The main phase of the Caledonian orogeny (from about 425 to 400 million years ago) is called the Scandian phase in Scandinavia and the "Grampian phase" in the British Isles. It was caused by the collision between Laurentia and Baltica. The Iapetus Ocean first closed in the north, then in the south. Therefore, the collision between Baltica and Laurentia took place a little earlier than that between Avalonia and Laurentia. Continental collision started in the Mid Silurian and mountain building took place in the Early Devonian (from 420–405 million years ago). In North America, the collision between Avalonia and Laurentia is called the Acadian orogeny.