Towards the end of the Carboniferous Period around 300 million years ago, movements in the crust plate on which Great Britain sits allowed molten rock, or magma, to be injected under pressure into the rock layers present over much of northern England. This molten rock then cooled, crystallised and solidified to form the Great Whin Sill – a mass of dolerite which, although mostly underground, can be seen exposed at the surface in many places today.
It extends for around 1,500 square miles and underlies much of south and east Northumberland and the Durham Coalfield but its maximum known thickness of around 70 metres is under the Pennine mountains.
The study of Geology began in the British Isles and many of the terms used to describe British geological features are still used throughout the world. Sill is one of those terms.
Quarrymen of Northern England used the term 'sill’ to describe a more or less horizontal body of rock. ‘Whin’ was applied to dark & hard rocks and the quartz-dolerite that makes up the Whin Sill is such a rock. Because it is harder than the surrounding rocks it often occurs as outcrops and some of Hadrian’s Wall as well as Bamborough and Dunstanbrough castles are sited on it as are many waterfalls such as High Force and Low Force.
Where the molten rock cools relatively rapidly lots of small crystals form and the resulting rock is a fine-grained basalt. Significant contraction forces build up as it cools and while a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form. The extensive network of cracks that develop may result in the formation of columns - very small (less than 1 cm diameter) if the cooling is very rapid, larger diameter columns if the cooling is slower. These structures are often erroneously described as having hexagonal jointing but although the average number of sides of the columns is indeed six, polygons with three to twelve or more sides can be observed.
One of the most famous examples of columnar basalt in the world is the Giant's Causeway in N. Ireland, where the huge nature of the hexagonal columns gives the impression that they were artificially constructed. Here, on the beach near Alnmouth, the causeway is a much lower affair, hence my originally christening it “The Midget’s Causeway”.
However, I'm told this is no longer politically acceptable so it's now a "mini Giant's Causeway" - and I hope that tall people will not be offended !!
To log this cache post a photo of yourself standing on the “mini Giant’s Causeway” and e mail me to say what slope or dip does the Whin Sill have at this location (i.e. how many degrees from the horizontal and in which direction N,S, E or W does it dip)?