Advanced Geomorphology

Advanced Geomorphology

Montri Choowong, Ph.D

Department of Geology Faculty of Science

Chulalongkorn University Bangkok, Thailand

First version as lecture note distributed to student in 2006 © 2008 Montri Choowong This book is prepared for “Advanced Geomorphology” and “Geomorphology” courses of the Department of Geology, Faculty of Science, Chulalongkorn University. Some modifications, transmission or reproduction may be made with the permission from author in writing form.

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Preface This book was developed from a lecture note over the years while teaching “Advanced Geomorphology” and “Geomorphology” in the Department of Geology, Faculty of Science, Chulalongkorn University. The book itself is not intended as a complete account of “all you need to know about geomorphology or its related sedimentology”. But rather offer definitions, explanations of those parts of geomorphology and related sedimentology to which students most often find difficult to understand. It is intended primarily for postgraduate and undergraduate students. Geomorphological study in the present day becomes very important in terms of how to explain the processes of the recent geological changes. I hope all students who read this book will appreciate by now that the book will help acquire the knowledge of the basic essentials of geomorphology, leading to advance it with some other related subjects like sedimentology, even remote-sensing. Also, the benefit of the book is that student can easily find references at the end of each chapter for their continue reading and researches. Apart from my written chapters, this book is partly updated from some selected content in book of a training course “Introduction to Quaternary Geology”, edited by Dr. Narong Thiramongkol and Dr. J.A.M. Ten Cate. Both senior editors combined all essential basic knowledge of geomorphology and Quaternary Geology together under training program sponsored from The Committee for Co-ordination of Offshore Prospecting in East Asia (CCOP) held in January 1984. In that book contains two volumes with a large number of experts written in their easy reading styles. I use some major parts in that book for several years, developed it as my own lecture note. I found that it has direct benefit to students, if I can include more advanced research result into it. There are six parts, Part I starts with a brief introduction to basic principles underlying the study of forms, landforms. Definitions and terms of some essential geomorphological terms are described and summarized. The use of geomorphological map and some of its applications is also introduced. Coastal landforms and range of coastal variation and sediment dynamic on the coast is at the end of the Part I. Part II deals with the description of sedimentary environment and processes to which they often mentioned to relate with geomorphological processes and landforms. Summary of example research findings in palynology and paleontology studies in special relation to the variation in stratigraphy is the main theme of Part III. Among landform development, the Quaternary process is of significant to involve with the most recent evolution of landforms. Therefore, Part IV deals directly with the process normally occur in Quaternary period. Special attention is paid to the change in climate condition leading to the change in global sea-level variation. At the end of this part, the most up-to-date “Quaternary Geology in Thailand” chapter was introduced that is based solely on the on-going publication in book series “Geology of Thailand” that will be officially published by the Geological Society of London. This part is one sample of what Thai geologist did in the part in the theme of Quaternary research. Part V includes some techniques that often apply for basic and advanced geomorphology. Students

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will learn the field and laboratory concept to be able to conduct their future researches. Drilling and sampling method are among the significant technique students strongly need to know. In the last Part VI, the weathering product from the change in climate and physical properties of surface material will de discussed in terms of how soil developed through time. Special emphasis is on the place where tropical climate is dominated.

Montri Choowong, Ph.D

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Acknowledgements Apart from my written chapters, permission to make this book possible, definitely, has come directly from Dr. Narong Thiramongkol, main editor of CCOP training course manual. Dr. Narong has encouraged me to modify part of his book to be appropriately updated for my teaching in Geomorphology and Advanced Geomorphology courses. He cheered me up every time we met and guided to include some more up-to-dated data, for example, Quaternary and geomorphology of Thailand. Until now, I have been spent almost 5 years developing this book, starting from make it as a lecture note. Surely, this is not a final version, but it will be developed again and again by the next coming years. Thanks are also to the original articles by several experts I selected and modified their written to include in this book. They include Dr. J.A.M. Ten Cate, Dr. J.A. Okkerman, Dr. R. Hillen, Dr. I. Khemruenromna. People who made this book possible include my former M.Sc students. They are Pannipa Tien, Rottana Ladachart and Rattakorn Songmuang (passed away). Without their will and power, I cannot finish this for sure. Also, this book tributes to Rattakorn. The comment from students has made a lot of improvement arisen to the book. I learned a lot on what are the main points students often find difficult to understand. Those comments are of my appreciation and inspired me to modified and also updated figures and tables in all chapters. Thanks are to students indeed. Finally, I am, of course, solely responsible for any shortcomings or errors that may remain in the text. As stated earlier, this book is not the final version of “Advance Geomorphology”, it rather provides a summary of what students need to know as a basis. More up-to-date idea and result of more recent research will surely be included in the future. However, with my strong intension to do so, I wish this version of the book will be able to inspire geology students who eager to know more about “Geomorphology” as we named this book “Advanced Geomorphology” as its name implied.

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Contents Page

Preface i Acknowledgements iii Part I: Geomorphology 1 1.1 Definitions: landforms and forms 1 1.2 Processes 10 1.3 Landforms in the humid tropics 19 1.4 Geomorphological maps and some of its applications 32 1.5 Range of coastal variation and sediment dynamic on the coast 37 Part II: Sedimentology 42 2.1 Sedimentary environment I 42 2.2 Sedimentary environment II 64 2.3 Sedimentary processes 77 Part III: Palaeotology 90 3.1 Palaeotology and stratigraphy 90 3.2 Palynology 98 Part IV: Quaternary 107 4.1 Introduction to Quaternary 107 4.2 Climate and climatic changes 110 4.3 Sea level (short, medium, long and very long-term) 117 4.4 Quaternary geology in Thailand 126 Part V: Geomorphological techniques 156 5.1 Geomorphological survey and mapping 156 5.2 Grain size determination and mineral separation techniques 164 5.3 Drilling and sampling 174 Part VI: Soil process 180 6.1 Main soil groups 180 6.2 Weathering and tropical landform evolution 189 Author Biography 204

Advanced Geomorphology Montri Choowong, Ph.D

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1.1 Definition: Landforms and Forms

1. Landforms In landform studies slope is implying any inclined geometric element of the

earth's surface. The ground surface is composed of sloping and flat elements (linear elements), and of convex and concave elements, several of which may combine in a single landform.

