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P1: TIX/XYZ P2: ABCfm JWST032-Thomas December 17, 2010 15:16 Printer Name: Yet to Come

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Arid ZoneGeomorphology

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Arid ZoneGeomorphology

Process, Form and Changein Drylands

Third Edition

Edited by

David S. G. ThomasSchool of Geography and the Environment

University of Oxford, UK

A John Wiley & Sons, Ltd., Publication

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This edition first published 2011 C© 2011 by John Wiley & Sons, Ltd.

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Library of Congress Cataloguing-in-Publication Data

Arid zone geomorphology : process, form and change in drylands / edited by David S. G. Thomas. – 3rd ed.p. cm.

Includes index.ISBN 978-0-470-51908-0 (cloth) – ISBN 978-0-470-51909-7 (pbk.)

1. Geomorphology. 2. Arid regions. I. Thomas, David S. G.GB611.A75 2011551.41′5–dc22

2010037270

A catalogue record for this book is available from the British Library.

This book is published in the following electronic formats: ePDF 9780470710760; Wiley Online Library 9780470710777; ePub 9780470975695

Set in 10/12pt Times by Aptara Inc., New Delhi, India.

First Impression 2011

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http://www.wiley.com/wiley-blackwell


For Alice

And in memory of my fatherFrederick Thomas

The Geographer who first inspired meand who passed away when this book was being completed

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Contents

List of contributors xviiPreface to the first edition xixPreface to the second edition xxiPreface to the third edition xxiii

I Large-scale controls and variability in drylands 1

1 Arid environments: their nature and extent 3David S.G. Thomas

1.1 Geomorphology in arid environments 31.2 Arid zone distinctiveness and the quest for explanation 41.3 Arid zones: terminology and definitions 5

1.3.1 Terminology 51.3.2 Definition 5

1.4 The age of aridity on Earth 71.5 The distribution of arid zones 81.6 Causes of aridity 9

1.6.1 Atmospheric stability 91.6.2 Continentality 91.6.3 Topography 91.6.4 Cold ocean currents 10

1.7 Climate variability 101.8 Dryland ecosystems 10

1.8.1 Arid zone geomorphology 121.9 Arid zone geomorphology and people 12

1.10 Organisation of this book 13References 14

2 Tectonic frameworks 17Helen Rendell

2.1 Introduction 172.2 Tectonic setting of drylands 182.3 Uplift and erosion, subsidence and sedimentation 182.4 Lengths of record 202.5 Existing erosional and depositional records in arid environments 21

2.5.1 Drainage patterns and fluvial systems 212.5.2 Playas 222.5.3 Desert pavements 222.5.4 Aeolian sequences 23

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viii CONTENTS

2.6 Selected examples of the geomorphological impact of active tectonics in aridenvironments 232.6.1 Tectonic disruption of fluvial systems 232.6.2 Tectonic controls on alluvial sedimentation 23

2.7 Conclusions 24References 24

3 Climatic frameworks: legacies from the past 27David S.G. Thomas and Sallie L. Burrough

3.1 Introduction 273.2 The significance of arid zone fluctuations in the past 27

3.2.1 Ancient arid zones 273.2.2 The development of aridity in the Mesozoic and Cenozoic 283.2.3 The Quaternary Period 293.2.4 Sedimentary records 293.2.5 Marine sediments and palaeoaridity 323.2.6 Rock varnish 343.2.7 Geomorphological evidence of arid zone change 353.2.8 Arid zone contraction 363.2.9 Ecological evidence 38

3.3 Dating arid zone fluctuations 393.4 Climatic interpretations and issues 39

3.4.1 Aridity during glacial times? 413.4.2 Drivers of late glacial tropical aridity 42


4 Dryland system variability 53David S. G. Thomas

4.1 A framework for dryland diversity 534.2 Geomonotony: how unvarying are the ‘flat’ drylands of the world? 554.3 Within-dryland diversity 574.4 Summary issues 58

References 59

5 Extraterrestrial arid surface processes 61Jonathan Clarke

5.1 Introduction 615.2 What does ‘aridity’ mean beyond Earth? 615.3 Why should planetary scientists understand terrestrial arid geomorphology? 625.4 What can terrestrial geomorphologists learn from a solar system perspective? 635.5 Mars: water-based aridity 64

5.5.1 Overview 645.5.2 The history of atmosphere–surface interactions 645.5.3 Martian water cycle 665.5.4 Surface images 665.5.5 The geomorphology of Mars 685.5.6 Summary 72

5.6 Titan: methane-based aridity? 725.6.1 Methane cycle 735.6.2 Surface images 745.6.3 Lakes 745.6.4 Rock breakdown: process and form 75


CONTENTS ix

5.6.5 Aeolian landforms 755.6.6 Fluvial systems 755.6.7 Summary 76

5.7 Venus: extreme aridity 765.7.1 Surface–atmosphere interaction 765.7.2 Surface images 765.7.3 Rock breakdown 775.7.4 Aeolian landforms 785.7.5 Summary 79

5.8 Future Directions 79References 79

II Surface processes and characteristics 83

6 Weathering systems 85Heather A. Viles

6.1 Introduction 856.2 What makes arid environments unusual in terms of weathering systems? 876.3 Theoretical underpinnings of weathering systems research 886.4 Current weathering study methods 906.5 Linking processes to form in arid weathering systems 926.6 Explaining the development of weathering landforms in arid environments 956.7 Weathering rates in arid environments 976.8 Arid weathering and landscape evolution 976.9 Scale and arid weathering systems 98

