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CONTENTS

PREFACE

ACKNOWLEDGMENTS

1 INTRODUCTION

1.1 Outline and Approach

1.2 Field Behaviour

1.3 Safety

1.4 Ancillary Skills

1.5 A Few Words of Comfort

2 FIELD EQUIPMENT

2.1 Hammers and Chisels

2.2 Compasses and Clinometers

2.3 Handlenses

2.4 Tapes

2.5 Map Cases

2.6 Field Notebooks

2.7 Scales

2.8 Protractors

2.9 Pencils and Erasers

2.10 Acid Bottles

2.11 Global Positioning System (GPS)

2.12 Other Instruments

2.13 Field Clothing

3 GEOLOGICAL MAPS AND BASE MAPS

3.1 Types of Geological Map

3.2 Topographic Base Maps

3.3 Geographic Coordinates and Metric Grids

3.4 Position Finding on Maps

3.5 Magnetic Declination

3.6 Planetable Mapping

3.7 Aerial Photographs

3.8 Satellite Imagery

4 METHODS OF GEOLOGICAL MAPPING

4.1 Traversing

4.2 Following Contacts

4.3 Exposure or Green Line Mapping

4.4 Mapping in Poorly Exposed Regions

4.5 Superficial Deposits

4.6 Drilling

4.7 Geophysical Aids to Mapping

4.8 Large-scale Maps of Limited Areas

4.9 Underground Mapping

4.10 Photogeology

5 FIELD MEASUREMENTS AND TECHNIQUES

5.1 Measuring Strike and Dip

5.2 Plotting Strike and Dip

5.3 Recording Strike and Dip

5.4 Measuring Linear Features

5.5 Folds

5.6 Faults

5.7 Thrusts and Unconformities

5.8 Joints

5.9 Map Symbols

5.10 Specimen Collecting

5.11 Field Photography

5.12 Panning

6 ROCKS, FOSSILS AND ORES

6.1 Rock Descriptions

6.2 Identifying and Naming Rocks in the Field

6.3 Litho-stratigraphy and Sedimentary Rocks

6.4 Fossils

6.5 Phaneritic Igneous Rocks

6.6 Aphanitic Igneous Rocks

6.7 Veins and Pegmatites

6.8 Igneous Rocks in General

6.9 Pyroclastic Rocks

6.10 Metamorphic Rocks

6.11 Economic Geology

7 FIELD MAPS AND FIELD NOTEBOOKS

7.1 Field Maps

7.2 Field Notebooks

8 FAIR COPY MAPS AND OTHER ILLUSTRATIONS

8.1 Fair Copy Maps

8.2 Transferring Topography

8.3 Transferring Geology

8.4 Lettering and Symbols

8.5 Formation Letters

8.6 Layout

8.7 Colouring

8.8 Cross-sections

8.9 Overlays

8.10 Computer Drafting of the Fair Copy Map

8.11 Text Illustrations

9 CROSS-SECTIONS AND THREE-DIMENSIONAL ILLUSTRATIONS

9.1 Cross-sections

9.2 Plotting and Drawing Cross-sections

9.3 Three-dimensional Illustrations

9.4 Models

10 GEOLOGICAL REPORTS

10.1 Preparation

10.2 Revision and Editing

10.3 Layout

10.4 Introduction

10.5 Main Body of the Report

10.6 Conclusions

10.7 References

10.8 Appendices

APPENDIX I: SAFETY IN THE FIELD

1.1 Emergency Kit

1.2 Distress Signals

1.3 Exposure

1.4 Lightning

1.5 Health in Warm Climates

1.6 Students in the Field

APPENDIX II: ADJUSTMENT OF A CLOSED COMPASS TRAVERSE

APPENDIX III: GEOLOGICAL PLANETABLING

Appendix IV: FIELD EQUIPMENT CHECKLIST

APPENDIX V: USEFUL CHARTS AND TABLES

REFERENCES AND FURTHER READING

SUBLIMENTAL IMAGES

INDEX

The Geological Field Guide Series

Basic Geological Mapping, Fourth edition John Barnes

The Field Description of Metamorphic Rocks Norman Fry

The Mapping of Geological Structures Ken McClay

Field Geophysics, Third edition John Milsom

The Field Description of Igneous Rocks Richard Thorpe & Geoff Brown

Sedimentary Rocks in the Field, Third edition Maurice Tucker

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PREFACE

This book is a basic guide to field techniques used in geological mapping. It is meant to be kept in camp with you and even carried in your rucksack in the field. In addition, because no piece of geological mapping can be considered complete until the geology has been interpreted and explained, chapters are provided on drawing cross-sections; on preparing and presenting ‘fair copy’ maps; and on presenting geological diagrams from your fieldwork suitable for inclusion in your report. A report explaining the geology is an essential part of any field project and a brief chapter on the essentials for writing and illustrating it concludes this book. Some emphasis, too, is given to field sketch-mapping because many reports lack those large-scale detailed maps of small areas which can often explain complex aspects of the geology that cannot be shown on the scale of the field map being used, and which are difficult to describe in words. Attention is also given to field notebooks which are, in many cases, deplorable.

