Usk Inlier

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The Usk Inlier is a domed outcrop of rock strata of Silurian age in Monmouthshire in south-eastern Wales. It is located in the countryside between the towns of Caerleon and Pontypool and the village of Raglan. The longer axis of the dome or 'pericline', often referred to as the Usk Anticline, is aligned north-south. The inlier is largely surrounded by a sequence of Old Red Sandstone rocks of Devonian age, though both these and the Silurian rocks are largely obscured by superficial deposits. [1] [2]

Dome (geology) Deformational feature in structural geology of symmetrical anticlines that intersect each other at their respective apices.

A dome is a feature in structural geology consisting of symmetrical anticlines that intersect each other at their respective apices. Intact, domes are distinct, rounded, spherical-to-ellipsoidal-shaped protrusions on the Earth's surface. However, a transect parallel to Earth's surface of a dome features concentric rings of strata. Consequently, if the top of a dome has been eroded flat, the resulting structure in plan view appears as a bullseye, with the youngest rock layers at the outside, and each ring growing progressively older moving inwards. These strata would have been horizontal at the time of deposition, then later deformed by the uplift associated with dome formation.

The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.

Monmouthshire County

Monmouthshire is a county in south-east Wales. The name derives from the historic county of Monmouthshire of which it covers the eastern 60%. The largest town is Abergavenny. Other towns and large villages are Caldicot, Chepstow, Monmouth, Magor and Usk. It borders Torfaen and Newport to the west; Herefordshire and Gloucestershire to the east; and Powys to the north.

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Old Red Sandstone assemblage of rocks in the North Atlantic region

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St Brides Bay bay in Pembrokeshire, Wales

St Brides Bay is a bay in western Pembrokeshire, West Wales.

Tortworth village in the United Kingdom

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Geology of Scotland

The geology of Scotland is unusually varied for a country of its size, with a large number of differing geological features. There are three main geographical sub-divisions: the Highlands and Islands is a diverse area which lies to the north and west of the Highland Boundary Fault; the Central Lowlands is a rift valley mainly comprising Paleozoic formations; and the Southern Uplands, which lie south of the Southern Uplands Fault, are largely composed of Silurian deposits.

The Geological Conservation Review (GCR) is produced by the UK's Joint Nature Conservation Committee and is designed to identify those sites of national and international importance needed to show all the key scientific elements of the geological and geomorphological features of Britain. These sites display sediments, rocks, minerals, fossils, and features of the landscape that make a special contribution to an understanding and appreciation of Earth science and the geological history of Britain, which stretches back more than three billion years. The intention of the project, which was devised in 1974 by George Black and William Wimbledon working for the Governmental advisory agency, the Nature Conservancy Council (NCC), was activated in 1977. It aimed to provide the scientific rationale and information base for the conservation of geological SSSIs (Sites of Special Scientific Interest, protected under British law. The NCC and country conservation agencies were established in 1990 when JNCC became established and took over responsibility for managing the GCR site assessment process, and publishing accounts of accepted sites.

The Silurian Lagerstätte preserved in the limestone Wenlock Series of Herefordshire, England, offers paleontologists a rare snapshot of a moment in time, about 420 Mya. In the formation, layers of fine-grained volcanic ash punctuate a sequence of carbonate muds that were accumulating in a marine environment on the outer continental shelf. In this fine-grained matrix, soft-bodied animals and delicate, lightly sclerotized chitinous shells are often preserved in three dimensions, as calcitic fossilizations within calcareous nodules. Calcitic fossilization is an unusual feature.

The geology of Wales is complex and varied; its study has been of considerable historical significance in the development of geology as a science. All geological periods from the Cryogenian to the Jurassic are represented at outcrops, whilst younger sedimentary rocks occur beneath the seas immediately off the Welsh coast. The effects of two mountain-building episodes have left their mark in the faulting and folding of much of the Palaeozoic rock sequence. Superficial deposits and landforms created during the present Quaternary period by water and ice are also plentiful and contribute to a remarkably diverse landscape of mountains, hills and coastal plains.

