Rock control in geomorphology

ROCK CONTROL IN GEOMORPHOLOGY

Skeleton 920

Lake

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6

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MUSKOKA, ant, Canada Contour interval 100 feet, elevations in feet above mean sea level (Courtesy of the Department of Mines and Technical Surveys, Canada)

12

14 km

ROCK CONTROL IN GEOMORPHOLOGY

by EIJU

YATSU

SOZOSHA

8, 3-chome, Kandanishikicho, Chiyodaku, Tokyo

1966

Copyright 1966 Sozosha

PREFACE

ALL RIGHTS RESERVED This book or any part thereof must not be reproduced in any form without the written permission of the publisher.

Printed in Japan

Geomorphologists too long have shied away from investigating . basic processes associated with landform origin and development. Few have gone beyond regarding them in amateurish or superficial ways. In doing so, they have delayed unduly the time when their field becomes truly scientific and useful to persons in allied disciplines. How many engineers, geologists, mineralogists, pedologists, ecologists, foresters, chemists, physicists, or agriculturists turn to geomorphological literature for information that might aid them in solving problems related to the earth's land surface? Should they do so, how often would their efforts be rewarded by finding useful information? It is a sad commentary, but in all probability their harvest would reveal few grains of corn embedded in huge volumes of chaff. This is a condition that Professor Yatsu hopes to correct. The intent of this short book by no means is an attempt to explain or catalogue a wide variety of examples where rock control has produced or is reflected in scientific land forms. Rather, it emphasizes the importance of physiochemical and mechanical processes that affect rock properties, and hence, secondarily, resulting land forms. It deals with fundamental, but largely unsolved, problems in rock dynamics. A style that at times is deliberately provocative is used in the hope that it will stimulate open and youthful minds to develop a truly scientific geomorphology. The fact that the text asks more questions than it answers points up the need for more penetrating research than geomorphologists are likely to deem essential. The hope is that a science hardly advanced beyond the swaddling clothes stage will mature and on the basis of merit attain its potential of being sound, respected, and useful. It should not continue to follow degenerative paths toward becoming little more than an aid in describing scenery. This viewpoint is similar to one that I expressed in presidential addresses before the Association of Americ:m Geographers and the Geological Society of America. My concluding statement in the Hitchcock Lectures at the University of California in 1965 repeated the hope.

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The effect of the influence of William Morris Davis, who is usually considered as the father of American geomorphology, was to divert students away from truly scientific research, such as was rather characteristic of the nineteenth century, into superficial inquiries centered on stages of development in cycles of erosion, a scheme that Davis advocated for many decades with much precise logic. Research was concerned with end products, hypothetical forms of the surface, while the investigation of datails of morphological processes lapsed into almost complete somnolence. Professor Yatsu specifically commends Filip Hjulstrom, Sundborg, Strahler, Wolman and Miller, Leopold and Langbein, Chorley, Glover, Dury, and Hack for their contributions toward developing a more meaningful geomorphology. But the emphasis in the book centers on the contributions of people such as H. W. Anderson, P. F. Kerr, F. J. Turner, F. Birch, D. Griggs, T. V. Karman, A. W. Skempton, L. Bjerrum, R. Grim, M. A. Melton, E. Penner, D. J. Varnes, and Karl Terzaghi. In addition many Japanese are cited, most of whom have published in English. While some of these names will be recognized by' American geomorphologists, probably few have intimate knowledge of their contributions. It is high time that they do! In progressing from superficial "explanatory descriptions," advocated by Davis, toward more meaningful studies of land forms, many geomorphologists, indeed, have struck out in new directions. Climatic geomorphology, currently in vogue in Europe and to some extent in America, is regarded by Professor Yatsu as but one interesting approach, rather than an end in itself. Its practitioners are likely to be concerned with broad features of landscapes and to regard rock types and structures as interesting only because they influence forms of micro-relief. Interpretations of aerial photographs, and we might add, other forms of remote sensing, although useful, will never provide the information needed to understand the origin of land forms. Dependence on mathematical elegance offers little more than the highly logical deductive approach of Davis, at least until sufficient knowledge has accumulated concerning all processes involved. This requires detailed observation and analysis, both on the ground and in the laboratory, where it requires the use of modern equipment. To develop anything like an ultimate geomorphology it is

