The Silurian Issue 2 December 2016...The Silurian Issue 2 December 2016 which was then cleaned and levelled by hand using sand cement mortar to fill the cracks and crevices. Blanket

The Silurian Issue 2 December 2016

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Contents

3 Learning From Experience.Nick Platt.

5 Sea Level Changes in theG e o l o g i c a l P a s t . ColinHumphrey.

6 Bill's Rocks and Minerals:Halite. Bill Bagley.

8 Joseph Bickerton Morgan:Aspiring Geologist andNetworker. Julian Lovell.

11 Geological Excursions:Excursion 3. The RingHole. Tony Thorp.

13 Geological Excursions:Excursion 4: Tan Lan.Colin Humphrey.

14 F o s s i l s i n t h e N e w s“LottieTM – Fossil Hunter”!Sara Metcalf.

15 Diamonds and the AmateurGeologist. Tony Thorp.

16 Geology in the HafrenForest. Colin Humphrey.

The Magazine of the Mid WalesGeology Club

www.midwalesgeology.org.uk

Cover Photo: Clywedog Dam ©Richard Becker

All photographs and other illustrations are by theauthor unless otherwise stated.

All rights reserved. No reproduction withoutpermission.

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Submissions

Submissions for the next issue by the beginningof May 2017 please.

Please send articles for the magazine digitally aseither plain text (.txt) or generic Word format(.doc), and keep formatting to a minimum. Donot include photographs or illustrations in thedocument. These should be sent as separate filessaved as uncompressed JPEG files and sized to aminimum size of 1200 pixels on the long side.List captions for the photographs at the end ofthe text, or in a separate file.

Hunstanton - base for the club's 2017 FieldWeekend. Photo ©Richard Becker

In this issue we report a talk given to the club earlier inthe year by Prof. Jerry Davies, a former ChiefGeologist for Wales with the British GeologicalSurvey. He and his colleagues considered the wayrocks near Llandovery could be interpreted to reveal thecycles of sea level change 440 million years ago, andhow a signal of global change caused by periodiccollapse of the Antarctic ice sheets could bedistinguished from changes wrought by local factors.

In the last article of this issue we have endeavoured toexplain how this changing sea level is reflected in rocksof some of the quarries in Hafren Forest. It is a soberingthought that global sea level changes we think wereabrupt all those years ago were actually slow comparedto the 3.4 +/- 0.4mm per year that NASA tells us ishappening now.

Elsewhere in this issue Tony Thorp discusses diamonds(are they really forever?); and one of our membersrecalls his experience as a junior site engineer asClywedog dam was beginning to rise from the ground.There is also a short and rather tragic biography ofVictorian local geologist Joseph Bickerton Morgan.

All this and more, along with regular pieces from Saraand Bill..

Michele Becker

http://www.midwalesgeology.org.uk/


Learning from Experience:my involvement with the construction of

Clywedog Dam

My formal studies were over, as was an apprenticeshipwith Vickers Armstrong. Work in a factory seemed sorestrictive and the inverse of the country life I was usedto. Civil engineering seemed more attractive thanelectrical engineering. So in the autumn of 1963 I headedinto the hills of Mid Wales, full of excitement, and withsome anxiety, to cut my teeth on my first major project.

We had to walk a couple of miles from the start of theaccess road construction to the V- shaped valley of theClywedog, with it's rocky river bed and steep sidescovered with rough grass and rocky outcrops. I soonlearned that my studies did not have much use in this verypractical environment. Surveying and setting out gavesome insight into the shape of things to come. Anexperienced practitioner guided us into the intricacies ofthe theodolite, and other techniques. There was nosatellite geodetic assistance in those days. All calculationswere done by triangulation with log tables.

The dam was designed as a double curvature concretebuttress structure, the buttresses fanning out from a radialpoint upstream of the dam and on the side of the valley asit curved around to the north. The central buttresses wouldbe the most structurally significant with foundations deepinto the rock of the valley floor. The dam was to be 72metres high from the valley floor.

Over the early weeks, local tracks were converted intohaul roads, fields were levelled, foundations were laid foroffices and other people started to arrive. I was sooninvolved in the planning of preparatory works for theoverhead cableway, the concrete batching plant, a baileybridge across the river, the quarry and crushing plant, andthen pipework for the river diversion to allow foundationconstruction in the dry. Access roads were cut into thehillside to give access to the buttress foundation atdifferent levels on the hillside, to the concrete batchingplant platform and the radial cableway runway and pivotpoint. As the work progressed, I was discovering anotherworld of Welsh farms and villages and people and girls,

mixed with beer and dancing, a controlled and orderedworkplace and a wild and unfettered social life. Ourpresence impinged on the local community, but we didn’tnotice and didn’t really care. There were four of us lads,carefree bachelors, living in caravans and squats, allstraight out of university. We bought a sailing dinghybetween us and raced and sailed in extreme conditions.Our bolthole was Newquay where we camped and drankinto the night. Worked hard and played hard!

But memory plays tricks. The general run of activities isforgotten but a few incidents achieve a longevity beyondtheir importance or relevance. They are exaggerated anddifferentiated due to personal involvement and becomethe jewels in the memories. And, so it is, that certainevents became part of the life experiences that make mewhat I am. So to the anecdotes.

It started with arriving at “the coal face”, the world oflogistics, the supply of human resources, of schedules tobe met, of programmes to be accessed. As new recruits,we were involved in the peripheral discussions but notinvolved in the decision making. The start was gettinginto the ground, “cutting the first sod”. There was littletopsoil to remove and the top rock layers were weatheredand friable. Below this, the rock had to be blasted withcare to prevent disturbing the lower levels of rockfoundation. It was done in two stages,first a general blastto outwith 2ft above the foundation levels, (we were stillusing Imperial units then). This remaining rock, known as“the Popped”, was used to obtain a general foundation

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The first foundations coming out of the ground.

The excavations have started.

Excavations on the south bank.


which was then cleaned and levelled by hand using sandcement mortar to fill the cracks and crevices. Blanketgrouting of the sub foundation rock was carried out on a10ft grid pattern and some 10 to 20ft into the rockensuring most of the cracks and fissures were filled withmortar. Later, a grout curtain was drilled and grouted 50ftand more into the rock at the up stream toe of the dam tolimit underseepage due to the high working water pressurein the valley floor. At the same time, a 10ft diameter steelpipe was installed at the side of the lower foundations inorder to carry the river flow during the dam construction.An upstream cofferdam guided the river flow through thepipe and disgorged below the location of the stilling basin(where flow velocity was reduced). The original designhad two pipes but a decision to install just one would berued later.

The overhead cableway spanned the valley to deliverconcrete, plant and equipment to most areas of damconstruction. Three derricks were installed, one at the

upstream face and ateach flank downstreamof the dam to cover areasnot within reach of thecableway. A rock cuttingwas made above thesouth location of thesouth abutment, abovethe batching plant andwas on a radial curvewith its centre at themast on the north flank.Later, this was the targeto f s o m e W e l s hn a t i o n a l i s t s w h oc o n s i d e r e d t h econstruction of the damas an English invasionT h e i r e x p l o s i v e sdelayed but did not stopthe works.

A rock cutting was created for the concrete batching plantand the access to it, which eventually became the accessroad to the control house. A 4 cubic yard capacity, splitdrum mixing plant was elevated on a structural steel frame

with a chute to the side that could fill a 4 cubic yardbottom opening skip. This skip would then be in thecoverage of the overhead cableway (or blondin as it wascalled, after the French tightrope walker) and concretecould be delivered to almost every area of the dam.

The quarry was surveyed and the sandstone approved foruse as aggregate in the concrete mixes. A large jawcrusher was erected away from the proposed rock face.Conveyors raised the general mixed stone to the screenswhich separated the different sizes of stone into bins andstorage areas. Concreting sand was imported by road fromEllesmere. The quality and gradings of these ingredientswas checked by the laboratory set up for the purpose ofquality control. The other major ingredient of concretewas a special coarse ground cement produced at Aberthawin South Wales, brought to Llanidloes station by rail andthen in road tankers to be air blown into 50 ton silosabove the batching plant.

For me, a great many new experiences were absorbed andstored for future reference; a fast moving and everchanging scene which challenged my previous views andadded excitement to life.

