Severnside FIELD TRIPS 2015 - 2016

Page 3 Forest of Dean 7 Garth Hill 10 Black Mountain 15 Pembrokeshire 18 Wye Valley 20 Cardiff Building Stones 25 Weston-Super-Mare 29 Wigpool Syncline 31 Fforest Fawr Geopark 34 Malvern Hills 38 Hereford Building Stones 42 Brecon Beacons

This little book contains extracts from ―Sabrina Times‖, the newsletter of the Severnside Branch of the Open University Geological Society. The field trips included were all undertaken during 2015 and 2016 and the write-ups have been provided by members of Severnside Branch. Thanks are due to those members, to the Branch and Events Organisers and especially to the various leaders on the trips.

Introductory Day 15th March 2015 Soudley Valley - Forest of Dean

Leader: Dave Green FGS

For this years Introductory Day we returned to the Soudley Valley Geology Trail in the Forest of Dean. During the course of the day we visited sites covering nearly 100 million years of the Earth‘s history. We began at Soudley Railway Cutting revealing the unconformity between the Lower Old Red Sandstone (Brownstone) and Upper Old Red Sandstone (Quartz Conglomerate) and then gradually worked our way through the geological succession ending at an exposure of the Coal Measures/Pennant Sandstone.

Geological Structure of the Forest of Dean

West East Soudley Valley

Coal Measures

Carboniferous Limestone Tintern Sandstone Group

Quartz Conglomerate

Under the excellent guidance of Dave Green we tried our hand at mapping the exposures. Strike and dip were measured at each site and found to be consistent: Strike = 192° Dip = 64° West. At each site these details, together with the lithology, were transferred to the locality map provided by Dave and throughout the day we built up a picture of the geology in the area. It soon became very evident how much work is involved in creating a comprehensive geological map of an area.

Lines on dial aligned with North — South lines on map

Dial set to 192°

Strike marked along edge of compass

Transferring a measured strike of 192° to a map

Introductory Day 15th March 2015 Soudley Valley - Forest of Dean

Severnside went up a hill and came down a mountain 12th April 2015

Leader: Stephen Howe

A small group gathered on the edge of Taffs Well Green on a cool, cloudy-bright, April morning to explore the southern rim of the South Wales Coalfield. As we set off across the road and through Garth Wood, Stephen set the tone of the day by pointing out the clues we were to follow, the early spring growth barely covering the thin soil over shallow exposures. The weather had been dry and we were surprised by a wet patch over the path as we entered the wood. Stephen pointed to the source of water, a small spring erupting from the junction of beds and a patch of flow stone, deposited lime, all surrounded by lime-loving hart‘s tongue fern and wild garlic (ramsons). The path took us through a wood dominated by beech, whose shallow, spreading roots can thrive in the thin lime soils. Stephen led us to the remains of the Barry Railway Viaduct which had carried a branch line in the early 20th century to carry coal to Barry Dock avoiding the monopoly of Cardiff Docks. The path took us passed an exposure of Friars Point Limestone, the upper member of the Black Rock Limestone Group, part of the Pembrokeshire Limestone succession (formerly the Carboniferous Limestone). See figure 1

Brigantian Not Present Asbian

Holkerian Hunts Bay Oolite 180 m.

Cefnyrhendy Oolite 20 m.

Arundian High Tor Limestone 65 m.

Caswell Bay Mudstone 5 m. Gully Oolite 25 m. Chadian

Friars Point Limestone 80 m.

Black Rock Limestone Group Brofiscin Oolite 14 m. Barry Harbour Limestone 32.m Courceyan Cwmyniscoy Mudstone 43m Avon Group (Lower Limestone shale group) Castle Coch Limestone 25m Tongwynlais Formation 40m

Fig. 1 Pembrokeshire Limestone Succession (by S. R. Howe)

As we climbed the hill we reached the level of quarries which exploited the succession. As we walked north we rose through the succession to the Gully Oolite, a thick bedded grey limestone, then the Caswell Bay mudstone, which had been ignored by the quarrymen. The path led us by the High Tor Limestone, a thick bedded bioclastic limestone with cross bedding. The last big quarry (Fig. 2) exploited the Hunts Bay Oolite, 180m thick and like all the other lime- stones of this succession it was heavily dolomitised. Now worked out and infilled with spoil, the quarry gets its name, Cwarr Glas, from the blue colour imparted to the pool by limestone.

Fig.2 Cwarr Glas Quarry where Hunts Bay Oolite was quarried. 7

Severnside went up a hill and came down a mountain (contd.) 12th April 2015

Within the quarry was a large block ignored by the quarrymen as it was fissured and filled with flowstone and stalactites deposited from flowing water (Fig.3). Further along the road we passed the tunnel entrances to iron mines under the hill. One was strongly secured as the cave system behind is extensive and dangerously flooded.(Fig.4) In its time the mine was a source of limonite and haematite, for its iron content, ochre, as a dye, a storehouse for ammunition in WWII and as a Victorian tourist attraction. The mine was served by horse drawn trams and their route was still discernible through the trees.

Fig. 3 Examining pothole fissures in block left in Cwarr Glas.

Fig. 4 The locked entrance to the extensive iron mine.

Across the road we reached Coed y Bedw, a wooded valley floor where Stephen pointed out the clues to a change. Here the beech, ramsons and hart‘s tongue fern had been replaced by holly and Hard fern more suited to the darker acid soil over the Millstone Grit. Streams in the valley bottom also showed signs of the underlying geology. One, which issued from a spring at the junction of the limestone and Millstone Grit, contained lithified white twigs formed by the calcite-rich water. Another (Fig. 5) flowing out of the nearly buried adit to the Coed y Bedw drift mine, was red from the iron ore. As we walked up along the side of the valley we could see further indications of the industrial heritage, stone sleeper blocks from the tramway, the overgrown remains of the mine manager‘s house and waste tips from small coal mines now being exploited by the farmer. These drift mines were exploiting the few thin coal seams in the Upper Coal Measures, Pennant Sandstone Formation.

Fig. 5 A red stream flowing from Coed y Bedw drift mine.

Severnside went up a hill and came down a mountain (contd.) 12th April 2015

As we ascended Garth Hill [Allegedly the inspiration for the film ―The Englishman who went up a hill and came down a mountain] a cool wind had increased which did not encourage lingering. We paused long enough for Stephen to point out how the landscape had been shaped by the geology and the action of ice (Fig. 6). The last glacial had cut through the Pen- nant Sandstone Formation, which underlies Garth Hill, and accentuated the offset caused by faulting. Ponding of the ice had widened the valley at several points in response to the differences in resistance of the underlying geology. Our walk back down the valley took us through an abandoned village now engulfed in a conifer plantation. We followed the track of another tramway through the shadowy remains of old bell pits. On our return we passed a recently created memorial to the men and boys who died in a mining accident, (Fig.7). This park and the evidence shown us by Stephen in the surrounding area indicates the long industrial history of Taffs Well which is closely tied to the geology.

Fig.6 Walking along the top of Garth Hill.

Fig. 7 Memorial to the loss of life in the Lan Mine Disaster, 1875

Report by Averil Leaver

Traverse of the Black Mountain 24th May 2015

Leader: Geraint Owen

Introduction This field trip to the Black Mountain in south-west Wales was led by Dr Geraint Owen, an associate professor in the Geography Department at Swansea University. During the trip we examined the rock types, fossils, rock succession, landscape and industrial uses of the rocks of the Black Mountain area north of the South Wales coalfield. The Black Mountain lies in the western part of the Brecon Beacons National Park, due north of Swansea, on the northern edge of the South Wales coalfield. Note that it should not be confused with the Black Mountains (plural) in the eastern part of the National Park. We explored four separate locations during the traverse, as shown on Fig. 1 We all met at a lay-by on the west side of the A4069 north of Brynamman (Location 1). Once assembled, Geraint described the geology of the area and the proposed itinerary. His original plan was to travel northwards from Location 1, across the Black Mountain summit, finishing the trip at Carreg Cennen Castle, Location 4. This would take us from the younger rocks in the south through progressively older rocks as we headed northwards. However, low cloud and a cool breeze in the morning persuaded us to drive to Carreg Cennen Castle after visiting the first two locations so that we could enjoy our lunch Fig. 1 Locations visited in the comfort of the excellent tea-room at the castle. Suitably refreshed, we then retraced our steps to visit the third location, where the trip ended. In the interests of logic and geological clarity, the description for each location is written in the order shown on Fig. 1, rather than in the order we visited them on the day.

Geological Context The South Wales coalfield lies in the centre of a large syncline broadly trending east-west. The Black Mountain is situated on the northern rim of this syncline, where the strata dips southwards. Although there are no longer any large-scale working underground coal mines in South Wales, a few open-cast mines are still operational. Coal occurs as thin seams in the sandstones and shales of the Coal Measures.

SOUTH NORTH

Fig. 2

Traverse of the Black Mountain (contd.) 24th May 2015

Geraint provided a geological cross-section and description of the Black Mountain strata - see Fig. 2. Rock exposures show a complete succession from the Coal Measures through the Millstone Grit (now re-named as the Marros Group in South Wales), Carboniferous Limestone, Upper and Lower Old Red Sandstone, to Silurian and Ordovician rocks of the Tywi Valley. Because the strata dip southwards, the age of the surface rock increased as we moved northwards from Brynamman.

Location 1 The unit exposed at the first location was the Farewell Rock (Fig. 3), a sandstone lying at the base of the Coal Measures, forming the boundary between the Coal Measures and the older Millstone Grit (Marros Group). The Farewell Rock was given this name by miners, since digging deeper than this bed would yield no further coal, i.e. they would bid ―farewell‖ to further riches. Around 312 million years ago the local environment was dominated by swamps, estuaries and deltas.

The iron oxide within the Farewell Rock gave it a yellowy-brown colour. The sandstone also contained mica, making it flaggy and easy to break. Cross-bedding was clearly seen in some parts of the exposure, and showed that the direction of flow was southwards, in the direction of dip. Various bits of evidence, including the presence of some plant fossils, pointed towards a fluvial environment for the deposition of the Farewell Rock. Fig. 3 Farewell Rock

A few metres north of the first exposure, we found a lens-shaped section of laminated mudstone sandwiched between layers of sandstone in the Farewell Rock (Fig. 4). This was evidently an infilled palaeo-channel formed when a high-energy fluvial event scooped out the channel in the sandstone. The channel was subsequently abandoned and filled with stagnant water in which the laminated mudstone formed, before further deposition of sandstone covered the channel.

Fig. 4 Farewell Rock with palaeo-channel Fig. 5 Twrch Sandstone erratic

Before moving on to the next location, Geraint pointed out a glacial erratic of Twrch Sandstone sitting on the glacial till covering the Farewell Rock (Fig. 5). It should be noted that Twrch Sandstone was formerly known as the Basal Grit. The Twrch Sandstone outcrops further to the north, and this boulder was undoubtedly transported southwards by glaciation around 20,000 years ago when the ice-sheet extended as far south as the Gower Peninsula.

Location 2 Our next stop on the Black Mountain traverse was at an area of old industrial workings about 0.5 km north of location one. We had now moved from the Farewell Rock into the older Shale Group of the Millstone Grit (Marros Group). The shales here belonged to the Bishopton Mudstone Formation, also formed in a marginal coastal plain with lakes and swamps periodically inundated by the sea. After a short walk from the car park, Geraint encouraged us to hunt for fossils in an eight metre high scree slope consisting of shale (Fig. 6). Scrambling around on the loose surface, we all found evidence of black shelly material in the crumbly, laminated mudstone. The lucky ones found good examples of goniatites and bivalves. As many of the fossils found in this horizon were cephalopods (a class of mollusc), the sediment was clearly deposited in a marine environment. There was no evidence of burrowing, indicating oxygen-poor water on the sea floor. The fossils here only occurred within a thin 20 centimetre band of Fig. 6

Traverse of the Black Mountain (contd.) 24th May 2015 mudstone. A marine band such as this normally contains particular fossils that allow its unique recognition, and are usually named after a particularly characteristic fossil. Geraint explained that this band was characterised by the goniatite Gastrioceras subcrenatum.

