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INTRODUCTION TO
APPLIED GEOMORPHOLOGY
FOR
SOIL SCIENTISTS (Geopedologists)
Note:
This syllabus is made to give an outline of the material treated in the foreseen module on
geopedology. The participants are advised to refer to the distributed books and albums,
while taking their own notes.
Abbas Farshad
Earth Systems Analysis (ESA)
Surface Processes Group (Geohazards), ITC, Enschede
The Netherlands
Revised: Aug. 2006
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1. Introduction:
Soils are three-dimensional bodies covering the earth surface, meaning that they are
integral parts of landscapes. This is exactly where geomorphology plays a vital role in the
study of soils. Geomorphology is a branch of geology, which deals with the form of the
earth, the general configuration of its surface, and the changes that take place, as in the
evolution of landforms. This definition, at the same time, highlights the fact that
geomorphology is stemming from geology, which is not difficult to understand, knowing
that geologic structure is a dominant controlling factor in the evolution of landforms.
The content of the series of lectures given here, within the module foreseen for
geopedologists is determined on the basis of understanding soils and learn how to look
for them in order to map them. Before getting into the main body of the real soil-related
lectures, geomorphology has to be dealt with. In order to deal with geomorphology some
basics of geology must be treated. In order to visualize landscapes aerial photograph and/
or Landsat data (combined with DEM) are an important tool. Relief, size, shape, texture
and greytone are used to analyze drainage patterns, vegetation, land use, etc as clue for
extracting information about soils and their distribution. Geology, geomorphology and
the distribution of soils in the landscape, in different altitudinal levels and climatic
conditions help understand the soil formation, which will be dealt with in a few lectures
(General Pedology). This series of lectures will be continued with a few lectures on soilgeomorphology (Geopedology), where physiographic rules (related with erosion and
sedimentation, etc) are treated. Through a number of lectures the inter-relationships
between landforms and soils are dealt with. The result is very striking when seeing that
participants are quickly able to carry out aerial photo-interpretation, applying
geopedologic approach, that is, a procedure to be followed when mapping soils. How to
map soils is the subject of a series of lectures on soil survey methodology, which will be
lectured partly parallel with the above-mentioned subjects and for a greater part at the end
of the module.
2. Objectives:
The objectives of this series of lectures are:
a.
To learn how to recognize material and its origin, in which landforms are
developed
b.
To learn how to recognize landscapes and their components (geoforms)
c.
To create awareness of the prevailing contemporary processes (geomorphic and
pedogenic) working on them
d.
To provide sufficient technical vocabulary (jargon for proper naming of what is
visualized)
e.
To enable the participants to carry out aerial photo-interpretation on the basis of
which soil map can be made, after having done the required fieldwork
f.
To take a glance through some of the recently developed techniques in 3-D
visualization, which should help digital mapping of geoforms.
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CHAPTER 1: THE GEOLOGIC FOUNDATION; PETROGRAPHY AND
TECTONICS
The formation of our planet has been the subject of investigation since long time. Plate
tectonics, which became accepted about 20 years ago, now provides the conceptual
framework that allows geologists to understand much about the nature of mountain
building and other processes that shape our planet's surface (Montgomery, 1997; http://
www. geology.com; http://observe.arc.nasa/earth/tectonics2.html). Once the original
piece, which later became the earth ball, separated from the huge "dust ball", the earth
crust was soon formed by cooling of the surface. Although the process of cooling has
been going on continuously, the earth still contains liquid material (magma) in the outer
core, which has been either exposing as lava eruptions in different parts of the world
(volcanic activity), or injected in between the solid crusts' layers (Fig. 1).
Fig. 1: A chemically differentiated earth ball
In this way igneous (extrusive and intrusive) rocks are formed (Fig.2). On the other side,
either through heat, generated by volcanic activities, or through pressure exerted from top
heavy layers, metamorphic rocks are formed. Through weathering, transportation of the
products and the sedimentation in huge water bodies (e.g., oceans) sedimentary rocks
were formed (Fig.3).
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Fig. 2: The ways magma moves out of its origin
Fig. 3: Formation of sedimentary rocks
The building stone of rocks are minerals, which are chemical compounds with
characteristic internal structures determined by a regular arrangement of atoms or ions of
different elements, such as kalium, Natrium and Calcium (Fig. 4).
Fig. 4: Halite (NaCl); crystalizes in cubes.
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In summary, three types of rocks are distinguished (http://geology.smith. edu/msa/
teaching.html):
1. Igneous rocks (http://www.fi.edu/fellows/payton/rocks/create/igneous.htm and
http://csmres.jmu.edu/geollab/Fichter/IgnRx/IgHome.html): The igneous rocks can
form in three ways:
a.
by crystalization in different depths of the earth's crust
b.
as out-flowing or out-blown lava or ashes
c.
as minor intrusions (e.g., dykes)
When igneous rocks are classified on the basis of chemical composition of the minerals,
forming the magma, the percentage of free silica (SiO2= quartz) is used, in which case
three groups of Acid, Intermediate and Basic rocks are distinguished. Examples of these
three groups are granite, diorite and gabbro, respectively. The basic composing minerals
in these rocks are quartz, feldspar (e.g., orthoclase = KalSi3O8) and mica [e.g., biotite =
K(Mg, Fe)3 AlSiO3 (OH)2]. Some geologists consider a fourth group, namely ultrabasic, such as perioditite, with the main mineral being the peridot, which is a gem variety
of olivine.
Another very straightforward classification of the igneous rocks is on the basis of the
depths in the earth's crust where the rocks have been formed, namely extrusive,
intermediate and intrusive rocks (Table 1). Extrusive rocks (also known as volcanic
rocks), such as basalt, have been formed on the earth's surface, where a basic magma has
been exposed to the air, that is, cooled off quickly. Intrusive rocks, such as gabbro, have
been formed somewhere deep in the earth's crust, and the intermediate group
characterized by intermediate size of crystals have been formed in shallow depth,
between layers of the earth's crust. This at the same time implies that extrusive rocks are
characterized by very fine to crypto (hidden= not visible with naked eyes) crystals and
the intrusive rocks contain coarse crystals. In other words, the speed with which magma
has been cooled off determines the degree of crystalization.
Table 1: A simple classification of (some well known) igneous rocks
Extrusive
(volcanic); finely
crystalized
Intermediate; with
medium sized
crystals
Intrusive; with
relatively larger
crystals
Acid; leucocratic
Intermediate;
mesocratic
Basic; melanocratic
Rhyolite
Andesite
Basalt
Micro-granite
Micro-diorite
Micro-gabbro
Granite
Diorite
Gabbro
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As seen in the above table, acid rocks are light-coloured (leucocratic), intermediate rocks
are mesocratic, and basic rocks are dark in colour (melanocratic). Obviously, this is a
very important morphologic characteristic helping recognition of the igneous rock
specimen in the field. Leucocratic rocks contain more felsic minerals (e.g., feldspar,
quarts) whereas the presence of more mafic (ferro-magnesian bearing) minerals (e.g.,
amphibols, mica) give darker colour to the rock.
