On the characteristics and origin of closed depressions in loess-derived soils in Europe—a case study from central Belgium

A closed depression (CD) is a landform where the hillslopes encircle a common sediment depository, and the sediment eroded from the surrounding hillslopes is trapped in the system. Despite the fact that closed depressions (CDs) have been regularly reported in the literature, little research on their origin exists. Therefore, a case study was made of the morphological characteristics and genesis of CDs in central Belgium that are still visible in the landscape. Through field surveys, 26 CDs were found in a 43-km2 study area (southwest of Leuven). Most CDs were situated on a plateau, far from villages but close to important rural roads. rphological features of the CDs (i.e., diameter, area, maximum depth and steepest slope) were measured. The results show that there were significant differences between the morphology of the CDs under forest and under cropland; CDs under cropland were larger, shallower and had gentler slopes compared to those under forest. Based on these morphological features, a hypothetical evolution scheme of CDs was formulated. Through a detailed literature search, eight out of 26 CDs were identified as having an anthropogenic origin; they were former quarries (mines) for calcareous sandstone or iron sandstone. In four other CDs, augerings were made, and detailed studies of their soil profiles within and in the vicinity of CDs indicated that they are quarries for calcareous loess. of the four CDs, with a catchment area of 0.588 ha, the soil erosion rate was calculated, using soil sequences phosphate content and charcoal analysis. Since 1950 AD, the mean annual soil loss due to water and tillage erosion within the catchment of the CD ranged between 5.5 and 9.8 ton/ha or 0.37–0.65 mm. Based on 14C-dating, it was calculated that mean annual soil loss since 430 AD amounted to 2.1 ton/ha or 0.14 mm.