A salt marsh is a coastal marsh that forms on mud flats. They usually form in very sheltered inlets and estuaries, or behind spits (places where fine sediment accumulates). Salt marshes form as vegetation builds up on these mud flats .
Sand dunes are created by strong winds and not by coastal erosion or deposition. As sand is blown up a beach is forms small hills. These are often rooted together by long-rooted grasses such as marram grass. Marram grass is usually planted to reduce the erosion of the otherwise unstable sand dunes.