LANDFORMS AND THEIR EVOLUTION

Small to medium tracts or parcels of the earth’s surface are called landforms. Several related landforms together make up landscapes. Each landform has its physical shape, size, and materials and it is a result of the action of certain geomorphic processes and agents. A landmass passes through stages of development somewhat comparable to the stages of life- youth, maturity and old age. Geomorphology deals with the construction of the history of the surface of the earth. Earth's surface changed due to erosion and the process of deposition. Each geomorphic agent produces its assemblage of landforms. Most of the geomorphic processes are imperceptible. Erosional or destruction and Depositional or construction- of landforms are produced by geomorphic agents. It depends on type and structure i.e. folds, faults, joints, fractures, hardness and softness, permeability and impermeability. There are some independent controls like the stability of sea level and Tectonic stability of landmasses. Any disturbance in any of these three controlling factors can upset the systematic and sequential stages in the development and evolution of landforms.

• Youth: Streams are few with poor integration and flow over original slopes. Showing shallow v-shaped valleys with no floodplains or very narrow floodplains along trunk streams. Waterfalls and rapids may exist where local hard rock bodies are exposed.
• Mature: Streams are plenty with good integration with v-shaped valleys in deep depth. Trunk streams are broad enough to have wider floodplains within the valley. Youth streams, waterfalls and rapids have disappeared.
• Old: Smaller tributaries during old age are few with gentle gradients. Streams meander freely over vast floodplains showing natural levees, oxbow lakes, etc. Divides are broad and flat with lakes, swamps and marshes. Most of the landscape is slightly above sea level.

2. Erosional landforms

• Valleys: Valleys are depressed areas of land–scoured and washed out by the conspiring forces of gravity, water, and ice. Some hang; others are hollow. They all take the form of a "U" or "V.", gorge, canyon, etc. 
• Gorge: It is a deep valley with steep to straight sides and forms in hard rocks. A gorge is almost equal in width at its top as well as its bottom.
• Canyon: It is characterised by steep step-like side slopes and may be as deep as the gorge. The canyon is wider at its top than at its bottom. The canyon is a variant of the gorge. It is commonly formed in horizontal bedded sedimentary rocks.
• Potholes and plunge pools: Over the rocky beds of hill streams, more or less circular depressions called potholes. Such large and deep holes at the base of waterfalls are called plunge pools.
• Incised or Entrenched Meanders:  Because of active lateral erosion, streams flowing over gentle slopes develop sinuous or meandering courses. Very deep and wide meanders found in hard rocks are called incised or entrenched meanders.

3. Depositional landforms

• Alluvial fans: These are formed when streams flowing from higher levels break into foot slope plains of low gradient. The load becomes too heavy for the streams to carry over gentler gradients and gets dumped and spread as a broad low to high cone-shaped deposit called an alluvial fan.
• Deltas: Deltas are like alluvial fans but develop at a different location. The load carried by the rivers are dumped and spread into the sea or along the coast. As the delta grows, the river distributaries continue to increase in length.
• Floodplains, natural levees and point bars: Deposition develops a floodplain just as erosion makes valleys. Sand, silt and clay are carried by slow-moving waters. A river bed made of river deposits is an active flood plain. The floodplain above the bank is inactive, containing two types of deposits namely flood and channel deposits. The flood plains in a delta are called delta plains.
• Natural levees are found along the banks of large rivers with low, linear and parallel ridges of the courses.
• Point bars are known as meander bars. They are found on the convex side of meanders of large rivers and are sediments deposited. Rivers build the point bars on the convex side; the bank on the concave side will erode actively.
• Meanders: In delta plains and large floods, rivers rarely flow in straight courses. Burhi Gandak River near Muzaffarpur, Bihar, has several oxbow lakes and cut-offs. Meander is not a landform but is only a type of channel pattern because of the propensity of water flowing over very gentle gradients to work laterally on the banks. The unconsolidated nature of alluvial deposits makes up the banks with many irregularities which can be used by water exerting pressure laterally. Coriolis force acting on fluid water deflects it like it deflects the wind. There is active deposition along the convex bank and undercutting along the concave bank or cut-off bank. The convex bank is known as a slip-off bank. As meanders grow into deep loops, the way gets cut off due to erosion at the inflexion points and is left as oxbow lakes.
• Braided channels: The main water channel splitting into multiple, narrower channels are said to be braided rivers or braided channels. It consists of a network of river channels separated by small, and often temporary, islands called braid bars. 

