For most oceans the basic feature of the elevation of ridges is a uniform relation between subsidence and age directly associated with the creation and cooling of the oceanic plate. In the North Atlantic we show that superimposed upon this subsidence curve are long-wavelength (>400-km) residual elevation anomalies up to 1200-m amplitude between 50¿N and 10¿N that are linearly related to regional variations in the free air gravity field. It is unlikely that this relation can be accounted for by sources in the lithosphere and hence it may be evidence of mass excesses or flow in the upper mantle. Further, the correlation between free air gravity and residual depth is two dimensional and passes through the origin with the same sign and gradient as were predicted by investigations of convection in a Newtonian fluid. We assume a correlation between long-wavelength residual elevation and free air gravity anomalies. Hence where the free air gravity field is close to zero, there should be no residual elevation anomaly. Thus the absence of a large free air gravity anomaly in the North Pacific, other than close to the Hawaiian Island chain, permits us to extend the uniform depth versus age relation from 80 to 150 m.y. B.P. The very small increase of depth with age in the Cretaceous and the uniform heat flow through old oceanic crust are evidence that the oceanic plate has a finite thickness, which can be detected in crust older than 80 m.y. B.P.