Sabrina Times July 2013 Open University Geological Society Severnside Branch

SABRINA TIMES JULY 2013 OPEN UNIVERSITY GEOLOGICAL SOCIETY SEVERNSIDE BRANCH Branch Organiser’s Report Dear Members In this issue: We are now well into the summer and finally some warmer weather has arrived. Our Ffos-y-fran postscript 2 programme so far this season has been varied, culminating at the end of June with a chilly day on the Black mountain with Tony Ramsay. I drove there in the fog and Coln Gravel Quarry 3 battled my way past hundreds of valiant cyclists who were heading up hill. On my way home, the weatherman referred to ‘the warmest day of the year so far’ - clearly, Llanmadoc, Gower 5 he had not checked out Carmarthenshire!! But the geology was good. Llansteffan and 6 During the rest of the season we have a joint trip with Oxford Branch in September th Amroth to Huntley Quarry in Gloucestershire. Our Introductory Day is scheduled for 17 November to Barry and will be led by Geraint Owen. The day of lectures is also Black Mountain 8 taking place in early December in Cardiff with several speakers already booked. Events & National 10 On the National programme this year’s Symposium is being organised in Dublin. I News have booked my place and I am extending my visit to give me the opportunity both to explore Dublin a bit and also to travel to the West coast. I will be staying in Contacts and 12 Galway so can have a look at the Burren and also Connemara. The Symposium Editorial programme itself is very interesting with lectures on ‘All sorts of everything’. Why don’t you come along and make a long weekend of it? It is always both an Branch Committee 13 informative occasion and good fun. We would like to encourage you to consider taking the Newsletter electronically in future so, if I have your email address, I will send you a copy of this by email. Taking it that way will save the Branch both time and money. You just need to let John de Caux know that you no longer want a paper version. Within the committee, we have been discussing the future and how to encourage more members to assist us with running the Branch. We feel that it is time to get some fresh blood in! If you would like to help, please get in touch. Best wishes Janet Belemnite and large Ammonite fossils from Calcium Carbonate deposits - See report Coln Gravel quarry - See report on Page 3 on Page 8 D13 Ffos-y-fran …….. a postscript I couldn’t go on the Ffos-y-fran field trip, and have never been precisely there, but reading the accounts in the last Sabrina Times reminded me of a highly enjoyable period in my life: the period that brought me to study geology. As some of you know, I came to geology through dry stone walling. I had hoped that most of my work would be rebuilding big old “mountain walls” of Old Red Sandstone in the Brecon Beacons National Park, and had my share of these. But I also found myself building new walls of a sort that I dubbed “instant heritage”, contracted among others by the Merthyr Tydfil Borough Council. Many local authorities in South Wales, faced with the post-industrial wastelands in their areas, decided in the ‘80s that they would try to improve them by making visual links with the pre-industrial rural past. There still are a remarkable number of field walls on farms above the valleys, built of various Coal Measures sandstones. Authorities like Merthyr and Blaenau Gwent embarked on a large programme of building new walls around their bailiwicks as part of large programmes of tidying up derelict or just visually depressing areas. These walls have been built almost exclusively of Pennant gritstone from quarries such as Gelligaer south of Merthyr and Gwrhyd at Rhiwfawr above the Swansea valley. (This was not too appropriate when the Pennant stone was used right out of its area, as on the approach roads to Cardiff airport.) The best of these walls have given good new landscape features to dignify the surrounds of a bleak village (e.g. Fochrhiw, two miles east of Ffos-y-fran, Figure 1) or to mark the entrances to a new park on reclaimed former industrial land (e.g. Dowlais, a mile north-west of Ffos-y-fran, Figure 2). Of the worst of these walls, it’s better not to speak! Philip Clark Fig. 1 Fig. 2 2 Cotswolds - Coln Gravel Quarry near Lechlade - 28th April 2013 Leader Neville Hollingsworth Warmer… and colder… than now! Sixteen branch members and friends met up at the entrance to Coln Gravel on a chilly but dry late April morning. Leader Neville Hollingsworth introduced us to the quarry manager who gave us a health and safety briefing before we walked half a mile or so down into the pit where he