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.
- © 1983 The Geological Society