Annual Summer Weekend 2014
Minehead, Somerset, 30 May – 2 June
North Devon-Somerset Coast
The 2014 Summer Weekend was spent near Minehead, exploring coastal sections from Kilve in west Somerset to Lynton, just over the border in Devon. Seventeen people enjoyed a long weekend brilliantly led by Chris Darmon, of Down to Earth magazine, and Geosupplies, well known as a field trip leader, and as chairman of the Youth Hostel Association, and president of Herefordshire and Worcestershire Earth Heritage Trust. We began on Friday evening with a geological briefing.
The district lies at the juncture of two different geological regions. To the west, in Devon, is rock formed from thick marine sediments deposited during the Devonian period, uplifted and folded on an east-west alignment by the late Carboniferous Variscan Orogeny. By the end of the Carboniferous period the region was mountainous and eroding. To the east, in Somerset, is younger rock, formed from Triassic and Jurassic sediments deposited by a rising sea encroaching westwards onto the eroding Devonian landscape (Carboniferous and Permian are missing in an unconformity). The lacustrine environment of the Triassic was followed by shallow marine limestones of the Jurassic. These younger rocks display a north-south alignment, overlying the east-west alignment of the older rocks.
Saturday was spent on the younger rocks to the east, around Kilve and Watchet, beginning with a single 1916 oil retort, which only briefly distilled oil from early Jurassic shales (ST 1451 4464) of the Blue Anchor Fm. Volume commercial oil extraction requires the organic content to have been matured (cooked) in situ at exactly the correct temperature: too high and it volatilises and escapes; too low and it fails to migrate to reservoirs. Bedded black shales were beautifully exposed on the beach. They smell oily when struck, and fizz furiously with acid. Sharp contacts with interbedded limestones testify to ancient sudden environmental change. Pronounced jointing shows in limestone beds but not in the incompetent shales. Loose blocks of limestone on the beach are imprinted with Psiloceras ammonites to 25 cm diameter.
Altrnating limestone and shales jointing present in the lomestones
Burrows on the base of limestone beds suggest oxic bottom water. Were the shale beds deposited in deeper anoxic waters? There looked like burrows on the shale beds; if so, then sea level change did not account for the different depositional environments. Walking west (1411 4430) we speculated about reasons for these sharp alternations in lithology. One possibility is pulses of stratification anoxia during the shale intervals, caused by sudden global warming due to release of methane sequestered on the sea floor as methane hydrate. Frequent clasts of petrified wood in the limestone confirmed shallow marine conditions locally during deposition. Bedding lines on the foreshore provided geometric evidence of faulting in the limestones, and fault drag could be discerned in some cases.
Red and tea-green marls
After lunch in Watchet we drove to Blue Anchor Bay (010 450). Triassic limey muds on the foreshore – the red & tea green marls - result from oxidising and reducing environments. Formed in salty playa lakes, these marls contain evaporites, here with extensive and spectacular sheets of pink alabaster (fine-grained gypsum, hydrous calcium sulphate), a secondary product of the halite crystals which would have precipitated first. The low cliffs of red mudstone, are part of the Mercia Mudstone Grp.
Fault placing pale grey limestone beds next to red Mercia Mudstone
At the east end of the bay a fault places pale grey limestone beds beside this red Mercia Mudstone, with fault drag showing. Is this the Triassic/Jurassic boundary? Perhaps not, because a little further east round the corner is more gypsum, suggesting that we are not quite finished with the Triassic. The day’s geology finished in the café at the west end of the promenade.
Sheets of pink alabaster
Sunday was spent further west on the Devonian rocks at Lynton, close to the fatal 1953 Lynemouth floods (car park 7108 4974). The ‘Valley of Rocks’ is a hanging valley, now mostly dry, curiously running along the coast of the Bristol Channel but a km or so inland. Originally the northwards East Lyn river failed to reach the coast and turned west to discharge a mile or two further along the coast, perhaps at Lynton, or perhaps further along at Lee Bay. Coastal erosion and glacial meltwater finally connected the East Lyn directly to the sea and left the Valley of Rocks high and dry, 145m above the East Lyn. Evidently once glacial, it ends at Lee Bay, where the base of the cliff shows an early raised beach of sand and pebbles, topped by river deposits of sand and partially rounded stone, and then by glacial head with large angular stones and a preferred orientation. This was a major fluvial and glacial discharge before the upstream end of the valley broke through to the sea.
View from North walk
During the day we walked up the natural rock castles, noting jointing; and the cleavage in less sandy beds (7101 4991). An excellent cliff path (North Walk) along the coast was built in 1817 for the ‘genteel’. Looking north, the headland is comprised of Devonian sandstone – the Hangman Fm, extensively used as a building stone in Lynemouth. We also walked the west section of the path, noting the effect of sea level change on cliff shape, and shelly marine fossils pathside, high above the present sea. After lunch (7053 4979) we examined more tors, including Castle Rock. These tors are sedimentary, not igneous, horizontally bedded and with widely spaced joints, making them stable and erosion resistant, so they tend not to tumble, but stand high above the surrounding surface, which was carried away by solifluction during the permafrost.
Monday morning was spent in seaside Minehead, the name possibly a corruption of Mynydd, alluding to the hill on which the old town stands. Many buildings use the local stone: Devonian ORS, Permo-Triassic NRS, and Jurassic limestone. Half a dozen old lime kilns are situated near the harbour. Several times, both day and night, we looked across the Bristol Channel from Minehead and guessed at the towns we saw: Barry and Penarth slightly to the east, perhaps Porthcawl slightly west. But we were cautioned to remember that there may have been substantial Variscan strike slip along the Bristol Channel, so that the land opposite was slid into place from elsewhere.
This year’s Summer Weekend was spent at Orielton Field Studies Centre, just south-west of Pembroke, where twenty members enjoyed good food and comfortable accommodation. Arrangements were excellent, even extending to a minibus for transport. Seven years ago we spent the weekend on the geology of north Pembrokeshire; so in 2013 it was time to see the Old Red Sandstone and Carboniferous rocks of the spectacular coastal sections of south Pembrokeshire.
The weekend began for some with a Friday afternoon visit to Pembroke Castle, built on and of Carboniferous Limestone exposed in the Pembroke Syncline. Beneath the castle is the Wogan, a vast natural cavern in the limestone, giving the castle occupants a basement access direct to the banks of the river Pembroke. The weekend proper began with supper on Friday followed by an introductory talk by our leader Sid Howells. South Pembrokeshire was folded by the late Carboniferous Variscan Orogeny when Africa collided with southern Europe, and the district was folded on a WNW axis. The southern coast is a Carboniferous limestone, exposed in the Bullslaughter Bay Syncline. This passes north into the Orielton Anticline, exposing older Devonian Old Red Sandstone, and even older Silurian and Ordovician in a few places. North of this lies the Pembroke Syncline, which again exposes the younger Carboniferous Limestone.
