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Page 1: Discipline course -1 Paper Lesson Developer: … The movement of weathered material under the influence of gravity and ... wasting is the downslope movement of soil or rock material

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Discipline course -1 Semester -1

Paper – Geomorphology Lesson- Mass Wasting

Lesson Developer:

Dr.Prabuddh Kr. Mishra

College /Department: Bhim Rao Ambedkar College University of Delhi

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Table of Contents

3. Geomorphic Process

3.2 Mass Wasting

1.1 Meaning and concept

1.2 Factors affecting mass waiting

1.3 Classification of Mass movement

Summery

Exercise

References

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Mass Wasting

1.1 Meaning and concept

The movement of weathered material under the influence of gravity and with or without

the influence of rainwater is called as mass movement or wasting. In other words, mass

wasting is the downslope movement of soil or rock material under the influence or gravity

without the direct aid of other medium such as water, air or ice, water and ice, however, are

frequently involved in mass wasting by reducing the strength of rock of soil and by

contributing to plastic and fluid behaviour of soil. Mass movements include both

detachment of rock materials and their down slope transport. The collective term for

gravitation or downslope movements of weathered rock debris is mass wasting.

R.J. Chorley has remarked that, "the relation between mass wasting and tectonics is a

relatively clear one. Where rocks are shattered, reliefs are high, this is where mass

movement is common and in fact, the denudation of high mountains may be the result of

mass wasting rather than fluvial or glacial process.”

Mass movement is a common phenomenon in all high and steep hill country and it can also

occur on very low-angle slopes. Very large land slides are most common in the technically

and seismically active belts of rising mountains chains where steep slopes, rapidly incising

rivers and glaciers in valley floors, jointed and fractured rock masses on slopes, severe

physical weathering, fluctuations in groundwater pressure, and many joints dipping steeply

out of the slopes, all contribute to instability.

1.2 Factors which results in mass wasting

1. Weathering reduces the shearing strength of materials by physical and chemical

changes through granular disintegration, hydration and expansion, saturation and

loss of compactness, drying and cracking etc. Original rocks materials may be

subject to mass wasting because of their composition. For example, rocks and

materials like schist, sand, gravel shale, soil etc. are easily sheared permitting fall,

slide, flow, creep or subsidence.

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2. The texture and structure of materials may also be conducive to mass wasting

because of their roundness or compaction.

3. Movements may takes place along the places of structural weakness such as

bedding planes, foliation, cleavage, fractures and Joints and faults.

4. Changes in environmental conditions including climatic factors and vegetational

cover may result in mass-wasting, (less vegetation cover, more lose will be the soil

and hence mass wasting also will be more).

5. Slope gradient is, always an important factor. Thus, a complex of factors in usually

involved in mass wasting, resulting in a variety of mass movement.

The role of water in the reduction of shearing strength is particularly important. This is often

referred to as lubrication of rock on weathered rocks debris. This really achieved through

the introduction of inter-granular forces by pore water pressure and the buoyant effect of

water. However, rapid mass wasting frequently takes place after heavy rains or rapid

melting of snow cover and does emphasize the effect of water on gravity movements (Fig.

4).

To watch video on mass wasting click on following web link: https://www.youtube.com/watch?v=yXaUbzVh4bI

1.3 Classification of Mass movement

Shapre (1938) made one of the first attempts at classification of the various types of mass

wasting and though several others have been proposed since then, his classifications are still

widely used. He recognized four major types of mass wasting which he called slow flowage,

rapid flowage, landslides and subsidence.

Hutchinson proposed a classification of mass movement on slopes based on mechanisms of

movement, morphology of the mass and the rate of movement and has identified as four

major types: creep, freeze, thaw movements, landslides mass movement involving sinking.

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A wide range of variations in terms of rate, direction and type of movements in noted in

mass movement in different places, having varying environmental conditions. In reality

mass movement have long preparatory period and there are certain precursor events which

herald the occurrence of mass movements but these are generally unnoticed. Most of mass

movements occur in mountainous areas and hence it is not possible to notice the precursor

events such as restlessness of animals, deserting of hives by bees etc. 'Hence, if a landslide

comes as a surprise to eye witnesses, it would be more accurate to say that the observers

failed to detect the phenomena which preceded the slide (R.J. Chorley).

