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: ''This article describes the geology of the historic county of Monmouthshire. It includes the modern administrative county and the 'principal areas' of Torfaen, Newport and Blaenau Gwent together with small parts of Cardiff and Caerphilly east of the Rhymney River.'' The geology of Monmouthshire in southeast Wales largely consists of a thick series of sedimentary rocks of different types originating in the Silurian, Devonian, Carboniferous, Triassic and Jurassic periods. The oldest rocks, of Silurian age, occur as a broad, northeast to southwest aligned anticline in the heart of the county. The central portion of this zone, between Usk and Pontypool, comprises the outcrop of the older shales, limestones and sandstones and, surrounded as it is by outcrops of younger rocks, is traditionally referred to as the Usk Inlier. These younger rocks are a mix of sandstones and mudstones of Devonian age and commonly referred to as the Old Red Sandstone or, colloquially the 'ORS'. The oldest rocks of the ORS sequence (and hence lowest in the sequence), the mudstones of the Raglan Mudstone Formation, are also assigned to the Silurian period, though were once considered to be Devonian. Towards the eastern, southeastern and western margins of the county are successive layers of rocks of Carboniferous age. The oldest of these and hence the lowest, resting directly on the ORS are various formations of the Carboniferous Limestone. These in turn are overlain, in the west, by the sandstones and mudstones of the Marros Group (formerly referred to as the 'Millstone Grit series') and lastly by the sandstones, mudstones and coal seams of the South Wales Coal Measures. Along the southern coastal strip are rocks of Triassic age which unconformably overlie the Devonian and Carboniferous rocks. An area of countryside at Llanwern, east of Newport is characterised by rocks of Jurassic age, the youngest solid rocks which occur within the county. Similar though smaller outcrops of Jurassic rocks can be found at Goldcliff on the Severn Estuary and also, concealed beneath more recent sediments, near the mouth of the Ebbw River south of Newport. There are a range of different types of superficial deposits of Quaternary age overlying the solid rocks ranging from estuarine alluvium along the coastal strip, through riverine alluvium in the floors of the major river valleys to glacial till and glacial sands and gravels. These 'drift' deposits also include peat, head and landslipped masses of both bedrock and superficial material.〔British Geological Survey 1:50,000 scale geological maps 214, 215, 232, 233, 249, 250, 263〕〔British Geological Survey 1:250,000 scale geological map ''The Rocks of Wales/Creigiau Cymru'', 1st edn, Solid. NERC 1994〕 == Silurian == There is an inlier of rocks of Silurian age in the heart of the county. They consist of a thick sequence (over 600m / 2000 ft) assigned to the Přídolí, Ludlow and Wenlock series. The oldest parts of the succession, the Wenlock Shales and Wenlock Limestone of early authors are now referred to as the Glascoed Mudstone, Ton Siltstone and Usk Limestone. Together these comprise the local Wenlock succession. Above these are the former Lower Ludlow Shales, now the Lower and Upper Forest Beds, and the Aymestry Limestone, now the Lower Llanbadoc Beds. The Upper Llanbadoc Beds and Lower, Middle and Upper Llangibby Beds (collectively forming the larger part of the former Upper Ludlow Shales) complete the Ludlow series. All of these rocks are of marine origin, their lithologies suggestive of a variety of deep and shallow water environments during deposition. The uppermost Silurian rocks are the Downton Castle Sandstone and the overlying Raglan Mudstones, both of Přídolían age. The Přídolían rocks were deposited in various coastal and floodplain environments.〔Barclay, W.J. 1989 ''Geology of the South Wales Coalfield. Part II, the country around Abergavenny'', 3rd edn. ''Mem Br Geol Survey'' Sheet 232 (England and Wales) (ch2)〕 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 Monmouthshire」の詳細全文を読む スポンサード リンク
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