Excursion to the Forest of Dean, Wye Valley, and South Wales
Total Page:16
File Type:pdf, Size:1020Kb
,142 F. W. RUDLER ON THE SOUTH WALES COAL-FIELD. probability, that the change may have been brought about by II gradual rise of temperature towards the north-west, connected with the thickening of the strata in that direction. He assumes that originally the coal-seams in the north-west of the field were covered by a much greater thickness of beds than in the south-east, and being thus buried deeper beneath the surface they must have been exposed to a higher temperature, which would tend to induce more or less complete de-bituminization of the coal. EXOURSION TO THE FOREST OF DEAN, WYE VALLEY, AND SOUTH WALES. MONDAY, AUGUST 6TH, TO SATURDAY, AUGUST 11TH. Directors: THE PRESIDENT; WILLIAM GALLOWAY, of Oardiff; W. F. GWINNELL, F.RMet. S.; J. STORRIE, Ourator of the Cardiff Museum; T. H. THOMAS, RO.A., President of the Cardiff Naturalists' Society; DR. O. T. V ACHELL, of Cardiff ; and E. WETHERED, F.G.S., F.O.S., F.RM.S., Hon. Sec. Ootteswold Naturalists' Field Olub and Cheltenham Nat. Hist. Soc. (Report by 'l'HE DIRECTORS.) Monday.-Gloucester had been chosen as a convenient centre for the work to be undertaken on Monday and Tuesday, and here the party was met, early on Monday morning, by Mr. Wethered, who acted as Director during the day. From Gloucester the train was taken, on the Ross and Hereford line, to Mitcheldean Road, at the extreme north-eastern corner of the Forest of Dean. A move was at once made in the direction of Baily Hill, and on the way thither an exposure of Old Red Conglomerate was seen. The Director here called a halt for the purpose of explaining the structure of the surrounding country. He pointed out the features of the Old Red Oonglomerate, which, he remarked, pointed to an alteration of the conditions which had allowed of the deposition of the Old Red Sandstone beneath, and to a coming change which brought in the Oarboni ferous rocks. For this reason he asked them to mark well t.his Oonglomerate, as he regarded it as the upper limit of the Old Red and Devonian rocks of the west of England. The beds which intervened between it and the true Carboniferous strata were, in his opinion, transitional, and should be so regarded. EXCURSION TO THE FOREST OF DEAN. 54;) Some old Roman workings for iron ore were then visited, which had been excavated in the Carboniferous Limestone. Baily Hill Quarry was the next place of interest in the pro gramme. It is opened in the Carboniferous Limestone, which appears to be all dolomitized. Proceeding further, across the dip of the strata, the Lower Limestone Shales were seen in two quarries. The Director described the features of the beds and also the chief fossils which occur in them. The" deep cutting" on the road from Drybrook to Ross was then visited. Here the 'I'ransition Beds, i.e., those between the Old Red Conglomerate and Lower Limestone Shales, are very well exposed. There is certainly no better exposure in the west of England. The many-coloured sandy beds and green shales which constitute this series were traced from the Lower Limestone Shales to the Old Red Conglomerate. After luncheon at a wayside inn, another quarry opening in the dolomitized Carboniferous Limestone was visited, and then some exposures of the Upper Limestone Series were examined. In these the" Crease" and "Whitehead" Limestones were shown, and resting on the latter in the Morse railway-cutting at Drybrook came the Millstone Grit. Leaving Drybrook, the party turned in the direction of Mitcheldean, where the Wilderness Cement Works Were to be visited. On the way some further beds of Millstone Grit were pointed out. They were of special interest inasmuch as they greatly resembled the Old Red Conglomerate in lithological structure, containing" veinstone" quartz pebbles which could not be distinguished from those in the Old Red Conglomerate. Mr. Colchester-Wemyss now took the lead, and conducted the geologists to the Wilderness Cement Works, Limited, of which he is the Chairman. On arriving there it was discovered that Mr. and Mrs. Colchester-Wemyss had provided hospitality in the shape of a substantial tea. The refreshment having been partaken of and the hospitality acknowledged by the President, Mr. Colchester-W emyss conducted the party over the quarries in which the Limestone is worked for making cement. These were all in the Upper Series, that is to say, in the Crease and Whitehead Limestones. From the Cement Works the party proceeded to Mitcheldean Station, whence they returned by rail to the headquarters at Gloucester. t<:orr t'l c>< Cl ~ ,--,----, o<P- !:>: o>-3 >-3 ~ l'l "J Old 7led,S , o ~ U2 , .." '.