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Editor’s Notes

Carole Gregory, our Newsletter Editor, is taking a holiday break for two issues so I‟m „filling-in‟. Ricki Bull, who has been helping with the content and managing the layout/production, because of other challenging commitments, is unable to continue so I‟m taking over this role permanently. Many thanks to Ricki for her fantastic efforts over several years of production and my personal appreciation for the training she has given me to enable this edition to be published. Carole will be back with the March 2011 edition.

The programme for 2011 is nearly complete so get ready with your „new‟ diaries. As a result of the New Year Bank Holiday don‟t forget the AGM is on the slightly later date, January 10th. The evening also includes a short lecture by Roger Lloyd followed by the usual socializing. Bring a plate of food for the buffet and drinks will be provided by the Society.

Chris Fone

General News Items

New ideas about the reason for Mount Etna’s location There has been much speculation about the root cause of Mount Etna‟s volcanicity as its chemistry is associated with intraplate volcanics as oppose to the subduction derived volcanicity linked to the nearby Aeolian Isles. The RGS visited both locations in 2004. The latest theory resulted from a fluid dynamic modelling of the mantle flow around the complicated tectonic district of Sicily and S. Italy. The subduction of the Ionian oceanic plate below the Tyrrhenian Sea between Calabria in S Italy and Sardinia produces the Calabrian arc; this stretches from Sicily into the Mediterranean and back around to near the heel of S Italy. Dr Wouter Schellart demonstrated that upper mantle flow around the particularly narrow Ionian subduction plate spills out of the edge in the west and locally decompresses. This leads to partial melting of the mantle material which plumes and is the source of the development of Mount Etna and the adjacent Sicilian volcanic zone. For more information see http://www.sciencedaily.com/news/earth_climate/geology

Field Trips and Reports

RGS Field trip to North , 17th- 19th September 2010

The purpose of the field trip was to understand the volcanic processes which resulted in the massif, and was concentrated on the area to the north of Dolgellau. The trip was led by Graham Hall who led a number of successful field trips to North Wales in the early history of the RGS. On the journey there, we managed to fit in a visit to a quarry in Shropshire.

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Bayston Hill Quarry near Shrewsbury

Figure 1 - Bayston Hill Quarry Bayston Quarry specialises in the production of high quality, high value aggregates. The aggregates are derived from greywackes and conglomerates, which are part of the Bayston- Oakwood formation within the Wentnor series of the Longmyndian (late Precambrian). The unconformity between the Precambrian and the Carboniferous overburden has been removed to allow an extension of the quarry towards the NE and we were able to examine this face. Carboniferous green/grey shales with slickensides and sandstones with calcite veins were observed. No fossils were found. Peter Worsley gave an overview of the area during the last Ice Age with ice from the Irish Sea direction colliding with ice from the Wrexham direction, terminating at the eastern edge of Shrewsbury, and depositing glacial sediments in close proximity with a completely different provenance David Ward thanked our host Ben Matthews for an interesting, enjoyable visit and we set off to find gold in Wales!

Tan y Grisiau Figure 2 - Graham explains a point on dykes

At our first location, Graham Hall explained that the whole Snowdonia massif is of volcanic origin and that an extensive body of magma built up in the area which fed a large volcano. Eventually the caldera collapsed and further magma was released at the caldera margins as cone sheets. Graham offered an explanation of the various types of rock that could be expected, given the high temperature regional metamorphism that took place. The style of volcanic discharge would be affected depending on the water (steam) content of the discharging magma.

After viewing the microgranite and a dyke (see Figure 2) we moved onto the second location, a small quarry in the vicinity. Graham explained that the top of a magma chamber would be the last to solidify, and would contain a concentration of unusual minerals after the usual granite minerals were removed. In this quarry, calcite crystals containing Uranium and Thorium silicates had previously been located.

At our third site at the dam on the Tan y Grisiau reservoir, we examined the metamorphism of shales at the contact with the microgranite. At a distance of 100m from the contact we observed high quality building slate with good cleavage. Proceeding towards the contact, the cleavage became progressively less clear. The explanation was that the granite intrusion altered the shale, and as this preceded the metamorphic event which created the cleavage, the slates were unable to form.

Our next stop was to the north of the reservoir. The Cambrian slates are overlain by the Lower Ordovician sandstones, then the Moelwyn volcanics were deposited by the eruptions of Manod, some 5 miles to the south. This occurred during the Argenic epoch. The land then subsided and the volcanics were overlaid by the Ordovician slates. We examined a rhyolite sill which sits on top of the Lower Ordovician sandstone cutting through the Moelwyn volcanics, and autobreccias, which form when viscous lava develops a crust which is then broken up by the continual flow of the lava, solidifying to give a „plum-pudding‟ consistency. Reading Geological society Newsletter 178 November 2010 4 5

The nearby Cwmorthin Slate Quarry is in the Ordovician slates. There were good exposures of the volcanics and rhyolitic rocks on the upward track. Looking back toward the lake fine examples of glacial action could be seen on the rounded shoulders of the valley.

