PHYSICAL GEOGRAPHY Made Simple

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PHYSICAL GEOGRAPHY Made Simple

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PHYSICAL GEOGRAPHY Made Simple

Richar

d H . Bryant , BA , Ph D MAI " SIMPL E BO

O K S

Made Simple Books

A n imprin t o f Heineman n Professiona l Publishin g Lt d Halle y Court , Jorda n Hill , Oxfor d OX 2 8E J OXFOR D LONDO N MELBOURN E AUCKLAN D SINGAPOR E IBADA N NAIROB I GABORON E KINGSTO N Firs t editio n 197
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Britis

h Librar y Cataloguin g i n Publicatio n Dat a

Bryant

, Richar d H .

Physica

l geograph y mad e simple. - 2n d ed . (Mad e simpl e books , ISS N 0265-0541
) 1 . Physical geography I . Titl e II . Serie s

910'.0

2 GB54. 5 ISB N 0 43
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0 1

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d an d boun d i n Grea t Britai n b y

Richar

d Cla y Ltd , Bungay , Suffol k

Preface

Man y ne w an d stimulatin g development s hav e bee n apparen t i n recen t years in the study of physica l geography. These are not just a question of new information, but reflect the introduction of fres

h concepts and frameworks in the subject. Almost inevitably, this has produced an increasing diversity of

specialis t textbook s tha t mak e i t difficul t fo r th e generalis t t o kee p abreas t o f al l aspect s o f th e discipline . Ther e ha s als o bee n a tendenc y fo r th e mai n part s o f physica l geography - namel y th e stud y o f landforms , weathe r an d climate , water , an d soils , plant s an d animals - t o evolv e thei r ow n approache s an d objectives . Thi s boo k makes an attemp t t o offe r som e redress to these trends. It tries to brin g togethe r withi n on e volum e muc h o f th e modern thinking in the subject, an d t o expres s thes e idea s i n a simpl e an d concis e manner . I t adopt s th e vie w tha t physica l geograph y i s concerned with the natural environment as a whole , i n whic h it s physica l an d biologica l component s ar e linke d withi n on e vas t system . Th e boo k als o give s mor e prominenc e t o som e o f the rather neglected part s o f th e subject, particularly biogeography, thus modifying the traditional imbalanc e withi n physica l geograph y toward s th e stud y o f landforms .

Student

s o f geograph y i n school s o r i n college s shoul d find thi s boo k a valuabl e introductio n t o moder n physica l geography . However , n o specialis t knowledg e is assumed on the part of the reader, and the book should therefore b e o f particula r interes t t o th e layman . I t i s hoped that this book, together with it s companio n volume ,

Economic and Social Geography Made Simple; will encourage readers to dig deeper into the subject of geography, and some guidance has been given in this respect in the lists of suggested further reading at the end of each chapter. Many friends and colleagues have given freely of their time and advice during the writing of

thi s book , an d i t i s a pleasur e t o tak e thi s opportunit y o f thankin g them, particularly: George and Eileen Booth, Elizabeth Dawlings, Haze l Faulkner , Marti n Harris , an d Pete r White ; D r Ia n Bailli e an d D r Kevi n O'Reilly ; and Dr Jean Emberlin for her help with the biogeography section .

RICHAR

D H . BRYANT ,

Department of

Geography, The Polytechnic of North London.

Revision

s hav

e been made to parts of the text to incorporate recent evidence and ideas, particularly in respect of Chapters Two and Eighteen. Similarly, some of the diagrams and tables have been amended. All lists of Suggested Further Reading have been brought up to date.

CHAPTER ONB

INTRODUCTIO

N T O PHYSICA L GEOGRAPH Y

Physica

l geograph y ma y b e define d a s th e integrate d stud y o f th e natura

l environment on or close to the Earth's surface. Human geography, on the other hand, is concerned with man's activities over the surface of the Earth. The nature of the relationship between man and the natural environment is inevitably a complicated

one ; it varie s fro m plac e t o place , an d i t ha s change d throug h time . I n th e contex t o f our present awareness of the need to conserve th e environmen t fo r th e bes t us e o f al l life , includin g man , th e traditiona l divisio n betwee n th e tw o part s o f geograph y ha s frequentl y becom e on e o f emphasi s rathe r tha n o f substance . I f physica l geograph y deal s wit h th e natura l environment , wha t i s mean t b y thi s expression ? First , w e shoul d not e that , strictl y speaking , environmen t mean s 'tha t whic h surrounds' : i n it s broades t sens e thi s include s al l energ y an d matte r capabl e o f influencin g man, from the astronomic to the subatomic level . Bu t i n practica l terms , suc h subject s a s astronom y an d nuclea r physic s ar e beyond the immediate concern of the physical geographer. He is primarily intereste d i n th e visibl e natura l environment , althoug h th e basi c principle s o f physic s an d chemistr y ar e fundamenta l i n explainin g ho w th e environmen t operates . Second , w e shoul d b e awar e tha t larg e part s o f th e Earth' s surfac e canno t no w b e describe d a s trul y natural , becaus e o f widespread interference b y man . I n man y case s th e apparentl y wil d part s o f th e countryside of Britain an d othe r heavil y populate d part s o f th e glob e ar e onl y semi-natural , an d i n others , the y are highly artificial. An evaluation of man's impact on the natural environmen t i s a them e w e shall return to at the end of the book. Nevertheless, i n whateve r setting , i t i s importan t tha t w e mak e th e attemp t t o understan d ho w natura l processe s operat e i n orde r t o appreciat e ou r environmen t mor e completely .

Physica

l geograph y has been described not so much as a basic science, but as a n integratio n o r overvie w o f a numbe r o f eart h and life sciences which give insigh t int o th e natur e o f man' s environment . Th e questio n the n is , wha t science s shoul d b e selecte d t o achiev e thi s objective ? First , w e nee d t o conside r th e for m o f th e Earth' s relie f features . Th e scientifi c stud y o f landform s i s know n a s geomorphology : thi s concern s itsel f no t only with the analysis of the shape of landforms, but also with the erosional an d depositiona l processe s a t wor k o n the m an d thei r evolutio n throug h time . Thes e aspect s o f th e environmen t ar e examine d i n Par t On e o f th e book . Som e o f th e majo r physiographi c feature s o f th e Earth , suc h a s mountai n chains , continenta l plain s an d ocea n basins , ar e a resul t o f interna l Eart h forces . Hence , certai n aspect s o f geology , th e stud y o f rocks, are relevant to physica l geography . Roc k typ e an d structur e ar e als o importan t a s variable s whic h influenc e th e effectivenes s o f wind , rai n an d weatherin g processe s o n landforms . Chapte r Tw o outline s th e essentia l geologica l backgroun d t o landfor m study . 1

2 Physical Geography Made Simple

A secon d majo r concer n o f th e physica l geographe r i s th e atmospheri

c environment. Meteorology, the study of weather processes, together with climatology, the analysis of climate or average weather, make up Part Two of this book. The distinction between these two atmospheric sciences is largely arbitrary: the climate of any particular place can only be understood through a knowledge of atmospheric processes.

