Editing Fjord (section)

==Formation==
[[File:Glacier in eastern Greenland.jpg|thumb|A glacier in eastern Greenland flowing through a fjord carved by the movement of ice]]
[[File:Fjord genesis.png|thumb|Illustration of how a fjord is created]]
[[File:Odda frå fly.jpg|thumb|[[Sørfjorden (Hardanger)]] with [[Sandvinvatnet]] and Odda Valley can be clearly seen as continuation of the fjord. [[Odda (town)|Odda]] sits on the [[isthmus]]. [[Folgefonna]] on the right hand.]]
A true fjord is formed when a [[glacier]] cuts a [[U-shaped valley]] by [[ice segregation]] and [[Abrasion (geology)|abrasion]] of the surrounding bedrock.<ref name= Murton2006>{{Cite journal |last=Murton |first=Julian B. |author2= Peterson, Rorik |author3=Ozouf, Jean-Claude |title=Bedrock Fracture by Ice Segregation in Cold Regions |journal= [[Science (journal)|Science]] |volume=314 |issue=5802 |pages=1127–1129 |date=17 November 2006 |doi=10.1126/science.1132127 |pmid=17110573 |bibcode=2006Sci...314.1127M |s2cid= 37639112 }}</ref> According to the standard model, glaciers formed in pre-glacial valleys with a gently sloping valley floor. The work of the glacier then left an [[Overdeepening|overdeepened]] U-shaped valley that ends abruptly at a valley or trough end. Such valleys are fjords when flooded by the ocean. Thresholds above sea level create freshwater lakes.<ref name="Aarseth" /> Glacial melting is accompanied by the rebounding of Earth's crust as the ice load and eroded sediment is removed (also called [[isostasy]] or glacial rebound). In some cases, this rebound is faster than [[sea level rise]]. Most fjords are [[Overdeepening|deeper than the adjacent sea]]; [[Sognefjord]], [[Norway]], reaches as much as {{convert|1300|m|abbr=on|0}} below [[sea level]]. Fjords generally have a sill or shoal (bedrock) at their mouth caused by the previous glacier's reduced erosion rate and [[terminal moraine]].<ref name= Alley2003>{{Cite journal |last=Alley |first= R.B. |author2=D. E. Dawson |author3=G. J. Larson |author4=E. B. Evenson |author5=G. S. Baker |title=Stabilizing feedbacks in glacier-bed erosion |journal=Nature |volume=424 |issue=6950 |pages=758–760 |publisher=Nature PublishingGroup |date=14 August 2003 |doi=10.1038/nature01839 |pmid=12917679 |bibcode= 2003Natur.424..758A |s2cid=4319448 }}</ref> In many cases this sill causes extreme currents and large saltwater rapids (see [[Tidal rapid|skookumchuck]]). [[Saltstraumen]] in Norway is often described as the world's strongest [[Tide#Current|tidal current]]. These characteristics distinguish fjords from [[ria]]s (such as the [[Bay of Kotor]]), which are drowned valleys flooded by the rising sea. [[Drammensfjorden]] is cut almost in two by the [[Svelvik]] "ridge", a sandy moraine that was below sea level when it was covered by ice, but after the post-glacial rebound reaches {{cvt|60|m|||}} above the fjord.<ref>Jørgensen, Per: ''Kvartærgeologi''. Landbruksforlaget, 1995.</ref>

