SABRINA TIMES December 2017

OPEN UNIVERSITY GEOLOGICAL SOCIETY SEVERNSIDE BRANCH

Branch Organiser‟s Report Hello everyone, We had perfect weather for our last field trip of the year on 15th October, when Dilys Harlow led us on a walk that included the large limestone quarries on Gilwern Hill near Abergavenny, and the disused (but impressive) Clydach Ironworks in the Clydach Gorge. During the walk we had an excellent view of the massive construction work being carried out for the upgraded„Heads of the Valley‟ road (A465) on its route through the Clydach Gorge. Our Day of Lectures on 25th November at the Museum of Wales in Cardiff was well attended, which was not really surprising given the credentials of our four speakers. Dr Chris Berry told us of his long, and finally successful, quest to find the exact location of the Devonian plant fossil site discovered by Norwegian botanist Ove Høeg in Spitzbergen. Prof Maurice Tucker described the various theories of how dolomite is formed, and proposed the idea that microbes and viruses have a large part to play in the dolomitisation process. Staying with the microbe theme, Prof Sue Marriott explained how microbial mats and films most probably produced some of the puzzling structures seen in sedimentary rocks of the Old Red Sandstone. And Prof Paul Wright described the trials and tribulations of modelling the deposition process of the carbonate reservoir in the large oil field off the coast of Brazil. GA Conference Quite a few Severnside members attended the annual Geologists‟ Association Conference in October, which this year was held at the Museum of Wales in Cardiff. With a theme of “past, present and future climates”, we were treated to ten excellent talks from leading UK experts.Courtesy of Steve Howe, slides from Dr Colin Summerhayes‟ talks on “Climate Evolution” and “The Anthropocene” can be downloaded from the South Wales GA website at http://www.swga.org.uk/ GA2017.html. Geology Videos I mentioned in a previous newsletter that members of the Mainland Europe branch had produced two introductory videos on geology. They have now produced three more videos, describing how to classify basalt, sandstone and limestone. All five videos can be accessed from their OUGS website, and are well worth a look. You can also access the videos using this YouTube link: https://www.youtube.com/channel/UCZ3qKE489Ew95iDh_e5FT0g. Membership Renewal Form You will have seen in the Society Newsletter that, because of the new Data Protection Regulations coming into force in 2018, the Society needs to have your formal consent in order to communicate with you by email. Please complete the new membership renewal form (see Society Newsletter November 2017), tick the appropriate boxes, and send the signed form to our membership secretary Janet Hiscott. Thank you ! Future Events Jan Ashton-Jones has been busy organising our events for 2018. Our AGM on January 20th will include two talks by Museum Curators. The annual Aberystwyth Weekend takes place on 24-25thFebruary, and our Introductory Day field trip is in Portishead on 18th March. More details are on the OUGS website at http://ougs.org/severnside/events/.

I hope you‟ll be able to attend our forthcoming events, and look forward to seeing you there!

Best wishes,

Norman Nimmo-Smith Inside this issue: Gilwern Hill & Clydach Gorge 2

Gregory Rift, Tanzania 6 Events 9 MERRY CHRISTMAS AND A HAPPY NEW YEAR TO ALL OUR READERS Contacts and Editorial 11 AGM Agenda 12