Saturday started on the sands of Freshwater East Bay [SS016978], an anticline within the wider Orielton Anticline. The cliffs show steeply dipping beds near the core of the anticline. We studied deposition patterns in the watery sand of an ebb tide and then walked across the bay, down-sequence from Younger ORS on one side, through Silurian mudstone and sandstone in the middle, and back up-sequence into the ORS. East along the coast for several miles are high ORS cliffs with a series of embayments where conjugate fault sets nearly normal to the fold axis are points of weakness and hence erosion.
At the far north of the bay is a pronounced conglomerate, the base of the Lower ORS, not to be confused with the extensive Ridgeway Conglomerate which marks the top of the Lower ORS. These sediments were deposited in semi-arid, sometimes waterlogged, Devonian braided rivers and coastal plains. We saw the evidence: red sandstones deposited in oxidising conditions, lie beside greenish sandstones deposited in a waterlogged environment; pronounced dessication cracks preserved in the mudstone; fine sandstones intercalated with thin lenses of grits due to surges of coarser material in riverine conditions; calcretes (carbonate nodules) formed as limestone precipitated from evaporated flood soils on exposed sand banks. Quartz and pink (potassium) feldspar grains were probably sourced from igneous rock to the north-west.
A few miles south-west the Carboniferous Limestone appears in the Bullslaughter Bay Syncline and we had lunch there at the picturesque Stackpole Quay [SR993958]. Axial core of syncline
At Quay Cove we saw the axial core of a tightly folded plunging asymmetric syncline and Sid Howells excavated the foreshore beach stone to uncover a huge Lithostrotion (colonial rugose coral); it has to be re-exposed at every visit! There are also brachiopods (productids), solitary corals (Caninia), large crinoid ossicles and very large worm casts (Palaeophycus?).
Next came Stack Rocks  in the Upper Carboniferous, on the MOD range. Caves are often eroded in softer horizons on either side of promontories, and frequently they meet in the middle to form natural arches, and then collapse to leave a tall stack just offshore. Green Arch
These form splendid safe havens for colonies of nesting seabirds. West of the stack lies the Flimston Fault, running south-east across the Bristol Channel. We could see, looking along the headland, where a lateral displacement of some 500 metres had shifted the rocks to produce a visibly different fold structure on the other side.
We moved on to the Devil’s Cauldron, Flimston Bay and, finally, a brief visit to Freshwater West , where the Flimston Fault runs along the wave-cut platform. Here, at Great Furznip, the Skrinkle Sandstones of the Upper Devonian lie unconformably above the Ridgeway Conglomerate of the Lower Devonian, and below the Lower Limestone Shales which mark the start of the Carboniferous. Back at Orielton we finished the day with supper and a second talk from Sid Howells.
Most of Sunday was spent walking the lovely beach from Saundersfoot Harbour , across the barnacled boulder bed of Monkstone Point, to Trevayne , in Carboniferous sandstones of the Middle and Lower Coal Measures. Once again, looking west into the cliffs cross-sections of numerous short range folds, faults and low angle thrust structures are presented. Near the harbour in the cliff faces there are ironstone nodules, formed by chemical replacement as iron in the pore waters precipitated as siderite (iron carbonate). The folding varies from tight upright folds to recumbent folds with thin anthracitic coal seams, which perhaps lubricated the folding.
Ladies Cave Anticline
Not far south of the harbour is the Ladies Cave Anticline , the most frequently illustrated geological feature of the district. Alternating thick sandstones and shales have allowed a sharp asymmetrical chevron fold to remain protruding well above the foreshore sand. The outer shape is protected by thick sandstone beds. The inner part has been eroded and hollowed out by hard quartzite storm beach boulders. A similar anticline appears fifty metres north along the beach, but now fully collapsed and only just emerging from the sand. Anticlinal rocks emerging from the sand become more widely separated towards the sea, showing the structure plunges north. The less steep side of the asymmetrical anticline dips roughly south, showing the Variscan Orogeny which caused it occurred to the south. After crossing Monkstone point, Tenby comes in sight and we made the long, steep climb up the cliff steps to the minibus waiting at the top.
Next stop Lydstep Point , on the Carboniferous Limestone. As so often on the South Pembrokeshire coast, west-east faults have led to progressive land displacement westward, and conjugate fault sets very roughly aligned north have caused embayments – and beautiful sandy havens. Caldey Island lies some 3km west and was probably once a fault-shifted peninsula, now separated from the mainland by erosion. On Lydstep Point we saw the Gash Breccia, a wide horizon in the vertically dipping limestone. It appears in many places and is thought to be unique to this district. Was it caused by limestone cave collapse, or brecciation during the Variscan Orogeny, some sort of fault crush perhaps, or something else? Much discussion has not reached agreement, or even consensus. Some of the blocks are bus sized, some of rock crushed much finer.
The final stop was 1 km west at Skrinkle Haven . From our vantage point, the nearest half of the cliff of this small bay is Carboniferous Lower Limestone Shale; the other half is the Skrinkle Sandstone which forms the Upper Devonian of the district. The Middle Devonian has been lost in an unconformity. The near side of the bay (north) is eroded on a fault within the shale, leaving a large, sheer wall-like cliff. At the far end of the bay the promontory is caused by the Ridgeway Conglomerate, marking the top of the Lower ORS. Unfortunately the far cliff face was in strong shadow, obscuring the boundary, but the nature of the fallen rock below each unit was clearly different. Sunday ended with supper and a quiet evening in the common room.
Colin at Manorbier
Monday morning we visited the shore of Manorbier Bay , in sight of the castle, to look at a cross-section of vertically dipping ORS, some 500 metres south of the Ridgway Conglomerate. Each of six beautifully exposed deposition cycles begins with a coarse, mud rip-up lag in red silty sandstone, and then fines upwards through channel fills of grey sandstone with parallel lamination, and some rippled lamination in the upper part. Each shallowing cycle is then topped by purple silty mudstone, with calcrete nodules formed from the soils which began to be established on the sand bars of a braided river, before avulsion (sudden change in direction) inundated the new vegetation and began another cycle of deposition. Most members left after the morning, with a few stalwarts staying on to search for fossils further up the succession. All agreed, one of our very best annual weekends.<
Amongst other things, we saw impressive exposures of Carboniferous Limestone with cyclothems, paleokarstic surfaces, fossil corals, brachiopods, plant debris, fluvial deposits of Silurian pebbles on an eroded Carboniferous surface and massive cross bedding in sandy oolitic limestone - or was it a limey sandstone? There were lots of things to puzzle out.
Parking for several cars is at O.S. ref SJ 234432.
From parking, take the path up the old incline, NW, and along the old tramway (You can see the occasional sleeper still.) to location 1. (SJ 231433) From here we get a good view of the local geology and the Dee valley. Castell Dinas Bran, facing you, is on Silurian (Ludlow) as is the lower ground below the road. The base of the Carb. limestone is shown on the map as faulted along the line of the road, which may be so, but the base is also a major unconformity, corresponding to it being above sea level during the Devonian.
