A dramatic coastline with spectacular beaches, a large limestone cavern and rocks that hold the secrets of early life and insights into mountain building processes
This raw and exposed piece of coastline, located in the UNESCO North West Highlands Geopark, unexpectedly provides a fertile triangle of land that supports a community, all thanks to the underlying Durness limestone. The site is also home to some of the oldest rocks in Scotland – Lewisian Gneiss – and provides evidence of what happens at depth in the Earth’s crust when mountains are forming above.
The Lewisian Gneiss has been through several very ancient mountain building events, which have formed a fascinating colourful striped rock, which shows many folds and stages of formation, including intrusion of magmas.
The Durness limestone, which is well exposed around Smoo Cave and Balnakeil beach, formed when the area was a beach in a shallow warm sea region. Bacteria were very active in the shallow seas, forming flat mats and large clumps called stromatolites. These life forms had already been releasing oxygen into the atmosphere for many millions of years, and essentially made it possible for more advanced life forms, such as our own, to eventually evolve. Some of these advanced life forms, such as various forms of shellfish and trilobites, had already started to appear when this limestone was forming and were living in the same sea as the stromatolites. You can see the curved stromatolite structures when you walk along the beach at Balnakeil, below the golf course.
The cave at Smoo is formed in the same limestone. The mineral that forms limestone dissolves very slowly in slightly acidic water, creating vast cave structures. Indeed, some of the best caving in Scotland is to be had in this limestone further south. At Smoo, past movements have created cracks or ‘faults’ in the rock, allowing water to seep through the rock and dissolve it, eventually forming this huge cavern. The layered limestone can be seen in the walls of the cave and the faults in the roof. Look out also for the swallow hole where the river disappears into the cave. The entrance to the cave is easily accessible via some steps, and boat tours into the cave are available.
The shape of the coastline around Durness reflects a set of ancient fault lines, where blocks of rock have moved vertically, so that Durness limestone is adjacent to older Cambrian quartzite, which in turn is adjacent to the much older Lewisian Gneiss. These faults are associated with the rifting of northern Scotland less than 300 million years ago, and the formation of the sedimentary basins that lie off the north coast, west of Orkney.
At Sango Mor and Fairaid Head, there are rocks that were once part of a huge low-angled fault in the roots of a mountain chain – the Moine Thrust. These rocks were crushed and then ‘streaked out’ with the huge amounts of sliding that took place when the Moine metamorphic rocks slid westwards over younger rocks, moving well over 100 kilometres in total. The easiest place to see the rocks close to the road is on the beach at Sango Bay.
Text contributed by Laura Hamlet & Pete Harrison, North West Highlands UNESCO Global Geopark
Find out more
Pop into the Rock Stop at Unapool to purchase a set of six ‘Pebble Routes’ – the Durness guided drive can give you an overview of the history and geological story of this section of coast.
100 Great Geosites: https://www.geolsoc.org.uk/GeositesSangoSmoo
This web page is published by the Scottish Geodiversity Forum under a Creative Commons ‘Attribution Non-commercial’ (CC BY-NC) licence, which permits non-commercial reuse provided the original work is properly cited.