Southern Uplands geological foundations

The broad rolling hills of the Southern Uplands of Scotland parallel the Scotland-England border. They are bounded on the north by the Southern Uplands Fault and their southern edge coincides with one of Britain's most fundamental geological divides. This is the so-called Iapetus Suture, the trace of the ancient Iapetus Ocean, which once divided two continents. To the north was the continent of Laurentia, containing northern Scotland, the north of Ireland, Greenland and North America. To the south lay Eastern Avalonia the continental landmass that included England, Wales and southern Ireland. Around 500 million years ago the ocean was more than 1000 kilometres wide and over the next 80 million years it closed with the drifting of the Earth's tectonic plates.

As the two landmasses drifted together closing the ocean, the ocean crust sunk downwards into the Earths mantle. At one point however, a portion of the oceanic crust instead of sinking downwards, was heaved upward onto the continental margin where it can be seen today at Ballantrae. As collision continued, layers of sediment that had accumulated on the ocean floor were scraped off and piled up against the Laurentia continental margin. This pile of sandy and muddy sediment, which was later compressed to become greywacke (coarse, muddy sandstone) and shales, forms the foundations of the Southern Uplands.

The Iapetus Ocean finally closed around 425 million years ago, bringing together Laurentia and Eastern Avalonia. The trace of the junction lies approximately along the line of the Solway and the Scotland-England border.