It is assumed that readers of this book have already had at least one year of university or equivalent geology, and have already been told what to look for in the field. Geological mapping cannot, however, be taught in lectures and the laboratory: it must be learnt in the field. Unfortunately, only too often, trainee geologists are left largely to their own devices, to sink or swim, and to learn to map for themselves with a minimum of supervision on ‘independent’ mapping projects. It is hoped that this book will help them in that task.

John W. Barnes, Richard J. Lisle, 2003

ACKNOWLEDGMENTS

It is impossible to name everyone who has been associated with the produc­tion of this book. Among those who deserve special mention with the text or diagrams are the late D. V. Ager, M. G. Coulson, F. R. Cross, A. R. Gar­diner, R. H. Graham, S. J. Matthews, the late T. R. O. Owen, P. Styles, and C. Tomlinson, all at that time of the University of Wales Swansea; also P. J. Brabham and A. Rogers of Cardiff University; M. H. de Freitas of Imperial College; and also my partner on eleven international mapping programmes in Asia and Africa, the late Edgar H. Bailey of the United States Geological Survey. Appreciation is also due to the Swansea University Earth Science Departmental drawing office staff, and particularly to the late Mrs V. M. Jenkins who typed the original drafts and to my late wife for reading the typescript and original proofs. I also wish to thank Richard Lisle for his help in preparing this fourth edition.

John W. Barnes, 2003

1

INTRODUCTION

There are many kinds of geological map, from small-scale reconnaissance surveys to large-scale detailed underground mine maps and engineering site plans, and each needs a different technique to make. Here, however, we are concerned only with the rudiments of geological mapping. The intention is to provide basic knowledge which can be built upon. We cannot tell you everything you need to know but we hope we can stimulate your imagination so that you can adapt your methods to most prevailing field conditions and to the scale and quality of your topographic base maps and, where necessary, to develop and devise new methods of your own. As a geologist, you must also remember that accurate geological maps are the basis of all geological work, even laboratory work, for it is pointless to make a detailed investigation of a specimen whose provenance is uncertain. As Wallace said in a 1975 Jacklin lecture: ‘There is no substitute for the geological map and section – absolutely none. There never was and there never will be. The basic geology still must come first – and if it is wrong, everything that follows will probably be wrong.’

1.1 Outline and Approach

This book is arranged in what is hoped is a logical order for those about to go into the field on their first ‘independent’ mapping project. First it describes the equipment you will need; then you are introduced to the many types of geological map you may have to deal with some time during your professional career. A description follows of the different kinds of topographic base maps which may be available for you to plot your geological observations on in the field. Methods to locate yourself on a map are also described and advice is given on what to do if no topographic base maps at all are obtainable.

The next three chapters describe methods and techniques used in geological mapping, including a brief description of photogeology; that is the use of aerial photographs in interpreting geology on the ground. A further chapter is devoted to the use of field maps and those much neglected items, field notebooks.

The last three chapters concern ‘office work’, some of which may have to be done whilst still at your field camp. They cover methods of drawing cross-sections and the preparation of other diagrams to help your geological interpretation. Advice is also given on preparing a ‘fair copy’ geological map which shows your interpretation of the data from your field map. However, a geological map is not, as is sometimes supposed, an end in itself. The whole purpose is to explain the geology of the area and your map is only a part of that process: a report is also needed to explain the geological history of the area and the sequence of geological events. Chapter 10 is a guide on how to present this important part of any geological mapping project.

