The rock surrounding Lundy Island formed around 360 million years ago. At that time, the area that is now the Bristol Channel was the bed of a shallow, tropical sea. Rivers and streams running off the surrounding land carried mud into this warm sea where it sank to the bottom. Over millions of years these muds were compressed and formed into rock known as shale. When the main, granite body of Lundy rose up, was intruded these shales were converted by heat and pressure into very soft slates, which collapse easily. You can see these rocks along the south-east of the island, below the Castle and on the shore at the Landing Beach, Rat Island and in the Devil’s Kitchen.

The main body of Lundy Island formed around 59 million years ago. Molten rock forced its way up through the shales, converting them to the soft slates which remain at the south of the island, and solidified to form granite. The granite contains semi-precious minerals such as amethyst, topaz and beryl and small amounts of tin and copper minerals. The island is similar to the landforms of Bodmin and Dartmoor, but the granite of Lundy is much younger. The Devil’s slide on east side of island is one of the longest single slabs of granite in Europe and it is an internationally famous rock climb.

Within the granite there are a number of ‘dykes’. These dykes are igneous bodies, formed by later activity, that cut across both the granite and the slate. A unique, fine-grained granite dyke formed at Lundy and is particularly visible in the huge slab of rock that forms the cliff below the castle and cutting across the landing beach and is known by geologists as ‘Lundyite’. Granite has long been quarried from Lundy and it was used to make many of the buildings in the village today.

During the ice ages that followed, the ice scraped away some of the granite to form the flat top to the island. The glaciers may have flowed around Lundy, leaving it as an island in a sea of ice. Following each ice age, there were warmer periods when the glaciers melted, causing changes in sea level. You can still see evidence of this on the east side of the island, where there are slight steps up the cliff (at about 15, 32 and 60 metres above the sea-level today) showing where waves formed old shorelines.

The island is still geological active today. Where the granite pushed through the rock, the slate is very soft and at times whole sections of slate collapse. Weathering of the shoreline along some of the numerous dykes has formed at least 37 sea caves around the coast. These are excellent pupping sites for Lundy’s grey seals. There is also an area called ‘The Earthquake’ on the west of the island. Apparently this formed on the same day as the Lisbon earthquake in 1756, when a section of the granite, up to 150 yards inland, opened-up to form great slots and chasms in the rock.