Mass movements are generally classified on the basis of causative factors.

a) Rate of Movement;

b) Direction of movement;

c) Lubricating substance; eg. water, ice etc.

The direction of mass movement of rock waste down slope may be (i) vertical (ii) Diagonal (iii) Lateral.

Vertical mass movements are further divided into (a) rock fall, (b) collapse earth fall.

Diagonal mass movements is divided into (a) soil creep (b) rock creep 9 (c) talus creep (d)

rock slide (e) debris slide (f) slump (g) debris flow, (h) mud flow (i) solifluction (i) avalanches

etc. Lateral mass movement includes (a) block slide (b) spread (c) cambering (d) sacking etc.

Several types of mass wasting/movement are distinguished are as follow (Table 1 and Fig. 2).

Table.1 Classification of Mass Wasting

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(Source: http://itc.gsw.edu/faculty/bcarter/physgeol/mass/type1.htm)

Landslide

All types of mass movement of rock waste including soils and ice are collectively called as

landslides. Which are variously classified on different bases i.e. direction of movement, type

and rate of movement, nature of materials, presence or absence of lubrications.

Mass movements wherein a mass of rock or weathered debris moves downhill along

discrete shear surfaces in defined as slide. Slides are promoted by a host of controlling

variables such as nature of slopes (vertical and cliff slope is essential for slides), moderate

lubrication by water, earth tremors, gravity, vertical and steeply included rock beds, base

removal etc. Slides are more frequent in certain locations having favourable conditions viz.

(1) steep hill slopes or steep valley sides of streams (2) fault scarps (3) rejuvenated fluvially

eroded valleys (4) sea coasts (5) alluvial river valleys (6) degraded hills and mountains.

On the basis of nature of materials direction and rate of movement slides are divided into:

slumps, rock slides, debris slide and earth slide.

i. Slump

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Slump Rock Slump Debris Slump Earth Slum p

Slumping involves intermittent sliding of rock fragments, rock blocks or soils down slope

along a curved plane caused by rotational movement and displaced blocks (whether rock

blocks or soil blocks) cover very short distances. Slump is promoted by undertaking of slope

base (with hill slope or valley side slope of streams) by streams, sea waves and by human

activities. In fact 'Slump is the form of slide most common in thick, homogeneous, cohesive

materials such as clay. The surface of failure beneath a slump block is spoon shaped concave

upward or outward.

Slumping is consuming a large chunk of rich agricultural lands every year along the Ganga

Valley in U.P. and Bihar. Based on the nature of material involved slumps is subdivided into

rock slump, debris slump and earth slump.

ii. Rock slide

Rock slide is also known as rock glide or block glide. It is most significant of all types of slides

wherein large rock blocks slide down the hill slopes. 'Rocks slides may be very large and

catastrophic in mountainous regions where the large available relief permits accelerations

of rock debris to velocities as great as those of rock fall and rocks avalanches. Rock slides

include rapid movement of materials down slope. The cross ventre slide of 1925 in

Hong Kong- flats that should not have been built-

In June 1972 about 200 mm of rain fell in one day with similar amounts falling

on the following days. The result was disastrous. A landslide developed on

one of Hong Kong’s slope and within a few minutes it had cut across two

roads and demolished a high rise block of flats causing more than 50 deaths.

The landslide was triggered by the rapid convergence of rain water in the soil

on the upslope of the flats, causing the soil to became saturated and unstable.

(Bunnet, 1985)

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Wyoming, U.S.A. and Turtle mountain slide of 1903 in Alberta, Canada are typical examples

of devastating landslides (Fig.1).

Fig.1 The rock slide at Frank, Alberta, Canada (1903)

(Source: http://www.physicalgeography.net/fundamentals/10x.html)

iii. Debris Slide

It is more extensive and occurs at larger scale than slump but there is little amount of water.

Debris slide is promoted because of two basic factors (a) Saturation of rocks due to water

(2) Sudden down slope movement of unconsolidated mantle rocks. The materials involved

in debris slide are a mixture of soils and rock fragments (boulders). The debris collects at the

foot hill or the base of the valleys and forms interesting morphological features. Debris falls

are especially common along the undercut banks of streams.

