The denudational process comprises several key components. Weathering initiates the breakdown of rocks and minerals directly on the Earth’s surface, occurring in three main forms: physical (or mechanical) weathering, which involves disintegration without chemical alteration; chemical weathering, which involves decomposition through chemical reactions, particularly prevalent in humid climates; and biological weathering, caused by living organisms. Following this initial breakdown, erosion takes over, dislodging and carrying away the weathered material. Finally, transportation moves these eroded materials across distances, culminating in deposition, where the material is laid down in new locations, thereby building up new topographical features.
Various natural agents drive these denudational processes. Running water, primarily rivers, plays a significant role, shaping distinct landforms across their upper (young), middle, and lower (old) courses through varied erosion and deposition patterns. Wind is particularly effective in arid regions, causing deflation and abrasion and creating features like sand dunes. Glaciers, massive ice bodies, exert considerable erosional and depositional force in cold environments. Lastly, waves constantly reshape coastal areas through their erosive, transportive, and depositional actions. It’s crucial to understand that while weathering is an integral part of denudation, it’s not the entire process; denudation encompasses weathering alongside erosion, mass movement, and the subsequent transportation and deposition of materials, all working collectively over extended periods to alter the Earth’s surface.
I. Short Answer Questions
Question 1.
What do you mean by denudation ?
Ans:
Denudation is a comprehensive term in geography and geology that refers to the long-term wearing away and stripping of the Earth’s surface by natural processes. Imagine it as the constant, slow-motion sculpturing of the land, driven by various forces that break down, move, and redistribute geological material.
Here’s a breakdown of what that entails:
- Moving (Erosion and Transportation): Once material is broken down, it’s carried away by various agents. Think of rivers carrying sediment, wind blowing sand, glaciers dragging rocks, or waves pounding coastlines and shifting debris. This movement is what defines erosion.
- Relocating (Deposition): Eventually, the transported material loses energy and is laid down in a new location. This can form new landforms, like river deltas, sand dunes, or coastal beaches.
- Gravity’s Role (Mass Movement): Sometimes, large masses of earth and rock move downslope under the direct influence of gravity, such as landslides or mudflows. This is another significant component of denudation.
Question 2.
Name important factors which affect work of a river.
Ans:
Rivers are dynamic forces of nature, constantly shaping the landscape through erosion, transportation, and deposition. Several key factors dictate how a river performs these tasks:
Factors Influencing River Work
- Volume of Water (Discharge): The amount of water a river carries is paramount. More water means more energy for the river to erode its bed and banks, transport sediment, and deposit material, especially during floods.
- Gradient (Slope): This increased velocity translates to greater erosional power, particularly for cutting deeper into the landscape. Gentle slopes, conversely, encourage deposition.
- Nature of Underlying Rock/Soil: The type of material the river flows over significantly impacts erosion. Softer rocks are easily eroded, while harder rocks resist the river’s force, sometimes leading to features like waterfalls where resistant layers meet less resistant ones.
- Sediment Load: A river’s ability to erode and carry more material is limited by the amount of sediment it’s already transporting. If it’s overloaded, its erosional capacity drops, and it will start depositing.
- Vegetation Cover: Plants along river banks and in the surrounding area stabilize the soil, reducing erosion and runoff. A lack of vegetation, often due to deforestation, can drastically increase erosion and sediment in the river.
- Climatic Conditions:
- Precipitation: The amount and intensity of rain or snow directly influence a river’s discharge and potential for flooding, thus affecting its erosional power.
- Temperature: Can influence weathering processes like frost action, particularly in colder climates.
- Base Level: This is the lowest point a river can erode to, typically sea level. Changes in base level, perhaps due to tectonic activity, can dramatically alter a river’s gradient and thus its erosional or depositional behavior.
- Human Activities: Our actions profoundly impact rivers. Dams alter flow and trap sediment, deforestation and urbanization increase runoff and erosion, and mining and agriculture can add significant sediment and pollutants. Channelization changes a river’s natural course and velocity.
Question 3.
What is the main function of a river in its upper course in land formation ?
Ans:
Rivers in their upper course are characterized by their powerful ability to sculpt the landscape through intense vertical erosion. This means the primary action of the river is to cut deeply downwards into its bed rather than widening its banks.