We then walked a short distance to a terrace, where Geraint pointed out a cross-section of ice wedge casts filled by fine- grained sediment cutting into glacial till (Fig. 7). These are indicators of previous permafrost conditions where repeated freezing and thawing at the surface of permafrost created polygonal patterns of ice-wedges (i.e. a periglacial environment). The soil is forced apart by the expansion of ice and, when it melts, sediments fall in from above to form the fossil ice- wedge cast. Seen from above, ice wedges usually produce a polygonal pattern on the surface.

Fig.7 Ice wedge clasts Fig. 8

On our way back to our cars, we examined a small quarry where nodules of ironstone had formed within the shale beds (Fig. 8). The ironstones were relatively heavy owing to their iron content, and varied in size from 4 cm to 16 cm. The concretions were formed by the precipitation of iron compounds (e.g. oxides or carbonates) that combined with the shale to produce nodules. In the past these were smelted to produce iron, but are rarely used for that purpose nowadays.

Location 3 (a) Heading further north along the A4069 to the highest point of the traverse we entered limestone country. An information board at the car park described the industrial history of the extensive quarries on the eastern side of the road. The Carboniferous Limestone was used to make lime, and many large limekilns were built adjacent to the quarries, a few of which are still visible today (Fig. 9). The lime was used in both agriculture (for neutralising acidic soil) and industry (for making plaster and mortar). The last working quarry was closed in 1958, bringing to an end over 200 years of lime-making on the Black Mountain.

Fig. 9 Lime Kiln Fig. 10 Tufa deposits

The limestone had formed in warm shallow seas with carbonate deposited on platform, shelf and slope areas. We spotted many marine fossils, including corals, brachiopods and crinoids, in the numerous limestone blocks scattered around. The presence of burrowing indicated bottom dwellers (benthic animals) in well oxygenated water. Adjacent to the car park we came across a slag heap containing the fused remains of limestone and coal from a limekiln. The limestone (calcium carbonate) had been altered to quicklime (calcium oxide), which is more reactive than limestone. Rainwater washing through the slag therefore produced an extremely alkaline solution. This subsequently formed tufa as the calcium carbonate precipitated out in streams running away from the slag heaps (Fig. 10). The yellowy-brown colour of the deposits was apparently caused by algae

Traverse of the Black Mountain (contd.) 24th May 2015

Location 3 (b) We then crossed the A4069 to the western side of the summit and wandered into an area of small quarries. The white colour of the rocks here contrasted starkly with the grey limestone on the eastern side of the road. While we were scratching our heads at this sudden change, Geraint explained that the exposures here were of the younger Twrch Sandstone (Basal Grit). A major fault (the Cwmllynfell Fault) running north-south, parallel to the road, had caused the rocks on the western side to drop by about 100 metres, which meant that the limestone and sandstone now lay side-by-side at the surface. The Twrch Sandstone seen here is a fine-grained, very hard, white rock composed of more than 95% quartz grains, and is therefore classified as a quartz arenite. Because the sedimentary sandstone was cemented by quartz, it is known as an ortho-quartzite to distinguish it from a metamorphic quartzite. The Twrch Sandstone formed from rivers depositing mainly sand and gravel material in channels to form river terrace deposits. The high silica content of the sandstone made it ideal for manufacturing bricks to line high-temperature furnaces. The rock could also be used in glass-making.

At one of the small quarries we examined a block of Twrch Sandstone about a metre across that had disaggregated, i.e. its outer layer had suffered from sub-surface chemical weathering and had become ―rotten‖ when fluids percolated down through cracks in the rock (Fig. 11). The rotten material was crumbly and hence much easier to extract than the un-weathered hard sandstone. No doubt the quarrymen focussed on removing the softer material.

Fig. 11 Disaggregated Twrch Sandstone

Another block of Twrch Sandstone pointed out by Geraint had the Fig. 12 Fossil tree imprint fossil imprint of a tree on its surface (Fig. 12). The original tree had presumably become buried in the soft sand and eventually decomposed, leaving an impression of its outer bark in the rock.

The final feature we investigated before moving on was a shake hole (or doline) in the Carboniferous Limestone adjacent to the Cwmllynfell

Fault described previously. The hole was about 10 metres deep and 30 metres across, formed when the roof of an underground cavity had collapsed. A number of similar shake holes had formed along the line of the fault, where cavity growth had probably been enhanced by acidic drainage from the Twrch Sandstone.

Location 3 (c) We then moved a few hundred metres northwards to a lower level on the Black Mountain where we examined an exposure of rock belonging to the Grey Grit of the Upper Devonian Old Red Sandstone, which lay beneath the younger Carboniferous Limestone. This rock was a terrestrial deposit of quartz conglomerate, comprising a mixture of poorly-sorted clast sizes of quartz in a matrix, formed in an environment dominated by rivers (Fig. 13). Fig. 13 Quartz Conglomerate

Traverse of the Black Mountain (contd.) 24th May 2015

We followed the outcrop in a westerly direction until it abruptly ended, which once more caused us to scratch our heads. Geraint came to our rescue yet again and explained that the Cwmllynfell Fault had displaced the rocks a few hundred metres northwards at this point. (Fig. 14) Having spotted the continuing outcrop of quartz conglomerate in the middle distance, we were able (with a little imagination) to trace the fault line running in a north-south direction.

Fig. 14 North-South displacement of Quartz outcrop

Location 4 Carreg Cennen Castle was the most northern point of our traverse, and our journey there took us into the lower landscape of the Old Red Sandstone, with a corresponding contrast in scenery. Whilst locations 1, 2 and 3 were in wild, open moorland, the castle was in the midst of green fields, trees and pleasant valleys. Looking back in a south-easterly direction towards the Black Mountain, and hence the northern face of the South Wales Coalfield syncline, we could clearly see the Old Red Sandstone at the base, with the Carboniferous Limestone in the middle and Twrch Sandstone (Basal Grit) on top.

But the puzzling feature here was that Carreg Cennen Castle was perched on top of a block of Carboniferous Limestone, (Fig.15) surrounded on all sides by much older Old Red Sandstone deposits. How could this possibly be? As usual, Geraint supplied the answer. Thrust faulting had pushed a block of limestone to the surface at this point. On a geological map we could see similar isolated limestone fault blocks within the Old Red Sandstone, running in a line ENE-WSW along the Carreg Cennen Disturbance, one of several narrow zones of intense deformation in South Wales with a trend broadly similar to those produced during the Caledonian Orogeny (pre-Carboniferous). The present structure may be due to reactivation of the Caledonian fault line during the later Variscan Orogeny.

Fig. 15 Carreg Cennen Castle Summary The traverse allowed us to examine progressively older rocks as we headed over the Black Mountain, from the Coal Measures of the Carboniferous in the south to the Old Red Sandstone of the Devonian period in the north. Faulting had displaced some of the rocks after their initial deposition, which led to a more complex geological structure that kept our brains busy throughout the day. Geraint did an excellent job in describing the various geological features on the ground and answering the many questions that arose. I, for one, returned home a much wiser person !

Report by Norman Nimmo-Smith

Pembrokeshire 27-28th June 2015

We are indebted to Pete Wood of the Oxford Branch for the following report of a joint Severnside/Oxford field trip to Pembrokeshire:

During the weekend of 27th to 28th June 2015, Severnside branch and Oxford branch ran a joint field trip to Fishguard in South Wales. Led by OU tutor John Downes, the aim of the trip was to examine the Middle Ordovician igneous rocks of Pembrokeshire.

The tectonic situation At the beginning of the Ordovician period the microcontinent of Avalonia lay in the southern hemisphere. To its northwest lay the larger continent of Laurentia and between the two was the Iapetus Ocean. Throughout Ordovician times Avalonia drifted northwest slowly closing the Iapetus Ocean through the development of subduction zones. In early Ordovician times volcanism associated with these subduction zones created an island arc composed of basic, intermediate and acid rocks with a calc-alkaline chemistry. In Pembrokeshire these rocks form the Treffgarne Volcanic Group that outcrop just to the south of the weekend‘s field area.

By Mid-Ordovician times crustal stretching behind the Treffgarne arc created a back-arc basin (see Figure 1) into which turbidity currents laid down a thick sequence of sediments. The chemistry of the volcanism in the basin differed from the arc by being more tholeiitic in character and comprising just acid and basic rocks. No intermediate rocks occur. These volcanic rocks, named the Fishguard Volcanic Group, were the main subject of the weekend‘s investigations

Figure 1

Figure 2

The sedimentary rocks The Cwm-yr-Eglwys Mudstone Formation is a series of hemipelagic mudstones and thin turbidites laid down in the back-arc basin. Figure 3 They were examined on the coast both west and east of Fishguard. To the west, at the village of Parrog, (O.S. reference SN 04581 39770) a small exposure of folded, dark grey mudstones revealed fine scale cross bedding indicative of current deposition. (see Figure 2). To the east, at Newport Sands, (O.S.reference SN 05402 40857) the cliffs revealed a sequence of turbidite sandstones and mudstones deformed in the Caledonian orogeny into chevron folds. (see Figure 3).

Pembrokeshire 27-28th June 2015 (contd.)

The Fishguard Volcanic rocks The Fishguard Volcanic Group was examined at several localities along the coast of the Pen Caer Peninsula, west of Fishguard. At Strumble Head, near to the lighthouse on the headland (O.S. reference SM 89625 41344), folded but otherwise well preserved basaltic pillow lavas are exposed (see Figure 4). This is the Strumble Head Volcanic Formation, a member of the Fishguard Volcanic Group, which has an accumulated depth of 1000 metres. The pillows were studied in some detail to identify the glassy rims of the chilled margins contrasting with the slightly coarser interiors.

Figure 4 Figure 5 At Y Penrhyn, on the eastern end of the Pen Caer peninsula, the acid component of the Fishguard Volcanic Group was investigated. The Goodwick Volcanic Formation was observed to be a sequence of rhyolitic breccias formed when the congealed crust of acid lava flows was disrupted by movement of molten material in the interior of the flow, a process called autobrecciation. The exposure was not very good, consisting of a series of smooth, curved surfaces barely protruding through the grass slopes on the cliff tops. However, our leader showed us a loose block incorporated into the dry stone wall of a sheep fold (O.S. reference SM 94868 40117) that revealed the rock consisted of rhyolite clasts set in a rhyolite matrix (See figure 5)

The Fishguard Intrusive rocks Dolerites of the Fishguard Volcanic Group were seen in a number of locations. At Pen Caer (O.S. reference SM 89832 41315) a dolerite intrusion could be seen in the sea cliffs revealing a strong vertical jointing. Looking eastwards from here, across the bay of Pwll Bach towards the headland of Carreg Gybi, our leader demonstrated the sequence of sediments and volcanic rocks that shaped the cliff faces.

Looking northeast from the summit of Garn Fawr (O.S. reference SM 89683 38870) a series of dolerite crags could be traced across the landscape forming a classic ―V‖ shaped outcrop of an anticline whose axis was plunging towards the northeast. This was the Llanwnda anticline.

The Pen Anglas headland (O.S. reference SM 94930 40504) is formed of a resistant dolerite intrusion (see Figure 6). In the view it is possible to make out the polygonal jointing patterns on the flat surface in the centre of the frame, directly below the Figure 6 stone pillar on the headland summit.

Pembrokeshire 27-28th June 2015 (contd.)

The intrusions of the Preselli Hills The Preselli Hills lie about 10 miles southeast of Fishguard. They are formed by Lower Ordovician, Didymograptus Shales overlain by the Fishguard Volcanic Group. Dolerite sills intrude both the shales and the overlying volcanics giving rise to isolated high rising crags such as Carn Menyn (O.S.reference SN 14578 32540) (see Figure 7) with its strong jointing picked out by weathering.

