The Argyll Group is a thick sequence of

metamorphosed Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is prec ...

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s that outcrop across the

Central Highlands of Scotland The Grampian Mountains (''Am Monadh'' in Gaelic) is one of the three major mountain ranges in Scotland, that together occupy about half of Scotland. The other two ranges are the Northwest Highlands and the Southern Uplands. The Grampian rang ...

, east of the

Great Glen The Great Glen ( gd, An Gleann Mòr ), also known as Glen Albyn (from the Gaelic "Glen of Scotland" ) or Glen More (from the Gaelic ), is a glen in Scotland running for from Inverness on the edge of Moray Firth, in an approximately straight ...

, as well as appearing in the north of Ireland. It is a subdivision of the Dalradian Supergroup and is itself divided into four units; from oldest to youngest these are the Islay, Easdale, Crinan and Tayvallich subgroups.

Islay Subgroup

The lower boundary of the Islay Subgroup and hence the Argyll Group as a whole is defined at the base of the

Port Askaig Port Askaig ( gd, Port Asgaig) is a port village on the east coast of the island of Islay, in Scotland. The village lies on the Sound of Islay (Caol Ìle) across from Jura. Economy Port Askaig has a hotel, a petrol station and shop next to the ...

tillite image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...

, a diamictite which displays limestone clasts overlain by quartzite and granite clasts. The tillite is thickest in its type area and on the

Garvellachs The Garvellachs (Scottish Gaelic: Garbh Eileaich) or Isles of the Sea form a small archipelago in the Inner Hebrides of Scotland. Part of the Argyll and Bute council area, they lie west of Lunga and northwest of Scarba and have been uninhabited ...

. The

Schiehallion Schiehallion (; gd, Sìth Chailleann, ) is a prominent cone-shaped mountain in the Breadalbane region of the Scottish Highlands, in the county of Perthshire. It rises to and is classed as a Munro. Schiehallion has a rich botanical life, int ...

Boulder Bed of the eastern Grampians correlates with this tillite. The tillite is overlain by the Bonahaven Dolomite and then the Jura

Quartzite Quartzite is a hard, non- foliated metamorphic rock which 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 tec ...

which can reach up to 5 km thick but which is more typically between 500 and 1000m thick. Psammites and pelites occur within the sequence in the

Ladder Hills The Ladder Hills ( gd, Monadh an Fhàraidh) are a range of hills in northeast Scotland which straddle the border between Aberdeenshire and Moray and form a part of the Grampian Mountains. The highest point of the range is Càrn Mòr at , which is ...

area.

Easdale Subgroup

The Easdale Subgroup consists of a basal

Scarba Scarba ( gd, Sgarba) is an island, in Argyll and Bute, Scotland, just north of the much larger island of Jura. The island was owned by Richard Hill, 7th Baron Sandys, and has not been permanently inhabited since the 1960s. It is now covered in ...

Conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** ...

overlain by the

Craignish Craignish (Scottish Gaelic, ''Creiginis'') is a peninsula in Argyll, on the west coast of Scotland. It lies around south of Oban, and north-west of Lochgilphead. The peninsula is around long, and is aligned along a north-east to south-west orie ...

Phyllite Phyllite ( ) is a type of foliated metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica achieves a preferred orientation.Stephen Marshak ''Essentials of Geology'', 3rd ed. It is primarily compo ...

. It also includes pelites, semipelites, psammites and some

volcaniclastic Volcaniclastics are geologic materials composed of broken fragments ( clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...

rocks. Extending between Jura, Knapdale and the

Moray Firth The Moray Firth (; Scottish Gaelic: ''An Cuan Moireach'', ''Linne Mhoireibh'' or ''Caolas Mhoireibh'') is a roughly triangular inlet (or firth) of the North Sea, north and east of Inverness, which is in the Highland council area of north of Sc ...

coast, the unit can reach to 3 km thick.