D35 Gilwern Hill & Clydach Gorge 15th October 2017

Leader: Dilys Harlow Gilwern Hill and Clydach Gorge are situated near Abergavenny, as shown on the map in Fig. 1. Our leader was Dilys Harlow, author of the excellent book “The Land of the Beacons Way” published in 2014 by South Wales Geologists‟ Association. On a pleasantly warm but cloudy day, members met at the normally unfrequented car park at Keepers Pond on the Blorenge to find it teeming with bikes, cars and police. Our trip coincided with a police and bikers club event and also the rare visit of a Rock Thrush which had brought out “twitchers” by the hundred. (Apparently this bird normally frequents warmer climes in southern Europe – a sign of global warming?) Fortunately Dilys consulted the warden and the police and found convenient hard parking for our base near the Lamb and Fox pub. Interestingly, this building and one other are all that remain of the sizeable village of Pwll Du, in its heyday Fig. 1 Map showing locations visited during the field trip a populous village housing workers in the ironworks and quarries nearby; but demolished in the slum clearances of the 1960s. Regional setting The area lies on the limestone North Crop of the South Wales coalfield, which forms a gentle syncline or basin to the south. In the north, older rocks are exposed as one goes away from the coalfield, so the less easily eroded Carboniferous and Devonian Old Red Sandstones form high ground north of the coalfield. Industrial past The gorge at Clydach has been a communications route for centuries, and the nearby sources of timber (for charcoal) and iron ore (as ironstone concretions or nodules in the coal measures) would have led to early iron production; but it was in the mid-1700s that coke started to be used to smelt iron and this led to vast expansion. Coal (for coke), iron ore, limestone (as flux in blast furnaces) and water (to drive the bellows for blast) were all close by. Geology and industrial archaeology are closely related here and this trip was very much about both disciplines. As we walked to our first location we passed the portal of the Pwll Du Tunnel. This was the longest horse powered tramway tunnel in Britain (1875m). It started as a coal mine near Blaenavon, but was extended in the early 19th century to carry limestone from Pwll Du to the blast furnaces at Blaenavon, and pig iron in the other direction to the local forges and wrought ironworks.

Gilwern Hill East ( or Graig ) Quarry Contouring round Gilwern Hill, we came to our main geological location, Graig Quarry (Fig. 2). This showed a series of bedded limestones with shale partings, near horizontal, but dipping gently to the south (Fig. 3). A stratigraphic column of the rocks at this locality is shown in Fig 4. Some of the rocks we investigated in the quarry are shown in Figs 5, 6, 7 and 8.

Fig. 2 The large Graig Quarry on Gilwern Hill

2 Gilwern Hill & Clydach Gorge 15th October 2017 (contd.)

Fig. 4 Outline stratigraphic column as exposed in Graig Quarry

Fig. 3 Limestone beds at Graig Quarry. The thickest bed is about 1.5m high

Fig. 5 Dark grey Gilwern Oolite with a Fig. 6 Fossils in Gilwern Oolite conglomeritic top containing locally-derived clasts filling channels. The Clydach Halt Member is the rubbly palaeokarst at the very top. (Photo by Chris Simpson)

Fig. 7 Erosive surface at base of Fig. 8 Uraloporella bed (oncoid-rich) at

Cheltenham Limestone Member base of Penllwyn Oolite member, forming (photo by Roger Thomas) the roof on the man-made cave (photo by Chris Simpson) 3 Gilwern Hill & Clydach Gorge 15th October 2017 (contd.)

Dilys explained the processes going on at the time of deposition. These are Lower Carboniferous rocks, deposited when the British Isles were in the tropics, with the Brabant landmass to the north and a marine basin to the south. Sea level changes were occurring, partly eustatic due to advancing and retreating ice sheets on Gondwana, then over the south pole; but more importantly, due to local tectonics. At the time there was probably movement occurring on Acadian (Caledonoid) faults to the north, while Variscan subduction was occurring to the south, causing local back arc extension. It would have been a seismically active time. There would have been the odd earthquake, with occasional exposure and deposition on erosive surfaces, and some soft sediment deformation and loading. The scene would have been a plain below the limestone Brabant foothills, sometimes marine, sometimes lagoonal, and sometimes exposed with playa-like lakes and river channels developing before being flooded with the next marine transgression. A discussion ensued on how oncoids and the ooids in oolitic beds were formed. Ooids (sand sized grains formed of concentric layers of calcium carbonate) must have been formed in warm shallow seas where particles were being rolled around as lime was being deposited. Oncoids, larger structures a few centimetres in diameter, generally spherical, (see Fig. 9), must have been at least occasionally mobile as they have no attachment. Perhaps growing in less disturbed waters, and possibly with associated bacterial or algal growth.

Fig. 9 Limestone fragment found by Jan, showing the cross-

section of two oncoids Gilwern North Quarry Our walk took us past Gilwern North Quarry which we did not enter, but we examined a rust coloured boulder of ankeritic siltstone nearby. Dilys noted that dolomite and ankeritic siltstone occur in the lowest beds exposed and were used for ballast and construction, as opposed to being used for flux in iron smelting.

