Field trip to the 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 .

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 and then to quarries in the region of May Hill, near Huntley, . 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 , The Vale of Gloucester and the Upper Severn Estuary. It crosses the 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. All rights Reserved We set out from Awre Village, southward along the old fishery track to the river bank. Along the way, we passed a salmon fishery hut with a large collection of conical wickerwork fish traps or “Salmon Putches” standing outside. See Figure 6.

Figure 6 Salmon Fishery Hut with “Putch” Fish Traps

The peninsula is largely covered by estuarine alluvium, but down at the water’s edge the underlying bedrock is well exposed. At Point ‘A’ (Figure 1), we immediately found an abundance of Lower Jurassic fossils, including numerous large ammonoid impressions and several species of bivalves (e.g. gryphaea). This is the Blue Lias. A fairly deep water sediment. Calcareous muds, interbedded with algal bloom sediments. According to Dave, just 60m of thickness here. We established ourselves, without question, as standing on the Lower Lias of the Lower Jurassic. See Figure 7.

Figure 7 Lower Jurassic (Lias) fossils from Awre Peninsula

Ideally, we would have walked south-westward along the western bank of the river to where it crosses the fault line. However, the small tributary of Blackpool Brook spills out here and prevented our crossing. Before we returned to the cars, Dave pointed out the high ground to the west of the fault line, at Hagloe. This was our next destination. We travelled by road to Hagloe and parked at Hagloe House. After a short walk downhill to the flood terrace, across the railway tracks and through a small thicket, we came out onto the estuary. At this point (Point ‘B’, Figure 1), the fault line leaves the west bank at a right angle to the bank and disappears beneath the river. It reappears on the opposite bank at Tites Point. Here, this major fault has minimal expression. You would hardly know it is there, but for the Lower Jurassic Blue Lias just a short step away from the Lower Devonian Sandstone on the west side. See Figure 8.

Figure 8 Dave Green describes the Hagloe Fault

The Blue Lias close to the fault line dips gently eastward and shows symmetrical, silt-infilled flaser bedding. This indicates a widely fluctuating flow regime. The Devonian sandstone showed calcrete nodules. We found river pebbles that had been fluvially transported north from the Brittany region (refer Kidderminster formation). Dave explained that these had been transported as far north as Cheshire, then carried south again by glaciers to this region.

South of the river, the fault continues southward as the Berkeley Fault, through the Vale of Berkeley. Dave explained that during the Mesozoic era, there had been down-faulting in the east, thus forming a graben structure beneath the Severn Vale. During Triassic and Jurassic times, this had filled with sediments that became the Mercia Mudstones (Triassic) and Lias Clay (Jurassic) of the region (syn-sedimentary faulting). See Figure 9.

Figure 9 Sketch View of the Geology & Scenery of the Severn Vale Full acknowledge is given to Dave Green for this excellent sketch. On the opposite bank, to the south, at Tites Point, some higher ground was visible. This is an inlier of much older rocks. There, Silurian Ludlow beds are faulted against the Upper Cambrian Tremadoc series. Interestingly, the fault plane dips to the west in the Vale of Berkeley and also in the region of the Malvern Hills, but dips to the east at Awre. It was a reverse (thrust) fault in late Carboniferous times, but became a normal fault in Permian times.

Location 2 - Huntley Quarry (OS 370853,219660)

After a lunch stop in Blaisdon, we travelled north to our second location, Huntley Quarry. This is a small quarry on the southern slope of Bright’s Hill, to the west of Huntley village. Here, the line of the fault (here, called the Blaisdon Fault) cuts into Bright’s Hill. The quarry exposes the line of entry. The A40 skirts the southern edge of Bright’s Hill and makes for easy access. This is a “Geology Reserve” opened in 2007. Dave has worked on it for some time, with a local RIGS team, trying to stabilise the fault contact.

This quarry is the only known exposure of the Huntley Quarry Beds. They are west of the fault line. They have recently been dated using acritarchs (loosely or unclassified microfossils), to Late Ordovician - Early Silurian times (445 – 439 Ma ago). The sandstones and siltstones of the Huntley Quarry Beds contain lava and volcanic ash fall deposits. They appear sub-horizontal near the road, but show upward “drag folding” and complete overturn against the fault plane, supposedly due to tectonic pressure during the “reverse fault” phase. It became a normal fault in Permian times. East of the fault, the Huntley Quarry Beds are deeply buried and probably correlate with a thick sequence of lavas beneath the Cotswolds. See Figure 10.

Figure 10 Huntley Quarry - Overturned Beds

On the east side of the quarry, the younger Mercia Mudstones and Bromsgrove Sandstone rest up against the Huntley Quarry Beds, along the Blaisdon Fault line. These were brought down by rifting of the Worcester Basin during late Palaeozoic - early Mesozoic times. East of the quarry, near the road, we looked at a horizontally bedded exposure, supposedly of the Mercia Mudstone. This showed faint aeolian truncated foresets. Under the hand lens, it contained frosted grains and no mica flakes; a Triassic desert environment.

Location 3 - Hobbs Quarry (OS 369473,219475)

About 1.5 km west of Huntley Quarry is the location of the HobbsQuarry SSSI. This quarry reveals some intriguing patch reefs in the Upper Silurian Wenlock Limestone that is exposed there. Dave explained that there are two beds of limestone in this section, separated by a thin bed of shale. The bed we could see in the quarry was the lower of the two, containing brachiopods and the narrow patch reefs that were so strikingly visible on approach. See Figure 11.

Figure 11 Hobbs Quarry, Patch Reefs in the Upper Silurian Wenlock Limestone

Closer examination showed the reefs to be formed as thin towers with bedded sediments overlapping their margins and apparently draped between them. The reef towers show no internal form/structure to suggest the organisms that formed them. There were no signs of corals or algal mats. Just a fine-grained, micritic limestone core. The information board describes the reefs as “ballstone reefs” that grew as the surrounding sediment thickened. See Figure 12.

Figure 12 Hobbs Quarry, Patch Reef Towers

As we concluded our visit to Hobbs Quarry, this brought our field weekend to the Forest of Dean to a close. Our introduction to the local geology along the major fault in the Vale of Gloucester had been very well prepared and highly informative. Dave Green is indeed a splendid Trip Leader. Thank You Dave.

Prepared by Julian Chard and Tom Miller