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High elevation regions cover a relatively small portion of Earth’s surface, but have an out-sized influence on regional hydrology, continental sediment transport, and global climate. What factors control the lifespan of mountains, and what are the mechanics of the transition to orogenic collapse? The Cenozoic morphology and evolution of the Cordilleran orogen is widely studied and debated in both North and South America, particularly the timing and rates of uplift, the links to possible mantle delamination, and the controls on extensional collapse. At least part of this discrepancy is due to the inherent complexity of atmospheric circulation and moisture sourcing in continental interiors, such as in the modern Rocky Mountains and Altiplano, where the distribution of hydrogen and oxygen isotope ratios (δD/δ18O) of precipitation does not correlate with temperature. Here we reconstruct past elevations across the northern and southern Basin and Range of the western U.S. and the Central Andes of southern Peru, using hydrogen isotope ratios (δD) of paleo-precipitation preserved in volcanic glass from widespread air-fall ashes and ignimbrites. Data span from the paleo-Pacific shoreline across the Cordilleran orogen and are paired with new sanidine 40Ar/39Ar geochronology and detailed chemo- and litho-stratigraphy, providing a novel approach to interpreting changes in δD values over space and time. Paleogene data in the western U.S. show that Pacific-sourced moisture crossed a high elevation Cordilleran hinterland prior to reaching what are now the Rocky Mountains and Great Plains. Mixing with Gulf-sourced moisture likely took place over the central and southern Rocky Mountains, making applications of the modern lapse rate or a Rayleigh modeled lapse rate inaccurate as both use a single moisture source. Data from west of the Sevier foreland, dominated by Pacific-sourced moisture, however, reveal the locations of peak Paleogene elevations. Across the southern Peruvian Altiplano, we observe a similar pattern of paleo-air-mass mixing from relatively higher and lower elevation moisture sources.