Geology and man, Wales

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From: Howells, M F. 2007. British regional geology: Wales. Keyworth, Nottingham: British Geological Survey.

Siâmbr Pentre Ifan, Preseli: Neolithic dolmen constructed with slabs of cleaved tuffaceous rock (Fishguard Volcanic Group) derived locally (MFH P662442.
Glaciofluvial sand and gravel, Pantgwyn Mawr Quarry, near Cardigan. P662440.

This guide has described the geological evolution of Wales over the past 700 million years, drawing particular attention to the major physical events and the initiation of fauna and flora. For many, the most important event would be at the end of this time span in the Quaternary era, when Homo sapiens evolved. The earliest evidence of human occupation is from Pontnewydd Cave in the Elwy valley, where teeth of Lower Palaeolithic (Old Stone Age) hunters and artefacts have been recovered; the association with many bones of bears and wolves suggest that the cave was used as an abattoir during a slightly warmer phase, some 230 000 years ago. In Paviland Cave in Gower, bones, referred to the Red Lady, dated at some 29 000 years, yielded protein that indicates a diet containing fish and vegetables, and suggest a marked development from the carnivorous habits of the Neanderthal community. In nearby Minchin Hole Cave, some researchers have suggested a matching cultural development, with fragments of ivory from mammoth tusks that are claimed to have been shaped, and ochreous bands on the walls that are claimed to be decorative graffiti, but others disagree. It is apparent that, by this time, human evolution was advanced, far closer to now than to its beginnings, and it is reasonable to assume that by Late-Devensian — early Holocene times, about 11 000 calendar years BP, the human population was beginning to be more established, moving inland from the coastal fringes as the influence of the ice waned. Although details of their communal living are restricted, there is a wealth of information through to Neolithic (5000 to 4000 years BP), Bronze and Iron age times, in the mounds, cromlechs and stone and hut circles that enrich the countryside (P662442). Pollen records after 3500 years BP show how cultivation encroached onto higher ground. Some 2000 years ago, the Roman legions entered Wales and their legacy, in the road pavements, for example through Bwlch y Ddeufaen and north from Caersws, and the forts, as at Caerleon and Caernarfon, are still discernible, and are now preserved by CADW (Welsh Historic Monuments Executive Agency).

The aboriginal Welsh population was profoundly influenced by the terrain. For example, deviations along the route of Offa’s Dyke, such as between Kington and the Clwydian Hills, reflects both the topography and, therefore, the geology. However, their main concern would have been for secure places to live. The main barriers around the edges of their forts and the houses within them would have been built of timber from the surrounding forests. However, there is evidence in the higher ground, as in Cwm Dulyn, north-east Snowdonia, of hut circles that were probably constructed entirely from the many glacial erratics in the vicinity. The Romans, being the first serious civil engineers to occupy the country, were the first to quarry and fashion local rocks to build and decorate their larger constructions.

For early man, rock such as flint, which could be fashioned into hand axes and arrow heads, represented wealth to be sought after and traded. In south-east England, the Chalk was a rich repository for such flints and the absence of similar rocks in Wales necessitated alternative solutions. There is evidence that fine-grained siliceous rocks were similarly utilised, for example in the ‘axe factory’ above Penmaenmawr fragments worked from the chilled margin of the intrusion were found, and there are many examples of the Ordovician air fall tuffs and metamorphosed siltstone in implements recovered from the Elwy valley caves. Such tuffs continued to be quarried for honestone across Snowdonia into the 18th century, for example the quarry that now channels the footpath south from Ogwen Cottage. The wealth of weather-resistant rock, in outcrop and glacial erratics, with variable shapes and sizes, facilitated their incorporation into the standing stones, stone circles and cromlechs, which developed out of the social and cultural aspects of the societies. The monuments are particularly prominent in western Pembrokeshire and, as at Pentre Ifan, their remnants are impeccably preserved. The bluestone lintels at Stonehenge were derived from the flanks of the Preseli ridge in Pembrokeshire, and how they were transported, by ice, hard labour or a combination of both, is still a matter of debate.

