Editing Valley (section)

===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}}