Flow

A flow is usually faster than a creep and it generally takes place on slopes whose materials

consists of fine particles. In other words, diagonal down slope movement of rock fragments

and soils along sliding plane with enough water is called flow. Flow involves, down slope

raped movement of rock debris or soils saturated with water like viscous fluid. Dry flows in

sand or silt are known, but most flows are saturated with water.

Flow is further divided into:

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i. Debris flow

ii. Mud Flow

iii. Earth flow

iv. Solifluction flow

The difference between debris flow, earth flow and mud flow is related to size of particles

and amount of water. The three terms form a series of progressively higher water content

(i.e, water content increased from debris flow to mud flow) but are often used

interchangeably. Debris flows have 20-80% particles coarsen than sand sizes, whereas earth

flows and mud flows are 80% or more mud and sand. Mud flow in the most end member of

the series.

i. Debris flow

It involves down slope movement of enormous amount of various soils and boulders either

separately or mixed together and occurs mostly along river valley sides. Debris flow occurs

mostly due to viability of water, presence of loosely deposited soils and fine rock materials,

lack of vegetation cover, clay minerals in the soil etc. Debris flows range in size from a few

meters to over 1000m in width and may be ten meters thick in places; more commonly they

are 1 to 5 m thick. Debris flow is most common on gully heads is the riverine tracts of major

alluvial rivers.

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Fig. 2 Types of Mass wasting

(Source: http://www.gly.uga.edu/railsback/1121MassWstgFourplex2.jpeg)

ii) Mud flow

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Mud flow more rapidly enough to be perceptible to the eye, have higher water content than

earth flow, and are usually confined to channel. Mud flow is more common along valley

sides of alluvial rivers and the debris (mud) so produced is transported by rivers (Fig. 3).

The necessary conditions which promote mud flow include (a) steep and vertical slope; (b)

presence of unconsolidated materials, on the upper surface so that when they mixed with

water becomes viscous fluid and slippery; (c) Intermittent supply of sufficient water as

lubricant; and (d) absence of vegetation.

Based on the above factors, Elliot Black welder (1928) considered arid regions as most

favourable from mud flow. C.F.S. Sharpe (1938) has divided mud flow into 3 categories on

the basis of spatial characteristics eg. (a) mud flow of arid regions (b) Alpine mud flow and

(c) volcanic mud flow.

Mud flow also occurs in tundra regions during the early

summer when the frozen soil thaws and turns into a

semi-liquid state thus enabling it to slide over the still

frozen subsoil. This sometimes also called as solifluction.

Fig. 3 Mudflow in the San Juan Mountains, Colorado (Source: http://www.physicalgeography.net/fundamentals/10x.html )

iii) Earth flow

Did You Know Herculaneum, at the foot of Mt.

Vesuvius, was devastated by mud

flow at the same time as Pompeii

was buried beneath volcanic ash

during the eruption of the

volcano in AD 79.

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It is promoted by excessive water received mostly through rainfall so that the materials are

over saturated. Earth flow is more common on hillsides or valley side (having alluvium, rich

in clay minerals). Earth flows are likely to be confused with slumping and mud flows. They

are often accompanied by slumping but they differ from slumping in that there is no

backward rotation of the mass. They differ from mud flows in that (a) they are slower,

seldom being perceptible to the eye except to observations extending over several hours or

days; (b) they are not confined to channels as mud flows are; (c) there is a lower water

content than in mudflows; and (4) they are not specifically characteristics of dry region as

one mudflows, but they are more common in humid areas. Earth flow form on terraces

hillsides. They are common in Appalachian Plateau region.

d) Solifluction (Gelifluction)

Anderson (1906) from a study of "mud and glaciers" of Bear Island in the North Atlantic and

the 'Stone rivers' of the Falkland Islands in the South Atlantic, concluded that they were a

product of slow flowing from higher to lower grounds of masses of soil or earth saturated

with water." This method of gravitative transfer of mantle rock He called solifluction (solum,

Soil + fluere, to flow). Solifluction is also similar to higher latitudes. There are 4 conditions

one which promote it and (a) a good supply of water from the melting of snow and ground

ice (b) moderate to steep slopes relatively free of vegetation (c) the presence beneath the

surface of perennially frozen ground as it is called Scandinavian; and (e) rapid production by

weathering.

