Fundamentals of Geomorphology PDF

 FUNDAMENTALS OF GEOMORPHOLOGY Second Edition This extensively revised and updated edition continues to present an engaging and comprehensive introduction to the subject, exploring the...

 FUNDAMENTALS OF GEOMORPHOLOGY Second Edition This extensively revised and updated edition continues to present an engaging and comprehensive introduction to the subject, exploring the world’s landforms from a broad systems perspective. It reflects on the latest developments in the field and includes new chapters on geomorphic materials and processes, hillslopes and changing landscapes. Fundamentals of Geomorphology begins with a consideration of the nature of geomorphology and the geomorphic system, geomorphic materials and processes, and the quest of process and historical geomorphologists, it moves on to discuss: Structure: landforms resulting from, or influenced by, the endogenic agencies of tectonic and volcanic processes, geological structures, and rock types. Process and form: landforms resulting from, or influenced by, the exogenic agencies of weathering, running water, flowing ice and meltwater, ground ice and frost, the wind, and the sea. History: Earth surface history, giving a discussion of Quaternary landforms and ancient landforms, including relict, exhumed, and stagnant landscape features, the origin of old plains; and evolutionary aspects of landscape change. Fundamentals of Geomorphology provides a stimulating and innovative perspective on the key topics and debates within the field of geomorphology. Written in an accessible and lively manner, it includes guides to further reading, chapter summaries, and an extensive glossary of key terms. The book is also illustrated throughout with over 200 informative diagrams and attractive photographs, including a colour plate section. Richard John Huggett is a Reader in Physical Geography in the University of Manchester.  ROUTLEDGE FUNDAMENTALS OF PHYSICAL GEOGRAPHY SERIES Series Editor: John Gerrard This new series of focused introductory textbooks presents comprehensive, up-to-date introductions to the fundamental concepts, natural processes and human/environmental impacts within each of the core physical geography sub-disciplines. Uniformly designed, each volume contains student-friendly features: plentiful illustrations, boxed case studies, key concepts and summaries, further reading guides and a glossary. Already published: Fundamentals of Soils John Gerrard Fundamentals of Hydrology Tim Davie Fundamentals of Geomorphology Richard John Huggett Fundamentals of Biogeography. Second Edition Richard John Huggett Fundamentals of Geomorphology. Second Edition Richard John Huggett  FUNDAMENTALS OF GEOMORPHOLOGY Second Edition Richard John Huggett Routledge Fundamentals of Physical Geography  First published 2007 by Routledge 2 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN Simultaneously published in the USA and Canada by Routledge 270 Madison Avenue, New York, NY 10016 Routledge is an imprint of the Taylor & Francis Group, an informa business This edition published in the Taylor & Francis e-Library, 2007. “To purchase your own copy of this or any of Taylor & Francis or Routledge’s collection of thousands of eBooks please go to www.eBookstore.tandf.co.uk.” © 2007 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 Data A catalogue record for this book is available from the British Library Library of Congress Cataloging in Publication Data A catalog record for this book has been requested. ISBN 0-203-94711-8 Master e-book ISBN ISBN 978-0-415-39083-5 hbk ISBN 978-0-415-39084-2 pbk ISBN 978-0-203-94711-1 ebk for my family  CONTENTS Series editor’s preface ix Author’s preface to second edition xi Author’s preface to first edition xii Acknowledgements xiv PART I INTRODUCING LANDFORMS AND LANDSCAPES 1 1 WHAT IS GEOMORPHOLOGY? 3 2 THE GEOMORPHIC SYSTEM 31 3 GEOMORPHIC MATERIALS AND PROCESSES 49 PART II STRUCTURE 95 4 LARGE-SCALE TECTONIC AND STRUCTURAL LANDFORMS 97 5 SMALL-SCALE TECTONIC AND STRUCTURAL LANDFORMS 116 PART III PROCESS AND FORM 151 6 WEATHERING AND RELATED LANDFORMS 153 7 HILLSLOPES 166 viii CONTENTS 8 KARST LANDSCAPES 183 9 FLUVIAL LANDSCAPES 220 10 GLACIAL AND GLACIOFLUVIAL LANDSCAPES 246 11 PERIGLACIAL LANDSCAPES 277 12 AEOLIAN LANDSCAPES 296 13 COASTAL LANDSCAPES 316 PART IV HISTORY 349 14 QUATERNARY LANDSCAPES 351 15 ANCIENT LANDSCAPES 378 Appendix: The Geological Timescale 409 Glossary 410 References 419 Index 448  SERIES EDITOR’S PREFACE We are presently living in a time of unparalleled change and when concern for the environment has never been 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 human activity in such issues that is of most concern, this activity affects the operation of the natural processes that occur within the physical environment. Most of these processes and their effects are taught and researched within the academic disci- pline of physical geography. A knowledge and understanding of physical 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 fundamental nature of the physical processes that act on or just above the surface of the earth. The volumes in the series are Climatology, Geomorphology, Biogeography, Hydrology and Soils. The topics are treated in sufficient breadth and depth to provide the coverage expected in a Fundamentals series. Each volume leads into the topic by outlining the approach adopted. This is important because there may be several ways of approaching individual topics. Although each volume is complete in itself, there are many explicit and implicit references to the topics covered in the other volumes. Thus, the five volumes together provide a comprehensive insight into the totality that is Physical Geography. The flexibility provided by separate volumes has been designed to meet the demand created by the variety of courses currently operating in higher education institutions. The advent of modular courses has meant that physical geography is now rarely taught, in its entirety, in an ‘all-embracing’ course but is generally split into its main components. This is also the case with many Advanced Level syllabuses. Thus students and teachers are being frustrated increasingly by lack of suitable books and are having to recommend texts of which only a small part might be relevant to their needs. Such texts also tend to lack the detail required. It is the aim of this series to provide individual volumes of sufficient breadth and depth to fulfil new demands. The volumes should also be of use to sixth-form teachers where modular syllabuses are also becoming common. Each volume has been written by higher-education teachers with a wealth of experience in all aspects of the topics they cover and a proven ability in presenting information in a lively and interesting way. Each volume 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 as boxed case studies and summary notes. x SERIES EDITOR’S PREFACE References to important previous studies and results are included but are used sparingly to avoid overloading the text. Suggestions for further reading are also provided. The main target readership is introductory-level undergraduate students of physical geography or environmental science, 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 AUTHOR’S PREFACE TO SECOND EDITION The first edition of Fundamentals of Geomorphology was published in 2003. I was delighted that it was well received and that I was asked to write a second edition. Anonymous reviewers of the first edition suggested that some rearrangement of material might be beneficial, and I have taken most of their suggestions on board. Cliff Ollier also kindly provided me with many ideas for improvements. The key changes are new chapters on geomorphic materials and processes and on hillslopes, the reorganizing of the tectonic and structural chapters into large-scale and small- scale landforms, and the splitting of the single history chapter into a chapter dealing with Quaternary landforms and a chapter dealing with ancient landforms. I have also taken the opportunity to update some information and examples. Once again, I should like to thank many people who have made the completion of this book possible: Nick Scarle for revising some of the first-edition diagrams and for drawing the many new ones. Andrew Mould for persuading me to pen a new edition. George A. Brook, Stefan Doerr, Derek C. Ford, Mike Hambrey, Kate Holden, Karna Lidmar-Bergström, David Knighton, Phil Murphy, Alexei Rudoy, Nick Scarle, Wayne Stephenson, Wilf Theakstone, Dave Thomas, Heather Viles, Tony Waltham, Jeff Warburton, and Clive Westlake for letting me re-use their photographs; and Neil Glasser, Stefan Grab, Adrian Hall, Heather Viles, Tony Waltham, and Jamie Woodward for supplying me with fresh ones. And, as always, my wife and family for sharing the ups and downs of writing a book. Richard John Huggett Poynton October 2006  AUTHOR’S PREFACE TO FIRST EDITION Geomorphology has always been a favourite subject of mine. For the first twelve years of my life I lived in North 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 my father was playing