The Carboniferous sea slowly transgressed over the eroded Silurian landscape during the Early Carboniferous (Dinantian) ~337Ma, covering S. Wales and later, N. Wales. The early Dinantian is therefore missing. Along the road below, "Basement Beds" outcrop sporadically in stream beds, these represent the "first washings" and are red shaley rocks.
The beds below loc 1 are the Upper Ty-Nant Lst Fm with the Eglwyseg Lst coming in at loc. 1 . Above the small scarp, in the gap, there is a sub-aerial eroded paleokarstic surface filled with a purple and green pebbly shale with Silurian pebbles showing imbrication.(Pics 1&2)
The next higher quarry face shows nice cyclothems, perhaps better appreciated at loc. 2, where there are half a dozen. These are produced by repeated transgression and regression of sea level. Wales was at the time sub-equatorial and water was locked up in a S. polar ice cap, with periodic inter-glacials. Periodic emergence is indicated by paleokarstic surfaces, with more rubbly beds, rust coloured calcretes, dessication cracks etc. (In one place, further round by Ty-Nant there is actually a thin coal seam. ) The bottom bed is a massive cream limestone (biopelsparite) bed with a hummocky paleokarstic top surface and rubbly layers. Above that in the next massive bed is an interesting "pseudobreccia" which can be examined in fallen blocks.
Following round to the east, higher beds can be examined as far as the end quarry, beloved by climbers. There are examples of colonial corals, burrows, etc. Above the quarry is an example of a limestone pavement.
A path can be followed back to the car parking for a cuppa and a sandwich, before taking a path to the ESE towards loc 2, (SJ 240428) Bron Heulog Quarry. This large quarry has a sheer face exposing nearly 200 feet of the Eglwyseg Limestone and shows really impressive cyclothems.
SAFETY NOTE Falling rocks from 200 ft are dangerous. Keep clear of the faces and wear your hard hat!
At the east end of the quarry impressive lime kilns are well preserved. (watch the unprotected edge!) Here lower beds of the Eglwyseg Lst are exposed (normally not seen as they erode easily) comprising green and grey marls and rubbly micrites with grey fetid biosparites and calcareous shales.
On the way back clean specimens of all lithologies can be found in the broken blocks, including examples from the Sandy Lst Fm (loc 3) and the overlying Cefn-y-Fedu Sst.
Location 3 (SJ 227455) is reached a mile or so east along the road but there is good parking so it is excusable to drive. This is in the Sandy Lst Fm and shows remarkably good large scale cross stratification, with conglomerates with quartz pebbles, nice deltaic structures. With a lens they are lovely oolitic sandy limestones and limey sandstones. Are the regular beds tidal? monsoonal? or what? This is a textbook face so, please, no hammers, but there is plenty more round the corner if you did not pick up some at loc. 2.
Refs G.A. Guide "No.6 Geology around the University Towns - Liverpool 1986
Brit Regional Geology North Wales. (At present O/P. Mine is prehistoric and dated 1948 price 3s.6d. but I think there is an issue about 1976.
"Pingo" is the Eskimo word for a hill and applies to the large number of almost circular hills found in the Canadian North West Territories round the Mackenzie Delta. They rise to up to 60 metres high by 600 metres wide and some have a central crater which may contain a circular pond. Within each there is a core of ice which builds up as a large lens pushing up a dome of sediment above it. Eventually the dome splits and the ice is exposed and can melt in the summer.
In Britain all we have left after the last ice age are circular ramparts filled with boggy sediment. They are rare in Wales. In the field at Tylwch we saw a good example with traces of another nearby.
Note that the pingo is on private land so do not traspass without permission, there is little point because there is nothing on the ground and it is best seen from the path which overlooks it to the north.
Do be careful because there are lots of farm ponds which are just that, Pic 6 shows one near Guilsfield!
M.R.Dobson 1995 "The Aberystwyth District" G.A.Guide No. 54 ISBN 0900717785 pp77-80
Cave R. and Haynes, B.A. 1986 Geology of the Country between Aberystwyth and Machynlleth Mem. Brit.Geol. Surv. Sheet 163 pp119-120
Leaflet No. 7 "The Temple Lead Mine" available, with others, from the M.W.Mining Museum, Ponterwyd, Aberystwyth. This museum is well worth a visit and is 2km west of Ponterwyd on the A44.
The walk starts at Yspyty Cynfyn, (SN752791) on the A4120, 2km south of Ponterwyd, where there is parking, (avoiding obstructing the church or barn entrance!). It follows a classic section described by O.T.Jones in 1909, defining the graptolite zones of the Cwmere and Derwenlas formations at the bottom of the Llandovery Series (Base of the Silurian). It includes the remains of Temple Mine which is a significant mine on the Castell fault. All the underground workings are dangerous and should not be entered. That said, hard hats are not required for the walk, just good boots and sandwiches.
Set off to the right of the churchyard, in the wall there are built in the standing stones of an old stone circle. This gave it's name to "Temple Mine". We follow the path a few hundred yards and down into a steep sided dingle where we cross Parson's bridge.
This structure replaces an earlier plank bridge which must have been a bit "hairy". It was used by the parson to visit his flock in Ystumtuen, which was a mining community above the gorge. The volume of water in the river was much greater before the power station took the majority of the flow, but it is still impressive. Note the enormous potholes. These are about as good as you get.
We bear right over the bridge, and 100 yards further on find the ruins of the dressing floors of Temple Mine. Dating from the 1880's, the ore bins, circular buddles, wheelpit and crusher base are clearly visible. The waste tips went straight into the river so there are few minerals etc to be found here. The ore used to be hauled up an incline to meet a track to Ystumtuen. The path up the slope is now overgrown and difficult to find. Continuing along the path, we are walking along the line of the tramway between the main adit and the dressing floors. The leat which fed the 14ft waterwheel is a few feet to the left.
Shortly we come to the main adit on the left with a very substantial wheelpit on the right below the path. This housed the waterwheel which ran plant, pumps and the winding gear.
50 yards on we cross a small tributary. This is an important location as both the tributary and the Rheidol follow the Castell Fault. This is a major feature along which mineralisation occurred. There is an old adit (Nantymoch level) about 50 yds up the tributary but it is quite a scramble to get to it.
The path now follows the old leat and graptolites can be found in rock exposures along it. You need to get your eye in for them as the slatey cleavage does not follow the bedding and the graps are on the bedding plane. The importance of these organisms is that they evolved rapidly and therefore make good zone fossils. In the Aberystwyth area in particular, we have a situation where, along the coast, we have a section along the cliffs where, in the north, the dip is to the south and, in the south, it is to the north. (i.e. basin-like) Because the sediment was deposited by north flowing currents, one stratum could well be a sandstone in the south and a mudstone in the north. In about 1909 graptolite zones were used to establish which Aberystwyth Grits in the south corresponded to particular Borth Mudstones in the north.
It was the noted geologist, O.T.Jones who mapped the area using graps for zoning.
