The Whin Sill
The Whin Sill is a well known geological feature that underlies much of north east England. Where it outcrops it helps create some of Cumbria’s, Durham’s and Northumberland’s most dramatic natural features including the huge gully of High Cup Nick, the cliffs beneath Hadrian’s Wall and the rugged Farne Islands in the North Sea. It was emplaced around 300 million years ago during the late Carboniferous period as hot volcanic magma that forced its way into the cracks in the sedimentary rocks around it forming ‘dykes’ in the vertical fissures and ‘sills’ in the horizontal.
At times the heat from the magma altered the chemistry of the neighbouring rock, notably in the case of the ‘sugar limestones’ that can be found around Cow Green in Upper Teesdale. Subsequent erosion of the softer surrounding rock has exposed large areas of the Whin Sill and within County Durham that exposure has been particularly prevalent in Teesdale where the most frequently visited site is probably at High Force waterfall in Upper Teesdale.
In Weardale exposures are fewer and further between but one place where the Whin Sill can be seen is at Burtreeford (pictured) near Cowshill in Upper Weardale where it has been thrust violently upwards to outcrop as a dyke known as the ‘Burtreeford Disturbance’. Here it has been exploited by the now abandoned quarry at Copt Hill for the hard-wearing dolerite (or ‘whinstone’ as it was known to the quarrymen) which is ideal for road building aggregate.