Torridonian Sandstone
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The Torridon Group is a series of
Tonian The Tonian (from , meaning "stretch") is the first geologic period of the Neoproterozoic era (geology), Era. It lasted from to Mya (million years ago). Instead of being based on stratigraphy, these dates are defined by the International Commissi ...
(lower
Neoproterozoic The Neoproterozoic Era is the last of the three geologic eras of the Proterozoic geologic eon, eon, spanning from 1 billion to 538.8 million years ago, and is the last era of the Precambrian "supereon". It is preceded by the Mesoproterozoic era an ...
)
arenaceous Arenite (from the Latin ''arena'', "sand") is a sedimentary clastic rock with sand grain size between 0.0625 mm (0.00245 in) and 2 mm (0.08 in) and containing less than 15% matrix. The related adjective is ''arenaceous''. The e ...
and
argillaceous Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay mineral ...
sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...
s, which occur extensively in the
Northwest Highlands The Northwest Highlands are located in the northern third of Scotland that is separated from the Grampian Mountains by the Great Glen (Glen More). The region comprises Wester Ross, Assynt, Sutherland and part of Caithness. The Caledonian Cana ...
of
Scotland Scotland is a Countries of the United Kingdom, country that is part of the United Kingdom. It contains nearly one-third of the United Kingdom's land area, consisting of the northern part of the island of Great Britain and more than 790 adjac ...
. These
strata In geology and related fields, a stratum (: strata) is a layer of Rock (geology), rock or sediment characterized by certain Lithology, lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by v ...
are particularly well exposed in the district of upper
Loch Torridon Loch Torridon () is a sea loch on the west coast of Scotland in the Northwest Highlands. The loch was created by glacial processes and is in total around 15 miles (25 km) long. It has two sections: Upper Loch Torridon to landward, east of Ru ...
, a circumstance which suggested the name Torridon Sandstone, first applied to these rocks by
James Nicol James Nicol, Nichol, Nicholl or Nicoll may refer to: * James Nicol (minister) (1769–1819), Scottish poet * James Dyce Nicol (1805–1872), Scottish politician * James Nicol (geologist) (1810–1879), Scottish geologist * James Henderson Nicol ...
.
Stratigraphically Stratigraphy is a branch of geology concerned with the study of rock layers (strata) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks. Stratigraphy has three related subfields: lithostrati ...
, they lie
unconformably An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...
on
gneiss Gneiss (pronounced ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. This rock is formed under p ...
es of the
Lewisian complex 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, North Atlantic Craton. These rocks are of ...
and sandstones of the lithologically similar
Mesoproterozoic The Mesoproterozoic Era is a geologic era that occurred from . The Mesoproterozoic was the first era of Earth's history for which a fairly definitive geological record survives. Continents existed during the preceding era (the Paleoproterozoic ...
Stoer Group The Stoer Group is a sequence of Mesoproterozoic sedimentary rocks that outcrops on the peninsula of Stoer, near Assynt, Sutherland. The dominant lithology is sandstone with breccias and conglomerate (geology), conglomerates developed near the bas ...
and their
outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets. Features Outcrops do not cover the majority of the Earth's land surface because in most p ...
extent is restricted to the
Hebridean terrane 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 Archa ...
.


Lithology

The dominant
lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
of the Torridon Group is red and brown
sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...
, often
arkosic Arkose () or arkosic sandstone is a detrital sedimentary rock, specifically a type of sandstone containing at least 25% feldspar. Arkosic sand is sand that is similarly rich in feldspar, and thus the potential precursor of arkose. Components ...
, with subsidiary amounts of
shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
, particularly towards the top of the sequence, with coarse conglomerates and
breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...
s locally at the base. Some of the materials of these rocks were derived from the underlying
Lewisian gneiss 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 Paleopr ...
, upon the uneven surface of which they rest, but the bulk of the material was sourced from rocks that are nowhere now exposed. Upon this ancient
denuded Denudation is the geological process in which moving water, ice, wind, and waves erode the Earth's surface, leading to a reduction in elevation and in relief of landforms and landscapes. Although the terms erosion and denudation are used interchang ...
land surface the Torridon Group sequence rests horizontally or with gentle dip. Some of the peaks, such as
Beinn Eighe is a mountain massif in the Torridon area of Wester Ross in the Northwest Highlands of Scotland. Lying south of Loch Maree, it forms a long ridge with many spurs and summits, two of which are classified as Munros: Ruadh-stac Mòr at ...
, are capped with white
Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...
quartzite Quartzite is a hard, non- foliated metamorphic rock that was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tecton ...
, giving them a distinctive appearance when seen from afar. Some of the quartzite contains
fossilized A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
worm burrows and is known as ''pipe rock'', which is approximately 500 million years old. The Torridon Group
landscape A landscape is the visible features of an area of land, its landforms, and how they integrate with natural or human-made features, often considered in terms of their aesthetic appeal.''New Oxford American Dictionary''. A landscape includes th ...
is itself highly denuded by
glacial A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...
and
alluvial Alluvium (, ) is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluvium is also sometimes called alluvial deposit. Alluvium is ...
action.


