SABRINA TIMES July 2019 Open University Geological Society Severnside Branch Branch Organiser’s Report Hello everyone, This year’s programme of events continues apace, with one overseas trip and two day trips being held since publication of our last newsletter. The industrious efforts of our trip organisers often go unnoticed and I’d just like to take this opportunity to thank Michelle, Jan, Kath and Teresa for producing a splendid programme this year. A group of OUGS members visited the Solnhofen area of southern Germany in May. Although the main theme of the visit was the primitive dinosaur-like bird Archaeopteryx and included visits to museums and quarries (for fossil- hunting), we also spent time at the Ries impact crater and the attractive walled town of Nordlingen. But the headline news was that Severnside member Terry Blake shot to fame during a fossil-hunting session in a quarry when he discovered a very fine specimen of the lobster Eryon cuvieri, apparently the first specimen of this species to be recorded from the Mörnsheim Formation of the Late Jurassic! Terry generously donated his rare find to the Bürgermeister-Müller Museum in Solnhofen so that it could be professionally prepared and made available for research and public display. Well done Terry. We hope to include some reports of this very successful trip in future editions of Sabrina Times. In June we spent a very full day with leader Dr Gareth George at the Mumbles and Caswell Bay areas of the Gower Peninsula, when we examined a variety of Carboniferous Limestone exposures and structural features associated with the Variscan Orogeny. Gareth was as enthusiastic as ever, and many a find was preceded by the words “boy oh boy, just look at this!” The geology of this area is very well described in Gareth’s excellent book “The Geology of South Wales: A Field Guide”, and it was a real treat to have Gareth as our personal guide for the day. Later in June we explored the coastline at the small village of Amroth in the company of Dr Geraint Owen. This section of the coast to the north of Saundersfoot in South Pembrokeshire is also of Carboniferous age, but the rocks are younger and have the added attraction of coal deposits. A report of the trip, kindly written by Mark Turner, is included in this newsletter. The detailed programme for the OUGS Symposium in August is now available on the website at: https://ougs.org/ society-events/19/ougs-47th-annual-symposium-milton-keynes/. With a number of wide-ranging talks by research staff at the Open University, along with a choice of interesting workshops and lab tours to suit all tastes, this promises to be a memorable event. A booking form is available to download from the website, so book today if you haven’t already done so! A new date for your diary is our annual Day of Lectures, which this year is being held on Saturday 7th December at the National Museum of Wales in Cardiff. Michelle is currently busy organising the speaker programme, so keep an eye on the OUGS website for further details of the event. Finally, our next field trip takes place on the picturesque coast of West Wales on Saturday 20th and Sunday 21st July, and will be led by Dave Green. There will be lots of good geology on offer, and day visitors are very welcome. Full details of the trip are provided on page 10 of this newsletter. Best wishes, Norman Nimmo-Smith In this issue: Woolhope Dome 2 Amroth 7 Events 10 Contacts 11 Jurassic lobster Eryon cuvieri (fossil length 8 cm) D43 Woolhope Dome 13th April 2019 Leader: Dave Green Introduction The Woolhope Dome is a Silurian inlier surrounded by younger Devonian rocks, located a few miles east of Hereford, the county town of Herefordshire. It is an upland area of folded and faulted Silurian rocks that have been exposed by the erosion of the younger Devonian rocks of the Old Red Sandstone. The sketch in Fig. 1 shows the geology and scenery of the Woolhope Dome, with its three rims of resistant limestone that stand up as ridges separated by more easily eroded clay vales. The high outer rim is formed of Aymestry Limestone. Fig. 2 shows the sedimentary rock units of the Woolhope Dome. Fig. 1 Sketched view, looking west, of geology and scenery in the Fig. 2 Succession of sedimentary rocks of the Woolhope Dome (courtesy of Dave Green) Woolhope Dome During the day we investigated the geology that created the circular ridges and undulating landscape of the Woolhope Dome, and found a number of marine fossils in the different limestone exposures we encountered along our route. In the morning we traversed the main Wenlock, Ludlow and Pridoli rock units on the eastern side of the dome and observed