The Open University Geological Society Wessex Branch Newsletter Website http://ougs.org/wessex December 2015 CONTENTS Branch Organiser’s Letter Page 1 Season’s Greetings Lyme Regis fossiling, 13 Sept 2015 Pages 2-3 to all readers Anglesey, 23-25 September 2015 Pages 4-9 Hope to see you at our AGM and Rock of Ages, a note by Tony Cross Page 9 day of lectures in Wool on Minerals guide no. 17 – Musocovite Page 10 Saturday, 23 January 2016. Wessex Branch committee Page 10 OUGS Symposium 2016, Exeter Page 11 Other organisations’ events Page 12 2016 AGM notice and programme Page 13 Branch Organiser’s Letter Forthcoming Wessex Branch events Page 13 Dear All OUGS events listing Page 14 I hope you have enjoyed your geology over the past 12 months. Do look later in the newsletter many of them the only concrete involvement for future events and in particular the advert for they have with the branch, we have decided to the AGM and lecture Day on 23rd January 2016 continue to do so. The cost of printing and in Wool near Wareham (page 13) and the postage is just covered by our branch grant. th th Symposium in Exeter 8 to 11 July 2016 Our branch over the last 20 years has ended up (page 11). Thanks very much for the offers of with a surplus in the accounts so we are help with the symposium. The cut-off date for sponsoring the cost of the boat along the the symposium is much earlier than in previous Heritage Coast for the symposium in 2016 as years due to our contract with Exeter University well as paying for one of our very supportive so book early to ensure you are not field trip leaders, Alan Holiday. disappointed. Best Wishes I look forward to meeting our new members and catching up with as many of you as Sheila Alderman, Branch Organiser Wessex possible during 2016. In the meantime, here is xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx my report for 2015. Tel. xxxxxxxxxxxxxxxxxxxxxxxxxxxx E-mail: [email protected] WESSEX BRANCH REPORT 2015 We have had 16 successful events since January including the AGM and lecture day, local field trips and residential field trips. We find that by having something each month members get in the habit of attending events. The programme for 2016 is nearly finalised – we want to be ahead of things because the committee, alongside SW and Severnside branches, continues to be busy organising the Symposium in Exeter 8th to 10th July 2016. Very many thanks to the Wessex Committee and members for making us such a friendly and interesting group. We still post Footnotes to almost 60% of our members and, as this is for Colin Morley et al at the Grand Canyon October 2015 Photo by Chris Crivelli Wessex Footnotes December 2015 Page 1 WESSEX OUGS FIELD TRIP TO LYME REGIS, 13 SEPTEMBER 2015 Lower Jurassic Geology of Monmouth Beach Leader: Sam Scriven Report by Heather Sloane Twelve OUGS members met Sam Scriven, Jurassic Coast Earth Science Advisor, at the Lyme Regis Lifeboat Station on a clear, sunny morning. It was just possible to view the range of the Jurassic Coast from the eastern boundary with the Cretaceous at distant Isle of Portland, passing the remnants of ice age deposits that formed Chesil Beach, crumbling blue cliffs with the sandstone-topped Golden Cap, to the boundary with the Triassic at Pinhay Bay in the west. We set off westward across the former site of the Lyme Regis Cement Works to explore the Lower Jurassic rocks and fossils, find the Triassic boundary and view evidence of Lyme Regis’ industrial past. Lower Jurassic geology formed around 200 Ma, when shallow seas at the western edge of the Tethys Ocean covered the British Isles, located in tropical latitudes. At this time floods of basalt erupting at the Atlantic Ridge were starting to separate the continents of Eurasia and Laurentia. The climate was less arid than the Triassic, so sea levels started to rise. The cliffs show significant rhythmic bands of limestone, marl and shale as evidence of fluctuating sea levels associated with this tectonic activity. Recent theories also attribute the band variance to climate changes influenced by Milankovitch Cycles. Rhythmic banding Photo by Linda Morley Blue Lias limestone formed in tidal seas, supporting both vertebrate and invertebrate marine ecosystems. Slabs were seen containing multiple bivalves (many fragmented), ammonites and belemnites, also trace fossils as evidence of benthic and pelagic ecosystems. Charmouth Mudstone layers have yielded fossils of whole fish, ichthyosaurs and plesiosaurs that settled on the sea bed. Lack of scavengers and deposition in still, cold waters created low-oxygen conditions that preserved specimens with soft parts, including skin and intestine content, as well as scales and skeletal structure. Shale beds are fissile, parallel layers of fine-grained clay, dark grey in colour indicating anoxic conditions and containing no evidence of benthic fauna. There was a considerable amount of beef-in-shale on the beach in varying thicknesses which provided specimens showing its formation. “Beef” is the name given to dark calcareous