Nick Platt

Member's Photo

Torquirhynchia inconstans

This is an Upper Jurassic rhynchonellid brachiopod,approx. 165 million years old, from the LowerKimmeridge Clays, and was found in Weymouth, Dorset.Like all brachiopods it has an asymmetric commissure,thought to be an adaptation to life in tidal environments.

Janey Haselden

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The Bailey Bridge access to the north bank.

The 'Blondin' carriage on the south bank.


Sea Level Change in theGeological PastA talk by Professor Jerry Davies

Professor Jerry Davies, former chief geologist for Waleswith the British Geological Survey, and now honoraryprofessor at Aberystwyth University, gave the February2016 evening lecture to the club. Titled ‘Sea level changein the geological past’, the speaker examined the influenceof glacioeustasy (effect of polar ice and glaciers on globalsea level changes) during the early Silurian in Mid Wales.The rocks surveyed lie in a band some 20 km north-east tosouth-west, close to the town of Llandovery and are theinternational type section for the Llandovery, and one ofthe best studied early Silurian sequences in the world.The Type Llandovery area lay on the south-east margin ofthe Lower Palaeozoic Welsh Basin, close to the WelshBasin Fault Zone which controlled the subsidence of thebasin at that time, and thus influenced its sedimentation.Plate movement, and therefore tectonism, was ongoingthrough the Llandovery associated with the closure of thenorthern section of the Iapetus Ocean.

A new architectural model for the Type Llandoverysedimentary succession was determined, based on fifteentraverses across the area. It takes the form of a cross-section containing the different rock formations. This wasthen developed into a chronostratigraphical model, across-section in which the different rock formations andfacies (recognisably different bodies of rock) could bepresented with time on the x axis, not in years but ingraptolite biozones. This work was based heavily onstratigraphical logging of the lithology and sedimentarystructures of the district, and on extensive biostratigraphy.All this formed the basis for inferring the way in whichsea level, or more precisely the local drainage datum,called base level, was changing through the time interval.

Interpreting the geology to identify changes in the sealevel or base level is called sequence stratigraphy.

Deposition during the latest Hirnantian stage (end-Ordovician period) to early Telychian stage (early Silurianperiod, Llandovery epoch) can be grouped into threemajor cycles, called simply ‘sequences’, each of whichcomprises a period of prograding, when the shorelineadvanced seaward by substantial sedimentation during atime of falling sea level, culminating in a sea levellowstand; plus a period of progressive flooding duringrising global sea level, culminating in a sea levelhighstand. In the former, an unconformity can be detectedproximal to the shoreline and inferred in the deeper basin,and in the latter a maximum flooding surface can beidentified. The low frequency, large amplitude sequencesare inferred to be glacioeustatic in origin. Around 20parasequences ( lower order f luctuat ions) aresuperimposed on the base level sequences, as shown inthe diagram. The parasequences may have their origin inregional forcing factors. Smaller fluctuations (not shown)may sometimes be detected superimposed on theparasequences.

Determination of the extent and timing of these base levelchanges in the Llandovery Type area is based onbiostratigraphy, including graptolites in deeper water andother fossils in shelf and slope areas, where differentspecies of brachiopods in particular are useful depthindicators. This work was reported in Davies et. al.(2013) in Geological Magazine*. It also highlighted anumber of changes to the accepted geological architectureof the Type Llandovery area, including progradesequences not previously identified in the record.Comparison of Type Llandovery area marine base levelchanges with those in other districts of the UK withLlandovery rocks, including North Wales, the LakeDistrict and the Southern Uplands of Scotland, showed asimilar pattern of sequences. This begins to suggest aglacioeustatic cause.

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The next step was to take the new Type Llandovery areamodel global, and see the extent of correspondence withmarine base level curves around the world, where 62datasets were obtained from the literature. These includedboth global and regional base level curves interpretedfrom the rock record, in a variety of tectonic settings,together with records of glacial activity, isotope data,changing patterns of facies and fauna, and climate models.The results have now been published by Prof Davies andhis colleagues**.

All the datasets have been adjusted to fit into the twelverecognised UK graptolite biozones for the time period,with each biozone being divided into three, giving 36 timeslices where available data on base level change fromwidely different sources could be compared. Where thereis a high correspondence of data from different datasetsaround the world it is likely that eustasy was an importantfactor in shaping the local events. Conversely, wherethere is a lower correspondence of sources it is likely thatregional influences were dominant, and glacioeustasy lessimportant. Prof Davies adopts a ‘eustasy index’ toquantify the relative importance of eustasy in base levelchange, though he stresses that a low eustasy index doesnot mean that eustasy was unimportant, merely that it wasless important than regional factors which may havecaused local base level movements to diverge from globaltrends.

The striking result of this analysis is the proof that themajor deepening events in the Llandovery Type area, doindeed correspond with major deepening eventselsewhere, with south polar ice sheet disintegration beingthe dominant forcing factor. But during majorprogradation sequences in the Llandovery, which occurredduring falling base level, regional forcing factors weremore important than eustasy. Prof Davies suggested thateach time the South Polar ice sheet disintegrated, it did soquickly, with the rise in eustatic sea level being clearlyrecorded in regional base level changes. However, theadvances of South Polar ice and consequent falls in globalsea level may have occurred more slowly, and as sedimentsource areas were exposed and eroded, the epeirogenicresponse (subsidence of sedimented basins andprogressive uplift of eroded areas) quickly outstrippedeustasy as the dominant forcing factor. Moreover, therecord of regressive events (falling base levels) was lesswell preserved because of continuing erosion.

Colin Humphrey

* Davies, J.R., et. al., 2013. A revised sedimentary andbiostratigraphical architecture for the Type Llandoveryarea, central Wales, UK. Geol. Mag. 150, 300-332.

** Davies, J.R., et. al., 2016. Gauging the impact of glacioeustasy on a mid-latitude early Silurian basin margin, mid Wales, UK. Earth-Science Reviews 156 2016 82-107.

Bill's Rocks and Minerals

Halite (Common salt).

Halite, or common salt is only one of a number ofevaporite minerals, some of which are well known to us,for instance, calcite, gypsum, dolomite, borax, anhydriteand magnesite. Perhaps not so well known are, sylvite,kainite, langbeinite, and several more.

The first thing to establish is the occurrence of evaporitemineral deposits, in other words, how and where they areformed. They are formed as the result of the evaporationof salt rich bodies of water, and there are two types ofdepositional environments, marine and non-marine.

Dealing first with marine environments. Typically seawater carries about 3.5% by weight of salts, but the saltswon’t start precipitating until the water has evaporated toabout half it’s original volume. For this to happen the seamust be closed off, either by tectonic activity or bylowering sea levels due to glaciation. As the body ofwater evaporates, the salts or minerals in suspensionbecome more and more concentrated until they are forcedto precipitate and form crystalline structures. The first andbetter known minerals to precipitate from the shrinkingbody of water are the carbonates calcite and dolomite,followed by the sulphates gypsum and anhydrite, then the

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chlorides sylvite and halite. Once the sea has completelydried and the sedimentary layers of salts are deposited thelowered sea basin creates conditions which allowscoverage by other sediments such as mudstone andsandstone.

Much of the salt mined from the completed sedimentarylayers is from salt domes which may be as large as fivemiles in diameter. The mechanism that creates salt domesis quite simple. Pressure exerted by overlying sedimentson the comparatively fluid salt layer forces it to migrateupwards through those mud and sandstone sedimentarylayers, deforming them as it does so, creating salt domesand salt columns, and also creating conditions for oilreservoirs. Most salt domes and columns do not reach thesurface, but when they do salt glaciers can be formed. Arecommended site to learn more about salt domes is to befound at http://geology.com/stories/13/salt-domes/

T o d a y a b o u t2 5 % o f s a l tproduction isfrom the forcedevaporation ofsea water invery large saltpans. The other7 5 % o fproduction isfrom historicdeposits, bothmarine and nonmarine.

Many non marine deposits of salts are in the form ofplayas or salt flats, and they are the result of falling waterlevels in shallow inland basins which periodically fall byevaporation, resulting in the precipitation of their saltcontent. Usually found in desert areas, and often in closeproximity to coastal regions, these basins or playas derivetheir salt content from ground water, and can cover vastareas. Although the basins are very shallow, often only a

few metres deep,they hold vastamounts of saltsimply becausethey are so large.The largest saltflats in the worldare at Salar deU y u n i i nBolivia, coveringan area of 4000square mi le s .Not so large, butstill quite substantial are the probably better known saltflats at Bonneville, Death Valley, and Salt Lake in theU.S.A.

