east midlands geological society - complete issues/mg15_3_2002… · geological history and the...

East Midlands Geological Society

President Vice-PresidentTony Morris Dr Richard Hamblin

Secretary TreasurerAlan Filmer Mrs Christine Moore

Editorial BoardDr Tony Waltham Tony MorrisDr John Carney Mrs Judy RigbyDr Andy Howard Mrs Judy Small

CouncilDr Beris Cox Janet SlatterJack Brown Dr Ian SuttonMiss Lesley Dunn Neil TurnerDr Peter Gutteridge Dr Alf WhittakerDr Andy Howard John WolffMrs Sue Miles

Address for CorrespondenceThe Secretary, E.M.G.S.Rose Cottage, Chapel Lane,Epperstone, Nottingham NG14 6AE0115 966 3854 [email protected]

The Mercian Geologist is published by the EastMidlands Geological Society and printed byNorman Printing Ltd (Nottingham and London) onpaper made from wood pulp from renewable forests,where replacement exceeds consumption.

No part of this publication may be reproduced inany printed or electronic medium without the priorwritten consent of the Society.

Registered Charity No. 503617

© 2002 East Midlands Geological SocietyISSN 0025 990X

Cover photograph: The eastern face of BardonHill Quarry, Charnwood Forest, exposing twoTriassic palaeovalleys that were cut into a hillside ofPrecambrian rocks and filled by red beds of theMercia Mudstone Group [photo: John Carney, withkind permission of Aggregate Industries UK].

Contents

Mercian News 150The RecordGeobrowserFrom the ArchivesBill Sarjeant 1935-2002

Book Reviews 156

Ian Jefferson, Mike Rosenbaum, Ian Smalley 157Mercia Mudstone as a Triassic aeolian desert sediment

Trevor D. Ford 163Dolomitization of the Carboniferous Limestoneof the Peak District: a review

Jonathan D. Radley 171The late Triassic and early Jurassic succession at Southam Cement Works, Warwickshire

Reports 175Edgehill Quarry – Jonathan RadleyHaute Provence – Alan FilmerWieliczka Salt Mine – Tony Waltham

Lecture Reports 180Gold in Britain – Bob LeakeDeep geology of Britain – Alf WhittakerThe Isle of Eigg – John HudsonIs the past the key to the future? – Chris Lavers

Excursion Reports 184Millstone Grit of South DerbyshireThe Yorkshire CoastFountains Abbey and Brimham RocksNottingham Sandstone Caves

Corrigenda 191

SupplementThe Geology of the Matlock Minesby Trevor D. Ford

VOLUME 15 PART 3 JULY 2002

Melton Mowbray Earthquake

At 4.25 pm on Sunday, 28th October, 2001,the East Midlands was affected by a sizeableearthquake. It has been termed the ‘MeltonMowbray Earthquake’, although its epicentre wasactually located farther north, just west of Eastwell(SK770283). The UK Seismic Monitoring andInformation Service, based at the British GeologicalSurvey offices in Edinburgh, determined its localmagnitude to be 4.1 ML, a figure revised from thevalue of 3.8 that had been calculated immediatelyafter the event. According to the EuropeanMacroseismic Scale (EMS), its intensity of just over5 denotes a strong earthquake… ‘felt indoors by manypeople, outdoors by a few, with a strong vibrationcausing windows, doors and dishes to rattle, hangingobjects to swing, doors to open and close and top heavyobjects to fall over’. Such effects were widelyexperienced in an area between Mansfield,Northampton and Kings Lynn and some minorstructural damage was also reported, involving fallenchimneys and cracked walls.

Contrary to media reports, this was not the‘worst earthquake in 250 years’ in these parts. Thathonour goes to the Derby Earthquake of 11thFebruary, 1957, with an intensity of 6-7 EMS(magnitude 5.3 ML) and an aftershock of 5 EMS(magnitude 4.2 ML). A chart of main shocksexperienced throughout the UK landmass wouldnevertheless place the Melton Earthquake currentlyat number 20. It was therefore an important event,although to keep it and the Derby Earthquake inperspective, it should be noted that the ten largestearthquakes in the world since 1900 had magnitudesof 8.5-9.5.

What caused the earthquake is a difficultquestion to answer. Seismologists at BGSEdinburgh calculate that one solution may be thatthe shock was generated at a depth of 11.6 kmbeneath the epicentre by a dextral, oblique slip typeof movement (ie extensional combined with dextralstrike-slip motion). This motion occurred alonga northwards-dipping fault, the orientation ofwhich was east-west. Such a depth would place theearthquake focus within early Palaeozoic basementrocks lying below a major rift structure known asthe Widmerpool Gulf. The latter is a deep,sediment-filled graben of Carboniferous age that isconcealed by Triassic and Jurassic beds in this partof the Midlands. We know that the Widmerpoolgraben was controlled by very large faults, ofbroadly east-west orientation, extending deep intothe basement. We also know that the moderntectonic regime of Britain is controlled by factorsthat include pressures exerted by the south-easterly‘drift’ of the Eurasian Plate, away from the Mid-Atlantic Ridge. Putting these lines of evidence

together, it seems possible that the orientation of theWidmerpool graben faults, or of similar easterlystructures in the basement, would enable them toabsorb modern intraplate strains by allowingportions of the crust to slide laterally past each other.It could be this process, operating at depth andinvolving no surface rupture, that periodicallygenerates earthquakes of the type we experiencedlast year.

Bingham Trails Heritage Association

As part of the Society's charitable objectives ofencouraging education, research and conservationin geology in the East Midlands we haveprovided funding of £250 and intellectual supportto the Bingham THA. Support by EMGS andother local and regional bodies enabled the BTHAto obtain lottery money administered by theLocal Heritage Initiative Fund. Phase one has seenthe publication of five free leaflets (including oneon geology), a display in the Old Court House andBingham Library of a 1:10,000 geological mapand other exhibits, and creation of a website(www.binghamheritage.org.uk). The geologicalleaflet, written by Andy Howard, is veryimpressive; it should bring a greater geologicalawareness to the residents of Bingham, andalso provide useful publicity for the Society inan area where there is little surface geology to beseen.

Nottingham's medieval caves

It is unusual for a journal to include review of apaper in any other journal, but it is appropriatein this case because of the Society's interestand involvement with Nottingham's sandstonecaves. The paper of note is Nottingham'sunderground maltings and other medieval caves:architecture and dating by Alan MacCormick, onpages 73-99 of volume 105 of the Transactionsof the Thoroton Society of Nottinghamshire,dated 2001. This is a significant paper on all theolder caves (pre-dating 1950) known underNottingham. These include the 28 cave maltingsthat are Nottingham's speciality, unmatchedelsewhere. It also covers the 27 other dated medievalcaves, with illustrations and discussion on thecarved pillars and ornamental detail that distinguishthem from the larger numbers of later and morefunctional caves. Appendices include listings of allthe cave sites, and also a valuable series ofcontemporary records of the older caves. Alan'svery welcome paper may not be an easy read(though there are more than 40 drawings andphotos), but it is a fascinating and authoritativeaccount that adds greatly to the documentation ofold Nottingham.

MERCIAN NEWS

150 MERCIAN GEOLOGIST 2002 15 (3)

M E R C I A N N E W S

The Society logo

Unusually for a society or organisation, the EMGShas thrived since its formation in 1964 without alogo. But the Society's imminent guidebook on thegeology of the East Midlands has required a logo togo on the cover alongside that of our co-publishers,the Geologists' Association. Accordingly, memberswere asked for ideas, which were then considered byCouncil. Hammers were rejected on conservationgrounds, mammoths are used elsewhere and thechoice appeared to be which fossil was mostidentified with the East Midlands. Charnia was thefront runner, but a draft appeared to be toobotanical, the palaeontologists lost to thesedimentologists and the Hemlock Stone waschosen. Various modern forms of lettering were triedbut eventually a traditional form of letters encirclingthe stone were chosen in a similar arrangement asused by the Geologists' Association.

Looking back in the Mercian Geologist, thesecretary was surprised to find only a singlereference to the Hemlock Stone - in Volume 1 part1, of 1964, in a report by Dr Frank Taylor (still anactive member of the Society) of an excursion he ledto localities in the area west of Nottingham. TheStone gets just six lines in the report. Happily,Frank Taylor's views expressed in 1964 are stillsupported by geologists currently working in thearea (see page 154 of this issue).

The Society has welcomed 18 new members, andmembership stands at nearly 400 at the end of 2001.Special tributes were paid to three members whohad recently died. Ben Bentley was unable tocomplete his term on Council due to illness; hisenthusiasm and energy, particularly in staffing theSociety display at numerous events and enthusinggeology to all who passed by, will be greatly missed.Edna Colthorpe and Josie Travis were foundermembers from 1964; both worked hard in the earlydays of the Society to ensure its smooth operationand to lay the foundations of its success today, andboth were regular attenders at Society lectures untilthis year.

Field meetings

The programme was disrupted by access problemsdue to the outbreak of foot and mouth disease, butfour excursions were possible.

In July 2001, Peter Gutteridge led a trip to the areaaround the National Stone Centre in Derbyshire,and Keith Ambrose led an evening visit to theMillstone Grit of South Derbyshire.

In September, Andy Howard led a weekendexcursion to Staithes and Cayton Bay, and NeilAitkenhead led a trip by coach to Fountains Abbeyand Brimham Rocks.

Indoor meetings

In March 2001, after the AGM, Dr A.R. Helmsleylectured on hay fever in the Palaeozoic.

In April, Prof. J.D. Hudson talked on thegeological history and the history of the geology ofthe Hebridian island of Eigg.

In October, Dr Sarah Davies talked on forests,floods and fires in the Carboniferous with insightsfrom the sediments of Nova Scotia, Canada.

In November, Dr D.T. Aldiss presented his lectureon the geology of the Falkland Islands.

In December, Dr R.L. Leake talked about newperspectives on gold in Britain and Ireland.

In January 2002, the lecture was by Dr AlfWhitaker on the deep geology of Britain.

In February, Prof. Peter Doyle entertained us withhis lecture on belemnites, the mystery thunder bolt,revealing new findings that featured in the nationalmedia the following week.

Events

The Society was represented at the Geologists'Association Reunion in Liverpool and at theCreswell Crags Archaeology and Geology RoadShow. Unfortunately some regular events werecancelled due to the foot and mouth outbreak.

Alan Filmer, Secretary

MERCIAN NEWS/THE RECORD

MERCIAN GEOLOGIST 2002 15 (3) 151

T H E R E C O R D

Editorial

This issue of Mercian Geologist is rather slimmerthan the first two issues of the volume merelybecause those two were very large; we are now downto smaller issues to bring roughly the right numberof pages within the volume of four issues.Compensation appears by way of the supplement onthe geology of the Matlock mines, which is a jointpublication with the Peak District Mines HistoricalSociety. The EMGS secretary has also launched anew initiative with his item on page 177 of this issueon some geological sites in France; Societymembers are invited to contribute to this series on"Holiday Geology", especially with accessible sitesand sights in Europe.

Geology goes for gold

The victory of the UK women’s curling team at thisyear’s Winter Olympics not only gripped the nation,but also gave a measure of quiet satisfaction to thosewhose only recollection of geology undergraduateteaching was being told that curling stones comefrom Ailsa Craig. To find out if this is still the case,we made enquiries north of the Border and havebeen assured that the finest quality stones are ofmicrogranite obtained solely from that island in theFirth of Clyde.

This alkaline rock is extremely hard and resistantto erosion, which is why it forms upstanding featuressuch as Ailsa Craig, as well as Rockall Island in theAtlantic. At Ailsa Craig it contains a distinctiveriebeckitic arfvedsonite variety of amphibole, and itrepresents a small pluton intruded during the earlyTertiary magmatism (at 62-52 Ma) of westernScotland. Although it is true that the bodies of somecurling stones are now made of granite quarriedfrom Wales, their most important part, that incontact with the ice, must be made of Ailsa Craigmicrogranite, and in the trade this is called an‘Ailsinsert’.

With the sport now extending worldwide (Braziland Israel were the latest to catch the craze),specifications are being tightened and a run on theexisting stone resource is expected. The maincurling stone quarries are on the northeast side ofthe island where the rarer, red (as opposed to blue)variety is particularly prized. They ceased working in1973, but thousands of tons of the rock remain as astockpile. As shown in the picture, roughly hewnstones are prepared on the island for shipment toworkshops on the mainland.

Big in Patagonia

New research is suggesting that the ‘Walking withDinosaurs’ series may have been made a little toohastily. Apparently there were much larger creaturesthat could have been illustrated, and as reviewed inthe New Scientist (September 2000, p.23) these havebeen known about for at least 15 years. Theirexistence has not been widely publicized because todate all the finds come from the former southerlycontinent of Gondwana, whereas the dinosaurhunters’ favourite stamping grounds have been insequences originally from the more northerly,Laurasian continent, in localities such as the Isle ofWight. Many Gondwanan dinosaurs fall into a newcategory, the ‘titanosaurs’, and include such speciesas Gigantosaurus, a carnivore from the Cretaceous ofPatagonia, which at 14 m long and 8 tonnes wasbigger than the largest T. Rex yet found, althoughapparently only half as intelligent. Even this creaturemay have given a wide berth to Argentinosaurus,45 m long and weighing 100 t, the largest animal

GEOBROWSER


G E O B R O W S E R

ever to walk the Earth. These more primitivedinosaurs apparently survived for 50 million yearsinto the Cretaceous of South America, as well as inother parts of Gondwana such as India and Africa.What is intriguing is that close Laurasian relativesare now being found in Europe, including the Isle ofWight. So it may be that the dinosaurs of NorthAmerica and Eurasia, although better known, wereactually the unusual ones, generated in isolation,and it is the southern hemisphere species thatrepresented the evolutionary mainstream during theCretaceous.

Resurrected fossils and an old bug

While new fossils are constantly being found, itseems that other finds are more in the nature ofrediscoveries that have prompted palaeontologists tore-examine many of the ‘established’ extinctionevents. A good example is the early Cretaceous(Cenomanian-Turonian) extinction among theechinoderms. Instead of the 71% of speciespreviously supposed to have perished, examinationof much younger strata, well above this time

Blocked-out curling stone ‘biscuit’ being rolled to theforeshore for transport by boat (photograph from BGSReport Vol. 16, No.9, 1987).

boundary, now suggests that only some 17% did notmake it (Science 2001, p.1037). Basing themagnitude of the extinction on groups of shallow-water taxa may have been the reason for the previouserror, because after the seas retreated these creatureswere not widely preserved. They only returned tothe fossil record some 20 million years later, afterglobal sea levels were re-established and shallow-water environments became better represented inthe rock sequence. Other extinctions are now beinglooked at, to see if there has been a similar biastowards the sampling of species from certainenvironments only. Perhaps the most spectacularexample of a ‘lazarus’ organism is the 250 millionyear old microbe discovered in New Mexico. Asreported in a Times article (19 October 2001), butnot yet confirmed, this early Triassic bacterium wasfound as spores sealed in a salt crystal, which wasretrieved from 600 m below the ground by drilling.It was revived from suspended animation by using agrowth promoter of amino acids, and belongs to onefamily of microbes that presently thrive in the salt-rich environments of the Dead Sea.

Delphi is not just archaeology

The testimony of certain ancient authors has linkedthe Delphic oracle of Greece to specific geologicalphenomena that have included toxic gaseousemissions, a spring, and a fissure in the bedrock.Many archaeologists and geologists have previouslyridiculed these theories, geologists believing thatsuch gases could only have been derived fromvolcanic activity, for which there is no evidencearound Delphi (nor the Gulf of Corinth in general).Support for the ancient ideas is now at hand,however, due to recent work (Geology, 2001; p.707)showing that the Temple of Apollo, in which theoracle or ‘Pythia’ resided, is situated directly over anextensional fault. That structure controlled thelocations of several springs, two of which rose withinthe Temple itself. Waters analysed from the oneremaining spring have included traces of ethylene,most probably incorporated by passage through thehighly bituminous strata beneath Delphi.

As the article explains, and we hesitate to makethis generally known, the inhalation of ethylene caninduce a sensation of ‘floating or disembodiedeuphoria, with a reduced sense of inhibition and ….a frantic thrashing of the limbs’. It was the highpriest Plutarch, quoted in a further recent article(Geology, 2000; p.651), who provided theexplanation for the oracle’s eventual demise. Henoted that things were never the same since thedestruction wrought at Delphi by the famousearthquake of 373 BC in the Corinthian Gulf. Theoracles did struggle on, but by AD 381 the‘euphoric’ powers of these ladies had declined tosuch an extent that the attraction was closed, soending a tradition that had been in existence since atleast 1400 BC.

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F R O M T H E A R C H I V E S

An archive photograph of East Midlands geology fromthe British Geological Survey collection.

The Hemlock Stone

One of the East Midlands’ most well-knowngeological landmarks, the Hemlock Stone is ofcourse very familiar to Mercian Geologist readers,having adorned the cover of each individual part ofVolumes 13 and 14. The Hemlock Stone now sitsproudly in the centre of the Society’s new logo, so itseems fitting to choose the Stone as the subject ofthis issue’s ‘From the Archives’.

Previous archive features have used images fromthe British Geological Survey’s own collection ofsurvey photographs. BGS is also the custodian of theBritish Association for the Advancement of Sciencearchive of geological photos, which numbers over9000 images dating from 1861 to 1945. This photoof the Hemlock Stone, taken in 1890, is from thatcollection. Like almost all photos or drawings of theStone, it shows the pillar from the south, itsnarrowest and most spectacular perspective. Thepillar is in fact several metres broad along its north-south axis, but it is an imposing feature from anyviewpoint.

The Hemlock Stone (BAAS photo #1488, British GeologicalSurvey Library). We have no information on the people in thephotograph, nor on the event they were attending. Can anyreaders help? If so, we will publish a note in the next issue.

The Hemlock Stone lies on the eastern side ofStapleford Hill (at NGR SK499386), to the west ofNottingham. The hill is underlain by Permo-Triassic sandstones of the Sherwood SandstoneGroup. The Lenton Sandstone Formation (formerlyLower Mottled Sandstone) forms much of theslopes, with the basal beds of the Nottingham CastleSandstone Formation (formerly Bunter PebbleBeds) forming a thin capping on the Hill. Almost alldescriptions mention that the Hemlock Stone iscomposed of ‘Bunter Pebble Beds’, but the reality israther different. The sandstone platform on whichthe pillar stands, plus the lowermost 2 m part of thepillar itself (up to the heads of the tallest figures inthe photograph) is in fact in the Lenton SandstoneFormation, a deep red-brown, very fine-grainedcross-stratified sandstone. The rest of the pillar(abou 7 m) is in the overlying Nottingham CastleSandstone. This is a yellowish grey, medium tocoarse-grained, cross-stratified sandstone withcommon large mudstone clasts. Extraformationalquartzite pebbles, typical of this sandstone, becomecommon towards the top of the pillar. TheNottingham Castle Sandstone part of the pillar isstrongly cemented by the mineral baryte (bariumsulphate), but the underlying Lenton SandstoneFormation is not, and retains the characteristic, veryfriable texture seen elsewhere in this formation inthe Nottingham area.

Theories abound about the origin of the HemlockStone, ranging from the supernatural to thescientific. Many have been documented in a well-researched booklet on the Hemlock Stone by R WMorrell, a former Secretary of the EMGS. Familiarold chestnuts such as druids, ley lines and demonicactivity have all appeared in various interpretations.The views of medieval scholars are the mostentertaining. They maintained that the Devil hurledthe Stone into place from Castleton in Derbyshire,in irritation at the chiming of local church bells.Most readers will no doubt discount this theory onintuition alone, but sticklers for a scientificrefutation should note that there is no Triassicsandstone at Castleton.

