Allen Fraser (UK)
In September 2009, the Shetland Islands were awarded the accolade of becoming the thirty-fifth European Geopark. This is fantastic news for the isles. It acknowledges the importance of Shetland’s incredible geology and creates opportunities to promote it to an international market and develop partnerships with other members.
When visiting, the best place to start your journey into Shetland’s ancient past is at Shetland Museum, in Lerwick. Here, displays take you back into the mists of time, revealing vanished landscapes and the amazing events behind them. All across Shetland, the rocks and landscapes tell an endless story – of oceans opening and closing, of mountain building and erosion, of ice ages and tropical seas, volcanoes, deserts and ancient rivers, of land use, climate change and sea level rise, and of minerals and miners.
Around 360mya, a walk through where Lerwick is now, would have meant a wade across fast-flowing rivers, in a climate like that in Death Valley, California. How do we know? Well, if you take a stroll around Lerwick, and walk from the Knabb to the Sletts and out to the Sands of Sound, you can see for yourself. Here, flat-lying beds of thick, buff-coloured sandstone begin to acquire rounded pebbles and cobbles of pink and white quartz.
These sandstone beds tell us that fast flowing rivers once deposited their loads in the area and that flash floods occasionally scoured the riverbed, leaving trains of far-travelled cobbles and pebbles embedded in the sandy layers. These rivers were fed by run-off from high mountains to the west, which carried sediments east to be deposited in lakes.
Looking at the Shetland landscape today, you might be confused by this talk of mountains. However, if you make your way either north or south from Lerwick, you cannot help but notice the ridges of hills that make up the ‘spine’ of Shetland. These hills are the eroded remnants of the ancient Caledonian Mountain chain that was thrown up some 400mya. This chain would have matched today’s Himalayas in height and grandeur.
The scree, which once mantled those mountain slopes, now forms low hills around Brindister, while the sediments that were laid down in the lakes are found at various places along the east coast from Bressay to Sumburgh. The great thicknesses and variety of these sediments are seen to best effect in the dramatic sea cliffs of Bressay and Noss. Plant leaves and other debris swept out into the lakes can be found as fossils on Bressay, while the fossil remains of fish that swam in the lakes appear among the classic rock formations at Exnaboe. The rivers that fed the lakes in this area were not so fast flowing as those near Lerwick and meandered between fields of sand dunes, which show up in the cliffs of southeast Mainland.
The Caledonian Mountain Chain was forced into being when an ancient ocean, called Iapetus, closed as three continents collided. Aeons after this collision, a mighty river cut through the mountains and a small fraction of that river’s course is seen as the steep-sided valley at Quarff. The grey and brown metamorphic rocks, which form these hills, can be seen in the sheer face of the roadstone quarry above Scalloway. Originally, these rocks were sands and muds laid down in the deeper part of the Iapetus Ocean, which existed some 600mya.
The Iapetus Ocean lay within the tropics – in fact, when the Iapetus closed, Shetland was placed in a vast desert continent, somewhere near the Equator. Mud, rich in calcium carbonate, was deposited in its shallower parts to become limestone. The heat and pressure of mountain building transformed the limestone into a calcite marble that now forms the fertile floors of the crofting valleys of Tingwall, Whiteness and Weisdale. As well as providing the best of Shetland’s agricultural soil, these calcium rich rocks were also quarried and burned with peat in kilns to make lime for the building trade. The increased demand for grander buildings in the eighteenth and nineteenth centuries required the use of building lime and ruined examples of kilns can be found at Fladdabister.
Going further back in time, when the ancient ocean of Iapetus began to form, it may have been rather like the modern Red Sea. The continental crust of the earth split and drifted apart, with water filling the space and volcanic magma erupting beneath the water to form the seafloor (or oceanic crust). Some of these rocks were rich in the mineral olivine and reacted with hot seawater to become serpentine and, eventually, soapstone (or steatite). A quarry track gives a wonderful section cut through serpentinite and steatite at Catpund, near Cunningsburgh.
At the Burn of Catpund, the easily worked steatite was extensively quarried in Viking times to produce various artefacts, both for local use and export. Chisel marks and hollows, where bowls had been fashioned and extracted, can be seen on many outcrops in the burn. The Vikings called the soapstone ‘klebber’, meaning ‘loomweight stone’, because it was so frequently used to make these vital everyday objects. The word is still used in Shetland today. These rocks, which once lay deep beneath the floor of the ancient Iapetus Ocean, now form the eastern half of the islands of Unst and Fetlar. Geopark Shetland’s new ophiolite trail on Unst and Fetlar will take you on a walk across what was once oceanic crust from just beneath an ancient ocean floor, progressing downwards until you reach what was once the Earth’s mantle.
