The Cowlitz Formation in southwestern Washington is exposed along Olequa Creek between Winlock and Olequa and preserves an important Paleogene record of early Tertiary life in western Washington. Brought to notice through fossil discoveries made in 1911 during studies of the region’s Tertiary paleontology, it was examined as part of the broader Eocene sequence alongside the Tejon Formation. Its rocks include sandstone, shale, conglomerate, minor shaly limestone, and many interlayered beds of tuff and basaltic lava, indicating partly marine and partly brackish-water origins. Fossils from the formation include shark teeth and 45 marine invertebrate forms, 30 of them then newly identified.