In physical geography, a dune is a hill of sand built by either wind or water flow. Dunes occur in different shapes and sizes, formed by interaction with the flow of air or water. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind. The valley or trough between dunes is called a slack. A "dune field" is an area covered by extensive sand dunes. Large dune fields are known as ergs. Some coastal areas have one or more sets of dunes running parallel to the shoreline directly inland from the beach. In most cases, the dunes are important in protecting the land against potential ravages by storm waves from the sea. Although the most widely distributed dunes are those associated with coastal regions, the largest complexes of dunes are found inland in dry regions and associated with ancient lake or sea beds. Dunes can form under the action of water flow and on sand or gravel beds of rivers, estuaries and the sea-bed. The sand dunes of Death Valley National Park are excellent places for nature study and recreation. All of the dunes in Death Valley National Park are protected as wilderness, off-road vehicle travel is not permitted anywhere in the park. Sand boarding is only allowed on the Mesquite Flat Dunes and prohibited on all other dune systems in the park to protect sensitive plants and animals. The slip faces are on the concave sides of the dunes.