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 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 ...

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 that outcrop across the Central Highlands of Scotland, east of the

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

, 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 Tillite Formation, 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: ''Na Garbh Eileacha'') or Isles of the Sea form a small archipelago in the Inner Hebrides of Scotland. The islands include Garbh Eileach, Dùn Chonnuill and Eileach an Naoimh. Part of the Argyll and Bute counc ...

. The

Schiehallion Schiehallion (; , ) is a prominent cone-shaped mountain in the Breadalbane, Scotland, Breadalbane region of the Scottish Highlands, in the county of Perthshire. It rises to and is classed as a Munro. Schiehallion has a rich flora, interesting ...

Boulder Bed of the eastern

Grampians The Grampian Mountains () 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 range extends northeast to so ...

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 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 ...

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 area.

Easdale Subgroup

The Easdale Subgroup consists of a basal

Scarba Scarba () is an island, in Argyll and Bute, Scotland, just north of the much larger island of Jura. The island has not been permanently inhabited since the 1960s. Until his death in 2013 it was owned by Richard Hill, 7th Baron Sandys; its ...

Conglomerate overlain by the

Craignish Craignish (Scottish Gaelic, ''Creiginis'') is a peninsula in Argyll and Bute, 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 ...

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

. 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 Knapdale (, ) forms a rural district of Argyll and Bute in the Scottish Highlands, adjoining Kintyre to the south, and divided from the rest of Argyll to the north by the Crinan Canal. It includes two parishes, North Knapdale and South Knapdale. ...

and the

Moray Firth The Moray Firth (; , or ) is a roughly triangular inlet (or firth) of the North Sea, north and east of Inverness, which is in the Highland council area of the north of Scotland. It is the largest firth in Scotland, stretching from Duncans ...

coast, the unit can reach to 3 km thick.

Crinan Subgroup

The Crinan Subgroup is formed by the Crinan Grits Formation, a submarine fan deposit. These rocks which are between 1.5 and 3 km thick, extend the width of Scotland from

Kintyre Kintyre (, ) 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 Loch Tarbert, Argyll, East and West Loch Tarbert, Argyll, West Loch Tarbert in t ...

Fraserburgh Fraserburgh (; ), locally known as the Broch, is a town in Aberdeenshire (unitary), Aberdeenshire, Scotland, with a population recorded in the United Kingdom Census 2011, 2011 Census as 13,100. It lies in Buchan in the northeastern corner of th ...

. Their base is characterised by a coarse quartzite in

Argyll Argyll (; archaically Argyle; , ), sometimes called Argyllshire, is a Shires of Scotland, historic county and registration county of western Scotland. The county ceased to be used for local government purposes in 1975 and most of the area ...

, 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. Lit ...

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 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 ...

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 Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

s and pelites are seen around

Tayvallich Tayvallich (pronounced ; ) is a small village in the Knapdale area of Argyll and Bute, west of Scotland. The village name has its origins in Gaelic, and means the "house of the pass". The village is built around a sheltered harbour on Loch Sw ...

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 ro ...

and

Limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

Formation and the overlying Tayvallich

Volcanic A volcano is commonly defined as a vent or fissure 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 fo ...

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 () is a freshwater loch in the central Scottish Highlands, highlands of Scotland, in the Perth and Kinross and Stirling (council area), Stirling Subdivisions of Scotland#Council areas, council areas, the largest body of fresh water in ...

Limestone Formation extends from

Campbeltown Campbeltown (; 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 fishing port. The 2018 populatio ...

Glen Isla The River Isla () is a tributary of the River Tay in Angus and Perthshire, Scotland. It rises in the Grampians before flowing down through Glen Isla into the Valley of Strathmore. After a course of 46 miles (74 km), it falls into the Ta ...

.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