Wessex Branch Newsletter
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The Open University Geological Society Wessex Branch Newsletter Website http://ougs.org/wessex July 2015 Branch Organiser’s Letter CONTENTS Branch Organiser’s Letter Page 1 Dear All Mendips weekend, 18-19 April 2015 Pages 2-6 I attended the OUGS AGM in Bristol in April and the Minerals guide no. 15 – Hydrozincite Page 6 Branch Organisers meeting in June. You will be Cerne Abbas, 10 May 2015 Pages 7-8 aware that there is a proposal that associate members of OUGS (ie those who have not studied or Cullernose Point, Northumberland Page 9 taught at OU), who pay the same fee as full Wessex Branch committee Page 9 members, should now be given the same rights (eg be able to vote at AGMs and hold posts on the Other organisations’ events Page 10 committee). The next OUGS Newsletter will include Forthcoming Wessex Branch events Page 11 different views about this. For your information, OUGS events listing Page 12 Wessex Branch has a number of very active and supportive associate members - as have other branches. Once you have had time to consider this October and November. Contact Jeremy if you would please let me know on [email protected] what you like to attend field trips on [email protected] think. I’m looking forward to our residential trip to Anglesey The other point of discussion was health and safety in September (waiting list only). Mark Barrett has on field trips. A good point was made by someone also booked our trip to Mull, 14th to 21st May 2016. who is an events organiser and also a geology leader Dr Ian Williamson is the leader. Contact Mark about - undertake your own risk assessment at any site the residential trips on [email protected] you visit, by standing back and observing potential I have a title for our AGM and lecture Day on hazards and take necessary action to protect yourself Saturday 23rd January 2016: “Mars, Mull and in addition to noting what the leader says and what Mountains”. There will be an advert in the next is written on the risk assessment form. See page 12 Footnotes. Dr Susanne Schwenzer will talk about of this newsletter for information about insurance. Mars and the Curiosity mission (unless she is called Our two-day trip to the Mendips in April and geo- away on another Mars project). Dr Ian Williamson, walk around the Cerne Abbas area in May went who is leading our residential trip to Mull in May 2016, extremely well. The trip to see three chalk pits will cover the geology of Mull and Dr Tom Argles, our around Dorchester was really informative. Many OUGS President, will give a lecture on an aspect of thanks to our leaders Dave Green, Kelvin Huff and the geology of the Himalayas. Rory Mortimore. Also thanks to you as participants The 2016 Symposium will be at Exeter University for your enthusiasm and interest that makes it all from Friday 8th to Monday 11th July 2016, with worthwhile for the leaders and organisers. potential field trips for a few days before in Devon I’m putting the finishing touches to the trip I am and after in Dorset. The title is HOW THE WEST WAS leading up Ham Hill on Saturday 4th July in memory MADE. Prof Iain Stewart hopes to give our of Hugh Prudden, who led a number of trips there for introductory lecture and we have a number of us. We will to be able to visit the working quarry and confirmed lecturers from what was Camborne School stone masons yard in the morning, after meeting of Mines, Plymouth University, the Met Office and outside the Prince of Wales pub on Ham Hill. BGS. July 17th to 19th is the Symposium in Newcastle. I Do look at the website hope to see many of you there. http://ougs.org/index.php?branchcode=wsx where Colin will post any changes to events and also On August 2nd Jeremy will lead a trip to Worbarrow news items. near the abandoned village of Tyneham on the Dorset coast between Lulworth Cove and Best Wishes Kimmeridge. Sheila Alderman, Branch Organiser Wessex On September 13th Sam Scriven from the Heritage xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx Coast Team will take us to see the ammonite xxxxxxxxxxxxxxxxx (evenings only) graveyard at Lyme Regis. We also have trips in E-mail: [email protected] Wessex Footnotes July 2015 Page 1 WESSEX OUGS WEEKEND FIELD TRIP TO THE MENDIPS, 18-19 APRIL 2015 Leader: Dave Green Reports by Andy Mitchell and Mark Barrett SATURDAY, 18 APRIL 2015: BURRINGTON COOMBE Burrington Coombe [ST 477 588] By way of introduction, Dave Green explained that the upper Old Red Sandstone formed the core of an east/west asymmetrical anticline, flanked by Carboniferous deposits, which was created as a result of the Gondwana/Laurasia collision and the consequent Variscan orogeny, To view this part you