Wonderful Wear

I know the River Wear starts at the confluence of several streams at the eponymous Wearhead, but we are going a little way up one of thse streams to Killhope. We will pick up a tale of lead mining there -  the geology related to the mineral wealth of the North Pennines will be the thread that joins a lot of the stories in first part this episode. Then after a little Whin Sill and some elegant Carboniferous monuments its time for some younger rocks – rocks for which Durham is world famous – The Permian

Transcript

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Episode 6. The Wonderful Weir. There's been a bit of a gap between episode 5 of the river trails and this final one. We went to the Alps to help teach our granddaughter to ski. It was a success. The weather has turned warm since we left, and suddenly there are loads of lambs in the field beside us. So you may hear them or their mother baying. I know the river Weir starts at the confluence of several streams at the eponymous Weir head, but we are going a little way up one of those streams to Killop. We will pick up a tale of lead mining there. The geology related to the mineral wealth of the North Pennines will be the thread that joins a lot of the stories in the first part of this episode. Then, after a little windsill and some elegant carboniferous monuments, it's time for some younger rocks. Rocks for which the whole of Durham is world famous. The Permian period. There are many places in Durham that could have been chosen to illustrate aspects of lead mining and the lives of those who worked in those mines. Killlop Museum, at the head of Weirdale, is the perfect place to tell their story. The museum is closed at the moment. I hope it opens again. How and where lead was formed and its relationship with rocks has been described elsewhere in these podcasts. This story is more about life in the remote northern Pennine Hills that were the centre of lead mining in Britain a hundred and fifty years ago. Killop, like similar mines in adjacent Northumberland and Cumbrian valleys, is almost five hundred meters above sea level, a place with a harsh climate and impoverished soils. The lead ore found in veins that crisscrossed these hills had been mined for centuries, and had made a select few families incredibly rich. For others, the majority, life could be punishingly hard. These were the miner farmers, working in the mines but also needing to eke out a living on isolated steadings with a small house, a barn, and a few animals. Many still dot the hillsides today. Despite the philanthropy of some mine companies, the vagaries of lead mining industry and the hazardous conditions underground meant that minor farmers were no strangers to poverty and ill health. Large families and high rates of child mortality were constants. Some chose to look for a better life elsewhere and emigrated to North America and to Australia. At Killip, you can read the letters these families exchanged in the eighteen fifties. They are disarmingly factual and poignant. People in these upland valleys were not embraced by an Anglican church focused on its softer valley parishes below. They were converted by Wesley to Methodism, and when this faith lost its missionary zeal became primitive Methodists. When you visit the area you will see many examples of the chapels and schools they built, some with their original internal architecture and furniture. Killop Mine opened in 1853 and was closed in around 1910. Restoration began in 1980, and today you can experience a trip into the mine, see the washing floor and crushing mill, the water wheel where the ore, Galina, was processed before it went for smelting. But perhaps the most evocative things are the stories of the daily lives of the people who lived and worked high in the Pennine Hills 150 years ago. Hello from Cows Hill, five kilometers downstream from Killip. The Bertreford disturbance might sound like a rowdy night in a Weirdale pub that got out of hand, but it's not. The disturbance is a very big bit of geological disruption in the North Pennines. This combination of folding, faulting, and general dislocation of the rocks stretches around forty kilometers from Loondale in the south to Allenheads in Northumberland in the north. On the basis of theoretical modelling of the hidden Weirdale granite and the rocks around it, scientists believe the disturbance goes deep. It is a major feature of the geological structure of Northern England, perhaps penetrating thousands of meters into the Earth's crust. Despite its size and significance, it's a shy and retiring feature at the surface, with little evidence of its presence or magnitude, which explain why it's not well understood. When did the Bertreford disturbance take place? Well, recent research has looked at large and small faults and structures in the North Pennines and dated the rocks by measuring the decay of radioactive elements. It concluded that this huge zone of bending and breaking is around 300 million years old. This is about the same time as the intrusion of the windsill. The Durham University scientists who did the research think the Bertreford disturbance may have been a major conduit for hot mineral-rich fluids which formed the North Pennine ore fields, and that this is all linked with the injection of the windsill. These connections upset the geological apple cart because they challenged decades of thinking about the way that mineral deposits here and elsewhere in the world were formed. Less than 500 meters from Bertreford, you can see all the rocks that are part of this radical geological interpretation. In the river weir upstream of Cows Hill, the Carboniferous limestones and sandstones are tilted by as much as 50 degrees from the horizontal. In the old cocktail quarry is the dolrite of the Windsill, and the extensive spoil heaps in the valley of Sedling Burn provide graphic evidence of the minerals and their