cricket. H. W. (‘Masher’) Martin, the head of geography and geology at Hertford Grammar School, whose ‘digressions’ during classes were tremendously educational, aroused my first formal interest in landforms. The sixth-form field- trips to the Forest of Dean and the Lake District were unforgettable. While at University College London, I was lucky enough to come under the tutelage of Eric H. Brown, Claudio Vita-Finzi, Andrew Warren, and Ron Cooke, to whom I am indebted for a remarkable six years as an undergraduate and postgraduate. Since arriving at Manchester, I have taught several courses with large geomorphological components but have seen myself very much as a physical geographer with a dislike of disciplinary boundaries and the fashion for overspecialization. Nonetheless, I thought that writing a new, student-friendly geomorphological text would pose an interesting challenge and, with Fundamentals of Biogeography, make a useful accompaniment to my more academic works. In writing Fundamentals of Geomorphology, I have tried to combine process geomorphology, which has dominated the subject for the last several decades, with the less fashionable but fast-resurging historical geomorphology. Few would question the astounding achievements of process studies, but plate-tectonics theory and a reliable calendar of events have given historical studies a huge boost. I also feel that too many books get far too bogged down in process equations: there is a grandeur in the diversity of physical forms found at the Earth’s surface and a wonderment to be had in seeing them. So, while explaining geomorphic processes 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, are an important aspect of geomorphology today. The book is quadripartite. Part I introduces landforms and landscapes, studying the nature of geomorphology 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 century ago. 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 time over which it has been evolving. Part II looks at tectonic and structural landforms. Part II investigates process and form, with chapters on weathering and related landforms, karst landscapes,  AUTHOR’S PREFACE TO FIRST EDITION xiii 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 processes involved 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 some great achievements of modern historical geomorphology. There are several people to whom I wish to say ‘thanks’: Nick Scarle, for drawing all the diagrams and handling 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 draft of the work that led to several major improvements, particularly in the overall structure; any remaining shortcomings and omissions are of course down to me. A small army of colleagues, identified individually on the plate captions, for kindly providing me with slides. Clive Agnew and the other staff at Manchester, for friendship and assistance, and in particular Kate Richardson for making several invaluable suggestions about the structure and content of Chapter 1. As always, Derek Davenport, for discussing all manner of things. And, finally, my wife and family, who understand the ups and downs of book-writing and give unbounded support. Richard John Huggett Poynton March 2002  ACKNOWLEDGEMENTS The author and publisher would like to thank the following for granting permission to reproduce material in this work: The copyright of photographs remains held by the individuals who kindly supplied them (please see photograph captions for individual names); Figure 1.4 after Figure 3 from Claudio Vita-Finzi (1969) The Mediterranean Valleys: Geological Changes in Historical Times (Cambridge: Cambridge University Press), reproduced by permission of Cambridge University Press; Figure 1.6 after Figure 4 from R. H. Johnson (1980) ‘Hillslope stability and landslide hazard – a case study from Longdendale, north Derbyshire, England’ in Proceedings of the Geologists’ Association, London (Vol. 91, pp. 315–25), reproduced by permission of the Geologists’ Association; Figure 1.10 after Figure 6.1 from R. J. Chorley and B. A. Kennedy (1971) Physical Geography: A Systems Approach (London: Prentice Hall), reproduced by permission of Rosemary J. Chorley and Barbara A. Kennedy; Figure 1.14 after Figure 3.10 from S. A. Schumm (1991) To Interpret the Earth: Ten Ways to Be Wrong (Cambridge: Cambridge University Press), reproduced by permission of Cambridge University Press; Figure 3.1 after Figures 3.3 and 3.5 from G. Taylor and R. A. Eggleton (2001) Regolith Geology and Geomorphology (Chichester: John Wiley & Sons), Copyright © 2001, reproduced by permission of John Wiley & Sons Limited; 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 and E. M. Campbell; Figure 5.9 reprinted from Earth-Science Reviews 69, J. W. Cole, D. M. Milner, and K. D. Spinks, ‘Calderas and caldera structures: a review’, pp. 1–26, copyright © 2005, with per- mission from Elsevier; Figure 5.24 after Figure 4.9 from M. A. Summerfield (1991) Global Geomorphology: An Introduction to the Study of Landforms (Harlow, Essex: Longman), © M. A. Summerfield, reprinted by permis- sion of Pearson Education Limited; Figure 8.1 after ‘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, reproduced by permission of Poole’s Cavern and Country Park; Figures 6.6, 6.7, and 6.9 after Figures 9.3, 9.13, and 9.30 from D. C. Ford and P. W. Williams (1989) Karst Geomorphology and Hydrology (London: Chapman & Hall), reproduced with kind permission of Springer Science and Business Media and Derek Ford; Figure 9.4 after Figure 14.1 from F. Ahnert (1998) Introduction to Geomorphology (London: Arnold), reproduced by permission of Verlag Eugen Ulmer, Stuttgart (the original German language publishers);  ACKNOWLEDGEMENTS xv Figure 10.6 slightly adapted from Figure 6.9 in A. S. Trenhaile (1998) Geomorphology: A Canadian Perspective (Toronto: Oxford University Press), reproduced by permission of Oxford University Press, Canada; Figure 14.1 after Figure 6 from Warburton and M. Danks (1998) ‘Historical and contemporary channel change, Swinhope Burn’ in J. Warburton (ed.) Geomorphological Studies in the North Pennines: Field Guide, pp. 77–91 (Durham: Department of Geography, University of Durham, British Geomorphological Research Group), reproduced by per- mission of ]eff Warburton; Figure 14.3 after Figure 2 from Quaternary International 79, J. Rose, B. S. P. Moorlock, and R. J. O. Hamblin, ‘Pre-Anglian fluvial and coastal deposits in Eastern England: lithostratigraphy and palaeoen- vironments’, pp. 5–22 copyright © 2005, with permission from Elsevier; Figure 15.9 after Figure 16 from P. Japsen, T. Bidstrup, and K. Lidmar-Bergström (2002) ‘Neogene uplift and erosion of southern Scandinavia induced by the rise of the South Swedish Dome’ in A. G. Doré, J. A. Cartwright, M. S. Stoker, J. P. Turner, and N. White (eds) Exhumation of the North Atlantic Margin: Timing, Mechanisms and Implications for Petroleum Exploration (Geological Society, London, Special Publication 196), pp. 183–207, reproduced by permission of the Geological Society, London, and Peter Japsen; Figure 15.6 after Figure 10 from D. K. C. Jones (1999) ‘Evolving models of the Tertiary evolutionary geomorphology of southern England, with special reference to the Chalklands’ in B. J. Smith, W. B. Whalley, and P. A. Warke (eds) Uplift, Erosion and Stability: Perspectives on Long-term Landscape Develop- ment (Geological Society, London, Special Publication 162), pp. 1–23, reproduced by permission of the Geological Society, London, and David K. C. Jones; Colour Plate 12 after Figure 2 from Geomorphology 71, Glasser, N. F., Jansson, K. M., Harrison, S., and Rivera, A. ‘Geomorphological evidence for variations of the North Patagonian Icefield during the Holocene’, pp. 263–77, copyright © 2005, with permission from Elsevier. Every effort has been made to contact copyright holders for their permission to reprint material in this book. The publishers would be grateful to hear from any copyright holder who is not here acknowledged and will undertake to rectify any errors or omissions in future editions of this book.  Part I INTRODUCING LANDFORMS AND LANDSCAPES  1 WHAT IS GEOMORPHOLOGY? Geomorphology is the study of landforms and the processes that create them. This chapter covers:
historical, process, applied, and other geomorphologies
the form of the land
land-forming processes and geomorphic systems
the history of landforms
