![Myklebustdalen-sanddal-fjordane](https://upload.wikimedia.org/wikipedia/commons/d/dc/Myklebustdalen-sanddal-fjordane.jpg)
U-shaped valleys, also called trough valleys or glacial troughs, are formed by the process of
glaciation
A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate be ...
. They are characteristic of mountain glaciation in particular.
They have a characteristic U shape in cross-section, with steep, straight sides and a flat or rounded bottom (by contrast, valleys carved by rivers tend to be V-shaped in cross-section). Glaciated valleys are formed when a glacier travels across and down a slope, carving the valley by the action of scouring. When the ice recedes or thaws, the valley remains, often littered with small boulders that were transported within the ice, called
glacial till
image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...
or
glacial erratic
A glacial erratic is glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word ' ("to wander"), are carried by glacial ice, often over distances of hundre ...
.
Examples of U-shaped valleys are found in mountainous regions throughout the world including the
Andes
The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
,
Alps
The Alps () ; german: Alpen ; it, Alpi ; rm, Alps ; sl, Alpe . are the highest and most extensive mountain range system that lies entirely in Europe, stretching approximately across seven Alpine countries (from west to east): France, Swi ...
,
Caucasus Mountains
The Caucasus Mountains,
: pronounced
* hy, Կովկասյան լեռներ,
: pronounced
* az, Qafqaz dağları, pronounced
* rus, Кавка́зские го́ры, Kavkázskiye góry, kɐfˈkasːkʲɪje ˈɡorɨ
* tr, Kafkas Dağla ...
,
Himalaya
The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 100 ...
,
Rocky Mountains
The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico ...
,
New Zealand
New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
and the
Scandinavian Mountains. They are found also in other major European mountains including the
Carpathian Mountains, the
Pyrenees
The Pyrenees (; es, Pirineos ; french: Pyrénées ; ca, Pirineu ; eu, Pirinioak ; oc, Pirenèus ; an, Pirineus) is a mountain range straddling the border of France and Spain. It extends nearly from its union with the Cantabrian Mountains to ...
, the
Rila and
Pirin
, photo=Pirin-mountains-Bansko.jpg
, photo_caption=Pirin scenery in winter
, country= Bulgaria,
, parent=
, geology= granite, gneiss, marble, limestone
, area_km2=2585
, range_coordinates =
, length_km=80 , length_orientation= north-s ...
mountains in
Bulgaria
Bulgaria (; bg, България, Bǎlgariya), officially the Republic of Bulgaria,, ) is a country in Southeast Europe. It is situated on the eastern flank of the Balkans, and is bordered by Romania to the north, Serbia and North Macedo ...
, and the
Scottish Highlands
The Highlands ( sco, the Hielands; gd, a’ Ghàidhealtachd , 'the place of the Gaels') is a historical region of Scotland. Culturally, the Highlands and the Lowlands diverged from the Late Middle Ages into the modern period, when Lowland S ...
. A classic glacial trough is in
Glacier National Park in
Montana
Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columb ...
, USA in which the
St. Mary River runs. Another well-known U-shaped valley is the
Nant Ffrancon
The Nant Ffrancon Pass in Snowdonia, North Wales, is the long steady climb of the A5 road between Bethesda, Gwynedd, and Llyn Ogwen in Conwy. The summit at is at Pont Wern-gof, about one-third of a mile beyond the eastern end of Llyn Ogwen. Fr ...
valley in
Snowdonia
Snowdonia or Eryri (), is a mountainous region in northwestern Wales and a national park of in area. It was the first to be designated of the three national parks in Wales, in 1951.
Name and extent
It was a commonly held belief that the na ...
,
Wales
Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...
.
When a U-shaped valley extends into saltwater, becoming an inlet of the sea, it is called a
fjord
In physical geography, a fjord or fiord () is a long, narrow inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Alaska, Antarctica, British Columbia, Chile, Denmark, Förden and East Jutland Fjorde, Germany, ...
, from the Norwegian word for these features that are common in Norway. Outside of Norway, a classic U-shaped valley that is also a fjord is the Western Brook Pond Fjord in
Gros Morne National Park in
Newfoundland, Canada.
Formation
Shape
Formation of a U-shaped valley happens over
geologic time, meaning not during a human's lifespan. It can take anywhere between 10,000 and 100,000 years for a
V-shaped valley
A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over ...
to be carved into a U-shaped valley.
These valleys can be several thousand feet deep and tens of miles long.
Glaciers will spread out evenly in open areas, but tend to carve deep into the ground when confined to a valley.
Ice thickness is a major contributing factor to valley depth and carving rates. As a glacier moves downhill through a valley, usually with a stream running through it, the shape of the valley is transformed. As the ice melts and retreats, the valley is left with very steep sides and a wide, flat floor. This
parabolic shape is caused by glacial erosion removing the contact surfaces with greatest resistance to flow, and the resulting section minimises friction.
There are two main variations of this U-shape. The first is called the Rocky Mountain model and it is attributed to alpine glacial valleys, showing an overall deepening effect on the valley. The second variation is referred to as the Patagonia-Antarctica model, attributed to continental
ice sheet
In glaciology, an ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than . The only current ice sheets are in Antarctica and Greenland; during the Last Glacial Period at La ...
s and displaying an overall widening effect on its surroundings.
Valley floor
The floors of these glacial valleys are where the most evidence can be found regarding glaciation cycles. For the most part, the valley floor is wide and flat, but there are various glacial features that signify periods of ice transgression and regression. The valley can have various steps, known as
valley step
A valley step (german: Talstufe or ''Talschwelle'') is a prominent change in the longitudinal slope of a valley, mainly in trough valleys formed by glaciers.
