The Alkali Canyon Formation is a geologic unit in Oregon’s Arlington basin, named for nearby Alkali Canyon and formally defined in 1981 after being distinguished from rocks long grouped within the Dalles Formation. It preserves fossils dating from the Tortonian to Zanclean stages of the Neogene, within the Hemphillian North American land-mammal age. Made of basalt gravel and tuffaceous sediment, it consists largely of material eroded from volcanic rocks on the northern flank of the Blue Mountains. Lying above the Columbia River Basalt Group and mostly beneath Quaternary loess, it records the filling of the Arlington basin and changing landscapes and animal life in north-central Oregon.