On the basis of absolute height (height above sea-level), relative height (difference in height between two forms), and external shape of the form we may distinguish for example the following list of landforms:

I. Positive forms A. Mountain

A1 Very high mountain A1a Rel. height > 800 m A1b Rel. height 500 < h < 800 m; abs. height > 2000 m A2 Moderately high mountain A2a Rel. height 500 < h < 800 m; abs. height < 2000 m A2b Rel. height 500 < h < 500 m; abs. height > 2000 m A3 Low mountain A3a Rel. height 300 < h < 500 m; abs. height < 2000 m A3b Rel. height 200 < h < 300 m; abs. height > 2000 m A3c Rel. height 200 < h < 300 m; abs. height < 2000 m

B. Hill B1 High hill B1a Rel. height 100 < h < 200 m; abs. height > 2000 m B1b Rel. height 100 < h < 200 m; abs. height 500 < h < 200 m B1c Rel. height 100 < h < 200 m; abs. height < 500 m B2 Low hill B2a Rel. height 50 < h < 100 m

C. Hillock C1 High hillock C1a Rel. height 25 < h 50 m C2 Low hillock C2a Rel. height 5 < h < 25 m

D. Rise D1 High rise D1a Rel. height 1 1/2 <h < 5 m D2 Low rise D2a Rel. height 1 1/2 <h < 1 1/2 m

E. Plateau E1 High plateau E1a Rel. height > 200 m E1b Rel. height 30 < h < 200 m; abs. height > 500 m


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E2 Low plateau E2a Rel. height 30 < h < 200 m; abs. height < 500 m E3 Plateau-like form E3a Rel. height < 30 m

F. Terrace A subdivision of terrace forms could be based on the height above the main

stream for example; F1: < 5 m; F2: 5 < h < 30 m; etc.

G. Fan-shaped form H. Slope (slope-angle > 2°), see paragraph 2: slopes K. Slightly inclined surface (slope-angle 1/2 < degree < 2°) II. Neutral forms L. Plain III. Negative forms M. Depression N. Valley

A subdivision of valleys could be based on the depth of the valleys for example N1: < 5 m depth; N2: 5-30 m depth, etc. the type of valley can also be indicated for example V-shaped valley, flat-bottom valley, etc.

2. Slopes

A knowledge of slope angles is necessary for the study of present-day processes and to understand the development of the relief. Measurement and direct representation of slopes are needed for most geomorphological studies.

The sources of slope data are either estimates or measurements in the field, on air photo's, or on a map. Slope angles can be easily determined from contour maps (slope lengths and differences in height):

tg A = h P Where A is the slope angle in degree, h is the difference in height in metres

and P is the horizontal projection of the slope distance in metres. In the following slope categories, notes are given on a) landforms, b)

characteristic present-day and partly past geomorphological processes, and c) economic values:


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0°-2° plain and slightly inclined surface a) floodplains, terraces, slopes of watersheds, planation surfaces,

slightly undulating areas b) sheet wash absent or relatively weak, beginning of soil erosion c) no obstacles to walking, road and railway transport; best conditions

for the construction of houses, settlement and industries; mechanization of agriculture and forestry possible

2°-5° Gently inclined slope a) some dune areas and valley sides, slopes of terraces, undulating

areas, fan-shaped forms b) mass movements start (solifluction, rill-wash and sheet-wash, soil

erosion). Some soil protection in agricultural regions necessary c) transport difficulties for wheeled vehicles; cultivation still possible

with large tractors. Cultivation in the direction of contours advisable. Difficulties with irrigation plants. Still favourable conditions for the construction of settlements and industries

5°-15° Strongly inclined slope a) valley sides, slopes of terraces and plateaus, dunes b) strong erosion (sheet and rill erosion), mass-movements of various

kinds. Soil protection in agricultural areas is necessary c) difficulties in transport, and in road building. Ploughing without

contour terraces impossible. Difficulties for tractors. Problems in construction of settlements and industries

15°-35° Moderately steep slopes a) typical valley sides in mountainous areas, inactive coastal cliffs b) intensive denudation processes, great menace by soil erosion c) limit of cultivation; walking exhausting; limit for road transport with

special vehicles; above 25° no possibility for agriculture or house building, and predominantly forest

> 35° Very steep slopes a) undercut slopes of valleys, slope of hogbacks and cliffs, canyons b) intensive denudation and erosion c) mostly forest, but also limit of utilization in forestry; very difficult to

walk; cultivation not possible; debris fall Slopes are full of irregularities. A slope, which appears smooth from a distance, may be rough in reality. If the slope is being compared with slopes in contrasted morphogenetic regions, the overall characteristics may be more important, but if it is being compared with other slopes in the same valley or some general areas, then the detailed characteristics will be significant. Savigear (1967) has suggested that problem of scale in slope studies may be tackled by recognizing that a hillslope from divide to stream channel consists of a number of components, which are themselves composed of a number of smaller forms called units (figure 1).


