14С dating of fragments of yedoma sequences in the north of Western Siberia, δ18О values of ice wedges of isolated fragments and reconstructed approximate mean January air temperatures during the periods of ice wedge formation. Reconstructions of mean January temperatures (Tmean Jan., °C) were carried out by comparing the isotope composition of modern ice veinlets (δ18Oiv) and mean January temperatures (t °J) for the period of ice veinlet growth, i.e., for the last 60-100 years (Vasil'chuk 1991). As a result of this comparison, the equation (1) is obtained: Tmean Jan=1:5×δ18Oiv (± 3 oC). The range of ±3°C indicates the average range of variations of the reconstructed temperatures (Vasil'chuk,1991).