The secular drift of the earth's rotation pole, which is evident in ILS data, is a memory of the planet of the last deglaciation event of the current ice age. In order to fit the observed rate and direction of drift with an earth model whose viscosity profile is capable of reconciling the free air gravity and relative sea level data of postglacial rebound, requires a lithospheric thickness which is substantially in excess of that which might be considered an appropriate average thickness for the entire planet. This required thickness, which is greater than 250 km, is nevertheless in accord with independent estimates of the thickness of the continental lithosphere.