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3. Erosional and depositional zones of slope components As water removes material and carries it down a slope, it tends to erode the

upper section of the slope but if the removal of material at the base of the slope is not equal to the rate of material from above, it may deposit material at the foot of the slope, thus creating an upper erosional zone and a lower depositional zone on any given slope.

Fringing may hillslopes in the humid tropics are zones of colluvium, material either washed downslope by water or carried by mass movement.

Even on a uniform lithology, the contrasted effects of erosion on the upper part of a slope and accumulation on the lower part would be expected to produce differences in form. The depositional and erosional parts of slopes may be regarded as two separate components.

Erosional slopes contain an upper convexity where soil creep (a type of mass movement) is dominant and a lower concavity principally formed by concentrated flow. Such a convex-concave slope is represented by the lowest of the four schematic slope profiles in figure 1 (Baulig, 1940).

This simple slope is but one of a variety of slope forms to be found in humid regions. Lithological variations along a slope greatly affect slope form (in humid tropical areas less than in humid temperate areas). Also vegetation influences slope: loose sand and gravel debris have an average angle of repose of about 33°, while natural forested slopes on this material have a mean angle of 41°.

These lithological and biotic factors introduce complications into the simple convex-concave type of slope.

To accommodate this variety of slopes, Dalrymple et al. (1968) has developed a hypothetical 9-unit land surface model (figure 1). Figure 2 gives examples of the application of the 9-unit land surface model.

Figure 1 Theoretical combinations of slope convexity and concavity (after Douglas, 1977)


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Figure 2. Applications of the 9-unit land surface model to slope profiles from different environments (after Douglas, 1977)


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On the interfluve (unit 1 of the 9-unit model) pedological processes involving

the vertical movement of subsurface water are dominant. Unit 2 is a seepage zone where through flow and thus lateral eluviation is important. The convex unit 3 is dominated by soil creep, while unit 4 is the free face of rock outcrop affected by rockfalls. Unit 5, the transport mid-slope, is similar, but not equivalent to the straight, or constant slope mentioned previously. The dominant process is the transport of material across the unit. The deposition of colluvial material from above dominated the straight or concave unit 6, while unit 7 (the equivalent of a river floodplain) is dominated by the deposition of alluvial material brought down the valley by a river. The erosive and transport actions of the river dominate the channel wall (unit 8) and the channel bed (unit 9).

While slopes are of crucial significance in all landform studies, they should not be studied in isolation, but should be related to the whole morphological environment, which in humid landform areas means that slope processes and forms must be related to river processes and fluvial landforms.

Considerations of the relationship between slope angle and slope stability inevitably lead to questions of the way in which slope forms change with time. Such changes are so slow that observational techniques can hardly provide direct evidence of the pattern of slope evolution. Experiments in slope evolution have been carried out (physical model experiments and mathematical models). In the humid tropics, Nossin (1964) and Swan (1970) have found that in West Malaysia slopes receded through the migration of the weathering front back into the hillslope. 4. Fluvial forms as example

Rivers act as a transporting agent for both excess of water and sediment. Furthermore they are an erosional and depositional medium creating characteristic landscapes and morphological features. The character and appearance of a river normally changes markedly on its way from the mountainous "hinterland" to the relatively flat coastal/deltaic plain.

The uppermost part is the collecting subsystem or sediment source area. Part

two is the transfer-zone, where for a stable channel input of sediment can equal output. The lowermost part is the area of deposition.

4.1 The collecting subsystem

The collecting subsystem is formed by the tributaries in the headwaters of a drainage basin. Here water and sediment are collected and transported through valleys (V-liked valleys, flat-bottom valleys, asymmetric valleys, etc.).

A drainage pattern is the arrangement of natural lines (main streams and their tributaries) in an area. The various types of drainage patterns are related to local geology and geological history.

The drainage density is the relative spacing of drainage lines and is depending on rainfall, vegetation, rock permeability, infiltration capacity, and relief.


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The drainage basin (or catchment area) of a river is the entire tract of country drained by that river and its tributaries. The boundary line between adjacent drainage basins is the divide (or watershed).

An alluvial fan is a body of detrital sediments built up by a steam or processes of mass wasting. The shape of the fan resembles a segment of a cone. Most fans have a concave upward profile. The angle of dip rarely exceeds 10°, the lower parts are generally much less steep. The channel system runs from apex to foot and sweeps systematically over the fan surface. 4.2 The transporting system

Rivers can be classified based on various characteristics, for example the origin of water (snow-, glacier-, and rain-rivers) or the changes in water discharge (intermittent, permanent). The most common subdivision of rivers is the one based on their channel pattern. Among the recognized channel patterns are: meandering, braided, straight and anastomosed. The former two are by far the most common.

A stream is called meandering if its single channel has a sinuosity (ratio of channel length to valley length) of at least 1.5. In and near bends, a meandering channel is asymmetric in cross-section; greatest depths occur near outer banks. In between bends, the cross-sections are more or less symmetrical. The deeps at bends are called pools, the shallows in between bends are called riffles. At low water stages riffles tend to be eroded, their material is collected in the pools. When discharge increases again, pools are scoured, anew bringing the eroded sediment onto the (downstream) riffles.

Braided channels are single channel bed-load rivers, which at low water have islands of sediments, or relatively permanent vegetated islands exposed in the channels. They have a much greater width-depth ratio than meandering channels. The small absolute depth of the channels implies that the highest velocities, occurring at or near the water surface, are also close to the bed. It means that the traction power on the bed is high.

