High-relief erosional shorelines are often characterized by hard bedrock in the coastal zone, in areas of active uplift or volcanic activity. Sea cliffs form when wave action erodes the base of exposed rocks. As the base is removed, the above rock eventually collpases into the water and is removed by the wave action. In this way, active sea cliffs are constantly advancing landward. Within sea cliffs, sea caves form if less resistant material is scoured from the rock face. If a sea cave penetrates all the way through to create a tunnel, a sea arch is formed.