It has been generally recognized that the flows in Mare Nubium and southern Oceanus Procellarum are relatively shallow. Much of Oceanus Procellarum and the central craters of the circular maria are filled with thicker lavas (Baldwin 1949). There are two lines of reasoning applied here. Some 94 percent of the moon's craters were premare (Baldwin 1964). Therefore, the lavas of the maria covered a preexisting cratered surface and, where the flows were thin, the rims of buried craters often project above the inner surface. Second, the lava surface is depressed below the mean sphere and the regions which do not show projecting crater rims are depressed the greatest amount (Baldwin 1949). If the lavas caused the surface to sink by an isostatic mechanism, they must be thicker in the lowest areas than they are where the crater rims are seen.