Geologic Maps: A Practical Guide to Preparation and Interpretation

236 Pages·2017·30.78 MB·English

by  Edgar W. Spencer

MAP ABBREVIATIONS
The United States Geological Survey has established standard symbols for use on the maps it
publishes. These are used on most other maps also. The standard abbreviations for formations
consist of a capital letter to indicate the period in which the unit was formed, followed by one or
more lowercase letters to indicate the name of the formation (i.e., Ob is the symbol for the
Beekmantown Formation of Ordovician age). Standard letters for periods are as follows.
Era Period Symbol
Cenozoic Quaternary Q
Tertiary T
Mesozoic Cretaceous C
Jurassic J
Triassic ^
Paleozoic Permian P
Pennsylvanian *
Mississippian M
Devonian D
Silurian S
Ordovician O
Cambrian _
Precambrian p_
An explanation of the symbols used on the map accompanies a geological map (generally beside
the map). The explanation usually includes the following information:
1. Name of the map.
2. Scale of the map, shown both as a fraction and as a bar scale.
3. Name of the author of the map.
4. A stratigraphic column showing the rock units and sediments recognized in the map area.
These are placed in a column with the youngest sediment or sedimentary rock unit at the top.
Others follow in order of age. Igneous and metamorphic rocks are usually shown at the bottom
of the column.
5. All other symbols (e.g., strike and dip of beds, faults, foliations, etc.) used on the map are
defined.
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Geologic Maps
A Practical Guide to Preparation and Interpretation
Third Edition
Edgar W. Spencer
Washington and Lee University
WAVELAND
PRESS, INC.
Long Grove, Illinois
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For information about this book, contact:
Waveland Press, Inc.
4180 IL Route 83, Suite 101
Long Grove, IL 60047-9580
[phone]
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Contents
About the Author vii
Preface ix
Maps and Images Used in the Study of Earth 1
1 Types of Information You Can Obtain from Maps and Images 1
Topographic Maps 1
Geologic Maps 2
Base Maps 2
Oblique Aerial Photographs 3
Vertical Aerial Photographs 3
Orthophotographs 3
Remote Sensing Images 4
Landsat Satellite Images 6
Google Earth 6
Side-Looking Airborne Radar (SLAR) Images 7
Geologic Maps 7
Geologic Cross Sections 10
Geologic Block Diagrams 10
LiDAR Maps 11
Structure Contour Maps 12
Tectonic Maps 12
Land-Use Maps Derived from Geologic Maps 13
Professional Uses of Geologic Maps 13
Geologists 13
Civil and Environmental Engineers and Engineering Geologists 13
Planners and Architects 14
Soil Scientists 14
Base Maps 15
2 Map Projections 15
Mercator Projection 16
Transverse Mercator Projection 16
Universal Transverse Mercator (UTM) Coordinates 16
Polyconic Projection 18
Lambert Conformal Conic Projection 18
Planimetric Maps 19
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iv Contents
Topographic Maps 19
Location 20
Location by Means of the Public Land Survey 20
Ground Distance and Map Distance 21
Scales of Quadrangle Maps 21
True and Magnetic North—Declination 22
Bearings and Azimuths 22
Preparing a Topographic Profile 25
Selecting Graph Paper for Topographic Profiles 27
Preparation of Geologic Maps 29
3 Preliminary Preparations 29
Define the Map Area 29
Collect and Review Existing Information 29
Select a Base Map 30
Making a Reconnaissance Survey of the Area 30
Obtain Permission to Enter Private Property 30
Collecting and Recording Observations 31
Decide Where to Collect Data 31
Record Observations 32
Determine Your Location 32
Determining Location with GPS 34
Precautions When Using a GPS Receiver 34
Use of Drones in Mapping 35
Geographic Information System (GIS) 35
Describing the Outcrop 36
Stratigraphic Units Used on Geologic Maps of Bedrock 36
Making Measurements with Compasses 37
Pointers on Making Measurements 40
Common Problems in Measuring Strike and Dip 40
