Geology of Monmouthshire

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.

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.

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.[5]

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.[7][8]

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.[10] There are no indications that Cwm Craf on its northern side held a glacier but it is perhaps periglacial in origin.

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. Slips are especially common around Coity Mountain and within the Sirhowy and the two Ebbw valleys. On the northeastern fringe of the coalfield, there are slips in the vicinity of Blorenge, the most well-known of which is the Tumble through which the B4246 road climbs towards Keeper's Pond.

A notable example of mass movement is Mynydd Henllys, west of Cwmbran where a large section of the mountain has slipped northwestwards into the post-glacially deepened valley of the Nant Cam, leaving a 2 km long degraded fissure feature high on the eastern slopes. It owes its origin to a mass of sandstone slipping in rotational fashion on a surface approximating to the Brithdir coal seam and associated mudstones and seatearths.[11]

The Black Mountains and nearby lesser hills formed from the Old Red Sandstone are formed from interlayered sandstones and less competent mudstones with the whole typically dipping southwards. The rocks are fractured in places and valley sides have been oversteepened by the passage of glacial ice. Various combinations of these circumstances has led to large numbers of landslips having taken place over millennia. Major slips are apparent on the western flanks of Ysgyryd Fawr and also affecting its southern ridge. Slips occur on either side of Cwm Coedycerrig with the largest affecting the southeastern slopes of Gaer on the north side. The landslipped material fills the valley floor. Smaller slips are evident on the northern slopes of the Sugar Loaf though the largest on that hill is at Graig where a spur extends towards the River Usk. Cwmyoy village famous for the crooked nature of St Martins Church sits on a landslip extending down from Hatterrall Hill which is still moving, albeit slowly. Nearby is the large sandstone cliff of the Darren, the back wall of another large slip. Further slips are evident within the Vale of Ewyas including the ground which rises to the east above Llanthony Priory.

Further east in lowland Monmouthshire, though affecting the same rock strata, landslips are recorded around Graig Syfyrddin and towards the Monnow valley as at Coed y Pwll northwest of Skenfrith.[12]

The deep incision of the Wye into the landscape on Monmouthshire’s eastern border has left slopes open to landslip. Two significant examples are seen between St Arvans and Tintern where Black Cliff and Wyndcliff represent the back walls of broad slips which extend down to the riverbank. Both crags are formed from strata of the Black Rock Limestone Subgroup (traditionally referred to as the 'Lower Dolomite'). It sits atop the less competent strata of the Avon Group (traditionally the 'Lower Limestone Shale') which has collapsed under the loading.[13][14][15] The A466 road runs through the middle of both slips.

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.[16][17]

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.[18]

Within the coalfield, as a result of mining activities, there are a number of areas of artificially made ground, consisting for the most part of spoil from both opencasting for coal and deep mining. A high profile example is that of the Canada Tips, an area of excavation and deposition of Coal Measures rocks north of Blaenavon associated in particular with opencasting during the Second World War. It is now included within the Blaenavon Industrial Landscape World Heritage Site. Other areas are formed by material such as slag from former ironworks.