There are three main types of salt extraction, deep shaftmining, solution mining, and solar evaporation.

In deep shaft mining, the salt from ancient buried depositsand salt domes is found in a compact form as rock salt,with crystals rarely observed, and usually quitediscoloured as the result of mineral impurities. Untreated,it is mixed with sand and used as a road treatment inwinter conditions. It can also be refined and used as afood additive.

In solution mining, halite is extracted from salt beds byerecting wells which inject water to dissolve the salt,making a brine solution which is then pumped away to therefinery plant for evaporation. This type of salt productionis the main source of salt for food additives and industrialprocesses.

Solar evaporation is a natural process which createsseasonal opportunities for salt harvesting. Salt gatheringusually happens once a year when the evaporated salt bedsreach a specific thickness. This type of salt production canonly occur in areas with low rainfall and a lot of sun, forinstance, Australia and some Mediterranean countries.

Halite crystals, although rare in deep mined deposits, areplentiful in playas and salt beds because they are open toair, allowing unrestricted growth. Although halite is

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Salt pans at Gozo. Creative Commons by Paul Stephenson.

Death Valley salt flats. Creative Commons by Vadim Kurland.

http://geology.com/stories/13/salt-domes/


typically colourless or clear it is not uncommon for it tohave a light pink colour because of the presence of theorganism Halobacterium ssp. which live in the brine. Onthe Mohs scale halite has a hardness of 2.5 which is aboutthe same as your finger nail, and the crystals are cubicwith perfect cleavage, and a conchoidal fracture. Halite crystals have an interesting habit of sometimesdisplaying “ hopper growth”, a habit which is also known

in several other minerals. Hopper is a descriptive term ofthe appearance of dished faces in the crystal which happenbecause of a combination of fast growth, and preferentialgrowth on the corners and edges of the crystal.

For an informative site on salt production and saltevaporates a visit to the website below is recommended. www.slideshare.net/hzharraz/topic-6-evaporite-salt-deposits

Bill Bagley

To see more of Bill's minerals visit http://midwalesgeology.org.uk/bagley-collection.php

Joseph Bickerton Morgan –

Aspiring Geologist and Networker.

The “tradesman” from Welshpool who achieved successin the late Victorian world of the “Gentleman Scientist”.

Joseph Bickerton Morgan came from a very ordinarybackground. He was born on 26th June 1859 at RoseCottage, Daywell, near Oswestry. His father, ArthurJ o s e p h M o r g a n , w a s f r o m L l a n s a n t f r a i d d ,Montgomeryshire and at the time of his son's birthdescribed himself as a farmer. Joseph's mother wasHannah White Bickerton who had been born in Liverpool.Joseph had an elder sister, Mary Jane. She worked athome as a mantle maker and remained unmarried. Sadly,Hannah Morgan died on the 5th April 1861, aged 35, as aresult of Tuberculosis, the disease which would also claimthe life of her son, just as his career as a geologist wasburgeoning.

By 1871 the Morgan family had moved to Welshpool,where Arthur had set up shop as a grocer, living at 27Severn Street. He also had a new wife, Ann, fromLlanraeadr-ym-mochnant. The business was obviouslysuccessful. Ten years later they had moved to largerpremises at number 30. Arthur now described himself asa confectioner and the census records the young Joseph asa baker. This situation appears to have caused Josephconsiderable distress, as is very apparent in one of hisletters to Professor Charles Lapworth, whose help Josephmanaged to obtain in order to further his interest in thestudy of geology. By 1891, when we find evidence thatJoseph was gaining significant recognition as a geologist,the census records him merely as a grocer's assistant! Hisachievements and a scientist's curiosity and enthusiasmare reflected in his letters to Lapworth. He surely foundthis description frustrating and a very bitter pill toswallow.

It is difficult to determine exactlywhen Joseph began to show aninterest in geology or, indeed, whatstimulated it. His letters andhandwriting reflect a good standardof education, a valuable asset in anage when rates of illiteracy werevery high. He would have leftschool at the age of 13 and itappears that the demands of theshop left little time for self-improvement. The main amateurinterests of the day were in 'living'

natural history or in the field of archaeology. Geology asan academic study was still in its infancy, still thepreserve of the enthusiastic few. Were there local factorswhich might have caught Joseph's attention? Did he meetanyone in Welshpool who inspired him and kindled a firewithin him that was to become his consuming passion?

In researching the story of Joseph Bickerton Morgan,(shall we now just refer to him as JBM as those of us whohave got to know him affectionately do?) documentaryevidence is scant. However, I suggest the names of twoinfluential local men. Firstly Morris Charles Jones andsecondly the Rev. John Edward Vize, Vicar of Forden, thequintessential “Gentleman Scientist”.

Morris Charles Jones was born in Welshpool, educated inLondon and went on to become a solicitor, practisingsuccessfully in Liverpool until he retired in 1880. He thenserved as a Justice of the Peace for Montgomeryshire. Hewas very interested in Archaeology and undertook anextensive excavation of the site of Strata Marcella Abbey.However, from the perspective of this article, his greatestcontribution to mid-Wales came in 1867, when, togetherwith T.O. Morgan of Aberystwyth, he was instrumental infounding the Powysland Club. The first annual meetingwas held on 1st October 1868, with the Earl of Powis inthe Chair. His wide-ranging interests were reflected in theclub's annual publication, the Montgomery Collection,

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30 Severn Street, Welshpool, where JBM lived and worked as a baker.

Hopper crystals. Creative Commons R. Weller/Cochise College.

http://midwalesgeology.org.uk/bagley-collection.php%20

http://www.slideshare.net/hzharraz/topic-6-evaporite-salt-deposits


which he edited up to the time of his death. On thenomination of the Earl of Powis he was appointed agovernor of Bangor College. Now here was a man withinfluence and connections!

Morris Charles Jones recognised JBM's ability andappointed him as an honorary assistant curator at thePowysland Museum in Welshpool, where he was giventhe task of re-organising the collections. The proceedingsof the annual general meeting for 1889 record theexpansion of the geological section which had been addedto and arranged by JBM. In this regard, his work washighly praised and through Morris Charles Jones mayhave come to wider attention. JBM was also responsiblefor developing some of the courses which were run by themuseum. These were seen as an important aspect of lifein Victorian times for those who wished to improvethemselves in days when education was valued as a wayto progress out of poverty and when any form of furthereducation for many was no more than a distant dream.

The Rev. John Edward Vize, Vicar of Forden for 40 years,was a member of the Powysland Club and so is likely tohave had personal contact with JBM. He was a classicexample of a Victorian English scientist, being aneducated man with time to spare from his clerical duties toinvestigate the world around him. He is best known forhis research on fungi. In his day he was regarded as one

of the worlds leading mycologists. Vize was also veryinterested in the subject of conchology. He appears tohave built up a very large collection of shells, aconsiderable number of which he donated to thePowysland museum. One of JBM's first tasks was toclassify and arrange this collection. In doing this he isagain likely to have had first-hand contact with Vize andwould have received great encouragement from theclergyman, who was known to have a great enthusiasmfor his subject.

The shell collection is no longer housed at the PowyslandMuseum. Investigations a few years ago revealed thatsome of the collection is now on display in the museum atLlanidloes. The museum also stores a quantity of materialwhich is thought to be the rest of the collection. Sadlyhousing and storage conditions at some point over theyears have degraded the collection.

JBM joined the Conchological Society of Great Britainand Ireland at a meeting held on the 6th May 1886. Hemaintained his membership until 1893. He did not publishany papers in the Journal of Conchology, neither is anobituary recorded.

Exactly when JBM started to correspond with ProfessorCharles Lapworth is not known but evidence from archivematerial held at the University if Birmingham's LapworthMuseum shows that it was under way by October 1882.The archive contains ten letters from JBM to Lapworth inwhich he seeks help in identifying fossil specimens,advice on areas in which to research the Silurian andOrdovician strata, and on methodology. The generalcontext suggests that a number of other letters werewritten to Lapworth but have not survived. Unfortunatelyall of JBM's papers have been lost, depriving us of thereplies received.