[Note - Dolomitisation is common in Carboniferous limestones. Dolomite CaMg(CO3)2 is a double carbonate of calcium and magnesium, not just a mixture. In the same way, iron can be included in the lattice, forming ankerite, CaFe(CO3)2 . Dolomite and ankerite form a solid solution in any proportion. Dolomite is preferred for construction as it is slightly harder than calcite and more resistant to attack by acid. It will only fizz with dilute hydrochloric acid if powdered or heated.] We followed the path round the hill down to a disused limeworks with remains of a sizeable pair of kilns, where we had a well-earned lunch break overlooking the viaduct at Clydach Halt that carried the now dismantled railway line. Ironworks The party then divided and half decided to continue downhill to visit the famous ironworks, while the rest climbed back to the starting point at a leisurely pace. We continued our descent down a steep path which followed the old incline to the ironworks (Fig. 10). This was well worth visiting and comprises the ruins of three blast furnaces constructed at the end of the 18th century, when coke replaced charcoal as fuel. It remained in production until the 1860s. The site was cleared and part restored by archaeologists and students in the 1980s, but has sadly become somewhat overgrown since. Fig. 10 Clydach Ironworks (photo by Roger Thomas)

4 Gilwern Hill & Clydach Gorge 15th October 2017 (contd.)

Fig. 11 Smart‟s Bridge

Nearby is a delightful small cast iron bridge, “Smart‟s Bridge”, constructed in 1824 to span a river (Fig. 11). Cast iron girders carry a roadway with cast hoof grips for horses and smooth grooves for the wagon wheels. Now a peaceful backwater, it is difficult to imagine the noisy, smelly hellhole it once was. However, we said goodbye to shades of the past and tackled the long ascent back the way we came. While catching up the rest of the party, we passed a possible “Farewell rock” at the base of the workable Coal Measures, but this apparently is not necessarily always the same formation, as it depends on the last contact with workable coal. We then took a direct path back to our base, passing old workings which had been the first “Open Cast” operations in the country, having been worked by Canadians during the last war. Returning to our parking spot after the strenuous climb, we had enjoyed a really interesting day which had demonstrated the close relationship of geology and industry. Many thanks to Dilys for her leadership and for reconnoitring the often obscure footpaths.

References: Dickson, W. J., and Wright, V. P. 1993 Carboniferous Limestone of the North Crop of the South Wales Coalfield, in Geological Excursions in Powys, Central Wales. Eds Woodcock, N. H. and Bassett, M. G. pub: NMU, Cardiff. Barclay, W. J. Geology of the South Wales Coalfield, BGS Memoir, Pt II, The country around Abergavenny.

Report by Tony Thorp

5 Tanzania - The Gregory Rift

Barbara Wood, one of our Severnside members, visited northern Tanzania in July 2017 to stay with friends at Arusha and enjoy a bit of sight-seeing. This part of northern Tanzania is full of interesting geology and Barbara has kindly written the following account of the area. Read on…

Fig 2 Map of northern Tanzania

Fig 1 Map of East Africa showing the movement of tectonic plates (courtesy of Rob Gamesby) The Gregory Rift is the eastern branch of the East African Rift fracture system (Fig. 1). The rift is being driven by a thermal plume which is causing the separation of the Somali and Nubian plates. It is composed of a series of faults and grabens from the Gulf of Aden, where it is widest, and extends southwards as far as northern Tanzania where it narrows to a few kilometres. This part of the rift was named after Professor John Walter Gregory (1864 - 1932), a British geologist and explorer who studied the area in 1892-3 and 1919. The Tanzanian portionincludes the highest mountain in Africa, Mount Kilimanjaro; the active volcano Oldoinyo Lengai; and the dormant volcano Mount Meru (Fig. 2). Some of the lakes are fresh water but many, like Lake Natron, are saline. A major episode of faulting approximately one million years ago has given rise to the present day geomorphology.

