A meeting of two continents in Scotland
Last Updated: 6th Dec 2023By David Carter
The Great Glen Fault from space.
A long time ago, in fact even longer than that, a very very long time ago, 410 million years if you want to be a bit more precise (give or take a few million), our beautiful world looked completely different to what it does today!
What is termed as the Caledonian Orogeny (a mountain-building cycle recorded in the northern parts of the British Isles, the Scandinavian Caledonides, Svalbard, eastern Greenland and parts of north-central Europe) encompasses events that occurred from the Ordovician Period to Early Devonian, roughly 490 to 390 million years ago.
An orogeny is a geological term describing the formation of mountain ranges. The process takes place at a convergent plate margin when plate motion compresses the margin.The Caledonian Orogeny is named for Caledonia, the Latin name for Scotland. The designation was first used in 1885 by Austrian geologist Eduard Suess (20th August 1831 - 26th April 1914); no, not that “doctor”, author and cartoonist - different spelling too!
Longer before then even, Earth used to consist of a supercontinent that, to all intents and purposes, united pretty much all of its landmasses. Called Rodinia, from the Russian родина (rodina), meaning motherland or birthplace, Gondwana formed the bulk of it towards the end of the Precambrian Supereon and the Proterozoic Eon. As part of our planet’s gradual yet ever changing process this supercontinent broke apart, with two pieces (albeit absolutely massive pieces) forming Laurentia from the western sections and Baltica from the northern margins of Gondwana. Eventually thousands of miles of ocean would separate Laurentia and Gondwana.
A "consensus" reconstruction of the supercontinent Rodinia 900 million years ago.
Laurentia slowly (incredibly slowly) but surely drifted westward away from Gondwana and then migrated northward. A vast body of water called the Iapetus Ocean eventually opened up between Laurentia, Baltica and Gondwana. Over countless millennia the Iapetus Ocean spread further and ultimately Laurentia and Baltica moved away from each other too. Baltica drifted northward as well, which separated it from the northern margin of Gondwana to the south by the appearance of the Tornquist Sea or Tornquist Ocean about 600 to 450 million years ago.
Laurentia, also called the North American Craton.
Baltica (in white, at the centre of the image) with the Rockall Plateau (also in white, to the left). Outline of present-day Europe for reference.
Sometime between the Late Precambrian Supereon and Early Ordovician Period (scientists seem to be are hedging their bets here, but it was an awfully long time ago) the Avalonia microcontinent, which had developed as a volcanic arc on the northern margin of Gondwana and eventually rifted off to become a drifting microcontinent, started to drift northwestward to form the Gondwana northern margin (Amazonia and northwest Africa) close to the original position of Baltica which had been to its north. It does sound confusing I know, but please bear with me!
The rifting of Avalonia involved the opening and spreading of the Rheic Ocean to its south, which also separated it from Gondwana. Consequently, Avalonia drifted towards the palaeocontinents Laurentia and Baltica, which by then were further north, and the whole process also involved the consumption of both the Iapetus Ocean and the Tornquist Ocean along its northern margin.
The microcontinent Avalonia migrated from Gondwana and collided with first Baltica and later Laurentia. This image shows this process at 480 million years ago, or thereabouts!
Gondwana 420 million years ago. View centred on the South Pole.
Location of the Acadian/Caledonian mountain chains in the Early Devonian Period (approximately 416 - 359 million years ago). Present day coastlines are shown for reference. Red lines are sutures, capitalised names are the different continents/super-terranes joined during the Caledonian Orogeny.
Ultimately, Avalonia ended up in the interior of Pangaea, which itself had been assembled over millions of years from the earlier continental units of Gondwana and Laurasia or Laurussia (also called Euramerica), later joined by Kazakhstania and Siberia. Laurasia was the more northern of two large landmasses that formed part of the Pangaea supercontinent around 335 - 175 million years ago, the other being Gondwana. It separated from Gondwana around 215 - 175 million years ago during the breakup of Pangaea, drifting farther north after the split and finally broke apart with the opening of the North Atlantic Ocean around 56 million years ago. The name is a portmanteau of Laurentia and Asia. When Pangaea (which is the most recent supercontinent known to have existed and also notably the first to be reconstructed by geologists) broke up, Avalonia's remains were divided by the rift which then became the Atlantic Ocean.
The supercontinent Pangaea in the early Mesozoic Era (at 200 million years ago).