The Nottingham Castle Sandstone was originallydeposited as a pebbly, fluvial sand by a major,possibly seasonal, river in a semi-arid continentaldrainage basin, perhaps like the Murray-DarlingBasin of present day Australia. Baryte is anauthigenic cement that was precipitated in localisedzones within the formation during burial diagenesis,partly by corroding and replacing detrital feldsparsand grains. Pore-filling carbonate cements,principally ferroan calcite and dolomite, wereformed at about the same time and probablypervaded the entire formation. Most of thesecarbonate cements were subsequently removed byreducing, meteoric groundwaters when theNottingham Castle Sandstone was exhumed fromits overlying cover rocks by erosion during theTertiary and Quaternary periods. This has left thefamiliar, weakly cemented, friable pebbly sandstone

seen at most exposures today. The baryte cement,due to its lower solubility, resisted this solutionprocess, leaving patches of more strongly cementedsandstone within an otherwise friable rock.

Though geologists agree that the strong barytecement accounts for the preservation of the pillar,debate continues about the agency responsible forremoving the surrounding, weaker sandstone. Wasnatural erosion responsible, or was the pillar leftbehind after ancient quarrying activities? Accordingto Morrell, the earliest scientific attempts to explainthe origin of the Stone were by William Stukeley inthe late 18th century, who was the first to putforward the quarry remnant theory. James Shipman,the foremost amateur geologist in the East Midlandsin the late 19th century, favoured natural erosion,especially glacial action, as the cause, an explanationlater followed by the Geological Survey in 1908.The current, ‘official’ BGS view, expounded in theDerby Sheet memoir and also favoured by FrankTaylor in an early Mercian Geologist article, is thatthe Hemlock Stone is a quarrying artefact.

Those favouring the natural erosion theory mainlyallude to the lack of any documented quarryingactivity in the vicinity. However, even a casual strollaround Stapleford Hill reveals copious evidence offormer quarrying on all sides of the Hill and aroundthe Hemlock Stone itself. This includes several oldquarry faces and spoil heaps in various states ofdegradation, indicating a long history of quarrying.Extraction seems to have favoured the LentonSandstone Formation, possibly for use as amoulding sand, but the Nottingham CastleSandstone was probably also won in lesserquantities. It is easy to visualise how, once quarryinghad exhumed the baryte-rich sandstone from itssofter rock surroundings, it would have beenimpossible to work the Stone pillar further for fear oftoppling it, with possibly catastrophic results.Interestingly, the baryte-cemented upper part of thepillar still bears a coating of industrial grime thatprobably pre-dates modern air pollution controls,indicating negligible erosion. The lower part, in thefriable Lenton Sandstone, has no grime coating andis actively eroding at present. This will eventuallyundercut the pillar and cause the entire upper part tofall en masse - but visitors are safe at present!

Andy Howard, British Geological Survey

Literature

Frost, D. V. & Smart, J. G. O., 1979. The geology of the countrynorth of Derby. Memoir Geological Survey G. B., sheet 125.

Gibson, W., Pocock., T.I., Wedd, C.B. & Sherlock, R.L., 1908. Thegeology of the southern part of the Derbyshire andNottinghamshire Coalfield. Memoir Geological Survey G. B.

Morrell, R.W., 1988. Nottingham’s enigmatic Hemlock Stone. 3rdedition. APRA Press, Nottingham.

Shipman, J., 1891. The geology of Stapleford and Sandiacre.Transactions Nottingham Naturalists Society for 1891, 10-19.

Stukeley, W., 1776. Itinerarium Curiosum. Vol 1, p.53.Taylor, F.M., 1964. The geology of the area west of Nottingham.

Mercian Geologist, 1, 73-4.

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VALE


VA L E

Bill Sarjeant 1935-2002

Well known to many members of the Society,William Antony Swithin Sarjeant, DSc., FRSC wasboth a micropalaeontologist of international reputeand a very sociable eccentric with a diversity ofinterests. In 1956, he gained a geology degree atSheffield University, his home town, when, becausehe was born on St. Swithin’s Day, he added his thirdforename by deed poll in time for it to appear on hisdegree certificate. He then stayed on to gain a PhDon the topic of An investigation of the palaeontologyand stratigraphical potentialities of the micro-plankton(dinoflagellates and hystrichospheres) in the UpperJurassic. In his time at Sheffield, he edited the SorbyRecord for the natural history society and also theStudents’ Union newspaper.

Academic positions followed at the Universities ofKeele, Reading, Nottingham and Oklahoma, untilApril 1972, when he became Professor of Geology atthe University of Saskatchewan in Saskatoon, wherehe stayed for the rest of his life.

His research work focused on the study of marinemicrofossils and on the history of the earth sciences,fields in which he was widely published andrecognized. He later expanded his studies to includefossil footprints. Also in 1972, he was awarded aDSc by Nottingham University. His submissionconsisted of 119 items, including 66 papers on fossilmicro-plankton and 31 papers on his other interestsin mineralogy, petrology and trace fossils. A citationat the time stated that … his contribution during thelast 20 years to the study of fossil micro-plankton isprobably unequalled. This despite losing much ofhis research material in a fire at NottinghamUniversity, which cooked his palynological samplesbeyond redemption. In the 1980s, he compiled amassive bibliography of the history of geology, in tenvolumes.

He was a founder member of the East MidlandsGeological Society in 1964. He led the Society’sinaugural field excursion to the Dudley CanalTunnel in the same year and addressed the Societyon “The Geology of Iceland”. He edited theMercian Geologist from 1964 to 1970, andpresented the first foundation lecture to the Societyin 1971. His papers in the Mercian covered CaltonHill asbestos, Derbyshire gypsum and Triassic fossilfootprints, among many others.

Bill was also a founder member of both the PeakDistrict Mines Historical Society and the AmericanAssociation of Stratigraphical Palynologists, and wasa member of twelve other learned societies. In 1995he was elected to Fellowship of the Royal Society ofCanada.

Outside geology, he wrote fantasy fiction under thename of Antony Swithin and was an avid collector ofbooks. He was an authority on Sir Arthur ConanDoyle, and published a critical analysis of theauthor’s “The Terror of Blue John Gap”, withreference to Castleton localities. Folk music was anenduring passion, and he became deeply involved inheritage preservation when in Saskatoon.

He was described in a reference in his earlier yearsas “A man of loyalty and wide interests (extendingfrom poetry to mineral lore), he is a thoroughlydecent chap and an asset to the department; peoplereact differently to his marked streak of naivety andquite unconscious proneness to drop the occasionalbrick”. He was also well known at the Miners Armsin Brassington for his ability to devour entire bowlsof pickled onions.

Bill Sarjeant died on July 8th, leaving his wifeMargaret “Peggy”, daughters Nicola, Rachel andJuliet and two grandsons.

The Pennines and adjacent areas (BritishRegional Geology, fourth edition) by N.Aitkenhead, W. Barclay, A. Brandon, R. A.Chadwick, J. I. Chisholm, A. H. Cooper and E. W.Johnson, 2002. British Geological Survey,Nottingham. 206 pages B5, 29 colour photos, 45figures, 0 85272 424 1, £18.In the mid-1900s the first 18 of the 20 very popularBritish Regional Geological guides were published,mostly with two or three editions and many withlarge numbers of reprints. The original Penninesguide was first published in 1936, with its thirdedition in 1954. Since then there have been manyreprints but until now, no new edition. This was veryunfortunate as it was in those last forty years of thecentury that the science of geology made significantstrides with the advent of plate tectonics and manyother important advances. These dramaticallychanged our understanding of our own regionalgeology. Our wait for a fourth edition of the‘Pennines and adjacent areas’ has at long last beenanswered, and that wait has been well worthwhile.

The area covered by the guide extends fromNottingham northwards to the Stainmore gap, andfrom the Irish Sea coast in the west across to theVale of York and the Trent valley in the east.

The present guide is divided into ten chapters,each with different authors who are past and presentstaff of the BGS. Many of those authors are wellknown to EMGS members as they have, in variousways, made large contributions to the activities ofthe society in recent years.

The opening chapter sets the scene in a review ofcurrent understanding of the broad aspects of thegeology of the region resulting from research in platetectonics, basin dynamics, sequence stratigraphyetc. This chapter is followed by seven more instratigraphic sequence from the pre-Carboniferousto the Neogene and Quaternary. Carboniferousrocks, which crop out across a vast part of theregion, are rightly afforded four of these chapters,and the considerable influence of the Quaternary onthe final moulding and geomorphology of the regionis given extensive coverage. All chapters are writtenconcisely, but in a very readable way, and it issurprising how much detail is presented in a limitedspace. The variations of the sedimentary sequencesacross the region are dealt with expertly, and thepalaeogeographic reconstructions help to stimulateinterest in the geological history.

The penultimate chapter on the structure shouldenhance aspirations of gaining a well roundedknowledge of the geological events that havedeveloped the present geology. Complementing andadding to the story of the region the last chapter isused to demonstrate how those of us who live in theregion have been influenced directly or indirectly bythe geology. The variety of the mineral resources,

their exploitation and influence on the growth ofsettlements and the potential hazards resulting fromthe underlying geology are all developed.

This guide is an excellent update on the thirdedition, and enhances the literature concerning ourlocal area. It is well written throughout, and with thenumerous contributing authors it has had to beextremely well edited to make it the cohesivepublication it is. The text is augmented with wellchosen and numerous text figures and tables. Thecover plate of Gordale Scar is most attractive andthere are 28 further plates, with variable colourquality, that enhance the guide.

The BGS are to be congratulated for thispublication which I thoroughly recommend. It is aquality text for background and general reading, andfor those who wish to pursue aspects of the subjectfurther there is a very good bibliography. The sellingprice is higher than for earlier guides, but there is awealth of content and, with a separate solidgeological map included in the back pocket, it is verygood value.

Ian Sutton

Rocks and Scenery of the Peak District, byTrevor Ford, 2002. Landmark: Ashbourne. 96pages of A5, 39 diagrams, 46 colour photos, 184306 026 4, £7.95. Most EMGS members will have heard one ofTrevor Ford's talks or have accompanied him on afield trip and will be aware of his great ability toconvey complex scientific concepts in a way that anyinterested person can understand. In this concisebook on the Peak District, Trevor has drawntogether many strands of his work, and carried thiscommunication skill to a wider readership. Thegeological processes that have shaped the area togive rise to what can be seen today are clearlyexplained in 18 short chapters, each dealing with arock type, structure or event, for example,limestones, volcanics, the ice age, minerals andmines, and of course caves. This book is a valuablenew overview, presenting information (muchpreviously only available in specialist publications)in a very accessible style.

Alan Filmer

REVIEWS


R E V I E W S


Mercia Mudstone as a Triassic aeolian desert sediment

Ian Jefferson, Mike Rosenbaum and Ian Smalley

Abstract. It has been suggested that at least parts of the Mercia Mudstone in the EnglishMidlands are closely related to loess, or loess-like sediments. It seems likely that the MerciaMudstone is an ancient form of parna, the aeolian desert sediment formed in the AustralianQuaternary. This is essentially a form of loess having silt-sized particles and an opendepositional packing. Loess-like systems tend to collapse when loaded and wetted, and thisappears to have occurred in the mudstone. The collapse behaviour of the Mercia Mudstone isprobably the strongest indication of a loessic origin. Study of the Mercia may throw some lighton the nature of parna - the most characteristic of the Australian loesses.

The original idea appears to be due to Bosworth(1913), that the Mercia Mudstone Group (which heof course knew as the Keuper Marl) was possibly inpart a loessic or loess-like deposit that had beenformed by arid desert processes. In the recent reporton the Mercia Mudstone prepared by the BritishGeological Survey (Hobbs et al, 1998) this idea isstill given some credence, as one of the ways inwhich the Mercia Mudstone was formed. The ideadeserves more discussion, from two points of view,as part of a further consideration on the mode offormation of the Mercian, and as a means ofproviding more data on loessic processes associatedwith hot deserts. The desert loess problem has beendiscussed for many years, in particular since theSmalley & Vita-Finzi paper of 1968. They claimedthat there were no specifically desert processeswhich could produce the fine material needed forloess deposits - and this claim is still being discussed(Sun, 2002). Bosworth saw loess as a desertsediment. Today, loess is seen more as a glacialphenomenon, related to cold phases of theQuaternary, rather than as a desert material. Thenature of ‘desert’ loess and the nature of desert-related loessic processes is still being questioned(Wright, 2001) and the nature of the MerciaMudstone may throw some light on nearlycontemporary processes.

Mercia Mudstone

The Mercia Mudstone Group is a sequence ofpredominantly mudrock strata that underlies muchof southwestern, central and northern England andon which many urban areas and their attendantinfrastructure are built. Although it causes fewserious geotechnical problems, some difficulties doarise as a result of volume change (Popescu et al,1998). It is significant to the construction industrybecause it is frequently encountered in civilengineering activities involving foundations,excavations and earthworks (Nathanial &Rosenbaum, 2000). Its nature is such that itsproperties may vary between a soil and a rockdepending on its detailed lithology and its state ofweathering. These descriptive statements aresupported by the recent report by the British

Geological Survey (Hobbs et al, 1998) on theMercia Mudstone in the context of a series ofmonographs on the Engineering Geology of BritishRocks and Soils; another excellent source ofpractical data is Chandler and Forster (2001).It appears that the Mercia Mudstone was depositedin a mudflat environment in three main ways:● settling out of mud and silt within

temporary lakes,● rapid deposition of sheets of silt and fine sand

by flash floods, ● accumulation of wind-blown dust on wet

mudflat surfaces.This last depositional mode has allowed a

comparison between the Triassic sediment andQuaternary loess. In fact it seems likely that parts ofthe mudstone, notably the outliers in Nottinghamand Leicester, are like the parna deposits which areobserved in south-eastern Australia (defined andnamed by Butler, 1956).

The overall disposition of the Mercia Mudstoneacross the country is controlled by the long-termtectonic trends (Smalley, 1967), and lies parallelto, and probably just to the south of, the major tiltaxis. Land to the south and east of the Mercianoutcrop is subsiding, largely as a result of long-termtectonic trends, but possibly with a contributionfrom isostatic recovery following the last glacialretreat.

Structure and nature

The Mercia Mudstone has been reported (Hobbs etal, 1998) as having a two-stage structure, formedfirst by the aggregation of clay-sized particles intosilt-sized units, and then by the agglomeration ofthese when weakly bonded by various cements. Theprimary ‘intra-ped’ structure is stronger than thesecondary ‘inter-ped’ structure. This is very like thesituation in southeatern Australia where the parnaconsists of a silt-sized material, often found in dunestructures. The silt-sized particles are aggregates ofclay units. The Triassic deserts could have provideda depositional environment very similar to that inPleistocene Australia. Parna behaves in many ways

like loess, has the classic, open, airfall structure, andalso mantles the landscape.

The Mercia Mudstone is a heavily over-consolidated and partially indurated clay/mudrock.It has been credited with ‘anomalous engineeringbehaviour’ and ‘unusual clay mineralogy’throughout. The former is usually attributed toaggregation of clay particles into silt-sized peds orclusters. The clay mineral composition is dominatedby illite (typically 40-60%) with additional mica,chlorite, and corrensite (swelling, mixed layer,chlorite-smectite), with minor smectite, palygorskiteand sepiolite.

Collapse and subsidence

L. J. Wills, who devoted much time to the study ofthe Triassic in England, appeared to suggest thatmudrocks in the Nottingham-Leicester area wereperhaps the most likely to be of a loessic nature(Wills, 1970). He pointed out that the ‘loess’ schoolof thought was initiated by Bosworth in his famousmonograph for the Leicester Literary andPhilosophical Society in 1913, although Bosworthdoes not actually say much about this idea.

In most parts of England and Wales the MerciaMudstone has been subjected to only mild tectonicdeformation. Dips are generally <5º, except in thevicinity of faults, though steeper radial dips occurlocally around the flanks of contemporaneouslandmasses such as the Mendips. Examining thisaspect, Bosworth showed how the mudstonesubsided after deposition (Fig. 2). The Bosworthcalculation showed a structural rearrangementfollowed by a collapse of about 25% - whichcompares reasonably well with the 15% subsidenceobserved when classic loess suffers fromhydrocompaction.

The study of the mudstone collapse may throwsome light on to the nature of the collapsemechanics in loess and loess-like materials. There isstill a large amount of interest in the problem ofhydrocollapse and subsidence in loess soils - inparticular in the Russian language literature(Trofimov, 2001). The Russian investigators see the‘development of collapsibility’ as a critical problemin the study of collapsing loess, and it is possible thatcollapse of parna and Mercia Mudstone mayprovide useful additional information for thisdebate. Loess collapse is influenced by the clay inthe system (Rogers et al, 1994). In theparna/mudstone system, where the particles are allclay mineral material (despite their aggregated siltsize), the particle contacts should all be mobilizable.

IAN JEFFERSON, MIKE ROSENBAUM AND IAN SMALLEY


Figure 1. Outcrop of the Mercia Mudstone Group (afterHobbs et al, 1998).

Figure 2. The original Figure 33 from Bosworth (1913)showing subsidence in the Mercia Mudstone; a roughdiagram, but possibly one of key significance. CitingBosworth - “thus taking α = 35º and θ = 10º, values observedin several cases, we obtain R = 0.25”.

The 25% collapse may be an example of the idealcollapse in aeolian silty sediments. This is asignificant observation and could prove to be thecritical link between the two systems (Fig. 3). Theall-clay particles may behave in an interestinglydifferent manner from the quartz silt particles; theyshould certainly have different shapes. The quartzsilt particles are remarkably flat (Assallay et al, 1998)but the parna/mudstone particles could be muchrounder - as they accumulate, no breakage isinvolved. These could form more collapsiblestructures, giving >20% collapse, rather than thenormal 15% in classic loess deposits.

The Mercia Mudstone is found to be ‘water-softened’ where its upper boundary acts as anaquiclude below sandstone or permeable fill. Hereit can be expected to have a low strength and highdeformability. The factors that cause water-softening in the lithified deposit could be similar tothose that cause hydro-compaction in a recent loessdeposit. In each case the effect of water at theparticle contacts allows strains to develop; in theMercia Mudstone this is seen as high deformability.In the loess there is also high deformability - whichtakes the form of structural collapse or hydro-compaction.

Loess and deserts

If loess is a deposit of wind-blown dust (silt), and ifdeserts are places where dust-storms are observed, itseems intuitive that deserts and loess go together.But the relationship is not as close as it may appear.Since Smalley & Vita-Finzi (1968) raised the issue,there has been the large problem of ‘making thematerial’. There are no obvious sources, in drysandy desert regions, of the silt-sized quartz particlesthat constitute most of the world’s loess deposits.Sun (2002) is close to solving the ‘desert loess’problem by showing that the great deserts to thenorth and west of the Chinese loess deposits act as‘holding-areas’ or large silt reservoirs. These supplysilt for the thick loess deposits, but are themselvessupplied with silt material from the mountains ofHigh Asia.

If there were an alternative method of forming silt-sized particles then desert sources might look morepromising; the alternative would have to provide away of avoiding the need for large geo-energies tofracture quartz particles to provide silt. A non-comminutive method of making silt might be toagglomerate clay-sized particles; these could then bemoved by the wind and deposited as loess-likesediments. It appears that this is what happens inparna deposits (the study of which is rapidlydeveloping) and it may have occurred in at leastparts of the Mercia Mudstone. The Mercianoccurrence is interesting and important because itoffers an extra window on to a currently importantproblem in loess sedimentology.

