Dr Tamsin Edwards, a lecturer in Environmental Science at the OU, started by referring to an article in Nature (528.115-118): ’Western Antarctic Ice Sheet enters irreversible decline’. Her questions were: How long will it continue? How fast? She emphasised the importance of topography, by showing an aerial view of Antarctica, with the Transantarctic Mountains dividing the continent. To the west are the Antarctic Peninsula, the Amundsen Sea, the Ross Sea and the Weddell Sea.
In places the ice sheet is over 2,000m thick with a base well below sea level. Weertman, Mercer and Hughes have done work on ice-sheet instability, finding that what happens under the ice sheet depends on the bedrock and type of bed. The point where the ice sheet becomes marine is the grounding line; if warm water makes the ice sheet unstable, the grounding line retreats, and this can lead to catastrophic sea level rise. There was a widespread rapid grounding line retreat of Pine Island glacier 1992-2011. The maps showing the Pine Island and Thwaites glaciers can be found at: AntarcticGlaciers.org.
Western Antarctica is the world’s largest Marine Ice Sheet, and at greatest risk of MISI (Marine Ice Shelf Instability, but there is also increased risk in the north and east. Graphs of retreat rate of the ice against the basal friction coefficient showed that the lower the friction, the faster the retreat. After a discussion of the ice loss in Western Antarctica (59Gt/y from 1992 to 2011), another graph showed sea level rise plotted against present day rate of mass loss Gt/yr??, allowing the models to be tested against observation.
Based on the information available, a sea level rise of 30cm is predicted for 2100, and 72 cm by 2200. In summary, there is potential for a large and rapid contribution to sea-level rise from the Amundsen Sea, but ice losses elsewhere will be limited by high bed topography. However current knowledge of retreat rates is limited. A number of questions about time-scales, ocean temperature etc. followed. We were directed to OpenLearn in the Department of Environment, Earth and Ecosystems.
About 20 of us gathered at the Natural History for this outing. We were pleased to welcome some members from other regions. We divided into 2 groups, ours was led by Jenny.
Through a secure door and up to the third floor there is a room the size of an aircraft hangar where there are more than 60 rows of cabinets. Inside the cabinets are hundreds of low and wide mahogany-faced drawers, each labelled, these contain fossils. Most of these have been taken from their original collections, examined, classified and placed in boxes and drawers according to age and type, e.g. all trilobites in one place, ammonites in another etc. There are exceptions to this, for example those collected by Charles Darwin and William Smith.
Our group’s first stop was with one of the curators, Jill Darrel, who talked to us about corals. She showed us a timeline which showed the first corals were about 485Ma at the end of the Cambrian. It showed extinctions at the end of the Permian and the Cretaceous. The coral types are not expanding now because of global warming and sea level rise; there is a biotic response to environmental change. SE Asia is a very diverse area for fossils. In Indonesia there is extraction of Brown Coal for exporting to China. The mud left behind is a good source of fossil coral, it is well preserved. 4 tons of it have been sifted!! Jill showed us some interesting corals, Brain Coral, Vase Coral and Mushroom Coral (free floating, not cemented to the bottom).
Examining the corals with Jill Darrell
We then had a brief look at some fossil Sponges, the vast majority being Cretaceous. The part of the sponge that is found are the spicules (thin rod-like elements) which support the soft body and are found as welded or cemented elements in various shapes e.g. cup-shaped composed of calcium carbonate, silica or a substance called spongin.
Richard Fortey talked to us about Trilobites. These are the most common fossil arthropods and have a tough exoskeleton of CaCO3 (calcium carbonate), which enabled them to survive the harshest conditions and increase their potential for fossilisation. He showed us Silurian Trilobites found in the Wenlock limestone and known as ‘Dudley’ bugs. The Trilobite’s body divides transversely into head, thorax and tail, which articulate with each other. The thorax consists of segments defined by thoracic grooves, the number of grooves is an important species identifier and is also useful for spotting fakes! They also have large compound eyes with many separate lenses. The Cambrian trilobite Paradoxides was the first catalogued trilobite. He then showed us a magnificent example of a whole group of large trilobites in slate from Morocco – the group were living together, probably gathered together for reproduction and killed by a local catastrophe. Similar gathering together for mating is found today with Horseshoe Crabs. Finally, Richard showed us some fossils of soft-bodied organisms from the Burgess Shale.
Quizzing Richard Fortey
Jenny has been working in the museum as a volunteer and showed us some graptolites which she had been working on. Graptolites are good zone fossils and are important for dating Palaeozoic rocks because they changed rapidly through time and many genera had worldwide distribution.
A very enthusiastic young lady called Zoe Hughes then told us about Brachiopods. Brachiopods were originally assumed to be bivalves, but in a brachiopod the two valves are of unequal size and not symmetrical about a medial plane. Thomas Davidson was known as the ‘godfather’ of Brachiopods and wrote a series of 24 beautifully bound monographs, one of which we saw. Zoe showed us a brachiopod which had been ‘prepped’ out, i.e. acetic acid had been dropped on the original to reveal its internal structure. She then took us to an area where special fossils are kept, here we saw: -
Barbara Cumbers and the Brachiopods
This ended a fascinating and informative afternoon. Many thanks to Di for arranging it and to the curators for the talks.
Sam is now at Royal Holloway, and currently writing up his Open University PhD. This was on the Caucasus, and included sampling and dating to better understand their geological history. It was a good talk, illustrated with plenty of photographs and diagrams and very well explained; the interest was proved by the number of questions at the end.
His first question was, why there are volcanoes in the Caucasus and not in the Himalayas? But first he explained that there is better access to the Himalayas so they are better understood. Although the Caucasus are nearer, they were in the USSR and have current “security issues”. Georgia does have good wine and dramatic scenery, but his field work was near South Ossetia, and in an area with poor roads.
This area is a collision zone 600km wide, with crust up to 60km thick. The early crust was formed between two branches of the Tethys, in an opening back arc basin due to convection behind the subduction zone. Eventually there were two subduction zones, with the South Armenian block between the two arms of the Tethys in the late Cretaceous. The Sevan and Zagros sutures were closed by the northward movement of the African plate, which collided around 13Ma. The sediments in the back arc basins now form the Caucasus, including younger Georgian volcanoes, which are unusual as they formed after the subduction ended. The crust shortened by 20km, with normal faults and steep folds, followed by uplift between 18 and 13Ma (or 5Ma, or both) which raised the marine sediments to form mountains, including Mount Kochek (about 2000m ago).
The collision was still continuing at 5Ma, as the Anatolian plate moved west, with north-south compression and east-west shearing. There are more volcanic rocks found in the Lesser Caucasus (to the north) compared to the Greater Caucasus (to the south), the volcanic rocks are mostly 2Ma, with the oldest being 9Ma, including spectacularly well preserved flora in the tuffs (trees and leaves). Then more volcanic activity formed Mount Kazbek, with eruptions dated at 800ka; a stratovolcano with lava flows from the summit, and also flank volcanoes from 10ka. This was formed after the end of the subduction and collision and on the thickest crust – why?
The style of volcanism, with columnar jointing in flows in paleo valleys (some over 4km long), also stacked lava flows, over twenty in 200ka, suggests it was very active – and the lava flows are found on top of the glacial deposits. The lava was intruded through sediments and thick crust suggesting the deep back arc basin rocks were exposed at the surface. There is evidence for explosive eruptions in the middle of the lava flows, from the ends of events, perhaps, and angular breccia in lava flows with columnar jointing, which is unusual. Mounts Kazbek and Ararat are similar, in age, tectonics and rock types, suggesting a similar formation mechanism. The geochemistry is mostly andesite, suggesting a continental arc, and the spider plots of trace elements when compared to primitive mantle show some samples much enriched in mobile fluid elements, and some significantly depleted. This is a common subduction pattern in arc blocks: a subduction signature. As Mount Kazbek has an arc signature, is there an arc under the Greater Caucasus, though there are no active volcanoes now? There are paleo hills from volcanic eruptions, hot springs such as in Bodrum (the source of Stalin’s favourite mineral water!), and some lava dates to 6ka. Much geophysical surveying has been carried out, as the area is rich in oil. Is this interpretation right; is a sinking slab and active subduction needed for these?
Subduction stopped and the collision started at 15Ma, thus the sinking crust is cooling and no longer receiving water, with hydrated lithosphere “frozen” beneath the crust for over 10Ma. The subducting slab levelled off along the mantle to end up beneath the Greater Caucasus, with the melt rising through the crust, still retaining its chemical signature, and erupting at Mount Kazbek, with similar occurring at Mount Ararat over the Zagros suture.
There may not be an ark under Mount Ararat but there is an arc signature and a flood of water from the mantle!
Professor Gideon Henderson FRS is a geochemist, Head of the Department of Earth Sciences at the University of Oxford, with a particular interest in the geochemistry of stalagmites with reference to climate change. The composition of the atmosphere in the past can be reconstructed using ice cores from Antarctica and Greenland, and coral reefs allow us to reconstruct sea level changes over 30,000y, but what about areas far from ice sheets and coral reefs: continental interiors?
Hence the importance of caves and stalagmites, caused by the re-precipitation of limestone dissolved by rainfall. With annually banded layers, stalagmites can be dated using the decay of naturally occurring uranium. Stalagmites need 3 things to form: carbonic acid (from the dissolution of limestone), rainfall and oxygen. If the ground above is frozen by permafrost rainfall cannot penetrate and stalagmites cease to grow. Thus they can be used as a proxy for past climates, by the use of a mass spectrometer.
A major factor is the Siberian permafrost, which contains 1700G tonnes of carbon, twice that in the atmosphere at present. The first example was a cave at Okhotnichya, right on the edge of the permafrost boundary, where the record goes back 400,000y. From 3 caves at different latitudes it seems that 1° warming is not enough to cause melting in the far north, but 1.5° is. The more northern cave is Ledyanaya Lenskaya with 1° warming. There is evidence that the permafrost used to extend further south.
Heading further south rainfall over time has been studied in the Heshang Cave in Wuhan (China) with stalagmites whose growth can be studied at very high resolution over 9,000y, enabling scientists to chart how much it rains over time. A comparison with Dongge Cave reveals an 8.2ky event as the driest period in the Holocene, whose best explanation is a change in ocean circulation, caused by catastrophic ice failure in the North Atlantic, affecting the Asian monsoon.
It remains necessary to make very careful records of these stalagmites, since they are not well-conserved in situ.
It was a pleasure to welcome Tony back as our speaker. With many years’ experience as an OU tutor at Summer school, he told us this was the first time he had attempted this sort of exercise. We arrived to find examples of different rocks laid along the benches, mainly limestone and sandstone.
We started with a familiar slide of the rock cycle, reminding us of origin of igneous, sedimentary and metamorphic rocks. This talk would be mostly sedimentary, but with some reference to granites. He then outlined the requirements for a successful building stone: appearance, availability, appropriateness (to the locality) and last, but important, affordability.