should visit the Members only area with its push from the south creating a series due to copyright considerations. of periclines – all separated by synclines. Our aim was to walk a transect of the Carboniferous and Upper Devonian on the northern limb of the Blackdown Pericline. Log in using your membership number and surname Location 1 – Quarry 1 [ST 477 590] Here we saw an exposure of the Clifton Down Limestone (CDL) with near-vertical dipping. It was a very fine grained variable biosparite/micrite with red stylolites and some crinoid and coral debris – indicative of a sheltered, low energy, lagoonal environment to the south of St Georges Land. The limestone produced a sulphurous smell when broken up (Stink Stone). The sulphur, on mixing with hydrothermal chlorides, gave rise to the lead ore deposits and mining history in the Mendips. The Clifton Down Limestone here was used for road base material and concrete supply. We saw evidence of blasting on the rock surfaces (plumose surface texture). The Clifton Down Limestone had been deeply buried, severely tilted and recrystallised. Quarry 1 Photo by Andy Mitchell Wessex Footnotes July 2015 Page 2 Location 2 – Quarry 2 [ST 476 588] The Burrington Oolite (BO) formed as a result of high energy wave action in water less than three metres deep. Older than the Clifton Down Limestone, the Burrington Oolite exhibits slickensides created by the action of competent rocks sliding over strata in order to accommodate folding. The central part of the quarry face is oolitic with two distinct dolomitic ribs forming local breaks in deposition with evidence of recrystallisation, pink staining and erosional surfaces. At the top of the north quarry edge was evidence of an unconformity, with red Triassic terrestrial conglomeritic flow deposits representing a wadi overlying part of the Burrington Oolite. Quarry 2 Photo by Andy Mitchell The group then walked along the B3134 up the valley between the Barrington Oolite, past The Rock of Ages, to Avelines Hole (cave) where there appeared to be a confusion of bedding planes and jointing, and further on to the Black Rock Limestone (BRL). The dips at Quarry 1 were 65 NE becoming less steep with a 57NE dip at the start of the Black Rock Limestone, where evidence of localised calcite replacement by silica was seen in fossils. The Black Rock Limstone represents open shallow clear water seas and contains a good variety of fossils (Productus, Spirifer, crinoids, colonial corals etc). Rod Ward (public domain) Rock of Ages (legend has it that a local Continuing along the B3134, the road turns and follows the preacher wrote the hymn of this name strike of the Black Rock Limestone to join a footpath leading while sheltering from a storm here) away from the B3134 uphill to our third location. Location 3 – East Twin Swallet [ST 479 580] East Twin Swallet is a cave where the Eastern Twin Stream meets the interface between an impermeable and a permeable rock formation - the Lower Limestone Shale (LLS) and the Black Rock Limestone containing thin impermeable chert beds. The path continues uphill following the stream where near-vertical dipping shales can be seen in the stream bed. We continued up the hill towards the core of the anticline to find the Upper Old Red Sandstone (UORS), a red quartz sandstone (Devonian under the Carboniferous). The red colour is due to oxidisation in a dry environment. The sandstone was most likely freshwater river deposits, close to a marine area – perhaps beach-type sands. The path swings back west along the Old Red Sandstone and back onto the strike of the Carboniferous sequences. A break in the trees enabled a view of the Great Scarp and Corner Buttress with its optical illusion of apparent dips and faulting on the scarp face. Also worth noting were the vegetation differences where the well-drained Old Red Sandstone is typical of bracken versus the limestone thin soil grasslands. At the 48S dip arrow on the map (ST475 580), the path joins the Western Twin Stream and follows the dip down towards the B3134 to see another swallet feature Great Scarp apparent dips near the pump house where the stream goes Photo by Andy Mitchell underground. Wessex Footnotes July 2015 Page 3 Location 4 – Volvo Car Park, Gurney Slade [ST 622 491] After lunch we made a short visit to see an unconformity (~120 million year gap) of a Triassic dolomitic conglomerate (infill wadi deposit) and very irregular lower Carboniferous limestone surface (old land) having striking differences in colour. Trias conglomerate Photo by Andy Mitchell Location 5 – Velvet Bottom [ST 502 555] Here lead mine workings dating from Roman times have been re-worked at least three times since and reach maximum production in the mid-17th century.