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necessary to solve essential, fundamental problems underlying the dynamics of rock deformation. This is something where most progress to date has been made by specialists in other fields, many of whom have been engaged in penetrating inquiries. It is not enough to know the physics and chemistry of bedrock, nor how rock behaves when subjected to mechanical tests. Research must emphasize underlying causes, extending into the physical chemistry of minerals on the scale of examining their crystal lattices, and into changes related to time as well as to local environments. Rock alterability ordinarily depends on the fact that minerals are stable in the environments where they originated, but are altered readily when the environment is changed. In the case of a metamorphic rock formed at considerable depth and later brought to the surface, for example, it is necessary to realize that such generalizations must be accepted with caution, because numerous exceptions may be cited. While it is generally true that sedimentary rocks of the orthoquartzite series are more stable than those of the arkose-graywacke series in surface environments, specific examples must be subjected to a variety of inquiries and tests before one knows whether the rule is being followed. Professor Yatsu is by no means suggesting that geomorphology must be developed by people in other disciplines. He notes that while the main contributors of fundamental information to date have been people such as chemists, geophysicists, pedologists, and mechanical engineers, once the geomorphologists get down to business and think in terms of fundamental processes, their contributions will surpass in value those of allied specialists. It is sincerely hoped that this text will be found on the shelves of all institutions where geology, geomorphology, or engineering are included in the curriculum. Its content should be understood by all advanced students in geomorphology and by civil or architectural engineers concerned with the design of structures resting on the ground, the location of highways, railways, tunnels, and dams, as well as mining engineers involved in planning underground workings or open-pit excavations. Its bibliography contains about seventeen pages of useful references, and in itself is an excellent guide to the acquisition of useful information. While serving as a visiting professor in the Department of

IV

Geography of Louisiana State University, Professor Yatsu at no time hesitated to exceed the "call of duty." His enthusiasm inoculated students fortunate enough to participate in his course on geomorphology, and on many occasions they requested him to present extra-curricular lectures and discussions; requests that he invariably accepted. I felt honored when he asked me to contribute this preface to the revised edition of a manuscript which originally was written for the purpose of presentation to his students and faculty members in our department, as a token of appreciation. If the seed planted by Professor Yatsu results in serious consideration of processes involved in rock alterability and bears fruit, we may anticipate the development of a truly scientific geomorphology, a goal that is both highly desirable and by no means out of reach. The book is not a compendium. Although it provides many examples where rock control is evident as influencing landform development, its aim is to suggest directions that investigators may follow with profit. If it results in the realization of many of Professor Yatsu's ideals, a compendium may be anticipated at some future date; and in all probability the professor, with his boundless energy and extreme enthusiasm, may be expected to be a major contributor. R. J. Russell

AUTHOR'S PREFACE The purpose of the present book is to point out some of the basic principles necessary for an exact understanding of rock control problems in geomorphology. It is intended for use as a supplementary reading. Some puzzling expressions and therefore humorou~ narrations are used to encourage and stimulate young students In geomorphological thinking. The book has been developed from my lectures in geomor?h~logy . at the Louisiana State University where I was kindly Invited In 1965. I wish to express my gratitude to the faculty ~:mbers for their kindness and generosity. In July 1966, I JOIned the Department of Geography, University of Ottawa. In this c.omfortable home-like atmosphere, I finished the manuscript of thiS book by supplementing the temporary printing with some explanations and figures. lowe a particular debt of gratitude to the faculty here. I am most grateful to Professor Richard J. Russell, director the Coastal Studies Institute, Louisiana State University, who kIndly honoured me by writing the preface to this book and by enco~raging me to publish it. I would also like to express my gratitude to the following persons: Mr. Kogi Yamaguchi, former teacher of the Ibaraki Normal School, Dr. Yanosuke O~~uka, .late Professor of the University of Tokyo, Professor Filip HJulstrom, l)"niversity of Uppsala, Dr. Yokichi Mino Emeritus Professor of the Tokyo University of Education Dr' Fumio Tada, Emer~tus Professor of the University of T~kYo: Professor Jean Tncart, University of Strasbourg, Professor Arthur N. ~t~ahler, Columbia University, Professor Harley J. W~lker,. LOUISiana. ~tate University, Professor J. Ross Mackay, University of BntIsh Columbia, Professor Mark A. Melton University of British Columbia, Professor Harishankar Prasad Srivastava, University of Ottawa, and Professor Hiroshi Nakano Chuo University, all of whom have exerted a great influenc~ on my work and life. I wish to acknowledge the reasonable criticism by my colleague, Dr. Takasuke Suzuki, Lecturer of Chuo University concerning "The Rock Control Theory" Applied Geography:

