may have travelled half a mile or more from their parent-cliffs. Occasionally a clay-band inosculating with the brec­ cias will show ripple-marks, or more rarely footprints, sun-cracks and rain-pittings, from which we gather that the low-shelving foreshore was liable to be laid open to the air. The Rheetic shales, on the other hand, give place to calcareous marls, sandy oolitic limestones, limestones composed of shell-debris, and finally massive sandstones. Though fragments of Carboniferous Limestone occur throughout most of the littoral Rhsetic, they are not thoroughly rolled, nor are there any shingle-beds. The littoral type of the Lias includes massive oolitic limestones with much chert, and a cavernous rock called Sutton Stone, com­ posed of shell-debris, cemented, but not completely infiltrated, with carbonate of lime. Galena in isolated crystals is common and glauconite occurs. The included fragments are generally water-worn, but there are some bands of angular material. The deposits suggest that neither the Rhretic nor Liassic seas were ever much troubled by powerful tidal currents or heavy surf; there is no indication of either in the littoral Keuper. So complete is the story told by these shore-deposits that it is possible to reconstruct the geography of the region at succes sive stages from Keuper to Lower Lias times. Early in the Keuper period a continuous sheet of water lay to the eastof Cardiff. Westward it was broken by islands, the form of which is closely reproduced in the limestone-inliers of Cadoxton and Michaelston, while a group of smaller rocks was dotted about near Barry and Sully. Farther west again a large tract of land, separated by a strait from the Michaelston island, extended past Cowbridge to the sea near Southerndown. The northern margin of this polynesia coincided approximately with the southern margin of the coalfield. Later on, continued subsidence of the land led to invasion of the whole area by the Rhretic sea. The invasion was simultaneous over a large region, as we may judge by the abrupt change into a Rhretic type of sediment and to a Rhretic fauna at the same horizon everywhere. We may therefore take all the points at which Rhsetic overlaps Keuper as having been situated on a Triassic contour-line, which defined the areas still 844 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. remaining above water when the invasion took place. The map forming Fig. 133 (showing Triassic geography) gives the distri­ bution of land and water as thus determined. Other maps, less complete, could be prepared, showing the gradual disappearance of the last of the islands during the period of the Lower Lias. Cardiff.*-Good exposures of the marl are to be found in a cutting on a mineral branch of the Taff Vale Railway on the west side of the Rhymney and in a brick-pit north of Roath. The base is, or used to be, visible in the Cardiff Railway, east of the Heath, but is shown to perfection in the Maendy Brick­ pit (p. 832). The littoral type is quarried at Radyr and has been much used, under the name of Radyr Stone, for building. The quarries show about 60 ft. of rock containing many fragments and pebbles from the Old Red Sandstone and sub-angular fragments of Carboniferous Limestone. The floor of the quarry is formed by the platform of Old Red marl on which the Radyr Stone rests. The Penarth cliffs show the upper part of the marls only, but give an unrivalled view of the passage of the red marls into the" tea-green beds," and of the Rhsetic Shales above. The red marls are remarkably homogeneous, but contain a few green bands or spots, and occasional lenticular masses of gypsum arranged along bedding-planes. At Lavernock, and in the low, rocky coast beyond it, the Keuper Marl assumes the littoral type. It comes into view on the west (upthrow) side of the Lavernock fault, and is continu­ ously exposed thence to near Barry. The unconformable super­ position of this rock upon the Carboniferous Limestone is beau­ tifully shown upon both Barry and Sully Islands. The follow­ ing section was measured upon the latter: ft. Red marls with mottled sandstone 18 Red and white limestone .... 14 Red marl with calcareous concretions and fine breccia . 10 KEUPER Red marl and marl-rock 14­ I mpersistent limestone. . I Red marl-rock and fine breccia. . 8 Limestone crammed with fragments of Carboniferous Limestone. .. 2 CARBONIFEROUS I L' di I f 0 0 LIMESTONE . j rmestone 'ppmg at ang es 0 4-0 to 50 These breccias and littoral deposits vary from yard to yard. but the section quoted shows the types of rock commonly devel­ oped. The limestones, except of course the included fragments, are all inorganic; many, if not all, are formed of fine limestone­ detritus. Some cuttings on the Taff Vale Railway, between Sully and Lavernock stations, also give excellent views of littoral Keuper.