Lapworth encouraged JBM to examine the rocks in theChirbury and Shelve area which he and others wereresearching. From the letters we can conclude that JBMwas a regular visitor to the area and would havecontributed to the information shown on the Shelve map.The map is approximately six foot square, hand paintedand lettered on linen. The Chirbury area is important indefining the geology of the Marches. The Chirbury seriescontains the Spywood Grit and the Aldress, Hagley andWhittery Shales noted by JBM. These beds are importantin unravelling the geological sequence as they lie at theinterface between the Ordovician and Silurian periods.Hence this area became the centre of attention, with visitsin turn from Sedgwick, Murchison, Salter, Rupert Jones,Lapworth, Foster and Watts, as the sometimesacrimonious debate over exactly where the division lieswas gradually resolved. Here our man from Welshpoolwas walking in the footsteps of, and rubbing shoulderswith, the greatest names in British geology. Baking beblowed – this is progress towards academia!

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Thamniscus, the tiny Bryozoan colonial fossil discovered by JBM near Welshpool. ©Cindy Howells, NMW.


JBM was obviously very encouraged by Lapworth andcontinued to map the strata of the region and search fornew fossil specimens. The year 1885 saw anotherachievement for a developing geologist. George RobertVine published a paper in the Quarterly Journal of theGeological Society which contained descriptions of anew species of Phyllopora, P. tumida from the Caradocrocks of Llanfyllin and of Thamniscus antiquus from theBala volcanic ash of Middletown Hill, both collected byJBM. Vine was a corset and stay maker in Sheffield whospecialised in the study of Bryozoans. Between 1877 and1893 he published about seventy five papers onBryozoans. He is principally remembered as the author ofthe Order of Cryptostomata. However, Vine, a Chartistand Methodist preacher, was regarded as something of afirebrand, with rather subversive views who never becamepart of the geological establishment.

JBM continued his researches of the Ordovician andSilurian interface in the area around Powis Castle and theMontgomeryshire marches. He painstakingly assembledfossil evidence until he reached a point where he felt ableto challenge the currently established thinking. Hisfindings were gathered into a brief paper which waspresented to a British Association meeting held in Leedsin 1890. Here he is actually challenging the great SirRoderick Murchison. He was obviously very confident inhimself and the quality of his researches. His paperargued the need to changethe boundaries and thepoints at which this shouldbe done. He finished bystating the need to redrawthe map, something hewould do in his leisure time.Remember, he was still atthe bakery! In a GeologicalSociety obituary attributedto Wi l l i am WhiteheadWatts, the writer expressesthe hope that the submissioncan be expanded usingJBM's papers. Sadly for usthere is no evidence that thisc a m e a b o u t , a n d a spreviously stated, the papersare now lost.

In the autumn of 1889 JBMapplied for Fellowship of theGeologica l Soc ie ty o fLondon. Here again, on hisapplication form, we seefurther evidence of JBM'sability to make contacts andnetwork at the highest level.His nomination paper wassigned by Clement Le NeveFoster, Charles Lapworth,T . R u p e r t J o n e s a n d

Griffiths Williams, some of the foremost names in Britishgeology. It is difficult to overstate the significance of this.It follows on from the establishment of JBM's relationshipwith Charles Lapworth which had blossomed over anumber of years. Lapworth saw in JBM a prodigioustalent which he nurtured and encouraged. He was a manof standing in the world of geology and must have hadevery confidence when introducing the young new-comerto his colleagues. The letters of JBM show that he visitedthe Wotherton area with Le Neve Foster in June 1889. Atthis time a new mine shaft had been sunk and time wasspent examining some of the spoil. Intriguingly, JBMdoes not tell us which Le Neve Foster accompanied him.Was it Clement, who at the time was Inspector of Minesfor the North Wales District, or was it his younger brotherErnest Le Neve Foster, who was a mining engineer? Weshall never know. Joseph Bickerton Morgan was electedto Fellowship of the Geological Society of London on the4th December 1889.

The other signatories are also significant. Thomas RupertJones qualified as a doctor but schooling in Somerset hadstimulated an interest in fossils and he soon turned hisattention full-time to geology. By 1849 he was assistantsecretary to the Geological Society of London and in 1862became Professor of Geology at the Royal MilitaryCollege, Sandhurst, where he was an authority onforaminifera. He was elected a Fellow of the Royal

Society in 1872. He was alsoa winner of the GeologicalSociety Lyell medal. Quitean endorsement.

Griffiths Williams started hiscareer as a schoolmaster (ashad Lapworth) and becameHeadmaster of the HigherGrade School in BlaenauFfestiniog but soon becamevery well-known in NorthWales as a geologist. In1895 he was appointedAssistant Inspector of Minesfor the North Wales Districtunder Clement Le NeveFoster. Quite how hebecame a friend of JBM isnot clear, possibly throughthe latter 's decision toexhibit at the NationalEisteddfod in Carnarvon.T h e i r f r i e n d s h i p w a ssuff ic ien t ly s t rong forWilliams to be executor toJBM's Will.

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JBM's application for Fellowship of the GSL.Image courtesy of the Geological Society


At last, it is all coming together for the young man fromWelshpool. In 1892 JBM decided to devote himselfentirely to science. Again with the friendship and supportof Charles Lapworth, he obtained a free scholarship to theRoyal College of Science, later to become ImperialCollege, London. He applied himself single-mindedly tohis studies and in just one year succeeded so well that heobtained the first prize at the college, together with theMurchison Medal and a gift of instruments and books.Success at last as he began working as a Demonstrator,the first step in his new academic career. Now he reallywas Joseph Bickerton Morgan, Geologist.

However, his devotion to his studies and the years of toilin the bakery in Welshpool had taken their toll. Hesuffered a haemorrhage of the lung and an attack ofpneumonia, from which recovery was very slow. He wasadmitted to the Royal National Hospital in Ventnor wherehe appeared to be making good progress towards a fullrecovery but another haemorrhage set in and he diedpeacefully on the 8th March 1894, taken by thetuberculosis which had ended his mother's life at almostthe same age.

The obituaries were fulsome in their appreciation of hislife and praised his achievements. He had demonstratedthat wherever you started it was possible to dream dreamsand with tenacity turn them into reality. It is safe to saythat JBM would have become one of the giants of Britishgeology.

Julian Lovell

Next time: A real “Gentleman Scientist”, John EdwardVize of Forden, Vicar and Mycologist.

Geological Excursions Sites to visit.Excursion Number 3The Ring Hole

Near Dolfor, landsliding in superficial deposit, slumping in the Montgomeryshire Trough

Location: Grid ref. SO120837On the north side of the B4355 road from Dolfor to Knighton, about three miles from Dolfor. There is a convenient lay-by just opposite and some “Danger landslip” signs nearby.

The Ring Hole itself is a deep semicircular declivitywithin a few yards of the road, formed by landslip withinthe great thickness of clayey till or boulder clay. Beforeexploring the Ring itself, it is worth looking at thesurrounding landscape. You are on a bleak plateau atsome 450m surrounded by a number of rounded hills, ofabout 500m height. This is held to be part of the MidWales “peneplain”, a proposed pre-glacial erosion surface,established during Mesozoic times and since then uplifted,dissected and eroded by ice and water. The generallyrounded nature of the topography results from the uniformsedimentary nature of the Silurian bedrock.

During the Devensian glaciation, this area would havebeen subject to a great thickness of ice sheet whichdeposited masses of boulder clay on its retreat. Thisplasters the surface to such an extent that it masks thetopography of the bedrock beneath it.

The Ring is best descended by taking the path round thewest (left) side, from which the whole structure can beseen. It is some 150m wide and over 50m deep. (Fig 1)The near side shows a near vertical face in boulder claywith scree slopes below. Visiting this site in winter I haveseen well developed pipkrakes (ice needles) on theboulder clay faces. Pipkrakes develop in wet clays if the interior is just abovefreezing and the surface is below. Capillary water movestowards the surface and freezes there, producing icecrystals which grow outwards as they are fed by water

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The Murchison Prize. Image courtesy of Imperial College

Fig. 1


from within the clay. This blows particles away from thesurface. The crystals can be inches long, a bit like hoarfrost; but the process is different as hoar grows from itsends, not its roots. Weathering is eroding these faces,producing loose scree slopes at their bases, sloping atabout 28 degrees. The screes comprise the pebble/bouldercomponent out of the boulder clay as the clay particleshave been washed out. They remain bare as water is notretained.

Some 50 m along the face edge there is a good path downto the base of the cliffs, along which you can examine thetill and scree. The scree is mainly composed of sandstoneand siltstone, similar to the local rocks, sometimesshowing the typical brownish weathering and finelamination of the Bailey Hill Formation. It is not veryrounded and has not travelled very far.

