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FUNDAMENTALS OF GEOMORPHOLOGY

Fundamentals of Geomorphology is an engaging and comprehensive introduction to geomorphology, andexplores the world’s landforms from a broad systems perspective with an emphasis on change. The richvariety of landforms found on the Earth’s surface are described and discussed, paying attention to the rolesof geomorphic processes and historical events in understanding their development and to how particulargeomorphic processes affect, and are affected by, humans.

Beginning with a consideration of the nature of geomorphology and the quest of process and historicalgeomorphologists (both of whose views are presented in the book), the author looks at the geomorphicsystem, moving on to discuss:

• Structure: landforms resulting from, or influenced by, the endogenic agencies of tectonic and volcanicprocesses, geological structures, and rock types.

• Process and form: landforms fashioned by the exogenic agencies of weathering, running water, flowingice and meltwater, ground ice and frost, the wind and the sea.

• History: Earth surface history, giving a discussion of the origin of old plains; relict, exhumed, andstagnant landscape features; cycles of landscape change; the work of rivers, wind, and the sea in the pastand the evolutionary aspects of landscape change.

Fundamentals of Geomorphology provides a stimulating and innovative perspective on the key topics anddebates within the field of geomorphology. Written in an accessible and lively manner, it includes guidesto further reading, chapter summaries, and an extensive glossary of key terms. The book is also illustratedthroughout with over 200 informative diagrams and attractive photographs, including a colour platesection.

Richard John Huggett is Reader in Geography at the University of Manchester.

ROUTLEDGE FUNDAMENTALS OF PHYSICALGEOGRAPHY SERIES

Series Editor: John Gerrard

This new series of focused, introductory textbooks presents comprehensive, up-to-date introductions tothe fundamental concepts, natural processes and human/environmental impacts within each of the corephysical geography sub-disciplines. Uniformly designed, each volume contains student-friendly features:plentiful illustrations, boxed case studies, key concepts and summaries, further reading guides and aglossary.

Already published:Fundamentals of Biogeography

Richard John Huggett

Fundamentals of SoilsJohn Gerrard

Fundamentals of HydrologyTim Davie

Fundamentals of GeomorphologyRichard John Huggett

FUNDAMENTALS OF GEOMORPHOLOGY

Richard John Huggett

Routledge Fundamentals of PhysicalGeography

London and New York

First published 2003by Routledge

11 New Fetter Lane, London EC4P 4EE

Simultaneously published in the USA and Canadaby Routledge

29 West 35th Street, New York, NY 10001

Routledge is an imprint of the Taylor & Francis Group

© 2003 Richard John Huggett

All rights reserved. No part of this book may be reprinted or reproduced or utilised in any form or by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying and recording,

or in any information storage or retrieval system, without permission in writing from the publishers.

British Library Cataloguing in Publication DataA catalogue record for this book is available from the British Library

Library of Congress Cataloging in Publication DataHuggett, Richard J.

Fundamentals of geomorphology / Richard John Huggett.p. cm. – (Routledge fundamentals of physical geography)

Includes bibliographical references and index.1. Geomorphology. I. Title. II. Routledge fundamentals of physical geography series.

GB401.5 .H845 2002551.41–dc21 2002075133

ISBN 0–415–24145–6 (hbk)ISBN 0–415–24146–4 (pbk)

This edition published in the Taylor & Francis e-Library, 2005.

“To purchase your own copy of this or any of Taylor & Francis or Routledge’scollection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.”

ISBN 0-203-47063-X Master e-book ISBN

ISBN 0-203-77887-1 (Adobe eReader Format)

for my family

CONTENTS

Series editor’s preface ixAuthor’s preface xiAcknowledgements xiii

PART I INTRODUCING LANDFORMS AND LANDSCAPES 1

1 WHAT IS GEOMORPHOLOGY? 3

2 THE GEOMORPHIC SYSTEM 35

PART II STRUCTURE 65

3 TECTONIC AND STRUCTURAL LANDFORMS I 67

4 TECTONIC AND STRUCTURAL LANDFORMS II 92

PART III PROCESS AND FORM 111

5 WEATHERING AND RELATED LANDFORMS 113

6 KARST LANDSCAPES 132

7 FLUVIAL LANDSCAPES 171

8 GLACIAL AND GLACIOFLUVIAL LANDSCAPES 209

9 PERIGLACIAL LANDSCAPES 236

10 AEOLIAN LANDSCAPES 253

11 COASTAL LANDSCAPES 277

PART IV HISTORY 311

12 ANCIENT LANDSCAPES AND THEIR IMPLICATIONS 313

Appendix 343Glossary 344References 353Index 375

CONTENTSv i i i

SER IES ED ITOR’S PREFACE

We are presently living in a time of unparalleled change and when concern for the environment has neverbeen greater. Global warming and climate change, possible rising sea levels, deforestation, desertification,and widespread soil erosion are just some of the issues of current concern. Although it is the role of humanactivity in such issues that is of most concern, this activity affects the operation of the natural processesthat occur within the physical environment. Most of these processes and their effects are taught andresearched within the academic discipline of physical geography. A knowledge and understanding ofphysical geography, and all it entails, is vitally important.

It is the aim of this Fundamentals of Physical Geography Series to provide, in five volumes, the fundamentalnature of the physical processes that act on or just above the surface of the Earth. The volumes in the seriesare Climatology, Geomorphology, Biogeography, Hydrology, and Soils. The topics are treated in sufficient breadthand depth to provide the coverage expected in a Fundamentals series. Each volume leads into the topic byoutlining the approach adopted. This is important because there may be several ways of approachingindividual topics. Although each volume is complete in itself, there are many explicit and implicitreferences to the topics covered in the other volumes. Thus, the five volumes together provide acomprehensive insight into the totality that is physical geography.

The flexibility provided by separate volumes has been designed to meet the demand created by thevariety of courses currently operating in higher education institutions. The advent of modular courses hasmeant that physical geography is now rarely taught, in its entirety, in an ‘all-embracing’ course but isgenerally split into its main components. This is also the case with many Advanced Level syllabuses. Thusstudents and teachers are being frustrated increasingly by lack of suitable books and are having torecommend texts of which only a small part might be relevant to their needs. Such texts also tend to lackthe detail required. It is the aim of this series to provide individual volumes of sufficient breadth and depthto fulfil new demands. The volumes should also be of use to teachers of sixth forms, where modularsyllabuses are also becoming common.

Each volume has been written by higher education teachers with a wealth of experience in all aspects ofthe topics they cover and a proven ability in presenting information in a lively and interesting way. Eachvolume provides a comprehensive coverage of the subject matter using clear text divided into easily

accessible sections and subsections. Tables, figures, and photographs are used where appropriate as well asboxed case studies and summary notes. References to important previous studies and results are includedbut are used sparingly to avoid overloading the text. Suggestions for further reading are also provided. Themain target readership is introductory-level undergraduate students of physical geography or environmentalscience, but there will be much of interest to students from other disciplines and it is also hoped that sixth-form teachers will be able to use the information that is provided in each volume.

John Gerrard

SER IES ED ITOR’S PREFACEx

AUTHOR’S PREFACE

Geomorphology has always been a favourite subject of mine. For the first twelve years of my life I lived inNorth London, and I recall playing by urban rivers and in disused quarries. During the cricket season,Saturday and Sunday afternoons would be spent exploring the landscape around the grounds where myfather was playing cricket. H. W. (‘Masher’) Martin, the head of geography and geology at HertfordGrammar School, whose ‘digressions’ during classes were tremendously educational, aroused my first formalinterest in landforms. The sixth-form field-trips to the Forest of Dean and the Lake District wereunforgettable. While at University College London, I was lucky enough to come under the tutelage of EricH. Brown, Claudio Vita-Finzi, Andrew Warren, and Ron Cooke, to whom I am indebted for a remarkablesix years as an undergraduate and postgraduate. Since arriving at Manchester, I have taught several courseswith large geomorphological components but have seen myself very much as a physical geographer witha dislike of disciplinary boundaries and the fashion for overspecialization. Nonetheless, I thought thatwriting a new, student-friendly geomorphological text would pose an interesting challenge and, withFundamentals of Biogeography, make a useful accompaniment to my more academic works.

In writing Fundamentals of Geomorphology, I have tried to combine process geomorphology, which hasdominated the subject for the last several decades, with the less fashionable but fast-resurging historicalgeomorphology. Few would question the astounding achievements of process studies, but plate-tectonicstheory and a reliable calendar of events have given historical studies a huge boost. I also feel that too manybooks get far too bogged down in process equations: there is a grandeur in the diversity of physical formsfound at the Earth’s surface and a wonderment to be had in seeing them. So, while explaining geomorphicprocesses and not shying away from equations, I have tried to capture the richness of landform types and the pleasure to be had in trying to understand how they form. I also discuss the interactions between landforms, geomorphic processes, and humans, which, it seems to me, is an important aspect ofgeomorphology today.

