Newsletter, Winter 2017
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NEWSLETTER, WINTER 2017 ----- WESTMINSTER ABBEY: SANCTUARY PAVEMENT SATURDAY 13th FEBRUARY By Lesley Exton Those able to secure a place on this over subscribed trip met up with Ruth Siddall, our leader for the afternoon outside Westminster Abbey (Fig 1). After negotiating around the tourists in front of the pavement, our party went round the rope barriers designed to keep the public out. We carefully walked along the edge of the pavement and found somewhere to sit so we could remove our shoes. Then in stocking feet we walked onto the Sanctuary Pavement for a more unusual building stone ‘walk’ examining a remarkable example of Italian Medieval decorative stonework. The Sanctuary Pavement lies at the heart of the Abbey, designed to commemorate the new shrine for the bones of St Edward the Confessor (situated behind the High Altar). It is in the Cosmatesque style, which resembles a ‘patchwork quilt’ of stones of different shapes fitted together to make a pattern. The Cosmati were a family of decorative stone masons living in Rome in the 12th Century who established this method of paving, which typically involves a framework enclosing areas of polychromy stonework. Fig 1. The group with Ruth outside Westminster Abbey. [Photo: Lesley Exton] 1 In this case it is a quincunx within a quincunx, surrounded by a border or roundels and tombs. However, it is not known if the paviours responsible for laying the Sanctuary Pavement were members of the family. The design is supposed to represent the cosmos, so the earth is at the centre. The spirals around the roundels interconnect and so can be walked while completing the rosary. There is an inscription running round the centre originally done with inlaid brass letters but most have been lost over the centuries so it is difficult to read now. The majority of the stones (marbles and porphyries) were looted from Roman sites and re-cut to form the pieces we could see. The framework is Purbeck Marble (green), in Italy you would usually find Carrara marble used; however, it doesn’t survive very well in Britain’s much damper climate. There were three phases of construction and/or repair. It was originally laid down in the 13th Century. The main stone varieties used are green and purple porphyry, the former Lapis Lacedaemonius comes from Greece while the latter Imperial Porphyry comes from Egypt’s eastern desert. Tadcaster limestone (yellow) a British limestone is used as a contrast. Both opaque and transparent coloured glass is also used and would have sparkled in candlelight. The first phase of restoration took place in the 17th/18th C and used stones that would be stock in trade at these times. They ignored the pattern using larger pieces and creating a more abstract pattern, so the repairs can be easily identified. The third phase occurred in the 19th C when Sir George Gilbert Scott replaced the eastern margin of the floor adjacent to the altar. He used both new and recycled stone in his designs. Our time there went all too quickly as we looked at the different stones used, including many one- offs; e.g Africano (red and black marble breccia), Egyptian Gabbro and alabaster. Many thanks go to Ruth for a very interesting and unusual ‘building stone walk’. Unfortunately, photography of the pavement was not allowed, however, hopefully, Ruth will have plenty to show us when she comes to speak to us about the pavement in January 2018. INDUSTRIAL GEOLOGY NEAR RIDGE, HERTFORDSHIRE SATURDAY THE 19th MARCH 2016 By Nick Pierpoint After the HGS party had assembled at the South Mimms service station on the M25 we had a trip briefing and a Costa before setting out in tight convoy to Pinks Farm between Ridge and Ridge Hill which in turn is nestled in-between Potters Bar to the East and Shenley to the West in Southern Hertfordshire. The aim of the trip was to seek evidence for the geologically based industries around Pinks Farm. Hertfordshire does not feature heavily in William Smith’s Memoirs with only six lines of text compared with the seven lines assigned to Rutland! To capture our attention Mike Howgate (trip leader) brought to our notice three blue dots on a William Smith map in the vicinity of Ridge. What were they recording? We were going to find out what these represented 200 years ago. With a William Smith map and a map of the Shenley Chalk Mine produced by the Chelsea Speleological Society we established our bearings, as we examined an old ‘well’ and two shafts (Figs 2 & 3) which provided access to subterranean chalk workings. With the aid of a torch light tethered to a piece of string we were able to see the chalk, as it was lowered down a mine shaft. One of the shafts is home to a couple