methodological isms INTRODUCING GEOMORPHOLOGY to major tectonic plates, and their ‘lifespans’ range from days to millennia to aeons (Figure 1.1). The word geomorphology derives from three Greek Geomorphology was first used as a term to describe the words: gew (the Earth), morfh (form), and log oV morphology of the Earth’s surface in the 1870s and 1880s (discourse). Geomorphology is therefore ‘a discourse on (e.g. de Margerie 1886, 315). It was originally defined as Earth forms’. It is the study of Earth’s physical land- ‘the genetic study of topographic forms’ (McGee 1888, surface features, its landforms – rivers, hills, plains, 547), and was used in popular parlance by 1896. Despite beaches, sand dunes, and myriad others. Some work- the modern acquisition of its name, geomorphology is a ers include submarine landforms within the scope of venerable discipline (Box 1.1). It investigates landforms geomorphology. And some would add the landforms of and the processes that fashion them. A large corpus of other terrestrial-type planets and satellites in the Solar geomorphologists expends much sweat in researching System – Mars, the Moon, Venus, and so on. Landforms relationships between landforms and the processes act- are conspicuous features of the Earth and occur every- ing on them now. These are the process or functional where. They range in size from molehills to mountains geomorphologists. Many geomorphic processes affect,  4 INTRODUCING LANDFORMS AND LANDSCAPES Figure 1.1 Landforms at different scales and their interactions with exogenic and endogenic processes.  WHAT IS GEOMORPHOLOGY? 5 Box 1.1 THE ORIGIN OF GEOMORPHOLOGY Ancient Greek and Roman philosophers wondered hollowing out softer rocks. During the Renaissance, how mountains and other surface features in the many scholars debated Earth history. Leonardo da natural landscape had formed. Aristotle, Herodotus, Vinci (1452–1519) believed that changes in the levels Seneca, Strabo, Xenophanes, and many others dis- of land and sea explained the presence of fossil marine coursed on topics such as the origin of river valleys shells in mountains. He also opined that valleys were and deltas, and the presence of seashells in mountains. cut by streams and that streams carried material from Xenophanes of Colophon (c. 580–480 BC) speculated one place and deposited it elsewhere. In the eighteenth that, as seashells are found on the tops of moun- century, Giovanni Targioni-Tozzetti (1712–84) recog- tains, the surface of the Earth must have risen and nized evidence of stream erosion. He argued that the fallen. Herodotus (c. 484–420 BC) thought that the valleys of the Arno, Val di Chaina, and Ombrosa in lower part of Egypt was a former marine bay, reput- Italy were excavated by rivers and floods resulting from edly saying ‘Egypt is the gift of the river’, referring the bursting of barrier lakes, and suggested that the to the year-by-year accumulation of river-borne silt in irregular courses of streams relate to the differences the Nile delta region. Aristotle (384–322 BC) conjec- in the rocks in which they cut, a process now called tured that land and sea change places, with areas that differential erosion. Jean-Étienne Guettard (1715–86) are now dry land once being sea and areas that are argued that streams destroy mountains and the sedi- now sea once being dry land. Strabo (64/63 BC–AD ment produced in the process builds floodplains before 23?) observed that the land rises and falls, and sug- being carried to the sea. He also pointed to the effi- gested that the size of a river delta depends on the cacy of marine erosion, noting the rapid destruction nature of its catchment, the largest deltas being found of chalk cliffs in northern France by the sea, and the where the catchment areas are large and the surface fact that the mountains of the Auvergne were extinct rocks within it are weak. Lucius Annaeus Seneca (4 BC– volcanoes. Horace-Bénédict de Saussure (1740–99) AD 65) appears to have appreciated that rivers possess contended that valleys were produced by the streams the power to erode their valleys. About a millennium that flow within them, and that glaciers may erode later, the illustrious Arab scholar ibn-Sina, also known rocks. From these early ideas on the origin of landforms as Avicenna (980–1037), who translated Aristotle, arose modern geomorphology. (See Chorley et al. 1964 propounded the view that some mountains are pro- and Kennedy 2005 for details on the development of duced by differential erosion, running water and wind the subject.) and are affected by, human activities. Applied geomor- parts of the world, some landforms survive from millions phologists explore this rich area of enquiry, which is and hundreds of millions of years ago. Geomorphology, largely an extension of process geomorphology. Many then, has an important historical dimension, which is the landforms have a long history, and their present form domain of the historical geomorphologists. In short, does not always relate to the current processes acting modern geomorphologists study three chief aspects of upon them. The nature and rate of geomorphic processes landforms – form, process, and history. The first two change with time, and some landforms were produced are sometimes termed functional geomorphology, the under different environmental conditions, surviving last historical geomorphology (Chorley 1978). Process today as relict features. In high latitudes, many land- studies have enjoyed hegemony for some three or four forms are relicts from the Quaternary glaciations; but, in decades. Historical studies were sidelined by process 6 INTRODUCING LANDFORMS AND LANDSCAPES studies but are making a strong comeback. Although terms, borrowed from biology, are misleading and much process and historical studies dominate much modern censured (e.g. Ollier 1967; Ollier and Pain 1996, 204–5). geomorphological enquiry, particularly in English- The ‘geographical cycle’ was designed to account for the speaking nations, other types of study exist. For exam- development of humid temperate landforms produced ple, structural geomorphologists, who were once a by prolonged wearing down of uplifted rocks offering very influential group, argued that underlying geological uniform resistance to erosion. It was extended to other structures are the key to understanding many landforms. landforms, including arid landscapes, glacial landscapes, Climatic geomorphologists, who are found mainly periglacial landscapes, to landforms produced by shore in France and Germany, believe that climate exerts a processes, and to karst landscapes. profound influence on landforms, each climatic region William Morris Davis’s ‘geographical cycle’ – in which creating a distinguishing suite of landforms (p. 13). landscapes are seen to evolve through stages of youth, maturity, and old age – must be regarded as a classic work, even if it has been superseded (Figure 1.2). Its Historical geomorphology appeal seems to have lain in its theoretical tenor and Traditionally, historical geomorphologists strove to work in its simplicity (Chorley 1965). It had an all-pervasive out landscape history by mapping morphological and influence on geomorphological thought and spawned the sedimentary features. Their golden rule was the dictum once highly influential field of denudation chronology. that ‘the present is the key to the past’. This was a The work of denudation chronologists, who dealt mainly warrant to assume that the effects of geomorphic pro- with morphological evidence, was subsequently criticized cesses seen in action today may be legitimately used to for seeing flat surfaces everywhere. infer the causes of assumed landscape changes in the past. Before reliable dating techniques were available, Walther Penck such studies were difficult and largely educated guess- work. However, the brilliant successes of early historical A variation on Davis’s scheme was offered by Walther geomorphologists should not be overlooked. Penck. According to the Davisian model, uplift and pla- nation take place alternately. But, in many landscapes, uplift and denudation occur at the same time. The con- William Morris Davis tinuous and gradual interaction of tectonic processes and The ‘geographical cycle’, expounded by William denudation leads to a different model of landscape evo- Morris Davis, was the first modern theory of land- lution, in which the evolution of individual slopes is scape evolution (e.g. Davis 1889, 1899, 1909). It thought to determine the evolution of the entire land- assumed that uplift takes place quickly. Geomorphic scape (Penck 1924, 1953). Three main slope forms evolve processes, without further complications from tectonic with different combinations of uplift and denudation movements, then gradually wear down the raw topog- rates. First, convex slope profiles, resulting from wax- raphy. Furthermore, slopes within landscapes decline ing development (aufsteigende Entwicklung), form when through time – maximum slope angles slowly lessen the uplift rate exceeds the denudation rate. Second, (though few field studies have substantiated this claim). straight slopes, resulting from stationary (or steady-state) So topography is reduced, little by little, to an exten- development (gleichförmige Entwicklung), form when sive flat region close to baselevel – a peneplain – uplift and denudation rates match one another. And, with occasional hills, called monadnocks after Mount third, concave slopes, resulting from waning develop- Monadnock in New Hampshire, USA, which are local ment (absteigende Entwicklung), form when the uplift erosional remnants, standing conspicuously above the rate is less than the denudation rate. Later work has general level. The reduction process creates a time shown that valley-side shape depends not on the simple sequence of landforms that progresses through the interplay of erosion rates and uplift rates, but on slope stages of youth, maturity, and old age. However, these materials and the nature of slope-eroding processes.  