Practical part 1 dealt with Portland Stone, an Upper Jurassic (Tithonian) oolitic and bioclastic limestone, formed in shallow marine conditions. Its use was illustrated by the Cenotaph in Whitehall, St Paul’s Cathedral, and the Whitbread Tower in Bury. It comes in three layers: the Portland Base bed, the Portland Whit bed and the Roach.
Practical Part 2 dealt with Red and Pink Granites, starting with Shap, but including Peterhead and Helmsdale etc. with a reference to Shap blue, emplaced into Silurian sedimentary and Ordovician volcanic rocks. Dark patches within the granite are referred to as enclaves, whereas there are also xenoliths: blocks of Silurian country rock. Whatever the method of dating employed the calculated age comes out as 393my. Examples of the use of granite included the Voortrecker Monument in South Africa, the Mormon Temple in Salt Lake City, and Marshall College.
Finally, we came to the Red Sandstone, starting with a natural arch in Utah, often Permian / Triassic, but can also be Proterozoic (Oregonian), Cambrian (Caerburdy), Devonian (Arbroath) or Cretaceous (Hunstanton Red Rock, a red carbonate). It needs only 4-5% iron oxide to give a dramatic colour.
Tony ended with useful websites:
and 3 testing laboratories:
He also recommended P.A.Floyd: Building Stones and Stone Buildings of Staffordshire.
We were then turned loose on the rocks with hand lenses to see for ourselves the different qualities of the different rocks. Many thanks to Tony for donating most of his samples, some of which were taken by individuals, and a representative selection by Di Clements for the use of members of London Branch for study or teaching purposes. These can be borrowed, as can fossil casts etc. by contacting her at: firstname.lastname@example.org
Diana Smith led us on a tour of Pembrokeshire geology and “In the footsteps of Gerald of Wales”, with readings from Giraldus Cambrensis (c. 1146 – c. 1223). Gerald was the Welsh-Norman Archdeacon of Brecon and historian, and his travel writing includes Welsh history and geography. Thus we visited both geological and historical locations, and learnt the history as well as the geology. His books are worth dipping into, though not always kind about the Welsh. We were based in Haverfordwest, in the Mariner’s Hotel.
The aim of the trip was to explore the geology of Pembrokeshire in West Wales north and south of the Landsker Line. This line is not one of geology but of culture, above the line the influences on language and place names are that of Celtic Wales, the castles are few and the churches are small and simple. South of the Landsker Line the English influence is thought to link back to the Norse settlers in the 12th century. Culture was important too as we were going to be sharing the thoughts of Gerard of Wales as we followed part of his journey while exploring the geology.
Our day started, after breakfast of course, with a short stop in a lay by on the A487 to look through a hedge across flat land to Roche Castle standing above the landscape on an outcrop of Precambrian flinty Rhyolite 1,400m thick. The rhyolite is between 613 and 625 Ma old with the chemical signature of weakly metamorphosed mantle similar to that of the Uriconian group found in Shropshire. We did not get close enough to study it in detail but its height above the landscape is a testament to the resistance to weathering of these Precambrian rocks.
This of course is why the site was chosen for the castle. It was built in the second half of the 13thcentury by a Norman knight Adam de Rupe as part of the defences across the Landsker line. Since its beginning the castle has had a varied history with different owners who have all put their mark on it. It is now a first class hotel. More information can be found on www.castlewales.com
The second part of our journey was to stop in the car park south of Newgate beach to get an overview of the ‘berm’, a storm beach that has been in existence for about 2000 years and is still undergoing constant change due to longshore drift. The 6000-year-old fossil forest at the south end of the beach, which is periodically uncovered, was seen by Geraldus Cambrensis (Gerald of Wales) during his tour of Wales in 1188 AD, described in his book and shared with us through a reading by Di Smith. The sediments are diverse; they are made up from tillites, including glaciated boulders, from as far afield as North Wales, the Lake District and Scotland all left behind after the retreat of an ice sheet. We then transferred to the west end of the beach for a closer look at composition and structure of the local sediments. There we found well-bedded silts, sands and muds with bedding planes and surface features which suggested that the beds were originally horizontal. The conjugate joints and the 500 dip were evidence for tectonic movement following the Caledonian trend. On the beach there were 500Ma Cambrian sediments faulted against Middle Coal Measures, an unconformity of approximately 2Ma and in the cliff much younger Quaternary deposits. A site that brings home to us the huge time scales that we can find in our geological record. We indulged in light refreshments to give ourselves time to reflect and think before we moved on to our next stop.
The next part of our day started at Caerfai Bay from where we walked along the coast path above St Non’s Bay passing St Non’s well on the way to St David’s; a good mixture of geology, myth and legend. The objective of this walk was to recognise the different Cambrian lithologies, determine their direction and amount of dip and finally to see how the geology shaped the making of St David’s Cathedral. At the back of the bay the rocks are Precambrian. Towards the front of the bay the Cambrian strata is younging with a very steep dip of 700to 800 which is indicative of these rocks being the southern limb of the St David’s anticline. We found what could have been the basal conglomerate of the Caerfai group with clasts of 30 cm, not confirmed because it was not in situ. We saw sandstones with mica matching the description of the St Non’s sandstone laid down in water and some of the reddish Caerfai bay shales. Further along the path there were some of the sandstones of the Solva group, probably in situ, but it was difficult to determine where they were in the succession. The Caerbwdy sandstones were the focus of our attention as we walked around St Non’s Bay. These are massive beds up to 3m thick of purplish fine to medium grained, poorly sorted sandstone with micas and feldspars and interstitial clays with evidence of bioturbation. We took particular notice of these beds as we would be looking for them later in the day. The beds coarsen towards the top and have some lithic fragments which can be green, people do not know why. Taken together this is evidence that the beds were deposited in tranquil areas below wave base but shallow enough to maintain the oxygen levels. We followed the path along cliff edge with indents formed by the erosion of the less resistant shales all the time moving into older and older rocks. At the back of the headland were the dark green slates and tuffs of the volcanic Ogofgolchfa Group of the Pebidian. We had walked 200 million years between the top of the basal conglomerate of the Cambrian and the top of the Precambrian – quite something when you think about it.
But we were not finished yet. We reached the site of Chapel of St Non. She holds a very important role in the history of Wales. She was born around AD475, lived as a nun and was raped by Prince Sant of Ceredigion and subsequently gave birth to a baby boy who became St David, patron saint of Wales. We also saw the holy well close to the site of the chapel. It was thought to have healing powers, particularly for complaints to do with eyes and is still visited today by people hoping for healing and good luck. All too soon we left this peaceful place and made our way back to St David’s and lunch full of enthusiasm to see the great Cathedral and find some of the rocks that we had seen on our walk.
Driving into north Pembrokeshire we stopped on a flat plain to look at a solitary hill, which Di described as a monadnock. She explained that the name comes from Mount Monadnock, an isolated plug of Ordovician igneous rock in New Hampshire. The 180m high Welsh volcanic hill, Carn Llidi, certainly stood out and it has an interesting provenance, being part of a layered gabbro intrusion.
During the Ordovician vast quantities of lava and ashes poured into the basin areas of Wales and the Lake District. Their sources were the island arc volcanoes that formed while the Iapetus Ocean was closing and as Eastern Avalonia containing southern Britain split from Gondwanaland and moved northwards. So fragments of ancient ocean floor have travelled about 9000 miles to become part of the mosaic of the Pembrokeshire rocks.
In the St David’s Head area, as part of the Llanvirn sequence, distinctive sheet-like layers of gabbroic and related rocks intruded into the Ordovician sediments. Petrological and geochemical examination suggests a complex evolution, with multiple magma injections and magma chamber crystallisation. Typical Caledonian NE/SW folding (End-Silurian) produced a tight NE trending syncline of this complex, with two linear opposing limbs of layered intrusion. The northerly limb outcrops for about 2km along the north-east coast of St David’s Head and the southerly, parallel and inland, contains Carn Llidi. Similar intrusions include Mynydd Preseli, the ‘Stonehenge’ Bluestone quarry site.
The present landscape evolved from uplift of the area and the subsequent peneplanation of the marine platform, dating from the Tertiary. Erosion of the wave cut platform to around 60m above sea-level left high stands of resistant igneous rock. We also paused to look at Llanvirn Farm. This is the area where Hicks (1875-81) first defined a series of rocks, from specific graptolites and trilobites, that distinguished it from the earlier Arenig and the later Llandeilo. Thus the Llanvirn, a further Ordovician epoch was added. Generally during this epoch dark mudstones with acid tuffs were the main sediments in low energy deep marine conditions
The Abereiddy region is the type area for the Llanvirn and lies on the north limb of the St David’s Anticline. The local structure of Abereiddy Bay has been shown to be a parasitic syncline with an overturned north limb and, unlike Caledonian folding elsewhere, shows the strike of the beds to be near E/W. The axis of this Llanrian Syncline runs through the bay, hence the sequence to the north is regionally inverted. The youngest rock, the Llandeilo-Caradoc Shales, in the core of the fold has eroded to form Abereiddy Bay with its distinctive black sands.
We parked on the shore car park, (Figure 1), which is in a glacial meltwater channel, and becoming smaller as the sea has already demolished the protective sea walls. Nearby cottages have this local shale on the roofs. These ‘slates’ are poor quality resulting from the low grade regional metamorphism. Tar or pitch coating, to ensure weatherproofing from driving rain, has to be maintained.
Figure 1. Glacial meltwater channel
Watching the tide, we went across the Llandeil-Caradoc shale to the southern margin of the beach. Here slippery rock faulted into gullies made progress difficult but we could see the older Caerhys Shale Formation (Upper Llanvirn). This contains the 'tuning fork' graptolite Didymograptus murchisoni which existed for a short time from about 470 to 464 mya. Fortunately, plenty of fallen material was available on the beach, which we successfully sorted. The bedding and cleavage are more or less aligned here giving good specimens on broken rocks, with some that have been stretched by tectonic forces. These graptolite planktonic organisms were well distributed floating in the oceans and then preserved in dark anoxic fine sediments on the sea floor. Their evolution in the Ordovician has made them useful zone fossils, (Figure2).
Figure 2. Graptolites
Walking south, on the road uphill, we stopped by the wall to our right. Di used the structure of some blocks as a model to tell whether a slate bed is inverted. If the cleavage is less steep than de bedding, then the stratus is overturned. On the other side of the road the slate outcrop was not so easy for us to decide. The bed is right way up as were all those on the south shore, (Figure 3).
Figure 3. Effect of ice action
Interestingly this contorted exposure shows the effect of ice action. Freeze and thaw opened up cleavage and removal of overburden caused slumping, the soil profile seen above has built up at much later date. Llanrian Volcanics (Upper Llanvirn) south of the Bay, include th Murchisoni ash band, Abereiddy Tuff, (Figure 4) which consists of lapilli (basaltic pyroclastic fragments around 3mm diameter) in well bedde grey fine-grained tuff. This is very similar to a particular volcanics facies found in the Lake Distric where it is green.
