LEADING ISSUES
IN
GEOMORPHOLOGY
BY
DR. HI JIMOH
DEPARTMENT OF GEOGRAPHY
UNIVERSITY OF ILORIN, NIGERIA
ALL Rights Reserved
ISBN 978 – 36018 -7 -3
(C) H.I. Jimoh
No part of this book may be reproduced stored in any retrieval
system, or transmitted in any form or by any means, electronic,
mechanical, photocopying recording, or otherwise without the
prior permission of the author in writing.
Haytee Press and Publishing Company Nigeria Ltd.
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DEDICATION
TO THE SWEET MEMORIES OF:
Jimoh Uhuache Omenesa (Father), Habibat Jimoh (Mother), and
Dr. R.O Oyegun (Teacher and Friend)
Words cannot quantify my level of appreciation for making me
what I am today. May the Almighty Allah grant to them good
eternal life – Amen. I wish these loving parents and Teacher had
lived long enough to enable me appreciate their inputs into my
life today. Acceptably, the Almighty Allah knows best.
ACKNOWLEDGEMENT
The expanding frontiers of the discipline of Geomorphology have
been championed by a number of energetic scholars whose works
have made landmarks in this subject. This current book has not
only envied these efforts but has equally benefited tremendously
from a number of these scholars. The works to which this current
approach in geomorphologic studies have benefited greatly
include those of Bunnett, R.B and Okunrotifa. P.O (1984),
Carson, M.A and Kirkby. M.J (1972), Coates, D.R (1958),
Faniran, A and Ojo. O (1980), Goudie, A.C (1989), Gregory, K.J
and Walling, D.E (1973), Hammond, R and McCullagh, P.S
(1974), King, C.A.M (1966), Monkhouse, F.J (1981), Pritchard,
J.M (1979), Selby, M.J (1979), Sparks, B.W (1972), Strahler, A.N
and Strahler, A.H (1973), Thornbury, D.W (1969), Small, R.J
(1979), Young, A (1972), Professors G.E.K Ofomata and K.O
Ologe, Mr. Tunde Malik (a former teacher who is the architect of
most of the diagrams in this book) among others, I m indeed most
grateful to these forerunners in geomorphologic studies.
FOREWORD
This book “Leading issues in Geomorphology” is a bold
attempt to succinctly highlight the basic issue involved in the
study of Geomorphology – a branch of Physical Geography. The
author has employed a simplified approach in writing the book.
As a matter of fact the need to allay fears of undergraduates
would appear to have motivated the author in writing the book. It
is therefore not surprising that the experienced teacher of
Geomorphology has presented the book in a rather simplified
manner both in content and language without loosing the details.
In essence the perceived difficulties in this aspect of geography
has been removed.
Starting with the relationship between geomorphology and
other sciences, the author gave details of the historical
development of the subject. Rocks, structural landforms and
weathering processes are discussed. Geomorphic processes are all
covered using simple, understandable and easily comprehensible
language.
The theoretical aspect of the subject is not left out as
chapters fourteen and fifteen concentrates on some quantitative
and modeling aspects of geomorphology.
I commend the book to the readership of undergraduates
in particular and Environmentalists in general.
J.F Olorunfemi. PhD
Professor of Geography
University of Ilorin.
CONTENTS
Dedication……………………………………………………..ii
Acknowledgement ……………………………………………iii
Foreword………………………………………………………iv
Contents………………………………………………………..v
Chapter One: The Lithosphere……………………………….1
Introduction ……………………………………………………1
Internal structure of the earth ………………………………….1
Chapter two: Geomorphology and other Sciences……………3
Types of Geomorphology ……………………………………...3
The growth and development of Geomorphology……………...4
Chapter three: Rocks ……….…………………………………7
Classification of Rocks ……….………………………………..7
Igneous rocks …………………………………………………..7
Sedimentary rocks …………………………………………….8
Metamorphic rocks …………………………………………..10
Chapter four: Structural landforms …………………………..12
First order landforms……………………………………………12
Second order landforms…………………………………………12
Third order landforms…………………………………………..14
Chapter five: Weathering ……………………………………...16
Mechanical weathering …………………………………………16
Chemical weathering …………………………………………...16
Biotic weathering……………………………………………….17
Factors of weathering…………………………………………..17
Rates of weathering…………………………………………….17
Importance of weathering………………………………………17
Chapter six: Fluvia processes………………………………….19
Mechanisms of river erosion……………………………………19
Erosion………………………………………………………….19
Transportation…………………………………………………..20
Deposition………………………………………………………20
The long profile of a river system ……………………………..20
CHAPTER ONE
River capture
Chapter seven: Glaciations
The mechanisms of glacier erosion
Highland glaciated features
Lowland glaciated features
Chapter eight: Waves and currents
Mechanisms of waves erosion
Coastal features of deposition
Chapter nine: The Aeolian processes
Mechanisms of wind erosion in the deserts
Features of wind erosion
Landforms of wind deposition
Chapter ten: Landforms classifications
Chapter eleven: Cycle of erosion
Interruptions of cycle of erosion
Chapter twelve: Mass wasting
Factors that favours mass movement
Types of mass movement
Chapter thirteen: Quantitative analysis in Geomorphology
Natural analogue system
Conceptual models
Model testing
Appraisal of models in Geomorphology
Chapter sixteen: The study of World landforms
Structural regions of the World
Topographic regions of the World
Erosional and Depositional landforms of the World
Bibliography
THE LITHOSPHER
Introduction
The lithospheric layer of the earth’s crust is a vital seam in
the discipline of physical Geography. Issues commonly discussed
relate to the internal structure of the earth. Evolution of rocks,
landform process. Weathering cycle, geomorphic processes and
mass wasting among others. These issues are equally very
fundamental in the study of Geomorphology.
Internal Structure of the Earth
Earth crust is a thin layer usually of about 17km in
thickness. It is this thin layer that contains the continents and the
ocean basins. Also, the earth crust constitute as the source of soil
and other sediments. Salts as well as gases in the atmosphere, all
free water of he ocean, free water in the atmosphere and lands.
The earth is made up of several concentric layers of shells
of various materials. The layers are different from each other in
terms of composition. Size and structure too. The outer layer of
the earth is the earth curst, which is otherwise known as the
lithosphere. The lithosphere on he other hand is made up of two
parts of sial and sima. The sial is discontinual and it is made up of
silica and alumina and therefore collectively called as sial. It has a
density of about 2.7. Sima is the second part of the lithosphere. It
is made up of basaltic rock and composed essentially of silica,
magnesia and iron. It is a continual layer and has a density of 3.0.
Since the sial is lighter than the sima, the continent can be said to
be floating on the sea of denser sima. The next layer, which is
immediately below the lithosphere, is known as the mantle or
mesosphere. It is about 2900km thick and composed mainly of
very dense rocks and olivine. Olivine is indeed a heavy silicate of
iron and magnesium is termed as ferromagnesium silicate, which
is mantle is made up of upper and lower mantles. Separating the
upper and lower mantles is a plastic layer known as the
asthenosphere. The next layer is the Mohorovicic discontinuity,
and Dr. Moho discovered it in 1909. This layer exists between the
sima layer and mantle. At this layer, the speed of propagation of
earthquake waves suddenly accelerates from about 5.0 to 8.1 km
separated from the mantle by the Gutenbrg discontinuity. The
core is metallic, consisting of iron and nickel (ni fe). The
temperature of the earth is estimated to be about 3700. The
average density of the barysphere is estimated to be about 10.5.
The barysphere consists of inner and outer core. The inner core
has a density of about 16 to 17 and a diameter of about 2600 to
2700km and is a solid mass. The mass of the earth is calculated to
be about 5.976 x 1021 tonnes. The outer core on the other hand, is
in a liquid state (See fig. 1).
Fig 1: Illustration of the internal structure of the earth.
CHAPTER TWO
GEOMORPHOLOGY AND OTHER SCIENCES
The prime concern of the discipline of geomorphology is
on landform studies or landform attributes. Recently, this
discipline has metamorphosed from mere observation and
reporting to a more rigorous science. For example, it has grown
from being purely qualitative to a more quantitative issues. This
therefore means that there are marked overlaps in geomorphology
with other sciences such as Geology, Geodesy, Geography,
Hydrology, Geophysics, Engineering, Agriculture and Chemistry
among others.
In some universities, Geomorphology is offered in the
Department of Geography located either in the Faculty of
Business and Social Sciences or Faculty of Science3. but in others
in the Department of Geology. This therefore means that, a would
be mainstream Geomorphologist must have a working knowledge
of geology, mathematics, physics, biology, chemistry, agricultural
science among others. This is because, the subject of
geomorphology dos not recognize disciplinary boundary, as it
tends to borrow concepts from other sciences to address
geomorphological problems. In essence, every geomorphologic
problem requires interdisciplinary approach.
Types of Geomorphology
This can be divided into two broad types as follows:
(a)
Static Geomorphology
This aspect of geomorphology studies landform perse.
Thus, knowledge of geology is required for prospective static
geomorphologist.
(b)
Dynamic or Process Geomorphology
It requires an understanding in mathematics, statistics etc,
importantly, for any meaningful study in geomorphology;
geomorphologist must work closely with other scientists for
meaningful results.
The growth and Development of Geomorphology
The growth period of the discipline of geomorphology is between
the 18th and 19th centuries. Actually, it was geologist,
mathematicians ad hydrologists that contributed immensely o the
growth and development of this discipline. Some of the great
scholars whose works have provoked the growth and development
in geomorphology are as presented below.
1.
James Hutton 1726-1797
(a)
He laid the foundation of modern geomorphology.
(b)
He authored a book titled “The Theory of the Earth” in
1785. By 1795, he authored another book trying to explain
features especially as observed on the earth surface.
However, his major weakness was that he used too many
obscured languages. Hence his work never received much
attention. His former student john playfair remedied this
situation.
(c)
Playfair represented his teacher’s work in a more simple
form by 1805 and was titled “Illustration of the
Huttonatian principles of the earth”. As a result of this,
many readers acknowledged the major contributions of
Hutton.
(d)
John Playfair in his book described the power of rivers to
erode and transfer eroded materials etc.
Playfair equally believed that rivers move from sources to
sea and could develop a graded profile.
2.
Charles Lyell (1797-1875)
(a)
He formulated the theory of unforntarianism i.e the
present is a key to he past.
(b)
3.
(a)
(b)
(c)
(d)
(e)
4.
(a)
(b)
(c)
5.
(a)
He authored a text titled “The principles of Geology”.
However, his work didn’t received wider audience
because it focuses mainly on marine erosion whereas what
was in vogue then was works on soil erosion.
C.G Greenwood (1857)
His tenet of discussion was essentially on the effects of
rain and rivers.
Also, he appropriately addressed the sub-aerial erosion.
He authored a text titled “Rays and Rivers”.
He described the powers of rainwash and compared that
with the actual work of rivers
He exhaustively discussed the concept of base level (The
level below which the land surfaces cannot be educed by
running water).
G.K. Gilbert
His contri8butions were both impassive in content and
methodology. As a matter of fact he had a power of
deductive reasoning.
His methodology proceeded in five stages namely: He
observes, arrange the observations in sequential order,
invent hypotheses to account for the characteristics of the
features he has observed, made deductions of
consequences that should follow his hypotheses and he
test the consequences against new observations.
He authored a text titled “the Geology of the Henry’s
mountain”.
He attempted to described and measure some
geomorphological features such as volume, velocity and
the gradient of rivers, and the relationship of these to one
another.
W.M. Davies (1850-1934)
He put the random ideas in geomorphology together
systematically and devised terminologist. For example, he
6.
(a)
(b)
7.
advanced the concept of the “Cycle of erosion” in
different climatic environment. However, his cycle of
erosion concept came under serious criticism due to his
imprecise assumption of sudden uplift and presence of
run-off to initiate erosion processes.
Walter Penck
He proposed the concept of parallel retreat of slopes,
which is in at variance with he postulates of W.M Davies
in respect of slope from
He authored a text titled “Die morphologist analyse,
1922”. He however died young which really affected his
career.
Modern Geomorphologist
The modern Geomorphology was spear headed by
Engineer Robert Hutton who initiated the quantitative
revolutions in geomorphological studies. This effort
involves the giving of figures to observations against he
qualitative assessment of geomorphological phenomena.
