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Ü Structural Landforms S3 STRUCTURAL ü LANDFORMS C. R. Twidale S&v * ' - NN . ^ . - % - ia w sT4C! P^% „ * * * a i t V - W**5uM ■ fS-T fc: 'r. Jjgg* • r_-i •'.-« g w Up to the end of last century, landforms were viewed largely as expressions of the structure of the earth’s underlying crust. But such interpretations were concerned for the most part with generalities and broad effects; the subtleties of structural factors became overshadowed first by cyclic ex­ planations and then by the modern emphasis on process and climatic geomorphology. This book arose from the neglect of struc­ tural factors in geomorphological interpre­ tation. Nowadays it is recognised that details of jointing and faulting, both past and present, of the stresses in folds, of past conditions of sedimentation, all play an important part in the determination of present landforms. Moreover, today’s geo­ morphologists must think in terms not only of distribution—length and breadth—but also in terms of vertical and temporal change. The author brings this new thinking into Structural Landforms and the result is a book of great interest and importance to students of geography and geology, to teachers and professional geomorphologists. It is particularly rich in photographs and line figures, and includes an excellent biblio­ graphy. Price in Australia $5.00 This book was published by ANU Press between 1965–1991. This republication is part of the digitisation project being carried out by Scholarly Information Services/Library and ANU Press. This project aims to make past scholarly works published by The Australian National University available to a global audience under its open-access policy. AN INTRODUCTION TO SYSTEMATIC GEOMORPHOLOGY The following volumes have been published: 3 Land forms of Cold Climates J. L. Davies 4 Coasts E. C. F. Bird 6 Volcanoes Cliff Ollier An Introduction to Systematic Geomorphology VOLUME FIVE Structural Landforms Landforms associated with granitic rocks, faults, and folded strata C. R. TWIDALE 1971 AUSTRALIAN NATIONAL UNIVERSITY PRESS CANBERRA © Charles Rowland Twidale 1971 This book is copyright. Apart from any fair dealing for the pur­ poses of private study, research, criticism, or review, as permitted under the Copyright Act, no part may be reproduced by any process without written permission. Inquiries should be made to the publisher. National Library of Australia Card no. and ISBN 0 7081 0733 8 Library of Congress Catalog Card no. 70-10505 Registered in Australia for transmission by post as a book For Armin Alexander öpik INTRODUCTION TO THE SERIES This series is conceived as a systematic geomorphology at university level. It will have a role also in high school education and it is hoped the books will appeal as well to many in the community at large who find an interest in the why and wherefore of the natural scenery around them. The point of view adopted by the authors is that the central themes of geomorphology are the characterisation, origin, and evolution of landforms. The study of processes that make land­ scapes is properly a part of geomorphology, but within the present framework process will be dealt with only in so far as it elucidates the nature and history of the landforms under discussion. Certain other fields such as submarine geomorphology and a survey of general principles and methods are also not covered in the volumes as yet planned. Some knowledge of the elements of geology is presumed. Four volumes will approach landforms as parts of systems in which the interacting processes are almost completely motored by solar energy. In humid climates (Volume One) rivers dominate the systems. Fluvial action, operating differently in some ways, is largely responsible for the landscapes of deserts and savannas also (Volume Two), though winds can become preponderant in some deserts. In cold climates, snow, glacier ice, and ground ice come to the fore in morphogenesis (Volume Three). On coasts (Volume Four), waves, currents, and wind are the prime agents in the complex of processes fashioning the edge of the land. Three further volumes will consider the parts played passively by the attributes of the earth’s crust and actively by processes deriving energy from its interior. Under structural landforms (Volume Five), features, immediately consequent on earth move­ ments and those resulting from tectonic and lithologic guidance of denudation are considered. Landforms directly the product of volcanic activity and those created by erosion working on volcanic vii viii Introduction to the Series materials are sufficiently distinctive to warrant separate treatment (Volume Six). Though karst is undoubtedly delimited lithologically, it is fashioned by a special combination of processes centred on solution so that the seventh