Figure 4. Abereiddy Tuff
Retracing our steps across the car park we joined the Pembrokeshire Coast Path, walking uphill through National Trust land. The path became steeper as we crossed the band of Castell Limestone Formation (Llandeilo) that has resisted erosion. This impure limestone was formed while water depth allowed reef growth. We reached the edge of the flooded quarry known as the Blue Lagoon, (Figure 5).
Figure 5. Blue Lagoon with diving platform
Good quality slate (Llanrian Volcanics) was successfully extracted, from the 1850's until 1901, and some buildings still remain including one that housed a hydraulic lift. The sea later broke through creating a sheltered harbour up to 25m deep in parts and was useful for local fishing boats. Now coasteering is popular here and high diving competitions (with up to 30m boards erected) take place. The greenish colour of the water derives from minerals leaching from the rock and slightly contradicts the name of the bay! From the viewing platform we could see the bands of shale in the cliffs corresponding to the Caerhys Shale Formation on the south side of the bay, (Figure 6), remembering that on this norht side all the beds are inverted. On our return walk we looked in vain for an outcrop of the Castell Limestone in the vegetation but without success. Fortuately, we found the ice cream van to finish a fine morning.
Figure 6. Shale bands in cliffs
We arrived in Porthgain in time for a fine lunch at the Sloop Inn. These days Porthgain is a small coastal hamlet with a sheltered harbour used for fishing boats and pleasure craft. However, it was once a bustling industrial village which owed its existence to the surrounding geology. Now the villagers rely on tourism and the export of crab and lobster to Europe. Slate quarrying began in the mid-19th century and Porthgain shipped out not only their own slate but that of nearby Trefin and Abereiddi. We climbed 70 steps up the side of the cliff to the west of the harbour to view the now disused slate quarry. The verically bedded mudstone shales belong to the Penmaen Dewi Shale formation of the Ordovician which rest unconformably on the Upper Cambrian Lingula Flags. The slate extracted here was not of the best quality for roofing as its porosity required an additional coating of lime mortar to seal the roof. The harbour pilot's house and some of the older cottages in the village still have their lime mortared roofs. The slate was also used as slab for flooring, window sills, lintels, doorsteps etc. The slate did not cleave well because of its silty nature and no more was used after 1910.
Figure 7. Maiden Castle, Treffgarne
Brick making began in 1889 using slate waste from the quarry. A 137m long tunnel was driven from the slate quarry to the quayside below enabling the waste to be brought directly to the brick works where it was crushed and refined by a water powered mill. Up to 50,000 bricks per week were produced in Porthgain. The brick making shed is now used as a restaurant named, appropritely, The Shed, whilst the restored brick drying shed appears to have no use except as a target for window breaking vandals! This industry ceased in 1913. Roadstone quarrying began in 1889 at a coastal cliff site a short distance west of the slate quarry. The stone although called (granite) was an Ordovician dolerite intruded into Ordovician sediments of the Arenig series. We walked along the coast to this two-benched quarry viewing it first from the top and then walking down the disued tramway onto the first bench. Benching is a good quarrying technique allowing the quarrymen to excavate deeper and faster. The invention of dynamite in 1867 also made the quarrying of this rock a viable proposition. A steam powered crusher located above the harbour reduced the rock to various grades and dropped it down into a giant hoppers constructed in the locally made brick on the west side of the harbour where it was stored before being shipped to such places as Somerset, Devon, Kent, London and Ireland. In 1909 exports totalled around 4,000 tonnes per month. However, demand for roadstone dwindeled as more vehicles started using rubber tyres and despite remodelling the harbour to allow larger ships to enter, the road stone trade at Porthgain ceased in 1931. The National Trist acquired the workings in the 1950s which remains as an impressive monument to the past industrialisation of Porthgain.
Burning limestone with culm (anthracite dust, shattered coal and clay) to produce a sweetener to improve the acidic soils had been practised in Pembrokeshire since the Middle Ages and most coastal villages with a harbour would have had one or more lime kilns. Porthgain was no different and there is still a pot kiln on the east side of the harbour, the oldest industrial building in the village. Before industralisation Porthgain's fishing harbour was also used by small boats bringing in limestone and coal or culm. During the industrialised period it was the larger boats carrying away the slate, roadstone and bricks from Porthgain which brought in limestone from quarries near Milford Haven and coal (anthracite) from South Pembrokeshire. Before the development of Portland Cement, lime was the basis of the mortar used for building purposes. Lime also had antiseptic properties when used as a wash for both internal and external walls.
Figure 8. Maiden Castle, Treffgarne
We left Porthgain after an interesting afternoon's exploration and headed towards Maiden Castle, part of a Precambrian rhyolite outcrop of about 600Ma. On our way to the outcrop we passed an Iron Age hill fort which unfortunately was too overgrown to explore. At the outcrop we were shown some auto brecciated rhyolite which occurs when rhyolite is highly extruded, breaks up internally and forms rhyolite clasts within a rhyolite matrix. Poll Carn is a similar outcrop in a nearby field. These rhyolitic outcrops could be called monadnocks as they have resisted erosion for some 600Ma, probably standing as islands in the sea. Monadnocks are defined as isolated rock hills, ridges or small mountains that rise abruptly from a gently sloping or level surrounding plain, the word coming from Mount Monadnock in New Hampshire, a plug of Ordovician igneous rock. Given half a chance Maiden Castle and similar outcrops will still be there in another 600Ma.
On the Saturday morning we drove from the hotel to the National Trust car park at St Martin's Haven on the Marloes peninsula, which
lies somewhat to the south of west of Haverfordwest.
The trust owns and manages much of the land on this part of the peninsula, and St Martin's Haven contains the landing stage for boat trips to Skomer Island.
Skomer is an important breeding sanctuary for seabirds, a national nature reserve, a Site of Special Scientific Interest, and a Special Protection Area, surrounded by a Marine Conservation Zone. It is owned by Natural Resources Wales, but managed by the Wildlife Trust of South and West Wales. It is also a scheduled ancient monument because of sites of archaeological interest dating from the iron Age and earlier, and is the type locality of the Skomer volcanic Group.
Our geological goal for the morning was to examine some of the rocks of this Group, but it is difficult to do this on Skomer itself. Landing on the island is restricted and subject to a significant fee, and most of the exposures are on cliffs which are not safely accessible for groups without special equipment. Accordingly, we were to look at available exposures on the peninsula (the western part of which was once joined to, and provides a geological continuation of, Skomer and its associated islets). We then took a boat trip across to the island shores, to catch sight of the cliffs and the wildlife. First we went down to ensure our booking for the boat trip, which had to be made on the day. We then re-assembled in the car park to hear from Diana about the geological background before we moved off. The rocks in this area are of early Silurian (Llandovery) age, and represent an inlier which was thrust up during the Variscan orogeny.
During early Silurian times, sea levels were generally rising, following a fall during a late-Ordovician glaciation, and this locality was in a shelf area of the Welsh basin. Referring to a map from Gareth George's Field Guide to the Geology of South Wales, however, Diana said that the waters were relatively shallow (as determined by fossil assemblages): they were supplied with relatively coarse sediment from land close by to the South. At the time, the Iapetus Ocean was closing by subduction beneath Avalonia, and this, together with local rifting which was forming half-grabens, gave rise to intermittent volcanism creating volcanic islands. (It appears from the literature that geochemical analysis of the lavas shows the signatures both of island-arc and of intra-plate magmatism.)
The Skomer Volcanics are the youngest volcanics in the Welsh basin: they constitute a diverse suite of lavas, and ashes derived from them, being mainly extrusive, or intrusive at shallow depth, but occasionally pyroclastic. Their strike is along the east-west axis of the Marloes peninsula: on the mainland they are found as far east as St Ishmael's, and in the west they are exposed on Grassholm Island and The Smalls, respectively some 11 and 25 km west of Skomer. They total some 900 m in thickness on the islands, but thin out to 200 m on the mainland. They range from mafic to felsic, and include the eponymous altered form of andesite, skomerite. However basaltic lavas are the most common. Some of these take the form of pillow lavas, but others may have been weathered sub-aerially following deposition. That would be consistent with the lavas having been extruded in a relatively shallow marine environment, and forming islands.
Moving away from the car park, and walking downhill towards a faulted area, occupied by a path, and which opens out by the sea shore to form St Martin's Haven, we stopped by an exposure of blocky brown tuff, perhaps andesitic which showed columnar cooling joints. Diana described these volcanics as the last gasp before the closure of the Iapetus. Reaching the path at the bottom, we passed up through a gate, and saw a boulder of mafic ash with clasts, but not in situ, and another erratic boulder which looked as if it had come from a flash-flood deposit.
We then walked uphill, on the land known as the Deer Park, apparently from an abortive experiment to stock deer in the 1920s, to the site of an old Coast Guard look-out, now only occasionally manned by Coast Watch, which stands on a basaltic flow extending westwards to the tip of the peninsula. This offered a fine view of Skomer and of Midland (or Middleholm) Island, the much smaller island which lies in front of it. We also had a view over to the more distant island of Skokholm to the south. The names of these islands tend to be Norse, in consequence of the significant pillaging and trading activities of the Vikings in this area. Skokholm is geologically distinct, being formed of Old Red Sandstone. The Old Red Sandstone is a litho-stratigraphic group mainly associated with the Devonian, but which began to be laid down in the Silurian.
Referring to a sketch-map of the local geology prepared for his book on Pembrokeshire geology by John Downes, who would be leading us the following morning, Diana said that the north of the peninsula was formed by predominantly andesitic lavas, with a terrace of basaltic lavas on high ground in the middle, and sedimentary rocks in between, and to the south. However, the area was much sliced up by faulting, to create a more complex picture, and at the tip of the peninsula, volcanic rocks tended to form the coastal headlands, while sedimentary rocks were eroded to form the inlets.
We noted from our view, that the land on the peninsula to the south tended to be lower-lying, no doubt as a result of being composed of sedimentary rocks which were unprotected by the lavas and more prone to erosion. Some of us had also noted a sweet fragrance blown on the wind; it was the perfume of a brilliant yellow waterfall of ‘prostrate broom’, draped just below the cliff edge, one of its rare occurrences in the area. We looked at the polygonal jointing on the basalt at our feet. This had on it much lichen, which likes iron and magnesium. Diana reminded us that the jointing lay at right angles to the cooling surface. The surrounding soil was also very brown, characteristic of the breakdown of mafic rocks.
Moving towards the southern shore, some of us saw a chough, a very rare protected bird. We next saw an arch eroded out of a basaltic-looking flow in an inlet. Looking over a cliff further on, we saw on a headland a distinct sequence, dipping in a southerly direction, with massive rocks at the base, followed by a well-bedded series in the middle, and then lumpy blocks at the top. Capping the promontory was what appeared to be a very tough sedimentary rock in situ, rich in quartzite. Diana commented that the volcanism was only intermittent, over fairly long periods of time.
