One of the smallest of the Great Plains ecoregions, the Sand Hills cover an area of about 61,000 km² and are located almost entirely within the state of Nebraska.  The Sandhills contain the largest dune system and one of the largest continuous expanses of native grassland left in North America. The Nebraska Sandhills can be distinguished from other tallgrass and mixed-grass ecoregions by a combination of physiography, soils, elevation, surface water characteristics, and natural vegetation.  The Sandhills consist of various dune types and interdunal valleys, dominance of Cenozoic sands, and a distinct soil type (entisols).  The western and portions of the northern Sandhills are dotted with small lakes and wetlands, including several high-quality sandhill fens. The hills contain a distinct grassland association dominated by sand bluestem (Andropogon hallii), Calamovilfa longifolia, and needle-and-thread (Stipa comata). The federally-listed (threatened) Hayden’s penstemon is endemic to this area. Very little of the Sandhills has been plowed; historically, fire and grazing by migratory herds of bison  played a major role in shaping the landscape. Today, the major disturbance factor is cattle grazing on the large ranches that cover much of the region.