<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> 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>
what are the two sides of a sand dune
what are the two sides of a sand dune
stoss ( upflow ) side `` slip face '' in the lee side
stoss ( upflow ) side `` slip face '' in the lee side