This dominant downward cutting is a direct result of the river’s environment in its upper reaches. Flowing through steep, mountainous, or hilly terrain, the water gains significant speed and energy. This high velocity allows the river to efficiently carve into the underlying rock and sediment.
The most prominent landform created by this relentless vertical erosion is the V-shaped valley. As the river continuously deepens its channel, the steep valley sides are simultaneously exposed to the elements, undergoing weathering and mass movement (such as rockfalls). The swiftly moving water then carries away this eroded material, maintaining the distinctive narrow, deep, and steeply sided “V” profile of the valley.
Furthermore, the variations in rock resistance within the riverbed can lead to the dramatic formation of waterfalls and rapids. When a river encounters a layer of hard, resistant rock overlying softer, more easily eroded rock, the softer material is worn away more rapidly.Rapids, on the other hand, develop where the river flows over a series of resistant rock outcrops, causing the water to become turbulent and swift.
Question 4.
Name two important land forms of river erosion.
Ans:
V-shaped Valleys: These distinctive valleys are a testament to the powerful erosive force of a river in its youthful stage. Imagine a deep, incisive cut into the landscape, with slopes that converge sharply towards the riverbed, much like the letter ‘V’. The river’s primary mode of operation here is aggressive downward erosion, relentlessly grinding into the underlying rock with the aid of abrasive sediments carried by its swift current. Concurrently, gravity plays a crucial role: as the river deepens its channel, the exposed valley sides become unstable. Processes such as landslides and the slow, steady creep of soil and rock cause material to slump or fall into the river, where it’s then transported downstream. This continuous removal of material from the flanks, combined with the persistent downcutting, meticulously crafts the characteristic ‘V’ profile.
Waterfalls: A waterfall is a striking geological feature where a river’s flow plunges abruptly over a vertical or near-vertical drop in its bed. These dramatic cascades typically arise from differential erosion, specifically where a resilient layer of rock overlies a softer, more easily eroded stratum. As the river courses over this geological arrangement, it preferentially wears away the less resistant rock beneath the harder caprock. This differential erosion leads to the undercutting of the more durable rock, creating an overhang. Eventually, due to the forces of gravity and the sheer weight of the unsupported rock, this overhang fractures and collapses.
Question 5.
How is a waterfall formed ?
Ans:
Waterfalls, striking features of riverine landscapes, form where a river’s course encounters a sudden, steep drop in elevation. Their creation is a fascinating interplay of varying rock resistances and the relentless erosive force of flowing water.
Envision a river traversing terrain made up of diverse rock strata – some exceptionally hard and durable, others comparatively soft and easily eroded. The river, a tireless natural agent of change, selectively excavates the less resistant rock situated downstream of a more resilient layer. This preferential erosion of the softer material beneath the tough caprock leads to the formation of an indentation, leaving the harder rock projecting outwards as an unsupported overhang.
As the softer rock continues to yield to the relentless watery assault, and its underlying support progressively weakens, the cantilevered, harder rock layer eventually succumbs to the forces of gravity, fracturing and collapsing into the turbulent plunge pool. This event causes the waterfall’s crest to shift upstream. Through the cyclical repetition of this undercutting and collapse over vast geological periods, the waterfall gradually retreats backward, leaving in its wake a deep, often narrow valley or gorge – an enduring testament to its upstream migration.
Question 6.
State the main function of a river in its middle course.
Ans:
In its middle course, a river’s dominant activities transition to transportation of sediment and lateral erosion. While some initial signs of deposition may emerge, and the intensity of downward (vertical) erosion considerably diminishes, the river’s main purpose becomes the conveyance of the material it collected in its upper reaches. Simultaneously, it actively erodes its banks from side to side. This sideways erosion contributes to the expansion of the valley and the development of distinct features such as meanders. The river’s gradient flattens, resulting in a reduced flow velocity compared to its upper course, yet it retains sufficient energy to move a substantial volume of sediment.
Question 7.
How are meanders formed ?
Ans:
Rivers in their mature stages often develop meanders, graceful curves shaped by the interplay of water flow, erosion, and deposition.
The process starts with slight irregularities in the riverbed, directing the faster current (the thalweg) towards one bank. This initiates a continuous cycle:
- Concurrently, on the inner bank, the reduced water velocity leads to deposition of sediment, forming a crescent-shaped point bar.
