Scotland’s geological timeline
Scotland packs an incredible range of geology within a small area. Scotland’s geological timeline spans over three billion years and captures a vast range of geological processes and phenomena. These include the formation of continental crust and its deformation during ancient orogenic episodes, through to the record of past Earth environments, to the sculpting of the landscape during glaciations, to human occupation and the exploitation of Earth resources. This is a short account of Scotland’s geological journey. The record is arranged as a tapestry across the country – naturally divided by the major fault lineaments that define much of the physiography of Scotland’s mainland. But it is a record that is very much enhanced by the geology of Scotland’s islands and, especially, its offshore land. We can follow the story through geological time and place. To start our story, we need to close the North Atlantic and locate Scotland as a part of the Laurentian continent.
The dawn – and the far NW
The story, as preserved on Scotland, begins in the late Archaean. The country’s oldest rocks lie within the Lewisian complex, preserved in the far NW of the mainland and on the Western Isles – the Outer Hebrides. These rocks include classic Archaean crust-forming rocks, gneisses derived from a range of plutonic rocks (tonalite-trondhjemite-granodiorite, TTG), and metabasic intrusions that date from around 3.2 Ga. These units include narrow infolded tracts of supracrustal units – metasedimentary and metaigneous rocks that include banded iron-stones along with regional basic dyke swarms, reworked by penetrative deformations that correlate with mobile belts elsewhere in Laurentia. The youngest events of these, locally termed “the Laxfordian”, dates at around 1.8 Ga, after which the region was denuded, bringing mid-crustal rocks to the Mesoproterozoic land-surface. The Lewisian complex has long been a proving ground for unravelling crust-forming and deformation processes, with dramatic, readily accessible outcrops.
The crust of the far NW of Scotland is overlain by siliciclastic successions informally known as the “Torridonian”. The oldest part, the Stoer Group, dates from around 1.2 Ga. It includes some of the earliest preserved non-marine life on Earth and dramatic meteoroid impact deposits. The younger Torridon Group, at 1 Ga, is the dominant succession, with at least 6-7 km of chiefly fluvial-alluvial sandstones. These fill and blanket a spectacular palaeotopography with at least 600 m relief. These Proterozoic successions are seeing renewed research as terrestrial analogues for investigating early landscape processes on Mars.
The Proterozoic successions are overlain by a transgressive shallow-marine succession of Cambro-Ordovician age. These represent the feather-edge of the continental margin strata deposited on the flanks of the Iapetus ocean. They have widespread correlations elsewhere on Laurentia and Baltica.
All this geology lies NW of the Moine Thrust, the edge of penetrative deformation and metamorphism associated with the Caledonian orogeny in the early Palaeozoic.
Mountain building
Much of the Scottish Highlands comprise strongly deformed rocks and granites that chart Caledonian mountain building, the orogeny that developed through the protracted closure of the Iapetus Ocean. Continental collision developed in two distinct episodes. The earliest, named the Grampian (after the part of the Scottish Highlands that lie to the SE of the Great Glen Fault) is Ordovician in age. The youngest is termed Scandian and is represented by the Moine Thrust and associated structures, to the NW of the Great Glen Fault. But both tracts have earlier histories that we should address before considering these Palaeozoic mountain building episodes.
The rocks between the Moine thrust and Great Glen Fault are generally referred to as the Moine – a term strictly applied to Neoproterozoic metasediments, the oldest parts of which are broadly time-equivalent to the Torridon Group and similarly underlain by much older Lewisian basement. Locally the Lewisian rocks are eclogitic, dated at around 1000 Ma and correlated with Grenvillian orogenesis in North America. They are a rare fragment of high-pressure metamorphism preserved from Proterozoic times. This has led to interpretations of Torridon Group and Moine rocks as molasse derived from eroded Grenvillian mountains.
