The ocean between worlds
Beneath the rolling hills and estuary flats of the Anglo-Scottish border lies the scar of a vanished ocean. This scar, known as the Iapetus Suture, marks the collision zone where two ancient landmasses fused together around 420 million years ago. The land that now forms Scotland was part of a continent called Laurentia, which also included North America and Greenland. England and Wales were part of a separate microcontinent named Avalonia, which drifted north from near the South Pole.
Between them lay the Iapetus Ocean, which began to form around 600 million years ago and at its peak was over 1,000 kilometers wide. For millions of years, these two pieces of proto-Britain existed on opposite sides of a vast ocean basin in the Southern Hemisphere. The slow, relentless process of plate tectonics drove them towards each other. As the ocean floor was forced downwards, or subducted, beneath the continents, the Iapetus Ocean shrank. This geological drama, known as the Caledonian Orogeny, culminated in the final continental collision during the Silurian Period. The impact crumpled the continental edges, creating a mountain range once as high as the Himalayas. Today, the suture line runs from the Solway Firth on the west coast, through the Cheviot Hills, and out to the North Sea.
Evidence in the stone
The proof of this continental crash is not a crack in the ground but is written in the rocks and fossils on either side of the line. The most definitive evidence comes from Ordovician-era trilobites, extinct marine arthropods. Fossils found in Scotland are distinctly North American in character, belonging to what is known as the Laurentian faunal province. Trilobites from the same period found in England's Lake District are of a European or "Avalonian" type, with different species. These fossil records show that the creatures evolved in complete isolation, separated by a deep ocean.
The bedrock itself tells the same story. The geology of the Scottish Highlands is similar with that of North America. To the south, in England, the rocks of Avalonia show different origins. The collision zone is a complex belt of faults and deformed rock. In Northumberland National Park, the immense pressures of the collision generated the granite of the Cheviot Hills and folded Silurian-era rocks. Though now mostly hidden by younger rocks and glacial deposits, deep seismic surveys confirm a major structural boundary dipping northwards deep within the Earth's crust, the root of the ancient mountain belt.