 |
|
|||
|
Environmental Change and Mineral Formation in Wales A Trip Through
Time Pre-Cambrinan Era:
Primeval Wales The oldest rocks in Britain occur in NW Scotland and are over 3000 million years old. Here in Wales, the oldest outcropping rocks are therefore geologically young, at only 700 million years old! Precambrian rocks occur on Anglesey and the Llyn peninsula, in SW Pembrokeshire and in places along the Welsh Borderland. Such ancient rocks are known to geologists as the "basement" and underlie most land areas in various forms and at varying depths, depending on the subsequent geological history of each area. They include both sedimentary and volcanic rocks, indicating that late Precambrian Wales was a turbulent place to be! Precambrian rocks in Wales contain mineral veins in places, and on Anglesey there are low-grade chromite deposits of probable Precambrian age. But they are too small to be worth exploiting. What is more interesting is that Precambrian volcanic rocks have been shown by drilling to underlie the area popularly known as the Harlech Dome in north Wales. It has been conjectured that these rocks may be the source-terrain for the gold occurring in the mineral veins of the nearby Dolgellau Gold-belt. Cambrian-Silurian
Periods: The Welsh Basin Throughout this time, known collectively as the Lower Palaeozoic, the building blocks of the landscape of Wales were being laid in place as thousands of metres of sedimentary and volcanic rocks accumulated in a marine basin - the Welsh Basin. This ranged westward across Wales from Shropshire, although the seas sometimes came and went over the low-lying landscape of the Midlands and beyond. To the north of Wales at this time lay the great Ocean of Iapetus, on whose north-western shores, many, many miles away, lay Scotland and Northern Ireland. This was an important time in the formation of Welsh mineral deposits. In shallow Cambrian seas, chemical precipitates periodically accumulated. Sometimes these were mixed with mud and other sediment, but there is one band, traceable around NW Wales, which exceeded 1m in thickness in places and was worked as a source of manganese. The ore, a close-grained, hard, heavy splintery rock, consisted of a mixture of manganese carbonates and silicates with some other minerals. It was mined in many locations around the Harlech Dome, such as at Hafotty, near Barmouth. Later again, in Ordovician times, similar chemical sediments accumulated, but in this case iron was the dominant metal. The ores were oolitic ironstones, a little similar to the well-known Jurassic ironstones of the E Midlands, but metamorphosed and deformed so that their mineralogy changed giving new minerals like magnetite that are economically important. This is beautifully revealed in sectioned samples. Iron-ores were worked in many areas with mines around Cadair Idris providing some interesting examples. Volcanoes in Snowdonia When there is volcanic activity, mineralisation tends to occur. This is mainly because volcanic areas are hot - and so act as powerful convective engines, driving hot groundwater in their circulation through the nearby rocks. When these waters reach surface, they may explode into the air on land as geysers, or form the now well-known "black-smokers" on the seabed. Black-smoker activity in what is now NE Anglesey was particularly intense, resulting in the formation of huge quantities of iron, copper, lead and zinc sulphides. This great mass of sulphides was worked for copper from the Bronze Age onwards at Parys Mountain, which by the late 18th Century was Europe's biggest copper-mine. It still attracts interest, and much work was done in the late 1980s, including the sinking of a major new shaft. Further south, near to Dolgellau, the mountain of Rhobell Fawr marks the site of another old volcano. Here, activity spanned the Cambrian-Ordovician boundary, and resulted in the formation of one of the oldest porphyry-copper deposits in the world - a huge (200 million tonne) mass of intrusive diorite impregnated throughout with copper and iron sulphides. Coed Y Brenin was discovered in the early 1970s by Riofinex Ltd, and is only exposed in a few places being largely buried under glacial deposits. The new Forest Enterprise geology trail visits the best outcrops. Copper had been worked in the Coed Y Brenin area long prior to the Riofinex discovery. Nearby is the remains of the Turf Copper mine, where a peat-bed impregnated with secondary copper minerals was stripped, burnt in a kiln and the ashes smelted to great profit. And not far away is the famous Glasdir mine, where a great pipe-like mass of shattered rock shot through with copper-ore veinlets was worked until 1914. Here, the now universal method of sulphide ore separation, flotation, was discovered and perfected in the early 1900s. In the same area, there is the Dolgellau Gold-belt. This tract of ground, curving in a south-westerly direction from Trawsfynydd around to Barmouth, contains perhaps the best-known metalliferous deposits in Wales - being the source of the Welsh gold used for Royal wedding-rings over the years. The gold occurs in very localised - but extremely rich - pockets in quartz veins also containing sulphides of iron, copper, lead, zinc, arsenic and cobalt plus tellurides of bismuth, lead and silver. Mineral exploration in the area still continues and the small amounts of gold found are used in jewellery manufacture. Further north, the Snowdon area, a major Ordovician volcanic centre, hosted an important 18th and 19th century copper mining industry. Arsenic was also produced in small amounts and, near Deganwy, antimony ores were worked in a related, but unusual deposit. The copper-mines may still be seen around Snowdon, and one, Sygun Mine, is open to the public. The copper-lodes were formed when seawater circulated through the rocks around the Snowdon volcanic caldera, was heated. The water leached metals from the rocks it passed through and redeposited the metals as sulphide ores in fractures closer to the surface. Turbidites - the
