The Moinian or just the Moine, formerly the Moine Supergroup, is a sequence of Neoproterozoic metasediments that outcrop in the Northwest Highlands between the Moine Thrust Belt to the northwest and the Great Glen Fault to the southeast and one part of the Grampian Highlands to the southeast of the fault. It takes its name from A' Mhòine , a peat bog in northern Sutherland. [1]
The metamorphic rocks that are now known informally as "the Moine" were originally interpreted as of Silurian age, as they lie in sequence with Cambrian to lower Silurian sedimentary rocks (now known to be lower Ordovician at the youngest and part of the Ardvreck Group). This view, espoused particularly by Roderick Murchison, a geologist known as the "Master of the Silurian", was opposed by James Nicol, who thought that the contact (or "zone of complication" as he called it) was tectonic in nature and that the metamorphic rocks were older and not in stratigraphic sequence with those below. This difference of opinion led to the Highlands controversy, which pitted groups of geologists against each other, particularly between the Geological Survey and academic researchers. [2] It was eventually settled in favour of Nicol's view with the recognition of the Moine Thrust. [3]
During the controversy, the metamorphic rocks above the contact were known as "gneisses and slates" or "gneissose schists". In the classic memoir The Geological Structure of the North-west Highlands of Scotland, Peach and Horne and their colleagues used the terms "eastern schists" and "Moine schists" interchangeably. [3] Eventually these rocks, now generallyknown as the "Moine Series" were mapped all the way to the Great Glen Fault in areas not covered by the Old Red Sandstone. [4]
The Moine was not subdivided in a regional way until 1969, when three units were proposed; the Morar, Glenfinnan and Loch Eil divisions. [5] The contact at the base of the Glenfinnan division against the Morar division was identified as tectonic in 1970, the Sgurr Beag Slide (now known as the Sgurr Beag Thrust). [6] At about the same time, a pre-Caledonian metamorphic and tectonic event affecting the Moine was identified, known as the "Morarian" or "Knoydartian". [7] By 1994 the term "Moine Supergroup" was defined to include the Morar, Glenfinnan and Loch Eil groups but not to include the Grampian Highlands sequences to the southeast of the Great Glen Fault that had been proposed as part of the Moine. [8]
Similarities in lithology and sedimentary facies between the Torridon Group and the Morar Group had long been noted. A normal stratigraphic contact between the Morar and Glenfinnan groups was interpreted on the Ross of Mull, making it the only non-tectonic contact between the groups, which elsewhere is formed by the Sgurr Beag Thrust. A re-examination of this boundary has, however, identified a zone of high strain, meaning that the boundary is everywhere tectonic across the Highlands. Detrital zircon geochronology, combined with the dating of igneous intrusions and metamorphic events, has supported the Torridon to Morar correlation. The Glenfinnan and Loch Eil groups (and the correlative Badenoch Group) all contain detrital zircons that give ages younger than 950 Ma, the timing of the Renlandian Orogeny, which is known to have affected the Morar Group. [9]
These observations have led to a revised stratigraphy for the Neoproterozoic sequences of the Highlands. An older Wester Ross Supergroup has been defined, consisting of the Torridon, Morar, Sleat, Iona groups and possibly the Tarskavaig Group and various groups on Shetland. This sequence was deposited in the interval 1000–960 Ma. A younger Loch Ness Supergroup has been defined, consisting of the Glenfinnan, Loch Eil and Badenoch groups. This sequence was deposited in the interval 900–870 Ma, after the Renlandian Orogeny and before the Knoydartian Orogeny. [9]
The Moine Thrust Belt or Moine Thrust Zone is a linear tectonic feature in the Scottish Highlands which runs from Loch Eriboll on the north coast 190 kilometres (120 mi) south-west to the Sleat peninsula on the Isle of Skye. The thrust belt consists of a series of thrust faults that branch off the Moine Thrust itself. Topographically, the belt marks a change from rugged, terraced mountains with steep sides sculptured from weathered igneous, sedimentary and metamorphic rocks in the west to an extensive landscape of rolling hills over a metamorphic rock base to the east. Mountains within the belt display complexly folded and faulted layers and the width of the main part of the zone varies up to 10 kilometres (6.2 mi), although it is significantly wider on Skye.