Acknowledgement 98References 98

7 Desert soils 101David L. Dunkerley

7.1 Introduction: the nature and significance of desert soils 1017.2 Taxonomy of desert soils 103

7.2.1 A note on terminology of near-surface features in desert soils 1047.3 Some distinctive aspects of desert soil development 1047.4 Stone-mantled surfaces and desert pavements 1057.5 Inorganic seals at the soil surface 106

7.5.1 Raindrop properties and raindrop impact seals 1067.5.2 Factors known to be significant in the formation of raindrop impact seals 1097.5.3 Depositional seals 1097.5.4 Effects of seals on infiltration and erosion 1107.5.5 Biological soil crusts 1117.5.6 The habitats or niches exploited by microphytic plants in drylands 1127.5.7 The organisms forming biological soil crusts 1137.5.8 The classification of biological soil crusts 1147.5.9 Effects of biological soil crusts on infiltration and overland flow 1157.5.10 Effects of biological crusts on soil stability and erosion resistance 1197.5.11 Possible effects of climate change on biological soil crusts 121

7.6 Vesicular soil structures 1217.6.1 Comparing the infiltrability of biological, raindrop impact and

vesicular surfaces 1227.6.2 Spatial heterogeneity of desert soils 123



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8 Desert crusts and rock coatings 131David J. Nash

8.1 Introduction 1318.2 Sodium nitrate deposits 132

8.2.1 General characteristics and distribution 1328.2.2 Micromorphology, chemistry and mode of formation 133

8.3 Halite crusts 1358.3.1 General characteristics and distribution 1358.3.2 Micromorphology and chemistry 1368.3.3 Mode of formation 137

8.4 Gypsum crusts 1378.4.1 General characteristics 1378.4.2 Distribution 1398.4.3 Micromorphology and chemistry 1408.4.4 Modes of formation 140

8.5 Calcrete 1418.5.1 General characteristics 1418.5.2 Distribution 1438.5.3 Micromorphology and chemistry 1468.5.4 Mode of origin 148

8.6 Silcrete 1518.6.1 General characteristics 1518.6.2 Distribution 1528.6.3 Micromorphology and chemistry 1538.6.4 Mode of formation 156

8.7 Desert rock coatings 1588.7.1 General controls on formation 1588.7.2 Rock varnish 1598.7.3 Silica glazes and iron films 162

8.8 Palaeoenvironmental significance of crusts 163References 165

9 Pavements and stone mantles 181Julie E. Laity

9.1 Introduction 1819.2 Surface types: hamadas and stony surfaces 181

9.2.1 Hamada 1819.2.2 Stony surfaces: gobi, serir, gibber plains and

desert pavements 1829.3 General theories concerning stony surface formation 185

9.3.1 Deflation 1869.3.2 Concentration by surface wash and rain splash 1869.3.3 Upward migration of stones 1879.3.4 Accretion of aeolian fines 1889.3.5 Desert pavement formation by aeolian aggradation and development of an

accretionary mantle 1889.4 Stone pavement characteristics 189

9.4.1 Setting 1899.4.2 Surface clast concentration and characteristics 189

9.5 Processes of pavement formation 1909.6 Processes of clast size reduction in pavements 192

9.6.1 Pavement soils 192


CONTENTS xi

9.7 Secondary characteristics of pavement surfaces and regional differences inpavement formation 194

9.7.1 Presence of calcium carbonate and carbonate collars 1949.7.2 Pitting 1949.7.3 Development of varnish 1949.7.4 Embedded clasts 1949.7.5 Clast orientation 1959.7.6 Clast rubification 1959.7.7 Development of ventifacted surfaces 195

9.8 Secondary modifications to pavement surfaces 1959.8.1 Patterns in pavement 1959.8.2 Animal burrowing, vegetation and stone displacement 1959.8.3 Regeneration of surfaces by rainfall and runoff events 1979.8.4 Earthquakes 1989.8.5 Off-road vehicle disturbance 1989.8.6 Removal of stones for agriculture 198

9.9 Ecohydrology of pavement surfaces 1989.9.1 Infiltration in pavements and runoff potential 1999.9.2 Ecohydrologic relationships and vegetation associations 199

9.10 Relative and absolute dating of geomorphic surfaces based on pavement development 2009.10.1 Changes in surface characteristics 2009.10.2 Pavement characteristics and geomorphic surface ages 2019.10.3 Pavement surfaces as a tool in geomorphic assessment 201


10 Slope systems 209John Wainwright and Richard E. Brazier

10.1 Introduction 20910.1.1 Contexts of slope systems 209

10.2 Badlands 21210.2.1 Processes and rates of badland evolution 219

10.3 Rock slopes 22210.3.1 Bare rock or slick-rock slopes 22210.3.2 Distinctive landforms of rock- and debris-mantled slopes 226