Crustal fragments of Avalonia underlie the southwest of Great Britain, southern Ireland, and eastern coast of North America. It is named for the Avalon Peninsula in Newfoundland and is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States. However, when the term "Avalon" was first used by Canadian geologist Harold Williams (14th March 1934 - 28th September 2010) in 1964 he was including only Precambrian rocks in eastern Newfoundland.
The Avalonian part of Great Britain almost exactly coincides with England and Wales. Elsewhere in Europe, parts of Avalonia can be found in north-eastern France, the Ardennes of Belgium, north Germany, north-western Poland, also south-eastern Ireland, plus the south-western edge of the Iberian Peninsula on either side of the Straits of Gibraltar because Iberia was later rotated away as the African part of Gondwana strike-slipped past it. This last movement caused the Alpine Orogeny which included the raising of the Pyrenees during the Miocene Epoch (23.03 - 5.333 million years ago) and Pliocene Epoch (5.333 - 2.58 million years ago).
Avalonia today, highlighted in yellow.
The Caledonian event itself was just one of several episodic phases of mountain building that have occurred in Earth’s long history. In the grand scheme of things, we humans are fairly insignificant. In fact, as the leading Roman lyric poet during the time of the first Roman emperor Augustus wrote in his Odes of Horace:
“Pulvis et umbra sumus.” (We are but dust and shadow.)
~ Quintus Horatius Flaccus (65 BC - 8 BC)
~ Quintus Horatius Flaccus (65 BC - 8 BC)
Over time, as on previous occasions, the world changed. Pangaea (also Pangea) drifted apart, the dinosaurs came and went (they were around somewhere between 243 and 233.23 - 66 million years ago), and Earth gradually became what it is today.
A world map on the Winkel tripel projection, a low-error map projection adopted by the National Geographic Society for reference maps.
What is now the U.K. had been separated by the Caledonian Orogeny between the two earlier palaeocontinents - the north of Scotland having been on Laurentia, and the rest on Gondwana (or a fragment of it in the form of microcontinent Avalonia).
While I am still in a literary mood and keeping the lyrical ambience going, as the renowned English playwright, poet and actor most eloquently put it in his play Richard II:
“This royal throne of kings, this sceptred isle,
This earth of majesty, this seat of Mars,
This other Eden, demi-paradise,
This fortress built by Nature for her self
Against infection and the hand of war,
This happy breed of men, this little world,
This precious stone set in a silver sea
Which serves it in the office of a wall
Or as a moat defensive to a house,
Against the envy of less happier lands,
This blessed plot, this earth, this realm, this England*.”
~ William Shakespeare (1564 - 1616)
(*later joined by Wales, Scotland and Northern Ireland)
This earth of majesty, this seat of Mars,
This other Eden, demi-paradise,
This fortress built by Nature for her self
Against infection and the hand of war,
This happy breed of men, this little world,
This precious stone set in a silver sea
Which serves it in the office of a wall
Or as a moat defensive to a house,
Against the envy of less happier lands,
This blessed plot, this earth, this realm, this England*.”
~ William Shakespeare (1564 - 1616)
(*later joined by Wales, Scotland and Northern Ireland)
Map of the United Kingdom of Great Britain and Northern Ireland, with Eire and northern France (part) - comprised of various satellite images.
The Great Glen Fault is a huge valley in the Scottish Highlands, eroded by glaciers more than 10,000 years ago and visible from space. It cuts diagonally across the Highlands from Fort William to Inverness and it originated towards the end of the Caledonian Orogeny. The immense glaciers that carved the valley to below present day sea level formed a series of deep lakes, the largest and most famous being Loch Ness. Just when exactly the famous lake monster first arrived has yet to be determined!
Loch Ness Monster - the "surgeon's photograph" of 1934, now known to have been a hoax.
Like other major fault zones around the world, the Great Glen has a long history of reactivation. Although it is known the north west and south east sides of the fault are moving in opposite directions, there is currently no agreement on how far they have moved. Because the geology on either side of the fault cannot be matched up prior to the Devonian Period, it is thought that the displacement could be at least as far as the exposed length of the fault on mainland Scotland. The Great Glen has been recognised as a major fault zone for well over a century, but it was not until the work of Scottish geologist William Quarrier Kennedy (30th November 1903 - 13th March 1979) in the 1930s (and published in 1946) that its significance was recognised when he showed that the fault had moved sideways - as a so-called strike-slip (or wrench) fault. The fault is mostly inactive today although occasional moderate tremors have been recorded over the past 150 years.
Map of the Great Glen Fault and other late Caledonian strike-slip faults in Scotland and northwestern Ireland.
