Northwest sea-board geological foundations

The geological foundations of the Northwest sea-board of Scotland, including the Western Isles, Coll, Tiree, Rum, Skye and the Northwest Highlands from Cape Wrath to Kyle of Lochalsh, are built from rocks of three very different ages. These foundations are separated from the neighbouring Northern Highlands by a fault known as the Moine Thrust.

Lewisian rocks

The oldest rocks of the Northwest Highlands are up to 3 billion (3,000 million) years old and are known as Lewisian rocks. They are the oldest rocks in Scotland and some of the oldest in the world. The Lewisian rocks formed over a huge span of time (up to 2 billion years), through burial, compression, folding and heating of large amounts of cooled molten rock, and much smaller volumes of muddy, sandy and rare carbonate (e.g. limestone) sediments.

Torridonian rocks

By around 1,000 million (1 billion) years ago, the Lewisian rocks were already ancient and had been eroded down into a hilly landscape. Across this landscape huge rivers flowed, around 1200 to 800 million years ago, depositing layers of red sandstone, muds and pebbly conglomerates. This sequence of river sediments is known as the Torridonian, and once formed a blanket up to 7.5 kilometres thick covering the ancient Lewisian landscape.

Like the Lewisian, the Torridonian rocks were later partly eroded away. Today much of the old Lewisian landscape has been re-exhumed with hills of Torridonian rock, such as Slioch and Suilven, perched on top. Around 550 to 500 million years ago in the late Cambrian and early Ordovician geological time periods, the area was covered by a shallow sea and a sequence of pure white sands (now quartzite), some muddier sediments (now soft, nutrient rich Fucoid beds), and finally a thick sequence of carbonate rocks (now magnesium-rich limestone know as Durness dolomite) were laid down on the eroded surface of Lewisian and Torridonian rocks. These Cambro-Ordovician sediments have now also largely been eroded away. Today they are only found in a thin strip along the east edge of the area, adjacent to the Moine Thrust, and on the tops of hills such as Beinn Eighe and Foinaven.