Until 410 million years ago, the area of land now recognised as Scotland was separated from England by an ocean wider than the present-day North Atlantic - the Iapetus Ocean. When the two halves of Britain, which were part of separate larger continental land masses, began to drift towards each other, so the Iapetus Ocean began to close inexorably. The seaway between the converging continents narrowed until they collided and mountains were squeezed up in place of the vanished ocean. The two ancient continents, originally on opposite sides of the vast ocean, were now joined along a line known as the Iapetus Suture which runs almost parallel to Hadrian's Wall.
