In the late Precambrian Era and into the Cambrian Period, the basement rock of Scotland was part of proto-North America, having accreted to that Continent as a series of exotic terranes. The Iapetus Ocean separated this continental mass from that of England and Western Europe.
Then, about 450 million years ago (MYA), the Iapetus Ocean closed and the two continents collided, triggering the Caledonian Orogeny; much later, about 150 MYA, the Atlantic Ocean opened and Scotland remained with Europe. Today, ancient rocks that preceded the Caledonian Orogeny underlie most of Scotland but surface primarily across the Western Islands and the Highlands; the latter region is divided into the Northwest Highlands and the Grampian Mountains by the Great Glen Fault. As these Highlands crumpled upward, volcanism, erosion and displacement of emplaced metamorphic crust produced the complicated geology of that region. South of the Highlands Boundary Fault, the Central Lowlands represent a Rift Valley, characterized by Paleozoic sediments, volcanic intrusions and more recent glacial deposits. Finally, the Southern Uplands, bordering England, are comprised of uplifted and eroded Silurian crust from the Iapetus Ocean.
Acting on this underlying mosaic of Precambrian and Paleozoic bedrock, the Pleistocene Glaciers (1.8 million to 10 thousand years ago) plowed down the higher peaks, gouged out canyons and valleys and then left behind layers of till as they receded northward. In addition, their meltwaters scoured the terrain and rising seas drowned the coastline, separating the western and northern islands from the Scottish mainland.
Then, about 450 million years ago (MYA), the Iapetus Ocean closed and the two continents collided, triggering the Caledonian Orogeny; much later, about 150 MYA, the Atlantic Ocean opened and Scotland remained with Europe. Today, ancient rocks that preceded the Caledonian Orogeny underlie most of Scotland but surface primarily across the Western Islands and the Highlands; the latter region is divided into the Northwest Highlands and the Grampian Mountains by the Great Glen Fault. As these Highlands crumpled upward, volcanism, erosion and displacement of emplaced metamorphic crust produced the complicated geology of that region. South of the Highlands Boundary Fault, the Central Lowlands represent a Rift Valley, characterized by Paleozoic sediments, volcanic intrusions and more recent glacial deposits. Finally, the Southern Uplands, bordering England, are comprised of uplifted and eroded Silurian crust from the Iapetus Ocean.
Acting on this underlying mosaic of Precambrian and Paleozoic bedrock, the Pleistocene Glaciers (1.8 million to 10 thousand years ago) plowed down the higher peaks, gouged out canyons and valleys and then left behind layers of till as they receded northward. In addition, their meltwaters scoured the terrain and rising seas drowned the coastline, separating the western and northern islands from the Scottish mainland.