* If The Country around Cardiff." Mem. Geol. Survey. 1902, chap. v. I o., If Bridgend," 1904, chap. v. SOUTH WALES.

~ ~ n '-.., ~ V) ;; ~

~ •~ , 0 " •~ ? :--

[j[(

~ " .•> ...., '" 0 I ,.. "0 >. '"~ ,b ,. s ~ ~ ~ " ,.. E ~ <> ,.. ~ " Z '"~ • ~ :-- .,....l .. ~ ~ ;: 846 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME.

The" tea-green marls" were formerly included in the Rhsetic group,* but have been placed with the Keuper Marl in the recent revision of South Wales, t on the.ground that they do not contain the characteristic fauna of the Avicula contorta-zone, and are separated from those shales by a plane that is always well-defined and frequently shows signs of slight erosion, while, on the other hand, they graduate imperceptibly down into the red marls. In 19°5 Mr. Richardson, t as one result of an exhaustive examination of the Rhsetic sections in Monmouthshire and Glamorganshire, claimed that the uppermost few feet of the green series ought to be classed with the Rhsetic on the strength of their containing Ostrea bristovi, together with Avicula contorta and other characteristic Rheetic forms. For these few feet he proposed the name" Sully Beds." The contention has not 'been made good. At the base of the Avicula contorta-Shales there is a little impersistent con­ glomerate occupying hollows in the surface of the green series. It is true that such characteristic Rhsetic fossils as Protocardium rha:ticum, Avicula contorta, and Pecten valoniensis were found by Mr. F. T. Howard below this basement conglomerate, but they lay in a film of black shale a quarter of an inch thick which rested on "a worn surface of the tea-green marls."§ The film clearly formed part of the black shales, not of the green, series. At St. Mary's Well Bay Avicula contorta was found byMr. Richardson "in a thin layer at the top of the' Sully beds,' " and the find is associated by him with that of Mr. Howard. Pre­ sumably it occurred under similar circumstances. At Cadoxton " Pteria ( Avicula) contorta and N atica ( ?) " are recorded by Mr. Richardson in a table, and in the text other Rhretic lamellibranchs are referred to, but not specified. The only mollusc left therefore beyond doubt is the Ostrea bristoui. This certainly occurs in the "tea-green marls." Though a specimen was obtained many years ago its exact horizon was not recorded, and it was rediscovered by Mr. Richardson, in the "Sully beds." But whereas this fossil is abundant in the "Sully beds," it is not known in the Rhsetic, The claim, there­ fore, that the Rhretic fauna extends down into the "tea-green marls" has not been made good so far as regards the molluscs. The other fossils yielded by the "tea-gr.een marls" include Gvrolepis alberti, Hybodus minor, and Saurichthys, from Gold­ cliff; teeth of Spha:rodus, Pala:osaurus, T'rematosaurus, and Mastodonsaurus, from Lavernock. The" tea-green marls" are admirably laid open midway be-

* H Notes on the Rhretic Beds and Lias -of Glamorganshire," by H. B. Woodward. Proc. Geol.. Assoc., vol. x (1888), p. 5-3!. t II The Country around Cardiff." Mem. Geol. Survey, 1902, pp. 38-39, etc, t Quart. Jou rn, Geol. Soc.. vol. Ixi (IgOS), p. 374; io., p. 335, and see ante. p. 332. § Trans. Cardiff Nat, Soc., vol. xxix (1896-7), p. 66. SOUTH WALES. tween Penarth and Lavernock Point, at that point, and again in St. Mary's Well Bay. In the last named Ostrea bristovi is abundant in a rock-band immediately below the base of the Rhsetic black shales. The littoral type is assumed by the "tea-green marls" a few miles west of Cardiff, in the neighbourhood of some of the islands shown in Fig. 133. The marls become more rocky and inosculate with breccia, but preserve a green or yellow tint, which distinguishes them from the littoral type of the red marl. Generally the littoral aspect is assumed first by the older beds, as would naturally be the case with a subsiding land-area. Thus a littoral type of green marls may be overlain by Rhsetic shale of normal aspect. At Pencoed the reverse is the case; the green marls present a normal appearance, though the Rhsetic shales have passed into the condition of massive sandstones. GtrdJer.*-At Port Eynon a small outlier of Triassic con­ glomerate has survived denudation owing to its being situated in a slight hollow in the plateau, where shales crop out. The boulders are large and rounded, and consist chiefly of Carboni­ ferous Limestone and chert. Half a mile west of Mewslade Bay a chasm, due to collapse along a line of fault, intersects the cliff from top to bottom. The collapsed masses at the bottom are imbedded in a deep-red sandy marl, variegated with streaks and spots of green. Obviously the 'chasm was collapsing in Triassic times. Elsewhere in Gower the remarkable perfec­ tion of the plateau and an occasional hsematite-vein are all that is left to testify to the former extension of the Trias.

