Mountain Building
A’ cruthachadh beinn
The mountains we see in Lochaber are the eroded roots of the Caledonian mountain chain, created over 400 million years ago. This ancient mountain range had peaks as high as parts of the present-day Himalaya, and extended for over 5,000 kilometres across a vast continent.
The range was split apart by the opening of the Atlantic Ocean. Today the eroded remains of the Caledonian range are found in eastern North America, Greenland, Ireland, Scotland Norway and Spitsbergen.
Mountain-building is a result of the constant movement of plates on the surface of the Earth, driven by heat from the Earth’s core. The series of powerful earth movements that built the Caledonian mountain range is known as the Caledonian Orogeny.
The rocks that underlie most of the Highlands southeast of the Great Glen are known as Dalradian rocks. They started out as sediment that was laid down between 750 and 500 million years ago on the edge of an ancient continent called Laurentia.
This continent was separated by an ocean from the continent of Baltica (the foundations of Sweden, Finland and Russia), and from a smaller continent called Avalonia (where the rocks of England and Wales were forming).
500 million years ago, the three continents began to move together, gradually closing up the ocean. The colliding rocks of the continents and intervening ocean crumpled, folded and pushed on top of each other, and eventually became welded together into one huge new continent.
The enormous forces created when continents collide can fold, thrust and sometimes melt the existing rocks. What were originally sedimentary rocks were transformed by high pressures and temperatures into a range of metamorphic (or 'changed') rocks which include quartzites, schists, marbles and slates.
In extreme cases, the rocks melted to form magma, which built up in huge chambers below the Earth’s surface.
