polje, (Serbo-Croatian: “field”), elongated basin having a flat floor and steep walls; it is formed by the coalescence of several sinkholes. The basins often cover 250 square km (about 100 square miles) and may expose “disappearing streams.” Most such basins have steep enclosing walls that range from 50 to 100 m (165 to 330 feet) in height, giving rise to the name “blind valley.” The flat floor of a polje is characteristically covered with a soil composed of the residues of limestone solution. These areas may constitute the only arable part of the rock wasteland in a karst region. See also karst.