Moving back to the top, we walked back down to the path leading to St Martin's Haven. While waiting to be allowed onto the boat, some of us looked at the displays relating to the Marine Conservation Zone displayed in offices at the Haven. The vessel which was to take us to see Skomer, the Dale Princess, was not large, and was well packed with our party and some others. However, it seemed well equipped, and its wheelhouse boasted a modern electronic chart which constantly displayed the boat's position. In addition to carrying out his nautical duties, the skipper was able to furnish us with a fluent commentary on the wildlife. After leaving St Martin's Haven in a northerly direction our course took us to the west, past Jack Sound and Midddleholm Island.
We learned that Jack Sound, with its strong tidal streams, had played a critical role in the development of Skomer as a breeding ground for seabirds, after the islands were cut off from the mainland by rising sea levels at the end of the last ice age. That acted as a natural barrier to prevent ground predators (such as rats, snakes and foxes) coming from the mainland. Humans also exploited that barrier in medieval times by using the islands as a breeding ground for rabbits. The rabbit population is now significantly diminished, but their burrows provide nesting accommodation for two of the most significant species of seabirds to breed on Skomer in the summer, puffins and Manx Shearwater. There are apparently about 10,000 breeding pairs of puffins on Skomer and Skokholm between mid-April to July, so it is one of the most significant colonies in the UK. There is a much larger colony of Manx Shearwater, about 300.000, and together with 40.000 on Skokholm, this is the largest breeding colony in the world. However, they are not easy to see, being nocturnal. They have a remarkable migration pattern, leaving after fledging to spend five years off the coasts of South America before returning for breeding.
Skomer also has large populations of other birds, including guillemots, razorbills, kittiwakes and gulls. We passed Middleholm, where we saw razorbills, kittiwakes and fulmars, and then the Neck of Skomer, which is joined to the rest of the island by a very narrow isthmus separating North Haven from South Haven. It seems likely from the geological map that the area now occupied by the two Havens was predominantly filled by sedimentary outcrops vulnerable to erosion, and Diana opined that the isthmus was not long for this world. We put into North Haven, hove to, and the engine was switched off for a few minutes, the better for us to enjoy the birdlife. However, we were reminded that nature is "red in tooth and claw" when we saw a greater black-backed gull take a puffin.
On the way back, we learned about the Skomer Vole, a sub-species which apparently developed from common voles after their accidental introduction by Man during the Iron Age. It seems they thrive on bluebell seeds, and on bracken, which is a carcinogen for most animals. On our return to St Martin's Haven, we went down to the eastern side of the shingle beach, where there are basaltic flows of aa-aa. (On the other side of the bay, and of the fault which controls it, is an outcrop of a conglomerate of reworked rhyolites and tuffs). Among the lava flows was a pyroclastic flow containing reddened basaltic blocks with calcite veining, and coarse ash with epidote and very nice lapilli. Having looked at this, we had lunch on the beach before returning to the cars.
The weather continued bright and sunny and was perfect for a hike along the top of the cliffs at Castle Martin. The group had been joined by John Downes who was to be our joint leader for the day with Diana Smith.
From Stack Rocks car park the group proceeded in an easterly direction. These spectacular cliffs were formed during the Lower Carboniferous (or Dinantian) time between 354 and 327 million years ago. In Pembrokeshire, the carbonate succession is over 1000 metres thick in the area we were visiting but reduces to 500 metres further northeast near Tenby. During the Late Carboniferous/Early Permian times, powerful Variscan tectonic forces folded and faulted the rocks of South Pembrokeshire. Our walk started with a view of the ‘Green bridge of Wales’ where John pointed out the Stackpole Limestone, well bedded with a slight north dip. The Arch has been created by sea erosion. Across the top of the cliffs there is a coating of Terra Rossa. As limestone weathers, the clay contained in the rocks is left behind, along with any other non- soluble rock material. Under oxidizing conditions, when the soils are above the water table, iron oxide (rust) forms in the clay to give that red to orange colour.
All along the cliffs at this point guillemots were nesting and we continued our walk a few hundred yards to Stack Rocks (Elegug* Stacks). The Terra Rossa is sometimes up to 1 metre thick in places and a small fault coming inland was present with the stacks themselves showing a flat surface. The plateau surface was clearly visible here as an indication of previous sea activity. Large numbers of guillemots, fulmars and gannets find the stacks a useful nesting site. Continuing east we came upon a promontory known as Flimston Castle on which stand the ramparts of an Iron Age fort. This area is dominated on the west side of the promontory by the Devil’s Cauldron. This is a classic development of an inlet along a geological fault. John explained, that this was a swallow hole, or a blow hole in which the sea surges through during stormy weather. It probably results from karstic weathering when the Carboniferous Limestone in the area was exposed at the surface and subjected to the effects of acid rain.
Walking down the path on the east side of the promontory towards the sea John was able to position us for a magnificent view of a Gash Breccia deposit which marks the line of the Flimston Fault. This may have been formed, in Triassic times, when a limestone cavern collapsed and floods infilled the void with clay and large boulders. Some of the larger vehicle-sized boulders were clearly visible. There is an alternative possible explanation that the breccia could be created by tectonic forces. The nearby folds give evidence of such movement.
The Flimston fault is a major wrench fault that runs NW-SE from Freshwater West and offsets the Bullslaughter synclinal axis by some 750 metres. The Stackpole Limestone is exposed on the coast west of the fault and the Crickmail Limestone and Bullslaughter limestone outcrop in cliffs. John drew our attention to the handout which showed the E-W axis of the Bullslaughter Bay syncline and the strata dipping steeply southwards. He then pointed out the beds further east dipping northwards showing the syncline axis was now lying to the north of Langstone Down.
*Elegug is the welsh for Guillemots
Figure 9. Flimston Castle
On Sunday afternoon we went on to Manorbier Bay, parking by the birthplace of Gerald of Wales in 1146. This is a fortified mansion (though more of a castle) built of the overlying Carboniferous Limestone, and was where Gerald grew up until joining the Benedictine Order in Gloucester. It would be an ideal place for a child, with castle, sea, cliffs and woods to explore.
Di reminded us that this trip was ‘Following in the footsteps of Gerald of Wales’ and therefore we took a few minutes to listen as she read from Gerald’s journal, giving his impressions of the locality. His glowing recommendation of Manorbier finish by him declaring “You will not be surprised to hear me lavish such praise upon it, when I tell you that this is where my family came from, this is the place where I myself was born. I can only ask you to forgive me”. We walked along the coast to the south to see sedimentary structures in the steeply dipping Moor Cliff Formation in the Milford Haven Group of the Old Red Sandstone (Upper Silurian), this is a marly, red sandstone. It formed in a semidesert environment with alternating wet and dry seasons: marl deposited in shallow lakes and watercourses in wet periods and aeolian dune bedded sands deposited in the semi desert conditions
We passed Kings Quoit, a cromlech with a dramatic location overlooking the bay. Then on to look at the three tuff beds which strike across the bay. The first is the Pickard Bay Tuff, and seen as a cleft in the cliff, originally deposited as a volcanic air fall, then the Townsend Tuff, which is thicker at two metres, then the Rook's Cave Tuff, narrower at 300mm wide. Some of us got down into the trench where the Rook's Cave Tuff has been eroded out and managed to find evidence of burrows and faecal pellets in the underlying mudstone (this was mostly done by feeling the surface). We could also see how this ash band had been offset by a sinistral strike-slip fault. The tuffs are preferenially eroded as they are weaker than the sandstone. The most prominent is the Townsend Tuff, a marker horizon with the Freshwater West Formation which can be traced across the whole of South Wales and the Welsh Borders.
Nearby calcretes can be seen in the sandstones, these are perpendicular to the bedding plane as they are formed by capillare action, from the calcium rich waters from the overlying limestones. These mineral rich waters leach down in wet seasons, and then are drawn up by capillary actions in dry seasons, filling the voids left by roots and burrowing animals. Cross bedding structures can also be seen, as can pseudo-anticlines caused by locally greater hydrostatic pressures due to locally increased porosities. The headland is known as Priest's Nose, an unkind jibe at Gerald, perhaps?
Figure 10. Gerald of Wales
We finished the afternoon with an ice cream, and we thanked John Downes for some very well explained and super geology. His book on the area is well worth a look, see my brief review elsewhere in LP. (page 20)
On the Monday morning we went into the Pembrokehire Coalfield and to Saundersfoot, into the Westphalian Carboniferous and to see the effects of Variscan Orogeny. This was once a prosperous coalfield, producing anthracites, from the fourtheenth to the twentieth centuries.
At its peak, the last colliery, Bonville's Court, produced 30,000 tons a year and the seat earths were also used for brickmaking. At Saundersfoot colliers were being beached at 30-40 a tide, filled and refloated. In 1829 the harbour was rebuilt and was then no longer tidal. Unfortunately, while the coal burned well and has a high calorific value, and was the only coal Queen Victoria would allow in her Royal Yacht, the thin seams are badly faulted and hard to work, and the last deep mine closed in 1930, with drift mines working until 1949.
The coalfield deposits are in a synclinal basin, with preferential erosion of the softer deposits; the setting of the Saundersfoot area is analogous to Lulworth Cove, with fault controlled erosion by major rivers. The mudstones and sandstones were laid down as muds and sands close to a river mouth during the Carboniferous Period, around 350Ma. In the sequence are plant fossils and thin coal seams, formed in the tropical swamps. Twenty million years later, as southwest England joined to the UK, these rocks were crumpled into north-south synclines and anticlines by the Variscan thrusts. The Lady's Cave fold is a classic straight sided anticlinal fold, and often pictured in textbooks.
Figure 11. De la Beche map of Pembrokeshire
We ended with a vote of thanks for Diana Smith for leading, and Sue Vernon for organising our tour of Pembrokeshire in the footsteps of Gerald of Wales - thoroughly enjoyable with a plethora of geology and history, and a most excellent hand out. And then we finished with hot chocolate and cake!
As Head of Marine Geoscience at the National Oceanographic Centre in Southampton, Dr Angus Best has made a particular study of the effects of global warming on methane gas hydrates, particularly within the Arctic Circle where the greenhouse effect is most pronounced.
He started by defining methane gas hydrates as ‘Fire Ice’, a type of solid clathrate with no chemical bonds, that form in high pressure and low temperature conditions using Van der Waals forces. Their main occurrence is at Continental margins, though they can exist on land. They can be stable in the water column and on the sea bed depending on pressure and temperature. They can be very solid and form mounds near vents on the sea bed. Angus showed a video clip of methane rising from the sea bed.
Most of the evidence comes from seismic reflection, using a BSR (Bottom Seismic Reflector). The hydrates are a major energy resource, at least as large as conventional reserves. While a very dense source of energy, they are not without danger and can cause instability on the sea bed, triggering slides such as Storegga (8ky) with accompanying tsunami.