2. Sedimentary rocks (http://encyclopedia.thefreedictionary.com/sedimentary+rock)
Rocks formed by the accumulation of sediment in water (aqueous deposits) or from air
(eolian deposits). The sediment may consist of rock fragments or particles of various
sizes (conglomerate, sandstone, shale); of the remains of products of animals or plants
(certain limestones and coal); of the product of chemical action or of evaporation (salt,
gypsum); or of mixture of these materials. Some sedimentary deposits (tuffs) are
composed of fragments blown from volcanoes and deposited on land or in water. The
sedimentary rocks can, therefore, be put in the following groups:
a.
clastic sedimentary rocks: formed by fine particles, remnants of other rock types
(examples are: sandstone and conglomerate). Tuffs are pyroclastic rocks, because
their building material is ash and other volcanic materials deposited in water.
b.
chemical sedimentary rocks: formed by precipitation of aquatic solutions
(examples are: limestone, gypsum)
c.
organic sedimentary rocks: formed by living material (peat, coal)
Contrary to the classification of igneous rocks, which is straightforward, classification of
sedimentary rocks is not so easy as the latter group is more complex. For instance,
following the above grouping, where should we put the organic limestone, which is
formed by limy parts of corals, algae, snails, etc? Is it a chemical rock or it is an organic
rock? Please be aware that the classifications given here are very simplified versions of
the original ones. For further details you are referred to different books on petrography.
Sedimentary rocks are characterized by being layered, also called stratified or bedded.
Each layer is called a bed or stratum. Sedimentary beds as deposited lie flat or nearly
level. However, later (orogenic) movements (folding and faulting which lead to the
formation of mountains) disturb the original setting of the layers, leading to the formation
of anticlines, synclines, etc (Figs. 6,7 & 8;further explanation will be given in chapter on
tectonics and structural landforms) As the sedimentary rocks can be formed in different
environments, such as marine and glacial, and by means of different agents (water, wind
and ice) some physical properties such as "sorting" and "degree of rounding" and
"stratification" can help recognize the origin of sedimentary rocks.
Sorting: sediments which show no or little variation in grain size are considered sorted.
A very good example of sorted sediment is "beach deposits", whereas glacial deposits
are good examples of entirely unsorted.
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Fig. 5: Examples of grading curves on semi-logarithmic paper (after Bell, 2000)
A very common term used by geologists is facies, which implies sorting in one and the
same sedimentary package. Where the original sediments were deposited
simultaneously in a continuum of sedimentary environments so that no sharp
demarcation is formed the whole sequence will be considered as a facies. Take the
example of rivers entering a water body (the sea) depositing sand, silt and clay. The
sedimentation of the river load will be stopped somewhere but deposition of calcite
from seawater will be active. Imagine that these sediments have turned to rocks,
meaning sandstone, siltstone, claystone and limestone will all be in one layer, one
grading into the other. This is the sedimentary facies.
Degree of rounding: depending on the distance sediments have transported the
constituting grains can be angular, subangular, subrounded or rounded. River deposits,
for instance, are rounded whereas colluvial deposits contain angular grains.
Stratification: this is a very obvious characteristic of the sedimentary rocks as
sedimentation takes place layer after layer, in some cases with interruptions. This process
also brings with the law of superposition, which is a simple principle. In undisturbed
sediments, the youngest stratum is at the top, superpositioning older strata. This law is
very often used to determine the relative age of layers. Other terms which might be used
to describe morphology of sedimentary rocks, and help interpretations, are cross-bedding
and ripple marks. Cross-bedding is an inclined bedding within a thick sedimentary layer
that is broadly horizontal. Cross-bedding can be used to determine the flow direction.
This is also the case with the ripple-marks. Symmetric ripple marks are formed by water
washing back and forth whereas asymmetric ripple marks are formed by currents flowing
consistently in one direction (Montgomery, 1997).
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3. Metamorphic rocks
(www.msnucleus.org/membership/html/jh/earth/metamorphic/lesson4/metamorphic4a.ht
ml):
Under the influence of high temperatures (resulted from intrusion of magma) and high
pressure (resulted, for instance, from orogenic movements) both igneous and sedimentary
rocks can be transformed into metamorphic rocks. In geology, the former case (under the
influence of heat resulted from the intrusion of magma) is termed as contact
metamorphism, whereas when the process takes place under high pressure the term
regional metamorphism is applied. In the process of metamorphism, no addition of other
substance takes place. It is simply a solution (liquefying) of the existing material and a
recrystalization. This implies that the chemical composition of the metamorphic rock is
the same as that of the former rock, from which it originates. Take the example of
limestone, where the main mineral is calcite. When limestone is metamorphosed marble
is the result, where the main mineral is the same, i.e., calcite.
Depending on the original rock, from which the metamorphic rock is resulted, two groups
of metamorphic rocks are distinguished, that is ortho- and para-.
-Slates are relatively weakly metamorphosed shales. They are split into thin plates. The
planes of splitting form a structure called shaly cleavage, which is a new structure and not
merely stratification or bedding. The term foliation is used for metamorphic rocks to
express the morphologic feature, looking like stratification.
-Phyllite is a stronger form of metamorphism exerted on shale.
-Schists are the most advanced grade of metamorphic rock. The foliation in schist is very
well pronounced. The term schistosity which is also used to describe this morphology in
the metamorphic rocks is stemmed from the name of the rock. The most common type of
schist is the micaschist, that is, when there are a lot of mica (biotite, muscovite, etc) is
available.
-Quartzite is an metamorphic rock derived from sandstones. The main mineral is quartz
(SiO2).
-Marble is a metamorphic rock derived from limestones. The main mineral is calcite
(CaCO3).
-Gneiss when derived from strata that have been in closed contact with intrusive magmas
is termed orthogneiss whereas if it is resulted from granite it is called paragneiss (granite
gneiss). They vary in appearance, mineral composition and structure. In general, gneiss
is characterized by being banded; alternation of bands of ferro-magnesian bearing (mafic)
minerals (e.g., biotite) and felsic (quartz and feldspar).
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Fig. 6: The structure of sedimentary rocks (From: Lobeck, 1939)
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Fig. 7: Kinds of faults (from: Lobeck, 1939)
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Fig. 8: Grabens and horsts (from: Lobeck, 1939)
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Looking to the brief explanation about the rock formation, one can realize that there are
two forces having been acting to have contributed to the formation of the earth surface,
namely endogenous and exogenous forces. Example of activities falling under the
endogenous force is to find out in the text when discussing about igneous rocks derived
from magma, while the example of exogenous activity is hidden in the explanation on
sediments and sedimentary rocks, which will be dealt with while lecturing.
We have already briefly referred to the endogenous forces when talking about magma
and its mobilization, which may lead to eruption. These forces when applied on the
already existing layers of the earth crust, lead to breakage (fault) and bending (fold), a
combination of which is called geologic structures.
Folding: In the deep crust, where temperature and pressure is high folding may occur as a
result of the exertion of endogenous forces on the already existing layers. If folds are
arched upward anticlines are formed whereas bowed down folds are termed synclines.
