Geomorphologic concepts by William D Thornbury

1. RATHINAVEL K
Reg No : 3382014
Ist M.Sc Applied Geology
University of madras
Guindy campus
Chennai 25

2. 2
• What is geomorphology ?
• Concepts in geomorphology
• Why do we need concepts ?
• Explanation of concepts
• Quick review of concepts
Contents

3. • As a word, geomorphology is composed of three Greek roots which
are “Geo, Morph & Logy” that mean the “Earth, Form & Science” respectively.
• As a science, geomorphology is one of the branches of geology,
dealing with the study of landforms of the earth, including submarine forms,
their formation, developments and relation to the other geological factors.
• Geomorphology, studies in particular the lithosphere, and
interactions with the atmosphere and hydrosphere, to understand the
interconnection of various system processes.
Geomorphology

4. 4
• Geomorphology come about as a result of dissatisfaction with
the term Physiography.
• Geomorphologists are particularly interested in the potential
for feedbacks between climate and tectonics mediated by geomorphic
processes.
➼ geomorphic agents, such as river, wind, glacier and sea-waves, etc.
➼ geomorphic processes ;
Erosion
Transportation
Deposition

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Why do we need Concepts ?
• Any discipline has concepts that are key for its progress. For
geomorphology these need to be explicitly stated and consistently
understood for what they are.
• They are a number of concepts will aid in the evaluation of much
that follows in the subject “geomorphology", but also not only ones used
in the interpretation of the landforms (landscapes).
• Some fundamental concepts are enumerated by
W.D. Thornbury which comes into use in the interpretation of landscapes.

6. Concept no 1
➼ This is the important principle of geology and is known as the
Principle Of Uniformitarianism. It was first enunciated by James Hutton in
1785. According to Hutton "The present is the key to the past".
➼ According to him geologic processes operated throughout
geologic time with the same intensity as now ,We know that it is not true.
“ The same physical processes and laws that operate
today operated throughout geologic time, although not
necessarily always with the same intensity as now ’’.
Explanation:

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• Tectonic activities, climate, glaciers for example prove that the
processes are not the same intensity
• Stream action in valley has the same intensity in the past and
now

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➼ There are numerous examples which shows that the intensity of various
geologic processes has varied through geologic time.
Dendrochronology

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10. 10
“ Geologic structure is a dominant control factor in
the evolution of landforms and is reflected in them
”.
Concept no 2
• It is including such phenomena as:
a- rock attitude (dip and strike).
b- presence or absence of joints, faults, folds and bedding planes.
c-physical hardness of the constituent of mineral
d-permeability or impermeability of rocks.

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Control of geologic structure on landform
Geological structures are,
1. Folded structure
2. Faulted structure
3. Horizontal structure
4. Vertical structure
5. Domal structure

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FOLDED STRUCTURE

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HORIZONTAL STRUCTURE

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FAULTED STRUCTURE

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UNICLINAL STRUCTURE

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DOMAL STRUCTURE

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VERTICAL STRUCTURE

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“ To a large degree the earth's surface
possesses relief because the geomorphic
processes operate at different rates ” .
Concept no 3
• Lithology and structure are major factors causing differential
wasting of the earth surface
• Temperature, moisture, altitude, exposure, topography
configuration, the amount and type of vegetation cover are factors causing
changes in geomorphic processes within narrow limits.

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• The rate of all weathering, all mass-wasting, all erosion, and all
deposition varies appreciably within rather narrow limits in relation to the
influence of local conditioning factors.
• variation in the climate condition between a (valley floor and hill top),
(bare ground and hay vegetable cover) may be reflected the:
•Amount of precipitation
•Amount of soil moisture
•Rate and type of evaporation
•Temperature fluctuation above and below the freezing line.