(Ref: Jones, O.T. 1909 The Hartfell-Valentian succession in the district around Plynlimon and Pont Erwyd Qt. Jl. geol.Soc.Lond.,65, 463-537)
By a small oak, the path bears up and left. The leat continues to a steep cliff where it was carried in a timber box section, chained to the cliff face. The path approaches the river again before a scree slope and a stile into a field. Most of O.T.Jones' sections are in this 400m section south of the stile. Graps are mostly located in the rusty weathering rocks, rather than the pale ones.
It has to be said that there are hundreds of species of grap, so they are not easy to identify. However, some from here are quite pretty, being pyritised.
The public path goes north through Brynbras farmyard and bears right, following their track to the George Borrow Hotel. As the most interesting bits are in the south, we returned that way. An interesting feature in the north is that the river meanders. It was thought that these were "incised meamders" formed when sea level was higher and the river was mature. Sea level then went down and the river was rejuvenated, cutting the meanders deeper as it did so. It is now thought that the more likely explanation is that the river burst out above an ice blockage during the ice age. The meanders can be seen from the road, in various places.
The return path up the ravine above Parson's Bridge always seems steeper and longer going up than coming down!
Simon J.S.Hughes, 1980 "British Mining, No. 17" Northern Mine Research Soc. (The most complete guide)
M.R.Dobson 1995 "The Aberystwyth District" G.A.Guide No. 54 pp89ff ISBN 0900717785
D. Bick 1991 "The Old Metal Mines of Mid-Wales" Pt 4 24-25
The Cwmystwyth mine (SN 805746) is at the west end of the old mountain road to Rhayader, just off the B4574.
Cwmystwyth is one of the largest of the old mines, to the north of the road are most of the workings and remains of the dressing floors.
Mineral specimens (Galena, sphalerite) can be found both sides of the road in the extensive spoil tips. Further to the north east is Copa Hill which has remains of uncertain pre-historic date with the best examples of the old "hushing" process. This involved damming water and releasing it to clear the topsoil exposing any signs of veins.
The roads which wind around the exceptionally beautiful and peaceful reservoirs of the Elan and Claerwen valleys reveal the Cerigwynion Grits which ended the Ordovician here, and the mudstones which began the Silurian. Rising and falling sea-level 440 million years ago produced a range of different types of deep-water sediment deposition. This sandstone and mudstone succession was deposited as a series of gently sloping lobes with slumps, debris flows and a wide range of turbidites. An unusual feature is the record of a very different depositional environment, the Caban Conglomerates, an erosive high energy submarine channel flow which cut through the sandstone/mudstone slope-apron and left its own story in the rock. Local folding (Rhiwnant Anticline) followed by natural erosion and man’s rock cutting and has left different rock facies conveniently juxtaposed.
CARN OWEN – Sunday 19 May 2013
John Mason led a sizeable group to Carn Owen and Esgairhir mine on a cold sunny day to do what members seem to like best: searching for fossils and collecting minerals. Carn Owen lies just west of Nant y Moch reservoir so we were able to enjoy a long drive around the reservoir on the way out and back. Carn Owen is a ridge close to the NW tip of Nant y Moch reservoir. The hard Upper Ordovician sandstones of the Drosgol and Brynglas Formations are exposed along the axis of a NNE aligned pericline (dome structure), with the younger, softer, shaley Cwmere Formation rock exposed around the flanks. The contact between the two is strikingly obvious in the change of slope. We have been to the lead mine before but this time we were taken to a spoil tip near the top of the tramway incline at SN 7308 8802. Tipped here on Ordovician sandstone is a huge pile of rusty weathering Cwmere shales with abundant Nomalograptus persculptus, the characteristic graptolite of the lower Cwmere, together with numerous small orthocones.
Next call was the nearby and more recent rock quarry at SN 732 881, a highly disturbed sandstone channel displaying great contortions of the bedding, which also incorporate huge mudstone diapirs resulting from upward squirting of still soft mudstone as large bodies of sand flowed onto them. This melange is thought to result from collapses of the south-eastern levee (banking) of the submarine channel. Finally, in a small quarry at SN 734 881 we saw the contact between the uppermost Brynglas sandstones and the lowest Cwmere mudstones, and discussed the reasons for the oxic condition of the former and the anoxic condition of the latter. Then we lunched in the sunshine.
The afternoon was spent at Esgair Hir mine, located just north of Nant y Moch, at SN 733 913. There were at least a dozen phases of mineralisation as superheated mineral rich fluids were released upwards from their deep reservoirs by movement on faults. It began work in 1693 and continued sporadically until 1904, producing a total around 2000 tonnes of lead ore and some 700 tonnes of copper ore. But it is best known for the small quantities of a silver ore, tetrahedrite, found in the lead ores, allowing silver to be reclaimed during smelting. We found on the spoil tips: chalcopyrite, marcasite, malachite, pyrite, galena, sphalerite and goethite, and various secondary minerals which have formed in recent times from weathering of the original primary minerals, including cerussite, pyromorphite, azurite, linarite and ferroan dolomite; over a dozen minerals in total, not bad for two hours searching, and everyone found a few minerals. Then John told us that he has found around forty minerals in total on these very tips over many visits!
Our visit to Middletown Quarry last Sunday was most enjoyable, perfect weather and unusual rocks. It is within the interesting Breidden inlier, north west of Welshpool which comprises an heterogeneous mix of Ordovician rocks all within a small area.
The main quarry is mostly within the Lower Rhyolitic Tuff member of the Middletown Formation (Ordovician, Caradoc). It is a rhyolitic tuff/breccia which is strikingly green - even brighter green if wet. The bedding reflects the different volcanic events and shows variation in colour and in the grading of the clasts within it. The green colour is thought to be due to the replacement of minerals within pumice by chloritic minerals. The image is of explosive eruptions of an island volcano producing ash which is carried down into deeper water by turbiditic flows.
The Lower Rhyolitic Tuff member is overlain by the Graptolitic Shale member and the Upper Rhyolitic Tuff member in the upper benches of the quarry. Although we could see the Graptolitic Shale Member, which is a dark "Black Shale", we could not approach it and had to content ourselves with examining different heaps around the quarry. Some was heavily pyritised, like the black colour, showing it originated in anoxic conditions. Graptolites were hard to come by, however.
One strange rock in the main quarry raised questions regarding its derivation. It as a conlomerate in which the clasts were "cobbles" of bentonite. Presumably derived from a soft sediment mass flow of partially lithified sediment in which ash had already been degraded.
At the top of the quarry and on the hilltop above, an andesitic conglomerate with andesitic cobbles overlies the Middletown Formation. This is in the Builthy Formation and may indicate that the volcanoes were becoming more intermediate in nature.
In the afternoon, after discussing the surrounding geology from the vantagepoint of the hilltop, we explored more of a section across the Breidden inlier by walking across the valley towards Rodney's Column, noting the change of slope and the dolerite exposures as we came to the intrusion exploited by the nearby Criggion Quarry.