Occurrence

Their outcrop extends in a belt of variable breadth from
Cape Wrath Cape Wrath (, known as ' in Lewis) is a cape in the Durness parish of the county of Sutherland in the Highlands of Scotland. It is the most north-westerly point in Great Britain. The cape is separated from the rest of the mainland by the Ky ...
to the Point of the peninsula of
Sleat Sleat ( ) is a peninsula and civil parish on the island of Skye in the Highland council area of Scotland, known as "the garden of Skye". It is the home of the clan '' MacDonald of Sleat''. The name comes from the Scottish Gaelic , which in tur ...
in
Skye The Isle of Skye, or simply Skye, is the largest and northernmost of the major islands in the Inner Hebrides of Scotland. The island's peninsulas radiate from a mountainous hub dominated by the Cuillin, the rocky slopes of which provide some o ...
, running in a NNE–SSW direction through
Caithness Caithness (; ; ) is a Shires of Scotland, historic county, registration county and Lieutenancy areas of Scotland, lieutenancy area of Scotland. There are two towns, being Wick, Caithness, Wick, which was the county town, and Thurso. The count ...
,
Sutherland Sutherland () is a Counties of Scotland, historic county, registration county and lieutenancy areas of Scotland, lieutenancy area in the Scottish Highlands, Highlands of Scotland. The name dates from the Scandinavian Scotland, Viking era when t ...
,
Ross and Cromarty Ross and Cromarty (), is an area in the Highlands and Islands of Scotland. In modern usage, it is a registration county and a Lieutenancy areas of Scotland, lieutenancy area. Between 1889 and 1975 it was a Shires of Scotland, county. Historical ...
, and
Skye and Lochalsh Skye and Lochalsh () was a local government district, created in 1975 as one of eight districts within the Highland region in Scotland. It include the Isle of Skye and the Lochalsh area on the mainland. The main offices of the council were in P ...
. They form the isolated mountain peaks of
Canisp Canisp (Scottish Gaelic: ''Canasp'') is a mountain in the far north west of Scotland. It is situated in the parish of Assynt, in the county of Sutherland, north of the town of Ullapool. Canisp reaches a height of and qualifies as a Corbett a ...
,
Quinag Quinag () is an 808 m high mountain range in Sutherland in the Scottish Highlands, with an undulating series of peaks along its Y-shaped crest. The name Quinag is an anglicisation of the Gaelic name ''Cuinneag'', a milk pail, reflecting its ...
and
Suilven Suilven is a Graham mountain in Scotland. Lying in a remote area in the west of Sutherland, it rises from a wilderness landscape of moorland, bogs and lochans known as Inverpolly National Nature Reserve. Suilven forms a steep-sided ridge som ...
in the area of
Loch Assynt Loch Assynt () is a freshwater loch in Sutherland, Scotland, north-east of Lochinver. Situated in a spectacular setting between the heights of Canisp, Quinag, and , it receives the outflow from Loch Awe, Inchnadamph, Lochs Awe, , and Loch Leit ...
, of
Slioch Slioch () is a mountain in the Scottish Highlands situated in Wester Ross, eight kilometres north of the village of Kinlochewe. Slioch reaches an elevation of and towers above the southeastern end of Loch Maree to give one of the best known a ...
near
Loch Maree Loch Maree () is a loch in Wester Ross in the Northwest Highlands of Scotland. At long and with a maximum width of , it is the fourth-largest freshwater loch in Scotland; it is the largest north of Loch Ness. Its surface area is . Loch Maree c ...
, and other hills. They attain their maximum development in the
Applecross Applecross ( , 'The Sanctuary', historically anglicized as 'Combrich') is a peninsula in Wester Ross, in the Scottish Highlands. It is bounded by Loch Kishorn to the south, Loch Torridon to the north, and Glen Shieldaig to the east. On its wes ...
,
Gairloch Gairloch ( ; , meaning "Short Loch") is a village, civil parish and community on the shores of Loch Gairloch in Wester Ross, in the North-West Highlands of Scotland. A tourist destination in the summer months, Gairloch has a golf course, a ...
and
Torridon Torridon () is a small village in the Northwest Highlands of Scotland. The name is also applied to the area surrounding the village, particularly the Torridon Hills, mountains to the north of Glen Torridon. The village lies on the shore of Loch ...
districts, form the greater part of Scalpay, and occur also in
Rùm Rùm (), a Scottish Gaelic language, Scottish Gaelic name often Anglicisation, anglicised to Rum ( ), is one of the Small Isles of the Inner Hebrides, off the west coast of Scotland, in the district of Lochaber. For much of the 20th century the ...
,
Raasay Raasay (; ), sometimes the Isle of Raasay, is an island between the Isle of Skye and the mainland of Scotland. It is separated from Skye by the Sound of Raasay and from Applecross by the Inner Sound, Scotland, Inner Sound. It is famous fo ...
,
Soay Soay (pronounced "soy") is the name of several Scottish islands. It is Sòdhaigh (sometimes anglicised "Soaigh") in Scottish Gaelic, and comes from the Old Norse ''so-ey'' meaning "island of sheep". It may refer to: * Soay, Inner Hebrides off south ...
and the
Crowlin Islands The Crowlin Islands () are a group of uninhabited islands in the Inner Hebrides of Scotland. They lie between Skye and the Applecross peninsula on the mainland. The individual islands are: * Eilean Mòr (big island) * Eilean Meadhonach (middle ...
. They are also found beneath much of the
Sea of the Hebrides The Sea of the Hebrides (, ) is a small and partly sheltered section of the North Atlantic Ocean, indirectly off the southern part of the north-west coast of Scotland. To the east are the mainland of Scotland and the northern Inner Hebrides (i ...
overlying the Lewisian gneiss.C.Michael Hogan, (2011
''Sea of the Hebrides''
Eds. P. Saundry & C.J.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment. Washington DC.


