Normally this week we would be out on the fells celebrating European Geoparks Week by showing you Upper Teesdale's extraordinary geology first hand. Since we are unable to do this, here is a digital alternative to our Geoparks event which was planned for this weekend. The event was in collaboration with AONB and so we have linked some of their geology resources and self-guided walks. We hope you can continue to learn, explore and enjoy the diverse landscape of Upper Teesdale this year.
A brief geological history of Upper Teesdale
If we go back around 450 million years ago, that's when the oldest rocks here were formed by volcanic eruptions amid a large ocean. These are Ordovician and Silurian slates and volcanic rocks, which are only exposed as 'Pikes' along the east fringe of the Northern Pennines, and can be seen near Middleton-in-Teesdale. Throughout the next 100 million years or so, the land was submerged beneath a shallow tropical Carboniferous sea. Beds of limestone began to form, and a depositional cycle of sand and mud resulted in a repeating sequence of limestone, mudstone and sandstone.
This sequence is largely responsible for the way the landscape looks today, producing terraced hillsides where the limestone and sandstone have resisted erosion, unlike the mudstones. A bedrock of a granatic batholith, also known as 'Weardale Granite', acted as a boyant basement which prevented the areas subsiding and kept it at such high altitude. As a result a much thinner succession of Carboniferous limestones, mudstones and sandstones accumulated on this bedrock than in the adjoining areas. This area is known as the ‘Alston Block’.
About 295 million years ago tectonic movement allowed a large amount of magma from the Earth's core up through the Earth's crust. This magma did not break the surface, but instead spread between the layers of existing Carboniferous rock and altered them with its heat. The magma cooled and crystallised to form the dolerite 'Whin Sill' with which we are familiar today. In places the maximum thickness of this sill reaches 70m, and is exposed at High Force waterfall where it has been eroded by the River Tees over time. The surrounding limestone was super-heated to such an extent that it was altered into a marble known locally as 'sugar limestone', and the shales into ‘whetstone’. This sugar limestone is exposed in several places on Widdybank and Cronkley Fell. Soils formed on this rock help support some of the important populations of rare alpine plants which grow there.
In more recent geological history, the landscape was shaped and changed dramatically by glacial landforms and deposits from the last ice age, which was at its greatest extent around 22,000 years ago. Ice moved across the landscape creating valleys, smoothing the fells and dales into what we know today. After the last glaciation the fells were bare rock, and meltwater flooded the valleys. Slowly the landscape was colonized by arctic plants and trees, and around 7,500 years ago peat began forming on the uplands, preserving evidence of ancient forests and early human activity.
A mining landscape
When the Whin Sill formed and heated the surrounding rock, it also heated mineral rich waters within the rock. These waters began to move and circulated through faults and fissures in the rock, and as they cooled the dissolved minerals crystallised. These are the veins and deposits famous as the North Pennine Orefield.
These mineral veins were discovered around 2,000 years ago during the Iron Age, and have been a huge foundation for the area's economy for many centuries since. The height of lead mining was in the 18th and 19th century when minerals such as galena (lead ore), sphalerite (zinc ore), iron ores, fluorite, and barium were commercially mined. These mines left their mark on the landscape, leaving a legacy of settlements, old shafts and spoil heaps, some of which make for unique habitats and add to the biodiversity of the area.
Quarrying was also an important industry shaping the landscape and economy in recent history. Most of the quarries extracted limestone and Whin Sill dolerite for building materials and aggregates to be used all over the country. Force Gath Quarry near High Force Hotel is still active, whereas many others in the area are now disused.
Self-guided geology walks
We hope that some of you may be able to go out and enjoy walks which will allow you to appreciate the unique geology of Upper Teesdale. Here we include a few routes where you can admire the incredible landscape and formations.
North Pennines AONB have some great guides to Geotrails to follow on their website here.
Reserve Warden
Charlotte Pink
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