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LESS EROSION = HIGH ALTITUDE LAND
MORE EROSION = LOW ALTITUDE LAND
POTENTIAL ENERGY = m x g x h (where m= mass, g= gravity and h= height)

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“ Geomorphic processes leave their distinctive
imprint upon landforms, and each geomorphic
process develops its own characteristic
assemblage of land forms ”.
• The term ‘process’ applies to the many physical and
chemical ways by which the earth's surface undergoes modification.
• In general endogenetic processes (originate from forces
within the earth crust such as diastrophism and volcanism) tend to
build up or restore areas which have been worn down by the
exogenetic processes (results from external forces like weathering,
mass wasting, erosion);
Concept no 4

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Each geomorphic process develops distinctive landform
1. Endogenetic Processes: Volcanic eruption, Tectonic movement,
Earthquake, Tsunami
2. Exogenetic Processes:
i) Fluvial Process
ii) Glacial Process
iii) Aeolian Process
iv) Coastal Process etc.
• landforms have their individual distinguishing features
depending upon the geomorphic processes responsible for their
development, for example:
A/ flood plains. Alluvial fans and deltas are products of stream action.
B/ Sinkholes and cavern are produced by ground water.
C/ Moraines are produced by glaciers.

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“As the different erosional agents act upon the
earth's surface there is produced an orderly
sequence of land forms” .
• The land forms possess distinctive characteristics depending upon
the stage of their development.
• This idea was most stressed by W. M. Davis and out of this idea grew
his concept of geomorphic cycle and its concomitant stages of youth, maturity,
and old age culminating in a topographic surface of low relief called peneplain.
Concept no 5

26. • A corollary to the concept of completed geomorphic cycle; is that of a
partial cycle is accepted as a concept.
• Partial cycles are more likely to occur than completed ones, for much of
the earth's crust is restive and subject to intermittent and differential uplifts.

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• An orderly sequence of landforms by the act of
geomorphic agent

28. “Complexity of geomorphic evolution is more
common than simplicity”.
➼ Usually most of the topographic details have been produced during
the current cycle of erosion, but there may exist within as area remnants of features
produced during prior cycles. Commonly we are able to recognize the dominance of
one cycle.
➼ Leland Horberg (1952) On the basis of origin of history, the landforms
are divided into five groups:
(1) Simple,
(2) Compound,
(3) Monocyclic,
(4) Multicyclic And
(5) Exhumed or Resurrected
Concept no 6

29. 29
• Simple landscapes are those which are the product of a single dominant
geomorphic process. One energy and one erosional cycle
• Compound landscapes are those in which two or more geomorphic
processes have played major role in the development of existing topography.
More than one energy
• Monocyclic landscapes are those that bear the imprint of only one cycle
of erosion; one erosional cycle

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• Multicyclic landscapes have been produced during more than
one cycle of erosion. Much of the earth's topography bears the imprints
of more than one period of erosion.. More than one erosional cycle,
e.g- dissected plateau, accordant summit level, multi planation surface
• Exhumed or resurrected landscapes are those which were
formed during some past period of geological time, then buried beneath
a cover mass of igneous or sedimentary origin, then still later exposed
through removal of the cover. Due to denudational process, multi
erosional cycle

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“ Little of the earth's topography is older than tertiary
and most of it no older than Pleistocene” .
• The geologic structures are in general much older than topographic
features developed upon them except in area of late Pleistocene and recent
diastrophism
• Most of the details of our present topography probably do not date
back of the Pleistocene, and certainly little of it existed as surface topography
back of the tertiary.
Concept no 7

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• Age of the earth’s Topography
90% landforms are Tertiary
and before Pleistocene era
e.g.- Mountain, river, valley,
lake etc

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34. 34
“ Proper interpretation of present-day landscape is
impossible without a full appreciation of the manifold
influences of the geologic and climatic changes during
the Pleistocene ”.
• Pleistocene have had far-reaching effects upon present-day
topography. Glaciation directly affected many million square miles, perhaps as
much as 10,000,000 square miles, but its effects extended far beyond the
areas actually glaciated.
• Glacial outwash and windblown materials of glacial origin extended
into areas not glaciated, and climatic effects were probably worldwide in
extent.
Concept no 8

