Severnside FIELD TRIPS 2013-2014

Severnside FIELD TRIPS 2013-2014 Page 3 Dolyhir Quarry 5 Ffos-y-fran and Blaen Morlais 8 Cotswolds - Coln Gravel Quarry near Lechlade 10 Llanmadoc, Gower 11 Llansteffan and Amroth, Carmarthenshire 13 Black Mountain 15 Four Quarries in a Day 17 Introductory Day - Barry island 19 Introductory Day - Clevedon 22 Nash Point 25 Builth Wells Anti-clinal 27 Severn Crossings This little book contains extracts from “Sabrina Times”, the newsletter of the Severnside Branch of the Open University Geological Society. The field trips included were all undertaken during 2013 and 2014 and the write-ups have been provided by members of Severnside Branch. Thanks are due to those members, to the Branch and Events Organisers and especially to the various leaders on the trips. 2 Dolyhir Quarry 3rd March 2013 Ice-cold in Dolyhir 10:30 am on Sunday 3 March and 14 of us met up with Dr Tom Cotterell of the National Museum of Wales by the entrance to Dolyhir Quarry near Kington in Radnorshire. We had a bit of a whip round to find a couple of spare hard hats – and even a bit of spare clothing on this raw, overcast morning and then set off up the ramp and into the quarry (Figure 1). This area was originally quarried for limestone in the late 1800s: the Silurian (Wenlock) Dolyhir Limestone and its underlying basal rudite originally extended right across the present quarry site. We were introduced to the Dolyhir Limestone shortly after entering the quarry: at the top of the face on our right (south side of the quarry) it is sitting unconformably on the underlying Neoproterozoic Strinds Formation (Figure 2). The Neoproterozoic rocks are the quarry’s present reason for existence as they have suitable qualities for roadstone: for example they are impact resistant and don’t easily Figure 1 Severnsiders making their way up the become smooth and polished. entrance ramp to Dolyhir Quarry There are two formations: Strinds and Yat Wood: they have been studied since the 1830s when Roderick Murchison worked in the area and (despite confusing faulting) this has shown that the Yat Wood Fm is the older (Figure 3). These are both volcaniclastic: they are interpreted as alluvial plain deposits derived from Avalonian island arc material. Both Figure 2 Exposure of Neoproterozoic Strinds Figure 3 A view of the NE face of Dolyhir Quarry formations are cut by east-west Formation overlain unconformably by Silurian shows the Strinds Fm (dark grey with clearly Dolyhir Limestone with rudite at its base. striking Ordovician dolerite dykes but these are truncated visible bedding at a steep angle) and the Yat at the unconformity below the Wood Fm (buff colour). Silurian limestone. We could see a particularly wide (c 20 m) dyke on the far (eastern) side of the quarry (Figure 4) and had a chance later in the day to see this close up. Our route took us next to the western side of the quarry to look at the Dolyhir Limestone. This is a well bedded reef limestone with some shaly horizons (Figure 5). The quarry manager had placed some particularly fossiliferous blocks to one side and we were able to check these over for coral fossils. (Figure 6 a, b, c and d). Figure 4 Strinds Fm and Yat Wood Fm in the The shale beds have trilobites east face of the quarry are cut by a 20 m wide, and brachiopods but we didn’t Figure 5 Well bedded exposure of Dolyhir vertical, dolerite dyke which strikes E-W. see those. Limestone Figure 6 (a), (b), (c), (d) Quarried blocks of Dolyhir Limestone contained numerous fossil corals. 3 Dolyhir Quarry 3rd March 2013 (contd.) The basal rudite (Figure 7) is, surprisingly in view of its name, quite clay-rich in places although in others there were numbers of rounded pebbles. The limestone appeared in places to have sunk into the top of the clay rich areas. We theorised about a possible environmental setting where the Neoprotero- zoic rocks were uplifted and eroded, some pockets of pebbles forming on beaches which were then submerged allowing finer material to be deposited on top. The silicic input must have stopped to allow carbon- ate deposition producing the limestone. After lunch in the (marginally) more sheltered lee of these Figure 7 Clast-rich example of the Dolyhir blocks we moved on around Limestone basal rudite. the north side of the quarry, getting good views of the sur- rounding countryside on the way (Figure 8) to see the Strinds and Yat Wood Fms and their relationships with each other, with intruding dykes, and with the limestone (Figure 9). Figure 8 View east across Dolyhir Quarry towards Hanter Hill and Hergest Ridge. Dolyhir