Landforms and Their Evolution

DEFINITION OF LANDFORMS AND EVOLUTION

**Landforms** are small to medium-sized tracts or parcels of the earth's surface, each with distinctive physical shape, size, and materials. They result from the action of specific geomorphic processes and agents.

**Landscape** refers to several related landforms together forming large tracts of the earth's surface. A landscape is a combination of multiple landforms acting in unison.

**Evolution of landforms** implies the stages of transformation of either a part of the earth's surface from one landform type into another, or transformation of individual landforms after their formation. Every landform has a history of development and changes through time—comparable to life stages: **youth, mature, and old age**.

**Key exam concept**: Landforms are dynamic, not static. They continuously change due to:

Continued action of geomorphic processes (weathering, erosion, deposition)

Changes in climatic conditions

Vertical or horizontal movements of landmasses

Changes in intensity or type of processes acting upon them

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RUNNING WATER AS A GEOMORPHIC AGENT

In **humid regions with heavy rainfall**, running water is the most important geomorphic agent in land degradation. It operates through two components:

1. **Overland flow**: Sheet flow of water across general land surfaces

2. **Linear flow**: Concentrated flow in streams and rivers within valleys

Relationship Between Gradient and Stream Action

**Steep gradients**: Vigorous youthful rivers with rapid erosion, minimal deposition, V-shaped valleys

**Gentle gradients**: Slow-moving rivers with active lateral erosion and significant deposition; rivers develop meanders

**Gentler river channels**: As channels become gentler due to continued vertical erosion, downward cutting becomes less dominant and **lateral erosion of banks increases**, reducing hills and valleys to plains

Sheet Erosion and Valley Formation

**Overland flow causes sheet erosion**. Water flowing over irregularities concentrates into paths, removing materials through friction of the water column:

Narrow rills (small channels) → gradually develop into **gullies** → **gullies deepen, widen, and lengthen** → network of valleys

**Early stages**: Down-cutting dominates; waterfalls and cascades removed

**Middle stages**: Slower bed-cutting; severe lateral valley-side erosion; divides lower

**Final stage**: Low relief lowland with faint topography, except for **monadnocks** (low resistant remnants standing out)

**Peneplain**: A plain formed by stream erosion representing the final stage of landscape reduction. The term means "almost plain."

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STAGES OF LANDSCAPE DEVELOPMENT IN RUNNING WATER REGIMES

YOUTH STAGE

**Stream characteristics**: Few streams with poor integration; flow over original slopes

**Valley shape**: Shallow **V-shaped valleys** with **no floodplains or very narrow floodplains** along trunk streams only

**Stream divides**: Broad and flat with marshes, swamps, lakes

**Meanders**: If present, develop over broad upland surfaces; may eventually entrench into uplands

**Notable features**: Waterfalls and rapids present where hard rock bodies exposed

**Landscape appearance**: Steep, rugged, high relief terrain

MATURE STAGE

**Stream characteristics**: Plenty of streams with good integration

**Valley shape**: Still V-shaped but deep; trunk streams broad enough to have **wider floodplains**

**Floodplain streams**: Meanders confined within valleys

**Stream divides**: Sharp (no longer flat and broad); marshes and swamps disappear

**Notable features**: Waterfalls and rapids disappear

**Landscape appearance**: Moderate relief with well-defined valleys and divides

OLD AGE STAGE

**Stream characteristics**: Fewer, smaller tributaries with gentle gradients

**Valley shape**: Broad, gentler valleys

**Meanders**: Rivers meander freely over vast floodplains with natural levees and **oxbow lakes**

**Stream divides**: Broad and flat with lakes, swamps, marshes

**Overall landscape**: Most at or slightly above sea level; very low relief; gentle slopes dominant

**Deposition dominance**: Depositional features far exceed erosional features

**Exam important**: Use these three stages to analyze river landscape development in case studies of Indian rivers.

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EROSIONAL LANDFORMS DEVELOPED BY RUNNING WATER

VALLEYS

**Definition**: Valleys are elongated depressions in the landscape formed by the continuous action of streams, beginning as rills and developing into major drainage features.