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VI

No.5, 1964. The present book is, so to speak, a collection of vindication and apology for those papers. In this sense, he prompted me to write it and kindly helped me in reading the proof. I highly appreciate the valuable suggestions from Professor David Erskine of the University of Ottawa in preparing the manuscript. I am greatly indebted to many authors for their data, tables, and figures that I have referred to, in this book. Those, whose sources are not referred to have been prepared by myself. Finally, I wish to acknowledge my debt to Mr. Kazuyoshi Izawa, Sozosha Publishing Co., for his generosity in taking charge of a pUblication that runs a risk of being financially unprofitable and for his valuable help in correcting the proof.

Department of Geography University of Ottawa July 28, 1966 EIJU YATSU

CONTENTS

Preface .................................. .................... ..... ...... ...... . i Author's Preface ·· ···· ........... ... .... ... .. ... .. ...... ...... .. ..... . ...... V I.

SIGNIFICANCE OF ROCK CONTROL IN GEOMORPHOLOGy··· ······ ·· ······ ·· ······ ···· ·· ·· ··· ·················· ..... . 1.1. Geomorphology and its Methods ··· ·········· ··········· ··· ······· ·· 1 1.1.1. Geomorphology··· ............ .. ............... ..... .. ... ........ . 1.1.2. Domains of Geomorphology and its Methods ........ . 1.2. Rock Control Problems .... .. ..... . ..... . . .. .. . . ... .. .... .. .. .. .. ...... 4 1.2.1. Rock Control Problems and its Former Researches 4 1.2.2. Air Photo Interpretation of Geological Structure .. ' 10 1.2.3. Rock Control in Relation to Processes ............ ······11 1.2.4. An Approach to the Rock Control Problems ·········12

II. EROSION AND SURFACE MATERIALS OF THE EARTH··· 15 2.1. Alterability of Rocks······ ·· ······ ·········· ······ ····· ·· ···· ···· ··· ·· ·· ·· 15 2.1.1. Alterability······································ ········ ·· ··· ·· ·· · '15 2.1.2. Methods of Studying Alterability·········· ·· ··· ····· · ······17 2.1.3. Some Problems on the Alteration···· ........ ... .. ..... . ····18 2.2. Erosibility······ .. . ........... ... ...................................... .. ..... '19 2.2.1. Soil Properties and Soil Erosion··· ·· · .................. .. .... 20 2.2.2. Landslips and Slope Rupture················· ····· ·· ·· ···· ·· · 23 2.2.3. Glacial Erosion and Erosibility of Rocks .............. . 34 2.2.4. Denudation and Erosibility of Rocks .... .. ............... 34 2.2.5. Wear of Rocks············· ··· ·· ····· ·······························35 III. MECHANICS OF SOLID ROCKS .... ..... ..... .......... .. ...... . .... .. 45 3.1. Strength of Solid Rocks .. .. ............................................ 45 3.1.1. Definition of Strength ... .... .. ....... .. ............ .. .......... 45 3.1.2. Classification of Disintegration .. ... .. .. ......... ... ......... 45 3.2. Failure of Rocks by Static Load·· ··· ········ ·········· ·· ···· ···· ··· 46 3.2.1. Rheological Behavior of Rocks .............................. 46 3.2.2. Theories of Fracture of Rocks ... .. ... .......... ............ 47 3.2.3. Time Dependance of Deformation and Fracture ·· ···· 47 3.2.4. Conditions which influence the Strength of Rocks··· 51

Vill

3.3. Impulsive Fracture and Others··· ·········· ·· ····· ................... 51 3.4. Cohesion of Rocks .. ········· .. ·· .. ·· ··· ·· ·· ··· .. ·· ········ ···· ·· ... .. ..... 60 3.5. Plasticity of Rocks and Salt Domes ........ · ................. . ...... 61 3.5.1. Plasticity of Rocks· ...................................... · ........ 61 3.5.2. Salt Domes .................... ...... ..... .. ...... .. ... ... ... . ... ... 64 3.6. Wear ....... ......................................... .. ... ....... ... ............ 66 3.6.1. Friction and Wear · ............ .... .. · .. · .. · .. ...... .... .......... 66 3.6.2. Various Wear Mechanism ................. .. .. ............... 67 3.7. Rock Mechanics and Geology ................ · .. · ...... · ............ 70