There can be little doubt that Bronze and Iron age man recognised clays that could be fired into domestic pottery and used to channel water, that they used coal and peat for fuel, and that metalliferous minerals were incorporated into their culture. Consequently, it would be surprising if the largest mining districts, such as those at Cwmystwyth, Minera and Parys Mountain, did not have a history that extended back for some 4000 years. Most of the non­ferrous metalliferous mines were worked mainly along veins for lead and zinc, which, in places, included both silver and gold in association. Elsewhere, as in the mines in the core of the Snowdon massif, and at Parys Mountain and in Coed y Brenin, copper was the main incentive, and the deposits were related to the late stages of Ordovician volcanic activity. In the Dolgellau and Dolaucothi districts, gold is associated with black mudstones, as is uranium at Tremadoc. Manganese, in both mudstone and as a replacement of ooidal ironstone, was worked on Llŷn and at Harlech. It was probably during the Roman occupation that the extent of the mineral wealth began to be appreciated, and there is hardly an account of a metalliferous mine in Wales that does not make reference to it being first developed during Roman times, or earlier, although the evidence, in many instances, is, at best, questionable. Arguments for the great antiquity of copper, lead and zinc mining at Parys Mountain on Anglesey, across the Snowdon massif and in the Central Wales Mining District are based mainly on mining techniques, which are extremely difficult to date, or on implements, such as hammer stones, which are similarly difficult to restrict to a certain period. Even so, on such evidence, several sites across Wales, including those recently excavated on Great Orme, have been assigned to the late Bronze Age or early Iron Age. However at its peak from the 18th through the 19th century, the mining of the veins in these districts, and those in the Lower Palaeozoic rocks in the Llanrwst and Llanfair Talhaiarn districts was intense and labour intensive. The mines hosted in Carboniferous Limestone, in south Wales, in the outcrop close to the north Wales coast, in the Vale of Clwyd, and at Minera and Llandegla, developed in similar fashion.

Even though there is evidence, as in the Blaenavon area, that coal was being extracted and sideritic iron ore was being smelted in the 15th and 16th centuries, the main development of the industries in both south and north Wales, and at the same time the development of the north Wales slate quarries was directly related to the industrialisation and colonial expansion of Britain in the late 18th century and through the 19th century. It is now almost impossible to envisage the speed of change in those sparsely populated valleys – the sinking of the mines for coal, the development of the railway system for transport to the docks and the urban areas of England, the movement of the labour force from England and the rural areas of central Wales, the expansion and development of the metalliferous industries, the opening up of quarries for sandstone to build the houses, ironstone for smelting, silica sand for furnace bricks, and limestone for flux for the furnaces. The key factor was not just that all of these essential geological elements were there in abundance to feed the force of the industrial revolution, but that they were there in close proximity. The line of the current Heads of the Valleys road cuts a swathe through the heartland, and just one sector, between Blaenavon and Merthyr Tydfil, displayed the fortuitous juxtaposition of the Ebbw, Sirhowy, Rhymney and Taff valleys, with pit heads by the score, and a concentration of the smelting and iron foundries at Blaenavon, Ebbw Vale, Tredegar, Merthyr Tydfyl and Dowlais. The broad extensive limestone plateau between Trefil and Mynydd Llangattock lay a stone’s throw to the north, and the surface water supply was sufficient for the steel and coal ‘masters’ industrial processes. Access to international markets from the ports at Cardiff and Barry was just a day trip away to the south. The importance of this industrial landscape in the early 19th century has been recognised internationally by its recently awarded World Heritage status, and consequently the remnants of the successive layers of its relentless development are guaranteed preservation. The site includes the Big Pit, the colliery museum of the National Museum of Wales, still with some ex-miners as guides. This industrial scenario, based entirely on the geology, extends seamlessly westwards through the Neath, Gwendraeth and Amman valleys, and further into Pembrokeshire. The patterns of development of these labour-intensive industries persisted, with very little change until the middle of the 20th century. The south Wales coal mining industry reached its peak in 1913, when 56 million tons of coal were produced and the industry employed 232 800. There is not a village in the coalfield without graphic evidence of the harshness of the mining industry which shaped their communities, none more so than Senghenydd where 81 men were killed in 1901 and 439 men in 1913, in underground gas explosions at the Universal Colliery. Following nationalisation in 1947, the National Coal Board began to develop new collieries, but by 1955 a programme of pit closures began, and after 1985 mass closures had reduced the mining industry in south Wales to a shadow of its former importance. During this last phase the legacy of the industry continued to impact on the communities and sometimes, as in Aberfan, in a most devastating fashion — on 21st October 1966 a spoil tip that had been sited on a spring line was mobilised into a slurry that engulfed the village school killing 116 children and 28 adults. Today, deep mine activity is at Tower Colliery, Hirwaun, following a successful employees take over, and the Nant Hir No. 2 Colliery, Seven Sisters, also under private licence. Work is in progress in three colleries, Aberperwn (Neath), Black Barn (Torfaen) and Blaencwn in consideration of their possible renaissance. Such activity is difficult to reconcile with those assurances, such a short time ago, that their immediate demise was inevitable. Open cast activity is currently restricted to four sites in the Vale of Neath but applications for development continue to be made. In north Wales, the pattern was repeated in the Flint and Denbigh coalfield, and by the 1980s all coal and steel production had ceased. Even the levels of brick and pottery clay extraction, based on the Coal Measures in the vicinity of Buckley, was reduced to the scale of a cottage industry, somewhat similar to the state of the slate industry in Caernarvonshire and Merionethshire.

