South Stack () is an island situated just off Holy Island on the northwest coast of Anglesey, Wales. Geology South Stack is an island known as a sea stack. It was formed by the wave erosion of sedimentary rocks that once connected the island to the mainland. The area is known geologically as the South Stack Formation. Its strata includes sandstones and interbedded shales which have been contorted by large folds and crumples. The folds can be seen in the seacliffs (best viewed from the steps leading down to the lighthouse). Thick beds of pure Holyhead Quartzite, which forms the bulk of Holyhead Mountain, lie above these folded sedimentary rocks. Recent studies suggests the rocks appearance was caused by large scale earth movements which is not a part of the normal sedimentary process; however, this theory remains controversial. Similar evidence of upthrusts can be seen in the cliffs to the south of South Stack. The fascinating geology of South Stack has been chosen as one of the top 100 geosites in the United Kingdom by Geological Society of London, for its display of small-scale folding. Lighthouse South Stack Lighthouse, which was completed in 1809, is sited above the sea on South Stack. Its lamp tower is -tall and the lighthouse complex covers . There are over 390 stone steps and 10 metal steps down to the footbridge. Originally the only way to cross to the island was in a basket suspended from a hemp cable. In 1828 an iron suspension bridge was built, it was replaced in 1964 with a steel cable truss bridge. However, in 1983 it had to be closed to the public due to safety concerns. After a new aluminium bridge was built, the lighthouse was reopened to the public in 1997. It is now a popular destination, attracting thousands of visitors every year. Nature reserve The island's cliffs are part of the South Stack Cliffs RSPB reserve which are home to an estimated 8,000 nesting birds during the breeding season. An RSPB visitor centre (with bird hide) is located at Elin's Tower on the mainland. Birds and marine life seen from the centre include choughs, peregrine falcons, and kestrels as well as harbour porpoises, grey seals, Risso's dolphins and bottlenose dolphins. Tidal wave project In 2019, proposals to develop a 'West Anglesey Demonstration Zone' tidal energy project at South Stack were submitted under the Transport and