Chapter XV. the Neozoic Rocks of Gloucestershire and Somerset
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CHAPTER XV. THE NEOZOIC ROCKS OF GLOUCESTER SHIRE AND SOMERSET. Bv L. RICHARDSON, F.R.S.E., F.L.S., F.G.S. N Gloucestershire and Somerset the Neozoic rocks are those I ranging from the Bunter to the Coralli an inclusive, the Selb ornian, Chalk, tr aces of Eocene, and Pl eistocene depo sits-the last including the sporadicall y-distributed patches of sand of the Chelt enh am and Gloucester areas, the so-call ed " Northern Drift " pebbles of the same parts, the gravel of local origin, the alluvium of the Severn-side, and the Burtle, Peat, and Forest Beds of Somerset. The Kimeridge, Portland, Pur beck, Wealden, Oligocene, Miocene, and Pliocene are absent. During the fifty years of active existence of the Geologists' Associati on, parties of its members have visited these counties on several occasions, usually combining the study of the Paleeo zoic rocks with the Neozoic, and rarely, if ever, restricting their attention to th e one. BUNTER. Bunter Beds occupy a small area around Brom esberrow,* a village about half-way between Newent and Ma lvern , and appear from beneath the Keup er rocks in the neighbourhood of the Quantock Hills, encroaching upon or faulted against the Palreozoic rocks of that tract. t In the former area the soft red sandstones belong presumably to the Upper Bunter: no pebble beds as old as those that occur around the Quantock Hills have been found there, nor have they been proved in the deep borings in or near the Moreton Valley. T he " Bromesberrow Beds," as the sandstones are locall y called, must apparently be paral leled with the " Upper Sandstones " of the Quantock region. KEUPER. In Gloucestershire this series is divisible int o two parts the Lower Keuper Sandstones, or "Waterstones," and the Upper Keuper Marls. The Lower Keup er Sandstones are well exposed in a quarry at Huntley, in close proximity to the great fault that throws them into juxtaposition with the Silurians of May Hill. In point of • M em . Geol. Surf/., "0 1. ii, pt. t ( 848). p , 11 4 . t M em . Ce DI. Su,v ., " T he Geolog y of the Quunrock H ill s, &c." ('9 08), pp. 46-<>4 . 330 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. time these beds are no doubt represented in the neighbourhood of the Quantock Hills, but are difficult to separate there. There is some variation in the lithic characters of the com ponent beds of the Upper Keuper from place to place. The highest subdivision, the" Tea-Green and Grey Marls," are best considered separately, for while the" Tea-green Marl " portion is of approximately the same lithic structure throughout this country, the Grey Marls have a less considerable geographi cal extent. In the Bristol Channel littoral the line of demarca tion between the "Tea-green" and "Grey Marls" is not at all sharply defined; indeed, it was on account of the sections in this neighbourhood that the question arose as to whether the" Tea green" and" Grey Marls" were Rheetic or Keuper. Originally, Etheridge divided the marls of grey, green, and black colours, which occur between the Keuper Red Marls and the Rhretic Black Shales, into two parts, calling the lower the "Tea-green Marls," and the upper the "Grey Mads." The former he classed with the Keuper, the latter with the Rhretic. Other authors did not feel disposed to accept this ruling, and when the question was again raised in subsequent years by Prof. Boyd Dawkins discovering Microlestes' teeth and charac teristic Rhsetic fossils in the "Grey Marls," Etheridge either forgot or abandoned his old ideas and referred the" Grey Marls," and the "Tea-green Marls" as well, to the Rheetic. Prof. Dawkins, however, had only found the organic remains in the top-portion of the" Grey Marls," and if Etheridge had simply used this information as substantiating his early claim for their Rhretic age, no doubt the matter would have been settled long ago. Buthedidnot: he grouped "Grey"and"Tea-green Marls "asRhretic. The late Edward Wilson, accustomed to work on the Rhsetic Series of the Midlands, could not understand how Etheridge came to the conclusion that the "Tea-green Marls" should be regarded as Rhretic. In the Midlands there is a sharp line of demarcation between them and the overlying Black Shales, and no signs whatever of any transition. By Etheridge lumping the fossiliferous" Grey Marls" with the" Tea-green," and employing the latter term for the joint series, Wilson did not suspect that there were beds on the Bristol Channel littoral, between the "Tea-green Marls" and Black Shales, that were absent from the Midlands, and that, in the former district, constituted true transition-beds, which it was difficult to apportion between the two series. But such certainly seems to be the case: the fossiliferous top-portion of the" Grey Marls "-the Sully Beds-are unques tionably Rhretic, but how far down they extend is a matter that must be decided upon by the downward extent of Rhsetic fossils. Up to the present the greatest thickness that has been assigned to them is 13 ft. r in., and this was recorded on the Somerset side at Blue Anchor Point. Therefore the "Tea-green and Grey THE NEOZOIC ROCKS OF GLOUCESTER AND SOMERSET. 