<P> In physical geography, a dune is a hill of loose sand built by aeolian processes (wind) or the flow of water . 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 stoss (upflow) side, where the sand is pushed up the dune, and have a shorter "slip face" in the lee side . The valley or trough between dunes is called a slack . A "dune field" or erg is an area covered by extensive dunes . </P> <P> Dunes occur in some deserts and along some coasts . 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 (fluvial processes), and on sand or gravel beds of rivers, estuaries and the sea - bed . </P>

What are the two sides of a sand dune