This extensive sand dune region along the northeast coast of Brazil in the state of Maranhão measures more than 40 miles (65 kilometers) in length. These dunes, classified as a parabolic or blowout dune formation, usually form adjacent to beaches that have a large supply of sand and strong prevailing onshore winds. The dunes are gradually encroaching westward because of the prevailing easterly winds. The small city of Barreirinhas is barely visible on the eastern side of the Rio Preguiças. A sand bar and its northwestward development at the mouth of the Rio Preguiças result from the warm south equatorial current flowing northwestward along the northeast coast of Brazil.