What is Stratified or-Glaciofluvial Deposits?

These are the deposits of streams formed by the melting of glaciers. These deposits are sorted and stratified by the action of water from melting ice. While the melt water deposits the coarse material near the end of the glacier, the finer material is carried further away.

The water of the streams flowing out of glaciers are often milky-coloured due to the presence of rock flour which is a product of glacial abrasion. The milky-coloured water is, therefore, known as Glacial-milk.

The important depositional features of glaciofluvial origin include outwash plain, valley trains, eskers, kames and kame terraces.

Outwash Plains

Beyond the terminus of most mountain glaciers the gradient are com­monly less steep and the valleys are wider which help in the deposition of rock material carried by streams fed by glacier melt-water.

The deposits are formed as alluvial fans, which coalesce into a gently sloping extensive plain known as ‘Outwash plain.’ These are composed of stratified sand, gravel and shingle.

While the coarser fragments like gravels and shingles are usually deposited near the edge of the terminal moraines; the accumulation of sandy deposits are formed farther away. The outwash material may extend for hundreds of kilometres down major valleys.

Valley Trains

Outwash sand and gravel occuring on valley floors form what is known as valley-train. They appear as terraces beginning from the terminal moraines and extending down the valley.

Eskers:

An esker is a sinuous ridge extending in the direction of the movement of glacier. These are steep-walled ridges of assorted and stratified gravel and sand. Their length ranges from a few hundred metres to several tens of kilometres and are from 3 to 30 metres or more in height.

These are a few tens of metres in width. Some eskers are more or less straight, while others are shaped more like a river-valley.

Even though there is a divergence of opinion regarding the origin of eskers, mostly these are believed to have been formed from the deposits laid down by the water flowing in the superglacial, englacial or subglacial stream channels.

The glacial streams, as we know, transport the load of the glacier which comprises sand, gravel and pebbles and deposit them in the glacial channels in the manner very much similar to that of rivers. When the glacier finally melts away, the deposits of the glaciofiu- vial streams are left standing as a ridge on the glacier floor.

Some authors believe that with the emergence of the sub-glacial streams into the frontal region of the glacier its velocity decreases resulting in the deposition of material carried by it at the margin of the glacier in-the form of a narrow delta. More and more new deltas are formed with the continuous retreat of the glacier.

Such deltas may coalesce to form a continuous ridge known as esker. But this process of formation fails to explain the rise of an esker on to an elevation and therefore this opinion does not receive much acceptance. Eskers are also known as Osseroross.

Kames

These are small rounded hills, composed of stratified sand, gravel, pebbles and fine clays with an average height of 10 to 12 metres. They are often found near terminal moraines. The presence of fine clays indicates that these are formed in stagnant water and are also associated with immobile ice.

Due to intensive thawing there forms numerous depressions and basins on the surface of such ice, which are filled with water and are thus turned, into small lakes. The surficial melt waters carry sediments from the top surface of the giacier into these depressions.

As the ice melts, the material that formerly filled depressions on top of the glacier gradually sinks down. With the complete melting of the glacier, these deposits take the form of small, irregular and rounded hills, known as kames.

Kame Terraces

These are ice-contact features in which the deposits are laid down against an ice surface. It is formed by the fillings of depressions between the glacier and the sides of the trough. Sometimes these are confused with lateral moraines but the composition of the material distinguish one from the other.

Apart from the above important depositional features, features like crevasse infillings, kettles etc. are associated with glacial deposition.

Surficial melt water fills the crevasses with the material carried by them which form short and straight ridges known as crevasse-fillings. This is a special variety of kame.

Kettles are depressions developed on the surface of glacial deposits. These are either circular or elongated in shape and vary in size between wide limits.

These hollows are formed by melting of blocks of ice that might have been enclosed or burried within the glacial drift during the process of deposition. Such depressions mark the former location of the ice block.

Sometime a few of such depressions may form lakes or swamps. These features are produced by stagnant glacier ice. The outwash plains are often pitted with such depressions.