WHAT IS GEOMORPHOLOGY? 7 (a) Youth (b) Maturity (c) Old age Figure 1.2 William Morris Davis’s idealized ‘geographical cycle’ in which a landscape evolves through ‘life-stages’ to produce a peneplain. (a) Youth: a few ‘consequent’ streams (p. 135), V-shaped valley cross-sections, limited floodplain formation, large areas of poorly drained terrain between streams with lakes and marshes, waterfalls and rapids common where streams cross more resistant beds, stream divides broad and ill-defined, some meanders on the original surface. (b) Maturity: well-integrated drainage system, some streams exploiting lines of weak rocks, master streams have attained grade (p. 229), waterfalls, rapids, lakes, and marshes largely eliminated, floodplains common on valley floors and bearing meandering rivers, valley no wider than the width of meander belts, relief (difference in elevation between highest and lowest points) is at a maximum, hillslopes and valley sides dominate the landscape. (c) Old age: trunk streams more important again, very broad and gently sloping valleys, floodplains extensive and carrying rivers with broadly meandering courses, valleys much wider than the width of meander belts, areas between streams reduced in height and stream divides not so sharp as in the maturity stage, lakes, swamps, and marshes lie on the floodplains, mass-wasting dominates fluvial processes, stream adjustments to rock types now vague, extensive areas lie at or near the base level of erosion. Source: Adapted from Holmes (1965, 473) 8 INTRODUCING LANDFORMS AND LANDSCAPES According to Penck’s arguments, slopes may either Eduard Brückner and Albrecht Penck’s (Walther’s recede at the original gradient or else flatten, accord- father) work on glacial effects on the Bavarian Alps and ing to circumstances. Many textbooks claim that Penck their forelands provided the first insights into the effects advocated ‘parallel retreat of slopes’, but this is a false of the Pleistocene ice ages on relief (Penck and Brückner belief (see Simons 1962). Penck (1953, 135–6) argued 1901–9). Their classic river-terrace sequence gave names that a steep rock face would move upslope, maintain- to the main glacial stages – Donau, Gunz, Mindel, Riss, ing its original gradient, but would soon be eliminated and Würm – and sired Quaternary geomorphology. by a growing basal slope. If the cliff face was the scarp of a tableland, however, it would take a long time to disappear. He reasoned that a lower-angle slope, which Modern historical geomorphology starts growing from the bottom of the basal slope, replaces Historical geomorphology has developed since Davis’s the basal slope. Continued slope replacement then leads time, and the interpretation of long-term changes of to a flattening of slopes, with steeper sections formed landscape no longer relies on the straitjacket of the geo- during earlier stages of development sometimes surviv- graphical cycle. It relies now on various chronological ing in summit areas (Penck 1953, 136–41). In short, analyses, particularly those based on stratigraphical stud- Penck’s complicated analysis predicted both slope reces- ies of Quaternary sediments, and upon a much fuller sion and slope decline, a result that extends Davis’s appreciation of geomorphic and tectonic processes (e.g. simple idea of slope decline (Figure 1.3). Field stud- Brown 1980). Observed stratigraphical relationships fur- ies have confirmed that slope retreat is common in a nish relative chronologies, whilst absolute chronologies wide range of situations. However, a slope that is actively derive from sequences dated using historical records, eroded at its base (by a river or by the sea) may decline if radiocarbon analysis, dendrochronology, luminescence, the basal erosion should stop. Moreover, a tableland scarp palaeomagnetism, and so forth (p. 354). Such quantita- retains its angle through parallel retreat until the erosion tive chronologies offer a means for calculating long-term removes the protective cap rock, when slope decline sets rates of change in the landscape. in (Ollier and Tuddenham 1962). It is perhaps easiest to explain modern historical geo- morphology by way of an example. Take the case of the river alluvium and colluvium that fills many valleys Eduard Brückner and Albrecht Penck in countries bordering the Mediterranean Sea. Claudio Other early historical geomorphologists used geologi- Vita-Finzi (1969) pioneered research into the origin cally young sediments to interpret Pleistocene events. of the valley fills, concluding that almost all alluvium Slope recession or backwearing Slope decline or downwearing (Penck) (Davis) Time 6 5 4 3 2 1 1 2 3 4 5 6 Pediplain Peneplain Figure 1.3 Slope recession, which produces a pediplain (p. 381) and slope decline, which produces a peneplain. Source: Adapted from Gossman (1970)  WHAT IS GEOMORPHOLOGY? 9 (a) (b) ( g) (c) (d ) Roman dams (e) (f ) Younger fill Older fill Bedrock (predominantly limestone) Calcareous crust Figure 1.4 A reconstruction of the geomorphic history of a wadi in Tripolitania. (a) Original valley. (b) Deposition of Older Fill. (c) River cut into Older Fill. (d) Roman dams impound silt. (e) Rivers cut further into Older Fill and Roman alluvium. (f ) Deposition of Younger Fill. (g) Present valley and its alluvial deposits. Source: After Vita-Finzi (1969, 10) and colluvium was laid down during two episodes of breached or found a way around the dams and cut increased aggradation (times when deposition of sed- into the Roman alluvium. Rivers built up the third iment outstripped erosion). Figure 1.4 is a schematic deposit, which contained Roman and earlier material reconstruction of the geomorphic history of a valley in as well as pottery and charcoal placing in the Medieval Tripolitania (western Libya). The key to unlocking the Period (AD 1200–1500), within the down-cut wadis. The history of the valleys in the area was datable archaeo- deposition of this Younger Fill was followed by reduced logical material in the fluvial deposits. Vita-Finzi found alluviation and down-cutting through the fill. three main deposits of differing ages. The oldest contains Wider examination of alluvia in Mediterranean val- Palaeolithic implements and seems to have accumulated leys allowed Vita-Finzi to recognize an Older Fill dating during the Pleistocene. Rivers cut into it between about from the Pleistocene and a Younger Fill dating from about 9,000 and 3,000 years ago. The second deposit accu- AD 500–1500. The Older Fill was deposited as a substan- mulated behind dams built by Romans to store water tial body of colluvium (slope wash) under a ‘periglacial’ and retain sediment. Late in the Empire, floodwaters regime during the last glacial stage. The Younger Fill was 10 INTRODUCING LANDFORMS AND LANDSCAPES a product of phases of erosion during the later Roman (e.g. Leopold et al. 1964). Stanley A. Schumm, another Imperial times, through the Dark Ages, and to the Middle fluvial geomorphologist, refined notions of landscape sta- Ages. Vita-Finzi believed it to be the result of increased bility to include thresholds and dynamically metastable erosion associated with the climate of the Medieval Warm states and made an important contribution to the under- Period or the Little Ice Age, a view supported by John standing of timescales (p. 27). Stanley W. Trimble worked Bintliff (1976, 2002). Other geomorphologists, includ- on historical and modern sediment budgets in small ing Karl Butzer (1980, 2005) and Tjierd van Andel and catchments (e.g. Trimble 1983). Richard J. Chorley his co-workers (1986), favoured human activity as the brought process geomorphology to the UK and demon- chief cause, pointing to post-medieval deforestation and strated the power of a systems approach to the subject. agricultural expansion into marginal environments. The Process geomorphologists have done their subject at matter is still open to debate (see p. 363). least three great services. First, they have built up a database of process rates in various parts of the globe. Second, they have built increasingly refined models for Process geomorphology predicting the short-term (and in some cases long-term) Process geomorphology is the study of the processes changes in landforms. Third, they have generated some responsible for landform development. In the modern enormously powerful ideas about stability and instability era, the first process geomorphologist, carrying on the in geomorphic systems (see pp. 19–21). tradition started by Leonardo da Vinci (p. 5), was Grove Karl Gilbert. In his treatise on the Henry Mountains Measuring geomorphic processes of Utah, USA, Gilbert discussed the mechanics of flu- vial processes (Gilbert 1877), and later he investigated Some geomorphic processes have a long record of mea- the transport of debris by running water (Gilbert 1914). surement. The oldest year-by-year record is the flood Up to about 1950, when the subject grew apace, impor- levels of the River Nile in lower Egypt. Yearly readings tant