Quarterly Journal of the Geological Society of London/Volume 33/On Coal-pebbles and their Derivation
50. On Coal-pebbles and their Derivation.
By H. K. Jordan, Esq., F.G.S.
The author commenced by referring to the discovery by the late Sir William Logan, as early as 1840, of a pebble of cannel coal in a layer of indurated clay at Penclawdd, and of coal-pebbles interstratified with the Pennant Sandstones in considerable abundance (Proc. Geol. Soc. vol. iii. p. 276). The late Sir Henry de la Beche also discovered coal-pebbles in several areas of the coal-field both in Monmouthshire and Glamorganshire (Mem. Geol. Surv. vol. i. pp. 193, 194).
The author noticed the division of the coal-bearing strata of the part of the Welsh coal-field east of the Vale of Neath into two groups. The higher, consisting of the sandstones known as the Pennant Grit, with comparatively few coal-seams, varies from 3000 feet in thickness at the Vale of Neath to 900 feet at its eastern outcrop near Pontypool; the lower, consisting chiefly of argillaceous shales with numerous seams of coal and ironstone, is about 1600 feet thick to the lowest workable coal-seam. According to his observations, coal-pebbles are not found in the shales of the lower group, but they occur towards the base of the Pennant Sandstones, sometimes associated with granite pebbles.
Sir W. Logan's pebble of cannel-coal was found in a layer of indurated clay overlying a seam of ordinary coal; and it was supposed to have been derived from a coal-seam about 2000 feet lower in the series. This view the author regards as untenable, as it would imply the elevation and denudation of the lower scams in some adjoining area of the original coal-field before the deposition of the higher Coal-measures—an assumption which is negatived by the structure of the district. His own opinion is that the pebbles are derived either from the seam of coal above which they are found, or from the destruction by erosion of a seam of coal which once existed approximately in the position where they are found, the erosion in either case being effected by the strong water-currents which distributed the grains of sand and other material upon the coal-seam.
The author instanced the sandstone forming the roof of the "Rock Fawr" seam near Bridgend as containing in some areas very large quantities of detrital coal and coal-pebbles up to the size of a hen's egg; and these pebbles, which consist, of the same coal as that in the seam below, have their angles but slightly rounded. In some seams the coal is overlain by thin beds of shale, coal, and fire-clay, upon which the roof of the seam reposes; but in places these overlying beds disappear, sometimes from large areas, bringing the sandstone roof into direct contact with the coal, which is generally thinner at these parts; and the author concludes that this lesser thickness of coal, and the disappearance of the shales, &c, are due to the erosive action to which he ascribes the origin of the coal-pebbles.
When these "coal-conglomerates" are found without any coal-seam immediately beneath them, the author infers that the seams from which they were derived have been entirely broken up, and converted into pebbles and fine detritus, the characters of the pebbles and the friability of the material of which they are composed being, he thinks, incompatible with the notion of their having been transported from a distance.
The greater frequency of coal-pebbles in the south than in the north and east parts of the coal-field he attributes to the occurrence of stronger water-currents, probably resulting from more rapid subsidence, in the former—in support of which he instances the increase which takes place in the thickness of certain strata in a south and west direction.
With regard to the occurrence of a pebble of cannel-coal at Penclawdd above a seam of bituminous coal, as recorded by Sir W. Logan, the author remarks that several seams of coal in South Wales have for their upper layer a thin bed of cannel-coal; and he infers that the Penclawdd seam was of this kind, and that its original superficial layer of cannel-coal was broken up and eroded as above suggested.
As an example of the effects of a water-current, the author stated that at a colliery in the Forest of Dean the "Coleford-Hill-Delf " seam, which had a thickness of about 5 feet, was in one part of the colliery found to be entirely "washed out," the sandstone roof resting directly upon the underclay. In an adjoining area the coal was from 8 to 12 feet thick; and the author was of opinion that the coal, after its formation, but before the deposition of the roofing sandstone, had been entirely removed from one locality and piled up in the other by the action of a strong current of water. The thick coal, although unusually friable, was free from any admixture of roof-material.
From the preceding considerations the author infers that, previous to the deposition of the roof-material, the coal was to a great extent consolidated, although perhaps only partially indurated; and he points out further that the hardness of coal is not, as sometimes supposed, dependent upon pressure, as hard and friable coal may be found in contiguous beds of the same seam, and the highest seams in the South-Wales Coal-field yield harder coal than others 3000 feet lower in the series.
Mr. Moggridge stated that he was well acquainted with the district referred to, and had always thought that these pebbles resulted from beds broken up and much rolled. There is a bed of good cannel-coal in the district, four miles from and at a higher elevation than Penclawdd, so that pebbles from it might have been deposited at the latter place. Thus we should not have far to go for the source of the pebbles.
Prof. Morris remarked that the subject brought forward by Mr. Jordan was one of great interest. He thought there might be three sources for these pebbles:—first, floating wood encased in sandstone and carbonized;secondly, the breaking up of submerged forests, when the fragments became imbedded in sands and clays; and, thirdly, the breaking up of old coal-beds, and the distribution of the fragments through younger deposits.
The Author, in reply, stated that he could not accept Mr. Moggridge's explanation, because the difference in the level of the deposits referred to was due to changes subsequent to the formation of the beds containing coal-pebbles. Nor could he adopt the explanations suggested by Prof. Morris, because, had the pebbles been so derived, it would be reasonable to expect their occurrence, more or less, throughout the entire series of Coal-measures, whereas they they were found associated only with seams of coal which had coarse sandstone roofs.