CHAPTER THREE
ROCKS
Rocks are made up of aggregates of different types of minerals.
Common among these minerals are the iron oxides (4% of rock
minerals). Calcites and dolomite (9%), Kaolinite or clay minerals
(18%), Quartz (28%) and the Feldspar group (33%). Due to these
variations in the rock’s minerals constituents they therefore tend
to vary in terms of texture, structure, colour, permeability and
degree of resistance to activities of denudations. Another
difference between rocks relate to their mode of occurrence.
Generally, all rocks can be classified into three major
groups of igneous, sedimentary and metamorphic rocks. About
75% of the total land surface of the continent consists of
sedimentary rocks while the remaining 25% represent igneous and
metamorphic rocks.
Classification of Rocks
Rocks of all grades and types can be classified on the basis
of mode of origin, appearance and composition as follows.
Igneous Rocks
They result from the cooling and solidification of magma (Molten
rocks) from beneath the earth’s crust. They are crystalline in
nature, do not occur in strata or layers, do not contain fossils of
animals, microbes and plants, and rich in heavy minerals. On the
basis of mineral composition, igneous rocks can be grouped into
acid, intermediate or basic rocks. Acidic igneous rocks contain a
high proportion of silica, for example granite. The basic igneous
rocks contain a greater proportion of basic oxides such as iron,
aluminum or magnesium and thus denser and darker in colour.
Igneous rocks can equally be divided into two main groups based
on origin:
(a)
Plutonic rocks
These rocks have cooled (igneous rock) extremely slowly
at great depths and have large crystals of over 1.25mm long.
Examples of this rock type include granite, gabbro and diorites;
and they may be exposed to the surface through the activities of
denudation and erosion.
(b)
Hybabyssal rocks
This category of igneous rock has cooled near to the
surface. Most often, they contain medium sized crystals or large
crystals sat in beds of finer crystals. However, it may be exposed
to the surface by activities of denudation
(c)
Volcanic Rocks
These are molten rocks that are poured out of lava or
volcano onto the surface. They solidify quickly on exposure due
to temperature variations. This mode of cooling promotes the
presense of small crystals. Columbia-snake plateau in USA and
the Cameroon Mountain are good examples of this type of
igneous rocks.
Sedimentary Rocks
This rock result from the accumulated sediments, usually
over a long period of time under water. The constituents
composed mainly deposited minerals and fragments produced by
both mechanical and chemical weathering of former rock masses
or by organic action. Usually, materials forming this rock types
are squeezed, cemented and hardened by pressure of overlying
beds, the cements being or the partially dissolved grains
themselves. Silica, carbonates and irons oxides are the typically
cementing.
Sedimentary rock has a characteristic of strata formation,
non-crystalline in nature; contains fossils of animals, microbes
and plants. The general texture of sedimentary rocks depends on
the circumstances under which they are laid down. Thus, wind
sorted grains produce rocks of similar grain size and texture while
ice sorted materials form rocks of irregular grain size and
coarseness. A close inspection will often reveal a rocks’s origin.
For example, sandstone contains quartz grains, which indicates a
river origin if sharp and angular or a wind blown, or Aeolian
origin if rounded and smoothed among others.
Sedimentary rocks may be classified into three major
groups or categories according to origin and composition as
follows:
(a)
Mechanically formed Sedimentary Rock
This rock type originates from the accumulation of materials
derived from other rocks that have been cemented together. The
texture, composition and colour of this rock vary tremendously.
This is usually due to the nature and constituents of the parent
materials. For example, when large pebbles are firmly cemented
to form a rock it is called breccia or conglomerates when
fragments are angular. Examples of sedimentary rock derived in
this way are common in the arid regions of Northernwestern
Nigeria, Southwestern Niger Republic, and Senegambia among
others.
(b)
Organically formed sedimentary rocks
It is derived from the remains of living organisms.
Example of animal’s remains includes corals or shellfish, whose
fleshy parts have been decomposed, leaving behind the hard
shells. The calcareous rocks such as limestone and chalks are in
this category of sedimentary rocks. Carbonaceous rocks equally
fall into this class of sedimentary rock types. Anthracite,
bituminous and lignite coals are forms of carbonaceous rocks.
Examples of calcareous and carbonaceous rocks are common in
the Carboniferous and Fossiliferous region of Pennines.
Calcareous rocks are equally common in Nigeria, part of Senegal,
soputhwestern plains of Ghana, while carbonaceous rocks are
common in Enugu and Okaba in Nigeria among others.
(c)
Chemically formed sedimentary rocks
It originates from the chemical precipitate of solution of
one kind or another, which latter coagulates to form this type of
sedimentary rock. Rock salts are derived from strata, which once
formed the beds of seas or lakes. Gypsum or calcium sulphate is
obtained from the evaporation of salt such as the Dead Sea, which
have a very high salt content. In a similar way, potash and nitrates
may be formed.
Metamorphic rocks
This rock type results from the alterations in the physical
or chemical properties of any mass of rock. That is, all rock types
whether igneous or sedimentary may become metamorphic or
changed rocks under great heat and pressure. Thus, the original
character and appearances of rocks may be greatly altered. For
examples, shale changed to schist’s, limestone to marbles,
sandstone to quartzite, igneous to granulites, granite to gneiss,
clay to slate, and coal to graphite. The change in the rock’s
original character and composition may be due to intense heat
created by an igneous intrusion (thermal or contact
metamorphism). The change may equally be due to heat or
pressure created during rock movement such as during folding or
faulting, by stress due to pressures and sharing during mountain
building (regional metamorphism) or by the movement of fluid
chemical elements in the rock. Finally, metamorphism
recrystallizes minerals into larger grains alters and rearranges the
grains, and further combine chemicals to create new minerals.
Generally, all rocks types have their modes of origins, thus
their roles in the evolution of landscape depends not only in their
structure, texture and compositions but, the time element available
to the activities of denudation to act on the rocks.
CHAPTER FOUR
STRUCTURAL LANDFORMS
The term structural landforms may be applied to
landforms that owes their origin and uniqueness to the
composition of and processes within the lithosphere or to the
variable responses of rocks at the surface of the earth to
weathering and erosion as a result of differences in physical and
chemical characteristics. In other to understand these landforms, it
is more rewarding to classify them according to hierarchy or order
of sizes. In essence, there are first order, second order and third
order.
First order Structural landforms
This is at the world scale. They consist of the continental
landmasses and their associated continental shelves on the one
hand and the ocean basins on the other and each measurable in
millions of km2. these landforms reflect fundamental differences
between the relatively light soil rocks which form the continental
land masses and the much denser basaltic sima on which they are
floating like rafts on a sea and which appears to form the floors of
the ocean basins.
Second order structural landforms
This is at the continental scales. Thy are the major relief
units into which the continental land mass such as Africa,
Australia or South America, may be divided and the area usually
measurable in hundreds of thousands of km2, but may be larger or
very much smaller. These landforms have resulted from the
operation of endogenetic (or internal) forces of epeirogenesis,
orogenesis and vulcanicity and may be described, together with
the first order structural landforms as techtonic landforms. For
examples, the continent of Africa may be broken down into its
second order structural landforms as follows:
i.
The Atlas Mountains in the Northwest and the cape
ranges
in the extreme south. These are fold
ii.
iii.
iv.
v.
Mountain, which were produced during the Alphine
and the vey much older Harcynian and Orogenesis,
respectively.
A number of large basin produced by regional
downwarping of the crust into which sedimentary
rocks have accumulated (e.g the Chad basin, the
Congo basin, the Sudan basin, the El jouf basin and the
Kalahari basin.
Coastal lowlands, which areas of crustal depression
carrying a sedimentary cover. They occur around the
margins of the continent and are widest in Somalia,
Mozambique, Nigeria, Senegal, Mauritania and former
Spanish Sahara.
Plateau and high plains (with some hilly mountains
terrain), which are areas of regional up warping or
swells where the ancient basement complex
metamorphic and igneous rocks are exposed and
which separate the basins and coastal lowlands (I and
ii above) from one another; and
The East African rift valley system and its
associated
volcanic landforms of which by far
the largest is the lava
plateau of Ethiopia. These
landforms are due to the very
intense nature of
epeirogenitic movements here, resulting in large-scale
faulting of the crust. In Nigeria, the second
order
structural landforms consist of swells or areas up
regional unwrapping, and basin troughs and coastal
lowlands that are areas of downwarpping.
The swells form the highest parts of the country within
which the following major physiographic units may be
recognized:
(a)
the high plains of Hausaland;
(b)
the Jos Plateau;
(c)
the high plains and ranges of Yoruba land, and
(d)
the eastern highlands;
The downward areas from the lower-lying part of the
country. The major physiographic unit recognizable within
them are shown as:
(e)
the Sokoto plains (which are part of the lullemmeden
Basin);
(f)
the Chad plains (which are part of the Chad Basin)
(g)
the Gongola Trough;
(h)
the Benue Trough;
(i)
the Niger Trough;
(j)
the plains and downlands of South Eastern Nigeria;
(k)
the lower Niger Trough;
(l)
the Niger Delta: and
(m)
the coastal plains of Southwestern Nigeria. These areas
have since cretaceous times, suffered repeated
downwarpping or have simply lagged behind as they
served as receptacles for the vast quantities of debris
eroded from the swells.
Third Order Structural Lanforms
Measurements are in thousand of km2. Landforms have
been produced from broadly similar groups of lithologies and
which have a character different from those of landforms
developed on adjacent groups of lithologies. The part of the Chad
basin, which lies within Nigeria is a good area to use as an
example. In that area, the type of terrain developed on each of the
five major groups of lithologies, namely: the intrusive igneous
and the metamorphic rocks, the cretaceous sediments, the Kerri
Kerri sandstone, the Chad sediments and the extrusive volcanic
area, broadly speaking, distinctive and may be described as third
order structural landforms. They owe their origin to the fact that
each group of rocks has responded differently to the sub-aerial
processes of weathering and erosion as a result of differences in
arrangement on the earth’s surface. Thus, the areas of igneous and
metamorphic rocks from the higher-lying uplands of the area and
are characterized by often Rocky Mountains and hilly terrain as
well as by undulating or rolling plains and plateau of generally
high drainage density. The cretaceous sediments form distinctive
cuesform hill ranges and undulating to rolling plains, which have
a relatively low drainage density (except on the more argillaceous
members where the plains are generally flat and surface drainage
is poor).
The Kerri Kerri sandstone forms an extensive, rolling,
waterless plateau while the area covered by Chad sediment are
monotonously flat plains of deposition. Finally, the extrusive
volcanic are associated with scarp-bound plateau, which may or
may not carry scoria and cinder cones.
There are subtle as well as not-so-subtle chemical and
physical differences between the individual rocks types making
up each of the above groups of rocks and within individual rock
types themselves. These between-rocks and within-rock
differences are exploited by the agents of weathering and erosion
to produce fourth and lower order structural landforms, which
range in size from hundreds of km2 down to fractions of kms.
These landforms include individual plans, plateau and mesas, hill
ranges and massifs, ridges and valleys, inselbergs and
escarpments, scuestas and vales. For examples, the cretaceous
sediments of the Chad Basin and the Gongola valley include
sandstone of which are ferruginized; shales, mudstone and some
lime stones.
CHAPTER FIVE
WEATHERING
Weathering is a process in which new secondary minerals
are synthesized from the Products of he break down of primary
rock minerals. Weathering activities can be divided into physical
(mechanical), chemical and biotic weathering.
(a)
Mechanical Weathering
This is a weathering process that leads to the breaking
down of rocks into smaller fragments without changing
their composition. Examples of this type of weathering
includes salt weathering, dirt cracking, cavitations fire
weathering etc. this type of weathering is most effective in
the tropical regions.
(b)
Chemical Weathering
This type of weathering affects the chemical and
according to the type and the degree or intensity of the
completely dissolved; some are changed chemically, while
product. The process of chemical weathering includes
solution and reduction processes.
It is important to note that mechanical weathering aids
chemical weathering just as chemical weathering participate in the
physical breakdown of rocks. Rocks that are more heavily
fractured by physical processes are more likely to be intensely
acted upon by chemical processes, given suitable environments.