As the outer bend erodes and sharpens, it intensifies the current’s impact, further accelerating erosion and subsequent deposition. This continuous feedback loop transforms minor curves into distinct meanders.
Meanders are not static; they constantly migrate laterally across the floodplain as the cut bank erodes. This lateral movement is key to shaping wide, flat floodplains.
Several conditions favor meander formation, including gentle slopes, the presence of fine sediments, moderate water flow, and the availability of a floodplain for migration.
Question 8.
Name two landforms in the lower course of a river.
Ans:
Two common landforms found in the lower course of a river are:
- Deltas: The river’s velocity decreases significantly, causing it to deposit the sediment it has been carrying, forming a network of distributaries and accumulating land.
- Oxbow Lakes: These are crescent-shaped lakes formed when a meander (a bend in the river) becomes more exaggerated over time. Eventually, the neck of the meander is cut off as the river takes a straighter course, leaving the old bend isolated as a lake.
Question 9.
Where is wind erosion most predominant ?
Ans:
Wind erosion is most predominant in arid and semi-arid regions, commonly known as deserts and areas bordering them.
Here’s why these environments are particularly susceptible:
- Lack of Vegetation: Deserts naturally have sparse or no vegetation. Plant cover acts as a protective barrier, holding soil in place with its roots and reducing wind speed at the ground level. Without this cover, the soil is exposed and vulnerable.
- Dry, Loose Soil: Arid regions receive very little rainfall, leading to dry and unconsolidated soil particles.
- Strong Winds: Many desert environments experience strong, persistent winds. These winds have the energy to detach and transport large quantities of fine soil particles (like sand and dust).
- Absence of Moisture: Moisture in the soil helps bind particles together, making them heavier and more resistant to wind. In dry environments, this binding agent is largely absent.
Question 10.
What are known as Deflation Hollows ?
Ans:
Deflation hollows, also known as blowouts, are shallow, basin-shaped depressions formed by the erosional action of wind. They are a characteristic landform primarily found in arid and semi-arid regions, as well as coastal areas with loose, unconsolidated sediments like sand.
Here’s a breakdown of their formation and characteristics:
Formation:
- Loose Material: Deflation hollows typically begin in areas with loose, dry, and fine-grained material, such as sand, silt, or weathered rock fragments, that is not held together by significant vegetation.
- Wind Erosion (Deflation): Strong, persistent winds lift and carry away these fine particles. This process, known as deflation, literally “blows out” the material from the surface.
- Turbulence and Eddies: As the wind blows, it creates turbulent eddies and localized currents that concentrate the erosional force in specific spots. These eddies can scour out initial small depressions.
- Deepening and Widening: Once a small depression forms, it becomes more susceptible to further wind erosion. The wind can funnel into the hollow, increasing its velocity and efficiency in removing more material. This continuous removal of particles causes the hollow to deepen and widen over time.
- Armor Layer: The process of deflation often continues until the wind reaches a layer of heavier, coarser material (like gravel or pebbles) that it cannot lift and transport. This coarser material forms a protective “armor layer” or “desert pavement” at the base of the hollow, which resists further erosion and prevents the hollow from deepening indefinitely.
Characteristics:
- Shape: They are typically saucer-shaped or basin-shaped depressions.
- Location: Most commonly found in deserts, semi-arid regions, dry lake beds (playas), and coastal dune systems.
- Vegetation (Limited): Due to the constant wind erosion and often arid conditions, vegetation within deflation hollows is usually sparse or absent, although some may develop around oases if the water table is reached.
- Armored Surface: The floor of a deflation hollow often consists of a lag deposit of coarser sediments, forming a “desert pavement” that protects the underlying material from further wind erosion.
Question 11.
Name one chief landform of wind deposition ?
Ans:
sand dune
Question 12.
Name two chief types of sand dunes.
Ans:
The two primary forms of sand dunes are:
- Barchan Dunes (or Crescentic Dunes): These distinctive dunes exhibit a crescent or arc shape, with their bowed, convex side facing the direction from which the wind blows. Their two pointed “horns” or tips extend downwind.