rocks that make up Central Wales At its peak, the submarine debris-flows, or turbidity currents, may have been travelling as fast as 50mph. As they slowed down, the particles of sediment began to settle out, coarsest first, to form the layered strata called turbidites, so common in Mid-Wales and seen particularly well in the sea-cliffs at Aberystwyth, every sand-silt-mud sequence representing a single submarine avalanche. In southern Central Wales, there is an unusual, if not unique, turbidite-associated gold-arsenic deposit. This was mined at Ogofau near Pumpsaint, both by the Romans, who worked on a massive scale, and by more modern mining companies. The ore consists of pyrite and arsenopyrite in which extremely fine-grained god is disseminated. The Romans worked the near-surface part of the deposit, where the sulphides had been oxidised, making the gold easier to recover. The site, also known as Dolaucothi, is now owned by the National Trust, and is open to the public. LOWER PALAEOZOIC LIFE The deeper seas of the Welsh Basin were populated in their upper layers with primitive planktonic organisms, such as the now extinct graptolites, which commonly occur as fossils in the shales of Mid and North Wales. Towards the Welsh Borders, the sea was shallower, allowing more diverse life to thrive. In mid-Silurian times, shallow, warm seas allowed coral reefs to flourish. Around the reefs, the shallow sea-beds teemed with life. Fossil remains of this bygone sea-floor community - shells of various types and the curious trilobites, crustaceans superficially resembling large woodlice - occur commonly in the Borderlands area. By late Silurian times the seas were retreating and estuarine and coastal conditions were established along the Welsh Borders. Newcomers, including the first primitive fish and land plants, occupied this environment. In the tidal lagoons in this area lived the eurypterids or sea-scorpions, primitive ancestors of the land scorpions of today - but reaching a length of three metres on occasion! LATE SILURIAN-EARLY
DEVONIAN PERIODS: THE ACADIAN OROGENY By the start of the Devonian period, the Welsh Basin was at an end. The landmasses formerly separated by the Iapetus Ocean had collided, and the earth movements this collision caused forced the remains of the Welsh Basin into a new range of mountains - the Welsh Caledonides. This was a slow process - remember that the Himalayas are still rising by 1-2 cm per year - but involving immense tectonic forces. As the marine sediments of the Welsh Basin were being deformed, every grain of sediment was compressed under phenomenal pressure. Under these conditions, the clay minerals making up the muddier sediments became soluble and readily recrystallised, becoming aligned at right angles to the direction of maximum pressure. Thus the rocks developed a grain - a cleavage. Due to the flaky nature of clay minerals, like mica, the rocks largely made up of them developed a particularly strong, pervasive, parallel cleavage, making it easy to split them into thin sheets. Thus were the foundations of the Welsh slate industry laid. Wales is renowned for its slate, one mineral product that is still actively mined today. Slate was the economic foundation of many of North Wales' larger villages and towns: places like Corris, Blaenau Ffestiniog, Llanberis and Bethesda owe their existence to a great extent to this product of tectonics. Welsh slates vary from place to place, both in age and in appearance. The highest-grade roofing slates - with their distinctive purple and green colouration - come from Cambrian rocks between Llanberis (home of the Welsh Slate Museum) and Bethesda and represent the work of greater tectonic pressures and metamorphic conditions than their Silurian and Upper Ordovician counterparts in the Corris area (home of King Arthur's labyrinth). Some of the largest workings were in Ordovician slates in the Ffestiniog area, where the Llechwedd Slate Caverns are open to the public and you can start to appreciate the vast amount of work undertaken by the men and women of yesteryear. The Central Wales Orefield consists of an extensive area of mineral-lodes hosted by Ordovician and Silurian turbidites. The mineralisation has been shown to have formed in several phases of activity, starting in Devonian times and continuing through the Carboniferous into the Permo-Triassic. The early veins were of particular importance as a source of silver in Civil War times, when mines such as Darren, Cwmsymlog and Goginan were in their heyday. Later veins carried less silver but much lead and zinc, and an excellent example is exposed underground at the Llywernog mining Museum. Other minerals found in Central Wales include suphides of cobalt and nickel and rare gold. Mining in Central Wales reached its peak in the 1850s, but had ceased by the Second World War. DEVONIAN RIVERS