Figure 7

These dolerites occur throughout the Preselli Hills but in the Carn Menyn area they have a distinctive texture that has earned them the description of ―spotted dolerites‖ (see Figure 8). They are actually altered rocks. Figure 8 Just as basalts on the sea floor are altered to spilites through interaction with seawater so these dolerites have also been altered. Sodium from seawater (probably from waterlogged, unconsolidated sediments) has entered the rock and calcium has been leached out resulting in the plagioclase feldspar changing from labradorite to albite/oligoclase. The rock has taken on the bluish tinge typical of the spilite clan. The characteristic texture and chemistry of these rocks has revealed that Carn Menyn is the original outcrop of the ―bluestone‖ standing stones at Stonehenge.

Acknowledgements Thanks are due to John Downes for his ever cheerful and knowledgeable leadership and his amazing ability to find geological gems down the most unlikely of tracks.

Thanks are also due to Jan Ashton-Jones and Janet Hiscott of Severnside Branch for undertaking the administrative duties that made the trip possible.

Wye Valley 13th September 2015

Leader: Elliott Carter

It was a lovely September morning as we stood by the side of the B4260 Wilton Road leading up into Ross. There was a good group of us come to be introduced to the geological history of the lower Wye Valley and to be shown how it has shaped the landscape.

We were all strung out along the pavement looking at a road cutting which may have once been a quarry. It is now a Site of Special Scientific Interest (SSSI) and has been kept cleared of most vegetation and has a display board. Elliot described the depositional environment for this sandstone from the Brownstone Formation of the upper Lower Devonian. During this period subduction of the Iapetus Ocean created large mountain ranges behind the subduction zone. These shed erosion material into the area, carried south by large rivers. The cross bedding that we could see (Fig. 1) shows the variety of material brought by these changing braided river systems. The variety of Fig. 1 Wilton Road SSSI showing cross bedding pebbles embedded within the sandstone give an indication of the from a braided river system source of material and these have been mapped by JR Allen.

A short drive brought us to a car park above the Biblins. From here we took a footpath through the woods and into King Arthur‘s Quarry (Fig. 2). We were now up into the Carboniferous Gully Oolite. The oolite limestone was deposited in a shallow sea with little contamination from runoff. The seawater would have been saturated with calcium carbonate and, with small waves agitating the water, oolites were produced. This quarry has been used as a source of aggregate as the limestone has been altered by burial, warming and recrystallization producing a hard grey rock. It is now used by rock climbers. The alteration has destroyed many features in the limestone but some cross bedding was still discernible (Fig. 3).

Fig. 2 Walking into King Arthur‘s Quarry Fig. 3 Altered Gully Oolite, King Arthur‘s Quarry, in which some cross bedding can still be seen.

Further along the quarry wall a change in material had caused the quarrymen to leave a corner of rock. Elliot pointed out the encroachment of a band of sandstone in the upper level (Fig. 4) which would have contaminated the aggregate. This sandstone indicated a change in environment with sea level falling to bring the coastline closer and river borne sands into the area.

Fig. 4 King Arthur‘s Quarry showing change in depositional environment giving a sandstone feature

Wye Valley 13th September 2015 (contd.)

Taking a footpath downhill we passed several short tunnels into the limestone which Elliot explained may have been carved by the River Wye when it created the valley it now occupies. In the process of down-cutting the river left benches and excavated the valley walls along lines of weakness. Natural England hold the PhD paper from the 1930‘s which plotted the course of the Wye at different levels in the valley and it is available to be studied in their offices. The Cave known as Arthur‘s Cave (he did get everywhere in the West of England and Wales) was probably constructed in the same way as there is evidence of a large fault which was a line of weakness (Fig. 5). This cave was excavated in the 19th Century by Rev. Symond and found to contain Prehistoric flint tools and animal remains! The path led down to the Seven Sisters Viewpoint where we had lunch overlooking the River Wye (Fig. 6). Here the river has cut a deep, steepsided valley through the resistant Carboniferous limestone and the Upper Devonian sandstone and conglomerates. Later we would look out from Symonds Yat Viewpoint to a different landscape (Fig. 7) where the river has taken a wide meander through the softer Sandstone of the Lower Fig. 5 King Arthur‘s Cave Devonian. The overall pattern of developed as a result of water the Wye is of a series of looping action along a fault meanders that cross back and forth from the harder limestone to the softer sandstone ignoring the difference in resistance. This is a superimposed river pattern. The original meanders formed on a level, wide floodplain then as sea levels fell during the Ice Age the river cut down through the rock and the differences in resistance showed only in the width of the valleys. We continued on to the riverside near Biblins youth campsite where we Fig. 6 Wye Valley from Seven Sister‘s viewpoint studied the cliff (Fig.8a). Once we found our way through the vegetation a closer inspection revealed the reason for the shiny surface to the rock. Rain water, saturated in calcium carbonate (CaCO3) from its passage through the limestone, had seeped out of the cliffs and the CaCO3 was precipitated out of solution and deposited on the surface of the rock as Tufa or flowstone giving some strange forms (Fig. 8b).

Fig. 7 Wye Valley from Symonds Yat viewpoint

Fig. 8a The Biblins – recent deposition of flowstone observed from path along Wye

Fig. 8b Closer inspection of flowstone

Elliot had given us a good introduction to the geology behind the impressive landscape of the Wye valley and had shown us how it had been Report by changed. Averil Leaver BSc (Hons) Geoscience 19

Cardiff Building Stones 25th October 2015

Leader: Linda Fowler

Introduction This field trip to Cardiff was led by Linda Fowler, and explored the stones used in a number of buildings (old and new) in the Cardiff Bay waterfront area, which is situated in South Cardiff. We also walked along the Cardiff Bay Barrage to examine some in situ rocks in the tall cliff at Penarth Head. This report is based on Linda‘s commentary during the day and her written notes. Most of the pre-lunch part of our outing is described in Leaflet 13 ―Building Stones of Cardiff‖ produced by the Geologists‘ Association - South Wales Group. A map of the Cardiff Bay area is shown in Fig. 1, and pinpoints the key parts of our route during the day. A more detailed view of the Waterfront area is shown in Fig. 2. This shows the route we took during the morning as we zig-zagged around various streets examining a variety of stone.

Fig. 1 Map of Cardiff Bay

We all met at the Havannah Street car park adjacent to the Wetlands area of Cardiff Bay. Once assembled, Linda explained the proposed itinerary for the day and gave a potted history of Cardiff over the last 200 years or so. Cardiff came to prominence with the building of docks and other infrastructure for export of the vast amounts of coal mined from the valleys of South Wales. The docks were built between 1830 and 1907, with the first one opening in 1839. The last cargo of coal left Cardiff in 1964 and, since 1987, the area has undergone considerable redevelopment. Fig. 2 Map of our route in the Waterfront area of Cardiff Bay This included the building of a barrage across Cardiff Bay and substantial renovation of the waterfront and docks. Wetlands Reserve Before setting out on our walk, we enjoyed the view of Cardiff Bay from the car park, looking out across the man-made Wetlands Reserve opened in 2002 when the Barrage was finished. This was previously an area of raised salt marsh, but has now changed to a freshwater environment, losing the intertidal areas. The Wetlands Reserve is a great spot for bird watching. Interestingly, the Cardiff Bay Barrage incorporates a specifically designed ―fish pass‖ to allow migratory salmon and sea trout to return to the rivers Taff and Ely. The ―fish pass‖ allows the fish to swim at all states of the tide from the estuary up and into the Bay. Geologically, Cardiff is built on Triassic Mercia Mudstone, which was deposited in a hot desert environment between 200 and 250 million years ago.

Ocean Reach Building A short walk from the car park took us to our first building of the day, the relatively new Ocean Reach apartment block. It is clad in two types of stone, one green and one yellow (Fig. 3). We quickly identified the yellow stone as a medium-grained sandstone. Variations in iron content had produced different coloured patterns in the individual sandstone blocks. Linda pointed out a number of coloured bands in the sandstone, a feature known as Liesegang Rings. Fig. 3 Examining the two types of facing They are produced when the pH and Eh conditions of pore water stone on the Ocean Reach building chemistry allow deposition of iron, which in turn causes colouring of the sandstone.

Cardiff Building Stones 25th October 2015 (contd,)

The identity of the green polished facing slabs was not quite as obvious. It had a sedimentary-like structure with graded bedding and rip-up casts (Fig. 4). After a lot of discussion and helpful hints from Linda, we were persuaded that the slabs were composed of volcanic tephra from ash up to lapilli size which had been deposited in water and subsequently metamorphosed into a slate. The green colour resulted from the presence of chlorite, and the rock was probably Upper Ordovician from the Lake District.

Mount Stuart Dry Docks On our way to the Pilotage Building we passed the Mount Stuart dry docks, built Fig. 4 Rip-up casts in the green slate for the maintenance and repair of ships. In later years it was used by British of the Ocean Reach building Rail car ferries, Royal Navy ships, and tankers.

Pilotage Building The Pilotage Building is the oldest building in the area, and was used as the administrative centre for the pilots who operated in the Bristol Channel. Despite the misleading sign on the wall, the building dates back to the 1880s. It is now used as a restaurant (Fig. 5). The building is constructed from grey sandstone, most probably Carboniferous Pennant Sandstone which crops out widely in the South Wales Coalfield. A pair of polished granite columns of unknown origin grace the entrance door.

Fig. 5 Grey sandstone was used in the Fig. 6 A wedge-shaped Pinna fossil in the oolitic lime- construction of the Pilotage Building stone of the old National Provincial Bank building

113/116 Bute Street Our next stop was at the old National Provincial Bank building at 113/116 Bute Street, built in the 1920s. It is constructed from prestigious white Jurassic Portland Stone, an oolitic limestone originating from the Isle of Portland in Dorset. With the aid of a hand lens, we could clearly see broken shells and ooids. There was a very fine example of the distinctive bivalve fossil known as Pinna on the stone surface. Viewed end-on it has a wedge-shaped cross-section (Fig. 6). Portland Stone formed in a marine environment on the floor of a shallow, warm sea probably near land. Back-and-forth movement of the shallow sea allowed the spherical ooids to grow on the sea floor around small nuclei created from precipitated calcite. The ooids were then cemented by more calcite to form a hard limestone.

54 Bute Street We then walked a short distance to Pascoe House at 54 Bute Street, built in 1875 for a coal company. We quickly spotted three different types of stone used in its construction; these were coloured grey, red, and yellow-buff. A fourth polished stone was used as an ornamental pillar on either side of the entrance door (Fig. 7). The buff coloured stone had been carved and used to decorate the building. With the aid of a hand lens we saw that this stone contained ooids and fossil shells, similar to the Portland Stone seen in the previous building. It was therefore an oolitic limestone and, as Linda explained, was in fact Middle Jurassic Bath Stone extracted from a quarry near the city of Bath in Somerset. Fig. 7 Examining the four types of stone used in the construction of Pascoe House

Cardiff Building Stones 25th October 2015 (contd.)

The red stone used along the The red stone used along the bottom of the building and in the arches above the ground-floor windows had a much coarser texture consisting of angular clasts of various sizes within a fine-grained matrix (Fig. 8). This was clearly a breccia. Linda explained that it was local Radyr Stone from the Upper Triassic, formed in a hot dry climate where occasional torrential rain led to flash floods. These carried rock debris along wadis which was then dumped as alluvial fans at their mouths. Over time, the fragments were cemented together with sand and calcium carbonate. The Radyr Stone, which was quarried just north of Cardiff between the years of 1850-1910, helps to confirm date the building (1875). The grey stone used in the walls between the ornate windows was similar to that seen Fig. 8 Close-up of the Radyr earlier in the Pilotage Building, and was presumed to be Pennant Sandstone. Finally, we Stone breccia on Pascoe quickly concluded that the two polished ornamental pillars at the doorway were granite. House Linda explained that it was known as Bessbrook Granite and came from the Mountains of Mourne in Northern Ireland. However its age was a matter of debate, with one source (the GA Guide) quoting 50 million years old and another (Ruth Siddall from UCL) quoting 400 million years old – yet another example of the uncertainty associated with some rocks! If Ruth is correct, the rock would be Newry Granodiorite, intruded during the Caledonian mountain building phase. If the GA are correct, the rock would belong to a much later felsic intrusion into the Newry Granodiorite, the Paleocene Slieve Gullion ring-dyke complex.

