Cliffs, Terraces, Caves and Stacks

Cliffs, terraces, caves and stacks are all products of wave erosion — the grinding, hammering and undercutting action of the sea against the land. They appear where erosion, not deposition, is the dominant shore process, and they represent different stages in the slow demolition of a coastline.

Wave-cut cliffs and terraces are the two forms most commonly found where erosion rules the shore. Almost all sea cliffs are steep, and their heights vary widely — from just a few metres to 30 metres or even more. At the foot of such a cliff there is usually a flat or gently sloping platform, covered with rock debris that has fallen from the cliff face behind it. When this platform sits at an elevation above the average height of the waves, it is called a wave-cut terrace. In other words, the terrace is the worn-down remnant of the cliff's base, left stranded above the reach of ordinary waves.

The same forces that carve the terrace also hollow out the cliff itself. The lashing of waves against the base of the cliff, together with the rock debris that gets smashed against the cliff along with those waves, creates hollows in the rock. These hollows are gradually widened and deepened until they form sea caves. As erosion continues, the roofs of these caves eventually collapse, and the sea cliff recedes further inland — each collapse pushing the cliff line back and feeding more debris onto the terrace below.

As the cliff retreats, some remnants of rock may be left standing isolated as small islands just off the shore. These resistant masses of rock, originally parts of a cliff or hill, are called sea stacks. They survive because they are made of more resistant material than the surrounding rock, which has already been worn away.