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18
Caves, arches and stacks
As cliffs are worn back, the softer rocks are scooped out to make caves, leaving harder rocks as arches and sea stacks.
Holes, known as blowholes, may eventually be forced through the roof of the cave. Once this occurs, the pressure created by each wave is released through the blowhole as a jet of spray.
Arch
As caves become deeper, there is a chance they may break right through a headland. When this happens, they make a natural stone arch (see the picture on page 1).
Stack
The roof of the arch gradually gets worn higher and wider. Eventually the top of the arch collapses and leaves one wall of the arch standing on its own in the sea. This pillar of rock is called a stack, a needle if it is very thin, and a stump if it
is mostly worn down (pictures , 2, 3, 4 and 5).
F  Caves can tell you a lot about which rocks are soft, how thick the rocks are and in which direction they are sloping.
The cave shown here in silhouette ‘leans’ to the right because this is the way the rocks slope. The view looks out on to a sea stack.
Waves can pick out weaknesses in rocks that are difficult to see. This is why a cliff may look as though it is made of the same rock, but be worn away only in some places and not others.
You can tell where rocks have areas of weakness from the occurrence of deep narrow inlets (also called coves), caves (picture ), natural arches and pillars of rock called stacks standing in the sea (pictures 2, 3, 4 and 5).
Cave
Sea caves (pictures , 4 and 5) are deep, natural hollows or tunnels in a cliff. They are formed when waves pound on a band of weaker rock. This means that the weak rock can be worked away faster than its surroundings. Sea caves can form in any rock that is strong enough to support a cave roof without it collapsing.