The importance of glaciation in the modification of landscape in the South Wales coalfield has long been acknowledged. The effect of periglacial conditions has never been fully assessed, even though these are of enormous relevance to the geomorphology and to the prediction of the engineering characteristics of the superficial deposits.Examination of field relationships and sedimentology has proved the presence, in valley-floors, of glacial lodgement tills, meltout tills and associated water-lain deposits. On lower valley-sides, however, a large proportion of the drift has moved downslope by gelifluction or as mudflows. These redistributed sediments contain silt bands which can cause side-slope collapse in cuttings. They also extend over valley-floors and may conceal glaciolacustrine deposits which can contain water at artesian pressures.Higher valley-side slopes and hilltops show extensive modification by periglacial processes, and there are solifluction terraces and nivation hollows. A good understanding of depositional processes is required for any investigation of slope stability.
Summary
Abernant Colliery, in the South Wales Coalfield, disposes of 300 000 tonnes of washery waste each year onto the floor of the Upper Clydach valley. Rockhead is deeply buried by 20–60 m of superficial deposits which consist of a Lower silty stony clay, Middle bedded silts, sands and clays, and an Upper silty stony clay. The Lower silty stony clay, up to 30 m thick, is of variable composition and is probably a glacial melt-out till. The Middle bedded deposits, thickest in the middle of the valley, were probably laid down immediately after the ice had melted. The Upper silty stony clay, 5 to 15 m thick, may have been largely laid down in the form of valleyside mudflows at the same time as the Middle bedded deposits were being formed, but there is also some evidence of movement of surface layers by solifluction. The presence of water-bearing Middle bedded deposits has caused engineering problems in spoil-heap construction and a slip which occurred in 1981 was in part caused by high artesian pressures in silts and sand in these beds. Many simple, low-gradient valley floor slopes in South Wales conceal highly complex drift successions. While the near-surface material may be strong, it may overlie weak, water-saturated material at a few metres depth.
Drift deposits exposed in a large open cut are described. The basal member of the sequence consists of bedded sandy gravels, and these are overlain by a grey silty sand containing abundant striated pebbles and boulders. T h e uppermost deposit consists of a brown poorly sorted drift containing silty and sandy lenses. Analysis of the texture, index properties and densities of the sediments, together with the shape, lithology and fabric of the contained pebbles suggests that the basal gravels were deposited by subglacial meltwater, the grey drift is a lodgement till, and the uppermost brown rubbly drift was formed by solifluction of the till, with the addition of frost shattered sandstone from the hilltop above. It is suggested that the glacigenic sediments are of Devensian age, and the solifluction layer Late Devensian.
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