Editing Valley (section)

==Formation of valleys==
Valleys may arise through several different processes. Most commonly, they arise from [[erosion]] over long periods by [[fluvial|moving water]] and are known as river valleys. Typically small valleys containing streams feed into larger valleys which in turn feed into larger valleys again, eventually reaching the ocean or perhaps an [[endorheic basin|internal drainage basin]]. In polar areas and at high altitudes, valleys may be eroded by [[glacier]]s; these typically have a U-shaped profile in cross-section, in contrast to river valleys, which tend to have a V-shaped profile. Other valleys may arise principally through tectonic processes such as [[rifting]]. All three processes can contribute to the development of a valley over geological time. The flat (or relatively flat) portion of a valley between its sides is referred to as the valley floor. The valley floor is typically formed by river sediments and may have [[fluvial terrace|fluvial terraces]].

===River valleys {{anchor|River valleys}}=== <!-- {R to section} redirect target, "[[River valley]]" -->
[[File:Halikonjokilaakso.jpg|thumb|left|The valley of Halikko River in [[Halikko]], [[Finland]]]]
[[File:Palakaria-and-Belchin-3.jpg|thumb|right|220px|Valley of Palakaria river springing from [[Vitosha]] Mountain, seen in the background, in [[Geography of Bulgaria|Bulgaria]]]]
The development of a river valley is affected by the character of the bedrock over which the river or stream flows, the elevational difference between its top and bottom, and indeed the climate. Typically the flow will increase downstream and the gradient will decrease. In the upper valley, the stream will most effectively erode its bed through [[corrasion]] to produce a steep-sided V-shaped valley. The presence of more resistant rock bands, of [[fault (geology)|geological faults]], [[fracture (geology)|fractures]], and [[Fold (geology)|folds]] may determine the course of the stream and result in a twisting course with [[interlocking spur]]s.

In the middle valley, as numerous streams have coalesced, the valley is typically wider, the flow slower and both erosion and deposition may take place. More lateral erosion takes place in the middle section of a river's course, as strong currents on the outside of its curve erode the bank. Conversely, deposition may take place on the inside of curves where the current is much slacker, the process leading to the river assuming a [[meander]]ing character. In the lower valley, gradients are lowest, meanders may be much broader and a broader [[floodplain]] may result. Deposition dominates over erosion.<ref>{{cite book |last1=Monkhouse |first1=F.J. |title=Principles of Physical Geography |date=1971 |publisher=University of London Press Ltd |location=London |isbn=0340090227 |pages=152–157 |edition=Seventh}}</ref><ref>{{Cite book |last=Morisawa |first=Marie |url=https://www.worldcat.org/oclc/2968 |title=Classification of Rivers |publisher=Reinhold Book Corporation |year=1968 |editor-last=Rhodes W. Fairbridge |location=New York |pages=956–957 |oclc=2968 |author-link=Marie Morisawa}}</ref> A typical river basin or [[drainage basin]] will incorporate each of these different types of valleys.

Some sections of a stream or river valleys may have vertically incised their course to such an extent that the valley they occupy is best described as a [[gorge]], [[ravine]], or [[canyon]]. Rapid down-cutting may result from localized uplift of the land surface or rejuvenation of the watercourse as a result for example of a reduction in the base level to which the river is eroded, e.g. lowered global sea level during an [[ice age]]. Such [[river rejuvenation|rejuvenation]] may also result in the production of [[river terrace]]s.<ref>{{cite book |last1=Monkhouse |first1=F.J. |title=Principles of Physical Geography |date=1971 |publisher=University of London Press Ltd |location=London |isbn=0340090227 |pages=161–164 |edition=Seventh}}</ref>

{{for|various lists of river valleys|Category:River valleys}}

===Glacial valleys===
[[File:Tal-y-llyn-valley-Dolgoch-01.jpg|thumb|upright=1.35|U-shaped valley on the [[Afon Fathew]] near [[Dolgoch Falls|Dolgoch]], Wales]]
[[File:Glacial Valley MtHoodWilderness.jpg|thumb|upright|A glaciated valley in the [[Mount Hood Wilderness]] showing a characteristic U-shape, the bottom's rocky 'rubble' accretion and the broad shoulders]]
There are various forms of valleys associated with glaciation. True glacial valleys are those that have been cut by a glacier which may or may not still occupy the valley at the present day. Such valleys may also be known as glacial troughs. They typically have a [[U-shaped valley|U-shaped cross-section]] and are characteristic landforms of mountain areas where glaciation has occurred or continues to take place.<ref>{{cite web |title=Vale of Eden |url=https://www.britannica.com/science/glacial-valley |publisher=Britannica |access-date=20 December 2020}}</ref>

The uppermost part of a glacial valley frequently consists of one or more 'armchair-shaped' hollows, or '[[cirques]]', excavated by the rotational movement downslope of a cirque glacier. During glacial periods, for example, the [[Pleistocene]] ice ages, it is in these locations that glaciers initially form and then, as the [[ice age]] proceeds, extend downhill through valleys that have previously been shaped by water rather than ice. [[Abrasion (geology)|Abrasion]] by rock material embedded within the moving glacial ice causes the widening and deepening of the valley to produce the characteristic U or trough shape with relatively steep, even vertical sides and a relatively flat bottom.