A thir d componen t o f physica l geograph y i s th e stud y o f plan

t and animal distributions, normally called biogeography. The physical geographer needs to be conversant with the basic principles of botany and zoology, and particularly of ecology, which studies the relationships between plants and animals and their environment. This is dealt with in Part Three.

Thes e thre e aspect s determin e th e basi c framewor k o f th e book , bu t ther

e are also other disciplines which make significant contributions to the subject. The more important of

thes

e include pedology, the study of soils, which form an important environmental link between landforms, climate, and plants and animals; hydrology, the study of water on the Earth's land areas; and oceanography, which covers the study of

waves , tides and currents, as well as the biological characteristics of oceans. Th e subjec t clearl y embrace s a wid e rang e o f specialism s an d th e physica

l geographer cannot hope to be an expert in them all. But it would be very wrong to imagine that physical geography is simply of potpourri of mappable subjects. It is worth recording that many of the above specialisms, now sciences in their own right, grew out of an original physical geography of a century or more ago. The inevitable trend towards specialism has in no way altered the realities of

nature

. On the Earth's surface, land, air, water, soils, plants and animals all exist together, and the physical reality of

an y one place is made u

p of all these elements. Matter and energy pass continually from one to the other. Although the combination of features may vary from one place to the next, everywhere there exists a tendency towards dynamic balance or equilibrium, in which a change in one of

th

e elements leads to adjustment in the others. The value of physical geography is not only that it studies the important components of the natural environment, but that it concentrates on the connections between them. Modern physical geography tries to interpret the natural environment as a dynamic entity. One way of demonstrating this is to use a systems approach, which is outlined later in this chapter. There

i s

a strong requirement today for the *lateral thought' scientist, who studies the interactions between the various components of

th e environment, rather than concentrating on a single specialism. Recen t Trend s i n Physica l Geograph y Thi s boo k attempt s t o b e a s u p t o dat e a s possible , tempere d i t i s hope d b y an appreciation of some of the difficulties of teaching an

d learning the subject. Although not all new ideas or discoveries at the research level are worthy of instant assimilation into teaching, many important concepts become buried in an increasing complexity of specialist textbooks. As any teacher of first-year undergraduates will testify, there is frequently a lag of ten or fifteen years between the introduction of

ne

w concepts at degree level and these becoming incorporated within school syllabuses. A brief outline is given here of recent trends in the three main fields covered in this book, together with some comment on general changes in the subject.

Introduction to Physical Geography 3

Th e stud y o f landform s ha s undergon e a significan t chang e o f emphasi s in the last twent

y years. Any understanding of landforms depends on an appreciation of the relative roles of climate, geology, form, process and time as governing factors. In the first part of the twentieth century, much of geo-morphological study placed its emphasis on climate, geology and time. In particular, the subject was dominated by W. M. Davis' cycle of erosion (see Chapter Nine), which stressed the evolution of landforms through time, and suggested a classification of landforms based on their stage of development in the cycle. The biggest drawback with this approach was its inability to accommodate effectively the dynamics of present-day processes. In the 1950s and 1960s a strong reaction against Davisian ideas led to their replacement by

a

n emphasis on process/form studies, which are concerned with an examination of the relationship between landforms and contemporary processes. The process/form approach can be usefully placed in a systems framework, as illustrated in Chapters Four (slopes) and Five (rivers).

I n effect , th e Davisia n cycl e n o longe r provide s a n adequat e framewor k fo

r modern geomorphology, and it has therefore not been used in this book as a methodological basis for studying the subject. However, this does not mean that all Davisian ideas are unsound, or that time is not an important factor in landform study. The current emphasis on process studies has had the benefit of making geomorphology much more relevant to the rest of geography, not least in dealing with applied problems (Chapter Twenty-Five). It is more advantageous that we know, for instance, something of the discharge and sediment load of streams, than that they are 'young' or 'mature', as Davis described them.

I n th e cas e o f weathe

r and climate, there has been not so much a methodological shift, as a tremendous increase in knowledge about the upper layers of the atmosphere. Much of this had come about with the use of satellites and remote-sensing techniques. Weather study has been comprehensively transformed from a two-dimensional view to one

i

n three dimensions. This has had a major impact on climatology. Up to ten years ago climate study was primarily descriptive, simply listing climatic facts for particular regions, or preoccupied with climatic classification. Although these remain legitimate concerns for the geographer, modern climatology also lays much more stress on synoptic or dynamic aspects, such as the analysis of general circulation patterns, airstream characteristics and meso-scale weather systems. Detailed investigations into the global energy budget (Chapter Ten) have helped to revolutionise our view of the general circulation (Chapter Thirteen). Mid-latitude depressions and other meso-scale weather systems are now regarded as important mechanisms in maintaining the circulation rather than accidental disturbances within it. By contrast, fronts are regarded as secondary consequences, rather than the causes of circulation patterns. These new developments allow us a much improved insight into spatial and temporal variations of climate (Chapters Fifteen and Seventeen). The study of biogeography has for long been a neglected part of physical geography, especially at school level, where syllabuses have tended to be dominated by landform and weather studies. Many physical textbooks confine themselves to descriptions of the major vegetation and soil types of the world. This is a reflection of

tw

o characteristics that have symptomised much of traditional biogeography in the past: it has been almost solely concerned

4 Physical Geography Made Simple

wit h plants , an d i t ha s bee n dominate d b

y the zonal approach and the concept of climatic climax (see Chapter Nineteen). However, in the last decade, the subject has been rejuvenated by a reawakening of

interes

t in plant and animal ecology. Biogeography has begun to focus much more on ecological relationships and processes, especially on energy flow and nutrient cycling. This has served to re-emphasise the use of

th

e ecosystem as a fundamental conceptual framework. Hence ecological principles form the basis for the consideration of plants

an

d animals in Part Three of this book. This allows for a much firmer explanation of distributional irregularities, the traditional concern of the geographer rather than the ecologist.

