South Stack in Wales is located on the northwest coast of the Isle of Anglesey, Wales, near Holyhead and adjacent to Holy Island.
South Stack is a sea stack formed by wave erosion of sedimentary rocks that once connected the island to the mainland. The area in which it is located is geologically known as the South Stack Formation.
The formations here contain sandstone and interbedded shales, which are deformed by large folds and twists, and these folds are clearly visible on the sea cliffs next to the steps leading to the lighthouse. Thick layers of Holyhead Quartzite lie above these folded sedimentary rocks. The geology of South Stack was selected by the British Geological Society as one of the top 100 geological sites in the UK.
South Stack Lighthouse: Built in 1809, designed by Daniel Alexander, it is 28 meters high and is located 41 meters above the sea in South Stack, becoming a lighthouse for ocean-going ships in the Irish Sea!