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Chapter 6 Landforms And Their Evolution
Introduction To Landforms And Evolution
Following our exploration of Earth's formation, its internal structure, plate movements, and the nature of rocks and minerals, we now turn our attention to the dynamic surface we inhabit. The Earth's surface is constantly being shaped and reshaped by various forces.
The question arises: Why is the Earth's surface uneven? As we learned, the Earth's crust is not static; it moves both vertically and horizontally. The internal forces (**endogenic forces**) originating from within the Earth build up the crust, creating variations in elevation, such as mountains and plateaus. These forces were perhaps more rapid in the distant past but remain active today.
Simultaneously, the Earth's surface is exposed to external forces (**exogenic forces**), primarily driven by energy from the sun (insolation). These forces work to wear down elevated areas (degradation) and fill in depressions (aggradation). The continuous interplay between the land-building endogenic forces and the land-wearing exogenic forces is what results in the diverse and uneven topography of the Earth's surface.
Specific, small to medium-sized areas of the Earth's surface with distinct physical shapes are called **landforms**. Examples include valleys, plains, hills, and dunes. When several related landforms occur together over a larger area, they constitute a **landscape**.
Each landform possesses unique physical characteristics, composition, and is a product of specific geomorphic processes and agents (like water, wind, ice). Most geomorphic processes act slowly, meaning the formation and significant modification of landforms take considerable time (hundreds or thousands of years).
Landforms are not permanent; they have a beginning and continuously change over time due to the ongoing action of geomorphic processes. Factors like changes in climate or movements of the Earth's crust can alter the intensity or type of processes operating, leading to further modifications of existing landforms.
The concept of **evolution** in landforms refers to the stages of transformation that parts of the Earth's surface or individual landforms undergo over geological time. Just as living organisms pass through stages, landscapes and landforms can be thought of as having a history of development, evolving through stages often described conceptually as youth, maturity, and old age. Understanding this evolution requires considering both the specific geomorphic processes acting on a landscape and how these actions change over time as the landform itself is modified and its gradient changes.
Running Water
In regions that receive significant rainfall (humid regions), **running water** is considered the most dominant geomorphic agent responsible for shaping the land surface. Running water acts in two main forms: as **overland flow** spread thinly over broad areas (sheet flow) and as concentrated **linear flow** within defined channels like streams and rivers.
Most erosional landforms created by running water are associated with youthful rivers flowing strongly down steep slopes. On such gradients, downward cutting (vertical erosion) is very active. As a river flows over time, its channel gradient lessens due to continuous erosion. This reduction in slope decreases the water's velocity and energy, causing deposition to become more prominent. While some deposition might occur on steep slopes, it is generally small-scale compared to deposition by rivers on gentler slopes.
The flatter the channel gradient, the greater the potential for deposition. As continued erosion reduces the stream bed slope, lateral erosion (sideways cutting of banks) becomes more dominant than down-cutting. This lateral erosion widens the valley and gradually reduces the surrounding hills and valley sides, eventually leading to the formation of plains. The complete reduction of a high landmass to a flat plain is a theoretical end-stage, rarely perfectly achieved in reality due to ongoing endogenic uplift and the resistant nature of some rocks.
Overland flow initially causes **sheet erosion**, removing a thin layer of soil. Irregularities on the surface cause this flow to concentrate, forming small channels called **rills**. Rills deepen and widen into larger features called **gullies**. Gullies continue to grow, merge, and extend, developing into a network of **valleys**.
As a landscape develops under the influence of running water, it passes through distinct stages, broadly comparable to the stages of life (youth, maturity, old age). These stages describe the characteristic features of streams and valleys at different points in their erosional evolution.
Stages Of Landscape Development
Landscapes shaped by running water evolve through identifiable stages:
Youth Stage
Characterized by:
- Few streams with limited integration (not well-connected network).
- Flow over original, often steep slopes.
- Shallow, narrow **V-shaped valleys**.
- Floodplains are absent or very narrow, found mainly along major streams.
- Stream divides (areas between drainage basins) are broad and flat, often with marshes, swamps, and lakes.
- Meanders (loop-like bends) might occur on these flat upland divides, potentially becoming deeply cut into the upland (incised meanders).
- Features like **waterfalls and rapids** are common where resistant rock layers are exposed.
Mature Stage
Characterized by:
- Numerous streams forming a well-integrated drainage network.