**Valley types based on shape and dimensions**:

1. **V-shaped valley**: Narrow valley with steep sides at angles; characteristic of young, fast-flowing rivers with vertical erosion dominant (Example: Upper Sutlej River)

2. **Gorge**: A deep valley with **very steep to straight sides**, almost equal in width at top and bottom. Forms in **hard, resistant rocks** where vertical erosion is concentrated and lateral erosion is minimal.

3. **Canyon**: Deep valley with **step-like side slopes** (not vertical); **wider at top than at bottom**. A variant of gorge. Forms commonly in **horizontal bedded sedimentary rocks** where differential erosion occurs along bedding planes (Example: Colorado River canyon, USA)

**Key difference**: Gorges maintain uniform width; canyons widen toward the top due to step-like erosion of horizontal rock layers.

POTHOLES AND PLUNGE POOLS

**Potholes**: More or less **circular depressions** formed on rocky hill-stream beds through stream erosion aided by abrasion of rock fragments (pebbles, boulders).

**Formation process**:

Small shallow depression forms initially

Pebbles and boulders collect in the depression

Flowing water rotates these materials (abrasion)

Depression grows in diameter and depth

Series of potholes join, deepening the stream valley

**Plunge pools**: Large, deep and wide potholes at the base of waterfalls, formed by:

Sheer impact of falling water

Rotation of large boulders at the base

Concentrated erosional energy

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INCISED OR ENTRENCHED MEANDERS

**Definition**: Very deep and wide meanders cut into hard rocks, showing the river's ability to erode vertically even in resistant material.

**Formation conditions**:

Rivers originally flowing over gentle slopes develop meandering courses

River continues meandering as the land is uplifted or climate changes

River maintains its meandering pattern while cutting down vertically through rocks

Creates meander loops cut deeply into hard rock

**Key characteristic**: Meanders appear as **deep loops carved into solid rock** rather than shallow loops on soft alluvium. The concave bank shows steep scarps; convex bank shows gentle profile.

**Example**: Colorado River showing entrenched meanders in canyon form; various Himalayan rivers showing incised meanders on uplift.

RIVER TERRACES

**Definition**: **Surfaces marking old valley floor or floodplain levels**, indicating former river bed elevations.

**Composition**:

Bedrock surfaces without alluvial cover, OR

Alluvial terraces consisting of stream deposits on bedrock

**Formation**: Result from vertical erosion by streams into their own depositional floodplain materials.

**Terrace types**:

1. **Paired terraces**: Terraces occurring at the same elevation on both sides of river, indicating synchronized erosion on both banks

2. **Unpaired terraces**: Terraces at different elevations on opposite banks, indicating lateral migration of river channel

**Significance**: **Multiple terraces at different heights indicate former river bed levels**, helping reconstruct river history and climate changes. Each terrace represents a stable period in river development followed by renewed downcutting.

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DEPOSITIONAL LANDFORMS DEVELOPED BY RUNNING WATER

ALLUVIAL FANS

**Definition**: Cone-shaped depositional landforms formed when streams flowing from higher elevations break onto foot slope plains of low gradient.

**Formation process**:

Streams from mountain slopes carry **very coarse sediment load** (boulders, gravel)

Load becomes too heavy for streams to carry over gentler gradients

Materials **dumped and spread** as broad cone-shaped deposit

Streams shift position across fan, creating **distributaries** (multiple channels)

**Characteristics**:

**Humid areas**: Low cones with gentle slope from head to toe

**Arid/semi-arid areas**: High cones with steep slopes

Sediment poorly sorted; coarsest materials deposited first

**Example**: Hill streams on way to Amarnath, J&K (as shown in NCERT); streams emerging from Aravalli and Vindhya ranges.

DELTAS

**Definition**: Depositional landforms similar to alluvial fans but develop where rivers meet the sea, where the river's sediment load is dumped into the sea.