- Seif Dunes (or Longitudinal Dunes): Characterized by their extended, linear ridge-like structure, Seif dunes align themselves parallel to the dominant wind flow. They tend to develop in areas experiencing robust winds and a moderate supply of sand, often as a result of the interaction of two subtly varying primary wind directions.
Question 13.
State three chief characteristics of barchans.
Ans:
Here are the three main characteristics of barchans, also known as crescentic dunes:
Distinct Crescentic Form
Barchans are immediately recognizable by their unique crescent or U-shape, resembling a half-moon. The convex, or outward-curving, side of the dune faces directly into the wind’s path. Conversely, the concave, or inward-curving, side forms the slip face on the sheltered, leeward side. A key indicator of wind direction is that the two “horns” or tips of the crescent consistently point in the direction the wind is blowing, showcasing the movement of the sand.
Asymmetrical Slopes
These dunes possess a clear asymmetrical profile when viewed in cross-section. The windward slope, which is the side exposed to the incoming wind, is long and gently inclined, typically between 10 to 15 degrees. Sand is continuously driven up this gradual incline by the wind. In stark contrast, the leeward side, or slip face, is considerably steeper, reaching an angle close to the natural angle of repose for sand, usually around 30 to 34 degrees. As sand accumulates at the dune’s crest, it spills down this steep face, facilitating the dune’s forward movement.
Independent Migration
Barchans are not static features; they are remarkably mobile and traverse desert environments as independent formations. Propelled by steady, unidirectional winds, sand is lifted from the gentle windward slope, carried over the dune’s peak, and then deposited onto the steep slip face. This continuous cycle enables the entire dune to migrate across the desert floor. Their tendency to form where sand is scarce often allows them to move without merging into larger, more complex dune systems, although they can occasionally combine under specific environmental conditions.
II. Long Answer Questions
Question 1.
Briefly describe the factors which affect the work of a river.
Ans:
The effectiveness of a river in shaping the landscape through erosion, transportation, and deposition is influenced by several key factors:
- Volume of Water (Discharge): A greater volume of water in a river (higher discharge) translates to more energy. This increased energy allows the river to erode more material, carry a heavier sediment load, and deposit finer sediments further downstream. Conversely, rivers with low discharge have limited capacity for significant work.
- Gradient (Slope): A steeper gradient leads to faster flow, which in turn enhances the river’s erosional power and its ability to transport larger, heavier particles. As the gradient lessens, the river’s velocity decreases, promoting deposition.
- Velocity of Flow: This is a critical factor, as a small increase in velocity dramatically boosts a river’s competence (ability to transport larger particles) and capacity (total sediment load it can carry). Higher velocity enables stronger abrasion of the river bed and banks, efficient lifting and carrying of sediments, and better sorting of material during deposition.
- Nature of the Load: The amount, size, and type of sediment (the “load”) a river carries affect its work. A river heavily laden with coarse, angular sediments will have greater abrasive power, leading to more erosion of its bed and banks. Conversely, if the river is carrying a very fine, suspended load, its erosional power might be reduced, and it will tend to deposit these fine materials more readily.
- Nature of the Underlying Rock: The geological structure and composition of the rocks over which a river flows significantly influence its erosional patterns. Softer, less resistant rocks (like shales or sandstones) are easily eroded, leading to wider valleys and more rapid downcutting. Harder, more resistant rocks (like granite or basalt) impede erosion, often resulting in narrower gorges, rapids, or waterfalls, and dictate the long-term course of the river.
- Vegetation Cover: The presence or absence of vegetation along the river banks and in its catchment area plays a crucial role. Deforestation, on the other hand, can lead to increased runoff, higher sediment loads, and greater bank erosion, making the river’s work more destructive.
Question 2.
Describe chief landforms of river deposition.
Ans:
Rivers, as they descend from elevated regions to the sea, carry a significant volume of eroded material. Their energy to transport this load diminishes with decreasing velocity, particularly in their middle and lower stretches, leading to the laying down of sediments. This process sculpts the landscape, creating distinct landforms. Here are some of the prominent landforms shaped by river deposition:
Floodplains
These are expansive, flat tracts of land situated alongside a river channel, constructed from the accumulation of fine sediments like silt and clay during periods of high water. When a river exceeds its banks, the water spreads out, loses momentum, and releases its suspended particulate matter. Over successive flood events, this leads to a buildup of fertile alluvium, rendering floodplains exceptionally productive for agriculture.