Genera

l Trend s A s fa r a s physica l geograph y a s a whol e i s concerne d i t wil l b e apparen

t from what has already been said that the subject in recent years has become far more process-orientated - that

is , concerned with explaining the spatial and temporal variations in the environment in terms of th

e processes operating - rather than simply describing distributions. Far less emphasis is now laid on global classifications of phenomena, particularly those based on climatic indices. This movement towards more rigorous analysis and explanation is

par

t of a trend evident in the whole of geography. Some have called this change a quantitative revolution since statistical procedures are now as important a tool as maps in advanced geography, but it might be more accurately described as a theoretical revolution. In

brief

, it is characterised by a more systematic application of scientific method to the subject. The careful distinction between inductive and deductive reasoning, precise measurement and observation, model building and systems analysis (both further explained below) are some of

th e manifestations of this new approach.

Anothe

r significan t genera l tren d i

s that the subject is becoming increasingly applied. This is partly because it is better able to do so as a result of the methodological changes already mentioned, and partly because there is more demand for it to be so in the context of

th

e current desire for better environmental management. The last chapter in this book summarises some of the possible applications of physical geography in this respect. The net result of these trends is that the subject is much more integrated than it has been for some time. In particular the systems approach provides a viable common framework for several of

it s discrete parts. Model s an d System s Model s Sinc e th e natura l environmen t i s s o complex , w e hav e t o simplif y i t i n som e way in order to portray or understand it. Representations of reality ar e called models. W e ar e all familiar with scaled-down models of ships or aircraft which w e ca

n physically construct: these are examples of hardware models. The term 'model' can also be used to describe conceptual idealisations of reality, such

a

s a hypothesis, a law or a theory. The definition of a model proposed by R. J. Chorley and P. Haggett is that it is 'a simplified structuring of reality which represents supposedly significant features or relationships in a generalised form*. In recent years geographers have been making considerable use of models in the application and development of

theory : this is a trend which was first apparent in economic geography in the 1950s, but has now spread to

Introduction to Physical Geography 5

physica l geography . However , model s ar e b y no mean s ne w t o th e subject : a map, a classroom globe and a diagram of a frontal depression, are all examples of models. Model s use d i n physical geography vary widely in the amount of abstraction of reality in them . Some hardware models, such as large tank models of rivers, estuaries, an

d coasts, are also iconic models, closely imitating the real world in all respects but that of scale. Considerable technical problems are involved in scaling

dow n natural objects in this way. Much more widely used are analogue models, whic

h represent the real world by other properties. A kaolin model of a glacier, and a map, are both analogue models. Diagrammatic or mathematical models can also be regarded as analogue models, but involve even more abstraction, replacing objects

o

r forces by symbols or equations. Mathematical models in particular have become very important in geographical research since they

ca n be used to predict changes. Sinc e the y hel p u s t o organis e an d explai n data , model s ar e obviousl y ver

y useful teaching and learning aids, and models of various types are widely used in this book. Models should also help to identify gaps in our knowledge and point the way to further work. However, there are problems in their use which arise from the complexity and diversity of

reality . Models are simplifications of reality, an

d they are often very attractive; there is always the danger that they might be substituted for and accepted as reality. Also, since reality can be simplified in many ways, it is important that any model is considered as but one possible way of viewing the real world. Good models should be capable of being tested against the real world and modified if necessary. Many of the long-standing arguments in the study of physical geography have been about generalised models or laws which are not capable of being proved or disproved

System

s Wit h thes e genera l point s i n mind , w e ca n no w conside r on e o f th e mor

e significant recent developments in physical geography, namely the widespread adoption of models in which the real world is viewed as a vast system or set of interlocking

systems

. A system can be defined as a sei of objects or attributes (that is, characteristics of an object, such as size or shape) linked in some relationship.

W e hav e alread y stresse d tha t th e natura

l environment appears to operate as an entity, in which each component has connections with all the other components. It is impossible to build a model which takes in the whole world or even a substantial part of it, so we identify various environmental subsystems within which the connections are fairly strong. Thus weather systems, drainage systems, ecosystems and many others can be described. In analysing these, the systems approach focuses attention on the whole system and the interrelationships

withi n it, rather than on the individual parts.

Example

s o f system s familia r t o u s i n everyda y lif e includ e transpor t net works, th

e electricity grid system, or the domestic hot-water system of a house. These systems can be modelled symbolically by means of flow diagrams, consisting of

th e objects in the system conventionally represented by symbols, usually box-shaped, and the mas s o r energy flows in the system represented by lines

. Electric circuit diagrams, or the map of the London Underground, are examples of flow diagrams. Figs. 1.1 and 1.2 are very simple flow diagrams;

Figs . 5.1 and 10.4 are more complicated examples.

6 Physical Geography Made Simple

System

s ar e normall y regarde d a s bein g o f tw o types : closed , i n whic h no energy or matter crosses th

e external boundaries of the system, as in Fig. 1.1a, and open, in which external factors can affect the variables within the system (Fig. 1.1b). Apart from the universe, no natural system is truly closed unless we artificially make it so for the purposes of study. However, the degree of 'openness* of systems varies considerably. The earth and its atmosphere represent a partially open system, exchanging energy with outer space, but to all intent closed to material exchange. Other open systems exchange both

mas s an d energy. For instance, a drainage basin receives energy and mass from precipitation, sunlight an d th

e elevation of the land. These inputs pass through the system, doing work on the way to emerge as outputs of heat, water and sediment in the sea and atmosphere. A drainage system is typical of many

/ C [ A < \ . 1 B 1 1 (a )

Externa

l factor s / C I A * > (b ) \ . 1 B Fig . 1.1 . Tw o type s o f system: (a) closed; (b) open. othe r ope n system s i n physica l geograph y whic h ar e calle d process-respons e systems , becaus e th e flow o f mas s o r energ y (th e process ) cause s change s o

f responses in form (shape or arrangement) in the system. However, where plants or animals are involved, as in a forest or a pond (Chapter Nineteen), it is called an ecosystem.

