Tuesday, 16 August 2011

Walking the Whin Sill

We learned about The Whin Sill in school geography lessons, when we were coming to terms with the mind-boggling notion of fluid rock. Back then it bore the epithet 'Great' and for us Londoners it was a reinforcement of everything we were taught to believe about the north of England; a gritty, faraway wildnerness that spawned gritty, uncompromising peoples. These days I travel south to get to that not-so-faraway country. The wilderness has become familiar, although no less awe-inspiring. And the people? Well I guess they are the melting pot of humanity I have come to expect wherever I go, uncompromising only in their refusal to be stereotyped.

Irresistible force meets immovable object. The Tees finds a
weak point in the Sill at Cauldron Snout
Wherever it breaks the surface The Whin Sill makes for exciting scenery. My geography teacher was accurate in introducing us to it as one of the dominant and defining features of the landscape of northern England. From the Farne Islands in the east to the sudden Pennine escarpment in the west, it crops up in many of the region's most celebrated natural wonders. The river Tees hurtles over its ledges in a series of thunderous cataracts at High Force, Low Force and Cauldron Snout. It forms vertical, castellated cliffs at Holwick Scar and Falcon Clints and provides the setting for a geography master's dream, the near perfect glaciated valley of High Cup Nick. It delighted the Romans too; they built their famous wall along its ramparts at Hotbanks and Cuddy's Crags.

Made of a tough, dark igneous rock called dolerite, The Whin Sill was intruded into the horizontal strata of the surrounding bedrock some 295 million years ago, during a period of tectonic stretching. Wherever there was a gap the 1000 degree magma poured in like hot wax, bubbling up through a vent from unimaginable depths. Because it was not laid down on the surface in sediments but forced entry later, the Sill is not uniform, as generations of quarrymen and drillers have discovered. In some places it is hardly there at all, in others it may extend to a thickness of sixty or seventy metres.

Textbook dream. The Whin Sill forms a perfect band around
the glaciated High Cup Nick
It was quarrymen, in fact, who in the nineteenth century gave the Whin Sill its name. Whinstone is a local term for the rock which makes first rate road stone but which is generally thought too hard to fashion into blocks for house building. They called it a sill because it lies horizontally. It was therefore the original sill, before the term was coined for more general use by geologists. The typical columnar forms of the whinstone crags are a consequence of the magma cooling and fracturing along the vertical plane. Whinstone quarries are very much in evidence in this part of the country although many of them, like the mineral mines, are now thoroughly worked out. It is stll referred to as the 'Great' Whin Sill in some places, to distinguish it from the lesser 'Little Whin Sill' to be found outcropping in parts of Weardale.

The Whin Sill exposed: fracture columns of dolerite at
Holwick Scar, Teesdale
It's hard to be blind to the geology in this part of the land and, with an accustomed eye, it is possible to discern some remarkable shifts in the curves of the landscape, the pH of the soil and the flora over the length of a leisurely walk. One could hardly expect an event as cataclysmic as that creating the Whin Sill to leave the surrounding country rock unaffected. Above Cauldron Snout, superheating of the overlying limestone has altered its crystalline structure to produce an alkaline granular rock known rather wonderfully as 'sugar limestone'. I don't recall learning about that in Surrey school geography classes. Perhaps it didn't quite resonate with the craggy northern image my teachers were so keen to depict. Anyway, sugar limestone is home to some uncommon alpine plants like the beautiful blue Spring Gentian. Sadly there were none to be found on my last visit to the Cow Green Reservoir, which sounds like the perfect reason for going back.

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