10.4 Conclusion 228References 229

III The work of water 235

11 Runoff generation, overland flow and erosion on hillslopes 237John Wainwright and Louise J. Bracken

11.1 Introduction 23711.2 Infiltration processes 24011.3 Factors affecting infiltration 241

11.3.1 Controls at the surface–atmosphere interface 24111.3.2 Subsurface controls 246

11.4 Runoff generation 24811.4.1 Ponding and surface storage 24811.4.2 Flow hydraulics 25011.4.3 Pipes and macropore flow 25211.4.4 Scales of overland flow 252


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11.5 Erosion processes on hillslopes 25411.5.1 Splash 25511.5.2 Unconcentrated overland-flow erosion 25611.5.3 Concentrated overland-flow erosion 25611.5.4 Patterns and scales of sediment transport 259


12 Distinctiveness and diversity of arid zone river systems 269Stephen Tooth and Gerald C. Nanson

12.1 Introduction 26912.2 Distinctiveness of dryland rivers 27012.3 Diversity of dryland rivers 273

12.3.1 Higher energy dryland rivers: the Mediterranean region 27412.3.2 Moderate and lower energy dryland rivers: southern Africa 28012.3.3 Lower energy dryland rivers: Australia 284

12.4 Reassessing distinctiveness and diversity 28912.4.1 Downstream flow decreases and localised flood patterns 29112.4.2 Induration of alluvial sediments 29112.4.3 Channel–vegetation interactions 29312.4.4 Fluvial–aeolian interactions 293


13 Channel form, flows and sediments of endogenous ephemeral rivers in deserts 301Ian Reid and Lynne E. Frostick

13.1 Introduction 30113.2 Rainfall and river discharge 302

13.2.1 Storm characteristics 30213.2.2 Flash flood hydrograph 30413.2.3 Transmission losses 30513.2.4 Drainage basin size and water discharge 307

13.3 Ephemeral river channel geometry 30913.3.1 Channel width 30913.3.2 Channel bed morphology 309

13.4 Fluvial sediment transport 31113.4.1 Scour and fill 31113.4.2 Sediment transport in suspension 31413.4.3 Sediment transport along the stream bed 317

13.5 Desert river deposits 32013.5.1 Thin beds 32113.5.2 Predominance of horizontal lamination in sand beds 32213.5.3 Mud drapes and mud intraclasts 324


14 Dryland alluvial fans 333Adrian Harvey

14.1 Introduction: dryland alluvial fans – an overview 33314.1.1 Definitions, local occurrence, general morphology 33314.1.2 Global occurrence and distribution of dryland alluvial fans 33414.1.3 The role of alluvial fans within dryland fluvial systems 338


CONTENTS xiii

14.2 Process and form on dryland alluvial fans 33814.2.1 Sediment supply, transport and depositional processes 33814.2.2 Post-depositional modification of dry-region fan surfaces 34014.2.3 Alluvial fan sediment sequences and spatial variations 34214.2.4 Alluvial fan morphology and style 345

14.3 Factors controlling alluvial fan dynamics 34514.3.1 Passive factors: influence on fan morphology 34714.3.2 Dynamic controls 351

14.4 Alluvial fan dynamics 35814.4.1 Expressions of fan dynamics 35814.4.2 Interactions between the dynamic controls: case studies of alluvial fan response

to Late Quaternary environmental change 35814.5 Discussion: significance of dry-region alluvial fans 362

14.5.1 Commonly held myths and outdated concepts 36214.5.2 Significance to science 36214.5.3 Significance of dry-region alluvial fans for society 363Acknowledgements 364References 364

15 Pans, playas and salt lakes 373Paul A. Shaw and Rob G. Bryant

15.1 The nature and occurrence of pans, playas and salt lakes 37315.1.1 Playa and pan terminology 37415.1.2 General characteristics 37415.1.3 Origins and development of pans and playas 376

15.2 Pan hydrology and hydrochemistry 37915.2.1 Inflow and water balance modelling 38015.2.2 Geochemical processes and mineral precipitation 38215.2.3 The importance of groundwater: classification of playa and pan types 38415.2.4 Implications of climate change and human impacts on playa hydrology 385

15.3 Influences of pan hydrology and hydrochemistry on surface morphology 38615.3.1 Pan topography 38615.3.2 Surface dynamics: mapping pan surface morphologies using remote sensing 388

15.4 Aeolian processes in pan environments 38915.4.1 Wind action on the pan surface 39115.4.2 The emission of fine particles (dust): process and controls 39115.4.3 Lunette dunes 39215.4.4 Yardangs 394

15.5 Pans and playas as palaeoenvironmental indicators 39415.5.1 Identification and dating of pan shorelines 39415.5.2 Dating and stratigraphy of lunette dunes 39415.5.3 Stable isotope studies and pan hydrochemical evolution 395References 395

16 Groundwater controls and processes 403David J. Nash

16.1 Introduction 40316.2 Groundwater processes in valley and scarp development 404

16.2.1 Erosion by exfiltrating water: definitions and mechanisms 40416.2.2 Seepage erosion and valley formation 40416.2.3 Characteristics of drainage networks developed by groundwater seepage erosion 40816.2.4 Parameters promoting the operation of groundwater seepage erosion processes 411


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16.2.5 Groundwater seepage erosion and environmental change 41116.2.6 In situ deep-weathering and valley development 412