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IV. MECHANICS OF UNCONSOLIDATED AND FRACTURED ROCKS ..................... ... ....... ..... .. ...... .... ........... . ··· .. · .. · .. · ·· · ·· · 79 4.1. Mechanics of Weathered Bedrock and Systems of Large Debris ........ ..... ..... ........................... .. ... .......... ... 79 4.1.1. Application of Soil Mechanics to Weathered Bedrock and Systems of Large Debris ......... ... .... .. 79 4.1.2. Some Examples of Field Measurements .. · ............ · .. 81 4.1.3. Some Problems on Unconsolidated or Fractured Rock ............ .... .. ...... ........................ ............... ··· 81 4.2. Mechanics of Fine Debris such as Soils ............. ........... 94 4.2.1. Physical Properties of Soils ................ .. .................. 94 4.2.2. Mechanical Experiments on Soils ............ · .............. 95 4.2.3. Properties of Some Erosible Sandy Soil I, Shirasu··· 96 4.2.4. Properties of Some Erosible Sandy Soil II, Masa ... 98 4.2.5. Stability of Slope ......... .................................. .. 102 4.3. Mechanics of Micronized Debris such as Clays .......... .. 104 4.3.1. Clayey Soils ........ ........................ ...................... 104 4.3.2. Mechanics of Clay .................................... ·........ 104 4.3.3. Thixotropy and Mudflows ................................. 106 4.3.4. Properties of Some Clayey Soil I, Kanto Loam ... 107 4.3.5. Properties of Some Clayey Soil II, Post-glacial Marine Soils ........................... ..... .................... .. 111 V. CONCLUSION

...... .......... ..... .. ..... .... ........ .... ... .. ........ .. ... .

125

Author Index .............. .. .................................. .... ...... Subject Index ..... ... ............. ... ...... ... ... ...... .. ......... .. .....

127 129

SIGNIFICANCE OF ROCK CONTROL IN GEOMORPHOLOGY

., 1.1. Geomorphology and its Methods 1.1.1. Geomorphology. This natural science is concerned with land forms, namely with the forms of land surfaces. It is very difficult to define exactly the last two words of the preceding sentence. The term "land surface" is thought of as the boundary between the solid and the fluid such as the hydrosphere or the atmosphere. In this case, the word "boundary" implies its meaning not in the size of atoms or molecules but in macroscopic size. Consider the soil covering the bedrock of the earth; it is composed of soil particles, air and water. Water and air exist between soil particles or on their surfaces. Also, in the case of bedrock, air and water are included in cracks, joints and cleavages. These contact surfaces between the solid and air or water are not called the earth's surface. Therefore the word "land surface" is the one which covers the uppermost particles of soil or of bed rocks. Exogenic processes occur intensively at or near such boundaries between the solids and fluids. One portion of consolidated solids are separated; abstracted into fluids, namely liquids and air; transported by them; and then deposited. On the other hand, the fluids invade the solids and alter them. This is an aspect of the weathering phenomena. Altered solids change their kinetic characteristics and begin to deform themselves or to slide and creep. The rupture and slip of the earth slope are examples of such mass wasting phenomena. Endogenic processes derived from the energy of the earth's interior contribute much to the deformation and displacement of the surface of the earth. Orogenic or epeirogenic movements and volcanism play this role; they join in the deformation and displacement of the earth's surface together with the exogenic processes.

1.1.2. Domains of Geomorphology and its Methods. It is intended here neither to describe the history of the development of