Solifluction is more slower and continuous movement; it is not confined to any channel. It

develops under severe subarctic or alpine climates rather than under arid on semi arid

climates as do mudflows. The topographic effects of solifluction are not striking because it

operates over the whole surface rather than being concentrated in channels. Locally it may

result in terraces like forms upon slopes or the fillings of smaller basins.

Falls

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Instantaneous fall of weathered rocks materials large blocks from steep hill slopes or earthen

materials from steep and clifted valley slides of streams under the influence of gravity is

called fall. The size of rocks fragments depends on the size and pattern of rock joints. This

type of movement involves vertical displacement of materials without water. The velocity of

all is greatest of all other types of mass movement. According to A.L. Bloom, 'Fall is a distinct

landslide process, but it is rarely independent of subsequent events. On the basis of

materials fall is subdivided into rock fall, debris fall and earth fall.

Fig. 4 Mass wasting in diagrams

(Source: http://soilerosion0918.wikispaces.com/Mass+Wasting)

Rock falls (The free falling of rock blocks over any steep slope)

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Rock falls are relatively small landslides confined to the removal of individual and superficial

blocks from a cliff base. Rock fall is facilitated by granular and block disintegration of rocks

under the processes of mechanical weathering and limited action of oxidation in

sandstones. Rock falls are features of all high and steep rock slopes and are one of the most

important processes in rock slope erosion.

They develop as physical weathering especially stress release joint opening and hydro

fracturing decrease the cohesion along joints. When fractures are sufficiently extended

blocks can fall. In so doing they may dislodge other fragments and produce an accumulation

of debris at the foot of the slope. They are most frequent in mountains areas during spring

months when there is a repeated freezing and thawing. As rock falls from a considerable

height they spread their debris over a large area unless the depositional zone is confined to

valley walls. (Because large rock falls and slides occur on the Moon it seems that water and

air are not necessary lubricants for such movements). As they bounce and hit the wall they

dislodge other rocks, sometimes starting landslides and leaving a cloud of dust along their

path. The frequency of rock falls depend upon certain environmental conditions such as

aridity/humidity factor, lithological and structural characteristics of rocks nature of slope

and vegetation etc. (Fig. 5).

In humid areas rock falls are very common features but in hot arid areas they are of very

rare occurrence. They may result in loss of lives and severe damage to property.

Fig. 5 Rockfall from steep hillsides

(Source: http://www.physicalgeography.net/fundamentals/10x.html)

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Debris fall

The nearly free fall of earth debris from a vertical or overhanging face. It involves rapid rate

of fall of weathered rock materials (which are finer than the materials involved in rock fall),

down slope (it may be hill slope or steep valley side slope of streams) from great height. The

fallen materials collect at the foot hill or cliff base form small mounds and ridges. Debris falls

differs from debris slides in that the material falls from a vertical or over-hanging cliff or

bluff. Debris falls are especially common along the undercut banks of streams.

Earth fall

It involves down slope movement of fixer materials than debris fall. In humid climate

regions where there are steep slopes , the masses of soil saturated with water, overburden

or weak bedrock may slide downslope during a period of a few hours in the form of

earthfall.

Creep

Besides, the mass movement on steep slopes, there are many other forms of a still slower

nature. These become apparent on close inspection of almost hillside. Creep is the slow

movement down slope of soil and rock debris which is usually not perceptible except

through extended observation.

Sharpe (1938) recognized four types of creep, on the basis of materials involved in such

movement creep are divided into (1) soil creep (fine weathered rock debris as well as soil)

and (2) rock creep (unweathered joints blocks). It may be pointed out that the rate of

movement of materials under creep is slow as a few mm/year.

i. Soil Creep

Soil creep is also called as solifluction which occurs in a variety of climatic conditions ranging

from tropical humid to peri-glacial climates. Soil creep is down slope movement of soil. In

soil creep is a slow down slope movement of material which occurs, at rate of 0.15 mm/year

in most soil creep does aid in slope wash and sometimes produces Miniature scarps and

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shallow channels depressions on slope. Frost weaving in the most important contributory

process to soil creep.

