Hadrian's Wall stands on frozen magma.
For 15 miles, the Romans built it along the top of a cliff. That cliff is the edge of a buried sheet of dark rock.
About 297 million years ago, molten rock rose from deep below. It squeezed sideways between the layers of rock. It never reached the air.
There it froze, into a sheet as thick as a ten-story building. Geologists call it the Whin Sill.
How do we know it never erupted? The rock on top of it is baked. A lava flow only cooks the ground beneath it. The proof was found right here, along the Wall.
Then, over the ages that followed, rain, frost and grinding ice wore the softer rock away around it. The hard sheet stayed. Its broken edge became a cliff, facing north.
Nineteen hundred years ago, Roman builders found that cliff waiting. They set their wall along its top.
The Romans built the wall. Magma, long before them, drew the line.