Some geomorphological implications of the cryo-aeolian deposits in western Belgium

Abstract

In a north-western region of Belgium (Fig. 1), we are studying the interaction between cryo-aeolian deposits and geomorphology. The relief is smoothed by a cover of loess and aeolian sand (Fig. 2).
Typical slope values can be observed on loess-covered surfaces, which vary according to sand content: 5%, 6% or 7%, depending on the purity of the loess (Fig. 3A). Similarly, in convex and concave landforms, the dominant slope values are 2%, 3% or 4%. These dominant values are even more pronounced in the straight sections of the slopes (Fig. 3B).

There is a characteristic geometric form of slope profiles on loess-covered hillslopes. We present these slopes using their slope gradient curve (Fig. 4), which is more suitable than a classical profile for describing gentle slopes. These curves are defined by functions of the form y' = f(x).

On flat interfluves, the cryo-aeolian cover is often absent when the substrate consists of clay. We explain this phenomenon by the high sensitivity of clay to mass movements under periglacial climatic conditions: as vegetation is destroyed, there are no obstacles to trap cryo-aeolian sediments.

In the transitional zone between aeolian sands and loess, the boundary between different grain-size distributions is often linked to geomorphological context, particularly at the foot of slopes facing W, NW or N (Fig. 5) or within valleys oriented from SW–NE to W–E. This is due to the differing transport mechanisms of sand (by saltation) and loess (in suspension). The spatial distribution of these different grain-size fractions suggests that, during the late Würm period, the prevailing winds likely blew predominantly from the W–SW.