Chapter 7. Landforms and Their Evolution

Overview
• After weathering processes have had their actions on earth materials making up surface of earth, geomorphic agents like running water, groundwater, wind, glaciers, & waves perform erosion.
• Several related landforms together make up landscapes, (large tracts of earth’s surface).
• Due to changes in climatic conditions and vertical or horizontal movements of landmasses, either intensity of processes or processes themselves might change leading to new modifications in landforms.

Running Water
• In humid regions, which receive heavy rainfall running water is considered most important of geomorphic agents in bringing about degradation of land surface.
• The gentler river channels in gradient or slope, greater deposition.
• Overland flow causes sheet erosion. Depending upon irregularities of land surface, overland flow may concentrate into narrow to wide paths.

Youth
• 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.

Mature
• During this stage, streams are plenty with good integration.
• 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 valley.

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 & marshes. Most of landscape is at or slightly above sea level.

Erosional Landforms Valleys
• Valleys starts as small and narrow rills; rills will gradually develop into long and wide gullies; gullies will further deepen, widen & lengthen to give rise to valleys.
• A gorge is almost equal in width at its top as well as its bottom. In contrast, a canyon is wider at its top than at its bottom.

Potholes and Plunge Pools
• Rocky beds of hill-streams have more or less circular depressions known as POTHOLES.
• Large and deep holes formed at base of waterfalls, because of sheer impact of water and rotation of boulders, referred as Plunge Pools.

Incised or Entrenched Meanders
• In streams that flow rapidly over steep gradients, normally erosion is concentrated on bottom of stream channel.
• It is common to find meandering courses over floodplains and delta plains where stream gradients are very gentle.

River Terraces
• River terraces are surfaces marking old valley floor or floodplain levels.
• There can be several such terraces at different heights indicating former river bed levels.

Depositional Landforms Alluvial Fans
• Alluvial fans are formed when streams flowing from higher levels break into foot slope plains of low gradient.
• Usually, streams which flow over fans are not confined to their original channels for long and shift their position across fan forming many channels known as distributaries.

Deltas
• Deltas are like alluvial fans but develop at a different location.
• The load carried by rivers is dumped and spread into sea.

Floodplains, Natural Levees and Point Bars
• Deposition develops a floodplain just as erosion makes valleys.
• Floodplain is a major landform of river deposition. The flood deposits of spilt waters carry relatively finer materials like silt and clay. The flood plains in a delta are known as delta plains.
• Natural levees and point bars are some of important landforms found associated with floodplains.

Meanders
• In large flood and delta plains, rivers rarely flow in straight courses.
• Meander is not a landform but is only a type of channel pattern. This is formed because of (i) propensity of water flowing over very gentle gradients to work laterally on banks; (ii) unconsolidated nature of alluvial deposits making up banks with many irregularities which can be used by water exerting pressure laterally; (iii) Coriolis force acting on fluid water deflecting it like it deflects wind.

Groundwater
• The surface water percolates well when rocks are permeable, thinly bedded and highly jointed and cracked.
• Any limestone or dolomitic region showing typical landforms produced by action of groundwater through processes of solution and deposition is known as Karst topography, after typical topography developed in limestone rocks of Karst region in Balkans adjacent to Adriatic Sea.
• The karst topography is characterised by erosional and depositional landforms.

Erosional Landforms Pools, Sinkholes, Lapies & Limestone Pavements
• Small to medium-sized round to sub-rounded shallow depressions known as swallow holes forms on surface of limestones through solution.
• A sinkhole is an opening more or less circular at top and funnel-shaped towards bottom with sizes varying in area from a few sq. m to a hectare and with depth from less than half a metre to thirty metres or more.
• Gradually, most of surface of limestone is eaten away by these pits and trenches, leaving it extremely irregular with a maze of points, grooves & ridges or lapies.

Caves
• In areas where there are alternating beds of rocks (shales, sandstones, quartzites) with limestones or dolomites in between or in areas where limestones are dense, massive & occur as thick beds, cave formation is prominent.
• Water percolates down either through materials or through cracks and joints and moves horizontally along bedding planes. It is along these bedding planes that limestone dissolves and long and narrow to wide gaps known as caves are formed as result.

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

Stalactites, Stalagmites and Pillars
• Stalactites hang as icicles of different diameters. Normally they are broad at their bases and taper towards free ends showing up in a variety of forms.
• The stalagmite and stalactites eventually fuse to give rise to columns and pillars of different diameters.

