Chapter 7 Landforms And Their Evolution

After rocks and minerals on the Earth's surface are broken down by weathering processes, dynamic forces driven by various geomorphic agents begin to act. These agents, such as running water, groundwater, wind, glaciers, and waves, perform erosion. Erosion is the process of picking up and transporting weathered material, which causes changes to the Earth's surface by wearing down landforms.

Following erosion, the process of deposition occurs, where the transported material is laid down in new locations. Deposition also contributes to shaping the Earth's surface, often building up new landforms or filling in depressions.

A landform can be described as a distinct, individual feature of the Earth's surface, ranging in size from small to medium. Examples include valleys, dunes, or cliffs.

When several related landforms occur together to create a larger area with characteristic physical attributes, they form a landscape. A landscape represents a larger tract of the Earth's surface composed of a collection of interconnected landforms.

Each specific landform has its unique shape, size, and composition, resulting from the action of particular geomorphic processes driven by specific agents. Most geomorphic processes and agents operate slowly over long periods, so the development of landforms is a gradual process.

Every landform originates at some point in time and then undergoes transformations in its shape, size, and characteristics due to the continued influence of geomorphic processes and agents. Changes in climate or movements of the Earth's crust can alter the intensity or type of processes at work, leading to further modifications of landforms.

Evolution in this context refers to the stages of transformation that a landform or a region of the Earth's surface passes through over time, from its initial formation through subsequent changes. Just like living organisms, landscapes and individual landforms are sometimes described as passing through developmental stages often characterized as youth, maturity, and old age.

Therefore, two important aspects of the evolution of landforms are:

1. The history of development and change that each landform undergoes over time.
2. The progression of a landmass through identifiable stages resembling youth, mature, and old age, characterized by specific assemblages of landforms.

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Running Water

In areas receiving abundant rainfall (humid regions), running water is considered the most significant geomorphic agent for degrading the land surface. Running water operates in two main ways:

• Overland Flow: Water flowing as a sheet or thin film over the general land surface, particularly during rainfall events.
• Linear Flow: Water concentrated into defined channels, forming streams and rivers that flow within valleys.

Most erosional landforms created by running water are associated with fast-flowing rivers on steep slopes, typical of younger stages. As rivers flow over steep gradients, they actively erode downwards (downcutting). Over time, this downcutting reduces the gradient, causing the flow velocity to decrease. Lower velocities reduce erosive power and increase the potential for deposition. While some localized deposition might occur on steep slopes, it is far more significant on gentler slopes.

The flatter the gradient of a river channel, the greater the tendency for deposition. As erosion continues and slopes become gentler, downcutting becomes less dominant, and rivers begin to erode their banks laterally (lateral erosion). This lateral erosion widens valleys and contributes to the gradual reduction of hills and valleys towards forming plains.

Complete reduction of relief of a high landmass to a perfectly flat plain is theoretically possible over immense geological time under stable conditions, but usually, residual hills of resistant rock (monadnocks) remain. Such a near-flat plain formed by extensive stream erosion is termed a peneplain ("almost a plain").

Overland flow causes sheet erosion, removing a thin layer of surface material. Irregularities on the surface can cause overland flow to concentrate, forming small channels called rills. Rills deepen and widen over time to become gullies. Gullies continue to grow, connecting and expanding to form a network of valleys. This development of valleys involves downcutting in early stages, removing obstacles like waterfalls and rapids, followed by increasing lateral erosion in later stages, widening the valleys and reducing valley side slopes. Eventually, the divides between drainage basins are lowered, forming extensive lowlands.

The evolution of landscapes shaped by running water can be broadly described in stages:

• Youth: Characterized by few streams with poor connections (integration). Streams flow rapidly over original steep slopes, creating shallow, V-shaped valleys with little to no floodplains. Stream divides are broad and flat, often containing marshes, swamps, and lakes. Meanders might form on these flat uplands and can become deeply cut into the landscape (incised meanders). Waterfalls and rapids are common where resistant rocks are encountered.
• Mature: Streams are more numerous and well-integrated, forming an efficient drainage network. Valleys are still V-shaped but are deeper. Trunk streams develop wider floodplains within the valley, where they may begin to meander. The flat inter-stream areas of youth become dissected, and divides become sharper. Waterfalls and rapids are largely absent.
• Old: Smaller tributary streams are few, and gradients are very gentle. Streams meander widely across vast floodplains, developing features like natural levees and oxbow lakes. Divides between drainage basins are broad and flat, potentially with remnants of lakes or swamps. Most of the landscape is at or slightly above sea level, approaching a peneplain.

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Erosional Landforms

Running water creates various landforms through the process of erosion.

