Abstract Eocene nonmarine sedimentary rocks that occur in northwest and central Washington as a widespread series of outcrops are evidence of a meandering river system that existed prior to the mid-Tertiary uplift of the North Cascade Range. Arkosic strata appear to have initially been deposited in a basin that was later divided by strike-slip faulting, producing outcrops of the Swauk Formation on the eastern flank of the North Cascades, and the Chuckanut Formation to the west. Plant fossils are abundant in both formations, but the Swauk paleoflora has received little study. The Chuckanut Formation paleoflora records a marked shift in the region’s paleoclimate. The Late Paleocene to Middle Eocene Bellingham Bay and Slide Stratigraphic Members, which comprise the lower 6000 m of the formation, contain diverse assemblages of subtropical plant fossils. In contrast, the overlying 3000-m-thick Padden Member contains taxa indicative of a warm temperate paleoclimate. An unconformity may separate the Padden Member from older Chuckanut strata, and the age of the Padden Member has not been determined. The climate shift may have been a Late Eocene fluctuation, but the possibility that the floral changes represent the transitional Eocene-Oligocene cooling event cannot be discounted. Animal fossils from the Chuckanut Formation include aquatic mollusks and a soft-shelled turtle, and track impressions from a variety of birds and mammals.