ИСТИНА

Late Saalian glacial uplands with hilly or kame and kettle topography covered by a loamy mantle of varied thickness are widespread at the centre of the Russian Plain. A local gully system called Puzhbol at the NE part of the Borisoglebsk Upland has been studied since the early 1960-s when several peat exposures were discovered on its sides (Novenko et al., 2005). It has been used since as one of the main reference sections for reconstructing the postglacial landscape history of the entire Central European Russia (Rusakov et al., 2015). Yet the interpretation of origin and timing of the related sedimentary and erosional events remained controversial, especially due to several hiatuses in the Weichselian deposition sequence. 3D-modelling of a local sediment sink on the left gully side was based on a new series of vertical and horizontal cross-sections and cores up to 7 m deep. Supplementary cores have shown the previously missing part of the section consisting of a stratified loamy thickness above the basal peats. Coupled with grain size, LOI, and spore-pollen analyses and 14C dating of organic-rich deposits, it allowed revealing the fuller structure, stratigraphy, and chronology of the Late Pleistocene landscape and geomorphic change of the key site. That local section represents an infill of the MIS 6 glacial kettle hole that was initially occupied by a shallow stagnant water body in the Early Eemian, right after or even during the degradation of a small dead ice block (Shishkina et al., 2019). That shallow pond-like lake had generally persisted until the Late Weichselian time (MIS 5 - MIS 2) periodically drying up and transforming into a forested bog during the interglacial and interstadials and reverting back during the colder stages of the Weichselian. The Late Pleistocene to Holocene transition was associated with the most dramatic environmental changes and abrupt fluctuations. Response in the local morphodynamics first involved activation of slope mass movement followed by a gully incision reaching the site by regressive head knickpoint retreat. The observed sediment record provides evidence of at least 4 incision-infill cycles of linear erosion landforms of the Late Glacial to the Late Holocene age. The first incision into the gradually undulating surface composed of the lacustrine and colluvial loams and its subsequent infill by stratified silty colluvium occurred not later than 6.5 cal. ka BP (Belyaev et al., 2020). It is fixed by the next incision phase infilled by pedosediments of reworked humic, eluvial and sub-eluvial horizons of the upper slope soils enriched by pyrogenic charcoal. The third incision stage can be linked to regressive growth of the main gully head and its branches (ca. 1.4 cal. ka BP) triggered by the onset of cut-and-burn agriculture practices determining conditions favourable for active deposition of agrogenic colluvium. Modern gully incision was preceded at least by another infill phase and occurred between 1941-1968 AD. That sequence of erosional and depositional episodes demonstrates a rather drastic and recent evolution of the initially watershed landscape due to a progressive incision of a local gully. The latter has partially erased the sedimentary record of a quite prolonged lacustrine stage during the Weichselian concealing the full extent of postglacial levelling of the interfluvial topography.