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Valleys

Valleys are elongated depressions formed by the erosional action of streams and rivers. They evolve from small rills and gullies into larger features. Different types of valleys are recognized based on their shape and dimensions:

• V-shaped Valley: Formed by dominant downcutting, resulting in steep, straight valley sides that meet at a narrow bottom, resembling the letter 'V'.
• Gorge: A deep, narrow valley with very steep, often near-vertical sides (Figure 7.1). Gorges are typically cut into hard, resistant rocks. They maintain a relatively consistent width from top to bottom.

Image showing a deep gorge formed by the Kaveri River, characterized by steep, straight rock walls.

• Canyon: Similar to a gorge but characterized by steep, step-like side slopes (Figure 7.2). Canyons are typically wider at the top than at the bottom and often form in horizontally layered sedimentary rocks, where differential erosion of harder and softer layers creates the stepped profile.

Image showing a deep canyon with prominent step-like slopes and an entrenched meander of the Colorado River in the USA.

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Potholes And Plunge Pools

These erosional features form on the rocky beds of streams.

• Potholes: Roughly circular depressions carved into the bedrock stream bed. They are formed by the erosive action of turbulent water combined with abrasion from pebbles and boulders that get trapped in slight depressions and are spun around by the current. As pebbles rotate, they grind into the bedrock, deepening and widening the depression. A series of connected potholes can contribute to deepening the stream channel.
• Plunge Pools: Large, deep potholes that form at the base of waterfalls. They are created by the sheer force of the falling water impacting the bedrock and the abrasive action of boulders churned within the pool.

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Incised Or Entrenched Meanders

Meanders are typically characteristic of rivers flowing over flat areas like floodplains with gentle gradients. However, sometimes meander-like bends are found deeply cut into hard bedrock in areas of high relief (Figure 7.2). These are called incised or entrenched meanders. They form when a river that previously developed a meandering course on a relatively flat surface experiences uplift or a drop in base level, causing it to resume downcutting into the underlying rock while preserving its sinuous plan form.

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River Terraces

River terraces are flat or gently sloping surfaces found along the sides of river valleys, representing former levels of the valley floor or floodplain. They are essentially remnants of older, higher floodplains that have been dissected by the river as it cut downwards into its own deposits or bedrock.

• Terraces can consist of bedrock with a thin alluvial cover or be composed entirely of alluvial deposits (alluvial terraces).
• Multiple terraces at different elevations indicate successive stages of downcutting.
• If terraces occur at the same elevation on both sides of the river, they are called paired terraces. If they are at different elevations, they are unpaired, suggesting complex erosional histories or lateral channel migration combined with downcutting.

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Depositional Landforms

As running water loses velocity and energy, it deposits the sediment load it was carrying, leading to the formation of various landforms.

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Alluvial Fans

Alluvial fans (Figure 7.4) are cone-shaped or fan-shaped deposits of sediment formed where a stream or river exits a narrow mountain valley and flows onto a flatter plain or broader valley. As the stream leaves the steep gradient of the mountain channel and spreads out on the lower slope, its velocity decreases abruptly, causing it to deposit its coarse sediment load.

Image showing a fan-shaped deposit of sediment, an alluvial fan, formed where a mountain stream meets a flatter area.

• Typically, streams on alluvial fans are not confined to a single channel but spread out and divide into multiple smaller channels called distributaries, which shift across the fan surface.
• Alluvial fans in humid climates tend to be lower cones with gentler slopes, while those in arid/semi-arid climates often form higher cones with steeper slopes due to the nature of sediment load and flash floods.

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Deltas

Deltas are depositional landforms similar in shape to alluvial fans but formed where rivers enter a standing body of water, such as a lake, sea, or ocean (Figure 7.5). As the river reaches the standing water body, its velocity drops dramatically, causing it to deposit its sediment load.

Satellite view showing the branching distributaries and accumulation of sediment forming part of the Krishna River delta in Andhra Pradesh.

• Unlike alluvial fans, delta deposits are often well-sorted, with coarser sediments settling near the river mouth and finer silts and clays carried further offshore.
• Rivers typically divide into multiple distributaries within a delta as sediment deposition blocks the main channel.
• Deltas build outwards into the water body as sedimentation continues.

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Floodplains, Natural Levees And Point Bars

Floodplains are flat areas along the sides of river channels that are periodically inundated by floodwaters. They are major landforms created by the deposition of sediment during floods. When a river overflows its banks, the water spreads out over the adjacent land, its velocity decreases, and it deposits fine-grained sediments (silts and clays) across the floodplain surface.

• The area immediately adjacent to the river channel that is frequently flooded and where active deposition occurs is the active floodplain (often including the river bed itself).
• Areas further from the channel, flooded less frequently, are the inactive floodplain, composed of older flood deposits and abandoned channel deposits.
• Over time, as channels shift laterally or are cut off, old channel deposits (often coarser material) can also be found on the floodplain. Floodplains associated with deltas are called delta plains.

