23rd November 2017

Geology: Rock in the isles

What’s so special about Shetland’s geology? Marsali Taylor talks to Robina Barton about Shetland’s status as a Geopark, and tries “Shetland’s Volcano” walk for herself.

Andesite, rhyolite, basalt, felsite, gabbro and granite, pyroxene hypersthenite, igneous, sedimentary, metamorphic. I’m sure it’s the names that make geology sound impossibly difficult. It was one of the topics I took a rain-check on when training as a tour guide. I managed to take in the ideas that even the plainest Shetland quarry is a geologist’s heaven, and that we have some of the oldest rocks in the world, which started life as part of the American continent, floating down near the South Pole. I promised myself, though, that I’d go back to it, someday.

That day arrived with the promotion of Shetland as a Geopark. I began with an encouragingly slim SNH pamphlet, Orkney and Shetland: a landscape fashioned by geology. Just the maps at the front gave me a better idea of why Shetland’s geology is so special.

The map of Orkney is a sedate thing, mostly orange, with a few brown areas. The Shetland map, on the other hand, is an explosion in a paint factory. Bright green up the central spine, on the east of Yell and the west of Unst, with red on Fitful Head, Skeld and up the west of Northmavine; navy along the cliffs of Vementry, West Burrafirth and Foula; purple for Eshaness and Papa Stour. Orkney’s brown is repeated along the south east mainland and on Bressay; Yell is mostly pale yellow, with pink bits on the south. All the east of Unst and Fetlar is a rather lovely blue. Fair Isle’s the same paler orange as the central west side. Only Foula has the Orcadian orange.

Interesting stuff, but just how special is Shetland, in a world geology context? I asked Robina Barton of Shetland Amenity Trust, which, supported by geologists Allen Fraser and Jonathan Swale and a number of local agencies, is behind the organisation of Shetland’s Geopark bid.

“Shetland really is very unusual – Scottish geology is amazing by global standards, and here in Shetland we can boast all the main rock types found in the Scottish Highlands, except for the ancient sediments, which run down from Cape Wrath to Ullapool. It’s not just that we have dramatic scenery, like Eshaness and St Ninian’s Isle; the rocks we have tell the story of global events over hundreds of millions of years.”

Forgive me, geologists, or go on to the next article; this is going to be geology for real beginners (me). Once upon a time, there was an ocean, with three big islands floating on it: the future America (Laurentia), the future Europe and Africa (Gondwanaland) and the more clearly named Baltica. Interestingly, the future England was in Gondwanaland, while Scotland-to-be was in Laurentia; maybe that explains something.

The Lewisian gneisses found in South Yell and North Roe were formed on Laurentia between 3000 and 1500 million years ago. They were originally granite, lava and sediments but have been heated and stressed over time, which has altered them to gneiss. Only slightly younger, and also from Laurentia, were the Grenvillian rocks along the coast from Vementry to Sandness and in the east of Foula. The rest of Yell is layered mica schist and quartzite that was originally sandy sediments laid down on top of the ancient gneisses. Most of the Shetland mainland though, the map’s green bit, is Laurentia’s shores, sediments laid down at the edge of the continent only 600 to 700 million years ago.
Between 600 million and 400 million years ago, the three continents collided. Crash. The edges rumpled, squished, and created the Caledonian mountain belt: the mountains of eastern North America, Greenland, Ireland, Britain and Scandinavia, and the rocky hills of Shetland, only much bigger, mountains the size of Everest. A section of the crust from beneath the deep ocean, made of strange serpentine rock, was trapped in this continental collision and can now be seen in the ophiolite landscape of eastern Unst and Fetlar. The collision also melted rocks deep in the crust to produce the granite masses of Muckle Roe and Ronas Hill and create the volcanoes of Eshaness and Papa Stour.
Movement on the Walls Boundary Fault, which runs from Sand Voe up through Aith Voe (remember that earthquake recently?) and out at Fethaland, shuffled these rocks around, sliding the west of Shetland over a hundred kilometres past the eastern side. Go down that fault line and you’ll reach Scotland’s Great Glen, part of that same fault.

At that point, what would become Britain was still at the centre of one huge continent just south of the equator. It was a mountainous, desert area with a large freshwater lake which swelled or shrank in response to rainfall changes. The lake teemed with primitive fish and was bordered by layers of mud, gravel and sand and volcanic ash from periodic eruptions. These layers covered much of Orkney and the south-east of Shetland: hardened, they became the sandstone cliffs of Sumburgh, the Knab, Bressay and Noss.