Erosional landforms

Pools, sinkholes, lap and limestone pavements: Small to medium-sized round to sub-rounded shallow depressions are called swallow holes. It forms on the surface of limestone through the solution.

Sinkholes: It is an opening more or less circular at the top and funnels shaped. A great variation is seen in the sizes of sinkholes with areas from a few sq.m to a hectare and with depths from less than half a metre to thirty metres or more. The term doline is sometimes used to refer to the collapse sinks. The lapie field may eventually turn into somewhat smooth limestone pavements.

Caves: Cave formation is prominent in areas where there are alternating beds of rocks (shale, sandstones and quartzite) with limestones or dolomites in between or in areas where limestones are dense, and massive and occur as thick beds. Caves having openings at both ends are called tunnels.

Depositional landforms

Many depositional forms develop within the limestone caves. Carbon carbonate is deposited when the water carrying its solution evaporates or loses its carbon dioxide as it trickles over rough rock surfaces.

Stalactites, Stalagmites and Pillars: Stalactites rise from the floor of caves, due to dripping water from the surface or through the thin pipe, of the stalactite, immediately below it. Stalagmites may take the shape of a column, or a disc, with either a smooth, rounded, bulging end or a miniature crater-like depression. The stalactites and stalagmites eventually fuse to give rise to columns and pillars of different diameters.

5. 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 the valleys of the mountains are called glaciers. It moves because of the force of gravity. Uttaranchal, Himachal Pradesh and Jammu Kashmir are places to see glaciers in India. The erosion of glaciers is huge because of friction caused by ice.

Erosional landforms

Cirque; Cirques are the most common landforms in glaciated mountains found at the heads of glacial valleys. The accumulated ice cuts these cirques while moving down the mountain tops. 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 headward 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. The divides between cirque side walls or headwalls get narrow because of progressive erosion. The highest peak in the Alps is Matterhorn.

Glacial valleys / Troughs: These are trough-like and u- shaped with broad floors and relatively smooth and steep sides. Lakes gouged out of the rocky floor or formed by debris within the valleys. Elevation of hanging valleys is on one or both sides of the main glacial valley. The faces of divides or spurs of valleys are quite often truncated to give them an appearance like triangular facets. Very deep glacial troughs with seawater making up shorelines are called fjords or fiords.

Depositional landforms

The unassorted coarse and fine debris dropped by the melting glaciers is called glacial till. Glaciofluvial deposits are called outwash deposits.

Moraines: They are long ridges of deposits of glacial till. Terminal moraines are long ridges of debris deposited at the end of the glaciers. Moraines form along the sides parallel to the glacier's valleys. The lateral moraines form along the sides parallel to the glaciers. Moraine in the centre of the glacial valley is flanked by lateral moraines called medial moraine and may join with terminal moraine forming a horse-shoe-shaped ridge.

Eskers: Eskers are ridges made of sands and gravels, deposited by glacial meltwater. It is flowing through tunnels within and underneath glaciers, or meltwater channels on top of glaciers.

Out Wash Plains: When the glacier reaches its lowest point and melts. It leaves behind a stratified deposition material, consisting of rock debris, clay, sand, gravel etc. This layered surface is called a till plain or an outwash plain.

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 the drumlins are parallel to the direction of ice movement. They may measure up to 1 km in length and 30 m in height. One end of the drumlins facing the glacier called the stoss end is blunter and steeper than the other end called the tail.

6. Waves and currents

Some of the changes along the coasts take place very fast. In one place there can be erosion in one season and deposition in another. When the weaves break, the water is thrown with great force onto the shore. Strom waves and tsunami waves can cause far-reaching changes in a short period. The coastal landforms depend upon: the configuration of land and seafloor. Whether the coast is advancing seaward or retreating landward. Two types of coasts are considered to explain the concept of the evolution of coastal landforms.

1. High, rocky coasts
2. Low, smooth, and gently sloping sedimentary coasts.

High rocky coasts: The shores of these high rocky coasts do not show any depositional landforms. The erosional feature dominates here. In these types of coasts, the sea will be very close to the land without any coast or sometimes a narrow coast. Wave-cut platforms, cliffs, sea caves etc. are common here. The west coast is a high rocky retreating cost.