Fig 3 View of the rift wall showing the layers of lava flows Between the Kenyan border and Lake Manyara the escarpment trend is north-south. The faulting caused a disruption of older drainage directions and the formation of a series of enclosed sedimentation basins at the foot of the main escarpment, as at Lake Natron. This basin has small opposed faults facing the escarpment. There is only one rift wall in this area (Figs 3 Fig 4 Basalt flow with columnar and 4). jointing, lying on top of ash deposits

6 Tanzania - The Gregory Rift (contd.)

Oldoinyo Lengai is a strato-volcano, situated to the south of Lake Natron (Fig. 5) It has an elevation of 2,980m, a relief of 2,090m and a diameter of 12km. The eruptive history is from approximately 22,000 years ago to the present day. It is the only currently active volcano in northern Tanzania and is unique in being the world's only active carbonatite volcano. (Carbonatite is an igneous rock with a composition consisting of greater than 50% carbonate minerals). Thevolcano has been visited since the 1980s by climbers for both sport and scientific studies.

Fig 5 The active volcano Oldoinyo Lengai, with weathered carbonatite lava in the foreground.

The classic cone of Oldoinyo Lengai stands next to the Natron basin boundary fault escarpment, against which its pyroclastic deposits are piled. Today it is covered by sparse grass and scrub at lower levels. Prior to the 1917 ash eruption, the volcano was highly vegetated with abundant game. In late 2007 much grass was ignited and burned as lavas spilled down the western and northern slopes. The cone is incised by deep radial gullies and on the NE upper slopes by a major landslide scar. Debris from this landslip lies on the plains to the E and NE, and other previous landslides originating Fig 6 Small island in Lake Natron formed from landslide debris. on the northern slopes had a sufficiently long The white dots are flamingos, which favour saline lakes run-out to form low islands in Lake Natron, 16km north of the volcano (Fig. 6). Parasitic craters occur on the western and eastern lower flanks, and a north-south trending line of three small scoria cones can be seen on the northern flank.The summit is occupied by two small craters. The older inactive southern crater is now a shallow, ash filled bowl. The active northern crater is shown in Fig. 7. The activity observed during 1966-67 and in 2007 was of Vesuvian and Plinian type eruptions. The active crater is now occupied by a major ash cone which has spilled over the outer edge of the original crater. Small ash eruptions have been observed in July and August and a major eruption might be imminent! Carbonatite is a variable magma and is considered to be of mantle origin. The magmas are typically associated with under-saturated (very low silica) igneous rocks. They consist mainly of Ca, Mg and Fe carbonates. Natrocarbonatites contain Na and K carbonates in significant quantities. They are anhydrous and react quickly with moisture in the atmosphere to turn the greyish-brown lava and ash to a white crust. Streaks of this on the flanks of the volcano look like snow!(See Figs 5 and 7). These magmas contain concentrations of rare earth Fig 7 Aerial view of the active crater of Oldoinyo Lengai elements, the source of Niobium being very important, (courtesy of Michael Dalton-Smith) also the industrially important minerals apatite, barite and vermiculite.

More than 300 occurrences of carbonatite are known, nearly all of them continental.They are generally associated with rift systems, most notably the African Rift Valley.

7 Tanzania - The Gregory Rift (contd.)

Mount Meru stands to the north of Arusha town at the edge of Arusha National Park (Fig. 8). It is another major strato-volcano with an eruptive history from the Pleistocene to Holocene. It has an elevation of 4,568m, a relief of 3,100m and an approximate diameter of 25km. The bulk of the main cone is dominated by pumice and tephra, interpreted as the product of Plinian eruptions, interceded with nephelinite and phonolite flows. These range in age from 1.16 to 0.06Ma.

Fig 8 Mount Meru at sunset (photo taken by Barbara from the garden of her friend‟s home)

Erosion of the pyroclastic rocks has given rise to deeply incised radial valleys, those on the cone perhaps having been glacially initiated. The cone has been extremely unstable with major lahar deposits flowing on to the surrounding plains. A large debris avalanche deposit on the eastern side had a 35km run out and came to rest on the lower slopes of Kilimanjaro. The coverage was approximately 390 square kilometres and had an estimated volume of 28 cubic kilometres, making it the largest sub-aerial debris avalanche yet recorded. Part of the avalanche terrain is now occupied by the Momella Lakes in Arusha National Park, about 12km to the east of Mount Meru (Fig. 9).

Fig 9 The Momella lakes in Arusha National Park

Article and photos by Barbara Wood

8 Severnside Branch Events

2018 EVENT LEADER DETAILS Oriel Suite, National Museum of Wales, Cardiff. January 20th Severnside AGM Doors open at 10.00 AM and the AGM will start at 11.00 AM

Microscope work and Techniques in Structural Geology. Bring your own microscope or be prepared to share. February 24-25th Aberystwyth weekend Charlie Bendall Bed & Breakfast at the Park Lodge Hotel.

