The Whin Sill

Squeezed between the sedimentary rocks of North East England is a hard, volcanic, mass of rock known as the Whin Sill, so large that it underlies three counties – Cumbria, Northumberland and County Durham. Where it outcrops it provides the drama for some of the UK’s most recognisable landmarks including Hadrian’s Wall at Highshield Crags (for example), England’s largest waterfalls, Cauldron Snout and High Force in Upper Teesdale and the spectacular glacial valley of High Cup in Cumbria (pictured above).  

When the land masses of Laurussia and Gondwana collided at the end of the Carboniferous period ~300 million years ago (mya) producing the supercontinent of Pangaea, the resulting deformation and faulting which took place over a period of about 15 million years allowed basaltic magma from the earth’s mantle to force its way upwards into the overlying rock to various levels, moving vertically in what we now refer to as dykes and horizontally as sills.

This occurred around the beginning of the Permian period ~296 mya when the sedimentary Carboniferous rocks of what is now North East England were much closer to the equator. An estimated volume of 215 km³  of magma migrated over an area of ~4500 km² – from the Pennine escarpment in the west to the North East coast, with much more extending well under the North Sea where it is at its thickest.


High Cup Nick
The Whin Sill at High Cup Nick above the Eden Valley in Cumbria © The Durham Cow


Cauldron Snout
England’s longest cascade descends the Whin Sill, Upper Teesdale © The Durham Cow


Dine Holm Scar
The Tees meets the Whin Sill @ Dine Holm Scar © The Durham Cow


High Force Waterfall
The Tees plunges 22 m over the Whin Sill at High Force, Upper Teesdale © The Durham Cow


Forcegarth Quarry
Quarrying the Whin Sill at Forcegarth, Upper Teesdale © The Durham Cow


Bamburgh Castle
Bamburgh Castle and the Whin Sill, Northumberland © The Durham Cow


Highshield Crags
The Whin Sill & Hadrian’s Wall @ Highshield Crags © The Durham Cow


Lindisfarne Castle
The castle is built on an outcrop of the Whin Sill, Holy Island © The Durham Cow


Greenfoot Quarry
Former dolerite quarry on the Little Whin Sill near Stanhope, Co. Durham © The Durham Cow

From studies of the chemical and palaeomagnetic characteristics of the rock, and of the vitrinite levels in nearby coal seams, it is thought that the rock may have been intruded in three phases: first the Little Whin Sill (LWS), followed by the Great Whin Sill (GWS) and finally the minor sills and dykes which are thought to have acted as feeders to the main intrusions.

The molten rock, at a temperature somewhere around 1100°C, may have taken 60 years to crystallize fully and in that time caused adjacent sedimentary rocks to undergo metamorphic change to varying degrees. In parts of Teesdale, around Cow Green and Cronkley, the limestone has been changed to a crystalline ‘marble’ which crumbles easily and is known as sugar limestone. Where shale was the adjacent rock it occasionally resulted in a highly resistant rock containing quartz grains known as whetstone, ideal for sharpening cutting edges of metal tools. Sandstone is more resistant to heat but seams of coal local to the intrusion were converted to coke.

Apart from its photographic appeal, the tough rock, which is composed largely of dark-grey, fine grained, quartz dolerite, is quarried commercially for roadstone when it is usually referred as whinstone, an informal term used by quarrymen – ‘whin’ meaning ‘hard’. Such is the significance of this band of volcanic rock that even the geological term ‘sill’, which is defined as a “tabular igneous intrusion, mainly concordant with bedding”, originated in the quarries of Northern England.

Locations

NORTHUMBERLAND: Farne Islands; Alnwick; Dunstanburgh; Hadrian’s Wall (various locations) 

UPPER TEESDALE: River Tees at Cauldron Snout, Dine Holm, Forcegarth Quarry, High Force & Low Force; Cronkley Scar; Holwick Scars; High Force Quarry; Crosthwaite Quarry

CUMBRIA: Pennine Escarpment, particularly at High Cup Gill nr. Dufton; Cash Force Waterfall

COUNTY DURHAM: (Weardale) Copt Hill Quarry, Cowshill (GWS); Greenfoot Quarry and Briggen Winch near Stanhope; Turn Wheel Linn, on the Rookhope Burn near Eastgate (all LWS)

References

Bowden, Alistair. History in the Landscape: The Rocks, Landscape and Minerals of Weardale. The Weardale Society, 2008.

British Geological Survey (BGS). “Geology of Britain Viewer.” http://mapapps.bgs.ac.uk/geologyofbritain/home.html.

Bulman, Russell. Introduction to the Geology of Alston Moor. North Pennines Heritage Trust, 2004.

Geology North. The Whin Sill. Geology North. https://www.geologynorth.uk/the-whin-sill/. Accessed 05 10 2020.

Pickett, Elizabeth. Reading the Rocks (Exploring the Geology and Landscape of the North Pennines). North Pennines AONB Partnership, 2011.

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