Hornfels, Cumbria

The pale-coloured rock in the upper part of the image below is the edge of the Shap Granite, which intruded as hot magma into the surrounding rocks around 400 million years ago.

Heat from the magma intrusion has “baked” the darker rock (which was originally mudstone), causing it to re-crystallise in a new form – a hard, flinty-looking metamorphic rock called hornfels.

This type of metamorphism is called contact metamorphism because it takes place where rocks are heated up in contact with igneous intrusions.