Field Meeting in the Peak District11-13 June 1971

Report of the Director: TREVOR D. FORD

WHILST the main theme of the meeting was to study the relationships andnature of the silica-sand pocket deposits on the Carboniferous Limestoneplateau, now known as the Brassington Formation (Boulter, Ford.Ijtaba & Walsh, 1971), the Director expressed the hope in opening theMeeting that the non-specialists would find plenty of interest. The Direc­tor explained that whilst the silica sands had been exploited for refractorymaterials for over a century, there had been little study of them untilrecently, except by the late Dr. Courtenay Yorke. His private publicationsissued over a period from 1954 to 1961 were a valuable record of sectionsno longer available and of the previous opinions concerning the deposits.In recent years, however, there had been a reawakening of interest (seeFord & King, 1969), and one of the objects of the meeting was to re­examine the evidence which had led to conflicting views between theDirector, the Secretary (Dr. Walsh) and others present. In setting out to becontroversial, it is good to report that by the end of the Meeting a fairdegree of agreement had been reached. The Meeting was further madememorable by the discovery of a third locality with the rare Mio-Pliocenefossil flora, in a position which had been forecast on stratigraphicalgrounds.

Field work commenced with a visit to Wynn's Tor (SK 240602), adolomite tor near Winster. This striking crag feature is typical of manyproduced by the interplay of weathering processes during and since theLast Interglacial. The distribution of incoherent dolomite crystal 'sand'around the tors was described as the result of decalcification of dolomitisedlimestone by weathering in both interglacial and post-Glacial times.Nearby, at Donkey Pastures (243602), members collected an unusual pinkbanded variety of baryte.

North-east of Brassington, the disused Manystones Quarry (236551)provided a good section of the contact of dolomitised limestone resting onan irregular surface of unaltered limestone, with clear evidence that dolo­mitisation had proceeded down joints from above. Subsequent ground­water solution had opened up these joints and small caves along thedolomite-limestone contact, and some of these had been partially infilledby minerals such as galena, baryte and calcite, sometimes showing bedding.

Walking across to Harborough Rocks (243552),the party saw abandonedsilica-sand pits in proximity to the dolomite escarpment with its cave which

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had yielded Neolithic and Romano-British remains. From this vantagepoint the geographical relationship of the Bees Nest and Green Clay Pitscould be seen. The party then entered the Bees Nest Pit (by kind permissionof Steetley Refractories Ltd.) and saw the type-section of the BrassingtonFormation (now somewhat weathered and liable to further rapid deteriora­tion):

Kenslow Member: blue-grey plant bearing claysBees Nest Member: vari-coloured clays and thin sandsKirkham Member: white, yellow and red sands and pebble gravels.

This sequence is some 45 m. thick and it has now been recognised in threewidely separated localities, the other two of which were visited later in theexcursion, and there is now little doubt that the three pockets representthree separate foundered remnants of a once-continuous sheet of thesethree members. Other localities which apparently once had the same sectionare no longer available owing to back-filling or flooding. The explanationwas offered that the Brassington Formation was once a sheet of fluviatilesediments covering much of the southern part of the CarboniferousLimestone, and that its constituents were at least in part derived from theretreating Triassic escarpment, now some 8 km. to the south. BunterPebbles in a bleached condition were found in abundance in the KirkhamMember. It was explained that a tacheometric survey by Mr. D. M.Urbani had shown that the deposits had sagged after deposition into atleast three separate sites of solution of the underlying dolomitised lime­stone within the Bees Nest Pit alone. Boreholes had reached the bottom ofsome of these sags at depths approaching 60 m. The Director explainedthat there were large collapse caverns at depths of up to 160 m. at the baseof the dolomite in the nearby Golconda Mine (249551), which was nowclosed. Some of these caverns contained large masses of sand, presumablywashed in from nearby pockets full of the Kirkham Member.

Attention in the Bees Nest Pit focused on a number of outstandingproblems: (1) Were there unconformities between the members of theBrassington Formation? Only the topmost (Kenslow) Member can be datedby fossils, so it was important to know whether there were unconformitiesbetween the members. The consensus of opinion was that the contactswere no more than what might be expected between individual sedimentaryphases of a fluviatile regime. (2) What was the age of the deposits? Dr.Boulter explained that his work on the fossil flora was hampered by thelack of a suitable well-dated comparative Neogene flora. Floras of broadlycomparable age on the continent of Europe were obviously living in a lessoceanic climate and so differences in detail were to be expected. However,he had deduced that the flora was most probably of Mio-Pliocene boun­dary in age with a tendency toward the early Pliocene side of the boundary.