An increase in ocean temperature can cause hydrate dissociation, referred to in discussions of past climate change as the clathrate gun. Greenland ice cores show evidence of repeated episodes of methane release at times of ice melting. (Kennett et al.2000 PNAS) The continental slope is vulnerable to such gas escapes. Therefore, research cruises have been undertaken by the NOC and the University of Southampton with a key discovery in 2008 of ongoing gas release at the landward limit of the hydrate stability field, where the mean temperature at 300-45m water depth has increased by 1°C in 30y. Seismic surveys are used to work out how much hydrate is there, but so far the results are inconclusive.
Our intrepid team met in the car park near Gill's Lap, with local members Patrick and Christine Frost kindly coming along to wish us the best of luck, and then returned to their dry home. "What day is it?" "It's today," squeaked Piglet. "My favourite day," said Pooh." (A.A. Milne) This walk is one of Brian Harvey's Geowalks, first done in 1988 I believe, and Iain Fletcher led the geology while Di Clements read from the Great Bear's books at appropriate locations.
Iain explained our location at the top of the Weald, on the Hastings Beds, the lowest of the Cretaceous deposits in the Weald - and apologised for the lack of exposures and, in view of the weather, the lack of the views. We started off by heading into the Enchanted Forest, down to the Heffalump Trap (empty, I'm sad to report) past Roo's Sandy Pit to the Milne and Shepherd Memorial, an excellent viewpoint in better weather and down to some real geology in the old sand quarries.
Figure 1. In the Heffalump trap!
In early Cretaceous times the Weald was at 30 degrees north, and was an alluvial mud trap between the high ground of the London Platform and Portsdown High. The Weald was bounded by reactivated Hercynian thrust faults, reactivated as extensional faults, hence the basin. This area would have been freshwater pools, river banks and brackish lagoons, with muddy and sandy channels. Dinosaurs would have grazed on horsetail plants, distrubed by occasional earthquakes from the bounding faults. These raised the highlands and increased erosion and river flows. Rising sea levels later in the Cretaceous inundated the basin and the marine Greensand and Chalk deposits were laid down and deformed into the current anticlinal structure by the Alpine Orogeny. Then 1500m was eroded from this area, to leave the current profile.
Figure 2. Poohsticks!
Having looked at possible (to us) sites for Wol and Eeyore's houses, we then went past the Five Hundred Acre Wood, on an Exposition to the North Pol, past the Roman Road and then the rain stopped ("The nicest thing about the rain is that it always stops. Eventually." Eeyore), as we circled around Eeyore's Sad and Gloomy Place, and we came back to the car park and an ice cream
Figure 3. Leaving the Enchanted Forest
Thanks to Iain for the geology and Di for the literary input.
And a geologist's motto? "When you see someone putting on his Big Boots, you can be pretty sure that an Adventure is going to happen." A.A Milme, Winnie-the-Pooh
As a geotechnical engineer working with SKM, now Jacobs, Alex went to Guinea in West Africa to advise on the stability of the slopes and access roads constructed with a view to exploiting the iron ore reserves there. The ore deposits are at two locations in the south-east of the country: Simandou and Mt Nimba, close to the borders with Liberia and Sierra Leone and 650km from the capital Conakry. Access is via an air strip and dirt roads. There is a proposal to build a long and expensive railway line from Conakry, but execution will depend upon the viability of the mining, which is at present on hold.
A map of the geology of West Africa shows two large shield areas with the Taoudeni basin between. The ore deposits overlie the pre- Cambrian basement of the more southerly shield and are extremely rich, with 65% iron. The strata are highly folded, having undergone 6 phases of deformation. The four rock types vary from hard and massive to friable and the mountains are draped in colluvium, unconsolidated rock fragments and sediments brought down the steep slopes by erosion or creep. The talk was illustrated by photographs of the access road along the mountain side. Simandou is a ridge with a narrow access road to drilling pads on the Pic de Fon across the top. The southern part of the site at Oueleba differs in that the top is very flat. The ore body is itabirite, banded quartz haematite and haematite schist, with erosion slides and folding ever present. There is deeply weathered granitic gneiss, phyllite with calcareous mudstone, and colluvium over the phyllite. There is instability on all access roads.
The Nimba site, close to the Liberian border, was started in the 1960s when a US company approaching from Monrovia constructed adits into the mountain. This site is dome-shaped with radial drainage and very steep slopes and a height of 1670m. The site is less well- domeshaped with radial drainage and very steep slopes and a height of 1670m. The site is less well-developed, with very strong folding. There was a failure in 2006. The rock is laminated itabirite, steeply dipping, but with a lot of little kink folds, and there are many slips at the contact between the itabirite and the colluvium.
Throughout the talk was illustrated with photographs of the roads, rigs, storm damage, living conditions of the labour force, and chimpanzees. Questions elicited the information that the world price of iron has fallen too low for exploitation to be economically viable at present.
We often see examples of Kentish Ragstone in the building stones of London; from the Roman wall near Tower Hill to Norman buildings such as the Tower of London, Westminster Abbey and the Guildhall. Prior to the development of the railways the quarries in Kent were the most accessible source of building stone. Historically it was quarried from numerous quarries along the outcrop in Kent with a concentration in the Maidstone area. The River Medway which flows through town provided an easy route for transporting the stone to the capital. Whilst LOUGS have seen Kentish Ragstone used as a building stone and visited the fromer Quarry at Dryhill near Sevenoaks, we had not until now had the opportunity to look around an working quarry.
Hermitage Quarry is the only fully operational Kentish Ragstone quarry and we were fortunate to be able to visit. The quarry is operated by Gallagher and we were given a tour by the quarry manager starting in the western extension of the quarry which is currently being worked. We were able to view the alternating layers of 'ragstone' and 'hassock' in the qurry faces. (Figure 1). The ragstone is the hard grey limestone which is used for building stone and the hassock is comprised of sand and silt.
Figure 1. Alternating layers of ragstone and hassock
Kentish Ragstone is part of the Hyte Formation and is Cretaceous in age. It was deposited in a shallow marine environment and we were able to see dark grains of glauconite and a few fossils. We were also shown a large ammonite that had been found in the quarry. Hermitage Quarry is predominantly worked for roadstone and construction aggregate although there is still some demand for building stone for restoration and local buiding projects in the vernacular style.
A rig was drilling holes for explosives in the area to be worked the following week. The quarry manager described how the quarry targets specific beds for building stone. Gallagher have commissioned studies to correlate the material worked at historic pits to the beds within the current working areas at Hermitage Quarry. Matching the stratigraph is important for herigate restoration projects.
Figure 2. Building stone procesing area
Following blasting the stone is taken to an on-site processing area where it is cut using large circular saws. (Figure 2). We were able to view masons working the blocks and later saw a selection of building stone products that Gallagher produce. Any waste material from processing the building stone was used for construction aggregate.
Figure 3. A selection of the builgind stone products produced at the quarry
We were able to see the machinery used for screening and crushing the aggregate and the stockpiles of material within the quarry. A fascinating tour that allowed us to see the whole process from extraction to processing.
The group reassembled at 2.00, just as the weather decided to show what it could do. We gathered with Geoff under shelter as we looked at the Maidstone coat of arms which features an iguanodon, as the left supporter, the lion of England on the right and the head of a white horse, symbol of the County of Kent, rising from battlements at the top. The iguanodon is a reference to the discovery of elements of the skeleton in a quarry within the borough in the 19th century.
Accompanied by thunder we then examined County Hall, where the coat of arms was that of Kent, with two sea lions as supporters, the white horse in the middle, and a ship’s sails on top. County Hall had a base of hammered granite, with Portland Stone and Kentish Rag above. Challenged by Geoff to identify the stone used in a later extension, we found it was artificial stone!
Our next stop was the wall of Maidstone Gaol. Built of Kentish Rag from local quarries in 1810 it is still extremely impressive. A marine limestone with a lot of quartz, it has glauconite and ‘hassock’ or calcite cement. Fossils include belemnite and ammonite. The string courses are laid on slate, and there is also a lot of chert. Entering Brenchley Gardens, we examined the War Memorial, a Cenotaph designed by Lutyens in 1921 at 2/3 the dimensions of the original in Whitehall. It is built of Portland Stone, a fossiliferous oolitic limestone with very small ooliths. Our next stop was a strange tower, a finial from the House of Commons, saved from the 1941 bombing. Its material is magnesian limestone of Permian age from Bolsover Quarry in Derbyshire. Nearby was a Gingko tree, a living fossil, as the species goes back to the Carboniferous. Proceeding toward the bottom of the park we paused at St Faith’s Church dating from the 1830s and built of Kentish Rag in a style known as sneck, with long horizontal stones and large squared blocks, infilled with small squared stones. Passing the museum with its dinosaur we entered Havoc Square commemorating the Battle of Maidstone on June 1st 1648, with walls of local stone including glauconite. Here and in the park enthusiastic bands of drummers hampered Geoff’s efforts to instruct us.
Making our way through the town we crossed the River Medway to look at the Portland roach clad court building. On the way back into the town we paused to examine the granite on slabs on the bridge containing impressive large zoned feldspar crystals. We finished an interesting tour in the town centre spotting a shoe shop window display with what looked like it could be Kentish Rag.
With a PhD in Chalk micropalaeontology and many years’ experience as a consultant geologist, Haydon is currently Geological Adviser to the Chiltern Society and Senior Vicepresident of the GA. In this role he has been involved in negotiations about the route of the proposed high-speed rail link.
His talk opened with an overview of the geology of the Chilterns and the nature of Chalk: biogenic, built up from foraminifera, coccoliths etc., dating from 90My, in a global greenhouse environment with active sea-floor spreading and major flooding of Continental shelves, and containing large, oddly-shaped flints.
With reference to Reed Quarry at the northern end of the range, and the Aston Rowant cutting, he described the variation in thickness of the sections. He referred also to the case of the disappearing Misbourne River near Amersham, part of the Thames river system. It flows over an impermeable layer, but at times drops below it. The ‘preferred route’ for HS2 is in a tunnel straight up the Misbourne Valley. Haydon’s question is: ’Why put a tunnel under a valley?’
An alternative route has been suggested by Peter Brett Associates, but again there is a difficult problem as the chalk is heavily faulted, and the high density of solution features per 100km2 affects the aquifer. The geological maps are old, and to date no boreholes have been sunk as there can be no access to the land until the relevant acts have been passed. With reference to the geological cross-section, the proposal is to tunnel through the Upper Chalk, containing many large flints. It seems they have the geology the wrong way up: it would be easier to tunnel in the Lower Chalk as there are fewer flints. The depth to the competent solid chalk is at least 16m.
The preferred route risks the total loss of water flow in the Misbourne river system, loss of habitat, the potential pollution of the main water supply to the area and the Thames system as a whole.
