Today the cliffs are subject to erosion by wave action, which undermines the base of support causing the cliff to collapse under its own weight. This process creates a variety of coastal landforms characteristic of erosional coasts such as sea caves, sea stack, and sea stumps. Branaunmore, a 67-meter high sea stack at the foot of the Cliffs of Moher below O'Brien's Tower, was once part of the cliffs, but coastal erosion gradually removed the layers of rock that joined it with the mainland. A large sea arch can also be seen at Hag's Head below the Napoleonic signal tower and many smaller sea arches can be seen from sea level.