In order to investigate if high elevations existed in the Rocky Mountains before the Late Miocene, oxygen isotope analysis was conducted on 57 Tertiary smectite samples as a proxy for d¹⁸O_{precipitation}. These smectites were formed as a result of the weathering of volcanic airfall deposits which blanketed much of western North America during the Tertiary. Smectite-containing ashfall samples were collected from Eocene, Oligocene, and Miocene deposits along a transect which extended from the western Great Plains of western Nebraska and South Dakota to the continental divide in Yellowstone National Park and at modern elevations from ~900 to ~2800 meters. Oxygen isotope ratios of Eocene smectite samples from central Wyoming are approximately 2-6 permil heavier than calculated oxygen isotope values of theoretical smectite in isotopic equilibrium with modern precipitation. These relatively heavy isotopic values are consistent with average temperatures which were 10-20°C warmer in North America during the Eocene. Oxygen isotope ratios of Eocene samples from western Wyoming and Oligocene and Miocene samples collected throughout the study area increase ~10 permil linearly from sample sites located at the crest of the Rocky Mountains to sites in western Nebraska and South Dakota. These results closely mimic the distribution and values of calculated oxygen isotope ratios of theoretical modern smectite. We suggest modern atmospheric circulation patterns and the resulting distribution of d¹⁸O_{precipitation} have persisted since the early Tertiary. Because this climate system requires interaction with and modification by high elevation areas in the Rocky Mountains, these data from early Tertiary smectites suggests the Rocky Mountains were at high elevation (near modern elevations) since at least that time. This conclusion does not agree with the traditional views that the Rocky Mountains were uplifted in the Late Miocene and Early Pliocene, but it does agree with many recent studies which suggest high elevations in the Rocky Mountains have existed since the end of the Laramide Orogeny in the Early Tertiary.