Craft in the Bay We continued our stroll through the streets of Cardiff and arrived at the ―Craft in the Bay‖ building. Currently used as an exhibition gallery and craft shop, it is an old dock warehouse that was moved and enlarged about 15 years ago. On the surface of the slate pavement outside the building we spotted some yellow cubic crystals of pyrite (iron sulphide or ―fool‘s gold‖). Pyrite frequently appears in metamorphic rocks such as slate, and would have formed during the metamorphism of anoxic sedimentary muds containing organic matter (Fig. 9). Not far away was a granite bollard from one Fig. 9 Gold-coloured iron pyrite of the old docks (Fig. 10). This possibly came from crystals on the surface of a slate Bodmin Moor in Cornwall. paving slab adjacent to the ―Craft . in the Bay‖ building

Millennium Centre Fig. 10 Granite bollard from Our next stop was at the splendid Millennium Centre, a prestigious arts centre clad in one of the docks, now relo- Welsh slate. Five types of Welsh slate, each with its own distinctive colour, were used in the cated outside the ―Craft in construction of the building, the colours resulting from differences in their composition. the Bay‖ building (Fig. 11). All slate came from waste quarried material.

Roald Dahl Plass Adjacent to the Millennium Centre is Roald Dahl Plass, a public space built on the former Oval Basin. This was the oval-shaped entrance to the large West Bute Dock which turned Cardiff into a major coal port when it opened in 1839. Although the dock has now been filled in, when operational it allowed up to 300 ships to berth at any one time. The new architecture of Roald Dahl Plass still retains the old upper stonework of the Oval Basin dock Fig. 11 The Millennium Centre with its five different entrance. The dock walls types of slate Fig.12 Top of the Oval Basin dock were originally built of wall showing the granite blocks used Pennant Sandstone, and hard granite had been used to protect the top lip of at the top, and a block of red Radyr the dock wall from the battering of moored ships. We noticed, however, local red Stone beneath used for repair of the Radyr Stone had been used in places to repair sections of the dock wall, no doubt wall an economy measure (Figure 12). 22

Cardiff Building Stones 25th October 2015 (contd.)

Scott Harbour Compass Rose After a much longer walk than originally planned we arrived at Scott Harbour, named after Captain Scott, whose ship the Terra Nova sailed from Cardiff Docks to the South Pole in 1910. The new buildings on the dock side, built in 1997, are clad in Rapakivi Granite, a Finnish granite noted for its large rounded (―orbicular‖) orthoclase feldspar crystals containing specks of dark minerals (Fig. 13).

At one end of Scott Harbour we came across an ornate compass rose constructed from a number of decorative stones.

With Linda‘s help we identified gabbro, gneiss, migmatite and a number of differently Fig. 13 Close-up of the Rapakivi -coloured granites (Fig Granite used to face buildings at 14). Scott Harbour, showing the distinctive orbicular feldspar crystals

Fig. 14 The ornamental compass rose at Scott Harbour Atradius Building with its decorative polished stones Heading further along the dockside, we came to the Atradius Building, opened in 1995 to provide commercial office accommodation. It is clad in two types of decorative stone, which have apparently been described as granite. However, examination showed that they were something quite different. The lighter-coloured stone contained small red garnet crystals and was technically a ―garnetiferous gneiss‖ (Fig. 15). The darker stone exhibited spectacular banding typical of a migmatite formed by the partial melting of a gneiss (Fig. 16). Some of the features we saw in this stone, a ―migmatitic biotitic granodioritic gneiss‖ according to the GA leaflet, included boudinage and pegmatite.

Fig. 15 Close-up of the light-coloured Fig. 16 Close-up of the darker-coloured facing stone on the Atradius building, facing stone on the Atradius building, showing numerous red garnet crystals showing the coloured banding within the migmatite

Waterguard Pub Now a very fine pub, this building was built in 1870 as the HM Customs and Excise House using grey Pennant Sandstone and buff Bath Stone. A new addition to the building is faced with a fine-grained cream Magnesian Limestone from Yorkshire.

Norwegian Church Mentally exhausted by the vast amount of information imparted by Linda, we sat outside the Norwegian Church in the warm sunshine to recuperate, looking over the Bay and enjoying our lunch. The Church, established in 1885, was built to provide religious and social care to the thousands of Norwegian sailors employed by the Norwegian merchant fleet. Having fallen into disrepair, the Church was rescued by The Norwegian Church Preservation Trust in 1987 and carefully dismantled and re-erected on its present site. It is now used as an arts centre.

Cardiff Building Stones 25th October 2015 (contd.)

Barrage Coal After lunch we walked across the Cardiff Bay Barrage, stopping to look at information boards about coal mining and the coal trade on the way. Nearby were a couple of enormous lumps of coal that clearly showed the original bedding planes of these ancient deposits (Fig. 17).

Penarth Head Having crossed the Barrage to the Penarth side of Cardiff Bay we headed towards the tall cliff-face nearby at Penarth Head. This displayed horizontal layers of sedimentary Triassic and Jurassic rock (Fig. 18). Towards the base of the cliff were distinctive red and green mudstones of the Triassic Mercia Mudstone Group, with intermittent bands and nodules of white and pink gypsum. Many blocks of gypsum had fallen from the cliff-face onto the beach as a result of erosion and we came across two varieties, alabaster and satin spar. Further up the cliff were the marine sedimentary layers of the Penarth Group and Blue Lias Formation. Blocks from these layers were scattered across the beach and we soon found bivalve fossils in some of them, as well as slabs with wave ripples (Fig. 19).

Fig. 17 A large lump of coal from a Welsh mine, now displayed on the Cardiff Bay Barrage

Fig. 18 The cliff-face at Penarth Fig. 19 Our beachcombing finds at Head, showing horizontal layers of Penarth Head included bivalve fossils Triassic / Jurassic sedimentary rock embedded in rock, gypsum blocks, and slabs with wave ripples

Pierhead Building We returned to the Cardifff Bay Waterfront by water-bus and headed towards the impressive Pierhead Building, a Grade 1 listed building and our final stop of the day (Fig. 20). Opened in 1897 as the headquarters of the Cardiff Railway Company, the exterior of the Pierhead is finished in Ruabon glazed terracotta bricks, some of which are very ornate.

Summary It was amazing how much variety was packed into this field trip. We examined building stones from far and wide, collected local rock from Penarth Head, learned about the industrial history of Cardiff and had a cruise on a water-bus. Our thanks go to Linda for answering our many geological questions and for making this a most memorable day.

Report and photographs by Fig. 20 The Pierhead Building, clad in Norman Nimmo-Smith glazed terracotta bricks

Weston-Super-Mare 20th March 2016

Leader: Dave Green

Introduction Weston-Super-Mare in Somerset is well known as a seaside resort and for the areas of thick mud exposed at low tide beyond its sandy beach. But Weston-Super- Mare is also a fascinating geological location, as we found out during this field trip led by Dave Green. We visited two coastal sites to the north of the town and examined sedimentary rocks of Carboniferous age containing a variety of volcanic deposits. Our group of 16 people, which included four new members, met in the National Trust car park at Middle Hope, a promontory projecting into the Bristol Channel about two miles north of Weston-Super-Mare. Once assembled, Dave provided us with a hand-out before describing the geology of the area and the proposed itinerary for the day. In the morning we would visit Swallow Cliff (Site 1) on the north side of Middle Hope. After lunch we would head south to Spring Cove (Site 2), which is situated opposite Birnbeck Island. (Fig.1). Both sites should be visited around low tide.

Geological Context The rocks we examined were laid down during the Early Carboniferous period, about 350 million years ago. At this time Britain was situated near the equator, Fig. 1 Location of Sites 1 and 2 towards the edge of the continent of Laurussia and experienced episodes of volcanic activity. Subsequent folding of the rocks took place in the Late Carboniferous, during the Varsican mountain-building phase, when Laurussia collided with the continent of Gondwana to form the super- continent of Pangaea. It was during this time that the Mendip Hills were formed, lying lie close to the northern limit of mountain building. Erosion of Permian deposits resulted in an unconformity between Carboniferous and Triassic deposits. Similarly, rocks formed during the later Jurassic, Cretaceous and Tertiary periods were subsequently removed by erosion. The rock exposures we saw during the trip are shown in the stratigraphic column (Fig. 2), which covers a time period from about 350 to 340 Fig. 2 Stratigraphic Column million years ago. The letter ―v‖ indicates the existence of volcanic material in the sequence of rocks.

View from Middle Hope From the car park we ambled up the path to the top of Middle Hope, a limestone ridge, where we enjoyed commanding views over the Bristol Channel and surrounding countryside. To the west, a few miles offshore, lay the small islands of Steepholm and Flatholm. In the far distance to the south we could see a headland sticking out into the Bristol Channel. This was Brean Down, another limestone ridge, located south of Weston-Super-Mare. Dave explained that both Middle Hope and Brean Down are westward extensions of the Mendips that stretched out into the Bristol Channel. Both Steepholm and Flatholm are part of this extension. Below us, looking south, the flat surface of Sand Bay stretched for nearly two miles towards the wooded limestone ridge of Weston Woods. Showing us a cross-section from his hand-out, Dave explained that there was a syncline beneath Sand Bay that had been filled unconformably with Triassic and Jurassic sediments (Fig. 3). These had subsequently been covered with recent marine sediments from the last ice age.

Fig. 3 Cross-section showing north-south folding

Weston-Super-Mare 20th March 2016 (contd.)

As we were admiring the view and gazing at the Bristol Channel, Dave posed a question. Why are copious amounts of mud deposited at Weston and elsewhere in the fast-flowing tidal waters of the Bristol Channel? After all, isn‘t mud deposition normally associated with a low-energy environment? We struggled to explain this phenomenon. The answer, he told us, is a chemical process called Flocculation that occurs when muddy fresh water meets salt water. The salt causes the clumping of minute clay particles into larger masses that are too heavy to remain suspended and therefore sink to the sea-floor.

Swallow Cliff (Middle Hope) – Site 1 We continued our stroll across Middle Hope and descended to a beach on its north side, at Swallow Cliff. Dave pointed out beds of grey rock on the foreshore dipping at about 30 degrees to the south (Fig. 4). Upon closer examination we could see ossicles from crinoid stems on the surface (Fig. 5). Digging deep into his rucksack, Dave pulled out a bottle of dilute hydrochloric acid and dropped some onto the rock surface. Instant fizzing proved that it was indeed limestone, known as Black Rock Limestone. We noticed some lighter-coloured nodules within the limestone, the largest being about 10 cm in size. These did not fizz when acid was applied and Dave explained that this was chert, a variety of crystalline silica (Fig. 6). The silica had been deposited at the same time as the lime before becoming concentrated into nodules and may well have originated from sponges or other marine animals with silica skeletons.

Fig.6 Chert nodules in Black Rock Limestone

Fig.4 Dipping beds of Black Rock Fig.5 Ossicles from a crinoid stem

Limestone at Swallow Cliff (pound coin for scale)

We then examined an exposure in a low cliff-face a short distance away, at the back of the beach, consisting of southward dipping limestone beds separated by beds of a clay-like rusty-coloured material (Fig. 7). This turned out to be volcanic ash (tuff) erupted from the cone of an ancient offshore volcano and deposited on the sea-bed, on top of the limestone. The iron -rich minerals such as pyroxene in the basalt produced the rusty colour. Moving up the sequence exposed in the cliff-face, we could see a thick bed of similar material representing five metres or more of continual ash deposits, before quieter conditions returned and more limestone was deposited on top.

Fig. 7 Alternating beds of Limestone and Tuff

Weston-Super-Mare 20th March 2016 (contd.)

