3. 1. Spatial distribution and structure of closed depressions (CDs)
CDs are small landforms, commonly found in the European loess belt (Kołodyńska-Gawrysiak & Poesen, 2017). 5367 CDs have been mapped in the Nałęczów Plateau. The total area of the CDs bottoms is 7.26 km2, representing 1.46% of the study area. CDs are not evenly distributed in the studied region. In some places, their density reaches 1 CD·ha-1 (max. 1.9 CDs·ha-1). Areas where CDs density does not exceed 1 CD·ha-1 predominate in the Nałęczów Plateau (Figure 4). Fossil soils and colluvial sediments in 5 CDs were studied in detail and dated (Kołodyńska-Gawrysiak et al., 2017; Kołodyńska-Gawrysiak, Poesen & Gawrysiak, 2018; Kołodyńska-Gawrysiak, 2019). These CDs are all filled with a 80-90 cm thick Holocene soil-sediment sequence, consisting of two colluvial sediment layers (C1 and C2) and a fossil Colluvisol (S2) Ab2g-Cg (Table 1, Figure 3). This sequence overlies a fossil soil (S1) Ab1g-Bht-Btg-BtC-C that developed on loess in situ. On the slopes this soil has the horizons A-E-Bt1-Bt2-BtC-C-CHCl+ or predominantly A-Bt1-Bt2-BtC-C-CHCl+ as a result of soil erosion (Figure 3). Calcareous loess was found below the soil surface of the CD slopes (Figure 3). Below the CD bottoms, calcareous loess was not detected. The dating results indicate that fossil soil S1 developed during along period from the Late Vistulian until the Late Boreal or Early Atlantic Period (10130± 60 BP until 7.99 ± 0.65 BP). The older colluvial layer C1 is 30-60 cm thick. The formation of this layer occurred from the Late Boreal or Early Atlantic Period until the (Middle)Late Bronze Age or Early Iron Age (7.99 ± 0.65 BP until 3.67 ± 0.33 BP/2.37 ± 0.13 BP). The fossil Colluvisol (S2) Ab2g-Cg developed on the oldest colluvial layer C1 (Figure 3). This fossil Colluvisol developed from the Late Bronze Age or Early Iron Age until the Early or High Middle Ages (3.67 ± 0.33 BP/ 2.37 ± 0.13 BP until 7th–8thc/12th–14th c). At the top of the CDs infilling younger colluvia C2, 50-70 cm thick with Colluvisol have been found (Table 1, Figure 3). The younger colluvia C2 layers were deposited during the last several hundred years, i.e. since the Early Middle Ages until today (7th–8th c until today) (Kołodyńska-Gawrysiak, 2019).