Assynt Scotland

Ardvreck Castle on Loch Assynt, Assynt, Scotland Photographic Poster Print by Grant Dixon, 24x18

Assynt Scotland

Hills of Assynt Near Lochinver, Lochinver, Scotland. Hills of Assynt Near Lochinver, Lochinver, Scotland Photographic Poster Print by Grant Dixon, 24x18.

Like sphinxes, the Assynt peaks gaze eastwards towards rocks that for decades have posed a riddle for geologists.
When the strata of Ben More Assynt, Quinag, Canisp and Suilven were being formed, the Atlantic did not exist and the northwest of Scotland was part of the North American continent. Then, around 1000 to 800 million years ago, rivers, flowing from the west, spread
thick accumulations of sand over an ancient land surface. This formation is known as the Torridonian sandstone because it is prevalent in Torridon. The covered land surface was carved from Lewisian gneiss, which here dates from some 2800 million years ago.

The Torridonian rocks were themselves buried by sandstones, shales and limestones deposited from a sea that covered the area, 570 to 550 million years ago, in the Cambrian and Ordovician periods of geological history. Today the Assynt peaks remain as relict mountains of Torridonian sandstone, many capped by the marine deposits of the Cambro-Ordovician period, now isolated by erosion and rising from the re-exposed undulating platform of ancient Lewisian rocks.

The uppermost strata of the Assynt peaks pass eastwards under a series of schistose rocks, the Moine Series, which form much of the Northern Highlands.  Since the 1860s, however, the nature of the junction between the Cambro-Ordovician and Moine rocks has been in dispute, and it was not clear whether the Moine schists were younger or older than the Cambro- Ordovician rocks.

In order to prove the relationship of the Moine schists with the Lewisian, Torridonian and Cambro-Ordovician rocks to the west, Archibald Geikie, head of the Geological Survey in Scotland, sent two of his most able field geologists, Ben Peach and John Home, to the northwest. This led to one of the most exciting discoveries in British geology. Their accomplishment is commemorated by a monument near the Inchnadamph Hotel. In their 1907 monograph, Peach and Home recorded the results of years of field work, which, because of the nature of the terrain and weather, had sorely taxed their physical stamina. They proved that the disputed junction is a low-angled break along which the Moine schists have been thrust west-northwestwards, overriding the Cambro-Ordovician and, elsewhere, over the Torridonian and Lewisian also. This break they termed the Moine Thrust, which is well displayed in the National Nature Reserve at Knockan Cliff on the A835 south of Elphin. They found the Moine Thrust to be the most easterly of a series of such thrusts along which huge slices of country slid one over the other, resulting in a lateral movement of tens of miles.

The magnificence of their conclusion is nowhere more impressive on the ground for there you can see mountains that have moved for miles. At the thrusts the rocks have been milled and melted, while subsidiary faults at higher angles have been developed in the rocks below the major thrusts. This belt of thrusting can be traced from Loch Eriboll in the north as far south as the Isle of Skye. These events took place around 430 million years ago. The pressures that were then folding the rocks of the Caledonian mountains to the southeast forced them against the stable North American continental block to the northwest, causing the tremendous overthrusts described by Peach and Home in their monograph.