Specific depositional features found on floodplains include (Figure 7.6):

• Natural Levees: Low, elongated ridges that form along the banks of large rivers. They are built up over many floods as the coarsest sediment is deposited immediately as floodwaters spill out of the channel and lose energy. They are essentially raised banks.
• Point Bars (Meander Bars): Deposits of sediment that accumulate on the inside bend (convex side) of a meander. As water flows around a meander, the current is slower on the inside bend, causing sediment to be deposited, while erosion occurs on the faster-flowing outside bend.

Diagram illustrating the formation of natural levees along river banks during floods and point bars on the inside bend of a meander.

Natural levees and point bars differ in location and formation: Natural levees are parallel ridges along the river banks formed by flood deposits during overflow. Point bars are crescent-shaped deposits on the inner bend of meanders formed by channel deposition during normal flow as the river migrates laterally.

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Meanders

Meanders refer to the loop-like bends or sinuous curves in a river channel (Figure 7.7). While often discussed alongside depositional features, a meander is fundamentally a channel pattern rather than a landform built purely by deposition, although deposition is integral to its formation and migration.

Satellite image showing a river with prominent meanders, along with several cut-off meanders that have become oxbow lakes.

Meanders develop for several reasons, particularly on flat plains with very gentle gradients:

• Water flowing over gentle slopes has energy to erode laterally into the banks.
• The banks are often composed of easily eroded, unconsolidated alluvial deposits with irregularities that initiate bending.
• The Coriolis force, though weak, slightly deflects flowing water, contributing to the development of bends over long distances.

Once a slight bend forms, the water velocity is higher on the outside curve, leading to erosion (undercutting the bank - called the cut-off bank, forming a steep scarp). Velocity is lower on the inside curve, leading to deposition (forming a point bar - the slip-off bank, with a gentle slope) (Figure 7.8). This differential erosion and deposition causes the meander bends to migrate laterally across the floodplain and enlarge over time. As meanders grow into deep loops, the narrow neck between adjacent bends can be eroded through, cutting off the meander loop from the main channel. The cut-off meander loop then forms a crescent-shaped lake called an oxbow lake.

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Groundwater

Our focus here is on the geological work performed by water found beneath the Earth's surface – groundwater – in shaping landforms, rather than on groundwater as a water resource. Groundwater is an effective geomorphic agent primarily through chemical processes.

Groundwater is most abundant and active where surface water can easily percolate downwards. This happens readily in rocks that are permeable (allow water to pass through), thinly bedded, or extensively fractured and jointed.

Once underground, water moves vertically downwards and also horizontally along bedding planes, joints, cracks, or through the pore spaces within the rock material. This movement allows the water to interact chemically with the rock.

Physical erosion (mechanical removal of material) by moving groundwater is generally insignificant. However, groundwater's chemical action, particularly solution (dissolving minerals) and subsequent precipitation/deposition, is very important in specific types of rocks, notably limestones and dolomites, which are rich in calcium carbonate ($CaCO_3$).

Regions where limestone or dolomite are present, either exclusively or interbedded with other rocks, and where groundwater activity creates characteristic landforms through solution and deposition, are called areas of Karst topography. This type of landscape is named after the Karst region in the Balkans near the Adriatic Sea, where it is prominently developed.

Karst topography is characterized by both erosional features (like sinkholes and caves) and depositional features (like stalactites and stalagmites).

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Erosional Landforms

Groundwater erosion, predominantly through chemical solution, creates distinctive landforms in karst regions.

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Pools, Sinkholes, Lapies And Limestone Pavements

These features are formed on the surface of limestone terrains:

• Swallow Holes: Small to medium-sized, shallow, rounded depressions formed on the surface through solution of the limestone. Surface runoff can disappear directly into these holes.
• Sinkholes: Very common features in karst areas. Sinkholes are funnel-shaped depressions that are wider at the top and narrow towards the bottom. They can vary significantly in size and depth.
  • Solution Sinks: Formed primarily by the chemical dissolution of limestone by acidic rainwater and groundwater.
  • Collapse Sinks (Dolines): Form when the roof of an underground cave or void collapses, creating a large surface depression. Sometimes, sinkholes covered by soil can form shallow water pools; stepping on them can be hazardous if the underlying cave roof is weak. The term Doline is sometimes used interchangeably with sinkhole, particularly collapse sinks.
• Valley Sinks (Uvalas): Form when multiple sinkholes or dolines merge together due to the collapse of the ground between them or slumping along their edges, creating elongated, larger depressions or trenches.
• Lapies: Highly irregular surfaces characterized by a maze of sharp pinnacles, grooves, ridges, and channels etched into the limestone bedrock (Figure 7.10). Lapies form due to differential solution along closely spaced, often parallel or sub-parallel, joints and cracks in the limestone.