RHlETIC BEDS. t

This group excludes the "tea-green marls," as already ex­ plained, but includes the representatives of the White Lias. Like the Keuper, it occurs in a normal and a littoral type. The former prevails throughout the area included in the one-inch map, sheet 263, but the littoral type sets in to the west of it. For the normal type we may turn to the Penarth and Lavernock cliffs. The sections at the Penarth Docks, from which Etheridge obtained his measurements, are not easy to examine, and at Penarth Head the Rhsetic beds are faulted and only partly accessible. Better sections occur at Seven Sisters (The Stairs on the one-inch map), and at Lavernock Point, where the follow­ ing details were observable in 1901 :

... West Gower." Mem. Geot, Survey, 1907, chap. vii. f H. B. Woodward. .. Notes on the Rhsetic Beds and Lias of Glamorgan­ shire," Proc, Ceol. Assoc., voL x (1888), p. 529; Mem. Geol, Survey, l< Jurassic Rocks of Britain." vol. iii, J893, pp- 119-1:22: ib. II The Country around Cardiff," 1902, chap. vi; L. Richardson. .. The Rhsetic Rocks of Monrnouthshire." Quart. !OUYH. Geol. Soc., vol. lxi (19°5), p. 374. t' The Rhretic Deposits of Glamorganshire," io., p. 3851 and see ante, chapter xv, 848 GEOLOGI STS ' ASSOCIATION J UBILEE VOLUME.

ft. in. Limestone with Ostrealiassica .­ L OWE R. Ostrta-bed s. a hard lami nated sha le LIAS. (" slat y bed ") 9 in. th ick at its base. ... IPale-blu e shaly marl with Modio/a minima, Ostre a, P leurophorus, etc., in a ban d near the base . . . 6 6 Smooth limestone with part­ ings of sha le, probably the Sun-bed; Plica/lila int us­ str iata." P. ht t/a ngiensis,' Lima ualoniensis." Ostrea, W hite Lias. Modiola'" o 10 Pale-blue marl and ripple­ mar ked gr it Wh ite or pale - blue marl, White Lias supposed ; Pecten fJ aloniensis ,' Schizo­ dus Iwaldi,* Protocardium philippianum,' Cardium cloacsnum ." Car dinia, * lignite,' 2 ft. to " 2 6 Dar k shale, A vicula contorta. " P ecten oakmie nsis" . 5 6 " Beef " and limesto ne . o 3i Sha le, A uicula contorta;" Geruillia pr

