Glacial till is underlain by Cretaceous marine sedimentary rocks. Soils include frigid Borolls with poorly drained Aquolls in depressional areas. These soils are generally deep, and textures range from sandy to clayey. Potential Natural Vegetation. The natural prairie vegetation is dominantly western wheatgrass, needleandthread, green needlegrass, and blue grama.
Little bluestem is important on sloping and thin soils.
Northern reedgrass, prairie cordgrass, big bluestem, and slim sedge are important species on wet soils. Birds are typical prairie pothole inhabitants. This area is the most productive waterfowl breeding area in the country, especially for gadwall, mallard, pintail, blue-winged teal, and several diving ducks. Other typical pothole associated species are sandhill crane, northern harrier, piping plover, Wilson's phalarope, Franklin's gull, and marsh wren. Rare species include least tern and bald eagle.