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The ice-rich core formation within the core of the lithalsa in the Sentsa River valley (West Buryatia) was studied. The concentrations of stable oxygen and hydrogen isotopes in the ice were determined, the plant residues in the enclosing deposits were aged. It was found that the lacustrine loams enclosing the ice body were accumulated about 4700-7000 years ago. The ice core of the lithalsa was characterized by δD values from 141.9 to-159.8‰, δO from-19.52 to-21‰, and d within 4.1-17.29‰. Small variations in the isotope composition of the ice-soil material within the lithalsa core point to its formation in an open system with water inflow from outside. The values of δO and δD were much closer to those for recent middle-winter and late-autumn atmospheric precipitation, as well as for riverine water, which pointed to a Holocene age of the ice. [ABSTRACT FROM AUTHOR]