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Glaciated cover area 10,000,000 sq km. in Asia, Africa mainly North Pole
Two aspect are very important
1. Geology
2. Climate
● Idea of geologic and climatic changes during Pleistocene is important

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Geology and Climate

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Geology and Climate

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“ An appreciation of world climate is necessary to a
proper understanding of the varying importance of the
different geomorphic processes ”.
• Climatic variations may affect the operation of geomorphic processes
either indirectly or directly.
• The indirect influences are largely related to how climate affects the
amount, kind, and distribution of the vegetal cover.
• The direct controls are such as the amount and kind of precipitation, its
intensity, the relation between precipitation and evaporation and daily range of
temperature, whether and how frequently the temperature falls below.
Concept no 9

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The climatic factors especially precipitation and temperatures influence the
operation of the geomorphic processes

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Differencing climate is responsible for difference in geomorphic processes:
• Climate: Tropical(hot), Temparate(moderate), Polar(cold)
• Geomorphic processs : Fluvial, Aeolian, Costal, Glacial etc.

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Climate and Geomorphological Processes

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Climate and Geomorphological Processes

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“ Geomorphology, although concerned primarily with
present day landscapes, attains its maximum
usefulness by historical extension ”.
• Geomorphology concerns itself primarily with the origins of the present
landscape but in most landscapes there are present forms that date back to previous
geological epochs or periods.
• A geomorphologist is thus forced to adopt an historical approach if he is to
interpret properly the geomorphic history of a region.
• The paleo-geomorphology covers the identification of ancient erosion
surfaces and study of ancient topographies.
Concept no 10

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• Present day Landscape attains historical extension (Fossils,
Human Skeleton, Dendrochronology etc)

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Historical extension

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Quick review of concepts

49. 49
Concept
Description
Alternatives
1. The same physical processes and
laws that operate today operated
throughout geologic time, although
not necessarily with the same
intensity as now
Similar geomorphic processes that
operate today operated throughout
geologic time.
Uniformitariani
sm
2.Geologic structure is a dominant
control factor in the evolution of
landforms and is reflected in them.
Relatively frequent, modest-sized
processes dominate in the progressive
changing of the landscapes over long time
periods.
Gradualism
5. As the different erosional agents
act upon the earth's surface there is
produced an orderly sequence of land
forms.
Erosional agents on a planetary surface
produce an orderly sequence of landforms
as a function of time.
Orderliness
4. Geomorphic processes leave their
distinctive imprint upon land forms,
and each geomorphic process
develops its own characteristic
assemblage of land forms.
Landscapes reflect the influence of certain
climatic processes by developing a
characteristic assemblage of landforms.
Morphoclimatic
Zonation

50. 50
8. Proper interpretation of present-
day landscape is impossible without
a full appreciation
of the manifold influences of the
geologic and climatic changes
during the Pleistocene.
The geologic and climatic changes of the
Pleistocene were critical in the shaping of
most landscapes.
Legacy of the
Pleistocene
10. Geomorphology, although
concerned primarily with present
day landscapes, attains its maximum
usefulness by historical extension
Geomorphology is primarily concerned
with present- day processes that shape
present-day landscapes.
Modernism
6.Complexity of geomorphic
evolution is more common than
simplicity.
It is necessary to search for simplicity in
geomorphological systems.
Simplicity
7 Little of the earth's topography is
older than tertiary and most of it no
older than Pleistocene.
Most of the Earth´s topography is no
older than Pleistocene.
Youthfulness of
Topography

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•Principles of Geomorphology, IInd edition,(page 16-33)
Thornbury W.D. (2002),
CBS Publishers & Distributors, New Delhi, India
Reference
Guidance
•Dr. S. Umapathy,
Retd. Prof. Jawahr Science College, Neyveli (NLC)