exploitation. Middlehope Burn above Westgate is crossed by a geological fault, an east-west fracture in the bedrock that dislocates one side relative to the other. In common with many other faults in this part of the Pennines, this one is occupied by a mineral vein. There were major mine workings associated with the vein in Middle Hope Burn, and because they are typical of North Pennine mining processes and the remains are relatively well preserved, they are a scheduled ancient monument. The vein with the fault is known as the slit vein. You can tell you are approaching the fault by the way the sandstones start to slope at greater angles in the stream. Miners in the past were well aware of the association of veins with faults and rocks at strange angles, and they used them to locate the minerals. As well as mining along the length of the vein at the surface, they sunk a shaft 178 meters deep, one of the deepest in Weirdale. Five hundred meters east beside the Westgate to Rookope Road is West Rig Open Cut, an iron ore opencast working which clearly shows the line of the vein and the fault left as a prominent unquarried rib. The slip vein is composed of several minerals, including galena, that's lead, ciderite, iron, fluorite, and quartz. Little of the waste material from processing the ore is visible at the mine today. But if you hike a further 800 meters upstream, you will come across mine dumps that do contain these minerals. As well as its geological and industrial archaeological significance, Middlehope Burn Mine site has a botanical interest. Some of the plants that grow here, like spring sandwort and mountain pansey, are able to tolerate soils contaminated with heavy metals, and these calmonarian plants and the woodland ecology of the valley mean that it is a site of special scientific interest. There were a few rocky reasons to include the town of Stanhop in the Durham Rocks book and in this podcast, and at least I've managed to place it in the valley it properly belongs in, that of the Weir. It's here not because of the Great Windsill, but the Little Windsill. Yes, there is more than one sill. Up and downstream of old Stannup Bridge, the river Weir runs in a ruler straight, narrow channel eroded through the Little Windsill. Like its big brother, this sill is the same very hard rock called dolerite. It's a bit of the same molten magma that was also injected horizontally between carboniferous sedimentary rocks around 295 million years ago, at the very beginning of the Permian period. When geologists first started to study the windsill, a northern quarrimans term for a hard level rock, they debated whether it was lava or that had flowed on the surface or a molten rock intruded underground. They concluded it was an intrusion, a single layer of rock that stayed at a consistent horizontal level. Embarrassingly, many years later it became apparent that the windsill did not conform to its own definition. It transgressed, literally, it moved between different layers of strata. Worse, it seemed that there was more than one windsill, so today you will hear geologists talk of the windsill suite or the windsill complex. The little windsill is part of that. But with a collective area of at least four thousand five hundred square kilometres, most of it beneath the surface, these well connected bits of rock are England's most extensive igneous intrusion. Stanhope has other geological claims to fame. There is the vast Ashes limestone quarry and its waste heaps overlooking the town. More famously, there is the fossil tree stump built into the wall of the churchyard. The stump was discovered with others in 1915 in a sandstone quarry at nearby Edmund Byers Cross. Three hundred and twenty million years ago, this tree was growing in a tropical swamp when Britain was just a few degrees from the equator. After the tree died and rotted, river sand replaced the trunk and roots and left them as a cast in the sandstone. The name of the plant is Sigillaria, a forerunner of today's small club mosses. Back then they grew to thirty meters tall. We have passed Frosterly and its small mine working in an old limestone quarry. Its output is not industrial fluorite, commonly called floor spar, it is beautiful green and blue purple crystals for sale to mineral collectors. We glide past the beautiful homegrown Carboniferous sandstone architecture of Durham Cathedral too, both topics that have been covered in earlier podcasts. Our destination is another sandstone monument, sitting on a hill on the south side of the Weir Valley, Penshaw. This is a tale of two hills. They may share a worm, but they don't share rocks. The song that everyone in the Northeast knows will tell you that the Lambton worm wrapped its tail ten times round Penshaw Hill. But this song was only written in 1867, and an earlier local myth claims it was Wormhill, two kilometres away at Fatfield, that was squeezed. The rocks of Penshaw are much older than Wormhill, around 260 million years old, and from the Permian period, the rock is called the Razby Dollar Stone, a part of the magnesium limestone, with more magnesium than calcium. It was one sediment in a warm sea between 100 and 300 meters deep. The sea was called the Zechstein, and while its west coast was only a couple of kilometres away, its east coast was hundreds of kilometres away in Central Europe. Compared to the rocks around it, the Razby Dollar Stone is relatively hard, and as a result, millions of years of erosion have left it as a 30 to 60 metre west facing scarp, running south across Durham to Ferry Hill. Penshaw Hill is part of that escarpment. Wormhill's tail may be older, but its geology is a lot more recent. Geologists interpret it as a mound of sand and gravel that was deposited by a river flowing from an ice sheet around twenty thousand years ago. There are others who feel some or all of Worm Hill may have been constructed or moulded by our ancestors, but until somebody excavates