TVGS would like to thank Graham Worton for his excellent, clear and comprehensive guide to this unique and totally essential to visit area—thank you Graham—we hope to see you again!
Thank you too to Ingrid Darnley for contributing this report on the field trip.
Photos:
A small and enthusiastic group of TVGS members met up at the Dudley Canals Trust car park. On a baking hot day the prospect of spending an hour in the cool Dudley canal tunnels was certainly an inviting one and we weren’t disappointed. This trip had been long in the planning and we were all very much looking forward to it, especially as we had as a guide Graham Worton, Borough Geologist and Keeper of Geology at Dudley Musuem & Art Gallery.
The Dudley Canal trust is run completely by volunteers and what a grand bunch they were, the Black Country humour providing a great accompaniment to the stunning trip through the tunnels. As someone who lived in Dudley for more than 35 years I was ashamed to admit I had never been in the canal tunnel or to Wrens Nest before.
425 million years ago rich limestone and coal seams were created beneath the West Midlands. These valuable deposits laid the foundations for the Industrial Revolution with the Black Country at its heart., the exploitation of these resources resulted in the spectacular network of underground canal tunnels carved beneath the region to mine the raw materials that powered industrial growth.
The trip gave us an underground view of what we were to see at Wrens Nest as the tunnel underpins both Wrens Nest and Castle Hill. The insides of the hills were extensively exploited and became a maze of mines, tunnels, steps and railways.
Our boat entered the underground tunnels via the ‘Tipton Portal’ entrance. The Dudley canal tunnel was very important during Industrial Revolution and beyond as the mining companies used it to bring limestone out from the galleries where it was being dug out. The boats were loaded up and then the cargo was transported to ironworks in the Black Country, Birmingham and beyond. We had a trip of 45 minutes where we saw several disused caverns, some of the trip taking place in the dark with excellent multi-media and tableaux to re-create some of the scenes from long ago. The ‘Introduction to Geology’ film was excellent, the best I have seen in its class. Fortunately our way out was illuminated by the twinkling of John Nicklin’s (and friend’s) white legs walking along the tunnel walls, ‘legging’ the boat in the old traditional way. Seriously John, good on you for volunteering!
Graham Worton supplemented the boat captain’s commentary adding to the interest of the boat trip as a taster for what was to come at Wrens Nest. Leaving the boat, we drove a short distance to the Wrens Nest nature reserve, parking in the unlikely surroundings of the old Mons Hill Dudley Technology College.
Wren’s Nest National Nature Reserve.
See http://www.bcgs.info/pdf_files/wrens_nest_leaflet_2009.pdf
Introduction
The Wren’s Nest is a disused Victorian quarry and derives its name from the Old English word Wrosne, meaning “the link”. This may relate to its topographical position on the boundary between the Severn and Trent watersheds.
Wren’s Nest National Nature Reserve is a geological site of exceptional importance and is one of the most notable geological locations in the British Isles. It is internationally famous for its large numbers of beautifully preserved Silurian limestone fossils. In recognition of its geological significance, Wren’s Nest was declared a National Nature Reserve in 1956, the first National Nature Reserve for geology, and also a Site of Special Scientific Interest in 1990 because of its exceptional geological and paleontological features of Silurian age.
The folded layers of the Much Wenlock Limestone Formation (formerly the Dudley Limestone) that form the hills have been quarried above and below ground for hundreds of years. The rocks of Castle Hill, Wren’s Nest and their surrounds provided the raw materials to establish Dudley as the centre of what became known as the ‘Black Country’ owing to the juxtaposition of materials for the manufacture of iron:- coal, ironstone and limestone together with fireclay, a unique assemblage.
The limestone deposits were an important source of building stone, then for lime mortar and agricultural fertiliser. During the height of the industrial revolution up to 20,000 tons of rock were removed annually from Wren’s Nest and used in the many local blast furnaces. Mining and quarrying ceased in 1924, leaving the hills honeycombed with a network of underground workings and caverns. However, without the industrial revolution, the rocks at Wren’s Nest might never have been exposed or the fossils discovered. It was during the mid-1840s that many of the best fossils were found, including the ‘Dudley Bug’ trilobite, which became an important local symbol and was featured on the town’s coat of arms until 1974.
