Short Essay on the Deposition by Valley-Glaciers

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The transporting power of a glacier diminishes when the ice begins to melt and the glacier slows down resulting in the deposition of the material carried by the glacier itself. Deposition takes place mainly in the downstream parts of glaciers, where the load is dropped forming huge accumulations of varying shapes and characters.

In glacial deposits big boulders and finest rock materials are accumulated at the same place. Glacially transported and deposited materials together constitute what is known as Glacial-drift.

This may be stratified or un- stratified. When dropped directly by the glacier, there results a heap of glacial deposits consisting of an assemblage of rock debris of varying dimensions; whereas when reworked subsequently by the melt water streams issuing from the decaying and retreating glacier there results sorted and stratified deposits.

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Accordingly, glacial deposits are classified into two main types as:

(a) Unstratified and unsorted deposits.

(b) Stratified or Glaciofluvial deposits.

On the basis of their mode of formation glacial deposits may exhibit a variety of forms as described below:

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Depositional-Features Produced By Valley-Glaciers

Unstratified Deposits

The unstratified, unsorted debris dropped more or less in a random fashion by glaciers form deposits known as till.

Till makes up a group of topographic features called Moraines. The distribution of moraines marks the extent of now-vanished glacier.

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The essential character of a till is that it is formed of a mass of broken rock fragments mostly angular or subangular and of many different sizes and compositions. The term Boulder-clay is also used to describe such deposits. Consolidated moraines are called tillites.

Moraines are localized deposits of glacial debris formed either on the body of the existing glacier or at various places along the glaciated valley of an extinct glacier. Accordingly, two different types of moraines are recognized.

1. Moving Moraines.

2. Stationary Moraines.

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1. Moving Moraines

These moraines travel with the glacial ice. Depending on their position on the body of a mountain or valley glacier various types of moving moraines have been distinguished, as-indicated below.

(a) Surface Moraines

Weathering and mass-wasting processes which operate efficiently in mountain slopes above the level of glacier contribute to the rock debris carried by the glacier on its surface.

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Such accumulation of rock debris on the surface of the valley glacier is known as surface-moraine. Surface moraines are in turn divided into two types as

(i) Lateral Moraines.

(ii) Medial Moraines.

Lateral moraines are low-ridges of rock debris formed along the margins of the glaciated valley. The rock debris are scoured from the valley sides above the glacier due to glacial erosion and also by weathering, snow slides, avalanches and other types of mass-wasting.

Besides, the rock debris are also brought by streams along lateral gorges. These rock debris are carried on the surface of the glacier along the margins where they have been dumped from the valley sides, forming what is known as lateral moraines or marginal moraines.

When two glaciers join, their lateral moraines are dragged along and their coalescence give rise to longitudinal ridges in the middle part of the glacier.

Thus the lateral moraines of tributary glaciers subsequently become the medial moraines of the main glacier. Because of their central position they are quickly removed away and are there­fore transitory in existence.

(b) Englacial Moraines

The rock debris lying on the surface of a glacier may fall into crevasses during the movement of the glacier and are enclosed in the body of the glacier. This debris enclosed in the glacier’s body is known as Englacial moraine.

Sometimes there is an accumulation of rock debris, due to weathering processes, in the neve field where these are burried under the newly fallen snow and later on engulfed in the moving ice they reach the drainage area.

(c) Ground Moraines

These are also known as Sub-glacial Mo­raines or bottom moraines. As we know, when a glacier moves over the valley-floor rock-fragments are plucked out, dragged along and incorporated in the basal layers of ice.

As the glacier retreats, the materials carried by the glacier are dropped at random upon the valley floor forming what is known as Ground-moraine. These deposits are quite irregular in form and are typical examples of glacial till. .These deposits are thin and uneven.

2. Stationary Moraines

These are also known as deposited moraines. In such cases the rock debris is deposited after melting of the glacier. These moraines include the terminal and recessional moraines.

Terminal moraines are formed at the terminus of glaciers, where the ice front remains stationary for long period of time.

Various fragmental material are deposited at the terminal fringes of glaciers when the ice melts away as a crescent-shaped areas of tumultuous heaps consisting of materials of varying size and composition.

The convexity of the crescent point towards the down slope of the valley where they occur. These deposits are in the form of a low ridge up to about 30 metres in height, lying across the valley floor. These are also known as End Moraines.

Recessional moraines are formed where a glacier retreats in a halting manner. When there is frequent and prolonged cessations in the movement of the glacier within the process of its recession, several terminal moraines may form in a concentric fashion which indicate the intermittent retreat of the glacial front.

Successive pauses in the position of a glacier-retreat from its terminus produce successive moraines known as recessional moraines. These moraines may be pushed ahead to a new position if the glacier readvances.

Apart from the moraines the unstratified glacial deposits also include features like erratic blocks.

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