Sub-divisions

The Torridon Group is divided into four formations, the Diabaig, Applecross, Aultbea and Cailleach Head formations. The Torridon Group infills an irregular land surface with up to 600 m of topography locally, cutting down through the previously deposited Stoer group sediments, resting in many areas directly on the Lewisian. It has been suggested that there is significant unconformity within this group, between the Diabaig and Applecross Formations.


Diabaig Formation

The lowest part of this formation consists of a basal breccia containing clasts derived from the underlying Lewisian complex with the thickest developments in the paleovalleys. The breccias pass vertically and laterally into tabular sandstones. These are locally channelised and interfinger with grey shales containing thin beds of fine-grained sandstone with wave rippled surfaces. The shales show the effects of desiccation with mudcracks preserved by being filled by overlying sandstone layers. In the upper part of the formation, beds of massive sandstone with sharp bases appear, becoming more common and thicker bedded towards the top. Ripple-drift lamination at the top of the sandstone layers indicates deposition from easterly-flowing currents. This sequence is interpreted to be to represent the progressive infill of the topography by
alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to Semi-arid climate, semiar ...
s building out into ephemeral lakes. The more massive beds are interpreted to be lake
turbidite A turbidite is the geologic Deposition (geology), deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean. Sequencing ...
s.


Applecross Formation

This formation consists of coarse sandstones, both trough and planar cross-bedded. The orientation of the troughs suggest a paleocurrent flowing from the Northwest. The sandstones carry a distinctive set of pebbles, including
jasper Jasper, an aggregate of microgranular quartz and/or cryptocrystalline chalcedony and other mineral phases, is an opaque, impure variety of silica, usually red, yellow, brown or green in color; and rarely blue. The common red color is due to ...
and
porphyry Porphyry (; , ''Porphyrios'' "purple-clad") may refer to: Geology * Porphyry (geology), an igneous rock with large crystals in a fine-grained matrix, often purple, and prestigious Roman sculpture material * Shoksha porphyry, quartzite of purple c ...
. Most of the sandstone beds are affected by soft-sediment deformation structures suggesting
liquefaction In materials science, liquefaction is a process that generates a liquid from a solid or a gas or that generates a non-liquid phase which behaves in accordance with fluid dynamics. It occurs both naturally and artificially. As an example of t ...
, possibly as a result of
seismic activity An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
. The uppermost part of the formation consists of finer-grained sandstones, transitional to those of the overlying Aultbea Formation. At
Cape Wrath Cape Wrath (, known as ' in Lewis) is a cape in the Durness parish of the county of Sutherland in the Highlands of Scotland. It is the most north-westerly point in Great Britain. The cape is separated from the rest of the mainland by the Ky ...
the basal part of the formation shows a fanning of paleocurrent directions consistent with deposition from a large alluvial fan (~40 km radius) with its apex near the Minch Fault. The source area for this fan has been calculated as about 10,000 km2.


