In order to investigate if high elevations existed in the Rocky Mountains before the late Miocene, we examined oxygen isotope ratios of 63 Tertiary smectite samples as a proxy for the isotopic composition of precipitation. Of these samples, 51 were also analyzed for hydrogen isotope ratios. These smectites were formed as a result of the weathering of volcanic air-fall deposits that blanketed much of western North America during the Tertiary. Smectite-bearing ashfall samples were collected from Eocene, Oligocene, and Miocene deposits along a transect that extends from the western Great Plains to Yellowstone National Park at modern elevations from ∼900...