The book is quadripartite. Part I introduces landforms and landscapes, studying the nature ofgeomorphology and outlining the geomorphic system. It then divides the material into three parts:structure, form and process, and history. William Morris Davis established the logic of this scheme a centuryago. The argument is that any landform depends upon the structure of the rocks – including their

composition and structural attitude – that it is formed in or on, the processes acting upon it, and the timeover which it has been evolving. Part II looks at tectonic and structural landforms. Part II investigatesprocess and form, with chapters on weathering and related landforms, karst landscapes, fluvial landscapes,glacial landscapes, periglacial landscapes, aeolian landscapes, and coastal landscapes. Each of these chapters,excepting the one on weathering, considers the environments in which the landscapes occur, the processesinvolved in their formation, the landforms they contain, and how they affect, and are affected by, humans.Part IV examines the role of history in understanding landscapes and landform evolution, examining somegreat achievements of modern historical geomorphology.

There are several people to whom I wish to say ‘thanks’: Nick Scarle, for drawing all the diagrams andhandling the photographic material. Andrew Mould at Routledge, for taking on another Huggett book.Six anonymous reviewers, for the thoughtful and perceptive comments on an embarrassingly rough draftof the work that led to several major improvements, particularly in the overall structure; any remainingshortcomings and omissions are of course down to me. A small army of colleagues, identified individuallyon the plate captions, for kindly providing me with slides. Jonathan D. Phillips, for helping me to explainhis ‘Eleven Principles of Earth Surface Systems’ in simple terms. Clive Agnew and the other staff atManchester, for friendship and assistance, and in particular Kate Richardson for making several invaluablesuggestions about the structure and content of Chapter 1. As always, Derek Davenport, for discussing allmanner of things. And finally, my wife and family, who understand the ups and downs of book-writingand give unbounded support.

Richard HuggettPoynton

March 2002

AUTHOR’S PREFACEx i i

ACKNOWLEDGEMENTS

The author and publisher would like to thank the following for granting permission to reproduce materialin this work:

The copyright of photographs remains held by the individuals who kindly supplied them (please seephotograph captions for individual names); Figure 1.2 after Figure 3.10 from S. A. Schumm (1991) ToInterpret the Earth: Ten Ways to be Wrong (Cambridge: Cambridge University Press), reproduced by permissionof Cambridge University Press; Figures 1.3, 4.5, 4.8, 4.10, 4.16, and 4.17 after Figures 11.11, 11.25,11.26, 11.36, 16.3, and 16.16 from C. R. Twidale and E. M. Campbell (1993) Australian Landforms:Structure, Process and Time (Adelaide: Gleneagles Publishing), reproduced by permission of C. R. Twidale;Figure 1.5 after Figure 4 from R. H. Johnson (1980) ‘Hillslope stability and landslide hazard – a case studyfrom Longdendale, north Derbyshire, England’ in Proceedings of the Geologists’ Association, London (Vol. 91,pp. 315–25), reproduced by permission of the Geologists’ Association; Figure 1.9 from Figure 6.1 fromR. J. Chorley and B. A. Kennedy (1971) Physical Geography: A Systems Approach (London: Prentice Hall),reproduced by permission of Richard J. Chorley and Barbara A. Kennedy; Figure 4.12 after Figure 4.9from M. A. Summerfield (1991) Global Geomorphology: An Introduction to the Study of Landforms (Harlow,Essex: Longman), © M. A. Summerfield, reprinted by permission of Pearson Education Limited; Figure5.1 after Figures 3.3 and 3.5 from G. Taylor and R. A. Eggleton (2001) Regolith Geology and Geomorphology(Chichester: John Wiley & Sons), reproduced by permission of John Wiley & Sons Limited; Figure 6.1after ‘Plan of Poole’s Cavern’ from D. G. Allsop (1992) Visitor’s Guide to Poole’s Cavern (Buxton, Derbyshire:Buxton and District Civic Association), after a survey by P. Deakin and the Eldon Pothole Club, reproducedby permission of Poole’s Cavern and Country Park; Figures 6.6, 6.7, and 6.9 after Figures 9.3, 9.13, and9.30 from D. C. Ford and P. W. Williams (1989) Karst Geomorphology and Hydrology (London: Chapman & Hall), reproduced with kind permission of Kluwer Academic Publishers and Derek Ford; Figure 7.10after Figure 14.1 from F. Ahnert (1998) Introduction to Geomorphology (London: Arnold), reproduced bypermission of Hodder & Stoughton Educational and Verlag Eugen Ulmer, Stuttgart (the original Germanlanguage publishers); Figures 7.14, 7.18, 9.3, and 11.5 after Figures 8.3, 8.5, 8.13, 11.3, and 17.14 fromK. W. Butzer (1976) Geomorphology from the Earth (New York: Harper & Row), reproduced by permission

of Addison Wesley Longman, USA; Figure 7.19 after Figure 6 from J. Warburton and M. Danks (1998)‘Historical and contemporary channel change, Swinhope Burn’ in J. Warburton (ed.) Geomorphological Studiesin the North Pennines: Field Guide, pp. 77–91 (Durham: Department of Geography, University of Durham,British Geomorphological Research Group), reproduced by permission of Jeff Warburton; Figures 8.4 and11.6 after Figures 6.9 and 12.22 from A. S. Trenhaile (1998) Geomorphology: A Canadian Perspective (Toronto:Oxford University Press), reproduced by permission of Oxford University Press, Canada.

Every effort has been made to contact copyright holders for their permission to reprint material in thisbook. The publishers would be grateful to hear from any copyright holder who is not here acknowledgedand will undertake to rectify any errors or omissions in future editions of this book.

ACKNOWLEDGEMENTSx iv

Part I

INTRODUCING LANDFORMS AND LANDSCAPES

INTRODUCING GEOMORPHOLOGY

The word geomorphology is derived from threeGreek words: γεω (the Earth), µορφη (form), andλογος (discourse). Geomorphology is therefore ‘adiscourse on Earth forms’. It is the study of Earth’sphysical land-surface features, its landforms – rivers,hills, plains, beaches, sand dunes, and myriad others.Some workers include submarine landforms withinthe scope of geomorphology. And some would addthe landforms of other terrestrial-type planets andsatellites in the Solar System – Mars, the Moon,Venus, and so on. Geomorphology was first used asa term to describe the morphology of the Earth’ssurface in the 1870s and 1880s (e.g. de Margerie1886, 315). It was originally defined as ‘the genetic

study of topographic forms’ (McGee 1888, 547),and was used in popular parlance by 1896.

Despite the modern acquisition of its name,geomorphology is a venerable discipline. AncientGreek and Roman philosophers wondered howmountains and other surface features in the naturallandscape had formed. Aristotle, Herodotus, Seneca,Strabo, Xenophanes, and many others discoursed on topics such as the origin of river valleys and deltas, and the presence of seashells in mountains.Xenophanes of Colophon (c. 580–480 BC) speculatedthat, as seashells are found on the tops of mountains,the surface of the Earth must have risen and fallen.Herodotus (c. 484–420 BC) thought that the lowerpart of Egypt was a former marine bay, reputedlysaying ‘Egypt is the gift of the river’, referring to the

1

WHAT IS GEOMORPHOLOGY?

Geomorphology is the study of landforms and the processes that create them. This chapter covers:

� historical, process, and applied geomorphology� the form of the land� land-forming processes and geomorphic systems� the history of landforms� methodological isms

year-by-year accumulation of river-borne silt in the Nile delta region. Aristotle (384–322 BC)conjectured that land and sea change places, withareas that are now dry land once being sea and areasthat are now sea once being dry land. Strabo (64/63BC–AD 23?) observed that the land rises and falls, andsuggested that the size of a river delta depends on thenature of its catchment, the largest deltas beingfound where the catchment areas are large and thesurface rocks within it are weak. Lucius AnnaeusSeneca (4 BC–AD 65) appears to have appreciated thatrivers possess the power to erode their valleys. Abouta millennium later, the illustrious Arab scholar ibn-Sina, also known as Avicenna (980–1037), whotranslated Aristotle, propounded the view that somemountains are produced by differential erosion,running water and wind hollowing out softer rocks.During the Renaissance, many scholars debatedEarth history. Leonardo da Vinci (1452–1519)believed that changes in the levels of land and seaexplained the presence of fossil marine shells inmountains. He also opined that valleys were cut bystreams and that streams carried material from oneplace and deposited it elsewhere. In the eighteenthcentury, Giovanni Targioni-Tozzetti (1712–84)recognized evidence of stream erosion, arguing thatthe valleys of the Arno, Val di Chaina, and Ombrosain Italy were excavated by rivers and floods resultingfrom the bursting of barrier lakes and suggested thatthe irregular courses of streams are related to thedifferences in the rocks in which they are cut, aprocess that is now called differential erosion. Jean-Étienne Guettard (1715–86) argued that streamsdestroy mountains and the sediment produced in the process builds floodplains before being carried to the sea. He also pointed to the efficacy of marineerosion, noting the rapid destruction of chalk cliffsin northern France by the sea, and the fact that themountains of the Auvergne were extinct volcanoes.Horace-Bénédict de Saussure (1740–99) contendedthat valleys were produced by the streams that flowwithin them, and that glaciers may erode rocks. Fromthese early ideas on the origin of landforms arosemodern geomorphology (see Chorley et al. 1964 fordetails on the development of the subject).

