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The Lias Group or Lias is a lithostratigraphic unit (a sequence of rock strata) found in a large area of western Europe, including the British Isles, the North Sea, the low countries and the north of Germany. It consists of marine limestones, shales, marls and clays. In the past, the name Lias was often not only used 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 epoch of the geologic timescale. It is now known that the Lias is Rhaetian to Toarcian in age (over a period of ca. 20 million years between ) and thus also includes a part of the Triassic. The use of the name "Lias" for a unit of time is therefore slowly disappearing. ==Subdivisions== In southern England, the Lias Group is often divided into Lower, Middle and Upper subgroups. In Somerset the Lias is divided into the following formations (from top to base): *Helwell Marl *Doniford Shale *Quantock Beds *Kilve Shale *Blue Lias *St. Audries Shale *Aldergrove Beds It is underlain by the Late Triassic Penarth Group,〔(Somerset Geological TimeLine ), fossilgalore.com〕 and overlain, after a stratigraphic hiatus, by the Early Cretaceous Upper Greensand Formation. There are restricted outcrops of Lias rocks on the west coast of Scotland where, in the Sea of the Hebrides depositional basin on Skye, Raasay and Mull, the Broadford Beds Formation, Pabay Shale Formation and overlying Scalpay Sandstone Formation are assigned to the Lias Group.〔http://mapapps.bgs.ac.uk/geologyofbritain/home.html?〕〔http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=SCS〕〔http://www.bgs.ac.uk/Lexicon/lexicon.cfm?pub=PABS〕〔http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=BFB〕 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.〔See for example Wong (2007)〕 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, ISBN 3-00-010197-7 (PDF: 6,57 MB)〕 *Opalinuston *Dörnten-Formation *Posidonia Shale *Amaltheenton *Capricornu-Formation *Raricostaten-Formation *Arieten Sandstone *Liassicus Sandstone *Psilonoten Sandstone 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Lias Group」の詳細全文を読む スポンサード リンク
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