444 – 419 million years ago
During the Silurian, Scotland lay south of the equator on the southern edge of a continent called Laurentia, to the south of which lay an ocean called the Iapetus. On the opposite side of the ocean lay the continent called Baltica, with England on its northern shore. The two continents were slowly moving towards each other and as they did so, the ocean between them was closing. By the end of the Silurian, the two continents eventually collided and the ocean disappeared. It was at this point that Scotland and England came together.
Before the ocean had finally closed, northern Scotland lay above sea level and southern Scotland constituted the ocean floor. Sand and mud was washed into the ocean from the land and formed very thick sequences of shale and muddy sandstones (known as ‘greywackes’). The youngest Silurian rocks show the change from sedimentary rocks formed in a marine environment, through to rocks formed on land, in rivers and lakes – as the land emerged from the sea. Silurian rocks form a large part of the Southern Uplands. The fossil remains found include trilobites, shellfish and early jawless fish, most if not all of which are now extinct.