Beyond a thick limestone bed we came across a (younger) dark volcanic rock containing coarse clasts, representing another phase of eruption. This was a lapilli-tuff. The dark rock surface was covered in a striking patchwork of white calcite ―net veins‖ (Fig. 8) and just beyond this was a thick bed of basalt from a lava flow. Towards the foot of Swallow Cliff itself, those with keen eyes spotted features such as cross-bedding and graded bedding in a layer composed of a mixture of lapilli-tuff and lime (Fig. 9).

Fig. 8 ―Net Veins‖ of calcite in lapilli-tuff Fig. 9 Graded bedding near the foot of Swallow Cliff A more recent Quaternary feature pointed out by Dave at this site was a raised beach, lying about 7 metres or so above the present beach (Fig. 10). It was clearly visible in cross-section; a conglomerate of rounded stones and pebbles sitting within a light brown matrix. This raised shore platform represents a period of eustatic sea level rise, higher than present day, during the last warm interglacial period (the Ipswichian) about 100,000 years ago. We then retraced our steps across Middle Hope and returned to the National Trust car park for lunch and a short drive south to our second site at Spring Cove.

Fig. 11 Gully Oolite (left) and Caswell Bay Fig. 10 Raised beach at Swallow Cliff Mudstone (right) at Spring Cove

Spring Cove – Site 2 From the roadside at a derelict café opposite Birnbeck Island, we descended down a steep slope onto the beach at Spring Cove. Here we noticed that the rocks were dipping south at about 30 degrees, which we were relieved to see was consistent with the cross-section shown in Dave‘s hand-out. To get to the cove, Dave explained that we had clambered down the bedding plane of a limestone rock known as Gully Oolite. On its face we could see a number of circular holes which had apparently resulted from weathering when the limestone had risen above sea-level. Lying above the grey Gully Oolite limestone, chronologically speaking, was a hard purplish-red rock that failed the ―fizz‖ test when Dave applied his hydrochloric acid. This turned out to be a dolomitic mudstone known as Caswell Bay Mudstone (Fig. 11). With a bit of prompting from Dave we spotted solitary rugose corals (Caninia) on its surface (Fig. 12). Fig. 12 Coral Caninia in Caswell Bay Mudstone

Weston-Super-Mare 20th March 2016 (contd.)

These ranged from two to three centimetres in diameter and, interestingly, they passed the ―fizz‖ test, demonstrating that had not been dolomitised. Towards the top of the cliff the 350 million-year-old dipping Caswell Bay Mudstone had been levelled off by erosion to produce an angular unconformity, on top of which were Quaternary beach deposits (Fig. 13). These were a mere 400 thousand years old (Anglian).

We then moved further along the beach to the next rock unit in the chronological sequence, a limestone known as Birnbeck Limestone, no doubt named after Birnbeck Island just opposite. Within this massive grey rock was a thick bed of black rock full of angular fragments – this was a basaltic lava flow with a thickness of about 15 metres at this point. Within the brecciated rock we could discern the distinctive shapes of pillow lavas with glassy rinds, albeit some more convincing than others, but pillow lavas nonetheless (Fig. 14). This clearly indicated an underwater volcanic eruption. Dave pointed out that most of the pillow lavas had broken up when erupted onto the sea-floor, producing a lithified rock packed with breccia. When Dave hammered out some samples of the lava, some of us were surprised to see that it was reddy-brown in colour and not the dark colour often associated with basalt. He explained that hot fluids circulating through the rock on the sea-floor at the time of the eruption had altered the rock‘s composition.

Fig.14 Pillow lava surrounded by basaltic breccias Fig.13 Angular unconformity between Carboniferous rock and Quaternary deposits

Before climbing back out of Spring Cove to return to our cars, we spent some time looking at the Gully Oolite rock in more detail (Fig. 15). Surprisingly, despite its name, we couldn‘t see any sign of ooids in the samples of rock Dave hammered out for us. The rock had a crystalline structure that didn‘t ―fizz‖ with the application of acid. Dave explained that the limestone rock had been buried long enough for localised dolomitisation to take place, when calcium carbonate was converted to calcium magnesium carbonate. And so ended our interesting visit to Spring Cove.

Fig. 15 Gully Oolite rock with no sign of the original oolites Summary The sites we visited at Swallow Cliff and Spring Cove had a great variety of well-exposed geological features packed into a relatively small area, and we could have spent many more happy hours examining them in detail. The mixture of sedimentary and volcanic deposits made the sites even more special. Dave provided an excellent commentary throughout the day, patiently answering all our questions and our thanks go to him for making this a most enjoyable day-out at the seaside.

Report and Photos by Norman Nimmo-Smith

Wigpool Syncline 24th Apri 2016

Leader: Dave Green

We all gathered on a wooded corner between Drybrook and Lea on a cool but bright morning to explore another of the fold features that have been formed in this corner of the Forest of Dean. The Wigpool Syncline is, paradoxically, a hill formed over a plunging, downfolded feature associated with movement along the Malvern Fault. We were going to look at the sequence of rocks, from Old Red Sandstone to Carboniferous Limestone series, that have been exposed at the surface by the erosion of this syncline.

Sequence Rock Name Thickness (m) Triassic Keuper/ Mercia mudstones 30 Unconformity Carboniferous Upper Coal Measures Supra-Pennant Group 330 Pennant Group 200-250 Trenchard Group 15-130 Lower and Middle Coal Measures Absent- Unconformity Carboniferous Lime- Drybrook Sandstone and Lime- 230 stone Series stone Whitehead Limestone 15-50 ?Unconformity Crease Limestone 10-30 Lower Dolomite 75-130 Lower Limestone Shales 60-75 Old Red Sandstone Upper ORS Tintern Sandstone Group 100 Quartz Conglomerate 6-30 Unconformity Lower ORS Brownstones 830

Fig. 1 Generalised Stratigraphy of the Forest of Dean area (after Trotter, 1942)

We crossed the road and entered the Lea Bailey Light Railway site (http://www.leabaileylightrailway.co.uk/home.htm ). The metal doorway entrance to the mine workings, (Figs 2 & 3) originally for gold and later iron ore, was cut through our oldest exposure, the Lower Old Red Sandstone (ORS) Brownstones. The bed of brown/grey sandstone was measured to be dipping 40o to SE and to be a friable, poorly sorted, subangular grained sandstone which contained mud clasts. This was indicative of being deposited from a braided river close to an uplifting mountain source of sediments.

Fig. 2 Dave Green and Severnside at entrance Fig. 3 Behind the door - Lea Bailey Engine door to mine workings

Wigpool Syncline 24th Apri 2016 (contd.)

After crossing the railway track at the road we cut back and uphill along a forest track. Through the trees Dave pointed out the edge of an outcrop of the next strata of rock, some large blocks had fallen down the bank from it. Further up the track we intersected with it and found a well cemented quartz conglomerate containing rounded pebbles ranging in size up to 2 inches. The pebbles were of veined quartz showing a metamorphic source. We were able to identify mud clasts, cross bedding, scour surfaces and pebble lags (Fig. 4) indicating the source as a terrestrial river environment.

A short distance further on we were walking on a different surface. Close inspection showed a nodular, cemented calcrete or caliche. This limestone was deposited in a desert environment from water being drawn up through the soil by evaporation. The last exposure of the ORS on the path side was the Tintern Sandstone, a buff/greenish sandstone with small mud flakes. This is thought to be a beach deposit from the transition period as the sea level fluctuated at the start of the Carboniferous.

We found our last exposure of the morning in a small quarry filled with spoil heaps just off Fig 4 Exposure of Quartz the track. The fresh fine- Conglomerate showing mud clasts, grained rock fizzed when scour surfaces, lags of pebbles and subjected to acid, indicating a cross bedding limestone of the Lower Photo: Averil Leaver Limestone Shales. (Fig. 5) Dave suggested that these micrites and calcareous shales may have been quarried for local buildings or cement and that they were laid down in lagoons on an arid coast near the equator, an environment similar to that of the Arabian Gulf now.

Fig. 5 'A fine-grained, micritic limestone with

Dolomite crystal inclusions'

After a refreshing break for lunch in Weston under Penyard we returned to Wigpool Common for a geological mapping session. Using a detailed map of the area, Dave led us around Wigpool Common which sits on the top of the dipping Wigpool Syncline. Using the features of the landscape, the vegetation, a few small quarries and iron workings and the lumps of rock within the soil (brash) he demonstrated how to build a picture of the underlying geology. Some of us were able to practice measuring the dip and strike of the few exposures in the quarries and these, when added to the map, gave an indication of the dipping syncline. During our walk we were able to identify the remaining types of rock from the Carboniferous up to the Whitehead Limestone and noted how in the quarry on the nose of Syncline the Lower Dolomite was revealed to be in the centre of the hill. This confirmed that the younger strata from the centre of the syncline, which had been compressed during folding, were more resistant to erosion. As we dispersed at the end of an interesting day the hope was expressed that the mapping exercise could be repeated to reinforce the learning experience.

Report by Averil Leaver

Fforest Fawr Geofest

The Fforest Fawr Geopark is set in the Brecon Beacons National Park in South Wales, and a Geopark Festival takes place in May/June each year with walks, talks and exhibitions. I was able to join two of the walks this year, one from Ystradfellte and the other from Craig-Y-Nos. Geological Context The rocks exposed in the Fforest Fawr Geopark cover a time span of about 150 million years, from the Ordovician Period (~450 Ma) to the Carboniferous (~300 Ma). Ystradfellte and Craig-Y-Nos lie towards the south of the Geopark in Carboniferous Limestone country. The limestone is sandwiched between the Devonian Old Red Sandstone and the younger Twrch Sandstone (previously Fig 1 Simplified cross-section from north to south in known as the Basal Grit), as shown in Fig 1. The sedimentary strata the vicinity of Ystradfellte and Craig-Y-Nos were subsequently tilted gently southwards during the Variscan Orogeny. Walk from Ystradfellte The walk on 29th May was led by Dr Geraint Owen from Swansea University, and explored various features in the limestone landscape including shake holes, sinkholes, caves, quarries, limestone pavement, fossils and glacial action. Fig. 2 shows a typical view of the landscape, with the stepped rock layers on the horizon dipping south. Leaving the car park at Ystradfellte we walked uphill along a Fig 2 On the horizon, stepped layers of rock dipping footpath until we came to an old stone wall built from a mixture of south grey limestone and brown Old Red Sandstone (Fig 3). Whereas the limestone blocks were sourced from local bedrock, Geraint explained that the blocks of sandstone were derived from glacial erratics transported from the north about 20,000 years ago when the ice-sheet extended as far south as the Gower Peninsula. Some of the limestone blocks included fossil coral (Fig 4).

Fig 3 Collapsed stone wall of Carboniferous Fig 4 Fossil Coral in wall Fig 5 Geraint describing the clints and Limestone and Old Red Sandstone blocks grikes of the limestone pavement Climbing further up the slope we passed a number of shake holes, depressions in the ground caused by collapse of the surface layer of limestone. We were now in a karst landscape formed by the dissolution of the soluble limestone in acidic water. We reached a large expanse of limestone pavement and examined the slabs (clints) that were separated by vertical fissures (grikes) as a result of this chemical weathering process (Fig 5). Glacial abrasion had scraped away the topsoil to expose the bare rock underneath. We headed downhill and

reached the River Nedd Fig 6 Dry bed of the River Nedd Fechan Fechan in the valley below. As we walked beside the river we saw that the water suddenly disappeared down a sinkhole in the river bed. Geraint led us ―downstream‖ along the (now) dry river bed (Fig 6) to a very large sink-hole, where the river bed disappeared underground. Some of us clambered down the steep side of the hole to see a pool of standing water in the vertical shaft that led into the underground river system. (Fig 7). In times of heavy rainfall, when the flow rate increased sufficiently, the river temporarily flowed along its original path and into the sinkhole. The River Nedd Fechan eventually emerged at the surface further down the valley – something to explore on another day. Fig 7 Clambering down the sink-hole on the River Nedd Fechan 31

Fforest Fawr Geofest (contd.)

