THERE is accumulating evidence1,2 that the average location of the Earth's magnetic pole has moved relative to the deep mantle over the past 200 Myr. This phenomenon of 'true polar wander' may be caused by the advance and retreat of ice sheets3 or by mass redistribution in the Earth's interior due to changes in the pattern of mantle convection4,5. New analyses1,2,6 of polar-wander data show a significant shift of the pole in the late Cretaceous, but this period is thought to have been too warm for glaciation to have occurred. Thus, most of the true polar wander must be due to mass movements in the mantle. Here we show, by analysing the appropriate equations for polar wander, that both viscosity and chemical stratification in the mantle are important in determining the rate of polar wander. The dynamical effects of chemical stratification and high viscosity (>1023 poise) in the lower mantle are to decrease polar-wander speed to a level consistent with the averaged velocities inferred from palaeomagnetic data. Whole-mantle convection models, without non-adiabatic density jumps at 670 km depth and with viscosities less than 1023 P, would produce wandering rates in excess of 5° per Myr.