 Volcanic andisol soils are common, with fertile mollisols in the lower areas.The central and southwest areas of the county are generally flat floodplains, sculpted by torrents of prehistoric Lake Missoula.A series of dramatic floods known as the Missoula Floods took place 15,000 - 13,000 years ago, as several ice dams melted, forming a series of low steps such as the "Heights", "Mill Plain", "Fourth Plain" and "Fifth Plain".