The chronostratigraphic age assignments of the major late middle Eocene through early Oligocene terrestrial deposits in western North America are shown in Figures 1–5. Although not all areas are dated and correlated with equal precision, we feel that these correlations are considerably more accurate and highly resolved than those presented by Prothero (1985), Emry et al. (1987), and Krishtalka et al. (1987). These major improvements in geochronology are due primarily to the development of 40Ar/39Ar dating, providing both new dates, and the correction and rejection of certain K-Ar dates. In addition, magnetic stratigraphy provides even finer-scale correlation of localities whose general age is already known from biostratigraphy or radiometric dating.
When Prothero and Swisher (1992) summarized the chronostratigraphic controls on the Uintan known at that time, there were relatively few constraining data points, and few grounds for controversy. The primary data base consisted of Flynn's (1986) magnetic stratigraphy of selected Bridgerian-Uintan sections, plus recently obtained 40Ar/39Ar dates and the magnetic polarity pattern of the Uinta Formation in the Uinta Basin (this volume, Chapter 1). Based on the evidence available at the time, Prothero and Swisher (1992) suggested that the Bridgerian/Uintan boundary occurred early in Chron C20r (based largely on Flynn's data), and that the early/late Uintan boundary (Uinta B2/C boundary in the Uinta Basin) occurred in Chron C20n. They also discussed another possibility—that the early/late Uintan boundary occurred in Chron C19n— but rejected that alternative as less likely for the reasons discussed below.