Editing South Stack (section)

==Geology==
[[File:South Stack3.JPG|thumb|left|[[fold (geology)|Folding]] in the [[South Stack Formation]] can be seen in the rocks of the mainland's sea cliffs.]]
South Stack is an island known as a [[Stack (geology)|sea stack]]. It was formed by the [[wave erosion]] of sedimentary rocks that once connected the island to the mainland.<ref> Stacks are formed over time by wind and water, processes of coastal geomorphology. [http://www.britannica.com/ebchecked/topic/123147/coastal-landform/49793/sea-stacks "Sea stacks"] britannica.com  {{webarchive|url=https://web.archive.org/web/20090414225722/http://www.britannica.com/EBchecked/topic/123147/coastal-landform/49793/Sea-stacks |date=14 April 2009}}</ref> 

The area is known geologically as the [[South Stack Formation]]. Its [[Stratum|strata]] includes [[sandstone]]s and interbedded [[shale]]s which have been contorted by large [[fold (geology)|folds and crumples]]. The folds can be seen in the [[seacliff]]s (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 rock]]s. 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.<ref>{{cite web |url=http://angleseynature.co.uk/geology.html |title=South Stack Geology |website=angleseynature.co.uk |access-date=24 August 2020}}</ref>

The 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.<ref name="GeolSoc">{{cite web |url=http://apps.esriuk.com/app/GreatGeosites/3/wmt/view/5e0d44970b3a4b4dafb7da0404b2d8ed/index.html |title=The Geological Society's 100 Great Geosites |publisher=The Geological Society |year=2014 |access-date=14 October 2014}}</ref>