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Wessex Branch Newsletter The Open University Geological Society Wessex Branch Newsletter Website http://ougs.org/wessex December 2012 WESSEX BRANCH REPORT 2012 2012 has been a good year for Wessex OUGS - increasing membership and well attended events. We started the year as usual in January with our AGM and Lecture Day in Wool near Wareham in CONTENTS Dorset with a record attendance of 79. The theme Branch Organiser’s Letter Page 1 was volcanics with lectures by Dave McGarvie, Alan Vallis Vale, 12 February 2012 Pages 2-3 Holiday and Mike Widdowson. Wessex OUGS covers Minerals guide no. 2 - Chalcopyrite Page 4 Dorset, South Wiltshire and Hampshire with some of us just over the border in Somerset, Devon and West Snowdonia, Day 2, 12 June 2010 Pages 4-7 Sussex so it is excellent that so many of you are able Wessex Branch committee Page 7 to get together to attend events. Kimmeridge Museum lottery success Page 8 Jeremy Cranmer produced a good and varied Hidden Treasures of Horseshoe Canyon Page 8 programme of day trips as well as being custodian of Notice of AGM, 26 January 2013 Page 9 our library books and maps, which you can order David Batty, Sandy Tolmay Page 9 from the website. Colin Morley and Rhiannon Rogers continued to work efficiently as our web-manager Subscription renewal reminder Page 9 and treasurer respectively. Hilary Barton took over Other organisations’ events Page 10 as Footnotes editor and has produced 5 really Forthcoming Wessex Branch events Page 11 interesting copies this year. Thanks to all of you contributing reports and sending her useful OUGS events listing Page 12 information. Colin and Linda Morley continue to take which is world famous [see page 8 for latest news. groups to explore the geology of Arizona including the Basin and Range area of Arizona in March as well Ed.]. Dr Ian West showed us coastal defence and as Colorado Plateau and Grand Canyon in November. processes at Hurst Spit and Hurst Castle at the Mark Barrett, our weekend trips organiser, arranged entrance to the Solent. We had real hands-on geology a whole week studying the Assynt this May staying at at Poxwell near Weymouth when we helped to clear Inchnadamph in the NW Highlands with leader John away undergrowth and the path to Poxwell Quarry, a Mendum ex BGS. It was fantastic, I know so much RIGS site, and then walked around the pericline. At more now about hard rocks and mountain building. I the end of September Prof Chris Wilson explained the was sorry to miss the winter weekend in Oxford Corallian from Osmington Mills to Ringstead Bay. Natural History in November with lectures by several The second introductory day to Worbarrow in PhD students on a fossil theme. Thanks to Mark for November with George Raggett showed the dipping organising these longer events. Mark is arranging Portland Jurassic Portland beds and Cretaceous weekends in South Devon and Shropshire in 2013 Purbeck plus the, Wealden, Greensand and Chalk and there is a suggestion of Northumberland and with many fossils and geological structures. Our maybe Southern Uplands in future years. final trip of the year is fossilling with Sam Scriven at Charmouth near Lyme Regis on 16th December. We held an introductory day in February at Vallis Vale in the Mendip Hills with Alan Holiday. This will Many thanks to all the committee for being so supportive to me and to the branch. Our theme for be run again for new and “old” students alike. The th De La Beche angular unconformity between the lectures on 26 January 2013 is Geology’s Role in Jurassic and Carboniferous really shows what Sustainable Energy at the D’Urberville Centre, Wool geology is all about. In March we were fascinated by so I hope to see most of you there. We look forward Prof Rory Mortimore at the Winchester Anticline, St to what promises to be another interesting year in Catherine’s Hill overlooking the M3 Twyford Down 2013. cutting. In May we combined human habitation and Sheila Alderman, Branch Organiser Wessex archaeology with geology at the Uffington White November 2012 Horse and Wayland Smithy with Dr Jill Eyers on the Ridgway in Oxfordshire. In June, Steve Etches took xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx us around Kimmeridge Bay and showed us his Tel. xxxxxxxxxxxxxxxxxxxxx (evenings only) amazing collection of fossils from the Kimmeridgian E-mail: [email protected] Wessex Footnotes December 2012 Page 1 WESSEX OUGS FIELD TRIP TO VALLIS VALE AND TEDBURY CAMP, SOMERSET Introduction to Geology II, 12 February 2012 Leader – Alan Holiday Report by Carolyn Dent On a cold frosty Sunday morning a group of us, led Another trip next year by Alan Holiday, set out to visit localities in the Vallis This trip, again led by Alan Holiday, will be Vale. The Carboniferous Limestone was formed repeated on 24 February 