Nearer the brook (the infant River Teme) the Councilhave tried to stampon the toe of thelandslip by loading itwith large importedblocks of a doleriterock. (Fig 2) Is itf r o m C r i g g i o nQuarry? Clay isexuding below them,so th e y a r e no tparticularlyeffective!

The stream is now fairly innocuous, but as the ice meltedat the end of the ice age, it must have been a ragingtorrent carrying rocks and boulders with it. It has carved asignificant gorge into the sandstone of the Bailey HillFormation. Crossing over the stream, one can examine therocks exposed inthe gorge. Thesecomprise thinlybedded sands andsiltstones of theUpper Bailey HillFormation (LowerLudlow) whichcontain a largep r o p o r t i o n o f“dis turbed” ors lumped beds .Locally, there aremany such which can be seen along and near the A483.The degree of disturbance varies from little to completelyhomogenised with just the harder concretions distributedtherein. The rocks exposed here are gently dipping, withthe bedding more or less intact, but with some packets ofbeds contorted, waved and even rolled. There is onespectacular “sausage roll” fold near the toe of the landslip.(Fig 3). Some packets of beds are undisturbed (Fig 4) andothers are coherent, but folded on a metre scale. The precise mechanism involved is obscure, but the

movement must have occurred soon after deposition andbe related to the rheology of the freshly depositedwater/sand/silt slurry when subjected to disturbance,p o s s i b l y o f aseismic nature.( W e a r e s t i l ls t r u g g l i n g t om o d e l t h ebehaviour of suchcomplex fluidsw h i c h s h o we x o t i c v i s c o -p l a s t i c i t y a n dhistory dependentbehaviour!)

By studying the vergence* of the folds in slumped beds itis possible to deduce in which direction movementoccurred. It can be assumed that movement occurred inthe direction of vergence and that it would have beendownslope.

It is by this kind of study that the existence and position ofthe “Montgomery Trough” was determined. In the 1960sR.J.Bailey studied the palaeocurrents and slumping in thisarea and found that the slumping confirmed that there hadbeen palaeoslopes forming a trough aligned NE-SW inthis part of the Welsh Basin, while palaeocurrents weregenerally towards the NE. This sounds easy, but try todetermine whether the folds here verge to the left or theright. You may have to ask several people and average! Itis worth discussing whether such features could equallywell be produced by tectonic means and what otherevidence would be relevant.

From further downstream a series of terracettes can beseen which are held together by the strength of vegetationand have failed in a rotational mode. The gorge becomesmore extreme and you may have to walk at a higher level,but the bedding becomes a bit thicker and shows typicalbrownish weathering of Bailey Hill. At this point one canreturn to the road or, if sufficiently energetic, climbCilfaesty Hill, just to the east, which is a good viewpoint.

*Note on Vergence:Vergence is the geological term for the direction ofoverturning of an asymmetrical fold in which one limb isshorter and steeper than the other. In the figure below forexample the fold verges to the right.

Tony Thorp

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Fig. 2

Fig. 3

Fig. 4


Geological Excursions Sites to visit.Excursion Number 4Tan LanO. T. Jones's famous Ordovician shoreline.

Location: Tan Lan Cottage SO 057 547 .Permission from the owner is required for the visit.Relict early Ordovician shoreline in the Builth Inlier(based on a field trip September 2016 which was led byTony Thorp)

This excursion visits the southern Builth Inlier, just westof Carneddau, to examine early Ordovician (Llanvirn)sites made famous by Jones and Pugh’s 1949 paper AnEarly Ordovician Shoreline in Radnorshire’. Prof. OTJones (d.1967) is still regarded as Wales’s greatestgeologist. The 1949 paper interprets the present ruggedupland grass terrain as originally a high cliff face ofaltered submarine lava flows and pillow lavas, with aboulder bed beach, and with sea stacks. The paper evenreports an early soil (which, on the visit ,we did notfind!) on the clifftop. A sketch of this scene in the paperi s i m p o r t a n t a s o n e o f t h e f i r s t d e t a i l e dpalaeogeographical reconstructions – a device nowcommon in geological texts.

Park at Tan Lan Cottage adjacent to the large new quarry.A dolerite sill cuts the 'Glyptograptus teretiusculus Shales'of the quarry. OT Jones had much to say about thedolerites but not this quarry; it has only just been opened.This was the furthest west site of the day; the rock dipswest so this was above the relict beach and above theshallow water sandstone which enveloped it. By then thewater had become deeper. These black shales providedbiserial G. teretiusculus graptolites, a fine Monobolinaramseyii brachiopod, some other brachiopods andpyrolusite dendrites (MnO2).After visiting the quarry walk 500m south to NewmeadScar (SO 057 542). In thick woodland an old quarry face

of relict cliffs of OTJ’s Lower Spilite (today named BuilthVolcanic Formation) is exposed. This is sea floor pillowlava and effusive lava flows which were metasomaticallyaltered, and described in 1977 by BGS as spiliticandesites. The rock is highly vesicular, with the size ofvesicles expanding towards the outer surface of theeffusion. (We carefully edged west at the top of a steeptree-covered scree slope, holding on to the base of theoutcrop.) The dip is c30ºW so we were walking up-succession – in effect up the spilite of the cliffs.Eventually Tony pointed out the contact with OTJ’s GreyFeldspar Sands (now the Newmead Sandstone) which ingradually rising sea level eventually inundated all of theancient shoreline and remaining cliffs.

Walk 150m SE up the hill (east is back down thesuccession) to an example of OT Jones’s boulder beds(SO 0581 5464), where he conjectured (and sketched) astructure consisting of pebbles and boulders fallen fromthe cliffs and lying on a beach. There are large pebbles ofspilite embedded in ferruginous, feldspathic sand (itselferoded from spilite). Further up the hill, 200 m east, is OTJones’s wave-cut platform (SO 0599 5468), which laybelow his cliffs, and now appears in the field as mossyrough spilite exposures on the topographical slope. Tonypointed out a prominent crag on the hill 200m further east,another of OT Jones’s sites, explaining it was still boulderbeach there, but with spilite boulders over 1m diameterembedded in sand.

Roughly 1 km south from Tan Lan is the site of Jones’smost iconic interpretation: a line of sea stacks now closeto a long stone wall and scarcely identifiable on theground (SO 0584 5400). The cliffs were said by OT Jonesto face east to the sea and to the line of degraded seastacks, with the intervening vegetated ground revealed byWW2 slit trenches to be Grey Feldspar Sand. However,this must have been a typo because the cliffs surely wouldhave faced west to the sea and sea stacks; the main bodyof spilite, with the cliffs, lies east of the stacks. Hisdiagrams and sketches are not consistent in that respect. Itmight be fun sometime to excavate and check whether thefour ‘sea stacks’ meet the basic requirement, i.e. are theydefinitely spilite cliff material surrounded by the ‘beach’of Newmead Sandstone.

Colin Humphrey

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O T Jones's sketch of a shoreline near Tan Lan.

Vesicular lava flow, Newmead Scar.


Fossils in the NewsAlmost 30 years ago (not newsworthy, I know!) I arrivedat Durham University to read Geology. I was extremelypleased and quite amazed to see that our cohort (1987-1990) was almost 50:50, male to female. Sadly, thisgender equality in our fabulous science has not alwaysbeen the case and, as a serving teacher, I know how hardit is to attract girls into any science, let alone ‘rockbashing’!

However, all is not lost for the future, as I want torecommend to all Mid Wales geologists (and theirrelatives) a brilliant Christmas gift for the buddingpalaeontologist (girl or boy). Let me introduce “LottieTM

– Fossil Hunter”!

Lottie may be a littledoll, but she won amigh ty ‘Po s i t i veRole Model’ awardin 2015. She wasdeveloped by Britishacademics and she’smade in Ireland (byArklu Ltd.) to have ac h i l d - l i k e ‘ a g eappropriate’ body,wears normal, non-sexualised clothingand does things thatlittle (and big) girlslike to do – like fossilcollecting!

Fossil Hunter LottieTM comes with a backpack, hammer,chisels, sturdy footwear (BarbieTM never had shoes likethese!), a whole array of plastic ammonites and a book onMary Anning! She is utterly charming (Dave got memine last Christmas and I use her as an ambassador forwomen in science at school).

