Shoreline erosion
Some coasts are created by shoreline erosion. Waves pound the shore with water 
, sand and even rocks 
. This can undercut cliffs causing large sections of rock and sediment to fall into the water.
Sea stacks form when waves bend around a headland of rock that juts out into the sea. First a cave is eroded into the side of the headland. This gradually gets bigger until an arch forms. Once the top of the arch collapses an isolated pillar or stack is left behind.
Different rock types are eroded at different rates. Soft rocks will be worn away much faster than harder rocks.
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