A ver y significan t propert y o f ope n system s i s tha t th

e elements within them attempt to adjust themselves to the flow of energy and matter through the system towards a condition of equilibrium or steady state. Thus, if

w

e regard a hillslope as an open system (Fig. 4.3), a harmonious relationship will develop over time between the gradient, the infiltration capacity and the size of the sediment particles on the slope. Similarly, in ecosystems, animal populations will adjust closely to plant productivity. The effect of

thi

s adjustment, is to balance the input of energy and material to the output. However, equilibrium does not mean the system is static, it is performing work all the time, but the opposing forces are balanced or

fluctuating about a mean: the state can be alternatively referred to as dynamic equilibrium. A fundamenta l mechanis m i n maintainin g thi s stat e o f self-regulatio n i s that of feedback . This means that when one of the components in the system

Introduction to Physical Geography 7

changes , perhap s becaus e o f som e externa l factor , thi s lead s t o a sequenc e o f change s i n th e othe r component s whic h eventuall y affect s th e firs t componen t again . Th e mos t commo n typ e o f relationshi p i s calle d negativ e feedback , whereb y th e circui t o f change s ha s th e resul t o f dampin g dow n th e first change . Fo r example , i n Fig . 1.2 , whic h i s a simplifie d versio n o f a strea m channe l system , th e increas e i n strea m velocit y (A ) cause s greate r erosio n (B) , thereb y increasin g channe l widt h (C) ; bu t thi s i n tur n alter s th e hydrauli c radiu s o f th e strea m (D ) thereb y slowin g dow n strea m velocity . Thi s kin d o f negativ e feedbac k loo p i s ver y commo n i n physica l geography , an d i s th e mai n facto r i n promotin g self-equilibriu m an d conservatis m i n natura l systems . Positiv e feedback , whic h i s muc h rarer , occur s whe n th e feedbac k loo p aggravate s th e origina l change . Fo r instance , i n a glacie r system , a n increas e i n velocit y lead s

Run-of

f fro m siopes*-(External factor) l > I* 1 I 1 1 L A Strea m velocit y > k - D

Hydrauli

c radius + , j_ ... _ ii u-JM < + B

Erosio

n + 1 T C

Channe

l width Strea m channel system Fig . 1.2 . A n exampl e o f negativ e feedback . t o a n increas e i n erosio n an d overdeepening . Th e overdeepenin g onl y serve s t o accelerat e th e velocit y further . Bu t eve n here , check s wil l eventuall y oper ate : if the bedrock gradient becomes too steep, this will fundamentally alter th e slip-plan e field i n th e ice , suc h tha t erosio n cease s (Chapte r Six) . Positiv e feedbac k loop s i n natur e usuall y operat e i n shor t burst s o f destructiv e activity , bu t i n th e longe r term , negativ e feedbac k an d self-regulatio n ten d t o prevail . No t al l system s ar e i n equilibrium , sinc e th e spee d o f respons e i n natura l system s t o externa l chang e varie s a grea t deal . Loca l weather systems , suc h a s a land-and-se a breez e (Fig . 16.2) , exhibi t a rapi d respons e t o change s i n sola r radiatio n throug h th e day . Th e profil e o f a beac h (Chapte r Eight) als o re spond s withi n a fe w hour s t o change s i n wav e forc e an d direction . O n th e othe r hand , bedroc k slope s chang e characte r ver y slowly : i n Britai n an d else where , w e hav e glacia l feature s whic h hav e bee n modifie d littl e i n th e 10,00 0 year s o f postglacia l tim e an d sho w littl e equilibriu m wit h present-da y processes . Similarly , plan t an d anima l ecosystem s ofte n contai n feature s tha t ca n onl y b e explaine d wit h referenc e t o som e pas t environmenta l condition s (Chapte r Twenty-Two) . Th e timela g betwee n externa l chang e an d interna l adijustmen t i s know n a s th e relaxatio n tim e o f th e system . Th e relaxatio n tim e

8 Physical Geography Made Simple

i s importan t i n determinin g th e amoun t o f attentio n w e nee d t o pa y t o th

e historical (time) factor in physical geography. Thus in geomorphology, the landscape adjusts slowly enough for strong historical legacies to remain, and time is an important element to be considered in landforms (Chapter Nine). There are therefore important links between geomorphology and geology. At the other extreme, relaxation times in weather and climate systems are very short, and the approach here is almost entirely in terms of contemporary process. We should note, however, that climatic change itself (Chapter Seventeen) has had an important historical effect on soils, plants, animals and landforms. The value of historical studies is that they add the qualifications we cannot detect in our modern observations.

In summary , w e ca n sa y tha t th e system s approac h i s currentl y provin

g useful in physical geography as a framework for process studies. One of the chief values of systems thinking is its

flexibility. Systems can be applied at a variety of scales and complexity. On the other hand, the pitfalls of the approach are the same as those for models generally. There is always the danger that we might mistake the framework for reality, and set off trying to identify systems per se rather than use the concept as an aid to understanding.

Suggeste

d Furthe r Readin g Brown , E . H. , 'Th e conten t an d relationship s o f Physica l Geography' , Geographical

Journal,

141
, pp . 35^8
, 1975
.

Davidson

, D . A. , Science for Physical Geographers, Edwar d Arnold , London , 1978
.

Goudie

, D . A. , et. ai, The Encyclopaedic Dictionary of Physical Geography, Blackwell ,

Oxford

, 1988
.

Gregory

, K . J. , The Nature of Physical Geography, Edwar d Arnold , London , 1985
.

Hanwell

, J . D. , an d Newson , M . D. , Techniques in Physical Geography, Macmillan ,

London

, 1973
. King , C . A . M. , Physical Geography (Chapte r 1) , Blackwell , Oxford , 1980
. White , I . D. , Mottershead , D . N. , an d Harrison , S . J. , Environmental Systems, Unwi n Hyman , 1984
.

PART ONE

LANDFORMS

CHAPTE

R TW O ROCK S AN D RELIE F

Geologica

l consideration s pla y a n importan t rol e i n th e developmen t o f landform s i n tw o respects . First , landform s ar e a resul t no t onl y o f erosiona l an d depositiona l agent s actin g o n th e Earth' s surface , bu t als o o f tectoni c forces , originatin g withi n th e Earth . Th e broa d outline s o f nearl y al l th e

Earth'

s majo r relie f features , suc h a s mountai n ranges , ocea n trenches , basin s an d plateaux, ar e tectonicall y formed , althoug h detaile d feature s ma y b e th e resul t o f late r erosion . Moreover , tectoni c forces , i n upliftin g land , ar e a majo r provide r o f energ y int o landfor m systems . Thu s i t i s necessar y tha t w e kno w somethin g o f th e tectoni c natur e o f th e crus t o f th e Eart h an d o f th e processe s a t wor k i n it , particularl y i n relatio n t o plat e tectonics . Second , rock s var y considerabl y i n thei r resistanc e t o erosion , an d i t i s relevan t t o conside r som e o f th e structura l an d lithologica l propertie s o f rock s wher e the y affec t th e cours e o f landscap e development .

