PRINCIPLES OF PHYSICAL GEOLOGY ARTHUR HOLMES D.Se. F.R.S. Regius Professor of Geology and Mineralogy Universiry qf Edinburgh 95 Plates and 262 Text Illustrations I THOMAS NELSON AND SONS LTD LONDON EDINBURGH PARIS MELBOURNE TORONTO AND NEW YORK 1+ l~ !i r/'-07 First plll>1isl1cd September 1944 Reprinted 1945\/ PREFACE • LIKE many other teachers of geology and geography in this country, I have long felt the need far a thoroughly up-to-da,te book qn Physical Geology. I theref0re readily responded when the publishers invited me to write the sbrt of book that seemed to be required. The method of treatment adopted is one that twenty years' experience has shown to be successful in training students and- in holding and developing their interest, even when, as has often been the case, some of them have come to the subject without any preliminary acquaintance with scien­ tific principles ~A methods. For this reason it is hoped that the book will app~l )ioConly to students and teachers and to the senior classes in schools, but also to the general reader who wishes to see something of the "wild miracle" of the world we live in through the eyes of those who have "tried to resolve Its ancient mysteries. The earth's activities may be compared to an intricate interplay of combined operations, and the results, whether they be landscapes, natural catastrophes, or materials such as building stones and fuels, are correspondingly varied. It is inherent in the character of the subject that t}1.e full significance of anyone aspect can ,be properly appreciated only in relation to the whole. A broad preliminary survey has therefore been presented in Part I, to serve as an.introduction to the more detailed treatment that follows. Part II de.als with the outer earth and only turns aside from tracks already familiar to take in the· more interesting results achieved by re_cent progress. Part III is mainly concerned with the activities of the inner earth and their surface expressions. Thanks to the variety of . detective methods that have been developed during the present century, the depths are less inscrutable than formerly and the time is ripe for the incorporation into an elementary book of the many spectacular discoveries that have already been made. But even so, the frontiers of established knowledge are soon reached, and what we think we see by peering beyond is me~ely " PREFACE lnterpretation, and must on rio account be confused with the ;olid facts of observation. WHile I have not hesitated to intro­ duce current views, since these reveal the active growth of the subject, it should be clearly te~lized that topics such as the cause of mountain building, th,e source of volcanic activity and the possibility of continental drift remain controversial just because the guiding facts are still too few. to provide a foundation for more than tentative hypotheses. It is my hope that recogni­ tion of some of the outstanding problems may stimulate at least a few of my readers to co-operate in the attempt, to solve them. Professor Alfred Brammall, Dr. Leonard Hawkes and my wife (Dr. Doris L. Reynolds) have given generous assistance by reading the first draft of the book, and I gratefully acknowledge that the subsequent revision owes much to their constructive criticism and helpful sugges~ions. For any defects ·that still remain-whether of fact, treatment, judgment or style-I am, of course, entirely responsible. As befits the subject, special care has been taken to illustrate the book as ful~y and effectively as possible. Of the 262 text­ figures over two hundred have been specially drawn. Some of the figures are original, others are based on dIagrams already: published, and a few have been directly reproduced with due acknowledgment. In addition to the photographs procured from professional photographers and press agencies, many more have been. contributed by various friends and official organizations. It is a pleasure to record my cordial thanks to the following : The Director of H.M. Geological Survey, for fifty subjects selected from the Survey's unrivalled collection of British geological photographs. Copyright of these is reserved by the Crown, and this has been speCifically stated beneath eleven of the reproductions, to indicate that they are included by per­ mission of the Controller of H.