Diagram illustrating a range of erosional and depositional landforms commonly found in karst topography, such as sinkholes, uvalas, lapies on the surface, and caves with stalactites and stalagmites underground.

• Limestone Pavements: Extensive, relatively flat areas of exposed limestone bedrock that have been heavily etched by solution along joints, forming a pattern of blocks separated by widened cracks or grikes. This is essentially a large-scale form of lapies development, sometimes resulting in a smoother overall appearance after significant erosion.

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Caves

Caves are underground openings or passages formed by the dissolution of soluble rock, primarily limestone, by groundwater (Figure 7.10). Cave formation is common in thick beds of massive or dense limestone, or where limestone layers are interbedded with less soluble rocks (like shale or sandstone).

• Water percolates downwards through cracks and joints or through the rock material itself and then flows horizontally along bedding planes. The dissolution of limestone along these pathways creates elongated voids and passages that grow into caves.
• Extensive cave systems, or mazes, can develop at different levels corresponding to different soluble rock layers.
• Caves often have openings to the surface, where underground streams might emerge. Caves with openings at both ends are sometimes called tunnels.

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Depositional Landforms

Within limestone caves, various depositional features are formed as dissolved calcium carbonate precipitates out of the water. Carbonated water (rainwater that has absorbed carbon dioxide) dissolves calcium carbonate from the limestone. As this water trickles within the cave, it may lose some carbon dioxide to the cave air or evaporate, causing the dissolved calcium carbonate to precipitate and solidify.

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Stalactites, Stalagmites And Pillars

These are the most characteristic depositional features in limestone caves (Figure 7.11).

• Stalactites: Icicle-shaped formations that hang downwards from the cave ceiling. They form as water drips from the roof, leaving behind tiny deposits of calcium carbonate with each drop. They are typically broader at the top where they attach to the ceiling and taper downwards towards the tip.
• Stalagmites: Upward-growing formations that rise from the cave floor. They form as water drips onto the floor from the ceiling or from the tip of an overhead stalactite. The calcium carbonate precipitates from the dripping water on the floor. Stalagmites can have various shapes, including columns or rounded mounds.
• Pillars (or Columns): Formed when a stalactite growing downwards and a stalagmite growing upwards meet and fuse together, creating a continuous column from the cave floor to the ceiling. These can vary greatly in diameter.

Image showing calcium carbonate formations (stalactites hanging from the ceiling and stalagmites rising from the floor) inside a limestone cave.

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Glaciers

Glaciers are large masses of ice that move slowly over land due to the force of gravity. They can exist as extensive ice sheets covering continents (continental glaciers), broad lobes of ice spread over plains at the foot of mountains (piedmont glaciers), or as linear flows of ice down mountain slopes within existing valleys (mountain or valley glaciers) (Figure 7.12).

Image showing a glacier filling and moving down a mountain valley.

Glacial movement is very slow compared to water flow, typically ranging from a few centimeters to several meters per day or even slower. The movement is driven by the weight of the ice itself and the pull of gravity.

Glaciers are powerful agents of erosion due to their immense weight and the incorporated rock debris. Erosion by glaciers involves two main processes:

• Plucking: The glacier freezes onto rock surfaces and lifts out loosened blocks and fragments as it moves.
• Abrasion: The rock fragments embedded in the base and sides of the glacier grind and scrape against the underlying and surrounding bedrock, wearing it down like sandpaper.

Glacial erosion is strong enough to significantly modify even hard, unweathered rocks. Over long periods, glaciers can erode high mountain peaks into lower features and carve deep valleys. As glaciers retreat, they leave behind eroded landscapes and deposit the material they carried. Continued glacial action eventually lowers divides and reduces slopes until glacial movement ceases, leaving behind a landscape of low hills, depositional features, and extensive outwash plains formed by meltwater streams.

Figures 7.13 and 7.14 conceptually show some of the erosional and depositional landforms created by glaciers.

Composite diagram illustrating various landforms created by glacial erosion (e.g., cirques, horns, U-shaped valleys) and deposition (e.g., moraines, eskers, drumlins).

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Erosional Landforms

Glaciers carve out distinctive features in mountainous and formerly glaciated terrains.

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Cirque

Cirques are amphitheater-shaped or bowl-shaped depressions carved into mountainsides at the heads of glacial valleys. They are typically found where snow accumulates and compacts into ice, forming incipient glaciers. The ice then erodes the mountainside through a combination of freezing onto rock and plucking, and abrasive grinding as the ice moves within the bowl. Cirques are characterized by steep, often vertical or concave headwalls and sidewalls. After the glacier melts, a lake often occupies the basin, known as a cirque lake or tarn. Multiple cirques can form in a stepped sequence down a mountainside.

