A
stack or
seastack is a
geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, formed by wave erosion. Stacks are formed over time by wind and water, processes of coastal geomorphology. They are formed when part of a
headland is
eroded by
hydraulic action, which is the force of the sea or water crashing against the rock. The force of the water weakens cracks in the headland, causing them to later collapse, forming free-standing stacks and even a small island. Without the constant presence of water, stacks also form when a
natural arch collapses under
gravity, due to
sub-aerial processes like
wind erosion. Stacks can provide important nesting locations for
seabirds, and many are popular for
rock climbing.