Crusta

l Structur e an d Movemen t Th e Eart h i s mad e u p o f a serie s o f concentri c roc k zones ; namely , th e crust , mantle , oute r cor e an d inne r core . Th e crus t i s th e mai n concer n here . I t varie s greatl y i n thicknes s an d composition ; beneat h th e ocean s i t i s a s littl e a s 5 k m thic k i n places , bu t unde r som e mountai n range s i t extend s t o depth s o f 7 0 km . Rock s i n th e crus t fal l int o tw o mai n groups . Th e ocea n basin s ar e predominantl y underlai n b y basalti c rocks , containin g muc h iro n an d mag nesium , an d havin g densitie s o f betwee n 2- 8 an d 3 0 (wate r = 1-0) . I n contrast , th e rock s tha t mak e u p th e continent s ar e lighte r i n colou r an d weigh t (densitie s o f c . 2-7 ) an d ar e ric h i n silico n an d aluminium . Belo w th e crus t li e th e dense r rock s o f th e mantle . Th e uppe r par t o f th e mantle , t o depth s o f abou t 10 0 km , i s soli d an d togethe r wit h th e crus t form s a relativel y rigi d uni t know n a s th e lithosphere . A t depth s betwee n 10 0 an d 25
0 km , th e mantl e i s partiall y molte n an d capabl e o f slo w flowage; thi s i s th e asthenospher e o r low-velocit y laye r (Fig . 2.1) .

Difference

s i n densit y betwee n th e continenta l an d oceani c area s o f th e lithospher e ar e though t t o explai n wh y continent s stan d hig h abov e ocea n basins . Eac h continen t i s underlai n b y a roo t zon e o f simila r ligh t materia l projectin g dow n int o th e asthenospher e b y a n amoun t proportiona l t o th e heigh t o f the continental area. The term isostasy describes this state of balance. Ther e ar e importan t implication s t o landform s o f an y disturbanc e t o this structura l equilibrium . I f materia l i s move d b y erosio n fro m a n are a an d de posite d o n th e se a floor, i t wil l involv e isostati c adjustmen t t o bot h areas: a rise i n th e leve l o f th e are a subjec t t o erosio n an d subsidenc e o f th e se a floor.

Similarly

, th e additio n o f weigh t t o a continenta l area , i n th e for m o f ic e o r a larg e bod y o f water , for example, will cause the crust to sink slightly. There may 1 0

Rocks and Relief 11

Crus t

Lithosphere

X *

AsthenosphereS/S

B Mantl e Crust Mantl e Lithosphere

Asthenosphere^

^ 6,37 0 km Fig . 2.1 . Th e interna l structur e o f the Earth (left), with detail of upper layers (right). b e a considerabl e timelag , perhap s thousand s o f years , betwee n th e externa l chang e an d isostati c respons e o f th e crust . Muc h broa d crusta l warpin g toda y i s relate d t o isostasy . Measurabl e amount s o f isostati c uplif t ar e takin g plac e i n region s suc h a s Scandinavi a an d Arcti c Canada , whic h wer e depresse d beneat h ic e cap s durin g Pleistocen e glaciations . However , ther e ar e othe r mechanism s o f crusta l warping , whic h wil l b e reviewe d below .

Crusta

l Plate s I t i s no w almos t universall y accepte d b y eart h scientist s tha t part s o f th e crus t ar e capabl e o f movin g horizontall y roun d th e globe , causin g th e conti nent s slowl y t o chang e positio n i n relatio n t o eac h other . Thi s ide a wa s origin all y synthesise d unde r th e ter m continenta l drif t b y A . Wegene r i n 1915
.

Considerabl

e oppositio n t o thi s thesi s persiste d fo r man y years . I n th e las t fiftee n years , however , ne w discoverie s hav e confirme d hi s genera l idea s an d hav e le d ultimatel y t o a bod y o f theor y whic h w e no w cal l plat e tectonics . Th e concept s embodie d i n thi s g o a lon g wa y toward s explainin g th e distributio n an d origi n o f man y majo r relie f features . Th e basi c principl e o f plat e tectonic s i s ver y simple : th e lithospher e i s broke n int o severa l section s o r plate s (Fig . 2.2) . Eac h plat e i s capabl e o f movin g ove r th e asthenosphere , carryin g oceani c an d continenta l crus t alike . Eac h plat e move s a s a singl e independen t body . A t th e mid-oceani c ridge s i n th e Pacific , Atlanti c an d India n oceans , plat e boundarie s ar e characterise d b y th e creatio n o f ne w crus t fro m below , an d i n th e proces s calle d sea-floo r spreading , th e plate s migrat e slowl y awa y fro m thes e centra l ridges . Elsewhere , a s aroun d th e peripher y o f th e Pacifi c Ocean , plate s mov e pas t eac h othe r o r collide . A t man y zone s o f collision , on e plat e override s th e other , th e lowe r plat e bein g absorbe d int o th e mantle , thu s maintainin g tota l materia l balanc e ove r th e globe , makin g u p fo r th e ne w crus t comin g ou t o f th e ocea n ridges . A t th e plat e boundaries , majo r tectoni c landform s ar e created . Th e area s o f collisio n ma y b e mad e u p o f thre e elements : dee p trenche s o n th e ocea n floor,

12 Physical Geography Made Simple

Fig . 2.2 . Majo r lithospheri c plate s and relative movement on the Earth's surface. som e u p t o 1 1 k m deep , markin g area s o f plat e subductio n int o th e mantle ; arc-lik e row s o f volcani c islands ; an d mountai n ranges , wher e th e plat e appears to be crumpling and thickening. All plat

e margins, including the mid-oceanic ridge systems, are frequently areas of considerable earthquake activity and volcanism.

E i J } Movement s

Movement

s o f th e crusta l piate s caus e pressur e an d tension s t o buil d u p on rocks

, in many cases leading to deformation of the land surface. The general term diastrophism is sometimes applied to the bending, folding, warping and fracturing of the crust. It is important to distinguish between several types of movement and their results.

O n a broa d scale , eart h movement s ma y b e divide d int o tw o types . Epeiro

-genic movements are those involving forces acting along a radius from the Earth's centre to the surface, and are characterised by large-scale uplift or submergence of land areas. The movements involved are often so slow and widespread that no obvious folding or fracturing is produced in the rocks. The second type of earth movements

ar

e those generated by forces acting at a tangent to the surface of the Earth, as primarily involved in plate tectonics. Where such disturbances have been responsible for the formation of the great fold mountain

range

s of the world, they are referred to as orogenic movements. The creation of very complex fold structures, as sometimes involved in orogenesis, is called tectogenesis by some authors.