However, chemical weathering is much more effective in the
humid regions.
(c)
Biotic Weathering
This involves the breaking down of soil particles due to
the eating and burrowing activities of both micro and
macro animals and pressure exerted by plant roots and the
mixing and transfers of soil materials by animals.
Factors of Weathering
The level of effectiveness of the types of weathering (a-c)
discussed depends on the following factors:
(i)
Climate (rainfall and temperature).
(ii)
Activities of plants and animals.
(iii)
The topography.
(iv)
The time factor upon which factors (i-iii) have been
able to act on a given rock type.
Rate of Weathering
This refers to the rock constituents such as structure,
colour, acidity etc. thus, factors affecting the rates of weathering
depend on:
(i)
Rock mineral colour
(ii)
Rock structure
(iii)
Rock texture
(iv)
Presence of joints, fissures or cracks in a rock mass
(v)
Level of acidity in a rock.
These factors put together will greatly exsplain the
rate of weathering of any mass of rock in any region in the
world.
Importance of Weathering
(i)
It is a principal agent in the evolution of Aeolian
features and other landforms too.
(ii)
It contributes immensely to the formation of all soil
types in any region.
(iii)
The effectiveness of the hydraulic activities of fluvial
processes and the emergence of some features
associated with upper course of a river system and the
activities of waves and current generally depends on
weathering actions etc.
CHAPTER SIX
FLUVIAL PROCESSES
Rain falling unto the earth surface causes overland flow.
The emergence of overland flow depends on: volume of flow,
presence of slope, roughness of the surface, and the degree of
turbulence involved. Due to irregularities on the earth surface,
there is the tendency for water to concentrate on the linear paths
(rills). With time roughly parallel rills may develop. One of the
rills may attain dominance over others. This process is known as
micro piracy. With increase in the efficiency of micro piracy, a
stream is eventually formed. This overland flow of water
normally confines itself to a distinctive channel of its own. Better
still, a river is a mass of water moving down wards (higher to
lower elevation) in a natural linear channel of its own making.
Mechanisms of River Erosion
The geologic work of streams consists of three interrelated
activities of erosion. Transportation and deposition.
(a)
Erosion
streams erode in various ways and this depends on the
nature of the channel materials, and the tools with which the
current is armed. The force of flowing water alone, exerting
impact and a dragging action upon the bed, can erode poorly
consolidated alluvial materials such as gravel’s and, silt and clay.
This process is termed as a hydraulic action. Where swift current
against bedrock channel’s walls carries rock particles, chips of
rocks are detached. The rolling of cobbles and boulders over the
stream-bed will further crush and grind smaller grains to produce
an assortment of grain sizes. This process of mechanical wear is
termed as abrasion, which is the principal means of erosion in a
river system. Finally, is the chemical process of rock weatheringacid reactions and solutions are effective in removal or rock from
the stream channel and may be designated as corrosion. Effects of
this process are most marked in limestone, which is a hard rock
not easily carved by abrasion, but, yielding readily to the action of
carbonic acid in solution in the stream water.
Youthful Stage
This stage of a river profile is next to the mouth of the
river. Here, the river is fast flowing, energy of river is
concentrated on valley deepening, vertical erosion process
dominates, and the nature of the valley is deep steep sided Vshaped cross profile. The features associated with this stag are:
(1)
Pot Holes
These are either large or shallow but circular depressions
found along the riverbed. They are created due to the movement
of river over the uneven bed surface. Rock materials such as
pebbles in transit usually cut the river-bed into circular depression
known as potholes as they swirl. The potholes may later be
enlarged or deepened and widened into larger potholes, otherwise
known as plunge pools. The system of deepening is by abrasion.
Plunge pools are common at the base of very large water-falls.
For example, the Niagara falls has a very large plunge pool below
it
(see
fig.2).
Fig. 2: Potholes in river-bed
(2)
Interlocking Spur
A spur is a large and prominent projection of land into a
young river valley. It is usually carved out due to fast movement
of a river system. At this sage of a river system, the river flows
around the projection to interlock. The activities of erosion on the
projected land are concentrated on the concave banks of the bends
of the river. The spur is made up of hard and resistant rock
(Fig.3).
Cataracts are also a form of interruptions, which are less
pronounced than Waterfalls.
Waterfalls can be produced due to one or more of the
following reasons: A waterfall may be developed as a result of the
interruption by a bar of stratum of more resistant rock lying
transversely along the river course. The resistant rock has ability
to resist the activities of denudation much more than the
surrounding rocks within the river valley. An example of such
resistant rock stratum is sill or dyke among others. The rocks
down stream of these interruptions are usually softer and therefore
eroded faster away. As a result of this, the gradient of the river
course become steepened where it crosses the resistant rock, and
this lead to the formation of the waterfalls. Such hard and resistant
rock may be due to the lying horizontal, verticals and slanting
positions of the hard rocks. (see fig. 4a, b, c).
Fig. 3: Interlocking spurs
(3)
Rapids and Water Falls
Rapids and Waterfalls are concupicious features found at
sudden break point along a river course. They result from sudden
steep change of gradient of the river course. The river therefore
makes attempt to smoothen out the interruption and reach a
graded profile
Rapid have graded profiles than waterfalls. Rapids are
usually created as a result of gentle increase in the bed beds slope
of a river valley without prominent interruption of the river flow.
Example of waterfalls created in this way includes the Niagara
Falls found within the Hudson-Mohawk Valley between USA and
Canada. Also, is the formation of Boti falls near Koforidua in
Ghana.
The second way in which waterfalls can be formed may be
due to the flow of a river over the sharp edge of a Plateaus. An
example of such waterfall is on the Jos plateau, Nigeria. Another
example is the living stone falls on Zaire River.
The third way in which waterfall can also be produced
along the river course is as a result of the development of faultline
across a river valley. Such faults across the river valley might
bring a faultline scarp, which is less resistant on the down stream
against more resistant rock of the upper stream over which the
river flows. A good example of such waterfall is the Victoria Falls
on the Zambezi River, and also Kalambo falls on the Zambia.
this place. A major example of such hanging valley waterfalls is
the Yosemite falls found in California. USA.
Fifthly, waterfall results where river flows over the steep
edge of coastal cliff directly into the sea. A good example of this
type of waterfall is along the Davons in Great Britain. Also, is the
Lobe falls along the coastal areas of Cameroon.
Finally, waterfalls resulting at knick points. Knick points are
break point along a river valley that have been rejuvenated.
Alternatively, knick points are edge of the giant step, otherwise
called terraces, which marked the level of the old flood plain
tow3ards the upper stream. A major example of this waterfall is
the Charlotte falls on Orugu River in Sierra Leone.
Middle or Matured Stage
River (fig. 5).
Fourthly, waterfall may develop at the junction of a hanging
valley and the glaciated main river. In this situation, both the main
and the tributary valleys are normally occupied by ice. The
activities of erosion are more severe over the main valley when
compared with the tributary valley. This is because: the main
valley contains larger volume of ice. Eventually when the ice
melts away from the valleys the tributary valley remain ‘hang’
above the main valley. Frequently, waterfall easily develops in
This stage is better discussed with the lower course of the
river system. This is because, some of the characteristic features
found at his stage are also found in the lower course. That is, the
two stages gradually grades into each other. However, a matured
valley normally has a V-shaped cross profile, gentle gradient,
river bends, removal of spurs, and widening of valley floors due
to lateral erosion process.
The main activity of river at this stage is erosional process.
Also, depositional activity equally stars, but this is limited to the
lower course of the river system. The features associated with this
stage are discussed as follows:
(i)
Meanders
Meanders are common to both the middle and lower
stages of a river system. They emerged due to the decreasing
ability of a river to flow in a straight or direct course. This
decrease in river strength can be likened to its competence and
capability. Hence, the materials are deposited. In this view, river
currents flow round the bend of a meander, concentrating their
erosional energy on concave sides of the bed. The concave side is
therefore an area of maximum erosion, while the convex side is an
area of active deposition. As erosion on the concave and
deposition on the convex sides continues, the meanders become
increasingly more pronounced. Meanders are particularly
common on the lower courses of both rivers Niger and
Mississippi. Three types of meander exist: incised, entrenched and
ingrown meanders. Incised meander has been cut very deep into
the alluvial deposits as well as the bedrock. It results from the
rapid down cutting by a large river that has fully developed a
system of meanders and is able to maintain its course during
rejuvenation. The entrenched meander has steeps and symmetrical
sides. While, the ingrown meander has steep been produced by
vertical and lateral erosional processes.
A feature in association with meanders is known as the meander
terrace (fig. 6).
flood plain. The alluvium consists of fine rock materials i.e sands
and gravel brought as bed loads scoured from outside the bends
immediately upstream. As river enters into old age, the energy
(competence and capability) of the river to transport is greatly
reduced, and the water spread over a wide area, and the river
starts to move sluggishly. Consequently, alluvial deposits become
thick due to the accumulation of the materials. Various boring that
have been made into the alluvial deposits of the Nile River have
not been able to reach the bedrock. Areas of fertile flood plains
include the lower valley of Yang-tse Kiang, China (fig. 7).
Fig. 7: Illustration of flood plain
Fig 6: River meandering in its flood plain
Levee’s (Natural embankments).
Levees are natural mounds of alluvium deposited on both
sides of the river channel of the lower course. It usually marked
areas of the inner bank of the river valley. As alluvial deposition
continues within the inner banks, the deposits are capable of
raising the level of water within the levee’s making the water to
become higher than the general flood plain. The alluvial deposits
(iii)
(ii)
Flood Plain
The development of a flood plain normally starts from the
middle course of a river system but is most characteristic of the
lower course. Flood plain develops in areas where the river has
attained its widest valley limit. Thick deposits of alluvium, which
is usually covered when the river is in a great flood, constitute the
become increased between the levees especially when the river is
in flood. Eventually, it is possible for the river to break through
the levees. This can have effects on both agriculture and human
lives, because, at that stage, the river will flow over and above the
general flood plains. This situation is common within the Hwanho river in China. Other rivers that have built levee include the
Ganges, Mississippi and river Po of Italy (See fig 8).
the river as a result of lack of gap within the levee through which
it can join the main river. Eventually where the gap exists the
tributary river joins the main river. The area through which the
tributary river joined the main river through the levee is known as
deferred junction. This feature is common with river Yazoo, a
tributary of river Mississippi (Fig 9).
Fig. 9: Illustration of Deferred junction.
(v)
Fig. 8: Illustration of Levees.
(iv)
Deferred Junction
This feature is associated with levee. This is a junction
between the main river and the tributary river that has been
postponed as a result of the building up of levee on both sides of
Braided Channel
This feature is common to both the middle and lower
courses of a river system. They are formed when river deposits its
alluvia materials within the channel of the main river channel.
This often results in the breaking of the main river channel into
small sub-channels. This situation is common around Lokoja and
the Delta areas of the Niger River Nigeria(fig 10).
Fig 11: Development of an Ox-bow lake.
Fig. 10: Braided channels
(vi)
Ox-bow Lake
Ox-bow Lake is common to both the middle and lower
courses of a river system. In fact, Ox-bow lakes are old meanders
but now cut-off due to the deposition of alluvial materials and are
totally separated from the main courses of the river. They
originated from an acute meander where a narrow neck of land
separates two concave banks, which are being under cut. This
feature is common along the lower course of river Mississippi
(fig. 11).
River Capture
It is otherwise known as a river piracy or river beheading.
This is a situation where a river abstract the headstream of a
contiguous river thereby enlarges its own drainage are at its
neighbuor’s extent. Certain conditions favour this occurrence.
This condition includes: the presence of two rivers named ‘X’ and
‘Y’ and flowing side by side too, either of the rivers say stream
‘X’, must be more powerful than stream ‘Y’ either because it
experiences rainfall at its headstream or due to ice melting and
either stream must equally develop a subsequent stream say
stream ‘X’ develops subsequent stream toward stream ‘Y’. With
these situations, stream ‘X’ will advance towards stream ‘Y’ due
to sideward erosion. With time, stream ‘X’ might at last capture
or pirate or behead stream ‘Y’. The point of diversion is known as
elbow of capture. The stream that has lost its headwater reduces
considerably in volume, hence, become too small for its existing
valley; this stream is termed as misfit. Below the capture is known
as a wind gap.
