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    Ancient Foundations

    The Geology of the Outer Hebrides

    Author: Gareth Crapper
    More by Gareth

    Our Ancient Planet

    Until just over 200 years ago, it was widely believed that the Earth was only around 4,000 years old.

    However, the discovery that our planet is extremely ancient came relatively recently with the birth of geology as a scientific discipline.

    This new understanding was built on a series of breakthroughs, many of which happened right here in Scotland. In 1788, at Siccar Point in Southeast Scotland, James Hutton discovered Hutton’s Unconformity—a junction of two very different rock types with a gap of millions of years between them.

    There’s even a song about it by Karine Polwart – give it a listen here.

    The geology of the Outer Hebrides reveals a story billions of years in the making, etched into its rugged and ancient landscapes. From ancient mountains to volcanic cliffs, this extraordinary region offers a glimpse into Earth’s deep time. In this blog, we’ll explore the formation of Lewisian gneiss, the dramatic impact of tectonic shifts, and the unique coastal features shaped over millennia.

    Geologic time scale

    Geologic Time Spiral – International Commission on Stratigraphy. (Jarred C Lloyd, Creative Commons.)

    The Passage of Time

    We now know the Earth to be around 4.5 billion years old, created by the gravitational accretion of large and small rocks into a Hadean world of lava, poisonous atmospheres and meteor bombardment.

    Over extremely long times, the Earth cooled, water condensed to cover the planet in oceans, and from the soup of chemicals, simple life developed. By around 2,500 million years ago this life had, by photosynthesis, established a permanent oxygen-containing atmosphere. The start of this Proterozoic eon is an important benchmark we will return to later, as the ancient rocks formed during this period are visible in the Outer Hebrides. Between then and now these simple organisms evolved into multi-cellular creatures which learned to swim, walk, fly, and ultimately take vacations in Scotland.

    There have been dinosaurs (still with us as birds), mass extinctions, glaciations, sea-level changes and a whole host of other events that all led us to what some now call the Anthropocene epoch: the age of people.

    tectonic Plates diagram - geology of the outer hebrides

    Map of Earth’s principal tectonic plates (M Bitton, Creative Commons.)

    In parallel with these great changes, over unimaginably long timescales, has been plate tectonics shaping the face of the planet. Over the surface of the Earth are seven or eight plates of rock that have been in motion since the Earth formed.

    The Iapetus Ocean and Scotland’s Connection to Canada

    Lewisian gneiss, some of the oldest rocks on Earth, is a cornerstone of the geology of the Outer Hebrides and will be a striking feature of your trip. These rocks, named after the Isle of Lewis, where they were first described, are around 3 billion years old and can easily be seen along the coasts of places like Harris and Lewis. On your trip to the Outer Hebrides, you will become very familiar with this rock and the landscapes it gives rise to. They rank amongst the oldest rocks you can see on the surface of the Earth—a truly ancient landscape.

    The Iapetus Ocean Closing

    Around 550 million years ago, a huge ocean, the Iapetus, was just beginning to close, a process that would take around 200 million years. One of the closure events was the Caledonian Orogeny, when the proto-American and proto-European landmasses collided, closing the Iapetus Ocean and pushing up the mountains of Scotland into something resembling the Himalayas or Alps today.

    Incidentally, you can see from the accompanying figure that the mountains of the east coast of North America, and those of Scandinavia, are related to those of Scotland, being formed in the same event.

    Over the intervening years, these “high alps” of Scotland have been weathered down and eroded, particularly by ice ages, into the hills we see today—giving us a glimpse of how the Himalayas might look in a few million years.

    Positions of Ancient Continents and the Iapetus Ocean, 550 million years ago. (Public Domain.)

     

    geology of the outer hebrides diagram

    Closure of the Iapetus Ocean: location of the Caledonian/Acadian mountain chains in the Early Devonian Epoch. Present-day coastlines are shown for reference. Red lines are sutures, capitalised names are the different continents/super-terranes that joined during the Caledonian Orogeny. (Woudloper, Creative Commons.)

