The Chuckanut Formation of Northwest Washington is a 6000 m thick unit of fluvial sandstone that is Early Tertiary (Johnson 1984). Fossilized leaves are found throughout the formation and are useful in assessing paleotemperatures. Climate Leaf Assemblage Multivariate Program (CLAMP) uses leaf morphology as a proxy for climate. Large, smooth leaves with attenuate tips indicate warm, humid temperatures. Smaller notched leaves represent cool, dry climates. The Chuckanut Formation yields both types of leaf imprints. The basal Bellingham Bay Member contains large smooth leaves, where as the overlying Padden Member has smaller notched leaves. Fossils of the Bellingham Bay Member produce a mean annual temperature of (MAT) 15o, and a mean annual range in temperature (MART) from summer to winter of 10o. The Padden Member yields a MAT=12o, MART=18o, and 100-200 cm of precipitation per year (Mustoe and Gannaway 1997). Preliminary CLAMP data from a site near the basal contact of the Bellingham Bay Member suggest a MAT of 19o, a MART of 0o, and 200 to 350 cm of precipitation per year. Deposition of the Bellingham Bay Member occurred in a climate that was 4 o -7 o warmer than that of the Padden Member. In order to interpret the paleoclimate inferences that can be made from the Chuckanut's fossil record, we need to better constrain its age. We will present preliminary magnetostratigraphic results, combined with existing radiometric dates, which will better constrain the age of both the anomalously warm interval near the base of the Chuckanut and the shift to cooler climates between the Bellingham Bay and Padden Members.