[[Interlocking spur]]s associated with the development of river valleys are preferentially eroded to produce [[truncated spur]]s, typical of glaciated mountain landscapes. The upper end of the trough below the ice-contributing cirques may be a [[U-shaped valley end|trough-end]]. [[Valley step]]s (or 'rock steps') can result from differing erosion rates due to both the nature of the bedrock (hardness and jointing for example) and the power of the moving ice. In places, a rock basin may be excavated which may later be filled with water to form a [[ribbon lake]] or else by sediments. Such features are found in coastal areas as [[fjord]]s. The shape of the valley which results from all of these influences may only become visible upon the recession of the glacier that forms it.<ref>{{cite book |last1=Monkhouse |first1=F.J. |title=Principles of Physical Geography |date=1971 |publisher=University of London Press Ltd |location=London |isbn=0340090227 |pages=230–234 |edition=Seventh}}</ref> A river or stream may remain in the valley; if it is smaller than one would expect given the size of its valley, it can be considered an example of a [[misfit stream]].

[[File:Pirin ezera Pano Chairski ezera.jpg|upright=1.35|A panoramic view of two merging U-shaped valleys in [[Pirin]] mountain, [[Geography of Bulgaria|Bulgaria]]|thumb]]

Other interesting glacially carved valleys include:
* [[Yosemite Valley]] (United States)
* [[Side valley]]s of the Austrian river [[Salzach]] for their parallel directions and hanging mouths.
* That of the [[St. Mary River (Montana-Alberta)|St. Mary River]] in [[Glacier National Park (United States)|Glacier National Park]] in [[Montana]], United States.

====Tunnel====
{{main|Tunnel valley}}
A tunnel valley is a large, long, U-shaped valley originally cut under the glacial ice near the margin of continental ice sheets such as that now covering Antarctica and formerly covering portions of all continents during past glacial ages.<ref name=J>{{Cite journal  | last =   Jørgensen | first =Flemming   |author2=Peter B.E. Sandersen   | title = Buried and open tunnel valleys in Denmark—erosion beneath multiple ice sheets  | journal = Quaternary Science Reviews  | volume = 25  | issue = 11–12  | pages = 1339–1363  | date = June 2006  | doi = 10.1016/j.quascirev.2005.11.006     |bibcode = 2006QSRv...25.1339J }}</ref> Such valleys can be up to {{convert|100|km|mi|abbr=on}} long, {{convert|4|km|mi|abbr=on}} wide, and {{convert|400|m|ft|abbr=on}} deep (its depth may vary along its length). Tunnel valleys were formed by subglacial [[water erosion]]. They once served as subglacial drainage pathways carrying large volumes of meltwater. Their cross-sections exhibit steep-sided flanks similar to fjord walls, and their flat bottoms are typical of subglacial glacial erosion.

====Meltwater====
{{main|Urstromtal}}
In northern Central Europe, the Scandinavian ice sheet during the various ice ages advanced slightly uphill against the lie of the land. As a result, its meltwaters flowed parallel to the ice margin to reach the North Sea basin, forming huge, flat valleys known as [[Urstromtäler]]. Unlike the other forms of glacial valleys, these were formed by glacial meltwaters.

{{Wide image|Hooker Valley in Aoraki-Mount Cook National Park.jpg|650px|New Zealand's Hooker Valley at [[Aoraki / Mount Cook National Park]], with [[Hooker Glacier (New Zealand)|Hooker Glacier]]'s terminus at [[Hooker Lake]] in the background}}

===Transition forms and shoulders===
[[File:Paria View at Bryce Canyon NP.jpg|thumb|upright=1.35|Look from Paria View to a valley in [[Bryce Canyon]], [[Utah]], with very striking shoulders]]
Depending on the [[topography]], the [[rock type]]s, and the [[climate]], a variety of transitional forms between V-, U- and plain{{clarify|date=January 2021}} valleys can form. The floor or bottom of these valleys can be broad or narrow, but all valleys have a shoulder. The broader a mountain valley, the lower its shoulders are located in most cases. An important exception is canyons where the shoulder almost is near the top of the valley's slope. In the Alps – e.g. the [[Tyrol (state)|Tyrolean]] [[Inn River|Inn]] valley – the shoulders are quite low (100–200 meters above the bottom). Many villages are located here (esp. on the sunny side) because the climate is very mild: even in winter when the valley's floor is filled with fog, these villages are in [[sunlight|sunshine]].