Geomorphology investigates landforms and theprocesses that fashion them. A large corpus of geo-morphologists expends much sweat in researchingrelationships between landforms and the processesacting on them now. These are the process orfunctional geomorphologists. Many geomorphicprocesses affect, and are affected by, human activ-ities. This rich area of enquiry, which is largely anextension of process geomorphology, is explored byapplied geomorphologists. Many landforms havea long history and their present form is not alwaysrelated to the current processes acting upon them.The nature and rate of geomorphic processes changewith time and some landforms were produced underdifferent environmental conditions, surviving todayas relict features. In high latitudes, many landformsare relicts from the Quaternary glaciations, but, inparts of the world, some landforms survive frommillions and hundreds of millions of years ago.Geomorphology, then, has an important historicaldimension, which is the domain of the historicalgeomorphologists. In short, modern geomor-phologists study three chief aspects of landforms –form, process, and history. The first two aresometimes termed functional geomorphology, thelast historical geomorphology (Chorley 1978).Process studies have enjoyed a hegemony for somethree or four decades. Historical studies weresidelined by process studies but are making a strongcomeback. Although process and historical studiesdominate much modern geomorphological enquiry,particularly in English-speaking nations, othertypes of study exist. For example, structural geo-morphologists, who were once a very influentialgroup, argued that underlying geological structuresare the key to understanding many landforms.Climatic geomorphologists, who are found mainlyin France and Germany, believe that climate exertsa profound influence on landforms, each climaticregion creating a distinguishing suite of landforms.

Historical geomorphology

Traditionally, historical geomorphologists strove towork out landscape history by mapping morpho-

INTRODUCING LANDFORMS AND LANDSCAPES4

logical and sedimentary features. Their golden rulewas the dictum that ‘the present is the key to thepast’. This was a warrant to assume that the effectsof geomorphic processes seen in action today maybe legitimately used to infer the causes of assumedlandscape changes in the past. Before reliable datingtechniques were available, such studies were difficultand largely educated guesswork. However, thebrilliant successes of early historical geomor-phologists should not be overlooked.

William Morris Davis

The ‘geographical cycle’, expounded by WilliamMorris Davis, was the first modern theory oflandscape evolution (e.g. Davis 1889, 1899, 1909).It assumed that uplift takes place quickly. The rawtopography is then gradually worn down bygeomorphic processes, without further compli-cations from tectonic movements. Furthermore,slopes within landscapes decline through time –maximum slope angles slowly lessen (though fewfield studies have substantiated this claim). So,topography is reduced, little by little, to anextensive flat region close to base level – a pene-plain – with occasional hills called monadnocksafter Mount Monadnock in New Hampshire, USA,which are local erosional remnants, standingconspicuously above the general level. The reduc-tion process creates a time sequence of landformsthat progresses through the stages of youth,maturity, and old age. However, these terms,borrowed from biology, are misleading and muchcensured (e.g. Ollier 1967; Ollier and Pain 1996,204–5). The ‘geographical cycle’ was designed toaccount for the development of humid temperatelandforms produced by prolonged wearing down ofuplifted rocks offering uniform resistance to erosion.It was extended to other landforms, including aridlandscapes, glacial landscapes, periglacial land-scapes, to landforms produced by shore processes,and to karst landscapes.

William Morris Davis’s ‘geographical cycle’ – in which landscapes are seen to evolve throughstages of youth, maturity, and old age – must be

regarded as a classic work, even if it is now knownto be flawed (Figure 1.1). Its appeal seems to havelain in its theoretical tenor and in its simplicity(Chorley 1965). It had an all-pervasive influence ongeomorphological thought and spawned the oncehighly influential field of denudation chronology.The work of denudation chronologists, who workedmainly with morphological evidence, has sub-sequently been criticized for seeing flat surfaceseverywhere.

Walther Penck

A variation on Davis’s scheme was offered byWalther Penck. According to the Davisian model,uplift and planation take place alternately. But, inmany landscapes, uplift and denudation occur at thesame time. The continuous and gradual interactionof tectonic processes and denudation leads to adifferent model of landscape evolution, in which theevolution of individual slopes is thought to deter-mine the evolution of the entire landscape (Penck1924, 1953). Three main slope forms evolve withdifferent combinations of uplift and denudationrates. First, convex slope profiles, resulting fromwaxing development (aufsteigende Entwicklung), formwhen the uplift rate exceeds the denudation rate.Second, straight slopes, resulting from stationary (orsteady-state) development (gleichförmige Entwick-lung), form when uplift and denudation rates matchone another. And third, concave slopes, resultingfrom waning development (absteigende Entwicklung),form when the uplift rate is less than the denudationrate. Later work has shown that valley-side shapedepends not on the simple interplay of erosion ratesand uplift rates, but on slope materials and thenature of slope-eroding processes.

According to Penck’s arguments, slopes mayeither recede at the original gradient or else flatten,according to circumstances. Many textbooks claimthat Penck advocated ‘parallel retreat of slopes’, butthis is a false belief (see Simons 1962). Penck (1953,135–6) argued that a steep rock face would moveupslope, maintaining its original gradient, butwould soon be eliminated by a growing basal slope.

WHAT IS GEOMORPHOLOGY? 5


( ) Youtha

( ) Maturityb

( ) Old agec

Figure 1.1 William Morris Davis’s idealized‘geographical cycle’ in which a landscape evolvesthrough ‘life-stages’ to produce a peneplain. (a) Youth:a few ‘consequent’ streams (p. 97), V-shaped valleycross-sections, limited floodplain formation, large areasof poorly drained terrain between streams with lakesand marshes, waterfalls and rapids common wherestreams cross more resistant beds, stream divides broadand ill-defined, some meanders on the original surface.(b) Maturity: well-integrated drainage system, somestreams exploiting lines of weak rocks, master streamshave attained grade (p. 191), waterfalls, rapids, lakes,and marshes largely eliminated, floodplains common onvalley floors and bearing meandering rivers, valley nowider than the width of meander belts, relief (differencein elevation between highest and lowest points) is at amaximum, hillslopes and valley sides dominate thelandscape. (c) Old age: trunk streams more importantagain, very broad and gently sloping valleys, floodplainsextensive and carrying rivers with broadly meanderingcourses, valleys much wider than the width of meanderbelts, areas between streams reduce in height andstream divides not so sharp as in the maturity stage,lakes, swamps, and marshes lie on the floodplains,mass-wasting dominates fluvial processes, streamadjustments to rocks types now vague, extensive areaslie at or near the base level of erosion.Source: Adapted from Holmes (1965, 473)

1

PediplainPediplain PeneplainPeneplain

12 24 5 6

456 3 3

Slope recession or backwearing(Penck)

Slope decline or downwearing(Davis)

Time

Figure 1.2 Slope recession, which produces a pediplain (p. 315) and slope decline, which produces a peneplain.Source: Adapted from Gossman (1970)

If the cliff face was the scarp of a tableland, however,it would take a long time to disappear. He reasonedthat the basal slope is replaced by a lower-angleslope that starts growing from the bottom of thebasal slope. Continued slope replacement then leadsto a flattening of slopes, with steeper sectionsformed during earlier stages of development some-times surviving in summit areas (Penck 1953,136–41). In short, Penck’s complicated analysispredicted both slope recession and slope decline,a result that extends Davis’s simple idea of slopedecline (Figure 1.2). Field studies have confirmedthat slope retreat is common in a wide range ofsituations. However, a slope that is actively erodedat its base (by a river or by the sea) may decline ifthe basal erosion should stop.

Eduard Brückner and Albrecht Penck

Other early historical geomorphologists used geo-logically young sediments to interpret Pleistoceneevents. Eduard Brückner and Albrecht Penck’s(Walther’s father) work on glacial effects on theBavarian Alps and their forelands provided the firstinsights into the effects of the Pleistocene ice ageson relief (Penck and Brückner 1901–9). Theirclassic river-terrace sequence gave names to the mainglacial stages – Donau, Gunz, Mindel, Riss, andWürm – and fathered Quaternary geomorphology.

