LANDFORMS AND THEIR EVOLUTION  

Landforms  

i). After weathering, geomorphic agents operate the landforms to change. 

ii). Small to medium tracts or parcels of the earth's surface are called landforms. Several landforms together are called landscape. Each landform has its own shape, size and materials.  

iii). Geomorphological processes are slow but significant in the long run.  

iv). Every landform has a beginning , they change their shape and composition in course of time. Due to changes in climate, vertical and horizontal movements landforms change their shape. Each landform undergo three stages called youth , mature and old stages.  

v). Geomorphology is the science of landforms. Various geomorphic agents bring the changes to the landforms such as running water, moving ice, wind, glaciers, underground water, waves by erosion and deposition.  

vi). Most of the geomorphological processes are imperceptible. The study of the landforms reveals that the stage structure and process of landforms.  

vii). As the geomorphic agents are capable of erosion and deposition, two sets:   

a) Erosional or destructional. b) Depositional or constructional. 

viii). Many varieties of landforms develop by the action of each of the geomorphic agents depending upon especially the type and structure i.e. folds, faults, joints, fractures, hardness and soness, permeability and impermeability, etc. come under the structure of rocks. There are some other independent controls like : 

a). Stability of sea level, 

b). Tectonic stability of landmasses, 

c). Climate, which influence the evolution of landforms.   

ix). Any disturbance in any of these three controlling factors can upset the systematic and sequential stages in the development and evolution of landforms. 

Running Water  

i). In humid regions, which receive heavy rainfall, running water is considered the most important of the geomorphic agents in bringing about the degradation of the land surface.  

There are two components of running water:  

a). Overland flow on general land surface as a sheet. 

b). Linear flow as streams and rivers in valleys.  

ii). Most of the erosional features are formed in the upper course/youthful stage of the river. Depositional features are formed in the lower course /old stage of the river. Steep- the slope more the erosion/gentle -the slope more the deposition. Vertical Erosion is more in the upper course, lateral erosion is more in the lower course.  

iii). Overland Flow: It is also called as sheet erosion. Depending on the irregularities of the surface the sheet erosion concentrated into channels. Due to sheer friction of the water narrow channels are formed, they are called rills. Rills develop into gullies, gullies further deepen, widen, lengthen and unite to give rise to a network of valleys. In the early stages downward cutting makes waterfalls, cascades in the middle stages streams cut their beds slower and lateral erosion of valley sides becomes severe. Later stages, the flat valley is formed with monadnocks. This type of plain is called peneplain.   

Stages of the river  

Youth  

i). Streams are few during this stage with poor integration and flow over original slopes showing shallow V-shaped valleys with no floodplains or with very narrow floodplains along trunk streams.  

ii). Streams divides are broad and flat with marshes, swamp and lakes.  

iii). Meanders if present develop over these broad upland surfaces. These meanders may eventually entrench themselves into the uplands.  

iv). Waterfalls and rapids may exist where local hard rock bodies are exposed.  

Mature  

i). During this stage streams are plenty with good integration.   

ii). The valleys are still V-shaped but deep; trunk streams are broad enough to have wider floodplains within which streams may flow in meanders confined within the valley.  

iii). The flat and broad inter stream areas and swamps and marshes of youth disappear and the stream divides turn sharp.   

iv). Waterfalls and rapids disappear. 

Old  

i). Smaller tributaries during old age are few with gentle gradients. 

ii). Streams meander freely over vast floodplains showing natural levees, oxbow lakes, etc.  

iii). Divides are broad and flat with lakes, swamps and marshes. 

iv). Most of the landscape is at or slightly above sea level.  

