Stacks are a type of coastal landforms. These are formed as a result of the erosional activity of the sea waves. As sea waves strike the rocks continuously, it leads to the formation of cracks in them. Over time, these cracks become larger and wider and appear like caves. They are called sea caves. As these cavities become bigger and bigger only the roof of the caves remain, thus forming sea arches. Further, erosion breaks the roof and only walls are left. These wall like features are called stacks. Hence, the sea caves are turned into stacks over a long period of time.