Glaciers
• Masses of ice move as sheets over land (continental glacier or piedmont glacier if a vast sheet of ice is spread over plains at foot of mountains) or as linear flows down slopes of mountains in broad trough-like valleys (mountain and valley glaciers) are known as glaciers.
• The movement of glaciers is slow, unlike water flow.
• Glaciers move basically because of force of gravity.

Erosional Landforms Cirque
• Cirques are most common landforms in glaciated mountains. The cirques quite often are found at heads of glacial valleys.
• The accumulated ice cuts these cirques while moving down mountain tops.

Horns and Serrated Ridges
• Horns form through headward erosion of cirque walls. If three or more radiating glaciers cut headward until their cirques meet, high, sharp-pointed and steep-sided peaks known as horns form.

Glacial Valleys/Troughs
• Glaciated valleys are trough-like and U-shaped with broad floors and relatively smooth, and steep sides.
• The valleys may contain littered debris or debris shaped as moraines with a swampy appearance.

Depositional Landforms
• The unassorted coarse and fine debris dropped by melting glaciers is known as glacial till.
• Most of rock fragments in till are angular to subangular in form. Streams form by melting ice at bottom, sides or lower ends of glaciers.

Moraines
• They are long ridges of deposits of glacial till. Terminal moraines are long ridges of debris deposited at end (toe) of glaciers.
• Lateral moraines form along sides parallel to glacial valleys.

Eskers
• When glaciers melt in summer, water flows on surface of ice or seeps down along margins or even moves through holes in ice.
• These waters accumulate beneath glacier and flow like streams in a channel beneath ice.

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

Drumlins
• Drumlins are smooth oval-shaped ridge-like features composed mainly of glacial till with some masses of gravel and sand.
• One end of drumlins facing glacier known as stoss end is blunter and steeper than other end known as tail.

Waves and Currents
• Most of changes along coasts are accomplished by waves.
• When waves break, water is thrown with great force onto shore, and simultaneously, there is a great churning of sediments on sea bottom.
• The constant impact of breaking waves drastically affects coasts.

High Rocky Coasts
• Along high rocky coasts, rivers appear to have been drowned with highly irregular coastline.
• The coastline appears highly indented with extension of water into land where glacial valleys (fjords) are present.
• The hillsides drop off sharply into water.

Low Sedimentary Coast
• Along low sedimentary coasts, rivers appear to extend their length by building coastal plains and deltas.
• When waves break over a gently sloping sedimentary coast, bottom sediments get churned and move readily building bars, barrier bars, spits & lagoons.

Erosional Landforms Cliffs, Terraces, Caves & Stacks
• Wave-cut cliffs and terraces are two forms generally found where erosion is dominant shore process.
• The lashing of waves against base of cliff and rock debris that gets smashed against cliff along with lashing waves create hollows and these hollows get widened and deepened to form sea caves.

Depositional Landforms Beaches and Dunes
• Beaches are characteristic of shorelines that are dominated by deposition but may occur as patches along even rugged shores.
• Most of sediment making up beaches comes from land carried by streams and rivers or from wave erosion.
• Beaches are temporary features.

Bars, Barriers & Spits
• A ridge of sand and shingle formed in sea in off-shore zone lying approximately parallel to coast is known as an off-shore bar.
• Spits may develop attached to headlands/hills.

Winds
• The wind is one of two dominant agents in hot deserts.
• The desert floors get heated up too much and too quickly because of being dry and barren.
• Stream channels in desert areas are broad, smooth & indefinite and flow for a brief time after rains.

Erosional Landforms Pediments and Pediplains
• Landscape evolution in deserts is primarily concerned with formation and extension of pediments.
• Erosion starts along steep margins of landmass or steep sides of tectonically controlled steep incision feature over landmass.

Playas
• Plains are by far most prominent landforms in deserts.
• In basins with mountains and hills around and along, drainage is towards centre of basin and due to gradual deposition of sediment from basin margins, a nearly level plain form at centre of basin.

Deflation Hollows and Caves
• The weathered mantle from over rocks or bare soil gets blown out by persistent movement of wind currents in one direction. This process may create shallow depressions known as deflation hollows.
• Deflation creates numerous small pits or cavities over rock surfaces.

Depositional Landforms
• The wind is a good sorting agent.
• Depending upon velocity of wind, different sizes of grains are moved along floors by rolling or saltation and carried in suspension and in this process of transportation itself, materials get sorted.

Sand Dunes
• Dry hot deserts are good places for dune formation.
• Parabolic dunes form when sandy surfaces are partially covered with vegetation.

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