The Southern Uplands – mountains form when continents collide
The origins of the rocks that make up the range of rounded hills now known as the Southern Uplands go back in time to nearly 500 million years ago. Then the world was a very different place. Layers of sediment were accumulating in a deep ocean (known as Iapetus) which spanned the southern hemisphere. To its north lay a continent that was eventually to fragment into North America, Greenland and Scotland; to its south was another that was to form England, Wales, western Europe and Africa.
As the ocean floor subsided, thousands of layers of sand, silt and mud built up. Repeated massive influxes were triggered by earthquake shocks which set off large underwater avalanches, called turbidity flows, at the edge of the continents. Each flow gave rise to a distinctive deposit. As the coarser materials, pebbles and sand, settled out first they occur at the base, grading up to fine silt and then mud. The fine top of one flow is abruptly overlain by the coarse base of the next flow, commonly with marks where the sand-laden current scoured into the mud.
Around 460 million years ago, the continents started moving towards each other, squeezing the ocean and pushing the sea-floor sediments up to the surface. Some 40 million years later, the two continents collided; for the first time Scotland met England – and Scotland came out on top.
As the Scotland/America continent was thrust over the England/Europe continent, the ocean sediments were squeezed into folds, fractured as they were racked by earthquakes and hardened into rocks called greywackes (German for grey rock). They were pushed up as mountains, part of a range that stretched from the American Appalachians, through Greenland and Scotland, to Scandinavia.
Over millions of years these mountains were raised and then worn down to give the rounded hills we know today as the Southern Uplands. The evidence of their past can today be seen within the rocks themselves, each layer of strata telling its own story. Along the Berwickshire coast the folded rocks are beautifully displayed.
Fossil remains of the earlier life that teemed in the ocean and along its margins are hard to find, although graptolites, an extinct type of floating invertebrate, can be locally quite common. These evolved particularly rapidly into different forms and so they can be very useful for unravelling the relative ages of the folded layers of rocks. This was first demonstrated in 1878 by one of the early geologists, Charles Lapworth, as celebrated by a plaque on the wall of his lodgings, Birkhill Cottage on the Selkirk-Moffat road. He carried out his researches at nearby Dob’s Linn, now recognised as an internationally important Site of Special Scientific Interest.
Sediments that were laid down along the coasts of the continents have fossils from the rich life that lived in shallower water and around coral reefs: trilobites, corals, various shell fish and other now-extinct sea creatures.
The Southern Uplands stand up as hills today because their rocks were baked hard when they were buried and heated during the mountain-building episode. However, they owe their distinctive rounded heather-covered shape with deeply incised valleys to more recent events.