The Geopark Shetland displays at Hagdale also demonstrate how minerals formed in these ancient rocks, and where and how some of these were mined and processed. At the recently restored Hagdale Mill, you can see where chromite ore was extracted. It is the only restored crushing circle in the UK. On the neighbouring Keen of Hamar, rare plants can be found growing on the serpentine debris. At Funzie on Fetlar, you can see in three-dimensional detail how boulders on an ancient beach were squashed and stretched by enormous tectonic forces, as they pushed the bed of the ancient sea up and over the continental rocks.
Earth moving tectonic forces are also in evidence in many other parts of Shetland. A trek across Fethaland shows how great rock slices of vastly different ages and types have been torn up and thrust north-westward, by tectonic forces, to lie next to each other. At Ollaberry, you can follow, and step across, an ancient geological fault, similar to the San Andreas Fault. This was active hundreds of millions of years ago, when ancient continents collided and slid past each other.
From Mavis Grind to North Roe, you can see how huge masses of magma squeezed, forced and eventually punched their way up through the crust beneath an ancient continent. Ronas Hill and the cliffs of Muckle Roe were formed from these magmas, now exposed after millions of years of erosion. They get their dramatic red colour from the abundance of the mineral, potassium feldspar, within the rocks.
From North Roe, you can walk back across rocks that are hundreds of millions, then billions of years old, and see how the present landscape was formed by ice. You can stop at the ‘axe factory’, where Neolithic man made his tools, and then travel on to find the remnants of trees that once grew by a lake some 120,000 years ago, before the last Ice Age.
When Shetland lay within the desert continent, lavas from ancient volcanoes spewed out onto sands and rivers at the margin of the long vanished Lake Orcadie. These can be seen at Melby and Huxter, and on Papa Stour. If you look out across St Magnus Bay, you can speculate if it really started life as a meteorite impact crater. Alternatively, take a walk around Papa Stour and marvel at its geos, stacks and caves, then turn inland and ponder on how man has changed the landscape.
Geopark Shetland has produced a volcano trail leaflet that points you towards sites of volcanic interest and geological exhibits at Braewick and Stenness. Near Eshaness Lighthouse, you can stand in a volcanic cone surrounded by rocks that were blasted high into the air, as this cone grew on the side of a massive volcano, about 360mya. Then, you can follow one of the best coastal walks anywhere, during which you will cross progressively older lava flows that reveal, in graphic detail, the best exposure of the anatomy of a volcano in Britain.
This walk will take you to the Grind o da Navir, where the rock started life as massive, red-hot pyroclastic flows, which swept down the volcanic slope. There, you will see how the forces of nature still operate here in a big way today, where a spectacular amphitheatre is being hewn out of the rock by gigantic storm waves that carry huge blocks of rock far inland to form beach ridges many metres high. (For more on the volcano trail, see my article in Issue 11: On the trail of Shetland’s volcano.)
You can then take a whistle stop tour of geological sites from west to east, across the central mainland and visit a quarry of unusual granite type, at Bixter. This granite takes different forms, as it is exposed further south, at Hamnavoe and Spiggie. On Hildasay, it was quarried for building stone and may have found its way to Australia, as ballast on wool clippers. Heading east, you cross a boundary zone between rocks that began life on the floors of two different oceans at different times, now welded together by tectonic forces. It seems extraordinary to us how these forces caused fist-sized crystals to grow in a narrow zone of rock that can be traced over a distance of 80km.
A visit to Garths Ness, beneath the shadow of Fitful Head, will show you where hot springs concentrated minerals on the bed of an ancient ocean and where attempts were made to mine copper ore in the nineteenth century. Former inhabitants of Old Scatness and Jarlshof archaeological sites may have exploited the minerals of this area in earlier times. Mineral deposits like these eventually became buried deep in the rocks, between Bressay and Sandwick, only to be dissolved once more and carried upwards to form the veins of copper and iron that were mined at Sand Lodge.
Shetland is a dynamic landscape. It has been sculpted from this diverse geology by rivers, glaciers and the sea, over the last two million years. The major landforms from before the Ice Age have been masked, but not destroyed, by glacial erosion. Its coastline is stunning in its variety and character, with an outer coast of the most spectacular cliff scenery in the world, contrasting with an inner coast of tranquil voes and beaches. The richness of its geology and geomorphology is the foundation for the many layers of natural habitat and human history that make a visit to Shetland so memorable.
For more on the geology of Shetland, see my article in Issue 13 of Deposits: Shetland: an archipelago on the edge.
About the author
Allen Fraser is part of Shetland Geotours UK.