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Whin Sill
locality:-   High Cup Gill
civil parish:-   Dufton (formerly Westmorland)
civil parish:-   Murton (formerly Westmorland)
county:-   Cumbria
locality type:-   geological site
locality type:-   rocks
coordinates:-   NY74562617 (etc) 
1Km square:-   NY7426
10Km square:-   NY72

Click to enlarge
BVB85.jpg  High Cup Scar.
(taken 24.7.2011)  

Whin Sill is a horizontal sheet of solidied magma which had spread between layers of limestone and shale in the north Pennines about 295 Million years ago, late Carboniferous period. It is up to 90m thick. The igneous rock is called dolerite, or whinstone.
In the last stages of cooling the magma shrank, resulting in vertical cracks breaking the rock into rough columns, which can be seen in High Cup Gill.
Dolerite is very hard. It is made of crystals of: pyroxene, calcium, magnesium, iron silicates; feldspar, calcium, potassium, aluminum silicates; and iron oxide.
Quarrymen gave Whin Sill its name; whin because it is hard, sill because it lies horizontally. The sill part was adopted by geologists for similar igneous formations.

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