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Caersws Basin
Historic Landscape

Historic Landscape Characterisation

The Making of the Caersws Basin Landscape



The Caersws Basin occupies the floor of the upper Severn between Newtown and Llandinam centred upon the village of Caersws, at a height of between about 120-230 metres, at the point where it broadens to between 1-1.5 kilometres at the confluence of the Severn with the Garno, the Trannon and the Cerist. The valley of the Garno narrows to about 200 metres near Pontdolgoch towards the north, and the Severn valley narrows to about 300 metres near Llandinam and just begins to narrow slightly towards Newtown towards the east.

South of the river the hills rise gently at first to a height of about 220 metres and then with steeper slopes up to the moorland rising to about 430 metres on the south-eastern boundary of the historic landscape area. To the west the basin is bounded by two hilly spurs flanked by steep slopes, one between the Severn and Cerist which rises to a height of about 270 metres, and the second between the Cerist and Garno which rises to about 290 metres. The land generally rises more gently to the north to a height of about 250 metres but more steeply immediately north of Llanwnog where the hills reach a height of 350 metres.

Geology and soils

The solid geology underlying the whole of the area is composed of Silurian sandstones and slates in the central and south-western parts of the area, with shales and grits to the north on Alltwnnog and to the south-east on Allt y Gaer and Penstrowed Hill.

The valleys of the Severn, Garno and Trannon are deeply glaciated troughs. The main valley floors are underlain by thick deposits of clay overlain by gravelly layers deposited by glacial meltwaters, together up to 68 metres in depth, which are overlain by silty alluvial sediments deposited by river action with some gravel bars resulting from reworking of earlier gravels. Drumlins which survive as distinctive low hillocks composed of Boulder Clay were formed in the Cerist valley, in the stream valley to the west of Cefn Carnedd, and between Caersws and Llanwnog and to the north-east of Caersws in the direction of Gwynfynydd.

The soils in narrow zones along the floodplain of the Severn and Garno valleys belong to the Soil Survey’s Teme series, being deep stoneless permeable silty soils with gravelly subsoil in places, most suited to dairying and stock rearing on permanent and short-term grassland with some cereals where flood risk is low. The Cerist valley and the area along the Manthrig Brook north of Caersws towards Llanwnog belong to the Conway series, which are deep stoneless fine silty and clayey soils affected by groundwater and most suited to permanent grassland for dairying and stock rearing. Soils on the rising land below the hills north of Cerist, parts of the rising land encircling Cefn Carnedd between the Cerist and the Severn, the gently sloping ground in the Garno valley to the west and north-west of Llanwnog, and the lower western hillslopes of Penstrowed Hill in the area between Moat Farm and Bronfelin Hall belong to the Denbigh 1 series. These are well drained fine loamy and silty soils overlying rock, suited to dairying and cereals in the more lowlying areas and stock rearing, woodland or rough grazing on higher and steeper sloping ground. The soils on the gently rising ground to the north-east of Llandinam and to the north of Caersws and east of Llanwnog belong to Brickfield 3 and Cegin series respectively. The former are seasonally-waterlogged fine silty and clayey soils, though well drained fine loamy soils occur in places; they are most suited to stock rearing and some dairying on permanent grassland, with some cereals in the drier areas."

Rivers and streams

Parts of the valley floors of the major rivers are regularly flooded for periods, but flooding of the Severn valley has been reduced with the construction of the Clywedog dam in the 1960s and the improvements to flood defences near Caersws probably in conjunction with the construction of the Cambrian Railway in 1863 and Van Railway in 1871 and during the 20th century.

The floodplain and valley floor margin of this stretch of the Severn is of some interest and forms part of the upper Severn Geological Conservation Review (GCR) site. A detailed history of the river channel and valley floor evolution has been established from studies of ancient and more recent geomorphological features of the Severn valley floor that have developed in response to natural changes, such as the climate, as well as human activity.

The valley floor displays a series of river terraces, palaeochannels and alluvial fans of Holocene and Pleistocene age (periods up to about 12,000 BC) which demonstrate that active channel migration and alluvial sedimentation has taken place over the last 10,000 years. The earliest terrace probably dates to the Late Devensian period (about 23,000 to 8,000 BC) as an accumulation of thick, gravelly, glacial meltwater outwash deposits, buried by later deposits within the historic landscape area. There are suggestions that the river Severn at least periodically had a more braided course. Most of the floodplain has been relatively stable and with a single river channel probably since at least 2,000 BC and certainly since about AD 70, when the Roman road east of Caersws was built on its surface close to the present river channel. Sedimentation since that time has led to the Roman road east of Caersws being partly buried by several centimetres of fine-grained alluvium. Locally, the Roman road has had an effect upon sedimentation within the valley, the alluvial valley floor to the north of the road having a greater topographic variability than to the south. Studies have shown that the river channel of the Severn continues to be reworked by a complex of meanders which have been active since the 3rd to 6th century AD, which work themselves downstream in a zone between several metres to several hundred metres across in places. A clear demonstration of this is shown by the former river meander east of Caersws which has clearly destroyed a section of the Roman road north of the river leading to the Roman fort.

The gradient of the river in the section of the flood plain of the Severn within the historic landscape is less than in the areas both upstream and downstream. This, combined with increased sediment supply in the Severn valley in historic times due to a number of factors including the climatic deterioration associated with the Little Ice Age about AD 1590 to 1850, the extension of arable farming and the widespread enclosure of fields in the 19th century, and the intensive period of metal mining that occurred in the upper Severn catchment in the later 19th-century, led to increased rates of sedimentation in the floodplain of the river.

Artificial changes to the river channel of the Severn downstream of Llandinam were made in 1859 with the construction of the railway to Llanidloes. The river channel was also straightened by means of an engineered channel over a one kilometre stretch of the river near Llandinam Hall probably between 1840 and 1886, probably to avert the threat to property, though the river has since reverted to a meandering course. The course of the lower Cerist/Trannon was straightened between the early 1830s and the later 1840s probably in an attempt to ameliorate flooding. In about 1871 about 5 miles of the river Cerist was diverted into a deep artificial channel to enable the construction of the Van Railway west of Caersws. Substantial river engineering on the Trannon was undertaken in the late 1970s.

Grazing of the most valuable floodplain land right up to the river’s edge has been blamed for the loss of streamside trees and consequent damage to river banks in the Severn valley between Welshpool and Shrewsbury.

Environmental history

Analysis of peat deposits just to the west of Caersws on the flood plain of the river Severn near its confluence with the Cerist and Trannon has provided some evidence of early vegetation history for the Caersws Basin. The peat deposits represent a rapid accumulation in an area of impeded drainage near the confluence of the Severn and Garno in the Neolithic period about 3,500 BC over a period up to about 300 years. Following this there was a lapse of about 2,000 years between the uppermost peats and the construction of the Roman road west of Caersws which possibly served lead mines in the vicinity of Van and Dylife or was en-route to the minor fort at Cae Gaer near Llangurig. Plant remains during the Neolithic part of the sequence show a transition from an open-waterlogged habitat to fen carr with alder. Accumulating peats include regional pollen from mixed oak woodland in which oak, hazel, lime, pine and some alder appear to have been dominant. Large areas of relatively closed wood and scrub persisted in the vicinity of the site. Subsequently there was some drying out, with a rise in hazel and pine and a drop in alder, though later expansion of alder suggests the creation of alder fen and the persistence of mixed oak woodland, but there is still no certain indication of early human influence upon the environment at this period.

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