contributors to process geomorphology included at Cairo are available from the time of Muhammad, Ralph Alger Bagnold (p. 85), who considered the physics and some stone-inscribed records date from the first of blown sand and desert dunes, and Filip Hjulstrøm dynasty of the pharaohs, around 3100 BC. The amount (p. 73), who investigated fluvial processes. After 1950, of sediment annually carried down the Mississippi River several ‘big players’ emerged that set process geomorphol- was gauged during the 1840s, and the rates of modern ogy moving apace. Arthur N. Strahler was instrumental denudation in some of the world’s major rivers were in establishing process geomorphology, his 1952 paper estimated in the 1860s. The first efforts to measure called ‘Dynamic basis of geomorphology’ being a land- weathering rates were made in the late nineteenth cen- mark publication. John T. Hack, developing Gilbert’s tury. Measurements of the dissolved load of rivers enabled ideas, prosecuted the notions of dynamic equilibrium estimates of chemical denudation rates to be made in and steady state, arguing that a landscape should attain the first half of the twentieth century, and patchy efforts a steady state, a condition in which land-surface form were made to widen the range of processes measured does not change despite material being added by tec- in the field. But it was the quantitative revolution in tonic uplift and removed by a constant set of geomorphic geomorphology, started in the 1940s, that was largely processes. And he contended that, in an erosional land- responsible for the measuring of process rates in differ- scape, dynamic equilibrium prevails where all slopes, ent environments. Since about 1950, the attempts to both hillslopes and river slopes, are adjusted to each other quantify geomorphic processes in the field have grown (cf. Gilbert 1877, 123–4; Hack 1960, 81), and ‘the forms fast. An early example is the work of Anders Rapp and processes are in a steady state of balance and may (1960), who tried to quantify all the processes active be considered as time independent’ (Hack 1960, 85). in a subarctic environment and assess their compara- Luna B. Leopold and M. Gordon Wolman made notable tive significance. His studies enabled him to conclude contributions to the field of fluvial geomorphology that the most powerful agent of removal from the  WHAT IS GEOMORPHOLOGY? 11 Karkevagge drainage basin was running water bearing systems and climate that are forged through the stor- material in solution. An increasing number of hillslopes ages and movements of energy, water, biogeochemicals, and drainage basins have been instrumented, that is, had and sediments. Longer-term and broader-scale intercon- measuring devices installed to record a range of geo- nections between landforms and climate, water budgets, morphic processes. The instruments used on hillslopes vegetation cover, tectonics, and human activity are a focus and in geomorphology generally are explained in sev- for process geomorphologists who take a historical per- eral books (e.g. Goudie 1994). Interestingly, some of spective and investigate the causes and effects of changing the instrumented catchments established in the 1960s processes regimes during the Quaternary. have recently received unexpected attention from scien- tists studying global warming, because records lasting Applied geomorphology decades in climatically sensitive areas – high latitudes and high altitudes – are invaluable. However, after half Applied geomorphology studies the interactions of a century of intensive field measurements, some areas, humans with landscapes and landforms. Process geomor- including Europe and North America, still have better phologists, armed with their models, have contributed coverage than other areas. And field measurement pro- to the investigation of worrying problems associated grammes should ideally be ongoing and work on as fine with the human impacts on landscapes. They have stud- a resolution as practicable, because rates measured at a ied coastal erosion and beach management (e.g. Bird particular place may vary through time and may not be 1996; Viles and Spencer 1996), soil erosion, the weath- representative of nearby places. ering of buildings, landslide protection, river manage- ment and river channel restoration (e.g. Brookes and Shields 1996), and the planning and design of landfill Modelling geomorphic processes sites (e.g. Gray 1993). Other process geomorphologists Since the 1960s and 1970s, process studies have been have tackled general applied issues. Geomorphology in largely directed towards the construction of models for Environmental Planning (Hooke 1988), for example, predicting short-term changes in landforms, that is, considered the interaction between geomorphology and changes happening over human timescales. Such models public policies, with contributions on rural land-use have drawn heavily on soil engineering, for example in the and soil erosion, urban land-use, slope management, case of slope stability, and hydraulic engineering in the river management, coastal management, and policy cases of flow and sediment entrainment and deposition formulation. Geomorphology in Environmental Manage- in rivers. Nonetheless, some geomorphologists, includ- ment (Cooke 1990), as its title suggests, looked at the ing Michael J. Kirkby and Jonathan D. Phillips, have role played by geomorphology in management aspects of carved out a niche for themselves in the modelling depart- the environment. Geomorphology and Land Management ment. An example of a geomorphic model is shown in in a Changing Environment (McGregor and Thompson Figure 1.5 (see also p. 22). 1995) focused upon problems of managing land against a background of environmental change. The conserva- tion of ancient and modern landforms is an expanding Process studies and global environmental aspect of applied geomorphology. change Three aspects of applied geomorphology have been With the current craze for taking a global view, pro- brought into a sharp focus by the impending envi- cess geomorphology has found natural links with other ronmental change associated with global warming Earth and life sciences. Main thrusts of research inves- (Slaymaker 2000b) and illustrate the value of geomor- tigate (1) energy and mass fluxes and (2) the response phological know-how. First, applied geomorphologists of landforms to climate, hydrology, tectonics, and land are ideally placed to work on the mitigation of natural use (Slaymaker 2000b, 5). The focus on mass and energy hazards of geomorphic origin, which may well increase in fluxes explores the short-term links between land-surface magnitude and frequency during the twenty-first century 12 INTRODUCING LANDFORMS AND LANDSCAPES ( a ) Scarp retreat I ( c ) Scarp rounding I Debris apron 20 m 20 m ( b ) Scarp retreat II ( d ) Scarp rounding II 50 m 50 m Figure 1.5 Example of a geomorphic model: the predicted evolution of a fault scarp according to assumptions made about slope processes. (a) Parallel scarp retreat with deposition of debris at the base. The scarp is produced by a single movement along the fault. (b) Parallel scarp retreat with deposition at the base. The scarp is produced by four separate episodes of movement along the fault. In cases (a) and (b) it is assumed that debris starts to move downslope once a threshold angle is reached and then comes to rest where the scarp slope is less than the threshold angle. Allowance is made for the packing density of the debris and for material transported beyond the debris apron. (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) and beyond. Landslides and debris flows may become temperature rises into predictions of critical boundary more common, soil erosion may become more severe changes, such as the poleward shift of the permafrost and the sediment load of some rivers increase, some line and the tree-line, which can then guide decisions beaches and cliffs may erode faster, coastal lowlands may about tailoring economic activity to minimize the effects become submerged, and frozen ground in the tundra of global environmental change. environments may thaw. Applied geomorphologists can address all these potentially damaging changes. Second, Other geomorphologies a worrying aspect of global warming is its effect on natural resources – water, vegetation, crops, and so on. Applied There are many other kinds of geomorphology, includ- geomorphologists, equipped with such techniques as ing tectonic geomorphology, submarine geomorphology, terrain mapping, remote sensing, and geographical infor- climatic geomorphology, and planetary geomorphology. mation systems, can contribute to environmental man- Tectonic geomorphology is the study of the interplay agement programmes. Third, applied geomorphologists between tectonic and geomorphic processes in regions are able to translate the predictions of global and regional where the Earth’s crust actively deforms. Advances in  WHAT IS GEOMORPHOLOGY? 