Lottie follows on from the 2013 Lego femalepalaeontologist who was part of a set celebrating womenin science: the palaeontologist is joined by an astronomerand chemist. The Lego sets sold out within weeks andnow trade at high value on online auction sites tocollectors! Lottie and her good friend Finn (a boy doll)are available on all good on-line selling sites and havetheir own page http://uk.lottie.com/.

Sorry to harp on about women palaeontologists, butanother website that might be interesting to readers is thefantastic http://trowelblazers.com/ which has literallydozens of articles about women palaeontologists,archaeologists and geologists. Their latest feature is onWinifred Goldring, who was State Palaeontologist of NewYork from 1939 to the early 1950s. Winifred rode around

with her pick axe in a motorcycle sidecar and in the 1920sredesigned the Hall of Palaeontology at New York StateMuseum. Trowel Blazers also have a Facebook page andin their words they serve to celebrate women“archaeologists, palaeontologists and geologists who havebeen doing awesome work for far longer, and in fargreater numbers, than most people realise.” Long maythey continue!

Sara Metcalf

Member's Photos

Sara accidentally dressed to match the Gwna Mélangesequence at Porth Twr Bach bay on Llanddwyn isle onAnglesey.

The rocks are an amazing mixture of colours and textures:dark green lavas, bright rose pink quartzites, purplemanganese rich shales, blue-black intrusive dolerite dykesand honey coloured limestones. Her matching clothes areBerghaus, Craghoppers and Mountain Warehouse!!

David Hearn

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http://trowelblazers.com/

http://uk.lottie.com/


D i a m o n d s a n d t h eAmateur GeologistDiamonds are forever!

Well, not so sure about that. Since the mid 1950s, thesynthetic diamond industry has grown. Mainly this is forindustrial purposes, but a profitable sideline is of coursethe jewelry trade, where they are sold on the basis ofbeing environmentally preferable and not conflict related.As an extra, they can of course be made from you aftercremation if you don't mind paying a premium!

The collapse in value benefits amateur geologists like usbecause rock cutting, grinding and lapping is now so easy,we can all cut, polish and make sections using the smalltools found in most households. In the simplest case, thecheap diamond hones sold to sharpen your pocket knivesare ideal to plane off flat rock surfaces to show structure.Otherwise, diamond discs for small angle grinders costabout £5 at Lidl or on e-bay. With this tool one can cuthand specimens down to size, with little risk of shattering

the critical bits.Forabout £30 there isa water cooledmachine sold as a"Tile cutter" whichis preferable, butless convenient touse. I use mine outof doors, whilew e a r i n g awaterproof jacket!

With tools like these, your collection of rocks and fossilscan be kept as compact specimens. Ideally the rocks arebest displayed as hand size, with one face cut and polishedand one fresh broken. Fossils can be on appropriatelysized substrate, with little surplus material.

With rather more perseverance, thin sections can beproduced. The “traditional” method was to produce thefirst surface by cutting and then grinding usingcarborundum powder of progressively finer and finergrades. Then the second surface was treated similarly andreduced in thickness by measurement, using a microscopeand the birefringence of a convenient mineral like quartzfor the final reduction to the standard 30 micron thickness.

Carborundum is very messy to use as each grade must becompletely removed before the next is used. Bondeddiamond is a revelation in this respect, as the abrasiveremains attached. The best way to use diamond hones is towork in a “Kitty-litter” tray with half an inch of water.Even easier grinding and lapping can be carried out withdiamond lapping discs now available from China for only£7 each for a 6 inch disk, of anything from 100 grit to

3000 grit. It is a characteristic of amateur geologists’homes that their collections spiral out of control intooutbuildings and gardens. Using very basic equipmentamateurs can now cut to size, polish and section theircollections, not only adding interest, but reducing thephysical size to manageable proportions.

So, we are allbenefitting fromthe collapse int h e p r i c e o fdiamonds and,interestingly, inthe collapse int h e m o s tsuccessful cartelin the history ofmarketing. Thatis the control ofthe market by

de Beers and Ayers, their advertising agency. Therein liesanother story!

Tony Thorp

Part 2 to follow next time.

Member's Photo

Fossil Calamites (Giant horsetail) internal cast of stemshowing branch scars. Byrmbo fossil forest, Wrexham.

Richard Becker

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Lapping the easy way

Using a diamond hone.


Geology in the HafrenForestMembers of Mid Wales Geology Club have beenexamining the geology of Hafren Forest for three years.The numerous quarries there tell a detailed story ofchanging sea levels and depositional environments at theend of the Ordovician period and into the Lower Silurian.Most of the Forest roads are accessible to the public onlyon foot or bicycle, but Natural Resources Wales kindlygave permission for us to use vehicles for the duration ofthe study. We have been fortunate on occasions to enjoythe guidance of Professor JR Davies and of Professor WRFitches. The Forest is situated between Clywedogreservoir to the east, and Plynlimon to the west, andcovers 40 square kms. First planted in 1937, on landacquired as twelve upland farms, around 100kms of forestroad had to be built, requiring the opening of some 34quarries, and numerous road cuttings, providing asplendid opportunity to study the geology of the area.

All the rock was deposited in the Lower Palaeozoic WelshBasin from the late Ordovician, Hirnantian stage, to earlySilurian, Telychian stage, in the time period 446 – 433Ma. Sediment flowed into the basin from the MidlandPlatform to the east and from Pretannia to the south.Before the Acadian Orogeny the area lay at the bottom ofa prograding slope which formed near the the south-eastmargin of the basin. A combination of glacioeustasy(global sea level changes due to ice sheets) and regionaltectonics, caused large and frequent fluctuations in localsea level, which are well recorded in the rock exposures.The area lies across the NNE aligned Central WalesLineament, which was important in the fault controlledsubsidence of the Lower Palaeozoic Welsh Basin, and itssubsequent inversion.

The stratigraphy is detailed in BGS Sheet 164(Llanidloes) published for the first time in 2010. Itcomprises: the Upper Ordovician Drosgol and Bryn-Glas

formations of the late Hirnantian stage, overlain in theLower Silurian by the Cwmere, Derwenlas and RhayaderMudstone Formations of the internationally recognisedRhuddanian, Aeronian and Telychian stages (smalldifferences exist between boundaries of local andinternational stages). The main part of Hafren Forestincludes around 1km of vertical succession of rock (smallseparate outlying areas of the Forest expose UpperSilurian rock and are not considered here). The CentralWales Syncline runs nearly north-south through theForest, so the youngest rock (Rhayader Mudstone) liesalong this line. The strata then rise onto the PlynlimonDome to the west and onto the Clywedog dome (outsidethe Forest) to the east, with older and harder rocksexposed as inliers on both domes. Interpretation of sea level change and depositionalenvironment The most interesting parts of the succession are thosewhere the latest sequence stratigraphy infers changing sealevel, often in detail, and sometimes abruptly. Thetechnique of sequence stratigraphy has allowed, duringthe last two decades, new insights to be gained into thechanging depositional environments of this part of thesouthern Welsh Basin. During the late Ordovician andearly Silurian, eustasy (global changes in sea level) playedan important part in local patterns of sediment sourcingand deposition. However, during the Telychian stagetectonics became important in Wales, and this obscuredthe eustatic signal in the local sedimentary successionswhich followed.

Study of the Type Llandovery area (Davies 2012, 2016)some 30 miles south of Hafren Forest resulted in a newsequence stratigraphy for the Llandovery epoch of theSilurian in that district (see the article titled ‘Sea LevelChange in the Geological Past’ in this edition of TheSilurian). The correspondence of the sequences in theType Llandovery area with those in many other parts of

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Stratigraphic column, based on the sequence in the BGS sheet 164 (Llanidloes) showing selected localities.

Geological map of part of the forest. Copied from BGS sheet 164, annotated with rock types and with localities circled in red.


the world has provided new evidence of the effect ofglacioeustasy, as southern polar ice sheets waxed andwaned. The latest Ordovician and early Silurian sequencein Hafren Forest can now be confidently interpreted in thecontext of this new sequence stratigraphy of the nearbyType Llandovery area, because in Hafren Forest duringthis period, changing sea level was also strongly reflectedin the geological record.

Selection of sites, and accessRocks of the Rhuddanian and Aeronian stages of theLower Silurian are widespread in the Forest, and exposedin numerous places. Rocks of the Hirnantian stage of theUpper Ordovician are less widely exposed in the forest.They include localities exposing the Pencerrigtewion,Lluest-y-Graig and Mottled Mudstone Members and theirsubjacent and overlying rocks.

