[1] The UNESCO Geopark designation reflects the diversity of the island's geology, which encompasses solid rocks from the Precambrian to the Neogene with some Miocene sediments and extensive Pleistocene glaciation features from the Quaternary period.
South Stack exhibits particular folding and faulting that have made it a site of interest for many years, having been first identified as the oldest Precambrian rock then the youngest and now said to be from the Cambrian period.
The plate interior on the northern coast is composed of mudstones and sandstones, some containing ‘dropstones, the remnants of the Gaskier's Ice Age that occurred at the end of Precambrian times.
Originally geoconservation on Anglesey was administered by the Gwynedd and Môn RIGS group, but a decision was taken to apply for Geopark membership.
The Watch House was originally the waiting place for the pilots guiding sailing ships in and out of the tiny harbour.
Because of the site's 'spectacular, accessible and well-preserved exposures of late Neoproterozoic-Cambrian mélange with more than 200 years of study', the International Union of Geological Sciences (IUGS) included the 'Ynys Llanddwyn late Neoproterozoic-Cambrian Mélange' in the tectonics chapter of The First 100 IUGS Geological Heritage Sites published in October 2022.