How do coastal sand dunes form?
Dunes are formed when wind and waves transport sand onto the beach. The ultimate source of the sand is often glacial till that resides offshore, or is eroded from nearby cliffs by waves, and transported along the shoreline by longshore currents (see coastal sediment processes). Rivers can also transport sand to the coast from inland deposits. Waves wash this sand up on the beach, and also remove it during storms. One beach can vary quite substantially with the seasons. During the winter, stronger waves pick up more sand (but leave behind larger sediment), causing the beach to be steeper and coarser. In summer, gentle waves transport sand onshore and the beach takes on a more shallow slope with finer sand. The shape and size of dunes formed depends on the shape of the beach. On a shallow-sloped beach, the energy of waves is dissipated and the suspended grains of sand are deposited, resulting in greater dune potential. Steeper beaches do not dissipate this energy as well, and sand is mor
