On the Geology of the Bristol District

375 ON THE GEOLOGY OF TH E BRISTOL DISTRICT. By PROF. W. J. S OLLAS, M.A., F.R.S.E., F.G.S., &c. Geologists possess,in the Survey Memoir, by Mr. H. B. Woodward, II complete handbook to this neighbourhood. In it descriptions of the localities which the excursion will visit a,re given in full det ail; to repeat them here would be a work of supererogation, and, con sequently, I shall only attempt in this paper to pres ent a sketch in broad outline of the geological history of the neigh bourhood. The earliest rocks which we shall examine ar e th e Old Red Sand stone, exposed on the summit of the Mendips, and in th e cliff from P ortishead to Olevedon ; they were deposited in an area of depression which was formed after the great post-Cambrian and pre-Silurian folding of our country, and which extended from th e Cambrian rocks of Wales on the west and north , to the line of Malverns on the east, and the Bristol Channel on the south. Outside the two latter boundaries was th e basin in which the Devonian rocks were contemporaneously deposited to the east and south. The lowest beds of the formation are chiefly red marls and shales which lie conformably on the older Silurian strata ; above these beds we find red sandstones, with red marls and cornstones, and above these conglomerates with red marls and sandstones, which form the highest beds of th e formati on, and pass conformably into th e Carboniferous Shales, as may be seen below Cook's F olly in the Avon section. The passage from marls through sandstone into conglomerates shows that the basin was becoming shallower , owing, probably, to deposition proceeding at a more rapid rate than depression ; the succession of the Lower Limestone Shales over the Old Red Sandstone shows, on th e other hand, th at depression was taking place at that time faste r than deposition could keep pace with it, and this relation (depression greater than deposition) ap pears to have continued into the Carboniferous L imestone times. The Cambrian and pre-Cambrian rocks, which furnished sediments for the Old Red Sandstone, consisted of arenaceous, argillaceous, and calcareous st rata, togethe r with various in trusive and erupted ign eous rocks. The origin of the ordinary sedimentary rocks of the Old Red Sandstone, such as sandstones, marls, and conglome- 2 D 876 W. J. BOL LAS ON THE GEOLOGY OF rat es, presents us with no difficulty; not SO, however, the red oxide of iron, to which it owes its colour, and the layers of " corn stone" or nodular limestone which characterise th e lower and middle parts of it. The cornstones are clearly inorganic, and have probably been formed by th e mechanical denudation and re-deposition of pre-Silurian limestone beds. The ferric oxide may have been derived in part from th e older red Cambrian beds, in part from the oxidised soil of igneous rocks, and in part from carbonate of iron introduced in solution by the action of decaying vegetation, and subsequently converte d, by the loss of carbonic acid and the addi tion of oxygen, into peroxide of iron, which was precipitated in the Old Red Sandsto ne sea. No fossils have been found in the Old Red Sandstone of this area, except the fishes Holoptychius and Coccosteus, of which scales and bones have been discovered near Portishead. To them, however, great interest attaches, and especially to H oloptychius, which resembled the existing Ceratodus, a fish showing strong tendencies towards the Amphibian type. P robably Ifoloptychius, if not in the direct line with, was at all events, closely related to, the forms from which the Amphibi ans of th e succeeding forma tion were derived. The Carboniferous format ion in this area consists of the follow ing members, th e thi ckness of which, in the Bristol Coal-field and the Avon section, is given in feet :- Coal Measures 5,000 Millstone Grit 1,000 Carboniferous Lim estone 2,500 Lower Limestone Shales 500 -9,000 Th e area (Fig. 1), in which it was deposited was the outcome of that of the Old Red Sandstone, modified by the general depression which it bad und ergone, and by various local changes of level, in cluding both elevation and depression. It was bounded on th e north by the now well-known central peninsula which stretched across the middle of England; Newent on the Malverns, and Haverfordwest, South W ales, being two points on the southern coast of the peninsula. On the south it was bordered by a continent, the north ern coast- line of which passed somewhere through the north-east of France, and THE BRI STOL DI STRICT. 