~. Stationery, Office. The Director of the United States Geological Survey, and also the National Park Service and the Department of the Interior of the United States, for providing batches of photo­ graphs from which many striking and instructive subjects have been selected. I , VI PREFACE The Director-Gen~ral of the South Afriqm Air Force, for supplying the originals of Figs. 69 and 81 and Plate 87A, which are "air photographs published under the Union of South Africa Government Printer's Copyright Authority No. 489 of 12.11.42," and of Plates 30, 45A and 88, which were taken by the Aircraft Operating Company of South Africa .Limited. The Curator of the Belfast Municipal Museum and Art Gallery for permission to use three photographs (Plates 8A, 17A, and 35B) from the well-known series taken by the late Mr.. R. Welch. The Geological P.hotographs Committee of the British Association (Fig. 18 and Plate 74B) ; the Egyptian Government (Plate 62A and B); the Burma Forest Service (Plate 75B) ; and the Burmah Oil Company Limited (Plate 77B). A word of personal appreciation is due to Mr. F. N. Ashcroft for the privilege of using ten of his superb geological photographs. Other friends and correspondents to whom I am indebted for illustrative material are Capt:]. Brown, Mr. F.- A. Bannister, Dr. H. S. Bell, Dr. A. ]. Bull, Dr. A. M. Cockburn, Dr. R. M. Craig, Mr. A. D. Combe, Prof. R. A. Daly, Dr. D. Griggs, Dr. L. Hawkes, Prof. H. G. A. Hickling, Dr. ·W. F. Hume, Prof. A. Las;roix, Dr. C. E. Marshall, Mr. G. O'Neill, Prof. S. H. _Reynolds, the late Dr. R. W. Sayles, Mr. G. S. Sweeting, Dr. C. T. Trechmann, Dr. G. W. Tyrrell, Prof. W. W. Watts, Mr. E.]. Wayland and Dr. C. E. Wegmann. It will be noticed that the illustrations are not 'listed in the preliminary pages. When such lists become unduly long, as would here have been the case, they tend to defeat their pur­ pose. As an ~lternative which it is -hoped will facilitate easy reference, the plates and text-figures are included in the index. I ARTHUR HOLMES DURHAM EDINBURGH July 1942 May ~944 Vll CONTENTS 'PART I A PRELIMINARY SURVEY I INTRODUCTION. Interpretations of Nature: Ancient and Modern The Major Fields of Scientific Study . 3 The Scope and Subdivisions of Geology. 5 II THE SHAPE AND SURFACE RELIEF OF THE EARTH 8 The Outer Zones of the Earth 8 Continents and Ocean Floors 10 The Shape of the Earth . 13 Isostasy . 15 The Distribution of Land and S,ea 17 III THE CHANGING FACE OF'THE EARTH 22 Weathering, Erosion, and Denudation. 22 Deposition of Sediment . 24 The Importance of Time 25 Earth Movements .. 26 Volcanic and Igneous Activity 28 Metamorphism of Rocks . 30 Summary of the Geological Processes 30 Isostasy and GeologiCal Processes . 32 IV MATERIALS OF THE 'EARTH'S CRUST: MINERALS 35 Elements and Crystals 35 Rock-forming Minerals 38 J MATERIALS QF THE EARTH'S CRUST: COMMON "ROCKS . 45 ~ IGNEOUS ROCKS ' 45 , Granite 45 Basalt . 47 Classification of the Common Igneous Rocks 49/ j'SEDIMENTARY ROCKS • 53 Sandstone and Shale 53 Limestones . 55 j METAMORPHIC ROCKS • • • • 57 Marble and Crystalline l.imestones 57 - Slate . 60 Kinds of Metamorphism 61 Crystalline Schists. 63 , Gneisses and Migmatite; 64 The Cycle of Rock ~hange 65 .,_ VI ARCHITECTURAL FEATURES OF THE EARTH'S CRUST 69 Fracture, Flowage, and Flow. 69 Folds. 71 Joints, 75 Faults . 78 Modes of Occurrence of Igneous Intrusions 82 The Emplacement of Batholiths 90 ix COJ'ifTENTS VII ROCKS AS THE PAGES OF EARTH HISTORY. 93 The Key to the Past . 93 The Succession of Strata I 97 The Significance of Fossils .. 99 The Geological Time Scal<;. 101 Igneous Rocks and the Geological. Time Scale . 102 Earth Movements and the 'Geological Time Scale 106 PART II EXTERNAL PROCESSES AND THEIR EFFECTS VIII ROCK WEATHERING AND SOILS . 112 Weathering and Climate . 112 Disintegration by Temperature Changes 113 The Role of Animals and Plants 115 Chemical Weathering 116 Weathering Residues . 119 The Mantle of Rock-waste . 121 The Growth and Nature of Soils 122 IX UNDERGROUND WATERS Sources of Ground-water . The Storage and Circulation of Ground-water Springs and Wells. Swallow Holes and Limestone Caverns Hot Springs and Geysers . Deposition from Ground~waters X RIVER ACTION AND VALLEY_ DEVELOPMENT. Some General Considerations Rain Erosion. Soil-creep and Landslides . Erosion and Transport by Rivers . .. Lengthening and Deepening of Valleys Grading of Rivers. , Waterfalls. Widening of Valleys Meanders. Flood Plains Deltas XI DEVELOPMENT OF RIVER SYSTEMS AND ASSOCI,\TED LANDFORMS 173 Tributaries and Drainage Patterns 173 Shifting of Divides and River Capture. 175 Escarpments and Related Features"" " 179 Superimposed Drainage. 182 The Normal Cycle of Erosion . 185 The Isostatic Response to Den<tdation . 189 Uplifted Peneplains. 191 Interruptions in the Qycle of Erosion . 194 River Terraces. 195 Incised Meanders and Gorges . 197 Antecedent Drainage of the Himalayas 200 Alluvial Fans and Gones . 201 X / CONTENTS XII GLACIERS AND GLACIATION./ Snow Fields and the Maintenance of Glaciers 204 Types of Glaciers . 206 The Movement of Glaciers 2 0 9 Surface Features of Glaciers 211 Glacial Erosion 214 Corries and Associated Features. 218 Modifications of Valleys by -Glacial Erosion 220 Glacial Deposits . 226 Gfaciofluvial Deposits .