A number of ways exists for recognizing an incidence of
river piracy. This includes: presence of an elbow of capture,
presence of marked differences in the valley shapes above and
below the point of capture, presence of misfit, presence of short
reversed streams leading from the wind gap to the point of
capture, and presence of dry valleys or dry gaps linking the valley
of one stream to the other. This occurrence indicates the old
course of pirated stream. However, it must be bore in mind, that,
all these may not necessarily mean an evidence of river capture.
Some sharp bends along river courses may have nothing to do
with river capture. Therefore, two or three signs may combine to
indicate a rivers beheading. A major example of an area with this
occurrence is the upper part of river Sittang in Northumberland
(fig. 12).
Fig 12: Illustration of River Capture.
CHAPTER SEVEN
GLACIATION
Glacier is an exemplify phenomena of the Temperate Zone
of the world. It is formed when temperature falls below 32of or
Ooc. At this temperature, water or rain waters freezes into snow.
Frequent freezing of water into snow leads eventually to the
formation of ice due to the snow pillage. Thus, as soon as the ice
becomes compacted and hardened due to frequent snow that
accumulates at a rate faster than it melts then ice become so thick
that the lower layers become plastic, outward or downhill flow
commences, and an active glacier has come into being, the upper
end of the glacier constitute the zone of accumulation, has a
thickness of about 40 metres and characterized with brittles which
can degenerate into crevasses and the ice beneath behave as
plastic substance and moves by flowage, the lower end or of the
glacier lie in the zone of ablation (melting). Here, the rate of ice
wasting is rapid.
The Mechanisms of Glacier Erosion
The two major processes of glacier erosion are plucking
and abrasion. Plucking is the tearing away of rock masses by
means of ice freezing into cracks and protuberances. The frozen
ice attaches itself to the moving glacier, tearing rock masses as the
main body of glacier moves along. These processes are most
effective on faulted and well-jointed rocks that form the bedrock
of glacier valleys. Abrasion on the other hand refers to erosive
action of transported rock materials embedded on basal grinding,
polishing and scratching process.
Highland Glaciated Features
(i)
Cirque
It is also known as corrie, own cirque. It is a steep sided
rock basin, and semi-circular in Plan. That is, cirque is a
depression that has been cut into bedrock over which the glacier is
moving.
The development of corrie begins with the accumulation
of snow into glaciations hollow, a small depression. The hollow is
usually created by the first onset of snow accumulation and
gradually enlarges through the process of freeze-thaw. The
materials embedded in the glacier are used as instruments for
abrasion, thereby deepening the floor of the corrie. The process of
plucking helps to extend the sideward extension of cirque. The
total process by which the cirque is deepened and widened is
called the basal sapping. Many cirques contain lake, which might
be a temporary or permanent one, and such lakes are common in
the English Lake district. A major example of this feature, cirque
is the Teleki cirque on mountain Kenya. Africa.
(ii) Arete
An Arete is a jagged steep sided, knife edged like and
narrow rocky ridge which separate one corrie from the other
around a mountain top .It usually developed as a result of the
formation of two or more corrie on the adjacent slopes. Cirques
are usually enlarged by back wall recession. Which leads to the
reduction of the area of rock between two corries. This recession
process continues until eventually, a narrow, knife-edge like steep
sided and elongated piece of rock is left separating one corrie
from the other. The best example of this feature, is the Striding
edge on Helvellyn in Westmorland. Also corrie is common on
mountain Kenya Africa.
(iii)
Pyramidal Peak
This is a steep sided, pointing mountain top and with
various sharp edges radiating from the peak. It is developed at the
function of three or more cirques by the process of back wall
recession of the various corries on the opposite direction of a
mountain. The sides and the peaks of the pyramidal peak are later
sharpened by frost action. A major example of this feature is the
Matterhorn peak in Switzerland. Also, is the Point John in
mountain Kenya, Africa (see fig. 13) for features (I, ii and iii).
Fig. 13: Illustrations of Cirque, Arete and Pyramidal Peak
(iv)
U-shaped glacial through
This feature has a broad, flat-bottom and steep sided
glacial valley. It usually has a roughly U-shaped cross profile.
More so, this feature is the modified valley of a river that has been
over deepened by glaciations. The glacier is incapable of cutting
its own valley but occupies an existing river valley, which it
latter modifies. The plastic or solid nature of the glacier including
the rocky materials embedded in the glacier enables it to cut out
and straightened the valley sides producing what is known as
truncated spurs. In fact, the development of U-shaped glacial
through might have some relation with interglacial river erosion
as well as postglacial activity. U-shaped glacial trough may
contain finger or kettle lakes as it is the case with the Ullswater
lake in Great Britain. (fig 14).
main valley with a waterfall. Example of waterfall of a hanging
valley is the Yosemite falls of California, USA (fig. 15).
Fig. 15: Illustration of a hanging valley
(vi)
Fig. 14: U-shaped glacial trough.
(v)
Hanging Valley
This feature is a product of high tributary valley, which is
relatively shallow, and enters the main valley at a very steep
slope. The development of a hanging valley is due to over
deepness of the main glaciated valley, which contains larger
volume of ice than the tributary valley. Eventually, the increased
erosional activity of the main valley glacier makes the main river
valley to become over deepened, while valley is very shallow.
After the glacier has melted the main river valley become over
deepened, while the tributary valleys, valley remain “hanging”
above the main valley. Some hanging valley usually open into the
Roche Montonnee
This is a resistant residual rock hummock. It usually
projects above the general level of the valley bottom and ice
movement striates its surface. The upstream side of this feature is
usually smooth, polished and r4onded with a gentle slopping side
due to the process of abrasion and ice striation to. The leeward
side of this feature is usually rough, irregular and very steep due
to the processes of plucking and frost action. Roche Montonnee is
particularly common in Northern Wales, Great Britain. (fig 16).
Fig 17: Illustration of Crag and Tail
Fig. 16: Illustration of Roche Montonnee
(vii)
Crag and Tail
This is a mass of hard rock like granite with a precipitous
slope on the upstream side, which protect the softer area from
being affected by the on-coming ice. Glacier therefore moves over
and around the resistant rock mass, thereby, polishing and
moulding the rock mass. Some rock materials on the leeward side
together with some materials from the upstream side are later
deposited on the leeward side of the crag to form the gentle
slopping tail. The tail is usually sedimentary in nature, and
composed mainly of the rock materials eroded from the upstream
side by the glacier and deposited on the leeward side of the crag;
such is the Basaltic igneous Plug of Edinburgh Castle rock, or the
Edinburgh Castle of Scotland (Fig. 17).
Lowland Glaciated Features
(i)
Erratic
These are large masses of granitic rocks that are usually
carried away from their places of origin by an advancing glacier.
These hummock rocky materials are left stranded after the glacier
has melted. Their characteristic and possibly mineral constituents
differ greatly from other rocks in its new environment. This
feature is common in the middle parts of Great Britain i.e the
sharp granite bounders of Scarborough of Great Britain. Also, is
the Madison Boulders of New Hernpshire, USA.
(ii)
Boulder Clay or Till
This is a large mass of unsorted rock materials that spreads
out to cover an extensive area, which forms a monotonous plain.
The rock materials are usually deposited by the glacier, most
especially where the glacier starts to ‘waste’ (melt). The boulder
clay or Till is an equivalence of alluvium in river deposition and
loess in wind deposition. The boulder clay or Till is composed
chiefly of a mixture of clay, pebbles, stones and sandy materials.
These materials are usually spread out evenly at the snout of a
glacier that is gradually melting, thereby developing a gently
undulating relief form. Boulder clays or Till are common in the
Great North European plains and also in the Mid-West region of
the United States.
(iii)
Moraines
They are composed of masses of boulder clay, stone
materials that have been eroded upslope by the glacier and later
deposited down slope in various shapes and sizes. Moraines are of
various types, namely:
Lateral moraine
This is the type of moraine deposited on both sides of a
moving glacier. The materials are usually let behind in a straightline on both sides of the valley after the glacier has melted.
Terminal Moraine
This is the type of moraine deposited at the end of the glacier. It is
alternatively known as End moraine. Usually it marks the point of
maximum movement of the glacier.
Median Moraine:
Here, materials are deposited in straight line in the middle of
glacier. It results from the Joining up of the two inner lateral
moraines of the two glaciers that have come together to form one
mass.
Sub-glacier moraine
This is the type of moraine found at the bottom of the glacier
between the valley floor and the glacier bottom. Sub-glacier
moraine is not usually visible until after the glaciers have melted.
Finally, the englacial moraine, which is the type found, buried
inside a glacier itself. The englacial moraine is usually fund
scattered all over ht bottom of a glaciated valley after the glacier
has melted. Moraines are common in the Great North European
plains (see fig. 18).
Fig. 18: Illustration of Moraines
(iv)
Drumlin
These are smooth and elongated hummocky deposits of
glacier clay or Tills. They are commonly found at the snout of a
retreating glacier. Drumlin is initially formed beneath the ice and
later exposed after the glacier melting. It may be form due to the
re-moulding of a pre-existing drumlin by a new glacier. That is,
some glacier Till might have been deposited earlier by glaciers
that have completely melted. Later, the area is taken over by the
new glacier, which now moulds the pre-existing glacier Till into
series of hummocky deposits. It may equally be formed by remoulding of glacier Till in an advancing glacier by the glacier
into drumlins. Drumlins possess long axes hat lie parallel to the
direction of the glacier that have formed them. Drumlin usually
exist in-groups forming the characteristic features of ‘basket of
eggs topography’. Some might exist in small mounds while others
exist as large hummocky deposits that might be 2km long and 9m
high. Drumlins are common in Northern England, Midland valley
of Scotland and Northern Ireland (see fig. 19).
Fig. 19: Illustration of Drumlins
(v)
Outwash plains
this is a lowland area that fronts the glacier snout. It is
usually covered by numerous deposits of sand and gravel’s
essentially of fluvio-glacial origin that have been washed out from
the terminal moraine, only to be re-deposited by the melted water.
Outwash plains are produced in areas, which marked the
maximum glacial movement. In most cases, areas of outwash
plains are also areas of numero
us lakes. These lakes are usually temporary in nature. Areas of
outwash plains are suitable for agricultural practices. A major
example of this feature on a large scale is the Great North
Europeanplains (fig. 20)
Fig. 20: Illustration of Outwash plains
CHAPTER EIGTHT
WAVES AND CURRENTS
Waves emerge when winds blow over an open ocean or
sea surface. The blowing air is usually in layers. The surface layer
of the air exerts a frictional drag on the layer above it, and so on.
The top air layer has the least drag on it; which means that the
layers of air move forward at different speeds. The air tumbles
forward and finally develops a circular motion. This motion exerts
downward pressure on the surface at its front and an upward
pressure at its rear. The surface beings to take on the form of a
wave. (see Fig. 21).
Fig 21: formation of waves.
Mechanisms of Waves Erosion
The erosional activities of waves depends on the following
processes of erosion:
First, the hydraulic action. The sheer impact of water of
waves which break powerfully against the foot of cliffs is capable
of very great erosive effects most especially on coastal rocks
which have a well developed system of bedding planes, faults and
joints. Hydraulic action of waves may take the form of explosion.
As waves rush and break powerfully against rock joints and
cracks, their impact brings much pressure to bear on the rock.
Consequently, the air trapped in the rock cracks becomes
suddenly compressed. When the water recedes with the retreating
waves (backwash), the compressed air now expands very rapidly,
causing a large explosion. Frequent repetition of this action over a
long period of time enlarges rock joints and finally leads to the
break up of rocks.
Second, the action of corrosion. This is a process whereby
waves use materials procured by past wave actions as tools for
further erosive work. These “tools” includes fragments of
boulders and sands, which are usually pounded against the cliff
base. Corrosion is the most powerful process in coastal erosion.
Three, solvent action (Solution). This relates to the
chemical action of seawater. Seawater is capable of dissolving
certain rock materials along the coastland and carries them away
in solution. Action of this process is most noticeable over
limestone region.
