Basalt sill, Northumberland

Hadrian’s Wall was built by the Romans around 120 AD, along a natural line of north-facing cliffs formed by the outcrop of the Great Whin Sill. The sill is roughly 70 metres thick and slopes (dips) gently to the south beneath the overlying sedimentary rocks.

The cliff is very steep due to the near-vertical joints in the basalt. The diagram is a cross-section through the sill, showing how the rock breaks along joints to form the steep cliffs on which the wall is built.