Levin The Earth Through Time 10th c2013 txtbk

TENTH EDITION

The Earth Through Time

To see where we might be going,
we must understand where we have been.

Robert Tamarkin, 1993

TENTH EDITION

The Earth
Through Time

H A R O L D L. L E V I N

Professor of Geology
Washington University,
St. Louis, Missouri

VP & EXECUTIVE PUBLISHER: JAY O’CALLAGHAN
EXECUTIVE EDITOR: RYAN FLAHIVE
EDITORIAL ASSISTANT: JULIA NOLLEN
MARKETING MANAGER: MARGARET BARRETT
SENIOR PHOTO EDITOR: JENNIFER MACMILLAN
DESIGNER: KENJI NGIENG
ASSOCIATE PRODUCTION MANAGER: JOYCE POH

Front Cover Photo Credits:
Spinosaurus – # Corey Ford/Stocktrek Images/Getty Images, Inc.
Cretaceous scene – # Richard Bizley/Photo Researchers, Inc.

This book was set in 10/12 Janson by Thomson Digital and printed and bound by Courier Kendallville.
The cover was printed by Courier Kendallville.

This book is printed on acid free paper.

Founded in 1807, John Wiley & Sons, Inc. has been a valued source of knowledge and understanding for more than 200 years, helping people
around the world meet their needs and fulfill their aspirations. Our company is built on a foundation of principles that include responsibility
to the communities we serve and where we live and work. In 2008, we launched a Corporate Citizenship Initiative, a global effort to address
the environmental, social, economic, and ethical challenges we face in our business. Among the issues we are addressing are carbon impact,
paper specifications and procurement, ethical conduct within our business and among our vendors, and community and charitable support.
For more information, please visit our website: www.wiley.com/go/citizenship.

Copyright # 2013, 2010, John Wiley & Sons, Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval
system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as
permitted under Sections 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher,
or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc. 222 Rosewood Drive, Danvers,
MA 01923, website www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department,
John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030-5774, (201)748-6011, fax (201)748-6008, website http://www.wiley.com/go/
permissions.

Evaluation copies are provided to qualified academics and professionals for review purposes only, for use in their courses during the next
academic year. These copies are licensed and may not be sold or transferred to a third party. Upon completion of the review period, please
return the evaluation copy to Wiley. Return instructions and a free of charge return mailing label are available at www.wiley.com/go/
returnlabel. If you have chosen to adopt this textbook for use in your course, please accept this book as your complimentary desk copy.
Outside of the United States, please contact your local sales representative.

Library of Congress Cataloging-in-Publication Data

Levin, Harold L. (Harold Leonard), 1929-
The earth through time / Harold L. Levin – 10th ed.

p. cm.
Includes index.
ISBN 978-1-118-25467-7 (pbk.)
1. Historical geology—Textbooks. I. Title.
QE28.3.L48 2013
551.7—dc23

2012030204

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

This book is dedicated to my wife Kay, who has cheerfully
endured my preoccupation with preparing this new edition,
and to Noah, Lillie, Eli, Mollie, Natalie, Emily, Caitlyn,
Hannah, and Candis. May they have the wisdom to treat
the Earth kindly.

ABOUT THE AUTHOR

Harold (“Hal”) Levin began his career as a petroleum
geologist in 1956 after receiving bachelor’s and master’s
degrees from the University of Missouri and a doctorate
from Washington University. His fondness for teaching
brought him back to Washington University in 1962, where
he is currently professor emeritus of geology and
paleontology in the Department of Earth and Planetary
Sciences. His writing efforts include authorship of ten
editions of The Earth Through Time; four editions of
Contemporary Physical Geology; Life Through Time; Essentials of Earth Science; and co-authorship of Earth:
Past and Present, as well as nine editions of Laboratory Studies in Historical Geology; and Ancient
Invertebrates and Their Living Relatives.
For his courses in physical geology, historical geology, paleontology, sedimentology, and
stratigraphy, Hal has received several awards for excellence in teaching. The accompanying
photograph was taken during a lecture on life of the Cenozoic Era. The horse skull serves to illustrate
changes in the teeth and jaws of grazing animals in response to the spread of prairies and savannahs
during the Miocene and subsequent epochs.

