page_title: Geology of Monmouthshire
section_title: Silurian
hierarchical_section_title: Geology of Monmouthshire / Silurian
caption_attribution_description: English: Geological map of Monmouthshire redrawn after BGS maps here
context_page_description: 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, 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.
context_section_description: There is an inlier of rocks of Silurian age in the heart of the county, west of Usk and straddling the River Usk and known accordingly as the Usk Inlier. These rocks 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.
caption: Geology of Monmouthshire; map
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