13 the measurement of rates and in the understanding FORM of the physical basis of tectonic and geomorphic pro- cesses have revitalized it as a field of enquiry. It is The two main approaches to form in geomorphol- a stimulating and highly integrative field that uses ogy are description (field description and morphological techniques and data drawn from studies of geomor- mapping) and mathematical representation (geomor- phology, seismology, geochronology, structure, geodesy, phometry). and Quaternary climate change (e.g. Burbank and Anderson 2001). Field description and morphological Submarine geomorphology deals with the form, mapping origin, and development of features of the sea floor. Submarine landforms cover about 71 per cent of the The only way fully to appreciate landforms is to go Earth’s surface, but are mostly less well studied than into the field and see them. Much can be learnt from their terrestrial counterparts. In shallow marine envi- the now seemingly old-fashioned techniques of field ronments, landforms include ripples, dunes, sand waves, description, field sketching, and map reading and map sand ridges, shorelines, and subsurface channels. In the making. continental slope transition zone are submarine canyons The mapping of landforms is an art (see Dackombe and gullies, inter-canyon areas, intraslope basins, and and Gardiner 1983, 13–20, 28–41; Evans 1994). slump and slide scars. The deep marine environment con- Landforms vary enormously in shape and size. Some, tains varied landforms, including trench and basin plains, such as karst depressions and volcanoes, may be rep- trench fans, sediment wedges, abyssal plains, distributary resented as points. Others, such as faults and rivers, channels, and submarine canyons. are linear features that are best depicted as lines. In Planetary geomorphology is the study of landforms other cases, areal properties may be of prime concern on planets and large moons with a solid crust, for exam- and suitable means of spatial representation must be ple Venus, Mars, and some moons of Jupiter and Saturn. employed. Morphological maps capture areal properties. It is a thriving branch of geomorphology (e.g. Howard Morphological mapping attempts to identify basic 1978; Baker 1981; Grant 2000; Irwin et al. 2005). landform units in the field, on aerial photographs, or Surface processes on other planets and their satellites on maps. It sees the ground surface as an assemblage of depend materially on their mean distance from the Sun, landform elements. Landform elements are recognized which dictates the annual receipt of solar energy, on as simply curved geometric surfaces lacking inflections their rotational period, and on the nature of the plane- (complicated kinks) and are considered in relation to tary atmosphere. Observed processes include weathering, upslope, downslope, and lateral elements. They go by a aeolian activity, fluvial activity, glacial activity, and mass plethora of names – facets, sites, land elements, terrain wasting. components, and facies. The ‘site’ (Linton 1951) was Climatic geomorphology rests on the not uni- an elaboration of the ‘facet’ (Wooldridge 1932), and versally accepted observation that each climatic zone involved altitude, extent, slope, curvature, ruggedness, (tropical, arid, temperate for example) engenders a dis- and relation to the water table. The other terms were tinctive suite of landforms (e.g. Tricart and Cailleux coined in the 1960s (see Speight 1974). Figure 1.6 1972; Büdel 1982). Climate does strongly influence geo- shows the land surface of Longdendale in the Pennines, morphic processes, but it is doubtful that the set of England, represented as a morphological map. The map geomorphic processes within each climatic zone creates combines landform elements derived from a nine-unit characteristic landforms. The current consensus is that, land-surface model (p. 169) with depictions of deep- owing to climatic and tectonic change, the climatic fac- seated mass movements and superficial mass movements. tor in landform development is more complicated than Digital elevation models lie within the ambits of land- climatic geomorphologists have on occasions suggested form morphometry and are dealt with below. They have (cf. p. 389–90). greatly extended, but by no means replaced, the classic  14 Deep-seated mass movements Nine-unit land-surface model (after Dalrymple et al. 1968) Units1 (interfluves) and 7 (alluvial toeslopes) 20° Unit 6 (colluvial footslopes on sandstone) 10°–20° Slump Landslide (type 7 (colluvial footslopes on shale) 9°–14° earth flow undetermined) Units 8 (channel wall) and 9 (channel bed) not shown Superficial mass movements Geological formations I I Middle Grits 1. 2. 3. 4. II Kinderscout Grits III Grindlow Shales S IV Shale Grit 5. 6. II I 7. T 8. Other features T T F T 9. e T T T T 10. IV T S S INTRODUCING LANDFORMS AND LANDSCAPES 11. F III III c d F S S Land-surface model b for Longdendale S S S II 3 2 1 5 a S 6 3 4 5 N Longdendale Reservoirs 6 a Bottoms d Torside 5 98 7 0 1 2 km b Valehouse e Woodhead c Rhodeswood Figure 1.6 Morphological map of Longdendale, north Derbyshire, England. The map portrays units of a nine-unit land-surface model, types of mass movement, and geological formations. The superficial mass movements are: 1 mudflow, earthflow, or peat burst; 2 Soil slump; 3 Minor soil slump; 4 Rockfall; 5 Scree; 6 Solifluction lobe; 7 Terracettes; 8 Soil creep or block creep and soliflucted material. The other features are: 9 Incised stream; 10 Rock cliff; 11 Valley-floor alluvial fan. Source: After Johnson (1980)  WHAT IS GEOMORPHOLOGY? 15 work on landform elements and their descriptors as DEMs are, therefore, a subset of DTMs. Topographic prosecuted by the morphological mappers. elements of a landscape can be computed directly from a DEM (p. 170). Further details of DEMs and their appli- cations are given in several recent books (e.g. Wilson and Geomorphometry Gallant 2000; Huggett and Cheesman 2002). A branch of geomorphology – landform morphometry or geomorphometry – studies quantitatively the form of the land surface. Geomorphometry in the modern PROCESS era is traceable to the work of Alexander von Humboldt and Carl Ritter in the early and mid-nineteenth cen- Geomorphic systems tury (see Pike 1999). It had a strong post-war tradition Process geomorphologists commonly adopt a systems in North America and the UK, and it has been ‘rein- approach to their subject. To illustrate what this vented’ with the advent of remotely sensed images and approach entails, take the example of a hillslope system. Geographical Information Systems (GIS) software. The A hillslope extends from an interfluve crest, along a val- contributions of geomorphometry to geomorphology ley side, to a sloping valley floor. It is a system insofar as and cognate fields are legion. Geomorphometry is an it consists of things (rock waste, organic matter, and so important component of terrain analysis and surface forth) arranged in a particular way. The arrangement is modelling. Its specific applications include measuring the seemingly meaningful, rather than haphazard, because it morphometry of continental ice surfaces, characterizing is explicable in terms of physical processes (Figure 1.7). glacial troughs, mapping sea-floor terrain types, guiding The ‘things’ of which a hillslope is composed may be missiles, assessing soil erosion, analysing wildfire prop- described by such variables as particle size, soil moisture agation, and mapping ecoregions (Pike 1995, 1999). content, vegetation cover, and slope angle. These vari- It also contributes to engineering, transportation, public ables, and many others, interact to form a regular and works, and military operations. connected whole: a hillslope, and the mantle of debris on it, records a propensity towards reciprocal adjustment Digital elevation models among a complex set of variables. The complex set of variables include rock type, which influences weathering The resurgence of geomorphometry since the 1970s is rates, the geotechnical properties of the soil, and rates in large measure due to two developments. First is the of infiltration; climate, which influences slope hydrology light-speed development and use of GIS, which allow and so the routing of water over and through the hills- input, storage, and manipulation of digital data repre- lope mantle; tectonic activity, which may alter baselevel; senting spatial and aspatial features of the Earth’s surface. and the geometry of the hillslope, which, acting mainly Second is the development of Electronic Distance through slope angle and distance from the divide, influ- Measurement (EDM) in surveying and, more recently, ences the rates of processes such as landsliding, creep, the Global Positioning System (GPS), which made the solifluction, and wash. Change in any of the variables very time-consuming process of making large-scale maps will tend to cause a readjustment of hillslope form and much quicker and more fun. The spatial form of surface process. topography is modelled in several ways. Digital repre- sentations are referred to as either Digital Elevation Isolated, open, and closed systems Models (DEMs) or Digital Terrain Models (DTMs). A DEM is ‘an ordered array of numbers that represent Systems