Magnetostratigraphic and biostratigraphic data have been used to construct a chronologic framework for the deposition of the Hepburn's Mesa Formation in the Yellow-stone Valley of southwestern Montana. Deposition spanned an interval from ∼16.8 Ma to at least 14.3 Ma, and possibly to 13.8 Ma. The magnetic data help to constrain the Barstovian land-mammal age in the northern Rocky Mountains, as well as two interval zones defined within it. The major faunal boundary ages determined here are indistin-guishable from ages for these same boundaries at the Barstovian type locality in southern California. Consequently, observed differences in generic and species composition between these regions are likely to reflect biogeographic and ecological variability, rather than temporal separation. When used in conjunction with biostratigraphic data from Jackson Hole, Wyoming, these new dates improve the time constraints that can be placed on the duration (∼1 m.y.) of the "mid-Tertiary unconformity" in the northern Rockies and on the inception at ∼17 Ma of the interval of crustal extension that led to extensive intermontane-basin development during the Neogene.