How Scotland's geological foundations came together

About 600 million years ago, the North-west Seaboard, Northern Highlands and Grampian Highlands all lay at the edge of a continent called Laurentia, about 100km or more apart from one another. The Dalradian rocks of the Grampian Highlands were still only layers of sediment building up in the shallows of the Iapetus Ocean south of Laurentia.

Closure of the lapetus Ocean

The Iapetus Ocean began to open up about 800 million years ago, as the forces of continental drift pulled apart a large, ancient continent. About 500 million years ago, the same forces began to close the ocean once more, bringing together Scotland and England.

The closure of the lapetus Ocean brought about a series of continental collisions known as the Caledonian mountain-building event or Caledonian Orogeny. It was in the lapetus Ocean, as it closed, that the volcanic islands thought to be the foundations of the Central Belt formed.

Landmasses set to collide

As the Iapetus Ocean closed, three landmasses were on a collision course with Laurentia:

• the chain of volcanic islands that had formed in the ocean was nearest – it was heading towards the foundations of the Grampian Highlands
• Eastern Avalonia, the continent that contained England, lay behind the island chain, on the far shore of the ocean
• Baltica, the continent containing Scandinavian Europe, lay further east – it was set to collide with the Northern Highlands

The collisions that followed during the Caledonian Orogeny formed the Caledonian Mountains, a massive mountain range similar in scale to the Alps or even the Himalayas. The remains of this mountain range stretch from Norway to the Appalachian Mountains of North America.

Stages of the Caledonian Orogeny

The Caledonian Orogeny occurred in three stages.

1. The chain of volcanic islands collided with the Grampian Highlands about 480–460 million years ago. This is called the Grampian Event.
2. Baltica collided with the Northern Highlands about 440 million years ago, pushing together the Northern Highlands and North-west Seaboard. This is called the Scandian Event.
3. Eastern Avalonia ‘soft docked’ about 425 million years ago, as England softly collided with Scotland.

With the completion of the Caledonian Orogeny, about 425 million years ago, Scotland’s geological foundations were finally brought together.