Fro m th e poin t o f vie w o f landfor m development , th e differenc e betwee

n epeirogenic and orogenic crustal movements can be quite striking, and this is illustrated with reference to fold mountains and block mountains later in the chapter. In orogenic movements, structurally identifiable units are usually difficult to recognise, but the results of epeirogenic movements may be clearly defined in the

relief . Rock s var y considerabl y i n thei r behaviou r t o eart h movements . Unde r

Rocks and Relief 13

surfac e conditions , rock s ar e brittl e an d fractur e whe n subjecte d t o stres s an d pressure , causin g faulting . Mor e deepl y burie d rocks , subjec t t o highe r tem perature s an d pressure , ar e relativel y plasti c an d ma y respon d t o stres s b y foldin g rathe r tha n fracturing . Th e effect s o f foldin g an d faultin g o n th e dispositio n o f strat a ar e considere d late r i n th e chapter .

Earthquake

s ar e th e mos t prominen t evidenc e o f present-da y eart h move ments , an d ar e th e resul t o f deformatio n i n th e crust , whic h finall y rupture s abruptly . Earthquake s ar e importan t i n landfor m studie s becaus e the y ca n trigge r of f catastrophi c event s i n erosio n processes . Thes e includ e large-scal e landslide s an d mudflows , a s i n Per u i n 1970
. A numbe r o f spectacula r glacie r surge s hav e bee n observe d i n Alaska , an d som e ar e though t t o b e associate d wit h earthquakes . Tsunami s ar e seismi c se a wave s whic h ca n arriv e a t coast s wit h grea t force , causin g considerabl e damag e an d shorelin e changes , a s i n recen t decade s i n th e Hawaiia n islands . Majo r Tectoni c Landform s

Mountai

n Chain s

Examinatio

n o f a relie f ma p o f th e worl d wil l revea l a numbe r o f majo r mountai n chain s arrange d i n lon g linea r arcs . Notabl e amon g thes e ar e th e

Alpine-Himalaya

n syste m an d th e Circum-Pacifi c syste m comprisin g th e Andes , th e Rockie s an d islan d chain s o f eas t Asia . Thes e system s eac h con sis t o f serie s o f fol d structure s aligne d i n a roughl y paralle l manner . The y appea r t o hav e forme d a s a resul t o f th e movemen t o f plate s (Fig . 2.3a ) durin g th e earth' s geologica l history . Thes e episode s ma y mar k th e closur e o f forme r oceans . Nearl y al l fol d mountai n system s involv e considerabl e thicknesse s o f sedimentar y rock s tha t hav e bee n crumple d an d folded . Accumulation s o f ove r 8 k m o f sedimen t ar e involve d i n th e Appalachia n mountai n system , fo r instance . Th e sedi mentar y rock s her e an d elsewher e characteristicall y exhibi t sign s o f bein g Fig . 2.3 . Deformatio n wher e crusta l plate s collide : (a ) thickenin g o f plate s i n collision, forming a mountain range; (b) oceanic plate bending unde r a thicker continental plate, creating an oceanic trench. (Cross-sections.)

14 Physical Geography Made Simple

deposite d nea r continenta l edge s an d consis t o f sandstone-shal e alternation s sometimes known as

flysch. The thicknesses of material involved were once thought to indicate that the sediments accumulated in slowly subsiding areas of shallow water termed geosynclines. However, now that it is realised that turbidity currents can deposit sand in deep water, the thick sedimentary sequences involved in mountains are regarded as large wedges of material which accumulated on continental slopes at the outer edge of continental shelves. The alternations in these sediments are important to landforms in the features of differential erosion that they may produce.

Th e transformatio n o f thes e sediment s int o mountain s ma y hav e bee n th

e result of both compressional forces and isostatic uplift. In some mountain systems, the uplift also appears to be associated with the intrusion of large bodies of igneous rock. The granitisation of the root zone may lead to a reduced rock density and increased volume, which causes the whole orogenic belt to rise. However, by whatever mechanism the chain is formed, gravitational sliding of the upper central areas often seems to have accompanied uplift, creating nappe structures or complex folds. Many of the world's largest mountain chains exist beneath the sea. Some of

thes

e are revealed as island arcs, as in the West Indies, and in the west and south-west Pacific ocean. Associated with them are deep oceanic trenches on the convex sides of

th e arc. These features are a direct result of the movement of crustal plates. The ocean deep marks th e sit

e of the downward plunging of the lower plate into the mantle (Fig. 2.3b). These sites are known as subduction zones. Melting of rocks here in the mantle gives rise to surface volcanic activity which is the basis of

th

e island arcs. The mid-oceanic ridges form the longest mountain chains. The mid-Atlantic ridge rises 3 km above the floor of the Atlantic; it is connected with the Indian Ocean ridge, and thence with the Pacific-Antarctic

ridge , resulting in a continuous system some 40,000 km in length. Bloc k Mountains , Basin s an d Rift s Th e morphologica l result s o f large-scal e warpin g an d faultin g ar e n o les

s spectacular than those produced by orogenic movements. Recent faulting is often clearly defined in the

relief

. Large tracts of land broken up by faults of great vertical displacement may form block mountains, separated by intervening basins. The Basin and Range country of western North America is of this

nature

, a mosaic of sub-parallel faults and differentially uplifted and tilted blocks. Within the individual blocks, the attitude of the strata does not necessarily reflect the most recent earth movements, and may be highly contorted from a previous episode. An uplifted block may alternatively be called a

horst

; a dropped block is a graben (Fig. 2.4). These may be small, or form large elongated rift valleys. Horst-and-graben structure is well exemplified by the Vosges, the Black Forest (horsts) and the Rhine rift valley.

Mos t continent s posses s rif t valleys , th e mos t extensiv e bein g th e Eas

t African rift system. Present-day rift valleys occur along the crest of tectonic arches formed mainly during the Tertiary period; some, such as those at the southern end of the Red Sea, are still developing. There are several ways in which

a rift valley may form. In the simplest case, two parallel faults allow the valley floor to sink between two inward-facing scarps. More commonly, there exist a number of faults on each side of the valley, sometimes arranged

Rocks and Relief 15

Fig . 2.4 . Diastrophis m a t th e Earth' s surface : (a ) bloc k faultin g (uneroded) ; (b ) simpl e foldin g (uneroded) ; (c ) foldin g showin g inversio n o f relief . e n echelon . I n othe r cases , th e basi c rift-valle y for m ma y b e modifie d b y th

e steep downwarping of the strata on either side towards the valley floor, thus disguising the faulting. Volcanoes are frequently associated with rift valleys, taking advantage of the crustal weaknesses set up by faulting.

