Maps of aspherical structure in the vicinity of the core-mantle boundary have been constructed from a tomographic analysis of P' and P'AB travel times in the distance range 150¿--180¿. These maps reveal long-wavelength features not previously detected by mantle tomography. The peak-to-peak amplitude of these features is on the order of 1 s in vertical travel time, too large to be explained by conventional thermal boundary layer models or dynamically supported topography on the core-mantle boundary. We hypothesize the existence of one or more chemical boundary layers whose large-scale accumulations on the surface of the core may in some ways be analogous to continents at the earth's surface.