The approach here is practical: it is basically a ‘how to do it’ book. It avoids theoretical considerations. It is a guide to what to do in the field to collect the evidence from which geological conclusions can be drawn. What those conclusions are is up to you, but bear in mind what the geologist Lord Oxburgh has said; that making a geological map is one of the most intellectually challenging tasks in academia (Dixon 1999).

1.2 Field Behaviour

Geologists spend much of their time in the open air and more often than not their work takes them to the less inhabited parts of a country. If they did not like open country, presumably they would not have become geologists in the first place: consequently, it is taken for granted that geologists are conservation-minded and have a sympathetic regard for the countryside and those who live in it. So, do not leave gates open, climb dry-stone walls or trample crops, and do not leave litter or disturb communities of plants and animals. When you are collecting specimens do not strip or spoil sites where type fossils or rare minerals occur. Take only what you need. Always ask permission to enter land from the owners, their agents or other authorities; and this includes National Trust areas unless they are specifically known to be open to the public. Most owners are willing to cooperate if they are asked but are understandably annoyed to find strangers sampling their rocks uninvited. Bear in mind that upset landowners can inhibit geological activities in an area for years to come, and this has already happened in parts of Britain. Many other countries are less populated and have more open space, and the situation may be easier, but every country has some land where owners expect people to consult them before working there. If in doubt, ask! (See also the ‘Geological fieldwork code’ published by the Geologists’ Association.)

1.3 Safety

A geologist must be fit if he is to do a full day’s work in the field, perhaps in mountainous country, in poor weather, or in a difficult climate, either hot or cold. Geological fieldwork, in common with other outdoor pursuits, is not without physical hazards. However, many risks can be minimized by following fairly simple rules of behaviour, and discretion may often be the better part of valour when, say, faced with an exposure in a difficult position, for a geologist is often on his own, with no one to help him, should he get into difficulties. Experience is the best teacher but common sense is a good substitute. Field safety is more fully discussed in Appendix I from both the standpoint of the student (or employee) and his supervisor (or employer).

1.4 Ancillary Skills

A geologist should be able to swim, even when fully clothed. If you can swim, you are less likely to panic when you slip off an outcrop into a river; or from weed-covered rocks into the sea or a rock pool; or even if you just fall flat on your face when crossing a seemingly shallow stream. A ford often proves deeper than you thought and not all natural water is quite as pellucid as poets would have us believe. Such accidents happen to most of us sometime. If you are faced by something risky, play it safe, especially if you are on your own.

Geologists should also be able to drive. They sometimes have to ride, too. Horses, donkeys, and especially mules, are still used in some mountainous areas. They can save a great deal of tedious walking and backpacking, and mules in particular can clamber up astonishingly steep and rocky slopes. Field geologists spend a great deal of their time getting from place to place.

1.5 A Few Words of Comfort

Finally, some cheering words for those about to start their first piece of independent mapping. The first week or so of nearly every geological mapping project can be depressing, especially when you are on you own in a remote area. No matter how many hours are spent in the field each day, little seems to show on the map except unconnected fragments of information which have no semblance to an embryonic geological map. Do not lose heart: this is quite normal and the map will suddenly begin to take shape.

The last few days of fieldwork are also often frustrating for, no matter what you do, there always seems to be something left to be filled in. When this happens, check that you do have all the essential information and then work to a specific finishing date. Otherwise you never will finish your map.

2

FIELD EQUIPMENT

Geologists need a number of items for the field. A hammer (sometimes two) is essential and some chisels. Also essential are a compass, clinometer, pocket steel tape, and a handlens, plus a map case, notebook, map scales, protractor, pencils and eraser, an acid bottle and a jack-knife. A camera is a must and a small pair of binoculars can be most useful at times, as is a GPS instrument if it can be afforded (see Section 3.4.9). Sometimes a 30 m tape may be needed and a stereonet. If using aerial photographs you will need a pocket stereoscope; very occasionally a pedometer can be useful, although not essential. You will also need a felt-tipped marker pen and/or timber crayons for labelling specimens.

Finally, you need a rucksack to carry it all, plus a waterbottle, emergency rations, a first aid kit, perhaps an extra sweater, your mobile phone (see Appendix I), and of course your lunch.

Geologists must also wear appropriate clothing and footwear for the field if they are to work efficiently, often in wet cold weather, when other (perhaps more sensible) people stay indoors; inadequate clothing can put a geologist at risk of hypothermia (Appendix I). A checklist of what you may have to pack before a field trip is given in Appendix IV, but this is an exhaustive list to cover various types of geological fieldwork in various climates; refer to it before first setting out to your field area base. A more detailed description of the essentials is given in Figure 2.1.