The terms anticlinorium and synclinorium are used to imply the complex forms of
anticlines and synclines, respectively (example of the Kandy area, Sri-Lanka; will be
demonstrated while lecturing). Regarding the terminology, there are several terms such as
hinge, axis, and limb, to be further discussed using figures 9 and 10, below:
Fig.9: An anticline in the field. Notice that the foreground shows a planation surface
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Fig. 10: A syncline
Faulting: Under the influence of strong tension or compression, the layers that are
normally not so deep in the crust can break and displaced. If it is confined to breaking,
that is, without displacement the term joint is applied. But breaking layers, which are at
the same displaced too, lead to faults. Faults are projected as lines on the earth's surface,
at least when they are exposed. Besides the main line there are usually secondary fault
lines as well. The displacement of the strata on either side of the fault line can be either
horizontal or vertical. The plane formed between the two displaced sides is termed as
fault scarp, which is a shiny, groovy surface. As a result of faulting, next to the simple
faults, step faults, thrust faults, graben and horst can be formed (Figs. 6,7 and 8; also
consult Montgomery, 1997). Parallel with the endogenous forces, we also referred to the
exogenous forces, working outside the earth's crust, eroding, transporting and depositing
sediments, the origin of the sedimentary rocks. As a result of erosion, geological
structures are often occur incompletely, meaning that geologists should reconstruct them
using their remnants, which might be quite scattered. Amongst the structural features
used for this purpose, strike and dip are quite essential (Fig. 6E). Strike is a compass
direction, measured parallel to the earth's surface. Dip is displacement downward from
the horizontal, measured in degrees. Normally, these two are shown combined on
geological maps.
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Fig. 11: Dip and strike
The Geologic time scale:
Geologists have divided the life-span of the earth into a time scale (Table 2), which
consists of very long periods (era), periods and epochs.
Table 2: Geological time table
Era
Period
Epoch
Cenozoic
Quaternary
Recent-Holocene
Pleistocene
Pliocence
Miocene
Oligocene
Eocene
Paleocene
Tertiary
Mesozoic
Paleozoic
Precambrian
Cretaceous
Jurassic
Triassic
Permian
Carboniferous Devonian
Silurian
Ordovician
Cambrian
Millions of years before
the present
2
12
25
40
50
70
135
170
225
270
350
400
460
Late
500
600
Early
4600
For further details please refer to University of California Museum of Paleontology
website
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CHAPTER 2: GEOMORPHIC PROCESSES
2.1. Introduction
Geomorphic processes are characterized by lateral movement, that is, transportation of
surface material by gravity, water, ice and wind. However, as the surface material should
be produced before it can be transported, two major groups of processes can be
distinguished, namely the groups of processes producing the surface material, and the
other group of processes which are responsible for the removal of the material. To ease
discussing the geomorphic processes, different authors have tried to classify them. For
instance, Goosen (1967), while using the term physiographic instead of geomorphic,
considers two main groups, namely, erosion and sedimentation. He then makes use of the
agents and modes for further classification. Different sedimentation processes are
alluvial, lacustrine, marine, aeolian, glacial and volcanic. The modes he mentions for the
alluvial sedimentation are alluvial fan, braiding, meander river, delta and alluvial
overflow plain sedimentation. Another experienced lecturer of geomorphology (van
Dorsser, 1986) treats the subject by having separate chapters on weathering and mass
movement and on rivers and valleys. In both cases, the endogenous processes leading to
the formation of structural landforms are treated in different chapters.
2.2. Weathering and mass movement
The term weathering, stemming from weather, that is, the impact of weather on
consolidated material, is used to imply a complex process. It encompasses a variety of
physical, chemical and biological processes that lead to fragmentation, breaking large
pieces of rocks down into smaller and smaller fractions (ref. Video-film). For
instructional purposes the following three types of weathering are dealt with separately,
although in nature they work simultaneously:
2.21 Weathering (see also Juma, 1999):
a. Physical weathering:
A classic example for this type of weathering is where there is an alternation of frost
and thaw, known as gelivation. Or an alternation of hot and cold periods (for instance
hot summer followed by cold winter, or hot days being followed by cold nights) which
can lead to fragmentation (Fig. 12), sometimes in a very nice looking forms, such as
peeled-off rocks -- exfoliation.
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b. Chemical weathering:
The best known type of chemical weathering is the solution of the limestone, leading to
the formation of cavities. A very representative landscape where the solution process
plays a dominant role is the karst landscape (stereogramme below, fig. 13)
Fig. 13: An example of chemical weathering (karst landscape in Jamaica)
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c. Biological weathering:
This may be considered as an integrated process, as both physical and chemical are
involved. Take the example of a tree roots, which may lead to rock fragmentation. The
process in this case is on one hand activated through the oozing from the roots and on
the other hand works by thickening of roots (age).
The type and rate of weathering varies from one climatic regime to another. In humid
regions chemical and chemico-biological processes are generally much more significant
than those of mechanical disintegration. The rate of weathering also depends on the type
of material (rocks or unconsolidated material), its structure or bedding (horizontal layers
versus vertically exposed layers) and texture (coarser textured rocks weather easier), and
mineralogical composition (homogeneous versus heterogeneous).
2.2.2. Mass wasting:
This is a general term used for a variety of processes by which large masses of earth
material are moved by gravity either slowly or quickly from one place to another (Fig. 14
and Table 3). Mass movements are dominant sources of sediment deform the upstream
surface and its influence in downstream is shown in limiting the level arable land,
lessening water quality, water quantity, and aquatic habitat. The material moved can be
either unconsolidated, fairly fine or large, solid blocks or sheets of rocks:
Fig. 14: Slow downhill creep of soil and weathered mantle is evident (tilted pole)
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Fall: The material falling are not in contact with the ground surface below. Rockfall is a
common process on very steep slopes and cliffs in mountainous areas, although soil
blocks can also fall. The accumulated debris at the foot of the cliff is know as talus.
Rockslide: A sliding of masses of loosened blocks over a surface, which sometimes
have a lubricating function. After the melting of the large glaciers of the Ice Ages
enormous rockslides took place.
Slide (Fig. 15): The descent of a mass of earth or rock down a hill or mountain side is
termed landslide and rockslide, respectively. Aggregated soil particles -- when they are
reasonably associated in a way that water can still surrounds the aggregates -show the
risk of landslide. If the rock or soil mass moves over a short distance , the term slump is
used. Slumps in soil are often characterized by a rotational movement. It is good to notice
that (sloping) soils at flocculated state (e.g., in Mollic horizon) are the most stable soils.
Flocculated soil particles are arranged in all kinds of edge-to-edge and face-to-face
contacts. It is not at all difficult to visualize the effect of tree roots which provide lateral
support and vertically anchor and buttress the soil mass.