Crinan Subgroup

The Crinan Subgroup is formed by the

Crinan Crinan is a name of Gaelic origin and it has a number of contexts: * Crinan, Argyll Crinan ( gd, An Crìonan) is a small village located on the west coast of Scotland in the region known as Knapdale, which is part of Argyll and Bute. Before the ...

Grits Formation, a

submarine fan Abyssal fans, also known as deep-sea fans, underwater deltas, and submarine fans, are underwater geological structures associated with large-scale sediment deposition and formed by turbidity currents. They can be thought of as an underwater ver ...

deposit. These rocks which are between 1.5 and 3 km thick, extend the width of Scotland from

Kintyre Kintyre ( gd, Cinn Tìre, ) is a peninsula in western Scotland, in the southwest of Argyll and Bute. The peninsula stretches about , from the Mull of Kintyre in the south to East and West Loch Tarbert in the north. The region immediately nor ...

Fraserburgh Fraserburgh (; sco, The Broch or ; gd, A' Bhruaich) is a town in Aberdeenshire, Scotland with a population recorded in the 2011 Census at 13,100. It lies at the far northeast corner of Aberdeenshire, about north of Aberdeen, and north of ...

. Their base is characterised by a coarse quartzite in

Argyll Argyll (; archaically Argyle, in modern Gaelic, ), sometimes called Argyllshire, is a historic county and registration county of western Scotland. Argyll is of ancient origin, and corresponds to most of the part of the ancient kingdom of ...

, otherwise it is formed from pelites, semipelites and psammites.

Tayvallich Subgroup

The

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. Li ...

of this subgroup varies across Scotland but is dominantly limestone though this is replaced in the east by

psammite Psammite ( Greek: ''psammitēs'' "(made) from sand", from ''psammos'' "sand") is a general term for sandstone. It is equivalent to the Latin-derived term areniteU.S. Bureau of Mines Staff (1996) ''Dictionary of Mining, Mineral, & Related Terms.'' ...

and

quartzite Quartzite is a hard, non- foliated metamorphic rock which 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 tec ...

Lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or ...

s and pelites are seen around

Tayvallich Tayvallich (pronounced ; gd, Taigh a' Bhealaich ) is a small village in the Knapdale area of Argyll and Bute, in Scotland. The village name has its origins in Gaelic, and means the "house of the pass". The village is built around a sheltered harbo ...

in Knapdale, after which area the subgroup is named, where it consists of the Tayvallich

Slate Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. It is the finest grained foliated metamorphic rock. ...

and

Limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

Formation and the overlying Tayvallich

Volcanic A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often found where tectonic plat ...

Formation. The succession is between 100 and 250m thick but can reach up to 2.5 km in the east where the psammites and semipelites occur. The

Loch Tay Loch Tay ( gd, Loch Tatha) is a freshwater loch in the central highlands of Scotland, in the Perth and Kinross and Stirling council areas. It is the largest body of fresh water in Perth and Kinross, and the sixth largest loch in Scotland. Th ...

Limestone Formation extends from

Campbeltown Campbeltown (; gd, Ceann Loch Chille Chiarain or ) is a town and former royal burgh in Argyll and Bute, Scotland. It lies by Campbeltown Loch on the Kintyre peninsula. Campbeltown became an important centre for Scotch whisky, and a busy fishin ...

Glen Isla The River Isla ( gd, Abhainn Ìle) is a tributary of the River Tay in Angus and Perthshire, Scotland. It runs for 46 miles (74 km) through the Kirkton of Glenisla (Clachan Ghlinn Ìle) and Strathmore (An Srath Mòr). Gallery File:Foot ...

.Strachan, R.A., Harris, A.L., Fettes, D.J. & Smith, M. 2002 The Highland and Grampian Terranes. ''In:'' Trewin, N.H. (ed) ''The Geology of Scotland.'' The Geological Society, London pp 96-99

References

{{Reflist Geology of Scotland