ИСТИНА

The structure of the ground wedges near Nizino settlement (Leningrad region) has been studied in detail (Streletskaya, 2017). A research team from the Institute of the Earth Physics explored the same quarry in 2002 and found about ten soil wedges spaced at 30–50 m intervals. The wedges enclosed fossil soil fragments with radiocarbon ages from 10230±40 14Сyr BP [GIN_12172] to 9240±90 14С yr BP [IGAN _3265]. Authors argue that features of soft sediment deformation caused by large earthquakes in the Holocene (Nikonov, Rusakov, 2010). Large wedges and polygonal forms are major indicators of the periglacial environment. Relict polygonal topography found in the European lands and characterizes almost all of geomorphic levels associated with Neopleistocene. As mean annual ground temperatures or water content in the active layer increase, primary soil wedges transform into two-stage structures, with their upper and lower parts developing as soil and ice wedges, respectively, in the conditions of continuing frost cracking. The two-stage structure of fossil wedges is quite reliable evidence of permafrost and a marker of the past active layer thickness. Ice wedges melt during permafrost degradation and leave secondary structures (casts) in their place, while the host sediments experience deformation. Repeated thawing and freezing of fine-grained sediments in the permafrost zone leaves a mark on the relative percentage content of quartz and feldspar, because the two minerals have different grain size limits of cryogenic disintegration (Konishchev, Rogov, 1994). Correspondingly, the ratio of quartz-to-feldspar percentages in coarse silt and fine sand fractions (cryogenic contrast coefficient, CCC) has implications for cryogenic weathering of sediments. This ratio exceeds 1 for cryogenic conditions, the higher the CCC the colder climatic conditions. The coefficient of cryogenic contrast for enclosing, filling and underlying deposits of the ground wedges is close to or greater than 1, thus indicating to the active processes of cryogenesis. The ground wedges, sand wedges and ice wedges had formed since 15.5 thousand years ago as a result of frost cracking (Fig.). The results of the studies do not testify to the hypothesis of seismic origin of the ground wedges near Nizino settlement. The ground wedges have not been the relict seismodislocations due to the Holocene earthquakes. The study was supported by grant 16-05-00612 from the Russian Foundation for Basic Research and was carried out as part of the government contract “Changes in the Earth’s Cryosphere under Natural and Manmade Factors” NIRAAAA-A16-16032810095-6.