Historic Landscape Characterisation

The Making of the Clywedog Valley Landscape

THE NATURAL ENVIRONMENT

Topography and geology have played a considerable role in the land use, settlement and industrial history of the historic landscape area.

Topography and drainage

The historic landscape area extends from the moorland at the foothills of Plynlimon (Pumlumon) in the west at the head of the Clywedog valley, at a height of about 520 metres above sea level, down to the Severn valley at Llanidloes to the south-east at a height of about 160 metres. Towards the headwaters of the Clywedog the valley opens out into a shallow basin a kilometre or more across but lower down towards its confluence with the Severn it formed the dramatic, now flooded, steep-sided, deeply glaciated and serpentine valley, several hundred metres deep and only 150-500 metres wide, which inspired the name Ystradhynod, first recorded in the 1570s as one of the townships which encompasses the valley cutting through the upland plateau, which probably derives from the roots ystrad (‘river valley’) and hynod (‘remarkable’). It is joined from the north and the west by a number of equally steep-sided tributary valleys, notably the Afon Bachog, Afon Lwyd, Afon Biga, Nant Felen, Nant Pen-y-banc, and Afon Gwestyn. To the north-west the Afon Twymyn and its tributaries the Nant y Iâr, Nant Dropyns, and Nant Bryn-moel drain north-eastwards into a deep, 200-metre deep gorge created initially by glaciation and subsequently deepened by river erosion following the capture of what was formerly part of the Severn river system by that of the Dyfi. To the north-east the area is drained by tributaries of the Trannon, including the narrow and steep-sided valley of the Nant Cwmcarreg-ddu, and the broader valley of the Cerist and its steep-sided tributary the Nant Gwden.

The name of the river Clywedog (also applied to several other rivers in Wales) is thought to derive from the clywed (‘to hear’) and thus have the meaning ‘noisy’. Rushing water also explains the origin of the name Dylife which is derived from dylif (‘flood, deluge, torrent’).

Drainage patterns in the Cerist valley were affected by the canalization of the river undertaken in the 1870s when the Van Railway was constructed. Drainage in the Clywedog valley has clearly been profoundly affected by the creation of the Clywedog Reservoir in the 1960s. More locally, patterns of drainage in a number of areas were affected temporarily or permanently as a result of the creation of smaller reservoirs, leats and sluices for the exploitation of water power in support of corn milling, textile production and metal mining particularly during the 18th and 19th centuries and as a consequence of extensive afforestation schemes in th 20th century.

Geology and soils

The solid geology underlying the whole of the area is dominated by slates, shales, grits, sandstones and mudstones of Ordovician and Silurian age. The older Ordovician mudstones and grits underlie some of the central parts of the area, between the Afon Biga in the south-west, Fairdre Fawr in the north-west, Y Fan in the north-east and Cwm Deildre in the south-east. The uppermost beds consist of soft black mudstones which can weather to clay-like deposits, whilst the lower beds consist of hard grits divided by shale beds. The lower part of the overlying Silurian rock consists of black shales which readily weather to an orangey-yellow colour due to the high iron sulphide content together with thin beds of sandstone which are well exposed in the Clywedog valley and around Dylife.

In a subsequent period of earth movement (known as the Caledonian orogeny) t9he Silurian and Ordovician rocks were uplifted and folded and a number of north-south and east-west faults subsequently developed across the area. The east-west faults in particular attracted mineralisation which filled the fault and fracture voids with economically viable lead, zinc and copper ores such as galena, sphalerite and chalcopyrite together with less valuable minerals like quartz, calcite and barite. Two significant east-west faults cross the historic landscape area, one towards the north-west from Dyfngwm at the head of the Clywedog valley to Pen Dylife and then to Dylife itself, the second towards the south-east from Gwestyn and Aberdaunant in the valley of the Nant Gwestyn to Bryntail, Y Fan and Cwmdylluan and beyond in the Nant Gwden valley. The mineral veins vary in thickness from a few centimetres to a maximum at Van of 15 metres in width. The veins are exposed in rock outcrops in places but are typically steeply inclined along dipping fault planes.

In the river valleys the solid geology is masked by fluviglacial drift and later alluvial deposits. A variety of soil types have resulted from the weathering of the solid and drift geology which have historically had a profound impact upon the agricultural potential of the land. The soils on the upland plateaux in the west and south of the area (belonging to the Hafren soil type), on Bryn Moel, Pen Dylife, and Bwlch y Garreg-wen, and on Mynydd y Groes and Bryn Mawr are loamy though often wet upland soils frequently with a peaty surface horizon and thin underlying ironpan, best suited to moorland, rough grazing and conifer woodland. The soils on the hillslopes throughout much of the area (belonging to the Manod type) are well-drained fine loamy or fine silty soils best suited to stock rearing on permanent grassland and woodland. The upper basin of the Clywedog together with the valleys of the Afon Biga, Afon Lwyd, and Afon Bachog, includes soils (belonging to the Wilcocks 2 type) which are seasonally waterlogged loamy upland soils with a peaty surface horizon best suited to stock rearing on wet moorland of moderate grazing value, some permanent, improved grassland, and conifer woodland. The area of the lower Clywedog and lower Gwestyn valleys again has fine silty and clayey soils (belonging to the Cegin series), subject to seasonal waterlogging, best suited to stock rearing and dairying on permanent grassland. Soils on the lower upland to the north of the Clywedog Reservoir, between Fairdre Fawr and Bryn y Fan and the valley of the watershed of the Cerist (belonging to the Brickfield 3 series) are fine loamy soils overlying more clayey soils, again subject to seasonal waterlogging, which have been suited to stock rearing and some dairying on permanent grassland and some cereal cultivation in drier areas.

Environmental history

Some evidence of the environmental history of the area since about 11,000 years ago has been provided by studies of pollen sequences in peat deposits in the Pumlumon (Plynlimon) area. In the earliest, late-glacial phase of the sequence tree pollen is low, but includes alder, pine, birch and hazel, ferns, mosses. A second phase, corresponding to perhaps the Mesolithic and earlier Neolithic periods is dominated by oak woodland, with some elm, pine, alder, birch and hazel, woodland probably, depending upon aspect, initially extending to a height of up to about 600 metres above sea level. A subsequent phase, corresponding with the later Neolithic, Bronze Age and Iron Age periods saw a decline in oak and birch and an expansion of alder and hazel woodland with some blanket peat formation in wetter areas and minor clearance episodes beginning in perhaps the later Neolithic and Bronze Age periods. Later changes in flora and the proportions of tree and grass species and evidence for deforestation and woodland regeneration have been interpreted as reflecting historically attested episodes of land use during the medieval period and the 19th and 20th centuries.

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