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45-630: The geology of the Highlands is complex. Along the western coastal margin it is characterised by Lewisian gneiss , the oldest rock in Scotland. Liathach , Beinn Alligin , Suilven , Cùl Mòr , Cùl Beag , and Quinag are just some of the impressive rock islands of the significantly younger rich brown-coloured Torridonian sandstone which rests on the gneiss. Some of the peaks, such as Beinn Eighe and Canisp , are topped with later light grey or white Cambrian quartzite . Cambro- Ordovician limestone

90-708: A supereon , but this is also an informal term, not defined by the ICS in its chronostratigraphic guide. Eozoic (from eo- "earliest") was a synonym for pre-Cambrian , or more specifically Archean . A specific date for the origin of life has not been determined. Carbon found in 3.8 billion-year-old rocks (Archean Eon) from islands off western Greenland may be of organic origin. Well-preserved microscopic fossils of bacteria older than 3.46 billion years have been found in Western Australia . Probable fossils 100 million years older have been found in

135-628: A continuous body, disrupted by Laxfordian deformation. These two belts of metasediments flank the South Harris igneous complex, and form the largest outcrop of such rocks in the Outer Hebrides. Radiometric dating has shown these metasediments to be of Paleoproterozoic age, similar to the rocks of the Loch Maree Group. The relationship between these metasediments and Scourian gneisses remains unclear. This fault zone stretches

180-594: A minor amount of supracrustal rocks . Rocks of the Lewisian complex were caught up in the Caledonian orogeny , appearing in the hanging walls of many of the thrust faults formed during the late stages of this tectonic event. The main outcrops of the Lewisian complex are on the islands of the Outer Hebrides , including Lewis , from which the complex takes its name. It is also exposed on several islands of

225-735: A possible 2450 Ma red alga from the Kola Peninsula , 1650 Ma carbonaceous biosignatures in north China, the 1600 Ma Rafatazmia , and a possible 1047 Ma Bangiomorpha red alga from the Canadian Arctic. The earliest fossils widely accepted as complex multicellular organisms date from the Ediacaran Period. A very diverse collection of soft-bodied forms is found in a variety of locations worldwide and date to between 635 and 542 Ma. These are referred to as Ediacaran or Vendian biota . Hard-shelled creatures appeared toward

270-566: A very low population density. Significant settlements are Kyle of Lochalsh , Mallaig , Dingwall , Dornoch , and Ullapool . The city of Inverness , known as the "Capital of the Highlands", on the eastern margin of the Northwest Highlands, is by far the largest settlement in the region and is the administrative centre for the Highland Council area . Lewisian complex The Lewisian complex or Lewisian gneiss

315-754: Is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland , forming part of the Hebridean Terrane and the North Atlantic Craton . These rocks are of Archaean and Paleoproterozoic age, ranging from 3.0–1.7 billion years ( Ga ). They form the basement on which the Stoer Group , Wester Ross Supergroup and probably the Loch Ness Supergroup sediments were deposited. The Lewisian consists mainly of granitic gneisses with

360-591: Is also to be found in a narrow outcrop between Durness and Skye . The larger part of the region is formed from rocks of the Moine succession much of which is metamorphosed sandstones ( psammites ) and schist . These have over-ridden the gneiss and the Cambrian and Ordovician rocks of the northwest margin along the Moine Thrust Belt . The geological understanding of thrust faulting slowly developed in

405-572: Is banded grey gneisses, typically granodioritic , tonalitic or trondhjemitic in composition. Metasedimentary gneisses are relatively rare. The protolith for the Scourian gneisses are thought to be granitic, with subsidiary mafic and ultramafic plutonic rocks giving an overall bimodal character. Some variation in the age of the protoliths from different parts of the complex and their subsequent tectonic and metamorphic history suggest that there are two or possibly three distinct crustal blocks within

450-664: Is known to occur during the RNA replication of extant coronaviruses . Evidence of the details of plate motions and other tectonic activity in the Precambrian is difficult to interpret. It is generally believed that small proto-continents existed before 4280 Ma, and that most of the Earth's landmasses collected into a single supercontinent around 1130 Ma. The supercontinent, known as Rodinia , broke up around 750 Ma. A number of glacial periods have been identified going as far back as

495-668: Is the earliest part of Earth's history , set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian , the first period of the Phanerozoic Eon , which is named after Cambria , the Latinized name for Wales , where rocks from this age were first studied. The Precambrian accounts for 88% of the Earth's geologic time. The Precambrian is an informal unit of geologic time, subdivided into three eons ( Hadean , Archean , Proterozoic ) of