Erosional Landforms    

Valleys  

i) Valleys start as small and narrow rills. These rills wil progressively develop into long and wide gullies.

ii) The gullies will again deepen, widen and lengthen to give rise to valleys.  

iii) The valley types depend upon the type and structure of rocks in which they form. Depending upon sizes and shapes, several types of valleys like V-shaped valley, gorge, canyon, etc. can be recognized. 

iv) A gorge is a deep valley with very steep to straight sides.  

v) It is almost equal in width at its top as well as its bottom.  

vi) A canyon is characterized by steep step-like side slopes and might be as deep as a gorge.  

vii) It is a variant of the gorge.   

viii) A canyon is wider at its top than at its bottom.  

ix) It is commonly formed in horizontal bedded sedimentary rocks and gorges form in hard rocks.  

Potholes   

i) Potholes are cylindrical holes drilled into the bed of a river that varies in depth and diameter from a few centimeters to several meters.  

ii) They are found in the upper course of a river where it has enough potential energy to erode vertically and its flow is turbulent.  

  

Plunge Pools  

i) A sequence of such depressions ultimately joins and the stream valley gets deepened.    

ii) At the foot of waterfalls also, large potholes, quite deep and wide, form because of the absolute influence of water and rotation of boulders. 

iii) These large and deep holes at the base of waterfalls are called plunge pools. 

iv) These pools also help in the deepening of valleys.   

  

Incised or Entrenched Meanders  

i) Entrenched meanders are symmetrical and form when the river down cuts quickly.  

ii) The speed of the river downcutting gives less opportunity for lateral Thus giving them symmetrical slopes.   

iii) These are very deep and wide meanders can also be found cut in hard rocks. 

iv) It is common to find meandering courses over floodplains and delta plains where stream gradients are very gentle.   

River Terraces  

i) River terraces are surfaces marking old valley floor or floodplain levels.    

ii) They may be bedrock surfaces without any alluvial cover or alluvial terraces consisting of stream deposits. 

a). Paired terraces: The river terraces may occur at the similar elevation on either side of the rivers. 

b). Unpaired terraces: When a terrace is present only on one side of the stream and with none on the other side or one at quite a different elevation on the other side.   

iii) The terraces may result due to:  

a). Change in hydrological regime due to climatic changes.    

b). Sea level changes in case of rivers closer to the sea.    

c). Receding water after a peak flow.    

d). Tectonic uplift of land.   

Depositional Landforms  

  Alluvial Fans  

i) They are found in the middle course of a river at the foot of slope/ mountains.  

ii) 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.  

iii) Thus, they get dumped and spread as a broad low to the high cone-shaped deposits called an alluvial fan .

iv) Usually, the streams which flow over fans are not confined to their original channels for long and shift their position across the fan forming many channels called distributaries.  

v) Alluvial fans in humid areas show normally low cones with gentle slope from head to toe and they appear as high cones with steep slope in arid and semi-arid climates.  

  

Deltas  

i) Deltas are like an alluvial fan but develop at a different location.  

ii) They are found in the mouth of the river, which is the final location of depositional activity of a river.  

iii) Unlike alluvial fans, the deposits making up deltas are very well sorted with clear stratification.  

iv) The coarser material settle out first and the finer materials like silt and clay are carried out into the sea.  

  

Flood Plains, Natural Levees and Point Bars  

i) Deposition develops a flood plain just as erosion makes valleys.  

ii) A riverbed made of river deposits is the active flood plain and the floodplain above the bank of the river is the inactive floodplain.  

iii) 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.  

iv) The levee deposits are coarser than the deposits spread by flood water away from the river.  

v) For large rivers, the sediments deposited in a linear fashion at the depositional side of a meander are called as Point Bars or Meander Bars.  

Meanders and oxbow lakes:  

i) Meanders are loop-like channel patterns develop over the flood and delta plains.   

ii) They are actually not a landform but only a type of channel pattern formed as a result of deposition.  

iii) They are formed basically because of three reasons:   

a) Propensity of water flowing over very gentle gradient to work laterally on the banks;  

b) Unconsolidated nature of alluvial deposits making up the bank with many irregularities;   

c) Coriolis force acting on fluid water deflecting it like deflecting the wind.  

iv) The concave bank of a meander is known as cut-off bank and the convex bank is known as a slip-off.  

v) 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  

i) 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.  

ii) Similarly, when more and more such central bars are formed, braided channels are formed.  

iii) Riverine Islands are the result of braided channels. 