Natural Levees
These appear as slightly elevated, elongated ridges or embankments running parallel to the river’s course within a floodplain. Their formation is a result of floods. As the river overflows, coarser, heavier sediments, typically sand, are immediately deposited along the riverbanks. Finer sediments, conversely, are carried further across the floodplain. Through numerous floods, these deposits aggregate, forming natural levees that often serve as protective barriers against minor inundations.
Point Bars
As the river navigates a bend, water velocity is higher along the outer bank, leading to erosion, while it is lower along the inner bank. This reduction in velocity on the inner bank prompts the river to deposit its sediment, progressively building up the point bar.
Oxbow Lakes
These are crescent-shaped bodies of water that form when a meander in a river becomes disconnected from the main flow. Continuous erosion on the outer bank and deposition on the inner bank gradually narrow the neck of a meander. Eventually, often during a flood, the river may cut directly across this constricted neck, forging a new, straighter channel.
Alluvial Fans
These are fan-shaped deposits of sediment that develop where a swift-flowing river or stream exits a confined valley, such as a mountain canyon, and debouches onto a flatter, more expansive plain. With the abrupt decrease in gradient, the river rapidly loses energy, leading to the immediate deposition of its coarse sediment load, including gravel, sand, and boulders, in a radiating, fan-like pattern from the valley’s opening.
Deltas
Upon reaching a standing body of water, the river’s velocity sharply diminishes, causing the deposition of its entire sediment load. This sediment accumulates over time, forming new land that extends into the water body. Deltas are typically characterized by a complex network of distributaries, which are smaller channels branching off the main river. Their specific shape can vary, influenced by factors like the river’s sediment volume, wave action, and tidal currents, resulting in forms like arcuate, bird’s foot, or estuarine deltas.
Question 3.
Write a short note on the three stages of a river.
Ans:
Upper (Youthful) Course
This high-energy environment creates characteristic landforms such as V-shaped valleys, dramatic waterfalls, turbulent rapids, and winding interlocking spurs. The river efficiently transports coarse materials like boulders.
Middle (Mature) Course
As the gradient lessens, the river’s energy decreases. The focus shifts to lateral erosion and the transportation of finer sediments. This leads to the development of wider valleys and prominent, snaking meanders. Early floodplains begin to form, along with natural embankments called levees, built up by flood deposits.
Lower (Old) Course
Nearing sea level, the river’s gradient becomes very gentle, and its velocity drops significantly. Deposition becomes the dominant process, as the river sheds its remaining sediment load. This results in expansive floodplains, crescent-shaped oxbow lakes (formed from cut-off meanders), and large, often triangular deltas at the river’s mouth, where distributaries branch out to spread water and sediment.
Question 4.
Describe the work of wind erosion and state two important landforms of wind erosion.
Ans:
Wind, a potent sculptor in arid and semi-arid environments, carves distinctive features into the Earth’s surface through two primary mechanisms:
Deflation Deflation is the process by which wind dislodges and transports fine, loose particles such as dust and silt from the ground. Over time, this consistent removal of surface material can lead to the formation of shallow depressions called deflation hollows or blowouts, where the ground level has been noticeably lowered by wind action.
Abrasion Abrasion occurs when wind, carrying an abundance of sand and other abrasive particles, grinds against exposed rock surfaces. This natural “sandblasting” effect smooths and sculpts rocks, resulting in the creation of unique geological formations.
Prominent Wind-Eroded Landforms Two striking landforms shaped by wind erosion are:
Mushroom Rocks (Pedestal Rocks): These are easily recognizable rock formations with a resemblance to mushrooms, featuring a narrower base and a wider top. Their unique shape is a result of more intense wind abrasion occurring nearer to the ground, where the concentration of wind-borne sand is highest. The upper parts of the rock, being less exposed to this concentrated abrasive force, retain their greater width, creating the characteristic mushroom-like profile.
Yardangs: These are elongated, aerodynamic ridges of rock or consolidated material that are sculpted by sustained wind abrasion. They typically orient themselves parallel to the direction of the prevailing wind and can range significantly in size, from just a few meters to several kilometers in length.