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540-694: Is the summit of Càrn Eige at an elevation of 1,183 metres (3,881 feet) above sea level. By comparison, across the Great Glen to the south, the Grampians includes Ben Nevis which is highest point in the British Isles at 1,345 metres. The aurora borealis is sometimes visible on winter nights, especially at the climax of the 11-year cycle. Bordering the region to the northeast is the lowland area of Caithness . There are relatively few roads, and many are single lane (with passing places). The region has

585-521: Is thought that the Earth coalesced from material in orbit around the Sun at roughly 4,543 Ma, and may have been struck by another planet called Theia shortly after it formed, splitting off material that formed the Moon (see Giant-impact hypothesis ). A stable crust was apparently in place by 4,433 Ma, since zircon crystals from Western Australia have been dated at 4,404 ± 8 Ma. The term "Precambrian"

630-731: Is used by geologists and paleontologists for general discussions not requiring a more specific eon name. However, both the United States Geological Survey and the International Commission on Stratigraphy regard the term as informal. Because the span of time falling under the Precambrian consists of three eons (the Hadean , the Archean , and the Proterozoic ), it is sometimes described as

675-534: The Huronian epoch, roughly 2400–2100 Ma. One of the best studied is the Sturtian-Varangian glaciation, around 850–635 Ma, which may have brought glacial conditions all the way to the equator, resulting in a " Snowball Earth ". The atmosphere of the early Earth is not well understood. Most geologists believe it was composed primarily of nitrogen, carbon dioxide, and other relatively inert gases, and

720-688: The Inner Hebrides , small islands north of the Scottish mainland and forms a coastal strip on the mainland from near Loch Torridon in the south to Cape Wrath in the north. Its presence at seabed and beneath Paleozoic and Mesozoic sediments west of Shetland and in the Minches and Sea of the Hebrides has been confirmed from the magnetic field, by shallow boreholes and hydrocarbon exploration wells. Basement rocks of similar type are found at

765-522: The geologic time scale . It spans from the formation of Earth about 4.6 billion years ago ( Ga ) to the beginning of the Cambrian Period, about 538.8 million years ago ( Ma ), when hard-shelled creatures first appeared in abundance. Relatively little is known about the Precambrian, despite it making up roughly seven-eighths of the Earth's history , and what is known has largely been discovered from

810-482: The oxygen catastrophe . At first, oxygen would have quickly combined with other elements in Earth's crust, primarily iron, removing it from the atmosphere. After the supply of oxidizable surfaces ran out, oxygen would have begun to accumulate in the atmosphere, and the modern high-oxygen atmosphere would have developed. Evidence for this lies in older rocks that contain massive banded iron formations that were laid down as iron oxides. A terminology has evolved covering

855-401: The 1960s onwards. The Precambrian fossil record is poorer than that of the succeeding Phanerozoic , and fossils from the Precambrian (e.g. stromatolites ) are of limited biostratigraphic use. This is because many Precambrian rocks have been heavily metamorphosed , obscuring their origins, while others have been destroyed by erosion, or remain deeply buried beneath Phanerozoic strata. It

900-606: The 19th century in what became known as the " Highlands Controversy ". . Considering its high northerly latitude of about 57 to 58 degrees north and the mountainous terrain, the climate is mild, moderated by the warming influence of the North Atlantic Drift which is a powerful warm western boundary current within the Atlantic Ocean that extends the Gulf Stream northeastward. At low elevations and near

945-658: The Central and Southern Regions, including the Canisp Shear Zone. This basic dyke swarm cuts the banding of the Scourie complex gneisses and therefore postdates the main igneous, tectonic and metamorphic events that created them. Due to the degree of later metamorphism and deformation in other parts of the mainland outcrop, the only reliable radiometric ages come from the Central Region, giving an age for

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990-495: The Leverburgh and Langevat supracrustals. Radiometric dating suggests that the complex was intruded over a period from about 2.2–1.9 Ga, comparable to the age of the Loch Maree Group. The Ness Anorthosite, exposed on the northeastern tip of Lewis, is also found associated with metasediments and yields a similar Sm-Nd model age of about 2.2 Ga. It is considered possible that the South Harris and Ness bodies once formed part of

1035-492: The Loch Maree Group form two large areas of outcrop near Loch Maree and Gairloch in the Southern Region. The group consists of metasediments with intercalated amphibolites , interpreted to be metavolcanics with some basic sills . They were probably deposited at about 2.0 Ga, as they contain detrital zircons that give a mixture of Archaean and Paleoproterozoic ages. The Laxfordian was originally recognised from