MERCIA MUDSTONE AS A TRIASSIC AEOLIAN DESERT SEDIMENT


Figure 3. Collapsing systems in metastable airfall structurescompared - loess versus parna (and possibly MerciaMudstone).

loess

open airfall structuree = 1; PD = 0.5Q = angular comminuted

quartz particles ~30 µmsome clay #10%

short range bonds modified by clay at contacts

linear collapse 15%

cold formation environmentperiglacial, glacial source

Quaternary - loess

parnaMercia Mudstone

open airfall structuree = 1; PD = 0.5CA = clay aggregate

particles ~30 µm

initial short-range bondstransform quickly when wet

linear collapse 25%

hot formation environment desert fringe, dry lakessalt lakes, clay dunes

Quaternary - parnaTriassic - Mercia Mudstone

Figure 4. A classicexposure (now flooded)of parna in the PioneerPit in the Murrumbidgeefloodplain 340 km east ofBalranald. The 2 m ofpale soil with blockyjointing just below thedark top layer is the WillsParna.

Parna

We know relatively little about parna (Fig.4) - it is alate-comer among the loess-like materials. It is anaeolian loessic clay, named by Butler (1956) inAustralia, and it has little local geotechnicalsignificance. A recent study (Chen, 1997) of parnanear Wagga Wagga (in S.E. New South Wales) hasdescribed the Quaternary sedimentation, landformsand soils of the region, and a large step forward hasbeen made by the publication of a superb and timelyreview (which provoked this Mercian correlation) byMcTainsh and Hesse (2001). These authors want tocall their material ‘loess’, claiming that confusionsover terminology have prevented Australianinvestigators from playing a significant role ininternational loess science, and the desert loessdebate.

The sedimentary environment in southeasternAustralia over a period of around 17000-15000 BPcould have been very similar to that in parts ofEngland during the Triassic. The Wills map (1970)of the dried-out phases of the German and EnglishNeotrias shows salt lakes and ‘loessic’ regions (Fig.5). The Bowler map (1975) shows the parnaenvironment in part of Australia, with salt lakes andparna dunes (Fig. 6). The unconsolidated sedimentsof the Riverina Plain of New South Wales andnorthern Victoria are dominated by fluvial deposits,but interspersed through the materials, and spreadextensively across the plain, Butler recognized anaeolian clay - parna. These parna deposits occureither as discrete dunes (dune parna) or as thin,discontinuous but widespread sheets (sheet parna).The principal feature of both forms is their origin asaeolian clays transported not as separate clay-sizedparticles but as stable clay aggregates or pellets.Dare-Edwards (1983) proposed three mechanismsfor the production of clay pellets:● deflation of the aerated clay surface of a lake

floor, swamp or dune corridor, following theevaporation and efflorescence of saline water,

● aeolian erosion of pre-existing soil surfaces,● stripping of alluvial clays.

The production of aeolian clay dunes requires theseasonal filling of lakes and swamps by saline waterfollowed by the evaporative drying of these waterbodies. The Australians would like to call parnaloess, and there is no doubt that there are strikingsimilarities (Fig. 3).



Figure 5. The Neotrias(Mercia/Keuper) in Europe(from Wills, 1970).Reception areas weredescribed by Wills asshallow basins on thepeneplain, which was largelydry and loess-covered.

Figure 6. The landscapewith aeolian loessic clays inwestern New South Walesand northern Victoria (fromBowler, 1975) - a possiblemodel for parts of theTriassic in the EnglishMidlands.

The scenario of a wind-swept, unstable landscape,in which lakes and rivers are drying up, vegetationcover is sparse, saline groundwater tables are highand episodic flash flooding occurs, corresponds withreconstructions of palaeo-environments proposedfrom the period 18,000 to 15,000 B.P., insoutheastern Australia. It might also be viable for theEnglish Midlands in the Triassic, and Figures 7 and8 may therefore give some indication of theenvironments around Nottingham in the lateTriassic.

Discussion

There is a precedent for comparing the TriassicEnglish Midlands with Quaternary Australia. Talbotet al (1994), looking at problems of sedimentation inlow-gradient desert-margin systems, compared theLate Triassic of northwest Somerset to the lateQuaternary of east-central Australia. Thiscomparison appears to be valid, and it allowsexamination of a sedimentary system that deliverssilt-sized clay agglomerate particles. Bowler (1975)records the first appearance of clay pellets in aeoliansediments on the Walls of China (a lunette - acrescentic ridge of aeolian sediment on the lee-side

of a salt lake - located on figure 6) at about 26 000BP. The appearance of the clay pellets is related toa phase of increasing aridity and the drying out ofthe salt lakes in the landscape. The link between saltlakes and Mercia Mudstone is also shown on theWills map (Fig 5).

Comparisons between Mercia Mudstone andparna are made somewhat difficult by the variationin lithology in the former. We can recognise, in theEast Midlands, five main lithofacies in the MerciaMudstone Group:● Laminated silty mudstone - very finely

laminated, with ripple marks, shrinkage cracksand planar lamination. Notably the RadcliffeFormation and parts of the GunthorpeFormation. Definitely water deposited, possiblylacustrine.

● Deformed mudstone - as above, but withlamination highly disrupted by shrinkagecracks, soft sediment deformation, and growthand dissolution of evaporitic minerals, thoughtraces of lamination remain. Common in theGunthorpe and Edwalton Formations.Definitely water deposited.

MERCIA MUDSTONE AS A TRIASSIC AEOLIAN DESERT SEDIMENT


Figure 7. Saltbush plainsnorth of Balranald.

Figure 8. Eroded remnantsof a lunette, dated from about65 000 BP, overlie lacustrinesediments on the margins ofthe Mungo salt lake.

● Thin beds of dolomitic or siliceous, fine sand orcoarse silt, often occurring in units of severalbeds separated by mudstone partings - givingrise to the so-called ‘skerries’. Abundantlaminations, with ripples and slumped laminae.Common throughout the Gunthorpe andEdwalton Formations. Definitely waterdeposited, probably by flash floods.

● Fine sandstone, in beds up to 400mm thick,well laminated, interbedded with mudstone ofthe first two types, containing commonmudstone rip-up clasts. The SneintonFormation, Cotgrave Sandstone Member andHollygate Sandstone Member (= Dane HillsSandstone) fall into this category. Definitelywater deposited, probably on distal alluvialplains with periodic floods.

● Structureless mudstone. Most of the upper40m of the Mercia Mudstone Group (CropwellBishop Formation, above the HollygateSandstone, below the Blue Anchor Formation)is of this lithofacies, the best examples beingassociated with the more gypsiferous levelsmined at East Leake and Barrow on Soar.There is another 5m of this facies at the top ofthe Gunthorpe Formation. It occurs in thinbeds elsewhere in the Mercia MudstoneGroup, but is minor compared to the othertypes. This seems to be the strongest candidatefor a parna-type deposit; if our concept is goingto work, this appears to be the material to becompared to the parna.

The original Bosworth proposal still has merit; theonly change required is to replace loess with parna -then the fit appears to be reasonable. We need toknow more about the lithology of parna before moreexact comparisons can be made; the MerciaMudstone has been studied for generations but theparna is new on the scientific scene.

The most interesting aspect of the Mercianmaterial could be its capacity for subsidence. Thisis one of the defining parameters for loess andloess-like materials, the one that gives them theirmajor geotechnical interest, and in this sense parnaand classic loess behave in similar fashion.Bosworth’s rough calculation yields a 25% linearcollapse; the usually cited figure for loess collapseis 15%. There is a critical value of clay mineralcontent that allows a loess to collapse efficiently,and it appears to be about 20%. This ‘small clay’system (Rogers et al, 1994) allows deformationat the particle contacts when the system isloaded and wetted, with the initial requirementbeing an open, metastable structure for thestructure-forming primary mineral units. Moreeffective compaction might occur in the MerciaMudstone due to higher loads over longer times.The observation of structure collapse andsubsidence is the best evidence that MerciaMudstone initially had an open metastablestructure, and the parna seems to be a likelyanalogue.

AcknowledgementsWe thank the organisers and leaders of the INQUA LoessCommission Western Pacific Working Group field trip toS.E.Australia for demonstrating the realities of theAustralian Quaternary and the possibilities of the EnglishTriassic. We thank in particular Prof Jim Bowler of theUniversity of Melbourne, and Dr David Gillieson forproviding photographs. We thank the BGS forbibliographical assistance, and particularly Dr AndyHoward for guidance on the lithologies of the MerciaMudstone.

ReferencesAssallay, A.M., Rogers, C.D. F., Smalley, I.J. & Jefferson, I.F., 1998.

Silt: 2-62 µm, 9-4 phi, Earth Science Reviews 45, 61-88.Bosworth, T.O., 1913. The Keuper Marls around Charnwood.

Leicester Lit. & Phil. Society, 129p.Bowler, J.M., 1975. Deglacial events in southern Australia: their age,

nature, and palaeoclimatic significance. In Quaternary Studies, eds.R. P. Suggate & M. M. Cresswell, Royal Society of New Zealand75-82.

Butler, B.E., 1956. Parna - an aeolian clay. Australian Journal ofScience 18, 145-151.

Chandler, R.J. & Forster, A., 2001. Engineering in Mercia Mudstone.CIRIA, London, 187p.

Chen X., 1997. Quaternary sedimentation, parna, landforms, andsoil landscapes of the Wagga Wagga 1:100,000 sheet S. E.Australia. Australian Journal of Soil Research 35, 643-668.

Dare-Edwards, A.J., 1983. Loessic clays of southeastern Australia.Loess Letter Supplement 2, 3-15.

Hobbs, P.R.N., Hallam, J.R., Forster, A., Entwisle, D.C., Jones,L.D., Cripps, A.A., Northmore, K.J., Self, C. & Meakin, J.L.,1998. Engineering geology of British rocks and soils: MerciaMudstone. Brit. Geol. Surv. Tech. Rept. WN/98/4.

McTainsh, G. & Hesse, P., 2001. Australian dust: processes,sediments, and contributions to soils. Loess Letter 45, 17-24.

Nathanial, C.P. & Rosenbaum, M.S., 2000. Ground characteristicsfor blast furnace site selection above Mercia Mudstone at Redcar.In The engineering properties of the Mercia Mudstone Group; Proc.CIRIA Seminar, Derby, 1998; CIRIA Conference Record 2000/1,London, 123-136.

Popescu, M.E., Rosenbaum, M.S., Nathanial, C.P. & Schreiner, D.,1998. Regional scale assessment of expansive soils as a geohazard.Proc. 2nd Internat. Symposium on the Geotechnics of Hard Soils - SoftRocks, Naples, Italy; Balkema, 283-288.

Rogers, C.D.F., Dijkstra, T.A. & Smalley, I.J., 1994.Hydroconsolidation and subsidence of loess: studies from China,Russia, North America and Europe. Engineering Geology 37, 83-113.

Smalley, I.J., 1967. The subsidence of the North Sea basin and thegeomorphology of Britain. Mercian Geologist 3, 267-278.

Smalley, I.J. & Vita-Finzi, C., 1968. The formation of fine particlesin sandy deserts and the nature of ‘desert’ loess. Journal ofSedimentary Petrology 38, 766-774.

Sun J. 2002. Provenance of loess on the Chinese loess plateau. Earth& Planetary Science Letters (in press).

Talbot, M.R., Holm, K. & Williams, M.A.J., 1994. Sedimentation inlow-gradient desert margin systems: a comparison of the LateTriassic of N.W. Somerset (England) and the late Quaternary ofeast-central Australia. In Paleoclimate and Basin Evolution ofPlaya Systems, ed. M. R. Rosen, Geol. Soc. Amer. Spec. Paper 289,97-117.

Trofimov, V.T. (ed.), 2001. Loess mantle of the Earth and its properties.Moscow University Press, 464p (in Russian).

Wills, L.J., 1970. The Triassic succession in the central Midlands inits regional setting. Q. Jl. Geol. Soc. 126, 225-283.

Wright, J., 2001. Making loess-sized quartz silt: data from laboratorysimulations and implications for sediment transport pathways andthe formation of ‘desert’ loess deposits associated with the Sahara.Quaternary International 76/77, 7-20.

I. F. Jefferson, M. R. Rosenbaum and I. J. SmalleyGeoHazards Group, School of Property and Construction,Nottingham Trent University, Nottingham NG1 4BU



Some 50 km2 of the Carboniferous Limestoneoutcrop in the southern half of the Peak Districtshows evidence of alteration of the originallimestone to dolomite, locally known as dunstonefrom its dull brownish grey colour on weatheredsurfaces. The dolomitized area (Fig. 1) is less than atenth of the total White Peak limestone outcrop, butthe alteration has produced both distinctive rocksand landforms such as dolomite tors (Ford, 1963).Magnesium was introduced in mobile groundwaterslong after sedimentation and resulted in themetasomatic growth of dolomite crystals within thelimestone, so progressively obscuring thesedimentary fabric and fossils (Parsons, 1922).

While the character of the alteration and thedistribution are well known, there has been little

study of the cause of dolomitization or the processesthat might have been involved. Furthermore, twoschools of thought have emerged concerning thedate of dolomitization, broadly late Permian or lateCarboniferous: each would imply the operation of adifferent mechanism for dolomitization.

Dolomites also occur amongst the lowest bedsseen in the Wye Valley and were penetrated in thefew deep boreholes sunk towards the sub-Carboniferous basement (Cope, 1973; Dunham,1973; Chisholm & Butcher, 1981; Chisholm et al.1988). These dolomitic beds are associated withother evidence of sabkha-shoreline conditions to beexpected at the start of an early Carboniferousmarine transgression and are not regarded as part ofthe high-level dolomitization discussed here.


Dolomitization of the Carboniferous Limestoneof the Peak District: a Review

Trevor D. Ford

Abstract. Large areas of the Carboniferous Limestone of the southern Peak District havebeen dolomitized, particularly the more coarse-grained calcarenitic facies. After a summaryof the physical features of dolomitized limestone, its stratigraphic distribution andrelationship to mineralization, the evidence points to a late Carboniferous date ofdolomitization. Possible sources of the magnesium are from buried shales in adjacent basins,effectively as an early flush of hydrothermal fluids, from altered mafic minerals in volcanicrocks or both. It is proposed that a magnesium-rich fluid moved ahead of the hydrothermalmineral fluids and was exhausted before the mineral veins were infilled.

Figure 1. The southern White Peak showing the dolomitized limestone areas (stippled).

The Carboniferous Limestone inliers of Breedonon the Hill in Leicestershire have also beendolomitized (Parsons, 1918) but these are directlyoverlain by the unconformable Mercia Mudstone oflate Triassic age. It seems that a different process hasoperated there: it is not considered further herein.

The aims of this communication are to reviewthe constraints on the process of dolomitization inthe Peak District and to discuss possiblemechanisms.

Petrography

Dolomite is the double carbonate of calcium andmagnesium, CaMg(CO3)2 (Deer, Howie andZussman, 1992). It was named after the 18thcentury French geologist D. G. de Dolomieu (King,1995). In the Peak District it has developed aseuhedral to subhedral crystals within the coarser-grained limestones of the southern half of the WhitePeak (Parsons, 1922). The Limestone and DolomiteResources Reports produced by the BritishGeological Survey in the 1980s gave many analysesand several photomicrographs (Bridge & Gozzard,1981; Bridge & Kneebone, 1983; Cox & Bridge,1977; Cox & Harrison, 1980; Harrison & Adlam,1985) but most did not discuss the dolomitizationprocess. Indeed, Cox & Bridge (1977) simplycommented that the cause was unknown and thatmagnesian fluids could have moved downwardsfrom the Permian or upwards from below.

Together these reports show that the dolomitizedlimestone is never pure dolomite (dolostone inAmerican terminology) but that the degree ofalteration is generally around 50-80% with theremainder of the rock as interstitial calcite. Thecontent of MgO does not reach the theoreticalmaximum of 21%, and maxima are generally below20% (Cox & Harrison, 1980). This degree ofalteration, though incomplete, is enough to yield thetextures and colour of dunstone. Surface weatheringgives a very porous appearance but at depthinterstitial calcite reduces the porosity. Boreholesreported in the BGS resources surveys have shownthat the degree of dolomitization is highly variableboth vertically and horizontally, and that the coarsercalcarenites have usually been altered most.

Dolomitization has taken the form of the growth ofeuhedral to subhedral dolomite crystals in the bodyof the rock (Fig. 2). Their growth has resulted inpartial or almost complete loss of original textures;fossils were destroyed or left as ghost outlines. Somebeds contain silicified fossils and chert nodules andthese are unaffected, being left surrounded bydolomitized limestone.

With the dolomite molecule being smaller thanthat of calcite, there is only a little loss of calcium,which has apparently been redeposited elsewhere,such as in mineral veins. The remaining calcite isusually recrystallized in the interstices. However,calcite is more soluble and weathering processestend to remove it preferentially, eventually leaving a

loose dolo-sand residue. Periglacial sludging takesthe dolo-sand away leaving upstanding cores of still-solid rock known as dolomite tors: Grey Tor andWyn’s Tor near Winster provide good examples(Ford, 1963).

Distribution

As noted by Parsons (1922) and in the variousGeological Survey Memoirs (Frost & Smart, 1979;Aitkenhead et al., 1985; Chisholm et al., 1988;Smith et al., 1967), dolomitized limestone is largelyconfined to two strips of country in the southernWhite Peak, one from Long Dale to Winster and theother around Brassington and Carsington on thesouth flank. Well-known occurrences in the formerarea are Wynn’s Tor and Grey Tor above Winster,and in the latter area are Harborough and RainsterRocks. Each of these areas totals around 20-25 km2.A detached area of about 2 km2 caps Masson Hillabove Matlock, and there are a few scatteredoutliers.

A few old quarry faces and several lead mines showthat the dolomitized limestone lies on top ofunaltered limestone. The depth to the base of thedolomite is highly variable, but is usually not morethan 40 m. The deepest dolomitization baserecorded is in Golconda Mine (Ford & King, 1965)at about 120 m. Here it undulates along a NW-SEaxis in flat-lying beds, and gives rise to a falseconcept of an anticlinal disposition of the mineraldeposits that lie on the flanks of the “high” ofunaltered limestone. In Masson Hill, Matlock, thebase of dolomitization is generally along the line ofthe Masson, Rutland and Wapping Mines, butdescends sharply almost to river level in TempleMine (Ford, 2002).

TREVOR D. FORD


Figure 2. Photomicrograph of dolomitized limestone withdolomite rhombohedra, from Rainster Rocks (photo:G S Sweeting).

Dolomitization does not normally extend beneaththe Namurian shale cover to the east, indeed therather shaly facies of the highest limestone beds (theEyam Limestones) are generally not altered. Only inthe vicinity of Winster is dolomitized limestoneknown to extend beneath the shale cover for about1.5 km (Aitkenhead et al., 1985). However, thestripping back of the shale cover to its presentposition around Winster and Brassington is aPleistocene feature and the Millstone Grit coverwith the thick Edale Shales at its base extended overthe whole dolomite outcrop in pre-Pleistocenetimes.

The stratigraphic relationships of dolomitizedlimestones are shown in Table 1. In the LongDale and Gratton Dale area, dolomitization hasaltered the Bee Low Limestones of Asbian age.Further east, around Winster, it is mainly theMonsal Dale Limestones of Brigantian age whichare altered, but the alteration extends upwards intothe Eyam Limestones where the more massivelimestone beds of reef facies are present. BothAsbian and Brigantian beds have been dolomitizedaround Brassington. The presence or absence oflavas within the limestones sequence appears tohave had some influence, as lavas are few and farbetween in the Long Dale and Brassington areas,where Asbian beds are affected. On Masson Hill,where there are two thick lavas and several claywayboards in the Brigantian Monsal DaleLimestones, dolomitization rarely extendsdownwards into the underlying Bee LowLimestones. The distribution along the two flanks ofthe western extension of the Masson anticline raisesthe question as to whether the eroded upperlimestones between the two areas might have formeda dolomitization cap to the upwarp but no clearevidence of this has been found.