Barmouth – Gorllwyn

Leaving the car park a steep road took us to our first stop. Off to the right of the track, a short scramble led to an exposure of tectonic ripples in the Gamlan shales. These are caused, not by sand or water but by movements of blocks of rock relative to each other. Tectonic ripples are very similar in appearance to water ripples but are much longer.

We returned to the track for a short distance then turned up a little used path to the crag. Here in the grit are fine examples of turbiditic sediments including many scours filled with pebbles, and showing cross-bedding. These were formed by the Lower Cambrian proximal turbiditic sediments sliding into the deep sea and forming deltas. The source of the sediments is thought to be the Irish Sea landmass. During the Lower Paleozoic there was a great deal of block movements which could be the possible cause of turbidic action.

Prince Edward Mine

Prince Edward gold mine, east of Trawsfynydd, is situated in an area known as the Dolgellau Gold Belt; this runs in an arc round the east and south side of the Dome. It is one of a dozen or so such mines in this area initially worked in the 1850s. The mine is in Upper Cambrian shales, belonging to the Maentwrog Formation. These are deep-water sediments deposited in a subsiding basin. The underlying Precambrian basement was subsequently differentially uplifted along faults during the Caledonian orogeny (Late Silurian). Basic dolerite sills were emplaced in the Ordovician, roughly coeval with the emplacement of the Tan-y-Grisiau granite pluton. The subsequent geotechtonic structure is known as the Harlech Dome; the Cambrian shales are horizontal in the centre and dip steeply round the fault-bounded periphery. On the east side these are the NNW–SSE Trawsfynydd fault and the NE-SW Bala fault. Shearing along faults in the dolerite and shales allowed hydro-thermal fluids to move up through fractures and the mineralisation is found associated with the resulting quartz veins. This mineralisation from hydrothermal activity is dependent upon the rock through which the pressurised fluids have passed and the presence of gold is indicative of the fluids leaching volcanic rocks. Gold was deposited when the fluids cooled below the temperature required to retain the gold in solution and the fluids were also destabilised by reducing minerals such as carbon and iron pyrites.

Figure 3 - Plan of Mine

Following a plan of the mine (see Figure 3), we explored various side adits. The first right hand adit ended at a fault which limited the mine on the east side. New surveys had suggested that faulting had offset the quartz vein and along the next right hand adit there was evidence of recent mining activity.

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Figure 4 - Pillar and Stall Retracing steps, the exploration continued along other adits where, in some cases the pillar and stall method of mining could clearly be seen (see Figure 4). Some adits were flooded and along one was a recent rock fall, which all added to the ambience when we discovered we could explore no further along that adit and had to turn back to investigate elsewhere.

Various minerals were collected by the group; Iron Pyrite (fool‟s gold), Galena (lead sulphide), Chalcopyrite (copper pyrite), Sphalerite (zinc sulphide) and in many places hanging from the roof were brown and shiny stalactites of Limonite. No one owned up to finding gold!

Due to the inclement weather conditions, it was decided to make this our last stop. After thanking Graham for his excellent field leadership, the party returned to Reading.

Reports from John Hurst, Chris Hooper, Roger Lloyd, Hilary Jensen and edited by Edmund Shirley

RGS Field Trip to Cuckfield, Sussex on Sunday 17th October

A group of RGS members assembled on a fresh, sunny morning on Whiteman‟s Green near Cuckfield. Our leader for the day, Alison Barraclough, introduced the location by explaining the current group of football pitches were on the site of an old quarry used to supply the local building industry with Sandstone from the Wealden, Cuckfield Stone beds in the Tunbridge Wells Sand Formation. The area had been made geologically famous by Gideon and Mary Ann Mantell who, in 1822, discovered the fossilized tooth of a creature which he later named an Iguanodon. Gideon and his wife continued to collect what was to become a world famous collection of dinosaur fossils and develop the geological chronology of the Wealden sediments.

Figure 1 – Gideon & Mary Ann Mantell

The group then walked down through Cuckfield village to the Church, looking at the building style and examples of the use of the local sandstone. Alison, who had spent her childhood in the locality and still had strong family links, explained the history, industry and politics of the village. A major staging post on the main route from London to the south coast it had been eclipsed by the development of the rail-link to the east. Iron nodules in the local sandstone were smelted with charcoal from Tilgate Forest to create a major iron-works that subsequently moved north and west to more profitable areas of the UK.

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