Chapter XXXIII. South Wales

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.

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