London Branch enjoyed a beautiful September day at Folkestone, where fossil hunting was followed by an examination of cliffs and a series of interesting and informative mini-talks delivered by Geoff Downer on past collapses and the measures put in place to minimise any further slips.
Fig. 1 - Looking out over East Wear Bay
The day started with a walk down to East Wear Bay, and some excellent trace fossils were found immediately in the greensand rock of the Folkestone Bed as the group walked onto the beach. The Folkestone Bed was formed under marine conditions and trace fossils, including dinosaur footprints and crab balls, suggest that this was probably an intertidal zone. Meanwhile, the lower levels of Gault clay contain large quantities of illite and kaolonite, suggesting that the sediments were formed from weathered granite. The upper reaches are host to illite and smectite, the weathered remains of volcanic ash, with possible volcanic origins from the proto- Atlantic or a failed rift in the North Sea. Towards the top, increasing amounts of calcium carbonate and phosphates indicate that the clay was formed in an area of high biological productivity, possibly an upwelling zone in which nutrients are raised from deeper waters.
Fig. 2 - Trace fossils in the greensand of the Folkestone bed
The morning proved highly productive, with the group gathering hundreds of fossils in a couple of hours. Abundant ammonites and bivalves, many of which were in excellent condition, were found alongside fossilised wood, gastropods, belemnites, coral and scaphopods. Many of the ammonites had clearly-defined chambers and whorls and some had been pyritised. Having worked up a good appetite, the group decamped to the nearby promenade to enjoy a welldeserved lunch and the chance for members to admire each other's finds.
Fig. 3 - A superb ammonite in the Gault clay
Next, we walked to the cliffs to the east of Wear Bay, to discover the reasons for previous catastrophic slips and efforts to prevent further ones. Slips have been occurring on the Wear Bay cliffs since the 16th Century, but erosion has worsened since the first harbour wall was built in the 1820s, which not only trapped material, thus preventing it from going to Wear Bay, but also created an eddy, which created a beach in the shadow of the harbour. Simultaneously, waves were refracted by the harbour arm into Wear Bay – the perfect recipe for erosion at the bottom off the cliffs. With the tow weight removed, slipping increases. The second cause for slipping is the way in which impermeable Gault Bed is sandwiched between the predominantly greensand Folkestone Bed (below) and the Chalk Bed (above), both of which are host to aquifers. The pressure acting on the clay from above and below results in it oozing out, which creates vertical sets in the Chalk. These sets can then cause rotational slips, as the cliff slides downwards and out towards the sea.
Engineers have sought to reduce these risks by boring 2 metre diameter tunnels, which extend 250 metres into the cliff, draining water from the base of the Chalk and discharging into the sea, thus relieving pressure on the Gault. A long concrete promenade has also been constructed along the foot of the cliff, which acts as a tow weight to stabilise the cliff and prevent wave erosion.
Next, the group visited the railway line which was the site of a near-disaster on 19th December 1915, when a landslide occurred shortly before a train carrying soldiers heading to the battlefields of World War 1 was due to pass through on the way to Dover. Fortunately for the driver and passengers, the train managed to come to a halt before it hit the debris and derailed,
Finally, we took a walk up a short, steep incline to briefly visit the site of Folkestone's Roman villa, which was discovered in 1923/4, and remained a major tourist attraction until it was covered up again in 1957 to prevent damage. Since 2013, archaeologists have resumed work at the site, uncovering Iron Age buildings beneath the Roman foundations. A variety of quern-stones, used for grinding flour, had been left at the excavation site.
Finally, a satisfied gaggle of geologists made its way back to the car park, with fossilised treasures aplenty as souvenirs of a great day.
In the morning, we visited the Haslemere Educational Museum (http://www.haslemeremuseum.co.uk/) - this has an excellent gallery of geology (and history and archaeology), with plenty of exhibits. The second Director of the Geological Survey, Sir Archibald Geikie (1835 to 1924) retired to Haslemere, and supported the Educational Museum, the museum receiving his collections. These included his paintings, of which there are over 800, mainly postcard or A5 sized. He painted these in the field, as others would take photographs. They also have his geological hammer and notebooks, and his geological collections. These are mainly teaching sets, as he gave many classes and courses for the museum. Lindsay Moreton, the Collections Manager, welcomed us and explained the history of the museum. We were shown around the collections and library, and given a tour by the Museum’s geological volunteer, John Betterton, after whom is named the Cornish mineral Bettertonite!
The museum is well worth a visit, with plenty see and learn.
Fig. 1 - One of Gelkie's Scottish paintings
After a pleasant lunch in one of Haslemere’s several teashops, John Lonergan led us around Fernhurst Furnace, in the west of the Sussex Weald. This is the best preserved of the Wealden Iron furnaces, in fact in the forest can be found the landscape of the industry, with mine pits to the west of the pond bay, a shanty town located to the east and the partly excavated furnace, tail races and copious slag heaps to the south. The Furnace was in operation from the late sixteenth century until the 1780s.
Fig. 2 - Illustration of Fernhurst Furnace by M. Codd
The furnace site is under dual ownership of Robin and Carla Barnes, who own the east side of the structure and the Cowdray Estate, who own the other half, and on open days Robin leads tours of the site – these are well worth joining as the vegetation is cut back and the elements clearly labelled. The ore was mined locally from the Weald Clay, and the furnace ran in campaigns in the winter, as the major constraint was water for powering the bellows for the furnace. The wood was cut locally, and the area coppiced. There is no forge at this site, though other are nearby, and this site certainly cast cannon and the circular casting pit can be seen.
Fig. 3 - Model of Fernhurst Furnace
There is an open weekend every September, with guided tours,
refreshments, birds of prey and the Sealed Knot – well worth a visit,
see their website for details.
The London Geodiversity Partnership and the London Branch of the Open University Geological Society made the large Chalk Quarry at Riddlesdown the focus of its conservation day for 2016. The event appropriately fell during Earth Science Week, with the theme ‘Earth Science in Action’ which fitted nicely with our aim of improving access for research and study.
The quarry is owned and managed by the City of London Corporation and Matt Johnson prepared the base for a bonfire for the face we were to clear. He also kindly provided tea and coffee. Fourteen of us reported for duty including members of other London geological groups. Before starting work, Liam Gallagher, a chalk expert, talked to the group about the chalk in general and put the importance of the pit into context.
This is what the face in Riddlesdown looked like before we cleared it. Look at this lovely clear face at the end of the day's
hard work! Well done, volunteers - much better access! For more photographs of the day see the LGP Flickr account:
Before work got started
Description of the importance of the face
Welcome refreshment from Matt
After the face is cleaned
The fire in full burn!
Chris, Statoil Professor of Basin Analysis at Imperial College, gave us a lively talk on the importance of salt in daily life, starting with food and the de-icing of roads. He admired it for the perfect cubic form of halite. He referred to the importance of precipitation: the evaporation of 1,000m seawater yields 16m salt. Examples of salt basins included the Salar de Uyuni, the world’s largest salt flat at 10,582 sq. Km, in SW Bolivia in the rain shadow of the Andes. He referred to the Dead Sea and the Messinian salinity crisis in the Miocene.
He continued with the importance of salt as a preservative, and therefore its use as a currency, with the derivation of the word ‘salary’ and its use as payment during the American Civil War; hence Saltville, Virginia.
Via the Danakil depression in the Afar and the psychedelic colours of the Yekaterinburg mine in Russia, we arrived at rock salt sculptures and buried salt sculptures. A discussion of salt domes, seabed salt extrusions that can rise thousands of feet led to an account and video of the disaster in Lake Peigneur, Louisiana. Texaco had drilled a mine shaft 30m deep when the rig got stuck and toppled over. It had accidentally penetrated a salt dome housing a mine of the Diamond Crystal Salt Company. As it sank the salt dissolved, creating a sinkhole and whirlpool that widened to 900m within hours. The whirlpool swallowed two drilling rigs and eleven barges. It drained 13million m3 of water in 3 hours and reversed the flow of a 12-mile canal to the Gulf of Mexico. No lives were lost on the rigs or in the mine. The lake refilled within days, via a 150m waterfall, so that a former freshwater lake 11ft deep is now a saltwater lake 1300ft deep.
Dealing more seriously with the physical properties of salt, Chris touched upon their importance in the deformation of the Earth’s crust, in that all other rocks get stronger with depth, but salt not. He showed seismic sections of salt diapirs and mentioned their role in hydrocarbon play as reservoir rocks, referring briefly to the problems of drilling for salt, using seismics to illustrate the problem
We assembled at the Red Lion Hotel in Salisbury, in the late morning, dropping our bags and grabbing a quick lunch, before making our way to the Cathedral Close for our appointment at the Masons' Yard at 1400.
Some of us had had the opportunity to cast a brief glance at Diana's very comprehensive hand-out notes on the geology and building stones of the area, together with the history of the construction of the cathedral and of the City of Salisbury. Diana was to speak about these notes, after dinner and during the course of the weekend.
We were able to gather that, unlike most English cathedrals, Salisbury was effectively completed in one generation, between 1220 and 1258. Though it was subsequently added to, much of the later work was subsequently undone. Thus, although the cathedral did suffer from Puritan vandalism in the 17th century, and later restorations, it largely stands as a masterpiece of the Early English style as conceived by Bishop Richard Poore, who was mainly responsible for initiating its construction. The spire is 14th century, there is some Decorated and Perpendicular work, and as we were to discover, the Dean and Chapter see it as their responsibility to maintain the cathedral as a contemporary place of worship, rather than as a museum. There have therefore been significant recent additions of contemporary works of art, devised on themes suited to the setting.
The cathedral replaced a previous cathedral at Old Sarum. Archaeological investigations have shown that the precursor was itself a substantial church, and it was there that the rites and ceremonies of the Use of Sarum originally developed.1 However the site of the old cathedral was abandoned, and the church relocated in what is now the site of the modern city, because the clergy, who had ample landed estates in the area, and could thus afford to contemplate the relocation, resented the domination of the royal garrison at Old Sarum.
In terms of building stone, the new project required a great deal of new building stone2 in an area dominated geologically by the Chalk, which is in general not a good building stone. Diana's notes explained that the bulk of the new building stone, the so-called "Chilmark " stone, was actually quarried near Tisbury in the Vale of Wardour. This stone, which is basically a glauconitic limestone, but varies somewhat, along a spectrum from a calcareous sandstone to a sandy limestone, is roughly contemporaneous with Portland Stone, at the top of the Jurassic, but was formed in a more littoral and energetic environment. It would have been a great logistic feat to transport more than 60,000 tonnes of stone from the quarries some 25 miles or so away by ox cart over bad often very muddy tracks. Stone for restoration work is now obtained from a quarry at Chicksgrove, owned by Lovell Stone.
