[en] All three cesium isotopes, natural Cs-133 (stable) plus fission Cs-135 (half-life 2.3 Ma) and Cs-137 (half-life 30 y), were measured in two coastal sediment samples using a thermal ionization mass spectrometer equipped with a retarding potential quadrupole lens filter. The ¹³⁵Cs/¹³³Cs ratio was about 1x10^-9 while ¹³⁷Cs/¹³⁵Cs was about 0.5 owing to the decay during the past 30 years from their production ratio of about 1. This appears to be the first detection of fallout Cs-135 in nature. Being an isotope ratio, ¹³⁷Cs/¹³⁵Cs is a far more powerful chronometer-tracer than Cs-137 alone. In order to explore its potential, the authors present briefly two simple models to illustrate how this ratio may be used to quantitatively estimate recent sedimentation and erosion rates