The May 2014 fieldtrip was led by Tony Thorp on and around Gaer Fawr Hill near Guilsfield, on the western flank of the Guilsfield anticline. We were near the top of the Gaer Fawr Fm (Ordovician, Caradoc), a shallowing-up sandstone with abundant fossils, as seen in the small quarry at the nature reserve car park. This hard sandstone forms the hill. Above this in the steeply dipping sequence is the softer Dolhir Fm (Ashgill), which has now eroded to a valley. The Ordovician is then topped by the Nod Glas, a thin horizon of soft black shale, which we saw in the stream banks at the valley bottom. Above this in the sequence is the very hard Powis Castle Conglomerate, a beach deposit, now forming a ridge, along which we walked, looking across the eroded valley, at Gaer Fawr Hill. All these features were beautifully evident on a lovely dry day, with additional features to be seen in the stream bed (bentonites, evidence of faulting, and limestone). The day finished with a walk to the top of the iron age hill fort, through woods of stunning bluebell display.
On the 16th July, a dozen members of the Mid Wales Geology Club spent the evening exploring the geology of Gilfach Nature Reserve.
Fresh rock by the entrance
In some ways Gilfach geology is simple as all the exposed bedrock is from just one formation, the Rhayader Mudstones Formation. In the reserve, exposures are generally weathered and covered in lichen; but there is a comparatively fresh exposure just outside the entrance on the A470 road cutting, where we could examine it at close quarters. (Watching the traffic!) Here it was seen to be an attractive green-grey fine grained rock in which individual grains are too small to be visible to the naked eye, hence it is classified by geologists as a "mudrock". It was laid down some 430 million years ago in quite deep water and is part of the Llandovery Series of the Silurian System.
Cleavage, bedding and joints
The rocks show a marked directional “grain” at a steep near vertical angle dipping (or sloping) towards the northwest. This is nothing to do with the beds as they were deposited, but is “cleavage” developed much later by tectonic strain at moderate temperatures and pressures. It is less developed than in slates, so these mudrocks would be termed shales.
Picking out the signature of the original bedding was more difficult. It shows as more subtle changes in colour and texture which can be picked out dipping much nearer the horizontal. On some cleavage faces the bedding shows as a ripple effect when the direction of cleavage differs slightly as it meets different beds. This is known as “cleavage refraction” and is caused by the cleavage tending to align itself more towards the bedding direction of muddy beds as compared to sandier ones. The bedding here is quite thin, being from a few cm to tens of cm. Each bed represents an “event”, specifically a muddy flow from a shallower shelf area in the east into deeper water here. It could have been triggered by a storm or similar disturbance and is termed a turbidite. Each turbidite could have been followed by a quiet period in which very thin laminae may have been deposited from the undisturbed waters above. These are termed hemipelagites and may be dark grey, pyritic and carbonaceous if deposited in anoxic conditions, with no life surviving; or lighter grey and mottled with burrows if animals lived there.
Other near vertical faces are “joints”, which are produced much later as pressure on the rock is reduced as it is brought nearer the surface by earth movement and erosion.
One of the notable features of this rock is the presence of concretions or nodules. These commonly form in mudrocks after deposition and are discrete local patches cemented differently from what surrounds them. They can be cemented by a variety of minerals; but in this rock they are generally either calcareous or phosphatic. In the road cutting we found one which was about 150 mm long and egg shaped. It fizzed when we touched it with acid, so was calcareous (cemented by calcite) and it showed a cone-in-cone structure. Cone-in-cone structures are poorly understood. They comprise nested cones of fibrous calcite and probably take a long time to form, requiring that a high proportion of saturated pore water remains within the sediment for hundreds of thousands of years. This can occur if a bed is overpressured by being trapped between impermeable layers. As rock is much denser than water, under these conditions pressure can be greater than that due to a simple water column.
There were also many smaller phosphatic nodules disposed along the bedding. These are blacker than the surrounding rock, but get paler as they weather.
Folds and faults
Walking back into the reserve and up to the crags on the hillside facing the parking, we saw similar directional features. The cleavage was again steeply dipping in the same north westerly direction and this was to turn out to be true all over the reserve to within a few degrees. It is a “regional cleavage” and is remarkably consistent over a large area.
The bedding, however, told a different story. We could see it was dipping in different directions as we looked round. In some places it was folded into synclines (downward) and anticlines (upward). Displacements in the bedding occurred where small faults existed.
Some beds had been partially eroded away where lines of concretions had been preferentially eroded out.
The story of the rocks
The structure of these rocks tells of a complicated story going back some 430 million years to a time when Wales was attached to the north of small continent which geologists call Avalonia. This was somewhere south of the equator, drifting slowly north towards a larger continent, Laurentia, later to become North America. It was starting to feel a soft collision with Laurentia. This was complicated by it not being a straight head-to-head collision, but at an angle, with some transverse movement. Further complication was introduced by the probable impact of another continental fragment, Baltica, from the east. The three way collision did not produce simple concertina folds, but something more like a crumpled sheet of paper, which is what we see in the variously dipping bedding planes. Cleavage developed much later (about 390 million years ago) in the Devonian period when the collision pressure peaked just as today in the Himalayas, where the Indian sub-continent is in collision with Asia.
To the recent stuff – The Quaternary, or Ice Age
What went on more recently during the Ice Age? The Wye valley going north is wide and there are ice eroded cwms exiting into it, so it was ice filled; but to the south it is much narrower and gorge-like so ice could well have been forced up the Marteg valley. (Ice can, of course, flow uphill on occasion!) It may even have been forced north as well, to join the great ice stream exiting along the Severn Valley.
The craggy hill tops could have been exposed, particularly in later periglacial times, when they would have been exposed to freeze-thaw conditions, producing quantities of “head”, comprising debris of all sizes down to small flakes, quantities of which we found in the ditches alongside the road as we walked up the valley and down to the river.
In the river itself there are some of the best potholes you will find anywhere, some quite “Henry Moorish”! No doubt the cavities left by eroded concretions helped start the potholing process. Pebbles carried by the river in turbulent eddies when in spate complete the process by abrading, expanding and making the cavities circular.
Typically the deposits on the north of the river appear to consist of head while those on the south are grey till, deposited from ice. Perhaps the early river established its gorge to the north of the ice-filled valley, where the sun thawed the ice first.
In places we saw a level red layer below the top of the till which had resisted erosion by the river. This could be a lithified layer, hardened by deposits of iron oxides, forming a pan.
This was indeed an enjoyable evening walk along one of the loveliest river valleys in Mid Wales, with a geological story as a bonus. If the opportunity of a repeat performance were to arise, a full day would be well justified, with a visit to the visitor centre for tea and a cake!
Hendre quarry Ystrad Meurig
Thirteen members enjoyed a day of glorious April sunshine at Hendre quarry (by permission of Hanson), five miles south of Devil's Bridge, led by Dr David James. Hendre (SN 721693) is a working quarry, on the eastern flank of the Teifi Anticline, which is the western upfold of the first order fold structure of Central Wales. The main quarry is several hundred metres across, with faces all round, allowing a nearly 360 degree view of folds in the Ystrad Meurig Grits. Here the Hendre Lobe of sandstones, within the Lower Silurian, Derwenlas Formation mudstones, was sourced via a channel from the Caban canyon, near Rhayader, 25-30 miles away, on the shelf edge of
the Lower Palaeozoic Welsh Basin.