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Horns And Serrated Ridges

These features form from the erosion of cirques:

• Horns: Sharp, pointed mountain peaks formed when three or more cirques erode headwards into a mountain from different sides, leaving a prominent, steep-sided peak as a remnant. The Matterhorn in the Alps and Mount Everest in the Himalayas are classic examples.
• Serrated Ridges (Arêtes): Narrow, knife-edge ridges formed between adjacent cirques or parallel glacial valleys. They develop as the steep sidewalls of the eroding cirques or valleys intersect, leaving a sharp, often jagged ridge crest.

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Glacial Valleys/Troughs

Valleys shaped by glaciers are distinctly different from river valleys. Glacial valleys, often called glacial troughs, are typically U-shaped in cross-section (Figure 7.13). They have broad, flat floors and steep, relatively smooth sidewalls, unlike the V-shaped valleys carved by rivers.

• Glacial valleys are formed as the glacier straightens and deepens existing river valleys through erosion, removing interlocking spurs (ridges on valley sides) and creating steep, truncated spurs.
• The floors of glacial valleys may be covered in till or moraines or contain lakes (paternoster lakes) gouged out of the bedrock.
• Hanging Valleys: Tributary glacial valleys that enter the main glacial valley at a higher elevation. After the ice retreats, streams flowing from hanging valleys often plunge into the main valley as waterfalls. The faces of the divides between the hanging valley and the main valley are often cut off by the main glacier, forming triangular facets.
• Fjords (Fiords): Deep, narrow inlets of the sea found along high-latitude coasts. They are glacial troughs that were eroded below sea level and subsequently flooded by the sea after the glacier retreated.

Basic differences between glacial valleys and river valleys:

Feature	Glacial Valleys (Troughs)	River Valleys
Shape	U-shaped (broad floor, steep sides)	V-shaped (narrow floor, sloping sides)
Plan Form	Straightened, interlocking spurs removed	Often sinuous, with interlocking spurs
Long Profile	Stepped profile, with basins (tarns/lakes)	Generally graded profile, smoother slope
Tributary Junctions	Hanging valleys (joining main valley at higher elevation)	Tributary valleys join the main valley at the same level

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Depositional Landforms

Glaciers deposit the unsorted material they carry (glacial till) and sorted material deposited by meltwater streams (outwash deposits) to form various landforms.

• Glacial Till: Unsorted mixture of rock fragments, ranging from clay to boulders, deposited directly by melting ice. Rock fragments in till are typically angular or sub-angular.
• Outwash Deposits (Glacio-fluvial Deposits): Sorted and stratified sediments (sand, gravel, silt, clay) deposited by meltwater streams flowing from a glacier. These deposits are somewhat rounded at their edges, unlike till.

Some common glacial depositional landforms (also illustrated in Figure 7.14):

Diagram showing various features resulting from glacial deposition, such as different types of moraines, eskers, outwash plains, and drumlins.

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Moraines

Moraines are ridges or mounds of glacial till deposited by a glacier. They are formed by the accumulation of debris carried by the ice.

• Terminal Moraine: A ridge of till deposited at the furthest point (toe) of a glacier's advance. It marks the maximum extent of glaciation.
• Lateral Moraines: Ridges of till deposited along the sides of a valley glacier. They consist of debris that has fallen onto the ice from the valley walls or been plucked from the sides.
• Medial Moraine: A ridge of till formed in the middle of a glacial valley when two valley glaciers merge and their lateral moraines combine. They are less distinct than lateral moraines.
• Ground Moraine: A widespread sheet of till deposited more or less evenly over the valley floor as the glacier retreats. It can have an irregular or hummocky surface.

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Eskers

Eskers are long, winding ridges composed of stratified sand and gravel. They form within or beneath a glacier as meltwater streams deposit sediment in ice tunnels or channels. After the ice melts, the sediment deposit remains as a raised, sinuous ridge that roughly follows the path of the former ice channel.

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Outwash Plains

Outwash plains are broad, flat areas formed by glacio-fluvial deposits (outwash) from meltwater streams beyond the edge of a glacier or ice sheet (Figure 7.13). As meltwater streams flow away from the ice margin, they deposit sorted sediment, often in the form of coalescing alluvial fans, creating a gently sloping plain of gravel, sand, silt, and clay.