Richard John Aldridge was a British palaeontologist and academic, who was Bennett Professor of Geology at the University of Leicester.

Trichrug is a hill four miles east of Llandeilo and 3 miles south of Llangadog in the county of Carmarthenshire, southwest Wales. It lies within the Brecon Beacons National Park and Fforest Fawr Geopark. Its summit at a height of 415m above sea level is marked by a trig point. It is a Marilyn with a prominence of 191 metres.

An inlier is an area of older rocks surrounded by younger rocks. Inliers are typically formed by the erosion of overlying younger rocks to reveal a limited exposure of the older underlying rocks. Faulting or folding may also contribute to the observed outcrop pattern. A classic example from Great Britain is that of the inlier of folded Ordovician and Silurian rocks at Horton in Ribblesdale in North Yorkshire which are surrounded by the younger flat-lying Carboniferous Limestone. The location has long been visited by geology students. Another example from South Wales is the Usk Inlier in Monmouthshire where Silurian age rocks are upfolded amidst Old Red Sandstone rocks of Devonian age.

Lewisian complex suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland

The Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane and the North Atlantic Craton. These rocks are of Archaean and Paleoproterozoic age, ranging from 3.0–1.7 Ga. They form the basement on which the Torridonian and Moine Supergroup sediments were deposited. The Lewisian consists mainly of granitic gneisses with a minor amount of supracrustal rocks. Rocks of the Lewisian complex were caught up in the Caledonian orogeny, appearing in the hanging walls of many of the thrust faults formed during the late stages of this tectonic event.

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.

Skiddaw Group group of sedimentary rock formations

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Pen-y-crug mountain in United Kingdom

Pen-y-crug is a hill in the Brecon Beacons National Park in the county of Powys, south Wales. It is commonly referred to locally as simply 'The Crug'. The Welsh name signifies 'top of the mound'. It slopes are moderately gently on three sides; only to the west do they steepen somewhat. Its flat trig point adorned summit at 331m overlooks the valley of the River Usk to the south. To the southeast are panoramic views over Brecon whilst eastwards are the Black Mountains and south the Brecon Beacons.

Wrekin Terrane An inferred basement rock terrane of the southern United Kingdom

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Rhinns complex

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Pauline is a fossil genus of ostracods from the Silurian. Genus contains two species: Pauline avibella found in 425-million-year-old rocks in the Herefordshire Lagerstätte in England near the Welsh Border and Pauline nivisis, known from the Lower Silurian Pentamerus Bjerge Formation of north Greenland.

Milford Haven Group

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The geology of the Isle of Man consists primarily of a thick pile of sedimentary rocks dating from the Ordovician period, together with smaller areas of later sedimentary and extrusive igneous strata. The older strata was folded and faulted during the Caledonian and Acadian orogenies The bedrock is overlain by a range of glacial and post-glacial deposits. Igneous intrusions in the form of dykes and plutons are common, some associated with mineralisation which spawned a minor metal mining industry.


  1. Aldridge, R.J., Siveter, David J., Siveter, Derek J., Lane, P.D., Palmer, D. & Woodcock, N.H. Geological Conservation Review Series vol 19, British Silurian Stratigraphy. Joint Nature Conservation Committee, Peterborough
  2. British Geological Survey 1:50,000 scale geological map sheets 232, 233, 249 & 250 (England & Wales series)

Coordinates: 51°41′N2°55′W / 51.69°N 2.92°W / 51.69; -2.92

Geographic coordinate system Coordinate system

A geographic coordinate system is a coordinate system that enables every location on Earth to be specified by a set of numbers, letters or symbols. The coordinates are often chosen such that one of the numbers represents a vertical position and two or three of the numbers represent a horizontal position; alternatively, a geographic position may be expressed in a combined three-dimensional Cartesian vector. A common choice of coordinates is latitude, longitude and elevation. To specify a location on a plane requires a map projection.