Grand Canyon was incised in Campanian time by the California River, a mainstem river that transported arc detritus from the Mojave-Sierran arc of California to the orogenic foreland. The modern Geand Canyon landscape passes two fundamental tests of long-term, kilometer-scale equilibrium erosion since Laramide incision: (1) Perturb the modern landscape with hard rock, and it quickly erodes back very close its initial form, and no farther; the disequilibrium erosion rates are orders of magnitude greater than the long-term erosion rate; (2) The thermal history of the modern surface does not vary significantly with elevation. A key prediction of long-term equilibrium erosion is that Grand Canyon existed through essentially all of Cenozoic time, and so evidence of detritus derived from it should be found in Tertiary basins in surrounding lowlands. Certain orthoquartzite clasts within gravels in the upper part of the Sespe Fm., a mid-Tertiary delta complex in coastal southern California, appear to exhibit a combination of petrographic, geochemical, paleomagnetic and detrital zircon signatures that are unique to the Shinumo Quartzite, a ca. 1.1 Ga orthoquartzite known only from the Upper Granite Gorge region of Grand Canyon. This discovery corroborates earlier suggestions that an “Arizona River” drainage system, precursory to the formation of the modern Colorado River (but with a much more limited drainage area), was supplying detritus to the coastal borderlands in mid-Tertiary time, and that the post-Laramide drainage divide had locally migrated as far east as the cratonic interior of North America by the end of Laramide time. Intergration of inland drainages with the Arizona River six million years ago marked an eastward shift of the continental divide to its present position in Colorado.