In the 19th century, [[Jens Esmark]]  introduced the theory that fjords are or have been created by glaciers and that large parts of Northern Europe had been covered by thick ice in prehistory.<ref name="Holtedahl" /> Thresholds at the mouths and overdeepening of fjords compared to the ocean are the strongest evidence of glacial origin,<ref name= "Nesje, A. 1994">{{cite journal |last1=Nesje |first1=Atle |last2=Whillans |first2=Ian M. |title=Erosion of Sognefjord, Norway |journal= Geomorphology |date=1 February 1994 |volume=9 |issue=1 |pages=33–45 |doi=10.1016/0169-555X(94)90029-9 |bibcode= 1994Geomo...9...33N |url= https://dx.doi.org/10.1016/0169-555X%2894%2990029-9 |access-date=20 September 2021 |language= en |issn=0169-555X}}</ref> and these thresholds are mostly rocky. Thresholds are related to sounds and low land where the ice could spread out and therefore have less erosive force. [[John Walter Gregory]] argued that fjords are of [[tectonic]] origin and that glaciers had a negligible role in their formation. Gregory's views were rejected by subsequent research and publications. In the case of Hardangerfjord the fractures of the [[Caledonian orogeny|Caledonian fold]] has guided the erosion by glaciers, while there is no clear relation between the direction of Sognefjord and the fold pattern.<ref name="Holtedahl">{{cite journal | last= Holtedahl| first= H. | year= 1967| title= Notes on the formation of fjords and fjord-valleys| journal= Geografiska Annaler: Series A, Physical Geography| volume= 49| number= 2| pages= 188–203| doi= 10.1080/04353676.1967.11879749 | bibcode= 1967GeAnA..49..188H }}</ref> This relationship between fractures and direction of fjords is also observed in [[Lyngen (fjord)|Lyngen]].<ref>Randall, B. A. O. (1961). On the relationship of valley and fjord directions to the fracture pattern of Lyngen, Troms N. Norway. ''Geografiska Annaler'', 43(3/4), 336–338.</ref> Preglacial, [[tertiary]] rivers presumably eroded the surface and created valleys that later guided the glacial flow and erosion of the bedrock. This may in particular have been the case in Western Norway where the tertiary uplift of the landmass amplified eroding forces of rivers.<ref name="Holtedahl" />

Confluence of tributary fjords led to excavation of the deepest fjord basins. Near the very coast, the typical West Norwegian glacier spread out (presumably through sounds and low valleys) and lost their concentration and reduced the glaciers' power to erode leaving bedrock thresholds. [[Bolstadfjorden]] is {{cvt|160|m|||}} deep with a threshold of only {{cvt|1.5|m|||}},<ref name="Holtedahl" /><ref name="Aarseth">Aarseth, I., Nesje, A., & Fredin, O. (2014). ''West Norwegian fjords.'' Geological Society of Norway (NGF), Trondheim, 2014. {{ISBN|978-82-92-39491-5}}</ref> while the {{cvt|1300|m|||}} deep Sognefjorden has a threshold around {{cvt|100|to|200|m|}} deep.<ref name= "Geografisk">{{cite book | title = Geografisk leksikon| editor-first= Waldemar| editor-last= Brøgger| place= Oslo| publisher= Cappelen| year= 1963}}</ref><ref>{{Cite web |url=https://snl.no/Sognefjorden |title=Sognefjorden |website= snl.no| date=15 August 2021}}</ref> [[Hardangerfjord]] is made up of several basins separated by thresholds: The deepest basin Samlafjorden between Jonaneset ([[Jondal (village)|Jondal]]) and [[Ålvik]] with a distinct threshold at Vikingneset in [[Kvam Municipality]].<ref name="Holtedahl" />

[[File:Muldalsfossen.JPG|thumb|upright|Muldalsfossen waterfall drops several hundred meters from the Muldalen hanging valley to [[Tafjorden]].]]
[[Hanging valley]]s are common along glaciated fjords and [[U-shaped valley]]s. A hanging valley is a [[tributary]] valley that is higher than the main valley and was created by tributary [[glacier]] flows into a glacier of larger volume. The shallower valley appears to be 'hanging' above the main valley or a fjord. Often, [[waterfall]]s form at or near the outlet of the upper valley.<ref>{{cite web |title=Glossary of Glacier Terminology |website= usgs.gov |publisher=US Geological Survey |date= May 28, 2004 |url=http://pubs.usgs.gov/of/2004/1216/h/h.html |access-date= 2007-05-24 }}</ref> Small waterfalls within these fjords are also used as freshwater resources. Hanging valleys also occur underwater in fjord systems. The branches of [[Sognefjord]] are for instance much shallower than the main fjord. The mouth of [[Fjærlandsfjord]] is about {{cvt|400|m|||}} deep while the main fjord is {{cvt|1200|m|||}} nearby. The mouth of Ikjefjord is only {{cvt|50|m}} deep while the main fjord is around {{cvt|1300|m|||}} at the same point.<ref name="Nesje, A. 1994"/>