The strata within the Grampian mountains are chiefly metasedimentary – the Dalradian Supergroup. These date from late Neoproterozoic to Cambrian, are broadly marine and deposited on the rifted continental margin of Laurentia, prior to and following the opening of Iapetus further to the southeast. The Dalradian includes tillites sandwiched between carbonates – indicative of dramatically fluctuating palaeoclimatic conditions – part of the global evidence base for “Snowball Earth”.
The Grampian orogeny is thought to reflect collision of the Laurentian margin with an island arc and is marked by classic examples of regional metamorphism. Along the southern flank of the terrain the metasediments grade out from migmatites to much lower-grade units – the classic Barrovian zones. In the north, the presence of syn-tectonic mafic intrusions are responsible for the so-called Buchan metamorphic facies. These rocks are intruded by late-Caledonian granites that dominate the Grampian massif of the Cairngorms.
The Scandian episode is thought to be associated with the collision of Laurentia and Baltica, the NW Highlands at this time being located significantly to the northeast of the remainder of Scotland prior to its late- to post-Caledonian left lateral displacement along the Great Glen and other faults. The orogeny is strikingly marked by thrust tectonics, most famously represented by the Moine Thrust Belt (one of IUGS’s First One Hundred Geological Heritage Sites). Ductile equivalents, exhumed from deeper in the evolving orogen, are found within the Moine terrain along with classic examples of poly-deformed, refolded metasedimentary rocks. As with the Grampians, the NW Highlands are studded by late-orogenic intrusions which show that significant mountain building finished in late Silurian times.
The main components of the Caledonian orogeny are found, in close juxtaposition, in the Shetland archipelago. However, there is an important additional component: the islands of Unst and Fetlar comprise ophiolite assemblages and associated sedimentary melange, broadly correlated with similar oceanic fragments in Newfoundland. Some consider these ophiolites to have once covered the Grampian Highlands, providing the burial conditions necessary for metamorphism of the Dalradian successions.
The Southern Uplands
Aside from the geochemistry of the late orogenic granites, in the Scottish Highlands, evidence for the subduction of Iapetus ocean floor is surprisingly sparse. However, key evidence comes from southern Scotland. The Southern Uplands comprise thick, deformed successions of Ordovician-Silurian turbidites. The region includes Lapworth’s fundamental use of graptolite assemblages, erecting a zonal scheme to map the complex structure. Regional biostratigraphic mapping established the presence of large-scale thrust repetition and the interpretation of the Southern Uplands as a subduction-accretion complex. Fragments of former oceanic crust are found at Ballantrae.
Collectively then, the Caledonian system of Scotland preserves, with the exception of significant high-pressure (subduction) metamorphism, all of the key components of destructive plate boundary systems leading to continental collision.
Late-to-post Caledonian events
The Caledonian mountain ranges denuded rapidly, during and after orogeny – as recorded by continental clastics, long-known as the Old Red Sandstone. These Devonian (locally late Silurian) strata lie unconformably across their deformed substrate – examples of which in southern Scotland, including at Siccar Point, were described and interpreted by James Hutton, at the dawn of our science. Devonian rocks typically lie in fault bounded basins, in part governed by major late-to-post Caledonian strike-slip fault systems that dismembered and reassembled the orogen into its modern tracts of terrains. Two of these fault systems – the Great Glen and the Highland Boundary faults, define the morphology of Scotland. In Caithness (far NE Scotland) and Orkney, Devonian strata host famous fish beds, preserved in lacustrine sandstones.
Volcanism associated with late Caledonian magmatism, rifting and basin-formation includes the famous Glencoe caldera in Lochaber Geopark. The hot spring chert deposits of Rhynie in Aberdeenshire preserve some of the best Palaeozoic arthropod assemblages ever found.
Into the Carboniferous and rivers still feed off the Scottish Highlands, shedding sediment into obliquely-rifting basins. The largest extent of these lies within Scotland’s Midland Valley – the geographic Central Belt and home to the majority of Scotland’s population. This is because the Carboniferous strata include extensive coal measures, ironstones and oil shales. Subsidence and sediment supply broadly kept pace throughout the Carboniferous so that deposition was always never far from the coastline. But high-frequency tectonic-eustatic-controlled fluctuations in sea level are recorded in classic cyclothems, of which the coal measures are just a part. Tectonic subsidence was accompanied by volcanism, with small volcanic complexes and regional sill systems.