Typically, a valley formed by glaciers has a series of basins with intervening steps ...
s, and over-deepenings anywhere from ten to hundreds of meters deep.
These then fill in with sediments to create plains or water to create lakes, sometimes referred to as "string-of-pearl" or ribbon lakes.
Such water filled U-valley basins are also known as "fjord-lakes" or "valley-lakes" (Norwegian: ''fjordsjø'' or ''dalsjø'').
Gjende
Gjende or Gjendin is a lake in Vågå Municipality in Innlandet county, Norway. It is located in the Jotunheimen mountain range and also inside Jotunheimen National Park. The proglacial lake shows typical characteristics of glacial formation, ...
and
Bandak
Bandak is a lake in the municipalities of Kviteseid and Tokke in Vestfold og Telemark county, Norway. The lake, which is part of the Telemark Canal route, belongs to the Skien watershed. The river Tokke flows into the lake, and the outlet i ...
lakes in Norway are examples of
fjord
In physical geography, a fjord or fiord () is a long, narrow inlet with steep sides or cliffs, created by a glacier. Fjords exist on the coasts of Alaska, Antarctica, British Columbia, Chile, Denmark, Förden and East Jutland Fjorde, Germany, ...
-lakes. Some of these fjord-lakes are very deep for instance
Mjøsa (453 meters) and
Hornindalsvatnet
Hornindalsvatnet is Norway's and Europe's deepest lake, and the world's twelfth deepest lake, officially measured to a depth of . Its surface is above sea level, which means that its bottom is below sea level.
The village of Grodås lies at ...
(514 m). The longitudinal profile of a U-shaped, glaciated valley is often stepwise where flat basins are interrupted by thresholds. Rivers often dig a V-shaped valley or gorge through the threshold.
Surrounding smaller tributary valleys will often join the main valleys during glaciation periods, leaving behind features known as
hanging valley
A valley is an elongated low area often running between hills or mountains, which will typically contain a river or stream running from one end to the other. Most valleys are formed by erosion of the land surface by rivers or streams over ...
s high in the trough walls after the ice melts.
After deglaciation, snow and ice melt from the mountain tops can create streams and rivers in U-shaped valleys. These are referred to as misfit streams. The streams that form in hanging valleys create waterfalls that flow into the main valley branch. Glacial valleys may also have natural, often dam-like, structures within them, called
moraines. They are created due to excess sediment and glacial till moved and deposited by the glacier.
In volcanic mountain ranges, such as the
Principal Cordillera of the Andes, glacial valley floors may be covered by thick
lava flows
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...
.
Trough end
A glacial trough or glaciated mountain valley often ends in an abrupt
head known as the 'trough end' or 'trough head'.
[Blackie, ''Blackie's Dictionary of Geography'', Mumbai: Blackie, 2000, p. 344.] This may have almost sheer rock walls and spectacular
waterfall
A waterfall is a point in a river or stream where water flows over a vertical drop or a series of steep drops. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf.
Waterfalls can be formed in severa ...
s.
[Monkhouse, F.J., ''Dictionary of Geography'', London: AldineTransaction, 1970, p.359.] They are believed to have been formed where a number of small glaciers merge to produce a much larger glacier.[ Examples include: Warnscale Bottom in the Lake District, ]Yosemite Valley
Yosemite Valley ( ; ''Yosemite'', Miwok for "killer") is a glacial valley in Yosemite National Park in the western Sierra Nevada mountains of Central California. The valley is about long and deep, surrounded by high granite summits such as Hal ...
, and the Rottal and Engstlige valleys in Switzerland.[
File:Trollstigvegen - no-nb digifoto 20151105 00069 NB MIT FNR 06409A (cropped).jpg, The deep U-shaped Isterdalen valley in Norway ends abruptly giving rise to tall waterfalls. ]Trollstigen
Trollstigen (or trollstigvegen; en, The troll path) is a serpentine mountain road and pass in Rauma Municipality, Møre og Romsdal county, Norway.
It is part of Norwegian County Road 63 that connects the town of Åndalsnes in Rauma and the ...
road climbs the steep cliffs of the trough end.
File:Trollstigvegen - no-nb digifoto 20151105 00053 NB MIT FNR 06406 (cropped).jpg, Valley head or trough end at Isterdalen from a different angle showing Trollstigen road and waterfall
Marine troughs
Glacial troughs also exist as submarine valleys on continental shelves, such as the Laurentian Channel
The Laurentian Channel is a deep submarine valley off the coast of eastern Canada in the Gulf of Saint Lawrence.
The channel is of glacial origin and is the submerged valley of the historic Saint Lawrence River, running from a sharp escarpment ...
. These geomorphic features significantly influence sediment distribution and biological communities through their modification of current patterns.
History
Geologists did not always believe that glaciers were responsible for U-shaped valleys and other glacial erosional features. Ice is quite soft and it was unbelievable to many that it could be responsible for the severe carving of bedrock characteristic of glacial erosion. German geologist Penck and American geologist Davis were vocal supporters of this unprecedented glacial erosion.
Progress was made in the 1970s and 1980s on the possible mechanisms of glacial erosion and U-shaped valleys via models proposed by various scientists. Numerical models have been created to explain the phenomenon of carving U-shaped valleys.
See also
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References
{{Reflist
Geomorphology
Glacial erosion landforms
Glaciology
Valleys