Three parts of the succession are considered in this note.First is the Drosgol - Bryn-glâs boundary. At the top ofthe Drosgol Formation (DF) is the PencerrigtewionMember (PtM), now interpreted as the acme of aglaciostatic sequence lowstand (the Hirnantian lowstand).Overlying this is the Bryn-glâs Formation (BGF), whensea level began to rise again, restoring mud deposition.The basal member of the Bryn-glâs Formation in this partof the district is the Lluest-y-Graig Member (LyG), astratified mudstone interbedded with thin sandstones.

This soon gives way to the more widely encounteredslumped and destratified mudstone, the main part of theBGF. These rocks, including the upper and lowerboundaries of the PtM, are exposed in Quarries 8, 9 & 10,three points along a stretch of track some 2km longrunning roughly N-S near the western edge of the Forest.

Next is the Mottled Mudstone Member (MMb), thetopmost part of the BGF. Asecond pulse of transgressionnear the end of the BGF wasfollowed by a brief regressionwhich triggered re-ventilationof the southern Welsh Basinand re-colonisation of the seafloor by benthic (sea floor)creatures, which left evidenceof their burrows in the form ofdark mottling now seen in themudstone. This was quicklyfollowed by a third pulse oftransgression, re-establishinganoxicity and extinguishingbenthic life, thus preservinggraptolite remains. This isrecorded in the earliest CwmereFormation (CeF). The abovelocalities are in the Ordovician.

Finally, there are quarrieswhich record the frequent sealevel changes in the LowerSilurian, in particular in theDerwenlas Formation (DlF),where short term cycling of sealevel caused basin bottomwater to alternate repeatedlybetween dysoxia (part ia lanox ia ) , and pa r t ia l r e -

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Road map of part of the forest. Selected geological sites shown as red dots with Q numbers being quarries. F is Severn-break-its-neck. G is the DrinkingStone. Vehicles may use the road to the car park and onto Cwmbiga.

Quarry 8 - DF/PtM boundary. Pencerrigtewion Member is a sandstone lobe appearing as topmost Drosgol Fm. It represents a sea level lowstand.


ventilation causing more oxic bottom water. Most of theDlF succession is exposed in one quarry. Another quarryexposes part of it, with the merit of also exposing, byfaulting, the subjacent CeF, and the overlying RhayaderMudstone (Rhs). All the localities described below are onor close to the Severn Way, a national trail which leads tothe source of the river Severn.

Upper Drosgol Formation (DF) and Lower Bryn-glâsFormation (BGF) - Quarries 8,9,10Quarry 8 extends 100m along the north side of the road at[SN 832 891]. A dip of 50º, younging to east, exposes thetopmost DF dark mudstone at the western end of thequarry and, with a noticeable colour difference, thecontact [SN 8300 8912] with the fine sandstone Ptm

which forms thetop of the DF. Thecontact between theP t M a n d t h eo v e r l y i n g L y G(b o t tom o f t h eBGF) is seen 80 meast, dipping intothe wet, vegetatedquarry floor nearthe eastern end[SN8316 8917].During the DrosgolF o r m a t i o n t h edepositionalenvironmentm o s t l y l e d t o

muddy sediment being deposited but there were aroundeight sandstones, up to 10m each. Eventually globalcooling and South Polar ice sheet formation during thelate DF caused a sea level lowstand which ended themuddy phase of DF deposition and began in the PtM tostrip away and remobilise sandy sediments depositedpreviously proximal to the basin. The PtM was depositedas a sandstone lobe, here around 60m true thickness,spreading across the mainly muddy slope apron of the DF.

It is heterolithic, including fine, medium and coarsesandstone, sometimes conglomerate; even occasionallymudstone; some large mudstone rip-up clasts also appearin the sandstone here. Fragments of LyG at the far easternend of the quarry appear chaotically banded as thefractures break across the finely laminated mudstone, aneffect enhanced by weather bleaching of alternate layers.

Quarry 9, a small roadside exposure at [SN 8277 8795],1.1km south from Quarry 10, shows in cross-section theboundary between the top of the PtM and the bottom ofthe LyG – and hence between the DF and the BGF. Herethe bedding is nearly horizontal. The base of the quarry isa 1m thick bed of immature, feldspathic, medium grainsandstone which is the top of the PtM. The lithology isthe same as that seen at the bottom of the PtM in Quarry 8(although the PtM is not the same throughout). Abovethis is up to 10cm of thin-bedded, colour-bandedmudstone, the lowest LyG, recording the beginning of thefirst major pulse of transgression after the sequencelowstand, and therefore also the beginning of the BGF.Above these first mudstone beds is another sandstone bedc30cm thick, part of the LyG, which consists of thinmudstone turbidites interbedded with sandstone turbiditesgenerally less than 20cm.

Quarry 10 [SN 8279 8738] is 600 m south along the roadfrom Quarry 9. This is low in the BGF, just above theLyG. Sea level was rising during the BGF; it is slumpedand destratified due to high deposition rates andoversteepening, as well as possible disturbance due toisostatic movement on faults. Some stratification isdiscernible as slight colour-banding of freshly broken rockat the foot of the face, but there is almost no evidence ofbedding on the substantial brown weathered face. Ironstaining in fallen stone shows Liesegang rings. This is amudstone turbidite with irregular cleavage. The smooth,shiny surface on some fractures may be due to grainalignment during slumping.

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Quarry 9 - Upper boundary of PtM. Sharp boundary between the Pencerrigtewion sandstone and the overlyingLluest-y-Graig mudstone.

Quarry 10 - The Bryn-glâs formation. Featureless, slumped, and destratified mudstone seen from the southern end.

Quarry 8 - Lluest-y-Graig. Finely laminated mudstone with bedding picked out by weathering.


Drinking Stone and Severn-Break-its-neck are alsointeresting exposures of the Ptm sandstone, and are passedvery close to the Forest main access road. The DrinkingStone is clearly situated [SN 8675 8679] on the bankabove the road just over 1km before the main car park. Socalled because legend says its morning wetness is due tonightly drinking in the river below! It is a 5 m boulder ofPtm sandstone detached and slid from bedrock uphillsome 100m to the NW. Vegetation makes access difficultbut the west facing side of this boulder is a bedding under-surface presenting an interesting large area of softsediment load casting. Severn-Break-its-Neck [SN 86318674] is nearby, and is signed from the main access road.Here the river Severn has chosen a major fault as a west-east passage across this hard rock and onto the softer BGF

mudstone. A waterfall occurs as the river flows off theerosion resistant Ptm cap rock onto the softer rock below.A small bridge allows the bed rock to be viewed bothsides.

Mottled Mudstone Member (uppermost BGF), andlowest CeF – ‘Mottled Mudstone Corner’ The Mottled Mudstone Member (MMb) occurs at the topof the BGF and is exposed in a cutting on the north side ofthe Forest road at [SN 841 899], which we have called‘Mottled Mudstone Corner’.

This can be accessed after parking roadside at Cwmbiga[SN 858 890] and walking 2km west along the road. Itincludes a 30m length up-sequence from the topmostBGF, through the Mottled Mudstone Member and into theCwmere Formation. The MMb is widespread in thedistrict but, being thin (here less than 10m), it is rarely

well exposed. NaturalResources Wales kindlyremoved the overgrowthand, following a routinefire-hose drill using thenearby stream, a nearlypristine section of MMbhas been revealed! Thewest (left) end, nearestthe stream, strikes andd i p s 0 1 5 / 3 8 E , a n dexposes BGF mudstonewi th a sub-ve r t ica lcleavage. Here, at thevery top of the BGF, thenormal destratified ands l u m p e d l i t h o l o g yd i sa p p e a r s a n d t h ebedding reappears.

The BGF was mostly atime of rising sea level,the second major pulse

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Part of the Drinking Stone. Classic example of soft sediment deformation on base of a detached block of Pencerrigtewion sandstone.

Mottled mudstone road cutting looking North. MMb dips west. Topmost Ordovician. Bedded BGF, burrow-mottled MMb, and graptolitic CeF.