of hundred hibernating Daubenton bats which feed on damselfly 2 nymphs as they emerge from water. There was plenty of surface water around. We also heard stories of Mike’s adventure entering the caves down a free hanging caving ladder – not a trivial exercise. Figs 2 & 3: Access points to the chalk workings – Drawholes. [Photos: Nick Pierpoint] In this rural environment the chalk was mined using drawholes as open pits would destroy good arable land. Apparently mines were taxable assets whereas water wells were not, so mining also avoided prying eyes and perhaps helped reduce tax obligations! Could this be the reason mines do not feature in county records held in Hertford? It has been suggested one of the larger water-filled depressions near Pinks Farm is due to cave roof collapse or a plugged mine shaft. If this were the case one would expect to see more water in the cave, but as the cave is dry they do not appear to be hydraulically connected. Intriguingly there are several depressions in an adjacent field up to 25m across and 10m deep – are these collapse features or swallow holes? Several subtle concentric scar features suggest these are ‘active’ and getting deeper (Fig 4). Observational records suggest these are indeed swallow holes. The ponds represent a perched water table above the Chalk within the Lower Tertiary. The water table in this area lies deeper within the chalk below the base of the chalk workings. Fig 4: Collapse features or swallow holes? [Photo: Nick Pierpoint] 3 Nearby earth works on slightly elevated ground were chalk pits; some recent badger and mole activity has brought to the surface reddy brown clay material which is from either the Reading Beds or Upnor Formations. We were able to pick up several black coated, smooth, very well rounded to elongate pebbles most likely from the Upnor Formation, similar to those seen in Hertfordshire Puddingstone. The speleological map showed the chalk has been worked in a series of galleries with approximately 20ft of head chalk. These man made caves have been tunnelled in excess of 1,132ft long, up to 40ft high and 32ft wide, with over 23,000 tonnes being extracted for agricultural purposes. Local records suggest these significant workings started prior to 1815 and continued to around 1912. The penultimate site was in a flat field where London clay had clearly been worked for brick making. There was evidence of working faces and even remains of a drainage ditch which led to a series of drainage ponds. Why had these not drained? There was some discussion suggesting loess of Anglian origin (behaving as an impermeable layer) may have held a perched water table. These sites are a complex series of industrial workings, with evidence of chalk and clay extraction commencing in the early 1800’s and lasting over a century. The clay is either from the Upnor Formation or Reading beds and used for brick making, although no evidence of the kilns remains visible. The many depressions across the location varied from swallow holes, dew ponds and potentially collapse structures with ponded water. One was located very proximal to a septic tank! Our final stop was to a larger open Chalk pit which could well have represented one of the blue dots on the Smith Map (1815) near Ridge Hill. Smith used the blue dots to represent mines or mineral extraction as his maps were primarily for economic purposes. A fine suggestion made by Mike was for HGS to celebrate William Smith in 2019, which coincides with the bi-centenary of geological sections he constructed between Cambridge and London part of which pass through Hertfordshire. It was a truly fascinating few hours treading in the footsteps of William Smith and furthermore it was in Hertfordshire. The trip finished in the village of Ridge at the Old Guinea pub for an Italian lunch. Thanks to Mike Howgate for leading this trip on a dry but cool March morning. DENBIES WINE ESTATE AND DORKING’S MUSEUM CAVES SATURDAY 21st MAY By Nikki Edwards An overcast sky was an inauspicious start to what transpired to be a fascinating day in Surrey. HGS members gathered at Denbies winery, at 104 hectares, probably the largest vineyard in Europe, one mile north of Dorking, Surrey, with the aim of exploring the reasons for the siting of the vineyard, to sample its products, and visit Dorking Museum and South Street Caves. Professor Dick Selley gave a lecture on The Geologic and Climatic Control on Two Millennia of English Viticulture. It was Dick Selley who applied the French concept of ‘terroir’ to the site, recognising the favourable combination of topography and geology coupled with the beneficial 4 effects of global warming, who suggested to his farmer friend that there was a good chance of establishing vines on his land.