Groundwater  

i) The surface water percolates well when the rocks are permeable, thinly bedded and highly jointed and cracked.  

ii) After vertically going down to some depth, the water under the ground flows horizontally through the bedding planes, joints or through the materials themselves.  

iii) It is this downward and horizontal movement of water which causes the rocks to erode.  

iv) Physical or mechanical removal of materials by moving groundwater is insignificant in developing landforms. That is why, the results of the work of groundwater cannot be seen in all types of rocks. But in rocks like limestones or dolomites rich in calcium carbonate, the surface water as well as groundwater through the chemical process of solution and precipitation deposition develop varieties of landforms.  

v) Any limestone or dolomitic region showing typical landforms produced by the action of groundwater through the processes of solution and deposition is called Karst topography after the typical topography developed in limestone rocks of Karst region in the Balkans adjacent to Adriatic sea.  

vi) The karst topography is also characterized by erosional and depositional landforms.  

  

 Erosional Landforms  

Pools, Sinkholes, Lapies and Limestone Pavements  

i) Small to medium sized round to sub-rounded shallow depressions called swallow holes form on the surface of limestones through solution.  

ii) Sinkholes are very common in limestone/karst areas. A sinkhole is an opening more or less circular at the top and funnel-shaped towards the bottom with sizes varying in area from a few sq. m to a hectare and with depth from a less than half a meter to thirty meters or more.  

iii) If the bottom of a sinkhole forms the roof of a void or cave underground, it might collapse leaving a large hole opening into a cave or a void below (collapse sinks).  

iv) When sinkholes and dolines join together because of slumping of materials along their margins or due to roof collapse of caves, long, narrow to wide trenches called valley sinks or Uvalas form.  

  

Caves  

i) In areas where there are alternating beds of rocks (shales, sandstones, quartzites) with limestone or dolomites in between or in areas where limestones are dense, massive and occurring as thick beds, cave formation is prominent.   

ii) Water percolates down either through the materials or through cracks and joints and moves horizontally along bedding planes.  

iii) It is along these bedding planes that the limestone dissolves and long and narrow to wide gaps called caves result. 

iv) There can be a maze of caves at different elevations depending upon the limestone beds and intervening rocks.  

v) Caves normally have an opening through which cave streams are discharged. Caves having openings at both the ends are called tunnels.  

  

Depositional Landforms  

i) Many depositional forms develop within the limestone caves. The chief chemical in limestone is calcium carbonate which is easily soluble in carbonated water (carbon dioxide absorbed rainwater).  

ii) This calcium carbonate is deposited when the water carrying it in solution evaporates or loses its carbon dioxide as it trickles over rough rock surfaces.  

  

Stalactites and Stalagmites  

i) They are formed when the calcium carbonates dissolved in ground water get deposited once the water evaporates. These structures are commonly found in limestone caves.  

ii) Stalactites hang as icicles of different diameters. Normally they are broad at their bases and taper towards the free ends showing up in a variety of forms. While Stalagmites are calcium carbonate deposits which rise up from the floor.  

iii) When a stalactite and stalagmite happen to join together, it gives rise to pillars or columns of different diameters. 

Glaciers  

i) Masses of ice moving as sheets over the land or as linear flows down the slopes of mountains in broad trough-like valleys are called glaciers. 

ii) The movement of glaciers is slow, and they move basically because of the force of gravity.  

iii) Erosion by glaciers is tremendous because of friction caused by sheer weight of the ice.  

iv) The material plucked from the land by glaciers get dragged along the floors or sides of the valleys and cause great damage through abrasion and plucking.  

v) Glaciers can cause significant damage to even un-weathered rocks and can reduce high mountains into low hills and plains. 