- Valleys are still **V-shaped but are deeper** than in the youth stage.
- Trunk streams (main rivers) are large enough to have wider floodplains within the valley.
- Streams within the floodplain may flow in meanders, but these are often confined within the valley walls.
- The broad, flat inter-stream areas and swamps of the youth stage have largely disappeared.
- Stream divides become sharper and narrower.
- Waterfalls and rapids are typically eliminated by erosion.
Old Stage
Characterized by:
- Fewer smaller tributaries compared to the mature stage, with very gentle gradients.
- Streams **meander freely** across vast, wide floodplains.
- Common features on the floodplain include **natural levees** (raised banks), **oxbow lakes** (cut-off meander loops), and broad meander belts.
- Divides are broad and flat, often with lakes, swamps, and marshes.
- Most of the landscape is reduced to a peneplain (an almost flat plain) at or slightly above sea level, with occasional isolated remnants of resistant rock (monadnocks).
Erosional Landforms
Running water creates various landforms by eroding and carrying away rock and soil material.
Valleys
Valleys are linear depressions created by stream erosion. They begin as small rills and evolve into larger gullies and eventually full-fledged valleys. Valleys vary in shape and dimensions. Common types include:
- **V-shaped valley:** Characteristic of youthful streams where down-cutting is dominant.
- **Gorge:** A deep valley with very steep, almost vertical sides. Gorges often form in hard, resistant rocks. The width is roughly equal at the top and bottom. (This refers to Figure 6.1).
- **Canyon:** Similar to a gorge but characterized by steep, step-like side slopes. Canyons are significantly wider at the top than at the bottom. They often form in horizontally layered sedimentary rocks, where different rock layers erode at different rates, creating the step-like profile. A canyon can be considered a variant of a gorge. (This refers to Figure 6.2).
Potholes And Plunge Pools
These are depressions formed in the rocky beds of streams.
- **Potholes:** Roughly circular depressions formed by the swirling action of water and the abrasive grinding of pebbles and boulders trapped within depressions on the streambed. Once a small hollow forms, trapped sediment is rotated by the current, drilling and enlarging the hole. A series of potholes can deepen and join over time.
- **Plunge Pools:** Large, deep, and wide potholes that form at the base of waterfalls. They are created by the sheer force of the falling water (hydraulic action) combined with the abrasive action of boulders swirled around by the turbulent water at the base of the fall.
Incised Or Entrenched Meanders
Meanders are loop-like bends in a river channel. They commonly develop on flat areas like floodplains where the stream gradient is gentle and lateral erosion is dominant. However, deep and wide meanders can also be found cut into hard bedrock in upland areas, even where the initial gradient was relatively steep. These are called **incised** or **entrenched meanders**. They suggest that a meandering course was established when the river was flowing over a gentler surface, and subsequent tectonic uplift caused the river to downcut into the underlying resistant rock while maintaining its sinuous path. (This refers back to Figure 6.2).
River Terraces
River terraces are flat or gently sloping surfaces representing former levels of a river's valley floor or floodplain. They are essentially remnants of older, higher floodplains that have been abandoned as the river has cut down vertically into its own deposits or bedrock. Terraces can be composed of bedrock with little sediment cover or built from layers of river-deposited alluvium. Multiple terraces at different elevations indicate successive stages of down-cutting. Terraces can appear on one side of the valley or on both sides at the same elevation (**paired terraces**) or different elevations (**unpaired terraces**).
Depositional Landforms
When running water loses energy, it deposits the sediment it carries, creating depositional landforms.
Alluvial Fans
Alluvial fans (This refers to Figure 6.3) are cone-shaped or fan-shaped deposits of sediment that form where a mountain stream carrying a heavy load emerges from a steep mountain valley onto a flatter plain or valley floor. The sudden decrease in gradient causes the stream to lose energy rapidly and dump its coarse sediment load. Over time, this deposition builds up a fan shape. The streams on an alluvial fan often diverge into multiple channels called distributaries as sediment deposition blocks the main channel. Fans in humid areas are usually lower and more gently sloped than those in arid/semi-arid areas, which tend to be steeper and higher. (This refers to Figure 6.3).