Between 205 and 65 million years ago (Jurassic to Cretaceous eras, dinosaur time), stresses created what was to be the North Sea, the start of the continent breaking up, and then the Atlantic, separating North America and Europe from Africa, and, later, North America from Europe as volcanic eruptions created the Mid-Atlantic Ridge. By two million years ago, here we were, all ready for a number of ice ages, interrupted by years of weathering by wind, rain and sea, which, between them, wore the mountains away and shaped the Shetland we know today.

So that’s the basic geology. But what exactly is a Geopark?

“My background is in archaeology,” Robina said, “so I think of it as being like a World Heritage Site, but for geology. To become part of the European Geopark Network you must have significant geology, but it’s also about protecting and conserving that geology, and using it to bring economic benefit to the area. That can be through increased tourism, by raising the profile of Shetland internationally and by raising awareness among the Shetland community about what is on their doorstep. Shetland also has great potential for research, and for field trips for geology students. Because we have such diversity in a small area, they can see an awful lot in a short time.”

Having fun discovering rocks isn’t just for adults.

“An exciting project that’s in the pipeline is a geology unit we’re helping to develop with Baltasound Junior High School, Brae High School and the Anderson High. We’re devising a programme aimed at S1 and S2 pupils which we hope will cover tectonics and volcanics and include both classroom based learning and a field trip to Eshaness. We’ll be tailoring the ‘Shetland’s Volcano’ trail for schoolchildren, and once the material has been developed it could be made available to other schools within and beyond Shetland.

“We also want to do another trail, like the ‘Shetland’s Volcano’, but in the North Isles, telling the story of the Shetland ophiolite and bringing in sites like Hagdale, the Keen of Hamar and Funzie. Volcanoes are relatively easy to explain and visualise, but bringing the ophiolite to life might be a bit more of a challenge!”

There are seven Geoparks in the UK already, and 36 worldwide. Robina has enjoyed making closer links with the others. “So far we have developed contacts at Lochaber and NW Highlands Geoparks (Scotland), Forest Fawr Geopark (Wales), Marble Arch Caves Geopark (Ireland), North Pennines AONB Geopark (England), Gea Norvegica (Norway) and also with aspiring Geoparks from both Norway and Sweden. This is very useful in terms of exchanging ideas and potential joint projects. I particularly enjoyed visiting Magma, near Stavanger, as their application to the European Geopark Network (EGN) is at the same point as ours, and they too, as the name suggests, have major volcanic elements.

“The EGN is part of the Global Geopark Network endorsed by UNESCO, and the Geoparks all promote each other, so it’s getting Shetland known in new places. Eco-holidays are becoming more popular, too, so it’s a good time for us to promote what we have to offer. The EGN assessors visited Shetland in May, and they seemed very happy. We’ll find out if we’ve been admitted to the European Geopark Network at the Geopark conference next month, at Naturtejo Geopark in Portugal.”

The “Shetland Rocks!” leaflet suggests several walks that will let you discover Shetland’s geology for yourself, and the Geopark Shetland website, www.geoparkshetland.org.uk gives details, with pictures, of ”Shetland’s Volcano”, which starts at “The Wall”, at Mavis Grind. There’s even a virtual tour of The Wall, with details on the different rocks, but that was too much information all at once for me. I headed out to see how much a geology ignoramus could find out.
When I got to The Wall, at Mavis Grind, it all began to make sense. The Wall is a “slice” through Northmavine. In the centre, the red granite heart that is Ronas Hill, sliced on one side by the Melby Fault, and with a dark centre of mixed gabbro and granite; the very middle of the volcano. Ronas Hill didn’t rise up; it was the volcano’s lower furnace, solidified to fire-red granite, with all the rocks above it weathered away. To the left, banded sandstone of that lost desert lake, with black, pitted lava rocks layered above it; to the right, the underlying grey rocks of the collision, gneiss and mica schist, a slice of green serpentine and soapstone.

Even the three types of rocks made sense now. The lava flow rocks, the granite and whorled felsite are igneous, as in ignited. The ones below are sedimentary, from the sediment of ancient lakes. The, gneiss and schist are metamorphic; they’ve been altered by heat and stress and had their shapes changed or metamorphosed.