On Masson Hill, dolomitization is largely confinedto the more massive limestone beds between the twomain lavas. These beds are about 40 m thick, withseveral clay wayboards. Dolomitization is confinedto the beds above the “Little Toadstone” wayboard,some 6 m above the Matlock Lower Lava (Ixer,1978). Both the limestones below the LittleToadstone and the dolomitized limestonesimmediately above have been mineralized withfluorspar replacements, pipe-vein cavities lined withminerals, and minor scrins (Ixer, 1974, 1978). Anup-dip fault postulated by Ixer seems to have littleeffect on dolomitization, but the alteration dies outsuddenly down-dip of the Masson Pipe mineraldeposit (Ford, 2002).

The boundary of dolomite on limestone is usuallysharp, with the transition taking place in a fewmillimetres. The boundary is often along claywayboards, but Ixer (1978) noted that the ratio ofMgO fell to a minimum immediately above andbelow wayboards in the Masson opencast, implyingthat the reduced permeability either side of thewayboards restricted fluid migration. ManystonesQuarry, near Brassington (now partly back-filled

with mineral processing waste), showed adolomite/limestone contact undulating across theface with downward prolongations of dolomite onmajor joints (Fig. 3). Within a short distance tothe east the base of dolomitization plunges morethan 120 m to the main levels in the GolcondaMine. Together with the dolomitized HarboroughRocks, the total thickness of dolomitization there isaround 150 m - rather less than the estimate of200 m given by Harrison and Adlam (1985). Akilometre further east, an irregular dolomite/limestone contact is visible outside the GallowsKnoll tunnel on the High Peak Trail. Elsewhere afew sections show a gradation from dolomite tolimestone, apparently as grain-size decreases.Locally there is an alternation of dolomite andlimestone along the contact, where only the coarse-grained limestones have been dolomitized. The fine-grained facies, with much less porosity, remainslargely unaltered. Some contacts are nearly verticaland follow joints, demonstrating that the limestoneswere sufficiently lithified to have developed a jointsystem before dolomitization. Details of othercontacts are to be found in Parsons (1922) and inthe BGS resources reports.

The strip of dolomitization from Long Dale toWinster is roughly parallel to but not always incontact with the Bonsall Fault. The latter cuts offthe dolomitized limestone along part of its length,demonstrating that at least the later faultdisplacements were post-dolomitization. Neither theBrassington nor Masson Hill areas of dolomitizationare juxtaposed to the Bonsall Fault.

The distribution of dolomitized limestoneoutcrops is similar to but not identical with that ofthe Neogene (Brassington Formation) silica-sandpocket deposits (Aitkenhead et al., 1985). As thesetwo phenomena are separated by some 270 millionyears the process of dolomitization is not thought tobe associated with the deposition of the BrassingtonFormation.

Relationship to mineralization

Previous researchers are agreed that dolomitizationpreceded mineralization. Large and small veins(rakes and scrins) pass from one host rock to the

DOLOMITIZATION OF THE CARBONIFEROUS LIMESTONE OF THE PEAK DISTRICT: A REVIEW


Table 1. Stratigraphy of the dolomitized limestones.

stratigraphic unit dolomitizationBrigantian

Eyam Limestones limited Monsal Dale Limestones (Upper) extensiveMatlock Upper LavaMonsal Dale Limestones (Lower) extensive Matlock Lower Lava

AsbianBee Low Limestones patchy

HolkerianWoo Dale Limestones contemporary

other without any substantial change. Perhaps themost obvious example is the Great Rake on MassonHill and High Tor, extending from limestone hostrock in Riber Mine to dolomite above MassonMines (Ford, 2002). Flat, pipe and replacementmineralization is common at or just below thecontact of dolomitized and unaltered limestones,particularly along the Little Toadstone wayboard, asin parts of the Masson Mines (Dunham, 1952; Ixer,1974, 1978; Ixer & Vaughan, 1993; Jones et al.1994; Ford, 1999; 2002). In the Black Ox workingsof the Masson Mines, thin layers of galena partiallyfill bedding planes and micro-joints in pre-existingdolomite. In the Golconda Mine, near Brassington,the base of dolomitization also follows a claywayboard in places (Ford & King, 1965). It isnotable that the mineral veins do not includedolomite as a primary mineral though it isoccasionally present as adventitious lumps or dolo-sand derived from the walls. Dolo-sand in the centreof a baryte-lined vug in Golconda Mine post-datesgalena-barytes mineralization (Ford & King, 1965).

Studies of successive generations of cementationof limestones by Hollis (1998) and Hollis &Walkden (1996) have shown that the calcite cementwas deposited in four layers which they namedZones, each characterized by different trace elementchemistry. Both magnesium and the elements of theore minerals increase in late Zone 3 and in Zone 4 -which those authors regard as having been depositedduring deep burial in late Carboniferous times, atmuch the same time as the main mineralizationphase. While their studies hint at dolomitizationbeing partly contemporary with mineralization, theydid not discuss the relationship of their findings tothe dolomitized limestones discussed herein.

An attempt to obtain magnesium metal fromdolomite near Hopton in the 1960s proveduneconomic, as only about half the anticipatedyield could be obtained. One of the factors is

thought to have been the minor lead-zincmineralization on the many joints interfering withthe extraction process.

The date of dolomitization

As dolomitization preceded mineralization, the realquestion is - when did mineralization take place?Also, in turn, did mineralization precede or followthe folding of the Matlock anticline? Dunham(1952) argued, briefly, that dolomitization hadoccurred as a result of downward percolation ofmagnesian brines from a late Permian Zechsteinmarine transgression across the South Pennineregion, and several authors have accepted thisargument uncritically. However, among others,Frost & Smart (1979) and Cope et al. (1992)thought it unlikely that the Permian transgressionhad extended over the South Pennines. Ifdolomitization was late Permian, then thesubsequent mineralization must have been very latePermian or Triassic in age. An alternative possibilitythat dolomitization might have been due to Triassicgroundwaters (Aitkenhead et al., 1985) wouldnecessitate mineralization having been post-Triassic,for which no evidence has been presented.

Dolomitization clearly followed lithification of thelimestones sufficient for them to have a joint andfracture system. The distribution of dolomitizationsuggests that it was later than the folding of theMatlock anticline. The latter had been growing sinceDinantian times as shown by facies changes in theEyam Limestones and the thinning of the EdaleShales over the Matlock anticline (Smith et al.,1967; Ford, 2002). Upfolding culminated in theWestphalian, as part of the Variscan orogeny at theend of the Carboniferous, demonstrating a fairlyrapid sequence of events at this time – folding andfracturing - dolomitization – renewed fracturing –vein formation. Without reliable means of dating

TREVOR D. FORD


Figure 3. An old face inManystones Quarry, nearBrassington showing theirregular contact of darkerdolomitized limestone aboveand paler unaltered limesonebelow. The lower part of thissection is now buried bywaste material.

these episodes, it is difficult to constrain thesequence in a definite chronological framework buta sequence of events in late Westphalian toStephanian times, i.e. part of the Variscan orogeny,is logical. Burial history curves presented by Hollis (1998)

(after Colman et al, 1989) show that maximumdepth of burial of the limestones at about 2500 mwas reached in late Westphalian times and thatmineralization reached a climax then. FromStephanian times (latest Carboniferous) onwards,there was a steady reduction in the cover withwaning mineralization. Though Hollis did notdiscuss dolomitization, there is a clear implicationthat it preceded mineralization at some stage in theWestphalian.

Fluid inclusion temperatures generally rangingaround 100-120ºC have been obtained mainly fromfluorite crystals in the veins (Atkinson et al., 1982;Colman et al., 1989). They indicate that there was acover of around 2000 m of Upper Carboniferousstrata on top of the limestone at the time ofmineralization. Projection of the Permian base fromthe Mansfield area suggests that most of the CoalMeasures and Millstone Grit had been eroded offthe limestone by late Permian times, so the depth ofburial necessary for the fluid inclusion temperatureswas not available then. These stratigraphicarguments confirm the burial histories deducedfrom cementation generations by Hollis (1998) andHollis & Walkden (1996). Furthermore Triassicrocks lie unconformably on the limestone aroundAshbourne and Snelston, indicating completeremoval of the cover over at least that small area.How much cover remained on the dolomitized areasin the late Permian and Triassic may be uncertain,but it is difficult to argue for enough to account forthe depth of burial sufficient for the formation of themineral veins.

An as-yet unsubstantiated report (Willies, pers.comm.) of pebbles of vein-stuff in the SherwoodConglomerate of the Trent Valley region mayindicate that vein minerals were available for erosionfrom an unknown mineral deposit in the southernPeak District by Triassic times, again indicating thatmuch if not all of the Upper Carboniferous coverhad then been eroded away. .

In addition, if there had been a thick cover of CoalMeasures and Millstone Grit in late Permian times,their thick shales would have obstructed downwardpercolation of magnesian brines to the limestone.And why would such percolation have occurred insuch limited areas?

On the basis of alteration of wayboard claysadjacent to mineral veins, Ineson & Mitchell (1972)applied the K-Ar isotope dating method anddeduced that there had been episodic mineralizationfrom mid-Carboniferous to Triassic, with a climaxin the late Carboniferous. However, the datingmethod has its critics and perhaps not too muchsignificance should depend on these results.

On structural grounds, it seems thatmineralization was an episodic process culminatingin mid to late Carboniferous times (Quirk, 1986,1993). He argued that the changing directions ofstress fields from late Dinantian to end Westphalianyielded fracture systems with varying orientations asa form of ground preparation. Mineral infill of thefractures could have been contemporaneous or later.Plant & Jones (1989) regarded the main episode ofmineralization as being in a short period in the lateWestphalian (upper Coal Measures). This isconsistent with the timing of the “inversion” of theSouth Pennine sedimentary basin to an area of upliftas a result of the Variscan orogeny. Thehydrothermal fluids responsible for both thedolomitization and the mineral vein infills wouldhave been able to migrate from the basin into thegrowing upwarp before too much cover had beenremoved.

The argument that dolomitization was pre-Triassic because the fill of the silica-sand pocketswas Triassic (Kent, 1957) has been negated bydating of the latter as Neogene.

Source of the magnesium

Discounting the Upper Permian Zechstein seabrines as a source of magnesium on chronologicalgrounds, as discussed above, a possible source orsources must be sought elsewhere in the SouthPennine region. There are several possibilities, allwith problems arising.

As with hypotheses concerning the origin of thesolutes in hydrothermal fluids resulting inmineralization, a potential source of magnesium



Figure 4. A dolomitized limestone surface on RainsterRocks. The shadowed hollows are probably where calcitefossils were preferentially dissolved out, though a colonialcoral survives in the lower left.

could be in the Dinantian/Namurian shales ofadjacent basins. Few analyses of the clay minerals ofthe shaly basinal equivalent of the Dinantianlimestones and the overlying shales of Namurian ageanywhere in the Pennines have been published(Plant & Jones, 1989), and no significant figures formagnesium have been traced for shales in the SouthPennine basins. As with the deltaic sandstones of theMillstone Grit, the clay minerals there have beenderived from parent rocks in the Caledonianmountain belt containing some mafic minerals, so asmall proportion of magnesium could be expected.If this was expelled as an early fluid pulse duringburial diagenesis, a magnesium-rich fluid could havemigrated into the Peak District limestones ahead ofthe main hydrothermal fluid. Such a flush ofmagnesium-rich fluid could have penetrated thecoarser-grained limestones and given rise todolomitization before being exhausted; later pulsesof hydrothermal fluid then yielded the vein mineralsthat filled the fracture systems. It is thus possible tovisualize a magnesium-rich front travelling throughthe limestones and causing dolomitization. Once themagnesium source was exhausted, dolomite was notavailable to fill fractures and so does not occur in thevein mineral assemblages.

A second possible source of magnesium could bethe dolomites of the Woo Dale Limestones (thelowest exposed beds in the Wye Valley) (Aitkenheadet al., 1985) or the deeply buried dolomites found inthe few boreholes to the base of the CarboniferousLimestone at Woo Dale (Cope, 1973), Eyam(Dunham, 1973), Via Gellia (Chisholm & Butcher,1981) and Cauldon Low (Chisholm et al., 1988).The first two boreholes encountered some 50-100 mof dolomites at the base of the Carboniferoussequence resting on a basement of Precambrian orLower Palaeozoic rocks, and the others werethought to be close to basement before beingterminated. However, as these concealed beds near

the base of the limestone sequence are stilldolomites, it is difficult to see how they could haveprovided sufficient magnesian fluids while stillremaining dolomite. It is probable that the earlyCarboniferous marine transgression started with asabkha shore-line phase on the South Pennineblock, so any adjacent basins could have had athicker sequence of dolomites, but these have yet tobe proved by deep boreholes.

The third possible source of magnesium is fromaltered volcanic rocks. The exposed lavas arebasaltic, containing a large proportion of maficminerals, principally augite with subsidiary olivine.Alteration by hydrothermal fluids and by weatheringconverts these to clay minerals, mainly chlorites,releasing magnesium. But was there enoughmagnesium released to account for all thedolomitization? From the visible altered portions ofthe lavas probably not, but there is evidence ofconsiderable amounts of concealed basalticvolcanics further east. At Ashover, some 200 m ofbasalts and basaltic breccias were found in boreholeswithout reaching their base (Ramsbottom et al.,1962). There was much alteration in what wasthought to be a vent beneath the Ashover anticline,and magnesium could have been released frommafic minerals therein. Much further east, asubstantial volume of concealed volcanics was foundin the Coal Measures of the N. E. Leicestershirecoalfield, but having these as a source would requirea mechanism to transfer the magnesian brines, bothdown to the limestone through the Coal Measuresand Millstone Grit with their thick shales, and for aconsiderable distance to the Peak District. Takentogether with other as yet unknown concealedbasalts, the volcanics could have been a source ofmagnesium distinct from the usual hydrothermalmineral source in shales, but no mechanism fortransfer of magnesian fluids from the basalts to thelimestones has yet been proposed.

TREVOR D. FORD


Figure 5. Wyn’s Tor – a cragof dolomitized Brigantianlimestone crag near Winster.

Quantification of how much magnesium could beobtained from these sources and how much has beendeposited in the partially dolomitized limestones isimpossible owing to lack of knowledge of theefficiency of extraction and migration and thequantities of the potential source rocks. On balanceit seems that the shales must be the preferred source,with a possible contribution from altered volcanics.

The apparent restriction of dolomitization to thepresent limited areas of outcrop may be misleading,as a dolomitized limestone crest to parts of theMatlock anticline and its western extensions hasbeen eroded away.

Dolomitization in other areas

No comparable large-scale post-depositionaldolomitization of the Carboniferous Limestone hasbeen found in other British orefields (Ixer &Vaughan, 1993). Limited areas of contemporaryfine-grained dolomite are present in the North Walesand Mendip successions of the CarboniferousLimestone, but no such dolomite has been recordedin the North Pennines. Dolomite is present as a veinmineral in the latter though it is not known as a veinmineral in the South Pennine orefield. Widespreaddolomitization of the Waulsortian mud-moundfacies of the Carboniferous Limestone in southeastIreland has been taken to indicate a magnesium-richfluid front moving upwards from the adjacentMunster Basin (Hitzman et al., 1998). The fine-grained character of the limestone apparentlyformed no obstacle, as the mud-mounds haveabundant stromatactis cavities providingpermeability. While the general setting iscomparable to the Peak District, the limestone faciesand tectonic setting in Ireland are different. Abroadly comparable situation to that in Ireland hasbeen described in Cambrian rocks in Missouri(Gregg & Shelton, 1989).

That there is no comparable dolomitization inother British orefields suggests a distinctive sourceor mechanism in the Peak District, but none hasbeen determined. However, it should be noted thatnone of the other orefields has lavas interbeddedwith the limestones. While these may not have beenthe main source of magnesium they certainly wereguiding horizons with limited permeability whichcould have concentrated magnesian brines at certainlimestone horizons, as they did the mineralizingfluids later.

Conclusions

Dolomitization affected some 50 km2 of theCarboniferous Limestone in the southern PeakDistrict. The coarse-grained calcarenite facies seemto have been most altered with generally sharp buttransgressive boundaries, often guided by joints.Dolomitization penetrated to depths averagingaround 40 m with a maximum of around 150 m,with unaltered limestone below. Dolomitization israrely complete but has resulted in the distinctivebrownish dunstone. Both Asbian and Brigantianbeds are affected, with the distribution guided tosome extent by lavas and clay wayboards.Dolomitization was post-folding but pre-mineralization and, as the latter is now generallyregarded as late Carboniferous, so dolomitizationmust also have been late Carboniferous in date.Sources of magnesium are likely to have been theDinantian and Namurian shales of adjacent sedi-mentary basins, effectively providing an early flush ofmagnesium-rich hydrothermal fluids, but a contri-bution from altered basaltic volcanics is possible.

AcknowledgmentsThanks to Bob King, Lynn Willies, Nick Butcher, NoelWorley, Kip Jeffrey and others for useful discussions overthe years and to Tim Colman for his helpful reviewer’scomments.



Figure 6. Rainster Rocks –dolomitized limestones ofAsbian age.

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Bridge, D.McC. & Kneebone, D.S., 1983. The limestone and dolomiteresources of the country north and west of Ashbourne, Derbyshire.Institute of Geological Sciences Mineral Assessment Report 129,78pp.

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Ford, T.D., 2002. The geology of the Matlock Mines – a review.Mining History, 14(6), 1-34.

Ford, T.D. & King, R.J., 1965. Layered epigenetic galena-barytedeposits in the Golconda Mine, near Brassington, Derbyshire.Economic Geology, 60, 1686-1701.

Frost, D.V. & Smart, J.G.O., 1979. Geology of the country north ofDerby. Memoir of the Geological Survey, 199pp.

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Harrison, D.J. & Adlam, K.A.McL., 1985. Limestones of the Peak – aguide to the limestone and dolomite resources of the Peak District ofDerbyshire and Staffordshire. British Geological Survey MineralAssessment Report 144, 40pp.

Hitzman, M.W., Allan J.R. & Beaty, D.W., 1998. Regionaldolomitization of the Waulsortian limestone in southeasternIreland: evidence of large scale fluid flow driven by the Hercynianorogeny. Geology, 26, 547-550.

Hollis, C., 1998. Reconstructing fluid history: an integratedapproach to timing fluid expulsion and migration on theCarboniferous Derbyshire Platform, England, 153-159 in Datingand Duration of Fluid-flow and Fluid-rock Interaction, ed. J.Parnell,Geological Society Special Publication 144.

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Trevor D. Ford, Honorary Research Fellow, Geology Department, University of Leicester, LE1 7RH

TREVOR D. FORD


Warwickshire’s Jurassic outcrop is dominated by abroad low-lying terrain formed by argillaceous rocksof the early Jurassic (Hettangian up toPliensbachian) Blue Lias and Charmouth Mudstoneformations. The Charmouth Mudstone Formationis poorly exposed, however the upper part of theBlue Lias Formation (Rugby Limestone Member;Ambrose, 2001) has been extensively quarried forthe cement industry. Largely inaccessible sectionsoccur in several disused pits, as at Rugby and nearHarbury.

Currently (2002) the only working quarry is atLong Itchington, [NGR SP420630] 10 km E.S.E. ofLeamington Spa (Fig. 1). Here, the deep, extensiveexcavation at Southam Cement Works exposes earlyJurassic mudstones and limestones of the Blue LiasFormation (Saltford Shale and Rugby Limestonemembers; Hettangian up to Sinemurian; liasicus upto bucklandi Zone). Limestones of the underlyinglatest Triassic (Rhaetian) Langport Member of theupper Lilstock Formation (‘White Lias’) are alsoexploited and are crushed for roadstone. The quarryoriginated in the latter part of the nineteenth century(Woodward, 1893) and has been identified as aRIGS (Regionally Important Geological Site) by theWarwickshire Geological Conservation Group.Access is restricted since the closure of RugbyCement’s adjacent factory. Partial descriptions of the succession have beengiven by Clements (1975), Old et al (1987), Swift(1995) and Ambrose (2001). Swift (1999) providedphotographs of the Langport Member succession inthe quarry. The sedimentology of the Blue Lias was

investigated by Weedon (1986) and Wignall andHallam (1991). Aspects of the palaeontology andichnology have been documented by Clements(1975), Gilliland (1992) and Swift and Martill(1999). Jones and Gould (1999) featured LongItchington material in their important study ofoyster (Gryphaea) growth and evolution.Additionally, the site has been mentioned by severalother workers including Nuttall (1916), Arkell(1947) and Hallam (1968). Rocks and fossils fromthe site are held in the collections of WarwickshireMuseum.