their effect on the landscape. In the afternoon we visited the older rock sequence in the centre of the dome, and towards the end of the day we climbed a small hill to get a good overall view of the inlier from the south. Geological History Herefordshire was covered by a shallow sea for most of the Silurian Period (444 to 419 Ma) and the Silurian rocks of the Woolhope Dome represent lagoon, reef and near-shore deposits. Melting of ice sheets led to a rise in sea level during the early Silurian period and the sea gradually spread over Herefordshire from the west. A shallow-water shelf environment lasted until the late Silurian when uplift caused a marine regression and the appearance of the non-marine Old Red Sandstone. This uplift was related to the steady rise of the Caledonian Mountains in the north of Britain. At the end of the Silurian period, Herefordshire was on the coastal plains at the edge of the Old Red Sandstone continent. The sequence of sedimentary Silurian rocks was subsequently folded during the Variscan Orogeny to form a dome structure (an anticlinal pericline). Our itinerary allowed us to visit exposures from most of the 25 million years of the Silurian period. Our Itinerary We all met in the car park at the Butcher’s Arms pub in the small village of Woolhope, which is situated in the middle of the Dome (see Fig. 1). The pub was to be our lunch venue so we popped indoors to place our orders from their extensive menu before our leader Dave Green provided us with a handout and described the landscape and geology of the Woolhope Dome in great detail. The plan for the day was to visit a number of locations where we could see exposures of the underlying rocks amidst the green fields and woods that blanketed the area. In addition we would see how the folded rocks had formed the Dome’s “ring” structure of ridges and valleys. During the first part of the day we drove in a few cars to our first location at Rushall (SO 642 350) on the eastern edge of the Dome and then walked about 4km back to the pub. This meant that we walked backwards in time from rocks of the late Silurian (Pridoli Epoch) to the older rocks of the Wenlock Epoch. 2 Woolhope Dome 13th April 2019 (contd.) Rushall By the time of the late Silurian, the sea was shallowing across Herefordshire and the marine environment was replaced by coastal, lagoonal and alluvial flood plains. A roadside exposure at Rushall allowed us to see the contact between the Upper Ludlow Siltstones, representing deep water, and the Downton Castle Sandstone, representing shoreline deposits (Fig. 3). The muddier layers of the Upper Ludlow Shale are often very rich in fossils, especially brachiopods and bivalves, as shown in Fig. 4. The Downton Castle Sandstone is a durable building stone with thinly spaced bedding planes, and has been extensively quarried. At its base is the famous Ludlow Bone Bed, which comprises sand layers within mudstones. The sand layers contain black particles that are the bones and scales of early jawless fishes, as well as the carbonised remains of early plants. Fig. 3 Exposure at Rushall showing the contact between the Fig. 4 A rich find of marine fossils inside a lump of Upper Upper Ludlow Siltstones and the Downton Castle Sandstone. Ludlow Siltstones at Rushall (gloved fingers for scale) The thin Ludlow Bone Bed lies at the base of the Downton Castle Sandstone (30 cm hammer handle for scale) Aymestry Limestone From Rushall we then walked uphill to reach the high ridge of Aymestry Limestone forming the outer rim of Woolhope Dome, sandwiched between the softer sediments of the Upper and Lower Ludlow Siltstones. We walked northwards along the limestone ridge (known as Marcle Ridge) on a narrow road, enjoying the view eastwards across the valley underlain by the Raglan Mudstone Formation to the Ledbury Hills about 9km away. These hills also consist of the more resistant layers of Aymestry and Much Wenlock Limestones that were folded during the Variscan Orogeny. A short distance along Marcle Ridge at Hooper’s Oak (SO 631 354) we examined exposures of the Aymestry Limestone, which clearly showed its bedding planes dipping eastwards at about 20° (Fig. 5). Fig. 5 Exposure of dipping Aymestry Limestone at Much Wenlock Limestone the top on Marcle Ridge Turning off the road and heading westwards, we then walked downhill and across a valley underlain by clays derived from the softer Lower Ludlow Siltstones, then climbed uphill towards the ridge of harder Much Wenlock Limestone, which forms the inner ring of the Woolhope Dome (Fig. 6). At the top of the ridge we examined a small quarry (SO 624 355) excavated in the Much Wenlock Limestone (Fig.