Finally, the process of attribution. This is a reduction
process whereby the weathered rock materials within the waves
are further broken down and reduced in size due to the collision
etc. the swash and backwash wave motions enhance the collision
of the rock materials in transit. Later, these materials become
reduced to sands, which are later deposited to form beaches.
Strictly speaking, attrition is not an erosive process; rather it is a
process that leads to the reduction in the sizes of the erosional
tools.
Coastal Features of Erosion
These are series of coastal inlets and headlands, which lie
right angels to the sea and exposed to repeated wave actions. They
are formed as a result of coastal rock which posses alternating
bands of hard and soft strata i.e granites alternating with softer
sand and clay lying parallel to one another, and at right angles of
the coastline. The less resistant structures are worn out fast to
produce the inlets known as bays by wave actions while the more
resistant bars of rocks are least affected by denudation; they
therefore stand out prominently as capes or headlands.
Alternatively, capes and bays may be formed when a coastline
with similar rock type erodes irregularly due to the variation
within the rock itself. Submergence of the coastal area or earth
movement along the coast can also cause the development of
capes or headland and bays. Capes and bays are common to he
coast of Persia and bay of Bengal (fig. 22).
(ii)
Sea Cliff
A cliff is a steep slope of land or vertical rock surface over
looking the sea. The nature of cliff formation depends on: nature
of coastal rocks, presence of joints, fissures on coastal rocks
degree of rock resistance to wave action etc. The cliff formation
depends on the mechanical action of waves whereby a wedge
mass of rock is removed from the coastal area at the sea level.
Cliffs are initiated usually by the development of a notch created
by wave action on the coastal rock at the base of the cliff. Once
the notch has been initiated, the cliff retreats by continuous wave
attack on its base, and weathering at the top and slumping of the
underlying mass of rocks. This is due to the gradual increase in
the size of the initial notch combined with sub-aerial denudational
process. This eventually caused the collapse of the cliff. At the
end of the exercise, the cliff is left rising from the wave cut
platform developed by the grinding action of rock materials swept
to and fro by the breaking waves. Some cliff may equally be
formed due to the changes in sea level, while others might be due
to faulting on coastal rocks (fig. 23). Cliffs are common along the
coastal areas of Senegal, Algeria, Chana, and Madagascar among
others.
Fig. 22: Capes or headland & bays.
Fig 23: Formation of a Cliff
(iii)
Wave Cut Platform
This is a flat area of land that extends into the sea from the
base of a retreating marine cliff. Te formation of cliff starts with
the undercut of a cliff at the notch. As the cliff become Reduced
due to recession, the platform is gradually extended backwards.
At the same time, the corrasional effects of rock tools in the
passing waves also gradually lower the wave cut platform. As the
platform becomes wider and wider, and continues to deep gently
towards the sea, a stage might be reached when it stops growing,
eventually, it is covered by very fine sediments and shallow water
and its slope continues seaward by the debris cut away from the
land which forms a wave built terrace. At low tides, the wave cut
platform might be exposed to the surface, but later covered by
waves at high tides. A major example of this feature is the
Strandflat Platform found in the Western Norway (fog. 24).
Coastal caves are holes in the cliff face. They developed
as a result of the continuous enlargement of initial lines of
weakness
Fig. 25 Illustration of Cave, Gloup and Geo
(vi)
Natural Arch
This is the hard roof over hanging a cave that has been
driven through the two sides of a narrow headland projecting from
the cliff face. It might also be formed due to the joining up of two
different caves that have been developed on the opposite side of a
different caves that have been developed on the opposite side f a
narrow headland. Arch is usually short-lived because, they later
collapse to leave stacks behind. Natural arch is common i.e the
Needle Eye found near Wick. Northern Ireland
(viii)
Fig. 24 wave cut platform
(iv)
Caves
Stack
A stack is a pillar of rock left abandoned in the seaward
section of a collapsed arch. Stacks may be about 183 meters high,
and could as well be called island. They are useful for the
construction of lighthouses to guide ships at sea. An outstanding
example is he Oldman of Hoy found near Orkney, Great Britain.
Also, is the Needle of Isle of Wight (fig. 26 for vii and viii). In
coastal rocks (i.e joints, faults and bedding planes) by waves. The
formation of the coastal caves is usually initiated by the hydraulic
action of waves. Later, the process of abrasion or corrosion
enlarges the caves. As these processes attack areas of local
weakness along the costal rocks, hollows are created which later
become extended into caves. Examples of this feature are found in
the Flamborough Head in England.
(v)
Blow holes or Gloups
This is a vertical pit developed in well-jointed coastal
rocks through which a long cave opens to the surface some
distance inland. It s formed as a result of waves breaking into the
caves. The forces of compression of the breaking waves may
weaken joints in the roofs of the cave end so much that it may
later collapsed. The outlets connecting the cave with the outside
then becomes the blowhole or gloups. It derives its name from the
spray of water thrown into the air by waves, which surges into the
cave from the sea below. A major example of this feature is at
Holborn Head at Caithheness, Scotland (see fig. 24 for iv and v).
(v)
The Geo
This is a long, narrow and steep sided coastal inlet, which
runs from the cliff edge for quite some considerable distance
inland from the sea. It is usually initiated like the cave by
hydraulic action, which widens a line of weakness or joints
running from the cliff inland in the coastal rock due to repeated
compression and expansion of trapped air. This lead to the
formation of a long cave, which runs from coat inland, the roof of
which later collapses to form a long, narrow inlet called a Geo as
a result of repeated hydraulic action. Geo is later widened,
modified and extended by abrasion. A major example of a Geo is
the Huntsman’s leap of Southern Pembrokeshire found in Britain.
Also, is the Wife Geo in Scotland (see fig. 25).
Fig. 26: Illustration of arch, Geo and Stack
Coastal Features of Deposition
(i)
Beaches
A beach is a coastal accumulation of sand or shingles and
gravels founds along low-lying coastline. Also beaches are
products of both swash and backwash. The materials consist of
sands, clay and shingles. Fine alluvial materials, volcanic
materials, coral fragments and shells of animals, majority of
which are normally brought down by rivers. These materials are
usually deposited along the shore between the low and high water
marks.
Beaches exist in various sizes and length. This depends on
the amount and the rate of supply of materials and the activities of
destructive waves. Actual orientation of beaches is normally
related to he direction of wave approach, because, the shape of
beaches are consequence f the breaking waves which are capable
of smoothening out irregularities of beaches. Beaches are
particularly common along the coastline of West Africa e.g the
Victoria beach and Lighthouse beach in Lagos. Nigeria (fig. 27).
strong river or water current that sweeps deposited materials away
from the end of a spit as quickly as they arrived.
The seaward extension of it may result into a hook shaped
like type; most especially, if the spit has been built up by waves
which approach spit from several directions. Spit then becomes
known as multiple re-multiple re-curves represent stage of wave
movement; and also the progressive recession of the coastal
region inland. As erosion causes the retreat of the coast and
subsequent pushing of the spit towards the retreating coast, the
hooks developed pits are common along the mouth of river
Senegal in West Africa. Spits, which developed across the mouth
of a river, may force a river to divert its outlets or change the river
mouth to a lagoon. In some cases, the spit might have grown to
block the mouth of the bay, but later broken through by wave
action (fig. 28. a and b).
Fig 27. Illustration of beach/its elements
(iii)
Sand Spits
These are natural embankment of deposited sands and
shingle materials along the coast. These pilled up linear structures
usually have one of its end attached to the mainland and the other
end projecting into he sea. Some spits are formed parallel to he
coastline, while others grow at an angle.
Spits are developed due to the movement of materials by
long shore drifts across any indentation along the coastline. The
landward side of the spit may later be stabilized and strengthened
by the growth of vegetation on it. Once, a spit has been initiated,
it will continue to grow very rapidly until a stage is reached
whereby its further extension is halted by:- decrease in the supply
of materials, presence of a deep water channel and presence of
Fig 28: Illustration of Sand spit
(iii)
Tomboli or Tombolo
When a spit grows out from the coastland into the sea to
connect an offshore island with mainland. It is known as a
Tomboli or Tombolo. Alternatively, it may also be formed as a
result of the attempts of two opposite spits to converge, thereby,
tying the island to the mainland. A Major example of this feature
on a world scale is the Chesil beach in Dorset, which extends for a
distance of bout 16,000 meters along the coast and connects the
Isle of Portland with mainland of Southern Britain, thereby
enclosing the fleet Lagoon. In West Africa, an example is the
Lumely beach, which linked in Aberdeen Hill and Cape Sierra
Leone with mainland Sierra Leone thereby enclosing the
Aberdeen (fig. 29).
being gradually filled and silted up with deposited sediments,
while het outer edge of the beach ridges become extended as
deposition continues. Alternatively, the hook of a spit might have
grown to he extent that it has joined the mainland on the other
side and the enclosed lagoon later filled up. Major example of
Cuspate foreland is Cape Kenedy in Florida. USA. Also, is the
point a’Lariee of Madagascar (fig. 30).
Fig. 30: Developments of Cuspate Foreland.
Fig. 29: Illustration of Tomboli or Tombolo
(iv)
Cuspate Foreland
This is a large triangular shaped deposit of sand and gravel
projecting from the Coastland into the sea. When two spits grows
towards one another and meet off-shore, foreland is gradually
enlarged as a result of the bay which is enclosed behind the spits
(v)
Coastal Sand Dunes
These are sedimentary rock structure commonly found in
coastal areas where large expanses of sand are exposed and dried
out at low tides. Later, the prevailing winds blowing onshore
moves the sands onto the land. The blowing prevailing winds in
such coastal areas must be predominantly on-shore and strong
enough to move a large quantity of sand inland from the beach
area.
The growth and stabilization of the coastal dunes are
greatly influenced by vegetation. Plant roots hold the sand
materials together resulting into eh formation of sand ridges or
mounds. Some coastal dunes may attain considerable heights
thereby forming a continuous dune line behind the coastal beach.
Generally, coastal dunes formed under similar conditions
of wind flow as the desert dunes. Some of the coastal dunes may
be crescent in shape with their convex side facing the prevailing
wind. That is, taking the reversed form of a barchan. Coastal
dunes are highly mobile, and therefore, easily moved inland by
strong onshore winds. Their movements can be dangerous for
human settlement, farmland, and lines of communication. Such
lines of communications can be indicated on maps to assist the
travelers. Coastal sand dunes are common along the coastlands of
Belgium, Netherlands, Morocco and Madagascar.
CHAPTER NINE
THE EOLIAN PROCESSES
About 20 percent of the total land hot deserts occupy
surface of the world and majority of these deserts are confined to
between latitudes 15ON and 30OS of the equator. Major examples
of such deserts include Sahara desert. Arabian desert, Californian
desert. Atacama desert, and the Kalahari desert (Namibian desert)
In these desert regions, winds produce a variety of interesting
sequential landforms that are both erosional and depositional in
nature.
Mechanisms of Wind Actions in the Desert
Both the mechanical and chemical weathering processes
are operative in the desert regions. They are of immense
assistance in landscape development, because of their role in the
production of loose rock materials. This is because wind on its
own has little or no potent erosive power. It is only when it arms
itself with weathered rock materials procured by the processes of
weathering that wind is capable of large scale erosion by abrasion
in hot deserts.
Firstly, the deflation processes. This is the process by
which unconsolidated rock materials are blown away by winds in
hot deserts. The sandy materials blown into eh air create dust
storms in many parts of deserts. Dust storms are common in the
Dust bowl of USA. Air borne materials eroded by deflation
process might be transported for considerable distance and
deposited far from their origin outside the desert region in form of
loess. For examples, traces of the Sahara sand dusts are common
in Southern Italy. The process of deflation may equally assist in
the initiation of a large depression. This has been the case with the
Quattara depression in Egypt.
Secondly, the process of abrasion. Abrasion or corrosion is
the process whereby outstanding landmasses are eroded by the
sand blasting effects of rock materials carried as ‘tools’ for further
erosion. This process is particularly important for the formation of
desert features such as rock pedestals, Zeugen, Yardang, Mesas
and Buttes and Inselberg.
Finally, the process of attrition. This is not an erosive
process rather, it is a process whereby the rock materials carried
by the wind are reduced in size due to their constant collision with
one another as well as with obstacles on their way. In this way,
large rock fragment can become reduced to finer particles.
