Shetland Geology
We are very lucky to have been given permission to use this article written by local geologist and tour guide, Allen Fraser. Allen organises tours of a geological theme from his base on the Mainland of Shetland. His website ShetlandGeology.com contains a wealth of information as well as the ability to book onto one of his tours
A Journey through Shetland's Geology
by Allen Fraser of Shetland Geotours
Exploring Shetland can be a journey back into the mists of time to reveal vanished landscapes, if you know what to look for. These islands have many fascinating geological localities that turn back the pages of Earth History through almost 3 billion years. All across Shetland the rocks and landscapes tell amazing stories of oceans opening and closing, of mountain building and erosion, of ice ages and tropical seas, of volcanoes, deserts and ancient rivers, of land use, climate change and sea level rise, and of minerals and miners.
Lerwick
If you take a stroll around Lerwick's shores notice how 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 river bed leaving trains of far travelled cobbles and pebbles embedded in the sandy layers. Taking this walk some 360 million years ago would have meant a wade across fast flowing rivers fed by run-off from high mountains to the west that carried sediments east to be deposited in lakes.
The scree that once mantled those mountain slopes now form low hills around Brindister while the sediments laid down in these 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 and the fossil remains of fish that swum in the lakes appear among the classic rock formations at Exnaboe. The rivers that fed the lakes in this area meandered between fields of sand dunes that you can now see in the cliffs of South-east Mainland.
Prehistoric Mountains
If you wind 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 thrown up some 400 million years ago and would have equalled today's Himalayas in height and grandeur. Aeons later 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 that 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 an ancient ocean known as Iapetus that existed some 600 million years ago.
Lapetus Ocean
The ancient Iapetus Ocean lay within the tropics and 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 18th and 19th century required the use of building lime and ruined examples of kilns can be found at Fladdabister.
When the ancient ocean of Iapetus began to form it may have been rather like the Red Sea of today with volcanic magma erupting beneath the water to form the sea floor. Some of these rocks were rich in the mineral olivine and reacted with hot seawater to become serpentine and eventually soapstone. A quarry track gives a wonderful section cut through serpentinite and soapstone at Catpund near Cunningsburgh. In the past soapstone was a valuable resource and at the Burn of Catpund 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 burn.
Unst and Fetlar
The rocks that lay deep beneath this ancient ocean now form much of Unst and Fetlar. On Unst a walk over rocks of this ancient oceanic crust will take you downwards until you reach what was once the Earth’s mantle. Here too you can see how minerals formed in these ancient rocks and where and how some of these were mined and processed. The Keen of Hamar is almost unchanged since the Ice Age and is where rare plants grow on the serpentine grassland formed on the thin soil cover afforded by these rocks. At Funzie on Fetlar you can see in three-dimensional detail how massive boulders were squashed and stretched by enormous tectonic forces as they pushed the bed of the ancient sea up and over continental rocks.
Earth moving tectonic forces is also in evidence in many other parts of Shetland. At Ollaberry we can follow and step across an ancient geological fault of a San Andreas type that was active hundreds of millions of years ago as ancient continents collided and slid past each other. A trek across Fethaland shows how great rock slices of vastly different ages and types were torn up and thrust north-westward by tectonic forces to lie next to each other.
Magma Flow
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 formed from these magmas, now exposed after millions of years of erosion, and get their dramatic red colour from the abundance of the mineral potassium feldspar within the rocks.
A walk west from North Roe to Uyea is a journey across rocks hundreds of millions then billions of years old. Stop at the place where Neolithic man made his tools and see around you how ice formed the landscape 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.
Papa Stour
At Melby and Huxter and on Papa Stour you can stand on lavas spewed out by ancient volcanoes onto the sands and rivers at the margin of the ancient Lake Orcadie, then look out across St. Magnus Bay and speculate if it really started life as a meteorite crater. Take a walk around Papa Stour and marvel at its geos, stacks and caves then turn inland and ponder on how the hand of man has changed the landscape.
The Wild Cliffs of Eshaness
Eshaness lighthouse is surrounded by rocks that once were blasted high into the air as a cone grew on the side of a massive volcano. This is also the start of a fabulous coastal walk above dramatic cliffs 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 that swept down the volcanic slope. 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.
Take a whistle stop tour of geological sites from west to east across central Mainland and visit a quarry in unusual granite type at Bixter. This granite takes different forms as it shows itself 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. Marvel at how these forces have caused fist sized crystals to grow in narrow zone of rock that can be traced over a distance of 80 kilometres.
Copper Ore
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 19th century. The inhabitants of Jarlshof and Scatness may have exploited the minerals of this area in earlier times. Mineral deposits like these eventually became buried deep in the rocks beneath 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.
Rich Geology of Shetland
Shetland is a dynamic landscape that has been sculpted from this diverse geology by rivers, glaciers and the sea over the last few million years. The major landforms from before the Ice Age have been masked but not destroyed by glacial erosion. Our 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 our 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 information on Shetland's natural wonders visit the Shetland Geotours website at http://www.shetlandgeology.com
