Editing Stack (geology) (section)

==Formation==
[[File:Downpatrick Head 2021.jpg|thumb|Downpatrick Head, [[County Mayo]], [[Ireland]]. Clear horizontal bedding is visible, exposing 350 million years of geological history.<ref>{{cite web | title = Mayo - County Geological Site Report | work = Geoheritage | publisher = [[Geological Survey of Ireland]] | url = https://gsi.geodata.gov.ie/downloads/Geoheritage/Reports/MO050_Downpatrick_Head.pdf | access-date = 28 December 2023 }}</ref>]]

Stacks typically form in horizontally bedded [[sedimentary rock|sedimentary]] or [[volcanic rock]]s, particularly on [[limestone]] cliffs. The medium hardness of these rocks means medium resistance to abrasive and attritive [[erosion]]. A more resistant layer may form a [[Caprock|capstone]]. (Cliffs with weaker rock, such as [[claystone]] or highly jointed rock, tend to [[Slump (geology)|slump]] and erode too quickly to form stacks, while harder rocks such as [[granite]] erode in different ways.)<ref name="Schwartz_2006">{{Cite book |url=https://books.google.com/books?id=VWnxpAxp6TMC&q=sea+stack+formation+high+compressional+strength&pg=PA238 |title=Encyclopedia of Coastal Science |publisher=Springer Science & Business Media |year=2006 |isbn=9781402038808 |editor-last=Schwartz |editor-first=Maurice |page=238}}</ref>

The formation process usually begins when the sea attacks lines of weakness, such as steep [[joint (geology)|joints]] or small [[Fault (geology)|fault zones]] in a cliff face. These cracks then gradually get larger and turn into caves. If a cave wears through a headland, an arch forms. Further erosion causes the arch to collapse, leaving the pillar of hard rock standing away from the coast, the stack. Eventually, erosion will cause the stack to collapse, leaving a stump. This stump usually forms a small rock island, low enough for a high tide to submerge.