crystals growing between layers of shale, which have the fibrous appearance of beef. The crystals form as interlocking cones that frequently appear as circular indentations in the original shale layer, which are often mistaken for fossils. Cone-in-cone “beef” As the sea receded, an extensive Photo by Linda Morley limestone slab became exposed approximately 300m along the beach. This supported some rusting steel rails and fixings, with blocks or pulley reels lying as beach debris. These remain from a tramway and winch mechanism of an aerial ropeway used to move material quarried from the cliff for the production of cement. In 1756 John Smeaton, an engineer from Leeds, found that lime, when ground and burnt with a quantity of clay, produced a “water cement” that set underwater. Clay adds silica and alumina to lime burnt to produce cement which expands into cracks; this repels moisture and is now known as Hydraulic Cement. This innovative product was used in concrete for docks, harbours, bridges and Tram rails Photo by Linda Morley lighthouses as well as for render for damp proofing. In 1855 a cement factory was built on Monmouth Beach to produce hydraulic cement. Industrial Lyme: Paper 61 provides maps showing the extent of the cement works, with many photographs and additional information from historical records of Lyme Regis. This includes records of merchants trading clay for bricks and pottery, Blue Lias for building materials and lime burners, while both were utilised for cement. Wessex Footnotes December 2015 Page 2 Industrial removal of limestone and clay from the beach has undoubtedly contributed to the rapid cliff retreat, which is often wrongly blamed on fossil collectors. We continued along the beach to bed 29, the Ammonite Pavement, where Sam pointed out some features of these creatures in the limestone bedding: Females are large, males are smaller. Maturity can be determined by the number and width of the chambers; evidently some ammonites were mature. Smaller, inner, gas-filled segments of the coil were walled off to give buoyancy. Long, outer segments housed the mollusc. There are fewer ammonites to the west, in the same bed. We discussed possible reasons for so many ammonites to expire in this location: Specimens are whole, so no evidence of storm surge or tsunami. Ammonite pavement Mass breeding ‘orgy’. Photo by Linda Morley Lagoon cut off by climatic change. Lagoon cut off by tectonic activity, eg closure of the Tethys Ocean. Lack of food (plankton) attributed to a break in the eco-system: o Meteor impact, the seas became acid and plankton were first to suffer. o Atmospheric dust, lack of light for photosynthesis of plankton. Sam pointed out that some of the limestone beds showed a variety of extension cracks, evidence of movement within the layer. Suggestions for this include: Unloading of upper layers by excavation and erosion. Difference between competent and incompetent materials in different beds. Clay swelling and contracting with changing temperature and climate. Water pressure of storms. The beach also showed eroded boulders as evidence of Cretaceous Extension cracks geology, fallen from inland by landslides. These included Photo by Linda Morley greensand, chalk and chalky sandstone containing bivalve, brachiopod and crustacean fossils. Seven Rock Point is the peak of a syncline; this is evident as the orientation of bed erosion is noticeably reversed beyond this point. We progressed around the point towards Pinhay Bay and looked back to view the rhythmic change of sedimentary beds from the cliff to the shore line. At our furthest point, Pinhay Bay, Sam pointed out the distinct White Lias beds extending throughout the lower beds at the base Rhythmic banding Photo by Linda Morley of the cliff. This indicates lagoonal deposition of the Upper Triassic when sea levels started to rise and shallow seas covered the arid desert of Pangaea; the more typical red-bed Triassic sandstones can be seen further to the west. Fossils found on Monmouth Beach included small ammonites, a Junction between White and vertebra (possibly an ichthyosaur cervical or tail), chert sponges and Blue Lias Photo by Linda Morley fine examples of “beef” crystallisation. The rain held off, so we had another splendid Wessex field day. Our thanks go to Sam Scriven for his guidance and sharing his knowledge of the area. Heather Sloane References 1 Bull, Richard. Industrial Lyme: Paper 6. The Cement Industry in Lyme Regis & Charmouth. May 2015 http://www.lymeregismuseum.co.uk/images/stories/research/6_cement_industry_in_lyme.pdf Wessex Footnotes December 2015 Page 3 WESSEX OUGS FIELD TRIP TO ANGLESEY, 23-25 SEPTEMBER 2015 Leaders: Dr Margaret Wood, Stewart Campbell and David Wilson Reports by Tony Loftus, Kelvin Huff, Gwenda Brewer, Lyn Relph and Alan Holiday 23 SEPTEMBER 2015 (DAY 1): MARQUESS OF ANGLESEY’S COLUMN & LLANDDWYN ISLAND The day began with an explanation of Ocean Plate Stratigraphy at our hotel by field trip leader Dr Margaret Wood, Managing Director of GeoMôn (Môn is the Welsh word for Anglesey).