Dolerite (Whin Sill)
What is dolerite?
It’s a rock that was once molten magma, like granite, but because it occurs in smaller and thinner volumes it cooled faster and so the mineral crystals in it are much smaller and you can only see them with a hand lens.
How do I recognize it?
It’s a dark grey, hard rock. It usually has vertical joints and fractures (caused by its cooling). You can recognize it in the landscape because creates a prominent scarp.
Where do I find it?
Dolerite is what makes up the Whin Sill that is present in many parts of Northumberland. Because it is a very hard rock you can see it in the escarpment that Hadrian’s Wall is built along and along the coast at Bamburgh and Dunstanburgh.
How was it made?
Dolerite was once a molten rock at a temperature of around 1,200 degrees C. When this part of the Earth was being stretched during an episode of mountain building caused by movement of the tectonic plates 295 million years ago, the molten magma was injected into weaker places in existing sedimentary rocks (rocks like limestone and sandstone) as “sheets” up to about 50 metres thick.
What does a dolerite landscape look like?
The injection of the dolerite as sheets into sedimentary rocks and then the tilting of all these rocks south and eastwards by around 10 degrees by earthquakes, gives a very characteristic “scarp and dip” or “cuesta” landscape. You can see it best when driving along the B3618 Military Road. Dolerite also produces a very characteristic vegetation – called Whin Sill grassland.
Which plants like dolerite?
It is nutrient-rich, but hard, so releases nutrients slowly, and on its shallow soils in dry seasons the vegetation can be physically-stressed. Shallow soils and bare rock support a distinctive set of species. They include chives, and annuals such as annual knawel and long-stalked crane’s-bill. A particularly attractive herb is maiden pink.
What use is it?
Because of it’s hardness dolerite is used extensively for roadstone – the rock fragments that are mixed with asphalt in “tarmac”. More than 100 years ago it was also quarried and shaped into “sets” to make cobbled streets.