Why does Scotland have a giant line cutting through it? That’s the Great Glen Fault — a massive ancient fracture in the Earth’s crust formed around 400 million years ago during the collision of tectonic plates. The Great Glen Fault formed during the Caledonian Orogeny, roughly 420 to 390 million years ago, when ancient continents collided as the Iapetus Ocean closed. This immense tectonic collision created a vast mountain range stretching across what is now Scotland and fractured the Earth’s crust into major fault systems. One of the largest of these was the Great Glen Fault, a massive strike-slip fault where huge blocks of rock shifted past one another. The same mountain-building event also helped create the ancient Appalachian Mountains in North America, which were once connected to the Scottish and Scandinavian Highlands before the Atlantic Ocean existed. Scotland and parts of North America were once part of the same continuous mountain chain. Later, during the formation and breakup of Pangaea, new tectonic stresses reactivated the fault zone. Although this activity did not form new mountains in Scotland, it kept the structure geologically active and weakened over millions of years. Erosion eventually exploited this weakened line in the crust, carving out the long, straight valley now known as the Great Glen. The valley contains Loch Ness, Loch Oich, and Loch Lochy, and remains one of the clearest visible reminders of the ancient continental collisions that once linked Scotland to the Appalachian region of North America.(Historic Vids)