vi

PREFACE

Students enrol in an Earth History course for many why cooling occurred. Was it the result of changes in
reasons. Often it is simply to satisfy a college science the amount of radiation received from the Sun, a shift
requirement. Those of us who teach the course, how- in the Earth’s axis of rotation, changes in the compo-
ever, strive to provide a better reason. We hope to sition of the atmosphere, or major changes in the
instill in our students knowledge about how our planet distribution of continents? The Earth scientists test
became a haven for life; how change has dominated each of these ideas in the laboratory and examine rocks
Earth history; and how change will continue to chal- of the appropriate age to find the best answer. That
lenge us in the future. In the geologic past, change has answer is always tentative and subject to new discov-
been driven by natural forces. Now we humans, only eries. Usually, the answer relates to reciprocal actions
recently arrived on Earth, are an additional cause of of all the Earth’s systems: the atmosphere, biosphere,
change. More often than not, we cause change that is and solid Earth. Students will read of such an inte-
harmful. These are reasons why our students need to grated approach to answering questions about the
more fully understand this small and overcrowded geologic past in the pages ahead.
planet. They need to learn from its history, its catas-
trophes, and its successes. The Earth Through Time is designed for the under-
graduate student who has little previous acquaintance
This textbook chronicles the Earth’s story from the with geology. Students exploring the possibility of an
time the Sun began to radiate its light, to the beginning academic major in geology, however, can be confident
of civilization. It is a history that began 4,600 million that the text will provide the necessary background for
years ago after our planet had gathered most of its mass advanced courses. I have included basic information
from a rotating cloud of dust, gases, and meteorites. about minerals, igneous, and metamorphic rocks so
From that time to the present, the Earth has experi- that the text can be used either for a single, self-
enced climatic shifts from widespread warmth to ice contained first course, or for the second course in a
ages. The floors of the oceans have alternately two-semester sequence of Physical Geology followed
expanded and contracted. Continents have drifted by Historical Geology.
thousands of miles, coalesced or splintered apart.
Rocks have been thrust skyward to form lofty moun- cTHE TENTH EDITION
tains, and placid landscapes have been disrupted by The goal of The Earth Through Time is to present the
earthquakes or buried in floods of fiery lava. history of the Earth, and the science behind that
history, as simply and clearly as possible. We have
At least for the past 300 million years, life has existed strived to make the narrative more engaging, to convey
on our planet. Fossil remains of that life attest to biologi- the unique perspective and value of historical geology,
cal achievements and failures in coping with changing and to improve the presentation so as to stimulate
conditions. There are lessons to be learned from our interest and enhance the student’s ability to retain
biological history—lessons that will help us anticipate essential concepts, hopefully, long after the final exam.
and act wisely to dangers we may face in the future. In this tenth edition, we have greatly improved the
illustration program with about 140 new photographs.
Learning what has happened on Earth in the past is Also included are drawings prepared by the author.
sufficient reason to study Earth History. A course in These are often more instructive than a photograph of
Historical Geology, however, has value in many other the fossil itself. There is little doubt that good illus-
ways as well. As a science, it informs us about a way to trations help students learn and retain information.
answer questions, how discoveries are made, and how As the Russian writer Ivan Turgenev wrote in 1862,
to tell the difference between valid and faulty assump- “A picture shows me at a glance what it takes dozens of
tions. All of this comes by way of the so-called scien- pages of a book to expound.”
tific method. The term “scientific method” may sound The seventeen chapters in the tenth edition have
a bit formidable to students, but it is only the rational been organized into three major divisions. Part I,
way we ask questions, make educated guesses about Discovering Time and Deciphering Earth’s Amazing His-
the answers, and test those answers by observation or tory, explains the methods used in reconstructing Earth
experimentation. As one example, we might infer that
the demise of a particular group of ancient animals
resulted from cooling of the planet. This leads us to ask

