Stack Rocks are 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.