2.1 Hammers and Chisels

Any geologist going into the field needs at least one hammer with which to break rock. Generally, a hammer weighing less than about images kg (1images lbs) is of little use except for very soft rocks; 1 kg (2–2images lbs) is probably the most useful weight. The commonest pattern still used in Europe has one square-faced end and one chisel end. Many geologists now prefer a ‘prospecting pick’; it has a long pick-like end which can be inserted into cracks for levering out loose rock, and can also be used for digging in soil in search of float. Most hammers can be bought with either wooden or fibreglass handles, or with a steel shaft encased in a rubber grip (Figure 2.1). If a wooden handle is chosen (it does have some advantages: it is more springy), buy some spare handles and some iron wedges to fix them on with.

Figure 2.1 Tools for the field: (a) traditional geologist’s hammer in leather belt ‘frog’; (b) steel-shafted ‘prospecting pick’; (c) bricklayer’s ‘club’ hammer with a replaced longer shaft; (d) 45 cm chisel with 2.5 cm edge; (e) 18 cm chisel with 2 cm cutting edge

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Geologists working on igneous and metamorphic rocks may opt for heavier hammers. Although 2 kg/4 lb geological hammers are available, a bricklayer’s ‘club’ hammer, with a head shaped like a small sledge hammer, can be bought more cheaply; but replace its rather short handle by a longer one bought from a hardware store.

Hammering alone is not always the best way to collect rock or fossil specimens. Sometimes a cold chisel is needed to break out a specific piece of rock or fossil. Its size depends on the work to be done. A 5 mm (images inch) chisel may be ideal to delicately chip a small fossil free from shale, but to break out large pieces of harder rock a 20–25 mm (images inch) chisel is required (Figure 2.1). Perhaps geologists should follow the lead of mine samplers, whose job it is to break off rock and ore and who find a ‘moil’ more effective. This is a steel bar, usually a piece of drill steel, 25–30 cm long, sharpened to a point and tempered. One thing which you must never do is to use one hammer as a chisel and hit it with another. The tempering of a hammer face is quite different from that of a chisel head, and small steel fragments may fly off the hammer face with unpleasant results.

Some geologists carry their hammers in a ‘frog’ or hammer holster, as this leaves their hands free for climbing, writing and plotting. They can be bought or easily made from heavy leather (Figure 2.1). Climbing shops stock them for piton hammers although some may be too small to take a geological hammer handle. Note also that using a geological hammer is a ‘chipping action’ and comes under the Health and Safety at Work Act as needing the use of approved goggles. Courts would probably take a less than liberal view of claims for compensation for eye injuries suffered if goggles were not being worn.

2.2 Compasses and Clinometers

The ideal geologist’s compass has yet to be designed. Americans have their Brunton, the French the Chaix-Universelle, the Swiss have the Meridian, and there is also the Clar compass, popular in Europe. All are expensive. Many geologists now use the very much cheaper Swedish Silva Ranger 15 TDCL or the similar Finnish Suunto (Figure 2.2(a)). All the above have built-in clinometers. The Silva (Figure 2.3) and Suunto compasses, however, have a transparent base so that bearings can be plotted directly onto a map by using the compass itself as a protractor (Section 5.2). However, like the Brunton, the Silva and Suunto are needle compasses and are not as easy to take bearings with on distant points as are prismatic compasses which have a graduated card to carry the magnetic needle. Silva do make a prismatic card-compass (No. 54), but it lacks a clinometer (Figure 2.4(d)). All these compasses except the Brunton are liquid-filled to damp movement of the needle when taking a reading. The Brunton is induction-damped.

Figure 2.2 Compasses designed for the geologist: (a) Finnish Suunto compass, similar to the Swedish Silva Ranger 15 TDCL; (b) American Brunton ‘pocket transit’; (c) Swiss Meridian compass; (d) French Chaix-Universelle. The Brunton and Meridian can also be used as hand-levels

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Figure 2.3 The Swedish Silva Ranger 15 TDCL geologist’s compass. Like the Suunto, an excellent and reasonably priced instrument

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A compromise may be made by using a separate clinometer and a cheaper compass, such as the Japanese Europleasure Lensatic compass. This is liquid-damped and can be read like a prismatic (available from sports and camping shops). Another alternative is to buy a hand-bearing compass such as the Meridian or the very robust former British army compass (second-hand shops). The latter suffers from having no straight edge with which to measure strike with, but is excellent if you need to survey-in numerous distant points accurately.