Distinguishing river alluvial plains and glacial outwash plains:

Feature	River Alluvial Plains (Floodplains)	Glacial Outwash Plains
Sediment Source	Eroded material transported by rivers from their drainage basin	Meltwater streams carrying sediment from glaciers
Sediment Characteristics	Typically finer-grained (sand, silt, clay) in distal areas; well-sorted; rounded grains	Often coarser-grained (gravel, sand) near glacier; stratified; somewhat rounded grains
Location	Along sides of rivers, covering valley bottoms and broad lowlands	Beyond the snout of glaciers or ice sheets

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Drumlins

Drumlins are smooth, oval-shaped, elongated hills composed primarily of glacial till, sometimes with lenses of sand and gravel. Their longer axis is parallel to the direction of ice flow. Drumlins typically have a blunt, steeper end facing the direction from which the ice advanced (stoss end) and a gentler, tapering end pointing down-ice (tail). They are thought to form either by the streamlining of ground moraine beneath actively moving ice or by the deposition of till beneath stagnant or slowly moving ice. Drumlins act as indicators of past glacier flow direction.

The difference between till and alluvium lies in their origin and characteristics: Till is unsorted sediment deposited directly by glacial ice; its particles are typically angular. Alluvium is sediment deposited by running water (rivers or streams); it is typically sorted by particle size and its grains are usually rounded due to transport.

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Waves And Currents

Coastal areas are characterized by some of the most dynamic geomorphic processes, primarily driven by the action of waves and currents. These processes can lead to rapid changes in coastal landforms, with erosion and deposition potentially alternating with changes in wave energy.

Waves are generated by wind blowing over the water surface. As waves approach the shore and encounter shallow water, they break, releasing significant energy. The impact of breaking waves on the shoreline and the churning of sediments on the seabed cause erosion, transportation, and deposition.

Other factors influencing coastal landforms include the shape and configuration of the coastline and the adjacent seafloor, and whether the coast is experiencing uplift (emergence) or subsidence (submergence) relative to sea level. Assuming a stable sea level, coastal landforms and their evolution can be broadly understood by considering two contrasting types of coasts: high rocky coasts and low sedimentary coasts.

Generating forces behind waves are primarily wind and seismic events (causing tsunamis). Currents are driven by factors like wind, tides, differences in water density, and the Earth's rotation.

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High Rocky Coasts

High rocky coasts are often associated with areas that have recently experienced submergence or tectonic uplift. They are characterized by steep cliffs that drop directly into the water and irregular, often indented coastlines. Rivers in these areas may appear 'drowned', forming estuaries or, if formerly glaciated, fjords (narrow, deep inlets).

• Initially, depositional landforms are minimal or absent. Erosion dominates as powerful waves crash against the rocky shore.
• Wave action carves sea cliffs and, as the cliffs retreat inland due to erosion, leaves behind a gently sloping bedrock surface at the base called a wave-cut platform or wave-cut terrace.
• Sediment eroded from the cliffs eventually breaks down into smaller fragments and is transported and deposited in the offshore area, potentially forming a wave-built terrace seaward of the wave-cut platform over time as the coastline smooths out.
• With sufficient sediment supply from erosion and rivers, longshore currents (currents flowing parallel to the shore) and waves can deposit material as beaches along the shore or as submerged bars (elongated ridges of sand/shingle parallel to the coast) in the nearshore zone. Submerged bars that are exposed above water are called barrier bars.
• A barrier bar connected to a headland (a rocky point of land extending into the sea) and extending partway across a bay is called a spit. If bars and spits grow across the mouth of a bay, they can enclose a body of water, forming a lagoon. Lagoons may eventually fill with sediment to form a coastal plain.

The west coast of India is a typical example of a high rocky, retreating coast where erosional features are dominant.

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Low Sedimentary Coasts

Low sedimentary coasts are typically associated with areas that have experienced recent emergence or significant sediment supply from land. They are characterized by gently sloping land that meets the water and smooth, less indented coastlines, often with coastal plains and deltas built by rivers.

• Incursions of water might take the form of shallow lagoons or tidal creeks. Marshes and swamps are common along the shore.
• Depositional features are dominant along low sedimentary coasts.
• As waves break over the gently sloping seabed, they churn and move sediment, building bars, barrier bars, spits, and lagoons.
• Lagoons on these coasts tend to eventually become filled with sediment and vegetation, transforming into swamps and then coastal plains.
• Large rivers with high sediment loads build extensive deltas along low sedimentary coasts.

The east coast of India is an example of a low sedimentary coast where depositional landforms are prominent.

Key differences between a high rocky coast and a low sedimentary coast:

Feature	High Rocky Coast	Low Sedimentary Coast
Topography	Steep cliffs, irregular coastline, hills meet water	Gentle slope, smooth coastline, coastal plains/deltas
Dominant Process	Erosion	Deposition
Key Erosional Forms	Cliffs, wave-cut platforms, sea caves, stacks	Less prominent, limited cliffs/platforms
Key Depositional Forms	Limited initially, patches of beaches/bars later	Beaches, dunes, bars, barriers, spits, lagoons, deltas, marshes
Rivers	Often 'drowned' (estuaries, fjords)	Build coastal plains and deltas

Regardless of the coast type, severe storm waves and tsunamis can cause rapid and drastic changes, overwhelming the normal processes of erosion and deposition.

