Waves, like streams erode the coastal rocks with the help of rock fragments present in the water.
Some of the important features made through sea wave erosion are mentioned here:
The maximum impact of the sea waves is observed on the lower part of the coastal rocks and consequently the lower part of the rocks is eroded more rapidly than the upper part.
This results in the formation of a hollow under the rock and with the passage of time this excavation in the lower part of the rock becomes larger.
The upper part of the rock is thus left projecting out towards the sea.
After sometime, this projecting part falls into the sea under its own weight.
As a result a vertical wall is left.
This vertical wall is called a cliff.
In India a number of sea cliffs are found along the Konkan Coast of India.
When the upper part of the coastal rock is hard and the lower part is soft, the erosion is not uniform.
The lower part of the rock is eroded much faster than the upper part.
Due to differential erosion a hollow is created in the lower part of the rock.
When the waves pound against this hollow, air present in the hollow gets compressed.
When the wave comes out of the hollow, the pressure on air is also released and it expands.
Due to continuous compression of the air in the hollow, the rocks are subjected to a great pressure and they break.
In this process, the hollows in the lower part of the rock keep on enlarging.
When a part of coast extends to some distance into the sea, sea waves working from opposite directions cut a passage through the soft rocks.
In the initial stages, this passage is a narrow hole but it enlarges into a broad arch.
These broad door-like features are called sea arches or natural bridges.
When the roof of an arch is broken by erosion or under its own weight or due to any other reason a part of the original rock remains standing as a solitary mass.
It may be the rock forming the side of the arch.
This type of a feature is called a sea stack.
Stacks are of a number of types depending upon their shape and the nature of the rocks.
Sometimes they take the shape of islands but such islands are not permanent.
Small underwater stacks are known as stumps.