Coastlines are changing all the time as new waves roll in and out and tides rise and fall every six hours or so. Over longer periods coastlines are reshaped by the action of waves and the corrosion of salty water.
On exposed coasts where waves strike the high rocks, they undercut the slope to create steep cliffs and headlands. Often waves can penetrate into the cliff to open up sea caves or blast through arches. When a sea arch collapses, it leaves behind tall pillars called stacks which may be worn away to stumps.
Waves work on rocks in two ways. First, the rocks are pounded with a huge weight of water filled with stones. Second, the waves force air into cracks in the rocks with such force that the rocks split apart.
The erosive power of waves is focused in a narrow band at wave height. So as waves wear away sea cliffs, they leave the rock below wave height untouched. As cliffs retreat, the waves slice away a broad shelf of rock called a wave-cut platform. Water left behind in dips when the tide falls forms rockpools.
On more sheltered coasts, the sea may pile up sand into beaches. The sand has been washed down by rivers or worn away from cliffs.
When waves hit a beach at an angle, they fall straight back down the beach at a right angle. Any sand and shingle that the waves carry fall back slightly farther along the beach. In this way sand and shingle are moved along the beach in a zig-zag fashion. This is called longshore drift.
On beaches prone to longshore drift, low fences called groynes are often built to stop the sand being washed away along the beach.
Longshore drift can wash sand out across bays and estuaries to create sand bars called spits.
Bays are broad indents in the coast with a headland on each side. Waves reach the headlands first, focusing their energy here. Material is worn away from the headlands and washed into the bay, forming a bay-head beach.
A cove is a small bay. A bight is a.huge bay, such as the Great Australian Bight. A gulf is a long narrow bight. The world’s biggest bay is Hudson Bay, Canada, which has a shoreline 12,268 km long. The Bay of Bengal in India is larger in area.