The relaxation spectrum of a stratified viscoelastic mantle with phase transitions contains a class of extremely long decaying modes with relaxation times 0 (106 yr), exceeding the characteristic time scale of an individual glacial cycle. Consequently, the net motion of the rotation pole produced by the cyclic loading and unloading of ice sheets would display a nonequilibrium behaviour in the initial stages of an ice age epoch. We present here calculations of the net polar speed as a function of time for the late Cenozoic ice ages. These results show an initial transient net velocity of 0 (1° Myr-1) which, after a few million years, decays to a steady value of 0(0.1° Myr-1). We propose that the length of a typical ice age period around 0(107 yr), is controlled by the time scale required for the steady-state polar wander to drift sufficiently far such that the necessary conditions for the Milankovitch mechanism to operate can no longer be maintained.