Straight channels are those, which have a sinuosity of, less than 1.5. The "talwegs" (line jointing the lowest points along a valley) of straight rivers are sinuous in plan, moving from near one bank to near the other between bars of sediments arranged alternately along the banks. Also pools and riffles develop.

Anastomosed rivers form an interconnected network of low-gradient, relatively deep and narrow, straight to sinuous channels with stable banks composed of fine-grained sediment.

The most important factors influencing the channel pattern are: discharge, sediment load and gradient (slope). It is important to note that most of the channel configuration is decided at bankfull discharge (the stage above which the riverbanks are flooded).

The main features of braided rivers are: bar and channel (active, dry, and abandoned).

The main features of meandering rivers are: pointbar, channels (active and abandoned), natural levee, flood basin (or back-swamp) and washover fans.


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A river-terrace is a remnant of a former valley flat (flood plain) in which the river has incised its new channel. Thus, there is always a difference in elevation between a terrace and the present valley floor of a river, paired terraces are located at equal elevation on both sides of the valley. They result primarily from lateral erosion of the river with only minor vertical erosion before renewed incision took place. Non-paired terraces result from continued vertical as well as lateral erosion. Criteria used for correlation of terraces are: elevation, lithology, and soil formation and weathering. The most common causes for terrace formation are changes in climate (i.e. changes in river pattern, changes in sediment supply), eustatic changes in sea-level (i.e. change in base level) and tectonic movements.

Figure 3. The fluvial system (picture from

http://www.daac.gsfc.nasa.gov/.../GEO_PLATE_F-12.shtml


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References Baulig, H., 1940. Le profil d' equilibre des versants. Annales Geogr., 49, pp. 81-97. Chorley, R.J., ed., 1971. Introduction to fluvial processes. Methuen, London. Dalrymple, J.B., Bloug, R.J., and Conacher, A.J., 1968. An hypothetical nine unit

landsurface model. Zeitschr.f. Geomorphologie, N.F., 12, pp. 60-76. Demek, J., 1972. Mannual of detailed geomorphological mapping. Czezhoslovakia

Academy of Sciences, Akademia, Prague, 344 p. Douglas, I., 1977. Humid landforms. Australian Nat. Univ. Press, Canberra, 288 p. Leopold, L.B. and Wolman, M.G., 1960. River meanders. Geol. Soc. Am. Bull. 71,

pp. 769-794. Lewin, J., 1978. Floodplain geomorphology. Prog. Phys. Geogr. 2, pp. 408-737. Nossin, J.J., 1964. Geomorphology of the surroundings of Kuantan (Eastern

Malaysia). Geologie en Mijnbouw, 44, pp. 157-182. Savigear, R.A.G., 1967. The analysis and classification of slope profiles. In Macar,

P. (ed.), L' evolution des versants. V Rapport de la Commission pour l' etudedes versants de l' U.G.I. Liege (Belgium), pp. 271-290.

Schumm, S.A., 1977. The fluvial system. Wiley, new York. Swan, S.B. St C., 1970. Relationships between regolith, lithology and slope in humid

tropical region, Johore, Malaysia. Trans. Inst. Br. Geogr., 51,pp. 189-200. Questions

1. What are difference between braided system and meandering system? 2. What is the morphology of river terrace after long-term weathering and

erosion? 3. What are the causes of slope gradient difference on rocky highland?

Exercises

1. Group discussion on the pattern of drainage system difference in Thailand. 2. Short presentation and discussion on how to recognize difference in drainage

pattern from topographical map.


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1.2 Processes

1. Introduction

Geomorphological processes are all those physical and chemical changes, which effect a modification of the earth's surficial forms.

The present landforms are the result of endogenetic, exogenetic, and extraterrestrial processes.

Endogenetic is a term applied to processes originating within the earth; exogenetic is a term applied to processes originating at or near the surface of the earth; and extraterrestrial is a term applied to processes from outside the earth.

All the processes, which bring the surface of the earth lithosphere to a common level, are called "gradation". Gradational processes belong to two categories: those, which level down "degradation", and those, which level up "aggradation".

The three distinct degradational processes are: weathering, denudation and erosion.

Weathering is the physical disintegration and chemical and biogenetic decomposition of rock that produces an in situ mantle of waste and prepares sediments for transportation. Most weathering occurs at the surface, but may reach considerable depths in tropical areas. Denudation is the transportation of weathering products down slopes by mass movement and/or sheet wash. Erosion is the wear of solid rock and loose material by the impact of detrital fragments and particles carried by running water (concentrated and unconcentrated), groundwater, wind, glaciers or the sea, and the removal of the loosened material (erosion includes transportation). Mass movement or mass wasting is the transfer of weathering products down slopes under the direct influence of gravity. Sheet wash is the transfer of weathering product downslopes under the direct influence of concentrated run-off. 2. Endogenetic processes and forms 2.1 Process

Forces responsible for endogenetic processes originate at some depth within the earth crust. They elevate or built-up portions of the earth surface and prevent that the earth land will be reduced to sea level.

Geologists have long recognized that the earth has its own source of internal energy, which is manifested repeatedly by earthquakes, volcanic activity, and mountain-building, but it was not until the late 1960s that an unifying theory of earth dynamics was developed. This theory, known as plate tectonics, has radically transformed our thinking about the crust and its movements. Abandoned is the old idea that the crust is a fixed and rigid sphere, with movements being largely vertical. We now have evidence that the crust is in continual motion, with individual fragments or plates moving thousands of kilometers. As the plates move, split apart,


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collide, and descent back into the mantle, they create earthquakes, volcanism, mountain-building, and other features in the "solid" part of the surface.