You could feel the difference even in touching them: the bubbled, solidified lava, the sandy texture of the desert, the sandpaper harshness of the granite, flecked with molten black. One rock was banded with layers of glassy mica; another had melted and cooled in a smooth swirl, like marble cake.

On to Braewick beach. Here the interpretative board is a sculpture, two “drong” shaped pillars with clear layers of black lava and rust-red sand on a plinth of “beach”: boulders above, banded sandstone below. I clambered over the stiles and headed down past a field of quilted, lavender-blue scabius to the beach. It was an extraordinary colour, coral pink; as I got closer I saw that it was built up of rounded boulders, all different sizes, so steeply that I couldn’t see the waterline. Terns chittered at me as I came over the rise; the sweep of wet sand was the colour of damp peat ash.

The stones on the beach were amazing. I picked up one of green granite, marbled white – another with a thin white line through it, and splatches of white, exactly as if someone had been painting over it; another, half-buried in sand, was jade-green with a sprigged bird-foot pattern like a Chinese lino-print. Metamorphic rocks, two different colours mixed together, then hardened into these patterns.

At the western end, the sea has hollowed a cave into the sandy banks, and filled it up with coloured boulders, worn smooth by the sea. Past it, the rock is black, pitted with little bubbles from gas; this was the first lava from the Eshaness volcano, and it’s easy to find pebbles of pitted black lava on the beach. Beside it, sandstone that’s rust red from that desert environment, and rock formed at the bottom of the ancient lake, layered grey and red. Where the cliffs seem a jumble of boulders marks a mudflow.

Stennes Beach was the next stop. What I was looking for here were vesicles – bubbles formed by gas or steam in the lava, which, once the lava cooled, left cavities. Water filtering through to these cavities left minerals which became agates. Above the beach was another sculpture, a volcano sliced in half, and painted vivid flame colours. The board beside it had a wonderful old photo of the Stennes Haaf Station in its heyday, as well as a picture of a banded agate, and the information that amethysts were purple quartz, and also sometimes seen here.

I spent the next half-hour happily searching for agates (and amethysts). There was plenty of milk quartz, gleaming white, smooth and round as the bubble cavities it had formed in. Agate was harder to spot. I found a possible bit, then a definite bit, striped green and white; another with an orange sworl, still encased in a lava pebble. At the far end of the beach was a huge lump of banded rock, just covered with vesicles.

Finally, to Eshaness, which a leading geologist, Dr W Mykura, called “The best section through the flank of a volcano in the British Isles.” The cave of Kirn o Slettans was a side cone, built up by lumps of hot rock blasted outwards. The cone-shaped stack of Muckle Ossa was probably the lava feed channel, exposed by erosion; the knobbled side of Calders Geo shows the lava pillows. A little further, at Drid Geo and Breigeo, were individual lava flows, stacked one on top of the other. The sea has undercut the domed flows at Breigeo, collapsing the cliff face.

Further along still is the reminder of the plume of hot gas and blobs of lava you see in every picture of a volcano eruption: the red rock of the Grind o da Navir. When the cloud settled into a thick layer on the side of the volcano, the lava and pumice it carried welded together in twisted flame shapes – a close look even shows the individual drops.
Even with the little bit of knowledge I’d acquired, every stone now told a story. I kept picking up pebbles and seeing molten rocks being mixed deep below the Earth’s crust, or rivers depositing layers of sand in that desert continent, or fiery ash being thrown high in the air. The whole landscape had acquired a history.

Later, in the cafe, I met friends and explained what I’d been up to. It turned out that one of them had been on the South Mainland geology trip, finding fossils in a sandstone cliff near Grutness. He’d been so inspired that he’d taken his family back for a picnic and fossil hunt.

“It was amazing,” my friend said. “At first I couldn’t see any, then I got my eye in and found loads, blue-purple, and you could feel the scales of the fish with a finger.”

I told Robina how enthusiastic we’d got.

“That’s exactly what we want,” she said. “Finding fossils is great fun, although it’s important not to try to remove them and take them away. We would ask people to contact us at the Shetland Amenity Trust to let us know of any findings.

“Geology is full of surprises and it’s surprisingly easy to get folk interested. Once you talk to people they begin to look for themselves and see how the land around them tells the fascinating story of Shetland’s turbulent past. Geology created Orkney’s rich soil and made farmers of the Orcadians, while the poorer soils of Shetland caused Shetlanders to take to the sea for food. It’s fundamental to everything.”

Marsali Taylor