Fig. 15: Diagram showing a rotational slide or slump
Flow (Fig. 16): Landslides involving unconsolidated, non-cohesive material are termed
flows. In flows material moves in a chaotic, disorganized manner. Earth-flow is used
when the flowing material is fairly dry soil, whereas mudflow is used to connote the
muddy material. This can even happen in deserts, after a heavy rainfall. For soil
surveyors is good to know that mechanically the deflocculated materials are prone to
mudflow. Soil material is deflocculated when there is no interactions between soil
particles so that all particles are in suspension individually
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Fig. 16: Thin stream-like mudflows spreading out upon the piedmont
Table 3: Types of mass-wasting (terminology)
Type of material
Engineering soils
Type of movement
Bedrock
Predominantly Predominantly
coarse
fine
falls
Rockfall
Debris fall
Earth fall
Topples
Rock topple Debris topple
Earth topple
Rotational
Few units
Rock slump Debris slump
Earth slump
Rock block- Debris block- Earth blockSlides:
Translational
glide
glide
glide
Many units Rock slide
Debris slide
Earth slide
Lateral spread
Rock spread Debris spread Earth spread
Flows
Rock flow
Debris flow
Earth flow
(deep creep) (soil creep)
(soil creep)
Complex
Combination of two or more principle types of movement
Solifluction: When the upper parts of a frozen soil layer is melted and moved over the
still-frozen substratum, geomorphologists use the term solifluction. The dispersed soil
material show risk of solifluction. Dispersed mechanical state is when soil particles are
weakly associated (edge to face contacts). The following graph (Fig. 17) demonstrates the
role of clay (CEC) and the exchangeable sodium percentage (ESP) 2.3. Rivers and
valleys (Sparks, 1972)
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Fig. 17: Chart of the classification of dispersive soils, prepared for S. Africa (Bell, 2000)
2.3. Rivers and valleys (Sparks, 1972)
Rivers are (have been) forming to transport the unconsumed precipitation water.
Depending on the water supply rivers can be perennial (permanent), or intermittent
(periodical). The very hazardous river flows in arid regions are also known episodical
rivers. Looking at valleys, rivers then can be divided into two main types, namely the
braiding system and the meandering system. A braiding river system (Fig. 22) is
composed of different channels sandwiching small islands of gravel and coarse sands. A
meandering river (Fig. 19), on the other hand, is mostly made of one bending channel.
For soil surveyors it is very important to distinguish between these two systems, as the
soils of the river plains are different, in terms of texture, stratification, etc. depending on
the river system forming the plain.
Rivers when flowing in their valleys may be destructive, depending on the slope gradient,
composition and configuration of the river bed, sediment load (in the water). At the end
of their trip, where sedimentation starts, rivers can become constructive, for instance
when they contribute to the formation of alluvial fans and deltas (see
3.1.3 and 3.1.4)
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Chapter 3: Geoform systematics
In soil science as a discipline, often you'll be confronted with taxonomic classification
systems, characterized by one of the models of hierarchical, relational or network and its
constituting elements (Zinck, 1988/89).
In a two-dimensional hierarchical system there are three basic elements, namely
category, class and taxon (see lecture-notes on soil geomorphology).
The mechanism of the classification can be explained in a very simple example (see
table, below), where a population of objects is involved. Small and large coloured (red
and green) squares, triangles and circles form a population (universe), which can be
classified in different ways, depending on which of the attributes (colour, form and size)
are taken first:
Attributes
Colour
Size
Form
Red
Large
Square
Attribute states
Green
Small
Triangle
Circle
All geoforms (the smallest unit in the categorical system, including form and material, is
termed “geoform”), or all pedons (the smallest soil body to be classified) are the
individuals within the geomorphic and soil universes, respectively. The next step is where
the universe is dissected into categories. A category is a segment of a universe,
represented by a population of individuals of a given homogeneity. A category is
identified by a generic concept applicable to all individuals present at that level (e.g.,
colour, size, form in the above example). Subdivisions of categorical levels are classes,
like Ultisols and Alfisols, which are classes of a population of mineral soils. Base
saturation is the determinant attribute in this case. In geomorphology, a monoclinal relief,
for instance, can be classified into Cuesta, Creston, Hogback and Bar, according to the
dip angle (e.g., 1-10, 10-30, 3070, and 70-90). A concrete taxonomic unit being a
member of an established class at a given categorical level is the taxon (plural = taxa). A
taxon may cover only part of the allowed range of variation concerning the attributes
selected to define the class. On the basis of the same hierarchical system, which will be
further treated in soil geomorphology lectures, the following landscapes are
distinguished, on the basis of which we will continue the rest of these notes.
-valley (Va)
-plain (Pl)
-peneplain (Pn)
-plateau (Pu)
-piedmont (Pi)
-hilland (Hi)
-mountains (Mo).
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The valley Landscape
Valleys may be formed through tectonic movements, such as the Rift- Valley in
Africa or other smaller grabbens which occur all over the world, such as the one in
Petchabun, Thailand.
Normally, the term valley is associated with river. Often, valleys start with an incision
which will be further widened by lateral erosion, which might be resulted from the river
water hitting its banks. Valleys may be V-shaped and/or U-shaped depending on the
nature of the bedrock and the type of climate, that is, the amount of water involved.
Normally, three sections (also known as longitudinal profiles) can be distinguished in any
river: (1) a catchment area, comprised of many streams and tributary (smaller) valleys
that collect water; (2) a river bed (the main valley); and finally (3) where the river ends,
that is either in a water body (sea or lake) or on a vast low-lying area. The longitudinal
slope of the river valley is controlled by the so called 'erosion base level'. Rivers are
normally more erosive in areas higher than the local erosion base level. The closer the
river bed to the local base level the less erosive it is. The sediments resulted from
downcutting of the surfaces above the base level will be deposited around the areas close
or at the base level.
Very often, people apply the term 'valley' to elongated low-lying portions of land
sandwiched between higher areas. This means that smaller incisions might also be called
valleys, although often terms such as small or little valleys, tributary or lateral valleys are
used too. Valley, as it is used here is defined as "elongated, flat land portion intercalated
between two bordering, higher relief zones (e.g., piedmont, plateau, hill or mountain). A
valley is generally drained by a river, which may be confluenced by streams and/or
smaller rivers joining the main river (Fig. 18).
Fig. 18: Various concepts of “valley” (Zinck, 1988/89)
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*********************************
Notes on Fig. 18:
(1) Valley as an area where sediments of longitudinal origin, carried from catchment area, are deposited in
floodplain and terrace units occupying the valley bottom.
(2) Valley as an area covered by longitudinal as well as lateral sediments, including therefore piedmont
fans and glacis.
(3) Valley as an area directly influenced by human activities, including therefore the lower stretches of the
surrounding slopes.
(4) Watershed or hydrographic basin as delineated by water divides
(a) Piedmont
(b) Mountain
********************************
The inherited character of rivers, that is to be able to erode, transport and deposit,
dependant on the circumstances, leads to the formation of such geoforms as depositional
(or accumulation) terrace, levee, basin, etc. A river can have a straight, bending,
meandering or braiding channel. A meandering river flows in sinuous curves. The water
in the meander moves as a cork screw, leading to a movement in a perpendicular
direction, formed by the centrifugal force on the water in the bed. This type of flow
causes erosion in the outer side of the meander and deposition in the inner side. The
strongest erosion takes place a short distance away from the central part of the bend. A
meander tries to broaden and move downstream. A cutting of the meander can be formed.
In the cut-off part an oxbow lake can be formed. The zone in which the meanders are
formed is called meander-belt. Some of the geomorphologic terms you may encounter in
the literature are: meander scrolls, point-bars, swale, cut-off meanders, undercut bank,
slip-off slope, neck, meander-belt, etc (Figs.19, 20 and 21).
