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 those parts of Cardiff and Caerphilly to the 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 landslip masses of both bedrock and superficial material.British Geological Survey 1:50,000 scale geological maps 214, 215, 232, 233, 249, 250, 263British Geological Survey 1:250,000 scale geological map The Rocks of Wales/Creigiau Cymru, 1st edn, Solid. NERC 1994 Karstification continues in connection with the Carboniferous Limestone outcrop.
Silurian
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.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)
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. 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. Graig Syfyrddin forms a westerly outlier of this scarp.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) (ch3) South of Gaer Wood, west of Trellech, it forms a significant watershed with streams running west, draining into the Usk catchment.
The uppermost Devonian age 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.
Carboniferous Limestone The limestone of South Wales is subdivided into a number of individual formations, not all of which are present in Monmouthshire and some that are, are notably thinner here than is the case further to the west. Those which outcrop on the north and east facing scarp which forms the edge of the coalfield dip into the coalfield at increasingly steep angles as the outcrop is traced south from Blorenge through Pontypool to the northeastern edge of Cardiff. The outcrop was extensively quarried in the past particularly around Gilwern Hill and Blorenge, in connection with the former ironworking industry.
Marros Group Referred to traditionally as the Millstone Grit (Series), the group comprises a basal sandstone overlain by mudstones. The sandstone, formerly known as the Basal Grit but now known formally as the Twrch Sandstone outcrops around Blorenge but can be difficult to distinguish from the overlying sandstone.
South Wales Coal Measures The Coal Measures occupy the majority of the western part of the historic county, today's Caerphilly and Blaenau Gwent unitary districts and half of Torfaen. Though now defunct, the legacy of the coal industry in this area is considerable. A sandstone extends widely across the area near the base of the Lower Coal Measures and is overlain by coal and ironstone-rich strata. The uppermost Coal Measures are again dominated by sandstone, the Pennant Sandstone forming the tops of the hills between the 'coalfield valleys'. Recent changes in stratigraphic nomenclature place these sandstones within the Warwickshire Group.
Triassic The almost flat-lying sandstones of the Triassic unconformably overlie the older rocks which tend to dip to the south. The Triassic sandstones are often known as the New Red Sandstone thereby distinguishing them from the Devonian age Old Red Sandstone. The uppermost beds of the Triassic are the Rhaetian age 'Rhaetic' which occur in the Llanwern and Wilcrick areas and at Goldcliff.Welch, F.B.A. & Trotter, F.M. 1961 Geology of the Country around Monmouth and Chepstow, HMSO, London(Ch.11) At Black Rock near Portskewett are seaweed-covered outcrops of sandstone which form a part of the Mercia Mudstone Group. It was beneath these rocks and through the underlying Pennant Sandstone that the Severn Tunnel was driven between 1873 and 1886 from nearby Sudbrook.
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. The cliff exposure here was commented upon by Gerald of Wales in 1188: Not far hence is a rocky eminence, impending over the Severn, call . . . Gouldcliffe or golden rock, because from the reflections of the sun's rays it assumes a bright golden colour.
Origins of the Wye gorge The origins of the deeply incised gorge of the River Wye which forms Monmouthshire's eastern border continue to be discussed. One proposal, that of superimposed drainage is that during the Jurassic and Cretaceous periods, sedimentary rocks similar in part to those found in the Cotswolds, were laid down across the area, burying the older Carboniferous Limestone. During the succeeding Tertiary period, an early version of the Wye flowed across the area, which at that time formed a flat coastal plain, and developed large meanders as it did so. When the land in the area subsequently began to rise relative to sea level the river was able to maintain its course by cutting down into its bed, so incising its course. In time the younger rocks were eroded away entirely whilst the river began to cut down through the underlying Carboniferous rock succession, as revealed in the sometimes craggy sides of the gorge today. Another theory for its formation involves the erosive power of debris-laden glacial meltwater escaping from proglacial lakes.
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,Barclay, W.J. & Wilby, P.R. 2003 Geology of the Talgarth District; a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50,000 sheet 214 Talgarth (England and Wales). though was active again in the later Variscan orogeny. Three roughly north-south aligned reverse faults are recognised in the Chepstow-Caldicot area.
Quaternary
Glacial legacy