We saw turbiditic sandstones of the various stages of the Bouma classification, and some flute casts which proved the palaeocurrent was flowing roughly westwards away from the basin shelf into much deeper water; though the relative absence of mudstone intervals restricts the development
of flute casts in these grits. Grading could be seen in some thick sandstone units. Large blocks on the central quarry floor show beautifully colour-banded cross sections and allow hand lens examination of contacts and grading. Groove casts and prod marks are also visible on these blocks.
Often the rock is made up of a continuous sequence of sandstone units, but in places the sandstones are interspersed with dark mudstone in which graptolites have been found. Folding is well displayed in the quarry, with some folds traceable north-south across the quarry. The benches
provide ample opportunity for interpretation and discussion of fold structures, some of which contain confusing parasitic folds and complicated by minor faults. With folds so well displayed, their asymmetry reveals the dominant vergence in a south-east direction, recording the Caledonian Orogeny. There are good examples of the open, rounded structure of folds in competent, thick
sandstones, and tighter chevron folds in thinner sandstones. The all-round views of quarry faces allow cross sections of structures to be seen on one side, and large areas of slightly rippled and curved bedding to be seen on other sides.
Following in Darwin's Footsteps
Our September field trip, on the 20th, followed "In Darwin's footsteps" under the leadership of member Roy McGurn. Our meeting point was at the first location, Pen y Foel Lane, Llanymynech, where Roy explained the background to the dozen assembled members.
In August 1831, England's foremost geologist, Prof. Adam Sedgwick, set out on an expedition to map the "transition beds" of Wales, that had hitherto defied meaningful interpretation. The period was one of dramatic change in the understanding and concepts of geology. Although a biblical interpretation (according to Genesis) was not seriously regarded by many researchers by this time, the concept of some kind of creation was still strong.
For the previous 50 or so years, the theories of Abraham Gottleb Werner had held sway, that the rocks we see are the result of some sort of fluvial action, perhaps floods or deposits left by a retreating sea. The school of thought was broadly known as "Neptunism". Sedgwick himself had used his presidential address to the Geological Society to recant his belief in this theory that very spring.
The succeeding counter argument, perhaps broadly termed Plutonism, was the development of ideas presented by James Hutton in his two lectures to the Royal Society of Edinburgh as far back as 1788. Hutton had postulated the rock cycle, with rocks constantly being renewed from igneous rocks breaking through to the surface from great depth. Hutton could see no sign of a beginning , nor any sign of an end to this process, which finally challenged any concepts of a divine creation, which also postulated a day of judgement.
It could be said that Copernicus challenged the concept of the earth being at the centre of the Universe. (He was wise to publish his works when more or less on his death bed, as in his day such ideas would have amounted to Heresy.). Hutton challenged the idea of a biblical creation, again another heresy in an earlier time. In the summer of 1831, the third prophet who was to eventually challenge man's place at the centre of creation, began his journey here.
It is perhaps how fate works that the grandson of Hutton's friend, Erasmus Darwin, who invoked some ire himself with observations on the immutability of species was asked to accompany Prof. Sedgwick on his quest.
A few yards up the lane, they stopped and no doubt could see in the road cutting, steeply dipping shaley rocks on which Darwin could try out his newly acquired clinometer (a small plaque marks the spot). From this spot they could see the nearby limestone escarpment and quarries of Llanymynech Hill. These are almost flat and, if projected, would have been overhead at this point. These radically different dips would indicate an unconformity, as taken by Hutton to show a break in succession and maybe a period of erosion.
The second location was on Trevor Rocks, above Llangollen, with a view of Castell Dinas Bran and up the Dee valley. Sedgwick had business in Llangollen, with Robert Dawson, a surveyor, who was to furnish him with notes on the limestone of the Vale of Clwyd.Sedgwick's mission was very vividly set out for him here. The prominent Mountain Limestone, as it was known is easily identified by its distinctive lithology. It was known elsewhere for the "Herefordshire Beds" to occur beneath the Mountain Limestone, again with a fairly distinctive lithology. Below the Herefordshire Beds, into deepest and darkest Wales, the lithology was a lot less distinctive and geologists at the time were having great difficulty making any sense of it. It was generally referred to as the transition beds..
Sedgwick needed to find a sequence of strata that took him down into the transition beds and from which he could derive fossil sequences to identify them. Ideally following the maps of the time, he needed to follow a sequence down from the Herefordshire Beds. (= Old Red Sandstone). Sedgwick was using a later version of Greenough's map that had partially corrected the Herefordshire Beds error. The red sandstone in the Vale of Clwyd had identified as New Red Sandstone, a rock common in England and known to be some way above the Mountain Limestone. However a small sliver of exposed rock was still indicated below the Mountain Limestone and was known as the Old Red Sandstone.
At this location the scree covers the base of the limestone. Tantalisingly traces of red soil may be found in places along here, a possible indicator of red sandstone (Old or New) beneath.
As Dinas Bran is significantly higher than the base of the limestone, even if the latter is "extrapolated" towards it, there seems to be something else happening between the two. A small fault is marked on the modern maps which must take the southern side up a little, so any base to the limestone will have been above the hilltop and hence long eroded away.
Sedgwick was not going to find much to help him here.
The third location was at Velvet Hill, next to the road up to Horseshoe Pass, at the entrance to Valle Crucis Abbey. Darwin's enthusiasm for his new clinometer is evident, he records:
"Saturday 6th August Vale of Crucis.
The bank above the abbey consists of Clay slate, which breaks at regular intervals, striking nw by n, d,25 to the ne by n. at different parts road observed beds of diluvium very Shrops only no sand:also boulders of trap."
The entrance to the abbey contains a small well. This is likely to have been the original road and the well may well have been a stop Sedgwick made to water his horse ahead of the long climb up Horseshoe Pass. Darwin busied himself with his notebook and clinometer.
Darwin went on to note.
"Beyond Vale of Crucis on the road to Rurhven the Limestone is seen having a grand escarpment to the west: The contrast between this and the more regular slope of the Clay Slatew gives more grandeur to the views. The Greywacke generally covered by gorse, Heath and Fern: the limestone either bare or the verdure very green." However Darwin's observation shows how they were using geomorphology to identify the outcrop of limestone. Beyond the pass the escarpment changes, with the rounded, heath covered greywackes he noted, seeming to break into the line of limestone hills, and the limestone outcropping into the greywacke area.
Next Step. Darwin notes "About 1/2 mile beyond Daforn, a black bituminous Limestone organic remains veined quartz in parts reddish in one part strata exactly arched. The line N by E."
This note refers to a disused quarry at Plas Newydd where, indeed we found limestone crags, reddish in parts. No arch, but much limestone may have been removed.