of all kinds are open, closed, or isolated accord- the spatial distribution of elevations above some arbitrary ing to how they interact, or do not interact, with their datum in a landscape’ (Moore et al. 1991, 4). DTMs are surroundings (Huggett 1985, 5–7). Traditionally, an iso- ‘ordered arrays of numbers that represent the spatial dis- lated system is a system that is completely cut off from tribution of terrain attributes’ (Moore et al. 1991, 4). its surroundings and that cannot therefore import or 16 INTRODUCING LANDFORMS AND LANDSCAPES Channel Valley-side slope Interfluve Wind erosion and deposition Waste mantle Debris Debris transport production Debris production Debris transport ro nt n gf th eri a We Uplift or subsidence Figure 1.7 A hillslope as a system, showing storages (waste mantle), inputs (e.g. wind deposition and debris production), outputs (e.g. wind erosion), throughputs (debris transport), and units (channel, valley-side slope, interfluve). export matter or energy. A closed system has bound- destroy it. The events between the creation and the final aries open to the passage of energy but not of matter. An destruction are what fascinate geomorphologists. open system has boundaries across which energy and Systems are mental constructs and have been defined materials may move. All geomorphic systems, including in various ways. Two conceptions of systems are impor- hillslopes, may be thought of as open systems as they tant in geomorphology: systems as process and form exchange energy and matter with their surroundings. structures, and systems as simple and complex structures (Huggett 1985, 4–5, 17–44). Internal and external system variables Geomorphic systems as form and process structures Any geomorphic system has internal and external vari- ables. Take a drainage basin. Soil wetness, streamflow, Three kinds of geomorphic system may be identified: and other variables lying inside the system are endoge- form systems, process systems, and form and process nous or internal variables. Precipitation, solar radiation, systems. tectonic uplift, and other such variables originating out- side the system and affecting drainage basin dynamics 1 Form systems. Form or morphological systems are are exogenous or external variables. Interestingly, all geo- defined as sets of form variables that are deemed to morphic systems can be thought of as resulting from interrelate in a meaningful way in terms of system ori- a basic antagonism between endogenic (tectonic and gin or system function. Several measurements could volcanic) processes driven by geological forces and exo- be made to describe the form of a hillslope system. genic (geomorphic) processes driven by climatic forces Form elements would include measures of anything (Scheidegger 1979). In short, tectonic processes create on a hillslope that has size, shape, or physical prop- land, and climatically influenced weathering and erosion erties. A simple characterization of hillslope form is  WHAT IS GEOMORPHOLOGY? 17 ( a ) Form system ( b ) Flow or cascading ( c ) Process–form or system process–response 1 2 3 system Cliff Rockfall Cliff Covered Talus cliff face Talus at time 2 Lower Lower Time slope slope 3 2 1 Figure 1.8 A cliff and talus slope viewed as (a) a form system, (b) a flow or cascading system, and (c) a process–form or process–response system. Details are given in the text. shown in Figure 1.8a, which depicts a cliff with a the system processes. A hillslope may be viewed in talus slope at its base. All that could be learnt from this way with slope form variables and slope process this ‘form system’ is that the talus lies below the cliff; variables interacting. In the cliff-and-talus example, no causal connections between the processes linking rock falling off the cliff builds up the talus store the cliff and talus slope are inferred. Sophisticated (Figure 1.8c). However, as the talus store increases characterizations of hillslope and land-surface forms in size, so it begins to bury the cliff face, reduc- may be made using digital terrain models. ing the area that supplies debris. In consequence, 2 Process systems. Process systems, which are also the rate of talus growth diminishes and the sys- called cascading or flow systems, are defined as tem changes at an ever-decreasing rate. The process ‘interconnected pathways of transport of energy or described is an example of negative feedback, which matter or both, together with such storages of energy is an important facet of many process–form systems and matter as may be required’ (Strahler 1980, 10). (Box 1.2). An example is a hillslope represented as a store of materials: weathering of bedrock and wind deposi- Geomorphic systems as simple or complex tion add materials to the store, and erosion by wind structures and fluvial erosion at the slope base removes mate- rials from the store. The materials pass through the Three main types of system are recognized under this system and in doing so link the morphological com- heading: simple systems, complex but disorganized ponents. In the case of the cliff and talus slope, it systems, and complex and organized systems. could be assumed that rocks and debris fall from the cliff and deliver energy and rock debris to the talus 1 Simple systems. The first two of these types have below (Figure 1.8b). a long and illustrious history of study. Since at 3 Form and process systems. Process–form systems, least the seventeenth-century revolution in science, also styled process–response systems, are defined astronomers have referred to a set of heavenly bod- as an energy-flow system linked to a form system in ies connected together and acting upon each other such a way that system processes may alter the system according to certain laws as a system. The Solar form and, in turn, the changed system form alters System is the Sun and its planets. The Uranian 18 INTRODUCING LANDFORMS AND LANDSCAPES Box 1.2 NEGATIVE AND POSITIVE FEEDBACK Negative feedback is said to occur when a change in a system sets in motion a sequence of changes that eventually neutralize the effects of the origi- nal change, so stabilizing the system. An example occurs in a drainage basin system, where increased channel erosion leads to a steepening of valley- side slopes, which accelerates slope erosion, which increases stream bed-load, which reduces channel erosion (Figure 1.9a). The reduced channel erosion then stimulates a sequence of events that stabilizes the system and counteracts the effects of the orig- inal change. Some geomorphic systems also display positive feedback relationships characterized by an original change being magnified and the system being made unstable. An example is an eroding hillslope where the slope erosion causes a reduction in infil- tration capacity of water, which increases the amount of surface runoff, which promotes even more slope Figure 1.9 Feedback relationships in geomorphic erosion (Figure 1.9b). In short, a ‘vicious circle’ is cre- systems. (a) Negative feedback in a valley-side slope– ated, and the system, being unstabilized, continues stream system. (b) Positive feedback in an eroding changing. hillslope system. Details of the relationships are given in the text. system is Uranus and its moons. These structures regarded as a complex but rather disorganized system. may be thought of as simple systems. In geomor- In both the gas and the hillslope mantle, the interac- phology, a few boulders resting on a talus slope tions are somewhat haphazard and far too numerous may be thought of as a simple system. The condi- to study individually, so aggregate measures must tions needed to dislodge the boulders, and their fate be employed (see Huggett 1985, 74–7; Scheidegger after dislodgement, can be predicted from mechan- 1991, 251–8). ical laws involving forces, resistances, and equations 3 In a third and later conception of systems, objects of motion, in much the same way that the motion of are seen to interact strongly with one another to the planets around the Sun can be predicted from form systems of a complex and organized nature. Newtonian laws. Most biological and ecological systems are of this 2 In a complex but disorganized system, a vast num- kind. Many structures in geomorphology display ber of objects are seen to interact in a weak and high degrees of regularity and rich connections, and haphazard way. An example is a gas in a jar. This may be thought of as complexly organized systems. system might comprise upward of 1023 molecules A hillslope represented as a process–form system colliding with each other. In the same way, the count- could be placed into this category. Other examples less individual particles in a hillslope mantle could be include soils, rivers, and beaches.  WHAT IS GEOMORPHOLOGY? 