    Canada & Scotland

    Looking further back to the start of the Proterozoic Eon, around 2.5 billion years ago, volcanic processes created rocks familiar to us today, like lava, pumice, and ash.

    These ancient rocks, subjected to extreme heat, pressure, and remelting deep within the Earth, are now known as gneiss (pronounced “nice”).

    The Lewisian gneiss we see today was originally part of what is now Canada.

    However, the Iapetus Ocean’s closure and the proto-Europe collision brought these rocks to Scotland. The remainder of these ancient formations can still be found in modern Greenland and Canada.

     

    Polished Lewisian Gneiss at Kylesku showing layers of ancient re-melted rocks.

    Polished Lewisian Gneiss at Kylesku showing layers of ancient re-melted rocks. Photo credit: Gareth Crapper. 

    A sheep on Lewisian Gneiss, nice!

    A sheep on Lewisian Gneiss, nice! Photo credit: Gareth Crapper. 

    The Geological Time Scale

    Eon

    Era

    Period

    Millions of Years Ago

    What Was Happening in Scotland?

    Phanerozoic0You are here!
    CainozoicQuaternary2A series of ice ages. Covering all but the highest peaks of Harris and South Uist…
    Neogene24Erosion continued in warm temperate conditions.
    Palaeogene65The Atlantic Ocean starts to open…
    MesozoicCretaceous146Warm shallow seas cover much of Scotland.
    Jurassic208The Minch Basin was flooded to form a shallow sea in which animal life flourished.
    Triassic245Seasonal rivers flowed westwards.
    PalaeozoicPermian290Scotland is drifting north from the equator and again hot and dry.
    Carboniferous363Close to the equator, Scotland is covered by tropical forests laying down coal deposits…
    Devonian409As part of a desert continent, the Scottish Alps are eroded…
    Silurian439The Caledonian Orogeny builds the Scottish Alps…
    Ordovician510Ancient continents bearing the precursors to Scotland and England move towards each other…
    Cambrian544
    Precambrian (Cryptozoic)Proterozoic1000-2500Lewisian Gneiss formed and subject to repeated metamorphosis.
    Archean2500-3800
    Pre-Archean (Hadean)4600

    The Geological Time Scale, adapted by author from The Geology of Britain, Peter Toghill and The Outer Hebrides: A Landscape Fashioned by Geology, Kathryn Goodenough and Jon Merritt

    The Outer Hebrides – A Geological Time Capsule

    The Outer Hebrides are built on a basement of the ancient Lewisian Gneiss.

    However, the Shiant Isles and St. Kilda are relatively young, formed from volcanic activity 55 million years ago when the Atlantic Ocean started to open and separate Europe from America. The crust of the Earth thinned with this separation and some major structures developed: Arran, Mull, Skye, and Ardnamurchan. These hard granites and gabbros are erosion-resistant and account for the high cliffs of St. Kilda.

    On a trip to the Outer Hebrides, you can see the oldest and some of the youngest rocks on the planet!

    Visit the Outer Hebrides & St Kilda

    Wildlife THE OUTER HEBRIDES & ST KILDA

    Seabirds circling St Kilda

    The East-West Divide of the Outer Hebrides

    Running up the eastern side of the Outer Hebrides is the Outer Hebrides Fault, a defining feature of the geology of the Outer Hebrides.

    It is a fracture in the Earth’s crust resulting in large amounts of friction and remelting of the Lewisian Gneiss. This has created a network of dark, glassy veins in the rock, making it more erosion-resistant. Hence, a line of low hills follows the fault from Barra to North Uist.

    Arrivals Dictated by the Geology of the Outer Hebrides

    There are two ways to arrive on the islands – by ship or by plane:

    • Arriving by ship, you will come into the east side, which is characterised by extremely rocky terrain and in-cut lochs deep enough for large passenger ferries to negotiate (though with good marker buoys and considerable skill of the pilots).
    • If you arrive by plane, you will land on the west side, either directly on the beach (Barra) or just behind the beach (Benbecula). These are characterised by low-lying flat lands characteristic of the west. (There is another airport at Stornoway on the east, but this again is built by an area of beaches.)