Features of Wind Erosion
(i)
Rock Pedestals
This is a large mass of resistant and granitic rock that has
been undercut; and stands prominently on the desert landscapes.
Indeed, they are the grotesque shaped feature in the desert regions
of the world. It is formed due to the activities of abrasion on the
rocks usually from the base. Abrasion process is most effective on
the softer layers of ht rock mass. Exfoliation process equally
assists to smoothen out the rock. Eventually, a grotesque or
fantastic shape rock mass is left standing on the desert landscape.
Hence, the rock mass is referred to as rock pedestal. This feature
is common in the Sahara desert (fig 31).
Fig 31: Illustration of Rock Pedestals
(ii)
Zeugen
This is a tabular mass of resistant rock, standing
prominently in the desert. It is usually composed of alternating
layer of hard and soft rocks. These alternating bands of rocks
usually lie horizontal on top of one and another. Better still the
soft rock layer usually lie beneath a surface layer of more resistant
rock. The sculpturing effects of the process of abrasion wear them
into a furrow and ridge looking landscape. Also, differential
weathering enhances this activity. Zeugen is equally common in
the Sahara desert regions of the world (fig. 32).
Fig 32: Illustration of Zeugen.
(iii)
Yardang
It is made up of long ridge of resistance rocks alternating
with narrow furrows of soft rocks. Here, both the bands of hard
and soft rocks aligned vertically to the direction of the blowing
prevailing winds. The process of abrasion is effected in the course
of the blowing prevailing winds, assisting in wearing the soft
bands of rocks into narrow corridors of the hard layers.
Eventually, the bands of hard rocks remain standing high above
the soft bands that have been worn into narrow corridors. Yardang
are particularly common in the Atacama desert in USA. Also,
Yardang are common in the Sahara desert regions (fig. 33).
Fig 33: Illustration of Yardages
(iv)
Mesas & buttes
Mesa is a Spanish word-meaning table. Mesa is a
flattopped table like mass. The top layer is resistant to the
activities of denudation. Erosion therefore concentrates below the
flat layer. This is because, the bands of rocks below the cap is
usually soft. In this is view; erosion concentrates on the soft layer
below, while the flat top is able to resist both wind and water
erosion or denudation. With increase in the forces of denudational
processes, the size of the mesa may be greatly reduced such that
mesa becomes a very small hill with a round top. Thus, buttes are
smaller forms of mesa. Mesas and buttes are common in the
Sahara desert. But, the major example of this feature is the Table
Mountain in South Africa and the Front Range of Rockies in the
Untied States (fig. 34).
Fig. 35: Illustration of inselbergs
(vi)
Fig 34: Illustration of Mesas and Buttes
(v)
Inselberg
Inselberg is a German word meaning “Island Mountain”.
In fact, they are domed shaped and steep sided isolated residual
hills. Usually, inselbergs marked the ruminant hills that have been
exposed to denudation over a long period of time. Inselbergs are
composed mainly of hard and crystalline materials ie. Granite and
gneiss. Inselbergs are formed due to the smoothening out of the
slopes of c such initial hills by sheetwash erosion. In an
alternative, inselbergs may be formed as a result of the combined
actions of wind and water erosion. This feature is common in the
Savanna region of Northern Nigeria and Kalahari Desert (fig 35).
Deflation hollow or blow out
This is a very large depression created due to the blowing
away f unconsolidated sand materials in the deserts. They are
usually produced in areas of weak rocks; and such rocks might
have been affected by faulting or the rock might be very weak in
nature. In most cases, the causes of deflation hollow initially
faulting takes place in place are eventually moisture collecting in
this faulted region. The precipitating moisture helped to enhance
the activities of chemical weathering. When the moisture
eventually dries out, the already broken down rock fragments may
be blown out as wind blows over them. Activities of eddy currents
helps in blowing away the rock materials from the deflation
hollow. This further deepens and widens the hollow. It is possible
for the deflation hollow to reach the water table. When this
happens, deflation process stops leading to he formation of an
oasis. An example of a deflation hollow is the Quattara depression
in Egypt, which is reputed to be about 13.41 meters deep. In some
cases, the deflation hollow might be unable to reach the water
table sufficiently. In this case, the little water exposed to the
surface dried up due to the temperature extremities leading to the
creation of pan. A major example of pan, which contains some
salty materials, is the Etosha pan found in the Kalahari Desert
region of the world (Se fig 36).
Fig 36: Illustration of Deflation hollow.
Landforms of Wind Deposition
The materials carried away from hot deserts usually travel
for a considerable distance from their places of origin before they
are eventually deposited to form various features either within or
outside it. Features produced as a result of wind deposition are
collectively known as sand dunes. These dunes exist in various
shapes and sizes ranging from thin collection of sand to very large
barchan or barchans.
(i)
The barchan
Barchan is a crescent, newly developing moon like shape
or horseshoe shaped feature found in the desert regions of the
world. They are formed due to the accumulation of sand materials
at a point where a chanced obstacle exists and lie on the path of
the blowing prevailing winds. Examples of such obstacles include
vegetation, rocky materials, stones or even dead bodies (corpse)
of animals etc. as soon as deposition commences on such
obstacles, increased deposition of sands continues upon them, and
the dunes continue to grow in size.
The horns of barchan constantly point to the direction of
which the wind is blowing. The horns are formed due to the fact
that the wind has less frictional drag at the edges of the barchan
because there is less sand loads at the sides of barchan. This
features usually posses a sloping convex or windward side, while
the leeward side is concave and more sheltered. And eddy
currents helps in removing materials from the concave sides as the
crescent shaped f the barchan moves forward with increased sand
accumulation due to the slipping over of the sand materials.
Migrating barchan are dangerous to human lives, as such
this can be checked through afforestation, which serves as
windbreakers. Also, paths of migrating barchan are usually
plotted on maps and then avoided by travelers in the deserts.
Barchan are common in the Arab and Sahara deserts. Some
barchan may be as high as 25 meters and as wide s 400 meters. A
major example of a group of barchan is the Erg. De Djourab
found n the Sahara area in the Northern part of Chad Republic (fig
37).
tidy by the action of eddy current. Seif dunes are common in the
Sahara desert and in the Great Sand Sea of Egypt and Libya.
Fig. 37: Illustration of a barchan.
(ii)
Seif Dunes or Longitudinal Dunes
These are long, narrow, regular ridges of sand
accumulation found mainly in the sandy type of deserts. Seif
dunes have step sides and very sharp crests. Also, seif dunes
usually lie parallel in the direction of the blowing prevailing
winds. Further, narrow furrows otherwise known as corridors (fig.
38) usually separate the ridges from one another. The corridors of
seif dunes are usually swept clean by the actions of the eddy
currents towards the ridges. As a result of this, the sand ridges
becomes higher in elevation and possessed very steep sides
whereas the corridors are swept clean of sand materials. Seif
dunes vary in sizes and length from place to place i.e may be as
high as 100m and long as 100km.
The origin seif has generated much controversy. A number
of scholars believed that, seif dunes are formed as a result of
cross-winds suddenly becoming the prevailing winds in a
particular area that have been covered by group of barchans. Over
a long period of time therefore, the horns of the barchan are
changed to longitudinal shape while the corridor or furrows kept
Fig,38: Illustration of Seif or longitudinal dunes.
(ii)
Loess
Loess are air borne and fine sand materials that have been
blown out of desert areas and re-deposited very far away from the
desert regions. Whenever they are, they are yellowish in colour,
fragile to handle, but it can be very fertile for agricultural
practices. Also loess often spread over a considerable area. It is
usually deposited as soon as the energy of the wind decrease.
Loess is common on the loess plateau found in the Hwan-Hobasin in China.
CHAPTER TEN
LANDFORM CLASSIFICATIONS
Landforms of all categories must be defined in terms of
their outstanding characteristics such as the main factors, internal
and external, responsible for their present forms. One resultant
quality is their dimension and scale whether they are major or
minor features. Involved is what can be referred to as relative or
local relief the amplitude between the altitude of the highest and
lowest points in any particular districts, such as the height of a
ridge or a valley floor. Another crucial characteristic is the nature
of the gradients and slopes bounding a feature, whether these are
steep, gentle or intermediate, concave or convex, continuous and
uniform or interrupted by distinct break and change of slopes.
Further, the nature and characters of the bedrock, whether old or
young, homogenous or heterogeneous, arrangements by tectonic
forces and whether they are covered with superficial materials. In
addition, the consideration for the degrees of resistance to
denudation, the stage reached in the cycle of denudation and their
resultant shapes and outlines, whether smooth and regular, or
diversified and dissected. Also the manner in which the distinctive
landform patterns are arranged or spaced relative to each other
must be considered too.
Various method of landform classification is in existence.
Strahler, A.N divided landforms into the main groups of initial
forms and sequential forms. In this case, the initial forms of
landforms indicate that the original features produced by tectonic
force have been only slightly modified, while the sequential forms
relates to pronounced modifications. And where the initial forms
may have been destroyed virtually beyond recognition, retaining
only the merest vestiges of their foundations.
The sequential forms include erosional types (eroded
valleys and depositions), residual types (surviving parts of worndown initial form). And depositional type (a new set of land forms
built up by deposition from the much altered fragments of the
old).
Other forms of classifications are based on the actual
relief-forms. Preston E. James, requiring a physical background
for the regional surveys; divided the land-surface of each
continent into the nine categories of pain, high mountain,
mountain and basin, hilly upland and plateau, Hamada,
intermonte basin, low mountain, erg, and ice-covered areas,.
Using these landform elements, he produced surface configuration
map. Again, E. Raise standardized a set of physiographic symbols
to indicate the forth morphologic types he required.
The classification that follows is essentially a compromise
between initial tectonic causes, method and degree of external
modification and present relief form. The four major groups are
mountain, plateau, plains and valleys and basins.
Generally, landforms classification requires a clear
understanding of landforms attributes, features and qualities. This
is because; these landform parameters constitute the basis for their
classifications and the subsequent decisions.
CHAPTER ELEVEN
CYCLE OF EROSION
Many Geomorphologies have regarded the cycle of
erosion postulated by W.M Davies as the most fundamental
concept in landform study. In the opinion of Davies, the earth’s
landforms are all closely related. Further, he argued that with
time, during which the denudational processes affects them,
landforms undergo a progress change from initial forms through
“sequential form” to” ultimate forms”. This therefore mean that,
hill tops and interfluves will not maintain forever the same heights
and shapes but, will be gradually lowered by the processes of
weathering, rain wash and creep with the result that the slopes
will decline in steepness. Davies believes strongly that physical
landscapes can be analysed in tem of the three variables of
structure, process and stage. In this case, structure refers to the
underlying rocks and the manner of their disposition. Process on
the other hand includes mechanical and chemical weathering, and
mass movements, rain wash, river erosion responsible for the
actual shaping of the landform. While stage refers to time during
which denudational processes have been operating on a particular
structure. Thus, Davies adopted the terminology in describing the
life cycle of a fauna and referred to landscapes as being in the
stage of youth, maturity and old age.
In other for Davies to demonstrate his concept
convincingly he made the assumption of initial land uplifted from
beneath, the sea by earth movement, and the process was very
quick and also there was rain to produce the initial streams on the
surface.
(a)
The Stage of Youth
At the initial stage, the streams would cut rapidly downwards and
would in due course form deep valleys bounded by slops. The
processes of weathering and slumping operate on these slopes at
quite a very slow rate compared with the speed of the river
incision. Also, the valley cross profiles will be V-shaped.
Throughout this stage, the remain of the initial land surface would
be preserved on the watershed between the consequent streams.
Davies further envisaged that irregular, waterfalls would mark the
long profiles of this sage (Youthful). Rivers would be marked by
with waterfalls, Cataracts and rapids.
(b)
The Stage of Maturity
By the commencement of this stage, the rate of the
deepening of the V-shaped valley characteristic of youth would
have been slowed down considerably. Also the various streams
would throughout this stage have lowered their channels nearer
and nearer to what Davies referred to as “the base level erosion”
(The level below which rivers cannot erode) would be under cut
and driven back and this is usually achieved through soil creeps
and rain wash process acting over the entire surface of the slope.
