The Flint Hills, historically known as Bluestem Pastures or Blue Stem Hills, are a region of hills and prairies that lie mostly in eastern Kansas. It is named for the abundant residual flint eroded from the bedrock that lies near or at the surface. It consists of a band of hills extending from Marshall and Washington Counties in the north to Cowley County, Kansas and Kay and Osage Counties in Oklahoma in the south, to Geary and Shawnee Counties west to east. Oklahomans generally refer to the same geologic formation as the Osage Hills or "the Osage."