Deltas
Deltas are depositional landforms similar in shape to alluvial fans but formed where rivers enter a larger, standing body of water like a lake, sea, or ocean. As the river meets the standing water, its velocity drops sharply, and it deposits its sediment load. This material accumulates at the river mouth, building outwards. Unlike fans, delta deposits are typically better sorted, with coarser material settling near the mouth and finer silts and clays carried further into the water body. As the delta grows outwards, the river often divides into multiple smaller channels called distributaries. Deltas take various shapes (like triangular, arcuate, or bird's foot), depending on factors like wave action, tidal range, and sediment load. (This refers to Figure 6.4 showing a delta). (This refers to Figure 6.4).
Floodplains, Natural Levees And Point Bars
Floodplains are broad, flat areas adjacent to river channels that are built by sediment deposition during floods. They are major river depositional landforms. When a river flows over its banks during flooding, the water spreads out and its velocity decreases rapidly, causing it to deposit the sediment it carries. These deposits, usually fine-grained like sand, silt, and clay, build up the floodplain over time.
The area covered by river deposits that is actively flooded is the active floodplain. Areas above the typical flood level, also built by past flood deposits, are inactive floodplains. In flatter areas, rivers often shift their channels laterally, abandoning old courses, which get filled with sediment and contribute to the floodplain's development.
Key features of floodplains include:
- Natural Levees: Low, elongated ridges that form along the banks of a river channel, built up by coarser sediment deposited directly along the channel edge as floodwaters overflow. They often appear as subtle rises parallel to the river. (This refers to Figure 6.5).
- Point Bars (Meander Bars): Crescent-shaped deposits of sediment that accumulate on the inner, convex bank of a meander bend. As water flows around a bend, it is faster on the outside and slower on the inside, leading to erosion on the outer bank and deposition on the inner bank. Point bars consist of sorted sediment and generally have a smooth, gentle slope towards the channel. (This refers to Figure 6.5).
Meanders
While meanders were mentioned as a channel pattern associated with youthful landscapes (incised meanders), they are most characteristic of large rivers flowing across wide, flat floodplains and deltas where the gradient is very gentle. (This refers to Figure 6.6 showing meandering river). (This refers to Figure 6.6).
Several factors contribute to meander formation:
- The tendency of slow-moving water on gentle slopes to exert lateral pressure and erode banks.
- The composition of alluvial banks, which are often unconsolidated and have irregularities that channel water laterally.
- The **Coriolis force**, which slightly deflects flowing water (to the right in the Northern Hemisphere, left in the Southern Hemisphere), initiating slight curvatures that can develop into meanders.
Once a slight bend forms, water flows faster on the outer bank and slower on the inner bank. This leads to erosion and undercutting on the outer, **concave bank** (the "cut-off bank") and deposition on the inner, **convex bank** (the "slip-off slope"). This differential erosion and deposition causes the meander bend to enlarge and migrate laterally over time. (This refers to Figure 6.7). As meander loops become extremely exaggerated, the narrow neck of land between two adjacent loops can be eroded through during a flood, cutting off the meander loop and leaving it as a crescent-shaped lake called an **oxbow lake**. (This refers to Figure 6.7).
Groundwater
Our focus here is on the role of **groundwater** (water stored and flowing beneath the Earth's surface) as a geomorphic agent, rather than as a resource. Groundwater is particularly effective at shaping landforms in specific types of rocks due to its chemical properties.
Surface water infiltrates into the ground more easily when rocks are permeable (allow water to pass through), thinly layered, and contain many joints or cracks. Once underground, water moves vertically and horizontally through these openings or through the porous rock material itself. While groundwater can cause some mechanical removal of material, its primary geomorphic work is through **chemical processes**, specifically **solution** and **precipitation**.
The effects of groundwater's chemical work are most evident in rocks rich in **calcium carbonate**, such as **limestone** and **dolomite**. Carbon dioxide dissolved in rainwater forms carbonic acid ($H_2CO_3$), which is capable of dissolving calcium carbonate. As this acidic water moves through cracks and layers in limestone, it dissolves the rock, creating voids and passages. When conditions change (e.g., water evaporates or loses $\text{CO}_2$), the dissolved calcium carbonate can precipitate out of solution, depositing mineral formations.
A region characterized by distinctive landforms created by the chemical action of groundwater on limestone or dolomite is called **Karst topography**. This name comes from the Karst region in the Balkans, known for its classic limestone landscape. Karst topography includes both erosional and depositional landforms.