Recognition of these features in the aerial photographs, which is rather easy, help soil
surveyors to map soils. The relations between these geomorphic features and the soils
formed on them will be dealt with in later lectures on soil geomorphology.
A braiding river, on the other hand, is characterized by different channels around
alluvial little islands (Fig. 22a and b). The growth of an island begins as the deposition
of a central bar. The bar grows downstream and in height, and forces the water to pass
through the floating channels which deepen and cut laterally into the banks to carry the
flow. Braiding rivers are formed where a sudden change in slope exists and in rivers
with an important change in discharge.
Whatsoever the type of river (meandering or braiding), there are a number of
characteristic geoforms, such as terrace, levee, basin, etc which might be formed by the
river.
23
A
B
Fig. 19: Schematic presentation of some of the features related to the meandering
system
************************************
Notes on Fig. 19:
A: The left part (3 block-diagrams) depicts the widening of a valley by lateral cutting
of a graded stream permits the free growth and cutoff of meander bends
B: The right half of the figure demonstrates a number of features related to a meandering
river system: 1. Undercut slope; 2. Slip-off slope covered by point-bar
deposits; 3. Neck; 4. Spur; 5. Oxbow lake; 6. Fill; 7. Present river channel; 8.
Meander belt.
*************************************
24
Fig.20: The stereogram depicts a few meander-related morphologic features such as
Meander cut-offs, neck, and basin.
25
Fig. 21: Stereogram depicts some details of the point-bars (see also fig. 19)
3.1.1 The formation of terraces:
Due to an uplift of the region, a lowering of the sea-level, changing base-level or climatic
change, a river can give a renewed erosion or sedimentation. As the result of a renewed
26
erosion a new incision will take place. This incision continues up-stream, the so-called
backward erosion. The point from where this erosion took place is often marked by a
sudden change in longitudinal river profile, rapid or waterfall. If due to new erosion, a
valley floor becomes dissected, this old valley floor is transformed into a terrace.
Different terraces, one on a higher level than the other, can be formed when different
periods of new erosion existed. This type of terrace is called erosion terrace. When after
formation of a sediment cover in a valley a new incision follows, the infilling becomes
dissected, an accumulation terrace is formed. These two can be distinguished by their
sloping surface. Erosion terraces slope towards the river channel, whereas the
accumulation terrace either does not show any slope, that is, horizontal, or it slopes away
from the river channel (Fig 23). The latter slope is caused by the formation of levee,
overflow mantle and basins.
Mountain
Terrace
Floodplain
Fig. 22A: A valley; terrace and floodplain (the two relief-types)
27
Fig.22b: The stereogram depicts the change of river system, from braiding to meander.
28
It is understandable that in older river systems it may happen that a series of
accumulation terraces have been developed, in which case we talk of high, middle and
low terraces. Obviously the higher terraces are older than the lower ones.
Fig.23: Floodplain and accumulation terraces (young to old).
29
3.1.2 The formation of levee and basins:
Due to the shallowness and/or exceeding loads a river may overflow on its banks. In this
case the coarse material is deposited close to the river channel whereas the finer material
(silt and clay) is carried further on and deposited far away from the river channel.
Repetition of this event leads to the formation of thick stratified sediments, thicker and
higher close and along the river, that is the levee. Further components, after the levee, are
overflow mantle, and overflow basin (Table 4). This configuration forms the latitudinal
profile of a river valley. Although this construction is more visible in floodplains and on
lower river terraces, in some cases however remnants are seen on older terraces too (Fig
24).
Table 4: Alluvial and colluvial landforms (Zinck, 1988/89)
Erosional
Ablation surface
Rill
Gully Badland
(complex gully)
Depositional
Load excess facies:
Point bar complex
Distributary levee
River levee
Deltaic levee
Splay axis
Splay mantle
Crevasse splay
Splay fan
Splay glacis
Overflow facies
Overflow mantle
Overflow basin
Decantation facies
Decantation basin
Backswamp
Ox-bow lake
Infilled channel
Colluvial facies:
Colluvial fan
Colluvial glacis
30
Fig. 24: Levee remnants (Mississippi river)
Back to the longitudinal profile of a river, as it was mentioned earlier in the text, the third
section in this profile is when the river is ending, either spreading out its load on a vast
surface or entering into a water body.
31
3.1.3 Formation of alluvial fan
When a heavily loaded river or stream emerges from hills or mountains onto a low-lying
area, that is, when a marked change in gradient occurs, the river load will be deposited, in
a fan-like form (Thornbury, 1969). The apex of the fan corresponds with the end of the
river or stream valley, with its base (fan-base) away from the apex, depending on the
speed and quantity of the water and the amount of load. The material comprising a fan
varies in texture from coarse to fine and finer. The fan deposits in a fan, which is formed
at the end of a short river or a stream will be much coarser in texture than the deposits of
a fan formed at the end of a long river. In the former case, the occurrence of boulders and
gravel, at the apex, is common. In the latter case, the coarsest texture will be sand and/or
sandy loam integrating into silt and clayey textures. In mountainous areas, where minor
streams descend from higher parts, very steep fans are formed, known as alluvial cone. In
some cases, where gravity dominates colluvial cones would form. A series of adjacent
fans may in time coalesce to form an extensive surface at the foot of mountain, which is
called bajada or bahada. Some authors have called it piedmont plain, in particular when it
is extensive. The building of fans takes place largely during flood times, when great
volumes of water with accompanying alluvium debouch onto them. A typical fan is
nearly semi-circular in plan but where stream gradients are very steep, the fan may be
elongated in a direction perpendicular to the mountain or hill front against which it has its
apex. The radial profile of a fan is typically concave whereas its cross profile is convex.
The drainage pattern is distributary, also called dichotomic. For soil surveyors, it is very
useful to carefully study and understand the fan (-modal) sedimentation process, as this
governs quite a bit of the field of physiography and soils. Besides the slope gradient,
which is varying longitudinally (from apex to the fan-base) and laterally, other attributes,
important to soil surveyors, such as texture, drainage, etc follow a pattern, both
longitudinally and transversally (Fig. 25a, b, c).
Fig. 25a: Alluvial fan (Apex to base)
32
Fig. 25b: Schematic presentation of the way grains distribute (granulometry)
Fig. 25c: A schematic view of how fans coalesce laterally to form piedmont (bahada!)
3.1.4 Formation of delta
Somewhat analogous to fan, delta follows, more or less the same line of construction, that
is, delta forms when the river ends, this time in a water body. This however does not
mean that all rivers entering a water body, sea or lake, will form delta. Normally,
turbulent water bodies are not the right environment for deltas to be formed. If water is
too turbulent estuaries may form. Deltas are also not formed if rivers carry little load
33
(Selby, 1989). Although much can be said about deltas and delta formation but as a soil
surveyor it is more important to know about deltaic deposits (Fig. 26). Ideally, delta
formation begins with the deposition of beds that are close to the shore and, in all
probability, relatively coarse grained. The so-called early foreset beds are deposited
under water and have steeper slopes than the underlying floor. The inclination of the
slope depends upon coarseness of the material, the subaqueous floor profile, the stream
regime and the nature of current and wave action of the water body. The fine portions of
the load that can be carried by the river and water currents for great distances are very
broadly distributed, forming the so-called bottomset-beds (Thornbury, 1969; Selby, 1989)
Fig.26: A schematic example of sedimentation in water, formation of delta
3.2. The plain landscape
Large, flat, unconfined, low-lying land portion with low relief intensity (1-10m of
altitude differences) and gentle slopes (generally <3%) is termed plain (Zinck, 1988).