We climbed back up onto the hillside and found that our route lay at the very top of the Carboniferous Limestone layer, at its unconformable upper boundary with the Twrch Sandstone. Exposures of Twrch Sandstone quartz conglomerate lay on one side of the path (Fig 8). This boundary between limestone and conglomerate represented a drop in relative sea-level, resulting in erosion of the limestone, followed by deposition of sand and gravel debris in river channels. Our route now took us to the Cwm Porth car park. From here we walked down another dry river bed. Unlike the previous one, which still ran with water on occasions of heavy rainfall, this ancient river bed had clearly not experienced running water for a long, long time. Part way down the steep-ish path we encountered a hole in the bed-rock through which we could see an underground river. Continuing further down the trail we arrived at a point where the underground river (the River Mellte) appeared at the surface through the mouth of a large cave (Fig 9). Gareth reminded us that in a karst region the reappearance of a stream or river is known as a resurgence. To see where the River Mellte disappeared below ground, we walked back up the way we had come and then descended into a narrow gorge where we discovered that the river entered a cave at the bottom of a vertical cliff (Fig 10). This was our last attraction of the day and a short leisurely walk in the sunshine took us back to Ystradfellte, where we all enjoyed a well-deserved drink at the New Inn pub and thanked Geraint for an excellent geological outing.

Fig 8 Block of weathered Twrch Fig 9 The River Mellte emerging Fig 10 The River Mellte disappearing into the Sandstone quartz conglomerate from the cave system cave system

Walk from Craig-Y-Nos The walk on June 1st was led by one of our members, Dilys Harlow, and was organised with members of Brecon Beacons Park Society to celebrate the launch of the revised route of the Beacons Way. Dilys published a book in 2014, entitled ―The Land of the Beacons Way‖, that describes the scenery and geology along its route. Detailed descriptions of the short sections we walked from Craig-Y-Nos are included in her highly recommended book. We departed from the car park at Craig-Y-Nos Country Park and followed the Beacons Way eastwards up the limestone hillside towards Penwyllt. On the way we passed the lower entrance to the Ogof Fynnon Ddu cave system, one of the largest in the British Isles. After a further climb we arrived at Penwyllt and had a rest at the disused railway Fig 11 Remains of limekiln on left-hand side of station adjacent to a large limestone quarry, which had an old limekiln entrance to limestone quarry at its entrance (Fig 11). A number of quarries and limekilns were established in the area during the 19th century. There is also an old brickworks nearby where furnace bricks were made using silica extracted from local quarries. In its heyday, up to 500 people lived at Penwyllt.

We continued our walk upwards along an old tram road, passing some shake holes on the way. Further up the hillside we had a good view of the craggy limestone ridge of Cribarth on the other side of the Tawe valley, and the row of workers‘ cottages at Penwyllt we had just left behind (Fig 12). There are also limestone quarries on the summit of Cribarth.

Fig 12 Looking back towards the limestone summit of Cribarth 32

Fforest Fawr Geofest (contd.)

Resuming our climb we eventually reached the Ogof Fynnon Ddu National Nature Reserve. We found good exposures of limestone pavement at this spot and numerous fossil corals to examine whilst we stopped for lunch. (Figs 13 and 14).

Fig 13: Fossil coral on limestone Fig 14: Large block of fossil coral pavement

After lunch we retraced our steps back along the Beacons Way to Penwyllt before taking a different route down the hillside to join up with the Beacons Way at Pwllcoediog Farm on the far side of Craig-Y- Nos, thus allowing us to complete a circular walk. We passed an attractive exposure of Carboniferous Limestone on the way, as shown in Fig 15. This photo also shows a scarp of tough Twrch Sand- stone forming the high ground in the distance.

Fig 15: Walking past a large exposure of Carboniferous Limestone

A pleasant surprise awaited us back at Craig-Y-Nos Country Park. Dilys had baked a special cake to celebrate the launch of the revised route of the Beacons Way! Not only that, but she had cleverly decorated the top of the cake with a map of the Beacons Way crossing the National Park from east to west. Small chocolate buttons marked the route and a number of small flags pinpointed the various places visited on the Fig 16: Celebration cake of The Beacons Way way (Fig 16). Alan Bowring, Geopark Development Officer, said a few baked by Dilys words before the yummy cake was cut into pieces for us to enjoy. Mary Berry would have been very impressed – well done Dilys!

[Dilys Harlow‘s book ―The Land of the Beacons Way‖ is published by the South Wales Geologists‘ Association and is available from http://www.swga.org.uk/ ].

Report & Photos by Norman Nimmo-Smith

Malvern Hills September 4th 2016

Leader: Moira Jenkins Introduction A group of fifteen members assembled on a dry and warm Sunday morning at Swinyard car park, situated towards the southern end of the 9-mile length of the Malvern Hills. Our field trip to the Malvern Hills was led by Moira Jenkins, a geological consultant with the Herefordshire & Worcestershire Earth Heritage Trust, who lives in Malvern and is very familiar with the local geology. During the day we explored the area between Gullet Quarry (SO 762 381) and British Camp (SO 760 400), as shown in Fig. 1. The Malvern Hills include some of the oldest rocks in England and are mainly comprised of diorites and granites formed deep underground about 670 Ma (million years ago). These Precambrian rocks are known as the Malverns Complex, a very apt name given the somewhat complex geology of the area. Before heading off Moira showed us specimens she had brought with her - a dark mafic diorite and a pink felsic granite containing orthoclase feldspar. She also showed us a samples of local Silurian sedimentary rock exhibiting bioturbation and a fossil coral. The succession of local rocks that we investigated and discussed during the day are shown in Fig, 2. Gullet Quarry We walked uphill from the Swinyard car park to Gullet Quarry, the last working quarry on the hills which closed in 1977. This flooded quarry displays a massive face of Precambrian rock with a deep lake at the bottom (Fig. 3). The igneous rocks of the Malverns Complex Fig. 1 Map showing the locations exposed in the quarry have been visited (map reproduced with compressed and metamorphosed to permission of the Ordnance Survey) produce schist and gneiss. Pink pegmatite intrusions could be seen cutting through the well- foliated rock face. In the past it was possible to reach the foot of this rock face by walking down a ramp (shown in Fig. 3) to examine at close quarters the various features of gneiss, schist, fault planes, shear zones and igneous Fig. 2 Succession of rocks intrusions. Unfortunately, as a investigated during the trip result of two tragic fatalities in the quarry‘s lake a few years ago, the quarry has now been fenced off for safety reasons and access is no longer permitted. Geological features now have to be examined at a Fig. 3 Metamorphosed Precambrian rock distance - binoculars would have been useful ! in Gullet Quarry Gullet Pass Pit We continued along the track past Gullet Quarry and climbed higher up the Gullet valley. The valley runs along a fault line that had been cut down by erosion. Moira pointed out that the rock exposed on the surface of the path changed from igneous/metamorphic of the Malverns Complex to a sedimentary quartz conglomerate of Cambrian age. This junction between the Precambrian and Cambrian rocks is possibly an unconformity. A little further up the track we arrived at a small quarry known as Gullet Pass Pit which has an exposure of Cambrian quartz conglomerate belonging to the Malvern Quartzite Formation (Fig. 4). The surface had been cleaned last year by volunteers. This was as part of an ongoing geological site clearance project for Malvern Hills AONB and Malvern Hills Conservators where geo-conservation work is carried out at several sites each year. In the middle of the quarry was a large bedding plane dipping Fig. 4 Cambrian quartz conglomerate in Gullet Pass Pit

Malvern Hills September 4th 2016 (contd.) at about 30 degrees, apparently containing surface evidence of fossils and burrows and Moira encouraged us to clamber over the rock to find them (Fig. 5). Despite our valiant efforts we failed to find anything, probably because the subtle surface imprints only become apparent under suitable lighting conditions. The exposure here represents a shallow-water, near-shore Cambrian environment, dated at around 540 Ma, when the Cambrian sea flooded the landmass of the Precambrian Malverns Complex, eroding it and producing the sediment which was laid down as the inter-bedded sandstone and conglomerate of the Malvern Quartzite.

Fig. 5 Trying to find fossils and burrows in Fig. 6 Upturned Silurian beds near the Gullet

Gullet Pass Pit unconformity

Gullet Unconformity We retraced our steps back down the track and then climbed up a path on the side of Gullet quarry. On the way we spotted a peregrine falcon flying above the tall quarry face. Philip Clark found out later from a warden that that peregrines have had a very good year on the hills and had fledged nine youngsters. Continuing our climb we eventually reached a famous site where Silurian sedimentary beds with a basal conglomerate unconformably overlie the Precambrian rock. These beds are near-vertical having been pushed upwards during the Variscan Orogeny, but more of this story will be revealed later. The near-vertical Silurian beds comprise a mixture of sandstones, siltstones and mudstones deposited in a shallow sea. (Fig. 6). Ripple marks could be seen on some beds and rock fragments we examined showed signs of bioturbation and broken shells, which were possibly brachiopods. Moira also pointed out a shear zone in the Silurian strata a few metres from the unconformity. The nature of the contact seen here between the Precambrian rock of the Malvern Complex and the overlying Silurian beds has been debated for many years but is now considered to be an unconformable relationship (Fig. 7). Moira explained that the Silurian conglomerate at the unconformity represents an ancient pebbly beach deposited on Precambrian rock near to Precambrian sea stacks. This unit is known as Miss Phillips‘ Conglomerate; her discovery of the conglomerate in 1842 was an important step in uncovering the geological history of the area. Interestingly, Anne Phillips was the sister of John Phillips, who started his career as assistant to his uncle the great William Smith, creator of the first geological map.

Fig. 7 The vertical Gullet unconformity, Fig.8 Looking east across the Severn Valley from with igneous Precambrian rock to the right Swinyard Hill of the pen and Silurian conglomerate to the left.

Views from the Malvern Hills We then ascended onto the ridge of Swinyard Hill to be rewarded with a spectacular view looking east across the Severn valley towards the Jurassic Cotswold Hills in the far distance (Fig. 8). Whilst we enjoyed the view Moira described the

Malvern Hills September 4th 2016 (contd.)

Variscan Orogeny in the late Carboniferous period around 300 Ma resulted in the uplift of Precambrian basement rock along a number of fault planes and the creation of the East Malvern Fault running north-south along the eastern edge of the hills. Following a period of erosion crustal extension reactivated movement along fault lines to produce a graben in the Severn Valley whilst still leaving the uplifted Precambrian Malvern Hills in place. The subsiding Severn valley became progressively filled with Permian and Triassic sediments – these are collectively known as the New Red Sandstone. The topmost layer today is the Mercia Mudstone Group, deposited in a hot desert environment around 230 Ma. The total downward movement of the graben was over 2 km. The view westwards from the top of Swinyard Hill was quite different, with a landscape of limestone ridges and clay vales formed when the horizontal sedimentary Silurian rocks were squeezed and crumpled alongside the uplifted Precambrian basement during the Variscan Orogeny. This now explained why the Silurian beds seen at Gullet quarry were near-vertical. Thus the Malverns Hills and the East Malvern Fault mark the boundary between the Palaeozoic rocks to the west and the Mesozoic rocks to the east. Clutter‘s Cave

After a welcome stop on the ridge to enjoy our packed lunches we continued our stroll northwards along the ridge of the hills to a rock exposure in which a cave had been excavated by hand sometime in the dim and distant past. This location is now known as Clutter‘s Cave (Fig. 9). The rock was volcanic, part of the Precambrian Warren House Formation, found on the lower hills such as Broad Down and Hangman‘s Hill to the east of British Camp. These are the only extrusive igneous rocks in the Malvern Hills. The rocks at Clutter‘s Cave consisted of spilitic, sodium-rich basalt that had erupted under the sea 566 Ma. Pillow lavas could be seen around the cave entrance. Most of the pillows were deformed and hence were not as instantly recognisable as they are in places such as Anglesey and the Lleyn Peninsula, but even the non-believers in the group were eventually convinced once the rounded outlines of the more obvious pillows had been pointed out (Fig, 10). Moira explained that thin sections of the volcanic rock had shown gas bubbles (vesicles) that had subsequently been filled with epidote. We saw veins of calcium carbonate in the lava. We tried the hydrochloric acid test on Fig.9 The volcanic exposure at Clutter‘s some white veins on the path, but we were disappointed when the veins Cave failed to fizz. Thin sections of the Warren House Formation were made by Platt in 1933 (Ref. 1) and he described the rocks as mafic spilitic, intermediate keratophyre and felsic rhyolite lavas with volcanic tuffs and later dolerite dykes. The thin sections are still held by Aberystwyth University though the rock specimens from which they were taken have unfortunately disappeared ! Walking a short distance northwards we could see the line of the northern Malvern Hills stretching into the distance. From this viewpoint we could clearly see that the line of hills from British Camp southwards had been displaced westwards (Fig. 11). This had occurred during later tectonic disturbances in Permian times, around 280 Ma, towards the end of the Variscan Orogeny.