1080-686: The Scourie complex cut by post-Scourian granites. Laxfordian reworking is extensive and very little unmodified Scourian crust has survived. Amphibolite sheets, interpreted to be deformed members of the Scourie Dykes, are much less common than on the mainland. More of the outcrop area consists of supracrustal rocks, about 5% of the total. The relationship between the supreacrustal rocks and the Scourian gneisses remains unclear. The South Harris igneous complex consists mainly of anorthosite and metagabbro , with lesser amounts of tonalitic and pyroxene-granulite gneisses. These igneous rocks are intruded into

1125-441: The Scourie dykes. This event deforms a suite of post-Badcallian pegmatites dated at 2.49-2.48 Ga and predates most of the Scourie dykes, giving a possible age range of approximately 2.48 - 2.42 Ga. The deformation was accompanied by retrograde metamorphism down to amphibolite facies, similar to the later Laxfordian event. Distinguishing between these two events has proved difficult. Major Inverian shear zones have been identified in

1170-591: The base of both the Morar Group and the overlying Loch Ness Supergroup, sometimes with well-preserved unconformable contacts, and these are generally accepted as forming part of the Lewisian, suggesting that the Lewisian complex extends at least as far southeast as the Great Glen Fault . Lewisian-like granitic gneisses of Paleoproterozoic age of the Rhinns complex are exposed on Islay and Colonsay in

1215-446: The coast the climate is maritime. The summers are mild and wet with average daytime highs under 17 °C, or 62.6 °F. The winters are cool at low elevation with average night-time lows below 0 °C, or 32 °F, but become colder and snowier inland and at higher elevations with the highest tops exhibiting a tundra climate. The Northwest Highlands are, on average, not as cold as the higher, snowier Cairngorm mountains which lie to

1260-401: The current scheme based upon numerical ages. Such a system could rely on events in the stratigraphic record and be demarcated by GSSPs . The Precambrian could be divided into five "natural" eons, characterized as follows: The movement of Earth's plates has caused the formation and break-up of continents over time, including occasional formation of a supercontinent containing most or all of

1305-483: The deformed Scourie dykes form amphibolite sheets within the reworked gneisses. The original mineralogy of the dykes is also changed to an amphibolite facies assemblage, even where they remain undeformed. The early Laxfordian fabrics are cut by a series of granites and pegmatites , particularly in the Northern and Southern Regions dated at 1.7 Ga. Much of the Lewisian outcrop of the Outer Hebrides consist of rocks of

1350-453: The early years of the Earth's existence, as radiometric dating has allowed absolute dates to be assigned to specific formations and features. The Precambrian is divided into three eons: the Hadean (4567.3–4031 Ma), Archean (4031-2500 Ma) and Proterozoic (2500-538.8 Ma). See Timetable of the Precambrian . It has been proposed that the Precambrian should be divided into eons and eras that reflect stages of planetary evolution, rather than

1395-705: The end of that time span, marking the beginning of the Phanerozoic Eon. By the middle of the following Cambrian Period, a very diverse fauna is recorded in the Burgess Shale , including some which may represent stem groups of modern taxa. The increase in diversity of lifeforms during the early Cambrian is called the Cambrian explosion of life. While land seems to have been devoid of plants and animals, cyanobacteria and other microbes formed prokaryotic mats that covered terrestrial areas. Tracks from an animal with leg-like appendages have been found in what

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1440-450: The entire length of the Outer Hebrides, a distance of about 200 km, dipping 20°–30° to the ESE. The fault rock within the fault zone shows a long and complex history of movement with the development of fault breccia , mylonite and pseudotachylite , indicating faulting at a wide range of crustal levels. Despite the multiple reworking that has affected Lewisian-like gneisses within

1485-493: The highest elevations. Naturally, the area would be a vast birch, pine and montane shrub forest, such as those surviving in Glen Affric . The region has steep, glacier-carved mountains, glens and interspersed plains. Many islands (which also vary widely in geography) lie off the coast. Elevations of around 750 metres (2,500 feet) or over are common, as are mountains exceeding 3,000 feet or 914 m ( Munros ). The highest point

1530-541: The landmass. The earliest known supercontinent was Vaalbara . It formed from proto-continents and was a supercontinent 3.636 billion years ago. Vaalbara broke up c. 2.845–2.803 Ga ago. The supercontinent Kenorland was formed c. 2.72 Ga ago and then broke sometime after 2.45–2.1 Ga into the proto-continent cratons called Laurentia , Baltica , Yilgarn craton and Kalahari . The supercontinent Columbia , or Nuna, formed 2.1–1.8 billion years ago and broke up about 1.3–1.2 billion years ago. The supercontinent Rodinia

