Mima mounds are low, flattened, circular to oval, domelike, natural mounds found in the northwestern United States, Idaho, Oregon, and Washington, that are composed of loose, unstratified, often gravelly sediment that is an overthickened A Horizon. These mounds range in diameter from 3 to more than 50 m; in height 30?cm to greater than 2 m; and in density from several to greater than 50 mounds per hectare.