Soil creep is usually confined to the upper metre or so of most soils. Rates of soil creep are

usually measured by placing pins or acrylic rods in the walls of trenches which are then

refilled with soil, and by using tilting bars pushed in to the soil. Initial survey and then re-

survey of the positions of the instruments indicates if, movement has occurred.

Many types of evidences have been held to demonstrate the existence of soil creep

including tree curvature, turfrolls, cracks in the soil etc. Most of these phenomena can,

however also be produced by other processes such as wind, slope wash or tilting under the

weight of the object. The only reliable method of determining the presence of creep is by

direct measurement.

In environments with seasonal variations of soil moisture and soil temperature, soil creep is

an episodic process caused by hearing and setting movements in the soil produced by

solution freeze-thaw, warming and cooling and wetting and drying cycles.

ii. Rock Creep

Rock creep involves downhill movement of rock debris having relatively great depth (upto

300 m) but the movement is very slow and ranges between one meter to 10 m/year. Rock

creep involves down slope movement of individual rock blocks. It is distinguished from soil

creep by its great depth of isolation from daily and seasonal climatic conditions, (from land

sliding by the lack of a single clearly defined failure plane) and slow rate of deformation.

Large scale ground ripples, or bulging of the toe of a slope, can result from depth creep.

Surface creep can occur in both soils and rock. In rock it is usually the result of fractures

developing across small joint blocks near the ground surface, and then the tilting down

slope of the separated block.

The following condition promote rock creep (1) deformation of rocks through bending,

folding, bulging, fracturing, spreading, distortion and buckling of included rock beds of

varying resistance (2) mechanical disintegration of rocks etc.

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Mass wasting in itself and particularly in combination with running water is the most

important process of degradation. Running water and glaciers move material faster and

farther, but initially, at least, mass wasting processes provide the bulk of the supply to the

streams and glaciers from the slope of valleys. Naturally, there are many borderline cases

with a little more water some mudflows would be called streams and with a little more ice

some rock glaciers would be glaciers.

Fig. 6 Mechanism of Weathering

(Source: http://capone.mtsu.edu/cdharris/GEOL100/erosion/ternary.jpg)

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Did You Know

Social and Economical Impact of Mass Movements Mass movements produce a variety of effects on the surface of the earth. For example, a

landslide may fall across a river, damming the water and causing it to form a lake. However,

landslides can also suddenly rush down a steep slope can cause great destruction across a

wide area of habitable land and sometimes cause floods by damming up bodies of water. In

Alberta, Canada, in 1903, a mass of about 30.6 million cubic m slipped from the top of Turtle

Mountain and fell to the floor of the adjacent valley annihilating the town of Frank and killing 70

people. Similar landslides have occurred in other high mountains, such as the Rocky Mountains, the

Alps, and the Himalaya. In Portuguese Bend about 156 houses have been torn apart or moved to

another area because of the movement of the land. Slurry can flow rapidly down slopes or through

channels, and can strike with little or no warning at avalanche speeds. It can travel several miles from

its source, growing in size as it picks up trees, cars, and other materials along the way. In Colombia,

the eruption of a volcano near the town of Almero erupted sending mud flow slurry that wiped the

town from the face of the earth; from 25,000 people only 3,000 survived.

(Source: http://www2.fiu.edu/~longoria/natural/mass/mmain.htm)

)

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Summary

Denudation refers to weathering, mass wasting, erosion, and transportation.

Mass wasting refers to the movement of large quantities of weathered materials down

the slopes.

Mass wasting involves the processes of creeping, sliding slumping, flowing and falling.

Slopes are either straight or concave or convex.

Mass wasting on slopes can be triggered by earthquake, undercutting by streams,

heavy or prolonged rains, or volcanic eruption.

Types of downslopes movement are flows, if they move as viscous fluid: slides, if they

move over the ground: falls, if they travel in the air is free fall.

Creep is the slow downslope movement of overburden and, in some cases, bedrock.

Landslides are sometimes called as avalanches, that term is best reserved for

snowslides. Slides involving overburden are called debris sliders.

