The Great Estuarine Group is a sequence of rocks

Group (stratigraphy)

A group in stratigraphy is a lithostratigraphic unit, a part of the geologic record or rock column that consists of defined rock strata. Groups are divided into formations and are sometimes themselves grouped into "supergroups"....

 which outcrop around the coast of the West Highlands

Scottish Highlands

The Highlands is an historic region of Scotland. The area is sometimes referred to as the "Scottish Highlands". It was culturally distinguishable from the Lowlands from the later Middle Ages into the modern period, when Lowland Scots replaced Scottish Gaelic throughout most of the Lowlands...

 of Scotland

Scotland

Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...

. Laid down in the Hebrides Basin during the middle Jurassic

Jurassic

The Jurassic is a geologic period and system that extends from about Mya to  Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...

, they are the rough time equivalent of the Inferior

Inferior Oolite

The Inferior Oolite is a geological formation in Europe. It dates back to the Middle Jurassic.-Formation:The Inferior Oolite is an Oolitic limestone, a Sedimentary rock, meaning it was formed during lithication. The Inferior Oolite is rich in organic material...

 and Great Oolite Groups found in southern England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

.

This sequence of rocks was originally named as the ‘Great Estuarine Series’ by the geologist

Geologist

A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...

, John Wesley Judd

John Wesley Judd

John Wesley Judd was a British geologist.He was born in Portsmouth the son of George and Jannette Judd and educated at the Royal School of Mines, where he later became Professor of Geology....

 in 1878. There are outcrops on the islands of Skye

Skye

Skye or the Isle of Skye is the largest and most northerly island in the Inner Hebrides of Scotland. The island's peninsulas radiate out from a mountainous centre dominated by the Cuillin hills...

, Raasay

Raasay

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. It is most famous for being the birthplace of the poet Sorley MacLean, an important figure in the Scottish literary renaissance...

, Eigg

Eigg

Eigg is one of the Small Isles, in the Scottish Inner Hebrides. It lies to the south of the Skye and to the north of the Ardnamurchan peninsula. Eigg is long from north to south, and east to west. With an area of , it is the second largest of the Small Isles after Rùm.-Geography:The main...

, Muck

Muck, Scotland

Muck is the smallest of four main islands in the Small Isles, part of the Inner Hebrides of Scotland. It measures roughly 2.5 miles east to west and has a population of around 30, mostly living near the harbour at Port Mòr. The other settlement on the island is the farm at Gallanach...

 and Mull

Isle of Mull

The Isle of Mull or simply Mull is the second largest island of the Inner Hebrides, off the west coast of Scotland in the council area of Argyll and Bute....

 and on the Ardnamurchan

Ardnamurchan

Ardnamurchan is a peninsula in Lochaber, Highland, Scotland, noted for being very unspoilt and undisturbed. Its remoteness is accentuated by the main access route being a single track road for much of its length.-Geography:...

 peninsula. It comprises a series of shale

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

s, clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

s and sandstone

Sandstone

Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

s of non-marine origin.

The Group overlies the Garantiana Mudstone of the 'Bearreraig Sandstone Formation' and is itself overlain by rocks of the 'Skye Lava Group', erupted during the Palaeocene.

The lowermost (and hence oldest) unit of the Great Estuarine Group is the ‘Cullaidh Shale Formation’. Overlying this is the 'Elgol Sandstone Formation', the type locality of which is to be found at the village of Elgol

Elgol

Elgol is a village on the shores of Loch Scavaig towards the end of the Strathaird peninsula in the Isle of Skye, in the Scottish Highlands.-Name:...

 on Skye. These sandstones are interpreted as being deltaic

River delta

A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...

 in origin. Above the sandstone is the 'Lealt Shale Formation', a unit in which fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

s of creatures which lived in brackish

Brackish water

Brackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak," meaning "salty"...

 lagoons abound. These include mussels, sharks and, famously plesiosaur

Plesiosaur

Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...

s. The discovery of the first plesiosaur was made in 1844 by the geologist Hugh Miller

Hugh Miller

Hugh Miller was a self-taught Scottish geologist and writer, folklorist and an evangelical Christian.- Life and work :Born in Cromarty, he was educated in a parish school where he reportedly showed a love of reading. At 17 he was apprenticed to a stonemason, and his work in quarries, together with...

. The ‘Kildonnan Member’ of the Lealt Shale Formation contains stromatolites and various microfossils such as dinoflagellate

Dinoflagellate

The dinoflagellates are a large group of flagellate protists. Most are marine plankton, but they are common in fresh water habitats as well. Their populations are distributed depending on temperature, salinity, or depth...

s and acritarch

Acritarch

Acritarchs are small organic fossils, present from approximately to the present. Their diversity reflects major ecological events such as the appearance of predation and the Cambrian explosion.-Definition:In general, any small, non-acid soluble Acritarchs are small organic fossils, present from...

s. The succeeding ‘Lonfearn Member’ consist of shales and thin shelly and oolitic

Oolite

Oolite is a sedimentary rock formed from ooids, spherical grains composed of concentric layers. The name derives from the Hellenic word òoion for egg. Strictly, oolites consist of ooids of diameter 0.25–2 mm; rocks composed of ooids larger than 2 mm are called pisolites...

 limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

s with conchostracan fossils. It has also yielded dinosaur

Dinosaur

Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...

 footprints.

At the next stratigraphic

Stratigraphy

Stratigraphy, a branch of geology, studies rock layers and layering . It is primarily used in the study of sedimentary and layered volcanic rocks....

 level the ‘Valtos Sandstone Formation’ represents a further series of deltaic sandstones. Large calcareous

Calcareous

Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...

 concretion

Concretion

A concretion is a volume of sedimentary rock in which a mineral cement fills the porosity . Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from the Latin con meaning 'together' and crescere meaning 'to grow'...

s commonly occur within this formation. They are post-depositional in origin with individual nodules reaching more than a metre in diameter and cutting across the bedding

Bedding

Bedding refers to the materials laid above the mattress of a bed for hygiene, warmth, to protect the mattress, and for decorative effect. Bedding is the removable and washable portion of a human sleeping environment. It is more easily and economically replaced than the bed itself...

. The ‘Duntulm Formation' succeeds the Valtos Formation and is in turn succeeded by the ‘Kilmaluag Formation’, and then by the youngest unit of the Great Estuarine Group, the ‘Skudiburgh Formation’.