The Iapetus Ocean is slowly closing and by the end of the Silurian the two continental blocks collide producing the Caledonian Orogeny. This collision finally brings Scotland and England together. The Caledonian Orogeny thus lasted from about 500 to 360 million years ago. By this definition most Caledonian igneous rocks in Scotland range in age from about 500 million (early Ordovician) to around 390 million years ago (early Devonian) with related activity continuing to around 360 million years (end Late Devonian) in Orkney and Shetland.

At the end of the Caledonian Orogeny, by the early Devonian, Scotland was part of a huge continent straddling the equator called the Old Red Sandstone Continent. The Caledonian mountain range lay along the southern margin and included Scotland. The rapid erosion of these mountains provided the sediments for the formation of the Old Red Sandstone of the succeeding Devonian period.