Lilstock Formation (Langport Member)

The main quarry floor is a broadly planar (but inplaces hummocky) iron-stained surface, marking theeroded top of the Langport Member. Various pitsand trench sections within the quarry floor exposethe full thickness (about 2.5 m) of the LangportMember, below which lies an unknown thickness ofgrey-green mudstones of the Cotham Member(lower Lilstock Formation). Only the topmost 0.5 mor so of this latter member is exposed at present buta sump opened in the base of the quarry some yearsago showed at least 7 m of the Cotham Member(A. Swift, pers comm).

The base of the Langport Member is marked by aseam of the oyster-like bivalve Atreta intusstriata(Emmrich). This shell bed encloses bored andAtreta-encrusted limestone pebbles; some ofcryptalgal appearance and resembling thestromatolitic ‘landscape marble’ of the Bristol area.Grazing traces attributable to regular echinoids(Gnathichnus pentax Bromley) have been identifiedon several pebbles. Above the Atreta shell layer, thelower half of the member is dominated by pale-greyto cream coloured, irregularly bedded, stylolitic,fine-grained limestones (micrites) with a few thinshale seams. Many of the beds are burrowed, andrecognisable trace fossils include small U-burrows(Arenicolites). Some units are quite fossiliferous,yielding small bivalves (including Gervillella sp.,Plagiostoma sp. and Plicatula cf. hettangiensisTerquem), gastropods, echinoid spines andoccasional solitary corals (Montlivaltia rhaeticaTomes). The top metre of the member is largely apoorly sorted breccio-conglomerate of micrite/bioclastic limestone blocks in a cemented micriticmatrix (Fig. 2). Shallow, channel-like structuresoccur in this upper division.


The late Triassic and early Jurassic succession atSoutham Cement Works, Warwickshire

Jonathan D. Radley

Abstract. Southam Cement Works Quarry, Long Itchington, exposes beds ranging from theCotham Member of the late Triassic Lilstock Formation up into the Rugby LimestoneMember of the early Jurassic Blue Lias Formation. The lithologies and fauna are describedand interpreted in the context of Triassic and Jurassic palaeoenvironmental change.

Figure 1. Locationof Southam CementWorks Quarry

Blue Lias Formation

The benches and walls of the main quarry exposethe Saltford Shale Member (c. 15 m), overlain bythe lower part of the Rugby Limestone Member(c. 25 m) (Fig. 3). The basal Hettangian planorbisammonite zone is unproven (Old et al., 1987) andthe Saltford Shale Member (liasicus up to angulataZone) rests sharply on the eroded surface of theLangport Member. Runnel-like depressions on thissurface preserve a veneer of black mudstonecontaining oysters, echinoid spines and boredlimestone pebbles. Above, the Saltford Shale isdominated by dark grey laminated mudstone with afew beds, lenses and nodules of fine-grainedlimestone. Sharp-based siltstone lenticles about 7.5m above the base of the shale preserve arthropod‘resting’ and burrow traces (Isopodichnus). A layer ofsmall oyster and serpulid-encrusted limestonenodules occurs about 2.5 m higher. Nodules andlenticles of laminated silty limestone towards the topof the member occasionally preserve concentrationsof spar/sediment-filled schlotheimiid ammonites(including the zonal species Schlotheimia angulata(Schlotheim)), together with small bivalves(?nuculids), fish scales and other fine-grainedskeletal debris. The ammonites are commonlyimbricated (Fig. 4). Some are encrusted with oystersthat display xenomorphic sculpture. WarwickshireMuseum’s holdings of Saltford Shale material fromthis site also include nautiloids and partly articulatedichthyosaur, plesiosaur and fish remains. Wignalland Hallam (1991) recorded minute high-spiredgastropods.

The overlying Rugby Limestone Member marksthe development of typical ‘Blue Lias’ facies -regularly alternating limestone, marl and shalymudstone beds (Fig. 3). The limestones are mostlypale grey in colour, laterally persistent and seldommore than 0.25 m thick. They are fine-grained,commonly sharp-based, bioturbated, and oftenhighly fossiliferous. Enclosed fossils are largelyuncrushed. Among the ammonites, schlotheimiids

dominate the lower beds (angulata Zone) and arereplaced upwards by species of Vermiceras andCoroniceras (bucklandi Zone; Clements, 1975).Nautiloids (Cenoceras sp.; commonly oysterencrusted) are common. The bivalve fauna isdominated by Plagiostoma giganteum J. Sowerby andAntiquilima succincta (Schlotheim) (sometimesdisarticulated and oyster-encrusted), together withsmall Gryphaea arcuata Lamarck and Liostrea sp..Pleurotomariid gastropods are recorded.Rhynchonellid brachiopods (Calcirhynchia calcariaS.S. Buckman) are abundant in the so-calledRhynchonella Bed, approximately 8 m above thebase of the member. Crinoid debris and fossil woodoccur in some beds. Clements (1975) providedadditional details of fossils. Trace fossils in thelimestones include Chondrites, Thalassinoides,Kulindrichnus and Diplocraterion. The latter are verycommon at several levels, notably within the WormBed (Clements, 1975) some 10 m above the base ofthe member. The dark grey marls and mudstonesyield rich microfaunas dominated by ostracods,foraminifera, echinoid debris and holothuriansclerites (Clements, 1975). The true shales tend tobe poorly fossiliferous.

JONATHAN D. RADLEY


Figure 2. Loose block of conglomeratic Langport Memberlimestone. Ruler is 15 cm long.

Figure 3. Southern end of Southam Cement Works Quarry.Sections in the foreground are about 2.5 m high and exposepale limestones of the Langport Member. The main cliffexposes alternating pale limestones and darker mudstones ofthe Rugby Limestone Member, above dark mudstones of theSaltford Shale Member. This is nearly 40 m high.

Interpretation of environments

The succession was deposited near the south-western end of the East Midlands Shelf, north westof the partly emergent London Platform. The Valeof Moreton Axis lay a few tens of kilometres to thesouthwest of Long Itchington (Ambrose, 2001),marking the eastern margin of the Severn Basin. TheLangport Member is largely absent in the Severndepocentre, suggesting emergence in latest Triassictimes (Swift, 1995). Thus, the Warwickshireoutcrop may equate to part of a central Englishembayment. The basal Atreta bed marks theestablishment of a mollusc-dominated fauna duringthe initial stages of the transgression recorded by theLangport Member. Above, the Arenicolites-burrowedsurfaces and corals indicate shallow-waterconditions. While of marine aspect, the low faunaldiversity may suggest slightly abnormal salinities.The micrites may signify that warm, calciumcarbonate-rich, predominantly low-energyBahamian-type environments prevailed duringdeposition of the Langport Member (Hallam,1960).

The matrix-supported conglomeratic limestoneswithin the higher part of the Langport Membersuccession resemble muddy debris flows. Similarbeds in the Langport Member of S W Englandsignify reworking of partly lithified rock in a shallowmarine or emergent setting, attesting to lateRhaetian eustatic sea-level fall (Wignall, 2001).Long Itchington material may have a similar origin,but requires further investigation.

Deposition of the Blue Lias Formation wasinitiated during a phase of eustatic sea-level rise(Hallam, 2001), marking establishment of theshallow, epicontinental Jurassic sea over extensiveareas of southern Britain. Following localisedsediment starvation (planorbis Zone) and generationof a basal shell/pebble bed, the Saltford ShaleMember of eastern Warwickshire marks a mid-Hettangian transgressive pulse onto the LondonPlatform (Donovan, Horton and Ivimey-Cook,1979). Its dark, laminated and poorly fossiliferouscharacter indicates deposition in generally anoxic

environments, punctuated by brief oxygenationevents. Wignall and Hallam (1991) attributed theanoxia to rapid deepening, resulting in poorcirculation beneath a stratified water column.Siltstone lenticles, reworked limestone nodules andimbricated ammonite concentrations (Fig. 4)demonstrate weak current influence and a possiblemechanism for periodic oxygenation. Stormgeneration is supported by the abundance ofcomparable storm beds (‘tempestites’) in shallowwater facies of the British Lower Jurassic (Hallam,1997).

Sedimentary environments of the present dayGerman Bight (North Sea) were taken as areasonable analogue for those of the British earlyJurassic by Elliott (1997) and Hallam (1997). In theGerman Bight, thin, silty, storm-flow deposits arewell developed within mud facies slightly belowmaximum storm wave base, at depths of around 20-30 m (Aigner, 1985). A comparable bathymetryseems plausible for the deposition of the SaltfordShale at Long Itchington.

The rapid transition to the relatively fossiliferousand calcareous beds of the Rugby LimestoneMember marks overall increased benthicoxygenation. Relative shallowing is supported by thecessation of sediment onlap onto the LondonPlatform at this time (Donovan et al., 1979). Thecyclic alternations of limestone, marl and shaleconstitute the well-known ‘Blue Lias’ facies of theBritish early Jurassic. Weedon (1986) studied thepetrology of these beds at several sites includingLong Itchington. He concluded that the limestonesare diagenetically ‘overprinted’ calcareous, marlysediments (also see Hallam, 1964). Darker marlsand shales were taken to signify periodicallyincreased terrigenous mud influx, that drownedcarbonate production and resulted in pooreroxygenation at the sea floor.

Weedon (1986) presented evidence to suggest thatthe rhythms resulted from changes in orbitalprecession and obliquity, affecting climate(Milankovitch cyclicity). He took the shaly intervalsto signify wetter phases and increased runoff.

THE LATE TRIASSIC AND EARLY JURASSIC SUCCESSION AT SOUTHAM CEMENT WORKS, WARWICKSHIRE


Figure 4. Fragmented limestone nodule from the Saltford Shale Member enclosing imbricated schlotheimiid ammonites (Warwickshire Museum Collection). The pencil provides scale.

However, Hallam (1986) drew attention to thewholly diagenetic origin of at least some Blue Liaslimestones, potentially weakening the case forMilankovitch cyclicity. Interestingly, Elliott (1997)established the presence of taphonomic andpalaeoecological rhythms in the Blue Lias, which areout of phase with the lithological cyclicity. He alsodocumented several previously undetectedsedimentary hiatuses, further weakening theMilankovitch model. Elliott’s work did not extend toLong Itchington, where there is scope for furtherinvestigation along these lines.

The rich ichnofauna and macrofauna of the RugbyMember limestones at Long Itchington indicategreater benthic oxygenation. The dominantepifauna suggests that the calcareous substrates werefirm and cohesive. The abundance of bivalves isconsistent with a generally shallow marineenvironment (Hallam, 1997), although commonammonites and absence of micritisation suggest anoffshore, possibly subphotic setting. Oxygen isotopeprofiles obtained from growth increments onGryphaea shells indicate an annual temperaturecycle of warmer and cooler conditions (Jones andGould, 1999).

Acknowledgments

The staff of Rugby Cement (Southam Works) are thankedfor access to the Long Itchington Quarry. Andrew Swift(Leicester University) and an anonymous referee kindlyprovided comments on the manuscript.

References

Aigner, T., 1985. Storm Depositional Systems. Lecture Notes in EarthSciences. Springer-Verlag, Berlin.

Ambrose, K., 2001. The lithostratigraphy of the Blue LiasFormation (Late Rhaetian-Early Sinemurian) in the southern partof the English Midlands. Proc. Geol. Assoc., 112, 97-110.

Arkell, W.J., 1947. The Geology of Oxford. Clarendon, Oxford.Clements, R.G., 1975. Report on the geology of Long Itchington Quarry

(Rugby Portland Cement Company). Department of Geology,University of Leicester.

Donovan, D.T., Horton, A. & Ivimey-Cook, H.C., 1979.Transgression of the Lower Lias over the northern flank of theLondon Platform. Journ. Geol. Soc., 136, 165-173.

Elliott, C.J., 1997. Taphonomy of Lower Jurassic Shell Beds and theiruse as a tool in Basin Analysis. Unpublished PhD Thesis, Universityof Birmingham.

Gilliland, P.M., 1992. Holothurians in the Blue Lias of SouthernBritain. Palaeontology, 35, 159-210.

Hallam, A., 1960. The White Lias of the Devon Coast. Proc. Geol.Assoc., 71, 47-60.

Hallam, A., 1964. Origin of the limestone-shale rhythm in the BlueLias of England - a composite theory. Journal of Geology, 72,157-169.

Hallam, A., 1968. The Lias. In: Sylvester-Bradley, P.C. and Ford,T.D. (eds) The Geology of the East Midlands. Leicester UniversityPress, 188-210.

Hallam, A., 1986. Origin of minor limestone-shale cycles:Climatically induced or diagenetic? Geology, 14, 609-612.

Hallam, A., 1997. Estimates of the amount and rate of sea-levelchange across the Rhaetian-Hettangian and Pliensbachian-Toarcian boundaries (latest Triassic to early Jurassic). Journ. Geol.Soc., 154, 773-779.

Hallam, A., 2001. A review of the broad pattern of Jurassic sea-levelchanges and their possible causes in the light of currentknowledge. Palaeogeography, Palaeoclimatology, Palaeoecology, 167,23-37.

Jones, D.S. & Gould, S.J., 1999. Direct measurement of age in fossilGryphaea: the solution to a classic problem in heterochrony.Paleobiology, 25, 158-187.

Nuttall, W.L.F., 1916. Geological Section. Report of the Rugby SchoolNatural History Society for the year 1915, 56-85.

Old, R.A., Sumbler, M.G., & Ambrose, K., 1987. Geology of thecountry around Warwick. Memoir British Geological Survey.

Swift, A., 1995. A review of the nature and outcrop of the ‘WhiteLias’ facies of the Langport Member (Penarth Group: UpperTriassic) in Britain. Proc. Geol. Assoc. 106, 247-258.

Swift, A. 1999. Stratigraphy (including biostratigraphy). In: Swift, A.and Martill, D.M. (eds) Fossils of the Rhaetian Penarth Group.Palaeontological Association, London, 15-36.

Swift, A. & Martill, D.M. (eds), 1999. Fossils of the Rhaetian PenarthGroup. Palaeontological Association, London.

Weedon, G.P., 1986. Hemipelagic shelf sedimentation and climaticcycles: the basal Jurassic (Blue Lias) of Southern Britain. Earth andPlanetary Science Letters, 76, 321-335.

Wignall, P.B., 2001. Sedimentology of the Triassic-Jurassicboundary beds in Pinhay Bay (Devon, SW England). Proc. Geol.Assoc., 112, 349-360.

Wignall, P.B. & Hallam, A., 1991. Biofacies, stratigraphicdistribution and depositional models of British onshore blackshales. In: Tyson, R.V. & Pearson, T.H. (eds) Modern and AncientContinental Shelf Anoxia. Geol. Soc. Special Publication 58,291-309.

Woodward, A.S., 1893. The Jurassic rocks of Britain: Volume III. TheLias of England and Wales (Yorkshire excepted). Memoirs of theGeological Survey of Great Britain.

Jonathan D. RadleyWarwickshire Museum, Warwick CV34 4SAand School of Earth and Environmental Sciences, University of Portsmouth, PO1 3QL

JONATHAN D. RADLEY


Edgehill Quarry, Warwickshire

The Lower Jurassic escarpment of Edge Hill, insoutheastern Warwickshire is capped by theMarlstone Rock Formation. Here, this unit consistsof about 8 m of unusually pure calcitic chamositicironstone, weathering to limonite (Whitehead et al,1952). It thins to the northeast and southwest,accompanied by increasing sand content. Rareammonites collected from the Marlstone prove a latePliensbachian to early Toarcian (spinatum up totenuicostatum Zone) age (Howarth, 1980). It hasbeen quarried around the scarp-top settlement ofEdgehill since mediaeval times for building stone,ornamental stone and aggregate (Edmonds et al,1965).

Hornton stone is now quarried at just one site, onthe crest of Edgehill about 700 m north of the A422road, at SP372468. This site has recently beendeepened, and has an active face over 100 m long.Old quarries northwards along the escarpment arenow backfilled, but other exposures survive nearby,notably within the former Banbury Ironstone Fieldto the southeast, and on the Burton Dassett ridge tothe northeast. The active Edgehill quarry section hasbeen selected as a RIGS (Regionally ImportantGeological Site) by the Warwickshire GeologicalConservation Group. Permission to visit the siteshould be sought from Hornton Quarries Limited attheir Edgehill works.

Lithologies and palaeontology

Excavations within the quarry floor have recentlyprovided sections down to the uppermost part of theunderlying Dyrham Formation (formerly MiddleLias Silts and Clays; Cox et al, 1999), seen to athickness of approximately 0.5 m. Iron-stained clayswere exposed, grading up into marl and richlyfossiliferous bioclastic and oolitic ferruginouslimestone. These beds are rich in pectinid bivalvesand worn belemnite rostra. One particular clay-limestone interface preserves a ‘belemnitebattlefield’ (Doyle & Macdonald, 1993). Thiscomprises a dense accumulation of variably wornbelemnite rostra interspersed with crushed bivalves(Fig. 1).

Overlying these beds, the widely distributedpebble bed at the base of the Marlstone RockFormation (Edmonds et al., 1965) is developed as a10-15 cm hard shelly pebbly ironstone bed,containing numerous pebbles and flattened cobblesof claystone, siltstone and shelly ironstone, up to 20cm in length. The pebble bed matrix yields a wellpreserved fauna of disarticulated pectinid bivalveshells (including large Pseudopecten equivalvis),oysters (including bilobate Gryphaea sportella), fullyarticulated deep-burrowing bivalves in life position(Pholadomya ambigua), abundant belemnite rostra

(Passaloteuthis and Parapassaloteuthis) and ‘nests’ offully articulated rhynchonellid and terebratulidbrachiopods (Tetrarhynchia tetrahedra and Lobothyrispunctata). Many oysters and belemnite rostra areserpulid-encrusted and extensively bored. Somebear grazing traces attributable to regular echinoidsand gastropods (ichnofossils Gnathichnus pentax andRadulichnus).

Above the quarry floor, the Marlstone RockFormation is seen in low faces up to 5 m in height,grading up into stony subsoil (Fig. 2). The lowerpart of the ironstone is intensely weathered andargillaceous, yielding scattered disarticulatedbivalves and further accumulations of fullyarticulated, well preserved Lobothyris punctata. Themain mass consists of the typical finely bioclastic,jointed, locally cross bedded oolitic ironstone,containing scattered wood fragments and a fewother fossils including the body chambers of largenautiloids. Much of the rock is rusty and limonitic,with sporadically distributed blue-green, chamositic‘cores’. Cross sections through bundled, tube-likeburrows are seen on loose blocks. The quarryworkshops provide an opportunity to inspect sawnslabs of the ironstone from several sites in theEdgehill-Hornton area. These show that the rock isextensively bioturbated. Recognisable trace fossilsincluding dumb-bell shaped cross sections throughDiplocraterion and/or Rhizocorallium.

REPORT


R E P O R T

Figure 1. A belemnite rostral accumulation (known as a‘belemnite battlefield’) at the top of the Middle Lias DyrhamFormation.

