A trip down the Tees

The first of our journeys is along the River Tees. The Tees has its headwaters way up in the Pennines, in the Carboniferous rocks just east of Cross Fell, but downstream of Cow Green Reservoir it cuts through some of the oldest rocks in Northern England – that’s why its the first or the rivers in this series. On our way downstream we will explore some different bits of the Whin Sill and some rocks it baked, the making pencils from ancient slates and take a closer look at High and Low Force, iconic sites of the Northern landscape. But let’s head for Widdybank Fell near the dam at Cow Green.

Transcript

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Series four River Rocks Series three of these podcasts was a journey the Roman Rock Trail That started in the west on the Solway and in seven episodes took us east to South Shield's Roman Fort. This fourth series will also take us on a journey. And not one but four down the four major rivers in the north, the Tees, the Tyne, the Eden, and the Weir. We'll stop off at around thirty to forty places that have something worth knowing about the geology those rivers cut through. Episode one A Trip Down the Tees The first of our journeys is along the River Tees, but before that I owe you an apology. You might have noticed that this episode was posted early today, and then not there. If you downloaded that first attempt, you will possibly have heard I managed to include Stanup on the River Tees, and not the River Weir. I really don't know why. I could blame Microsoft Word, but I suspect it's just me having a senior moment. Sorry. Anyway, I've corrected it, and Stan Up is no longer on the Tees, but will appear in its rightful place on the River Weir in episode six. But back to the Tees. It has its headwaters way up in the Pennines in the Carboniferous Rocks just east of Crossfell. But downstream of Cowgreen Reservoir, it cuts through some of the oldest rocks in northern England. That's why it's the first of the rivers in this series. On our way downstream, we will explore some different bits of the windsill and some rocks it's baked, making pencils from slates and take a closer look at high and low force, iconic sights of the northern landscape. But first, let's head for Widdybank Fell, near the dam at Calgreen. If you walk from the car park at Calgreen Reservoir to Cauldron Snout Waterfall, you cross the western edge of Widdybank Fell. The fell is one of the top five botanical hotspots in Britain. It's no coincidence that one of its rocks is special too. Locally called the Sugar Limestone, it is a pure limestone of Carboniferous Age. It's called the Melbourne Miscar limestone, and it has been baked by the intrusion of the windsill dolorite beneath it. Molten magma at around 1100 degrees C has changed the limestone into a crumbly sugary marble. Natural weathering reduces it to something that looks like white, coarse sand. The process of changing rocks by heat and or pressure is called metamorphism, and this is one of the only few places in the county where you can see a metamorphic rock. Look out for flushes where water flows out at the base of the limestone and at the top of the impervious windsill. These and other local habitats support a variety of rare plants, such as spring gention, yellow saxifrage, alpine rush, bird's eye primrose, dwarf milkwort, Teesdale Violet, Spring Seg, and Hoary Rock Rose. These are plants that endure in the harsh climate of the Pennine Uplands and may have survived for perhaps 1200 years since vegetation started to recolonize the fells after the last glaciation. These plants are vulnerable, so please keep to the marked footpath. The dramatic waterfall at Cauldron Snout highlights the natural toughness of a special rock, the windsill, a quality that has made it a prime choice for prehistoric stone tools. Cauldron Snout is in Upper Teesdale. It is stunning, but many other spectacular northern locations, High Force, High Cupnick, Hadrian's Wall, and Dunstanborough, all make the same point. They are just a few of the outcrops of the windsill, a molten rock that was squeezed into older Carboniferous rocks of northern England 295 million years ago, and it solidified as a dark grey, compact, fine-grained igneous rock called the dolerite, sometimes quartz dolerite. The windsill's hardness means it resists erosion more than other rocks, so not only does it create outstanding landscapes, it is also quarried extensibly for hard wearing tarmac for our roads. So it's no surprise that it was a rock that prehistoric people used to make their tools. The extent of the windsill in northern England is vast, from Holy Island to Hadrian's Wall and from the Cumbrian Pennines to Teesdale. If you include the parts of the windsill you can't see below other rocks, it covers at least four thousand five hundred square kilometres, and it's England's biggest igneous intrusion. That huge geographic extent and durability meant ice sheets and rivers carried cobbles and boulders of Windsill much further afield, into Yorkshire and the East Midlands. And because of that, it was also possible for Stone Age people to find and make tools from these erratics and boulders of Windsill Dollerite. The use of dolerite for stone implements is common in the Neolithic and early Bronze Ages, and archaeologists who study them have identified the rock as a distinct category within their classification. It's called group 18. Unlike flint, a rock universally used for tools, which is brittle and glass-like and breaks naturally with a curved surface and sharp edge, dolerite is fine-grained, interlocking mineral crystals. Attempting to shape dolerite as you would a flint by napping, that's flaking, is much less successful than pecking or grinding it with another rock to create a tool. So unsurprisingly, many of the implements made of dolerite are not axes, but hammers, axe hammers, and mace heads, all with a hole pierced in them to take a shaft, something that would be near impossible with flint. While many of these dolerite implements were working tools, others appear to have had a ceremonial function, and the distribution of group 18 implements across England and Wales likely indicates