The island is formed from rocks of the late Precambrian Gwna Group, part of the Monian Supergroup. The rocks are a mélange, often called Gwna Mélange, which have an unusual mix of clasts of all sizes up to 100m across. These are also very varied types, including both sedimentary and igneous rocks. Blocks of sheared granite in the melange can be seen in the northwestern coastal cliffs of the island. In other places there are clasts of quartzite, limestone, sandstone, mudstone, jasper and basalt. The deposit is an olistostrome, a very large underwater landslide possibly caused by an earthquake some time after 614 million years ago.