SIGNIFICANCE OF ROCK CONTROL IN GEOMORPHOLOGY

Geomorphology and its Methods

geomorphology nor to treat in detail the methods of geomorphology. In this connection, several textbooks are very easily accessible so that the author wishes to leave the trifling and delicate details to them, and he intends only to summarize his personal opinions about the problems. Such a brief epitome might be necessary, preliminary to the following discussions of section 1. 2. Considering the development of geomorphology, it was a topology of forms of land surface and had devoted itself to the definitions and classifications of land forms, just as phytology, which used to be taxonomy or plant morphology in its first stage. It is a matter for congratulation that our geomorphology has developed into comparative embryology as phytology has done into embryology and then into physiology. But alas! Our geomorphology has strayed into that fanciful theory of Davis. What a deplorable thing this is for the appropriate development of geomorphology! The principal role of gemorphology is, of course, in studying the evolution of land forms. The history of the development of land forms is thought of as consisting of two forces-an endogenic process or endogenic factor, for example, crustal movements and intrusion of igneous body; and an exogenic process, such as erosion, transportation and sedimentation. These two forces, moreover, have changed their intensity in the course of the development of the earth. Exogenic processes are especially influenced by climatic environments which change as time passes. It is true that there is a land form of fault. Everybody agrees readily and without any hesitation that a shatter zone is weak and likely to be erosible. Then, why is the shatter zone weak? How is it eroded? On what does this weakness depend? Furthermore, what is the mechanism of erosion? In answering these questions, many geomorphologists cheat their students, flourishing their abstract explanations or their own imaginations without any intention of scrutinizing the true nature of the matter. Do they not have any ability of understanding the true nature of matters? Or do they have any eagerness to do so? Are they not ashamed of themselves as scientists? Professor Filip Hjulstrom (1935) should be worthy of our greatest respect, for he has built up the school of Uppsala with his lofty ideal for science and integrity, doing researches into exogenic process and its application to geomorphology, especially

to fluvial geomorphology, while a crowd of geomorphologists became apostles of Davis. So we look forward to the researches by young students of this school (Sundborg, 1956). Recently a group of American geomorphologists have intensively developed analytical and dynamic researches in this science, and it seems to be a resurgence of Gilbert's idea (1914) and the influence of a late American engineer, Horton (1945). Their contribution in two decades should be highly appreciated (Strahler, 1952 ; Wolman and Miller, 1960; Leopold and Langbein, 1962; Chorley, 1962; Glover, 1964; Dury, 1964; Hack, 1965) . It is true that climatic geomorphology, which has been quite in vogue recently, has contributed greatly to the study of geomorphology. It is, of course, a very excellent approach to this science. However, the writer wonders whether this method, popular in recent times, is not merely a systematization of simple observations, and he feels that there is some limitation to this approach. In the domain of geomorphology, there are many phenomena very difficult to observe or measure in a short while, so it is natural that we should use the historical approach to the study of geomorphology. Most of the researchers occupied with this kind of historical approach, however, are sadly lacking in the sense of modern science, and have no knowledge of 19th century chemistry and do not even understand the Mechanics of Newton. And how many of these researchers there are in the world! Their theory is composed of mere deductions. They fall into a chain reaction of empty imaginations, though trying to shake themselves out of Davis' theory. The writer contributed to the first number of Applied Geography on how applied geomorphology should be, under the title of "Les caracteres fondamentaux de la geomorphologie appliquee" (Yatsu, 1959). At present, some researchers of applied geomorphology seem very enthusiastic about drawing maps of land form classification or geomorphological maps and seeking the way of their expression. It might be very handy to the users of those maps. But, if they were too much concerned with the way of expression and too much fascinated with the beauty of color in maps, they might be artists. It is as if they were practicing flower arrangement(Ikebana) . Geomorphological maps would be of little use unless they contained not only classifications of land forms, but also dynamic and physical characteristics of the matters that compose land forms. Those

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SIGNIFICANCE OF ROCK CONTROL IN GEOMORPHOLOGY

would be, as it were, maps needed for travelers or useless guidebooks for those engineers who are to do some work with them. It might be better for geomorphologists to be indulged in fantasy with those maps on their laps, to make some guesses at land forms looking at air photograph, to observe sometimes outcrops in the fields, or to take a survey of the whole panorama of land forms from the top of mountains. But be careful! They need to be careful not to degrade into studying the "Science of Scenery}' It is necessary to introduce dynamics into geomorphology so that this science may rank among modern sciences. It is natural, of course, that geomorphology cannot be explained in every respect by dynamics, but at least, dynamics is essential to the explanation of processes of exogenic agents. Geomorphology should be much obliged to mathematicians and hydrodynamists for their participation in the study of it. In their case, it is absolutely necessary that they should use their mathematical approaches but only after they have understood the full intrinsic 'phenomena of geomorphology. If not, their approaches would be a science of fantasy. The system of the study of geomorphology has no need for smartness or elegance. It is the fundamental nature of land forms that should be made clear (Yatsu, 1964). 1.2. Rock Control Problems 1.2.1. Rock Control Problems and its Former Researches. In brief, rock control theory is concerned with the influences of rock properties on the formation of land forms. The problem of rock control has been considered as a geomorphological concept for a very long time, and the writer believes it began with the start of geomorphology; therefore it is not a new concept. However, according to many books and reports on this subject, it is, strictly speaking, treated as structure control such as distribution of strata, folds, faults, unconformities and so forth, although the reports sometimes deal with the physical and chemical properties of rocks which compose layers. Their treatment of this difficult subject is considerably rough, and some of those researchers, even while saying with a nonchalant air that geological structure is a dominant control factor in the evolution of land forms and is reflected in them, have no intention