The basic elements of the plate tectonic theory are quite simple. The lithosphere, which includes the oceanic and continental crust, and the upper mantle is rigid, whereas the underlying asthenosphere yields to plastic flow (figure 1). Although there are many unsolved questions, radiogenic heat in the upper mantle is considered to be the basic source of energy.

The heat causes the material in the asthenosphere to move slowly in a convection cell, with the hot material rising to the base of the lithosphere where it than moves laterally, cools, and descends to become reheated, beginning the cycle again. Where the convecting mantle rises, it arches the lithosphere to form the mid-oceanic ridge and, as it moves laterally, it pulls the rigid lithosphere apart (sea-floor spreading). The lithosphere is thus broken into a series of fragments or plates, which are several thousand kilometers in diameter. As the plates move apart, molten rock from the hot asthenosphere rises into the rift zone and cools to form new crust.

The continental blocks, composed of relatively light granitic rock, float passively on the denser lower part of the lithosphere, sometimes splitting and sometimes colliding. Plates containing dense oceanic crust move via the descending convection currents down into the asthenosphere at the deep oceanic trenches and are consumed. By contrast, plates containing light continental crust cannot sink back into the mantle. Instead, continental margins adjacent to the descending plates are deformed into linear folded mountain belts.

The boundaries of plates thus coincide with ridges and trenches, and mark zones of earthquakes and volcanic activity. According to the plate tectonic theory, the lithosphere is moving at a rate of 2 to 16 cm per year.

Figure 1. Generalized global tectonic model (http://www.tulane.edu/~sanelson/geol111/pltect.htm).


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2.2 Forms

Surface forms created by endogenetic processes can be divided into three groups of forms. a) Neotectonic forms caused by tectonic movements in the crust e.g. fault scarps,

crests of anticlinal arches, slope of horsts and "grabens", earthquake displacements and salt domes represented by topographic highs.

b) Volcanic forms include all forms due to volcanic eruptions. Central eruption build/volcanic cones of various types. Fissure eruptions take place along a fissure or series of fissures. Many types of volcanoes can be recognized (see e.g. Rittmann, 1960). Examples of destructional forms are: volcanic rift opened by an explosive eruption, and craters. Examples of constructional forms are: lava slopes and lava plateaus (from fissure eruptions) and outer flanks of volcano (from central eruptions).

c) Forms resulting from deposition by hot springs they include all forms due to hydrothermal activities as geyser cones and mud volcanoes.

3. Exogenetic processes and forms 3.1 Aggradation or deposition

This process contributes to the general leveling (leveling-up) of the earth's surface. Deposition, except where groundwater is involved, results from a loss in transporting power of running water; waves, currents, tides and tsunami (a Japanese term for a long-period wave caused by any large-scale disturbance on the sea-floor of short duration, such as volcanic eruption or earthquakes); wind, and glaciers.

Deposition from groundwater results from changes in condition of pressure and temperature. Deposition by a glacier as it melts may be considered a special type of loss in transporting power. 3.2 Degradation

The three degradational processes are weathering, denudation and erosion. 3.2.1 Weathering

At least four variable factors influence the type and rate of rock weathering. There are two main weathering processes: physical and chemical processes. In the lecture note: "weathering and tropical landform evolution" attention will be paid to the subject "weathering".

3.2.2 Denudation

Two denudational processes may be distinguished: mass movement and sheet wash.

A. Mass movement or mass wasting The classification of types of mass wasting of Sharpe (1938) seems still to be

best available and has become into rather general usage:


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Slow flowage types

Creep The slow movement (+1 m/yr) downslope of soil and rock debris which is usually not perceptible except through extended observation. Soil creep: downslope movement of soil Talus creep: downslope movement of talus or scree Rock creep: downslope movement of individual rock blocks Rock-glacier creep: downslope movement of tongues of rock waste

Solifluction The flowing (+ 10 m/hour) downslope of masses of rock debris which are saturated with water and not confined to definite channels.

Rapid flowage types

Earthflow The downslope movement (10-100 m/hour) of water-saturated clayey or silty earth material.

Mudflow The movement (100-1000 m/hour) of water-saturated rock debris down definite channels.

Debris avalanche

The extremely rapid flow or slide of rock debris in narrow tracks down steep slopes.

Torrential mud cascades

The surge (1-5 km/hour) of soil and rock debris in a water-stream down narrow track. Large blocks carried among ill-sorted material.

Landslides The movement of relatively dry masses of earth debris

Slump The downward slipping of one or several units of rock debris usually with a backward rotation with respect to the slope over which movement takes place.

Debris slide The rapid rolling or sliding of unconsolidated earth debris without backward rotation of the mass.

Debris fall The nearly free fall of earth debris from a vertical or overhanging face.

Rockslide The sliding or falling of individual rock masses down bedding, joint or fault surfaces.

Rockfall The free falling of rock blocks over any steep slope Subsidence the downward displacement of material into subsurface voids with no lateral movement. B. Sheet wash

This is the removal of finely weathered particles in off-flowing films of water or in a network of shallow rills not yet concentrated into definite channels.


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C. The slope denudation system

Any slope may undergo some components of the various forms of denudation activity mentioned before. Although biological activity and creep may be continuous, much denudation activity is episodic in occurrence, from the rare earthquake event of great magnitude to the impact of raindrops from rainstorms. The processes operate between the upper end of the slope, which may be a hard caprock, a plateau surface, or a ridge crest, and the base of the slope. Slope forms will change as a result of the operation of the denudation system depending of the variable: lithology, structure, tectonic activity, slope angle, biological activity, climate and man. 3.2.3 Erosion

There are five major media which are capable of securing and transporting loose material. These media or agencies are: running water; groundwater (excluding underground streams); waves, currents, tides and tsunami; wind; and glaciers.