16.3 Groundwater and pan/playa development 41316.4 Groundwater and aeolian processes 414

References 418

IV The work of the wind 425

17 Aeolian landscapes and bedforms 427David S.G. Thomas

17.1 Introduction 42717.2 Aeolian bedforms: scales and relationships 427

17.2.1 Scale effects in aeolian bedform development 43017.3 The global distribution of sand seas 430

17.3.1 Sand sea development 43217.3.2 Sediment supply in sand seas 43217.3.3 Sandflow conditions and sand sea development 43517.3.4 Sand sheets 435

17.4 The global distribution of loess 43717.4.1 Loess production and distribution 43717.4.2 Peridesert loess 437

17.5 Dynamic aeolian landscapes in the Quaternary period 43917.5.1 Dating aeolian landscape change 443


18 Sediment mobilisation by the wind 455Giles F. S. Wiggs

18.1 Introduction 45518.2 The nature of windflow in deserts 456

18.2.1 The turbulent velocity profile 45618.2.2 Measuring shear velocity (u∗) and wind stress 45718.2.3 Measuring aerodynamic roughness (z0) 46018.2.4 The effect of nonerodible roughness elements on velocity profiles 462

18.3 Sediment in air 46418.3.1 Grain entrainment 464

18.4 Determining the threshold of grain entrainment 46618.5 Surface modifications to entrainment thresholds and transport flux 468

18.5.1 Surface crusting 46818.5.2 Bedslope 46918.5.3 Moisture content 469

18.6 Modes of sediment transport 47118.6.1 Suspension 47118.6.2 Creep 47118.6.3 Reptation 47218.6.4 Saltation 472

18.7 Ripples 47318.7.1 Ballistic ripples 474

18.8 Prediction and measurement of sediment flux 47518.9 The role of turbulence in aeolian sediment transport 478



CONTENTS xv

19 Desert dune processes and dynamics 487Nick Lancaster

19.1 Introduction 48719.2 Desert dune morphology 48719.3 Dune types and environments 487

19.3.1 Crescentic dunes 48719.3.2 Linear dunes 49019.3.3 Star dunes 49419.3.4 Parabolic dunes 49419.3.5 Zibars and sand sheets 494

19.4 Airflow over dunes 49419.4.1 The stoss or windward slope 49519.4.2 Lee-side flow 497

19.5 Dune dynamics 49719.5.1 Erosion and deposition patterns on dunes 49719.5.2 Long-term dune dynamics 500

19.6 Dune development 50219.7 Controls of dune morphology 503

19.7.1 Sediment characteristics 50319.7.2 Wind regimes 50319.7.3 Sand supply 50419.7.4 Vegetation 50519.7.5 Controls of dune size and spacing 50519.7.6 Dune trends 508

19.8 Dune patterns 50919.9 Conclusions 511

References 511

20 Desert dust 517Richard Washington and Giles S. F. Wiggs

20.1 Introduction 51720.1.1 Dust in a geomorphological context 51720.1.2 Measuring dust 52220.1.3 Modelling dust 52420.1.4 Distribution of dust 525

20.2 Key source areas 52620.2.1 Bodele Depression, Chad 52620.2.2 Saharan Empty Quarter 52820.2.3 China 53020.2.4 Southern Africa and Australia 531

20.3 Temporal changes in dust 53220.3.1 Observational record 532

20.4 Future climate change 53220.5 Conclusions 532

References 533

21 Wind erosion in drylands 539Julie E. Laity

21.1 Introduction 53921.2 The physical setting: conditions for wind erosion 540

21.2.1 Processes of aeolian erosion 54021.2.2 Yardangs 54121.2.3 Yardang formative processes 545


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21.2.4 Inverted topography 55321.2.5 Ventifacts 553


V Living with dryland geomorphology 569

22 The human impact 571Nick Middleton

22.1 Introduction 57122.2 Human impacts on soils 571

22.2.1 Terracing and rainwater harvesting 57122.2.2 Irrigated agriculture 57322.2.3 Accelerated erosion 57422.2.4 Grazing 575

22.3 Human impacts on sand dunes 57622.4 Human impacts on rivers 576

22.4.1 Large dams 57622.4.2 Urbanisation 57722.4.3 Changes in vegetation 578

22.5 Cause and effect: the arroyo debate continues 57822.6 Conclusions 579

References 579

23 Geomorphological hazards in drylands 583Giles F. S. Wiggs

23.1 Introduction 58323.2 Aeolian hazards 583

23.2.1 Blowing sand and active dune movement 58323.2.2 Human disturbance of stable surfaces 585

23.3 The aeolian dust hazard 58623.4 Agricultural wind erosion 58723.5 Drainage of inland water bodies 58923.6 Fluvial hazards 59323.7 Conclusions 594

References 595

24 Future climate change and arid zone geomorphology 599Richard Washington and David S. G. Thomas

24.1 Introduction 59924.2 Climate change projections: basis and uncertainties 59924.3 Overview of global climate change projections in the context of arid zones 600

24.3.1 Methods of establishing climate change impacts in arid zones 60224.4 Climate change and dunes 60324.5 Climate change and dust 60524.6 Climate change and fluvial systems 60724.7 Conclusions 607

References 608

Index 611


List of contributors

Dr Louise Bracken, Department of Geography, DurhamUniversity, Science Laboratories, South Road, DurhamDH1 3LE, UK

Dr Richard Brazier, School of Geography, University ofExeter, The Queen’s Drive, Exeter EX4 4QJ, UK

Dr Rob G. Bryant, Department of Geography, Universityof Sheffield, Sheffield S10 2TN, UK.

