ИСТИНА

Lake Seliger is a system of 24 semi-isolated bays (so-called Plyosy), which stretch for 60 km from north to south on the Valdai Upland. This territory is the main watershed of the East European Plain, which divides the river runoff between the basins of the Caspian and Baltic Seas. The Valdai Upland is in the margin zone of the last glaciation. This area has a typical post-glacial landscape with marginal moraines, kamas, eskers and kettle holes. The Valdai Upland gave the name to the last glacial epoch in the Russian geological systematic - the Valdai glaciation. Traditionally, the Lake Seliger is considered relict lake, which remained after degradation of a huge proglacial lake. But bottom sediments and topography previously have not been studied in sufficient detail. In winter of 2018, the bottom sediments of Lake Seliger were drilled from ice. Drilling was carried out on 5 profiles in the southern part of the lake. Bathymetric and GPR work was also carried out for all profiles. In total, 14 borehols were drilled. Received and delivered to the laboratory 43 m of cores. For samples from reference cores, the loss on ignition, the magnetic susceptibility and the particle size distribution were determined. 15 radiocarbon dates were obtained. In all boreholes at the bottom of the lake, 2-3-meter, and in some cases 6-meter lake mud, have been discovered. The upper part of the mud has a dark gray color due to enrichment with organic matter (30-60%). This is the Holocene sapropel (gyttja). The lower layers of mud in many boreholes have a light gray or blue-gray color, because they contain little organic matter (3-10%). This is a sign of formation in a cold climate - at the end of the last glacial epoch. Everywhere under the mud coarse sands occur. It is deposits of a fairly fast water flow. Many bays of the lake have an elongated and meandering shape and it looks like flooded river valleys. The topography of the bottom of these bays also indicates the fluvial origin. Based on the study of sediments and topography of the bottom and shores of the lake, the following conclusions were made: Lake Seliger is a flooded system of river valleys which appeared after the retreat of the last glacier. The transformation of the river system into the lake took place gradually at the end of the Late Pleistocene, during the Belling-Allred interstadial (about 14.5–12.5 ky BP). The likely reason for the transformation of the river system into the lake system is the differential glacio-isostatic movements of the earth's crust which changed the surface topography. Acknowledgments. This research was supported by the project of the Russian Science Foundation 17-17-01289.