is rightly famous, and has been given SSSI status, on the basis of more than 85 different rare minerals which have been found here but finding these is a real skill: they are tiny – microscopic! – and tend to be found on a particular style of joint surface which Tom showed to us. Despite being coached by Tom we didn’t have any success in finding such things as Harmotone (a barium zeolite) or Ewaldite (a barium/strontium car- bonate) and were generally pretty happy to find calcite in big enough sizes to demonstrate its 3 cleavages (Figure 10), barite, malachite (Figure 11) and azurite (Figure 12) of more easily seen dimensions. Many of the rare minerals are found in small scale “alpine style fissures” where fluid has circulated through fractures, redepositing elements sourced from nearby host rock. Figure 9 Bedded shale and silt of the Strinds Fm overlain by Dolyhir Limestone. As well as being an angular unconformity it looked as though the limestone had been thrust across the shale beds from left to right, dragging the tops as it went. Figure 10 Massive calcite vein material Figure 11 Hand specimen of malachite Figure 12 Hand specimen of crystalline showing typical rhombohedral cleavage. (image 1.7 cm wide). azurite (image 1.7 cm wide) Many thanks to Tom for a fascinating and expert tour of the quarry. Linda Fowler 4 Ffos-y-fran and Blaen Morlais 18th March 2013 We are able to present two write-ups for the day......there must have been a lot of “persuading” that morning ! 1 This was a half-day visit to what is, in spite of its name, an open cast coal mine. My first thought on reading that a mine was called a 'Land Reclamation Scheme' was 'spin', but in fact the name is not really out of place. Merthyr Tydfil is just up the valley from Aberfan, and the location for both ironstone and coal mining activities over several centuries. The result of all this activity was a large area of waste land to the south east of the town, including illegal landfill sites, polluted watercourses, bare spoil heaps, and numerous old mine shafts and adits, mostly unmapped. Following the Aberfan disaster in 1966, local authorities surveyed their local areas for safety and land reclamation purposes. In the 1980s the East Merthyr Reclamation Scheme was launched, of which the current site is phase 3. We were first driven to a viewpoint above the active workings, with Merthyr in the background. To start the workings a large hole was dug (called a 'Box Cut') and the material placed into three 'overburden' heaps. These are 'hydroseeded' (sprayed with a mix of water, grass seed and fertiliser) to green them up. Then one side of the working void is dug out (the right hand side, in this picture), the coal extracted and the remaining rock put on the other side of the void (the left here). Thus as extraction proceeds the hole itself moves to the right. They are also working down the dip, so the last bit of extraction will be from a deeper pit than shown here. When mining finishes the overburden will be put back into the hole, and topsoil and vegetation will be restored. Managing the site includes such things as installing bird scarers, to ensure no birds nest on an area about to be dug out! Fig. 1 A composite image of the workings at Ffos-y fran. View includes Merthyr Tydfil and the snow covered Brecon Beacons in the distance. The seams being worked are mostly the Middle and Lower Coal Measures. There is an approximate ratio of rock to coal of 11:1. We were told that the yellow excavators and trucks removed only rock. The red trucks removed the coal. At any time there were far more yellow vehicles visible than red ones! Although not obvious from these photos, the yellow trucks are the size of a house, and the excavators would tower above the average family home. Our second stop was up a very steep, rather loose track to a bit of coal seam. Marc, our guide, demonstrated his ability to extract the Landrover from soft ground (Fig. 2) before showing us the coal seam (Fig. 3) Fig. 2 Fig. 3 5 Ffos-y-fran and Blaen Morlais 18th March 2013 (contd.) And one of the party decided to be Cinderella................. 95% of the coal from Ffos-y-fran is taken to the power station at Aberthaw, on a dedicated railway line. The properties of the coal vary both along a seam and from seam to seam, so the coal extracted is piled up according to properties near the rail loading area, and then the correct amounts are taken from different piles and 'blended' to produce a mix with the properties that Aberthaw was designed to operate with.

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