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Acceptance of the absence of unconformities between the members meantthat the underying sands were probably of Mio-Pliocene age also. (3) Wasthere any other evidence to support consanguinity of the members? Mr.Ijtaba explained that his work on the consolidation history of representa­tive clay beds suggested that the members were all of the same age, thoughdiscussion indicated that peri-glacial effects on the clays might mean thathis results had little significance.

On the whole it was agreed that geomorphological arguments supporteda concept of consanguinity of the members of the Brassington Formation.Furthermore some members argued that there was a wider significance inthat the Formation was unlikely to have been deposited at its presentaltitude of 274 to 335 m. above sea-level and that considerable upwarpingof the South Pennines must have taken place in and since the early Pliocene.

The next problem discussed was that concerning the 'drift' overlying theBrassington Formation. In the north-east corner of the Bees Nest Pit, astreaked-out silty clay with scattered blocks and pebbles, which hadpreviously been called a till, was regarded by Dr. Straw as a solifluctiondeposit, though the presence of rare far-travelled erratics confirmed theformer presence of till in the area. The question of the blocks of NamurianShales was next studied on the south face of pit and it was clearly seen thatthe shale was unconformably overlain by the Kirkham Member, and thusthat it was part of a shale cover of unknown extent on the limestone andbeneath the Brassington Formation, which had foundered with it. Theformer interpretation that such shale blocks were erratics in the till couldno longer be supported, at least in this instance. Immediately under theshale a mass of angular chert gravel could be seen to be sandwichedbetween the shale and the dolomite wall of the pocket, and the opinion wasexpressed that this might represent an insoluble residue of the Ds limestones,removed from this area by sub-aerial weathering in early Namurian times.Again such masses of chert gravel had formerly been regarded as erraticsin the till, but such a concept was here hard to support.

The party next visited Kirkham's Pits, west of Brassington (216541),by kind permission of Hoben Quarries Ltd. These actively worked pitsshow the same three members as the Bees Nest Pit, 2 km. to the east. Thecontacts between the base of the Brassington Formation and the pit wallshere showed lilac-stained Namurian shales, in places altered (hydro­thermally?) to white halloysite 'snowballs' encased in manganese-richearthy material, formerly worked as a pigment under the name of 'wad'.The halloysite had been tried for porcelain manufacture by Duesbury, ofCrown Derby fame, in the late eighteenth century, apparently withoutsuccess. The plant-bearing clay was seen near the south-west corner of thesouth pit, where it had been exposed in the working face only a few daysearlier. Numerous pieces of Taxodiaceous wood were seen as well as small

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leaves, and most members left with samples for washing out more delicateplant fossils.

The day closed with visits, firstly to Hulland for a fine exposure of theBunter Pebble Beds (280456) in demonstrating the nature of the parentmaterial of the Brassington Formation, and to Blackwall (257497), for thehill-top outlier of sands and gravels which had previously been called eitherBunter or an early Pleistocene terrace. Opinion of the Meeting was that itwas weathered Bunter, though the old records of erratics here indicateda former cover of Pleistocene gravel, now represented only as remaniepebbles.

During the evening some members visited the underground workings ofthe Royal Mine, Matlock Bath (292579), and saw cavities in the workedout fluorspar deposits filled with laminated sands and silts of presumedPleistocene date.

The following morning opened with a visit to the disused opencastfluorspar workings on the summit of Masson Hill (284592), at an altitudeof 335 m. above sea-level. After explaining the history of fluorspar miningthe Director drew attention to the ancient solution caves filled withlaminated silts and sands, which had been exposed during mining. Thesehad never been studied in detail but the Director pointed out that suchfilled caves could be found all through Masson Hill from the present highlevel, some 244 m. above the River Derwent, down to only 30 m. aboveriver-level. Clearly a very different hydrologic regime was required, and itcould be that the high-level sediments were deposited before the cutting ofthe Derwent Gorge, or when ice blocked the gorge at an early stage ofdevelopment.