vii

viii c Preface

history, and the important contributions of early geol- (which students can refer to when they read
ogists. Students will learn how rocks are dated and about geologic events at particular localities), a
then used to construct a geologic time scale in the third Periodic Table and Symbols for Chemical Ele-
chapter of Part I. Part II, Rocks and Fossils and What ments (useful as as a reference when reading the
They Tell Us about Earth History, describes the nature sections on mineral composition and radioactive
and origin of Earth materials, and how rocks and elements), Convenient Conversion Factors,
fossils reveal events of the geologic past. In Part III, Exponential Scientific Notation, Rock Symbols,
The History of Planet Earth and Its Inhabitants, we a simplified Bedrock Geology of North America,
examine the actual history of our planet, from its fiery and a Table of Common Rock-Forming Silicate
birth to the unfolding of the modern world. New Minerals.
information has been provided relating to how scien-
tific hypotheses are validated, the history of discoveries Most chapters include one or more general interest
relating to radioactive dating, how volcanic activity “boxes.” Enrichment boxes briefly examine a topic of
relates to atmospheric change and extinctions, the general interest that is related to material in the chap-
“snowball Earth” controversy, laggerst€atten, new ters. Geology of National Parks and Monuments
energy sources, the role of HOX genes in evolution, boxes provide information about where the geology
the Great Ordovician Biodiversity Event, and an imag- described in the chapter can actually be visited.
inative walk through a Carboniferous rainforest. We
thoroughly reviewed the previous edition, updated cPEDAGOGICAL SUPPLEMENTS
information, added new discussions, and objectively To help students understand, retain, and appreciate
deleted material rarely used in today’s historical geol- the information in their Historical Geology course,
ogy courses. The Earth Through Time is accompanied by an exten-
sive set of supporting materials. These supplements
cHOW DOES THE EARTH THROUGH include:
TIME HELP STUDENTS LEARN? Lecture PowerPoints, Test Banks prepared by
Marc Willis. Student Study Guide, Chapter Quizzes,
The Earth Through Time has a number of features to Web Links, Chapter Tutorials, Instructors’ Manual
engage the student and promote learning. prepared by David King.
Overhead Transparencies for use in the labora-
Questions for Review allows students to test their tory and lecture hall are available to instructors on
understanding of material in a chapter and to further request. (John Wiley and Sons, Inc., may provide comple-
process what they have learned. In this edition we mentary instructional aids and supplementary packages to
have added over seventy-five new questions. those adopters qualified under our adoption policy. Please
contact your sales representative for more information.)
Chapter Summaries. Each chapter ends with a Student Companion Website (www.wiley
summary of essential concepts, affording students .com/college/levin). This website features simple-to-
the condensed “meat” of a topic. If the summary use and highly effective study tools that will help
statement is not fully understood, it is a cue to revisit students isolate and retain key information from the
the topic in the chapter. text, prepare efficiently for exams. The website
includes:
Technical terms are printed in boldface the first
time they are used. A list of Key Terms is provided at Student Study Guide prepares students for tests
the end of the chapter, along with the page number and quizews by providing a concise chapter sum-
on which they are defined. If encountered later in mary, key terms, and self-quiz with answer key.
the text, the student can see the term defined again
in the book’s Glossary. Chapter Quizzes with immediate results on com-
pletion of each quiz.
So that students can anticipate what lies ahead, each
chapter begins with a list of Key Chapter Web Links allow students to explore external
Concepts. resources for topics examined in each chapter.

Caption questions occur beneath many text figures. Chapter Tutorials permitting the student to dis-
These draw attention to geologic or fossil features sect each chapter with a thoroughly notated
and help clarify information provided in the text. outline.