**Formation process**:

Rivers carry load to sea level

Load not carried away far into sea or distributed along coast

Sediments spread and accumulate as **low cone into the sea**

**River distributaries continue to increase in length** as delta grows

**Characteristics of delta deposits** (vs. alluvial fans):

**Very well sorted sediments** with clear stratification

Coarsest materials settle first (closest to river mouth)

Finer fractions (silts, clays) carried far into sea

Progressive fining from apex to seaward margin

**Indian examples**:

Ganges-Brahmaputra Delta (world's largest)

Krishna River Delta, Andhra Pradesh

Godavari Delta

Mahanadi Delta

**Exam important**: Deltas are critical agricultural areas in India due to fertile alluvial deposits.

FLOODPLAINS, NATURAL LEVEES, AND POINT BARS

**Floodplain definition**: A major landform of river deposition; the relatively flat areas bordering rivers that are inundated during floods.

**Two types of floodplain deposits**:

1. **Active floodplain**: River bed made of river deposits; submerged during normal flow

2. **Inactive floodplain**: Above riverbank; inundated only during floods

**Inactive floodplain deposits** (two types):

**Flood deposits**: Fine materials (silt, clay) from spilled waters

**Channel deposits**: Coarse deposits from abandoned or cut-off channels where lateral shifting occurred

**Natural levees**:

**Definition**: Low, linear, parallel ridges of **coarse deposits** along riverbanks

**Formation**: Coarser sediments deposited near channel during overbank flow; become higher near channel, lower away from channel

**Structure**: Often cut into individual mounds

**Location**: Characteristic of large river floodplains

**Significance**: Provide natural barriers reducing flood spread inland; may be built upon or reinforced for flood protection

**Point bars** (meander bars):

**Location**: Found on **concave side of meanders** of large rivers

**Composition**: Mixed sizes of sediments

**Formation**: Deposited in linear fashion by flowing waters along bank

**Shape**: Almost uniform in profile and width

**Opposite feature**: Concave bank (outside meander) shows erosion and scarp formation

**Key difference exam question**: Natural levees are **coarse deposits along outer channel banks** forming ridges; point bars are **mixed sediment deposits on inner (concave) meander sides**.

MEANDERS

**Definition**: Loop-like channel patterns developed by rivers, not a landform itself but a **type of channel pattern**.

**Why meanders form** (three reasons):

1. **Propensity of water** flowing over very gentle gradients to work laterally on banks (lateral erosion exceeds vertical)

2. **Unconsolidated nature** of alluvial deposits making bank materials easily erodible with many irregularities

3. **Coriolis force** acting on fluid water, deflecting it (like deflecting wind)

**Development process**:

**Initial stage**: Slight irregularities along banks slowly transformed into small curvature

**Deepening**: Curvature deepens due to:

Deposition on inside (convex) of curve

Erosion/undercutting along outside (concave) of curve

**Without erosion/deposition**: Meander tendency reduced

**Large river meanders**: Active deposition on convex bank; undercutting on concave bank

**Meander morphology**:

**Concave bank**: Known as **cut-bank or cut-off bank**; shows steep scarp; erosion dominant

**Convex bank**: Shows long, gentle profile; deposition dominant (point bar)

Both banks have different slopes and sediment characteristics

**Meander growth and oxbow lake formation**:

As meanders grow into deep loops, they eventually get cut-off

**Cut-off mechanism**: Erosion at inflection points (where meander curves reverse)

**Oxbow lakes**: When meander loops are abandoned, they form crescent-shaped lakes

**Process**: Cut-off channels gradually get filled with sediments; become isolated from main river

**Example**: Burhi Gandak River near Muzaffarpur, Bihar showing multiple oxbow lakes and cut-offs (visible in satellite imagery).

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GROUNDWATER AS A GEOMORPHIC AGENT

**Focus**: Not on groundwater as resource, but on its role in **erosion of landmasses and evolution of landforms**.

**Groundwater movement and percolation**:

Surface water percolates well in **permeable, thinly bedded, highly jointed and cracked rocks**

**Vertical movement**: Water goes down to certain depth

**Horizontal movement**: Water flows horizontally through bedding planes, joints, or materials themselves

**Erosive action**: This downward and horizontal movement causes rock erosion

**Limitations of groundwater erosion**:

**Physical/mechanical removal** of materials by moving groundwater is **insignificant** in developing major landforms

Results of groundwater work **cannot be seen in all rock types**

Exception: **Limestone and dolomite rocks**

**Limestone/Dolomite erosion by groundwater**:

Rocks rich in calcium carbonate

**Surface water AND groundwater** through **chemical solution and precipitation processes** develop distinctive landforms