Palaeo-environments

The onshore, British, mid to late Pliensbachian isrepresented by a shallowing-upward marinesuccession in both basin and shelf settings (Sellwood& Jenkyns, 1975; Hesselbo & Jenkyns, 1998).Situated at the south-western end of the EastMidlands Shelf (Cox et al., 1999), southernWarwickshire clearly demonstrates this regionalpattern. The higher slopes of the Edge Hillescarpment are formed by the silty clays, silts, sands,and impure limestones of the Dyrham Formation,‘coarsening up’ from the Lower Lias clay lowlandsof the Warwickshire Feldon. This episode ofshallowing has been attributed to regional hinterlandand/or sea-bed uplift (Hallam & Sellwood, 1976;Hallam, 1988). The fauna of the uppermostDyrham Formation at this site points to a welloxygenated marine environment. The wornbelemnite rostra (Fig. 1) accumulated during aphase of low sedimentation.

Capping the Dyrham Formation, the pebble bedat the base of the Marlstone is taken to represent theculmination of the shallowing episode. Itsgeneration appears to have involved wave and/orcurrent erosion of bedrock, followed by and perhapsconcurrent with, concentration of worn lithoclasts,shells and belemnite rostra in an intermittently high-energy environment. The hard substrate grazingtraces are significant in this respect, suggestingwidespread growth of algae and cyanobacteria in ashallow-water, photic setting (Bromley, 1994).

Thereafter, the Marlstone apparently signifiesslight deepening, allowing net accumulation ofbioturbated chamositic oolitic sediment. Localised

cross-bedding indicates wave and/or current activityabove storm wave base. The brachiopodaccumulations are comparable to the lifeassemblages documented from the Marlstone ofLeicestershire by Hallam (1961). These may havebeen preserved by rapid burial beneath ooidbedforms. Historically, the Marlstone has beencentral to a debate concerning the origin of ooliticironstones. It is now generally thought that theabundant iron was derived from terrestrial lateritesoils, indicating a warm, humid Pliensbachianclimate (Hallam & Bradshaw, 1979). The fossildriftwood confirms the influence of nearby land,probably the western margin of the LondonPlatform (Cox et al., 1999). The unusually pure,relatively thick ironstone development at Edgehillsuggests deposition in a semi-isolated basin, broadlycoincident with A.H. Cox and Trueman’s (1920)Edgehill syncline.

References

Bromley, R.G. 1994. The palaeoecology of bioerosion. In: Donovan,S.K. (ed.) The Palaeobiology of Trace Fossils. The John HopkinsUniversity Press, Baltimore, 134-154.

Cox, A.H. & Trueman, A.E., 1920. Intra-Jurassic movements andthe underground structure of the southern midlands. GeologicalMagazine, 57, 198-208.

Cox, B.M., Sumbler, M.G. & Ivimey-Cook, H.C., 1999. Aformational framework for the Lower Jurassic of England andWales (onshore area). British Geological Survey Research ReportRR/99/01.

Doyle, P. & Macdonald, D.I.M., 1993. Belemnite battlefields.Lethaia, 26, 65-80.

Edmonds, E.A., Poole, E.G. & Wilson, V., 1965. Geology of theCountry around Banbury and Edge Hill. Memoirs of the GeologicalSurvey of Great Britain. HMSO, London.

Hallam, A., 1961. Brachiopod life assemblages from the MarlstoneRock-Bed of Leicestershire. Palaeontology, 4, 653-659.

Hallam, A., 1988. A reevaluation of Jurassic eustacy in the light ofnew data and the revised exxon curve. In: Sea-Level Changes-AnIntegrated Approach. SEPM Special Publication 42, 261-273.

Hallam, A. & Bradshaw, M.J., 1979. Bituminous shales and ooliticironstones as indicators of transgressions and regressions. JournalGeological Society, 136, 157-164.

Hallam, A. & Sellwood, B.W., 1976. Middle Mesozoicsedimentation in relation to tectonics in the British area. Journal ofGeology, 84, 301-321.

Hesselbo, S.P. & Jenkyns, H.C., 1998. British Lower Jurassicsequence stratigraphy. In: Mesozoic and Cenozoic SequenceStratigraphy of European Basins. SEPM Special Publication 60,561-581.

Howarth, M.K. 1980. Toarcian correlation chart. In: Cope, J.C.W.,Getty, T.A., Howarth, M.K., Morton, N. & Torrens, H.S. Acorrelation of Jurassic rocks in the British Isles Part One: Introductionand Lower Jurassic. Geological Society, London, Special Report14. Blackwell, Oxford.

Sellwood, B.W. & Jenkyns, H.C., 1975. Basins and swells and theevolution of an epeiric sea (Pliensbachian-Bajocian of GreatBritain). Journal Geological Society, 131, 373-388.

Whitehead, T.H., Anderson, W., Wilson, V. & Wray, D.A., 1952.The Liassic Ironstones. Memoirs of the Geological Survey of GreatBritain.

Jonathan RadleyWarwickshire Museum and Portsmouth University

REPORT


Figure 2. Exposure of the Marlstone Rock Formation in theEdgehill quarry.

Réserve Géologique de Haute Provence

The Provence region of southeast France receivesmany British visitors who enjoy the weather, theperched villages, the lavender fields and thedelightful ambience. Haute Provence is less visitedand lies northeast of Provence, bounded to the northby the Dauphine Alps. It is an area of rugged highcountry dissected by many gorges.

The Réserve Géologique de Haute Provenceoccupies 190 000 ha. Its aims are to conserve thegeological assets of the area while making a selectionof them available to visitors. Entry points are markedby large coloured road-side signs that show anammonite. The two main towns of the reserve areDigne les Bains and Castellane, each of which has alarge geological museum devoted to different aspectsof the reserve. There is also a more modest museumin the small town of Barrême (of the LowerCretaceous Barrêmium).

The rocks of the reserve are mainly limestones andshales from the Upper Jurassic, Cretaceous andTertiary. All have been folded on a grand scale bythe Alpine orogeny.

The museum at Digne les Bains is built on a clifffaced with tufa, above the river just to the north ofthe town. From the car park, three trails lead upthrough woodland to the museum, and these pass bysculptures on the theme of rocks and water by pastartists in residence. At the museum, the stream thatpours down the hillside is actively depositing tufa,and also provides the water element in the theme of

the reserve. Each gallery in the museum attempts torelate modern living plants and creatures with thosein the fossil record. There is a room with walk-through aquaria and fossil fish displays, and anotherfor plants old and new. The gallery devoted toammonites themes on modern paintings that areinspired by the fossils on display.

The minor road heading due north out of Digne,close to the museum, reaches 23 km to the village ofBarles. It passes several geological localities, each afew kilometres apart, and each with the familiarammonite sign, a small car park and interpretativenotices (that are mostly bi-lingual).

The first geology-stop, maybe 5 km out of Digne,is a spectacular roadside exposure of a large beddingplane, dipping at about 40°, that contains countlesslarge ammonites.

The next signed stop, a few km further on, has atrail 2 km long through an attractive gorge and up ahillside to an in situ fossil ichthyosaur exposed on abedding plane. The fossil is protected by a glassbox, and has a diorama worthy of Alan Dawnbehind it.

The third site is at the roadside and has birdfootprints in Tertiary sediments (though closeinspection reveals that this is a glass-reinforcedplastic replica of the real material that is now safelyin a museum). The road lies in the Clues de Barlesgorge, which cuts through vertical beds so that topsand bottoms are exposed for inspection.

The next locality offers a short walk on the dry bedof the Bes River, to reach some infilled paleo-channels that are now tilted to the vertical; the sitereveals small-scale depositional features.

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H O L I D AY G E O L O G Y

Overall and close-up views of the roadside ammonite-rich bedding plane north of Digne les Bains.

The last site on the road to Barles has a shortcircular path through woodland, where a number ofreplica plant fossils have been mounted adjacent toliving trees that are considered to have someconnection.

The road south from Digne les Bains is the N85 toCastellane, 54 km away. It passes through the villageof Barrême, where signs point to the geologicalmuseum, housed in the waiting room of the stationon the narrow gauge line from Digne to Nice. Thereare various interpretative boards and lots of fossils(mainly ammonites, needless to say). So abundantare large specimens of the partly uncoiled type thatsome villages along the route have mounted them onplinths on road roundabouts.

At the Col de Leques, 10 km short of Castellane,there is a sign to the Sirènes. A walk of 2 km leads toa valley where the fossilised remains of numerousancestral manatees (sirènes in French) have beenfound. These animals, which today are best knownin Florida, probably gave rise to the mermaid legendthat has its counterparts in many other cultures.About 40 million years ago, a large number ofmanatees died and sank to the sea bed - which wasa surface of Jurassic rocks. The bodies were buriedin mid-Eocene sediments, which therefore lieunconformably on the Jurassic rocks, and the wholesequence was uplifted and tilted to about 40° duringthe Alpine orogeny. After the last ice age a smallstream has cut a valley by exploiting thisunconformity, and has gradually removed a sectionof the Tertiary sediments - to leave the fossilmanatees lying on the old Jurassic sea bed just a fewmetres up the valley side. An elevated boardwalkgives access to the exposure, and the fossils areprotected behind glass screens. Two notice boardsdescribe the life and death of the sirènes, and alsothe legends around them and the wider geologicalframework.

The road from the col continues south toCastellane, where the museum (which we did notvisit) concentrates on the manatee fossils, withreconstructions of the creatures, and a feature ontheir associated legends.

Alan Filmer

Wieliczka Salt Mine, Krakow

Krakow draws the lion's share of visitors toPoland, and justifiably so. The old city has asplendid collection of fine buildings that sufferedvery little damage during the wars of the last century.It is a lovely place to visit, and also has the perfectescape from the urban world in the TatrasMountains just to the south. The geologicallyminded visitor has an added attraction in theWieliczka salt mine, tucked away in the suburb ofthe same name on the southeast side of the city. Thishas been developed for tourist access on a grandscale, and it offers a spectacular and fascinatingunderground tour.

Miocene salt stretches for about 10 km beneathWieliczka, roughly east-west in a nappe structureparallel to the Carpathian front (just to the south).Within this length, the salt is 500-1500 metres thick.Any traces of an early classic diapiric salt dome(formed simply by overburden stress) are now wellhidden in a structure that is thickened by spectacularplastic flow under tectonic lateral compression. Thelower part of the orebody is bedded and folded, butis nearly solid pure salt, except for some interbeddedclay, siltstone, gypsum and anhydrite. The upperpart is a large-scale breccia, a classic residual depositleft by partial dissolution and extensive collapse;blocks of salt many tens of metres across havesurvived within it. Over the salt, a cap of gypsumand clay appears to be a recent dissolution residue astypically occurs on salt domes at outcrop.

Brine springs at Wieliczka were exploited morethan 5000 years ago, but it was only in 1250 AD thatthe source rock salt was first seen during the diggingof a new brine well. Rock mining soon started andthe first deep shaft was sunk before 1500. Theindustry expanded, and there are now 2000 minedchambers, reached by 200 km of tunnels, beneath26 shafts, that descend to a depth of 327 m. Miningended in 1997, and the site is now just preserved forits underground visitors.

The tour route starts with a spiral staircase for 45m down the Danilowicz Shaft. Many of the tunnelsfrom there on are lined with old or new timbers orcolliery arches, but some have bare walls of cleansalt. They break out into a succession of chambers,each of which exploited a single massive block of saltwithin the breccia zone. The first notable room wasone of many turned into a chapel by the miners.It has some excellent statues, arched doorways andtall pillars, all carved out of solid salt - the saltstatues are a much-lauded speciality of Wieliczka.One of the bedrock pillars is out of true, and looksto be the only sign of salt squeezing along the touristtrail. Another large room has a flat roof, stabilisedwith rock bolts, with long straw stalactites of puresalt, comparable to those of calcite in a naturallimestone cave.

HOLIDAY GEOLOGY


H O L I D AY G E O L O G Y

The highlight of the tour is the Chapel of theBlessed Kinga, a room 54 m long. It was excavatedin 1862-1880 by working over 20,000 tonnes of saltfrom a single block in the breccia zone. Now it is hasa salt floor, a huge salt altar at one end, bas-reliefcarvings in walls of translucent salt, and brilliantchandeliers of salt crystals. Coal mines were neverlike this. The tunnel beyond cross-cuts downthrough beds of salt dipping at 50° and impure withclay and gypsum. The walls also expose someexcellent breccia beds, each only 2 m thick, with saltfragments each 50 mm across. They would appearto be left from phases of Miocene dissolution andcollapse that interrupted salt precipitation.

The tour continues down through spaciouschambers floored with deep brine lakes, and thenthrough rooms with monumentally massive timberroof supports. Brine streams still flow in channelsthrough and along some galleries; they were animportant part of the mine production, but now arejust a source of beautiful cubic salt crystals. Anunderground restaurant occupies one of thechambers, and there are yet more large rooms withmuseum displays that are not always open. From therestaurant room, it's 135 m straight up the KingaShaft in an original triple-deck miners' cage - back todaylight.

Tony Waltham

HOLIDAY GEOLOGY


The Chapel of the BlessedKinga in the Wieliczka saltmine, with walls, floor, roof,carvings and chandeliers allmade of salt.

A simplified geological cross section through the Wieliczka salt body.

Gold in Britain and Ireland

Summary of lecture presented to the Society on Saturday8th December 2001 by Dr Bob Leake, of B.G.S.Finding gold by panning alluvial sediment has oftenbeen the first step in exploration that eventually ledto the discovery of gold-bearing mineralisation.Systematic study of alluvial gold grains began atBGS in the mid-80s after the acquisition of anautomated electron microprobe machine capable ofmapping the distribution of elements withinindividual grains. Initial work on alluvial goldfrom South Devon showed a great deal ofinternal compositional heterogeneity to bepresent, particularly in palladium and silvercontents, which often revealed how the grain hadgrown over time. In addition, microscopic inclusionsof different varieties of selenide mineral wereobserved to be associated with some of the types ofgold.

The interpretation of the chemical characteristicsof the South Devon alluvial gold provided thecrucial clues that show that oxidising solutionscirculating within Permian red beds wereresponsible for transporting gold, palladium,platinum and other elements. Deposition thenoccurred when these fluids penetrated into morereducing rocks below the Permian unconformity, orwhere they became mixed with more reduced fluids.On the basis of this model, exploration was switchedfrom South Devon, where the Permian cover hadlargely been removed by erosion, further north tothe Crediton Trough, filled with a thick sequence ofPermian red beds. Alluvial gold similar to that inSouth Devon was found at many localities, and goldmineralisation was found subsequently in situ inassociation with alkali basalt within the Permian redbed sequence. Further application of the model alsoled to similar gold being found in association withthe Mauchline and other Permian red bed basins inSouthern Scotland.

Following the success in studying alluvial goldfrom Devon, BGS began a similar study of materialfrom other parts of Britain and Ireland incollaboration with Rob Chapman of LeedsUniversity. Internal chemical heterogeneity wasfound in gold from other localities, but not to theextent of that present in Devon gold. However, awhole range of other opaque inclusions comprisingvarious sulphides, sulpharsenides, arsenides,sulphosalts, tellurides and other minerals werefound to differ from area to area. The nature ofthese inclusions together with the general chemistryof the gold grains reflected different styles ofhost mineralisation and different geologicalenvironments. Gold from one particular site orarea usually has a definite signature with a distinctrange of composition and a constant inclusionassemblage.

There are three main types of alluvial gold in theSouthern Uplands of Scotland, the silver contents ofwhich are plotted in the graph below, together withsome minor types. The Shortcleugh type ischaracterised by its common arsenopyriteinclusions; it is dominant in the Leadhills districtand also in Galloway, where it has some connectionwith centres of minor igneous activity. Similar goldalso occurs within a comparable geologicalenvironment in the Mourne Mountains of NorthernIreland. The Megget type occurs over a wider areaof the Southern Uplands, and is characterised byinclusions of base metal sulphides and minortetrahedrite. but little arsenopyrite. Similar goldoccurs in South Mayo, Ireland, associated withshear zones in rocks similar in age to those of theSouthern Uplands. The third type is characterisedby inclusions of selenides but not sulphides, and isassociated with red bed basins and their immediatecontact rocks.

Establishing the nature of the different types ofalluvial gold to be found in Britain and Ireland isalso vital in solving the old archaeological problemof the likely sources of gold used to make the manyearly Bronze Age artefacts that have been found,particularly in Ireland. Similarly, comparison ofartefact and natural gold may reveal the nature ofprocesses used in the Bronze Age to extract andrefine gold prior to working.

LiteratureLeake, R. C., Bland, D. J., Styles, M. T. and Cameron, D. G., 1991.

Internal structure of of Au-Pd-Pt grains from South Devon,England, in relation to low temperature transport and deposition.Trans. Inst. Min. Met., 100, B159-B178.

Leake, R. C., Chapman, R. J., Bland, D J., Condliffe, E. and Styles,M. T., 1997. Microchemical characterization of gold fromScotland: Trans. Inst. Min. Met., 106, B85-B98.

Chapman, R. J., Leake, R. C., Moles, N. R., Earls, G., Cooper, C.,Harrington, K. and Berzins R., 2000. The application ofmicrochemical analysis of alluvial gold grains to the understandingof complex local and regional gold mineralization: a case study inthe Irish and Scottish Caledonides. Economic Geology, 95, 1753-1773.

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Silver content of the three major types of gold in southernScotland (numbers of grains in brackets)

Deep geology of Britain

Summary of lecture presented to the Society on Saturday12th January 2002 by Dr Alfred Whittaker, formerly ofBritish Geological Survey.The deep subsurface geology of Britain was a majorresearch topic of the last two decades of the 20thCentury. New and improved types of relevant datafrom British and international deep drilling projects,in combination with related ultra-deep seismicreflection profiling, led to the development ofvarious national programmes and muchinternational discussion and collaboration.

The UK programme was initiated by the BritishGeological Survey whose Deep Geology Unitcarried out onshore seismic reflection surveysrecorded to 12 seconds of Two Way Travel Timeand giving data to the base of the continental crust.The seismic programme was later extendedconsiderably by the offshore seismic surveys carriedout by the BIRPS group at Cambridge.Interpretations divided the British crust verticallyinto three zones of different reflection character. Thelowermost crustal zone was characterised by laterallypersistent strong reflections, and was interpreted ascomprising rock material subjected to ductile typesof deformation, while the two overlying (andprobably seismogenic) middle and upper zonesshow brittle deformation characteristics. The upper(sedimentary, ‘layer cake’) zone displays manynormal growth faults that commonly overlie low-dipping seismic reflection features of the middlezone (in turn interpreted as important thrust faultsand detachment surfaces). The structuralrelationships in the upper and middle zones indicatereactivation of faults in both normal and reversesenses following successive periods of tectoniccompression and tension during Phanerozoic time.

In addition, many boreholes (in the depth range2-3 km) were sited, prognosed and managed by theDeep Geology Unit, sometimes as stratigraphic‘step-out’ wells to encourage oil prospection in partsof Britain and funded by the Department of Energy.Also drilled were geothermal exploration andproduction wells as part of the UK GeothermalProgramme. The development of an internationalprogramme of continental scientific drilling in the1980s and 1990s brought about much scientificcollaboration and ultimately proposals for a Britishprogramme discussed in depth at a GeologicalSociety meeting in London in 1987.

Related projects, now belonging to the currentInternational Continental Scientific DrillingProgram (ICDP), were the 12km-deepSoviet/Russian borehole in the Kola peninsula,which proved unexpected mineralisation andhydrological activity at much greater depths thanwas previously thought possible, and the Germandeep drilling project (KTB) which drilled to 9 kmdepth in the early 1990s. The lecture ended with abrief description of the Chicxulub ICDP drillingproject (which began in December 2001) located inthe Yucatan peninsula of the Gulf of Mexico. At thetime of writing the Chicxulub borehole was at adepth of 500m, about half way to the expectedCretaceous-Tertiary (KT) boundary, rich in theplatinum-group mineral iridium. The drilling targetis a well-defined impact crater presently consideredto be the site where a major asteroid or meteorite fellto Earth 65 million years ago and caused the massextinction at the KT boundary.