Erosional Landforms  

Cirque  

i) Cirques are the most common of landforms in glaciated mountains. The cirques quite often are found at the heads of glacial valleys.  

ii) The accumulated ice cuts these cirques while moving down the mountain tops. 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.  

iii) 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  

i) Horns form through headward erosion of the cirque walls.   

ii) Horns form when three or more radiating glaciers cut the headward until their cirques meet high, sharp pointed and steep-sided peaks.   

iii) The splits between Cirque side walls or headwalls get narrow because of progressive erosion and turn into saw-toothed ridges occasionally mentioned to as aretes with very sharp crest and a zig-zag outline.    

iv) Horns formed through headward erosion of radiating cirques are:    

a) The highest peak in the Alps Matterhorn  

b) The highest peak in the Himalayas Everest   

  

Glacial Valleys/ Troughs  

i) They are U-shaped and trough-like with broad floors and comparatively smooth and steep edges.    

ii) The valleys may comprise scattered debris or debris moulded as moraines with swampy    iii) The very deep glacial troughs occupied with sea water and making up shorelines in high latitudes are known as fiords/ fjords 

Depositional Landforms  

i). The unassorted coarse and fine debris dropped by the melting glaciers is called glacial till.  

ii) Most of the rock fragments in till are angular to sub- angular in form. Streams form by melting ice at the bottom, sides or lower ends of glaciers.  

iii) Some amount of rock debris small enough to be carried by such melt-water streams is washed down and deposited. Such glaciofluvial deposits are called outwash deposits. Unlike till deposits, the outwash deposits are roughly stratified and assorted.   

iv) The rock fragments in outwash deposits are mostly rounded at their edges.  

  

Moraines  

i) These are long ridges of deposits of glacial till.  

ii) 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. There can be many lateral moraines on either side in a glacial valley.  

iii) Many valley glaciers retreating rapidly leave an irregular sheet of till over their valley floors. Such deposits varying greatly in thickness and in surface topography are called ground moraines. The moraine in the centre of the glacial valley flanked by lateral moraines is called medial moraine.  

  

Eskers  

i) The esker is one of the most striking landforms of fluvioglacial deposition.    

ii) They are usually formed of washed sand and gravel and they vary in shape and size.  

iii) When glaciers melt, the water flows on the surface of the ice or leaks down along the margins.    

iv) These waters amass underneath the glacier and flow like streams in a channel beneath the ice. Such streams flow over the ground with ice forming its banks.  

v) Very coarse materials like stones and blocks along with some minor segments of rock debris transported into this stream settle down in the valley of ice underneath the glacier and after the ice melts can be found as a winding ridge called Esker. 

Outwash Plains  

i) It is also known as a sandur.    

ii) It is a plain formed of glacial sediments deposited by meltwater outwash at the limit of a glacier.   

  

Drumlins  

i) These are smooth oval shaped ridge-like topographies composed primarily of glacial till with masses of gravel and sand.   

ii) It forms due to the dumping of rock debris underneath heavily loaded ice through fissures in the glacier.    

iii) The long axes of drumlins are parallel to the direction of ice movement.    

iv) Drumlins give an indication of the direction of glacier movement.  

v) The Stoss end is the steeper of the two ends and used to face into the ice flow.  

 

 

Coastal Landforms  

i) When waves break, the water is thrown with great force onto the shore, and simultaneously, there is a great churning of sediments on the sea bottom.  

ii) Constant impact of breaking waves drastically affects the coasts. Storm waves and tsunami waves can cause far-reaching changes in a short period of time than normal breaking waves. As wave environment changes, the intensity of the force of breaking waves changes.  

iii) Other than the action of waves, the coastal landforms depend upon:  

a). The configuration of land and sea floor;  

b). Whether the coast is advancing (emerging) seaward or retreating (submerging) landward.  

 iv) Assuming sea level to be constant, two types of coasts are considered to explain the concept of evolution of coastal landforms:  

a). High, rocky coasts (submerged coasts);  

b). Low, smooth and gently sloping sedimentary coasts (emerged coasts).  