Appendices include a Classification of Living Flashcards permit the student to display either
Things that helps students place fossils described term or definition to lock in information for
in the text in the text within their taxonomic group. examinations.
The appendices also include the aforementioned
Glossary, a map of the Physiographic Provinces Geography Reference Sites provide geographic
of the United States, a World Political Map sites to supplement information in the text.

cACKNOWLEDGMENTS Preface b ix
Most of the changes in this edition resulted from the
suggestions of an insightful and diligent group edition. No matter how difficult, Jennifer always man-
thoughtful and dedicated professors who reviewed aged to find just the right image of a fossil or significant
the text and whose names are below. In particular, rock outcrop. As the project moved through prepara-
my sincere thanks must include those at John Wiley tion of final pages, Associate Production Manager
and Sons. Joyce Poh coordinated various aspects of production
Ryan Flahive provided the stimulus and encourage- from far-away Singapore. Finally, the author is grate-
ment to move the project forward on schedule. What- ful for the opportunity to work with so fine a publisher
ever problem arose, Ryan always had an instant as John Wiley and Sons, Inc.
solution. The countless details associated with chan-
neling the book into production were efficiently Many changes in this edition are the result of
handled by Editorial Assistant Julia Nollen. For assist- incisive comments and suggestions of the diligent
ance in bringing the book to the public, the author is group of reviewers listed below:
indebted to Marketing Manager Margaret Barrett.
The skill and unflagging assistance of Senior Photo Todd Feeley
Editor Jennifer MacMillan was indispensible to this Torrey Nyborg
Bruce Robertson
Kerry Workman Ford
Jane Matheney-Rood

Student Companion Site: Quizzes, CONTENTS
study guide & WEB links

PART I Discovering Time and What Occurs When Atoms Decay? 37
Deciphering Earth’s Amazing History The Principal Radioactive Timekeepers 41
How Old is Earth? 45
CHAPTER 1
PART II Rocks and Fossils and What
The Science of Historical Geology 1 They Tell Us About Earth History

Why Study Earth History? 2 5 CHAPTER 4
Geology Lives in the Present and the Past 2
A Way to Solve Problems: The Scientific Rocks and Minerals: Documents That
Record Earth’s History 4 9
Method 3
Minerals as Documents of Earth History 50
E N R I C H M E N T Scientific Discoveries Must be Tested Minerals and Their Properties 50
Common Minerals that Form Rocks 52
Three Great Themes in Earth History 7 Earth’s Three Great Rock Families and How They
What Lies Ahead? 9
Formed 57
CHAPTER 2 Igneous Rocks: “Fire-Formed” 58
Sedimentary Rocks: Layered Pages of History 67
Early Geologists Tackle History’s Metamorphic Rocks: Changed without Melting 72
Mysteries 1 3
CHAPTER 5
The Intrigue of Fossils 14
An Early Scientist Discovers Some The Sedimentary Archives 8 1

Basic Rules 15 Tectonic Setting is the Biggest Factor in Sediment
European Researchers Unravel the Succession of Deposition 82

Strata 17 Environments Where Deposition Occurs 83
Neptunists and Plutonists Clash 18 What Rock Color Tells Us 89
Uniformitarianism: James Hutton Recognizes that What Rock Texture Tells Us 91

the Present is Key to the Past 18 E N R I C H M E N T You Are the Geologist 93
The Principle of Fossil Succession 20
The Great Uniformitarianism–Catastrophism What Sedimentary Structures Tell Us 94
What Four Sandstone Types Reveal About Tectonic
Controversy 21
The Principle of Cross-Cutting Setting 98
Limestones and How They Form 99
Relationships 21 Organizing Strata to Solve Geologic Problems 103
Evolution: How Organisms Change Through Sea-Level Change Means Dramatic Environmental

Time 23 Change 106
Earth History in America 24 Stratigraphy and the Correlating of Rock

CHAPTER 3 Bodies 107
Unconformities: Something is Missing 109
Time and Geology 2 9 Depicting the Past 112

Finding the Age of Rocks: Relative Versus Actual GEOLOGY OF NATIONAL PARKS AND MONUMENTS
Time 29 Grand Canyon National Park, Arizona 118