Karst Topography
Karst landscapes are shaped by the solubility of carbonate rocks, resulting in unique surface and subsurface features. (This refers to Figure 6.8 showing various karst features).
Erosional Landforms
Groundwater erosion, mainly through solution, creates various depressions and openings on the surface and underground.
- **Swallow Holes:** Small to medium-sized, typically circular to oval, shallow depressions on the limestone surface formed by solution.
- **Sinkholes:** More prominent depressions, common in karst areas. They are typically circular at the surface and funnel-shaped downwards, varying significantly in size (from a few square meters to a hectare) and depth (less than a meter to over thirty meters). Some sinkholes form purely by solution (**solution sinks**). Others form when the roof of an underground cave or void collapses (**collapse sinks**). Sinkholes can sometimes be hidden by a thin soil layer, appearing as shallow pools (**doline** is a term sometimes used for collapse sinks or larger solution features), posing a hazard. Surface runoff often disappears into swallow holes and sinkholes, flowing as underground streams.
- **Valley Sinks (Uvalas):** Larger depressions formed by the merging of multiple sinkholes or dolines due to the collapse or slumping of the land between them, creating elongated, trench-like features.
- **Lapies:** As solution activity continues, large areas of the limestone surface can become highly irregular, etched with a maze of sharp points, grooves, and ridges. These forms, known as lapies, are created by differential solution along parallel or sub-parallel joints. Extensive lapie fields can eventually be smoothed into areas called **limestone pavements**.
Caves
Underground **caves** (or caverns) are significant erosional features in karst areas. They form when groundwater dissolves limestone, particularly along bedding planes, joints, or faults. Water often seeps down through cracks and then moves horizontally along less permeable layers or zones of weakness, dissolving the soluble rock along these paths. Caves can vary in size and extent, forming complex networks at different levels. They typically have openings where underground streams can emerge, and caves with openings at both ends are sometimes called tunnels.
Depositional Landforms
Within limestone caves, groundwater can also create depositional features. This happens when the water carrying dissolved calcium carbonate loses its dissolved $\text{CO}_2$ (due to changes in pressure or air circulation) or evaporates, causing the calcium carbonate to precipitate out of solution and build up mineral formations. (This refers to Figure 6.9 showing cave deposits). (This refers to Figure 6.9).
Stalactites, Stalagmites And Pillars
- **Stalactites:** Icicle-shaped deposits that hang downwards from the cave ceiling. They form as calcium carbonate-rich water drips from the roof, losing $\text{CO}_2$ and depositing a tiny ring of calcite. Successive drips build the formation downwards. Stalactites are usually wider at the top where they attach to the ceiling and taper towards their tip.
- **Stalagmites:** Deposits that rise upwards from the cave floor. They form from calcium carbonate precipitated from dripping water that falls from the ceiling (often from the tip of a stalactite). The water hits the floor and precipitates calcite, building the formation upwards. Stalagmites can have various shapes, from columns to discs.
- **Pillars (or Columns):** Formed when a stalactite and a stalagmite grow towards each other and eventually meet and fuse, creating a single column extending from the cave floor to the ceiling.
Glaciers
Glaciers are large masses of ice that move slowly over land due to gravity. They can exist as vast, continuous sheets covering continents (continental glaciers, or piedmont glaciers if they spread over plains at mountain bases) or as linear flows confined to valleys in mountainous regions (mountain or valley glaciers). (This refers to Figure 6.10 showing a glacier). (This refers to Figure 6.10).
Types And Characteristics Of Glaciers
The movement of glaciers is much slower than water flow, ranging from centimeters to meters per day. This movement is driven primarily by the force of **gravity**, causing the ice mass to slowly deform and slide, especially when the ice thickness and slope are sufficient.
Glaciers are powerful agents of erosion. Their immense weight causes considerable friction as they move. Material is eroded from the underlying and surrounding rock through two main processes:
- **Abrasion:** The grinding and scraping action caused by rock fragments embedded in the base and sides of the glacier rubbing against the bedrock.
- **Plucking:** The glacier freezes onto fractured bedrock and, as it moves, pulls away (plucks) loosened blocks of rock.
Glaciers can cause significant erosion even on unweathered, solid rock, dramatically reshaping landscapes and reducing high mountains into lower forms over geological time. As glaciers melt and retreat, they deposit the material they have carried (glacial till), leaving behind characteristic depositional landforms.