Uluru: a case study

It is perhaps easiest to explain modern historicalgeomorphology by way of an example. Take Uluru(Ayers Rock), arguably the most famous landform inAustralia and one of the best-known in the world,ranking alongside the Grand Canyon, the Matter-horn, and the Niagara Falls (Box 1.1). On seeingUluru, no geomorphologist can fail to wonder howit formed. A picture of its history has been piecedtogether by geomorphologists, partly by a process ofelimination (Twidale and Campbell 1993, 247–51).Uluru cannot be the result of faulting of the rocks inwhich it is formed, since its scarps are not related tofaults in the bedrock. Nor is it the product of a

resistant lithology (the composition of the rock ofwhich it is made), because similar bedrock underliesthe adjacent plains, although the rocks comprisingUluru were folded during the Middle Palaeozoicorogeny (mountain-building episode) and later,which may have imparted a degree of resistance toerosion. Some investigators deem Uluru to be thelast remnant of scarp retreat that has eaten away thesurrounding bedrock. A more believable explanationis that Uluru began beneath the land surface and wasexposed in stages (Figure 1.3). There is evidence thatCretaceous topography was a surface of low relief,with a bedrock valley occupying the area between


123

4

Block collapsesafter undermining

Original surface

( ) Early Mesozoica

( ) Late Mesozoicb

( ) Late Cenozoicc

( ) Presentd

Figure 1.3 Possible evolution of Uluru (Ayers Rock)from the Early Mesozoic era to the present. Thenumbers in the Late Mesozoic enlarged section arestages of undermining.Source: After Twidale and Campbell (1993, 251)


Uluru is an ‘island mountain’ or inselberg sitting863 m above sea level and rising 300–340 mabove the surrounding desert plain (Twidale andCampbell 1993, 247–51) (Plate 1.1a). It is madeof green–grey arkosic sandstone beds formedduring the Cambrian period. Uluru’s red colour isdue to a thin coating of iron oxide. The sandstonebeds dip almost vertically. The summit rollsbroadly, with ribs and corrugations associated withminor ridges running parallel to the strike of the beds and with dimples associated with theformation of many basins. The steep sides of Uluruare fretted and sculpted, with many huge caves,especially on the southern slope, and with flaredslopes up to 4 m high, again well developed on thesouthern base (Plate 1.1b). Other curious featuresinclude ‘The Brain’, which results from a breachin the outer skin of Uluru and the exposure of thedipping beds, and the ‘Kangaroo Tail’, which isone of several sheet fractures up to 2 m thick. Thesurface is mostly covered with thin rock flakes orscales. The plains surrounding Uluru are formedon alluvial (river-deposited) and aeolian (wind-deposited) sediments. To the south, they are quitethick (tens of metres), but to the north and west,they are very thin (absent or just a few metresthick) and, in several places, the steep bedrockwalls give way to rock platforms and even, inLittle Ayers Rock, blocks and boulders resting ona small bedrock dome. Some 24 km west-north-west of Uluru lie the Olgas, a group of domes andturrets rising from the desert plains and formed inconglomerate. Borings between Uluru and theOlgas have revealed that the plain is underlain bya broad and shallow valley cut in Cambrian bedsand filled with later sediments, the oldest of whichare swamp deposits of Late Cretaceous age.

Box 1.1

ULURU (AYERS ROCK) , CENTRAL AUSTRALIA – A FAMOUS LANDFORMWITH A HISTORY TO REVEAL

Plate 1.1 Ayers Rock, central Australia. (a) Generalview. (b) Caves in the rock face. Notice the large cavetowards the base.(Photograph by Kate Richardson)

the Olgas and Uluru in Late Cretaceous times, and low hills on either side of the valley marking the present sites of the Olgas and Uluru. Waterrunning off these hills would have rotted the bedrock underneath the adjoining plains. The weath-ering front (the depth at which weathered andunweathered rock meet) would have become steepand deep. In the zone where the water tablefluctuated, intense weathering would have produceddeep indentations in the bedrock, which are seentoday as gaping caves on the southern boundingslopes. Later, the plains were lowered, which exposedthe upper parts of the steep bounding slopes,including the deep indentations that now stand35–65 m above the level of the plain. At this time,the plain was some 4 m higher than at present.Flared slopes were initiated beneath it, and thesewere exposed when the plain reached its presentlevel. Uluru has thus taken about 70 million yearsto attain its present form.

Process geomorphology

Process geomorphology is the study of the processesresponsible for landform development. In themodern era, the first process geomorphologist,carrying on the tradition started by Leonardo daVinci (p. 4), was Grove Karl Gilbert. In his treatiseon the Henry Mountains of Utah, USA, Gilbertdiscussed the mechanics of fluvial processes (Gilbert1877), and later he investigated the transport ofdebris by running water (Gilbert 1914). Up to about1950, when the subject grew apace, importantcontributors to process geomorphology includedRalph Alger Bagnold (p. 255), who considered thephysics of blown sand and desert dunes, and FilipHjulstrøm (p. 178), who investigated fluvial pro-cesses. After 1950, several ‘big players’ emerged thatset process geomorphology moving apace. Arthur N.Strahler was instrumental in establishing processgeomorphology, his 1952 paper called ‘Dynamicbasis of geomorphology’ being a landmark publica-tion. John T. Hack, developing Gilbert’s ideas,prosecuted the notions of dynamic equilibriumand steady state, arguing that a landscape should

attain a steady state, a condition in which land-surface form does not change despite material beingadded by tectonic uplift and removed by a constantset of geomorphic processes. And he contended that,in an erosional landscape, dynamic equilibriumprevails where all slopes, both hillslopes and riverslopes, are adjusted to each other (cf. Gilbert 1877,123–4; Hack 1960, 81), and ‘the forms and pro-cesses are in a steady state of balance and may beconsidered as time independent’ (Hack 1960, 85).Luna B. Leopold and M. Gordon Wolman madenotable contributions to the field of fluvial geomor-phology (e.g. Leopold et al. 1964). Stanley A.Schumm, another fluvial geomorphologist, refinednotions of landscape stability to include thresholdsand dynamically metastable states and made animportant contribution to the understanding oftimescales (p. 30). Stanley W. Trimble worked onhistorical and modern sediment budgets in smallcatchments (e.g. Trimble 1983). Richard J. Chorleybrought process geomorphology to the UK anddemonstrated the power of a systems approach tothe subject.

Process geomorphologists have done their subjectat least three great services. First, they have built upa database of process rates in various parts of theglobe. Second, they have built increasingly refinedmodels for predicting the short-term (and in somecases long-term) changes in landforms. Third, theyhave generated some enormously powerful ideasabout stability and instability in geomorphic sys-tems (see pp. 19–20).

Measuring geomorphic processes

Some geomorphic processes have a long record ofmeasurement. The oldest year-by-year record is theflood levels of the River Nile in lower Egypt. Yearlyreadings at Cairo are available from the time ofMuhammad, and some stone-inscribed records date from the first dynasty of the pharaohs, around3,100 BC. The amount of sediment annually carrieddown the Mississippi River was gauged during the1840s, and the rates of modern denudation in someof the world’s major rivers were estimated in the


1860s. The first efforts to measure weathering rateswere made in the late nineteenth century. Measure-ments of the dissolved load of rivers enabledestimates of chemical denudation rates to be madein the first half of the twentieth century and patchyefforts were made to widen the range of processesmeasured in the field. But it was the quantitativerevolution in geomorphology, started in the 1940s,that was largely responsible for the measuring of process rates in different environments. Sinceabout 1950, the attempts to quantify geomorphicprocesses in the field have grown fast. An earlyexample is the work of Anders Rapp (1960), whotried to quantify all the slope processes active in asubarctic environment and assess their comparativesignificance. His studies enabled him to concludethat the most powerful agent of removal wasrunning water bearing material in solution. Anincreasing number of hillslopes and drainage basinshave been instrumented, that is, had measuringdevices installed to record a range of geomorphicprocesses. The instruments used on hillslopes andin geomorphology generally are explained in severalbooks (e.g. Goudie 1994). Interestingly, some of theinstrumented catchments established in the 1960shave recently received unexpected attention fromscientists studying global warming, because recordslasting decades in climatically sensitive areas – highlatitudes and high altitudes – are invaluable.However, after half a century of intensive fieldmeasurements, some areas, including Europe andNorth America, still have better coverage than otherareas. And field measurement programmes shouldideally be ongoing and work on as fine a resolutionas practicable, because rates measured at a particularplace may vary through time and may not berepresentative of nearby places.

Modelling geomorphic processes

Since the 1960s and 1970s, process studies havebeen largely directed towards the construction of models for predicting short-term changes inlandforms, that is, changes happening over humantimescales. Such models have drawn heavily on

soil engineering, for example in the case of slopestability, and hydraulic engineering in the cases of flow and sediment entrainment and deposition in rivers. Nonetheless, some geomorphologists,including Michael J. Kirkby and Jonathan D.Phillips, have carved out a niche for themselves inthe modelling department. An example of a geo-morphic model is shown in Figure 1.4.

Process studies and global environmentalchange

With the current craze for taking a global view,process geomorphology has found natural links with other Earth and life sciences. Main thrusts ofresearch investigate (1) energy and mass fluxes and(2) the response of landforms to climate, hydrology,tectonics, and land use (Slaymaker 2000b, 5). Thefocus on mass and energy fluxes explores the short-term links between land-surface systems and climatethat are forged through the storages and movementsof energy, water, biogeochemicals, and sediments.Longer-term and broader-scale interconnectionsbetween landforms and climate, water budgets,vegetation cover, tectonics, and human activity area focus for process geomorphologists who take ahistorical perspective and investigate the causes andeffects of changing processes regimes during theQuaternary.