Wales was the world leader in the production of roofing slates throughout the 19th and early 20th centuries. Small slate quarries had been opened across the whole of the Lower Palaeozoic outcrop, but the main enterprise lay in Snowdonia, and the development of the rail network, including the narrow gauge, and the ports at Penrhyn, Caernarfon and Dinorwic, ensured the growth of villages such as Bethesda, Llanberis, and Blaenau Ffestiniog. At the turn of the 19th century, some 14 000 men were employed in the industry, but a gradual decline into the middle of the 20th century accelerated into almost total demise with the availability of cheap slates from overseas and the development of the clay and composition roofing tile industry; currently only 300 to 400 are employed. Penrhyn Quarry at Bethesda and Dinorwic Quarry at Llanberis are probably the most impressive excavations, but whereas the former currently operates a small-scale production, Dinorwic is the site of the power generators for the pumped storage hydroelectric scheme that was developed in the 1970s. Today, the scale of this great industry is most clearly reflected in the waste tips which, in places, dominate the landscape; 95% of the extracted rock was rejected. However, many of the narrow gauge railways have been preserved to ferry the tourists up and down the valleys.

Apart from this extremely close link between geology, industry and the provision of employment, the most obvious link between geology and population is seen in the construction of the villages, towns and cities. The determining factor in the use of building stones is their availability. This is nowhere more obvious than in the standing stones, castles and the cathedrals, when, during their construction, transport for any significant distance would have been very difficult. Typical of locally sourced building stones are the Carboniferous Limestone of the Din Lligwy circular huts on Anglesey, the Silurian flags in Conwy Castle and Valle Crucis Abbey, the Cambrian grit in Harlech Castle, Devonian sandstone in Tintern Abbey, and Triassic sandstone in Caldicott Castle; the pattern is repeated consistently. Even in comparison with present-day construction the scale of many of these projects is difficult to understand: that it was 1271 when the ‘mass of masonry’ (Pennant Sandstone) used for building Caerphilly Castle was sourced, excavated and transported to the site. The local availability of different rock types is reflected in the buildings, as in the Precambrian and Cambrian rocks in St David’s Cathedral and the adjacent Bishop’s Palace in north Pembrokeshire, and in the Victorian and Edwardian suburbs of Cardiff and the villages in the Vale of Glamorgan where there are many examples of Carboniferous sandstone and limestone, juxtaposed with Triassic sandstone and Jurassic limestone. The availability of Lower Palaeozoic, Carboniferous and Triassic rocks along the north Wales coast is reflected in the towns between Bangor and Flint. However, whole areas may be virtually dominated by a distinct lithology, and the villages throughout the south Wales coalfield, built from the flags of the Pennant Sandstone, are the best example.

Cursory observation of the small towns and villages of central Wales would suggest a similar uniformity, with the dominant lithologies being blue-grey muddy siltstone and sandstone of the Silurian. However, local differences can be distinguished on closer examination, for example between Llandrindod Wells and Rhayader or Aberystwyth and Machynlleth. At Aberystwyth, the dominant flags incorporated in the buildings of the late 19th and early 20th century are from the turbidite sandstones of the Aberystwyth Grits that are exposed in the cliffs and on the foreshore, and were quarried in the vicinity. At Machynlleth, beyond the outcrop of the Aberystwyth Grits, the sandstone flags are less common, and grey muddy siltstone flags are dominant. To the east, to Llanidloes, Newtown and Welshpool, there is a progressive increase in the influence of Carboniferous and Triassic rocks, and of bricks and tiles sourced in Flintshire, Denbighshire and Shropshire. Similarly, eastwards from Llandovery, the point at which the influence of Old Red Sandstone begins in the buildings of Brecon, Hay-on-Wye, Abergavenny and the intervening villages lies almost directly on the geological boundary. In recent years these local differences, which so clearly reflected the local geology, are being encircled and overwhelmed by a tide of breeze blocks, pebble dash and composition tiles or, even worse, cheap imported slates.

The relentless expansion of the construction industry is matched in the cement and aggregate industries. Large cement works are sourced from quarries in Carboniferous and Jurassic limestones in the Vale of Glamorgan and Carboniferous limestones in Flintshire. High purity and dolomitic limestones are also quarried for metallurgical and refractory use. The Dinantian limestone is the major resource of hard rock aggregate, as in the quarries at Llandulas and Trefil, but locally, as at Old Radnor, the Silurian (Wenlock) limestone, unconformably overlying the Precambrian, makes a significant contribution. Even the shortest journey throughout the Principality passes quarries in a wide range of rock types that have been excavated in the past, if only for local use. In central Wales, the coarse grits in the Lower Silurian have been a rich source, but the recent closure of the Cerrig Gwynion Quarry near Rhayader has meant that Hendre Quarry in the Rheidol valley is the only current operation. The largest excavations of igneous rocks for aggregate are in the granophyre near Trefor on Llŷn and the microdiorite at Penmaenmawr, both with the facility to ship out the aggregate directly. Elsewhere, dolerites are worked from the quarries at Llanelwedd, near Builth Wells and diorites (Precambrian) at Johnston in Pembrokeshire.