Rock Control Problems

5

of studying the properties of rocks. Some have declared that the general features of land forms are determined by climatic environments and have insisted that rock control is only important in micro-configuration. Davis thought that structures, processes and stage are the dominant factors in the formation of topography (Davis, 1899). This concept was correct and adequate. His concept of the. term stage, however, was of imaginary time in a fanciful cycle. He deceived his followers, giving merely verbal descriptions to various processes which were easily accepted. This trend was an extremely regrettable digression and forced the advancement of geomorphology to be enormously retarded compared with the advancements of the other natural sciences. Many books juggle to substitute the concept of structure control for that of rock control, or consider the latter to be a factor of lower degree than the former, although they distinguish them from each other. Is it really adequate to depreciate the problems of rock control in comparison with structural control? To this question the writer wishes to reply with this antagonistic question. Imagine the thick deposition of completely homogeneous and isotropic strata followed by diastrophisms such as tilting, folding, faulting, etc. Consider that these strata do not lose the characteristics of their homogeneity and isotropy in spite of such crustal movements. Is any structure reflected in the topography in this case? It is common in nature that strata different in physico-chemical characteristics alternate, and that these strata are deformed by diastrophism to produce folds, faults, cracks, joints, shatter zones and so on, which give anisotropic parts to these strata. Furthermore, two kinds of rocks or strata of different strength are commonly in contact with each other unconformably. This is the reason why structures are reflected in land forms. In other words, it results from the anisotropy and non-homogeneity of the materials forming land forms that the structures give the important influences upon the topography. The existence of the anisotropy and non-homogeneity, namely differences in rock properties, enables us to acknowledge the structures. Is this not a correct opinion? It is more general and more adequate to state that the structures are reflected in land form because of the differ~nce in strength or erosibility of rocks; therefore

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SIGNIFICANCE OF ROCK CONTROL IN GEOMORPHOLOGY

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O'OOlm~~ J0.05 mm in diameter) divided by the aggregated silt plus clay; surface area is obtained by considering the particles as spheres and assigning mean diameters of 7. 5, 3. 5 and O. 9 mm to greater than 5. 0, 2. 0 to 5. 0, and O. 05 to 2. 0 mm particle-size classes respectively; aggregated silt plus clay is the ultimate silt plus clay minus the suspension percent. He applied the same method to the investigation of soil erosion in . western Oregon and northern California (1961) and concluded that the soil erosibility index is significantly related to soil-geologic rock

22

Erosibility

EROSION OF SURFACE MATERIALS OF THE EARTH 400

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Relation of calculated erosion to measured erosion for some south coastal watersheds in California (after Anderson, 1951).

Table 2. Soil-geologic sequences of physical soil characteristics and erosibility for major soil-geologic types: Santa Maria and Santa Ynez Basin, California (after Anderson, 1951) . Geology' Miocene Continental (sandstones) Quaternary terrace deposits (limestone) Upper Cretac· eous sediments (shale) Middle Mio·

