MONMOUTHSHIRE, a western border county of England, bounded east by Gloucestershire, north-east by Herefordshire north-west by Brecknock, west and south-west by Glamorgan shire (Wales), and south by the estuary of the river Severn. The county falls into two natural regions :—the basin of the Usk (after that river emerges from the gap between the Brecon Beacons, 2,907 ft., and the Black mts., 2.624 ft.), which forms the north-eastern and central portions of the county, and the basins of the river Rhymney and the river Ebbw which drain the south-east slopes of the Brecon Beacons, i.e., the eastern end of the south Wales coal-field. The river Monnow forms the north east boundary of the county, but its upper waters include a section of the Black mts., within the county boundary on the north side. After the junction of the Monnow with the Wye at Monmouth, the main river becomes the eastern boundary; the upper and middle river Rhymney form its western border. The Monnow, and the Wye as far as Tintern, pass over Old Red Sandstone rocks (a great series of red marls, sandstone and concretionary limestones), which occupy the north-east and cen tral parts of the county, grits and conglomerates of the highest beds giving rise to bold escarpments and lofty plateaux (e.g., The Sugar Loaf, 1,955 ft., Skirrid Fawr 1.596 ft., and Blorenge, 1,838 ft., which almost encircle the town of Abergavenny).
West of the town of Usk, the Old Red Sandstone plain is interrupted by an extensive anticline of older Silurian strata (Wenlock shale and limestone and Ludlow beds), a small inlier of the same rocks appearing at Rhymney near Cardiff. The Rhymney (down to Caerphilly), and the Ebbw (with the Sir howy and other tributaries) which joins the Usk below Newport, are parallel consequent streams draining the coal-field, which here consists of a series of north-north-west to south-south-east valleys, separated by high ridges of Pennant grit, and related to lines of faulting that are much later than the foldings of the coal-field. These valleys are deep and narrow, and often of U-shaped trans verse profile, due to scouring by glaciers from the Brecon Beacons. The Carboniferous limestone, millstone grit, and coal measures (lower coal series, Pennant sandstone and upper coal series) dip westward and succeed each other from east to west. The Coal Measures abound in coal-seams and ironstone and their valleys, densely populated with people formerly occupying higher pastoral districts, now forsaken, give rise to acute social and economic problems. Around the eastern edge of the coal-field a
narrow rim of Carboniferous limestone occurs, and is crossed by a western tributary of the Usk at Pontypool. Very striking land forms are seen in the valley of the Wye between Tintern and Chepstow, where Carboniferous limestone again appears on the western rim of the outlying Forest of Dean coal-field (Glos.). Rhaetic and Lias limestones and shales outcrop at Llanwern and Goldcliff, near Newport. Glacial gravel and boulder clay are found in the valleys, and a broad tract of alluvium borders the Bristol channel from Rhymney to Portskewet, forming a lowland way through Monmouthshire to the Vale of Glamorgan, part of the plain of Gwent.
History and Early Settlement.—Monmouthshire in early times was heavily forested and formed part of the difficult coun try of the lower Severn. There are evidences of the use of the route to the west (via the south Wales coastal plain) in the Bronze age; finds of flat axes and socketed celts being important in the southern section of the county. The westward route was used by the Romans, who built Caerwent and Caerleon (q.v.), the latter of which performed a service similar to that performed by Chester in the north, guarding the coastal route into Wales.
Monmouthshire, at the time of the Heptarchy, formed the Welsh kingdom of Gwent, and no permanent English settlement was effected in the district until close upon the middle of the th century. The incursions of the West Saxons began in the 7th century, and Brochmael and Fermael, kings of Gwent, acknowledged Alfred as their lord. In the 9th and loth centuries the district was frequently harried by the Danes, who in 915, under Ohter and Hwald, sailed round Wessex and Cornwall to the Severn, plundered the banks of the Wye and took prisoner the bishop of Llandaff. In 926 Aethelstan met the kings of the north Britons at Hereford and fixed the Wye as the limit of their terri tory. In 976 the Danes destroyed Caerleon, at this time the chief town of the district. The early 1 1 th century was taken up with interminable contests between the Welsh princes for the suc cession in south Wales, and the Welsh Chronicle relates that in 1047 the whole of South Wales lay waste, and in 1049, Griffith, the king of South Wales, assisted Irish pirates in plundering. In 1065 Harold conquered the district between the lower Wye and the Usk.