A great deal of Cretaceous Purbeck limestone, so-called "marble", was used for pillars in the interior and for flooring, large pillars of unpolished stone being decorated by polished columns. Significant quantities of calcareous tufa, as well as "clunch" (chalk rock) and brick were used unseen in the vault webbing. Also largely unseen, were significant amounts of stone recycled from the old cathedral, Caen stone, used as rubble, and Hurdcott Stone (a glauconitic Cretaceous sandstone from the Upper Greensand) used in the upper foundations, and in the wall of the Refectory restaurant.
As we were to discover, a number of other types of building stone were subsequently introduced into the fabric of the cathedral.
We were met at a corner of the cloisters by Gary Price, the Clerk of Works, who took us through a door into the Masons' Yard, formerly part of the garden of the old Bishop's Palace. He introduced himself and explained that the slightly quaint name of his office meant that he was the chief executive of the Works Department of the cathedral. Salisbury was now one of only nine of the 42 cathedrals in England and Wales to retain its own Works Department. The Department is mainly devoted to work on the cathedral, but does a small amount of work on external commission. Gary said he had been in the Department for some 30 years, 26 as a stone sawyer/cutter before taking up his present position. He said that he went to the quarry and chose the stone, but for the most part his present work was administrative.
Fig. 1 - Masons'yard
The Department was small, about 20, in contrast to the thousands who would have been employed in the construction of the cathedral over some 38 years in the 13th Century. Health and safety is now much better, and there have been no fatalities in his time. The Department had been more or less continuously in existence since the cathedral was built, and over much of the time had mainly been devoted to routine conservation work. In significant measure this was due to the solid foundations of the cathedral, not so much the built foundations, as the gravel (about 27ft) overlying Upper Chalk.3
However, by the 1960s it had become apparent that the fabric was in need of significant renewal. In 1985, there had been a major fund-raising campaign, which had included an abseil by the then clerk of works off the tower and the spire. The campaign, supported by the Prince of Wales, had raised some £6.5m. Extensive work had been then carried out on the spire tower and west front. The spire had proved to be in a particularly bad state, despite work in the 1950s in which the top 9m. of Chilmark had been replaced by new Clipsham stone, a building stone from the Upper Lincolnshire Limestone Formation, quarried in Rutland, and used in a number of important buildings, notably at the two ancient universities. Much of the stonework had weathered away, and daylight had been visible in some parts.
The major works programme is now about 85% complete, and the target date for completion is about 2019-2020. Currently work is focussing on the east end, the oldest part, which was in very bad condition from previous interventions and had proved to require double the amount of new stone that had been estimated. Further work will then need to be carried out on the Chapter House, where the Cathedral's copy of Magna Carta is kept, and on the old Bishop's Palace. It seems that there are plans, once the major works programme is completed, to open up public access to the Masons' Yard and place a new focus on education in conservation work.
Fig. 2 - Specimens of Portland stone
Gary took us further into the yard and we saw some pieces of Portland stone. He explained that, while the bulk of the external fabric was Chilmark Stone, Henry Clutton, who had worked as cathedral architect, had used Portland extensively for gargoyles and finials in the 19th century. Gary commented that Portland was harder than Chilmark, but more consistent, and thus in many respects easier to work with. He remarked that when struck, Portland would give a distinctive ring.
Fig. 3 - Gargoyle for repair
We moved to a spot outside the banker masons' shop from which we could look in at the masons at work. Gary explained the work with a chisel with an 8 Mohs tungsten-tipped edge, and a round mallet of beech or box. He showed us the carved initials which identified the work of individual masons.
Fig. 4 - Masons’ marks
We noted that the masons were masked and that sizeable extractor fans were employed, due to the high silica content of Chilmark.
Fig. 5 - Mason at work
We were then shown trays of specimens removed from the fabric for restoration or replacement. These included a piece of Victorian stonework damaged by air pollution, tracery which was being reinforced by spring-loaded pins, and a curious piece of what appeared to be French limestone containing in a hollowed-out part a deposit of gravels. Gary said that the small-scale conservation work included cleaning with dolomite, and the use of a special mortar made with lime and sand.
Fig. 6 Specimens for restoration
We moved past the sawmill, where stone was cut using a saw with industrial diamonds, and walked to the far end of the yard, where there some large blocks of stone, one weighing some 4.7 tonnes. Gary commented on the high porosity of the limestone, which could absorb a great deal of water, and noted that, as the stone was sold by weight, it was the practice of quarries to sell it in winter!
Fig. 7 - Huge saw with diamonds!
We looked back towards the cathedral, and Gary pointed out the creep of the spire towards the south-west, the direction of the prevailing winds, and said that the finials crept in the same direction. On the ground we saw a slab of Carboniferous gritstone from the Forest of Dean, used for drainage paving stones and other stonework outside the cathedral. Under wraps stood a Hepworth Crucifixion, and Gary explained that one of the subsidiary tasks of the Department was setting up and moving the works of contemporary art displayed in the cathedral.
Fig. 8 - South-west creep of spire can be seen in this picture
Thanking Gary for a most interesting and instructive visit, we adjourned for tea in the Refectory. After tea, we walked in the cloisters, and looked at the head of the Angel Gabriel set up in the cloister garth. This was carved with the grain in Purbeck freestone by Emily Young and set up on a bronze plinth. It was one of eight angel heads sculpted by her and exhibited in the cathedral for its 750th anniversary. The cathedral has frequent exhibitions of religious and quasi-religious works of art. For example in 2013, there was a "Messenger of the Spirit" exhibition featuring the works of Helaine Blumenfeld.
We walked out of the cloisters, and had a look at the West Front of the cathedral. We examined the pilasters in the doorway, a Victorian replacement for weathered Purbeck pilasters by George Gilbert Scott. These are commonly referred to as being made from Ashburton stone (like Purbeck "marble", a fossiliferous limestone), but Diana said that strictly speaking they were probably Ipplepen. We examined the traces of an ochre deposit on the West Porch (apparently man-made and due to what is called the Greco-Latin empirical treatment) and which may have been applied for protective as well as aesthetic reasons. Before moving away from the cathedral, we noted the drainage stones made in the Forest of Dean stone we had seen earlier in the Masons' Yard. Walking out into the Close, we saw a further use for this material in the new plinth recently made to replace the original concrete plinth provided for the Walking Madonna by Elizabeth Frink. The bronze statue is seen walking away from the cathedral, but is apparently intended to symbolise Our Lady (to whom the cathedral is dedicated) walking purposefully into the world after the Resurrection.
Diana pointed to the top of the spire, where Chilmark stone had been replaced by Clipsham, and then pointed out the Museum, where we would be going the next day, after our visit to the interior of the cathedral in the morning. We then returned to the hotel to unpack before dinner.
1 This was a variant on the Roman liturgy, which was later adopted as the national liturgical use in England under Henry VIII, and again under Philip and Mary, and which influenced some of the text of the Book of Common Prayer devised by Cranmer.
2 Its mass is estimated at over 100,000 tonnes.
3 Diana's notes explained the paradoxical advantage of the water table just underlying the surface of the gravel terrace. The gravels were about 15% by volume of water, which is incompressible, and thus provided a very firm foundation. However the cathedral was prone to flooding.
We spent some time by the very large and elaborate alabaster and marble monument to Lord Hertford (and his first wife by a clandestine marriage), which stands in the south-east choir aisle. It is really a monument to Jacobean funerary extravagance, perhaps to a preoccupation on the part of the man principally commemorated there4, and to the considerable wealth of a local landed family. His family are still significant landowners in Wiltshire and Devon, and their present head, the 19th Duke of Somerset, was recently elected as a cross-bench member of the small group of representative hereditary peers still allowed to sit in the House of Lords.
The lengthy Latin inscription gives the impression that Lord Hertford was an important proconsul of the English state. In reality, while brought up to expect high office, he had the misfortune to be destined to be only a minor footnote in English history.
He was the eldest surviving son (by the second marriage) of the Duke of Somerset who was the first Lord Protector in the reign of Edward VI, and was a boyhood companion of the young monarch. However, his father was deposed from the role of Lord Protector, and subsequently executed for plotting against his successor, Northumberland. The associated Act of Attainder prevented the young man from succeeding to his father's lands and titles. Some of the lands were restored under Mary, and Elizabeth created him an earl, with the same designation he had enjoyed as a courtesy title in his father's lifetime. He was soon in trouble again, after contracting a clandestine marriage with Catherine, the sister of Lady Jane Grey, the lady commemorated in the monument. This resulted in his imprisonment in the Tower until Lady Catherine died. The royal displeasure was incurred because it deprived the Crown of a potentially valuable matrimonial and diplomatic asset, and was also a political threat because of Catherine's proximity to the succession. Clandestine marriages with members of the royal household became rather a habit with Lord Hertford, because he contracted two further such marriages.
Happier times came with the accession of James I. The Latin text rather cryptically refers to him as having been that King's envoy to the "ArchiD.D.". This was Albert VII, Archduke of Austria, and Duke of Burgundy, under the latter title ruler of the Spanish Netherlands (in modern terms very roughly Belgium, Luxembourg, and some north-eastern parts of France close to the Belgian border). Following the Treaty of London of 1604, which concluded 19 years of war between England, and Spain and the Spanish Netherlands5, Lord Hertford was sent on a ceremonial mission to take the Archduke-Duke's oath of peace6. After that, he seems to have retired to his estates.
4 Lord Hertford erected three monuments elsewhere to various members of his family in his own lifetime.
5 Apart from the spat involving the Spanish Armada, Queen Elizabeth had supported the nascent Dutch Republic, northern provinces of the Low Countries which had rebelled under Calvinist leadership against Spanish domination.
6 Why was he sent on this mission after a somewhat chequered earlier career? The negotiations for the Treaty of London were carried on at Somerset House, and that may have given someone the idea. As the head of a leading Protestant family, Lord Hertford would also have bene a good choice as envoy to conclude the deal, in terms of winning over those with misgivings about making a peace treaty with the leading Catholic power of the day.
22 of us met at the main entrance and stopped at the admissions desk to peer through the glass door of the gift shop which is on the old site of the ‘plumbery’, where all the lead work for the Cathedral was done. Old lead would be melted down and poured out on sand tables to form new lead sheets. This keeps trades on site and reduces transport costs. When the Cloister was re-roofed in 1790 they replaced the lead with Welsh slate brought by boat around the coast and up the river. Once inside the Cathedral itself we headed for the East end as there was a wedding due at noon and Diana had to compete with the organist and the choir warming up. On the way through we stopped to look at a couple of plaque surrounds and tombs – the type of stone could be identified but not the source.
The weight-bearing pillars are made up of plain buffed drums of Purbeck Bluestone, an evaporid limestone, showing bedding; the decorative pilasters are made of the same stone as the pillars but instead of being buffed they are cold-polished using sheep fleece sprinkled with sharp sand which gives a different lustre; the tops and bases are hot polished with wax, the hot wax soaks into the stone and changes the colour to black. The very thin decorative pillars in the Holy Trinity Chapel have original iron tie-rods at the top. The ‘water holding bases’ were a decorative feature popular for a 50-year period up to about 1220.
