Iceland is built up of volcanic rocks with sedimentary interbeds, which have been piled up continuously since Miocene times. In the Pleistocene rock series, sediments of fluvial, lacustrine, marine and glacial origin and soils are very common and frequently thick. A sudden climatic deterioration took place at about 3 Ma BP. The Pliocene lusitanic marine fauna was replaced by a boreal fauna. Conifers and deciduous forest vanished and the flora became similar to the present one. From 3 to 2 Ma BP, inland ice caps were common during cold spells. From then on ice sheets reaching down to sea level have covered most of the country at least 12 times during glacials.