Limestone mining has in particular left a spectacular legacy of quarries and caverns, including Dark Cavern, Britain’s largest man-made limestone cavern, that, according to our guide could accommodate the Houses of Parliament and St Paul’s cathedral (twice!). There are ambitious plans to re-open it, old wharves and a defunct canal and to hold shows and entertainment in them as once was. The caverns were linked to the national canal system by the Dudley Tunnel, the earliest narrowboat canal tunnel in the world. Built in 1785 at just over a mile long, it cuts the English watershed and when first built was the longest canal tunnel in England.
Wren’s Nest and Castle Hill contain excellent exposures of middle Silurian (Wenlock and lower Ludlow series) rocks, including a definitive section through the Much Wenlock Limestone Formation. The Wenlock Limestone of Dudley contains the most diverse and abundant fossil fauna in the British Isles: over 600 species of marine invertebrate, representing some 29 major taxonomic groups. The site is the type locality for 186 species (more than any other British site); 63 of these recorded nowhere else. In general the majority of the limestones were deposited in shallow tropical or sub-tropical waters which were quiet or intermittently agitated, although there were periods of higher energy. At times the conditions were conducive to the development of patch reefs, with sub-tidal and lagoonal or similarly protected waters allowing reef growth just below wave-base.
The Wenlock Limestone of Dudley is a fossil lagerstatten, containing rare and important life assemblages, in the form of beds of articulated crinoids (sea lilies) superbly preserved under deposits of terrigenous mud and volcanic clay. Rare annelid (worm) and early plant remains have been found, containing soft tissue. Other features of the site include bioherms (fossil ‘patch’ reefs preserved ‘in situ’), and expansive ripple beds, which provide evidence of littoral zone conditions.
The famous geologist Sir Roderick Murchison visited Dudley Murchison visited Dudley around 1837. In 1839 he published an illustrated catalogue of Silurian fossils, ‘The Silurian System’, of which 65% were from the Wrens Nest limestone quarries. These original fossils including crinoids can still be seen on display in Dudley Museum and Art Gallery today.
In 1839 he was at Dudley (at around the time he documented our own Martley Rock gravel pit) and again in 1849 to address members of the British Association, inside Dark Cavern – by gaslight. An estimated 15,000 people attended each event, with Murchison being acclaimed ‘King of Siluria’ at Wren’s Nest, and carried out on the shoulders of the quarry workers for whom he had become a great friend and favourite.
http://www.dudley.gov.uk/see-and-do/museums/dudley-museum-art-gallery/dudleys-heritage/dudleys-bid-for-world-heritage-status
Graham has recorded a 3 minute YouTube recording about this at awww.youtube.com/watch?v=eIAMy-BmWmc
THE GEOLOGICAL TRAIL
Graham took us along the Wrens Nest Trail which is a walk of about 3 km.
In a description of this type it is impossible to do justice to the descriptions Graham gave throughout the trip which, whilst focusing on the geology, explained this within the context of the social and economic history of the area which was fascinating. What I have done is to try and give a brief overview.
Quarry trench, Mons Hill
This is where our tour started with a general introduction looking at the southern face of the quarry.
This man-made trench was excavated in the 18th century. Three distinct lithological units can be seen here. The oldest of these is the Nodular Member, the top of which forms the large bedding plane. This unit passes up, eastwards, into the Upper Quarried Limestone Member which comprises a sequence of alternating flaggy limestones and thin shales, overlain by a massive 4.2 metre thick bed of coarse grained blue-grey limestone. Above this is a 1.2 metre thick bed of nodular limestone with shale partings. Continuing eastwards, the sequence passes up through silty shales with thin nodular limestone bands into pale grey shales. The latter has two bands of green bentonite clay (formed from water-lain volcanic ash) several metres above the base. The entire sequence dips eastwards at an angle of about 60 degrees. Limestone was extracted along the strike of the beds, hence the linear shape of the workings.
The Nature Conservancy Council (N.C.C.) Cutting.