LIAS. A part of the Lower Lias only survives in South Wales. The zones represented are enumerated as follows by Mr. H. B. Woodward :* Zone of Ammonites rArietites] semicostatus. [ArietitesJ bucklandi. " [SclzlotlzeimiaJ angulatus. " " [PsilocerasJ planorbis (including the " " Ostrea-beds). At Lavernock the Lias can be examined both in cliff and foreshore, the sequence being continued upwards from the section given on p. 848. Lavernock Point is formed by the Ostrea-beds resting on the Rhsetic shales. The rocks dip to the south, and on rounding the point we find the Ostrea-beds dipping beneath the foreshore and succeeded by shales and limestones with abundant impressions of Ammonites (Psiloceras) planorbis, with A. belcheri, A. jolznstoni, and small examples of Lima gigantea. A few yards farther on these, in turn, dip beneath a mass of shales almost devoid of limestones, for which the name of Laver­ nock Shales is used by Mr. Woodward. Above these again, but in an inaccessible position, there lie about 25ft. of lime­ stones and shales. As we pass the axis of the syncline the strata flatten and then, rising gently, present the same sequence in reverse order. The Ostrea-beds are about 20 ft. thick and, in addition to an abundance of Ostrea liassica and Modiola minima, contain Plicatula spinosa, Lima (Radula) duplicata, L. punctata, Pleuromya crowcombeia, Monotis decussata, and Carpenteria, They merge upwards into the shales with A. planorbis, and with them form the zone of that ammonite. Fine specimens of Otozamites hennoquei, from Penarth, are preserved in the Cardiff Museum. Cardinia ovalis from the Lavernock Shales, and Mont­ livaltia from the limestones above, are recorded by Mr. Richard­ son. The shales were considered by Mr. Woodward to represent the Bucklandi- and Angulatus-zones, and the overlying limestones to belong to the Semicostatus-zone. The lowermost beds of the Lias are shown also in brick-pits at Lower Penarth, and the Lavernock Shales have been cut into to the east of Lavemock Station.

• Mem, Geol. Survey, If Jurassic Rocks of Britain." vol. iii (1893), p. 121. SOUTH WALES.

For an account of the littoral type of the Lias reference should be made to Mr. Woodward's" Notes on the Rhsetic Beds and Lias of Glamorganshire" (Proc. Geol. Assoc., Vol. x, 1888, p. 529), and to the Geol. Survey Memoirs, "The Jurassic Rocks of Britain," vol. iii (1893), and" The Country around Bridgend" (190 4)' THE RAISED BEACH. Though the existence of a Raised Beach in Gower has long been known, its age was in doubt until 1900, when Mr. Tiddeman a.rnounced that he had obtained proof that it lay below the glacial drift of the neighbourhood. * The beach lies at a, height of about 25ft. above Ordnance Datum and consists of three distinct members which occur in a definite order wherever they are present. (See Fig. 134.) I. The highest member, or "head," is a talus of angular fragments of limestone, obviously derived from the neigh­ bouring cliffs, and containing no other rocks. 2. Under this and mingling with it is a foxy-red sand, very variable in thickness and containing an occasional snail­ shell. This is evidently a blown sand. 3. The lowest member is a layer of well-rounded pebbles of limestone, crowded with shells of recent species such as Littorina littorea, L. rudis, Patella uuigata, Purpura lapillus, Cardium edule, Tellina balthica, Balanus. This shingle rests on a water-worn platform cut out in the limestone, 10 to IS ft. above the modem foreshore, and keeps at a fairly constant level. It varies from a few inches to upwards of 4 ft. in thickness. The "head" (I) and the shingle (3) are always firmly cemented by carbonate of lime; the sand (2) is occasionally almost loose. The sections in which proof of the pre-glacial age of the beach may be best obtained recur along the south coast of Gower, and the description of one is applicable almost equally to all. Upon the "head" (I) and sharply separated from it there lies a mass of very roughly stratified' gravelly drift, crammed with boulders more or less rounded and often striated. The boulders consist chiefly of Carboniferous sandstones and Coal Measure rocks with many of Old Red Sandstone, while, on the other hand, they include few, if any, of limestone. The deposit corresponds precisely in character with the glacial drift of the neighbouring parts of Glamorganshire, and upwards of 40 ft. of it are superimposed upon the raised beach in some of the coast-sections. The best localities for observing the complete sequence are: IS0 to 250 yards east of Rams Tor j between Rams Tor and Langland Bay j between Whiteshell Point and Caswell Bay j and from Caswell Bay to Hareslade j in SevenSlades j at • Geoi, Mag. dec, 4. vol. vii, (1g00), p. 441, 852 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. various points between Oxwich Point and Mewslade, and lastly on the 'Vanni's Head. In some of the more westerly occurrences there are a few pebbles of Old Red Sandstone in the shingle (3), and more' rarely some fragments which look like chalk-flints, but are possibly Carboniferous cherts. Both Old Red Sandstone and chert occur in situ in the Gower cliffs.

