Two limestone pillars, once part of another arch which has now collapsed. In spring and summer they are covered in guillemots, razorbills and other seabirds.
Stack Rocks are on the only part of the Pembrokeshire Coast Path on which cycling is permitted. It is also on Castlemartin firing range, so can be closed to the public at times. Check gov.uk/government/publications/castlemartin-firing-notice--2