Compiling Field Observations on the Base Map 42
Interpreting the Data 42
Field Checking Your Interpretation 45
Preparing the Final Map 45
Final Map Checklist 45
Identification and Description of Sedimentary Rocks 47
4 Field Description of Sedimentary Rocks 47
Stratification 47
Composition 48
Texture 48
Color 48
Sedimentary Rock Types 48
Environments of Deposition 49
Primary Features in Sedimentary Rocks 49
Textural Variations in Sedimentary Rocks 50
Primary Features Found on Bedding Surfaces 52
Use of Aerial Photographs in Mapping 55
5 Vertical Photographs 55
Stereographic Photography 56
Identifying Surficial and Bedrock Materials on Aerial Photographs 58
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Contents v
Interpretation of Surficial Geologic Maps 65
6 Uses of Surficial Geologic Maps 65
Types of Unconsolidated Materials Shown on Geologic Maps 66
Introduction to Geologic Maps of Bedrock 71
7 Primary Shape of Sedimentary Rock Bodies 72
Structure Contour Maps 77
Geologic Maps of Homoclinal Beds 83
8 Patterns of Homoclinal Beds on Geologic Maps 83
V-Shaped Contact Patterns on Geologic Maps 85
Determining the Strike and Dip of a Plane from
Three Points of Known Elevation on the Plane 86
Tracing Plane Contacts through the Topography 88
Layer Thickness and Width on Maps 93
Constructing Cross Sections of Homoclinal Beds 95
Unconformities 103
9 Unconformity Patterns on Geologic Maps 103
Folds on Geologic Maps 111
10 Fold Geometry 111
Fold Patterns on Geologic Maps 111
Hints on Reading Maps of Folded Strata 113
Constructing Cross Sections of Folded Rocks 115
Freehand Cross Sections 115
Balanced Cross Sections 115
Tracing Folds through the Topography 117
Structure Contour Maps of Folded Strata 117
Faults on Geologic Maps 127
11 Fault Nomenclature 128
Cross-Section Construction in Faulted Areas 130
High-Angle Faults 131
Patterns of High-Angle Faults on Geologic Maps 131
Patterns along Strike-Slip Faults 139
Low-Angle Faults 140
Igneous and Metamorphic Rocks 149
12 Appearance of Plutons on Geologic Maps 149
Nomenclature and Classification of Intrusions 150
Concordant Plutons 152
Discordant Plutons 152
Tectonic and Regional Maps 161
13
Appendix A: Safety in the Field 167
Appendix B: Geologic Maps and Explanations 169
Selected References 219
Index 220
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About the Author
Edgar Winston Spencer is the Ruth Parmly Professor of Geology Emeritus at
Washington and Lee University where he served as department head from 1959 until
1995. He grew up in Arkansas and went to college at Vanderbilt and Washington and
Lee University. While a graduate student at Columbia University, he worked at the
Lamont–Doherty Geological Observatory and taught at Hunter College. His disserta-
tion concerned the structure of the Beartooth Mountains in Montana. He continued
mapping and structural work in the Madison Mountains in Montana and later in the
Appalachians where he has done regional mapping in the Blue Ridge and in the Val-
ley and Ridge for the Virginia Division of Geology and Mineral Resources. He con-
ducted field seminars in the central western Valley and Ridge for the American
Association of Petroleum Geologists. In 1991, he received an outstanding faculty
award from the Virginia Council of Higher Education, and was given the Anna Jonah
Award for Outstanding Contributions to Virginia Geology by the Virginia Geological
Field Conference in 2013. He is a member of Sigma Xi and an honorary member of
Phi Beta Kappa and Omicron Delta Kappa. Spencer is the author of a structural geol-
ogy text and several introductory geology textbooks. More recently he wrote the Guide
to the Geology and Natural History of the Blue Ridge Mountains. He continues mapping in
the central Appalachian Mountains and has served as a guide with many alumni col-
leges at Washington and Lee University.
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