it, the jury is out. There are a few legendary worms in Northeast England history. There's the ladly worm of Spindelson Huff and the Sockburn worm. If the artist's paintings are to be believed, all these worms look more like dragons or serpents, wyvens, which go back into Anglo Saxon folklore. Lewis Carroll's Jabberwok was illustrated like this too. He wrote the poem while in the northeast and lived at Croft on the Tees as the boy. Like many Victorians, he was probably fascinated by these worms and the new science of geology that was discovering dragon-like dinosaurs in the rocks around Britain. We are right beside the river at Claxof, just opposite Castletown in Sunderland. We have moved into a different geological period, the one after the Carboniferous. It's called the Permian, and this bit of Permian is around 255 million years old. This riverside cliff starts at the bottom with rocks that used to be desert sands and finishes at the top with ones that were once coral reefs. What happened? Around 300 million years ago, our climate changed, and what is now Britain and northern Europe had drifted north away from the equator. A landscape of tropical swamps and lazy rivers became one of a hot, arid, rocky plain, one of the world's biggest ever deserts. Over almost 50 million years, more than 500 meters of Carboniferous rocks were all eroded away. You can see the evidence of that desert weathering in the red purple colour of those older Carboniferous rocks at Castletown. They're on the north bank, just above the river. On this rocky plain, sand was deposited, blown by the wind into long dune ridges. These dunes can be seventy meters high and up to two kilometers across, but in between them there may be little or no sand at all. These sands are now the yellow sandstone rock in the cliff, and their sloping layers and wind faceted round quartz grains are evidence of their desert dune origin. The sandstone grains are only held together with iron minerals, so they are soft and friable, and they weather quickly to become sand again. Although they have been used as building sand, the yellow Permian sands, the name that geologists give them, are relevant to us because underground they contain huge amounts of water. They are an important aquifer. The fact that they hold water also caused serious problems for coal miners who had to pass through them as they dug shafts and boreholes to reach the coal seams beneath. The solutions were pumping or freezing the rocks or leaving wide rock barriers. All these were expensive, and that meant that the operating lives of mines were reduced. These fossil sand dunes continue under the North Sea, and their capacity to hold fluids and gases means they are an important reservoir for oil and gas. It was research on shore by geologists in Durham that helped predict where these ancient dunes, and thus all the oil and gas, would be. The sharp divide between the yellow sands and the overlying grey magnesium limestone in the cliff at Klaxov shows how desert conditions rapidly came to an end as a salty sea called the Zekstein flooded the land from Poland to England and coral reefs grew where sand dunes once were. We are just north of the mouth of the River Weir, on the coast at Roker, and standing beside some pale yellow-brown rocks that don't look like your normal sedimentary rock. They look like a pile of cannonballs. The magnesian limestone is perhaps the most famous of Durham's rocks in the geological world. These rocks tell the story of barrier reefs and a warm sea that kept evaporating in the hot sun when our part of the world was just like Australia's west coast. Roker is just one of the many places along the Durham coast where you can see rocks that were once limey mud and animals living on and beside a reef in a tropical sea just 10 degrees north of the equator. The story began around 260 million years ago in the Permian period, when a northern ocean burst into a low basin between what is now Britain and Poland. It created a sea called the Zechstein, which was less than 300 meters deep, and along its western coast, where Durham is now, there were barrier reefs. In the water either side of the reefs, different types of limestone were deposited. Magnesium carbonate, dolomite, as well as calcium carbonate. As the seawater kept evaporating, salts like anhydrite and halite were deposited too. Fluctuating sea levels and instability in the reef zone meant huge areas of semi-solidified limestones would slump and slide down the slopes. In the millions of years that followed, some of the limestones were recrystallized into weird shapes, like the cannonballs you see at Roker. Despite the harsh, salty environment, animals thrived here and built up the reefs. There are many fossils in the Magnesian limestone, shellfish, algae, plankton, and primitive animals called bryozoa that lived in colonies and looked like twigs or fans. Like many rocks which are special and famous, the magnesium limestone has been studied intensively by geologists over more than 150 years. Our understanding of how it formed and the sequence of events has changed. Some of the names of the rocks changed with that improved interpretation. So if you hear terms like raise before and roca formations, don't worry, just think magnesian limestone. And that concludes our river trips. This journey concept got me thinking while I was on holiday. Before I retired I was fortunate enough to travel a lot, and I saw some brilliant geology in some very different parts of the world. In two of those countries, which are a bit bigger than the UK and had lots more space than people, I seem to remember that creative people had produced guides to their roadside geology. Two I remember distinctly were Namibia in Africa and Oregon in the USA. I know they might have a lot more bare rocks sticking out than us, but I've been wondering whether we might go on some road trips or even some rail trips next. What do you think?