Introduction to Outwash Plain
Outwash plains are wide, tenderly sloping sheets of glacial sediments deposited by meltwater outwash at the snout of the glaciers. As it flows, glaciers crush the underlying rock surface and carry the scrap along with it. The sediments are accumulated at the terminal of the glaciers over the outwash plain by the meltwater, with large stones accumulated near the terminal moraine, and finest particles are carried further.
Outwash plains are commonly found in Iceland where geothermal activity speeds up the melting of ice flow and deposition of sediments carried by melting water. The other name outwash plains are sandur (plural: sabdurs), sandar, and sandr.
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What is Outwash Plain?
An outwash plain is both an erosional and depositional surface formed by meltwater coming from the glaciers. These plains are generally identified by braided streams and found in the front of the glaciers. The streams are generally small and braided because the size of the sediment varies and the original stream gets split up. As the streams meander around, the erosion takes place left to right rather than up and down. These glaciers are generally formed beyond the terminal moraine deposited by the glaciers.
Define Outwash Plain
A wide, tenderly sloping sheet of outwash accumulated by meltwater streams flowing in front of or beyond a glacier, and formed by coalescing outwash fans is defined as an outwash plain.
Outwash Plains Formation
A fluvioglacial landform formed by both deposition and erosion by meltwater is a glacial outwash plain or sandur that generally has braided streams. Glaciers and icecaps contain large quantities of sediments and slits, that are picked up as they erode by the meltwater that carries the sediments away from the glaciers and deposit them on a broad plain. The materials in the outwash plain are size-sorted by the surface water of the melting glacier, with small particles like a slit, being the most distantly re-deposited, whereas the largest sediments are re-deposited to the original terminus of the glaciers.
Outwash Plains Occurrence
The Kerguelen Islands, Svalbard, and Iceland are the islands where the outward plains are found. Outwash plains are also most commonly found where geothermal activities below the ice caps accelerate the deposits of the sediments by meltwater.
Glacial Outwash
The considerable amount of water that flowed from melting ice deposited different kinds of materials, the most important of which are glacial outwash. Glacial outwash plains made up of outwash deposits are flat and consist of layers of sand and other fine sediments. The plains at the bottom of the glacial mountains or the reserve of continental ice sheets are covered with glacial-fluvial deposits in the form of broad flat alluvial fans which combine to form an outwash plain of sand, silt, and clay.
The thickness of the outwash can reach 100 m (328 feet) at the end of the glacier, although the thickness is usually much less, it may extend many kilometres in length. Outwash plain may be extended for miles beyond the margin of the glaciers.
Outwash sediments from the Wisconsin Glacial episode can be tracked down to the mouth of the Mississippi River, 1120 km (700 miles) from the nearest terminus of the glacial.
Did You Know?
An outwash, also known as sandur, is a plain formed by melting glaciers.
Outwash plain on the map is found in the southeast quarter.
The largest outwash plain in the world covering an area of 1300 km is Skeiðarársandur.
Outwash plains are commonly found in Iceland where geothermal activity speeds up the melting of ice flow and deposition of sediments by meltwater.
A terminal moraine, also known as end moraine is a type of moraine that is formed at the edge of the glacier, making its maximum advance.
FAQs on Outwash Plain
1. What is an outwash plain and how is it formed?
An outwash plain, also known as a sandur, is a broad, flat-to-gently-sloping landform created by the deposition of sediments from glacial meltwater. It forms when meltwater streams, flowing from the terminus (end) of a glacier, carry a large volume of sand and gravel. As these streams spread out and lose energy, they deposit their sediment load, building up the plain layer by layer with glaciofluvial deposits.
2. What are the main characteristics of an outwash plain?
The key characteristics of an outwash plain are:
- Sorted Sediments: It is composed of sand and gravel that have been sorted by flowing water, creating distinct layers (stratification). Coarser materials are dropped closer to the glacier, while finer particles are carried farther away.
- Braided Streams: The surface is often marked by a network of interweaving, shallow river channels known as braided streams.
- Location: It is typically found in front of the terminal moraine of a glacier.
- Topography: The landscape is generally flat or has a very gentle slope away from the glacier.
3. How does an outwash plain differ from a moraine?
The primary difference lies in their formation agent and sediment type. An outwash plain is formed by meltwater depositing sorted, stratified sediments (sand and gravel) away from the glacier. In contrast, a moraine is a ridge formed by the direct action of glacier ice, consisting of unsorted, unstratified material called glacial till, which is a mix of clay, sand, gravel, and boulders.
4. What is the key difference between the sediments in an outwash plain and glacial till?
The key difference is sorting. Sediments in an outwash plain are sorted by flowing water, which separates particles by size, resulting in layers of sand and gravel. This material is called outwash. Glacial till, on the other hand, is deposited directly by melting ice without the sorting action of water. Therefore, till is a completely unsorted mixture of various-sized materials, from fine clay to large boulders, all jumbled together.
5. Why are braided streams a common feature on outwash plains?
Braided streams are common because the meltwater from glaciers carries an exceptionally high sediment load. As the water flows over the gentle slope of the outwash plain, it loses velocity and cannot transport all the sediment. The excess sediment is deposited as bars or small islands within the channel, forcing the water to split and flow around them in multiple, interconnected channels, creating a braided pattern.
6. Can you give an example of a well-known outwash plain?
A classic and active example of an outwash plain is the Skeiðarársandur in Iceland, which is formed by the meltwater of the Vatnajökull glacier. Many parts of the Canterbury Plains in New Zealand are also extensive outwash plains formed during past glaciations and are now fertile agricultural regions.
7. Is an outwash plain considered a landform of erosion or deposition?
An outwash plain is fundamentally a depositional landform. Although the sediments were originally created by glacial erosion elsewhere, the plain itself is built up by the accumulation and deposition of these materials by meltwater streams. The dominant process in the formation of the plain is deposition.