AND CARBONIFEROUS SWAMPS By the middle of the Devonian period, the former Welsh Basin had therefore become part of a range of mountains. The climate was arid, with occasional rainfall eroding the mountains away and generating new sediments in the process. Rivers carried debris away towards the Midlands and South Wales, depositing sands over the plains. The sands eventually consolidated, to form what is today called the Old Red Sandstone. These strata give the characteristic red colour to the soils of parts of South and East Wales. The Devonian period has been called the "age of fishes". It marked the major evolutionary burst that established these creatures in their full diversity - bony fish, sharks, lungfish - in seas, lakes and rivers worldwide. And from the lungfish there evolved the first amphibians to crawl onto the land, where the colonisation by plants was already well underway. With the onset of the Carboniferous period, shallow warm seas invaded areas of NE and South Wales, once again allowing coral reefs to flourish and lay down the limestones in which Wale's caves were later formed. But by the Upper Carboniferous, and particularly in South Wales, extensive coastal deltas and swamps dominated the landscape. These were dense with tree-sized club mosses while ferns, seed-ferns, and other plant life lived on the raised river levees. Great thicknesses of peaty matter accumulated in these swamps, which, under the higher pressures and temperatures of deeper and deeper burial, were slowly converted into coal seams. In South Wales, Rhondda Heritage Centre and Big Pit are places to find out everything you need to know about coal and coal mining. Some impressive mineral specimens are also known from cavities in coalfield ironstones. Later Carboniferous rocks in South Wales include river-deposited sandstones containing pellets of coal. These show that, by that time, the strata deposited in the coal-swamps had been uplifted and was already being eroded. This is evidence for the early phases of the next mountain-building episode - the Variscan orogeny. At this time, the landmasses which are now Africa, South America, India and Australia were joined together as a supercontinent, By about 290 million years ago, this huge land-mass, which is known posthumously as Gondwanaland, had collided with North America, Europe and Asia to form a single supercontinent - Pangaea. PERMIAN EXTINCTIONS,
TRIASSIC DESERTS AND JURASSIC SEAS Pangaea was a short-lived supercontinent. It lasted for only 50 million years or so before fragmenting again. When it did break up, the fragments drifted gradually (via a few more collisions) into the familiar global map we see today. The formation of Pangaea had a major effect on life globally, Wales included. Back then, Britain lay close to the Equator, while Africa and South America were much nearer to the South Pole. Extensive ice-caps covered these southern lands, and with so much water trapped as ice, sea-levels dropped, bringing changes in marine salinity. These major environmental changes now left much of the area that is now the UK, including Wales, as a desert with expanses of sand dunes dotted with salt lagoons where areas of former sea had been cut off as global sea levels fell. Theories change from year to year for the end-Permian mass extinctions: both an asteroid impact and massive volcanic activity have been blamed for tipping this already precarious situation into a crisis where life itself came precariously close to disappearing. For, so far as we know, this was life on Earth's nearest miss. Just outside modern-day Wales, great sedimentary basins developed - areas where subsidence and sedimentation occurred. One was in the Irish Sea, and another in the Cheshire area. By the start of the Jurassic period, the land was widely subsiding and in, once again, came the sea, submerging all but the mountain areas. The transition from Triassic desert conditions to the Lower Jurassic seas is well displayed in rocks exposed along the South Wales coast between Penarth and Lavernock, near Cardiff. And, as in the old Welsh Basin of Ordovician and Silurian times, great thicknesses of marine sediments accumulated over the next few million years. Like history, geological time has a habit of repeating itself. Some very important mineral deposits were formed during this time, as high heat-gradients generated during basin subsidence generated metalliferous brines. These then migrated, up and along into the areas of older rocks. The limestone sequences of the Lower Carboniferous were a particularly favourable host for mineralisation, and so the "Pennine-type" lead-zinc-fluorite-baryte orefields of Wales were formed, namely the Halkyn-Minera district of NE Wales and the Welsh sector of the South Wales-Mendip Orefield. The unique copper deposits of the Great Orme, near Llandudno, are thought to have been formed at this time too, and have been mined since the Bronze-age. Iron-ores, such as those mined until quite recently at Llanharry, South Wales, are also a part of this major mineralising event. JURASSIC-LATE CENOZOIC