Applied geomorphology

Applied geomorphology studies the interactions of humans with landscapes and landforms. Processgeomorphologists, armed with their models, havecontributed to the investigation of worrying prob-lems associated with the human impacts onlandscapes. They have studied coastal erosion and beach management (e.g. Bird 1996; Viles andSpencer 1996), soil erosion, the weathering ofbuildings, landslide protection, river managementand river channel restoration (e.g. Brookes andShields 1996), and the planning and design oflandfill sites (e.g. Gray 1993). Other process geomor-phologists have tackled more general applied issues.


Geomorphology in Environmental Planning (Hooke1988), for example, considered the interactionbetween geomorphology and public policies, withcontributions on rural land-use and soil erosion,urban land-use, slope management, river manage-ment, coastal management, and policy formulation.Geomorphology in Environmental Management (Cooke1990), as its title suggests, looked at the role playedby geomorphology in management aspects of theenvironment. Geomorphology and Land Management ina Changing Environment (McGregor and Thompson1995) focused upon problems of managing landagainst a background of environmental change. Theconservation of ancient and modern landforms is anexpanding aspect of applied geomorphology.

Three aspects of applied geomorphology havebeen brought into a sharp focus by the impendingenvironmental change associated with globalwarming (Slaymaker 2000b) and illustrate the valueof geomorphological know-how. First, appliedgeomorphologists are ideally placed to work on themitigation of natural hazards of geomorphic origin,which may well increase in magnitude and frequencyduring the twenty-first century and beyond. Land-slides and debris flows may become more common,soil erosion may become more severe and thesediment load of some rivers increase, some beachesand cliffs may erode faster, coastal lowlands maybecome submerged, and frozen ground in the tundraenvironments may thaw. Applied geomorphologists


20 m 50 m

50 m20 m

( ) Scarp retreat Ia ( ) Scarp retreat IIb

( ) Scarp rounding Ic ( ) Scarp rounding IId

Debris apron

Figure 1.4 Example of a geomorphic model: the predicted evolution of a fault scarp according to assumptions madeabout slope processes. (a) Parallel scarp retreat with deposition of debris at the base. The scarp is produced by asingle movement along the fault. (b) Parallel scarp retreat with deposition at the base. The scarp is produced byfour separate episodes of movement along the fault. In cases (a) and (b) it is assumed that debris starts to movedownslope once a threshold angle is reached and then comes to rest where the scarp slope is less than the thresholdangle. Allowance is made for the packing density of the debris and for material transported beyond the debrisapron. (c) Rounding of a fault scarp that has been produced by one episode of displacement along the fault. (d)Rounding of a fault scarp that has been produced by four separate episodes of movement along the fault. In cases (c)and (d), it is assumed that the volume of debris transported downslope is proportional to the local slope gradient.Source: Adapted from Nash (1981)

can address all these potentially damaging changes.Second, a worrying aspect of global warming is itseffect on natural resources – water, vegetation, crops,and so on. Applied geomorphologists, armed withsuch techniques as terrain mapping, remote sensing,and geographical information systems, can contri-bute to environmental management programmes.Third, applied geomorphologists are able to translatethe predictions of global and regional temperaturerises into predictions of critical boundary changes,such as the poleward shift of the permafrost line andthe tree-line, which can then guide decisions abouttailoring economic activity to minimize the effectsof global environmental change.

FORM

The two main approaches to form in geomor-phology are description (field description andmorphology mapping) and mathematical represen-tation (geomorphometry).

Field description and morphologicalmapping

The only way fully to appreciate landforms is to gointo the field and see them. Much can be learnt fromthe now seemingly old-fashioned techniques of fielddescription, field sketching, and map reading andmap making.

The mapping of landforms is an art (seeDackombe and Gardiner 1983, 13–20, 28–41;Evans 1994). Landforms vary enormously in shapeand size. Some, such as karst depressions and volca-noes, may be represented as points. Others, such asfaults and rivers, are linear features that are bestdepicted as lines. In other cases, areal properties maybe of prime concern and suitable means of spatialrepresentation must be employed. Areal propertiesare captured by morphological maps. Morpho-logical mapping attempts to identify basiclandform units in the field, on aerial photographs,or on maps. It sees the ground surface as an assem-blage of landform elements. Landform elements

are recognized as simply curved geometric surfaceslacking inflections (complicated kinks) and areconsidered in relation to upslope, downslope, andlateral elements. They go by a plethora of names –facets, sites, land elements, terrain components, andfacies. The ‘site’ (Linton 1951) was an elaborationof the ‘facet’ (Wooldridge 1932), and involvedaltitude, extent, slope, curvature, ruggedness, andrelation to the water table. The other terms werecoined in the 1960s (see Speight 1974). Figure 1.5shows the land surface of Longdendale in thePennines, England, represented as a morphologicalmap. The map combines landform elements derivedfrom a nine-unit land-surface model with depictionsof deep-seated mass movements and superficial massmovements. Digital elevation models, which liewithin the ambits of landform morphometry and aredealt with below, have greatly extended, but by nomeans replaced, the classic work on landformelements and their descriptors as prosecuted by themorphological mappers.

Geomorphometry

A branch of geomorphology – landform mor-phometry or geomorphometry – studiesquantitatively the form of the land surface. Geomor-phometry in the modern era can be traced to thework of Alexander von Humboldt and Carl Ritter inthe early and mid-nineteenth century (see Pike1999). It had a strong post-war tradition in North America and the UK, and it has been ‘rein-vented’ with the advent of remotely sensed imagesand Geographical Information Systems (GIS) soft-ware. The contributions of geomorphometry togeomorphology and cognate fields are legion. Geo-morphometry is an important component of terrainanalysis and surface modelling. Its specific applica-tions include measuring the morphometry ofcontinental ice surfaces, characterizing glacialtroughs, mapping sea-floor terrain types, guidingmissiles, assessing soil erosion, analysing wildfirepropagation, and mapping ecoregions (Pike 1995,1999). It also contributes to engineering, transporta-tion, public works, and military operations.


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Digital elevation models

The resurgence of geomorphometry since the 1970sis in large measure owing to two developments: first,the light-speed development and use of GIS, whichallow input, storage, and manipulation of digitaldata representing spatial and aspatial features of theEarth’s surface; and second, the development of Elec-tronic Distance Measurement (EDM) in surveyingand, more recently, the Global Positioning System(GPS), which made the very time-consumingprocess of making large-scale maps much quickerand more fun. The spatial form of surface topographyis modelled in several ways. Digital representationsare referred to as either Digital Elevation Models(DEMs) or Digital Terrain Models (DTMs). ADEM is ‘an ordered array of numbers that representthe spatial distribution of elevations above somearbitrary datum in a landscape’ (Moore et al. 1991,4). DTMs are ‘ordered arrays of numbers thatrepresent the spatial distribution of terrain attri-butes’ (Moore et al. 1991, 4). DEMs are, therefore, a subset of DTMs. Topographic elements of alandscape can be computed directly from a DEM andthese are often classified into primary (or first-order)and secondary (or second-order) attributes (Moore etal. 1993; Evans 1980). Primary attributes arecalculated directly from the digital elevation data.Slope, aspect, plan curvature, and profile curvatureare all examples of primary attributes (Table 1.1).

Secondary attributes combine primary attributes.They are indices describing or characterizing thespatial variability of specific landscape processes.Indices of soil erosion potential and soil wetness areboth examples of secondary attributes (Table 1.1).DEMs, although they provide descriptors of land-surface form, may be linked to process studies sincethey allow the spatial variability of form-controlledprocesses to be mapped in detail. Further details of DEMs and their applications are given in sev-eral recent books (e.g. Wilson and Gallant 2000;Huggett and Cheesman 2002).

PROCESS

Geomorphic systems

Process geomorphologists commonly adopt a sys-tems approach to their subject. To illustrate whatthis approach entails, take the example of a hillslopesystem? A hillslope extends from an interfluve crest,along a valley side, to a sloping valley floor. It maybe regarded as a system insofar as it consists of things(rock waste, organic matter, and so forth) arrangedin a particular way. The arrangement is seeminglymeaningful, rather than haphazard, because it maybe explained in terms of physical processes (Figure1.6). The ‘things’ of which a hillslope is composedmay be described by such variables as particle size,


Wind erosion and deposition

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Figure 1.6 A hillslope as a system,showing storages (waste mantle), inputs(e.g. wind deposition and debrisproduction), outputs (e.g. wind erosion),throughputs (debris transport), andunits (channel, valley-side slope,interfluve).


