In profiles taken of old snow on Alpine glaciers, a new reference horizon for dating has been discovered in the form of atmospheric fallout from the Chernobyl nuclear accident. The dating method is based on the known time of deposition of the fission products immediately after the Chernobyl accident on April 26, 1986. In future, this reference horizon may be used for the determination of net accumulation over a long period. The peak value of gross beta activity from the Chernobyl accident is of the same order of magnitude as the peak value from atmospheric nuclear weapons tests in 1963.