1575-443: The main part of the swarm as about 2.4 Ga. Some dykes, which appear to have been intruded into cooler Scourian crust give ages of about 2.0 Ga, the same age as undated sills within the Loch Maree Group. Some of the main dyke suite show evidence of intrusion into hot country rock . Most of the dykes are quartz-dolerites in terms of chemistry, with less common olivine gabbro , norite and bronzite picrite . Supracrustal rocks of

1620-402: The mainland outcrop. The main metamorphic event in the Central Region was the 2.5 Ga granulite facies Badcallian event. The Northern Region lacks evidence of granulite facies and in the Southern Region an earlier 2.73 Ga event is recognised locally. This tectonic and metamorphic event postdates the main granulite facies metamorphic event in the Scourian complex but mostly predates intrusion of

1665-403: The metamorphic and structural development of the Lewisian as a series of discrete orogenic events that could be discerned in the field. They used a swarm of dolerite dykes, known as the Scourie dykes, as markers to separate the tectonic and metamorphic events into a Scourian event that occurred before the intrusion of the dykes and a later Laxfordian event that deformed and metamorphosed members of

1710-767: The metasediments of these two sequences, they show evidence of a common history, although with some important differences. The largest, the Glenelg -Attadale inlier , shows evidence of eclogite facies metamorphism within both of the tectonically juxtaposed units that make up the inlier, thought to be associated with crustal thickening during a Paleoproterozoic event at about 1.7 Ga and the Grenvillian orogenic event respectively. Precambrian The Precambrian ( / p r i ˈ k æ m b r i . ə n , - ˈ k eɪ m -/ pree- KAM -bree-ən, -⁠KAYM- ; or Pre-Cambrian , sometimes abbreviated pC , or Cryptozoic )

1755-425: The presence of deformation and metamorphism of the Scourie dykes. The Laxfordian can be divided into an early event before 1.7 Ga, associated with retrogression of the Scourie gneisses from granulite to amphibolite facies and a later event with local further retrogression to greenschist facies, part of which may be Grenvillian in age (about 1.1Ga). The early event is particularly associated with shear zones in which

1800-526: The same area. However, there is evidence that life could have evolved over 4.280 billion years ago. There is a fairly solid record of bacterial life throughout the remainder (Proterozoic Eon) of the Precambrian. Complex multicellular organisms may have appeared as early as 2100 Ma. However, the interpretation of ancient fossils is problematic, and "... some definitions of multicellularity encompass everything from simple bacterial colonies to badgers." Other possible early complex multicellular organisms include

1845-418: The same dyke swarm. Subsequent fieldwork, metamorphic studies and radiometric dating has refined their chronology but supported their original hypothesis. The oldest part of the Lewisian complex is a group of gneisses of Archaean age that formed in the interval 3.0–2.7 Ga. These gneisses are found throughout the outcrop of the Lewisian complex in the mainland. The dominant lithology of the Scourie complex

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1890-450: The southeast. However, on 30 December 1995, the UK's lowest temperature was recorded at Altnaharra, at −27.2 °C (−17.0 °F). This matched a similar recording at Braemar in the Grampians on 11 February 1895 and on 10 January 1982. Snow does not usually lie for significant periods at the coast or at low elevations but may lie for up to 105 days per year, persisting into the summer months, at

1935-730: The southern part of the Inner Hebrides. Similar rocks also outcrop on Inishtrahull off the north coast of County Donegal and in County Mayo where they are known as the 'Annagh Gneiss complex'. The first comprehensive account of the Lewisian complex was published in 1907 as part of the Geological Survey memoir The Geological Structure of the North-West Highlands of Scotland . In 1951 John Sutton and Janet Watson built on this work by interpreting

1980-482: Was lacking in free oxygen . There is, however, evidence that an oxygen-rich atmosphere existed since the early Archean. At present, it is still believed that molecular oxygen was not a significant fraction of Earth's atmosphere until after photosynthetic life forms evolved and began to produce it in large quantities as a byproduct of their metabolism . This radical shift from a chemically inert to an oxidizing atmosphere caused an ecological crisis , sometimes called

2025-852: Was mud 551 million years ago. The RNA world hypothesis asserts that RNA evolved before coded proteins and DNA genomes. During the Hadean Eon (4,567–4,031 Ma) abundant geothermal microenvironments were present that may have had the potential to support the synthesis and replication of RNA and thus possibly the evolution of a primitive life form. It was shown that porous rock systems comprising heated air-water interfaces could allow ribozyme - catalyzed RNA replication of sense and antisense strands that could be followed by strand-dissociation, thus enabling combined synthesis, release and folding of active ribozymes. This primitive RNA replicative system also may have been able to undergo template strand switching during replication ( genetic recombination ) as

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