An outwash fan is a fan-shaped body of

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand a ...

s deposited by

braided stream A braided river, or braided channel, consists of a network of river channels separated by small, often temporary, islands called braid bars or, in English usage, '' aits'' or ''eyots''. Braided streams tend to occur in rivers with high sediment ...

s from a melting

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

. Sediment locked within the ice of the glacier gets transported by the streams of meltwater, and deposits on the outwash plain, at the

terminus Terminus may refer to: * Bus terminus, a bus station serving as an end destination * Terminal train station or terminus, a railway station serving as an end destination Geography *Terminus, the unofficial original name of Atlanta, Georgia, United ...

of the glacier. The

outwash An outwash plain, also called a sandur (plural: ''sandurs''), sandr or sandar, is a plain formed of glaciofluvial deposits due to meltwater outwash at the terminus of a glacier. As it flows, the glacier grinds the underlying rock surface and c ...

, the sediment transported and deposited by the meltwater and that makes up the fan, is usually poorly sorted due to the short distance traveled before being deposited.

Formation

Outwash fans typically form from valley glaciers flowing downhill in the mountains. Thus, outwash fans are usually found in colder environments where glaciers are more prevalent and are often located where a valley or canyon empties out onto a flatter wider plain. Outwash fans form when melt-water from a retreating glacier deposits transported sediment in the shape of a fan along the outwash plain. Glaciers contain large amounts of sediment (i.e. sand, silt, clay) that is gathered through glacial erosional processes such as abrasion between the glacier and the underlying rocks, and through glacial plucking. As the glacier begins to retreat and

ablation Ablation ( la, ablatio – removal) is removal or destruction of something from an object by vaporization, chipping, erosive processes or by other means. Examples of ablative materials are described below, and include spacecraft material for a ...

increases, the melting ice deposits sediments from a single source at the

of the glacier. Melt-water from the glacier forming a

then carries the sediment away and redeposits it further away from the glacier. The sedimentation of the outwash fan is often poorly sorted due to the short distance covered by the transported sediment, however, water runoff from the glacier acts as a natural sorter. Finer materials such as silt and clay are redeposited further away from the glacier, while larger sediments such as pebbles and rocks remain closest to the glacier. Because the sediments rely on melt water for transportation, the ablation rate directly affects the development of an outwash fan. Other factors that affect fan morphology include the slope of the outwash fan and the depth and width of its distributive channels. Minor fans, unlike the more extensive outwash fans that transport larger sized material by high energy streams from underneath the glacier, are mainly made up of fine grained sediments (i.e. sand) and are deposited by low energy streams that drain debris along the glacier's surface.Krüger J. "Development of minor outwash fans at Kotlujökull, Iceland". ''Quaternary Science Reviews'', 16 (1997), pp. 649-659

References

*http://encyclopedia.farlex.com/outwash * * * Glacial landforms {{geomorph-stub