Fig. 10 Pillow lava at Clutter‘s Cave

Fig. 11 British Camp on the left has been displaced westwards relative to the line of the Malvern Hills on the right

Malvern Hills September 4th 2016 (contd.)

Volcanic Tuff Our next destination was an outcrop of rock on the eastern side of Hangman‘s Hill, not far from Pink Cottage. And yes, it is painted pink! Moira explained that this exposure (Fig. 12) was also from the Warren House Formation but, unlike the exposure at Clutter‘s Cave, lava at this location had erupted explosively onto a land surface and formed layers of both lava and tuff. When we examined the rock closely, we could see small vesicles in places, some of which contained green epidote (Fig. 13). On a lower section of the exposure we saw narrow grooves (slickensides) running across a flat surface, providing evidence of movement along a fault plane.

Fig. 12 Volcanic exposure on the east side of Hangman‘s Hill

Fig. 13 Vesicles (gas bubbles) on the volcanic exposure

Quaternary Gravels We then descended the east side of Hangman‘s Hill onto Castlemorton Common and headed in the direction of the car park. As we walked along Moira described how the large area of common land had evolved. During the Quaternary ice ages, under periglacial conditions, frost shattered rock fragments from the Malvern Hills were washed downhill. Because the ground was permanently frozen (permafrost), the gravels spread across the impervious surface rather than being carried away via rivers. The gravels further away from the hill merged with interglacial river terraces and, on the area of the common, only a thin soil has developed that is unsuitable for agriculture. Nowadays the common land is only used for grazing of cattle and sheep. We stopped at two streams trickling down the hillside to examine the gravels that had been washed out from deposits underlying the Common (Fig.14). Fig. 14 Gazing with interest at We soon reached Swinyard car park, where we thanked Moira for leading a very gravel in a stream bed (only interesting walk around the hills and answering our many questions. It was an geologists do this) excellent day.

Reference 1: Platt,J.I. 1933, The petrology of the Warren House Series, Geol Mag 70, 423-429.

Report and photos by Norman Nimmo-Smith

The Building Stones of Hereford September 18th 2016

Leader: Linda Fowler. A group of 12 members spent an informative and interesting day exploring Hereford and examining the range of stones used in the construction of the buildings within the old city walls. The city lies on Silurian mudstones, unsuitable as a building stone and so the earliest structures were constructed from local timber. These have not survived, but later examples from medieval time onwards were evident as we toured the city centre. Some had frames in-filled with lime plaster, while others had later brick inserts. The oldest surviving stone constructions used Devonian Old Red Sandstone from quarries near Ross on Wye, which could be easily transported along the River Wye directly to Hereford. Additionally, Lower Silurian Aymestry and Wenlock limestone were available by land transport from exposures in the Woolhope Dome (a pericline). Brickworks to the north supplied the city later. With the advent of canals and then railways, other building materials were transported from the Potteries and East Midlands. Today sources of modern decorative facings include Norway. The route we followed is shown in Fig.1. We started at Gaol Street car park, opposite the sturdy-looking Old Police Station and Gaol (A). They were Fig. 1 Our route around Hereford—the capital constructed of letters refer to the buildings we investigated micaceous, cross bedded Lower Devonian Old Red Sandstone blocks (Fig. 2). In contrast, the Shire Hall (B), constructed in 1815, has a frontage of massive Old Red Sandstone Doric columns reminiscent of the British Museum architecture (Fig. 3). The rest of the building is now Fig. 2: Old Police Station and Gaol built from faced in New Red Sandstone Old Red Sandstone and contrasting Matlock Peak Moor Sandstone of Carboniferous age. Opposite the Shire Hall stands the War Memorial (C), constructed of Carboniferous Limestone from Darley Dale, mounted on steps of Cotswold Pea Green Grit containing crinoids and brachiopod Fig. 3: Shire Hall with Old Red Sand- fossils (Fig. 4). The limestone is carved and stone columns engraved and supports 3 statues of First World War Servicemen constructed in much paler Portland stone. Along the street from the memorial is the Town Hall (D), its frontage decorated with impressive terracotta mouldings and facings in contrasting colours made by Royal Doulton (Fig. 5). The building base consists of cross bedded sandstone blocks. Opposite the Memorial, St Peter‘s Church (E) has been extensively renovated. It was originally built of Old Red Sandstone and has been repaired at various times (Fig. 6). In the 19th century it was repaired with sandstone blocks and dressing marks to make it blend in. Later, cement based blocks with imitation dressing marks have been inserted and, Fig. 4: War Memorial with more recently, smooth machine-cut sandstone blocks have Carboniferous Limestone been used. column Fig. 5: Colourful terracotta facing on the Town Hall

The Building Stones of Hereford September 18th 2016 (contd.)

Fig. 6: St Peter‘s Church Fig. 7: Rusticated Ashlar carving on Fig. 8: Attractive features in showing stone restoration limestone blocks sandstone facing at Primark

The group next moved past a 17th century half-timbered building (the Old House) with rendered infilling, built on Old Red Sandstone footings, to the shopping area near the Butter Market (F). On the way, a diversion inside WH Smith‘s entrance revealed a timbered wall infilled with plaster above the displays. The 1860 Butter Market (F) entrance is formed from limestone blocks whose surface has been carved to imitate weathering in a style known as Rusticated Ashlar (Fig. 7). The limestone contained hollow ooids, bivalves in a micrite matrix – a biomicrite. In contrast, the Marks and Spencer frontage (G) opposite is faced in polished black granite whose quartz crystals seemed, on close inspection, to be shattered. A diversion to view the sandstone block construction of Primark‘s frontage (H) revealed planar cross stratification and dewatering structures (Fig. 8).

Returning to opposite Marks and Spencer, the HSBC (I) frontage of pale rusticated Portland limestone contained ooids and

Fig. 9: Bivalve shells in limestone of Fig. 10: All Saint‘s Church (knife and Fig. 11: Polished Serpentinite on HSBC building fork for scale) shop pillar bivalves (Fig. 9). In contrast, the black polished Larvikite facings of Boots (J) contained feldspar phenocrysts, distributed increasingly close together up one panel in particular. A schiller effect could also be seen, all suggesting the source is Permian Norwegian granite from Larvik. The 1303 All Saint‘s Church (K) at the end of the street is another Old Red Sandstone building (Fig. 10). Because of earthquake damage in the 19th century, the tower was repaired first with Hollington stone and then in the 20th century with modern machine cut sandstone. Diverting to examine the cylindrical columns on the Carpet shop frontage (L) revealed black with purple, green and red polished Serpentinite (Fig. 11), while the adjacent doorway‘s similar style polished columns are pale grey limestone, with coral and bivalves, possibly Devonian.

The Building Stones of Hereford September 18th 2016 (contd.)

The route next led past the city walls (M), constructed of Old Red Sandstone blocks and river pebbles to the left bank (N) of the River Wye. The original walls date from 908 AD and eventually were 16 feet high, with 34 foot watchtowers. Amongst the Old Red Sandstone blocks graded bedding in the grit with the coarser quartz being angular was seen (Fig.12). It was also noticeably micaceous with weathered blocks varying in colour from shades of green to dull red. On a section of wall near the Cathedral was a blue plaque commemorating Nell Gwynne. She spent her early childhood in the town. As mistress of Charles II, her surviving son became the Duke of Albany. She was instrumental in the establishment of the Royal Hospital, Chelsea.

Fig. 12: Coarse sandstone blocks in the City Fig. 13: Wye Bridge constructed from Old Red Walls Sandstone

The Wye Bridge (O) is also an Old Red Sandstone construction (Fig. 13). It replaced the earlier wooden bridge in 1490 and was damaged in the Civil War. Today it has York (limestone) stone paving slabs. The first bridge was probably built c.800 AD. The group re-crossed the river to look at the Left Bank building (N), built of a pale cream shelly oolitic limestone containing 2-3 cm diameter bivalve fossils. Victorian Gothic style Hereford Museum (S) is constructed with an upper storey of dark grey, Forest of Dean Pennant sandstone over deep red Radyr stone pillars (Fig. 14). The latter is a coarse conglomerate, from the base of the New Red Sandstone Formation. In contrast, the arch around the door is decorated with animals carved into yellow/cream oolitic limestone containing minute gastropod fossils. The granite steps of The Old Post Office (T), now Pizza Express, contain Feldspar phenocrysts in hexagonal cross-section. This location led us conveniently to the Cathedral and a well-earned lunch stop, particularly for our guide, Linda! Adjacent to the Cathedral (U) is the New Library Building (Q), constructed of Millstone Grit from Elton in Derbyshire, built to house the cathedral‘s library of chained books and the Mappa Mundi in a controlled Fig. 14: The ornate Museum building environment. The Cathedral (Fig. 15), first constructed on this site pre 1055 was restarted in 1080 and consecrated in 1145. It is composed of gritstone. In 1786 the west tower collapsed as a result of an earthquake and was repaired by James Wyatt. It was restored in 1900, using Hollington stone. The paved area in front of the tower contains a modern image of a tree growing in the fields of Herefordshire, constructed in different coloured stones. The materials used are smooth, pale grey Pennant sandstone, Hereford Red, darker grey Millstone Grit and green Kirkstone Slate. The Mason‘s yard is located in the Cathedral Precinct, tucked into the side of the nave at its south east corner. We were able to see pieces of masonry in the process of transformation into replacement blocks to repair damaged parts of the building. The Cathedral‘s interior dates from 1200 but was extended and modified by James Wyatt after the tower collapse. Of geological interest are the Purbeck limestone columns with rip up mud clasts in their bases and the Fig. 15: Hereford Cathedral from the south- west

The Building Stones of Hereford September 18th 2016 (contd.) white life-size tomb effigy of a knight who had fought at Crecy and Poitier, intricately carved in alabaster (Fig. 16). The latter would have been sourced from Nottinghamshire. At the far end of the cathedral the tomb of Archbishop Jacob Atlay included another life-size alabaster figure. This tomb included red and black marble in its base, and it was noticeable how much colder marble was to the touch than alabaster.

Fig.16: Alabaster figure in Hereford Cathedral Fig. 17: Lumps of slag used to construct a buttress

Our tour then visited Castle Cliff (W), the last surviving part of Hereford‘s Norman castle. It was badly damaged during the Civil War. We then moved on along the bank of the River Wye to the oldest Hereford building, a house built of Old Red Sandstone and located next to the water-gate steps. They were once located in the town‘s defensive wall and were gated. Materials and goods were able to pass safely to and from river transport into the walled town at this point. Nearby, a metal Victorian suspension footbridge built in 1897 (X) now provides a pedestrian crossing point over the River Wye. By the steps down to the footbridge we noticed a wall with a buttresses made of what appeared to be lumps of slag (Fig. 17). The Nelson monument (Y) on Castle Green was erected in 1805 to honour this frequent visitor to Hereford. The green is the site of the various traditional fairs held in Hereford throughout the year. The grey base of the monument is of more weather-resistant Forest of Dean Carboniferous sandstone than the pale inset Portland limestone commemorative plaque, while the tall column is of Jurassic Bath Stone, an oolitic limestone (Fig. 18). St Ethelburt‘s Hospital (Z), now almshouses, is constructed of Old Red Sandstone and was our last location. It is one of many almshouses that have existed in Hereford to provide for sick, poor and elderly inhabitants. The oldest were established by religious orders or the Church. St Ethelburt‘s was established in 1225, rebuilt in 1805 in Gothic style. Hereford has more than 20 almshouse groups, now providing sheltered housing. The group enjoyed sunny weather while discovering a great deal about Hereford‘s history in their quest to examine the building stones. Our guide had obviously spent a great deal of time collating the information she gave us on a thoroughly enjoyable field visit. Fig. 18: Nelson monument with its Bath Stone column

Report by Janet Hellen Photographs by Norman Nimmo-Smith

Brecon Beacons October 16th 2016

Leader: Dr. John Davies Introduction A group of 17 OUGS members met in the Craig y Fro car park on the north flank of the Brecon Beacons, about 8 miles south-west of Brecon along the A470. In the company of our leader Dr John Davies, president of the South Wales Geological Association, the plan for the day was to explore the Old Red Sandstone (ORS) landscapes at a number of locations in the area.

