An outwash plain, also called a sandur (plural: ''sandurs''), sandr or sandar, is a

plain In geography, a plain, commonly known as flatland, is a flat expanse of land that generally does not change much in elevation, and is primarily treeless. Plains occur as lowlands along valleys or at the base of mountains, as coastal plains, and ...

formed of

glaciofluvial deposits Fluvioglacial landforms or glaciofluvial landforms are those that result from the associated erosion and deposition of sediments caused by glacial meltwater. Glaciers contain suspended sediment loads, much of which is initially picked up from the ...

due to

meltwater Meltwater (or melt water) is water released by the melting of snow or ice, including glaciers, glacial ice, tabular icebergs and ice shelf, ice shelves over oceans. Meltwater is often found during early spring (season), spring when snow packs a ...

outwash at the terminus of a

glacier A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...

. As it flows, the glacier grinds the underlying rock surface and carries the debris along. The meltwater at the snout of the glacier deposits its load of

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

over the outwash plain, with larger boulders being deposited near the terminal

moraine A moraine is any accumulation of unconsolidated debris (regolith and Rock (geology), rock), sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a gla ...

, and smaller particles travelling further before being deposited. Sandurs are common in Iceland where geothermal activity accelerates the melting of ice flows and the deposition of sediment by meltwater.

Formation

Sandurs are found in glaciated areas, such as

Svalbard Svalbard ( , ), previously known as Spitsbergen or Spitzbergen, is a Norway, Norwegian archipelago that lies at the convergence of the Arctic Ocean with the Atlantic Ocean. North of continental Europe, mainland Europe, it lies about midway be ...

Kerguelen Islands The Kerguelen Islands ( or ; in French commonly ' but officially ', ), also known as the Desolation Islands (' in French), are a group of islands in the subantarctic, sub-Antarctic region. They are among the Extremes on Earth#Remoteness, most i ...

, and

Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...

. Glaciers and icecaps contain large amounts of

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...

and sediment, picked up as they

erode Erode (; īrōṭu), is a city in the Indian state of Tamil Nadu. It is located on the banks of the Kaveri river and is surrounded by the Western Ghats. Erode is the seventh largest urban agglomeration in Tamil Nadu. It is the administrativ ...

the underlying rocks when they move slowly downhill, and at the snout of the glacier, meltwater can carry this sediment away from the glacier and deposit it on a broad plain. The material in the outwash plain is often size-sorted by the water runoff of the melting glacier with the finest materials, like silt, being the most distantly re-deposited, whereas larger boulders are the closest to the original terminus of the glacier. An outwash plain might contain surficial

braided stream A braided river (also called braided channel or braided stream) consists of a network of river channels separated by small, often temporary, islands called '' braid bars'' or, in British English usage, '' aits'' or ''eyots''. Braided streams t ...

complexes that rework the original deposits. They may also contain

kettle lake A kettle (also known as a kettle hole, kettlehole, or pothole) is a depression or hole in an outwash plain formed by retreating glaciers or draining floodwaters. The kettles are formed as a result of blocks of dead ice left behind by retreating ...

s, locations where blocks of ice have melted, leaving a depression that fills with water. The flow pattern of glacial rivers across sandar is typically diffuse and unchannelized, but in situations where the glacial snout has retreated from the terminal

, the flow is more channelized. Sandurs are most common in Iceland, where

geothermal Geothermal is related to energy and may refer to: * Geothermal energy, useful energy generated and stored in the Earth * Geothermal activity, the range of natural phenomena at or near the surface, associated with release of the Earth's internal he ...

activity beneath ice caps speeds up the deposition of sediment by meltwater. As well as regular geothermal activity,

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

activity gives rise to large glacial bursts several times a century, which carry down large volumes of sediment. The

Gaspé Peninsula The Gaspé Peninsula, also known as Gaspesia (, ; ), is a peninsula along the south shore of the St. Lawrence River that extends from the Matapedia Valley in Quebec, Canada, into the Gulf of St. Lawrence. It is separated from New Brunswick on it ...

that makes up the essential part of southern

Quebec Quebec is Canada's List of Canadian provinces and territories by area, largest province by area. Located in Central Canada, the province shares borders with the provinces of Ontario to the west, Newfoundland and Labrador to the northeast, ...