Finally, at this stage the processes of “divide wasting”
would have considerably reduced the slope angles. The ultimate
effect of the process of divide wasting is the reduction of relief to
a decrease in the vertical height separating interfluves summit and
valley floors.
(c)
The Stag of Old Age
The processes of landscape evolution would have become
extremely slowed down in operation at this stage. This
development undoubtedly would have resulted from the gradual
reduction of river gradients and an associated decline of stream
energy and the continued lowering in angle of valley side slopes
so that creep and wash would become less and less effective.
Finally, at this stage, the relief would almost totally have
been destroyed and the land surface would assume the form of a
very gentle undulating standing only a little above the base-level
erosion. Above the base-level erosion. Above the peneplain,
Davies refers to a few isolated hills as Monadnocks because
erosion processes could not consume those areas.
Interruptions of Cycle of Erosion
Davisian cycle of erosion may be interrupted in two main ways:
(a)
By a major change in the base level: and
(b)
Climatic changes.
The base level is the level below which stream cannot cut. But
in area of intense earth movement activities, the base level can
be uplifted or depressed. If the base level is uplifted then river
begins the wok of aggradations (filling-in), but if the base
level is depressed, a new set of landforms is superimposed on
the earlier ones. For example, rivers with incised valleys.
Sometimes change is due to the climatic event. For instance,
the world is now in a warm phase having been through a cold
phase. The sets of landforms that are been created today under
the prevailing climate are been superimposed on the inherited
landforms from cold time. A common form of climate change
is those caused by glaciations or desertification. Glaciations
occurring during cold period and desertification in warm
period. For example, the northern part of North Africa is
undergoing a desertification period. The Davisian cycle was
not totally acceptable to geomorphologist even to date. The
work have been criticized on the following grounds:
(a) The work is an over simplification of ideas.
(b) The assumption is not logical enough in the real world
situation.
(c) Davisian cycle of erosion is not only a pre-existing fact
but a tourism too, hence, no theory. However it is to Davis
credit that he stressed the following among others.
(d) The development of slope is fundamental in
geomorphology, and Davis did mention that slopes
changes in angle with time.
(e) Rivers and their profiles are also affected. Davis
extensively discussed the concept of grade. He also treated
the team of drainage system.
(f) He noted how rock types do affect landform formation and
could affect the cycle of erosion too.
(g) He discussed what he termed climatic accident. That is, he
noted that his own cycle could be interrupted by climatic
accident.
(h) He brought a systematic analysis to the discipline
(i) Finally, he stressed the development of meanders in the
course of the formation of landforms.
CHAPTER TWELVE
MASS WASTING
Mass movement is the displacement of soil and rock on
slopes under the influence of gravity with or without the aid of
water as a transporting agent. This movement is subject to a
variety of processes, some of which act very slowly, but
continuously over a long period of time. While others cause
sudden movements of large masses of materials causing
catastrophic incidence. Free rock fall may move at the rate of abut
160km per hour, while soil creep may cover only a few
millimeters per year.
Generally, mass movement may be a flow or a slide. Also,
they may either be rapid or slow in movement.
Factors that favour mass movement
The following factors/conditions favour the occurrence of
mass movement:
(a)
Presence of deep profile of regolith
(b)
Presence of incoherent or loose mantles
(c)
Presence of high void ratio in the soil and contents too.
(d)
Presence of sharp basal surface beneath the regolith
(e)
Activities of biotic elements
(f)
Occurrence of earth tremors such as earthquake and
volcanic eruptions.
(g)
Rock blasting and tree felling activities.
(h)
Deforestation processes.
Types of Mass Movements
Soil Creep
This is the slow downward movement of debris and soul
under the influence of gravity. Water, as an aid is not very
important. The movement is not perceptible. The only observable
(a)
features indicative of soil creep are found in vegetation and man
made structures. Evidence of soil creep is also seen in the bulging
of walls due to the accumulation of debris from the up slope side.
Fence, posts, telephone and telegraph posts and even trunk of tees
may lean down hill due to he soil creep.
The rate of soil creep depends on climatic condition; it is
rapid under humid conditions because of the presence of
lubricating water. It is also depends on the angles of slope, impact
of raindrops and rainwash, frost heaving activities or the activities
of burrowing animals and plants roots etc. (see fig 39).
Fig 39: Illustration of Soil Creep.
(c)
Fig 40: Illustration of Talus Cone
(b)
Talus Cone
In cliffy mountainous rocky areas, debris continuously
break loose from the mountain. Surface, rolls and slid down to
form a cone like hill at the base.
This cone like hill are referred to as Talus cone or scree
slope. The angle of such slopes is usually constant and it is
between 340-370. The bigger fragments fall further down to the
base while the finer materials builds up to form cone (see fig 40).
Solifluction
This is a mass of soil or earth saturated with water flowing
from higher to lower ground. It is found in high latitude areas i.e
in Alphine or sub-arctic climate where the ground is continually
permafrost and vegetation is absent. It depends on good supply of
water from melting snow and ground ice and moderate to steep
slope. Solifluction flows during the thawing period of summer.
The mechanism is the surface may be saturated due to water
obtained from the free-thaw action of ice. This lubricates the
upper surface and it starts to move.
(d)
Earth Flow
It is the slowest of all types of rapid flow. It operates on
gentle slopes in humid areas by beds of silty capped, in many
cases, by less slippery deposits such as sand. Because the capping
materials become more easily saturated than the capped, the
former starts to move. Sometime as slip may transform into a
flow. One notices that at the steeper portion, there is slipping of
rock because of the presence of shear surface. But, as the slopes
become more gentle downward, the movement become flow. In
many cases, slumping precedes earth flow, but there is no
backward rotation like in slumping, which is faster. A noticeable
earth flow is that of ST. Lawrence valley in USA or where glacial
clays overlie either bedrock or hard pan and are in turn covered by
sands or les slippery clay. It is common in the Appalachian
plateau region.
(e)
Mud Flow
It is more rapid than earth because of the steeper slope and
the higher water content. They usually follow former stream
channels thus; they can be called rivers of mud. They result when
water suddenly burst into an area of unconsolidated and loose
materials. Mudflows require abundant and intermittent supply of
water, absence of vegetation unconsolidated and deeply
weathered materials containing clay and silt to help in lubrication,
and moderately steep slopes. Mudflows occur in Wadis where
water suddenly burst into a plain where there is already a load of
unconsolidated materials, which becomes saturated and swept by
water. Mudflows are common occurrences in the desert of the
USA.
Alphine mudflow is the name given to mudflow found in
mountain areas where thee is sparse vegetation and intermittent
water supply derived in part from snow melt. The Shumgullion
mudflow, which dammed the Gunison River to form Lake
Cristobal in Western Colorado, is a goods example of Alphine
mudflow. Volcanic mudflows otherwise known as Lahars are
found in areas of recent eruptions where fine and unconsolidated
materials such as ash, cinders, dust etc are available and move
easily when saturated (see fig 41).
involve sliding and flowing. At the steeper portion of the slope,
the movement initially caused slippage, but later flattens out
where the accumulated momentum and a high water content
causes the mass to flow (see fig 42).
Fig 42: Illustration of Debris Avalanche.
Fig 41: Illustration of Mudflow
(f)
Debris Avalanche
It is a very rapid flow associated with mountain and humid
climate with vegetation cover. Steep slopes are required. It may
CHAPTER THIRTEEN
CHAPER FOURTEEN
RESOURCE ASPECTS OF FOLDS AND
FAULTED STRUCTURES
QUANTITATIVE ANALYSIS IN
GEOMORPHOLOGY
A number of natural resources occurred in association
with folds, faults and even faulted blocks. The bulk of these
resources are the well-priced types. For instance, the Appalachian
regions exemplify the resources of maturely folded region.
Several minerals are found in these regions such as coals, which is
very common in the Appalachian regions. The original
bituminous coal have been folded, squeezed and compressed, and
the pressure has converted it to anthracite or hard coal.
In a faulted area or structure, minerals such as petro
chemical solution tend to rise along the fault line and get to the
surface. Ground water can easily rise along the fault line and be
harvested on th surface. Springs either hot or cold are situated
along the fault lines, and such springs are usually of high
economic values as minerals such as gold, silver, could be
extracted from it. Also. Such springs can constitute as tourist
centres. This has been the case with the Geysers and hot springs
of the Yellowstone National Park. USA.
Petroleum can be trapped when a reservoir rock is brought
into juxtaposition with an impermeable rock.
The subject of geomorphology has been for a long time
purely descriptive but the advent of quantitative revolution in the
1950s has ushered in the use of statistical sledgehammer for
cracking geomorphological nuts. It has therefore become
fashionable to give figures to measured observations. Assigning
figures or numbers in this way to observation is what is implicit in
quantification.
Geomorphology however, late in catching up once it got
into it, it has become a powerful tool. The use of statistical
method is not itself the goal of geomorphological studies but,
statistical tools are very powerful and a means of reaching the
goal which is a better understanding of the world.
There are four statistical methods of very great use in
geomorphological studies.
(a)
Sampling
(b)
Statistical testing
(c)
Correlation analysis
(d)
Multiple regression analysis
At the onset of any geomorphological studies, the
statistical method of sampling must be employed. A sample
designed or sampling procedure must be adopted to ensure the
highest possible degree of objectivities. If you are dealing with an
area, the use of grids or coordinates is important. Sampling with
ensure that adequate number of sample is obtained and also that
time is not wasted in collecting a necessary date.
Mathematical statistics is concerned with the making of
inference from small sample about the characteristic of a fat
population whose absolute parameters can never be known.
The statistical test of significance is concerned with the
probability of being right or wrong in stating some hypotheses
concerning the relation of one or more sample to the population
from which it has been dawn. The individual measurement is
called a variate. Example of test of significance is well
documented in most works. For example, in 1948 and 1952
Strahler and Schuum made some observation of shape of the same
area. Strahler made 154 measurements and found a mean of 49.1
and standard deviation of 3.6 while Schuum made 149
measurements and found a mean of 48.8 and standard deviation of
3.5. The difference between the two mean was found to be (a-b) =
0.30. Statistical test were therefore applied to ascertain whether
this test was significant or not. The ‘t’ test was applied thus:
(t)
=
49.1-48.8=0.561
4.64/1/154- 1/149
Checking this (t) value, it was found that the probability of
this result occurring by chance was 50%. Before this testing, some
assumptions have been made regarding the difference in the
means of slope measured by these two researchers. For example,
it was assumed that the difference was as a result of weathering
between the slopes which was 0.30, but after the significance test
was conducted the result indicated that there was no justification
in assuming that the slope have been weathered back to a lower
angle during the period between surveys in 1948 and 1952.
observation in many natural phenomena has shown that the type f
distribution of individual in which is a sample frequently showed
is the so-called frequency distribution. If the research has a body
of raw data a rapid way of testing for normality is by graphical
way or by plotting the raw data in arithmetic probability paper.
On this paper a normal cumulative frequency distribution will
show a straight line.
In geomorphology, it is necessary to establish whether the
sample you’ve taken or the result you’re obtained from analysis
are valid. Significant testing in very vital in this regard. The
probability that our hypothesis is correct is given at various
confidence intervals such as 0.05%. At 95%, it means that our
recorded result is expected to occur at 19 out of 20 times.
Sometimes, it may be difficult by visual inspection of the raw data
to see the difference between two samples unless it is further
subjected to more rigorous statistics. For example, the chi-square
test can be used to bring out the difference between samples.
Another field in which statistic can be applied in geomorphology
is in correlating one variable with another. In this method, it must
be borne in mine that even when significant level is established
between two variables it does not necessarily implied that the two
variables are directly related. The established relationship is just a
first stage but valid. Physical reason must be adduced to explain
the relationship. Two variables can be plotted one against the
other in a scattered diagram (scattergram). If the result shows that
the two functions have a linear relationship, then straight line can
be fitted to the dots. The straight line often marks the sum of
square of distance from the line at minimum. This is why this
method is called the least square method and the line is called the
regression line. After putting th points, a wide scatter can occur,
then regression analysis and testing become necessary and
valuable. The correlation co-efficient can be computed and it
ranges from t1.0 (perfect positive correlation) to –1.0 (perfect
negative correlation.