Dr Sallie L. Burrough, School of Geography and Envi-ronment, Oxford University Centre for the Environment,South Parks Road, Oxford OX1 3QY, UK

Dr Jonathan Clarke, Mars Society Australia, Box 327Clifton Hill, Victoria 3068, Australia/Australian Centrefor Astrobiology, Biological Science Building, Universityof New South Wales, Kensington, NSW 2052, Australia

Dr David Dunkerley, School of Geography and Envi-ronmental Science, Clayton Campus, Monash University,Victoria 3800, Australia

Professor Lynne E. Frostick, Department of Geography,University of Hull, Hull HU6 7RX, UK

Professor Adrian Harvey, School of Environmental Sci-ences, University of Liverpool, Roxby Building, ChathamStreet, Liverpool L69 7ZT, UK

Dr Julie E. Laity, Department of Geography, Califor-nia State University, Northridge, 18111 Nordhoff Street,Northridge, CA 91330-8249, USA

Professor Nicholas Lancaster, Division of Earth andEcosystem Sciences, Desert Research Institute, 2215 Rag-gio Parkway, Reno, NV 89512-1095, USA

Dr Nick Middleton, School of Geography and Envi-ronment, Oxford University Centre for the Environment,South Parks Road, Oxford OX1 3QY, UK

Professor Gerald Nanson, School of Earth and Environ-mental Sciences, University of Wollongong, Wollongong,NSW 2522, Australia

Professor David J. Nash, School of Environment andTechnology, University of Brighton, Brighton BN2 4GJ,UK

Professor Ian Reid, Department of Geography, Lough-borough University, Loughborough LE11 3TU, UK

Professor Helen Rendell, Department of Geography,Loughborough University, Loughborough LE11 3TU, UK

Professor Paul A. Shaw, Faculty of Science and Agricul-ture, University of the West Indies, St Augustine, Trinidadand Tobago

Professor David S. G. Thomas, School of Geographyand Environment, Oxford University Centre for the Envi-ronment, South Parks Road, Oxford OX1 3QY, UK.

Dr Stephen S. Tooth, Institute of Geography and EarthSciences, Institute of Geography and Earth Sciences,Aberystwyth University, Penglais Campus, AberystwythSY23 3DB, UK

Professor Heather A. Viles, School of Geography andEnvironment, Oxford University Centre for the Environ-ment, South Parks Road, Oxford OX1 3QY, UK

Professor John Wainwright, Department of Geography,University of Sheffield, Sheffield S10 2TN, UK

Dr Richard Washington, School of Geography andEnvironment, Oxford University Centre for the Environ-ment, South Parks Road, Oxford OX1 3QY, UK

Dr Giles F. S. Wiggs, School of Geography and Envi-ronment, Oxford University Centre for the Environment,South Parks Road, Oxford OX1 3QY, UK

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Preface to the first edition

Arid environments may not be the most hospitable placeson Earth, but the 30 % or more of the global land sur-face that they cover does support an ever-growing humanpopulation and has fascinated travellers, explorers and sci-entists for centuries. Early geomorphological studies werefrequently carried out indirectly, sometimes even unwit-tingly, by those whose main purpose and motives lay else-where: inevitably, but with some notable exceptions, theiraccounts were descriptive and unscientific. Some wouldeven argue that these traits persisted and dominated desertgeomorphological studies well into the second half of thiscentury. Recent years have, however, seen an enhancedrigour in the investigation and explanation of landformsand geomorphological processes in arid lands. New datahave been gathered by techniques ranging in scale fromthe detailed monitoring of processes in the field to remotesensing from space; old theories have been questionedand new ones, based on evidence rather than surmise,have been proposed.

The idea for this volume grew out of these advancesand the absence of a recent book which encapsulesthem (Cooke and Warren’s Geomorphology in Deserts is15 years old and Mabbutt’s Desert Landforms is 11 yearsold). There have been valuable volumes produced in re-cent years that deal with specific topics of interest to desertgeomorphologists, but none (to my knowledge) that at-tempts a broader view of arid zone geomorphology. It ishoped that this book fills this gap.

The decision to invite others to contribute chapters wasmade easily. The geomorphology of arid environments isa huge topic, embracing much of the subject matter of geo-morphology as a whole: desert landforms consist of much

more than piles of unvegetated sand. Arid and semi-aridenvironments are very varied, too; involving the expertiseof others has therefore inevitably broadened and deep-ened the basis of the text. While there are inevitably gaps,these have hopefully been kept to a minimum. Many peo-ple have provided the help and inspiration needed to turnArid Zone Geomorphology from an idea to a book. An-drew Goudie introduced me to deserts, since which timemany people and funding bodies have enabled me to visitthem and to conduct research in them: I would particularlylike to thank the Shaws in Botswana and Sleaze and Val forshowing me Death Valley and other Californian hotspots.During the production of the book the contributors haveefficiently met the tasks I have set them, including refer-eeing other people’s chapters; Rod Brown provided ad-ditional help in this respect, too, while Chapter 12 alsopassed through refereeing within the US Geological Sur-vey. The cartographers of many institutions, but especiallyPaul Coles of the Geography Department, University ofSheffield, produced the diagrams. At the publishers, IaianStevenson and Sally Kilmister gave me valuable adviceand logistical help. Steve Trudgill inspired me to put abook together in the first place.

