Stacks are isolated pillars of rock which rise steeply from the sea. Around the Pembrokeshire Coast there are a number of stacks. Stacks were once part of the mainland. However, over time the sea has worked into weaknesses in the rock, known as joints and bedding planes, and worn it away. What is left is a stack that is separate from the mainland. Some stacks form from arches, after the arch part has collapsed. An example of this arch is the 'Green Bridge of Wales', which can be seen on the page about arches.
The top picture shows Stack Rocks, a pair of stacks found at Castlemartin, close to the Green Bridge and the Cauldron. The stacks are of Carboniferous Limestone, the dominant local rock type. The two stacks are important nesting sites for guillemots and kittiwakes, two of the many species of seabirds found on the Pembrokeshire Coast. The birds return to the stacks in spring, and can be viewed from the mainland throughout the spring and early summer. Locally the Stack Rocks are known as the Elegug Stacks, after the Welsh word for Guillemot.
Pen-y-Holt Stack, above, is found in Pen-y-Holt Bay, along the coast from Stack Rocks, and is in the MOD Army Range. It is only accessible through walks organised by the Pembrokeshire Coast National Park Authority. The presence of several submerged ledges offshore and a resistant platform beneath the stack itself, means that the stack is relatively protected from the full force of the strongest waves. Despite its precarious appearance, the stack is a refuge for vegetation above the limits of wave action.