Darwin goes on "1/2 mile further a tortuous valley through Clay Slate generally dipping to the E. About a mile from the Ruthven beds of sandstone"
This is Nant y Garth, the faulting brings up the Silurian beds again and forms the end of the Vale of Clwyd. At its end, the road opens into a plain with hills either side, especially a clear line of them to the north. We are into Triassic New Red Sandstone, but outcrops are rare although the soil is red.
Appropriately, we ended our trip at the Castle Hotel, now a Wetherspoons, where Sedgwick and Darwin spent the night. Darwin notes:
"Ruthin: takes its name from the new Sandstone on which it is built. The soil is for some miles about the town and the plain may be considered of that formation. In most places covered up by diluvium. Mile to the west of the town a quarry of worked. The rock is spotted with brown the stone at Cardeston. Overlying Magn conglomerate. It is very irregular strat but the rock on which the castle is built nearly horizontal seams. petrifications this there are some beds of Old red Sand striking the same ways as the overlying stone but dipping at a greater angle. A further is more clearly visible in a water"
The elusive ORS has finally reappeared and Sedgwick may have thought he had an opening into the lower succession, the notes stress this rock is UNDER the limestone and does have all the appearance of the beds found in Herefordshire. However the boundaries into the lower beds are generally faulted here, in its simplest form the Vale of Clwyd is a rift valley, down faulted during the Upper Carboniferous-Triassic. Sedgwick was not to find a succession into the transition beds anywhere in North Wales, as the unconformity Darwin didn't observe way back in Llanymynech represented a break in deposition and a period of erosion that covered the entire Devonian period and the beginning of the Carboniferous.
Members had enjoyed a fascinating trip and Roy's research and leadership were really appreciated. Most members started to return home at this point, but noted that Ruthin contained a remarkable assortment of building stones, particularly evident in St. Peter's Church, where Sedgwick and Darwin attended Matins on Sunday 7th August 1831. They perhaps viewed the pot-pourri of stone used in its construction.
Ruthin seems worth a full day trip in its own right. Perhaps we could follow more footsteps at a future date.
Land of My Fathers was a public outreach programme, centred on an exhibition with associated activities in Llanidloes in 2007/8, arranged by Mid Wales Geology Club in conjunction with the Welsh Mines Society. Funding assistance was provided by the National Lottery Awards-for-All, and by the Geological Society of London under their 'Local Heroes' scheme. The programme also celebrated the bicentenary of the Geological Society. Several other organisations and numerous professional geologists provided further assistance. The exhibition, A Geological History of Mid Wales and its Metal Mining ran for week in September 2007 at the Minerva Arts Centre, Llanidloes. It contained several hundred specimens of rocks, minerals and fossils, over fifty large posters, a variety of geological models, and 3-D maps from the British Geological Survey. More than 300 people visited the exhibition.
Seven associated talks were held, mostly in Newtown, running through into 2008, including presentations by Prof David James, Prof Neil Glasser, Prof John Cope, Prof Cynthia Burek, and Dr John Davies. The talks by Prof James and George Hall (President of the Welsh Mines Society) at Minerva during the exhibition week were opened by Lembit Opik MP and attended by 145 people. Several field trips were also arranged, and there were activity sessions for nearly 200 primary school children during the exhibition.
What happened in Mid Wales during 300 million years between its formation and the recent ice age?1
Professor John Cope (National Museum of Wales)
The first of Mid Wales Geology Club's Land Of My Fathers public talks on the geological history of the Welsh Basin was given at Plas Dolerw, Newtown on 23 May 2007. Professor John CW Cope from the National Museum of Wales spoke on 'The Missing Years', a still controversial view of what might have happened in Wales during the last 300 Ma (million years).
Professor Cope observed in 1994 that in the Tertiary period (post-65 Ma) the continental crust beneath the Irish Sea must have been uplifted at least 2 km. This contributed to the present regional south-eastward dip of Wales and England, so the rocks become progressively older north-westwards. Subsequent research suggested that the crust was underplated and thickened by rising magma, causing the uplift, though there is no evidence of surface volcanism, as in the Western Isles of Scotland.
A much earlier uplift of the Welsh Basin into mountainous terrain around 395 Ma is well understood, and these ancient rocks are exposed today. Central Wales then remained land for some 200 Ma. Those early mountains might have been 3 km high but were eroded back to sea-level by the Early Jurassic, 200 million years ago. Jurassic rocks have long been known from South Wales, but were not found further north until they were discovered in the Mochras Borehole near Harlech. In the borehole an extraordinarily thick succession of Lower Jurassic rocks, preserved from erosion by downfaulting, demonstrated that all of Wales probably had some Jurassic cover.
In the early Jurassic, a rising sea-level inundated most of Wales intermittently. In the latest Jurassic and Early Cretaceous Wales was probably land and at least some of the Jurassic sediments would have been eroded at that time, but the sea returned again strongly, depositing more sediment: during the Late Cretaceous, when at its highest, sea-level may have been more than 200 metres higher than it is now. Because Early Cretaceous erosion may have removed much of the Jurassic cover it is difficult to estimate the total thickness of these Mesozoic deposits, but the Cretaceous chalk alone is likely to have approached 1 km in thickness.
The Jurassic-Cretaceous climate was tropical, with average temperature around 25°C. The local seas teemed with life, including the large marine reptiles, plesiosaurs and ichthyosaurs. Some of the large dinosaurs which populated the land areas may also have lived in Central Wales at times of lower sea-level, but any such fossils have long-since been eroded.
The idea that the Chalk may have covered Wales was first suggested in 1902 by Strahan. He observed that the present rivers of SE Wales did not follow the structures deformed around 280 million years ago (during the Variscan orogeny) but instead cut across them. He thought they may have originated in the flatter overlying Chalk cover, cutting through to superimpose their drainage pattern on the rocks beneath. Although a Chalk cover for Wales now seems conclusive, there are no remnant patches of flint gravels as in SW England, probably due to the rapidity of erosion of the Welsh Chalk cover in the early Tertiary.
Rifting, and the opening of the northern North Atlantic began in the Cretaceous and continued into the Tertiary with clearly associated post-65 Ma volcanism. A present-day volcanic hot spot exists beneath Iceland; there was extensive volcanism off the west coast of Scotland in the Tertiary, and now it appears there was early Tertiary magmatic underplating beneath the Irish Sea. Magma, perhaps rising from the Earth's mantle, accumulated beneath the crust and the land domed upwards. Gravity anomaly maps reveal high-density rock beneath the continental crust, and indicate underplating of the crust by 8 km of basalt around the Isle of Man, reducing to some 4 km under North Wales.
The resultant Tertiary doming of land where the Irish Sea is now, led to some 2 km of Jurassic sediments and Cretaceous Chalk being stripped off by erosion. This view is supported by burial-depth studies using fission track analysis, (which indicates maximum rock temperature caused by earlier burial), and suggests that the Lower Palaeozoic (older) rocks now exposed must have had up to 2 km of newer rocks deposited on them, and subsequently eroded. The south-eastward regional dip of England and Wales is around 0.5 deg, which corresponds well with the loss of around 2 km by erosion on the uplifted western side, while the non-uplifted eastern side of Britain retains its Chalk today.
