On a regional scale (longer than flexural wavelengths) isostasy can be used to probe the 4-D evolution of the lithosphere as elevation responds to changes in lithospheric structure. For example, isostasy places significant constraints on the Phanerozoic evolution of the Southwestern US. During most of the Phanerozoic, the Western US was a continental platform. A section of shallow marine and continental sediments consistent with the global eustatic curve indicates that this lithosphere was essentially stable. No Precambrian or Early to Mid-Paleozoic precursors of the Colorado Plateau or southern Rocky Mountains were apparent. Late Paleozoic Ancestral Rocky Mountain formation, subsequent erosion and magmatic activity may have modified the lithosphere defining the Colorado Plateau. However, its bulk structure is constrained by isostasy in that it remained close to sea level into the Late Cretaceous (Campanian). Data from xenoliths erupted by Cenozoic volcanism and low heat flow in the interior of the Colorado Plateau provide evidence for a cool, relatively thick Colorado Plateau lithosphere at the time of Laramide southern Rocky Mountain compressional deformation that was capable of transmitting plate-boundary stresses from the west into the interior of the plate.