331 Marls" that must be given to theKeuper are the, original beds thus named, less the Sully Beds. The" Grey Marl" portion is not co-extensive with the" Tea green Marls." It is unrepresented, as far as can be seen, in Gloucestershire, but is present in many of the Somerset sections, and on the shores of the English Channel at Culverhole, near Lyme Regis. Below the Tea-green Marls come red marls, variegated with regular bands and anomalous patches of grey and green tints. Then comes, in the Staunton-Newnham area, at about 215 ft. below the Rheetic, a fairly persistent bed of sandstone, containing Estheria; ichthyodorulites of a species of H ybodus, and the remains of plants. Below, in the same area, come marls again of the same type as those above. * In the Bristol district, however, the lower portion of the Upper Keuper is considerably more arenaceous, and around Chewton Mendip intermittent bands of calcareous sand stone make their appearance. This change as regards lithic structure in the Bristol area is one that might have been expected, for beds of MilIstone Grit and Old Red Sandstone must have been exposed while the Triassic rocks were being accumulated, and as what must have been inlet-riven islands are approached, thick conglomerates-the beaches of the period -take the place of the marls, and indicate the former extent of the Keuper waters. t "The classic section of a Trias beach" is that at the abrupt turning of the Bridge-Valley Road at Clif ton, t and a similar accumulation, resting upon the verti cally-placed Old Red Sandstone beds instead of inclined Car boniferous Limestone, is seen in a cliff section between Portis head and Clevedon. § The accumulation varies but little from place to place, so that if one section has been seen that is sufficient; but it may be added that in the Mendip Hills it has yielded in the past rich stores of calamine; while at pre sent, at Winford, the earthy variety of hsematite, raddle, which is so useful in the manufacture of pigments, is worked. The normal Keuper Marls, sometimes with the Tea-green Marls above, can be seen at The Mythe Tute (Tewkesbury), Wainlode.] Garden,' Beachley or Sedbury/'* and Aust Cliffs,tt Milton Lane, near Wells,tt the railway-cutting near Shepton Mallet,§§ and along the Watchet Coast. II II * See L. Richardson, PrOC. Cotteswold Nat. F.e., vol. xv, pt. 2 (I905)~ pp. 93- 10 0 . + See R. Etheridge, Quart. [otirn, Geol. Soc., vol. xxvi (,870)' pp. '74-r9'. t Proc. Geot. Assoc., vel. vi, p. 398. ~ I sa., p p. 398-399. II L. Richardson, Proc. Cotteswold Nat. F.C., vol. xiv, pt. 2 (1903) pp. r28-'40. -,r I bid., r54-,65. ** L. Richardson, Quart. [ourn, Geol. Soc., vol. lix ('903), pp. 390-395. +t R. Short, Ibid., vol. lx ('904), pp, '78-'79. H P. B. Brodie, Ibid., vol. xxii ('866), pp_ 93-95. §j C. Moore, Ibid., vol. xvii (1861), p. SI5. 1111 Boyd Dawkins, Ibid., vol. xx ('864), pp. 396-412. 33 2 GEOLOGISTS' ASSOCIATION JUBILEE VOLUME. No trace of rock-salt has been observed in the marls of these counties, but ramifying veins of gypsum-some of a beautiful pink colour-traverse portions of the cliffs of Aust and West Somerset; while deposits of celestine, near Yate,* and at Abbots Leigh, add to the commercial assets of the Bristol dis trict. The cliffs at Austj and in West Somerset'[ have been visited by the Association, and so have the interesting celestine workings near Yate, § where the mineral is first located by long probing rods. The celestine is exported from Avonmouth to Germany in large quantities for use in sugar-refining. In brief, the process consists in the combination of the strontium with the sugar, the removal of the mother-liquor, and the breaking up again of the strontium mono-saccharate. RHlETIC. In Gloucestershire and Somerset the Rhsetic Series is more frequently exposed, has a greater superficial extent and thick ness, and is more interesting than in any of the other counties. Usually its position between the red ground of the Keuper and the clay-flats of the Lower Lias is indicated by a very marked ridge, at or near the top of which its beds crop out.