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About Geowalks - Articles
The following article is published in the About Scotland section of visitscotland.com.
Scotland's Geology
Scotland's geology is world-famous, both for the rich variety of our rocks and the place they hold in the development of new ideas. The rocks of Scotland have formed over a time span of billions of years, with different events forming a wide variety of rock types. This geological diversity is reflected in Scotland's scenery, in the way that the rocks have been sculpted over millions of years to give the Highlands and Lowlands, the firths and the islands, the glens, lochs and serrated mountain ridges.
Deep Time
Scotland's story stretches
back to the early history of the earth, with our oldest rocks formed an unimaginable
3 billion years ago. Stand on any rock in Scotland, and you are standing on a
piece of history, one offering a glimpse into Scotland's past. But more than that,
you can feel the slow, ceaseless change on a global scale which has created and
altered these rocks, and sense that we are but small flecks on an ever-changing
tapestry. This idea of 'deep time', of gradual changes over a huge time span,
and of continued gradual change, owes its genesis to a famous Scot, James Hutton
(1726 - 1797). Hutton's theory of the Earth challenged the accepted precepts of
the day, that the Earth was merely a few thousands of years old, and laid the
basis for the modern science of geology.
Scotland's
Geological Jigsaw
We can imagine Scotland as a jigsaw, with six main pieces,
slabs of continent formed at different times in different places. Go back beyond
500 million years, and the oldest rocks give us glimpses of a complex history,
with long-lost ocean basins, volcanic islands and chunks of continent that have
been altered and overprinted by more recent events. The recent history is clearer,
starting with the Caledonian Orogeny around 400 million years ago. Before that
time the rocks of Scotland, Scandanavia and North America were one continent,
while on the other side of the now-vanished Iapetus Ocean lay the rocks of England
and the rest of northern Europe. The Caledonian Orogeny was a continental collision
which closed the Iapetus Ocean, collided the rocks of England and Scotland, and
fused the main jigsaw pieces of Scotland together. Then quieter conditions prevailed
and much of Scotland was buried under layers of sediment, forming sandstone, coal
and other sedimentary rocks. Our tectonic activity wasn't over, though, for 60
million years ago the continent split apart, forming the North Atlantic in a spectacular
firestorm of erupting volcanoes along Scotland's western edge.
A
Brief Tour of Scotland's Geology
The Western Isles
Scotland's oldest
rocks - also the oldest in Europe - can be found in the Western Isles. These ancient
gneisses, used in the standing stones at Callanish in Lewis, have been deeply
buried and contorted during a long history. These rocks are also found on the
mainland, in a thin strip along the north-west coast. Here they are often buried
under younger sandstones and limestones, including the Torridonian Sandstones,
dating back to 1 billion years and the oldest sedimentary rocks in Scotland.
The
Moine Thrust
Just inland from the coast, we come to our first major discontinuity,
the world-famous Moine Thrust, which is best seen at Knockan Crag, just north
of Ullapool. This fault plane separates the gneiss and sandstones of the west
from metamorphic rocks, the Moine Schist, to the east. The schists were originally
ocean sediments, were caught up in Caledonian Orogeny, and pushed westwards over
sedimentary rocks. This juxtaposition of metamorphic rocks above sedimentary rocks
was a major puzzle to the Victorian geologists who first studied them, and solving
the puzzle represented a major leap forward in our understanding of mountain building
and faults.
The Great Glen
Travelling further
south, we reach the Great Glen between Inverness and Fort William, site of a major
fault line separating two blocks of metamorphic rocks with different histories.
To the east, these rocks are buried under the Old Red Sandstone, made famous by
stonemason Hugh Miller (1802 - 1856), who found many unique specimens of fossil
fish around Cromarty on the Black Isle.
The Cairngorms
South of the Great Glen, the Highland schists often contain large bodies of granite,
for example in the Cairngorm mountains. These granite masses were once molten,
with hot, liquid rock squeezing and melting its way towards the surface. Trapped
in the crust, the magma cooled slowly, forming the crystalline granite. This activity
was associated with the Caledonian Orogeny, and there were many volcanoes as well,
as for example at Glencoe. What we see in the Highlands now is the result of millions
of years of erosion that has removed the top of a mountain range, exposing its
roots.
The Lowlands
The rocks of the Scottish
lowlands have a very different story from the mountains of the north. In the midland
valley, some of Scotland's youngest rocks cover over the older crust. This is
the site of the first attempt, starting nearly 400 million years ago, to pull
the newly formed continent apart. The result was a rift valley, covered with a
rash of volcanoes and a hole filling with sediments being washed from the high
ground to the north and south. Stand anywhere in Scotland's central belt, and
the hills that pepper the landscape - Dunbarton Rock, the Campsies, Arthur's Seat,
North Berwick Law - are all the result of volcanic activity during the Carboniferous
Period. On the margins are older volcanoes such as the Pentlands and the Ochils,
for example, but they are all linked by a common theme of being much more durable
and resistant to erosion that the surrounding sedimentary rocks.
Southern
Scotland
The rocks of Southern Scotland, the Southern Uplands, again have
a different story to tell, illustrating the range and complexity of Scotland's
rocks. Here we find the remains of the Iapetus Ocean, the sediments that were
once on the ocean floor and have been crumpled up and attached to the rest of
Scotland during the Caledonian Orogeny. Here we find the remains of ancient sea
creatures, the graptolites and trilobites that lived in the ocean over 400 million
years ago. The rounded character of the Southern Uplands is in sharp contrast
to the more rugged Highlands, echoing their contrasting geology.
As you enjoy Scotland's famed tranquillity, perhaps even while sampling some of Scotland's national drink, it is worth reflecting how the underlying rocks give character to the landscape of Scotland, to many aspects of our culture, and perhaps even to its people themselves. And the rocks tell of an incredible history, of a piece of continent that has travelled the globe, been pummelled into mountains, rocked by volcanic eruptions and pulled apart along rifts. Along the way the climate has changed from tropical to glacial and everything in between. Today's peace masks a turbulent history, and current conditions are merely a chapter in a story that will go on and on into a turbulent future.
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