Lastly, but most importantly, Liz Thomas not onlysuffered me during the book’s gestation, but helped ina multitude of practical ways and provided a valuable,independent, geomorphological viewpoint. To all of theabove, my parents and any I have forgotten to mention,my sincere thanks.

David S.G. ThomasSheffield

August 1988

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Preface to the second edition

It is almost eight years since the text of the first edition ofArid Zone Geomorphology was written, and seven yearssince the book was first published. Coincidently, on theday of publication, 7 December 1988, the ‘Finger of God’,pictured on the cover of the first edition, collapsed. So,along with a substantially changed content, this secondedition has a new cover.

Since the first edition was produced, much has hap-pened in terms of both geomorphic research in arid envi-ronments (or drylands, or deserts: such terms are com-monly used interchangeably) and in general and non-scientific interest for such areas. Arid regions are areasof concern, because of population growth, the impactsof desertification and of natural phenomena, particularlydroughts. The impending impacts of global warming onthese areas and their peoples are also of growing concern.Scientists, including geomorphologists, are responding tothe need to know more about the nature and operation ofprocesses in drylands by conducting more research, bothfundamental and applied.

This new edition of Arid Zone Geomorphology aimsto reflect the changes and advances in geomorphologicalknowledge that have occurred, especially since the pub-lication of the first edition. This has been done in twoways. First, the chapters from the first edition have beenupdated, in some cases radically. Second, the content ofthe book has been expanded, with the number of chaptersall but doubled and arranged in a new framework of sixsections. This has been done to fill gaps in coverage orexpand areas of particular interest. In both cases, as withthe first edition, experts have been invited to write thechapters of this text rather than one person attempting tosummarise and review what amounts to a vast chunk ofgeomorphology. In the majority of cases the authors ofchapters from the first edition have rewritten their ownmaterial. In some cases where circumstances have pre-vented this, new co-authors have conducted the task. Fora few themes covered in the first edition, new authors havewritten material afresh.

In all, the production of this new edition has resulted in34 researchers from over 25 academic or research institu-

tions making contributions, all involved with research inthe fields on which they write. It is this wealth of expertiseand the wide-ranging and diverse experience of drylandsthat it represents that make this book. As editor I am in-debted to the cooperation of the contributors for meetingdeadlines and to those who have conducted last-minutetasks at my request. The willingness with which writing,updating, changing text, reviewing and other tasks havebeen taken up is enormously appreciated. The involve-ment of some new contributors to the second edition hascome about through conducting fieldwork in deserts inAfrica and Asia with them: Dave Nash and Jo Bullard,whose PhDs I had the privilege to supervise; and StephenStokes, Giles Wiggs and Sarah O’Hara. For others, listen-ing to their papers at conferences and meetings in Ahmed-abad, India and Hamilton, Canada, and even the UK, orcasual conversations over coffee or on fieldtrips, led meto ask them to contribute: David Dunkerley, Gerald Nan-son, Jacky Croke, Ed Derbyshire, Helen Rendell, LillanBerger, Vatche Tchakerian (and, indirectly, Julie Laity) allbecame victims in this way.

The production of this book has been greatly helped byKate Schofield, Sam Rewston and Sarah Harmston in theGeography Department office at Sheffield and Paul Colesand Graham Allsopp in Cartographic Services who haveproduced or updated many of the figures. Iain Stevensonand Katrina Sinclair at the London office of John Wiley& Sons, Ltd have eased production matters. To all thosenames above, the undergraduates, postgraduates andacademics who used the first edition and passed oncomments for possible future changes, and especially mywife Lucy, who painstakingly prepared the index and ourdaughter Mair, who has tolerated the production of thisvolume since her birth, my sincere thanks.

David S.G. ThomasSheffield

January 1996

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Preface to the third edition

It was a pleasant surprise when in 2008 the publishersrequested that I consider putting together a new editionof Arid Zone Geomorphology. It is now thirteen yearssince the second edition was published and, inevitably,the literature of desert and dryland geomorphology hasburgeoned in the intervening period. Research on pro-cesses has benefited from many technological advances,no greater than in aeolian geomorphology, where it is nowpossible to measure airflow and sediment movement in thefield over very short (in some cases subsecond) time pe-riods. The benefits of advances in reductionist researchhave been complemented by developments that allow thebigger picture to be better viewed in space and in time.There are now many options in satellite-based remotesensing, allowing surface conditions and the atmosphereabove drylands to be analysed, while ground penetratingradar is permitting the internal structures of some drylandlandforms to be viewed. Better reconstructions of pastdryland environments, including land surface responsesto global climate change, are possible due to a plethora ofproxy records being available and enhanced chronometriccontrol of the timings of change are possible due to theadvances in luminescence dating. There is therefore muchto include in an updated edition.