The term plain is a general term and used to label surfaces with the above morphologic
characteristics. This means that plains might be resulted from different processes (e.g.,
erosion, sedimentation). A plain may be formed as a result of several rivers joining to
form a complex fluvial system, that is, alluvial plain. A plain can also be formed by other
processes, for instance, under karstic regime (Fig. 38), aeolian or volcanic activities,
which may lead to the formation of flat, large surfaces.
34
3.2.1 Some notes on sea action and the resulted geoforms (coastal phenomena):
A coastal area is a frontier region between land and sea. This is an area where deltas
locate, but also beach, sand bars and dunes (Figs. 27, 28, 29, 30, 31, 32, 33, and 34). The
processes are those influenced by waves, currents and wind, however, crystallized as
special system of sedimentation and erosion. It is special because sea is the environment
where not only all rivers, glaciers, dusts (wind products) are ended in but it is also
influenced by tectonic and volcanic activities.
Fig.27: A schematic view of the beach profile Fig.31: Formation of spits
Fig. 28: Schematic view of the origin of the coastal currents: A=the vaws approach the
beach obliquely; B= the vawes approach the beach steeply (perpendicular to coast)
35
Fig. 29: Barrier islands
36
Fig. 30: A part of delta on aerial photograph; showing also interlocking drainage pattern
Fig. 31: Formation of spits
37
Fig. 32: An example of delta/ spit on aerial photograph (Lake Titicaca)
38
Fig. 33: Vertical section through a sea cliff shaped by wave action
39
Fig.34: An example of marine terraces; Livorno, Italy.
In brief, waves, tidal currents, and the wind are the most active agents affecting coasts.
The above figures show a few features and geoforms, which will be explained while
lecturing. Cliffed coasts (Fig. 33), spits (Figs. 31 and 32), marine terraces (Fig 34) and
coastal dunes (Figs. 35 and 36) are a few coastal forms to be named in this short series of
lectures on geomorphology.
40
Fig. 35: Coastal plain, Averno Portugal (Landsat TM; Fieldwork area of former Soil
Science Division, ITC)
41
Fig. 36: A part of the coastal plain depicted in fig. 35 showing beach dunes
3.2.2 Short notes on sedimentation, leading to the formation of plain:
Different types of sedimentation, which may lead to the formation of a plain, are alluvial,
Lacustrine, aeolian, glacial. Usually, when we speak of alluvial sedimentation, one may
think of river (braiding and/or meandering) sedimentation. Often, in these cases the
landscape is the valley. However, there are cases, where very extensive flat areas have
been formed, without a river being, involved. In these cases, the term plain is an
appropriate one (alluvial plain). In some other cases, where for instance the bottom of a
vast lake is exposed as a very vast flat area, or a level area has been formed by wind
action, or by glacial activities, lacustrine plain, aeolian plain or glacial plain will be the
appropriate terms to be used (see our notes on aeolian and glacial geoforms)
42
3.2.3 Short notes on karst:
Rock solution was earlier discussed in connection with weathering (Fig.13). In areas with
calcareous bedrock (limestone, dolomite and in some cases gypsum) and excessive
amount of rainfall karstic activities will lead to the formation of karst geoforms, such as
sinkholes, lapis, etc. The word karst is a comprehensive term applied to limestone or
dolomite areas that possess a topography peculiar to and dependent upon underground
solution and the diversion of surface waters to underground routes. The term comes from
the narrow strip of limestone plateau in Jugoslavia and adjacent portions of Italy
bordering the Adriatic Sea, where there exists a remarkable assembly of features
dependent upon subsurface solution. There are four conditions essential to full
development of karst (Thornbury, 1969):
1.
a (relatively) soluble rock such as limestone, dolomite and/or gypsum must be
present near the surface,
2.
rock structure should be very massive (dense), thinly bedded, highly joined (Fig.
37, A picture from El-Torcal, Antequera, Spain)
3.
the existence of entrenched valleys below uplands underlain by soluble and well
-joined rocks
4.
a good amount of rainfall.
As we do not intend to discuss the karst extensively, we will mention a few features,
which are very characteristic for karst areas:
Lapies: bare, etched, pitted, grooved, rather smooth surfaces in karst areas. Rill-lapies
is a type you see in Fig. 37, in El-Torcal, Spain.
Sinkholes: the most common feature in a karst landscape is the sinkhole and the
different forms of holes or depressed (negative relief) areas such as dolines, uvalas,
cockpits, etc
Karst towers, cones, and poljes are some other residual karst ggeoforms, which will
be explained while lecturing.
Table5: Karstic geoforms
Relief-type (subgroup level)
Conical karst (dome)
Tower karts
Labirinth karst
Hill (hum)
Polje (karst plain)
Dry vale
Canyon (collapse vale)
Landform (subfamily)
Lapies
Shallow hole (ponor)
Sinkhole (doline)
Intergrown sinkhole (uvula)
43
Fig. 37: A view of El-Torcal, Antiquera, Spain
44
45
3.2.4 Some short notes on glacial activities and the resulted geoforms:
Glaciers are major bodies of ice which can move under the influence of gravity. They are
of two main types, namely ice-sheets and valley glaciers. Obviously, in ice sheet ice
movement is not confined laterally, but in a valley, friction with the valley walls reduces
flow rate in the center of the glacier and at the surface, with a decline towards the walls
and base. Differences in pressure exerted on the ice, depending on the location on and in
the ice body, cause crevasses, which are the best ways for penetrating boulders, gravel,
and other fragmented rock pieces in the ice. This material --called the moraine-originates from the bottom of the ice (bed) and from the mountain surrounding the
glaciers. The material is transported under the ice, in the ice and on the ice and form, if it
is deposited, the different moraines -- ground morains, medial, lateral and terminal
moraine. Glacial deposits are composed of all sizes of material and they show no
stratification. In the inland ice areas the ice sometimes pushed ridges from the material
that was already present. An example of this can be observed in several places in the
Netherlands, for instance in an area near to Arnhem, about an hour driving from
Enschede. This types of ridges are called pushed ridges. Sometimes, the term 'pushed
moraine' is used which is not necessarily correct, at least not everywhere. In Hattem, near
Arnhem, for instance, the main body of the pushed material is a (braiding system) river
deposit. Terminal moraine is formed after the ice has melted off. In another round when
the ice is extended again, the terminal moraine becomes overridden by the ice, leading to
the formation of drumlins .
The meltwater of the ice transports material, so called fluvioglacial material, of different
sizes, but mainly clay, sand and gravel. The sand and gravel is deposited in front of the
ice with a horizontal stratification, called outwash. Besides the forms mentioned here,
there are many others, some of which are given in the following table (Table 6).
