We analyse the geomagnetic polarity time scale (GPTS) since the Upper Jurassic by displaying the successive lengths of polarity intervals as a function of their order of occurrence. The sequence consists of three segments. Between the Upper Jurassic and the Lower Cretaceous, segment A comprises intervals of short duration, with a mean duration of about 0.29 My, and no clear long-term evolution. Segment B begins around 130 Ma, displays a sudden increase of the duration of the magnetic intervals, an interval of maximum duration, the normal Cretaceous superchron, and a long and erratic sequence of intervals with decreasing average duration between 85 Ma and about 25 Ma. From 25 Ma to the present, segment C consists of intervals of short duration with a mean value of 0.23 My. This description suggests that the Earth's magnetic field could have experienced a fairly stationary regime until slightly before the onset of the Cretaceous superchron, when the regime has been rapidly and strongly perturbed before progressively returning to another stationary regime about 25 Ma ago. A geophysical explanation for this sequence of events could be that the geodynamo has been perturbed by the arrival of some cold material at the core mantle boundary. As this material would have heated up, the geodynamo would have been brought back to its stationary regime.¿ 1997 American Geophysical Union