High Rocky Coasts  

i) Along the high rocky coasts, the rivers appear to have been drowned with highly irregular coastline.  

ii) The coastline appears highly indented with extension of water into the land where glacial valleys (fjords) are present.  

iii) The hillsides drop off sharply into the water. Shores do not show any depositional landforms initially.   

iv) Erosion features dominate along high rocky coasts, waves break with great force against the land shaping the hill sides into cliffs. With constant pounding by waves, the cliffs recede leaving a wave-cut platform in front of the sea cliff. Waves gradually minimize the irregularities along the shore.  

v) Bars are submerged features and when bars show up above water, they are called barrier bars. Barrier bar which get keyed up to the headland of a bay is called a spit. When barrier bars and spits form at the mouth of a bay and block it, a lagoon forms. The lagoon would gradually get filled up by sediments from the land giving rise to a coastal plain. 

Low Sedimentary Coasts  

i) Along low sedimentary coasts the rivers appear to extend their length by building coastal plains and deltas. The coastline appears smooth with occasional incursions of water in the form of lagoons and tidal creeks.  

ii) The land slopes gently into the water. Marshes and swamps may abound along the coasts.  

iii) When waves break over a gently sloping sedimentary coast, the bottom sediments get churned and move readily building bars, barrier bars, spits and lagoons.  

iv) Lagoons would eventually turn into a swamp which would subsequently turn into a coastal plain.  

v) The west coast of our country is a high rocky retreating coast. Erosional forms dominate in the west coast.  

vi) The east coast of India is a low sedimentary coast. Depositional forms dominate in the east coast.  

  

Erosional Landforms  

Cliffs, Terraces, Caves and Stacks  

i) Wave-cut cliffs and terraces are two forms usually found where erosion is the dominant shore process.   

ii) Almost all sea cliffs are steep and may range from a few m to 30 m or even more.   

iii) At the foot of such cliffs there may be a flat or gently sloping platform covered by rock debris derived from the sea cliff behind. Such platforms occurring at elevations above the average height of waves is called a wave-cut terrace.  

Erosional Features Of Waves  

i). The lashing of waves against the base of the cliff and the rock debris that gets smashed against the cliff along with lashing waves create hollows and these hollows get widened and deepened to form sea caves.  

ii). The roofs of caves collapse and the sea cliffs recede further inland. Retreat of the cliff may leave some remnants of rock standing isolated as small islands just off the shore. Such resistant masses of rock, originally parts of a cliff or hill are called sea stacks.  

iii). Like all other features, sea stacks are also temporary and eventually coastal hills and cliffs will disappear because of wave erosion giving rise to narrow coastal plains, and with onrush of deposits from over the land behind may get covered up by alluvium or may get covered up by shingle or sand to form a wide beach.  

  

Depositional Landforms  

  Beaches and Dunes  

i) Beaches are characteristic of shorelines that are dominated by deposition, but may occur as patches along even the rugged shores.  

ii) Most of the sediment making up the beaches comes from land carried by the streams and river or from wave erosion. Beaches are temporary features.  

iii) Most of the beaches are made up of sand sized materials. Beaches called shingle beaches contain excessively small pebbles and even cobbles.  

iv) The sands lied and winnowed from over the beach surfaces will be deposited as sand dunes. Sand dunes forming long ridges parallel to the coastline are common along low sedimentary coasts.  