Table 1.1 Primary and secondary attributes that can be computed from DEMs

Attribute Definition Applications

Primary attributesAltitude Height above mean sea level Climate variables (e.g. pressure, temperature),

or local reference point vegetation and soil patterns, material volumes, cut-and-fill and visibility calculations, potential energy determination

Slope Rate of change of elevation – Steepness of topography, overland and subsurface gradient flow, resistance to uphill transport,

geomorphology, soil water contentAspect Compass direction of steepest Solar insolation and irradiance, evapotranspiration

downhill slope – azimuth of slope

Profile curvature Rate of change of slope Flow acceleration, erosion and deposition patterns and rate, soil and land evaluation indices, terrain unit classification

Plan curvature Rate of change of aspect Converging and diverging flow, soil water characteristics, terrain unit classification

Upslope slope Mean slope of upslope area Runoff velocityDispersal slope Mean slope of dispersal area Rate of soil drainageCatchment slope Average slope over the Time of concentration

catchmentUpslope area Catchment area above a small Runoff volume, steady-state runoff rate

length of contourDispersal area Area downslope from a small Soil drainage rate

length of contourCatchment area Area draining to catchment outlet Runoff volumeSpecific catchment Upslope area per unit width of Runoff volume draining out of catchment, soil

area contour characteristics, soil water content, geomorphology

Flow path length Maximum distance of water Erosion rates, sediment yield, time of concentrationflow to a point in the catchment

Upslope length Mean length of flowpaths to a Flow acceleration, erosion ratespoint in the catchment

Dispersal length Distance from a point in the Soil drainage impedancecatchment to the outlet

Catchment length Distance from highest point to Overland flow attenuationoutlet

Local drain Direction of steepest downhill Calculation of catchment attributes as a function direction (ldd) flow of stream topology. Computing lateral

transport of materials over a locally defined network

Stream length Length of longest path along Flow acceleration, erosion rates, sediment yieldlocal drainage direction upstream of a given cell

soil moisture content, vegetation cover, and slopeangle. These variables, and many others, interact toform a regular and connected whole: a hillslope, andthe mantle of debris on it, records a propensitytowards reciprocal adjustment among a complex setof variables. The complex set of variables includerock type, which influences weathering rates, thegeotechnical properties of the soil, and rates ofinfiltration; climate, which influences slope hydrol-ogy and so the routing of water over and through thehillslope mantle; tectonic activity, which may alterbase level; and the geometry of the hillslope, which,acting mainly through slope angle and distance fromthe divide, influences the rates of processes such aslandsliding, creep, solifluction, and wash. Change inany of the variables will tend to cause a readjustmentof hillslope form and process.

Isolated, open, and closed systems

Systems of all kinds may be regarded as open, closed,or isolated according to how they interact, or do notinteract, with their surroundings (Huggett 1985,5–7). An isolated system is, traditionally, taken tomean a system that is completely cut off from its

surroundings and that cannot therefore import orexport matter or energy. A closed system hasboundaries open to the passage of energy but not ofmatter. An open system has boundaries acrosswhich energy and materials may move. Most geo-morphic systems, including hillslopes, may bethought of as open systems as they exchange energyand matter with their surroundings.

Internal and external system variables

Any geomorphic system has internal and externalvariables. Take a drainage basin. Soil wetness,streamflow, and other variables lying inside thesystem are endogenous or internal variables. Precipi-tation, solar radiation, tectonic uplift, and other suchvariables originating outside the system andaffecting drainage basin dynamics are exogenous orexternal variables. Interestingly, all geomorphicsystems can be thought of as resulting from a basicantagonism between endogenic (tectonic andvolcanic) processes driven by geological forces and exogenic (geomorphic) processes driven byclimatic forces (Scheidegger 1979). In short, tectonicprocesses create land and climatically influenced


Table 1.1 (continued )

Attribute Definition Applications

Stream channel Cells with flowing water or Location of flow, erosion and sedimentation, flow cells with more than a given intensitynumber of upstream elements

Ridge Cells with no upstream Drainage divides, soil erosion, connectivitycontributing area

Secondary attributes

Wetness Index Index of moisture retention

where As is specific catchment and b is slope

Irradiance Amount of solar energy received Soil and vegetation studies, evapotranspirationper unit area

Source: Adapted from Huggett and Cheesman (2002, 20)

lntan

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weathering and erosion destroy it. The eventsbetween the creation and the final destruction arewhat fascinate geomorphologists.

Systems are mental constructs and have beendefined in various ways. Two conceptions of systemsare important in geomorphology: systems as processand form structures and systems as simple andcomplex structures (Huggett 1985, 4–5, 17–44).

Geomorphic systems as form and processstructures

Three kinds of geomorphic system may beidentified: form systems, process systems, and formand process systems.

1 Form systems. Form or morphological systemsare defined as sets of form variables that aredeemed to interrelate in a meaningful way interms of system origin or system function.Several measurements could be made to describethe form of a hillslope system. Form elementswould include measures of anything on a hill-slope that has size, shape, or physical properties.A simple characterization of hillslope form isshown in Figure 1.7a, which depicts a cliff witha talus slope at its base. All that could be learntfrom this ‘form system’ is that the talus liesbelow the cliff; no causal connections between

the processes linking the cliff and talus slope areinferred. Sophisticated characterizations of hill-slope and land-surface forms may be made usingdigital terrain models (Table 1.1).

2 Process systems. Process systems, which are alsocalled cascading or flow systems, are defined as‘interconnected pathways of transport of energyor matter or both, together with such storages ofenergy and matter as may be required’ (Strahler1980, 10). An example is a hillslope representedas a store of materials: weathering of bedrock andwind deposition adds materials to the store, anderosion by wind and fluvial erosion at the slopebase removes materials from the store. Thematerials pass through the system and in doingso link the morphological components. In thecase of the cliff and talus slope, it could beassumed that rocks and debris fall from the cliffand deliver energy and rock debris to the talusbelow (Figure 1.7b).

3 Form and process systems. Process–formsystems, also styled process–response systems,are defined as an energy-flow system linked to aform system in such a way that system processesmay alter the system form and, in turn, thechanged system form alters the system processes.A hillslope may be viewed in this way with slope form variables and slope process variablesinteracting. In the cliff-and-talus example, rock


CliffCliff

Lowerslope

Talus

Flow or cascadingsystem

CliffCliff

Lowerslope

Talus

Rockfall

1 2 3

123

( )b( )a Form system Process–form orprocess–response system

( )c

Time

Coveredcliff faceat time 2

Coveredcliff faceat time 2

Figure 1.7 A cliff and talus slope viewed as (a) a form system, (b) a flow or cascading system, and (c) or process–form or process–response system. Details are given in the text.

falling off the cliff builds up the talus store(Figure 1.7c). However, as the talus store increasesin size, so it begins to bury the cliff face, reducingthe area that supplies debris. In consequence, therate of talus growth diminishes and the systemchanges at an ever-decreasing rate. The processdescribed is an example of negative feedback,which is an important facet of many process–formsystems (Box 1.2).

Geomorphic systems as simple or complexstructures

Three main types of system are recognized under thisheading: simple systems, complex but disorganizedsystems, and complex and organized systems.

1 Simple systems. The first two of these typeshave a long and illustrious history of study. Since at least the seventeenth-century revolutionin science, astronomers have referred to a set ofheavenly bodies connected together and actingupon each other according to certain laws as asystem. The Solar System is the Sun and itsplanets. The Uranian system is Uranus and its moons. These structures may be thought of as simple systems. In geomorphology, a fewboulders resting on a talus slope may be thoughtof as a simple system. The conditions needed to dislodge the boulders, and their fate afterdislodgement, can be predicted from mechanicallaws involving forces, resistances, and equationsof motion, in much the same way that the motion


Negative feedback is said to occur when a changein a system sets in motion a sequence of changesthat eventually neutralize the effects of theoriginal change, so stabilizing the system. Anexample occurs in a drainage basin system, whereincreased channel erosion leads to a steepening ofvalley-side slopes, which accelerates slope erosion,which increases stream bed load, which reduceschannel erosion (Figure 1.8a). The reducedchannel erosion then stimulates a sequence ofevents that stabilizes the system and counteractsthe effects of the original change. Some geo-morphic systems also display positive feedbackrelationships, characterized by an original changebeing magnified and the system being madeunstable. An example is an eroding hillslope,where the slope erosion causes a reduction ininfiltration capacity of water, which increases theamount of surface runoff, which promotes evenmore slope erosion (Figure 1.8b). In short, a‘vicious circle’ is created and the system, beingunstabilized, continues changing.

Box 1.2

NEGATIVE AND POSIT IVE FEEDBACK

Slopeerosion

Infiltrationcapacity

Streambed load

Surfacerunoff

Valley-sideslope angle

Channelerosion

Slopeerosion

( ) Negative feedback loopa

( ) Positive feedback loopb

Figure 1.8 Feedback relationships in geomorphicsystems. (a) Negative feedback in a valley-sideslope–stream system. (b) Positive feedback in aneroding hillslope system. Details of the relationshipsare given in the text.

of the planets around the Sun can be predictedfrom Newtonian laws.

2 In a complex but disorganized system, a vastnumber of objects are seen to interact in a weakand haphazard way. An example is a gas in a jar.This system might comprise upward of 1023

molecules colliding with each other. In the sameway, the countless individual particles in ahillslope mantle could be regarded as a complexbut rather disorganized system. In both the gasand the hillslope mantle, the interactions aresomewhat haphazard and far too numerous tostudy individually, so aggregate measures mustbe employed (see Huggett 1985, 74–7;Scheidegger 1991, 251–8).