Although the fault continues on the North American side of the Atlantic Ocean it is no longer part of a contiguous fault as the complete fault was broken when the Mid-Atlantic Ridge formed 200 million years ago.
Illustration showing the various faults at 370 million years ago that now extend across the North Atlantic Ocean.
During the Palaeozoic Era (538.8 - 251.9 million years ago), the interactions between the continents of Laurentia, Baltica and Gondwana were governed by two major oceans: Iapetus and the Rheic Ocean. The Iapetus Ocean had opened in the Late Ediacaran Period (635 - 538.8 million years ago)/Early Cambrian Period (538.8 - 485.4 million years ago). It was named for Iapetus, in Greek mythology the father of Atlas (from which source the Atlantic Ocean ultimately gets its name), just as the Iapetus Ocean was the predecessor of the Atlantic Ocean. The Rheic Ocean, on the other hand, opened up in the Early Ordovician Period (485.4 - 470 million years ago) and although it has not received the same attention as Iapetus, it is arguably the more important ocean of the two. It was named after Rhea, sister of Iapetus, and existed until the Early Carboniferous Period (358.9 - 327 million years ago). It was the evolution of Rheic Ocean that dominates the geology of southern Europe, eastern North America and northern Africa following the closure of Iapetus in the Silurian Period (443.8 - 419.2 million years ago). It was also the Rheic Ocean that created the vast orogen of Ouachita-Appalachian-Variscan in the Late Palaeozoic and, in doing so, assembled the greater part of Pangaea. So many dates, such incredible timescales, much more yet to discover?
Loch Lochy in the Great Glen from the summit of Meall na Teanga.
Built in the early nineteenth century by Scottish civil engineer Thomas Telford (9th August 1757 - 2nd September 1834), the Caledonian Canal cuts through the Great Glen in several sections between the lochs to connect the Atlantic coast at Corpach near Fort William with the North Sea coast at Inverness through the Great Glen. The inland waterway is a sister canal of the Göta Canal in Sweden, also constructed by Telford. Only one third of the entire length of the route, 22 miles (35km), is man-made, the rest being formed by Loch Lochy, Loch Oich, Loch Ness, and Loch Dochfour.
The Caledonian Canal and Loch Ness.
‘Lord of the Glens’ leaving Corpach Sea Lock at the western end of the Caledonian Canal.
As an interesting aside, robot submarines, including one named ‘Boaty McBoatface’, were recently (2021) tested in Loch Ness in preparation for deep sea research expeditions. Designed to dive down to depths of 19,685ft (6,000m), these state-of-the-art machines are equipped with sensors for detecting marine life around icebergs and under the polar icecaps. Little ‘Boaty McBoatface’ (also known as ‘Boaty’) is the British lead boat in a fleet of three robotic lithium battery-powered autonomous underwater vehicles (AUVs) of the Autosub Long Range (ALR) class. They were launched in 2017 and carried on board the polar scientific research vessel RRS Sir David Attenborough as a focal point of the Polar Explorer Programme of the UK Government. Engineers ran the trials in Loch Ness because it offered a large enclosed space where it was easier to recover a lost sub than at sea. Loch Ness is more than 20 miles long (32km) and almost two miles wide (3km) at its widest point. The official maximum depth is more than 750ft (229m), although a tour boat skipper in 2016 stated that his sonar equipment had recorded a new deepest point of 889ft (271m).
A robot submarine just below the surface of Loch Ness.
The Blue Marble - NASA.
“Nature does not hurry, yet everything is accomplished.”
~ Lao Tzu (semi-legendary ancient Chinese philosopher from around the 6th century BC and author of the Tao Te Ching and the Zhuangzi, the two foundational texts of Taoism (Daoism).
~ Lao Tzu (semi-legendary ancient Chinese philosopher from around the 6th century BC and author of the Tao Te Ching and the Zhuangzi, the two foundational texts of Taoism (Daoism).
Sources
The information in this article has been curated from a variety of online sources, all freely available and in the public domain, with some added flavour from me.
Below is a sizeable handful of some of the main references utilised. That is to say it is by no means an exhaustive list because some small snippets also came from personal knowledge, other sources including various books and literature, as well as the vast and glorious internet!