Volcanoe

s

Volcani

c landform s ar e ver y diverse , mainl y a s a resul t o f chemica l differ

ences in the magma which feeds different sites. This not only creates different structural types of volcano, but also results in landforms that vary greatly in their resistance to erosion. Two major types of volcano are generally recognised. Outpourings of very fluid basaltic lava are usually accompanied by little violent eruptive activity. Individual lava

flows are normally only a few feet thick, but over a long period of time, repeated flows build up shield volcanoes (Fig. 2.5), as exemplified by those of the island of Hawaii. Here, Mauna Loa and Mauna Kea are made up of thousands of

flows to rise 9 km above the ocean floor. Cooler and more viscous andesitic lavas build volcanoes that are characterised by explosive eruptions and the ejection of a wide range of pyroclastic material (ash, cinders and rock fragments). Cone-shaped volcanoes result, otherwise known as strato-volcanoes.

Othe r type s o f volcan o sometime s recognise d includ e th e composit e type , consisting of several vents and parasitic cones. Extremely viscous acid or

16 Physical Geography Made Simple

Fig . 2.5 . Th e tw o majo r type s o f volcano. rhyolit e magma s resul t i n eithe r ver y violen t eruptions , o r th e creatio n o f a lava dome or plug which chokes the vent, and remains as resistant rock long after the

flanks of the volcano have been worn away. At the other extreme, some continental areas are covered with enormous accumulations of basaltic lava, made

u

p of many thin flows, which has emanated from fissures instead of a single vent. The resulting landform is a plateau rather than a cone. The Columbia plateau of the north-west United States, parts of the peninsula of India and portions of South Africa are of this nature.

Mos t activ e volcanoe s toda y ar e concentrate d i n severa l well-define d zones

, mainly along plate margins. The best known is the Pacific 'Ring of Fire', largely made up of explosive andesitic volcanoes. The mid-oceanic ridge systems are entirely volcanic, as are many individual oceanic islands. Some chains of volcanic islands - for example, the Hawaiian group - show a progression of increasing age away from the most recently active vent. This sequence is attributed to the gradual passage of the oceanic crust over a so-called hot-spot in the mantle underneath.

Roc k Structure s an d Landform s Whe

n tectonic activity ceases or becomes very slow, external weathering and erosion forces gradually become

th e dominant factors in the sculpturing of the landscape. Th

e influence of geology on the landscape passes to a more detailed level where even minor variations in the lithology and structure of

rock s may have an influence on the landforms. Join t Structure s Al l rock s develo p joint s a s the y consolidat e an d crac k unde r th e stresse

s set up by cooling or pressure changes. Both intrusive and extrusive igneous rocks develop columnar joints in response to contraction while cooling, well exemplified

b

y basalt. Sedimentary rocks usually develop joints at right angles to their bedding planes. Some jointing patterns

ar

e systematic (regular), while non-systematic joints are generally curved fractures which cross each other

Rocks and Relief 17

irregularly . Join t direction s hav e a profoun d influenc e i n guidin g th e cours e o f erosional processes, which becomes reflected in the alignments i n landforms. For instance, i

n many valley floors, streams follow joint directions, particularly in jointed igneous rocks and on flat-lying sedimentary rocks.

A striking influence on drainage lines in the upper Mississippi has been noted. Studies on East Yorkshire rivers in England have shown that joint directions in Millstone Grit and other rocks strongly influence valley alignment.

A t a more detailed level, cros s jointing in granite is fundamental to the shape of tors (see Fig. 3.1). I t i s o f considerabl e significanc e t o landfor m studie s tha t join t frequenc y i n most rocks apparently decreases with depth, and that many joints are formed only when pressure o n th e rock is relieved by surface erosion, perhaps in associatio n wit h uplift . Thi s proces s is known as unloading or the pressure-release mechanism.

In sedimentary rocks, bedding planes open up and joints are formed at right-angles to them. In rocks with no original structures, sheeting may occur, creating joints parallel

t o the existing land surface. Sheeting particularly affects granites, helping t o perpetuate the domed relief of the origina l granit e for m (se e pag e 21)
. Th e significanc e o f joint s i n facilitatin g erosio n i s therefor e complicate d b y the fact that the joints themselves seem to depend on erosion . This relationship may b

e self-perpetuating, since there is probably a tendency for distinct joint-orientated landforms, such as domes, canyons and cliffs, to persist. This is

a n example of feedback in geomorphology, whereby the form (the cliflF) controls the process o f unloading, which in turn determines the form.

Faultin

g A s use d earlie r i

n this chapter, a fault is a fracture in rock along which rocks have been relatively displaced. The amount of displacement, which can be horizontal or vertical, may be very small and hardly distinguishable from

a joint , o r it may be many tens of kilometres. It is possible to describe faults by several geometric o r genetic criteria. For our purposes, three types may be distinguished . I n a normal fault (Fig. 2.6), usually the result of tensional forces, NORMA L FAUL T (Cros s section ) REVERSE FAULT TEAR(TRANSCURRENT) (Cros s section ) FAULT (Pla n view ) Fig . 2.6 . Type s o f fault .

18 Physical Geography Made Simple

th e inclinatio n o f th e faul t plan e an d th e directio n o f downthro w ar e eithe r bot h t o th e lef t o r bot h t o th e right . I n a revers e fault , resultin g fro m com - pressiona l forces , th e bed s o n on e sid e o f th e faul t plan e ar e thrus t ove r th e other. A tea r o r latera l faul t i s on e i n whic h rock s ar e displace d horizontall y alon g a lin e o f fracture , wit h littl e o r n o vertica l movement . Th e amoun t o f vertica l displacement , i f any , o n a faul t i s referre d t o a s th e thro w o f th e fault ; th e amoun t o f latera l displacemen t i s know n a s th e heave . Th e surfac e expressio n o f a faul t depend s o n th e hardnes s o f th e rock s involve d an d th e siz e o f th e displacement . Norma l faultin g i n competen t rock s produce s a faul t scarp , a sharpl y define d cliff-lik e feature . Wit h th e passag e o f time , th e origina l faul t scar p ma y becom e subdued , bu t th e faul t plan e ma y hav e a contro l o n th e landscap e fo r lon g afterwards , especiall y i f rock s o f differen t hardnes s hav e bee n brough t togethe r o n eithe r sid e o f th e fault . Suc h feature s ar e calle d fault-lin e scarps . Wher e a softe r roc k wa s origin all y uplifte d i n respec t o f a harde r rock , i n time , a s th e weake r roc k i s mor e rapidl y erode d away , th e resistan t roc k ma y com e t o form th e highe r relie f o n th e fault-lin e scarp . Fault s frequentl y ten d t o occu r i n zones , wher e th e affecte d strat a ma y for m a dislocate d bel t perhap s severa l hundre d metre s wide , a s o n part s o f th e Sa n

Andrea

s fault . Thes e zone s ma y b e characterise d b y crushe d o r shattere d roc k know n a s faul t breccia , an d presen t situation s whic h favou r linea r erosion . Th e fjord s o f wester n Norwa y largel y coincid e wit h faulte d an d crushe d zones , indicatin g tha t fluvial an d subsequen t glacia l erosio n wer e largel y structurall y controlled . I n th e block-faulte d countr y describe d earlier , fault s ma y stan d ou t boldly , bu t equally , a larg e numbe r o f fault s exis t withou t an y trac e i n th e landform .

