SABRINA TIMES December 2018
Total Page:16
File Type:pdf, Size:1020Kb
SABRINA TIMES December 2018 Open University Geological Society Severnside Branch Branch Organiser’s Report Hello everyone, Our last event of 2018, the annual Day of Lectures at the National Museum in Cardiff was very successful with four excellent talks, including one from OUGS President Dr Susanne Schwenzer describing the Curiosity Rover on Mars and her scientific work as a mission team member. Prof. Philip Donoghue introduced us to the emerging field of molecular palaeontology now being used to establish evolutionary timescales. Cyclic caldera eruptions was the theme of Dr Wim Degruyter’s talk which included a fascinating description of the volcanic eruptions on Santorini. Also, Dr Ian Skilling’s talk showed us that all sorts of interesting things happen when eruptions of basaltic lava come into contact with bodies of water. Fortuitously the Soyuz space capsule that brought UK astronaut Tim Peake back to Earth from the International Space Station was on display at the National Museum while we were there, and was a bonus attraction. The capsule can be seen at the museum in Cardiff until 10th February 2019. There are some spaces still available at our annual geology workshop being held at Aberystwyth in February. This is always a golden opportunity to learn new aspects of geology in a friendly environment, so please contact organiser Janet Hellen if you wish to attend – details are on the website at: https://ougs.org/severnside/events/ You may remember that our first field trip of 2018, the Introductory Day at Portishead, coincided with a heavy snowfall the night before that prevented many members from reaching Portishead on the day. Well, I’m delighted to say that leader Mark Howson has kindly agreed to run the trip again on 24th March 2019. There is much interesting geology to see at Portishead, so fingers crossed for better weather next March! Finally, please don’t forget that our branch Annual General Meeting will be held at the National Museum in Cardiff on 24th February 2019. This is your chance to have a say in how your branch is run, and to let us know what sort of future events you would like to have. Details of the AGM are provided below. Best wishes, Norman Nimmo-Smith Severnside Branch AGM National Museum of Wales, Cardiff , CF10 3NP— Oriel Suite Sunday 24th February 2019 Doors open at 10 am, and the AGM itself will start at 11 am. Tea and coffee will be available from 10 am In this issue: There will be a lunch break when you will have the choice of eating your packed lunch in the meeting room; obtaining food at Llandrindod Wells area 2 the museum restaurant; or visiting one of the many local eateries British Camp, Malvern hills 5 Following the AGM, museum curator Cindy Howells will give a talk entitled “The Geological History of Wales”. Branch Library additions 7 The branch library will be available Events 8 A visitor car park is situated behind the Museum, off Museum Avenue. The charge is £6.50. (Exit token from the Museum Shop.) Contacts and Editorial 9 D40 Llandrindod Wells area 28 - 30 September 2018 Leader: Dave Green This report describes some of the sites visited during one day of a weekend trip to mid-Wales organised by the East Anglia branch and led by Dave Green. Builth Inlier We started off the day at Gilwern Hill (grid ref: SO 090 583), a few miles east of Llandrindod Wells, more or less in the middle of an area known as the Builth Inlier. Please note that this hill should not to be confused with the Gilwern Hill near Abergavenny! The Builth Inlier is the site of an ancient volcanic centre on the edge of the Welsh Basin and consists of volcanic rocks, igneous intrusions and marine sediments of Ordovician age (dated at about 465 Ma), surrounded by younger Silurian rocks. Subsea volcanism led to the growth of islands which rose up and were then eroded. The excellent views we had from Gilwern Hill clearly showed the lumpy, more rugged scenery formed by hard igneous rocks, and the undulating countryside underlain by softer sediments less resistant to erosion (Figs 1 and 2). Fig. 1 Looking south from Gilwern Hill towards the Brecon Beacons on the skyline, with lumpy igneous rocks in the middle distance. Fig. 2 Prominent crags of volcanic rhyolitic tuffs and agglomerate on Gilwern Hill 2 Llandrindod Wells area 28 - 30 September 2018 (contd.) The Builth Inlier is famous for its trilobites, and a few sharp eyes spotted a couple of examples in the track where we parked our cars. The trilobite fossil shown in Fig. 3 was found on a piece of rock extracted from a nearby quarry. Fig. 3 External fossil of a Trilobite on Gilwern Hill (pen Fig. 4 Searching for fossils in a small quarry on for