By the close of the Carboniferous and end of the Variscan orogeny in southernmost England, the continental crust that now underlies Britain was assembled forming a small part of the supercontinent, Pangaea. The region passes now into a tectonically more quiescent intraplate setting prior to the later opening of the North Atlantic.
Rifting and basins
Moving forward into the Permian and the Mesozoic, the geological record onshore Scotland is rather sparse, being largely limited to outcrops in the Inner Hebridean islands (e.g. Skye, Raasay) and the Moray Firth coast. It is in the offshore that these successions are volumetrically important – forming the fills to the sedimentary basins of the North Sea. Drilling and seismic imaging associated with extensive hydrocarbon extraction make these basins arguably the best-understood geological province in the world and the source of much of our understanding of continental rifting processes. The onshore outcrops are important for testing inferences drawn from subsurface information and especially in training generations of hydrocarbon geoscientists.
The Scottish Highlands continued to play an important role as a sediment source during the Tertiary for the North Sea and other basins around the coast.
Atlantic opening and a volcanic flourish
The Palaeocene marks a point of significant change in Scotland’s geological journey: the start of its separation from Laurentia, which it had been a part of for at least three billion years. At the onset of Atlantic opening, western Scotland experienced extensive volcanism, localised in a series of igneous complexes. The famous ones, forming the Hebridean islands of Skye, Rum, and Mull together with Ardnamurchan on the mainland and the Isle of Arran in the Firth of Clyde, are accompanied by at least five others now (largely) submerged offshore. The exceptional exposures in main centres have inspired fundamental insights of subvolcanic processes. The lava fields continue into Northern Ireland – forming not only the Mourne mountains but also the Causeway Coast World Heritage site. Much of the Scottish crust has likely been underplated by basaltic intrusions, generating regional topographic elevation and tilting to the southeast.
Ice and the modern landscape
Scotland is now one of the most tectonically stable parts of the world. Its geological history in the past few million years has been dominated by glacial processes associated with various northern hemisphere ice ages. Almost the entire area was covered, during the Last Glacial Maximum (Late Devensian, 31,000-16,000 years ago). The subsequent warming was interrupted at around 12,900 years ago, leading to glacial readvance (recognised as the Younger Dryas ). Collectively, these most recent parts of the glacial history have sculpted the Scottish Highlands, creating the dramatic landscapes. Isostatic unloading of the melted ice sheets is marked by a series of raised beaches along the coast. With the onset of the Holocene and rapid warming, the ongoing climate history is recorded, especially in Scottish lochs.
Humans
Although there is little evidence of human occupation in Scotland prior to the Last Glacial Maximum, it is likely that hunter-gatherer communities roamed the country from early Holocene times. There are extensive Neolithic settlements, including Skara Brae on Orkney, that date to at least 5,000 years ago, which used Devonian flagstones as convenient building materials. Industrial heritage is long-standing too, for example, using ironstones with locally generated charcoal to work iron – in numerous sites. But it was the exploitation of extensive coalfields that marks Scotland (and the world’s) move into an industrial age – from the late 18th century. James “Paraffin” Young was amongst the first to develop oil shales (1850s). Subsequently, the discovery of oil in Scottish waters of the North Sea and the development of giant fields such as Forties and Brent at the start of the 1970s led to the offshore oil boom. The basin is gradually being repurposed for offshore wind, with Scotland’s first large-scale windfarms being commissioned in 2017. Now-depleted oil and gas fields in the North Sea are being appraised as possible reservoirs to sequester CO2. Meanwhile, with the demise of deep coal extraction (in 2002), some of Scotland’s mines are being scoped to be repurposed for subterranean pump-storage, effectively gravity-batteries for Scotland’s expanding renewable energy sector.
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