    These transportation methods take advantage of the underlying geology of the islands.

    ferry coming into tarbert - showing the east west divide in the geology of the outer hebrides

    Ferry negotiating sea loch on Harris, coming into Tarbert. Photo credit: Gareth Crapper.

    plane on barra beach - showing the differences of the geology of the outer hebrides

    Plane after having landed on the flat beach on Barra. Photo credit: Gareth Crapper.

    Coffin Roads

    Historically there were many isolated settlements along the east, exploiting the rich seas around the bays. However, the ground is so rocky that there are a number of “coffin roads” across the island to the west coast where there is deeper sandy soil by the beaches.

    Modern Roads

    The roads on South Harris further demonstrate the different sides of the islands. Up the west coast, the road follows beautiful sandy beaches with spectacular views. On the east side, the “Golden Road” had to be constructed. It ducks between hills and weaves a complex course. The name comes from the supposed cost to construct it: it may as well have been made of gold!

    a white sand beach with view of a rainbow and an island

    West: Looking out to Taransay from Harris. Photo credit: Gareth Crapper.

    a beautiful white sand beach on a sunny day with azure blue waters and beautiful purple beach flowers in the foreground

    West: A calm moment at Huisinis. Photo credit: Gareth Crapper. 

    An island road that goes up and down along a rocky and loch landscape

    East: The better side of Harris? Cycling on the Golden Road. Photo credit: Gareth Crapper.

    A cyclist about to go down a hill on an island road on a misty day.

    East: Dropping into Geocrab: a roller coaster of a ride. Photo credit: Gareth Crapper. 

    Language

    The maps are also a great place to look for clues as to the different geology on either side of the island. Maps and place names offer a lovely little Gaelic introduction too. Browse the terms below to learn a few common Gaelic terms you might come across when poring over maps of Scotland.

    West

    • Tràigh – Beach: The beaches run up and down the west side, giving the coastline a straighter line.
    • Caolas – Strait: Often between the islands, such as South Uist, Benbecula and North Uist where the strait is crossed by causeways.
    • Machair  Field of wildflowers: Strictly a “fertile plain” of grassland, but where these occur in Gaelic-speaking regions, they are typically the (often west-facing) lowland coastal fields of spectacular wildflowers.

    East

    • Rubha – Peninsula: A series of peninsulas and points jut out into the Minch, incut by small narrow inlets and many extensive sea lochs, the most spectacular being Loch Seaforth on Lewis.
    • Àird – Point
    • Geodha – Narrow inlet
    • Loch – Body of water
    • Cnoc – Small rounded hill. Gaelic is reminiscent of Inuit; while the absolute number of words the Inuit have for snow is disputed, there are lots of words in Gaelic for hills. Cnoc is just a small, entry-level hill. (Check out a few here).
    • Siader – A pasture: Actually derived from Nordic, showing Viking heritage, siader appears as a place name highlighting that there was at least some flat, habitable ground nearby.

    Ancient Civilisations

    One of the spectacular attractions of the Outer Hebrides is the standing stones at Calanais on Lewis. The purpose of these structures is a subject of much speculation, but it is believed to be part of a Europe-wide culture that had its hub in the Orkney Islands. What is beyond doubt is that whoever built these structures used the materials readily to hand – Lewisian Gneiss.

    While the ancient people of Calanais used their local stones, it recently came to light that the UK’s most famous stone circle, Stonehenge, includes rocks brought 700 km from the north of Scotland to form the altar: there is clearly something very special about Scottish rock.

    standing stones iwht rainbow in the background

    The standing stones of Calanais. Photo credit: Gareth Crapper.

    Close up of standing stone

    Striations in the Calanais stones: layers of remelted rock in the Lewisian Gneiss. Photo credit: Gareth Crapper.

