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Somerset is a rural county in the southwest of England, covering . It is bounded on the north-west by the Bristol Channel, on the north by Bristol and Gloucestershire, on the north-east by Wiltshire, on the south-east by Dorset, and on the south west and west by Devon. It has broad central plains with several ranges of low hills. The landscape divides into four main geological sections from the Silurian through the Devonian and Carboniferous to the Permian which influence the landscape, together with water-related features. The low lying areas of the North Somerset Levels and Somerset Levels have been subject to thousands of years of flooding and man's attempts to control the flow of water. In the north of the county the Limestone of the Mendip Hills dominates the landscape, while in the south the Blackdown and Quantock Hills rise out of the levels. The highest areas are on Exmoor. The wide variety of landscapes has led to several areas being designated as Sites of Special Scientific Interest for geological reasons, and support a range of flora and fauna as can be seen from the List of Sites of Special Scientific Interest in Somerset == Rock ages == The oldest rocks are of Silurian age (443–419 million years ago), the most southerly known outcrop of rocks of this age in Britain. They make up a sequence of lavas, tuffs (volcanic ash), shales and mudstones in a narrow outcrop to the northeast of Shepton Mallet, in the eastern Mendip Hills. Rocks from the Devonian (419–359 million years ago) are found across much of Exmoor,〔(【引用サイトリンク】title=Geology of Exmoor )〕 the Quantocks (including Hangman Sandstone and Cockercombe tuff), and in the cores of the folded masses of the Mendip Hills. Carboniferous Period (359–299 million years ago) rocks are represented by the Carboniferous Limestone that forms the Mendip Hills, rising abruptly out of the flat landscape of the Somerset Levels and Moors. The limestones are very fossiliferous, and contain evidence of the abundant marine life that existed at the time of their creation, including fossil crinoids (sea-lilies), corals and brachiopods. At the end of the Permian (299–252 million years ago) and Triassic periods, the Variscan orogeny resulted in the formation of several mountainous areas including Dartmoor in the south, Exmoor and the Quantocks, and the Mendips. In the Taunton area Permian (299–252 million years ago) red sandstones and breccia outcrop, although rocks of Triassic age (252–201 million years ago) underlie much of Somerset and form the solid geology of the Somerset Moors and Levels.〔(【引用サイトリンク】 work=English Nature, Special Sites, Somerset Geology )〕 There are no glacial deposits. The Triassic rocks consist of red marls, sandstones, breccias and conglomerates which spread over the older rocks. The Dolomitic Conglomerate is an old shingle beach of Keuper Marl age. The Rhaetic Beds are full of fossils due to invasion of the Jurassic Sea. The Lias consists of clays and limestones, the latter being quarried and are famous for their fossils. Blue Lias was burnt locally to provide a source of lime for making lime mortar. It is still used as a decorative building stone. Blue Lias is believed to have been quarried on the Polden Hills as early as the 15th century and was quarried in Puriton from the early 19th century until 1973, when the local cement works closed.〔Dunning, R.W. (1992). ''The Victoria History of the Counties of England. A History of the County of Somerset. Volume VI: Andersfield, Cannington and North Petherton Hundreds (Bridgwater and Neighbouring Parishes)''. Oxford: Oxford University Press. ISBN 0-19-722780-5. Page 183.〕 Above the Lias is the Lower Oolite Series which are chiefly clays and oolitic limestone. The famous Bath Stone is obtained from the Great Oolite bed.〔Hudson, Kenneth (1971). ''The Fashionable Stone''. Bath: Adams & Dart. ISBN 0-239-00066-8.〕〔Bezzant, Norman (1980). ''Out of the Rock...''. London: William Heinemann Ltd. ISBN 0-434-06900-0. Page 143.〕 Oxford Clay is the chief member of the Middle Oolite Series;〔 and above this are the Upper Cretaceous rocks with Gault, Upper Greensand and Chalk. Alluvial flats and peat bogs occupy much of the centre of Somerset.〔 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「Geology of Somerset」の詳細全文を読む スポンサード リンク
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