Green Geology



Site of Special Scientific Interest (SSSI)

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Path, steps, slippery when wet

The Whin Sill - High Force Waterfall

Text and photographs:  Naomi Stevenson


Page updated March 2013

High Force Waterfall, in the village of Middleton-in-Teesdale, is a spectacular exposure of the Whin Sill – a quartz-dolerite sill emplaced in the late Carboniferous approximately 295 million years ago as a result of tectonic forces following the Variscan orogeny (mountain-building episode).


The Whin Sill, which underlies a large area of northern England and which extends out under the North Sea, is lens-shaped in cross section; the magma which it formed from came from feeder dykes and settled in a subsiding basin – hence the shape. It is, on average, 30 metres thick.


Emplacement of magma inevitable alters the country rock it is intruded into – at High Force, the sill is in contact with sandstone which has been dewatered and baked to a hornfels texture in the process.  As the dolerite cooled, it formed the columns which are so prominent here.


Like almost all of the British landscape, Teesdale has been changed as a result of agricultural and industrial activity.  The Whin Sill is still quarried for roadstone and aggregate.  The sill gives the area a tremendous sense of place so there has inevitably been much conflict between industry, tourism and conservation groups here.





Click to enlarge

High Force waterfall