Table 6: Nival -glacial - periglacial landforms Gelifraction crest Solifluction Fluvioglacial outwash "fan" (sandur) Proglacial "fan" Kame Esker Os (Osar) Patterned ground
Stone stripes Gelifraction scree "fan" Gelifraction scree talus Periglacial solifluction
Erosional (denudational)
Depositional (accumulative)
Nival
Perenne snow mantle
Snow avalanche "fan"
Snow avalanche corridor (track)
Nivation cirque
Glacial
Crevasse field
Polished and striated surface
(roch
moutonnees)
Glacial cirque
Threshold
Overexcavation hollow
Glacial trogh
Glacier ice
es
Inland ice
Dead-ice depression
Ground moraine
Frontal (terminal) moraine
Lateral moraine
Median (Central) moraine
46
Glacial shoulder
Hanging valley (gorge)
Horn (nunatak)
Push moraine
Drumlin
Blocks stream
P eriglacial
Fluvio-glacial mantle
3.3. Peneplain Landscape:
Gently undulating land portion, characterized by a pervasive repetition of low hills,
rounded or elongated, with summits of similar height, separated by a dense, reticular
hydrographic network. They form either by dissection of a former plain or plateau, or by
down-wasting and flattening of an originally rugged land surface (Zinck, 1988/89).
The peneplanation process, related to the concept of 'cycle of erosion' of W.M. Davis, is
well known to all who studied geology and/or geomorphology. Davis used the concept of
'base-level' to explain the process of peneplanation. He considered three cycles in the
process, namely, youth, maturity and old age. These phases reflect the a series of events
from the uplift of a landmass above sea-level through the formation of a stream network,
which helps down-wearing of the surface to the base-level. In this way, a plain-like or an
almost-plain (peneplain) forms, including, here and there, isolated hills, known in this
case as inselbergs. Some inselbergs rise straight from the level surface area (almostplain), but others are surrounded at their foot by a gently sloping rock platform called
pediment. This probably was the reason that Walther Penck criticised the peneplanation
process as was raised by Davis. Penck and later on, Lester King (King, 1967) believed
that slopes were eroded and retreat laterally, keeping their original slope angle. The
process is known as the pediplanation (Birot, 1965). Whatsoever the reason is and in
what manner peneplains are formed, for soil surveyors is important to realize that
peneplains are erosional surfaces, meaning that the soil forming factor 'parent material'
and 'time' (from the Jenny equation) play dominant role (Ref. Example photograph: from
fieldwork area of Ho in Ghana). Topographically, catena and/or toposequences will be
formed where the Rhue model can be of use to come to geoforms, that is, summit,
shoulder, backslope, footslope and toe-slope facets.
3.4. Plateau landscape:
Large, flat, unconfined, relatively elevated land portion which is commonly limited on at
least one side by an abrupt descent (escarpment) to lower land. It forms frequently by
tectonic uprising of a former plain, subsequently subdivided by the incision of deep
gorges or valleys. Plateaus can also have other origins, such as fluvial or volcanic. In any
case, plateaus are topographically table-like (Fig. 39). A very high river terrace of an old
river system, being now far away from the current river system is considered as a plateau
(Ref.: Example photo Spain, also Fig. 23).
47
Mesa
Escarpment:
-Scarp(Cliff)
-Debris talus
Fig. 39: Plateau; mesa and escarpment (two relief-types)
3.4.1 Short notes on volcanic activity and the resulted landforms:
As mentioned earlier, volcanism includes all natural processes resulting in the formation
of volcanoes, volcanic rocks, lava flows, etc. To geomorphologists, volcanic activities are
not necessarily destructive. Volcanism leads to the formation of a large variety of
landscape types including mountain, plateau, valley, etc. (Figs. 41, 42, 43 and 44). This
means that volcanic activity can be constructive (see the video-film).
Table 7: Volcanic geoforms
Relief-type
Depression
Cone
Landform
Crater
Caldera
Maar
Lake
Ash cone
Cinder cone
Spatter cone Shield volcano
48
Flat
Mesa
Cuesta
Hogback
Bar
Dike
Escarpment
Strato volcano
Cumlo volcano
Lava flow Pillow lava Fluvio-volcanic
flow Cinder field Ash mantle
Hanging lava flow
Sill
Longitudinal dike
Annular dike (ring-dike)
Volcano scarp
Neck
Volcanic plug
Fig. 40: A view of a compound volcanic cone
49
Fig. 41: Plan view of a composite cone
3.3.2 Short notes on tectonics and the resulted (structural) landforms:
Earlier in the text, folding and faulting were shortly discussed. Tectonic activity is one of
the fastest activities that can change the ace of the earth from one moment on to the other.
Reference should also be made to the chapter 3.6, on hilland and mountains, as many
hills and mountains are structural in origin (Table 8) Table 8: Structural landforms:
Fig. 42: Schematic cross section of Kali Konto (Indonesia) upper watershed
50
Fig. 43: Geomorphological sketch-map of a part of the Kali Konto area (see also figures
44, and 45; air photo’s depicting parts of the area surrounding the lake and the hilly area
to the east of the map)
51
Fig. 44: The aerial photograph covering a part of the lake (see fig. 43)
52
Fig. 45: The aerial photograph depicting the eastern part of the map of figure 43 (Kali
Konto area, Indonesia)
53
Relief-type
Primary
Cuesta (1-10)
Creston (10-30)
Hogback (30-70)
Bar (>70)
Mont (anticline)
Val (syncline)
Overthrust sheet
Klippe
Fault escarpment
Horst
Graben
Tilted fault-block
Derived
Monoclinal
Doubled cuesta
Flarion
Outlier hill
Orthoclinal (subsequent)
depression
Cataclinal (consequent)
depression
Anaclinal (obsequent) vale
Cataclinal gap
Landforms
Scarp
Debris talus
Sytuructural surface
Substructural surface
Floded (Jurassic)
Excated anticline
Hanging syncline
Combe
Cluse
Ruz
Chevron
Creston
Folded (Appalachian)
Truncated anticline
Bar
Hanging syncline
Folded (complex)
Creston of overturned flank
Escarpment of faulting fold
Faulted/ fractured
Faultline escarpment
Fault escarpment facet
For further understanding the meaning of the terms, given in the table above, see the
Example air photographs and/or satellite images (hard copies).
Fig. 46: A structural landscape composed of cuesta, mesa, butte and escarpment
54
3.5 . Piedmont landscape:
Any sloping surface at the foot of more elevated landscapes, such as plateau, hills or
mountains, is considered as piedmont (Zinck, 1988/89). Originally, piedmonts may be
degradational (erosional surfaces) and/or accumulational (as a result of deposition of
materials transported from a distance. In this context, alike other terms used here (e.g.,
peneplain, which is used because it means “almost a plain”) piedmont also is simply used
for the surfaces at the foot of mountain, as it means “foot of mountain” (pied=foot, and
mont=mountain). In practice, it will be noticed that piedmonts are formed as a result of
pedimentation and/ or joining fans (coalesced fans). Be sure not to confuse these two
terms ‘piedmont’ and ‘pediment’, as the former is a physiographic term, whereas
‘pediment’ is a geomorphologic term, relating to the pedimentation process (see under
3.3 the discussion on peneplain and pediplain). We also will discus the confusion often
raised on the difference between the terms ‘glacis’ and ‘pediment’. To simplify, the
following statement can be used: Pediments are glacis, but not all glacis are pediments,
knowing that we have erosion glacis (pediment) and accumulation glacis (Example
photo: Sbeitla, Tunisia).