Figures 6.11 and 6.12 illustrate common glacial erosional and depositional forms. (This refers to Figure 6.11 and Figure 6.12).
Erosional Landforms
Glacial erosion carves distinctive features into the landscape, particularly in mountainous regions.
Cirque
**Cirques** are among the most recognizable landforms in glaciated mountains. They are armchair-shaped or bowl-shaped hollows carved by glacial erosion at the head of a mountain valley. Cirques have steep, concave walls (often dropping almost vertically) at the back and sides, and a lip or sill at the front. After the glacier melts, a lake often occupies the basin within the cirque, called a **cirque lake** or **tarn**. Multiple cirques can form in a stepped sequence down a mountainside.
Horns And Serrated Ridges
- **Horns:** Sharp, pointed peaks formed when three or more cirques erode headward into a mountain from different sides, leaving a pyramidal peak between them. The most famous example is the Matterhorn in the Alps. (Mentioned as related to Everest in the Himalayas).
- **Arêtes (Serrated Ridges):** Narrow, knife-edge ridges formed when two cirques erode back-to-back, or when parallel glacial valleys erode into a mountain ridge from opposite sides, leaving a sharp, jagged crest between them.
Glacial Valleys/Troughs
**Glacial valleys**, also called **glacial troughs**, are carved by valley glaciers. Unlike river valleys, which are typically V-shaped, glacial valleys are characteristically **U-shaped**, with broad, flat floors and relatively smooth, steep sides. The glacier's erosive power reshapes the original V-shaped river valley it may have occupied. The floor may be covered with glacial debris or contain lakes scooped out by the ice. Smaller valleys carved by tributary glaciers may join the main glacial valley at a higher elevation, forming **hanging valleys**. The junction between a hanging valley and the main U-shaped valley often features truncated spurs, which are blunt, triangular faces resulting from glacial erosion. Very deep glacial troughs that have been flooded by sea water are called **fjords** and are common in high-latitude coastal areas.
The basic difference between glacial valleys and river valleys lies in their shape (U-shaped vs. V-shaped), the smoothness of their walls, the width of the floor, and the presence of features like hanging valleys and fjords (unique to glacial erosion).
Depositional Landforms
When glaciers melt, they deposit the unsorted mixture of rock fragments and sediment they carried, known as **glacial till**. Till consists of rock fragments of all sizes, typically angular to sub-angular because they haven't been significantly rounded by water transport. Meltwater streams flowing from glaciers can sort and transport some of the finer debris, depositing it as **glacio-fluvial deposits** or **outwash deposits**. Unlike till, outwash is roughly stratified and the particles (sand, gravel) are somewhat rounded. (This refers to Figure 6.12 showing depositional forms).
Moraines
**Moraines** are accumulations of glacial till that form distinct ridges or mounds. Types of moraines include:
- **Terminal Moraine:** A ridge of till deposited at the furthest point reached by the glacier (at its 'toe' or snout). It marks the maximum extent of the ice advance.
- **Lateral Moraines:** Ridges of till deposited along the sides of a valley glacier, formed by debris that accumulates along the edges of the ice flow.
- **Medial Moraine:** A ridge of till that forms down the center of a glacial valley when the lateral moraines of two confluent (joining) glaciers merge.
- **Ground Moraine:** A widespread, relatively thin layer of till deposited over the valley floor as the glacier retreats. It often creates an irregular, undulating surface.
Eskers
**Eskers** are long, winding ridges composed of stratified sand and gravel. They are formed by deposits from meltwater streams that flow within tunnels or channels under, within, or on top of a melting glacier. The stream channel walls are formed by ice. When the ice melts, the sediment deposited in the channel is left standing as a raised ridge. Eskers can be many kilometers long and follow the path of the ancient sub-glacial stream.
Outwash Plains
**Outwash plains** are broad, gently sloping areas in front of the terminal moraine of a glacier or ice sheet, formed by glacio-fluvial deposits carried and sorted by meltwater streams. These plains are composed of gravel, sand, silt, and clay, often spread out as coalescing alluvial fans. Outwash plain deposits are sorted and stratified, unlike the unsorted glacial till left directly by the ice.
The difference between river alluvial plains and glacial outwash plains lies in the origin of the sediment (river erosion/deposition vs. glacial erosion/meltwater deposition) and the sorting characteristics (alluvial plains can be more varied, outwash plains show distinct sorting by meltwater).
