Glacial Landforms: Erosional and Depositional

2 years ago

Atiqur Rahman

13 minutes

Glaciers and ice sheets are the world’s main storehouse of fresh water. They are the thick and large masses of slow-moving ice under the influence of gravity. They are formed by the compression of snow. The ice mass is mostly confined in areas where the temperature is below zero degrees Celsius. They are found in high latitudes or high altitudes.

In higher latitudes, glaciers are seen even on flat ground. In low latitudes, the glaciers are found in the higher altitudes. Ice accumulation is possible when there is a greater amount of snowfall in comparison to snowmelt. Glaciermoves downslope due to gravity. Therefore, slowly moving ice body known as glaciers.

Glacial Conditions

The glacial condition exists where the temperature is always below zero degrees Celsius. Most precipitation occurs in the form of snowfall in glacial areas. The annual amount of snowmelt is less than or equal to the annual snowfall over the area. If snowfall is more, the glaciated area is on the rise; but when it is equal to the snowfall, it is stagnant. When the snowfall is less than the snowmelt, the glaciated area is on the decline.

Glaciated areas always bear ice on the surface. It is not so only when the slope is too high to sustain the ice. The accumulated ice is on slides downward. In this way, it develops a large number of features.

Work of Glaciers

The glaciers perform their work in three different ways- erosion, transportation and deposition.

Glacial Erosion

Glacier performs its erosional works primarily in two ways- plucking and abrasion. When any irregularity is found in the way of a glacier, the solid mass of ice exerts enormous pressure. The pressure plucks and crushes the rock in the way. It becomes easier when the jointed rocks are found. This process is termed plucking or quarrying.

The plucked rocks or the fragmented rocks materials available keep on moving along with the moving ice. Once that material moves along the side of the glacier’s bed, they erode the surface. This leads to the scratching of the bed as well as the reduction of the moving materials in their size. This process is termed abrasion.

Both plucking and abrasion are very clear in the Figure.

Factors Affecting the Rate of Glacial Erosion

There are several factors affecting glacial erosion, like- the thickness of ice, topography, the geology of the area, temperature, velocity of the glacier, slope amount of ice accumulation and ablation etc. (Figure).

Glacial Transportation

The work of transportation by the glacier is performed in three ways. When the weathered rock materials fall on the glaciers, they are transported to the ice surface. Slowly and slowly, a part of the materials fall below the ice surface and are transported for a considerable distance. Some of the rock fragments move further downward and reach the bottom of the glacier.

Based on the position of the rock fragments, they are categorized into three. The first one is said to be surface (supraglacial), the second to be internal (englacial) and the third to be basal (subglacial).

Glacial Deposition

The eroded and transported load of the glacier is deposited when the carrying capacity is reduced. The carrying capacity is reduced due to slope reduction or melt of ice and further lowering the ice mass. In both cases, the deposition is evident.

The deposition by glaciers is very distinct because it is unsorted and ungraded deposits of the load of the glacier. Depending upon the place and material of deposits, they may be categorized into several groups about which, which will be dealt with while dealing with landforms.

Glacial Erosional Landforms

Cirque

The shape of the cirque is just like an armchair, and it is made by glacial erosion. When the glacier descends from the mountain slope, it moves faster due to the very steep slope.

Later when the slope lowers, the excessive accumulation of ice exerts enormous pressure, and the base is eroded by ice rotation, plucking and abrasion. In due course of time, the base is further eroded, and a depression is created (Figure).

It is called cirque, a French term meant for a circle or bowl shape. It is also known as Corrie in Scottish.

Successive Development of Cirque or Corrie

Aretes

When several cirques or corries are developed on different sides of a mountain peak, their separating boundaries are further narrowed and sharpened. The sharp knife or blade-edged boundaries are called arêtes. It is shown in a diagrammatic presentation in Figure and the real feature in Figure. Mountain climbers generally follow aretes to scale the peak.

Col

Col is a depression-like feature made between two peaks. Generally, col is made by the erosive action of glaciers. This depression is created by the moving glacier. It helps in passing the mountain from one side to another. Col is also known as pass or saddle.

A saddle is a seat fastened on the back of a horse for sitting and riding the horse. It is depressed at the centre but raised in the front and rear and hence, it is called so (Figure).

Horn or Pyramidal Peak

When the glaciers are developed from different sides of a peak, they erode the preexisting surface of the peak successively (Figure) and make it a sharp and pointed peak with a very steep slope. This gives rise to a pyramidal peak which is also termed a horn as it is very pointed (Figure).

Successive Development of Pyramidal Peak

Hanging Valley

A hanging valley is a tributary glacial valley of the main glacier. The main glacier has a huge amount of ice, hence, eroding the base greater. Therefore, the main glacial valley is much deeper. The tributary glacier has a less amount of ice, hence, less erosion. Therefore, the base of the tributary glacier remains above the main glacier valley (Figure).

When the glaciation is over, the tributary valley is seen above the main valley. That is why it is called a hanging valley.

Truncated Spur

A spur is a long tongue-shaped ridge dropping down in an inverted ‘V’, separating the two adjacent valleys. The valleys confluence with another main river. When the same area comes under the influence of glaciers (due to climate change), the main river turns into the main glacier. The volume of ice in the main glacier is very large, and it erodes its valley greater.