EVOLUTION OF WALES It is supposed that the seas at times completely covered the low-lying landscape of Jurassic-Cretaceous Wales. But by the onset of the Cenozoic era, 65 million years ago, the sea was definitely on the retreat. But what do we know about the dinosaurs? Did they ever live here? The answer is a definite yes, because fossilised footprints are known from Mesozoic rocks in the Vale of Glamorgan. But, by the beginning of the Cenozoic era, dinosaurs, along with many other life forms, had been wiped out in a second mass-extinction, which is currently blamed on climatic changes initiated by an asteroid impact in the area that today forms the Gulf of Mexico. Following this crisis - whatever caused it - a small, previously subordinate group of animals that had been hiding in the background for 140 million years literally inherited the Earth. They were the mammals. Throughout Cenozoic times, the climate of Wales was warm in comparison with today. For considerable periods it was sub-tropical. During this time, the sulphide-bearing ore-deposits of Wales were deeply weathered, with new, often colourful minerals forming, such as malachite and pyromorphite. Manganese oxide ores, worked in the Arenigs and locally in Central Wales, were probably formed at this time. The landscape was populated with the often vaguely recognisable ancestors of elephants, hippos, deer, cattle and many other familiar forms. But, only 1.6 million years ago, the climate began to change, heralding the onset of the ice age. PLEISTOCENE GLACIATION During the Pleistocene period, several cold spells, each lasting thousands of years, resulted in ice-caps over the Welsh mountains and were interspersed with periods in which the climate was about the same as, or even warmer than, the present day. During these warmer periods, lions, hippos, bears and other exotic animals inhabited Southern Britain. During the colder spells it was reduced to tundra. Glacial ice is an extremely powerful agent of erosion and it was the Pleistocene glaciations that were responsible for the final shaping of the Welsh landscape we see today. The Ice stripped off friable, weathered rocks, and removed the upper oxidised parts of many ore-deposits. Melt-water rivers, washing away the lighter sands and leaving behind the heavier minerals, including, in parts of North Wales, grains of gold, then reworked the eroded debris. It was this process that created the gold-placers of the Mawddach and other North Wales rivers; where in the past some sizeable gold nuggets have been found. Today they are very rare because of so much earlier prospecting! Before, during and after the Ice Age, Wales' limestone areas - chiefly in NE and S Wales - underwent a particular type of geological process peculiar to limestone. Rainwater is capable of dissolving limestone, made up as it is largely of calcium carbonate. The process is extremely slow in human terms but over relatively short spans of geological time - tens of thousands of years - it is remarkable how much solution can take place. The rainwater makes its way underground, down fault planes and joint-fractures; it may simply work downwards, or, finding a particularly marked geological contrast, for example a massive limestone band underlain by an impermeable shale-bed, it may work along. Either way, the product is cave systems. Wales has several important cave systems, for example in the Halkyn and Minera districts of NE Wales and, in S Wales, at Dan Yr Ogof, where caving is particularly popular and there are show-caves open to the public. Here, you can see the beautiful flowstone formations - wavy curtains of stalactites and stalagmites, formed where the same waters that dissolved the limestone in the first place have now redeposited the calcium carbonate in the form of aragonite. These are the products of thousands of years of slow, gentle chemical activity - something to contemplate when you stand before them. HOLOCENE PERIOD:
10000 years ago to the present Knowing when the first miners came to Wales is a difficult question to answer. We know that the ice had largely gone by 10,000 years ago and colonisation was already well underway. People, it seems, have always had a fascination with rocks and minerals and it seems scarcely possible that these early Welsh inhabitants would not have noticed the occasional unusual or colourful stone in their travels. But our modern scientific methods have demonstrated that, by just under 4000 years ago, there was quite an active mining industry in parts of Wales. These early miners were Bronze-Age people, who had migrated slowly N and W up through Europe, exploring for minerals as they went - and in particular for two metals - tin and copper. Tin only occurs in "academic" amounts in Wales, but with copper it is a different story. These people dug for copper at numerous localities from Great Orme in N Wales to Cwmystwyth in Mid-Wales: it seems that wherever there was significant copper mineralisation they had a go. Mining was done by shattering the rock with fire, then breaking and prising it out with heavy stone hammers and picks made from pieces of antler. The remains of such tools and the charcoal from the fires has provided our historians with a way of finding out just how old these early mines are - by Carbon 14 dating. Results are extremely consistent across Wales, implying an incoming wave of people who understood, to a sufficient degree, geology, prospecting, mining and metallurgy, nearly 2000 years before the coming of the Romans. The mining history of Wales and the quest for its various mineral products is the key to Underground Wales, as you will see as you browse through the pages of this site. Enjoy your visit! top |
||||