Low sedimentary coasts: The rivers appear to extend their length by building coastal plains and deltas. Coastal lines appear smooth with occasional incursions of water in the form of lagoons and tidal creeks. Large rivers carry lots of sediments and build deltas along low sedimentary coasts. The east coast is the sedimentary coast. Depositional forms dominate on the east coast.

Erosional landforms

Cliffs, Terraces, Caves and Stacks: Wave-cut cliffs and terraces are two forms usually found where erosion is the dominant shore process. Sea cliffs are steep and may range from a few meters to 30 or more meters. The foot of such cliffs may be flat or gently sloping platforms covered by rock debris derived from the sea cliff behind. Such platforms occurring at elevations above the average height of waves are called wave-cut terraces. The lashing of waves against the base of the cliff and the rock debris that gets smashed against the cliff along with the lashing waves create hallows. These hallows get widened and depend to form sea caves. Originally parts of a cliff or hill are called sea tracks. Sea tracks are temporary and eventually disappear because of wave erosion giving rise to narrow coastal plains.

Depositional landforms

Depositional landforms made by wind and good sorting of grains can be found.

Beaches and dunes: Most of the sediment making up the beaches comes from the land carried by streams and rivers or from wave erosion. Beaches are temporary features. The sandy beach which appears permanent may be reduced to a narrow strip of coarse pebbles in some other season. Most beaches are made up of sand-sized materials. Beaches are called shingle beaches to contain excessively small pebbles and even cobbles.

Sand dunes: Dry hot deserts are good places for sand dune formation. The sand lifted and winnowed from over the beach surfaces will be deposited as dunes. Sand dunes parallel to the coastline are very common along with low sedimentary costs.

Crescent-shaped dunes are called barchans with the points or wings directed away from wind direction i.e. downwind.

Parabolic dunes form when a sandy surface is partially covered with vegetation. These are reserved barchans with the wind direction being the same. Seif is similar to barchans with a small difference; it has one wing or point that happens when there is a shift in the wind conditions.

A longitudinal dune is from when the supply of sand is poor and wind direction is constant with long length and low height. Transverse dunes are aligned perpendicular to the wind direction.

Shaped dunes: when sand is plenty, quite often, the regularly shaped dunes coalesce and lose their characteristics. Most of the dunes in the desert shift and a few of them will get stabilised especially near human habitations.

Bars, barriers and spits: A ridge of sand and shingle formed in the sea in the offshore zone lying approximately parallel to the coast is called an offshore bar. The further exposed offshore bar is said to be a barrier bar.

Offshore and barrier bars are commonly formed on the mouths of the river or at the entrance of the bay. Sometimes such bars get keyed up to one end of the bay and are called spits. Splits may also develop attached to headlines or hills. The coastal offshore bars offer the first buffer or defence against storms or tsunamis by absorbing most of their destructive force. The barriers are beaches, beach dunes and mangroves to absorb the destructive force of storms and tsunamis. Mangroves and sediment budgets are distributed by human habitations which will lead to bear the first strike of a storm or tsunami.

7. Winds

The wind is one of the two dominant agents in hot deserts. Hot desert floors get heated up too much and too quickly because of dry and barren. The heated floor heated air directly above them. Winds move along the desert at high speed and obstructions in their path create turbulence. Strom winds are very destructive. Winds cause deflation, abrasion and impact. Deflation includes the lifting and removal of dust and similar particles from the surface of rocks. The weathered debris in deserts is moved by winds and rains or sheet wash.

Erosional landforms

Pediments and Pediplains: Landscape evolution in deserts is primarily concerned with the formation and extension of pediments. Gently inclined rocky floors close to the mountains at their foot with or without a thin cover of debris called pediments. The high relief in desert areas is reduced to low featureless plains called Pedi plains.

Playas: It is the basin from which water evaporates quickly. It has no vegetation. These types of lakes have the flattest form of geographical features in the whole world. It is also called a sink. The playa plain covered up by salts is called alkali flats

Deflation Hollows and Caves: The weathered mantle from over the rocks or bare soil gets blown out by the 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 the first shallow depressions are called blowouts. Some of the blowouts become deeper and wider fit to be called caves.

Mushroom, Table and Pedestal rocks: Some remnants of resistant rocks polished beautifully in the shape of a mushroom with a slender stalk and aboard and rounded pear-shaped cap above. Sometimes, the top surface is board-like a tabletop and quite often. The remnants stand out like pedestals.