LiteratureChadwick, R.A., Kenolty, N. & Whittaker, A., 1983. Crustal

structure beneath southern England from deep seismic reflectionprofiles. Journal Geological Society London, 140, 893-911.

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A north-south geoseismic crustal section across the Variscanfold belt of southern England to 18 km deep.

The Isle of Eigg: its geological historyand the history of its geology

Summary of lecture presented to the Society on Saturday7th April 2001 by Prof. John Hudson of LeicesterUniversity.In the late 19th and early 20th centuries, the islandof Eigg, in the Hebrides, was famous in Britishgeological discourse, out of all proportion to its size.Fossil wood, the “Eigg Pine”, had been describedfrom there as early as the 1830s. In the 1840s, HughMiller discovered the first Scottish plesiosaur andgraphically described his visits in his posthumous“Cruise of the Betsey” of 1858. Then ArchibaldGeikie, in 1865, interpreted the dramatic pitchstonerock of the Sgurr of Eigg as the fill of a pre-existingvalley, carved in the underlying basalts, and nowstanding proud of its surroundings. This providedhim with an object lesson in the power of erosion ata time when the erosional origin of valleys was notuniversally accepted. In 1906 Alfred Harkerchallenged Geikie’s interpretation, claiming that thepitchstone was a transgressive sill, not a lava flow;Geikie was outraged, and in 1914 E.B. Bailey sidedwith him. But perhaps because of this clash of thegeological titans of the day, the Sgurr lost itstextbook popularity, and petrologists who studied itsmineralogy and geochemistry were reluctant tocomment on its field relations.

In more recent times, these 19th centurydiscoveries and controversies have been re-investigated. Geikie’s interpretation of the Sgurrpitchstone has been vindicated. Ann Allwright’smapping has revealed a buried landscape, not just asingle valley, beneath the pitchstone. At its base thepitchstone contains pumice-shards, showing thatexplosive eruptions preceded the lava flow. Fossilwood occurs partly in the basal pitchstone butmainly in underlying conglomerates in the base ofthe old valley system, attesting to a vegetatedlandscape. Some of the wood occurs as charcoalfrom wildfires, perhaps started by volcanicactivity.

The Jurassic rocks, especially those now known asthe Kildonnan Member, contain remarkably wellpreserved fossils, ranging from protists to Miller’splesiosaur. Visitors are often surprised by theoccurrence of mollusc shells of pristine aragonitebeneath a substantial pile of basalt lava flows. Thisexcellent preservation has enabled the author andcolleagues to use isotopic geochemistry, as well asmore traditional palaeontology, to interpret theenvironment of deposition, which we believe to beshallow, brackish lagoons, fed mainly by freshwaterinflow, close to the coast and subject to occasionalinvasion by seawater and marine fauna. The ValtosSandstone, forming the cliffs on the west coast ofEigg, contains remarkable concretions cemented bycalcite. These developed in the subsurface, and the

larger ones took millions of years to grow. Erosion ofthe sandstone contributes sand to the presentbeaches, especially the famous “singing sands”.

These sandstone cliffs, and the gleaming whitesand, form an unforgettable picture as theforeground to the view of the mountains of Rum.The top of the Sgurr is one of Scotland’s supremeviewpoints; the whole framework of northwesternScotland, from the Outer Hebrides, via the Minch,Skye, Mull and the Small Isles, to the Caledonianmountains of the mainland, is laid out before you.The island of Eigg has much to offer geologists, andit is a delightful place to visit.

Reference

Emeleus, C. H., 1997. Geology of Rum and the adjacent islands.Memoir of the British Geological Survey, Sheet 60 (Scotland).

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The Sgurr of Eigg with the unconformity clearly visible wherethe massive pitchstone fills in the valley that had been cutinto the horizontal basalts.

Is the past the key to the future?

Summary of lecture presented to the Society on Saturday10th February by Dr Chris Lavers, of NottinghamUniversity.The principle of uniformitarianism - that the presentis the key to the past - has served earth scientists wellfor many years. The philosophy is quite odd,however, in the sense that it became entrenched at atime when geologists were beginning to suspect thatthe earth must be immensely old. It follows from thisthat the present must be no more than the finestshaving from the tip of geological time. Can wereally understand and interpret 4.6 billion years ofhistory using the present, any more than we canunderstand the Eiffel Tower by examining a sliver ofpaint from the top? Of course we must learn aboutthe past by studying the present, but we may alsolearn about the present, and perhaps also the future,by studying the past.

For example, two of the most pressingenvironmental problems that we currently face areglobal warming and biological invasions (the lattersometimes referred to as ‘global mixing’). How maywe judge the likely impact of these processes on thebiosphere? Computer modelling is one way but,different models yield different predictions, and thequestionable reliability of such techniques,particularly in relation to climate, is widelyacknowledged.

Another approach is to look to the past forguidance. In the case of global warming and mixingthe best analogue is arguably the Permian-Triassictransition. The potential for global mixing at the endof the Permian was great because the earth’scontinental landmasses had all coalesced to form thesupercontinent of Pangea. Recent research alsosuggests that a 6oC rise in temperature at equatoriallatitudes occurred at this time, a figure at the upperend of the prediction for average global warming by2100 by the Intergovernmental Panel on ClimateChange. Warming at higher latitudes is thought tohave been even greater, leading to the establishmentof a relatively uniform warm to hot climate acrossPangea. The change is thought to have been broughtabout by the release of carbon dioxide from theoxidation of coal-bearing deposits in the southernpart of Pangea, and latterly from the eruption of theflood basalts of the Siberian Traps.

The Permian-Triassic transition was marked bythe largest mass extinction in the history of life. Thebulk of extinctions on land probably resulted fromthe homogenisation of habitats across Pangea as thepole-to-equator temperature gradient flattened.This gradient also drives the circulation of theoceans, and it seems likely that circulation slowed somuch at the end of the Permian that the world’soceans became catastrophically depleted in oxygenand nutrients. The most extraordinary aspect of

both terrestrial and oceanic ecosystems in theearliest Triassic was the ultra-low diversity andextreme cosmopolitanism among the survivors.Ninety per cent of all terrestrial tetrapodassemblages at this time, for example, consist of theremains of just one type of animal: the dicynodontLystrosaurus.

Although global warming has been cited as thedeadly coup de grâce for Palaeozoic life, it isinconceivable that global biodiversity would havefallen so low at the end of the Permian hadcontinents and oceans been separated as they arenow. Isolation acts to protect and generatebiodiversity whatever the extraneous circumstances.We may not today be pushing the continentallandmasses together in a physical sense today, butour transportation of species around the globe ishaving much the same effect. In fact the rate ofglobal mixing is higher today than at any time inEarth’s past.

To contend that we are currently heading for aPermian-like environmental catastrophe would betoo extreme, but the parallels are clearly evident.The same processes, of global warming and globalmixing, are certainly in operation, and are widelyacknowledged to be key threats to the biosphere. Assuch, even if the late-Permian is rejected as ananalogue for our current environmentalpredicament, it at least seems sensible to take thisfascinating time in Earth’s history as a warning.

Literature

Lavers, Chris, 2000. Why Elephants Have Big Ears. Gollancz:London.

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Millstone Grit of South Derbyshire

Leader: Keith Ambrose (BGS)Wednesday 25th July 2001

The purpose of this excursion was to examineexposures of the inlier of Namurian strata, atMelbourne in South Derbyshire. This inlier hasreceived very little attention from past workers. Itwas briefly described by Fox-Strangways (1905,1907), who identified three subdivisions; an upperseries of fine-grained, thin bedded sandstones; amiddle series of massive, coarse, yellow, brown andred grits; a lower series of coarse conglomeraticbeds. He noted that tracing the beds any distancewas difficult and made no attempt to correlate thegrits with those in North Derbyshire, nor did he giveany indication of age. Mitchell and Stubblefield(1948) noted that the Dawsons (Carver’s) Rocksinlier was probably the Rough Rock, citing evidencefrom a brick pit at Worthington in support. Here,marine Namurian beds were proved with a faunasuggesting a horizon a little below the Rough Rock.Kellaway and Horton (1963) noted a Marsdenianfauna from boreholes near the Staunton HarroldReservoir and the Subcrenatum Marine Band,defining the base of the overlying Langsettian(Lower Coal Measures), was mapped at outcrop justto the south of Melbourne. This identified thepresence of the Rough Rock. Ford (1968) notedArnsbergian shales in the Ashby Borehole, sited justwest of the Namurian inlier. Monteleone (1973)reported an E1 (Pendleian) age for the beds atDiminsdale, based on palynology, but gave nodetails of the assemblage. Fulton and Williams(1988) suggested the inlier was of Kinderscoutian toYeadonian age.

Recent mapping by the British Geological Surveyand an appraisal of cored boreholes in the area hasprovided new evidence for the Namurianstratigraphy (Ambrose, 1997, in prep; Ambrose andCarney, 1997; Carney et al., 2001a, b). Theboreholes have proved several marine bands andidentified the Rough Rock, Chatsworth and AshoverGrits. These grits have in turn, been mapped out atthe surface. They have been worked for buildingstone in several places and this excursion visited twoof these quarries, one in the Ashover Grit and one inthe Rough Rock

The Millstone Grit sandstones have traditionallybeen regarded as deltaic in origin; the Ashover Gritwas deposited by a major braided river complex,probably flowing across a delta top. Sediment, richin feldspar grains and derived from a northerlysource, was deflected to the NNW along theWidmerpool Gulf by a landmass which lay to thesouth. This landmass, variably referred to as ‘StGeorge’s Land’, ‘Midland Barrier’, ‘Wales-BrabantIsland/Barrier’, ‘Midland Landmass/Massif’, is alsothought to have supplied sediment in the early

Namurian. Heavy mineral studies show two sourcesof sediment in the Millstone Grit of southDerbyshire. In the Melbourne Borehole, the lowestgrits and the Chatsworth Grit were all derived froma southerly source, while the Ashover Grit andRough Rock were derived form a northerly source.The same pattern was seen in the WorthingtonBorehole but with some suggestion of mixed sourcesin the Ashover Grit (Hallsworth, 1998).Palaeocurrent measurements taken from somesandstones show currents mainly to the north andnorthwest, with some to the west and northeast.

Locality 1 was at a quarry in the Ashover Grit atStanton by Bridge. This quarry provided the stoneused to build the bridge across the River Trent, here,linking it with Swarkstone on the north side of theriver. The Ashover Grit crops out extensively aroundStanton by Bridge where it is a single sandstonewhich is at least 33 m thick. To the east andsoutheast, the Ashover Grit splits into more thanone sandstone bed. Its correlation with the AshoverGrit of the Melbourne Borehole is based on thepresence of common, coarse, angular, pink K-feldspar grains; the sandstones above this level at

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Outline map of the geology of the Melbourne area, showingthe two localities visited.

outcrop and in the borehole are all devoid of pinkfeldspar.

The grit is well exposed in two quarries at Stantonby Bridge and has been worked in several others(which are now overgrown). It exposes about 8 m ofbuff to grey, fine- to very coarse-grained, poorlysorted, cross-bedded sandstone, which is commonlypebbly. The pebbles comprise rounded quartz andangular to subangular pink feldspars. Alternatingcoarser and finer foreset laminae with rapid upwardfining are seen in some beds. Individual sets varyfrom 0.3 to 1.2 m thick and many have erosive baseswith pebbly lags. Palaeocurrent data collected fromhere and a nearby quarry showed dominant trendsto the north, northwest, and west with minorvariations to the southwest, south, northeast andeast.

Locality 2 was a quarry on the south side ofMelbourne. The quarry, which provided much ofthe building stone used in the town, exposes about15 m of the Rough Rock, which forms theuppermost bed of the Millstone Grit. Recognition ofthis sandstone as the Rough Rock is based onevidence from earlier mapping carried out in 1963,prior to the construction of the Staunton Harroldreservoir. The Subcrenatum Marine Band wasidentified at outcrop (Kellaway and Horton, 1963)overlying the sandstone and marking the base of theoverlying Lower Coal Measures (Langsettian).

The main face exposes a uniform sequence of buff,with some red or orange-brown staining, fine- tomedium-, locally coarse-grained sandstones. Thesandstones are almost exclusively planar cross-bedded with sets generally 0.5-1.0 m thick. Locallythere are thinner sets and in parts of the quarry,there are at least three thick sets 2.0, 3.55 and 4.0 mthick. No parallel-lamination is exposed. Someparts of the quarry expose large-scale trough cross-bedding, seen only in the lower beds. Thesebedforms are not persistent on any one level andpass laterally into planar cross-bedding. The sets areup to 1 m thick and 3-4 m wide. The recognition oftrough cross-bedding may be a function ofaccessibility as the upper beds are not readily visibleor are absent.

In some of the sets, clearly defined coarser (coarse-to very coarse-grained) and finer laminae arepresent, indicating grainflow and grainfall processesrespectively. Bottom sets are well developed in manyof the sets and bases may be planar or gentlyundulating; some have a basal pebbly lag, consistingof angular to well rounded granules and smallpebbles up to 20 mm in diameter, of vein quartz andpink feldspar. The bottom sets of some units are alsocoarser. The upper of the two thick sets showsclearly defined rapid fining upward cycles 1-4 cmthick. In the uppermost beds, climbing ripplelamination is locally developed. Palaeocurrentmeasurements taken from planar forsets and troughaxes show flow directions varying from WSW toNE.

The only other palaeocurrent data from the RoughRock in this area has been published by Fulton andWilliams (1988), from the outcrop at Carver’sRocks, to the west of the present area. They show adistinct bimodal distribution to the west andnorthwest. Bristow (1993) considered the RoughRock to have been deposited by a braided river on adelta top.

References

Ambrose, K., 1997. Geology of the Melbourne area. BritishGeological Survey Technical Report WA/97/40

Ambrose, K., in prep. The stratigraphy of the Namurian (MillstoneGrit) of South Derbyshire, England.

Ambrose, K. & Carney, J. N., 1997. Geology of the Calke Abbeyarea. British Geological Survey Technical Report WA/96/17.

Bristow, C. S., 1993. Sedimentology of the Rough Rock: aCarboniferous braided river sheet sandstone in Northern England.291-304 in Best, J.L. & Bristow, C.S. (eds). Braided Rivers.Geological Society Special Publication No. 75.

Carney, J. N., Ambrose, K. & Brandon, A., 2001a. Geology of theLoughborough district: a brief explanation of geological sheet 141Loughborough. Sheet Explanation of the British Geological Survey,1:50 000 Series Sheet 141 Loughborough.

Carney, J. N., Ambrose, K. & Brandon, A., 2001b. Geology of thecountry between Loughborough, Burton and Derby. SheetDescription of the British Geological Survey, 1:50 000 Series Sheet141 Loughborough. 92pp.

Ford, T. D., 1968. The Millstone Grit. 83-94 in Sylvester-Bradley,P. C. & Ford, T D. (eds.). The Geology of the East Midlands.Leicester University Press.

Fox-Strangeways, C., 1905. The geology of the country betweenDerby, Burton-on-Trent, Ashby-de-la-Zouch and Loughborough.Memoir of the Geological Survey of Great Britain, Sheet 141.

Fox-Strangeways, C., 1907. The geology of the Leicestershire andSouth Derbyshire Coalfields. Memoir of the Geological Survey ofGreat Britain.

Fulton, I. M. & Williams, H., 1988. Palaeogeographical change andcontrols of Namurian and Westphalian A/B sedimentation at thesouthern margin of the Pennine Basin. 179-199 in Sedimentation ina Synorogenic Basin Complex: the Upper Carboniferous of NorthwestEurope. Besley, B. M. & Kelling, G. (eds). Glasgow and London:Blackie.

Hallsworth, C. R., 1998. Stratigraphic variations in the mineralogyof the Millstone Grit in the Melbourne area: implications forprovenance. British Geological Survey Technical Report WH/98/53C

Mitchell, G. H., & Stubblefield, C. J., 1948. The geology of theLeicestershire and South Derbyshire Coalfield, 2nd edition.Wartime Pamphlet Geological Survey of Great Britain, 22.

Monteleone, P. H., 1973. The Carboniferous Limestone of Leicestershireand South Derbyshire. Unpublished PhD Thesis, University ofLeicester.

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The Yorkshire Coast

Leader: Andy Howard (BGS)Weekend 22nd-23rd September 2001

This weekend trip had been organised at shortnotice to supplement the Society’s field excursionprogramme in 2001, which had been severelycurtailed due to countryside access restrictionsarising from the Foot and Mouth Disease epidemic.The aim of the trip was to demonstrate the widevariety of Jurassic sedimentary rocks exposed on theYorkshire Coast and their associated depositionalenvironments, including shallow marine stormdeposits, shallow water carbonates, ironstones andfluvial sandstones. Of particular interest was theprofound influence of sea level change on thedeposition of the sequence, and the trip providednumerous opportunities to compare and contrastthe sedimentary facies and fossils in adjacent marineand non-marine formations.

Cleveland coast

The Saturday was blessed with warm and sunnyweather. Twelve EMGS members met in the carpark at Staithes, and walked down to the harbour toview the shallow marine sandstones of the StaithesFormation. These are of Lower Jurassic age, andconsist of thin, well-laminated sandstone bedsinterbedded with bioturbated, argillaceoussiltstones. The sandstone beds contain abundantevidence of rapid deposition by storms, includinghummocky cross-stratification, wave ripples andsharp erosional bases with shell lags. Theinterbedded siltstones are thoroughly bioturbated bya diverse assemblage of trace fossils, reflecting lowerdeposition rates during fair weather conditions.

Engineering works on the breakwaters at Staithesharbour had temporarily created a tidal pool thatcould not be crossed until an hour and a half beforelow tide. This delay was taken up by an early lunch,leaving the entire afternoon to traverse the excellentexposures of the Cleveland Ironstone and WhitbyMudstone formations on the foreshore betweenStaithes and Port Mulgrave. The ClevelandIronstone includes several beds or seams of sideriticironstone containing ooids (formerly known as‘ooliths’) composed of the mineral berthierine, aniron-rich clay mineral. Each ironstone bed containsa diverse and often highly fragmented fauna ofmarine bivalves and also abundant trace fossils,including the distinctive U-shaped burrowRhizocorallium. Each ironstone bed was formedduring an extended episode of non-deposition andchemical alteration of the sea bed sedimentsassociated with a period of sea level rise. Each bedlies above an upward coarsening sequence of severalmetres of mudstone, siltstone and fine sandstone,reflecting periods of gradual sediment accumulationand basin infill between the sea level rises.

At Old Nab to the south east of Staithes, formerunderground pillar and stall workings in theCleveland Ironstone Formation have been exhumedby marine erosion. After examining these workings,the party moved on to the Whitby MudstoneFormation exposed near Port Mulgrave. Here, alarge landslip in 1993 had brought down hugeamounts of fresh ‘Upper Lias’ mudstone andironstone nodules from the upper cliff, providing afertile hunting ground for ammonite collectors.Members took the opportunity to gather somefossils, and were able to compare their finds withthose of the many dedicated fossil collectors stillscouring the landslip material. After searching theWhitby Mudstones for jet, a mineral formed fromthe compressed and altered remains of fossil wood,the party entered the small harbour at Port

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The Staithes Formationabove the village harbour.

Mulgrave. This was built in the 19th century to shipiron ore from mines at Grinkle, about 2 km inland.The ore was transported to the port through aninclined tunnel, the portal of which is still visiblenext to the harbour breakwater.

The party ascended the cliffs at Port Mulgrave,noting the landslips associated with a spring line atthe base of the Middle Jurassic sandstones thatformed the upper part of the cliff. They thenfollowed the pleasant cliff top path back to theStaithes car park.