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Erosional Landforms

Coastal erosion by waves and currents creates distinct features along shorelines.

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Cliffs, Terraces, Caves And Stacks

• Sea Cliffs: Steep rock faces along the coast formed by wave erosion undercutting the landmass (Figure 7.13 shows a conceptual cliff). Most sea cliffs are very steep and can be tens of meters high.
• Wave-Cut Platforms (Terraces): Flat or gently sloping bedrock surfaces formed at the base of a retreating sea cliff by wave erosion. They are exposed at low tide and covered at high tide, indicating the former position of the cliff.
• Sea Caves: Hollows or openings carved into the base of sea cliffs by the erosive action of waves, particularly hydraulic action (force of water entering cracks) and abrasion (rock debris grinding against the cliff). As erosion continues, caves can enlarge, their roofs may collapse, leading to the retreat of the cliff line.
• Sea Stacks: Isolated pillars or remnants of resistant rock standing just offshore (Figure 7.13 shows a stack). They form when erosion cuts through a headland, creating arches, and the roofs of these arches eventually collapse, leaving behind detached rock columns. Sea stacks are temporary features that will eventually be eroded away by wave action.

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Depositional Landforms

Coastal deposition occurs where wave and current energy is insufficient to carry the sediment load, leading to the accumulation of material.

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Beaches And Dunes

• Beaches: Accumulations of loose sediment (sand, pebbles, cobbles - shingle) along the shoreline. Beaches are dynamic and temporary features, constantly reshaped by wave action, tides, and currents. Sediment sources include rivers, coastal erosion, and offshore areas.
• Sand Dunes: Ridges or mounds of sand deposited by wind, typically found inland, just behind a beach. Wind lifts sand from the beach surface and deposits it where it encounters vegetation or other obstacles. Coastal sand dunes often form elongated ridges parallel to the coastline, particularly on low sedimentary coasts.

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Bars, Barriers And Spits

• Off-shore Bar: A submerged or partially submerged elongated ridge of sand and/or shingle located in the nearshore zone, roughly parallel to the coastline.
• Barrier Bar (or Barrier Island): An off-shore bar that has been built up above sea level by wave action and sediment accumulation. Barrier bars are often separated from the mainland by a lagoon or salt marsh.
• Spit: A type of bar or barrier bar that is attached to the land (headland) at one end and extends outwards into a bay or across a river mouth (Figure 7.15 shows a spit). Spits are built by longshore drift, which transports sediment along the coast.

Satellite image showing a spit, an elongated sand feature attached to the land at one end, extending into the sea near the Godavari River delta.

• Lagoons: Bodies of shallow water separated from the open sea by bars, barrier bars, or spits. Lagoons are formed when these depositional features partially or completely block the entrance to a bay or an area behind a barrier island. Over time, lagoons tend to fill up with sediment from land or the surrounding barrier features, eventually turning into coastal plains.

Coastal off-shore bars, barrier bars, beaches, dunes, and mangroves play a vital role in protecting inland areas from the destructive energy of storm waves and tsunamis by absorbing the impact. Disrupting these natural coastal defenses through human activity increases vulnerability to coastal hazards.

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Winds

Wind is a dominant geomorphic agent, particularly effective in arid and semi-arid environments (deserts). Desert surfaces heat up quickly and intensely, creating unstable air with upward movements and turbulence. Strong winds, especially storm winds, can move large amounts of sediment.

Wind performs geomorphic work through three main processes:

• Deflation: The lifting and removal of loose particles (dust, silt, sand) from the ground surface by wind. This process can create shallow depressions called deflation hollows.
• Abrasion: The grinding and wearing away of rock surfaces by wind-borne sand and dust particles. This is analogous to sandblasting and is most effective close to the ground where sand is transported.
• Impact: The simple force of wind-blown particles hitting a rock surface. This contributes to breaking down and eroding the rock.

While wind is a significant agent in deserts, many desert landforms are also shaped by mass wasting and, crucially, by running water, especially during rare but intense flash floods. Desert rocks weather rapidly due to large daily temperature swings (mechanical weathering) and some chemical processes, producing abundant debris. Flash floods act as powerful sheet washes, capable of moving larger particles and performing general mass erosion, particularly when vegetation is sparse. Wind is more effective at transporting finer materials like sand and dust. Stream channels in deserts (wadis or arroyos) are often broad, shallow, and temporary, flowing only after rainfall.

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Erosional Landforms

Wind and flash floods create distinctive erosional features in desert landscapes.

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Pediments And Pediplains

The evolution of desert landscapes often involves the formation and expansion of pediments and pediplains.