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type and that the surface-aggregation ratio is also related to vegetation and geographic zones in these regions (1961). Although his studies of course succeeded very much in clarifying the erosibility of soil, there remain still many problems. What determines the value of aggregated silt plus clay, suspension percent, ultimate silt plus clay, etc? What kind of clay minerals exist in this soil? How much is the cation exchange capacity of that soil? How about the absorbed ions? What is the aggregation? It seems to be indispensable for studying soil erosibility to solve the clay-mineralogical and physico-chemical problems on erosibility itself. If not, the erosion ratio is merely a simple, apparent, and statistical thing. There is much qualitative and indirect evidence that high fe rtility does reduce soil erosion losses. It is, as Peterson (1961) pointed out, partly because fertile soils can grow excellent vegetation cover that reduces erosion, and on the other hand, because highly fertile soils generally have better tillage than their depleted counterparts. Higher fertility moreover will reduce soil erosion through more organic residues, more active soil flora and fauna, and higher soil organic matter contents. Regarding wind erosion of soils, three major factors are involved; wind velocity, nature of the surface, and soil properties. Among the last two, the most important factors are the aggregation of soils which undergoes soil erosion and the conditions of the state of soils, especially soil water content, because erosion by wind only occurs in dry regions. Aggregation of soils depends upon the content of clay and organic materials, clay mineral species, and physico-chemical properties of soils. We will skip their considerations.

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Moisture Dispersion Erosion Relative Colloid equivalent Ratio Ratio erosibility2 Percent Percent

sion Percent

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66.1

36.0

25.8

'23.2

16.8

7.4'

15.2

94. 1

39.0

23.4

16.1

9.6

2.12

5.1

36.2

31.4

16.3

14.1

7.3

1.34

(shale)

'From Geologic Map of California, Jenkins, 1938. Parenthetical expressions were from field observations at th~ particular places where soil samples were taken. 'From Equation 1, Table 1, with Miocene Continental geology taken as 100. 'The standard deviation of the Dispersion Ratios of this soil·geologic type, with five replications, was 2. 1, that is about 9 percent of the mean, indicating little variation within the geologic type.

2.2.2. Landslips and Slope Rupture. Both terms indicate different types of slope failure. Civil engineers commonly combine them into one term, landslide, whose meaning is different from Sharpe's concept (1938). The word "landslip" is almost the same as "earth flow" in Sharpe's classification of mass wasting. In section 4. 2. 5., the reason why the writer challenges the use of Sharpe's terminology will be demonstrated. Landslips imply plastic deformation and flow of the mass wastes that compose the slopes of the protuberant parts of the earth. Even landslips with much aspect of flow are extremely different from the behavior of volcanic mud flows or mud flows in arid zones..

(

24

EROSION OF SURFACE MATERIALS OF THE EARTH

A slope rupture looks like the failure of brittle materials overpassing their elastic limits, while a landslip is similar to the plastic deformation and creep failure of ductile materials. Koide (1955) has classified the landslips in Japan into three groups: (1) Landslips in tertiary regions, (2) Landslips in shatter zones, (3) Landslips in thermal spring areas. His conventional classification is very convenient for practical use, and so agricultural or forestry engineers have voluntarily applied it to their work. In his classification, however, the overlapping conceptions induce terrible confusion and requires a traffic cop to control its usage, as the writer recently indicated (Yatsu, 1965 a). In the phenomenon of landslips, day minerals seem to play important and fundamental roles of rock control. Clay minerals neither occur nor participate in slope failures of the rupture type. Slip planes or slip surfaces sometimes exist very distinctly, irrespective of the forms they may assume, and along these planes occur clayey materials which some call landslip clay. The writer is rather inclined to name this "slip surface clay" and reserve "landslip clay" for the clay minerals in the landmass or waste of landslips and in the bedrocks, including the slip surface clay. When it is realized that from the mineralogical point of view, there is no difference between the clay minerals of slip surfaces and those in the waste or in bedrock, it becomes evident that it is probably more convenient to lump them together and call them landslip clays rather than to restrict this term to the clays occurring at the landslip surface. The appearance of slip surface clay indeed has a real aspect of plastic materials so apprehensible that geomorphologists, geologists and civil engineers have grown accustomed to paying attention exclusively and short-sightedly to these clays and not to understanding that they have nothing but some clayey aspects because of their being kneaded at the shear planes called landslip surfaces. Although geological conditions are acknowledged to be a predisposition to landslips, there are more important aspects than overly comprehensive and superficial concepts such as geologic conditions. What type of rocks in general form those landslip areas? Why? How are those rocks decomposed? What kind of surface-chemical

25

Erosibility

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Water content

(%)

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Compaction tests on masa (3-1ayer compaction, after Tanimoto and Nishi, 1963).

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100

Mechanics of Fine Debris such as Soils

MECHANICS OF UNCONSOLIDATED AND FRACTURED ROCKS

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CLAY CONTENT

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116

MECHANICS OF UNCONSOLIDATED AND FRACTURED ROCKS

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