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
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
|The Whin Sill exposed: fracture columns of dolerite at|
Holwick Scar, Teesdale