All this indicates earlier presence of around 2 km of Jurassic and Cretaceous cover over Central Wales. If Wales were covered by the Chalk seas, then all its rivers must have originated in post-Chalk time. Gravity studies suggest the centre of the Irish Sea uplift lay to the south of the Isle of Man, in which case the river systems would have developed radially from that point. Remarkably, many rivers of southern Britain and Ireland do have courses close to this direction.
Most of the long rivers in Wales and England also rise in the west and flow eastwards, whilst in Ireland, on the other side of the putative dome, the long rivers rise in the east and flow westwards. A soft Chalk cover would have allowed rivers to incise deeply and quickly, and penetrate the Lower Palaeozoic rocks. This established a permanent drainage system which persisted in the older rocks long after the Chalk had gone, helping to create the incised plateau appearance of so much of Wales today.
1Most rocks exposed in Mid Wales today are Ordovician & Silurian, 488 to 416 Ma, part of the Lower Palaeozoic. The time periods discussed in the talk are much younger:
The Metal Mines of Mid Wales: Where are the Lodes?
(Structural & Stratigraphic controls on the Central Wales Orefield)
Professor David James, 21 November 2007, Plas Dolerw, Newtown
Dr David James is visiting professor at Cardiff University. In November 2007 he gave an elegantly simple and splendidly illustrated talk to Mid Wales Geology Club in Newtown, on the factors determining the location of the ore lodes in the Central Wales Mining District. The rocks containing the ores, mostly lead and zinc sulphide, were deposited in the Welsh Basin in Ordovician and Silurian times. Metals in ionic form, widely disseminated in these rocks at depth, were scavenged by hot fluids flowing slowly under pressure to areas where they accumulated. Eventually the mineral-rich hydrothermal fluids used faults to escape suddenly upwards from depth, depressurise and cool, and precipitate the ores. Subsequent erosion exhumed the ore deposits close to the surface, making mining economic.
Professor James adopted an oil industry reservoir analysis approach to explain the location of the ore lodes. Sites are determined by differences in permeability of the deformed rock after fracturing; and differences in bed thickness due to variable sedimentation on the floor of the extending basin, which slipped on fault lines into half grabens, producing fault blocks dipping down-to-east.
Hydrothermal fluids arose mostly from dehydration of rocks during metamorphism, and partly perhaps from water retained in the pores of the original sea-floor sediments. Over-pressured fluids moved very slowly through the rock towards the surface when permitted to do so by the developing permeability caused by uplift during the Caledonian Orogeny. The rocks were strongly folded and the fluids accumulated in anticlinal apices (tops of upward folds) and in upward-pointing pinched-out strata. Fluid movement was so slow that sites of mineralisation developed 5-10 million years after rock deformation and metamorphism.
Sandstone beds were initially around 30 percent porous, with interconnected pores creating permeability, allowing fluids to pass. Softer mudstones were more porous, typically 40-50 percent, but without comparable interconnection of pores the original mudstone was less permeable. After lithification the rocks became denser and lost their porosity, so most of the later porosity was fracture induced. Fractured sandstones, common in the Ordovician, transmitted hydrothermal fluids. Shale horizons, more frequent and thicker in the overlying Silurian, were much less permeable, capping and sealing hydrothermal fluid reservoirs in the sandstone. Monograptus sedgwickii shales of the Silurian period were particularly effective as caps as they easily deformed plastically.
Faulting under extensional conditions released accumulated mineral-rich fluids from their host rocks by fracturing the impermeable sealing shales. Some faults in the orefield have throws of hundreds of metres. When fluid in the rock at the top of the fault is suddenly connected to the higher pressure of fluid in the deeper rock at the bottom of the fault, the rocks at the top often exploded, as seen in several striking photographs of orefield hydraulic brecciation shown by the speaker. Competent rock like sandstone is more prone to brittle fracture than less competent, softer, more plastic shale. Therefore faults tended to initiate in sandstone, which explains the mines in the Formations of Van, Cwmystwyth and Early Silurian Derwenlas.
The continental collisions of the Caledonian Orogeny subjected Ordovician and Silurian rocks of the Cambrian Mountains to compression and mild sinistral shear (the north-western edge along the Menai Strait was pushed down-left, and the Welsh Border was pushed up-right) leaving basin-bounding faults with NNE alignment. As the region relaxed around 390 million years ago in the early Devonian, compression changed to extension, reversing the shear direction (to dextral), opening dominantly ENE faults around the orefield area, and leading to the first of several stages of mineralisation (timings are known from study of lead isotopes in the ores).
Geomechanics predicts that the greater the fluid over-pressure in the fault host rock, the steeper is the dip of the fault in which the lode subsequently forms. The larger lodes often dip steeply, 75 degrees or more, on normal faults produced during extension. Understanding the sense of movement would normally be helped by observation of slickensides (scratch marks) indicating the direction of movement of rocks in the fracture zone. Unfortunately the character of the faults changed as tectonic conditions changed, obliterating the evidence of earlier fault movement.
A large volume of hydrothermal fluid was needed to deposit the larger lodes, possibly in a series of releases. The fracture channel probably occluded with ore deposit, thus sealing itself and allowing over-pressure to re-build. Many thousands, or even a few million years later, further movement on the fault could then release another accumulation of mineral-rich fluid. Evidence of previously brecciated rock, contained within a later brecciation, indicates a multi-stage process. Secondary porosity can also aid fluid movement, and is caused by dissolution of minerals cementing the rocks, with calcareous cement more easily dissolved than silica cement. This may explain why the silica-cemented rock of West Wales does not contain useful secondary porosity.
The preferential habitat of economic mineralisation in the orefield is therefore on faults with the biggest throws, in high points of the containing strata, above anticlinal sandstone reservoirs, especially in thick sandstones, and where potential shale seals were not thick enough to preclude all upward movement of fluids. This explains the rich lodes at Dylife, Van, Cwmsymlog, Frongoch, Logaulus and Cwmystwyth. Few ores are found on the Plynlimon Dome itself as its apex position permitted fluids to rise into Silurian rocks which have since been eroded. Between Llanidloes and Rhayader no good lodes have been found because fewer suitable fractures were initiated in the Silurian strata there, and the thicker shale of this district may have sealed any Ordovician fluid reservoirs at depth.
In discussion Professor James did not rule out future small-scale economic mining in the orefield. Shallow mines would be the most attractive because deep, wet mines need to be pumped and, although metal prices have increased, so have energy prices. Silver was once extracted from lead ore (galena) bearing tetrahedrite (a complex metal sulphide with some silver) especially at Cwmsymlog, and modelling might indicate where further deposits lie. It is also possible to re-work some spoil tips for minerals, but disturbing spoil tips would expose toxic waste and be uneconomic because of the consequent environmental obligation.