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Hagerman Fossil Beds National Monument (HAFO), Idaho, is internationally significant for the vertebrate fossils from its hundreds of fossil localities spanning more than a million years of the Pliocene. This study establishes the background for comparisons among localities in the Glenns Ferry Formation within HAFO by describing the nature of the fossiliferous deposits, using published data to revise age estimates for HAFO localities, and relating the relative differences in elevation of the fossil-bearing localities to particular time horizons. Fossils from the anthills and blowout localities are considered to be essentially in situ stratigraphically. Species of modern harvester ants do gather fossils from more than the immediate area, but the maximum vertical movement is probably within the resolution of elevation possible at most HAFO localities. The microstratigraphy of blowout localities is described here for the first time, with vertebrate fossils derived exclusively from layers of about 12-cm thickness. Fossils recovered as surface float should be excluded from stratigraphic comparisons. Based on a combination of paleomagnetic and radioisotopic studies, the maximum age for the top of the Glenns Ferry Formation exposed at HAFO is estimated at 3.11 Ma, and the minimum age for the lowermost exposure is estimated at 4.18 Ma. It is improbable that strata of the Glenns Ferry Formation exist at HAFO that are younger than 3.04 or older than 4.29 Ma. Finally, using marker beds and published stratigraphic sections, the differences in elevation needed to compare localities in the Glenns Ferry Formation at HAFO against a generalized composite section are established. Fossil-bearing sites within this framework can be placed in proper stratigraphic context, and faunal change thereby can be evaluated more precisely. [ABSTRACT FROM AUTHOR]