2.2.1 Compass graduations

Compasses can be graduated in several ways. The basic choice is between the traditional 360 (degrees) and the continental 400 (grads) to a full circle. Both are used in continental Europe and if you do buy a compass there, check it first. If you opt for degrees, you must then choose between graduation into four quadrants of 0–90° each, or to read a full circle of 0–360° (‘azimuth’ graduation). Here, we recommend azimuth, for bearings can be expressed more briefly and with less chance of error. Comparisons are made in Table 2.1.

Figure 2.4 Various other compasses: (a) Japanese Lensatic compass, with a good straight side for measuring strikes (some models have a clinometer), and can be read like a prismatic compass; (b) British army prismatic compass; very accurate, robust, and excellent for taking bearings, but very expensive and has no straight sides; (c) Swiss Meridian bearing compass; it has no clinometer; (d) Swedish Silva prismatic compass No. 54; (e) Japanese ‘universal clinometer’ made by Nihon Chikagasko Shaco, Kyoto (see also Figure 2.8)

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2.2.2 Using compasses

Prismatic compasses and mirror compasses are used in different ways when sighting a distant point. A prismatic is held at eye level and aimed like a rifle, lining up the point, the hairline at the front of the compass and the slit just above the prism. The bearing can then be seen in the prism, reflected and enlarged from the compass card. A mirror compass can be read in two ways. The Brunton company recommend that the compass is held at waist height and the distant point aligned with the front sight so that both are reflected in the mirror and are bisected by the hairline on the mirror (Figure 2.5). As the Brunton is undamped, do not wait until the needle has stopped swinging, wait until the swing is only a few degrees and read the average of the swing; it takes practice. This waist-high method is not easy to do with the Silva-type mirror compass, and many will find it more convenient to sight the distant point by holding the compass at eye level and reflecting the compass needle in the mirror. Some prefer to read a Brunton in the same way. Mirror compasses have a distinct advantage over prismatics in poor light such as underground.

Table 2.1

Quadrant bearing Azimuth bearing
N36°E 036°
N36°W 324°
S36°E 144°
S36°W 216°

Figure 2.5 The recommended way to use a Brunton compass when taking a bearing on a distant point (reproduced by courtesy of the Brunton company, Riverton, Wyoming)

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2.2.3 Clinometers

Few compasses incorporate a clinometer into their construction. Clinometers can be bought separately and a few types, such as the Finnish Suunto, have the advantage that they can also be used as a hand-level. Some hand-levels, such as the Abney (Figure 2.7(d)), can be used as a clinometer, although rather inconveniently. The Burgess level and angle indicator, designed for do-it-yourself handymen, makes a cheap and effective clinometer (and it is sold under other names). Rabone also market a cheap builder’s level which can be used as a clinometer. These DIY instruments (Figures 2.7(a) and (c)) often have a magnetic strip so they can be attached to metal gutters and downpipes. Remove it for obvious reasons.

Some builder’s ‘two-foot’ rules have a brass middle joint graduated every 5°. Although not accurate enough for normal dip measurements, they are useful for measuring lineations Section 5.4.2.

Clinometers can be easily made either by using the pendulum principle or, better still, the Dr Dollar design, as follows: photocopy a 10 cm diameter images-round protractor for a scale and glue it to a piece of Perspex after duffing out the figures and re-numbering so that 0° is now at the centre. Cement transparent plastic tubing containing a ball-bearing around it and fill each end with plasticine or putty to keep the ball in (Barnes (1985), and see Figures 2.6 and 2.7(b)).

Figure 2.6 A home-made clinometer (reproduced by permission of the Earth Science Teachers’ Association)

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Figure 2.7 A selection of clinometers: (a) Rabone adjustable spirit level; (b) Home-made clinometer (see Figure 2.6); (c) Burgess ‘level and angle indicator’; very cheap, if you can find one in a DIY shop (may be sold under other names); (d) Abney hand-level; can also be used as a clinometer; (e) builder’s ‘two-foot’ rule with level bubble and 5° graduation at hinge; useful for measuring lineations

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2.2.4 Lineation compass

The Japanese produce a most useful compass designed to measure trend and plunge of lineation simultaneously. The compass case is on gimbals so that it always remains level whatever the angle of its frame. It is effective in even the most awkward places (Figures 2.4(e) and 2.8). The design is derived from the German miner’s compass. The maker is Nihon Chikagasko Shaco, Kyoto.

