The energy of breakers expended along a coastline causes erosion and transportation of shoreline materials. Weak or soft shoreline materials are eroded rapidly by wave action to form marine scarps , while more resistant materials are eroded slowly to form marine cliffs . Beaches are thick wedge-shaped accumulations of sand shaped by the swash and backwash of water along the shoreline. The sands transported by littoral drift can form spits , bars and pocket beaches along a coast. Tides are regular fluctuations in sea level caused by the gravitational forces of the sun and moon.
Tides drive ebb and flood currents which redistribute fine sediments within bays and estuaries.
Coastlines of submergence result from partial drowning of the coast by a rise in sea level or sinking of the land. They tend to be deeply embayed with bold relief. Coastlines of emergence , where submarine deposits become exposed, usually have gentle relief and slope gently toward the ocean. Rapid emergence can produce raised shorelines and marine terraces . Global sea level has fluctuated substantially in the past and is presently rising slowly.