The widening of valley incisions by soil fall in a forested Keuper area, Luxembourg

In the gently undulating Keuper region of Luxembourg valley incisions in mixed oak‐beech forests are being widened by a process of soil fall. This process is optimized by a number of ecological conditions. Soil profiles examined in the Deifebaach catchment, located between Larochette and Ettelbruck, generally have an abrupt boundary between the silty surface soil (A2 horizon) and the dense clayey subsoil (B horizon). This boundary is marked by the formation of a dense root mat. The subsoil contains a large proportion of oriented clay and is subject to swelling and also to dispersion. Clay particles translocated downwards increase the density of the subsoil and decrease its permeability. The soils are very poorly drained, being waterlogged for much of the year, and have experienced some fragipan development. When the subsoil is exposed it disintegrates into an incoherent mass of aggregates and recedes beneath the root mat which supports the overlying material until it eventually collapses. The nature and amount of soil fall recession is examined along a 320 m reach of die Deifebaach. It was found that at least 30 per cent of the right bank and 40 per cent of the left bank are subject to recession by soil fall.