A timescale based on years BP (conventionally before 1950). Subdivisions on the scale are defined by particular units of duration (e.g. 106 years, 109 years) rather than reference points in actual rock successions. An example of such a subdivision is the placing of the boundary between the Archaean and the Proterozoic at 2500 Ma ago (i.e. 2500 × 106 years) ago.