pointed out the ‘no go zones’ before Neville told us about the geology. (Figure 1) The solid geology here is Jurassic, the 165 Ma old Oxford Clay (Peterborough Fm). There are no Late Jurassic, nor Cretaceous, rocks; the Quaternary cover is two terraces of river gravels which sit unconformably on the clay. One zone of the Oxford Clay here is richly fossiliferous: perhaps the best in the country: septarian nodules are packed especially with ammonites (Cosmoceras Jason, Reineckia) and nautiloids (e.g. Paracenoceras) and belemnites; there are bivalves (Gryphaea) in a horizon with shelly beds. There were also crocodiles and dinosaurs (Ichthyosaur, plesiosaur) in the Jurassic seas and their bones have been found here. Unconformably above the Jurassic rocks are sands and gravels which Figure 1: “This is your challenge, should you choose to make up the Thames valley river terraces. At this locality there are a accept it” – Neville points out the area we could lower and a higher terrace. The lower dates from a warm stage at 240 explore for fossils ka and hominids such as Neanderthal and Heidelbergensis would have been present. Checking the BGS iGeology map, this is shown as the Northmoor Sand and Gravel Member of the Thames Valley Formation; it is dominated by clasts of Middle Jurassic Limestone plus a small amount of Bunter quartz/ quartzite and some flint. Neville told us that the higher (which isn’t mapped on the iGeology scale) was deposited between 50 – 12 ka BC during a cold stage and there is evidence of a typical cold stage fauna of mammoths and woolly rhinoceros. I suspect that, like other gravels in the area, it could have been sourced from the ice cap close-by to the north. Being a bit challenged in the hip region I had plenty of thinking time while others were searching through the mud and gravel to see what they could find: thinking scientific method: ‘pose a question that can be tested’ – “The river terrace gravels are derived from Late Jurassic, Cretaceous, and later rocks which originally overlaid the area but have been eroded”. “Detrital material from eroded rocks to the north was transported south by glaciers during cold periods and then reshaped into river terraces during warmer interglacials.” I couldn’t test this myself, but certainly the clast types listed by BGS seem to conform. Checking the BGS map, the deposits show arcuate outcrops in places, reminiscent of meanders. After the briefing in the pit itself Neville led the way first to some ‘reject piles’ – material that didn’t contain sufficient gravel for the quarries but plenty of Oxford Clay, with abundant nodules. (Figure 2) I wished I’d brought a sledge hammer! The nodules contained ammonites in Figure 2: The reject pile, with nodules from the Oxford Clay abundance (Figure 3), many still displaying the nacreous lustre of aragonite rather than calcite. However they are extremely friable and therefore hard to extract. Figure 3 a, b and c: Ammonites from the Oxford Clay 3 Cotswolds - Coln Gravel Quarry near Lechlade - 28th April 2013 (contd.) From here the group made its way down a gravel slope into the pit itself (Figure 4) where more ammonites, belemnites, and a mammoth tusk (Figure 5) were amongst the finds. Figure 4: Fossil hunting in the gravel pit Figure 5a: Neville’s mammoth tusk: the ivory was a beautifully pure white but the tusk was very fragile and would need consolidating as Figure 5b: the cross-section of the tusk shows quickly as possible to avoid it crumbling. a concentric growth structure. We had to be out of the pit by 2pm, but enjoyed a fascinating morning and made some excellent finds. Our thanks go to Jan Ashton-Jones for the organisation, to Neville Hollingsworth for arranging the visit, and to Hanson Aggregates for allowing us onto the site. Linda Fowler Panorama of the part of the quarry where we spent the day 4 Llanmadoc, Gower - 11th May 2013 Leader - Steven Howe Our small group gathered in Llanmadoc car park on a cool, cloudy and windy day in hope of some spring sunshine. We were set on a circular tour to explore the North Gower exposures of the Old Red Sandstone (ORS) and Carboniferous Limestone (now known as the Pembrokeshire Limestone Group) that are exposed in an anticline forming Llanmadoc Hill and the northwest Gower coast Tors between Burry Holm and Whiteford Point. As we walked through the small collection of houses around the church Steven pointed out the difference between the source of the stone in the vernacular farmhouse and the mid 19th century restored church, the latter having stone from Bath as shown in Figure 1.

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