The processes by which these agencies acquire the loosened materials are: a) Hydraulic action the sweeping away of loose material by moving water b) Deflation the blowing away of loose material by the wind c) Scouring the removal of loose material by ice moving over a land/surface The processes by which earth surfaces are eroded by materials in transit (in

the agencies) are: a) Corrasion the mechanical wear of rocks by the effect of material being

transported by wind, ice, or running water b) Abrasion the mechanical wear of rocks by the effect of material being

transported by breakers on the coast c) Corrosion the removal of material by solution Attrition is the wear that rock particles undergo through mutual rubbing,

grinding, knocking, scraping and bumping with the result a decrease in size. Transportation may be accomplished by the agencies in five ways: a) Traction the transport of material on or close to the bottom (in water and

air) by rolling, pushing and sliding. Moving water can transport both small- and large- size particles in this way but wind can only transport material of much smaller size because its density is much lower. At extreme velocities wind can move pebbles by traction.

b) Saltation the transport of material by lifting in a steep upward movement and then followed by a more gentle downward gliding.

c) Suspension the temporary transport of rock particles by moving air or water. It is possible because the flow of air and water is mainly turbulent with upward currents which can lift and keep particles in suspension.

d) Solution a part of the load carried by moving water is in solution and becomes part of the fluid.

e) Floatation is a minor transporting process. Some inorganic materials such as rock pumice or sheets of mica may be carried in this way.


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Figure 2. A portion of the Green River basin and erosion pattern is seen from the Green River Overlook in Canyonlands National Park (http://images.google.co.th). Figure 3. Erosion induced by solution on the rocky coast (http://images.google.co.th)


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3.3 Exogenetic forms

Under this heading all forms are included directly created by exogenetic processes.

3.3.1 denudation forms

They include all destructional and constructional forms developed predominantly by transportation of weathering products down slopes without the decisive co-operation of other processes. These forms are mainly surface forms built of solid rocks in dissected regions such as uplands and mountains (fragments of structural surface, ridge formed by intersection of valley sides, hard rock ridges, summits, passes, etc.). Many forms continued by the structure of the rocks belong here, mainly those on rocks with contrasted resistance to denudational processes, such as edge of mesa of cuesta.

This group of forms includes also forms due to mass movements and sheet wash, e.g. destructional form: scar of landslide, and constructional form: landslide tongue.

Other examples of denudation forms are fragments of planation surfaces. 3.3.2 Fluvial forms

This group of forms includes constructional and destructional forms developed by the activity of running water in narrower or wider courses (concentrated) such as torrents, brooks rivers. Examples of destructional forms are: river bed of perennial stream, channels in seasonal streams, abandoned loops (cut-off, ox-bows, etc.), rapids and waterfalls, scarps of river-terraces, etc.

Examples of constructional forms are: river-built plains (levees, back-swamps, etc.), river terraces, alluvial fans, delta plans, etc. 3.3.3 Fluvio-denudational forms

These forms include all valley sides created by river erosion and denudation (mass movement and sheet wash), small valleys, etc.

3.3.4 Fluvio-glacial forms

They embrace all forms created by melt waters of glaciers during either their subglacial courses or after outflow from the glacier.

3.3.5 Karst forms These forms develop because of the solubility of certain sedimentary rocks (limestone, anhydrite, and gypsum) by percolating surface water, flowing streams and sea spray. On the basis of morphological processes they can be divided into five groups. a) Karst forms caused by solution of bedrocks, such as "Karren", solution sinkholes

(dolines), uvala, polje, ponors, etc. b) Karst forms reproduced in unsoluble rocks (pseudo-karst forms).


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c) Karst forms originating from solution and erosion by running water such as blind valleys, cupola karst (a corrosion relief), cone karst or tower karst (a corrosion - erosion relief), etc.

d) Karst forms resulting from deposition of calcium carbonate (Travertine, duricrusts).

e) Karst forms resulting from solution and marine abrasion such as point and ridges, tidal niches, a pitted abrasion platform, etc.

3.3.6 Suffosion forms

By the term "suffosion" we understand the underground removal of rock parts by underground water. These types of forms are rare and occur mainly on slopes with a thick weathering layer resulting in depressions and blind valleys. 3.3.7 Glacial forms

Many types of destructional and constructional forms can be distinguished (nunatakh's, drumlins, glacial striae, glacially eroded surfaces and valleys, ground moraine ridges, etc.).

3.3.8 Nivation and cryogenetic forms

These forms occur in recent and Pleistocene periglacial regions with regular freeze-thow-action (stone polygons, pingo's, cryoplanation terraces, etc.). 3.3.9 Thermokarst forms

They include all forms resulting from degradation of permafrost (the melting of ground ice and dead ice blocks). 3.3.10 Aeolian forms

These forms occur in all regions in which a thin or disturbed vegetation cover allows the wind either today or in the geological past to blow away the loose material (deflation and transport), use it as abrasion material on solid rock (corrosion), and to deposit it on another place. Examples of aeolian forms are: deflation hollow (destructional), irregular dunes and loess plateaus (both constructional forms). 3.3.11 Marine and Lacustrine forms

These forms occur in the border area between water and land. They are produced by the processes of wave action and shore currents induced by wind and tides. Examples of destructional forms are cliff faces, wave cut notches, sea arches and channels of tidal waters. Examples of constructional forms are beaches, beach ridges, tidal flats, marshes and mangrove swamp plains.


