Granite tor, Stowe's Hill, Bodmin Moor, Cornwall, UK. Around 290 million years ago (MYA) a large volcanic intrusion occurred that cooled into the granite. This was finally exposed in parts some 245 MYA and started to weather, but it was not until 2 MYA that the tors were formed, as they lay very close to the ice sheet of the Ice Ages . On the tors, erosion and weathering created stacks of tiers or pillows so typical of West Country moorland landscape. They are still very prone to frost damage. Deep below the surface, the volcanic intrusion is believed to extend from Devon to the Scilly Isles.