**Solution process**: Carbonated water (CO₂-rich rainwater) dissolves calcium carbonate

**Precipitation process**: Calcium carbonate deposits when water evaporates or loses CO₂

**Karst topography**:

**Definition**: Any limestone or dolomitic region showing typical landforms produced by groundwater action through solution and deposition

**Named after**: Karst region in Balkans adjacent to Adriatic Sea

**Characteristics**: Distinctive erosional AND depositional landforms specific to limestone

**Significance**: Important terrain type worldwide; found in India (parts of Meghalaya, Karnataka, etc.)

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EROSIONAL LANDFORMS IN KARST TOPOGRAPHY

POOLS, SINKHOLES, LAPIES, AND LIMESTONE PAVEMENTS

**Swallow holes**:

Small to medium-sized **round to sub-rounded shallow depressions**

Form on limestone surfaces through **solution action**

Water disappears underground through these holes

**Sinkholes** (dolines):

**Definition**: Opening more or less **circular at top, funnel-shaped toward bottom**

**Size variation**:

Area: Few sq. meters to a hectare

Depth: Less than 0.5 meter to 30+ meters

**Formation types**:

1. **Solution sinks**: Form solely through solution action of groundwater on limestone

2. **Collapse sinks**: Start as solution forms; if sinkhole bottom forms roof of underground void/cave, it collapses leaving large opening into cave below

3. More common: **Solution sinks** than collapse sinks

**Characteristics and behavior**:

Often covered with soil mantle, appearing as shallow water pools

**Hazard**: Stepping over such pools causes sudden subsidence (like quicksand in deserts)

**Underground flow**: Surface runoff flows down swallow and sink holes as underground streams

**Re-emergence**: Water re-emerges downstream through cave openings

**Valley sinks (Uvalas)**:

**Formation**: When sink holes and dolines join together due to:

Slumping of materials along margins

Roof collapse of caves

**Shape**: Long, narrow to wide trenches

**Development**: Result of progressive coalescence of individual sinkholes

**Lapies** (karren):

**Formation**: As surface limestone eaten away by pits and trenches, becomes extremely irregular

**Structure**: Maze of **points, grooves, and ridges**

**Cause**: Differential solution activity along parallel to sub-parallel joints in limestone

**Progressive change**: Lapie field may eventually turn into somewhat smooth **limestone pavements**

**Appearance**: Resembles stone pavement with blocks and joints visible

**Limestone pavements**:

Later stage of lapies development

Relatively smooth compared to early lapie terrain

Resistant calcite blocks separated by solution-widened joints

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CAVES

**Formation conditions**:

Alternating beds of rocks (shales, sandstones, quartzites) with limestones/dolomites interbedded, OR

Dense, massive limestone occurring as thick beds

**Cave formation process**:

**Percolation**: Water percolates down through materials or through cracks and joints

**Horizontal flow**: Water moves horizontally along bedding planes between rock layers

**Solution**: Limestone dissolves along bedding planes

**Result**: Long and narrow to wide gaps called **caves**

**Cave characteristics**:

**Multiple caves**: Maze of caves at different elevations depending on limestone beds and intervening rocks

**Opening**: Caves normally have opening(s) through which cave streams discharge

**Tunnel caves**: Caves having openings at both ends called **tunnels**

**Stream caves**: Underground streams flow through caves

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DEPOSITIONAL LANDFORMS IN KARST TOPOGRAPHY

**General principle**: Calcium carbonate (chief chemical in limestone) is **easily soluble in carbonated water** (CO₂-absorbed rainwater). It deposits when:

