¹³⁷Cs has been used as a marker with a maximum concentration in 1964 in relation with the great nuclear explosions. On the same sedimentary material a chronostratigraphy has been established by means of pollen analysis. In the case of the Lake Morat basin, Cannabis pollen is shown to be an excellent chronostragraphical marker. The growing of hemp was stopped around 1950. The sedimentation rates calculated by these two different methods are practically similar, the difference being of the order of the experimental error. In conclusion: one can therefore affirm that ¹³⁷Cs does not diffuse throughout sedimentary accumulation in a lacustrine environment as thought by some authors.