Introduction to the Geology of Portishead.

March 18th Portishead Mark Howson FGS This is our annual introductory day and will be particularly suitable for those with little or no previous experience of geological field trips. We shall investigate the sedimentary rocks that were laid down Ogmore and during the Carboniferous, Triassic and Jurassic periods along this April 28th Dr. Geraint Owen Southerndown scenic part of the South Wales coast.

A weekend trip to examine the minerals of North Wales led by the Senior Curator of Mineralogy at the National Museum of Wales. May 26-27th North Wales Tom Cotterell

Event Notes

EVENT DETAILS CAN CHANGE Any last-minute changes of times or arrangements are on the OUGS website.... www.ougs.org Or...... Contact Norman Nimmo-Smith - 01684 891859 [email protected] If you are not receiving email reminders from Norman regarding forthcoming events or if you get a new email address please let him

Transport If you are able to offer a lift to any event or if you have local knowledge regarding bus or train services etc. that might help those without their own transport to attend an event please let Janet know. Updates Why not try the following link to a service for keeping an eye on changes in the events list. http://www.changedetection.com

THERE IS A NOMINAL FEE OF £3 ON DAY TRIPS (UNLESS OTHERWISE STATED) TO COVER LEADERS EXPENSES. ALWAYS WEAR APPROPRIATE GEAR...WARM CLOTHING AND HIKING BOOTS. BEAR IN MIND THAT MANY SITES, ESPECIALLY QUARRIES, REQUIRE THE WEARING OF HARD HATS. ACTIVE QUARRIES REQUIRE HARD HATS AND YELLOW JACKETS. IT IS RECOMMENDED THAT PARTICIPANTS BRING HARD HATS TO ALL FIELD TRIPS - JUST IN CASE !

IMPORTANT Each person attending a field meeting does so on the understanding that he/she attends at his/her own risk. The OUGS has Public Liability Insurance cover for field and indoor meetings but Personal Accident and Personal Liability cover remain the responsibility and personal choice of the participant. There may be an element of appropriate cover included in house or travel insurance. Although OUGS activities are not particularly dangerous members are advised to check whether exclusions apply to activities in which they propose to participate in case they wish to arrange further cover. An annual travel insurance may be the best solution for any member who regularly attends field events. This is again a matter of personal choice. All members participating in overseas events will be required to have travel insurance for the duration of the event. Participants should be covered for Medical, Repatriation and Personal Liability expenses. The Personal Accident element remains the personal choice of the member. Disclaimer None of the information in any of the advertisements for field trips or other events in this newsletter constitutes a brochure under the Package and Travel Regulations (1992)

9 In Memoriam

Remembering John Lynham

John Lynham will be fondly remembered by many of the older Severnside branch members of the OUGS, and particularly any who went on the Lundy trips. Sadly, John passed away in August at the grand old age of 91. His funeral took place in Yeovil, where he last lived. John will be remembered for his energy and enthusiasm for geology and many other subjects as well. He was very well versed in the Tertiary geology of Lundy, and was involved with discussions with Dr Clive Roberts, who was writing up his PhD at that time. John had a terrific sense of humour, and was so convivial. He was always striding out at the head of a struggling rabble of less fit members, and at the same time carrying on an in-depth conversation, with nobody having the breath to reply. John lived life to the full, and will be leaving a big hole in the lives of his many friends and family.

Jan Boddy,

Elizabeth McIntyre

Elizabeth died on March 31st 2017, in her 85th year. When she was a biochemistry student at Liverpool in the early 1950s she shared lodgings with a girl studying geology, read Arthur Holmes‟ Principles of Physical Geology and fell in love with the subject. She also met Roy McIntyre, a mathematician, and they were married in 1953. Before their 4 children were born she worked as a biochemist in London and Bristol but later decided to be a post-person in Winterbourne so as to be home when the children came back from school. The family were keen orienteers. At one event a fellow orienteer, OU tutor and author Andrew Bell, discovered Elizabeth collecting rocks and suggested she join the OU, which she did in 2001 and soon persuaded Roy to study geology too. For years all their holidays have had a geological theme. She could tell you the geological history of just about any part of Britain, especially Scotland.

