Welsh Journals

Certain aspects of soils developed on Calcareous parent materials in South Wales By C. B. CRAMPTON (Soil Survey of England and Wales, Rothamsted Experimental Station, Harpenden, Herts.) INTRODUCTION The soils investigated occur within the Glamorganshire Coalfield and the peripheral lowland belt which includes the Gower Peninsula, the Vale of Glamorgan and its extension into Monmouthshire. An outline of the geology and the localities at which the soils were sampled are shown in the Sketch-Map. Over most of Gower outcrop Carboniferous strata, including the Carboniferous Limestone, although much of the surface is covered by boulder clay strongly influenced by the Irish Sea Drift as well as the underlying rocks (Griffiths, 1939). Old Red Sandstone ridges such as Rhossilli Down and Cefn Bryn rise abruptly from this plateau-like surface of boulder clay country to about 600 ft. Topographically, the Vale of Glamorgan can be considered as consisting of two plains, one at around 200 ft. being the outcrop of the Liassic limestones, the other at about 300 ft. and farther inland where the Carboniferous Limestone gives rise to the sharp slope traversing the Vale and separating the two plains. Poorly drained soils on the coastal plain have been strongly influenced by fine- textured drift from the Coalfield, whilst freely draining soils here have been influenced by the Irish Sea Drift (Crampton, 1961). Deep glacial deposits occurring over the Carboniferous Limestone cropping out in the north of the Vale, and extending into Monmouthshire over Old Red Sandstone rocks cropping out around the Coalfield periphery, contain Irish Sea Drift and material carried by ice off the Coalfield (Crampton, 1960). Some of the limestone bands (cornstones) within the Old Red Sandstone strata are sufficiently competent to give rise to shallow ridges projecting through the drift mantle. The broken scarp defining the southern margin of the Coalfield is formed by Old Red Sandstone conglomerates at some points, and the Carboniferous Limestone or Millstone Grit at others. The Pennant Series constitutes the more arenaceous division of the Coal Measures and tends to give rise to the bold north-facing scarps surrounding the core of the Coalfield, and to form the flat-topped ridges which coverge northwards to meet at Craig-y-Llyn at 1,969 ft. Shales within the more argillaceous division of the Coal Measures, the Lower Coal Series, tend to outcrop along valley bottoms or give rise to broad depressions, usually covered by drift. The principal collecting ground for ice was the Old Red Sandstone of the Fforest Fawr-Brecon Beacons range where powerful glaciers moved southwards across the formation, over the shallow, dissected ridges of the outcrops of the Carboniferous Limestone and Millstone Grit (Robertson, 1932). This ice was split by Craig-y-Llyn, one branch turning south-west down the Neath and Tawe valleys (Strahan, 1907), and the other turning south-east into the valleys of the Cynon and Taff (Gibson and Cantrill, 1917). Eventually they reunited and moved into the Vale of Glamorgan. During the last phase of glaciation in South Wales this was the only ice movement, the Irish Sea Ice failing to reach this area.