scale) Gilwern Hill Dave led us along a path to a small quarry on the western flank of Gilwern Hill (grid ref SO 087 587). The sedimentary rock at this spot consisted of re-worked volcaniclastics from the subsea eruption that eventually rose above the sea surface to form islands. Subaerial erosion produced marine sediments that now contain fossils such as brachiopods, trilobites, crinoids and sponge spines. We spent some time picking over the rock fragments in the quarry and hammering those that looked promising (Fig. 4). Although some of the lucky ones found a few fossils, we didn’t find as many as Dave had expected, probably because all the easy pickings had already been taken by previous visitors. Fig. 5 A group huddle to discuss yet another graptolite find in the embankment at Llandrindod Wells Fig. 6 One of the many Graptolite fossils found at Llandrindod Wells Llandrindod Wells We then drove away from the Builth Inlier to the outskirts of Llandrindod Wells (grid ref SO 058 601) where we had more success in finding fossils at an embankment on the edge of a housing estate (Fig. 5). Here we found many graptolite fossils of Silurian (Wenlock) age in the laminated silty mudstones of the Builth Mudstone Formation, deposited in a deep sea environment. One of the excellent graptolite fossils found during our rummaging around in the loose stones is shown in Fig. 6. 3 Llandrindod Wells area 28 - 30 September 2018 (contd.) Elan Valley In the afternoon we drove from Llandrindod Wells to Rhayader and then on to the Elan Valley, stopping at the Caban Coch reservoir (grid ref SN 924 646). Five reservoirs were built in this area to provide water for the City of Birmingham. We parked in a large quarry, now a public car park (Fig. 7), adjacent to the Caban Coch dam (Fig. 8). Fig. 7 Quarry adjacent to Caban Coch Dam exposing Fig. 8 Caban Coch Dam (courtesy of Wikipedia) rocks of the Silurian Caban Conglomerate Formation An information board in the quarry car park helpfully informed us that the exposed rock belonged to the Caban Conglomerate Formation. A large amount of this hard rock was removed from the quarry and used as core material for most of the dams. Dave explained that the rocks in the Elan valley formed from sediments of Ordovician and Silurian age washed into the deep marine Welsh Basin situated on the edge of the micro-continent of Avalonia as it drifted north, prior to its eventual collision with Laurentia. Some of the sediments on the continental shelf of Avalonia were re-deposited by submarine avalanches (turbidites) to deeper levels. The Silurian Caban Conglomerate Formation formed this way about 440 Ma from a mixture of pebbles, sand and mud that carved out a submarine canyon through finer turbidites as the high-energy mass flow cascaded down the continental slope, forming distinctly graded beds. Looking at the quarry face, we could see various beds of conglomerate, pebbly-mudstone and sandstone that had been subjected to faulting and folding. Fig; 9 shows a block of conglomerate lying on the quarry floor, and Fig. 10 shows typical bedding structures on the quarry face. Fig. 9 A block of Caban Conglomerate lying on Fig. 10 Bedding structures within the Caban the quarry floor (Dave’s boot for scale) Conglomerate Formation (pen for scale) I would like to thank Wendy Hamilton for organising this visit to mid-Wales, and Dave Green for taking us to some interesting places in the vicinity of Llandrindod Wells and explaining their geology. This is not an area of Wales that the Severnside branch have visited very often, and I hope that we’ll be able to organise some trips of our own here in the near future. Norman Nimmo-Smith 4 British Camp - Malvern Hills August 2018 Leader: Moira Jenkins Moira Jenkins led a trip to British Camp on the Malvern Hills during the Worcester Symposium in August, and has kindly provided this report describing some of the geological points of interest. British Camp, also known as Herefordshire Beacon, is an Iron Age Hill Fort on top of the ridge of the Malvern Hills. These are underlain by resistant Precambrian rocks, the Malvern Complex, dated at about 670 million years ago. These cooled deep in the Earth’s crust and have been pushed up by earth movements over hundreds of millions of years along a line of weakness in the Earth’s crust, the Malvern Line, and were eventually uncovered by erosion. There is a major fault, the East Malvern Fault, running down the east side of the hills, on which the total downthrow is about 2,500 metres. The hills are also crossed by east west faults and the British Camp area has also been moved during the Variscan orogeny along a thrust fault pushing it to the west out of line with the hills to the north, as seen in Figure 1.