Since the glacier is the moving solid ice, it erodes not only the bed but side also. The dropped-down spur is eroded, and the slope of the spur is interrupted. This interrupted/ broken spur is called a truncated spur (Figure).

‘U’ Shaped Valley

Glacial ice movement erodes its sides and makes the trough much wider. The glacier’s bed is flat, and both sides are almost vertical. This elongated wider trough is called a ‘U’ shaped valley (Figure). When the glaciation period is over, and ice is melted, the valley is exposed.

Fjord

Fjords are submerged glacial valleys under the seawater along the coast. The bottom of the valley is cut down because of heavy accumulated ice movement even below the sea level.

With the culmination of the ice age, the sea level also rose, and down cut glaciated valleys along the sea coast were drowned under the seawater. Because of this process, the coast is greatly dissected (Figure). This sort of coast is found in high latitudes and near polar areas.

Close-up View of Fjord along the Sea Coast in High Latitude
Source

Glacial Depositional Landforms

Moraine

The materials carried by the glacial ice are deposited in an ideal condition when the carrying capacity is reduced. These deposits are generally unsorted and un-stratified as it is deposited as and when the material reaches and dropped. Most of them are boulders of different sizes.

Depending upon the place of deposits, the moraines are classed into different groups. They are:

Lateral Moraine: The two sides of the glacier generally have exposed rocks. The weathered materials are dropped at the side of the glacier and are transported along. Materials are also supplied due to landslides as well. Carried materials also erode the side of the glacier, and furthermore, loads are generated. These materials are deposited along the side of the glacier, and hence, they are called lateral moraine (Figure).

Medial Moraine: When a tributary glacier or two glaciers meet with each other, the meeting side’s lateral moraines also join together and move further downward. Slowly and slowly, the joined lateral moraine reaches the middle/ near middle. When the ice melts, the joined lateral moraine is deposited and is called medial moraine (Figure).

Terminal Moraine: The deposits which occur at the down end of the glacier in a ridge-like accumulation. These debris are pushed forward by advancing the glacial tongue and dropped at the end. The terminal moraine spreads from one side or bank of the glacier to another, covering the entire width. It appears like a belt of small hilly ground with knobs and joints (Figure)

Recessional Moraine: Recessional moraine is a sort of terminal moraine. After the formation of the terminal moraine, when the glacier retreats, a new terminal moraine is formed upstream. The previous terminal is later called recessional (down) moraine (Figure).

Ground Moraine: A ground moraine is an irregular blanket of glacial loads deposited on the floor in a rolling to flat landscape after glacial melt/ retreat. As the term explains itself, it is the whole ground downside of the existing glacier where the glacial load is deposited (Figure 14). It is formed in both cases- continental glacier and valley glacier. Ground moraine is also known as till plain.

Glacial Till and Till Plain

The glacier carries huge amounts of loads with it. When the glacier melts/ retreats, the transported loads are deposited. This deposition is undulating to flat in look. The deposition of such materials at a certain location at a small scale is till (Figure), and the widespread area containing till is termed as till plain (Figure). This type of till plain is very extensive in continental glaciers but confined in the valley glacier.

Outwash Plain

The downside of the glacial melt is water associated with smaller pieces of ice floating below. The melted water carries gravel, sands, silts and clays downward and is deposited far away. It forms a sloppy but relatively flat plain known as an outwash (material washed/ carried by water) plain. It is different from till plain.

Till plain has unsorted materials directly deposited by the melt of ice and, hence, dropped without ordering the size of materials. The Outwash plain has sorted deposits as it is deposited by glacio-fluvial (small ice pieces of ice and flowing water) action.

Esker

Esker is a long, narrow and zigzag ridge-like structure generally made up of stratified sediments deposited by subglacial meltwater (Figure). Its height varies from 5 meters to 50 meters, and its width is 50 meters to 500 meters, whereas the length many as 500 meters to many kilometres. Esker is not necessarily a continuous feature but may be detached or broken. Literally, it is deposited by the meltwater of the glacier, and hence, they are sorted.

Kettle

A kettle is a shallow and small pothole or depression area where glacial brought sediment is deposited, and the remaining hollow part covers the water. It is formed and appears on the surface by retreating glaciers. Probably, this depression is created by the plucking, but later deposition of sediment and water accumulation give birth to the kettle. This water-filled body is known as Kettle Lake (Figure).

Kame

Kame is the accumulation in a mound-like shape made up of poorly sorted sand, boulders and gravels brought by glaciers at its terminus. A number of kames a formed when the glacier melts and terminates or recedes. A group of closely formed kames are termed a kame complex. Several kettle lakes are seen in a kame complex.

Roche Mountonnee

It is a French word meaning woolly rock, but in English, it means sheep-back or sheep-rock. Roche Moutonnee (Figure) is a smoothened bedrock of a glaciated region by abrasion on the upstream side and rough and steep surface to the downstream due to plucking/ scouring. Once the area is free from glaciers, the same feature appears on the surface.

Drumlin

Drumlin is an elongated oval-shaped deposited mound of small hill-like features formed by glaciers (Figure). It is made up of glacial till containing boulders, gravels and sand transported by glaciers and finally deposited in the above-mentioned shape. They are seen in the plain and are many in number. They vary in dimension from one to two kilometres in length, 400 to 600 meters in width and 15 to 30 meters in height.

Glacial Landforms: Erosional and Depositional

Glacial Conditions