• Pediments: Gently sloping, relatively smooth rock surfaces that extend outwards from the base of steep mountain fronts in deserts. They may have a thin cover of gravel or sediment. Pediments are formed by a combination of lateral erosion by intermittent streams flowing from the mountains and sheet floods spreading across the surface.
• The steep mountain front above the pediment (the scarp or free face) gradually retreats parallel to itself over time (backwasting) due to weathering and erosion at its base and sheetwash on the pediment. As the mountain front retreats, the pediment expands.
• Eventually, through the retreat of mountain fronts and expansion of pediments, the mountains are reduced to isolated remnant hills (inselbergs - German for "island mountains") standing above vast, nearly flat plains.
• Pediplains: Extensive, low-relief plains formed by the coalescence of multiple pediments over large areas, representing a late stage of desert landscape erosion and reduction.

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Playas

Playas are flat, central basins found in desert regions with internal drainage (where streams flow towards the center of a basin rather than to an ocean) (Figure 7.10 shows a playa conceptually). Sediment eroded from the surrounding mountains and hills is transported and deposited in the center of these basins, forming a nearly level plain.

• During periods of sufficient rainfall, these basins may fill with a shallow, temporary lake, also called a playa lake.
• Due to high evaporation rates in deserts, the water quickly evaporates, leaving behind a dry, flat surface.
• If the groundwater table is high or significant salt-bearing sediments are present, evaporation can lead to the accumulation of salts on the surface, forming a salt pan or alkali flat (a playa plain covered in salts).

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Deflation Hollows And Caves

• Deflation Hollows: Shallow depressions formed by the removal of loose sand, silt, and dust from the surface by persistent wind erosion (deflation).
• Wind abrasion can also create small pits and cavities on exposed rock surfaces. Repeated abrasion and deflation, particularly in softer spots on rocks, can lead to larger depressions called blowouts, which may enlarge into wind-eroded caves.

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Mushroom, Table And Pedestal Rocks

These distinctive rock formations (Figure 7.13 conceptually shows some wind-eroded shapes) are created by differential wind abrasion, which is most intense near the ground surface where wind-blown sand is concentrated.

• Wind abrasion wears away the lower parts of isolated rock outcrops more rapidly than the upper parts, leaving the upper portion supported by a narrower base or stalk.
• This process can sculpt rocks into shapes resembling mushrooms, tables, or pedestals. The surfaces may become polished by the abrasive action.

Erosional features carved out by wind action include deflation hollows, wind caves, abrasion pits/grooves, and mushroom/table/pedestal rocks. However, large-scale erosional features like pediments and the overall reduction of mountains to pediplains are primarily the result of a combination of stream action (sheet floods, lateral erosion by intermittent streams) and backwasting of slopes, facilitated by weathering, with wind playing a more dominant role in shaping smaller details and removing finer debris.

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Depositional Landforms

Wind is an effective sorting agent, transporting sediment based on particle size and wind velocity. Larger grains are moved by rolling or bouncing (saltation) along the surface, while finer particles (silt and dust) are carried in suspension. As wind speed decreases, particles are deposited, with coarser grains settling first.

Wind depositional landforms are well-sorted and can form wherever there is sufficient sand supply and consistent wind direction.

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Sand Dunes

Sand dunes are accumulations of wind-blown sand into mounds or ridges (Figure 7.16). They are characteristic landforms of sandy deserts and coastal areas, formed where wind energy decreases or obstacles cause sand to accumulate.

Diagram showing different common shapes of sand dunes (barchan, parabolic, seif, longitudinal, transverse) and the wind direction relative to their form.

Different types of sand dunes develop depending on factors like sand supply, wind direction and consistency, and the presence of vegetation or obstacles:

• Barchans: Crescent-shaped dunes with the concave side (horns or wings) pointing downwind. They form in areas with a constant, moderate wind direction and a limited sand supply on flat, hard surfaces.
• Parabolic Dunes: U-shaped or V-shaped dunes with the concave side (horns) pointing upwind, anchored by vegetation. They are essentially reversed barchans and are common in coastal areas or partially vegetated sandy environments.
• Seif Dunes: Long, linear ridges of sand that are parallel to the prevailing wind direction or result from winds blowing from slightly different directions. A seif dune resembles a barchan with only one wing extended into a long ridge.
• Longitudinal Dunes: Very long, straight or slightly wavy ridges of sand parallel to the prevailing wind direction. They form in areas with low sand supply and strong, consistent winds. They can be very long but are generally lower in height than barchans or seifs.
• Transverse Dunes: Long ridges of sand that are oriented perpendicular (at right angles) to the prevailing wind direction. They form in areas with abundant sand supply and a constant wind direction.

When sand supply is very high, different dune types may merge, forming complex dune fields. Many dunes in deserts are mobile, shifting position over time, while some become stabilized by vegetation, particularly near human settlements or coasts.

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