Cover beds are usually not regarded of use for relative dating. The examples discussed in this chapter demonstrate otherwise. Central European cover beds usually are of Pleistocene age, and they can be utilized for distinguishing older landforms, such as slope failures, covered by one or more cover beds, from those which are not covered by periglacial deposits. In the western USA, where intervening soil-forming episodes provide a stratigraphic framework for such deposits, the stratigraphic value of cover-bed and soil successions is tested on various types of landforms. However, dating landforms relatively by overlying cover beds calls for due consideration of erosion-induced hiatuses and of tectonically induced processes out of phase with those driven by climate.
Slope deposits that formed by unconcentrated dislocation and that cover slopes in a rather uniform way are referred to as “cover beds” in this book. They consist of materials from upslope but may contain admixed eolian matter. They are commonly multilayered with the individual layers separated by disconformities (lithological discontinuities). The main scope of this book is to provide a comprehensive state of the art of this particular type of slope deposits in the mid-latitudes. Being a major component of the near-surface ground, cover beds should be included as an integral part into the so-called critical-zone concept and may partially replace the existing “biomantle concept” for the mid-latitudes.
Slope deposits, which veil entire slopes or large parts of them in a rather uniform manner (cover beds), are ubiquitous in the subdued mountains of Central Europe. Here we provide an overview of the current state of knowledge on these deposits. The Central European cover beds are divided into (1) the upper layer that is ubiquitously distributed and displays a relatively constant thickness; (2) the intermediate layer the distribution of which is mainly restricted to flat relief, to slope depressions, and to lee-ward facing slopes; and (3) the basal layer, which is rather widespread again. Both the upper and intermediate layer contain intermixed loess, whereas the basal layer is free of loess and typically has a high bulk density. Aside from the loess content, the composition of the layers differs, reflecting varying portions of crushed and chemically weathered rock allocated from up-slope. This causes notable diversity depending on bedrock and, thus, induces remarkable regional differences. Cover beds were mainly formed by periglacial gelifluction. The upper layer formed in the Late Glacial possibly during several short episodes of activity. In contrast, the underlying layers may be diachronous; nevertheless, they display recurring vertical sequences. This is probably due to the fact that loess-free layers usually could not deposit as long as there was loess in the environs, which may have been inherited from older deposits. Thus, the last phase of surface wash, during which older loess was removed, determines the age of the lower layers.
Cover beds form a rather uniform veil of most slopes in Central Europe and, most likely, all over the mid-latitudes. Accordingly, their properties are of utmost importance for the formation of soils, for slope hydrology, and for slope dynamics. More research is, however, needed, as there is still insufficient information on the properties of cover-bed successions reaching deeper than 1 m, and there are still many areas where cover beds have not yet been studied at all. The potential use of cover beds for paleoenvironmental reconstructions is also still limited by the unsatisfactory techniques that are currently available for their numerical dating. Furthermore, better techniques for modeling the distribution and the properties of cover beds are required to forecast their influence on flooding events.
The occurrence and spatial distribution of cover beds are decisive for modern hillslope morphodynamics. This aspect is strongly associated with the geotechnical properties of cover beds and especially with their anisotropy. The impact of periglacial cover beds on mass movements, in particular landslides in subdued mountains, will be discussed in this section. The case studies in this chapter show that soil-physical and soil-mechanical properties significantly influence the forces in periglacial cover beds and, thus, directly control the slope stability. In contrast to long-lasting stable geomorphological factors, the reduction of shear strength, friction angle, and cohesion as well as the deformability may induce abrupt instability. Abrupt instabilities in periglacial cover beds particularly arise, when the soil-water content increases and, consequently, both the pore water and the hydrostatic pressures rise. In particular, we show that landslides to a large extent occur in hillslope sediments which are weakly consolidated and sensitive to water penetration.
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