Evolution and characteristics of landforms in the Fluvial, Glacial, Arid and Karst regions  

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Landform

Each landform has its unique physical shape, size, materials and is a result of the action of certain geomorphic processes and agent(s). Every landform has a beginning. Landforms once formed may change in their shape, size and nature slowly or fast due to continued action of geomorphic processes and agents. Due to changes in climatic conditions and vertical or horizontal movements of landmasses, either the intensity of processes or the processes themselves might change leading to new modifications in the landforms.

Evolution

It implies stages of transformation of either a part of the earth’s surface from one landform into another or transformation of individual landforms after they are once formed. That means, each and every landform has a history of development and changes through time. A landmass passes through stages of development somewhat comparable to the stages of life — youth, mature and old age.

Geomorphic Agents

Changes on the surface of the earth owe mostly to erosion by various geomorphic agents. Running water, ground-water, glaciers, wind and waves are powerful    erosional and depositional agents shaping and changing the surface of the earth aided by weathering and mass wasting processes. These geomorphic agents acting over long periods of time produce systematic changes leading to sequential development of landforms.

Fluvial landforms

The landforms created as a result of degradational action (erosion) or aggradation work (deposition) of running water is called fluvial landforms.

These landforms result from the action of surface flow/run-off or stream flow (water flowing through a channel under the influence of gravity). The creative work of fluvial processes may be divided into three physical phases—erosion, transportation and deposition.

The landforms created by a stream can be studied under erosional and depositional categories.

Erosional category

Valleys, gorge and Canyon

The extended depression on ground through which a stream flows throughout its course is called a river valley. gorge is a deep valley with very steep to straight sides. A canyon is characterized by steep step-like side slopes and may be as deep as a gorge.

At a young stage, The profile of valley  is typically ‘V’ shaped. As the cycle attains maturity, the lateral erosion becomes prominent and the valley floor flattens out. The valley profile now becomes typically ‘U’ shaped with a broad base and a concave slope.

Potholes, Plunge pools

Potholes are more or less circular depressions over the rocky beds of hills streams.Once a small and shallow depression forms, pebbles and boulders get collected in those depressions and get rotated by flowing water. Consequently, the depressions grow in dimensions to form potholes.Plunge pools are nothing but large, deep potholes commonly found at the foot of a waterfall. They are formed because of the sheer impact of water and rotation of boulders.

Incised or Entrenched Meanders

They are very deep wide meanders (loop-like channels) found cut in hard rocks.In the course of time, they deepen and widen to form gorges or canyons in hard rock.The difference between a normal meander and an incised/entrenched meander is that the latter found on hard rocks.

River Terraces

They are surfaces marking old valley floor or flood plains.They are basically the result of vertical erosion by the stream. When the terraces are of the same elevation on either side of the river, they are called as paired terraces.When the terraces are seen only on one side with none on the other or one at quite a different elevation on the other side, they are called as unpaired terraces.

Depositional Features

Alluvial Fans

They are found in the middle course of a river at the foot of slope/ mountains.When the stream moves from the higher level break into foot slope plain of low gradient, it loses its energy needed to transport much of its load.Thus, they get dumped and spread as a broad low to the high cone-shaped deposits called an alluvial fan.

Deltas

They are found in the mouth of the river, which is the final location of depositional activity of a river. The coarser material settle out first and the finer materials like silt and clay are carried out into the sea.

 

 Flood Plains, Natural Levees

Natural levees are found along the banks of large rivers. They are low, linear and parallel ridges of coarse deposits along the banks of a river.The levee deposits are coarser than the deposits spread by flood water away from the river.

 

 Meanders and oxbow lakes

  • They are formed basically because of three reasons: (i) propensity of water flowing over very gentle gradient to work laterally on the banks; (ii) unconsolidated nature of alluvial deposits making up the bank with many irregularities; (iii) Coriolis force acting on fluid water deflecting it like deflecting the wind.
  • The concave bank of a meander is known as cut-off bank and the convex bank is known as a slip-off
  • As meanders grow into deep loops, the same may get cut-off due to erosion at the inflection point and are left as oxbow lakes.

Braided Channels

When selective deposition of coarser materials causes the formation of a central bar, it diverts the flow of river towards the banks, which increases lateral erosion. Similarly, when more and more such central bars are formed, braided channels are formed. Riverine Islands are the result of braided channels.

 

Karst Topography

Any limestone, dolomite or gypsum region showing typical landforms produced by the action of groundwater through the process of solution and deposition is called as Karst Topography (Karst region in the Balkans).

Sinkholes

A sinkhole is an opening more or less circular at the top and funnel-shaped towards the bottom.When as sinkhole is formed solely through the process of solution, it is called as a solution sink.When several sink holes join together to form valley of sinks, they are called as blind valleys.

 

Caves

In the areas where there are alternative beds of rocks (non-soluble) with limestone or dolomite in between or in areas where limestone are dense, massive and occurring as thick beds, cave formation is prominent. Caves normally have an opening through which cave streams are discharged Caves having an opening at both the ends are called tunnels.