Many geographical problems are caused by a number of
interacting variables. For example, a simple phenomenon can
depend on several factors. In this case, multiple regression is
used. Multiple regression is an extension of bivariate correlation
method. The basic goal of multiple regression is to produce a
linear combination. That is how man factors (independent
variable) are responsible for our observation (dependent
variables). Such as linear combination can therefore be used to
predict, project or postdict.
Multiple regression has been used very extensively in the
study of soil erosion. Other multiple use techniques are: factor
analysis, canonical correlation, and discriminal analysis. Marrkov
chain all handled by computer.
Several researcher frowned at the use of too much
mathematics i.e. statistic in the study of geomorphology. For
example, Wooldridge. (1958) argued that, it is unnecessary to
over mathematicalise the geomorphological studies. He went on
to say that, not all the geomorphological attributes could be
subjected to measurement. For example, how do you measure
energy in geomorphological studies? How do you measure force?
This is because, the actions of these are identificable but their
measurements are difficult. However, in spite of these criticisms,
statistics is a good tool for geomorphological studies. This is
because; it helps in the formulation of theories, hypotheses and
laws on which any science depend in order to grow.
CHAPTER FIFTEEN
MODELS IN GEOMORPHOLOGY
Between the 18th and 20th century, a lot of
compartmentalization has come into the science of
geomorphology. Countries became associated with one particular
branches of geomorphology. A part from this, there grew a great
diversity in geomorphological studies with no single accepted aim
or procedural method. For example, the American
Geomorphologists today specialized in dynamic process
geomorphology. The French and German Geomorphologists
specialized in Pleistocene geomorphology and the Russian
specialized in applied geomorphology. The Swedish almost
entirely specialized in all geomorphological processes and East
European Geomorphologists specialized in geomorphological
mapping.
The great diversity and divergence in approach
necessitated a group of committed Geomorphologist to search for
a unifying concept, and what we now refer to as general system
theory was the result. In 1967. Chorley presented a
comprehensive map of geomorphoic activity.
The objective in all studies is to understand the real world,
but what we investigate and how we do it is dependent on how we
see it and what see in it. To achieve this, the term model was
adopted. A model is a simplified hypothetical description and
explanation of the interaction of phenomena on the earth surface.
Models can be static or dynamic. Example of static model are
those that represent the structure of the land. A peculiar example
of a static model is the map. A map is the picture of an area taken
at a particular time and representing that particular time. The
dynamic model on the other hand represent processes and
functions. For example, the hydrological cycle is an example of a
dynamic model.
Natural Analogue System
It is a form of translating what geomorphological
phenomena will see into some analogue or similar natural system
believed to be similar or better known. For example, we can
group objects together for the purpose of making some general
statements about them, like saying a tribe is Ibo, Yoruba or Hausa
or say somebody is like an ape. Such translations are often
inadequate and very subjective.
Our description is dependent because on how and what we
are seeing. Indeed, most traditional work in geomorphology lies
in this area of descriptive or verbal analogue.
Analogue models or systems are two types:
a.
Historical analogue:
This analogue grouped together geomorphic phenomena
with regard to their position in time sequence. This is
based on the assumption that what has happed before will
happen again or that what has existed in the past has a
relevance to what is existing now. for example, using
desert of today to illustrate the landforms of the past.
b.
Spatial Analogue
This model associate one set of phenomena with the other.
This is based on the assumption that if one area is
compared with another area believed to be similar to it, it
will enable us to make meaningful generalization about
the original area we are trying to compare our new area
with (see fig 43).
c
d
Fig 43: Illustration of a spatial analogue.
The same strata are in two areas; the only difference is that the
strata in area ‘Y’ has been overturned, but in classification. The
two areas belong to the same structural formation. It is on the
basis of this that we have structural provinces or physiographic
regions.
A geomorphological problem can be dissected into its
component part so that each part and its interaction with the other
part can be conveniently examined. This is a scientific approach
to the study of the physical world. The composition of the
separate unit and their interaction with one another is referred to
as a physical system and there are two major ones:
(a)
Hardware or scale modeling
The important structural elements in this type of model
can be substantiated. It is actually an imitative segment of the real
world and such model can be made of the same material with
which the real object is composed. However, the actual size
cannot be duplicated and so, hardware are actually scale model.
And this scaling down enables us to control any experience in
which these models are used. And this is the main reason why is
not frequently used in earth science studies. For example, a toy
plane placed at the window at airport represent a particular real
plane of a particular type. Some of the advantages of scale
modeling are that it saves time and can be subjected to the control
of the researcher.
(b)
Area X
a.
b
Area Y
a.
b
c
d
Mathematical Models
These models are abstractions in that they replace objects
forces or events by simple or complex expressions containing
mathematical variable, parameter and constants. There are two
types of these models:
i.
Deterministic Model
This model usually establishes exactly predictable
relationship between independent and dependent variable. For
instance, this model has been used in the study of slope
transformation. The only disadvantage is that no matter how
complex is the mathematics, it is impossible to incorporate all the
variables in one single equation and yet their unknown variable
may be so important in determining the result of the natural
processes we are trying to study. It is this inadequacy in this
model that led to the formulation of the stochastic model.
ii.
Stochastic Model
This is essentially a mathematical model and hence it
involves mathematical variables, parameters and constants. Also,
the model contains some random components. These random
components are added to the equation because of the
unpredictable fluctuation in the observation or experimental data.
One of the simplest stochastic models is the Marchow chain
process. The equation states that:
Yt + 1 = (YE + 1 where:
Yt = Situation of time of place ‘t’.
YE + 1 = Situation of time of place E + 1
E = Constant.
The equation shows that whatever new stage one desire an
answer is dependent on the former state of the object of study.
Another type of stochastic model is the Monte Carlo. This model
does not depend on the previous state of the object of study.
Geomorphologist frequently used the Marchov chains
models. For example, in 1959, Carl used the Marchov chain
model to discuss or study the development of limestone cave or
canverns.
Conceptual Models
These are the types of model that provide a general basis
for Geographical investigation. It also helps in making some type
of explanations. However, there is not much mathematics
involved. It consist mainly of a complex linkage structure which
both isolate, the components of the system and also give an
indication of the interrelationship between these components. The
emphasis here is placed on urbanization. Hydrological cycle is an
example of a conceptual model. Emphasis is mainly placed on the
organization of the structure. Within a super system, the various
components are isolated for such a study. For example, the
drainage basin is a physical system that can be isolated for such a
study. However, it will be necessary to know the space the system
has occupied and the stage it has reached. And then, the
relationship between input, system phase and space and the
output.
Many geomorphic processes have actually passed the path
of the super system and investigator may be interested in knowing
jus the output or may be concerned with the intricacies of the
internal working, or he may concentrate on the input of the
system. many times we don’t know enough of the system working
intricacies. For example, a system involving rainfall and erosion.
That is Rainfall
Groundcover soil
Soil
loss
Note that the arrows shows the processes involved in getting to
other stages. Sometimes, we require little or no information about
the systems phase and space. The stage. The stage at which the
system is studied and the level of details required differentiate the
various system especially the black box model. It is black box
because; we are concerned mainly with input and the output. And
what enters the system’s phase and the technicalities of the
operation are not our main system’s phase and the technicalities
of the operation are not our main concern. As far as the researcher
is concerned the mechanics of operations is black, period. That is
what Geomorphologist has done through the ages. But, recently,
the emphasis is now changing in Davis opinions that, landform is
a function of process, structure and stage to process. That is,
Geomorphologists are now trying to answer the question “How”.
In which case, in the study of soil erosion, he must appraise the
factors of soil erosion and the process responsible. He must
answer the question: why and how do certain processes operate to
produce a given output?
Model Testing
Testing of models involves a re-appraisal of response
predicted by the models against real world situation (see the
figure below)
Y = soil loss
TAR = Total amount of rainfall per month/yrs
C = Constant
To Predict, you can project your line on the graph. If a model
passes this stage is regarded as correct or satisfactory. But, many
models fail at this point. Unsuccessful model can either be
discarded or remodeled. If they are re-modelled, they must also be
re-tested. This is for the sole purpose of making conclusion and
interpretations of objectives to be reliable.
Appraisal of Models in Geomorphology
Models assist in prediction and can be assessed by
matching its predictions against the yardstick of observed data.
Also, models play crucial roles in the development of theory and
translate ideas contained in a conceptual model into the former
symbolic logic of mathematics.
Generally, models show the structure and pattern of events of
research. Also, they assist in removing incidental information and
explain the effect and roles of variables or events on each other.
Finally, models simplify situation, assist in making valid
statements and judgement, take into account the conceptual ideas
and maintain the usefulness of the geographical approach to
research questions among others.
CHAPTER SISTEEN
THE STUDY OF WORLD LANDFORMS
R.E. Murphy has attempted the classification of the landform of
the world. The Murphy’s system uses three levels if information
in successive application to identify a landform type in terms of
its geologic origin and rock composition. A classification based
on this premise is referred to as structural region or division of the
world. When classification is based on the configuration of the
surface they will talk of topographic regions, but when it is based
on the type process, which has sharpened a surface, we talk of
erosional or depositional landform regions of the world. Muphy
used these three categories for his classification. The entire
Muphy system uses sets of letter symbols.
The first letters represent the structural region; the second
letters represent the topographic classes, while the third letter
indicates the erosional or depositional landscape.
1.
Structural Regions of the World
Several structural regions are recognized and are designed
by capital letter A, C, G, L, R, S, & V and these letters are defined
as follows:
A = Alphine region, or system: This is a system of mountain
chains formed since the Jurassic period. They tend to form
a girdle round the world.
C = Caledonian/Hercynian or Appalachians: These are
reminant of mountain chains formed during the Paleozoic
and Mesozoic era.
G = Gondwana shields: These are areas of stable massive rocks
of earth crust and they lie south of the Great East West
portion of the Alphine systems and are made up of entirely
Precambrian rocks that cover the whole of the surface.
And where the Precambrian rocks are absent on the
surface, it is enough for more on than 320km. That is, the
L =
R =
S
=
V =
2.
P =
H =
T
=
M =
W =
D =
distance between the earth crust is less than 32okm. This
shield area has not been subjected to orogency or earth
movement since the Cambrian period.
Laurasian shields: This is just like the Gondwana shield
but, lie north of the East West Alphine system.
Rifted shield area: These are faulted block area forming
graben and host and other volcanic features.
Sedimentary cover: These are areas of sedimentary layer,
which have not been subjected to earth movement, and
they lie outside the crystalline shield area.
Isolated volcanic area: These are areas of volcanoes,
active or extinct. It is associated with volcanic features
but, it lies outside the Alphine and the rifted shield area.
Topographic Regions of the World
There are six classes of topographic regions and
designated by letters P, H, T, M, W & D.
Place: These are surface with relief less than 100m above
the sea level. They slope gently to the sea.
Hills or low tablelands: Here, the local relief is between
100 & 600m.
High table land: These are upland surface of about 1500m,
but local relief is less than 300m except where they are
dissected by widely separated canyons.
Mountains: These are areas of steep slopes with local
relief or more than 600m above the sea level.
Widely spaced mountains: These are mountains, which are
discontinuous (not in chains) and stands in isolation but
the interviewing space between the mountains have local
relief less than 150m above the sea level.
Depression: These are basins surrounded by mountains,
hills or tablelands. And these form abrupt or sharp
boundaries around the basin.
3.
h
=
d
=
g =
w
=
i
=
Erosional and Depositional Landforms of the World
The kind of geographic processes acting currently or
relatively recently in geologic time to shape the landscape
into its present form provides the basis of five classes of
areas indicated by the letters thus: h, d, g, w & I defined as
follows:
Humid landforms area: These are area in which the pattern
or permanent streams has a drainage density of one stream
for every 16km distance. These areas have not been
subjected to glaciation since the beginning of Pleistocene.
Dry/landform areas: The drainage density in this area is
less than one in 1km. Low glaciation since Pleistocene.
Most of the karst and the arid areas are included in this
zone.
glaciation area: These are areas covered by glacier ice at
sometimes since the Pleistocene time.
Wisconsin and wurm glaciated area: These are areas
covered by glacier ice during or since the Wisconsin and
wurm glaciation but today are free of ice.
Ice caps: These are area covered by glacier ice up till the
present time.
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