3 In a third and more recent conception of systems,objects are seen to interact strongly with oneanother to form systems of a complex andorganized nature. Most biological and eco-logical systems are of this kind. Many structuresin geomorphology display high degrees ofregularity and rich connections and may bethought of as complexly organized systems. Ahillslope represented as a process–form systemcould be placed into this category. Otherexamples include soils, rivers, and beaches.

Geomorphic system dynamics:equilibrium and steady state

The idea of dynamic equilibrium and steady state,as developed by John T. Hack from Grove KarlGilbert’s notion, was a condition in which land-surface form stays the same despite material beingadded by tectonic uplift and removed by a constantset of geomorphic processes, and that prevails in anerosional landscape where all slopes, both hillslopesand river slopes, are adjusted to each other (p. 9). In practice, this early notion of dynamic equilibriumwas difficult to apply to landscapes and other formsof equilibrium were advanced (Howard 1988)(Figure 1.9). Of these, dynamic metastable equi-librium has proved to be salutary. It suggests that,once perturbed by environmental changes orrandom internal fluctuations that cause the crossing

of internal thresholds (Box 1.3), a landscape willrespond in a complex manner (Schumm 1979). Astream, for instance, if it should be forced away froma steady state, will adjust to the change. However,the nature of the adjustment may vary in differentparts of the stream and at different times. DouglasCreek in western Colorado, USA, was subject toovergrazing during the ‘cowboy era’ (Womack andSchumm 1977). It has been cutting into its channelbed since about 1882. The manner of incision hasbeen complex, with discontinuous episodes ofdowncutting interrupted by phases of deposition,and with the erosion–deposition sequence varyingfrom one cross-section to another. Trees have beenused to date terraces at several locations. The terracesare unpaired (p. 199), which is not what would beexpected from a classic case of river incision, andthey are discontinuous in a downstream direction.This kind of study serves to dispel for ever thesimplistic cause-and-effect view of landscape evolu-tion in which change is seen as a simple response toan altered input. It shows that landscape dynamicsmay involve abrupt and discontinuous behaviourinvolving flips between quasi-stable states as systemthresholds are crossed.

The latest views on landscape stability (or lack ofit) come from the field of dynamic systems theory,which embraces the buzzwords complexity andchaos. The argument runs that steady states in thelandscape may be rare because landscapes are inher-ently unstable. Any balance obtaining in a steadystate is readily disrupted by any process thatreinforces itself, so keeping the system changingthrough a positive feedback circuit. This idea isformalized as an ‘instability principle’. This prin-ciple recognizes that, in many landscapes, accidentaldeviations from a ‘balanced’ condition tend to beself-reinforcing (Scheidegger 1983). This explainswhy cirques tend to grow, sinkholes increase in size,and longitudinal mountain valley profiles becomestepped. The intrinsic instability of landscapes isborne out by mathematical analyses that point tothe chaotic nature of much landscape change (e.g.Phillips 1999; Scheidegger 1994). Jonathan D.Phillips’s (1999, 139–46) investigation into the


nature of Earth surface systems, which includesgeomorphic systems, is particularly revealing andwill be discussed in the final chapter.

Magnitude and frequency

Interesting debates centre around the variations inprocess rates through time. The ‘tame’ end of this

debate concerns arguments over magnitude andfrequency (Box 1.4), the pertinent question herebeing which events perform the most geomorphicwork: small and infrequent events, medium andmoderately frequent events, or big but rare events?The first work on this issue concluded, albeitprovisionally until further field work was carriedout, that events occurring once or twice a year


( ) Static equilibriuma

( ) Stable equilibrium (recovery)b

( ) Unstable equilibrium (stabilization)c

( ) Metastable equilibriumd

( ) Steady-state equilibriume

( ) Thermodynamic equilibrium (decay)f

( ) Dynamic equilibriumg

( ) Dynamic metastable equilibriumh

Equilibrium state 1

Equilibrium state 2

Threshold

Maximum entropy

Figure 1.9 Types of equilibrium in geomorphology. (a) Static equilibrium occurs when a system is in balance over atime period and no change in state occurs. (b) Stable equilibrium records a tendency to revert to a previous stateafter a small disturbance. (c) Unstable equilibrium occurs when a small disturbance forces a system towards a newequilibrium state where stabilization occurs. (d) Metastable equilibrium arises when a system crosses an internal orexternal system threshold (p. 19), so driving it to a new state. (e) Steady-state equilibrium obtains when a systemconstantly fluctuates about an mean equilibrium state. (f) Thermodynamic equilibrium is the tendency of somesystems towards a state of maximum entropy, as in the gradual dissipation of heat by the Universe and its possibleeventual ‘heat death’ and in the reduction of a mountain mass to a peneplain during a prolonged period of no uplift.(g) Dynamic equilibrium may be thought of as balanced fluctuations about a mean state that changes in a definitedirection (a trending mean). (h) Dynamic metastable equilibrium combines dynamic and metastable tendencies,with balanced fluctuations about a trending mean flipping to new trending mean values when thresholds are crossed.Source: After Chorley and Kennedy (1971, 202)

perform most geomorphic work (Wolman andMiller 1960). Some later work has highlighted thegeomorphic significance of rare events. Large-scaleanomalies in atmospheric circulation systems veryoccasionally produce short-lived superfloods thathave long-term effects on landscapes (Baker 1977,1983; Partridge and Baker 1987). Another studyrevealed that low-frequency, high-magnitude eventsgreatly affect stream channels (Gupta 1983).

The ‘wilder’ end engages hot arguments overgradualism and catastrophism (Huggett 1989,1997a). The crux of the gradualist–catastrophistdebate is the seemingly innocuous question: havethe present rates of geomorphic processes remainedmuch the same throughout Earth surface history?Gradualists claim that process rates have beenuniform in the past, not varying much beyond their

present levels. Catastrophists make the counterclaimthat the rates of geomorphic processes have differedin the past, and on occasions some of them haveacted with suddenness and extreme violence,pointing to the effects of massive volcanic explo-sions, the impacts of asteroids and comets, and thelandsliding of whole mountainsides into the sea.The dichotomy between gradualists and cata-strophists polarizes the spectrum of possible rates ofchange. It suggests that there is either gradual andgentle change, or else abrupt and violent change. Infact, all grades between these two extremes, andcombinations of gentle and violent processes, areconceivable. It seems reasonable to suggest thatland-surface history has involved a combination ofgentle and violent processes.


A threshold separates different states of a system.It marks some kind of transition in the behaviour,operation, or state of a system. Everyday examplesabound. Water in a boiling kettle crosses atemperature threshold in changing from a liquidto a gas. Similarly, ice taken out of a refrigeratorand placed upon a table in a room with an airtemperature of 10°C will melt because a tempera-ture threshold has been crossed. In both examples,the huge differences in state – solid water to liquidwater, and liquid water to water vapour – mayresult from tiny changes of temperature. Manygeomorphic processes operate only after a thresh-old has been crossed. Landslides, for instance,require a critical slope angle, all other factorsbeing constant, before they occur. Stanley A.Schumm (1979) made a powerful distinctionbetween external and internal system thresh-olds. A geomorphic system will not cross anexternal threshold unless it is forced to do so by a

change in an external variable. A prime exampleis the response of a geomorphic system to climaticchange. Climate is the external variable. If, say,runoff were to increase beyond a critical level, thenthe geomorphic system might suddenly respondby reorganizing itself into a new state. No changein an external variable is required for a geomorphicsystem to cross an internal threshold. Rather, somechance fluctuation in an internal variable within ageomorphic system may take a system across aninternal threshold and lead to its reorganization.This appears to happen in some river channelswhere an initial disturbance by, say, overgrazingin the river catchment triggers a complex responsein the river channel: a complicated pattern oferosion and deposition occurs with phases ofalluviation and downcutting taking placeconcurrently in different parts of the channelsystem (see p. 19).