{:;; >-3 ~ A~ .' ,_ ,": ' ~, ,~. o '.z;udlCffY . '"t:I (%j B J~ "Yrenlock~ I/p. Uan()oYf1I;tj-, FIG . l. -O~ OIOGICAL MAP of th e FOR ~ ST OF DSAN and part of the WYll: VUUY, includi ng the country vis ited on Monday, Augu st 6 (N.E. of the Fores t of Dean), Tuesday, Ang Dst 7 (Wostbury .on-E;cvern and May Hill ), and Wednesday, Aug ust 8 (Wye Valley). Scale, four miles to one incb.-From the Geological Survey Maps. WYE VALLEY, AND SOUTH WALES. 545 Tuesday.-A journey of eleven miles on the South Wales Railway carried the geologists from Gloucester to Newnham. The line first crosses Olney Isle, an alluvial flat enclosed by two arms of the Severn. Telford's graceful one-arch road-bridge over the west arm attracted attention. The rail runs on the Lower Lias to Churcham, and then on the New Red Marl to Newnham. At Newnham Station they were met by Mr. Gwinnell, who acted as conductor during the day. From a neighbouring bridge he pointed out the Keuper Sandstone, which is well exposed in the midst of the red marls in the railway cutting. The old Roman road from the Forest of Dean to the Severn Ferry was crossed, a heap of pseudo-volcanic material derived from the blast furnaces and laid bere for road-metal attracting much attention on account of its structure, and especially by the presence of free sulphur, evidently due to changes subsequent to solidification of the slag. Aft.er a brief examination of a recent cuttingin tbe marls, the party proceeded along the terrace gardens, where a good view was had of the position of the great N. to S. fault and the striking contrast in the scenery on either side of it, the Old Red rising steeply beyond the line of fracture and being capped on tbe distant ridge by Carboniferous strata. Proceeding to the churchyard, situated on a vertical cliff of Red and Green Marl about 70 feet high, a splendid view was obtained of the great horse-shoe curve of the Severn, the Vale of Glo'ster, and tbe distant Cotteswolds. Here the Director described the topography and physiography, remarking especially on the erosive action of the stream and tides on the convex sides of its curve and the accumulation of alluvial deposits on the concave portions. The phenomena of the "bore," so well seen at this spot at spring tides, were also described. The base of the cliff at the ferry was next examined and seen to consist of many thin strata of various tints of red, green, and grey marls, of very unequal hardness, and consequently weathering unequally, producing a series of ledges. A walk of one mile along the river bank brought the party to Broad Oak, where the Mountain Limestone from the Avon gorge, being landed there for road-metal, was examined. Continuing along the river bank for another mile the 546 EXCURSION TO THE FOREST OF DEAN, celebrated Garden Cliff at Westb ur y-on-Severn was reached, just before which the Director, with the aid of a large map and vertical and horizontal sections of the cliff on a large scale, explained the general features. The state of the t ide compell ed Lower Lias. ) [onotis bed, Es th crl a zone. Up. Pecten bed. Low, Pect en bed . Avicula contortu find Cardium shal es. !Bono bed. ~~~~;.r'., Pullastra bed, -_>"<........ ~.-..../I -~ ,Z::- -::.:;-. Rhretic ma rls. ~_-:::... --::-:.- -..:. -~./- --:::\- ~;;~~;;E ~~ K eu p e r marls. Fig. 2. Vertical section of t he Rheetic Series at Garden Cliff, Wes tbury. on-Severn , Glouce stershire. (Re duced fr om secti on by Mr. R. Bther idge, F.R .S., in ' PrOD. Cotteswo ld Club,' 1865.) the party to proceed along t he cliff to the far end before descending, so that t he successive strata were examined in descending order from the beds of the Planorbis zone of the Lower Lias, through the Whit e Lias Series and Bl ack Rheetic Shales to the grey Rhoo tic Marls and red Keuper Marls. The " Monotis bed" and "Bone bed" were called upon to yield considerable spoil, many good specimens of reptilian and fish bones and coprolites, mostly brilliant with golden pyrites, being carried off. The prominent cornice-like ledg es, formed by the E stheria WYE VALLEY, AND SOUTH WALES. 547 Bed, and especially by the Upper and Lower Pullastra Beds (the latter strongly ripple-marked, and though but a foot thick pro jecting as much as seven feet), were especially noticeable. While seated on the fallen blocks from this cornice, or perched high up on the face of the cliff, the party was photographed. The hammers of the geologists were found useful in cutting foot holds on the headland, whose base was under water, and which had to be rounded to avoid a long detour.