19 when all slopes, both hillslopes and river slopes, are Geomorphic system dynamics: equilibrium adjusted to each other (p. 10). In practice, this early and steady state notion of dynamic equilibrium was open to ques- As defined by John T. Hack, a steady-state land- tion (e.g. Ollier 1968) and difficult to apply to scape is one in which land-surface form stays the landscapes. In consequence, other forms of equilib- same despite tectonic uplift adding material and a rium were advanced (Howard 1988) (Figure 1.10). Of constant set of geomorphic processes removing it. these, dynamic metastable equilibrium has proved An erosional landscape in dynamic equilibrium arises to be salutary. It suggests that, once perturbed by Figure 1.10 Types of equilibrium in geomorphology. (a) Static equilibrium occurs when a system is in balance over a time period and no change in state occurs. (b) Stable equilibrium records a tendency to revert to a previous state after a small disturbance. (c) Unstable equilibrium occurs when a small disturbance forces a system towards a new equilibrium state where stabilization occurs. (d) Metastable equilibrium arises when a system crosses an internal or external system threshold (p. 20), so driving it to a new state. (e) Steady state equilibrium obtains when a system constantly fluctuates about a mean equilibrium state. (f ) Thermodynamic equilibrium is the tendency of some systems towards a state of maximum entropy, as in the gradual dissipation of heat by the Universe and its possible eventual ‘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 definite direction (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) 20 INTRODUCING LANDFORMS AND LANDSCAPES Box 1.3 THRESHOLDS A threshold separates different states of a system. It change in an external variable. A prime example is the marks some kind of transition in the behaviour, oper- response of a geomorphic system to climatic change. ation, or state of a system. Everyday examples abound. Climate is the external variable. If, say, runoff were Water in a boiling kettle crosses a temperature thresh- to increase beyond a critical level, then the geomor- old in changing from a liquid to a gas. Similarly, ice phic system might suddenly respond by reorganizing taken out of a refrigerator and placed upon a table itself into a new state. No change in an external vari- in a room with an air temperature of 10◦ C will melt able is required for a geomorphic system to cross an because a temperature threshold has been crossed. In internal threshold. Rather, some chance fluctuation both examples, the huge differences in state – liquid in an internal variable within a geomorphic system water to water vapour, and solid water to liquid water – may take a system across an internal threshold and may result from tiny changes of temperature. Many lead to its reorganization. This appears to happen geomorphic processes operate only after the crossing in some river channels where an initial disturbance of a threshold. Landslides, for instance, require a criti- by, say, overgrazing in the river catchment triggers cal slope angle, all other factors being constant, before a complex response in the river channel: a compli- they occur. Stanley A. Schumm (1979) made a power- cated pattern of erosion and deposition occurs with ful distinction between external and internal system phases of alluviation and downcutting taking place thresholds. A geomorphic system will not cross an concurrently in different parts of the channel system external threshold unless it is forced to do so by a (see below). environmental changes or random internal fluctuations change is seen as a simple response to an altered input. that cause the crossing of internal thresholds (Box 1.3), It shows that landscape dynamics may involve abrupt a landscape will respond in a complex manner (Schumm and discontinuous behaviour involving flips between 1979). A stream, for instance, if it should be forced away quasi-stable states as system thresholds are crossed. from a steady state, will adjust to the change. However, The latest views on landscape stability (or lack of it) the nature of the adjustment may vary in different parts come from the field of dynamic systems theory, of the stream and at different times. Douglas Creek in which embraces the buzzwords complexity and chaos. western Colorado, USA, was subject to overgrazing dur- The argument runs that steady states in the landscape ing the ‘cowboy era’ (Womack and Schumm 1977). It may be rare because landscapes are inherently unsta- has been cutting into its channel bed since about 1882. ble. This is because any process that reinforces itself The manner of incision has been complex, with discon- keeps the system changing through a positive feed- tinuous episodes of downcutting interrupted by phases back circuit and readily disrupts any balance obtain- of deposition, and with the erosion–deposition sequence ing in a steady state. This idea is formalized as an varying from one cross-section to another. Trees have ‘instability principle’, which recognizes that, in many been used to date terraces at several locations. The ter- landscapes, accidental deviations from a ‘balanced’ con- races are unpaired (p. 236), which is not what would dition tend to be self-reinforcing (Scheidegger 1983). be expected from a classic case of river incision, and This explains why cirques tend to grow, sinkholes they are discontinuous in a downstream direction. This increase in size, and longitudinal mountain valley profiles kind of study serves to dispel for ever the simplistic become stepped. The intrinsic instability of landscapes cause-and-effect view of landscape evolution in which is borne out by mathematical analyses that point to the  WHAT IS GEOMORPHOLOGY? 21 chaotic nature of much landscape change (e.g. Phillips provisionally until further field work was carried out, 1999; Scheidegger 1994). Jonathan D. Phillips’s (1999, that events occurring once or twice a year perform most 139–46) investigation into the nature of Earth surface geomorphic work (Wolman and Miller 1960). Some later systems, which includes geomorphic systems, is par- work has highlighted the geomorphic significance of rare ticularly revealing and will be discussed in the final events. Large-scale anomalies in atmospheric circulation chapter. systems very occasionally produce short-lived super- floods that have long-term effects on landscapes (Baker 1977, 1983; Partridge and Baker 1987). Another study Magnitude and frequency revealed that low-frequency, high-magnitude events Interesting debates centre on the variations in process greatly affect stream channels (Gupta 1983). rates through time.The ‘tame’ end of this debate concerns The ‘wilder’ end engages hot arguments over gradual- arguments over magnitude and frequency (Box 1.4), the ism and catastrophism (Huggett 1989, 1997a, 2006). pertinent question here being which events perform The crux of the gradualist–catastrophist debate is the the most geomorphic work: small and infrequent events, seemingly innocuous question: have the present rates of medium and moderately frequent events, or big but rare geomorphic processes remained much the same through- events? The first work on this issue concluded, albeit out Earth surface history? Gradualists claim that process Box 1.4 MAGNITUDE AND FREQUENCY As a rule of thumb, bigger floods, stronger winds, where T is the recurrence interval, n is the number higher waves, and so forth occur less often than their of years of record, and m is the magnitude of the smaller, weaker, and lower counterparts. Indeed, graphs flood (with m = 1 at the highest recorded discharge). showing the relationship between the frequency and Each flood is then plotted against its recurrence inter- magnitude of many geomorphic processes are right- val on Gumbel graph paper and the points connected skewed, which means that a lot of low-magnitude to form a frequency curve. If a flood of a particu- events occur in comparison with the smaller number lar magnitude has a recurrence interval of 10 years, of high-magnitude events, and a very few very high- it would mean that there is a 1-in-10 (10 per cent) magnitude events. The frequency with which an event chance that a flood of this magnitude (2,435 cumecs of a specific magnitude occurs is expressed as the return in the Wabash River example shown in Figure 1.11) period or recurrence interval. The recurrence inter- will occur in any year. It also means that, on average, val is calculated as the average length of time between one such flood will occur every 10 years. The magni- events of a given magnitude. Take the case of river tudes of 5-year, 10-year, 25-year, and 50-year floods floods. Observations may produce a dataset comprising are helpful for engineering work, flood control, and the maximum discharge for each year over a period of flood alleviation. The 2.33-year flood (Q2.33 ) is the years. To compute the flood–frequency relationships, mean annual flood (1,473 cumecs in the example), the peak discharges are listed according to magnitude, the 2.0-year flood (Q2.0 ) is the median annual flood with the highest discharge first. The recurrence interval (not shown), and the 1.58-year flood (Q1.58 ) is the is then calculated using the equation most probable flood (1,133 cumecs in the example). n+1 T = m 22 INTRODUCING LANDFORMS AND LANDSCAPES 4,000 3 Q50 = 3,398 m /sec 3 Q25 = 2,973 m /sec Discharge, Q (m /s, also called cumecs) 3,000 3 Q10 = 2,435 m /sec 3 Q5 = 2,011 m /sec 2,000 3