The Chuckanut Formation is a geological sequence of marine rock found in northwestern Washington, USA, composing part of the Chuckanut Mountains. It is the southern continuation of the British Columbian Huntingdon Formation. The Chuckanut formation is known for tropical Tertiary Period fossils. In sections the formation reaches up to , marking it as one of the thickest sequences of fluvial sedimentary rock found in North America. A fossil turtle shell was recovered from the formation at Clark Point in 1960. The specimen was held in the private collection of the finders until 1981 when it was examined at Western Washington University and identified as an indeterminate member of the Testudinoidea superfamily. Reexamination of the fossil in 2000 showed specimen to belong to the Trionychidae f