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Hadrian's Wall was built on igneous rock in Northumberland. In 120AD the Roman Emperor Hadrian visited Northumberland to strengthen his defences of the Roman Empire. He found the granite cliff and realised its potential for defence. The line of igneous rock found here is a sill, formed 295 million years ago when magma cooled within the earth. Over time, the less resistant rock surrounding the sill became eroded, leaving a higher ridge of land perfect for defence.