The Lias Group or Lias is a lithostratigraphic unit (a sequence of

rock strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as e ...

) found in a large area of western Europe, including the

British Isles The British Isles are a group of islands in the North Atlantic Ocean off the north-western coast of continental Europe, consisting of the islands of Great Britain, Ireland, the Isle of Man, the Inner and Outer Hebrides, the Northern Isl ...

, the

North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian ...

, the

Low Countries The term Low Countries, also known as the Low Lands ( nl, de Lage Landen, french: les Pays-Bas, lb, déi Niddereg Lännereien) and historically called the Netherlands ( nl, de Nederlanden), Flanders, or Belgica, is a coastal lowland region in N ...

and the north of

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwee ...

. It consists of

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...

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 ...

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

marl Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. Marl makes up the lower part ...

s and

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay pa ...

s. ''Lias'' is a Middle English term for hard limestone, used in this specific sense by geologists since 1833. In the past, geologists used ''Lias'' not only for the sequence of rock layers, but also for the timespan during which they were formed. It was thus an alternative name for the

Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-J ...

epoch In chronology and periodization, an epoch or reference epoch is an instant in time chosen as the origin of a particular calendar era. The "epoch" serves as a reference point from which time is measured. The moment of epoch is usually decided ...

of the

geologic timescale The geologic time scale, or geological time scale, (GTS) is a representation of time based on the rock record of Earth. It is a system of chronological dating that uses chronostratigraphy (the process of relating strata to time) and geochr ...

. It is now more specifically known that the Lias is

Rhaetian The Rhaetian is the latest age of the Triassic Period (in geochronology) or the uppermost stage of the Triassic System (in chronostratigraphy). It was preceded by the Norian and succeeded by the Hettangian (the lowermost stage or earliest a ...

Toarcian The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toar ...

in age (over a period of 20 million years between ) and thus also includes a part of the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest per ...

. The use of the name "Lias" for a unit of time is therefore slowly disappearing.

Subdivisions

In southern

England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe ...

, the Lias Group is often divided into Lower, Middle and Upper subgroups. In Southern England the Lias is divided into the following formations (from top to base): * Bridport Sand *

Beacon Limestone The Beacon Limestone Formation, historically known as the Junction Bed, is a formation of early Jurassic age (Pliensbachian–Toarcian). It lies above the Dyrham Formation and below the Bridport Sand Formation. It forms part of the Lias Group. ...

* Dyrham Formation * Charmouth Mudstone * Blue Lias In the East Midlands Shelf the Lias is divided into the following formations (from top to base): * Whitby Mudstone * Marlstone Rock * Charmouth Mudstone * Scunthorpe Mudstone In the Cleveland Basin in Yorkshire the Lias is divided into the following formations (from top to base): * Whitby Mudstone * Cleveland Ironstone * Staithes Sandstone * Redcar Mudstone In

South Wales South Wales ( cy, De Cymru) is a loosely defined region of Wales bordered by England to the east and mid Wales to the north. Generally considered to include the historic counties of Glamorgan and Monmouthshire, south Wales extends westwards ...

only the Blue Lias is present. The Lias is underlain by the Late Triassic

Penarth Group The Penarth Group is a Rhaetian age (Triassic) lithostratigraphic group (a sequence of rock strata) which is widespread in Britain. It is named from the seaside town of Penarth near Cardiff in south Wales where strata of this age are exposed in ...

, and overlain by the Inferior Oolite in most of England and the

Dogger Formation Dogger may refer to: * Dogger Bank, a large shallow area in the North Sea between Britain and Denmark * Dogger Bank incident, the Russian attack on British fishermen in 1904 at the Dogger Bank area in the North Sea. * Dogger (boat), a type of ketc ...

Ravenscar Group The Ravenscar Group is a Jurassic lithostratigraphic group (a sequence of rock strata) which occurs within the Cleveland Basin of North Yorkshire and extends to both the Hambleton and Howardian Hills. The name is derived from Ravenscar on th ...

in the Cleveland Basin. In some areas there is a stratigraphic hiatus, and the rocks are overlain by Cretaceous marine sediments. There are restricted outcrops of Lias rocks on the west coast of

Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to ...

where, in the Sea of the Hebrides depositional basin on

Skye The Isle of Skye, or simply Skye (; gd, An t-Eilean Sgitheanach or ; sco, Isle o 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 ...

, Raasay and Mull, the Broadford Beds Formation, Pabay Shale Formation and overlying Scalpay Sandstone Formation are assigned to the Lias Group. In Dutch lithostratigraphy, the name Lias has no official status, however, it is often used for the lower part of the Altena Group in the subsurface of the Netherlands and the southern North Sea. In northern Germany, the Lias Group consists of nine formations (from top to base):Deutsche Stratigraphische Kommission (eds.)
''Stratigraphische Tabelle von Deutschland 2002''
(stratigraphic tables of the German commission on stratigraphy), Potsdam 2002, (PDF: 6.57 MB) * Opalinuston * Dörnten-Formation * Posidonia Shale * Amaltheenton * Capricorn Formation * Raricostaten-Formation * Arieten Sandstone * Liassicus Sandstone * Psilonoten Sandstone

References

Notes

Literature

* (2007): ''Jurassic'', in: {{aut, Wong, Th.E.; Batjes, D.A.J. & Jager, J. de (''eds.''): ''Geology of the Netherlands'',

Royal Netherlands Academy of Arts and Sciences The Royal Netherlands Academy of Arts and Sciences ( nl, Koninklijke Nederlandse Akademie van Wetenschappen, abbreviated: KNAW) is an organization dedicated to the advancement of science and literature in the Netherlands. The academy is housed ...

, pp. 107–125. Early Jurassic Europe Jurassic geology Jurassic England Jurassic Germany Lithostratigraphy of England Lithostratigraphy of Germany Jurassic System of Europe

Subdivisions

See also

References

Notes

Literature