The Formation of the Lias Cuesta (Luxembourg) in the Light of Present-Day Erosion Processes Operating on Forest Soils

ABSTRACT

The escarpment of the Lias cueasta in the southern part of Luxembourg is formed by the Lias sandstone. The lowland in front is underlain by poorly consolidated Keuper marls. In several places the equally “weak” Psilonoten marls form a secondary step between the main escarpment and the lowland. As this relief configuration cannot be accounted for by lithological differences between the underlying rock types, the hypothesis is tested here that it is due to differential erosion of the soil under “natural” conditions, i.e. deciduous woodland. It is shown that this hypothesis can be accepted when the erosion processes are viewed in their ecological context. Today, erosion by splash is one of the major processes responsible for lowering the forest floor in Luxembourg. It was found that, other conditions being equal, the efficacy of this process is determined more by the absence of a protective litter cover than by the erodibility of the soil material. Due to a much larger exposed surface area, especially during the summer months, the Keuper soils are indeed more subject to splash than the Psilonoten soils, whereas the soils of the Lias sandstone remain protected by a litter layer virtually throughout the year. Leaf consumption by earthworm, mainly Lumbricus terrestris, is the principal cause of the observed differences. The ecological conditions of the poorly drained Keuper soils are most favourable for this earthworm species.