The Highland Boundary Fault is a major fault zone that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east. It separates two different geological terranes which give rise to two distinct physiographic terrains: the Highlands and the Lowlands, and in most places it is recognisable as a change in topography. Where rivers cross the fault, they often pass through gorges, and the associated waterfalls can be a barrier to salmon migration.
The Caledonian orogeny was 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. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly 490–390 million years ago (Ma). It was caused by the closure of the Iapetus Ocean when the Laurentia and Baltica continents and the Avalonia microcontinent collided.
The Dalradian Supergroup is a stratigraphic unit in the lithostratigraphy of the Grampian Highlands of Scotland and in the north and west of Ireland. The diverse assemblage of rocks which constitute the supergroup extend across Scotland from Islay in the west to Fraserburgh in the east and are confined by the Great Glen Fault to the northwest and the Highland Boundary Fault to the southeast. Much of Shetland east of the Walls Boundary Fault is also formed from Dalradian rocks. Dalradian rocks extend across the north of Ireland from County Antrim in the north east to Clifden on the Atlantic coast, although obscured by later Palaeogene lavas and tuffs or Carboniferous rocks in large sections.
The Torridon Group is a series of Tonian arenaceous and argillaceous sedimentary rocks, which occur extensively in the Northwest Highlands of Scotland. These strata of the are particularly well exposed in the district of upper Loch Torridon, a circumstance which suggested the name Torridon Sandstone, first applied to these rocks by James Nicol. Stratigraphically, they lie unconformably on gneisses of the Lewisian complex and sandstones of the lithologically similar Mesoproterozoic Stoer Group and their outcrop extent is restricted to the Hebridean Terrane.
The Northwest Highlands are located in the northern third of Scotland that is separated from the Grampian Mountains by the Great Glen. The region comprises Wester Ross, Assynt, Sutherland and part of Caithness. The Caledonian Canal, which extends from Loch Linnhe in the south-west, via Loch Ness to the Moray Firth in the north-east splits this area from the rest of the country. The city of Inverness and the town of Fort William serve as gateways to the region from the south.
Knockan Crag lies within the North West Highlands Geopark in the Assynt region of Scotland 21 kilometres (13 mi) north of Ullapool. During the nineteenth century Knockan Crag became the subject of much debate when geologists noted that the Moine schists at the top of the crag appeared to be older than the Cambrian and Ordovician rocks such as Durness limestone lower down. Disagreements over the processes that could have caused this to occur were referred to at the time as the "Highlands Controversy". The argument was primarily between Roderick Murchison and Archibald Geikie on the one hand and James Nicol and Charles Lapworth on the other. Murchison and Geikie believed the sequence was wrong and that the Moine schists must be the younger rocks. The controversy was finally resolved by the work of Ben Peach and John Horne whose 1907 paper on the subject remains a classic text. Peach and Horne demonstrated that the situation resulted from the action of a thrust fault - this being the first to be discovered anywhere in the world. The older rocks had been moved some 70 kilometres to the west over the top of the younger rocks due to tectonic action.
The Lewisian complex or Lewisian gneiss 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 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 Hebridean Terrane is one of the terranes that form part of the Caledonian orogenic belt in northwest Scotland. Its boundary with the neighbouring Northern Highland Terrane is formed by the Moine Thrust Belt. The basement is formed by Archaean and Paleoproterozoic gneisses of the Lewisian complex, unconformably overlain by the Neoproterozoic Torridonian sediments, which in turn are unconformably overlain by a sequence of Cambro–Ordovician sediments. It formed part of the Laurentian foreland during the Caledonian continental collision.
The Colonsay Group is an estimated 5,000 m thick sequence of mildly metamorphosed Neoproterozoic sedimentary rocks that outcrop on the islands of Colonsay, Islay and Oronsay and the surrounding seabed. They have been correlated with the Grampian Group, the oldest part of the Dalradian Supergroup.