John has now retired from what was the Countryside Council for Wales. He is chairman of the Welsh Stone Forum and spends much of his time exploring Wales, mapping and co-relating the rocks that he finds.

As shown in Fig. 1, we visited 5 different locations during the day. In John‘s words, the day was to be a ―coach tour‖ as we drove from site to site. By doing this he would be able to describe how the landscapes in this part of the Brecon Beacons were a result of its geological history and the different layers of the ORS sediments.

Geological Context The ORS sediments were deposited during early stages of the Caledonian Orogeny, which in this area caused faulting rather than folding. Repeated tectonic movements caused these faults to remain active and large blocks to develop in the landscape (similar to Fig .1 Map showing locations visited in the Brecon Beacons those in Northern England, such as the Alston Block (map reproduced with permission of the Ordnance Survey) that some of us will have become familiar with on Summer Schools). These blocks were separated from each other by positive ridges caused by the major fault zones, in turn causing the sediments to vary in depth from one to another even when the source of the sediments was the same. There is much evidence for the sediments being deposited in various locations in a system of channels developed in a flat landscape.

John‘s belief is that, at the time these sediments were being

laid down, the region was very similar in nature and size to what can be seen today in the Indus Valley (Fig. 2), with rivers flowing south-west along the south-eastern side of the rising mountain- front. Large amounts of sediment were transported by these meandering and braided river channels spread across the plains. Fluvial fining-upward cycles from sandstones to mudstones can be identified. Away from the main channels, flood-plain deposits include palaeosols with calcretes, and the sequence is followed by two marine incursions of the Upper ORS. Channels frequently commence with coarse bases containing debris eroded from previously deposited palaeosols, followed by massive sandstones and flaggy tops of finer material, passing up into mudstones and calcretes at the top of the cycles. Fig .2 The present-day Indus Valley (image courtesy of BBC)

Naming of ORS rock layers (―stratigraphic nomenclature‖)

Many of our text books, and certainly the existing geology maps, all use nomenclature which is out-of-date and John provided us with details of the current names of the official formations, together with an explanation on how the old formations relate to them. The handouts that he provided demonstrate how it is possible to correlate formations right across the Anglo-Welsh basin from south Pembrokeshire in the west to Ross-on-Wye and Blakeney in the east, the sub- groups allowing the details of what were locally named members to be fitted into the big regional picture.

Brecon Beacons October 16th 2016 (contd.)

John‘s work has enabled him not only to correlate exposures across the whole of the Anglo-Welsh basin but also to identify additional formations not yet officially named by the ICS (see Note 1, page 6) due to their non-occurrence in Pembrokeshire. These are the Bryn Menyn Mudstone Member; Walford Mudstone Member; and Blaen Haffes Formation, all of which are within the upper half of the Cosheston Subgroup of the Daugleddau Group, previously known as the Brownstones.

Fig. 3 shows John‘s revised litho-stratigraphy for the ORS in the Anglo-Welsh basin, along with his new names for three of the units. See also Note 1 (page 6)

Fig 3: A New Litho-Stratigraphy for the Old Red Sandstone of South Wales

Location 1 Our journey started at the Craig-y-Fro car park on the A470, where John provided us with his handout and described the plan for the day. From the car park, John pointed out some Quaternary glacial features in the area. A small glacier had formed beneath the crags above the A470 road 11,000 years ago, and the road had been cut through mounds of moraine debris left behind when the ice melted.

Across the road from the car park was a small disused quarry exposing rock of the Senni Formation. A waterfall running down the faces of the quarry kept them clean. (Fig. 4). This quarry is a Site of Special Scientific Interest and is the Fig 4: Craig y Fro Quarry, with interbed- type location for two species of early plants (410Ma) found in the beds of the ded sandstone and argillaceous rocks Senni Formation, namely Gosslingia breconensis Heard and Krithodeophyton Edwards. Apparently David Attenborough visited the quarry whilst filming his ―Life on Earth‖ series, and used dynamite to expose fresh bedding planes in order to find some more plant fossils!

On the north side of the quarry John pointed out a coarse pebble bed of the base of a cycle with finer-grained material lying above it, both of which exhibited cross-stratification (Fig. 5). The pebble bed contained calcrete and mud clasts; the latter had been washed out from the exposed surface to leave numerous holes.

Fig 5: Craig y Fro quarry; pebble bed

Brecon Beacons October 16th 2016 (contd.)

Location 2 We drove a short way along the A470, parking our cars in another layby, where John described the landscape scenery looking north down the Tarrell valley (Fig. 6). In Devonian times we would have seen an upland area in the distance formed by the rising Cambrian Mountains. Erosion of these hills and mountains would have produced the terrestrial sediments of the ORS, with meandering and braided rivers flowing in a south-westerly direction. Running across the lower half of the valley in a NE-SW direction is the Swansea Valley fault zone, which was active during the Caledonian Orogeny and is part of the much larger Welsh Borderland Fault System. On the west side of the valley we could see the crags of Craig y Fro (Fig .7). The upper part of the crags is formed by the Lawrenny Cliff Formation, overlying the Senni Formation. Individual stacks of crags are separated by steep to vertical faults – these were sub-parallel, or branching faults associated with the Swansea Valley fault zone located a short distance to the north.

We then walked a short distance towards the Storey Arms Outdoor Education Centre thence down into the valley along the old military road, [passing a stone wall built by Philip Clark..Ed.] which is now part of the Taff Trail that runs from Cardiff to Brecon. Before doing so, John described more of the ORS ―layer cake‖ strata seen from this point. The famous Plateau Beds form the topmost layer of the hillside to the east, and this unit can also be seen on the summits of Pen y Fan and its neighbouring peak Corn Du. These rocks have recently been renamed by the BGS as the Pen y Fan Formation. Below this layer lie the rocks traditionally known as the Brownstones Formation but, as result of his work, John has proposed a new name for this particular unit – the Blaen Haffes Formation. So many new names to remember!

After walking a short distance down the old military road, which can be seen towards the bottom of Fig. 6, we arrived at a pebble bed exposed on the surface of the pathway. This first appearance in the succession of exotic pebbles, including rock and quartz clasts, were matrix-supported (Fig. 8), and John explained that this unit could be matched right across the Anglo- Welsh basin. This was the Lawrenny Cliff Formation, the lowest unit of the previously named Brownstones Formation, and the same bed which appears in waterfalls below the main road in Nant Gerdinen. The suite of clasts it contained showed a Mid-Wales origin.

Fig. 6: Looking north from the A470 at Craig y Fro, with Fig. 7: The crags of Craig y Fro, separated by vertical faults old military road in the foreground

A Break for Lunch at Location 3

It was now time for some lunch, and we drove back down the A470 to the National Park Visitor Centre to make use of its tea-room and toilets. Sitting outside on the terrace in the sunshine we had a lovely view of the two highest peaks in the Brecon Beacons, Pen y Fan and Corn Du. Before heading off to our next destination of the day John described the rock units which could be seen from the National Park Centre, from the Moor Cliffs Formation to the Pen-y-fan Formation. He also mentioned the effect that the fault zone known as the Swansea Valley Disturbance had on the course of the rivers, including the River Wye north-east of Llyswen.

Fig. 8: Pebble bed exposed on the surface of the old military road, showing quartz and rock grains

Brecon Beacons October 16th 2016 (contd.)

Location 4 Our journey to location 4 involved interesting driving along narrow roads. Of particular interest were the two steep hairpin bends (known as the Devil‘s Elbow) we negotiated as we climbed up out of the Senni Valley. But we all arrived eventually with no mishaps. From here we had a spectacular view northwards along the Senni Valley in the direction of Sennybridge (Fig .9). John pointed out that the Swansea Valley Disturbance (also known as the Cribarth Disturbance by the BGS) ran along the line of the valley, heading in a SW direction towards Craig y Nos, and that the disturbance had controlled the ORS sedimentation. Fig. 9: John describing the landscape looking north At this spot there were a number of rock exposures of the Lawrenny along the Senni Valley Cliff Formation close to the roadside, and we spent some time examining their features, including exotic pebbles. One of the exposures showed an excellent example of soft sediment deformation (Fig. 10). Before becoming solid rock, the sediment layers had become liquefied and formed cusps, probably in response to being shaken by earthquakes.

Location 5 For our final stop of the day, location 5, we drove a short distance south across the watershed and parked beside the River Llia (Fig. 11), which flows across the New Shipping Formation at this location. Once again John described the near-horizontal layer-cake strata clearly seen in the surrounding hillsides.

The sedimentary layers of the Brecon Beacons nowadays dip gently southwards as a result of compression during the Variscan Orogeny. This dip could be seen quite clearly along the course of the River Llia, where water slides had been created – at these slides, the river flowed downwards across a dipping bedding plane (Fig. 12). Because of the southern dip, the surface rocks become progressively younger along the course of the River Llia. Further south the river leaves the Devonian ORS behind and enters Carboniferous Limestone country. At Porth-yr-ogof it disappears underground into limestone caves for a short distance until it emerges and continues its journey to the sea. (Porth-yr-ogof is well worth a visit!).

Before heading home, we all thanked John for taking us on a fascinating geological journey across a small part of the Brecon Beacons National Park.

Fig 10: Cross-section of sedimentary rock Fig 11: John describing the landscape at location showing soft sediment deformation (top of pen 5, beside the south-flowing River Llia for scale)

Fig 12: ―Water slide‖ on River Llia, where the river flows across a dipping bedding plane (Photo: Linda Fowler) 45

Brecon Beacons October 16th 2016 (contd.)

Postscript - Maen Llia Standing Stone

As we drove from Location 4 to Location 5 we passed the standing stone of Maen Llia, situated only a short distance from the road (Fig. 13).

Partly moss-covered, this impressive stone is 3.7m high and is believed to date from the Bronze Age. On a clear day it can be seen from quite some distance down the Llia valley, suggesting that it may have been important as a territorial marker. Standing at an altitude of 573m, it is also thought to be the highest standing stone in South Wales.

Like most other standing stones in the Brecon Beacons, this monolith is made of Old Red Sandstone. The information board at the roadside stated that the stone is made from calcrete, although closer examination suggested that it was actually a conglomerate containing clasts of re-deposited calcrete (Fig. 14).

Fig 13: Maen Llia standing stone Fig 14: Close-up of the ORS calcrete conglomerate of the Maen Llia standing stone

Note 1: If you are unfamiliar with the International Commission on Stratigraphy (ICS): ‗Its primary objective is to precisely define global units (systems, series, and stages) of the International Chronostratigraphic Chart that, in turn, are the basis for the units (periods, epochs, and age) of the International Geologic Time Scale; thus setting global standards for the fundamental scale for expressing the history of the Earth‘. (Source: stratigraphy.org). Many geologists use both the new and old terms interchangeably which can be very confusing!

Report: Norman Nimmo-Smith and Janet Hiscott Photos: Norman Nimmo-Smith, except Fig 12 (Linda Fowler)