2013. It is some 350 ma ago before being folded and uplifted particularly suitable for OUGS students of during closure of the Rheic Ocean and associated S104, S276, etc but all are welcome. For Variscan Orogeny around 300 ma. Then the Triassic more information see page 11 of this newsletter. breccia was deposited 210ish ma, White Lias around 205 ma and the Inferior Oolite around 175 ma. This field trip was the second of two designed to complement learning on the OU S276 geology course, and the group was a diverse mix of students, ex-students, partners, and those who had come along to share their expertise. [Susan Graham’s report on the Part I trip to Worbarrow Bay on 26 November 2011 was published in the July 2012 edition of Footnotes. Ed] The first locality visited was along a rutted road marred by fly-tipping, behind which a low cliff gave us a taster of what was to come. There are three groups of rocks and two unconformities in this cliff. Layers of Carboniferous Limestone dipped towards us, above which the main angular unconformity, overlain by beds of the White Lias, was clearly visible. A parallel unconformity above the White Lias, overlain by beds of Inferior Oolite, was much less obvious to the casual observer. After walking through an industrial estate and squeezing along a wire fence past further evidence of fly-tipping we emerged in a pretty wooded river valley, the geology of which was to keep us Carboniferous limestone dipping enthralled for the rest of the day. As we followed Photo by Carolyn Dent the path of this river we passed several disused lime kilns, evidence of past exploitation of locally quarried limestone. Coal for the kilns was sourced from the nearby Somerset Coalfields at Radstock. Limestone is still being quarried from this area today. Large pieces of bright red Triassic breccia are visible by the path, a poorly sorted breccia containing a variety of sizes of angular clasts of limestone suspended in a red silty carbonate-rich matrix. This immature breccia was likely deposited during flash floods, and the red coloration is due to highly oxidised iron as FeO3, an indicator of an Poorly sorted red Triassic breccia oxygen-rich arid environment Photo by Carolyn Dent during deposition. The next exposure of interest was steeply dipping beds of Carboniferous Limestone. These provided an opportunity for practice with compass clinometers, with which we determined dip to be approximately 45 degrees to the north, interesting as most of us had visually estimated dip to be more like 70-80 degrees Using a compass clinometer to prior to measurement. Raised nodules of chert had proven estimate dip Photo by Carolyn Dent Wessex Footnotes December 2012 Page 2 relatively resistant to erosion compared to the limestone. Alan Holiday explained that the chert was formed where organisms had been burrowing into the limestone, where they caused locally acidic conditions in which silica is relatively insoluble and therefore precipitated out as chert. We proceeded down the path to a locality where a fault plane perpendicular to the bedding was exposed above us. Slickensides visible on the fault plane indicated the last movement on the fault to be E-W. This suggests that tectonic compression must have occurred in both E- W and N-S directions, as the dipping bedding planes form part of an anticlinal fold resulting from N-S crustal compression. If there is pressure from multiple directions during folding a plunging fold will be formed, rather than a simpler cylindrical fold when pressure is from just two directions. Examination of the geological map for this anticline shows that it dips to the east, so it is indeed a plunging fold. Fault plane above bedding planes We then scrambled up a snowy slope to reach a cave Photo by Carolyn Dent above the path, with good exposures of Lithostrotion coral fossils. Corals are good environmental indicators of shallow tropical seas during deposition, but are poor indicators of age. A short walk from this site led to Tedbury Camp, where the Inferior Oolite has been stripped from above the Carboniferous Limestone (by man), revealing the natural erosion surface of the unconformity itself. That this erosion surface was once under the sea is shown by the presence of numerous burrows of bottom living animals. These are found on the Jurassic wave-erosion surface, rather than on the limestone bedding planes, showing that they date from when erosion was occurring during the Jurassic rather than from when the rock was being laid down in the Carboniferous. An exposure at the edge of this surface facilitated viewing of the Carboniferous Limestone perpendicular to the bedding planes. The 45 degree dip can be much more accurately estimated when viewed side on than at the earlier exposure where beds were dipping towards us.
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