Water carrying it in solution **evaporates**, OR

Water **loses CO₂** as it trickles over rough rock surfaces

STALACTITES, STALAGMITES, AND PILLARS

**Stalactites**:

**Appearance**: Hang like **icicles** of varying diameters

**Shape**: Broad at bases, taper toward free ends; variety of forms

**Location**: Hang from cave ceiling

**Formation**: Calcium carbonate precipitation from dripping water

**Growth direction**: Downward

**Stalagmites**:

**Appearance**: Rise up from cave floor

**Shape**: May form columns, discs, or crater-like depressions; smooth, rounded, or bulging ends

**Formation**: Caused by **dripping water** from surface above or through thin pipe of stalactite above it

**Growth direction**: Upward

**Growth point**: Directly below stalactite

**Pillars/Columns**:

**Formation**: When stalactites and stalagmites **fuse/meet** vertically

**Result**: Continuous column of varied diameters extending from floor to ceiling

**Strength**: More structurally complete than individual formations

**Key exam points**:

Stalactites hang tight to cave ceiling (memory: "t" for tight)

Stalagmites might reach ceiling (memory: "m" for might reach)

Both require calcium carbonate-rich water and chemical precipitation processes

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GLACIERS AND GLACIAL PROCESSES

**Glacier definition**: **Masses of ice moving as sheets** over land or as linear flows down mountain slopes in trough-like valleys.

**Glacier types**:

1. **Continental glaciers**: Vast sheets of ice covering large areas

2. **Piedmont glaciers**: Sheets spread over plains at foot of mountains

3. **Mountain glaciers** (valley glaciers): Linear flows down mountain slopes in broad trough-like valleys

**Movement characteristics**:

**Speed**: Slow movement, unlike water flow

**Rate variation**: Few centimeters to few meters per day, or even less or more

**Cause**: Gravity force

**Process**: Ice under pressure becomes plastic and flows downslope

**Glacial work on landscape**:

**Debris transport**: Rock fragments (boulders, stones, sand) get dragged along floors/sides of valleys

**Erosional capacity**: Great damage through **abrasion** (rubbing/grinding) and **plucking** (ice-wedging)

**Resistance**: Glaciers can damage even **un-weathered rocks**

**Landscape transformation**: Can reduce high mountains into low hills and plains

**Glacial landscape evolution**:

**As glaciers move**: Debris removed, divides lowered

**Final stage**: Slope reduced so much that glaciers stop moving

**Remaining features**: Mass of low hills, vast **outwash plains**, other depositional features

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EROSIONAL LANDFORMS IN GLACIATED REGIONS

CIRQUES

**Definition**: **Amphitheatre-shaped or bowl-shaped hollow** formed on mountain sides where glacier originates and moves downslope.

**Formation process**:

Glacier accumulates in hollow on mountain side

Ice under pressure begins moving downslope

**Plucking**: Ice freezes onto rock walls and floor, pulling rock fragments away as glacier moves

**Abrasion**: Rock fragments in glacier grind and scrape bedrock as glacier moves

**Deepening**: Continued plucking and abrasion deepen hollow progressively

**Steepening**: Headwall (back wall) steepens to near-vertical

**Characteristics**:

Steep headwall at back/upper end

Gentler floor where ice accumulates

Often contain **cirque lakes** (tarn) after glacier melts

May be connected by ridge called **arête** if two adjacent cirques erode toward each other

**Horn** (pyramid peak) forms when three or more cirques erode around a peak

U-SHAPED VALLEYS

**Formation**:

Glacier flows downslope through river-cut valleys

**Valley transformation**: River-carved V-shaped valleys become U-shaped

**Process**: Glacier erodes sides and floor uniformly as it moves, widening and deepening valley

**Steep sides**: Nearly vertical to very steep sides from plucking and abrasion

**Flat floor**: Relatively flat floor from abrasion

**Differences from river valleys**:

River valleys: V-shaped with narrow floor

Glacial valleys: U-shaped with wide, flat floor

Glacial valleys: Much wider and deeper due to glacier's erosive power

HANGING VALLEYS

**Definition**: Valleys of smaller tributary glaciers that enter main glacial valley at a much **higher elevation** than the main valley floor.