Sheet flow refers to refers to the movement of individual particles downslope. Strictly,

this is not an aspect of mass movement.

Rain action produces gullies and earth pillars when condition are favourable, e.g.,

gullies form on sloping land free of vegetation when rainfall is fairly heavy; earth’s pillar

forms on slopes composed of clay and boulder where there is little or no vegetation.

The end product of weathering is called as regolith.

Water in regolith moves upward by capillary action and downward by infiltration.

Exercises

State the meaning of mass wasting. Name the three features that it can produce.

Illustrate with relevant diagrams.

Explain the factors which results in mass wasting.

Name the types of slumps and explain each in detail.

Differentiate between fall and flow.

Briefly explain the geomorphic importance of mass wasting.

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Briefly explain what could happen to the surfaces of steep slopes :

a) When belts of trees are cut down

b) After a prolonged period of frosty weather

c) When cutting is made for a road.

References

Birkeland P.W. 1984. Soils and Geomorphology. New York, Oxford University Press.

Robinson D.A., and Williams R.B.G., 1994. Rock Weathering and Landform Evolution. John Wiley & Sons, Inc., New York

Berner, E. B., and R. A. Berner. 1996. Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, N.J.: Prentice Hall. Bland, W., and D. Rolls. 1998. Weathering : An Introduction to the Scientific Principles . New York: Arnold.

Colman, S. M., and D. P. Dethier. 1986. Rates of Chemical Weathering of Rocks and Minerals. New York: Academic Press.

Ollier, C.and Pain, C. 2000. The Origin of Mountains ,Routledge.

S. Singh. 1998. Geomorphology, Prayag Pustak Bhawan, Allahabad.

Dayal, P. 1976. A Text Book of Geomorphology, Shukla Book Dept, Patna.

A.N. Strahler & A.R. Strahler 1978. Modern Physical Geography, John Wiley.

J.A. Steers, 1961. The Unstable Earth, Lyell Book Dept, Ludhiana.

Kale V. and Gupta, A. 2001. Elements of Geomorphology. Oxford University Press, Calcutta.

Bharatdwaj, K. 2006 Physical Geography: Introduction to Earth.Discovery Publishing House,Delhi.

Leong G.C. 2006 Certificate Physical and Human Geography, Oxford India.

Bunnett, R.B. 1965. Physical Geography in Diagrams, Pearson Education, India.

Bloom, Arthur L., (2003) Geomorphology: A Systematic Analysis of Late Cenozoic Landforms.

First Indian Reprint. Delhi: Pearson Education (Singapore) Pvt. Ltd.

Page 21: Discipline course -1 Paper Lesson Developer: … The movement of weathered material under the influence of gravity and ... wasting is the downslope movement of soil or rock material

Institute of Lifelong Learning, University of Delhi

The above lesson has been checked for plagiarism by the s/w used by the ILLL technical

team

Chorley, Richard J., Schumm, Stanley A. and Sugden, David E., (1984) Geomorphology.

London: Methuen & Co. Ltd.

Engeln, O. D. von, Geomorphology. (1960) New York: The Macmillan Company.

Suggested Readings

Thornbury, W. D. (2004) Principles of Geomorphology, CBS Publishers, Delhi.

Strahler, A. and Strahler, A. (2002) Physical Geography: Science and Systems of the Human Environment, John Wiley and sons, New York.

Web Links

1. http://www.google.co.in/imgres?biw=1366&bih=643&tbm=isch&tbnid=95lCG7CyHEM

YcM:&imgrefurl=http://web.arc.losrios.edu/~borougt/MassWastingDiagrams.htm&doc

id=ruqoQg8MjKaEAM&imgurl=http://web.arc.losrios.edu/~borougt/ClassificationofMo

vements.jpg&w=750&h=536&ei=p2QxUsqFIIT5rQehhoCABg&zoom=1&ved=1t:3588,

r:2,s:0,i:84&iact=rc&page=1&tbnh=190&tbnw=266&start=0&ndsp=18&tx=133&ty=80

2. http://www.physicalgeography.net/fundamentals/10x.html

3. http://www.gly.uga.edu/railsback/1121MassWstgFourplex2.jpeg

4. http://www.gly.uga.edu/railsback/1121Lxr30.html


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