Burrow-mottling in the MMb. Burrows show dark on wet surface. A sharp sea-level fall removed water column stratification, re-oxygenated bottom water, allowing re-colonisation.


of transgression after the lowstand, interpreted asglacioeustasy, with southern polar ice sheets melting inresponse to global warming. But during the latest BGF ashort sea level fall occurred, perhaps a few tens of metres,re-ventilating the basin bottom water, leading to re-colonisation of the sea bed, and burrowing. At first theorganisms are small and the dark burrow mottles (seenmost easily when wet) are only 1-3mm in diameter in thepale colour-banded rock, but quickly with increasingoxygenation, slightly larger organisms appeared, leavingburrows up to 10mm across. The boundary betweenunmottled BGF and definitely burrow-mottled MMb canbe determined to less than 1 metre. Traces of burrow-mottling begin 10m from the bridge, and cease after 20mwhere the lithology changes to rusty weathering,graptolitic CeF. The final exposure of MMb showsburrows up to 10mm.

Burrow mottling, and the Mottled Mudstone Memberitself, was ended by a third pulse of transgression whichonce again flooded the shelf, re-establishing anoxicconditions probably by a combination of stratification ofwarm top water flowing from the shelf, and upwelling atthe shelf margin causing increased organic productionwhich deoxygenated the water. As the global sea levelrose this time, it reached a point at which the southernWelsh Basin, periodically isolated from the IapetusOcean, again became connected to it, resulting in aninflux of ocean water and marine life. This event istermed the persculptus connectivity. It led to the finelylaminated Cwmere Formation, rich in pyrite withconsequent rusty weathering of exposed rock. Within 2mof the MMb the lithology is clearly anoxic and rustyweathering, with a regular fine cleavage and containingNormalograptus persculptus graptolites [SN 8414 8993],preserved because the sea floor had again become anoxic.The exposure reveals only a few metres of the CeF and istherefore the topmost Ordovician. The BGS map of theLlanidloes district (BGS 2010) shows the base of theLlandovery, which is therefore the Ordovician-Silurianboundary, as just above the base of the CeF. In the early1990s the base of the Llandovery was taken as first

appearance of the graptolite N. persculptus, but now it istaken as the first appearance of the later A. acensus and P.acuminatus graptolites. These have not been found in thelowest Cwmere in the district and hence the lowestCwmere is here assigned to the Ordovician.

Lower Silurian rocks - Quarries 18 &14Quarry 18 is on the south side of the road [SN8506 8633],800m on foot SW of the public car park; a right of waycrosses the river. Faulting and the angle of cut of thequarry juxtaposes Cwmere, Derwenlas and RhayaderMudstone Formations side by side. Looking south intothe quarry the CeF lies to the left (north-east), DlF is inthe centre and Rhs at the right (south-west). The CeF isidentified by its pale and extensive rusty weathering, andextremely friable nature: where the rock is well weatheredit can be rubbed to a mud between the fingers. The DlF inthe centre is mostly a pale grey and greenish, more oxicmudstone. The pale grey Rhs on the right shows itscharacteristically evident bedding, jointing and cleavagediscontinuities. At ground level in the Rhs to the right ofthe fault cleft, is an interval of darker millimetre-laminated shale with sparse Monograptus sedgwickiigraptolites, displaying their characteristically hookedthecae. This is the Sedwickii Shale, the appearance ofwhich determines the base of the Rhayader MudstoneFormation in the district.

The CeF records the third transgressional pulse after theeustatic lowstand which occurred during deposition of thePtm. The basin shelf was flooded again, altering themixing currents and stratifying the water column, leadingto dysoxic or anoxic bottom water. The overlying DlFrecords mostly more oxic conditions, though with periodiclower order sea level movements which led to intervals ofdysoxic bottom water, and some pyrite formation, withconsequent rusty weathering. The overlying Rhs beginswith an important transgression, anoxicity, and theSedgwickii Shales, followed by mostly oxic bottomwaters and bioturbation of the sediment. This eustaticsurge is indicated in the earliest Telychian part of thesequence stratigraphy diagram.

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Graptolites in MMb and Rhs. Above: biserial Normalograptus persulptus in the basalCeF at Mottled Mudstone Corner. Below: Monograptus sedgwickii from basal Rhs in Quarry 18.

Quarry 18 - Cavitation mineralisation in Rhs. Movement on the fault (direction arrows) causes cavitation and formation of white mineral bands, each consisting of numerous fine fibres parallel to the movement.


The curious juxtaposition of CeF, DlF and Rhs is the samesmall quarry deserves a mention. It is due to faulting. Amajor transverse fault thirty miles long runs very close tothis quarry and is extensively mineralised in the district.South-west it becomes the Castell Fault in Ceredigion,and to the east it passes through Severn-break-its-neckand on to the Van lode. A short splay on this fault runsapproximately north-east, from the visible cleft at middle-right of the quarry, passing in front of the left side of thequarry face. The Rhs lies on the viewer’s side of the fault,and the DlF and CeF lie on the other side, i.e. behind thefault. Normally the CeF-DlF boundary would liestratigraphically some 45m below the basal Rhs but aheave south of the fault has lifted it to the level of the Rhs.Pronounced subvertical slickensides appear on the DlF atthe back of the quarry face. Further evidence of tectonicmovement is suggested by a prominent pattern ofmicrocrystalline mineralisation in the Rhs, where slidingof joint faces caused slight cavitation, forming horizontallines normal to the direction of movement. Under handlens each of these lines is a short band of fibrousmineralisation (probably clay minerals like illite, smectiteand chlorite) with the fibres parallel to the axis ofmovement.

Quarry 14 [SN 8514 8751] is 1.3km north-west of thepublic car park on foot. It provides an unusual continuousexposure of several alternations between oxic and dysoxicsediments during the Upper DlF, and is seen asparasequences in the convolutus biozone of the Aeronianstage of the sequence stratigraphy. Thinly interbeddedmudstone turbidites dip c.20ºE and vary from pale grey tonearly black. The oxic horizons are grey and the dysoxicones are faintly rusty weathering and sparsely graptolitic,except for the very rusty weathered exposure of the top ofthe succession at the right (east) side of the quarry whichhas abundant graptolites, which may be M. triangulatus orM. convolutus. This also contains bentonite horizons, oneof 15mm, and much fine pyrite, where ferric compoundshave been reduced to sulphide. Thealternations result from minor regionalf luc t a t ions in mar ine base l eve lsuperimposed on a rising eustatic trend ofsea level – alternations of perhaps 10m.This was more easily seen before the facewas blasted in 2016. Now the face is veryragged and a large volume of loose rocklies against it. When this has beenremoved the feature may again becomevisible. At present study of the quarryremains difficult.

Three transgression-regression coupletscan be seen in the above rocks, and in thesequence stratigraphy diagram of the theType Llandovery area (previous article).Each rises to a maximum flooding surfaceand then falls to a lowstand: 1. the riseduring the late Ordovician (including theearliest CeF), followed by a fall in mid

CeF (Hirnantian-Rhuddanian couplet); 2. the rise duringlate CeF and fall in early DlF (Rhuddanian-Aeroniancouplet); and 3. the rise at earliest Rhs and subsequent fall(Aeronian-Telychian couplet). These sequences, and theparasequences superimposed on them, adjust the various,often differing and usually much simpler sequencestratigraphies proposed over the last twenty years or so,and provide valuable insights into the depositionalenvironments of the southern Welsh Basin.

Colin Humphrey

References

Davies, J.R., et. al., 2013. A revised sedimentary andbiostratigraphical architecture for the Type Llandoveryarea, central Wales, UK. Geol. Mag. 150, 300-332.

Davies, J.R., et. al., 2016. Gauging the impact of glacioeustasy on a mid-latitude early Silurian basin margin, mid Wales, UK. Earth-Science Reviews 156 2016 82-107.

Davies, J.R., et. al., 2009. Sedimentary and faunal events revealed by a revised correlation of post-glacial Hirnantian (late Ordovician) in the Welsh Basin, UK. Geol. Journal, 44, 322-340.

Davies, J.R., et. al., 1997. Geology of the country around Llanilar and Rhayader. British Geological Survey.

Cave, R., and Hains, B.A., 1986. Geology of the country between Aberystwyth and Machynlleth, British GeologicalSurvey

Sheet 164 (Llanidloes), 1:50 000, British GeologicalSurvey.

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Clearing the Mottled Mudstone Member. 23rd June 2015.

Photo ©Richard Becker.

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The Silurian Issue 2 December 2016...The Silurian Issue 2 December 2016 which was then cleaned and levelled by hand using sand cement mortar to fill the cracks and crevices. Blanket