This is a narrow cutting, about 35 metres long, which was excavated by the NCC in 1977 to provide a dip section through the Nodular Member. The eastern end of this cutting corresponds with the top of the Member. It passes through a sequence of progressively older beds, comprising nodules and lenses of limestone, separated by silty shales and mudstones.
At its western end the cutting joins a trench formed by quarrying of the Lower Quarried Limestone Member, a section of which can be examined in the north face of the workings. It is considerably thicker (16.2 metres) than the Upper Quarried Limestone Member, and comprises thickly bedded blue-grey limestone with shale partings, notably near the top and bottom. At the top is a 26 cm bed of limestone which was usually left to form the roof of underground workings. Colonies of stromatoporoids and corals (Coenites, Favosites) are preserved in position of growth towards the top of the Member.
The lowest, and therefore the oldest beds, are exposed in the north-western corner of the trench. They consist of greenish grey mudstones with limestone nodules and lenses. Below these, friable shales of the Coalbrookdale Formation (formerly Wenlock Shale) are just visible beside the steps.
Watershed Viewpoint (Murchison Point)
After walking across a plateau of wild flowers we looked out at the stunning view across Dudley at this, the watershed point. To the left (East) water drains into rivers which flow into the North Sea. To the right (West), water drains into the Atlantic. From the 1830′s onwards eminent geologist Sir Roderick Murchison made numerous visits to the area and this is said to be Murchison’s favourite spot. He encouraged local miners to establish a collection of Carboniferous Coal Measures and Silurian Limestone fossils. The Collection contained many new species and included the famous Calymene blumenbachii, or ‘Dudley Bug’.
After Murchison published his work on ‘The Silurian System’, in 1839 he encouraged and inspired local mine agents, industrialists, lay people, patrons and luminaries of the day to establish the Dudley and Midland Geological Society. Murchison inaugurated this, the Midland’s first geological society, in 1842. In 1975 the geological society’s third incarnation, the Black Country Geological Society (BCGS), was born.
Since September 2001 Dudley Museum and art Gallery has hosted regular rock and fossil fairs that draw large numbers of people from across the country. The next one in September 2013 will form part of the Dudley Museum Year of Geology celebrations, Graham highly recommends this to TVGS members! http://discover.dudley.gov.uk/events/dudley-rock-fossil-festival-2013/ Saturday 28 and Sunday 29 September at Dudley Town Hall and Dudley Museum and Art Gallery.
The Fossil Trench and Ripple beds
In the “Fossil Trench”, there is a huge rock face covered in ripple marks from the Silurian Much Wenlock Limestone Formation. The surface of the rock preserves ripples as seen on beaches today, in sand laid down around 400 million years ago over a period of three million years. The ripples were created by the wind and waves, layers of sand and mud built up on top, each with its own pattern. Today, as each layer erodes, the ripples underneath are exposed again. Such features can be seen today in the littoral zone (on sandy beaches and in river estuaries) and indicate wave or current action in shallow water. This demonstrates the principle of “uniformitarianism”: the idea that the natural processes we see happening now (e.g. volcanoes, erosion, etc) also happened throughout geological history.
The exposure is fenced off and we viewed it from a platform. Graham explained that a fossil dealer had cut out a 2M square section of a significant and beautiful crinoid formation, leaving a gap at the bottom of the flat rock face, so the rock above eventually slid down into the trench, At the moment, the rock face is unstable. In consultation with local residents it was decided to fence it off until the slope is stable enough to be reopened.
At the base of the limestone exposure bedding plane, the scree is rich in fossils including trilobites, brachiopods, corals, crinoids, bryozoans and cephalopods. There was an opportunity to explore the fossil collecting opportunity at the end of the tour.
http://www.dudleytunnel.co.uk/index.html
http://www.bcgs.info/pdf_files/Wren%27s_Nest_Geological_Handbook_Field_Guide.pdf
http://www.dudley.gov.uk/resident/environment/countryside/nature-reserves/wrens-nest-nnrwrens-nest-nnr/
http://www.dudley.gov.uk/see-and-do/museums/dudley-museum-art-gallery/
http://users.hal-pc.org/~hickmagp/bcgsweb/wren.htm