Geology of Monmouthshire

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.^[1][2]

Silurian

Geology of Wales and South West England; map

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.^[3]

Devonian

Within Monmouthshire, the Devonian consists entirely of the Old Red Sandstone. It is widespread in its occurrence from the Black Mountains in the north, wrapping around either side of the centrally located Usk Inlier and extending in a strip north of Newport towards Cardiff. Traditionally divided into three parts, the lowermost Red Marl Group, the middle Red Sandstone Group and uppermost Quartz Conglomerate Group, modern classification of the sequence recognises the St Maughans Formation (which directly overlies the Raglan Mudstone Formation), the Senni Formation and the Brownstones Formation (these three comprising the Lower Devonian) together with the Plateau Beds Formation and Quartz Conglomerate Group of the Upper Devonian.

The relatively hard-wearing sandstones of the Brownstones Formation form the greater part of the Black Mountains ridges of Hatterrall Hill and Chwarel y Fan and the conspicuous hills of Sugar Loaf, Ysgyryd Fawr and Bryn Arw. It is also responsible for Graig Syfyrddin. The Brownstones also give rise to a major landscape feature in eastern Monmouthshire, a sinuous though largely unbroken west and northwest-facing escarpment which runs from the Wye Valley at Monmouth south and southwest to Newport and forming the hills at Trellech, Devauden and at Wentwood.^[4]

The uppermost Devonian pebbly conglomerates of the Quartz Conglomerate Formation give rise in the east of the county to a secondary, though less pronounced scarp which provides this part of Monmouthshire with its highest elevation, the 306m / 1003 ft high flat-topped eminence of Beacon Hill.

Carboniferous

Carboniferous rocks occur most widely in the west of the county where the deeply dissected plateau of the South Wales Coalfield consists of the Coal Measures sandstones, mudstones and of course, coal seams - all of Westphalian age. The plateau is edged by the outcrop of the Namurian age Millstone Grit and the Dinantian age Carboniferous Limestone. The Carboniferous Limestone also occurs widely in the southeast of the county and along the sides of the Wye Valley.

Triassic

The almost flat-lying sandstones of the Triassic unconformably overlie the older rocks which tend to dip to the south. The uppermost beds of the Triassic are the Rhaetian age 'Rhaetic' which occur in the Llanwern and Wilcrick areas and at Goldcliff.^[5]

Jurassic

A small outlier of Jurassic age strata occurs east of Newport as does a rather smaller one on the coast at Goldcliff. They comprise rocks assigned to the Lias.

Igneous rocks

Igneous rocks are limited to a couple of very minor intrusions of monchiquite in the Old Red Sandstone to the southeast of Usk. There appears however to be a common (and mistaken) belief amongst non-geologists that Sugar Loaf is an extinct volcano. Nevertheless, it is formed entirely of sedimentary rocks but has been eroded into a shape which, particularly when viewed from the east, is reminiscent of a stratovolcano.

Structure

There are numerous faults which affect the Silurian, Devonian and Carboniferous rocks of the county. They are most numerous, or at least have been most intensively mapped, within the Coalfield area. The majority in this area are aligned northwest to southeast.

In the north, the major fracture known as the Neath Disturbance cuts north-east trending through the Old Red Sandstone to the north of Sugar Loaf and thence along the Monnow valley to its east. It is the southernmost major tectonic feature of the British Caledonide belt,^[6] though was active again in the later Variscan orogeny.

Quaternary

Glacial legacy

There is clear evidence for glaciation of the uplands in the west of the county having taken place on one or more occasions. The Vale of Ewyas in the Black Mountains and the Sirhowy Valley, Rhymney Valley, Ebbw Vale and the valley of the Ebbw Fach in the Coalfield bear the hallmarks of glacial excavation - U-shaped profiles and over-steepened sides. There are moreover stretches of glacial till spread across their flanks. Substantial moraines are evident in the Usk valley including the Nevill Hall/Llanfoist moraine south of Abergavenny and the Usk Moraine which is believed to represent the furthest advance of an Usk valley glacier during the last Ice Age.

The Punchbowl is a well-developed glacial cirque on the eastern side of Blorenge.^[7] There are no indications that Cwm Craf on its northern side held a glacier but it is perhaps periglacial in origin.

Landslips

Landslips are common on the steep sides of the coalfield valleys where a combination of glacial over-deepening and the presence of the Pennant Sandstone overlying weaker mudstones has provided favourable conditions for ground movement. Mining activity has reactivated ancient landslips in certain cases.

Major landslips are apparent on the flanks of Ysgyryd Fawr and around the Sugar Loaf and again on the steep slopes at Cwmyoy in the Vale of Ewyas.

Alluvium

The floodplains of each of the major river valleys within the county are floored by fluviatile alluvium. The most extensive is that of the River Usk though this narrows for a couple of miles south of Llanfihangel nigh Usk. Those of the River Monnow and Afon Lwyd are up to 350m wide in places whilst that of the Trothy is rather narrower. In contrast there are wide spreads of these deposits around the smaller Olway Brook from Raglan southwards. Alluvial flats are restricted within the Wye valley as the river has cut itself a narrow gorge. The Coalfield rivers show similarly restricted floodplain development, occupying as they do, valleys which have been cut largely by glacial action.^[8][9]

There is evidence of river terrace development along the course of both the Usk and the Wye.

There are extensive deposits of estuarine alluvium on the shores of the Severn. The bulk of this is in the form of silt of a blue/grey colour. The reclaimed estuarine flats behind the modern sea defences are as much as 5 km in width east of Newport. Tidal flats extend considerably into the Severn estuary.^[10]

See also

• Geology of the United Kingdom
• Geology of Wales

References