**Formation**:

Main glacier erodes main valley more deeply than tributary glaciers erode their valleys

When glacier melts, tributary valley is left "hanging" above main valley

Often form **waterfalls** where tributary streams drop to main valley

GLACIAL LAKES AND TARNS

**Formation**: Depressions in cirques or valley floors filled with water after glacier melts.

**Tarns**: Cirque lakes, formed in cirque depressions

**Moraine-dammed lakes**: Lakes held behind moraines across valleys

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DEPOSITIONAL LANDFORMS IN GLACIATED REGIONS

MORAINES

**Definition**: Accumulation of rock fragments of all sizes carried and deposited by glaciers.

**Moraine types by location**:

1. **Lateral moraines**:

Form along **sides of glacier** where rock fragments from valley walls fall onto glacier

Parallel ridges of debris on both sides of glacier

Become visible as glacier melts

2. **Medial moraines**:

Form where **two glaciers merge**, at contact zone between them

Appear as **stripe of dark debris** in center of merged glacier

Result from merging of lateral moraines from two glaciers

3. **Terminal moraine** (end moraine):

Formed where glacier **terminus (snout) stops advancing** or where it reaches its maximum extent

Large ridge of mixed debris (boulders, gravel, sand, clay) marking glacier's farthest advance

If glacier retreats and stops again, **recessional moraines** form parallel to terminal moraine

4. **Ground moraine**:

Fine material (till) left on landscape beneath glacier

Consists of pulverized rock, clay, silt, sand deposited as glacier melts

Forms gentle undulating terrain

**Moraine composition**: **Unsorted, unstratified material** (till) containing:

Boulders

Gravel

Sand

Silt

Clay

Unlike river deposits (sorted and stratified).

OUTWASH PLAINS (SANDURS)

**Definition**: Extensive plains formed by **meltwater and sediments flowing from glacier fronts**.

**Formation**:

Glacial meltwater carries sediments (sand, gravel) far from glacier

Water spreads out and **deposits sediments on wide plains**

Sediments **sorted and stratified** by flowing water (unlike moraines)

Multiple streams create braided river patterns across plains

**Characteristics**:

**Well-sorted** sediments (larger particles near glacier, finer downslope)

**Stratified** layers showing different deposition periods

**Braided drainage patterns** due to multiple channels in meltwater streams

Often very productive agricultural land

**Indian example**: Outwash plains at foot of Himalayas where glacial streams emerge.

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EXAM-IMPORTANT SUMMARY COMPARISONS

Running Water Landforms

| Stage | Valleys | Streams | Divides | Floodplains | Notable Features |

|-------|---------|---------|---------|-------------|------------------|

| **Youth** | V-shaped, shallow | Few, poorly integrated | Broad, flat | None/very narrow | Waterfalls, rapids; marshes/lakes |

| **Mature** | V-shaped, deep | Many, well-integrated | Sharp | Wide | No waterfalls/rapids |

| **Old Age** | Broad, gentle | Few, gentle gradient | Broad, flat | Vast | Oxbow lakes, meanders free |

Valley Types

**V-shaped**: Young, steep rivers; vertical erosion dominant

**Gorge**: Hard rocks; steep/straight sides; equal width top/bottom

**Canyon**: Sedimentary rocks; step-like sides; wider at top than bottom

Karst Landforms

**Solution features**: Sinkholes, swallow holes, lapies, limestone pavements

**Caves**: Solution along bedding planes; varied elevations

**Depositional features**: Stalactites (hang), stalagmites (rise), pillars (fused)

Glacial Features

**Erosional**: Cirques, U-shaped valleys, hanging valleys, tarns

**Depositional**: Lateral/medial/terminal/ground moraines, outwash plains

**Key difference**: Moraines are unsorted; outwash plains are sorted

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CRITICAL EXAM POINTS TO MEMORIZE

1. **Landform evolution** follows youth → mature → old age sequence with predictable changes in slope, stream patterns, and relief

2. **Running water** works through both erosion (vertical in youth) and deposition (lateral in mature/old age)

3. **Meanders form** only on gentle gradients where lateral erosion exceeds vertical erosion

4. **Deltas are sorted**, alluvial fans are unsorted depositional features

5. **Karst topography** specific to **limestone/dolomite** regions; requires chemical solution processes

6. **Glaciers erode U-shaped valleys** from river V-shaped valleys; create extensive depositional moraines

7. **Natural levees** = coarse ridges along banks; **point bars** = meander-side deposits

8. **Stalactites hang tight**; **stalagmites might reach** ceiling