Drumlins
**Drumlins** are elongated, oval-shaped hills composed of glacial till. They are smooth and streamlined features, typically occurring in groups or "swarms". The long axis of a drumlin is parallel to the direction of ice movement. Drumlins are generally asymmetrical; the end facing the direction from which the ice came (the **stoss end**) is blunt and steeper, while the down-ice end (the **tail**) is tapered and gently sloping. They are thought to form either by the streamlining of existing till deposits or by the deposition of till beneath a moving glacier. Drumlins are valuable indicators of the direction of past glacial flow.
The difference between till and alluvium is fundamental: **Till** is unsorted sediment deposited directly by a glacier, typically angular. **Alluvium** is sorted and stratified sediment deposited by running water, with particles usually rounded by transport.
Waves And Currents
Coastal areas are dynamic environments shaped by the constant interaction between land and sea, primarily driven by waves and currents. Coastal processes can be very rapid and result in significant erosion and deposition.
Changes along coasts can happen quickly, with erosion dominating in one season and deposition in another. **Waves** are the most important agents shaping coastlines. When waves approach the shore and break, they exert significant force on the land, churning and moving sediments on the seafloor. The continuous impact of breaking waves leads to erosion. Storm waves and tsunamis are particularly powerful and can cause dramatic changes in a short time.
Besides wave action, coastal landforms are influenced by the **configuration of the land and seafloor** and whether the coast is experiencing **emergence** (land rising relative to sea level, or sea level falling) or **submergence** (land sinking relative to sea level, or sea level rising).
Assuming sea level is relatively constant, coastal evolution can be understood by considering two broad types of coasts:
- **High, rocky coasts (often associated with submergence):** Characterized by steep cliffs and irregular coastlines.
- **Low, smooth, and gently sloping sedimentary coasts (often associated with emergence):** Characterized by beaches, dunes, and depositional features.
Coastal off-shore bars serve as natural barriers against storm surges and tsunamis, absorbing significant destructive energy. Disturbing the natural sediment supply or coastal ecosystems like mangroves can destabilize these protective features, making inland areas more vulnerable.
Coastal Types And Processes
Coastal morphology is determined by a combination of factors, including wave energy, sediment supply, geological structure, and sea-level changes.
High Rocky Coasts
On high rocky coasts, the coastline is often irregular and indented, sometimes with drowned features like fjords (glaciated valleys submerged by the sea). The land often drops steeply into the water, forming cliffs. Initially, depositional features are minimal, and **erosional processes dominate**.
Waves crashing against the cliffs erode the rock, gradually causing the cliffs to retreat inland. Erosion at the base of the cliff creates a notch, and eventually, the overhanging rock collapses. As the cliff retreats, a gently sloping platform, often covered by rock debris, is left behind at the base of the cliff, called a **wave-cut platform**. Continuous wave action tends to straighten irregular coastlines over time.
Erosion on rocky coasts provides sediment. This material, broken down and rounded by wave action, can be deposited offshore, potentially forming a **wave-built terrace** in front of the wave-cut platform. Sediment transported by waves and longshore currents (currents moving parallel to the shore) can also be deposited along the shore as **beaches** or as submerged ridges called **bars** in the nearshore zone. Bars that emerge above water at low tide are called **barrier bars**. A barrier bar connected to the mainland at one end is a **spit**. If bars or spits grow across the mouth of a bay, they can enclose a body of water, forming a **lagoon**, which may eventually fill with sediment to become a coastal plain.
Low Sedimentary Coasts
Along low sedimentary coasts, rivers often build outwards by depositing sediment, forming coastal plains and deltas. The coastline is generally smooth, with occasional lagoons or tidal creeks where water extends inland. The land slopes gently towards the sea. Marshes and swamps are common features in these areas, and **depositional processes dominate**.
On these gently sloping shores, breaking waves churn and move abundant bottom sediments. This sediment is readily deposited to form bars, barrier bars, spits, and lagoons. Lagoons gradually become shallower as they fill with sediment from land or the beach, eventually turning into swamps and then coastal plains. The stability of these features depends on a continuous supply of sediment. However, large storm waves or tsunamis can significantly alter these features regardless of sediment supply.