In some stress-tectonic regions of the [[Rocky Mountains]] or the Alps (e.g. [[Salzburg]]), the [[side valley]]s are parallel to each other, and are [[hanging valleys|hanging]]. Smaller streams flow into rivers as deep canyons or [[waterfall]]s.

===Hanging tributary{{anchor|Hanging_valleys}}===
[[File:Closeup of Bridalveil Fall seen from Tunnel View in Yosemite NP.JPG|thumb|upright|Bridal Veil Falls in [[Yosemite National Park]] flowing from a hanging valley]]
[[File:Ибърско езеро.jpg|thumb|Hanging valley, [[Ibar (lake)]] valley, [[Rila]] Mountain, [[Bulgaria]]]]

A hanging valley is a [[tributary]] valley that is higher than the main valley. They are most commonly associated with U-shaped valleys, where a tributary glacier flows into a glacier of larger volume. The main glacier erodes a deep U-shaped valley with nearly vertical sides, while the tributary glacier, with a smaller volume of ice, makes a shallower U-shaped valley. Since the surfaces of the glaciers were originally at the same [[elevation]], the shallower valley appears to be 'hanging' above the main valley. Often, waterfalls form at or near the outlet of the upper valley.<ref>{{cite web| title =Glossary of Glacier Terminology| publisher =U.S. Geological Survey| date =May 28, 2004| url =http://pubs.usgs.gov/of/2004/1216/h/h.html| access-date = 2007-05-24 }}</ref>

Hanging valleys also occur in [[fjord]] systems underwater. The branches of [[Sognefjord]] are much shallower than the main fjord. The mouth of [[Fjærlandsfjord]] is about {{Convert|400|m|sp=us}} deep while the main fjord nearby is {{Convert|1200|m|sp=us}} deep. The mouth of [[Ikjefjord]] is only {{Convert|50|m|sp=us}} deep while the main fjord is around {{Convert|1300|m|sp=us}} at the same point.<ref>Nesje, A., & Whillans, I. M. (1994). Erosion of Sognefjord, Norway. ''Geomorphology'', 9(1), 33-45.</ref>

Glaciated terrain is not the only site of hanging streams and valleys. Hanging valleys are also simply the product of varying rates of erosion of the main valley and the tributary valleys. The varying rates of erosion are associated with the composition of the adjacent rocks in the different valley locations. The tributary valleys are eroded and deepened by glaciers or erosion at a slower rate than that of the main valley floor; thus the difference in the two valleys' depth increases over time. The tributary valley, composed of more resistant rock, then hangs over the main valley.<ref>{{cite web|url=http://www4.uwsp.edu/geo/faculty/lemke/alpine_glacial_glossary/landforms/hangingvalley.html |title=Illustrated Glossary of Alpine Glacial Landforms - Hanging Valley |publisher=.uwsp.edu |access-date=2011-10-03}}</ref>

===Trough-shaped===
Trough-shaped valleys also form in regions of heavy topographic [[denudation]]. By contrast with glacial U-shaped valleys, there is less downward and sideways erosion. The severe downslope denudation results in gently sloping valley sides; their transition to the actual valley bottom is unclear. Trough-shaped valleys occur mainly in [[periglacial]] regions and in [[tropical]] regions of variable wetness. Both climates are dominated by heavy denudation.<ref>{{Citation |last1=Rowley |first1=Taylor |title=Chapter 13 - Periglacial Processes and Landforms in the Critical Zone |date=2015-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780444633699000136 |volume=19 |pages=397–447 |editor-last=Giardino |editor-first=John R. |access-date=2023-10-19 |publisher=Elsevier |last2=Giardino |first2=John R. |last3=Granados-Aguilar |first3=Raquel |last4=Vitek |first4=John D. |series=Developments in Earth Surface Processes |doi=10.1016/B978-0-444-63369-9.00013-6 |isbn=978-0-444-63369-9 |editor2-last=Houser |editor2-first=Chris|url-access=subscription }}</ref>

===Box===
Box valleys have wide, relatively level floors and steep sides. They are common in periglacial areas and occur in mid-latitudes, but also occur in tropical and arid regions.<ref>{{Cite book |url=https://books.google.com/books?id=UHRU_6nUSR4C |title=Encyclopedia of Geomorphology |publisher=Psychology Press |year=2004 |editor-last=Goudie |editor-first=Andrew |pages=98|isbn=9780415327381 }}</ref>

===Rift===
{{main|Rift valley}}
Rift valleys, such as the [[Albertine Rift]] and [[Gregory Rift]] are formed by the expansion of the [[Earth]]'s crust due to [[Plate tectonics|tectonic]] activity beneath the Earth's surface.