The format of this edition is much changed from thesecond, which itself was markedly expanded from the firstedition. First is that while some chapters are updates oftheir equivalents in the second edition, many are new,even if bearing the same or similar titles. The structureof the book is also changed. The introductory chapters inSection 1 are increased from two to five. This has beendone to allow a bigger picture to be developed early onprior to the presentation of thematic sections. In previouseditions long-term change was not presented until late inthe book, whereas now it is integrated through the volumeas a whole. This is borne of recognition that to under-stand landforms and landscapes fully, it is necessary notonly to have knowledge of the processes operating to-day but the inheritance that has occurred from the past.Thus process geomorphology and Quaternary period re-constructions are not artificially divided, as is the common

case, into separate research agendas, but instead are inte-grated as appropriate in individual chapters. Therefore thenature and role of long-term change on drylands, and theirformer extensions, are presented in Chapter 3 (as opposedto Chapter 26 in the second edition). The diversity of dry-lands worldwide is also considered in a separate chapter,while arid landscapes on planets other than our own arealso considered early on. This reflects the knowledge ofdrylands that is arising through extraterrestrial research.

Three sections, on surfaces, water and wind, then fol-low, with a concluding section on human aspects of dry-land geomorphology, including the potential impacts oftwenty-first century climate change. The regional chap-ters from the expanded second edition are removed andinstead illustrative case studies are included within indi-vidual chapters where relevant. This makes for a slim-mer book, more akin to the first rather than the secondedition.

In 2004 I moved to the University of Oxford af-ter 20 years at Sheffield University, where a significantdesert/dryland research group had been established. Ox-ford now has perhaps the UK’s biggest arid land researchgrouping and this is reflected in the authorship of some ofthe chapters. We remain in this book truly international inoutlook, however, and this is not simply reflected in wherethe UK-base contributors conduct their desert research– including in the North American arid zone, southernAfrica, North Africa, Australia and Arabia – but in inter-nationally renowned researchers from the US, Australiaand the West Indies contributing to this volume. I thankthem all for their efforts.

A book is not simply down to its contributors, however.In this regard it is key to note that without the effortsof Fiona Woods and Izzy Canning at John Wiley &Sons, Ltd there would not be a third edition. The samecan be said for Jan Burke at the School of Geographyand Environment at Oxford, whose assistance in thefinal stages has been immeasurable, I also thank AilsaAllen who drew many of the figures, Paul Coles atSheffield whose assistance in tracking down and passingon artwork from the second edition has been gratefully

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xxiv PREFACE TO THE THIRD EDITION

appreciated, and Lucy Heath for considerable help inthe dreaded task of preparing the index. There are twofurther groups to thank. First is the truly inspirational newgeneration of desert geomorphologists whose doctoraltheses I have had the privilege to supervise over the pasttwo decades and whose research appears in some ofthe chapters. Second are the 300+ undergraduates andMaster’s students who in fifteen years Giles Wiggs and I(and when at Sheffield, Rob Bryant) have taken on annual

dryland geomorphological field trips to Tunisia and theUAE. They have been a good sample of users of AridZone Geomorphology, and their comments and usage ofthe book have led to some of the changes in this edition.Thank you all.

David S.G. ThomasOxford

March 2010

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ILarge-scale controls and variabilityin drylands

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1Arid environments: their nature and extent

David S.G. Thomas

1.1 Geomorphology in arid environments

Aridity, a deficit of moisture in the environment, is a sig-nificant feature of a large part of the Earth’s land sur-face. Aridity is complex, and its environmental manifes-tations vary from place to place and through time, suchthat its definition, occurrence and environmental con-sequences are complex and require careful unravelling.Aridity is also complex and challenging for many lifeforms, since moisture is such a fundamental requirementfor many. Aridity does not simply equate with the conceptof deserts, and goes far beyond what are widely regardedas such, so that there is a great diversity within, and be-tween, arid environments. The purpose of this book is toprovide explanations for the diversity and nature of aridenvironments, through an exploration of the land-shapingprocesses that operate within them.

For much of history and for many human races, aridenvironments have been areas to avoid, though for thosethat have been, and continue to be, resourceful and ableto adapt, arid regions have proved to be environmentsthat can be effectively and successfully utilised. Lack ofsurface water, limited foodstuffs and climatic extremeshave generally made arid areas unfavourable places forhabitation, though for resourceful hunter–gatherer andpastoral–nomadic peoples, living at low population den-sities, these environments have proved to be places ofopportunity. In other contexts, the apparent scarcity ofkey resources may have driven innovation: it is perhapsno coincidence that early civilisations, in Mesopotamia,in Egypt and in parts of central Asia, developed strate-gies to cope with aridity, with early agriculture develop-ing, c.4000 years ago, in the Mesopotamian heartland.However, for populations from more temperate and bet-ter watered regions, aridity has often proved a significant

challenge. Even with the technological advances of thelate nineteenth and twentieth centuries that made traveland existence in drylands possible for a greater rangeof people, arid environments still provide major limita-tions to the range and extent of human occupations andactivities.