Large rivers with substantial sediment loads often build **deltas** where they meet the sea along low sedimentary coasts. The east coast of India is an example of a low sedimentary, depositional coast, while the west coast is a high rocky, retreating (erosional) coast.
Erosional Landforms
Despite the general classification, both rocky and sedimentary coasts can exhibit erosional features, although they are more prominent on rocky coasts.
Cliffs, Terraces, Caves And Stacks
These are common erosional features found on rocky coasts:
- **Sea Cliffs:** Steep slopes or vertical rock faces along the coast, formed by wave erosion at their base.
- **Wave-Cut Terraces:** Flat or gently sloping platforms at the base of sea cliffs, representing the eroded former level of the land.
- **Sea Caves:** Hollows or tunnels formed by wave erosion and abrasion at the base of a cliff. Continued erosion can enlarge caves, and if caves form on opposite sides of a headland, they can meet to form an arch.
- **Sea Arches:** Formed when a sea cave erodes through a headland, leaving an archway.
- **Stacks:** Isolated pillars or columns of rock standing in the sea, detached from the main cliff. They are remnants of headlands or cliffs where wave erosion has caused the connecting rock to collapse (sometimes the top of a sea arch collapses to form stacks). Stacks are temporary features that will eventually be eroded away.
Depositional Landforms
Depositional features are characteristic of areas where sediment supply exceeds wave erosion and transport, more common on low sedimentary coasts but also found in sheltered areas on rocky coasts.
Beaches And Dunes
- **Beaches:** Accumulations of sand, pebbles, or cobbles along the shoreline. Sediment comes from rivers, coastal erosion, or offshore sources. Beaches are dynamic and often temporary, changing shape and size seasonally with wave energy and sediment supply.
- **Sand Dunes:** Mounds or ridges of sand formed inland from beaches by wind picking up sand from the beach surface and depositing it. Sand dunes often form long ridges parallel to the coastline on low sedimentary coasts.
Bars, Barriers And Spits
These are linear depositional features composed of sand and/or shingle:
- **Off-shore Bar:** A submerged ridge of sediment formed parallel to the coast in the nearshore zone.
- **Barrier Bar:** An off-shore bar that has grown large enough to be exposed above sea level, forming an elongated island parallel to the coast.
- **Spit:** A linear depositional feature that extends from a headland or the mainland into a bay or across a river mouth. It is attached to the land at one end and extends into the water.
Barrier bars, bars, and spits can grow to partially or completely block the entrance to a bay, forming a **lagoon** behind them. Lagoons gradually fill with sediment from land or sea, potentially developing into coastal plains over time.
Winds
Wind is a significant geomorphic agent, particularly dominant in **hot deserts**, where vegetation is sparse and dry, unconsolidated material is abundant. Dry, barren desert surfaces heat up rapidly, causing air above them to warm, become less dense, and rise, creating turbulence. Obstacles on the ground create eddies and localized whirlwinds. Winds also flow across the desert floor at high speeds.
Wind Action And Processes
Wind performs its geomorphic work through three main processes:
- **Deflation:** The lifting and removal of loose, fine-grained particles (dust, silt) from the surface by wind. This can create shallow depressions (deflation hollows) or lower the overall land surface.
- **Abrasion:** The wearing away of rock surfaces by the impact and grinding action of sand and dust particles carried by the wind. This acts like natural sandblasting and is most effective close to the ground where sand particles are transported.
- **Impact:** The sheer force of the wind and the momentum of wind-borne particles hitting a surface, causing minor damage.
While wind is a defining agent in deserts, it's important to note that other processes, particularly **mass wasting** and **running water** (during rare, intense rainfall events), also shape desert landscapes. Desert rocks weather mechanically due to large diurnal temperature changes. When torrential rain falls, it causes sudden, powerful **sheet floods** or sheet wash that can easily remove large quantities of weathered debris. Thus, wind primarily moves fine material and creates specific features, but much of the large-scale erosion and sediment transport in deserts is accomplished by episodic flash floods. Stream channels in deserts (wadis or arroyos) are often broad, shallow, and flow only for short periods after rain.
Erosional Landforms
Wind and intermittent water action create characteristic erosional features in deserts.
Pediments And Pediplains
The evolution of landscapes in deserts often involves the formation and expansion of pediments. **Pediments** are gently sloping, rocky platforms located at the base of desert mountains or scarps, often covered by a thin layer of debris. They form as the steep mountain front retreats parallel to itself due to erosion. This retreat, known as **backwasting**, involves lateral erosion by sheet wash and intermittent streams at the mountain base undermining the slope above.