THE GOWER CAVES. The proof of the pre-glacial age of the raised beach throws light on the age of the contents of the Gower Caves. Many caves and fissures in the limestone have yielded bones, but the best results, from a geological point of view, were obtained in

FIG. I34.-GENERAL SECTION OF THE DRIFTS, GOWER COAST.-R. H. Tiddeman, rrr­ I f I ,I"'; , ~ !, -"/1A J" ., Q. ..--'"'"...... -.--r',--.-••...--:~ f/ I".d;L •• , F: • ••••: 'f I/• • ~~-"-ZS;i// i t : , E . •, 'I ', ,;:..:::~. a.D;' ._ • e .... s .. /;1/,'/1/,' // I I /.' p '. __U L (;, I -, 1 . ' -----_ .... _~-- A. Present Beach. E. Lower Head. B. Raised-Beach Platform. F. Glacial Beds. C. Raised-Beach. G. Upper Head Post-Glacial. D. Blown Sand. a.D.L. Sea Level. d 2 Limestone. Bacon Hole and Mitchin Hole. They were explored by Colonel Wood and Dr. Falconer, and described in "Palreontological Memoirs and Notes of the late Hugh Falconer, A.M., M.D., V.P.R.S." London, 1868. Bacon Hole, according to an account by Mr. Starling Benson, * contained the following deposits: Dark-coloured earth (ancient British pottery); stalag­ mite with limestone-breccia (Ursus, Bos}, ochreous cave-loam and dark sand (Elepltas antiquus, Rhinoceros leptorltinus, Hya:na, wolf, Ursus, Bos, Cervus, Meles taxus, and Putorius): Stalagmite; yellow sand abounding with shells of Littorina rudis and L. littoralis, with some of Clausilia nigricans which appeared to have been deposited after the cave had become freed from salt water by elevation of the coast. Mitchin (Minchin) Hole showed a somewhat similar

• Swansea Lit. and Sci. Soc., Annual Report for 1857, p. 10. SOUTH WALES. assemblage, and at the bottom a marine sand which yielded a considerable quantity of bones of Rhinoceros hemitechus [R. lcptorhinus, Owen], together with Elepltas antiguus, Bison priscus (common), Hya:na spelcca, Canis lupus, and Ursus spela:us. The marine sand of the caves was thought by Falconer almost certainly to correspond to the raised beach, for they differed so little in level there was hardly any room to doubt that they belonged to the same series of deposits and to the same period of upheaval. The marine sand therefore, with its mammalian remains, preceded the glaciation of this region.

GLACIAL DEPOSITS. The southern margin of an ice-sheet which had its ongm somewhere to the north of the coalfield, and travelled in a south­ south-easterly direction, lay near Cardiff. The origin is deter­ mined not only by the direction of the striee which cross the northern part of the coalfield, but chiefly by the nature of the boulders. These consist of Old Red Sandstone, Pennant, and quartz-conglomerates, and may be traced, with a gradually decreasing proportion of Old Red Sandstone, across the coalfield to the southern margin, where they were reinforced from the south crops of the Millstone Grit and Old Red Sandstone. The direction of the flow agreed in the main with that of the existing river-systems; the ice, like the rivers, made for the Bristol Channel, though, unlike the rivers, it did not everywhere reach it. In Monmouthshire it went round the great north-western shoulder of the coalfield by Usk ; in East Glamorganshire it moved south­ south-eastwards to near Newport, Cardiff, and Bridgend; in West Glamorganshire it travelled south-westwards to Swansea and Gower. That the ice did not always reach the Channel is proved by a fairly well-defined limit to the glacial deposits which runs from near Newport by Cardiff to Cowbridge. South of this line there is a conspicuous absence of all evidence of glacial action except for two or three boulders. The deposits thin off gradu­ ally at their southern margin, without any semblance of a ter­ minal moraine. Cardiff.*-Near Cardiff the drift generally is of a gravelly type and displays the usual features of mounds, ridges, and water-logged hollows. There are sections of it in the Cardiff Brick Co.'s pit at Maendy (p. 832), in railway-cuttings near St. Mark's Church, in the Taff Vale Railway near Whitchurch, and in many other places. Much of it is coarse and roughly strati-

