| South Stack Formation | |
| Type: | Geological formation |
| Age: | Furongian-Tremadoc ~ |
| Period: | Furongian |
| Prilithology: | Metamorphosed sandstone |
| Otherlithology: | Metamorphosed mudstone |
| Namedfor: | South Stack |
| Region: | Holy Island, Anglesey |
| Coordinates: | 53.3°N -45°W |
| Unitof: | Holy Island Group |
| Underlies: | New Harbour Group |
| Overlies: | not exposed |
| Thickness: | ~360- |
The South Stack Formation is a sequence of Cambro-Ordovician (Furongian to Tremadocian) metasedimentary rocks exposed in northwestern Anglesey, North Wales.[1] The outcrop of this formation at South Stack was chosen as one of the top 100 geosites in the United Kingdom by the Geological Society of London, for its display of small-scale folding.[2]