Geology of Durness
Geoolgy form the The Sutherland
Biodiversity Action Plan
The area has formations that span over 2,800 million years and include some of the oldest surface rocks in Britain. This has shaped not only the landscape, which is characterised by vast hills and mountains rising out of flatter peatland, rocks and lochs, but also Sutherland’s distinctive flora and fauna. Much of West Sutherland is underlain by the oldest of these rock types, Lewisian gneiss, which has eroded to form a characteristic mosaic of rocks, low hills, small lochs and peaty areas known as ‘knockand-
lochan’. The quartzite-capped giants of Foinaven, Arkle, Suilven, Canisp and Quinag rise up as huge sandstone masses of out of this ancient gneiss floor.
West Sutherland also has areas of limestone, which weathers to produce unusually fertile soils with their own very specific flora. A narrow outcrop of the Durness limestone runs from the North Coast south through the county, forming the steep crags at Stronchrubie and Knockan. At Inchnadamph you can see limestone pavements, underground water
systems and caves.
Much of North and Central Sutherland is composed of schists and granulites. The softer schists have weathered to produce vast plains, which over the centuries have been covered in blanket bog. Rising out of this flat landscape are the more resistant rock masses of Ben Hope and Ben Klibreck. The Moine Thrust, running north-south, marks the western edge of this area, providing evidence of immense disturbance of the earth’s crust some 400 million years ago.
In East Sutherland there are outcrops of younger, sedimentary rocks associated with Old Red Sandstone laid down when the area formed part of a vast freshwater lake known as Lake Orcadie, during the Devonian period. The “Fallen Stack of Portgower” is, in fact, evidence of a submarine landslide triggered by seismic activity on the Helmsdale Fault. This fault separates the more resistant metamorphic and igneous rocks, which form high ground, from the more fertile coastal plain. In the Brora area there are Jurassic rocks, including a coal seam which was once the basis for
Brora’s industrial growth.
During the last glaciation, Sutherland was covered in ice and stripped of its vegetation and topsoil. Evidence of this process can be seen in the many corries, which have been gouged out of the hills in the North West, and the glacial till or boulder clay, which was deposited in the valleys. Since then, much of the county has been covered in blanket peat, which influences much of the plant life inland. On the Eastern side of the county, there are many meltwater channels and eskers (valley ridges laid down by rivers underneath the ice).
GEOLOGY
from Durness Past & Present
The parish can be divided into well determined geological regions. Distribution is defined more by faulting than any other single factor. Faults have brought the limestone against the Lower Cambrian and Pre-Cambrian so that there are contrasting adjacent rocks types. Conclusive proof can be found in the area that north west Scotland was divided from the rest of Britain and most of western Europe and lay on the opposite side of the Lower Palaeozoic Proto-Atlantic Ocean. One of Britain's outstanding earth science sites, it is of essential importance in studies of transatlantic correlation, palaeogeography and faunas.
Figure 3.
The oldest rocks in the area are of the oldest in the world. Three major divisions of the pre - Cambrian system are found. The oldest is Lewisian comprising of rock between one thousand four hundred million years and two thousand six hundred million years old. The other two divisions Moine series and Torridonian, eight hundred to nine hundred million years old, distinguishable by their appearance. The youngest solid rocks are of Cambrian and lower Ordovician, approximately four to six hundred million years. An excellent section through some of the Cambro-Ordovician Rocks can be seen along the south shore of Balnakeil Bay. Limestones and Dolomites are present here and some of the former contains fossils and a large number of Chert Nodules, sub spherical masses of rock with a chemical composition similar to quartz. The compacting of sediment deposited in a low energy marine environment that is on a part of the seabed where there was little or no wave or current action formed these rocks. They are fine-grained (tiny particles) and muddy rocks. At Faraid Head, the rocks are associated with gigantic earth movements, which took place approximately three hundred and seventy five million years ago during the latter stages of the Caledonian Orogeny (mountain building movement). The rocks forming Faraid Head are metamorphosed but they had a different origin from the Lewisian Gneiss and were altered at a much later date. In Sango Bay, there is another small belt of rocks, which have been thrust over from the east and faulted down. The actual fault plane is the steep cliff that forms the eastern end of the bay.
The rocks at the western end of Sangobeg Sands are of the Lower Cambrian age and are known as pipe rocks because of the pipe like markings in the Quartzite (metamorphosed sandstone) of which they are composed. These markings are not true fossils but are called 'trace fossils' because they are impressions left in the original sediment by organisms. Worm like creatures with vertical burrows formed these 'pipes'. The rocks at the eastern end of the sands are Lewisian Gneiss with an age of about fourteen to eighteen hundred million years. Veins substantially thick of biotite and or hornblende can be found as garnets, amethyst, and iron ore minerals. Two or three times a year geology students are brought to Durness for two or three weeks from national and international Universities to study local rocks.
At approximately grid ref. 420650 there is a huge mass of almost pure Feldspar. Three or four vertical Pegmatite veins (very course grained crystalline rock) each approximately three hundred metres long and one hundred metres wide occur in the Lewisian Gneiss. There is a minimum of twelve million tones of pegmatite in about half a square kilometre of ground and the feldspar comprises about seventy percent of this. Feldspar is used in the glass and ceramics industries. Dolomite is utilised for magnesium production.