• 1/ The Country around Cardiff," It/em. Geot, Survey, 1902, PP. 78-81. lb. II Newport," 1899', pp. 8:l-8S. 854 GEOLOGISTS' ASSOCIATION JUBILEE yOLUME. fied, but it contains a few bands or pockets of sand mixed with coal-dust. A few fragments of what look like chalk-flints occur, but these are possibly Carboniferous or Liassic cherts. Very rarely a far-travelled erratic of a rock foreign to South Wales (except possibly Pembrokeshire) has been found. Such erratics occur more abundantly at Pencoed, near Bridgend, and in Gower. Taken with other circumstances, they prove that the Bristol Channel was invaded for a certain distance by ice from the westward, which belonged to a system altogether distinct from that of Brecknock and Glamorgan. In the Caerphilly basin the habit of the glacial sands and gravels to form mounds and ridges is admirably illustrated. The post-glacial work of the Rhymney river has lain in the excavation of ravines through the ridges and the levelling-up of the hollows to form its gradient. Gower and Swansea.*-Near Swansea and Neath the ice reached the Bristol Channel in great force. The hills near the latter town are glaciated to their summits, and prodigious quan­ tities of gravelly drift were brought down from the Old Red Sandstone and Carboniferous rocks to the north-west and shot upon the lower ground. Under a northern suburb of Swansea glacial graver was met in coal-workings at a depth of IS0 ft. below Ordnance Datum and 300 ft. below the surface of the ground. In the absence of the gravel this part of the Tawe Valley would form a ": sound." At Glais the gravel forms a remarkable dam-like moraine about 140 ft. high. The dam starts from the steep hill-side on the east of the Tawe Valley and runs for about a mile across the valley, leaving a narrow gap for the passage of the river. It presents a steep face to the south, but a gentler slope to the north, the direction from which the ice was travelling. .The dam as a whole indicates that the ice-foot lingered for some little time nearly in the same position; examined in detail, it can be seen to consist of a number of little subsidiary gravel­ ridges, which no doubt mark slight oscillations in the position of the ice-foot. The chief interest in the drift of Gower lies in its relation to the raised beach (p. 851). The far-travelled boulders referred to occur on the beach in Caswell Bay and elsewhere. One large block of grey granite lies on a slope 100 ft. above the sea at Pwll-du. In Rhossili Bay there is a grand exhibition of drift. An inter­ bedded layer of stratified sand and gravel yielded to Prestwich t Mya, Turritella tercbra, Nassa reticulata, and N. iucrassata (?).

* II The Country around Swansea," Mem. Geol. Survey. 1907. chap. x, lb. H'Vest Gower:' 1907, pp. :19-42. t Quart. [ourn, Geot. Soc ., vol. xlviii (1892), p- 29'. The deposit was correlated with the raised beach. The reasons for classing it as glacial are given in .. West Gower," Mev«. Geol. Survey. 1907, p. 41, SOUTH WALE S. 855 Shells are known in the drift of South Wales only in connection with the ice-flow from the westward ; none have been observed in the drift of northern derivation.

POST-GLACIAL DEPOSITS. The most interesting feature in the post-glacial deposits is the proof they contain of a considerable subsidence of the land since Neolithic times. In many part s of the coast th e stools, prostrate trunks and matted branches of trees may be seen at or below Ordnance Datum. The stools are in position of growth and rooted in a blu e alluvial clay, facts which ca n be readily ascertai ned in Swansea Bay west of Swansea Bay Stat ion. In dock-excavations at various pl aces " peat-beds " were found also at a much lower level, and th e great excavations for the Barry Docks, made while the survey of that region was in progress in 1896, gave an opportunity for a det ailed examination of a series of so-call ed" peats " and the associated strata." Barry Isl and was separated from the mainland on the north by the tidal estuary of the Cadoxton river, and on the east by a low col, over which the tid e had made a later entry into the estuary. The island was accessib le on foot by this col at low tide. The excavations in the estuary showed recent tida l dep osits of S crobicularia-clay and shingle with shells. These rested on a well-washed floor of blue silts full of sedges, interbedded with ostracod-marls and containing four peat-beds. T he lowest peat ­ bed lay at a depth of 35 ft. below Ordn ance Datum, and en­ closed the stools of large trees rooted in an underlying soil, in which were land -shells . The oth er peats, and the silts, resem­ bled swamp-deposits, but this lowest band had been undoubtedlv a lan d-surface, and it proved a subsidence of the land of not less than 55 ft. A fr agment of a polished flint-implement and two bone-needles in the uppermost peat proved that the subsidence was in progress in Neolithic times.