European interest in arid environments grew from thelate eighteenth century onwards (Heathcote, 1983), usu-ally associated with the quest for natural resources andcolonisation, or with attempts at religious conversion.Much of the early ‘Western’ scientific knowledge con-cerning such areas came not from specialist scientists butfrom those whose primary goals were associated withthese activities. It has been noted or implied (see, forexample, Cooke and Warren, 1973; Cooke, Warren andGoudie, 1993; Goudie, 2002) that early geomorphologicalresearch in arid areas was dogged by excessive descrip-tion, superficiality and secular national terminology. Thefirst characteristic, description, has often been criticised,especially at times when quantification has been a cen-tral paradigm in geomorphology. Yet description can bean important prerequisite of rigorous explanation, analy-sis and deeper investigation. This is no better illustratedthan by the work of Dick Grove and Ron Peel (e.g. Grove,1958, 1969; Peel, 1939), where careful description of landforms and landscapes preceded analysis and the quest forgeomorphic explanations of their development and thecontrols on the processes that shaped them.

In the case of early works, the descriptive componentis hardly surprising. For European writers with temper-ate world origins, desert landscapes must have repre-sented spectacular, bizarre and unusual contrasts to theplant and soil mantled landscapes of many of their home-lands. Before early descriptive accounts are totally pillo-ried, it should also be remembered that geomorphological

Arid Zone Geomorphology: Process, Form and Change in Drylands, Third Edition. Edited by David S. G. ThomasC© 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.

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4 CH1 ARID ENVIRONMENTS: THEIR NATURE AND EXTENT

accounts in the works of early Europeans were usuallybut by-products of the reasons for their being in desertsin the first place. This also helps to explain the sec-ond characteristic, the superficiality of early reports andstudies.

The third characteristic attributed above to early works,secularity, arose because national groups tended, untilrelatively recently, to confine their interests to particu-lar deserts. In Africa, Asia and Australia, early geomor-phological investigations were heavily influenced by thedistribution of the impacts of European colonialism. ThusFlammand’s (1899) account from the Sahara, Passarge’s(1904) two volumes on the Kalahari and numerous reportsfrom Australia (e.g. Sturt, 1833; Mitchell, 1837; Spencer,1896) reflect broader colonial interests of their time. Ithas been noted, and is now increasingly realised as solu-tions are sought to dryland environmental problems (e.g.Mortimore, 1989; Thomas and Middleton, 1994; Thomasand Twyman, 2004), that the environmental knowledgevested in indigenous populations was, and remains, con-siderable and meritorious. Yet this was usually either ig-nored or unrealised by Europeans entering arid environ-ments for the first time in the nineteenth and through muchof the twentieth centuries: such people often saw desertsthrough eyes more accustomed to their starkly contrast-ing points of origin, leading to a preoccupation in somecases with the spectacular and unusual landforms theyencountered in deserts.

There were, of course, exceptions to these character-istics, even in the nineteenth century. Perhaps most no-table were the investigations in the southwestern UnitedStates, often with a geomorphological slant, of JohnWesley Powell (1875, 1878) and Grove Karl Gilbert(1875, 1877, 1895), the latter regarded by many as thefather of modern geomorphology. Their activities weredriven by a governmental quest to expand the frontiers of(European) utilisation of North America and their workswere essentially early forms of resource appraisal. Someof the early accounts of the geomorphology of the Aus-tralian deserts had a similar basis; for example, ThomasMitchell wrote:

After summounting the barriers of parched desertsand hostile barbarians, I had at last at length thesatisfaction of overlooking from a pyramid of granitea much better country (Mitchell, 1837, p. 275).

We had at last discovered a country ready forthe reception of civilised man. (Mitchell, 1837,p. 171)

1.2 Arid zone distinctiveness and thequest for explanation

Early accounts of arid landscapes may, however, be ofrestricted geomorphological value for a different reason:their focus on unusual and spectacular features was oftenat the expense of representativeness (but also see Chapter4, showing that accounts could also focus on the monotonyof some dryland regional landscapes). The lack of reliable,systematic, information and data was one reason whytheory in arid geomorphology changed rapidly throughthe first six decades of the twentieth century. As Goudie(1985, p. 122) noted:

A prime feature of desert geomorphological researchover the past century or so has been the rapidity withwhich ideas have changed, and the dramatic way inwhich ideas have gone in and out of fashion. Thisreflects the fact that hypothesis formulation has of-ten preceded detailed and reliable information onform and process, and the fact that different workershave written about different areas where the relativeimportance of different processes may vary substan-tially.

Within these changing ideas was a view that arid en-vironments are distinct, even unique, in terms of the op-eration of geomorphological processes and their resultantlandscape outcomes. Early quests for synthesising expla-nations sought generalisations that were deliberately dis-tinct from those developed for other environments. Davies(1905) produced his cycle of erosion for arid environ-ments based on the belief that fluvial processes in drylandsproduced distinct outcomes at the landscape scale. Thisnotion of distinctiveness was clearly also present in mor-phogentic or climatic geomorphology models of explana-tion (e.g. Birot, 1960; Budel, 1963; Tricart and Cailleux,1969). While the very terms ‘drylands’, ‘arid zone’ andso on clearly imply a climatic delimitation of the extentof these environments, it is debatable whether sweepingmodels of desert geomorphic explanation are justified,for three reasons. First, notwithstanding that there maybe ‘a world of difference in the landscapes and geomor-phological processes that occur in these different climaticzones’ [within arid environments] (Goudie, 2002, p. 5) isthat drylands themselves are not internally homogeneous;indeed they are markedly diverse climatologically andtectonically (see Chapter 2), which affects seasonality,plant cover, landscape erodibility, sediment types, sedi-ment availability and so on. Second, today’s arid regions

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