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https://rock.geosociety.org/net/gsatoday/archive/2/11/pdf/i1052-5173-2-11-sci.pdf
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https://www.searchanddiscovery.com/documents/97019/index.htm
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https://www.unil.ch/files/live/sites/mcg/files/users/gborel/public/Peri-Gond_GSA.pdf
Blakey R. C (2003) "Carboniferous-Permian paleogeography of the assembly of Pangaea". In Wong, Th. E. (ed.). Proceedings of the XVth International Congress on Carboniferous and Permian Stratigraphy. Utrecht (vol. 10, p. 16). Utrecht, the Netherlands: Royal Netherlands Academy of Arts and Sciences
https://www.academia.edu/8298797/Carboniferous_Permian_paleogeography_of_the_assembly_of_Pangaea
DiPietro Joseph. A (2018) “Orogeny - an overview”. Geology and Landscape Evolution (Second Edition), pp. 59-77
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/orogeny
Allen Mike (2019) “The long and moving story of the Great Glen Fault”. East Midlands Geological Society, Mercian Geologist, vol 19 (4)
http://www.emgs.org.uk/Mercian/Mercian%20-%20Individual%20papers/Mercian%20Geologist%20volume%2019%202016-2019/Mercian%202019%20v19%20p216%20Moving%20story%20of%20the%20Great%20Glen%20Fault%20Allen.pdf
Strachan R. A, Woodcock N. H (2021) “The Tectonic Pattern of Britain and Ireland”. Encyclopedia of Geology (Second Edition), pp. 328-337
https://www.sciencedirect.com/science/article/abs/pii/B9780124095489124121
Robert Boris, Domeier Mathew, Jakob Johannes (October 2021) “On the origins of the Iapetus Ocean”. Earth Science Reviews, vol. 221
https://www.sciencedirect.com/science/article/pii/S0012825221002920?via%3Dihub
Science Direct | Supercontinent
https://www.sciencedirect.com/topics/earth-and-planetary-sciences/supercontinent
Encyclopædia Britannica (2015) “Pangea (ancient supercontinent)”.
https://www.britannica.com/place/Pangea
The Geological Society | Plate tectonics of the UK | Caledonian Orogeny
https://www.geolsoc.org.uk/Plate-Tectonics/Chap4-Plate-Tectonics-of-the-UK/Caledonian-Orogeney
The Geological Society | Great Glen Fault, Scotland
https://www.geolsoc.org.uk/Policy-and-Media/Outreach/Plate-Tectonic-Stories/Great-Glen-Fault
Gazetteer for Scotland (2016) “Great Glen Fault”.
https://www.scottish-places.info/features/featurefirst10625.html
British Geological Survey | Geological timechart
https://www.bgs.ac.uk/discovering-geology/fossils-and-geological-time/geological-timechart/
Britannica | Geologic time | Periods, Time Scale, & Facts
https://www.britannica.com/science/geologic-time
Discovering Fossils | How Great Britain Formed
http://www.discoveringfossils.co.uk/how_britain_formed.htm
WJEC | A Level Geology – Topic T4: Geological evolution of Britain Tectonic events (KI 1a, 1c, 1d, & 1e)
https://resource.download.wjec.co.uk/vtc/2021-22/el21-22_14-8e/eduqas/t4-geological-evolution-tectonic-events-ko.pdf
Geological Digressions | “Bits of North America that were left behind: How bits of ancient North America (Laurentia) were left behind in the Scottish Hebrides”.
https://www.geological-digressions.com/tag/caledonian-mountain-building/
Houseman Greg (2008) “Animation of the dispersal of Gondwanaland”. University of Leeds, U.K.
https://homepages.see.leeds.ac.uk/~eargah/Gond.html
Map showing Avalonia in the Ordovician Period
https://web.archive.org/web/20140903054817/http://cpgeosystems.com/450_Ord_EurMap_plates.jpg
Scottish Canals | Visit the Caledonian Canal
https://www.scottishcanals.co.uk/visit/canals/visit-the-caledonian-canal
Shipping Wonders of the World | The Caledonian Canal
https://www.shippingwondersoftheworld.com/caledonian-canal.html
YouTube (2021) “Why There’s a Straight Line Through Scotland”.
https://m.youtube.com/watch?v=2F5zkuXJNn8
NBC | News | Science News (July 3, 2013) “The Loch Ness monster legend: It's geology's fault”.
https://www.nbcnews.com/sciencemain/loch-ness-monster-legend-its-geologys-fault-6c10519234
BBC | News | Scotland (27 May 2021) “Robot submarine Boaty McBoatface in Loch Ness dive tests”.
https://www.bbc.co.uk/news/uk-scotland-highlands-islands-57267470#:~:text=Robot%20submarines%2C%20including%20one%20named,6%2C000m%20(19%2C685ft)
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