(Lower St-Lawrence and Gaspé areas) also contains several examples of paleo-sandar, dating from the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

ice melt.

The prototype sandur

One of the sandurs from which the general name is derived is , a broad sandy wasteland along Iceland's south-eastern coast, between the icecap and the sea. Volcanic eruptions under the icecap have given rise to many large glacial bursts (' in Icelandic), most recently in 1996, when the

Ring Road A ring road (also known as circular road, beltline, beltway, circumferential (high)way, loop or orbital) is a road or a series of connected roads encircling a town, city or country. The most common purpose of a ring road is to assist in reducin ...

was washed away (minor floods have also occurred since then). This road, which encircles Iceland and was completed in 1974, has since been repaired. The 1996

jökulhlaup A jökulhlaup ( ) (literally "glacial run") is a type of glacial outburst flood. It is an Icelandic term that has been adopted in glaciological terminology in many languages. It originally referred to the well-known subglacial outburst floo ...

was caused by the eruption of the volcano, with peak flow estimated to be compared to the normal summer peak flow of . Net deposition of sediment was estimated to be . The main braided channels of are the Gígjukvísl and rivers, which incurred net gains of respectively during the 1996 jökulhlaup. In the Gígjukvísl there was massive sediment deposition of up to , which occurred closest to the terminus of the glacier. The erosional patterns of can be seen by looking at the centimetre-scale elevation differences measured with repeat-pass laser altimetry (

LIDAR Lidar (, also LIDAR, an acronym of "light detection and ranging" or "laser imaging, detection, and ranging") is a method for determining ranging, ranges by targeting an object or a surface with a laser and measuring the time for the reflected li ...

) flown in 1996 (pre-flood), 1997, and 2001. Of the overall deposition during the 1996 jökulhlaup, nearly half of the net gain had been eroded 4 years after the flood. These two rivers on the sandur display drastically different erosional patterns. The difference in sediment erosion can be attributed to the wide trench near the terminus where the Gígjukvísl flows, in contrast with the , which has braided flows directly onto the outwash plain. The Gígjukvísl river is where some of the highest level of sediment deposit occurred and also where the largest erosion happened afterward. This indicates that these massive jökulhlaup deposits may have a large geomorphic impact in the short term, but the net change on the surface relief could be minimal after a couple of years to a decade. The observed change of from a diffuse to a channelized distributary system where it has the most observed sediment deposit has a significant impact on the development of the fluvial succession in the proximal zone. However, in order to have sustained active accretion across the entire sandur there needs to be a diffuse, multipoint distribution system. The system of accumulation on , which is a product of glacier retreat, can be seen as multiple regions of differing channel patterns that distribute sediment across the plain in dynamic configurations.

Fossil sandar

Fossil sandar (i.e. no longer active) are found in areas which were formerly glaciated. An example would be the Usk Valley of South

Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...

where, towards the end of the last ice age, the receding Usk valley glacier left behind a series of recessional moraines and sandar deposits down-valley of them. Many of the sandar surfaces are still visible, albeit degraded over succeeding millennia. Extensive sandar are also recorded in the eastern part of the

Cheshire Plain The Cheshire Plain is a relatively flat expanse of lowland within the county of Cheshire in North West England but extending south into Shropshire. It extends from the Mersey Valley in the north to the Shropshire Hills in the south, bounded by t ...

and beneath

Morecambe Bay Morecambe Bay is an estuary in north-west England, just to the south of the Lake District National Park. It is the largest expanse of intertidal mudflats and sand in the United Kingdom, covering a total area of . In 1974, the second largest ga ...

, both in northwest

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

. 'Valley sandur' deposits are recorded from various localities in that same region.

References

External links

NASA page about the Skeiðarársandur

A study of sandur formation in western Greenland
{{Authority control Glacial landforms Glaciology

Formation

The prototype sandur

Fossil sandar

See also

References

Further reading

External links