Her crowning achievement was her 3D geological maps. She devised a unique way of printing BGS maps at successive contour levels, pasting these onto thin polystyrene tiles and building up intricate 3d maps, complete with cross-sections. A map would typically take her about 500 hours over 3 months, that‟s 5 hours a day – in addition to OU courses! Many of her maps are now used to demonstrate local geology, including at Bristol University.

She was knowledgeable, patient, kind and great fun. Those who knew her will remember her with fondness and admiration.

Dilys Harlow

10 Severnside‟s OUGS Neighbours and other Organisations

It‟s worthwhile taking a look at what other branches are doing. Many of their events could be convenient to attend. OUGS Oxford Branch Organiser: [email protected] OUGS Southwest Branch Organiser: [email protected] OUGS Wessex Branch Organiser: [email protected] OUGS West Midlands Branch Organiser: [email protected]

Many of our members belong to other Earth Science related organisations which also do interesting events; Welsh Stone Forum http://www.museumwales.ac.uk/en/364/. Geologists Association [email protected] Affiliated Regions: South Wales www.swga.org.uk West of England www.wega.org.uk Bath Geology Society www.bathgeolsoc.org.uk Avon RIGS Group The Avon RIGS Group are re-launching their newsletter, Outcrop, as an on-line blog. The relevant websites are: www.avonrigs.org.uk http://avonrigsoutcrop.blogspot.com

South East Wales RIGS Group Contact Nigel McGaw 01656 654439 [email protected]

The Russell Society www.russellsoc.org.uk

Teme Valley Geological Society Based in Martley, Worcestershire http://www.geo-village.eu

OUGS Severnside Branch Committee

Norman Nimmo-Smith - Branch Organiser 01684 891859 [email protected] Averil Leaver - Treasurer 01446 418592 [email protected] Jan Ashton-Jones - Events co-ordinator 01432 860736 [email protected] Janet Hiscott - Librarian 01633 960845 [email protected] Jan Boddy 01793 762575 [email protected] Kath Addison-Scott 01179 538085 [email protected] Anthony Bukowski 02920 300080 [email protected] Linda Fowler 01432 860465 [email protected] Teresa Jenkins 07542 895244 [email protected] John de Caux - Newsletter Editor 01633 875955 [email protected]

Editorial

If you are one of the remaining members still receiving Sabrina Times by post why not get the much better colour version of this and future issues of the newsletter by email (in .pdf format). Just send a request to your editor at: [email protected] Contributions for the newsletter will be very welcome from members, as will any comments or suggestions for improvement. We are able to read or scan most data formats or documents. For photos please send the images as separate files, preferably in jpeg (.jpg) or bitmap (.bmp) format with notations in the text. For large files OneDrive or Dropbox may be used…...just email the editor a link.

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Severnside Branch Annual General Meeting

Open University Geological Society AGM of the Severnside Branch

Oriel Suite National Museum of Wales, Cardiff 20th January 2018 11.00 a.m. AGENDA

Apologies for absence

Matters arising from the minutes of the last meeting held on11th February 2017. (Published in March 2017 edition of Sabrina Times.)

Officers Reports: Branch Treasurer Branch Organiser Other Officers

Election of Officers Branch Organiser Branch Treasurer Committee members

Any other business

Tea and Coffee will be served from 10.00 a.m.

The Branch Library will be available

Following the AGM there will be two talks by experts from the National Museum :

Tom Cotterell (Senior Curator: Mineralogy & Petrology) - "Minerals of North Wales" Cindy Howells (Curator: Palaeontology) - "Archaeopteryx: The Bird-Like Dinosaur"

Membership Renewal

As mentioned in Norman‟s report (Page 1 ) it is necessary for all members to complete and sign a Membership Renewal Form to comply with forthcoming Data Protection Legislation. Forms can be downloaded from the OUGS website (A copy is enclosed with the postal version of this Sabrina Times) http://ougs.org/members/ Forms should be signed by all Members, Joint Members and Family Members. Renewals are due on 1st January 2018 and the cut-off date is 31st January 2018

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