- The Chuckanut Formation in northwestern Washington (named after the Chuckanut Mountains, near Bellingham), its extension in southwestern British Columbia (the Huntingdon Formation), and various related formations in central Washington (including the Swauk, Roslyn, Manastash, and Chumstick) are fluvial sedimentary formations of Eocene age, deposited from about 54 million years ago to around 34 million years ago. The nature of the deposits and included plant fossils indicate a low-lying coastal plain with a subtropical climate; the nature of the sediments indicates metamorphic sources in northeastern Washington.