Deposits of glacial or alluvial sand and gravel are common, and many have been worked from time to time for local use. However, commercially viable deposits, such as the fluvioglacial sediments at Pantgwyn Mawr (P662440) at the northern edge of Cardigan, at Pentir, south of Caernarfon, Bodfari and Wrexham are few. In recent years, dredging in the Bristol Channel has been particularly successful, especially for sand, and it is likely that this will be extended.

The influence that water had in the industrial development of Wales is matched by its current importance as a commercial commodity. The mountainous terrain at the eastern edge of the Atlantic Ocean ensures an average annual rainfall significantly greater than the rest of southern Britain. However, the nature of the geological foundation for most of the country directs most of the rainfall into surface drainage rather than into major underground aquifers, which would lie mainly in post-Carboniferous sequences.

The Triassic sequence in the Vale of Clwyd basin is an aquifer that has been drawn into the public water supply. The sandstones are variously porous and fracture flow is locally important. Superficial deposits conceal most of the sequence and away from the well defined valley margins the supply is mainly artesian. The main extraction is from a series of boreholes in Llannerch Park and some boreholes farther upstream are used seasonally to augment river flow. Similarly, there are many private wells into the Triassic sequence in the Cardiff district. The water-bearing strata of the Carboniferous Coal Measures was a problem in most mines throughout the coalfields and then, ironically, the high concentration of iron and sulphate as a result of the mining has markedly reduced its quality. The aquifers within the thick sequence of Dinantian limestone are important both in north Wales and around the periphery of the coalfield in south Wales. For example, the Schwyll Spring, near Bridgend is sourced from rainfall, stream sinks and sections of the drainage, and supports a significant abstraction licence, whereas wells in the vicinity of Castlemartin are less productive. In north Wales, Ffynnon Asaph (spring) is the key feature of the water supply to Prestatyn.

The Lower Palaeozoic strata through central Wales are weakly permeable and water storage is mainly confined to fractures. Successful wells are as speculative as those drawn from the shallow zone of weathering, particularly at the boundary between the superficial deposits and bedrock, which are such a common feature of the remote hill farms. In some of the larger valleys, sourced by large catchments such as the Rheidol, Teifi, Dyfi and Tywi, sand and gravel within the superficial deposits have provided a reliable source and the Lovesgrove boreholes in the vicinity of Aberystwyth are probably the most significant.

Overall, the vulnerability of groundwater in Wales to pollution from ill-considered farming practice is less than in many other parts of southern Britain. However, in areas of Carboniferous strata, acid and ferruginous discharges are common and the closure of the coal mines did little to control the problem, Similar discharges occur in the vicinity of the metal mines and is nowhere more graphically displayed than in the acrid pool, which reputedly devours abandoned cars rapidly, deep in the hole at Parys Mountain. Of course, it was the salinity and the brackish springs that put the wells into Builth, Llandrindod, Llangamarch and Llanwrtyd, in the early 19th century. The spa waters are derived from slow and deep circulation over very long periods of time — the discharge at Llandrindod Wells is derived from rainwater recharge in Pleistocene times. The most obvious manifestation of the surfeit of water in Wales are the numerous reservoir complexes that litter the valleys. Their construction has ensured the supply of water not only throughout the Principality but also to assuage the thirst of the industrial Midlands and north-west England.

In the 1970s, following the sinking of the Mochras Borehole at Llanbedr, and the discovery of oil and gas in the North Sea, hydrocarbon exploration off the coast of Wales was intense. The surveys yielded a great deal of information about the geology and the structures, but proved no significant concentrations of hydrocarbons. Nevertheless, the presence of the Kinsale Head field, off the south coast of Ireland, to the west, and the Douglas Oilfield and Morecambe Gas field, to the north, is likely to stimulate further exploration.

Bibliography

COULSON, M R (editor). 2005. Stone in Wales, materials heritage and conservation. (Cardiff: Cadw.)

GIBBONS, W, TIETZSCH-TYLER, D, HORAK, J M, and MURPHY, F C. 1994. Chapter 9 Precambrian rocks in Anglesey, south-west Llŷn and south-east Ireland. 75–84 in A revised correlation of Precambrian rocks in the British Isles. GIBBONS, W, and HARRIS, A L (editors). Geological Society of London Special Report, No. 22.

TIETZSCH-TYLER, D. 1996. Precambrian and early Caledonian Orogeny in south-east Ireland. Irish Journal of Earth Sciences, Vol. 15, 19–39