Fluvioglacial processes

deposition and its features & erosion

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The melting of ice produces a great deal of water which has the capacity to carry much debris. As water often flows under considerable pressure it has a high velocity and isvery turbulent. It can therefore pick up and transport a larger amount of material than a normal river of the similar size. It is now believed that this water and its load is responsible for the creation of subglacial valleys that are often deep and riddled with potholes.

When the meltwater discharge decreases, the resultant loss of energy causes the material being carried by the meltwater to be deposited. As with all water deposition, the heavier material is dropped first, resulting in the sorting of material.

Depsoits may also be found in layers (stratifed) as a result of seasonal variations in the meltwater flow.

The main features produced by fluvioglacial deposition are: eskers, kames and the outwash plain. Lakes on the outwah plain may have layered deposits in them called varves.

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Main features of an Esker:

  • they are long ridges of material running in the direction of the ice advance.
  • they are sinuous (winding) form, 5-20m high.
  • they consist of sorted coarse material, usually sand and gravel.
  • they are often statified.

Eskers are believed to be deposits made by subglacial streams. The channel of the stream will be restricted by the ice walls, so there is considerbale hydrostatic pressure which enables larger loads to be carried and also allows the stream to flow uphill for short distances. This accounts for the fact that some eskers run up gentle gradients. The bed of the channel builds up above the sourrounding land, and a ridge is left when the glacier retreats during deglaciation. In some areas the ridge of an esker is combined with mounds of material, possibly kames.

Such a feature is known as a beaded esker.

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Kames are mounds of fluvioglacial material.

They are deltaic deposits left when meltwater flows into a lake dammed up in front of the glacial snout by recessional moraine deposits.

When the ice retreats further, the delta kame often collapes. Kame terraces are frequently found along the side of the glacial valley and are the deposits of meltwater streams flowing between the ice and the valley side.

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Outwash plains (sandur)

Outwash plains are found in front of the glaciers snout and are deposited by the meltwater streams issuing from the ice. They consist of material that was brought down by the glacier and then picked up, sorted and dropped by running water beyond the position of the ice front. The coarsest material travels the shortests distance and is therefore found near to the glacier; the fine material, such as clay is carried some distance across the plain before being deposited. The deposits are also layered vertically which reflects the seasonal flow of meltwater streams.

Meltwater streams that cross the outwash plain are braided. This happens as the channels become choked with coarse material because of marked seasonal variations in discharge. On the outwash plain there is often a series of small depressions filled with lakes or marshes. These are known as kettle holes. It is believed that they are formed when blocks of ice, washed onto the plain, melt and leave a gap in the sediments. Such holes fill with water to form small lakes. Aquatic plants become established in the lakes abd this leads to over time to the development of a marshy area and then peat.

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Outwash plains continued...

Lakes on the fringe of the ice are filled with deposits that show a distinct layering. A layer if silt lying ontop of a layer of sand represents 1 years deposition in the lake and is known as a varve.

The coarser, lighter-coloured layer is the spring and summer deposition when meltwater is at its peak and the meltwater steams are carrying maximum load.

The thinner, darker-coloured and finer silt settles during autumn and through the winter as stream discharge decreases and the very fine sediment in the lake settles to the bottom.

Varves are a good indicator of the age of the lake sediments and of past climate as the thickness of each varve indicates warmer and colder periods.

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Erosion - Proglacial lakes and overflow channels.

Glacial meltwater has a great erosive power because of its volume and the large amounts of debris it contains. During deglaciation, lakes develop on the edges of the ice, some occupying large areas. Overflows from these lakes which cross the lowest points of the watersheds will create new valleys.

When the ice damming these meltwater lakes totally melts, many of these new valleys are left dry, as drainage patterns revert to preglacial stage. In certain cases, however, the post glacial drainage adopts them, giving rise to new drainage patterns.

Large meltwater lakes of this kind occured in the English Midlands (Lake Harrison), the Vale of Pickering in North Yorkshire and the Welsh Border (Lake Lapworth) at the end of the last glaciation.

The river Thames is thought to have to have followed a much more northerly course before the Quaternary glaciation - its modern course was formed when ice filled the northern part of its basin and forced it to take a different route.

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