  

Bars, Barriers and Spits  

i). A ridge of sand and shingle formed in the sea in the off-shore zone lying approximately parallel to the coast is called an off-shore bar. An off-shore bar which is exposed due to further addition of sand is termed a barrier bar.  

ii). The off-shore bars and barriers commonly form across the mouth of a river or at the entrance of a bay. 

iii) Sometimes such barrier bars get keyed up to one end of the bay when they are called spits. Spits may also develop attached to headlands/hills.  

iv) The barriers, bars and spits at the mouth of the bay gradually extend leaving only a small opening of the bay into the sea and the bay will eventually develop into a lagoon.  

v) The lagoons get filled up gradually by sediment coming from the land or from the beach itself (aided by wind) and a broad and wide coastal plain may develop replacing a lagoon. 

Winds  

i) Wind is one of the two dominant agents in hot deserts.   

ii) The desert floors get heated up too much and too quickly because of being dry and barren. The heated floors heat up the air directly above them and result in upward movements in the hot lighter air with turbulence, and any obstructions in its path sets up eddies, whirlwinds, updrafts and downdrafts.  

iii) Winds cause deflation, abrasion and impact.  

a). Deflation includes lifting and removal of dust and smaller particles from the surface of rocks.   

b). In the transportation process sand and silt act as effective tools to abrade the land surface.  

c). The impact is simply sheer force of momentum which occurs when sand is blown into or against a rock surface.  

iv) The desert rocks devoid of vegetation, exposed to mechanical and chemical weathering processes due to drastic diurnal temperature changes, decay faster and the torrential rains help in removing the weathered materials easily.  

v) Stream channels in desert areas are broad, smooth and indefinite and flow for a brief time after rains.  

  

Erosional Landforms 

Pediments and Pedi plains  

i) Gently inclined rocky floors close to the mountains at their foot with or without a thin cover of debris, are called pediments. Such rocky floors form through the erosion of mountain front through a combination of lateral erosion by streams and sheet flooding.  

ii) Once, pediments are formed with a steep wash slope followed by a cliff or free face above it, the steep wash slope and free face retreat backwards. This method of erosion is termed as parallel retreat of slopes through back wasting.   

iii) So, through parallel retreat of slopes, the pediments extend backwards at the expense of the mountain front, and gradually, the mountain gets reduced leaving an inselberg which is a remnant of the mountain.  

iv) The high relief in desert areas is reduced to low featureless plains called Pedi plains.  

  

Playas  

i) Plains are by far the most prominent landforms in the deserts.   

ii) In basins with mountains and hills around and along, the drainage is towards the centre of the basin and due to gradual deposition sediment from basin margins, a nearly level plain forms at the centre of the basin.  

iii) In times of sufficient water, this plain is covered up by a shallow water body. Such types of shallow lakes are called playas, where water is retained only for short duration due to evaporation and quite often the playas contain good deposition of salts. The play a plain covered up by salts is called alkali flats.  

  

Deflation Hollows And Caves  

i) 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.  

ii) 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 blowouts become deeper and wider fit to be called caves.  

  

Mushroom, Table, and Pedestal Rocks  

i). 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.  

ii). Sometimes, the top surface is broad like a table top and quite oen, the remnants stand out like pedestals.  

Depositional Landforms  

i) Depending upon the velocity of wind, different sizes of grains are moved along the floors by rolling or saltation and carried in suspension and in this process of transportation itself, the materials get sorted.  

ii) When the wind slows or begins to die down, depending upon sizes of grains and their critical velocities, the grains will begin to settle. So, in depositional landforms made by wind, good sorting grains can be found.  

iii) Wind is there everywhere and wherever there is a good source of sand and with constant wind directions, depositional features in arid regions can develop anywhere.   

Sand Dunes  

i) Dry hot deserts are good places for sand dune formation.  

ii) According to the shape of a sand dune, there are varieties of sand dune forms like Barchans, Seifs etc.

iii) Thecrescent-shaped dunes are called Barchans and they are the most common one.  

iv) Seif is similar to Barchans but has only one wing or point.  

v) 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. 

vi) 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. They may be very long and low in height. 

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