The Castle, Yesnaby
Key sites
Stacks
Definition: pillar-like eroded remnant of a coastal cliff, with margins often defined by major fractures. Stac Gaelic - a rock pillar
Stacks can be formed by the loss of the
keystone of an arch in response to marine erosion. Stacks can
also develop directly from erosion of the cliff face or headland
where the overlying rock is not strong enough to form a
supporting roof. Orcadian sea stacks offer challenging and
spectacular
climbing.
Stacks may form relatively rapidly through selective erosion
along weaknesses in otherwise resistant rock. Once isolated from
a host cliff the stack may become a relatively persistent
feature, although we know next to nothing about the survival
time of stacks. The stacks may have few joints and erode more
slowly in comparison with the retreating cliff. Secondary arches
may develop within the stack where strong local joint patterns
are exploited by marine processes. Eventually the stack will
collapse leaving a stack remnant or stump.