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- 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. (en)
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