The interpretation of free air gravity anomalies over present day centers of post-glacial rebound has always been controvesial. We show that the observed gravity anomalies over both Fennoscandia and Laurentia and the associated relative sea level variations since deglaciation are explicable with a sample Newtonian viscoelastic model of the earth if the earth model includes the density jumps in the mantle associated with the phase transitions at 420 and 670 km depth. The result has important implications for mantle convection since the observed free air anomalies are very sensitive to the viscosity of the mantle beneath the deepest phase transition.