Fforest Fawr Geopark James Cresswell (UK)
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Fforest Fawr Geopark James Cresswell (UK) Fig. 1. A geological map of the Fforest Fawr Geopark. Copyright the British Geological Society. n 2005, Fforest Fawr Geopark and outcrop in the extreme western resolved the dispute by defining the became the first geopark to be portion of the geopark, between the Ordovician. He recognised that the Iestablished in Wales. As of October towns of Llandeilo and Llandovery fossil fauna in the disputed layers 2013, it was one of 58 geoparks in (Fig. 1). The names of these towns was different from both the Cambrian Europe. The park has been sculpted by may sound familiar to those who and the Silurian. Lapworth divided nearly 500myrs of geological history know something about stratigraphy, up his Ordovician, which is named and contains the highest mountains because they have been used to name after the Ordovices tribe who lived in in southern Britain, the deepest cave stages within the Ordovician and North Wales and mid Wales during in Britain, the finest collection of Silurian periods. In fact, Wales as a Roman times, into the following waterfalls in Britain and 7,000 years whole has played a very major role series: Tremadog, Arenig, Llanvirn, of recorded human occupation. in the naming of geological periods, Caradog and Ashgill, all named after The geopark lies wholly within the with the earliest three periods places in Wales or near the border, Brecon Beacons National Park, since multicellular life began - the where rocks of these ages occur. occupying the central and western Cambrian, the Ordovician and the The Llanvirn is subdivided into the part of the park. Silurian - all having Welsh origins. Llandeilian and Abereiddian, with The Cambrian Period was named the Llandeilian named after the Ordovician after the Latin name for Wales, Fforest Fawr Geopark town, Llandeilo The oldest rocks in the geopark are Cambria, by Adam Sedgwick; while and the Abereiddian named after the of Ordovician age (485 to 443mya). the Silurian was named by Roderick Pembrokeshire village of Abereiddi. During this time, the geopark was Murchison in the 1830s after the In the UK, these divisions continue to very different from today. There was Silures tribe who inhabited southern be popularly used, but in some parts a great ocean known as the Iapetus Wales in Roman times. Originally, of the world, other stages have been separating England and Wales from there was no Ordovician Period. used. This led to the standardisation Scotland, in which lived many marine However, the Cambrian and Silurian of the Ordovician in 2008 by creatures, such as graptolites and of Sedgwick and Murchison were the International Stratigraphic trilobites. found to overlap in places, causing Committee. The Tremadoc, named Ordovician rocks were deposited at disagreement between the two after Tremadoc in North Wales, the margins of the Iapetus Ocean men. In 1879, Charles Lapworth was internationally accepted as the 32. earliest stage of the Ordovician, but In Llandovery, Wenlock and earliest known fossils of a primitive the Llandeilian is not now accepted Ludlow times, the geopark was still fish. as an international time period. under water, with sediments being However, Llandeilo does still possess deposited in the Iapetus Ocean. In Devonian another claim to geological fame. this ocean lived marine creatures, The Caledonian Orogeny, the name The very first trilobite described by such as brachiopods, which allowed given to the mountain building science was discovered just outside Murchison to define the period and its event that produced the Caledonian the town; it was collected in 1698 by stages. However, from the mid Silurian Mountains, lasted into the Middle Rev Edward Lhwyd and he described onwards, the Iapetus was closing. Devonian. The Caledonian Mountains it as “some kind of flat fish”. England and Wales, both part of the were a huge chain of mountains A good place to see Ordovician rocks in ancient micro-continent of Avalonia, that were probably Himalayan in the geopark is the largest prehistoric were colliding with Scotland, which height and stretched from what is hill fort in South Wales, Garn Goch, was part of the ancient continent of now Svalbard down the coasts of near the village of Bethlehem. Here, Laurentia. This started to form a huge East Greenland and western Norway, the rocks are coarse sandstones laid chain of mountains - the Caledonian through the British Isles, on to the down in a shallow sea 405myago, Mountains - and uplifted the rocks Appalachians in North America. which have since been tightly folded. of the geopark area out of the sea. Whenever a great chain of mountains The geopark has published a geotrail By the latest Silurian (the Pridoli), is thrust up, it will be eroded down that can give you a guided walk all the sedimentation taking place in and huge thicknesses of sediment around the geology and archaeology England and Wales was terrestrial; can be deposited in sedimentary (see box, Published geotrails by the this is why a site in the Czech Republic basins adjacent to them. The Fforest Fawr Geopark). defines this age, because it was still Devonian scene in the Fforest Fawr marine and consequently preserves Geopark would have been something Silurian marine fossils. The rocks deposited like modern day Pakistan, with a The rocks of Fforest Fawr Geopark in the geopark of Pridoli age are the huge mountain chain to the north are all sedimentary and pile on top of Raglan Mudstones. They are red and an arid plain to the south, over each other like a layer cake. Overlying in colour and are part of the Old which rivers periodically flooded the Ordovician rocks are rocks of Red Sandstone that continued to be depositing sediment. the Silurian Period. These rocks also deposited throughout the Devonian, Devonian rocks cover more of the occur in the northwest portion of the the period that succeeds the Silurian. geopark than rocks of any other age geopark (see Fig. 1), stretching in a The global stratotypes of the and all the high peaks in the geopark band to the east of the Ordovician Telychian and Aeronian are - the Black Mountain, Fforest Fawr rocks near the town of Llandovery to on private land, but a great and and the Brecon Beacons (Fig. 2) - near Llandeilo, with a smaller area accessible place within the geopark are made from Devonian Old Red occurring just to the north of Brecon. to look at Silurian rocks is Sawdde Sandstone. Many of the rocks are red As previously discussed, the Silurian Gorge. This site is an SSSI due to the in colour, as the name would suggest. Period was named by Murchison geology, so no hammering is allowed This is due to haematite that formed after the Silures tribe, who would on the bedrock. Here, the transition in these arid conditions. In other have inhabited the Fforest Fawr from the marine conditions in the places, the rocks are green and this Geopark during Roman times. Today, Ludlow to the terrestrial conditions is the result of a different oxidation the Silurian is divided up into four in the Pridoli can be seen. The state of the iron that may be due epochs: the Llandovery, the Wenlock, Ludlow sediments from the quarry to water percolating through the the Ludlow and the Pridoli. These here have also yielded some of the sandstone at a later date. In many names are internationally recognised as the official names for rocks of these time periods all around the world. The Llandovery is named after the Fforest Fawr Geopark town, the Ludlow and Wenlock are named after places in the nearby English county of Shropshire; and the Pridoli after a site in the Czech Republic. Each epoch is further subdivided into stages and two of the three Llandovery stages - the Aeronian and Telychian - have their global stratotypes within a few kilometres of the geopark boundary. The Aeronian Age lasted from 440.8 to 438.5mya and is named after Cemcoed-Aeron Farm. The Telychian lasted from 438.5 to 433.4mya and is named after the Pen-lan-Telych Farm. Both farms are in the area Fig. 2. Pen y Fan, at 886m, is the highest mountain in southern Britain and the Fforest surrounding Llandovery. Fawr Geopark. 33. places in the geopark, such as on the slopes of Bannau Sir Gaer on the Black Mountain, sedimentary structures (for example, cross bedding) are clearly visible. At a few rare sites, such as Heol Senni Quarry, fossil fish have also been found. The Devonian is also known for the remains of the planet’s first forests and, in certain places, such as near the summit of the Black Mountain, black organic material can be found within the sandstone (Fig. 3). Rocks of the lower Devonian are preserved as the St Maughans, Brownstones and Senni Formations. These are all sandstones and mudstones. However, the middle Devonian is missing so, presumably, Fig. 3. Organic remains of early forests in the upper Devonian on the summit of the Black there was a period of erosion after Mountain. they were deposited, removing them before the Upper Devonian Plateau Beds and Grey Grits were laid down. The Plateau Beds are so-called because they cap many of the highest peaks in the geopark. Fig. 4 shows the Plateau Beds lying unconformably on the Brownstones. The best places to see Devonian rocks in the geopark are found in the walks that climb the high mountain peaks. Pen-y-Fan, at 886m, is the highest mountain in southern Britain and the views from the top are magnificent. However, it is also a very popular mountain and, at certain times of the year, can be quiet busy.