Foldin

g Th e natur e o f folde d structure s become s particularl y significan t i n landfor m studie s wher e severa l contrastin g lithologie s ar e involved . Muc h depend s o n th e degre e o f dip . I n a simpl e serie s o f folds , anticline s (upfolds ) ma y b e distinguishe d fro m syncline s (downfolds) . Th e foldin g ma y b e symmetrical , o r wher e th e limb s o f th e fol d diffe r i n dip , the y ma y b e asymmetrical . Degree s o f increasin g asymmetr y ca n b e recognise d i n overturne d an d recumben t fold s (se e Fig . 2.4b) . I n som e case s majo r anticline s an d syncline s ma y hav e mino r fold s superimpose d o n th e flanks of themain structure, creating anticlinoriums an d synclinoriums . I n Britain , example s o f thes e structure s ar e provide d b y th e Weal d an d Hampshir e basin s respectively . Th e relationshi p betwee n a serie s o f fold s an d relie f ma y b e considere d direc t wher e anticline s an d syncline s for m hig h an d lo w relie f respectivel y (a s i n Fig . 2.4b) . Th e Jur a mountain s i n norther n Switzerlan d ar e a well-know n example . Quit e frequently , however , ther e i s a n inversio n o f relief , in which the axia l line s o f th e anticline s ar e deepl y eroded , an d th e syncline s for m th e hig h ground . On e explanatio n o f thi s cite s th e fracturin g o f th e ape x o f th e anticline a s a zon e o f weakness , leadin g t o rapi d erosio n o f thi s zone . I t seem s tha t erosiona l rate s ma y b e sufficientl y effectiv e durin g slo w rate s o f foldin g s o tha t th e syncline s becom e th e highes t point s i n th e are a withi n a shor t tim e afte r foldin g commences . Th e landfor m expressio n o f individua l limb s o f fold s depend s primaril y o n tw o factors : th e inclinatio n o f th e strata , an d th e arrangemen t o f varyin g

Rocks and Relief 19

Hthologies

. On e significan t effec t o f inclinatio n i s tha t th e relativ e erodibilit

y of a rock is expressed most accurately in the landscape when dips are vertical, and least accurately when dips are low. In the latter case, a resistant cap-rock may protect weaker underlying strata. Some examples of this in relation to scarplands are illustrated in Chapter Four (Fig. 4.7). Moderate dips accentuate the importance of gravity in erosional processes, particularly on sea cliffs and other similar situations where the lower end of a tilted block of strata is being eroded away, and the dips are seaward. The effect of varying Hthologies in tilted strata is to produce examples of differential erosion, of which the alternations of scarp and vale in the scarplands of south-east England are a well-known example.

Roc k Typ e an d Landform s Resistance to Erosion Th e resistanc e o f a n individua l roc k typ e depend s o n a larg e numbe r o f variables, including mineralogical composition, grain size, the nature of the cementing agent , individual grains and permeability. These factors are particularly relevant i

n relation to the effectiveness of weathering, and the response of a number of rock types to weathering is considered in the next chapter. But in all landscapes, the most important factor is not so much the absolute resistance of a rock, but its relative resistance in relation to the strata around it. It is this which creates the pattern of

hig

h and low relief. For instance, chalk is a relatively hard rock in relation to adjacent strata in south-east England, but in Northern Ireland, it is soft compared to surrounding basalts.

Th e detaile d effect s o f variation s i n relativ e resistanc e o f

a rock to erosion may be illustrated with reference to igneous intrusions. Dykes are a common form of intrusion which are usually discordant to the country rock. They are vertical or near-vertical structures, varying

i n thickness from a few centimetres up to about fifty metres in thickness. The relief effect of dyke

s varies greatly. Where they are harder than the surrounding country rock, they will tend to form a wall-like ridge (Fig. 2.7a), sometimes traceable for several kilometres across country. Where dykes are weaker than surrounding rocks, they will form a trough (Fig. 2.7b). Sometimes the rock in immediate contact with the dyke will have become metamorphosed so that it forms either raised (Fig. 2.7c) or channelled (Fig. 2.7d) margins to the dyke. Contrary to what is generally assumed, dykes in Britain rarely form continuous wall-like features, since they are often intruded into other igneous, or metamorphic rocks, of greater resistance than themselves.

Sills

, on the other hand, generally act as harder members of the rock series into which they have been intruded. Unlike dykes they are intrusions which are concordant with the bedding planes of the country rock, and they are usually intruded into weaker sedimentary strata. In addition, they are nearly all horizontal in attitude. Thus, although they

ar

e composed of the same range of materials as dykes, they are often bold relief formers, creating ledges or

scarp s i n man

y places. Whin Sill in northern England is a well-known example. It is intruded into Carboniferous sediments and for most of

it

s length forms a bold escarpment or small craggy hills. Where crossed by streams its presence is often indicated by waterfalls.

20 Physical Geography Made Simple

la ) (c ) (b ) (d ) rf: \c U S Fig . 2.7 . Exampl e o f th e varyin g relativ e resistanc e o f rocks : (a) dyke more resistant than country rock; (b) dyke less resistant; (c ) rock s metamorphosed by contact with the dyke stand higher than the dyke itself ; (d) dyke with channelle d margins .

Distinctiv

e Lithologie s I t i s sometime s asserte d i n landfor m studie s tha t distinctiv e lithologie s creat e distinctiv e landscapes . However , i t i s hope d tha t thi s chapte r ha s show n tha t ther e ar e certai n qualification s t o thi s idea . Roc k type s for m differen t kind s o f relie f i n differen t structura l situations . Moreover , a singl e roc k typ e ma y no t alway s behav e th e sam e wa y t o weatherin g i n a variet y o f climati c situation s (Chapte r Three) . Thu s i t i