3.6. Hilland and Mountain:
These two landscape units are rugged and elevated. Hilland is further characterized by the
repetition of hills and/or ridges, with uneven summit heights, separated by a rather dense
hydrographic network. Hilland can be mistaken with very high dissected glacis surfaces.
However, the major difference between the two is that in hilland tops (summits) are not
concordant.
Mountain is defined (Zinck 1988/89) as elevated, rugged, deeply dissected land
portion, characterized by:
.
•important relative height in relation to lower-lying, surrounding landscape units
.
•important internal dissection, generation high relief intensity
Mountains are often structural (result of orogenesis), but volcanism can also lead to
mountain forming. Mountains are often very complex and composed of hills/ ridges
and mini-valleys (vale) and/or mini-plain (glacis)
Hilland, on the other hand, can be structural, but also erosional, for instance, where
old very high river terraces are eroded (denudational surface). Another case is the
forms resulted form wind action, such dune-land, where hills (dunes) are aeolian
forms.
55
3.6.1 Notes on wind action:
Contrary to the water action, which follows a top-down trend, wind moves in all
directions. Wind-affected areas are mainly concentrated in wide areas in arid regions
and in coastal areas. The reason for this is that both areas are bold, free of dense
vegetation cover or other obstacles.
It is not difficult to visualize the processes that result from wind action, namely
detachment, transportation and sedimentation. The abrasive action of wind in arid
regions is a classic example of detachment, called also corraison, that is, the natural
sandblast action of wind-blown sand.
Concerning the process of transportation by wind, three types of movements are
distinguished: 1. Suspension; 2. Saltation; and 3. surface creep. Suspension is important
in the transport of finer material (dust). The fine particles of lime suspended in a turbulent
flow of air have been considered as an important cause to form calcic horizons in arid
regions. In general, this process has not been adequately studied. Saltation is a bounding
movement, resulting from impact and rebound of wind driven sand. Saltation can be
described as a trajectory-like path, resembling the way frogs move on the land. Surface
creep is resulted from the impact of sand grains moving by saltation. This is a slow
movement in the form of creeping on the surface.
Sedimentation takes place when the grains fall down because the moving force
becomes insufficient to carry them forward. Most of the material carried by
suspension is deposited in this manner.
In arid regions, where lack of water and sparseness of vegetation facilitate wind action,
the largest and most active sand areas occur, forming large physiographic units of which
the sand features are easily recognized on aerial photographs, even without a stereoscope
(Table 9). But aeolian deposits also occur in non-arid regions, superimposing other
landforms. These are recognized by their contrasting tones, shapes and windswept or
streamlined appearance (Example photographs: from Colombia, and Zambesi river
plain).
The aeolian sediments are best known as dunes, sandsheets, coversand and loess
blankets. Dunes, being the most common sand depositional geoform, consist of sand,
but in some cases crumbs of clay of sand-size may behave as sand grains, forming dunes
too. Two types dunes are distinguished: Longitudinal dunes (Fig. 47) are long, narrow,
more or less symmetrical ridges parallel to the prevailing wind. Their crests may be
rounded or sharp, with many peaks and depressiuons. They may be several thousands
meter long, free of vegetation, with a linear arrangement, which is clearly visible on air
photos (Fig.35 and 36; Aveiro, Portugal)
Transverse dunes always occur in clusters in association with large source areas of
sand. They are formed by reworking into a series of roughly parallel ridges and
56
through, straight or gently curving. The long axis of each dune is transverse to the
prevailing wind.
Barchans and parabolic dunes are sometimes considered as special types of transverse
dunes. These types look like horse-shoes and are not difficult to identify. The horns of
barchans point downwind whereas those of the parabolic dunes point into prevaing wind.
The latter type can be vegetated.
The internal structure of sand deposits is important to soil surveyors. Wind-blown
deposits may exhibit one or more of the cross-bedding, lamination, and quick-sand
structure features. -Cross-bedding results from the changes in wind direction and varying
angles of
deposition along the dune sides.
-Lamination is visible when the sand deposit-profile is wetted by water. Laminaes are
formed with different texture (fine and coarser sand-grains) as a result of varying
wind velocities during accretion. Accretion deposits are a result of a combination of
saltation and surface creep.
-Quick-sand or abrupt changes from well compacted to poorly compacted structure is
also believed to result from changes of wind velocities and the type of surface
(level or rough) on which the deposition occurs. Deposition by accretion occurs on
more level areas in hollow-like places, whereas deposition by encroachment takes
place when the surface upon which deposition takes place is not smooth but is
marked by an obstruction such as an abrupt rise or drop.
Table 9: Eolian landforms:
Erosional
Depositional
Yardang
Deflation basin (blow-out hollow)
Stony deflation surface (reg)
Rocky deflation surface (hamada)
Barkhane
Nebka
Parabolic dune
Longitudinal dune
Transversal dune
Star-shaped dune
Pyramidal dune
Reticulate dune
Eolian levee (associated with deflation basin)
Generalized sand mantle
Loess mantle
57
Fig. 47: Lonitudinal sand dunes on aerial photograph (stereogram)
58
4. A glance through recently developed techniques in 3-D visualization,
helping digital mapping of geoforms
A digital terrain model is a mathematical (or digital) model of the terrain surface (Li et al,
2005). The mathematics takes care of the interpolation process, which has been advanced
with increasingly efficient and cheap computation power and storage, availability of
digital contour, stream, and orthophotographic data (http:\\
www.ffp.csiro.au/nfm/mdp/softdem.htm). Li et al. (2005) classify the surface modeling
approaches as: 1. point-based modeling, 2. triangle-based modeling, 3. grid-based
modeling and 4. a hybrid approach combining any of two of the three approaches. The
required data for the digital terrain modeling may come either from field survey (eg., use
of conventional surveying instrument or GPS), from stereo pairs of aerial (or space)
images using photogrammetric techniques, or from digitization of the existing
topographic maps. The latter source is the most commonly used technique, although more
and more people make use of the freely available DEM’s, down loadable from SRTM
(Shuttle Radar Topography Mission) at http://srtm.usgs.gov/. However, this product (with
90m resolution, except for the USA, with 30m resolution) won’t satisfy those who need
high resolution data.
Almost all well known commercial GIS packages are equipped with a sub-module taking
care of generating DTM. ARCINFO, for instance, is equipped with ANUDEM, a
program developed in the Centre of Resource and Environmental Sciences of the
Australian National University in Canberra, which supports production of grid-based
DEMs using contourline map. Or in ENVI software, the sub-module “topography”
supports generating DTM using ASTER images. GRASS GIS software is also equipped
with a number of terrain analysis procedures, especially for hydrological modeling and
erosion mapping. There are also a few freely available packages, such as TARDEM and
TauDEM developed at the Utah Water Research Laboratory.
Fig. 48 : An example of DTM, Namchun, Thailand
59
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