PHYSIOGR APHY. Ri ven which drai n about two-thirds of Wales empty th eir waters into the Irish Sea by way of the Bristol Channel. They includ e the Upper Severn, t the Wye, the Usk , the E bbw, the Rh ymney, the T aff', the Neath, the Tawe, th e Loughor, the T owy, the Tar, and the Cleddau. The water-parting which separates these from the streams of West Wales , runs from Strumble H ead eastwards, gra duall y curving to north-eastwards, and finally to northwards through Cent ral Wales. It thus keeps

'" .')ua r/ . [ ou rn , Geol . S oc.. vol , Iii (1896), p . 474, and tI Th e Coun try around Cardiff," M n 1t. Ge o!. S urv ey . 1902, pp . 84-93 . t A conven ient t erm. f OT the Severn above tnc junction of the Warwickshire Avon 856 GEOLOGISTS ' ASSOCIATION J UBILEE VOLUME. nearly pa rallel to the west coast and at a small distance from it. The rivers as a general rule take a normal direction at right angles to the Welsh parting-for example, the Upper Severn, Severn, Wye, and Usk flow a little south of east, while the Cleddau and T fif run generally southwards. These facts are illustrated in the map fo rming P late XXXI, which is reproduced from Quart. Journ, Geol , S oc., vol. lviii , by permission of the Council. T he Lower Severn and the Bristol Channel constitute an intercepting line of drainage, in which the waters of all these rivers are collected, and carr ied back south-westwards and westwards. This intercepting line runs in a general way parallel to the Welsh pa rting, and like it curves from an east-and-west direction in th e Bristol Channel to a north-easterly direction farther north. Close on the south -east side of this intercepting line th ere run s a second main water-parting, from which flow the rivers of the east of England. These keep an eastward course till th ey fall either into the North Sea or the English Channel. The drainage-scheme thus resolves itself into two groups of rivers trending eastwar ds, with an intercepting line between them trending south-westwards . When and how was this scheme brought into operation ? The initiation of the river-system of the south-east of England has been discussed elsewhere, * and it will suffice here to state th at it appears' to ha ve originated on a slope of Upper Cretaceous and Tertiary strata. From the fact th at the average rise west­ wards of the chalk would carry it over Wales at a far higher level than it is likely to have attained , it is inferred that there was a roll-down, or, in other word s, an anticlinal axis, a short distance west of th e present chalk-escarpment. Such an anticline would provide the same explanation for the position of the water-parting between the Thames and Severn drainage systems, as that which has been proved for the subsidiary part­ ings, such as that between the Thames and Frome systems. It would explain also the existence of the intercepting line of drain­ age constituted by the Lower Severn and Bristol Channel. As a fact, the Upper Cretaceous Rocks still survive und er pa rts of th e main water-parting, and in such places are almost, if not quite, horizontal. Lastly, we may remember that th e tilting of the strata, which determined the courses of the Thames and Frome with their tributaries, has been proved to have taken place in post -Oligocene and pre-Pliocene times. On attempting to apply the same explanation to the river­ system of South Wal es, t we are met with the difficulty that what­ ever late Secondary strata may have once existed, there are none

* Mem.Geol. Survey, " G eolog y of the Isl e: of Wight," Ed. a, 1889, chap . xv. Proc, Geoi . Auoc., vol. xiv (1896), p. 405. t Q ua r t . [ owrn, Geol . S oc., vol. lv ii! ( J90~), p . 207 . I ,, , ,/ J •• I I ! .~ { . /1 $I J' ...... -;. :.