The shrine of the first Bishop of Salisbury, St. Osmond, is made of Tourney stone; Tourney was widely used in medieval times, it has a high silica content and comes out of the ground quite soft and easily worked and ‘cures’ on exposure to the air so pieces are worked on site and shipped out as completed items. The font at Winchester is Tourney.
The location of the original consecration crosses of the Cathedral are marked with carved roundels set into the wall. The ceiling has quadripartite vaulting – the ribs are built on a form, the upper surface is plastered, the forms are removed and the newly exposed surfaces were plastered with a calc-tufa infill which was then brightly painted.
The floor tiles are original medieval – the clay is rolled out, a design is impressed into the clay, a slip is applied to infill the impression and the tile is glazed. The length of Salisbury Cathedral is six times its width.
The Chantry Chapel of Edmund Audley is in the perpendicular style with a flat top from around 1500 made of blond Bath Stone, an oolitic spar-supported limestone. The bosses of the lattice in the Chantry ceiling have Audley’s symbol; the outside is decorated with the Tudor rose and pomegranate motifs of Henry VIII and Katherine of Aragon; the statue of the Virgin Mary has disappeared, probably desecrated during the Reformation. The Jesus and Mary statue in the Morning Chapel (St. Martin’s Chapel) is made from a hard and gritty silica-rich Purbeck whetstone – it rings if tapped with metal.
The ceiling of the crossing is of a different style to the Holy Trinity Chapel and Nave as it was reworked in the fashion of the time when the spire was added over 100 years after the Cathedral was completed. Brass rings mask the joints between different pieces of the pillars. The blind buttresses can be seen from where the spire was added. The oak for the stalls in the Quire was donated by Henry III. We sat in the Nave and listened to the music while watching wedding guests arrive and a bat or bats flitted around the ceiling.
The original font would have been in the traditional location at the back of the Cathedral, in 2008 a new William Pye feature font was consecrated in the unusual location of the centre of the Nave. The top is bronze and the base is clad in Purbeck stone.
Fig. 9 - William Pye font
We headed back out through the Cloister, with a quick stop to look at a grave marker exhibiting a burrow system trace fossils. In the Chapter House George Gilbert Scott’s refurbishment included a very good Victorian version of the medieval floor tiles as seen elsewhere in the Cathedral and made in the same way. Above the seats for the Chapter members is a frieze (originally brightly painted) made of Chilmark with repairs in Cobb Jurassic inferior oolite. The palmier (pillar) base is decorated, the weight bearing centre is full of Unio bivalves.
On the monument, the effigy of Lord Hertford's first wife is shown to his east, and at a higher level, to reflect her higher status as a member of the Royal Family. However, there is scant evidence, to say the least, that she is actually buried there.
After lunch we assembled in the yard in front of Salisbury Museum, formerly Sherborne Place the home of the Abbot of Sherborne. The window jambs and sills and part of the porch are made of Ham Stone, a beautiful mellow Jurassic limestone widely used in Dorset, from a river channel moving through a major sand bar. Among the flint in the wall is a greenish stone, Hurdcott Stone from the Lower Cretaceous, impervious, but it wears back very quickly. Diaper work adds interest to the walls.
Our main target for the afternoon was the Wessex Gallery, opened in 2014, to replace two former Pitt-Rivers galleries. It shows the history of the area from the early Beaker people, through the Roman era to the Norman Conquest. It starts with hunters and gatherers, goes on to Neanderthals and early humans, taking in the Amesbury Archer (2400-2200BC), Durrington Walls, Boscombe Down, Winterslaw Beaker burials, Monkton Up Wimbourne, Shrewton Barrow Cemetery, bronze age metal working, bracelets, chisels etc. There are sections devoted to Pitt- Rivers and other researchers.
Diana had provided us with a quiz to sharpen our eyes, so afterwards we sat and discussed our findings. The questions ranged from tools, made with jadeite, gneiss and eclogite, engraved slate, axe heads made of dolerite, beads of amber, wood, faience, an iron age quernstone made of dolerite. The Amesbury Archer probably came from the Alpine region, inferred from Oxygen isotope analysis of his tooth enamel. We linked to OU courses with dolerite from the Win Sill and jadeite and bluestone.
We followed this with a look at the exhibition: Constable in Context, which wound in and out of galleries and passageways. The high point was the big picture of the Cathedral, with a guide on hand to explain the view, not a view that one can actually see, but one that includes neighbouring buildings as well.
We then embarked on a tour of the Cathedral Close, looking first at number 67 with blue and red bricks, the blue having been fired at a higher temperature. Myles Place, a high-status stone house nevertheless has side walls of brick. Next door is the Walton Canonry, then Leadenhall (aula plumbea), built by Elias of Dereham, the ‘architect’ of the Cathedral. Then the South Canonry, a flint house with no quoins on the window frame. Bricks became standardised about 1661 with the introduction of a tax on bricks. Audley House had gauged bricks over the windows. The North Canonry has an example of case-hardening. Arundells, the former home of Ted heath, the Hungerford Chantry and finally Mompesson House completed our tour. By which time we were all exhausted, and the forecourt café having closed, back to the hotel and a well-earned rest.
We started at the Chesil car park by the old railway tunnel, and after Diana outlined Winchester’s history set off for King Alfred’s statue. Chesil or Cheese Hill comes from the Old English for gravel and this is where boats were moored on the River Itchen. This is likely to be where the Tournai marble fonts, for example, were unloaded. The Itchen cut down through the chalk as the Wealden Dome was pushed up by the Alpine orogeny, and is one of the classic English chalk rivers. It was the Itchen that made Winchester as the City of Kings, starting as the capital of Alfred’s Wessex. It was the fifth largest walled city, having been Venta Bulgarum in Roman times, and the Roman wall can still be seen in River Walk. After the Romans left it was abandoned for two centuries, the Old Minster being founded in 648AD.The High Street runs east-west, with burghage plots either side, not a chequerboard plan as at Salisbury. Which explains why it has a different feel, being much less spacious and on a hill. Winchester was laid out under King Alfred, as one of his fortified strongpoints to defend Wessex against the Danes.
We started at King Alfred’s statue, opposite St John’s House with its false top storey, with painted windows. This is part of the hospital for the poor, dated to 1289, but on a Saxon foundation. Between this and the Almshouses is St John’s Chapel. This is predominantly flint, with Quarr stone from the Isle of Wight, Malmstone, Portland stone, erratics from Guernsey, gabbro/dolerite and Clunch – and much of this is reused from the Nunnery Minster that was located opposite (now The Mayor’s House). Guernsey rocks are common in the High Street, and this highlights the use of the English Channel and River Itchen for the import of building stone to the city.
Fig. 10 - St John's Church wall; a study in flint, among other rocks
Next is the 1871 Town Hall, certainly striking and modelled on the Ypres Wool Hall in Belgium, roofed in Westmorland Slate (possibly Honister), with Scottish red granite columns, a porch of Permo Triassic red and dark sandstone (again probably Scottish), with an 1892 two-storey extension in flint and Bath stone.
Fig. 11 - Winchester Guildhall with band sandstone and Westmoreland slate
Within the Guildhall porch is a serpentenite war memorial. King Alfred’s statue is on two-mica granite plinth, probably Cornish and with a tectonic fabric, the upper section is finer grained than the lower. Walking up the High Street, Diana pointed out the white lines on the clay chimney pot, this was to indicate that they were made in Farnham; it is a band of white slip. We looked at the Debenham Larvikite, then past the 1713 Guildhall with its statue of Queen Anne. She planned to complete Charles I I’s palace, but died too soon. The Guildhall has a Portland stone facade.
Fig. 12 - Inside the Guildhall, mostly flint and timber, with Purbeck columns
Next on to the 1974 Law Courts, with flint cladding and Cotswold limestone. This is on the site of the old castle, and some of the ruins can be seen in the courtyard. This is where Charles II’s palace, designed by sir Christopher Wren, was planned. By this is a gem, the flint Guildhall, built 1222-1235 under Henry III, even then it cost over £500 – but then he also financed the cathedral. It is an aisled hall, with columns of Purbeck stone and was used as the Court of Justice until recently (Walter Raleigh was tried here, and in 1846 here was heard the last trial for duelling). Inside is the wooden Round Table, with Henry VIII’s portrait as King Arthur, but probably made for Edward I in 1290.
Outside is the Jubilee sculpture, by local sculptor Rachel Fenner, to commemorate Queen Elizabeth II's Golden Jubilee in 2002 on the middle level of the terrace in front of the Law Courts, which celebrates the rivers of Hampshire, with Moot Horns and an Overflow Stone. The main elements are Burgundy Chablis limestone, from the Great Oolite Formation.
Fig. 13 - Jubilee sculpture
From here we went toward the cathedral, past the tall sculpture with decorative lights which could be changed by text – and ended at the tombstone of an unfortunate soldier who died from drinking small beer. A lesson for us all; and we certainly learnt a lot of geology and history this weekend, and thanked Diana Smith for her leadership, and Yvonne Brett for her organisation. Worth another visit.
Fig. 14 - A sculpture that changes colour when texted!
Dr Paul Kenrick, Researcher in the Department of Earth Sciences at the NHM with a specific interest in plant palaeobiology and the development of soils, gave an interesting and informative talk on the important impact plants made on key Earth systems. Plants are the basis of terrestrial ecosystems and photosynthesis, producing oxygen, occurred very early in the biological life of our planet.
Life on land during the Archaic consisted of microbial mats, and then came the Great Oxygenation event. During the Phanerozoic there was a jump in oxygen production, as evidenced by the presence of charcoal in the fossil record. Plants modify Earth chemistry and therefore geology through the Carbon cycle. Carbon gets incorporated into plants and is then recycled through erosion, becoming CaCO?. Plants on land sequester that carbon, in particular through the rooting system. By about 390Ma in the middle Devonian, cryptogamic (i.e. non-seed- bearing plants) ground systems had developed: mosses, cyanobacteria, lichens.
Recognition of this sort of ecosystem depends on the fossil record, but also on geochemical method. Plants evolved onto land at one moment in time: vascular plants, mosses and algae. Spirogyra evolved in a gaseous medium, dependent on freshwater sediments. Early diversification of plants and animals took place in the Devonian; these plants had no leaves or roots.
There is inconsistency between the fossil record and the phylogenetic record. Cycles of marine and terrestrial sediment provide very fragmentary evidence, although spore capsules like those in modern mosses are found in charcoal.
The Rhynie chert in Oregon dates from the Devonian (407Ma), going back into the Silurian. It is a Devonian outlier with 53 bands of chert, the remnant of a hot spring system in a tropical environment, rich in silicates. It contains our earliest known terrestrial ecosystem, but one which is difficult to interpret, since the classification of some fossils, such as prototaxites, a huge organism up to a metre wide and 8m tall, remains enigmatic.