Continents Collide
Until 410 million years ago, the area of land now recognised as Scotland was separated from England by an ocean wider than the present-day North Atlantic - the Iapetus Ocean. When the two halves of Britain, which were part of separate larger continental land masses, began to drift towards each other, so the Iapetus Ocean began to close inexorably. The seaway between the converging continents narrowed until they collided and mountains were squeezed up in place of the vanished ocean. The two ancient continents, originally on opposite sides of the vast ocean, were now joined along a line known as the Iapetus Suture which runs almost parallel to Hadrian's Wall.
Scotland's geological diversity is largely derived from this continental collision. Prior to closure, the land which was to become Scotland was, in fact, a series of distinct geological 'terranes' separated by major geological faults.
As the Iapetus Ocean closed, so the disparate terranes slid together side by side to form one contiguous landmass and then joined with England. In geological terms, the 'union' of Scotland with England took place around 410 million years ago.
Colliding continents have given rise to some of Scotland's most familiar scenic views and landmarks. The Highland Boundary Fault, a terrane boundary, cuts across the south end of Loch Lomond. This Highland - Lowland boundary shown on the photograph reflects the major geological change from hard Dalradian metamorphic rock in the background to the pebbley conglomerates and softer sandstones of Devonian age in the foreground.
The line of the fault is marked by an intrusion of serpentine, forming much of Conic Hill and Inchcailloch Island. This very base-rich material was derived from great depths in the Earth's crust. The 'Highland Line' marks a sharp change in topography, weather, vegetation, wildlife and landuse. It also played an important part in Scotland's historical and cultural development.
A mountain range perhaps as high as the Himalayas was created as a result of the Scotland - England collision. Great masses of granite and related rocks were formed from molten magma injected deep into the Earth's crust. However, as the great peaks that must have existed then were worn down by ice and water over many millions of years, these granites and their darker, more base-rich equivalent rocks, known as gabbros, were exposed at the Earth's surface. The Cairngorms, Ben Nevis and many other significant hills and mountains throughout Scotland are carved from granite formed at this time. The altered rocks of Dalradian and Moine, comprising mainly schists and quartzites (the latter having the strength of steel), are also resistant to erosion, so have remained as relatively high ground since they were formed. And so the Scottish Highlands were born.
The relationship between geology and landscape is clearly reflected in the Landsat image below. The harder more resistant crystalline rocks of the Highlands form the higher ground, separated from the Midland Valley by the Highland Boundary Fault. Many of the other major displacements, such as the Great Glen Fault and Southern Upland Fault, are also clearly visible.