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3.3.12 Organogenetic forms They include forms related to the activities of animals and growing plants

(such as coral reefs and peat accumulations). The majority of the tropical lowland peats occur at or near sea-level, mainly in

the form of peat domes. Three main types of reefs are recognized. A fringing reef grows directly against the bedrock of a coast. A barrier reef lies off the coast and is separated from it by a lagoon usually too deep to permit coral growth. A reef ring enclosing a lagoon lacking an island of non-reef origin is called an atoll.

Other forms are e.g. beaver dams and termite mounds.

3.3.13 Anthropogenic forms They include all forms resulting from the activities of man e.g. pits and

quarries, dumps, embankments and polders ("made-ground" reclaimed from the sea or lake).

References Douglas, I., 1977. Humid landforms. Australian National University Press, Canberra,

288 p. Demex, J., 1972. Manual of detailed geomorphological mapping. Czechoslovakia

Academy of Sciences, Academia, Prague, 344p. Sharpe, C.F.S., 1938. Landslides and related phenomena. Columbia University

Press, New York, 137 p. Thornbury, W.D., 1964. Principles of geomorphology. John Wiley & sons Inc., New

York, 618 p.

Exercises

1. Each student selects one clear form or landform and then make short assay to explain how selected form or landform develops through time.

2. Each student gives an example of marine, transitional landforms in Thailand and discusses how those landforms responded to the ancient and recent climatic change.

3. Group discussion on the possibility to see organogenetic forms along the coastal zone of Thailand.


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1.3 Landforms in the humid tropics

1. Fluvial processes and forms 1.1 Distinctive forms of tropical river channels 1.1.1 Mobile stream channels

Mobile stream channels are cut from friable alluvium, regolith, or rocks whose texture is such that the materials can be moved by hydrodynamic forces, which means that the form of the stream channel is rapidly adjusted to them. Streams with mobile channels are normally developed in alluvial plains, but they also exist in friable rocks such as shales, poorly consolidated sandstones, and conglomerates. In the humid tropics, as opposed to other morpho-climatic zones, streams with mobile channels are exceptionally frequent even on solid rocks because of the importance of weathering.

Valley bottoms in the wet tropics are favourable to weathering because of their constant humidity. The presence of groundwater allows water to infiltrate the joints and fissures of the fresh rock and to weather it little by little. Even with poor groundwater circulation due to lack of permeability or lack of slope, weathering ends by being very effective in the long run. Various engineering works carried out for the construction of dams and bridges have generally revealed a considerable thickness of regolith below the surface of typical valleys.

Because of intense weathering many streams flowing through regions of solid rocks have mobile channels even if the alluvium only forms a thin veneer, 3 to 4 m thick (9 to12 ft), on the valley bottom. Such a disposition occurs when two conditions are simultaneously realised: a) an easy penetration of the water into joints and fissures; the ease of penetration is all the more important as the ground water circulation is difficult and slow; and b) a good susceptibility to weathering.

Weathering also determines the peculiar texture of the alluvium. The exceptional outcrops of fresh rock, the weakness of the forces of disintegration working upon them, the slowing down of overland flow by the comb-like effect of the plant cover all contribute to the fact that the hillsides provide the streams with only a very small amount of coarse debris. The only important coarse fractions come from earthflows that have stipped off coarse material from the lower layers of the subsoil transitional to the fresh rock. Furthermore, such earthflows should reach a riverbed. If they spread on a gentle slope or on the flood plain the material continues to weather before being reworked by the stream. 1.1.2 Rock channels

Rockbars are extremely frequent on tropical rivers, especially, it seems, in the forested zone. Contrary to what is the case in temperate latitudes, they do not necessarily reflect important breaks in the long-profile. For instance, the elevation of the Maroni (on the border between French Guiana and Surinam) at 125 km (78 miles) from the coast is only 20 m (66 ft) in spite of several dozen rapids. According to Bakker (1975b) the room (330 ft) contour interval is reached only at a distance of


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200 to 300 km (125 to 185 miles) from the river's mouth. Usually the rapids only modify the water flow at high stage. At low stage the rock outcrops appear in the channel and the water flows around them without forming important rapids.

Four kinds of inequalities of rock bars may be distinguished. a) Waterfalls b) Rapid with potholes c) Polished rapids

2. Littoral processes and coastal landforms The morphogenic processes responsible for the specific characteristics of the littorals of the tropics have diver’s origins. Some forms, like the familiar offshore bars, lagoons, and mudflats, are the direct result of the peculiar nature of tropical rivers. They result from the great abundance of fine debris carried down by the streams. Other forms result from the particularities of slope development, such as cliffs caused by landslides, or forms due to corrosion, especially through the crystallization of salt. Still others are tied to specific biotic activities, such as the construction of coral reefs and the formation of algal crusts. 2.1 Particularities related to the abundance of fine sediments

The extent of coastal lowlands in the tropics is considerable, on the average larger than in other latitudes, periglacial regions excepted. Even the crystalline basement may be fringed by fluvio-marine plains instead of ending in the sea cliffs, as does most of the east coast of Brazil from Rio Grande do Sul to Paraiba, the coast of West Africa from the Casamance to Cameroon, and a great part of the coasts of India, Java, the Malay peninsula, and Borneo. Mountains or hills seldom reach the sea in the form of rocky coasts, as in the west of Ivory Coast. Rocky promontories are usually small and isolated between fluvio-marine plains, as at Monrovia, or at Cabo Frio and Ilheos in Brazil. They anchor the coast and frequently cause a change in direction, as at Cabo Frio or Cape Palmas (West Africa), but represent only an infinitesimal proportion of the coastline. There is an enormous disproportion between rocky coasts and lowland coasts. It poses the problem of the provenance of the alluviated material. 2.1.1 Origin of the littoral deposits

The enormous mass of sediments deposited in fluvio-marine plains surely does not originate from a few sectors of sea cliffs through littoral drifting. As we have mentioned for the East Coast of Brazil, the disproportion is enormous. Furthermore, long stretches of depositional coasts exist in regions where there is not a single cliff capable of furnishing the debris, except minor rocky promontories that produce a few reefs, as, for example, the whole coast of the State of Bahia to the south of the Bay of Todos os Santos (Brazil).

