Editing County Donegal (section)

===Geology and terrain===
[[File:Donegalmap.jpg|thumb|Topographic map of County Donegal]]
[[Image:Glenveagh Valley (2009).jpg|thumb|left|Glenveagh Valley]]
Donegal can be divided into six main geological subdivisions.<ref>GSI., pp. 29</ref> Of these, the most expansive sections are the [[Lower Carboniferous]] limestones and sandstones of south Donegal, the [[Devonian]] granite of the [[Donegal batholith]], which extends for about 75&nbsp; km northeast to southwest from [[Ardara, County Donegal|Ardara]] to [[Fanad]] Head, and quartzo-feldspathic [[Dalradian]] rocks from the [[Precambrian]] era, which cover much of the rest of the county. The geology of Donegal is very similar to that of [[County Mayo]], and both counties are located within the [[Grampian Mountains|Grampian]] [[Terrane]]. The oldest rocks in Ireland are a [[Granite|granitic]] [[gneiss]] found on the island of [[Inishtrahull]], located ca. {{cvt|10|km|mi}} north-east of Malin Head. They are 1.78 billion years old, making them [[Paleoproterozoic]] in age.<ref name="Muir">{{cite journal |last=Muir |first=R.J. |author2=Fitches W.R. |author3=Maltman A.J. |year=1994 |title=The Rhinns Complex: Proterozoic basement on Islay and Colonsay, Inner Hebrides, Scotland, and on Inishtrahull, NW Ireland |journal=Transactions of the Royal Society of Edinburgh: Earth Sciences |volume=85 |issue=1 |pages=77–90 |url=http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8344451 |access-date=15 September 2012 |doi=10.1017/s0263593300006313 |s2cid=131108674 |archive-date=22 July 2015 |archive-url=https://web.archive.org/web/20150722123641/http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8344451 |url-status=live}}</ref> The oldest rocks on mainland Donegal are a quartzo-feldspathic paragneiss found around Lough Derg, which have been dated to 1.713 billion years ago.<ref>{{Cite web |url=https://www.gsi.ie/documents/Geoschol_Donegal_Geology.pdf |title=Geology of County Donegal |website=gsi.ie |access-date=22 July 2021 |archive-date=18 December 2017 |archive-url=https://web.archive.org/web/20171218114131/https://www.gsi.ie/documents/Geoschol_Donegal_Geology.pdf |url-status=live}}</ref>

Donegal is the most seismically active part of Ireland. The [[Great Glen Fault|Leenan Fault]] is a large [[Fault (geology)#Strike-slip faults|strike-slip fault]] that bisects the county from Lough Swilly to Donegal Bay, and dozens of tremors have been recorded along the fault since the late 19th century, although none have been larger than a magnitude 3 on the [[Richter magnitude scale|Richter scale]].<ref>{{Cite web |url=https://www.irishtimes.com/culture/heritage/why-does-donegal-get-a-disproportionate-number-of-irish-earthquakes-1.3854663 |title=Why does Donegal get a disproportionate number of Irish earthquakes? |website=irishtimes.com |access-date=22 July 2021 |archive-date=24 July 2021 |archive-url=https://web.archive.org/web/20210724135745/https://www.irishtimes.com/culture/heritage/why-does-donegal-get-a-disproportionate-number-of-irish-earthquakes-1.3854663 |url-status=live}}</ref><ref>{{Cite news |url=https://www.bbc.com/news/world-europe-48102567 |title=Earthquake close to Irish border in County Donegal |work=BBC News |date=30 April 2019 |access-date=22 July 2021 |archive-date=21 July 2021 |archive-url=https://web.archive.org/web/20210721202816/https://www.bbc.com/news/world-europe-48102567 |url-status=live}}</ref>

The county's landscape was carved out by glaciation at the end of the [[Pleistocene]] and the subsequent [[Holocene glacial retreat|retreat]] during the early [[Holocene]]. Donegal contains one of Ireland's three glacial [[fjord]]s (or [[fjard]]s) at [[Lough Swilly]], the others being [[Carlingford Lough]] and [[Killary Harbour]].{{sfn|Dundurn|2000|p=4}} Lough Swilly is the county's largest inlet and forms the western boundary of the [[Inishowen Peninsula]]. The thick ice sheet that once covered the region carved out deep basins in the Donegal uplands and many [[Tarn (lake)|tarns]] or corrie lakes developed in these depressions after the ice had melted, including Lough Maam below [[Slieve Snaght]] and Lough Feeane under [[Aghla More]]. Larger glacial lakes formed in the county's distinctive [[U-shaped valley]]s, such as [[Lough Beagh]] and [[Gartan Lough]] in [[Glenveagh]], and [[Dunlewey Lough]] and [[Lough Nacung Upper]] in the [[Poisoned Glen]]. Valleys such as these were the last areas to retain glaciers as temperatures rose.<ref>GSI., pp. 37</ref>

As the ice sheet thinned, topography became the dominant force driving the direction of ice and meltwater flow.<ref>GSI., pp. 38</ref> Erosion by glacial meltwater carved out large channels in southern Donegal which directed water and sediment to [[outwash fan]]s in Donegal Bay. The area south of Donegal town, where the [[River Eske]] flows into the bay, is an example of one of these outwash areas. Sea levels in the area began to stabilise around 5,000 years ago, and the balance of erosion and deposition along Donegal's coastline resulted in the development of many sandy beaches and [[Spit (landform)|spits]] interspersed with jagged sea cliffs.<ref>GSI., pp. 36</ref>