Figure 2.8 Japanese ‘universal clinometer’. Based on the ‘German miner’s hanging compass. Trend can be measured directly from the compass, which always stays horizontal, and plunge is read by the pointer hanging below the compass box

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2.3 Handlenses

Every geologist must have a handlens and should develop the habit of carrying it at all times, so that when he needs it he has it with him. A magnification of between 7 and 10 times is probably the most useful. Although there are cheap magnifiers on the market, a good quality lens is worth the extra cost in flatness of field and should last you a lifetime. To ensure that it does last a lifetime, attach a thin cord to hang it round your neck. Monocle cord is ideal if you can find it, as it does not twist into irritating knots. However, always keep a spare in camp, for your fieldwork could be jeopardised should you lose the only one you have with you.

2.4 Tapes

A short ‘roll-up’ steel tape has many uses. A 3 m tape takes up no more room than a 1 m tape and is much more useful. You can use it to measure everything from grain size to bed thickness, and if the tape has black numbering on a white background, you can use it as a scale when taking close-up photographs of rock surfaces or fossils. A geologist also occasionally needs a 10 m or 30 m ‘linen’ tape for small surveys. You might not need it every day but keep one in camp for when you do. Treat a tape with respect. Wind it back into its case only when clean, for dirt will wear off the graduations. If a long tape is muddy, coil it into loops between measurements. When you do eventually wind it back into its case, do so between fingers of your other hand or through a damp rag to wipe off the dirt. When finished for the day, wash and dry it before putting it away.

2.5 Map Cases

A map case is obviously essential where work may have to be done in the rain or mist; but even in warmer climes, protection from both the sun and sweaty hands is still needed. A map case must have a rigid base so that you can plot and write on the map easily; it must protect the map; and it must open easily, otherwise it will deter you from adding information to the map. If it is awkward to open, you will probably say ‘I will remember that and add it later’ and of course, being only human, you forget! The best map cases are probably home-made (Figure 2.9). Pencil holders make mapping easier, whether attached to your map case or your belt. Make your own.

2.6 Field Notebooks

Do not economise on your field notebook. It should have good quality ‘rainproof’ paper, a strong hard cover and good binding. It will have to put up with hard usage, often in wet and windy conditions. Nothing is more discouraging than to see pages of field notes torn out of your notebook by a gust of wind and blowing across the landscape. Loose-leaf books are especially vulnerable. A hard cover is necessary to give a good surface for writing and sketching. A notebook should fit into your pocket so that it is always available, but big enough to write on in your hand. A good size is 12 cm × 20 cm so make sure you have a pocket or belt-pouch to fit it. Try to buy a book with squared, preferably metric squared, paper; it makes sketching so much easier. Half-centimetre squares are quite small enough. A surveyor’s chaining book is the next best choice: the paper is rainproof, it is a convenient size, and it has a good hard cover. A wide elastic band will keep pages flat and also mark your place.

Figure 2.9 A map case made from a Perspex sheet attached to a plywood base by a nylon or brass ‘piano hinge’ (from a DIY), using ‘pop’ rivets. A wide elastic band is used to keep the map flat. Even simpler, use adhesive carpet tape for the hinge, but it will need frequent renewing

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2.7 Scales

A geologist must use suitable scales, most conveniently about 15 cm long: a ruler is just not good enough. Rulers seldom have an edge thin enough for accurate plotting of distances, and trying to convert in your head a distance measured on the ground to the correct number of millimetres on the ruler for the scale of your map just leads to errors. Scales are not expensive for the amount of use they get. Many are thinly oval in section and engraved on both sides to give four different graduations. The most convenient combination is probably 1:50 000, 1:25 000, 1:12 500 and 1:10 000. In the USA scales with 1:62 500 and 1:24 000 are needed. Colour code scale edges by painting each with a different coloured waterproof ink or coloured adhesive tape, even nail varnish, so that the scale you are currently using is instantly recognisable. Although triangular scales with six edges, each with a different scale may seem an even better bet and are excellent for the drawing office, their knife-sharp edges are easily chipped in the field (Figure 2.10).

Figure 2.10