Sedimentological studies indicate the following sources of the sediments: a) Streams, which bring mainly fine materials. In general fine sands and

clays predominate as a result of the decrease in current in the lower stream course, affected by the Flandrian transgression. Coarse sands are


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deposited in deltas where they form sand banks. Special circumstances are necessary to account for the presence of coarse sands and granules (2 to 4 mm), as, for instance, at the mouth of the Sassandra in Ivory Coast. This river clears important rapids 10 km (6 miles) from the ocean and then empties into a narrow estuary, where the current is strong during destructive spates that come down a long-profile rich in rapids for the last 100 km (60 miles). At the moment strong currents destroy the bar at the river's mouth and allow the arrival of granules onto the subaquatic delta constructed at the mouth of the passes. Most of the granules are ferruginous concretions of pedologic origin.

Deltas are more common than estuaries. Their very gentle gradients, even on large rivers, do not allow the transport of coarse sands out to sea. For example, the Senegal, although in the semiarid Sahelian zone, only transports medium and fine sands (medians of 200-300 microns) into the ocean in spite of a discharge exceeding 6,000 cu.m (200,000 cu.ft) at high stage.

b) Weathering of the continental shelf during marine regressions, especially during the Wurm. Even in regions of crystalline basement, as in Guinea, considerable areas of the continental shelf emerged down to depths of 60 to 80 m (200 to 265 ft) during the last glaciation. The duration of the regression, under climates assuredly somewhat different from the present ones, has been long enough to permit a considerable degree of weathering, which, judging from what may be observed on terraces and slopes dated to the same period, may have reached a thickness of 1 to 2 m (3 to 6 ft).

During the Flandrian transgression the ocean progressively invaded these plains, sometimes surmounted by residual reliefs, such as the Los islands off Conakry. The rising sea removed the weathered products, even scoured part of the decomposed rock, churned it all up, pushing most of it, little by little, before it during its advance. Whereas the clays were dispersed, the sands piled up at the end of the transgression into enormous offshore bars (Dunkirkian), always very well developed on inter-tropical coasts, especially in regions where rocks rich in quartz liberate much sand (granites, gneisses, certain mica-schists), as along the central part of the east coast of Brazil or in Ivory Coast (Tertiary sands).

2.1.2 Conditions of deposition

The considerable masses of clastics deposited on tropical shores are mainly fines, for three reasons: the nature of the fluvial sediments, the materials weathered on the continental shelf during regressions and, in some cases, the biotic conditions. Clays and fine sands predominate. Coarse sands are uncommon: apart from the mouths of certain rivers they are mainly found on beaches where high swells progressively concentrate them, dispersing the finer clastics out at sea. For


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example, the grade size graphs made at Recife by Ottmann et al. give the following medians:

180-300 microns immediately below lowest spring-tide level 140-600 microns on the shore 170-180 microns on the storm beach The storm beach sand is thrown up from the offshore during storms, and for

this reason it is finer. An important characteristic of the sediments of inter-tropical shores is their

very high mobility. It is due, first of all, to grade size, especially the usual absence of pebbles, which are less easily transported than sands or clays. But it is also promoted by other factors:

a) High temperature, which seems to affect the mobility of clays as a result of an appreciable decrease in the viscosity of the water that increases its turbulence and thus helps colloids to remain in suspension. The suspended load of rivers in flood colours the sea over great distances. The Amazon is held responsible for the muddy shores that extend from its mouth all the way to French Guinea. But it is difficult to determine exactly how much more suspended matter is carried and just how much more mobile colloids are with increased temperature. Such measurements have never been made, and we remain in the realm of hypotheses.

b) The tropical oceanic regime, which is generally charaterised by a high and much more constant swell than in other latitudes. The ocean normally does not become unruly as in the temperate zone subject to cyclonic storms although on certain coasts hurricanes and typhoons accompanied by surging seas locally produce catastrophes that destroy beaches and modify tidal inlets. But the swell is powerful enough throughout the year thoroughly to work the sand of exposed beaches. This constant working of the sand contributes to its fragmentation and wear and explains the concentration by elutriation of the coarser fractions on beaches where there is a tendency to erosion. The coarse sand, in turn, causes the appearance of extremely convex beaches with upper slopes of 15 to 20°, not counting micro-cliffs.

Nevertheless, a certain number of antagonistic elements intervene that reduce the mobility and facilitate the deposition of the sediments:

a) Mangroves: on the shores of estuaries the brackish waters are occupied by mangrove, which tolerates rivers degrees of salinity and, in the case of some species, even a temporary submersion by fresh water. These shrubs are remarkable well adapted to the environment: Avicennia seeds, for instance, are contained in fruits shaped like pointed bombs which when they fall stick in the mud, where they germinate.

Mangrove always grows on muddy ground subjected to frequent submersion usually by the tides but occasionally only by river floods, as certain Rhizophora of the lagoonal shores of Ivory Coast. It plays a comparable role, which only differs in details, on tropical shores to that of Spartina in the temperate zone. Although the water circulates easily

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