Aultbea Formation

This formation is similar to the Applecross formation except that the sandstones are fine to medium-grained and there are very few pebbles. Almost all of these sandstone beds show the contortions shown by the older formation. The Applecross and Aultbea Formations together consist of an overall fining-upward sequence of sandstones. Only the outcrops at
Cape Wrath Cape Wrath (, known as ' in Lewis) is a cape in the Durness parish of the county of Sutherland in the Highlands of Scotland. It is the most north-westerly point in Great Britain. The cape is separated from the rest of the mainland by the Ky ...
described above have a consistent radial pattern suggesting that the sequence was deposited in a bajada environment, by a series of smaller fans merging to form a braided river system.


Cailleach Head Formation

This formation is similar to the underlying Aultbea Formation, the main difference being in grain size, with this formation being noticeably finer-grained. The sequence is made up of 22 m thick cycles, each with a basal
erosion surface In geology and geomorphology, an erosion surface is a surface of rock (geology), rock or regolith that was formed by erosion and not by construction (e.g. lava flows, sediment deposition) nor fault (geology), fault displacement. Erosional surfaces ...
followed by dark grey shales with desiccation cracks, planar cross-bedded sandstones with wave rippled tops, overlain by trough cross-bedded
mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...
ceous sandstones. These cycles are thought to represent repeated progradation of deltas into a lake. A lack of
evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as oce ...
minerals suggest that the lakes had through drainage.
Acritarch Acritarchs () are organic microfossils, known from the Precambrian to the present. The classification is a catch all term used to refer to any organic microfossils that cannot be assigned to other groups. Their diversity reflects major ecolog ...
microfossils A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, ...
were described from here by Teall in 1907, the first
Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) 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 t ...
fossils described in Britain.


Age

The upper age limit for the deposition of this sequence is constrained by the age of the last tectonic and metamorphic event to affect the Lewisian complex and the depositional age of the Stoer Group on which it was deposited, for which ages cluster between about 1200–1100 Ma. The lower limit is provided by the age of the lower Cambrian quartzite that lies above it, about 544 Ma. Radiometric ages from the basal part of the Torridon Group sequence itself give ages of about 1000–950 Ma. This implies an age gap of at least 200 Ma between the deposition of the Stoer and Torridon groups, consistent with the paleomagnetic evidence of a major break. Ages of detrital zircons also provide some constraints on the sequence age. The lower part of the Sleat Group show ages consistent with derivation from Scourian and to a lesser extent Laxfordian rocks, with no dates after 1700 Ma. The upper part of the Sleat Group includes a large component of broadly Laxfordian age with almost no Archaean ages, with a lower limit of about 1200 Ma. In contrast the Diabaig Formation shows a small group clustered around 1100 Ma, the age of the
Grenville Orogeny The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, ...
. In the Applecross and Aultbea Formations there are many more zircons giving ages around 1100 Ma and even below 1000 Ma.


Depositional setting

Variations in thickness and
lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
in the Sleat and Torridon Groups were interpreted as reflecting deposition in a rift setting. Evidence from
seismic reflection Reflection seismology (or seismic reflection) is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth's subsurface from reflected seismic waves. The method requires a controlled seismi ...
data in the
Minch The Minch () is a strait in north-west Scotland that separates the mainland from Lewis and Harris in the Outer Hebrides. It was known as ("Scotland's firth") in Old Norse. The Minch's southern extension, which separates Skye from the middle ...
suggests that the Minch Fault was active throughout the deposition of the Torridon Group. This is consistent with the generally westerly derived pebbly material throughout the thickness of the Applecross Formation, suggesting a constantly rejuvenated sediment source in that direction. More recent work has suggested that although the Sleat Group was probably deposited in a rift setting, the scale and continuity of the Torridon Group, particularly the Applecross and Aultbea Formations, is more consistent with a
molasse __NOTOC__ In geology, "molasse" () are sandstones, shales and conglomerates that form as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse deposits accumulate in a foreland basin, especially on top of flys ...
type setting possibly related to the
Grenville Orogeny The Grenville orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, ...
, within a
foreland basin A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...
.


See also

*
Torridonian 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 sou ...


References

{{commons category Sandstone in the United Kingdom Geology of Scotland Caithness Sutherland Ross and Cromarty Skye and Lochalsh Stratigraphy of the United Kingdom Torridon