As the mountain front retreats and the pediment extends outwards, the mountain mass is gradually reduced. Eventually, only isolated remnants of resistant rock, called **inselbergs** (meaning "island mountains"), may be left standing above the plain. The coalescing and extension of pediments from multiple retreating mountain fronts create vast, low-relief plains called **pediplains**.
Playas
**Playas** are flat-bottomed, internal drainage basins found in arid and semi-arid regions. Rainfall in surrounding mountains or hills drains towards the lowest point of the basin, forming a temporary, shallow lake (the playa lake). Due to high evaporation rates, the water quickly disappears, leaving behind a dry, flat lakebed often covered in fine silt and significant salt deposits. A playa surface encrusted with salts is called an **alkali flat**.
Deflation Hollows And Caves
- **Deflation Hollows:** Shallow depressions formed by persistent wind deflation removing fine, loose material from the surface.
- **Wind-eroded Pits/Cavities:** Wind abrasion, especially near the ground where sand transport is highest, creates small pits or hollows on rock surfaces. Over time, these can enlarge into larger features called **blowouts**, and in softer rock or along zones of weakness, they can develop into small **caves**.
Mushroom, Table And Pedestal Rocks
In desert areas, differential erosion by wind (abrasion and deflation) can carve resistant rock outcrops into unusual shapes. If the lower part of a rock is exposed to more intense abrasion by wind-blown sand (which is typically concentrated near the ground), it erodes faster than the upper part. This process can create formations with a narrow base and a wider top, resembling a **mushroom** (mushroom rock), a **table** (table rock), or standing on a narrow support like a **pedestal** (pedestal rock).
Erosional features carved out by wind action specifically include deflation hollows, wind-eroded pits/cavities/blowouts, and mushroom/table/pedestal rocks. However, pediments and pediplains are formed by a combination of wind action and significant sheet wash/intermittent stream erosion.
Depositional Landforms
Wind is an excellent sorting agent. The size of particles transported by wind depends on wind velocity; finer particles are carried higher and further in suspension, while sand grains are moved along the surface by bouncing (saltation) or rolling. As wind speed decreases, particles are deposited according to their size and weight, resulting in well-sorted deposits.
Depositional landforms created by wind are common in arid and semi-arid regions where there is a good supply of sand and consistent wind direction.
Sand Dunes
Sand dunes are mounds or ridges of sand deposited by wind. They are characteristic landforms of sandy deserts and coastal areas. Their formation requires a source of sand, wind, and an obstacle (like a rock, plant, or even a slight irregularity in the ground) to initiate deposition. Dunes exhibit a variety of shapes and sizes depending on factors such as wind direction variability, sand supply, wind speed, and vegetation cover. (This refers to Figure 6.14 showing types of dunes).
Common types of sand dunes include:
- **Barchans:** Crescent-shaped dunes with horns (tips) pointing downwind. They form where wind direction is constant, sand supply is moderate, and the surface is flat and barren.
- **Parabolic Dunes:** U-shaped dunes with horns pointing upwind, anchored by vegetation. They are often found in semi-arid or coastal areas where sand is being deflated from a source area and vegetation is present to stabilize the arms. They can be considered 'reversed' barchans in terms of horn orientation relative to wind.
- **Seif Dunes (Longitudinal Dunes):** Long, linear ridges of sand aligned parallel to the dominant wind direction. They form in areas with less abundant sand supply and consistent wind direction, sometimes with secondary wind directions modifying their shape (a seif can resemble a barchan with only one extended arm).
- **Transverse Dunes:** Long, linear ridges of sand oriented perpendicular (at right angles) to the prevailing wind direction. They form in areas with abundant sand supply and relatively constant wind direction, often where a large sand source is oriented perpendicular to the wind.
When sand supply is very high, dunes can merge into complex forms, losing their distinct shapes. Most sand dunes are mobile and migrate over time as wind transports sand from the upwind side (stoss slope) to the downwind side (slip face). Some dunes can become stabilized by vegetation, particularly near human settlements.
Exercises
Multiple Choice Questions
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Answer The Following Questions In About 30 Words
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Answer The Following Questions In About 150 Words
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Project Work
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