they were widely traded. Identifying a prehistoric tool as windsill dolrite is relatively straightforward, using techniques that analyse its mineralogy, chemistry, and magnetic characteristics. But confirming the tool was fresh from a northern windsill outcrop, or a far travelled erratic, is a challenge as tough as the rock itself. Cauldron Snout and the many windsill outcrops in the Upper Tees Valley would be beautiful places to start that quest. The name must be literal, clint from the Norse word meaning a rocky cliff, and Falcon from the peregrines that occupy it still, or perhaps golden eagles that once soared above it. Falcon Clint's is in Upper Teesdale, just downstream from Cauldron Snout. The upper part of the cliff is made of the same windsill dolerite that forms the waterfall of the snout. Its characteristic vertical columns are very obvious. They were caused by the molten magma cooling and shrinking and cracking. The lower part of the cliff is marginally paler limestone. It has horizontal layers, and that was altered by to marble by heat caused by the injection of the windsill's molten magma around 295 million years ago. But this description is not just about the windsill and the limestone that became marble, it's about what lies beneath them, at the very bottom of the cliff. At the west end of Falcon Clintz, the Pennine Way footpath runs over a dark, rubbly looking rock. It's from the earliest part of the Carboniferous period, about 350 million years ago, and it's called a conglomerate, a rock made of pebbles and cobbles in a matrix of mudstone. It's the product of millions of years of erosion of an ancient mountain chain, when torrents of water carried, tumbled, and eventually deposited this mix of different sized debris. Sea level then rose, and many layers of limestone, plus sandstone and shale completely concealed the conglomerate. Millions more years of erosion, and especially the work of glaciers and rivers in the last 2.6 million years has exposed the rock at the surface. Whether you walk upstream or downstream to see falcon clints, you have a good chance of spotting some special birds, dippers, lapwings and oyster catchers, and maybe ring oozel and blackcock, and if you are very lucky, a peregrine falcon. Peeping through the Carboniferous strata in the valley of the River Tees beneath Cronkly Scar of the oldest rocks in Durham. These grey green slates are part of the Skiddo group of the Ordovician period of Earth's history. Geologists know these rocks as the Teesdale Inlier, where older rocks are surrounded by younger ones. Primitive marine fossils called graptolites help us confirm the age of the slates, between four hundred and eighty five and four hundred and sixty million years old. The rocks were once mud at the bottom of a deep ocean. The ocean was known as the Iaptus. That ocean was being squeezed between two continents and would ultimately cease to exist. Our part of Britain was near the South Pole then. Over millions of years, as our tectonic plate migrated north, the muds have been buried by thousands of meters of other sediments, and heated by molten rocks beneath. Pressure and high temperatures have changed them, and they became a slate, a typical metamorphic rock and one which splits readily. Beside the small outcrop of these rocks is a stone ruin. It is the site of a mill which manufactured slate pencils in the eighteen fifties. Slate pencils are so called, not because they were made of slate, but because they were for writing on slate tablets, which Victorian children knew well. From the pieces of pencils still lying around, it looks as if they have been produced by sawing the slates into square sectioned rods. In this part of Teesdale the local call these pencils widdies after the nearby Widdybank fell. Just upstream there are some very different and slightly younger Ordovician rocks, called tufts, ash ejected from a volcanic vent which has hardened into stone. This small outcrop is probably the same age and origin as the volcanic rocks of the Central Lake District. High Force is stunning. It is no surprise that it is one of the most iconic and visited locations in Durham. The word force comes from Foss, a Viking name for a waterfall. The river Tees drops twenty one meters over the hard dolorite of the windsill. Look beneath the windsill and you will see those darker sandstone and limestone layers. These three hundred and thirty million year old softer sedimentary rocks are slowly but steadily being eroded back by the river until they eventually the dolerite is undercut and collapses. All waterfalls are moving upstream. Nature is slowly reprofiling the river and smoothing out these steps. Two point five kilometers downstream is Low Force, a smaller but no less beautiful waterfall also flowing over the windsill. If you walk just downstream of Winchbridge on the south bank, beneath a large knoll of windsill, there is a huge tilted raft that's a block of sandstone and siltstone, two meters thick and over seventy meters long. It once formed the roof of the chamber of molten rock, and then fell into it and was changed, that's metamorphosed, into a rock geologist call a hornfells, and quarrymen and miners call Whetstone. The river Tees at Low Force was used as a location in the twenty nineteen war film nineteen seventeen. There is no windsill in northern France, and Teesdale is a long way from the soft Cretaceous rocks of Salisbury Plain, where much of the rest of the movie was shot. But to the irritation of many geologists, film directors are not too troubled by rock continuity. Watch the rapid transit of Robin Hood the Prince of Thieves, from the White Cliffs of Sussex to Sycamore Gap, or Hoothred's escape from the Icelandic beach in the Last Kingdom, in reality, Blast Beach south of Seam. It's okay, I know. Us rock folks suffer from imagination deficit disorder. There were too many places to fit our geological journey down the river Tees into one episode, so it's going to get two. The next episode will start further downstream, close to Barnard Castle, taking Pierce Bridge, Dinsdale near Darlington, and Teeside before finishing where the Tees empties into the North Sea at Hartley Pool Bay.