natural environment, pastures, historic buildings as well as fairly benign (for Norway) bathing temperatures. Natural history The island has an interesting geology, being composed mainly of Ordovician and some Silurian mudstone and limestone. The transition between rocks of the two periods is exposed on the south-eastern tip of the island, illustrating the sudden marine transgression that marks the shift between two important parts of Earths history. The sequence has been affected by both folding and thrusting as part of the Caledonian orogeny with the strata lying vertically or in some case becoming overturned. 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. The mostly sedimentary geology has given the island a very fertile soil, giving it Norway's highest biodiversity found in such a small place. A few plant species found on the island do not occur anywhere else in Norway. Due to the geology and natural diversity, the island has been declared a natural preserve. While no part of the island is off limit to the public, collecting plants and taking rock-samples is banned. Visiting Hovedøya The island is connected to Oslo by means of two boat routes (lines B1 and B4) which dock on the north side of the island. Service is year-round although it's only running during daylight and therefore is limited during the winter season. In summer time a small cafeteria right by the monastery ruins is open to the general public. Curiosities There is a fox living on the island that is often photographed by internet users and is known to occasionally steal things. References External links Katolsk.no Catholic webpage about the monastery OsloSurf.no entry on Hovedøya Hovedøya www.visitoslo.com Islands of Oslo