Excursion to the Frome District, Somerset: May 28Th to June 1St
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EXC URSION TO THE FROME DI STRICT, SO:lIE RSET. 209 that Dr. Abbott would continue his researches relating to this extremely interesting subject. Th e party left Tunbridge Wells by the 8. II train. R EFERENCES. Geological Survey Map, Sheet 6. Geological Survey, Hori zontal Sections, Sh eet 77. 1875. TOPLEY, W.-I< Geology of the Weald ." M'm. e,o!. S urvey, 1879. FAWCETT. W.-I< Excursion to Tu nbridge Wells." Ruord of Ex cursrons, pp. 38-44. 1887. WOODWARD. H. B.-I< Geology of England and Wal es." Fig. 57. and pp. 359-365. 1895. R. S. HERRIES and G. ABBOTT.- " Excursion to Tunbridge Well s." P rrc, Geol. A ssoc., vol. xiv, p. 198. 18g8. R. S. HERRIES.-I< Excursion to Crowborough." Proc. Geoi. Assoc., vol. xv, p. 450. EXCURSION TO THE FROME DISTRICT, SOMERSET MAY 28TH TO JUNE 1ST (WHITSUNTIDE), 1909. Director , L. RICHARD SON, F.R.S.E., F.L.S., F.G.S. Excursion Secretary : G. W. YOUN G. eR,pori b;'THE Dm EcToR.) T HE official party left Paddington on May zSth at 5 p.m., and arrived at Frome at 7.19 p.m., the headquarters being at the George Hotel, Frome. T he main object of the excursion was to study the Inferior Oolite of the Bath-Doulting District and its relations to the beds immediat ely above and below it. In the circular of the Whitsuntide Excursion to the Middle and South Cotteswolds, etc., of 1 908 , it was stated that " in all some twenty-seven sub -divisions have been made of the Inferior Oolite Series in the West of England . Between the Upper Lias and a bed in the Inferior Oolite called the' U pper Trigonia Grit I should come twenty of these. Three, however, are not represented at all in the region between Chipping Nort on and the Mendip Hills, and the others are not co-extensive with that tract; sometimes they are present, at others absent. This appearance and disappearance of the sub-Upper Tngonia-Grit Inferior-Oolite deposits has been found to be due to crust-pressures, which have from time to time thrown the deposited rocks into anticl inal and synclinal flexures-the rocks in the synclines being preserved according to their amount of depression, and those on the anti clines being removed according to their amount of elevation. Th e most intense flexuring that took place in Inferior-Oolite times preceded the deposition of the Upper Tngollia·Grit, and PROC. GKOL. Assoc. VOL. XXI. PART 4. 1909.] 16 210 EXCURSION TO THE FROME DISTRICT, SOMERSET. the geographical distribution of the pre-Garantiana Inferior Oolite sub-divisions has been mainly governed thereby. Thus at Leckhampton Hill the Upper Tngonia-Grit is parted from the , Freestone Series I by the Notgrove Freestone, Gryphite-Grit, Buckmani-Grit, and Lower Tngonia-Grit; at Birdlip the' Upper Trigonia-Grit' and' Freestone' are in apposition; in the neighbour hood of Swift's Hill, near Stroud, the 'Intervening Beds' are the same as at Leckhampton; but then, to the south of the Stroud Valley, they begin to die out, and from Selsley Hill to beyond Sodbury the Upper Trigonia-Grit and Freestone beds are in apposition again. Between the last locality and Bath even the , Freestone Series' disappears, and the'Grit' comes to rest upon the Sands of the Upper Lias." From Bath to the neighbourhood- of the Mendip Hills there is no Inferior-Oolite deposit of earlier date than this Upper Tngonia-Grit, and near Vallis Vale even this bed is overstepped by the equivalent of the Crypeus-Grit of the Cheltenham district, which there comes to rest directly upon the remarkably level planed and bored surface of the highly-inclined Carboniferous Limestone. On the south side of the Mendip Hills, however, in the railway-cutting at Doulting, the Upper Trzgonia-Grit comes in again, and its marked resemblance to the conglomerate-bed of Wellow and of Maes Knoll, Dundry, is distinctly noticeable. The Inferior-Oolite beds of the Bath-Doulting district between the Upper Trzgonia-Grit and the Fuller's Earth are the equivalent of the Crypeus-Grit of the Cheltenham district, but whereas in the latter district the whole deposit is rubbly as a rule, in the former this feature is the exception. The following sub-division of the Inferior Oolite of the Doulting district has been made: Approximate Sub-divisions, thickness in feet. ' i.-Rubbly-Beds; Terebra!ula globa!a, f DOULTING.. auctt., ~on Sowerby, common, etc.... 8 =Clyj;eus- 8~ BEDS. { 1I.-AnabaCla-Llmestones ... ... ... Grit. iii.-Douiting Stone; massive Freestone... 44 (Upper Coral-Bed and Dundry Freestone wanting.) vi.-" Conglomerate-Bed" [Upper Trigonia- UpP,er I 1:; '] .cngoma- Gnt I" Grit. The Members left Frome about 9.15 a.m, on Saturday, and drove to Maes Down-a hill about a mile and a half to the south of Doulting. Here the Director pointed out the geography of the country embraced within the view, calling particular attention to the ever-inspiring Glastonbury Tor and the long line of the Mendips, which, he said, marked an anticlinal axis along which movement EXCURSION TO THE FROME DISTRICT, SO~IERSET. 