Stalactites and stalagmites

They are formed when the calcium carbonates dissolved in groundwater get deposited once the water evaporates.These structures are commonly found in limestone caves.Stalactites are calcium carbonate deposits hanging as icicles while Stalagmites are calcium carbonate deposits which rise up from the floor.When a stalactite and stalagmite happened to join together, it gives rise to pillars or columns of different diameters.

GLACIERS

Masses of ice moving as sheets over the land (continental glacier or piedmont glacier if a vast sheet of ice is spread over the plains at the foot of mountains) or as linear flows down the slopes of mountains in broad trough-like valleys (mountain and valley glaciers) are called glaciers.

EROSIONAL LANDFORMS

Cirque

Cirques are the most common of landforms in glaciated mountains. They are deep, long and wide troughs or basins with very steep concave to vertically dropping high walls at its head as well as sides. A lake of water can be seen quite often within the cirques after the glacier disappears. Such lakes are called cirque or tarn lakes.

Horns and Serrated Ridges

Horns form through head ward erosion of the cirque walls. If three or more radiating glaciers cut headward until their cirques meet, high, sharp pointed and steep sided peaks called horns form.

 

Glacial Valleys/Troughs

Glaciated valleys are trough-like and U-shaped with broad floors and relatively smooth, and steep sides. There may be lakes gouged out of rocky floor or formed by debris within the valleys. There can be hanging valleys at an elevation on one or both sides of the main glacial valley. Very deep glacial troughs filled with sea water and making up shorelines (in high latitudes) are called fjords/fiords.

 

Depositional landforms

 

Moraines

They are long ridges of deposits of glacial till. Terminal moraines are long ridges of debris deposited at the end (toe) of the glaciers. Lateral moraines form along the sides parallel to the glacial valleys. The lateral moraines may join a terminal moraine forming a horse-shoe shaped ridge. deposits varying greatly in thickness and in surface topography are called ground moraines.

 

Eskers

When glaciers melt in summer, the water flows on the surface of the ice or seeps down along the margins or even moves through holes in the ice. These waters accumulate beneath the glacier and flow like streams in a channel beneath the ice. Such streams flow over the ground (not in a valley cut in the ground) with ice forming its banks. Very coarse materials like boulders and blocks along with some minor fractions of rock debris carried into this stream settle in the valley of ice beneath the glacier and after the ice melts can be found as a sinuous ridge called esker.

Outwash Plains

The plains at the foot of the glacial mountains or beyond the limits of continental ice sheets are covered with glacio-fluvial deposits in the form of broad flat alluvial fans which may join to form outwash plains of gravel, silt, sand and clay.

Drumlins

Drumlins are smooth oval shaped ridge-like features composed mainly of glacial till with some masses of gravel and sand. The long axes of drumlins are parallel to the direction of ice movement. They may measure up to 1 km in length and 30 m or so in height.

 

Arid Landforms

Wind is one of the  dominant agents in hot deserts. The wind action creates a number of interesting erosional and depositional features in the deserts.

 

EROSIONAL LANDFORMS

Pediments and Pediplains

. Gently inclined rocky floors close to the mountains at their foot with or without a thin cover of debris, are called pediments. through parallel retreat of slopes, the pediments extend backwards at the expense of mountain front, and gradually, the mountain gets reduced leaving an inselberg which is a remnant of the mountain. That’s how the high relief in desert areas is reduced to low featureless plains called pediplains.

Playas

Plains are by far the most prominent landforms in the deserts. In times of sufficient water, this plain is covered up by a shallow water body. Such types of shallow lakes are called as playas where water is retained only for short duration due to evaporation and quite often the playas contain good deposition of salts.

. Deflation Hollows and Caves

Weathered mantle from over the rocks or bare soil, gets blown out by persistent movement of wind currents in one direction. This process may create shallow depressions called deflation hollows. Deflation also creates numerous small pits or cavities over rock surfaces. The rock faces suffer impact and abrasion of wind-borne sand and first shallow depressions called blow outs are created, and some of the blow outs become deeper and wider fit to be called caves.

Mushroom, Table and Pedestal Rocks

Many rock-outcrops in the deserts easily susceptible to wind deflation and abrasion are worn out quickly leaving some remnants of resistant rocks polished beautifully in the shape of mushroom with a slender stalk and a broad and rounded pear shaped cap above. Sometimes, the top surface is broad like a table top and quite often, the remnants stand out like pedestals.

Depositional Landforms

When the wind slows or begins to die down, depending upon sizes of grains and their critical velocities, the grains will begin to settle.

Sand Dunes

Dry hot deserts are good places for sand dune formation. Obstacles to initiate dune formation are equally important. There can be a great variety of dune forms Crescent shaped dunes called barchans with the points or wings directed away from wind .Parabolic dunes form when sandy surfaces are partially covered with vegetation. That means parabolic dunes are reversed barchans with wind direction being the same.

Seif is similar to barchan with a small difference. Seif has only one wing or point. Longitudinal dunes form when supply of sand is poor and wind direction is constant. They appear as long ridges of considerable length but low in height. Transverse dunes are aligned perpendicular to wind direction. These dunes form when the wind direction is constant and the source of sand is an elongated feature at right angles to the wind direction.

 

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