The island is cross-cut by diabase dikes associated with a major rift in the Oslo area in the Permian. The diabase has been heavily quarried for building material, and the island has about 20 quarries dating from the medieval period through to the 19th century. The oldest rock on the island dates back to late Ordovician and the eastern parts is littered with erratic boulders deposited during the last ice Age, thus the island illustrates well the local geological events spanning nearly half a billion years.