Geology of Scotland
Sandstone, mudstone and limestone were laid down in the area which is now the Midland Valley. This occurred in shallow tropical seas on the edge of the once-great Iapetus Ocean. The Iapetus Ocean closed 480-430 million years ago, when the volcanic island arc collided with Laurentia. By the end of the Silurian period (c. 420 million years ago), the Iapetus Ocean had completely disappeared. The combined mass of three continents formed the "new" continent of Laurasia. This would become the northern part of the supercontinent Pangaea.