2 I I had taken place time after time. During the course of the excursion they would have the opportunity of studying some of the effects of these movements in the littoral deposits of certain Neozoic rocks, Moore's so-called" dykes," and markedly planed surfaces. Away from the Mendip . Hills the various deposits assumed their normal aspect, and here at Maes Down the Marl stone of the Middle Lias was present and well-developed. Its presence had been overlooked during the Geological Survey of the district, and was not shown therefore on the official map. An inspection was made of the quarry, and evidence of the basement-beds of the Upper Lias was also obtained in the form of many ammonites.* Entering next the Farmcombe Quarry'[ a magnificent section was seen showing the upper portion of the Doulting Stone, the Anab.uia-Limestones, Rubbly Beds and basal Fuller's Earth Clay. It was remarked that the Doulting Stone would be better seen in the next quarry to be visited-that called locally "Brambleditch "--but attention was directed to the fact that its topmost stratum had a waterworn and bored surface with oysters attached. Then come the Anabacia-Limestones. It is often somewhat difficult to separate them from the Doulting Stone on the one hand and the Rubbly Beds on the other. Typically, however, while the Doulting Stone is brownish, very sparry-as at the Chelynch Quarry-and not conspicuously oolitic, the Anabacia-Limestones are white and finely-oolitic, the oolite granules being noticeably even-sized. At the recently-opened Holcombe-Lane Quarry, about a quarter-of-a-mile to the east of the Farmcombe Quarry, these distinctions are very well marked.] From the Rubbly Beds the Anabacia-Limestones are differen tiable on account of their being typically not so brown, rubbly or fossiliferous. Crowds of Terebratula globata, auctt., moreover, occur in the Rubbly Beds. The Rubbly Beds are subject to some variation in thickness, in the neighbourhood of Midford being represented only by a thin rubbly layer which is hard to separate from the Fuller's Earth. At the Doulting-Bridge or Brambleditch Quarry the Doulting Stone was finely exposed, and from the succeeding Inferior-Oolite and Fuller's-Earth Beds many fossils were collected-the crowds of Ostrea knorri eliciting particular comment. The members of the party then viewed the section in the adjoining railway cutting from a bridge, and the Director pointed out where the bottom and conglomeratic bed of the Oolite rests upon the Lias j and the succeeding and thinner-bedded limestones of the Doulting Stone when compared with the superjacent massive beds of Doulting Freestone of commercial * Geot. M ae.. t906, p. 368. t Quart. [ourn, Geo/. Soc.. vol. lxiii (1907), pp, 38~-397. Geol. Mag. (1908), p. SU, I Traces of Fuller's Earth (Knorri-Clays), filling in fissures in the A nabaCla-Lime· stones, occur here. 212 EXCURSION TO THE FROME DISTRICT, SOMERSET. fame. Farther along the cutting typical Anabacia-Limestones are excellently exposed and have their top-layer noticeably bored. Then come the Rubbly Beds and Fuller's Earth, so that a complete view of the Inferior Oolite as developed in this neighbourhood is obtained. Pointing to the higher ground to the south the Director stated that in the sides of the Chesterblade Road, just before it topped the hill, were to be seen crowds of valves of Ostrea acuminata, Sow. The horizon at which this oyster was most abundant appeared to be just below the well-known Fuller's-Earth Rock. In the ditches alongside the lanes on that high ground lay innumerable specimens of Ornifhella omithoupha!a and its multitudinous varieties; while in the Alham-Lane Quarry the sequence of (I) Ostrea acuminata and (2) Omithe/la ornithocepha!a was corroborated by the former fossil being found in the days which had been deeply cut into by the wheels of the carts ascending the bank to this quarry, and the latter occurring the most abundantly in the rubble and top-soil of the excavation, Amongst the fossils that can be collected at Alham Lane are the following : Pseudomelania, Pleurotomaria, Natica (of N. bajodensis-group) , Serpula tricariuata, Sow., S. limax, Goldfuss, S. jJlimtilis, Diastopora mithelini (Lamarck), Hoiectypus hemi spherims (Agassiz), H. depressus (Leske), Rhynchonella smitht~ Walker, Terebratula globata. auctt, non Sow., Ornithella ornithocephala (Sow.) and varieties and mutations, Ceromya plicata, Agassiz, Chlamys vagans (Sow.), Cypricardia bathonica d'Orb., Gervillia acuta, Sow., Goniomya angulifera (Sow.), Gresslya peregrina (Phillips), Homomya gibbosa (Sow.), Luana (cast), Lima duplicata (Sow.), Limatula gibbosa (Sow.), Pieuromya sp., Pholadomya sp., Protocardia buckmani (Morris and Lycett), Ostrea costata, Sow.