Field Trip to the Forest of Dean 2 Nd – 3Rd May 2009
Total Page:16
File Type:pdf, Size:1020Kb
Field trip to the Forest of Dean 2nd – 3rd May 2009 This weekend trip was intended to combine the different experiences of viewing the iron- and coal-bearing rocks in situ, underground, in the Forest of Dean mining region, with a general overview of the stratigraphy and geological structures evident in the Vale of Gloucester. Saturday 2nd May Location 1 - Hopewell Colliery (OS 360351,211420) In the morning we all set off from The Malt Shovel in Ruardean, (where we were staying) to meet in the car park of the mining museum. Hopewell Colliery is an old Victorian drift mine, which has been kept open and working by Free Miners and for the visiting public. A mine is described as a drift mine when it burrows into the side of an incline along a horizontal coal seam, rather than tunnelling down vertically. This particular mine is located on Cannop Hill where there are exposed coal measures running in seams through the Carboniferous Sandstone rock layer. Free Miner Robin Morgan led our exploration of the mine. He gave us access to viewing these seams underground and a detailed insight into the history of extracting coal in the Forest of Dean. To become a Free Miner you have to be a male, born within the Hundred of St Briavels (roughly the Forest of Dean district) and to have worked down a mine for a year and a day. Edward II, in the 14th century, gave the local miners the right of mining freely in the forest after miners from the area helped to tunnel under the enemy’s castle fortifications. The shaft we walked down had a very low ceiling and with the additional few inches that the helmet gave, made it impossible to walk down without bumping your head. (Especially if like me, you are over 6ft tall). Figure 1 Girders and Arches It was made trickier by the wooden roof props that were installed, which gave the roof a very inconsistent height. Yet no matter how many knocks befell you, it was always good to ‘feel’ just how structurally sound the mine was! The traditional wooden roof props (girders) used vertical or inward leaning "Sets" that supported horizontal "Flats". These would span the width of a tunnel and be installed every few feet to support the roof. Over the flats would be driven horizontal poles called "Runners" to spread the roof loading. Later, steel arches were used, with greater strength and longevity, but more expensive. Further down the mine, Robin showed us a coal deposit that had been extracted. It was the first section we had seen mined because all the coal near the top had too much clay contamination making it poor quality and therefore uneconomical to work. The horizontal cut out followed the coal seam at a slight upwards angle and was barely tall enough for a man to slide up into on his back. Yet he was able to use a pickaxe to chip away at the coal and let the loose material roll down into the shaft. Finally we were led down a path towards an opening, which let the bright daylight shine in. On the rock wall of the passage, there were horseshoes hung on nails that had been found buried in the stream during clearance operations. These had been used by pit ponies in an earlier period of the mine’s history. Their job had been to tow the full coal carts to the surface. Once we reached the outside, it was just a small walk through a part of the forest back to the museum building, where we handed over the helmets and made our way to the Café for a cup of tea. Location 2 - Clearwell Caves (OS 357750,208250) In the afternoon we visited Clearwell Caves. For over 4000 years they used to be an iron ore and ochre mine, now a working mining museum and impressive show cave. There was an option to go down into the deep mines, roughly 200 feet down, this required the donning of overalls and helmets but unfortunately it was fully booked. So we took a guided tour of the nine caverns that composed the show cave at about 100 feet underground. We were very luckily to have Free Miner Jonathan Wright, who manages Clearwell Caves, as our tour guide. He is the son of Ray Wright a free miner who in 1964 began to explore the abandoned iron ore mines in the area; he found a whole underground world beneath the Forest of Dean. In 1968 he opened Clearwell Caves to visitors and over the years, enlarged the show cave to the present day total of nine caverns. As Jonathan showed us round the caverns he pointed out many of the mining artefacts left by the miners and explained how the ore became deposited in the natural cave system. Over many thousands of years, the pore water dissolved the limestone to form the caves. The pore water containing high levels of iron, deposited both forms of the minerals iron ore and Ochre. Iron ore, probably derived from seepage of iron rich solutions from the Carboniferous Coal Measures and Permian sandstones above. It passed down through the Whitehead Limestone and on reaching the different chemical environment of the Crease Limestone, precipitated out to form goethite, haematite and ochres in the joints. Ochres formed as the result of microbial activities on the iron compounds in the pore water. See Figure 2. Figure 2 Clearwell Caves, Whitehead & Crease Limestones As we progressed through the caverns we could see mining evidence all around us. Such as an iron wedge-like tool that was driven between the iron ore layer and the limestone beneath. A hammer knocked it further in, weakening the joint until the iron ore became detached. Another method of iron ore separation, known as fire-setting, was shown by the darkened rock at the base of the chamber, where fires were lit to heat up the rock. Water was then thrown onto the walls, so cooling them, the layer of iron ore expanded under the heat at a different rate to the limestone underneath. Then when cooled rapidly by the water, the sudden change in temperature caused fractures between the limestone and iron. Thus the iron ore became loose enough to be picked off. In the picture, you can see the markings on the walls from tools used to break off the iron ore deposited on the rock surface. Much of this surface is now covered with calcite overgrowth, giving the markings a true antiquity and authenticity. See Figure 3. Figure 3 Ancient Pick Marks on the Mine Walls The miners worked a 10-hour shift six days a week. The method that was commonly used to time the shift was to have 10 candles that roughly each took an hour to burn. After the first five, workers would sit down and eat lunch then after another five it was time to go home. The candles were held in “Nellies”, a twig with a lump of wet clay on the end in which the candle was pushed, the other end of the twig being held in the miners mouth. See Figure 4. Figure 4 14th Century Forest of Dean Miners with Nellies What probably surprised us the most was the use of child labour to carry the ore out of the mine, they carried 20lb. loads in a wooden box known as a “Billy” slung over their backs. Small stones were sprinkled on their backs to cause pain. They would then be pressured into moving faster so that the load could be taken off quicker. We passed a Pneumatic drill from a later era, imbedded into the rock wall. It was nicknamed the “Widow Maker”, dust from it caused lung infections and the vibrations caused muscles to come away from bones! After our return to the surface Jonathan showed us his collection of artefacts, fossils and minerals that had been found in the caves. These included a clay pipe, pieces of Crease Limestone, goethite (Brush Ore), calcite and haematite (Kidney Ore), as well as a selection of ochres, which vary in colour from yellow to purple, a unique colour from each natural pocket. Afterwards we all gathered back at The Malt Shovel for a meal and a discussion about the addenda for the next day. Sunday 3rd May After the previous day spent largely underground, our Sunday was spent very much in the open air. Dave Green took us out to the Upper Severn Estuary and then to quarries in the region of May Hill, near Huntley, Gloucestershire. Our plan was to look at local geological features along the line of the major fault system that runs north-south from Staffordshire, through the Vale of Gloucester, and south through the Vale of Berkeley. This ancient, tectonic fault system penetrates the lithosphere very deeply, Dave believes it penetrates all the way to the base of the crust. Along its length, its surface expression includes “The Wrekin”, The Malvern Hills, The Vale of Gloucester and the Upper Severn Estuary. It crosses the River Severn just south of the Awre Peninsula. Location 1 - Awre Peninsula (OS 370850,207950) Sunday morning we set out to meet at a splendid location on the Severn Estuary, on the Awre Peninsula. Our meeting point was the car park of the Red Hart Inn, Awre. Here, the fault system is exposed as it strikes southward across the eastern sweep of the river. See map, Figure 5 below. Figure 5 Geological Map of the Awre Peninsula Reproduced with the permission of the British Geological Survey © NERC.