Box 1.3

THRESHOLDS


As a rule of thumb, bigger floods, stronger winds,higher waves, and so forth occur less often thantheir smaller, weaker, and lower counterparts.Indeed, graphs showing the relationship betweenthe frequency and magnitude of many geomorphicprocesses are right-skewed, which means that a lotof low-magnitude events occur in comparisonwith the smaller number of high-magnitudeevents, and a very few very high-magnitudeevents. The frequency with which an event of aspecific magnitude occurs is expressed as thereturn period or recurrence interval. Therecurrence interval is calculated as the average

length of time between events of a givenmagnitude. Take the case of river floods.Observations may produce a dataset comprisingthe maximum discharge for each year over a periodof years. To compute the flood–frequencyrelationships, the peak discharges are listedaccording to magnitude, with the highestdischarge first. The recurrence interval is thencalculated using the equation

Box 1.4

MAGNITUDE AND FREQUENCY

Q1.58 = most probable annual flood = 1,133 m /sec3

Q2.33 = mean annual flood = 1,473 m /sec3

Q5 = 2,011 m /sec3

Q10 = 2,435 m /sec3

Q25 = 2,973 m /sec3

Q50 = 3,398 m /sec3

1.1 1.2 1.5 2 3 4

Recurrence interval (years)

5 10 20 30 40 500

1,000

2,000

3,000

4,000

Dis

char

ge,

(m/s

,al

soc a

lled

cum

ecs )

Q3

Figure 1.10 Magnitude–frequency plot of annual floods on the Wabash River, at Lafayette, Indiana, USA. Seetext for details.Source: Adapted from Dury (1969)

Tn

m= + 1

HISTORY

Historical geomorphologists study landformevolution or changes in landforms over medium andlong timescales, well beyond the span of an indi-vidual human’s experience – centuries, millennia,millions and hundreds of millions of years. Suchconsiderations go well beyond the short-termpredictions of the process modellers. They bring inthe historical dimension of the subject with all itsattendant assumptions and methods. Historicalgeomorphology relies mainly on the form of theland surface and on the sedimentary record for itsdatabases.

Reconstructing geomorphic history

The problem with measuring geomorphic processesis that, although it establishes current operativeprocesses and their rates, it does not provide adependable guide to processes that were in action a million years ago, ten thousand years ago, or even a hundred years ago. Some landform featuresmay be inherited from the past and are not currentlyforming. In upland Britain, for instance, hillslopessometimes bear ridges and channels that werefashioned by ice and meltwater during the last iceage. In trying to work out the long-term evolutionof landforms and landscapes, geomorphologists have three options open to them – modelling,

chronosequence studies, and stratigraphic recon-struction.

Mathematical models of the hillslopes predictwhat happens if a particular combination of slopeprocesses is allowed to run on a hillslope for millionsof years, given assumptions about the initial shapeof the hillslope, tectonic uplift, tectonic subsidence,and conditions at the slope base (the presence orabsence of a river, lake, or sea). Some geomor-phologists would argue that these models are oflimited worth because environmental conditionswill not stay constant, or even approximatelyconstant, for that long. Nonetheless, the models doshow the broad patterns of hillslope and land-surfacechange that occur under particular process regimes.A discussion of hillslope modelling is beyond thescope of this book, but the interested reader who isunfamiliar with the topic could do no better thantry the computer-assisted learning (CAL) module on Simulating Slope Development from theGeographyCAL software available at www.le.ac.uk/cti/Tltp/roll.htm.

Stratigraphic and environmental reconstruction

Fortunately for researchers into past landscapes,several archives of past environmental conditionsexist: tree rings, lake sediments, polar ice cores,mid-latitude ice cores, coral deposits, loess, oceancores, pollen, palaeosols, sedimentary rocks, and


where T is the recurrence interval, n is the numberof years of record, and m is the magnitude of theflood (with m = 1 at the highest recorded dis-charge). Each flood is then plotted against itsrecurrence interval on Gumbel graph paper andthe points connected to form a frequency curve(Figure 1.10). If a flood of a particular magnitudehas a recurrence interval of 10 years, it wouldmean that there is a 1-in-10 (10 per cent) chancethat a flood of this magnitude (2,435 cumecs inthe Wabash River example shown in Figure 1.10)

will occur in any year. It also means that, onaverage, one such flood will occur every 10 years.The magnitudes of 5-year, 10-year, 25-year, and50-year floods are helpful for engineering work,flood control, and flood alleviation. The 2.33-yearflood (Q2.33) is the mean annual flood (1,473cumecs in the example), the 2.0-year flood (Q2.0)is the median annual flood (not shown), and the1.58-year flood (Q1.58) is the most probable annualflood (1,133 cumecs in the example).

historical records (see Huggett 1997b, 8–21).Sedimentary deposits are an especially valuablesource of information about past landscapes. In somecases, geomorphologists may apply the principlesof stratigraphy to the deposits to establish arelative sequence of events. Colluvium for example,which builds up towards a hillslope base, iscommonly deposited episodically. The result is thatdistinct layers are evident in a section, the upperlayers being progressively younger than the lowerlayers. If such techniques as radiocarbon dating ordendrochronology can date these sediments, thenthey may provide an absolute timescale for the past activities on the hillslope, or at least the pastactivities that have left traces in the sedimentaryrecord (Box 1.5). Recognizing the origin of thedeposits may also be possible – glacial, periglacial,colluvial, or whatever. And sometimes, geomor-phologists use techniques of environmental

reconstruction to establish the climatic and otherenvironmental conditions at the time of sedimentdeposition.

Environmental reconstruction techniques havebeen given a fillip by the recent global environ-mental change agenda. A core project of the IGBP(International Geosphere–Biosphere Programme) iscalled Past Global Changes (PAGES). Itconcentrates on two slices of time: (1) the last 2,000years of Earth history, with a temporal resolution ofdecades, years, and even months; and (2) the lastseveral hundred thousand years, covering glacial–interglacial cycles, in the hope of providing insightsinto the processes that induce global change (IGBP1990). Examples of geomorphological contributionsto environmental change over timescales may befound in the book Geomorphology, Human Activity andGlobal Environmental Change edited by OlavSlaymaker (2000a).


A broad range of methods is now available fordating events in Earth history (Table 1.2). Someare more precise than others. Four categories arerecognized: numerical-age methods, calibrated-agemethods, relative-age methods, and correlated-agemethods. Numerical-age methods produceresults on a ratio (or absolute) timescale, pin-pointing the times when environmental changeoccurred. This information is crucial to a deepappreciation of environmental change: withoutdates, nothing much of use can be said about rates.Calibrated-age methods may provide approxi-mate numerical ages. Some of these methods arerefined and enable age categories to be assigned todeposits by measuring changes since deposition insuch environmental factors as soil genesis or rockweathering (see McCarroll 1991). Relative-agemethods furnish an age sequence, simply puttingevents in the correct order. They assemble the

‘pages of Earth history’ in a numerical sequence.The Rosetta stone of relative-age methods is the principle of stratigraphic superposition. Thisstates that, in undeformed sedimentary sequences,the lower strata are older than the upper strata.Some kind of marker must be used to matchstratigraphic sequences from different places.Traditionally, fossils have been employed for thispurpose. Distinctive fossils or fossil assemblagescan be correlated between regions by identifyingstrata that were laid down contemporaneously.This was how the stratigraphic column was firsterected by such celebrated geologists as William(‘Strata’) Smith (1769–1839). Although this tech-nique was remarkably successful in establishing thebroad development of Phanerozoic sedimentaryrocks, and rested on the sound principle ofsuperposition, it is beset by problems (see Vita-Finzi 1973, 5–15). It is best used in partnership

Box 1.5

DATING TECHNIQUES


Table 1.2 Methods for dating Quaternary materials

Method Age range (years) Basis of method Materials needed

Sidereal methodsDendrochronology 0–5,000 Growth rings of live trees or Trees and cultural

correlating ring-width materials (e.g.chronology with other trees ships’ timbers)

Varve chronology 0–200,000 Counting seasonal sediment Glacial, lacustrine, layers back from the present, marine, soil, andor correlating a past sequence wetland depositswith a continuous chronology

Sclerochronology1 0–800 Counting annual growth bands Marine fossiliferous in corals and molluscs deposits

Isotopic methodsRadiocarbon 100–60,000 Radioactive decay of carbon-14 A variety of chemical

to nitrogen-14 in organic tissue and biogenic or carbonates sediments

Cosmogenic 200–8,000,0002 Formation, accumulation, and Surfaces of landformsnuclides1 decay of cosmogenic nuclides

in rocks or soils exposed to cosmic radiation

Potassium–argon, 10,000– Radioactive decay of potassium-40 Non-biogenic argon–argon 10,000,000+ trapped in potassium-bearing lacustrine deposits

silicate minerals during and soils, igneous crystallization to argon-40 and metamorphic

rocksUranium series 100–400,0003 Radioactive decay of uranium and Chemical deposits

daughter nuclides in biogenic and biogenic chemical and sedimentary deposits except minerals those in wetlands

Lead-210


Table 1.2 (continued )

Method Age range (years) Basis of method Materials needed

Electron-spin 1,000–1,000,000 Accumulation of electrical charges Cultural materials, resonance in crystal lattice defects in terrestrial and

silicate minerals resulting from marine fossils, natural radiation igneous rocks

Chemical and biological methodsAmino-acid 500–1,000,000 Racemization of L-amino acids to Terrestrial and marine

racemization D-amino acids in fossil organic plant and animals material remains

Obsidian hydration 100–1,000,000 Increase in thickness of hydration Cultural materials, rind on obsidian surface fluvial gravels,

glacial deposits, clastic deposits in lakes and seas, silicic igneous and pyroclastic rocks

Lichenometry1 100–10,000 Growth of lichens on freshly Exposed landforms exposed rock surfaces supporting lichens

Geomorphic methodsSoil-profile 8,000–200,000 Systematic changes in soil Soils and most

development properties owing to weathering landformsand pedogenic processes

Rock and mineral 0

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