The source of the sediments next raised discussion. Opinions variedfrom (a) derived from a former outlier of Millstone Grit (unlikely);(b) from glacial outwash and loess (no erratics seen); or (c) from the formerspread of the Brassington Formation. In the absence of detailed minera­logical studies this problem could not be resolved.

Visiting the Low Moor Pits (189566)next, the party saw a large mass offoundered Namurian Shale lying stratigraphically below the KirkhamFormation sands, with an intervening zone of lilac-coloured weatheredshale and a mass of iron-cemented sandstone. Opinions on this varied from(a) a Triassic relic, (b) a Millstone Grit relic, and (c) a slumped iron­cemented part of the Kirkham Formation. The problem was not resolvedto the satisfaction of all present, but most members accepted the opinionof Dr. Kent that it was not Millstone Grit but an iron-cemented portion ofthe Brassington Formation. Overlying these was a scree of large and smallfragments of limestone derived from the higher flanks of the dry valley.This site showed how the pocket deposits were truncated by the dry valleysystem, which was largely of late Pleistocene age.

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Adjacent to the Low Moor pits, the dolomite tors of Roystone Rocks(197567) were pointed out, emphasising the contrast in the weatheringresponse of dolomite to limestone .

The party next moved on to Kenslow Pit (183615), by permission ofDerbyshire Silica Firebrick Ltd. , and saw the three members of theBrassington Formation in position yet again. Recent slumping of the sidesof the pit has obscured the section, but points noted included the largewood fragments in the Kenslow Member, the presence of shale clasts inone horizon of the Kirkham Member (remarkably like some seen in theTr ias of Cheshire), frequent iron-staining of the Kirkham Member andiron-cemented basal conglomerate to the Kirkham Member in the north­east corner of the pit, and the Namurian shale and chert breccia massesclose to the eastern wall of the pit. These had been regarded by some assolifluction deposits, but on this occasion opinion was that they could bemasses of the sheets of these sedimentary formations once covering thelimestone but which had been dragged down the sides of the solution col­lapses. Finally a number of striated erratics were seen on the westernslopes of the pit but the admixture of bricks, etc., caused suspicion thatthey had all been tipped here.

At Alport (222647) massive tufa deposits were seen, and the Directorpointed out that since they extended down to river level they were of latePleistocene age, but as they were now dry and dead, the catchment for theseepage had been removed. This might indicate, from topographicalrelat ionships, that the Lathkill Valley had truncated the catchment inrecent times, and thus that it was at least in part younger than the BradfordValley at this point of confluence. Such tufa deposits were widespread inDerbyshire but no detailed study of them had been made.

The final visit of the excursion was to the opencast fluorspar miningoperation at Raper Mine, Alport (217653). After explaining the nature ofthe ore deposit, the method of working and the problems of an opencastmining operation in a National Park, the Director drew attention to thetill. In places up to 6 m. thick, this contained a wide variety of striatederratics including Lake District granites and volcanics. Dr. Straw explainedits distribution on a terrace of the Wye-Derwent drainage, about 30 to61 m. above river-level (Straw, 1962). The periglacial frost-shattering ofstones in the top few feet indicated that there had been a further coldperiod since its deposition, and that the age of the till was probably in thePenultimate Glaciation.

The excursion closed at this point, with Mr. H. M. Montford expressingthe appreciation of the party to the Director, who in turn thanked allpresent for their contributions to the solution of at least some of the prob­lems believed to be present two days earlier.

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REFERENCESBOULTER, M. c. 1971. A Palynological Study of Two Neogene Plant Beds in Derby­

shire. Bull. Br. Mus. nat. Hist., 19 (7), 361-410, pis. 1-9.----, T. D. FORD, M.IJTABA & P. T. WALSH. 1971. The Brassington Formation;

a Newly Recognised Tertiary Formation in the Southern Pennines. Nature,231,134-6.

FORD, T. D. & R. J. KING. 1969. The Origin of the Silica Sand Pockets in the Derby­shire Limestone. Mercian Geologist, 3, 51-69.

STRAW, A. 1962. Glacial Drift in the Area round Bakewell, Derbyshire. E. MidldGeogr.,3 (2), 72-80.

YORKE, C. 1954-61. The Pocket Deposits of Derbyshire. Private Publications, Birken­head.

Trevor D. FordGeology DepartmentThe UniversityLeicester LEI 7RH