volume partakes also of the character of the first group of volumes. J. N. Jennings General Editor ACKNOWLEDGMENTS The production of any book involves far more than its writing. It entails much tedious and painstaking work, and I wish to take this opportunity to thank those several persons who, directly or in­ directly, and in numerous ways, have helped with the preparation of Structural Landforms. Many colleagues and friends have kindly supplied photographs, many of which, for reasons of space, have not been utilised. Only one request for illustrative material was directly refused, and, whether the prints supplied were used or not, I wish to thank those who went to so much trouble on my behalf. I am grateful to the many organisations and individuals who kindly gave permission to use photographs and illustrations, and whose assistance is acknow­ ledged in the text. Janet Saies and Janeen Nicol read the first draft, justifying their interest with the modest claim that if they could understand the text then anyone could. They could not follow all of it, and I am grateful to them for drawing my attention to ambiguous and con­ fused passages, for their expenditure of time, and their humour and enthusiasm. Tim Hopwood has drawn my attention to certain anachronisms in my use of structural terms, and George Sved has, over the years, done much to disembarrass my mind of some, though I fear not all, mechanically unsound ideas. Yngvar Isaacson and Bruce Curtis have generously made available their expert knowledge of certain North American areas, and I have profited greatly from discussion of particular problems in structural geomor­ phology with Jim Talbot, Edwin Hills, Rudi Horwitz, Armin Öpik, and Heli Wopfner. The onerous task of translating my rough sketches into line draw­ ings fell to John Heyward, of the Department of Human Geography, Institute of Advanced Studies, Australian National University. Apart from the field sketches, he is responsible for all the figures, and the skill and flair he has brought to his task are obvious to all. ix X Acknowledgments To John and to his superiors, who so generously and understand­ ing^ allowed me the use of his services, I am most grateful. My wife, Kate, quite apart from encouraging me to continue with the writing of the book in the face of more than usually difficult and trying circumstances, with her customary skill, patience and instinct, deciphered my scrawl, unravelled my amendments, and from it all produced a typescript. I am the more appreciative of her help because I know how difficult it was for her to fit in the typing between running a household, caring for a family, and sundry other good works. Most of all, however, I must express my gratitude to the General Editor of this series, J. N. Jennings. Joe has for many years now given me much help and good advice, but never so much as in the preparation of this book. For providing the opportunity to par­ ticipate in the series, for his liaison work between author and draftsman, for his tact, patience, encouragement, and understanding, for his many helpful suggestions, and, most of all, for his scholarly and critical mind, I thank him. Adelaide C.R.T. September 1969 CONTENTS page Introduction to the Series, J. N. Jennings vii Acknowledgments ix I INTRODUCTION 1 II JOINTS, BOULDERS, AND RELATED FEATURES 4 Geological character of granite 4 Jointing and its significance 9 Terminology 14 Outcrops with closely spaced fractures 18 Differential subsurface weathering and the origin of boulders 20 Boulders of subaerial origin 39 Minor features developed on boulders 41 III CHARACTERISTICS AND ORIGIN OF INSELBERGS 45 Shape in plan 50 Compartment weathering and the two-stage evolution of inselbergs 50 Other hypotheses of inselberg evolution 55 Inselberg profiles 59 Lamination: distribution and origin 77 Minor forms evolved on inselbergs 80 IV LANDFORMS ASSOCIATED WITH FAULTS 97 Geological aspects of faulting 97 Landforms related to faulting 109 Examples of fault-dominated landscapes 138 xi xii Contents' page V LANDFORMS EVOLVED ON FOLDED SEQUENCES 151 Varied resistance of rocks 151 Plateau, mesa, and butte assemblages 154 Types and geometry of folds 164 Tectonic forms: folded or warped land surfaces 173 Structural forms: forms eroded from folded sequences 187 Examples of folded terrains 195 Drainage patterns of fold belts 206 VI CONCLUSION 217 Bibliography 219 Index 233 FIGURES , ^ page 1 Occurrence of granitic rocks in Australia 5 2 Section through the Cornubian granite pluton 7 3 Cloos’s diagram showing the chief types of joints occurring in a batholith 11 4 Field sketches of Dartmoor tors 13 5 Granite outcrops of the eastern margin of the Mt Lofty Ranges, South Australia 18 6 Section showing all slopes topography developed on granite in Peru 19
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