Cayton Bay

In marked contrast to the previous day, Sundaydawned dull, cold and equinoctal; seven membersattended for the day. Cayton Bay, southeast ofScarborough, is essentially a buried valley floored byMiddle Jurassic rocks, and plugged by glacial till ofLate Devensian age. Marine erosion has cut into thesofter glacial till plug to form the central part of theBay, with the stronger Jurassic bedrock forming theheadlands, or ‘nabs’, at each end. The till isextensively slipped with several excellent examplesof classic, amphitheatre-shaped rotationallandslides, and the beach is strewn with severalWorld War II pillboxes that were originally built atthe top of the cliffs.

The day commenced with an examination of thelate Middle Jurassic sandstones of the OsgodbyFormation, which form the lower part of HighRed Cliff at the southern end of Cayton Bay.These sandstones contain a diverse fossilassemblage of marine molluscs and trace fossilssimilar to the Staithes Formation. But, unlike thatformation, the entire rock is thoroughly bioturbatedwith only traces of original sedimentary laminationremaining, suggesting that storms were of lesserimportance as an agent of deposition. The OsgodbyFormation contains almost spherical calcareousnodules over 1 m in diameter, each containing

beautifully preserved trace fossils. Lyingimmediately below the Osgodby Formation, theCornbrash Formation consists of a thin,bioturbated, ferruginous limestone with berthierineooids and abundant marine fossils includingthe distinctive, oyster-like bivalve Lopha marshii.The Cornbrash, which unconformably overliesthe non-marine mudstones of the RavenscarGroup, strongly resembles the ironstone seamsof the Cleveland Ironstone Formation. Like eachof them, it marks a very long episode of non-deposition associated with a major rise in sealevel.

After an extended lunch to wait for the outgoingtide, the party negotiated the boulder field at thesouthern end of Cayton Bay to view the MiddleJurassic rocks of the Ravenscar Group exposed atYons Nab. The Yons Nab Fault separates thesestrata from the younger rocks of High Red Cliff. TheRavenscar Group is mainly composed of marginalmarine, deltaic and fluvial sediments but includesthinner units of marine strata. Again, periodic sealevel change had a major influence on deposition ofthe sequence. The lowest strata seen at Yons Nabwere the marine, oolitic limestones of the MilleporeBed, a direct equivalent of the LincolnshireLimestone of Lincolnshire and Humberside.Unfortunately, the high state of the tide madethese rocks difficult to examine. The overlyingGristhorpe Member, however, was very wellexposed. This unit consists of marginal marinemudstones, siltstones and sandstones and containsabundant evidence of smaller scale oscillations insea level. Beds containing rootlets and plantfossils, including the famous Gristhorpe PlantBed, strongly indicate emergence and soilformation, whereas other beds with marine tracefossils indicate submergence and at least quasi-marine conditions.

The excursion concluded with an examination ofthe Scarborough Formation, another marine unitthat overlies the Gristhorpe Member. TheScarborough Formation is composed of well-laminated sandstones interbedded with bioturbatedmudstones and siltstones containing a marinebivalve and trace fossil assemblage. It stronglyresembles the Staithes Formation and was probablyalso deposited in a shallow marine setting influencedby major storms. These beds are overlain by a fluvialchannel sandstone with a sharply erosional base thatcuts down several metres into the underlying marinebeds, indicating that deposition of the formation wasterminated by a substantial fall in sea level. Thechannel sandstone displays an excellent example of‘epsilon’ cross-bedding, which is formed by lateralmigration of meanders within a sinuous fluvialchannel. On retracing their steps back to CaytonBay at the end of the trip, members were able tofollow this channel at a higher level in the cliff, anddetermine both the extent of its lateral migrationand the degree of downcutting into the underlyingScarborough Formation.

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The view down to Port Mulgrave.

Fountains Abbey / Brimham Rocks

Leader: Neil AitkenheadSunday 30th September 2001

Fountains Abbey

Having travelled to North Yorkshire by coach, theparty disembarked at the Fountains Abbey VisitorsCentre where the leader outlined the day’sprogramme. The abbey was built by the CistercianOrder between c.1130 and 1526 and made wealthyby its extensive sheep-rearing lands extending rightacross the Pennines. The carefully preserved ruinsand surrounding land are owned by the NationalTrust and maintained by English Heritage, and arenow a World Heritage Site.

The abbey lies in the incised meandering valley ofthe River Skell, part of which was landscaped in the18th Century with lakes and follies as well as‘artfully contrived views’. However, it was thescattered rock exposures in the valley that were to beour main objectives and required a walk of about 2.5km to the first exposure.

Generally the river has eroded down to 30-40 mbelow the adjacent terrain to expose bedrock in cliffsformed by the meander scars. However, the presentflow seems too weak to erode such a deep valley. Itseems likely that most of this erosion was by glacialmeltwaters, perhaps initially sub-glacially and thenpro-glacially. This occurred mainly during themelting of the Pennine ice-sheet towards the end ofthe last (Devensian) glaciation some 14 000 yearsago. However, since this area was near the marginsof both this ice-sheet and that of the Vale of York,meltwater erosion may have taken place over anextended time interval.

High relief moraines to the south of FountainsAbbey (e.g. How Hill with a relief of about 30 m atSE 276670) are thought to be marginal to the Valeof York ice-sheet, and the extensive cover of glacialtill on the ground adjacent to the Skell valley wasprobably deposited by that ice.

Around Fountains Abbey, rocks of the CadebyFormation (Lower Magnesian Limestone) of LatePermian age rest unconformably on rocks of theUpper Carboniferous Millstone Grit Group. Mostof this area lies in a lower sub-division of theformation characterized by deeper water facies incontrast to “patchily-developed littoral or sub-littoral dolomites” with abundant bivalves (notablyBakevellia) in places (Smith, 1974). This deeper-water facies generally comprises “slightly fetid,thinly bedded to flaggy calcitic fine-graineddolomites and dolomitic limestones with knobblybedding planes that are commonly coated with blackstylolitic residues”. Carbonate-lined cavities areabundant. Mottling similar to that in Durham iswidespread, and much of the rock has a tendency toautobrecciation and a chunky fracture.

Our walk took us down into part of the Skell valleyknown as The Valley of the Seven Bridges, whereseven meanders are each crossed by little hump-backed bridges. Locality 1 was next to the farthestdownstream of these bridges.

Locality 1

Rubbly thin-bedded dolomitic limestones with fairlyconstant bed thickness occur at river level. Therewas some discussion about the origin of the rubblybedding, which is sometimes attributed toburrowing or bioturbation. However, there seemedto be no evidence of the former presence ofburrowing organisms such as bivalves and it waspointed out that similar rubbly beds in Durhamwere thought by Smith and Francis (1967) to be theresult of pressure solution.

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Geological map of theFountains Abbey area,showing the route followedand localities visited. All theground outside the Skellvalley is covered with severalmetres of glacial till. Thestratigraphic column showsonly members of theKinderscoutian within theNamurian Millstone GritGroup (based on BGS maps).

A precipitous face of apparently massive thick-bedded dolomitic limestone is present with its baseabout 10 m above river level showing markedcontrast with the underlying thin-bedded facies.Access was not attempted due to its steepness but itwas noted that the base was sharp, undulating andpossibly disconformable, with a suggestion of crossbedding just above the base. A sample obtainedfrom just above the base on a previous visit appearedunder the hand lens to be a partially dolomitisedgrainstone in which a relict oolitic texture could justbe discerned.

Walking upstream to Locality 2, it was noted thatglacial till had slumped down almost to river level onthe north bank and there were numerous erratics ofCarboniferous sandstone on the bed of the river.

Locality 2

A meander scar extends for over 100 m, exposingsome 12 m of a similar facies to the lower beds atLocality 1, at a slightly lower stratigraphic level. Bedthicknesses range from 5 to 20 cm, varying onlyslightly along the strike. At one point a springseeping from the rock face has produced a largeprotuberance of moss-covered tufa.

Locality 3

After walking round The Lake, we arrived at a smallexposure by the path below the Octagon Tower.This comprises blocky grey fine-grained dolomiticlimestone (calcisiltite), and loose blocks containedtiny, white, coiled foraminifera.

Locality 4

The geological map (BGS, 1987) indicates that atsome point along the path we were following, wewould cross the unconformity where the CadebyFormation rests on the Carboniferous LowerPlompton Grit. However, fragments of dolomiticlimestone in the bank behind the folly known as the‘Temple of Piety’, indicated that we were still on theoutcrop of the Cadeby Formation.

Locality 5

A rubbly bank next to the junction of paths headingeast and west of Half Moon Pond was found to havesmall exposures of medium- to coarse-grainedsandstone. These are Millstone Grit just below thebasal Permain unconformity, the actual plane ofwhich was not exposed. However, its line can befairly easily traced as it descends into the valley atthis point.

It was pointed out that this unconformity is ofgreat geological significance. It represents a timewhen some 2000 m of late Namurian andWestphalian strata may have been removed duringan interval of folding, faulting, uplift and erosionlasting perhaps 30 million years. This Variscanorogeny resulted from the final collision of thecontinents of Gondwana and Laurasia to form thesupercontinent of Pangea. From late Carboniferousto early Permian times, the region movednorthwards from the equatorial forest belt to the

tropical desert belt. Then, rifting in what is now theNorth Sea, possibly combined with a rise in sea levelcaused by melting of the southern polar ice-caps,created the Zechstein Sea. The carbonate rocks ofthe Cadeby Formation were then deposited on thewestern margin of that sea.

Locality 6

The Lower Plompton Grit is one of several distinctsandstone units of Kinderscoutian (R1) age in northand west Yorkshire that amalgamate southwards toform the extensive outcrop of the Kinderscout Grit.Generally, “it is medium- to very coarse-grained,locally pebbly, cross-bedded, feldspathic sandstone”(Cooper & Burgess, 1993). It was deposited in thegreat river system that formed an extensive deltaprograding intermittantly southwards across thePennine Basin in Namurian and early Westphaliantimes.

The Grit is well exposed in a quarried cliff thatextends discontinuously for about 400 m beside thefootpath immediately north of Fountains Abbey.The advantage of siting the abbey so near to thisexcellent source of building stone becomes obviousto the visitor. The sandstone in the cliff is medium-to coarse-grained and well weathered to emphasizethe cross-bedding and reveal large ferruginousconcretions in places. The attractive colours of thestone – pale to yellow brown with a pinkish tinge inplaces – probably indicates its proximity to theoverlying plane of unconformity.

Before boarding the coach for Brimham Rocks,some members enjoyed their packed lunches amongthe abbey ruins, noting the rare examples ofcrinoidal Nidderdale Marble (Blacker & Mitchell,1998) in some of the buildings.

Brimham Rocks

Owned and managed by the National Trust,Brimham Rocks lie on a broad hilltop reaching aheight of 301 m on the north side of Nidderdale.This extraordinary landscape of sandstone tors isformed by the deep weathering of the LowerBrimham Grit - which is probably the lateralequivalent of the Lower Plompton Grit at FountainsAbbey, 7.5km away to the north-east. Wilson andThompson (1965) note that in the Kirkby Malzeardarea immediately to the north, the Lower BrimhamGrit ranges up to about 30 m thick. The medium- tocoarse-grained sandstone at Brimham displays avariety of cross-bedding sets. These can beinterpreted as having formed as shifting sand bars,sandbanks and dunes migrating downstream on thebed of a powerful river that deposited huge spreadsof sand in delta distributaries extending south towhat is now the Peak District. Tree ferns fell fromthe eroding banks of this river and became buried inthe sand. We saw the remains of one of these in theform of a sub-horizontal tube, 3.5 m long and 0.5 min diameter in the side of a tor.A few of the tors are surmounted by blocks ofsandstone that have been tilted to a near vertical

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position with respect to their bedding structures. Indiscussion, two explanations were put forward, thefirst being that the blocks had been let down by iceas it melted during a waning phase of the lastglaciation. The second explanation was that at somepoint in the erosion process, the uppermost part ofthe tor had become undermined and unstable, andthe perched block had simply tilted or toppled intoits present position. No concensus was reached!

The main discussion, however, was about how thetors had formed in the first place. In general, it iscommonly believed that tors remain after theintervening rock has been removed followingsoftening by chemical weathering particularly alongand adjacent to strong joint planes. Such planes areclearly present at Brimham and may have beenwidened by incipient cambering in this elevatedhilltop position, especially near the flanking westernand northern edges of the escarpment. About20% of the sandstone consists of feldspar, a mineralprone to chemical weathering especially in thewarm climate that prevailed during Neogene timeswhen the area was probably uplifted to its presentposition.Although it is generally accepted that almost the

whole region was ice-covered during the last glacialmaximum, there is little evidence of ice erosion atBrimham. The ice here may have only formed athin, semi-static cover with the main local ice streamflowing down Nidderdale. Once melting and retreathad started, the Brimham escarpments would havebeen amongst the first areas to be uncovered,exposing them to the full force of katabatic windsdescending from the main Pennine Ice Cap to thenorth-west. These winds, armed with ice crystalsand (increasingly as melting progressed) with sandand rock particles, would have been the main agent

for the erosion and removal of the softened rock andthe abrasive etching of the tors. Disaggregation ofthe sandstone would have been greatly enhanced bythe effects of repeated freezing and thawing. Otherplanes of weakness in the sandstone, such asbedding planes and poorly cemented beds, wouldhave been more susceptible to erosion - giving rise tothe strange shapes and features, including scatteredsmall potholes, that add to the attractiveness ofthe tors.

Although the day had started rather cloudy anddamp, it became clear and dry by the afternoon, andthe party felt that their long journey fromNottingham had been well worth the effort.

Acknowledgements

I am most grateful to Dr Anthony Cooper of the B.G.S.for his help in my preparations for this excursion. The textwas improved following helpful comments from TonyBenfield and Dr Albert Wilson.

References

Blacker, J. G. and Mitchell, M., 1998. The use of Nidderdale Marbleand other crinoidal limestones in Fountains Abbey, NorthYorkshire. Proceedings of the Leeds Philosophical and Literary Society,Scientific Section, X11(1), 1-28.

British Geological Survey, 1987, 1:50 000 Series Map Sheet 62,England and Wales (Harrogate), Drift Edition.

Cooper, A. H. and Burgess, I. C., 1993. Geology of the countryaround Harrogate. Memoir British Geological Survey, Sheet 62.

Smith, D. B., 1974. The stratigraphy and sedimentology of Permianrocks at outcrop in north Yorkshire. Journal of Earth Sciences Leeds,8, 365-386.

Smith, D. B. and Francis, E. A., 1967. The geology of the countrybetween Durham and West Hartlepool. Memoir of the GeologicalSurvey of Great Britain, Sheet 27.

Wilson, A. A. and Thompson, A. T., 1965. The Carboniferoussuccession in the Kirkby Malzeard area, Yorkshire. ProceedingsYorkshire Geological Society, 35, 203-227.

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The tors of Brimham Rocks.

Nottingham Sandstone Caves

Leader: Tony WalthamWednesdays 14th and 21st February 2001

This excursion was repeated, as each had to belimited to 25 people, but it was still over-subscribed,and it may be repeated again next winter. Itspopularity may have been due to the opportunity tosee the caves in the company of the author of theSociety's very successful publication SandstoneCaves of Nottingham (£4.50 from the Secretary); ormaybe that the trip was free.

There are hundreds of man-made caves cut in tothe Nottingham Castle Sandstone under the citycentre. One large group lies behind the oldsandstone cliff spanned by the Broad Marsh Centre,and part of these forms the Caves of Nottingham,open to visitors during opening hours of theshopping centre. A number of caves are seen alongthe underground trail, including the old tannery site,and exhibits have been arranged to illustrate how thecaves were used in times past, and then as air-raidshelters in 1939-1945.

The visit was arranged for Wednesday evenings,when members of the Nottingham Archaeologicaland Historical Society (NHAS) meet to excavatecaves that have been filled with rubble and debrisduring successive stages of site. During the walkthrough the tourist section of the caves, Tonyexplained their history, and pointed out theengineering structures created when building theshopping centre to avoid damaging those caves ofhistorical importance. It was particularly interestingto see a tiny part of the original Drury Hill survivingon the rock above caves that had been cut outbeneath the old street.

We then headed into the western caves whereNHAS archaeologists were working to clear cavesthat were used as stables in the 1800s, in order tofind more about their earlier history. From these, weemerged into the night through a newly opened holein the cliff, in a triangle of land that is now otherwiseinaccessible between various modern buildings. Wethen went back into the cliff into the three cavesunder the garden of Willoughby House (by kindpermission of the owner). These were the suitablymagnificent wine cellars of Lord Willoughby, and itis suspected that they were also used as socialdrinking rooms. Further west along the cliff, the oneremaining segment of the Black's Head caves wasvisited. Sadly, most of the caves here are now full ofconcrete, including a malt kiln and a tannery foundand then filled during emergency engineering work.Tony also told how he was not quite tall enough toacquire some modest wealth during exploration ofthe cave. Intrigued? Buy the book or put you namedown for the next trip.

Alan Filmer

Despite careful checking a few errors crept intoGraham Lott's masterly treatise on building stonesin the last issue of the Mercian Geologist, and hegratefully accepts the following corrections, whichwere kindly supplied by Neil Aitkenhead.Page 101, Table 2 - Quarries at Ashford in the Waterand Hognaston are in limestones of the BrigantianStage, not the Arundian Stage. Page 103, Table 3 - Quarries at Coxbench andHorsley are in the Rough Rock not in the CrawshawSandstone. Page 103, Table 3 and text paragraph 3 - Quarries atWhatstandwell are in the Ashover Grit not in theKinderscout Grit.Page 104, Figure 3 caption - The columns are ofCrawshaw Sandstone, not of Rough Rock. Page 104 - The last sentence in the final paragraphstarting “The same sandstone from the Coxbench orHorsley quarries …..” should be transferred to theend of the 4th paragraph on page 103.Page 110, Plate 1D caption - The stone is from theWidmerpool Formation, not Woodale Limestone. Page 121 - Add to the list of references:-Frost, D. V. & Smart, J. G. O., 1979. The geology of the country

north of Derby. Memoir Geological Survey G. B., sheet 125.

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NOTES TO CONTRIBUTORS

Scientific papers. These are accepted on theunderstanding that they have not been published (orsubmitted) elsewhere. Shorter reports, news items,reviews and comments are all welcome, especiallywhere they have a local interest.Submissions. All material should be prepared andsubmitted in a format as close as possible to that ofthe Mercian Geologist since 1992. Please submit twocopies of the full text and diagrams of scientificpapers; only single copies are required ofphotographs and of the texts of shorter items. Alltexts should be machine-written on A4 paper, single-sided, double-spaced and with ample margins. Thetext should be followed by a complete list of thecaptions of all the diagrams, maps and photographs,numbered in a single sequence. It is helpful if text isalso sent by e-mail, and is safer when cut and pastedinto a message; please avoid attached files in the firstinstance. Electronic files should have plain text withno formatting. Illustrations and tables are requiredin hard copy only.Abstract. Scientific papers should start with anabstract that states the essential themes and results.Maps and diagrams. These are essential to almostany paper, and are of prime importance because theyare conspicious and so tend to be studied morefrequently than their parent text. Please take careover their preparation, and follow the guidancenotes in the adjacent panel.Photographs. Black and white prints with goodcontrast are preferred. Any other format ofphotograph may be acceptable, but only afterdiscussion with the editor.References. Text references should be in the styleof (Smith, 1992) or related to work by Smith(1992); use (Smith et al., 1992) for more than twoauthors. All references are cited in an alphabeticallist at the end of the text; please copy the stylealready used in Mercian Geologist, and cite journaltitles in full.Copyright. Authors are responsible for clearingcopyright on any material published elsewhere.Offprints. 25 complimentary copies of scientificpapers are provided.Correspondence and submissions. All itemsshould be addressed to the editor, Dr TonyWaltham, Civ. Eng. Dept., Trent University,Nottingham NG1 4BU. The editor welcomesqueries and discussion concerning any intendedsubmissions; phone 0115 981 3833 or e-mail<[email protected]>.

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