SABRINA TIMES December 2017
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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.