The coastline as we know it is itself a feature caused by the erosion of waves and the deposits they carry. Rocky cliffs may form where the water meets the land as the waves scour parts of the coastline away, leaving a vertical face above water and a platform below the sea. It does this by cutting a notch out of the land near the bottom of the cliff, which leads to instability above the notch. The land above the notch is eroded away in a process known as mass wasting, while the land below the notch is left intact to create a platform. If tectonic action lifts the Earth's crust where the sea terrace is located, the terrace may emerge above the water level to form a marine terrace.
Coastal erosion is an important issue to be aware of, as the retreat of the land from the sea can destroy man-made structures and change the habitat of creatures that live in or by the sea. Sea waves play a large part in the erosion of coastlines and can create many types of land features within or next to the ocean.
Cliffs and Terraces
Sea caves are created by waves battering the coastline. The waves throw particles of rock at the land, which causes abrasion --- the process by which land is slowly rubbed away by friction between the coastline sediment and the headland rock. At weak points in the cliffs along a coastline, the abrasion rounds out a bowl-shaped feature at the bottom of the cliffs called a sea cave.
Sea arches are the next step along from sea caves that continue to be eroded by the waves. The feature is created when two sea caves on each side of a headland join in the middle, creating a tunnel through the cliffs. As this tunnel is eroded and enlarged, the headland begins to take on the shape of an arch, with a pillar of rock remaining in the sea and a slice of land connecting it at its uppermost point to the new headland, which has retreated from its original position.
Sea Stacks and Stumps
As sea arches age, they become sea stacks. These are features of coastal erosion that occur when the land that connects the pillar in the sea to the headland collapses, leaving a tower of rock in the ocean. These stacks themselves are eroded by the waves and become smaller, at which point they are described as sea stumps.
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