377 FIG L 378 W. J. SO LLAS ON TH E GEOLOGY OF near th e South of Corn wall and the South of Ireland. The South of E ngl and basin was in free communication with a wider European one. During a period of exceptional depression, the sea was clear th ough by no means deep, and beds of limeston e were deposited over nearly its whole extent. In thi s neighbourhood thick field-lik e growths of enerinites and rich banks of molluscs produced, by th eir growth and decay, the lower parts of th e limestone ; th ese at length yielded to a luxurious growth of corals, by which th e upper part of th e limestone was chiefly formed. Associated with the coralliferous beds are thick deposits of oolite, which may be seen in the Avon section, and thus th e association of coral reefs and oolitic deposits which exists at the present day, which existed in the Silurian period, and is so characterist ic of th e Jurassic forma tion, is again met with here. The quiet growth of the limestone was interrupted on one occa sion at least by an episode of ign eous activity, which has left its traces in th e thick beds of. volcanic ash associated with once vesi cular but now amygdaloidal basaltic lava, to be seen interbedded in th e limestone along the coast-se ction from 'Veston-super-Mare to Swallow Cliff. While th e Carboniferous Lim estone was forming over Somerset, a great river which I shall venture to call De la Beche's River,as th ose which entere d th ebasin of the North of England may be terme d Hull's Riv er and Phillips's River, flowed through the great southern con tin ent and emptied itself into our basin somewhere south of Corn wall and Devon; it soon brought to an end the conditions under which limestone had been formed over those counti es, building up by its deposit of sediment the great thickness of th e Culm Mea sur es. This continued accumulation of sediment brought the mouth . of the river continually farth er and farth er north , till at length its delta began to spread its seaward margin over Somerset and South 'Vales. Deposition was now in excess of depression, and the Mill-: stone Grit, with its th in bands of limestone and marine fossils (Producta scrobicula, &c.), is th e first sign of that enlargement of the delta which culminated in its extension over th e whole of th e western part of th e South English basin. On th e alluvial plain s or delt aic flats so formed, the Carboniferous vegetation extended itself from adjacent lands. The spores and debris from the Lyco pods, Ferns, Equisetums and Conifers accumulated on the g round, and in th e warm, moist climate, formed a fermenting mass, which both served as a hotbed, stimulating the germination of fallen THE BRISTOL DISTRICT. 379 spores, and led to such a complete decay of woody tissue as sufficed to obliterate nearly every trace of its structure. The more resinous spores and the bark of the trees resisted decay better, and are more frequently preserved. Wide tracts of the decayed and fermented mass were quietly de pressed beneath the water-level; shales and sands were strewn over it, and so far one coal-bed was completed. Deposition con tinued in excess of depression till a fresh land-surface arose, and a fresh coal supply was laid down; depression again ensued, and this series of processes was repeated as many times at least as there are coal beds (44) in our coal-field. At length the great geosynclinal which had been in progress throughout the whole of the Silurian, Old Red Sandstone, and Carboniferous times, and which extended from Ireland into Bel gium and Germany, was complete; tremendous pressure then set in from the South, and in the basin of the South and West of England thrust the great mass of Siluro-Carboniferous deposits against the buttresses afforded by the Pre-Silurian rocks of Wales on the West and North, and the Malvern range on the East. Thus, from an approximately horizontal position, they were folded up into great rolling curves (synclinals and anticlinals), sometimes sharply curved and even bent back on to themselves (reversed anti clinals), and fractured and displaced (faults both normal and re versed). The synclinals, or rather basins, exist now as the South Wales, Forest of Dean, Bristol, (Fig. 2,) Nailsea, and various other coal-fields, most of which are now concealed uuder newer strata; the anticlinals which separate the basins are to be seen in the Mendip range, which extends from Frome on the east to the Steep Holme on the west; in Broadfield Down which extends westwards to Swal low Cliff and the Flat Holme;' and in the curved ridge of the Durdham, Clifton, and Leigh Downs.

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