    Coastal Geology and Machair

    Strictly, the Gaelic word machair means any fertile, low-lying grassy plain; however, it has come to mean the specific grasslands rich in wildflowers best seen on the west shores of the Outer Hebrides. The shallow sloping seabed to the west is ideal for shellfish, and their shells wash up on the beaches. These shells break down into fine white carbonate-rich sand, which neutralises the acidic peat soil: the result is a special environment, known as machair, in which a variety of spectacular wildflowers grow.

    purple blue hare bells close up in a grassy field

    Hare bells. Photo credit: Gareth Crapper.

    wild thyme close up in grassy field. Very purple.

    Wild thyme. Photo credit: Gareth Crapper.

    Common Blue butterfly

    Common Blue butterfly. Photo Credit Gareth Crapper.

    In addition to plant life, it is a haven for birds and insects. Just two examples are the corncrake and the great yellow bumblebee:

    • The corncrake is in steep decline due to modern farming practices, but in the Outer Hebrides the grassland management is traditional, and meadows are commonly mown from the “inside-out” which allows the corncrakes to escape to the margins. Extremely difficult to spot, their distinctive rasping call lets you know they are around.
    • The great yellow bumblebee, once widespread, is now the UK’s rarest bee, found only on the margins of northwest Scotland (and one peninsula in Ireland). This bee requires the kinds of flowers present in the machair but has also been forced north by climate change, meaning this is the only place to see it.

    Summary

    A simple understanding of the Earth’s history can lead to a whole new appreciation of the Outer Hebrides. Particularly impressive is the extensive outcropping of Lewisian Gneiss once you understand how truly old it is.

    This deep time writ large over the landscape showcases the fascinating geology of the Outer Hebrides and gives a proper measure of how ancient our planet is.

    The geology has also driven the population of the islands, with fertile sea inlets, coffin roads and shallow beaches producing special habitats.

    Hopefully, knowing more about the geology of these special islands will enrich your visit. It might even spark your interest in visiting some of the other amazing geological sites in Scotland—there’s so much to see.

    Visit the Outer Hebrides With Us

    Glossary

    • Gabbro:A coarse-grained igneous (volcanic) rock typically found in Scotland from the Paleogenic volcanic episode around 50 million years ago.
    • Gneiss: A coarse-grained metamorphic rock (changed by remelting, often multiple times at depth). It usually shows compositional banding where the constituent rocks have separated during remelting under pressure. While gneisses generally are a common rock form, the Lewisian Gneiss is particularly ancient and of volcanic origin.
    • Granite: Granite is a coarse-grained intrusive igneous rock that is often seen where it has been injected into older rocks. Most recognisable as a pink rock with large crystals, it can have a wide range of compositions and colours.
    • Iapetus Ocean: An ancient precursor ocean to the Atlantic. The closing of the Iapetus led to Scotland and England combining and building the mountains of eastern North America, Scotland and Scandinavia. Subsequent to this the Atlantic Ocean opened leading to the separation of Europe and North America and the formation of the volcanic hills such as those seen on Mull and Skye.
    • Oregony: A mountain-building process caused by the coming together of two continental plates.
    • Plate Tectonics: A theory based on the existence of a series of continental plates that move over long timescales over the surface of the Earth.
    • Unconformity: A junction of two very different rock types, typically with a gap of millions of years between them.

    Meet the Author: Gareth Crapper

    During a career in the chemical industry, I always had ‘the outdoors’ as a place to get away to. Originally, this was via hillwalking until I was introduced to climbing. This change led to trips to Scotland, the Alps, and the Himalayas. A new job in southern England cut this short, so I got on my bike and explored the Chiltern Hills. Back north now, I’ll ride anywhere on anything. Recently, I have been lucky enough to work as a bike guide in Ireland, England, and Scotland. My favourite rides are at a social pace, off the beaten track, and with a hidden gem café in my sights. Failing that, an al fresco bacon butty and posh coffee, when the mini stove comes out of the backpack, is the perfect addition to a ride.

    View profile More by Gareth

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