The Badenoch Group is a sequence of metamorphosed Tonian age sedimentary rocks that outcrop across the Central Highlands of Scotland, east of the Great Glen. This rock sequence has formerly been referred to as the Central Highland Migmatite Complex and the Central Highland Division.
The Highlands controversy was a scientific controversy which started between British geologists in the middle of the nineteenth century concerning the nature of the rock strata in the Northwest Highlands of Scotland. The debate became contentious, even acrimonious, because of some of the personalities involved and because it pitted professional geologists of the Geological Survey against academic and amateur geologists. An initial resolution was achieved by about 1886 but the great complexity and scientific importance of the discovery of the Moine Thrust Belt and the geological processes involved in its creation led to field work continuing for a further twenty years culminating in the 1907 publication by the Geological Survey of a book of fundamental geological significance: The Geological Structure of the North-West Highlands of Scotland.
This article describes the geology of the Cairngorms National Park, an area in the Highlands of Scotland designated as a national park in 2003 and extended in 2010. The Cairngorms National Park extends across a much wider area than the Cairngorms massif itself and hence displays rather more varied geology.
The Torridonian is the informal name given to a sequence of Mesoproterozoic to Neoproterozoic sedimentary rocks that outcrop in a strip along the northwestern coast of Scotland and some parts of the Inner Hebrides from the Isle of Mull in the southwest to Cape Wrath in the northeast. They lie unconformably on the Archaean to Paleoproterozoic basement rocks of the Lewisian complex and unconformably beneath the Cambrian to Lower Ordovician rocks of the Ardvreck Group.
The Loch Ness Supergroup is one of the subdivisions of the Neoproterozoic sequence of sedimentary rocks in the Scottish Highlands. It is found everywhere in tectonic contact above the older Wester Ross Supergroup. It is thought to be unconformably overlain by the Cryogenian to Cambrian Dalradian Supergroup.
The Wester Ross Supergroup is one of the subdivisions of the Neoproterozoic sequence of sedimentary rocks in the Scottish Highlands. It lies unconformably on medium to high-grade metamorphic rocks and associated igneous rocks of the Archaean and Paleoproterozoic age Lewisian complex or locally over the Mesoproterozoic sedimentary rocks of the Stoer Group. The contact between the Wester Ross Supergroup and the next youngest of the Neoproterozoic sequences in the Scottish Highlands, the Loch Ness Supergroup, is everywhere a tectonic one.
The Knoydartian Orogeny is a Tonian tectonic and metamorphic event, or group of events, that is recognised in the rocks of the Wester Ross and Loch Ness supergroups of the Scottish Highlands. It is dated to about 820–725 Ma, predating the deposition of the Cryogenian to Cambrian Dalradian Supergroup. It is named after Knoydart, one of the localities where the event was first recognised.
The Renlandian Orogeny is a Tonian tectonic and metamorphic event that is found in East Greenland, on Svalbard, on Ellesmere Island and in Scotland. It takes its name from Renland in East Greenland, where the event was first recognised.
The Sgurr Beag Thrust is an important tectonic structure within the Neoproterozoic metasedimentary sequences of the Scottish Highlands. The thrust, or similar structures correlated with it, form the boundary between rocks of the Glennfinnan Group and the underlying Morar Group. It divides the Wester Ross Supergroup from the Loch Ness Supergroup. The history of this structure remains poorly understood although it is thought to be at least partly of Caledonian age.
The Morar Group is a sequence of Tonian sedimentary rocks that have been subjected to a series of tectonic and metamorphic events since their deposition. Originally interpreted to be lowest (oldest) part of a "Moine Supergroup", this sequence now forms part of the Wester Ross Supergroup. They lie unconformably on Archean to Paleoproterozoic basement of the Lewisian complex. The contact with the overlying Glenfinnan Group of the Loch Ness Supergroup is everywhere a tectonic one, formed by the Sgurr Beag Thrust or related structures.