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Lewisian gneiss 2900–1100 million years old

Lewisian gneiss: Most of the Outer Hebrides and the North West Highlands Geopark have a bedrock formed from Lewisian gneiss. These are among the oldest rocks in the world, having been formed up to 3 billion years ago. About 2900–2700 million years ago north-west Scotland, together with parts of Greenland and North America, made up the ancient continent of Laurentia, which was being built up as igneous rock deep in the earth’s crust and then metamorphosed at very high temperatures. These rocks, with irregular light and dark layers, were intruded by later basaltic dykes and granite magma.

Torridonian sandstone 1000–750 million years old

Torridonian sandstone: Many of the most spectacular mountains in north-west Scotland are composed of Torridonian sandstone, a coarse-grained purplish-red sandstone. Sediments were laid down upon the gneiss by broad, shallow rivers, where the water flowed in many small channels separated by sand-bars. These sand grains and pebbles would have come from an eroding mountain range whose roots are now on the other side of the Atlantic. Deposition over 200 million years was followed by uplift and tilting downwards to the west. By 540 million years ago, erosion had formed a near-horizontal surface.