:.. -.: n ....1 ...... :-..

...... ~ '. ,J> ••• -? c ...: '" o

C "T .

'o'0

". .

. 1: . 1, . ~ \ ..,.•.--- ... - ..1.,.1

I -J 0 \~

~

ell ce. ~

e o :')J ;;.1 SOUTH WALES. 857 left now. Of the land -surface on which the rivers commenced to run not a scrap survives. But there are certain peculiar ities in some of the rivers which are highly instructive. We see in the first place that the rivers are wholly ind ependent of the pre-Triassic distur bances and of the north- and-south faults which recur throughout the coalfield. Not only do they cross and recross the folds and faults, but they ignore also the pre-Triassic geography which was dominated by these struct ures. To illustrate this point they have been inserted on the map of Triassic geography for ming F ig. 133. The Ogmore and E wenny cross the site of a Triassic stra it and flow from it on to and across an island, while the D awen rises in the strait and crosses the same island. The El y in one pa rt of its course follows what may have been a Triassic gap , but on the other hand the Cadoxton river, ri,ing on the site of a Triassic strait, leaves it to cut right across the Michaelston crag. Obviously, the Triassic land was effectually cloaked by later formations when these rivers selected their courses. On the other hand, a closer examination of the Welsh river­ system reveals the fact that there are certain disturbances which have absolutely controlled th e drainage wherever they occur. For example, the Neath, T awe, Upper Loughor, and T owy furnish nota ble exceptions to the eastward tend ency of the Welsh rivers. They flow pa rallel, not at right-angles, to the main water-parting, and act as interceptin g lines to rivers flow­ ing in the normal south-eastward direction, so as to carry th e water back south-westwards. They thus rep eat faithfully, though on a small scale, the phenomena of the intercept ing line of the Lower Severn. Eve rv one of these abn ormal lines of inter­ cepting drainage coincides with a belt of disturbance. A series of such belts enters the coalfield from the east-north-eas t, form­ ing offshoots from the great line of disturbance which traverses Wales from Church Stretton to Pernbrokeshire. The belts recur at fairly regul ar intervals, and each is characterised by contor­ tion and over-thrusting of the strata . Every one traps the rivers and hold s them unt il the disturbance loses intensity. These facts suggest that the Welsh ri ver-system may have been initiated by earth-movements similar to those which deter­ mined the Thames and Frome syste ms. As a renewal of the old east-and-west movement in post-Oligocene times defined the courses which were to be followed by the Thames and Frome, so a renewal of the west-south-w est movement at some unknown date determined the position of the Welsh parting, the normal south-eastward flow of the rivers from it, the exceptional diver­ sion from that direction along lines of acute folding and over-thrusting, and lastly the great intercepting line of the Severn and Bristol Chann el. It would seem probable that the initiation of the two systems, east and west of the Severn resp ec- 858 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. tively, was contemporaneous, and if proof were forthcoming that there had been post-Oligocene movement along the Welsh dis­ turbance, this hypothesis would be greatly strengthened. No evidence for or against such a supposition has yet appeared. The effect on the river-system of the occupation of South Wales by ice appears to have been trifling. The valleys are deeply cumbered with drift, but all the main lines were reoccu­ pied by the rivers at the close of the glacial epoch. The process of re-excavation has been accompanied by the shooting of gravel on to the low ground, either in deltas or in terrace-like flats, such as that on which the main part of Cardiff is built. Two small valleys possibly had a different origin. One of them is a deep cwm, about half a mile long, which cuts north­ wards into the Pennant scarp at Cockett, leaving a narrow ridge at its head to connect the hills on either side of it. It contains no stream proportionate to its size, but was possibly excavated by an effluent from the ice banked up on the northern side of the range, thus constituting an "overflow valley," such as those described by Prof. Kendall. The other example occurs in the same neighbourhood. A narrow valley occupied by the London and North-Western Railwav traverses the scarp from the north to the sea at Black Pill. The valley is continuous, but is not now a through line of drainage. The water from the northern part of it flows northwards into Afon Llan, while that from the southern part runs southwards to Black Pill, the parting betweeu the two flows being an undefined peaty swamp.