A Scale of Geologic Time 30
Actual Geologic Time: Clocks in the Rocks 34
Radioactivity Provides a Way to Date Rocks 36

x

CHAPTER 6 Contents b xi

Life on Earth: What Do Fossils Reveal? 1 2 5 Following Accretion, Earth
Differentiates 228
Fossils: Surviving Records of Past Life 126
The Primitive Atmosphere—Virtually No
E N R I C H M E N T Amber, the Golden Preservative 129 Oxygen 229
E N R I C H M E N T The Mazon Creek Lagersta€tte 131
The Primitive Ocean and the
Figuring Out How Life is Organized 132 Hydrologic Cycle 232
Evolution: Continuous Changes in Life 133
The Case for Evolution 141 Origin of Precambrian “Basement” Rocks 232
The Origin of Life 238
E N R I C H M E N T Earbones Through the Ages 142
GEOLOGY OF NATIONAL PARKS AND MONUMENTS
Fossils and Stratigraphy 144 Voyageurs National Park 246
Fossils Indicate Past Environments 151
How Fossils Indicate Paleogeography 155 In Retrospect 247
How Fossils Indicate Past Climates 158
An Overview of the History of Life 159 CHAPTER 9
Life on Other Planets: Are We Alone? 163
The Proterozoic: Dawn of a More Modern
CHAPTER 7 World 2 5 1

Plate Tectonics Underlies All Earth Highlights of the Paleoproterozoic
History 1 6 9 (2.5 to 1.6 billion years ago) 253

Earthquake Waves Reveal Earth’s Mysterious E N R I C H M E N T The 18.2-Hour Proterozoic Day 255
Interior 170
Highlights of the Mesoproterozoic
Earth’s Internal Zones 172 (1.6 to 1.0 billion years ago) 257
Earth’s Two Types of Crust 175
Plate Tectonics Ties It All Together 177 E N R I C H M E N T BIF: Civilization’s Indispensable
Drifting Continents 178 Treasure 258
Evidence for Continental Drift 179
Paleomagnetism: Ancient Magnetism Locked Into Highlights of the Neoproterozoic
(1.0 to 542 million years ago) 259
Rocks 182
Today’s Plate Tectonics Theory 184 Proterozoic Rocks South of the Canadian
What Happens at Plate Margins? 189 Shield 260
What Drives Plate Tectonics? 194
Verifying Plate Tectonics Theory 195 E N R I C H M E N T Heliotropic Stromatolites 262

E N R I C H M E N T Rates of Plate Movement 201 Proterozoic Life 263

Thermal Plumes, Hotspots, and Hawaii 202 CHAPTER 10
Exotic Terranes 202
Broken, Squeezed, or Stretched Rocks Produce Early Paleozoic Events 2 7 5

Geologic Structures 205 Dance of the Continents 277
Some Regions Tranquil, Others
GEOLOGY OF NATIONAL PARKS AND MONUMENTS
Hawaii Volcanoes National Park 210 Active 277
Identifying the Base of the Cambrian 281
PART III The History of Planet Earth and Early Paleozoic Events 281
Its Inhabitants Cratonic Sequences: The Seas Come in, the

CHAPTER 8 Seas Go Out 282
The Sauk and Tippecanoe Sequences 284
The Earth’s Formative Stages and the Way Out West: Events in the Cordillera 287
Archean Eon 2 1 5 Deposition in the Far North 288
Dynamic Events in the East 289
Earth in Context: A Little Astronomy 216
GEOLOGY OF NATIONAL PARKS AND MONUMENTS
E N R I C H M E N T The Origin of the Universe 221 Jasper National Park 290

A Solar System Tour, from Center to Fringe 221 E N R I C H M E N T A Colossal Ordovician Ash Fall: Was it a
Killer? 292

The Caledonian Orogenic Belt 296

E N R I C H M E N T The Big Freeze in North Africa 297

Aspects of Early Paleozoic
Climate 299

xii c Contents GEOLOGY OF NATIONAL PARKS AND MONUMENTS
Grand Staircase–Escalante National
CHAPTER 11 Monument 407

Late Paleozoic Events 3 0 3 Gondwana Events 410

The Seas Come in, the Seas Go Out 306 E N R I C H M E N T Chunneling Through the Cretaceous 411
Unrest Along the Western Margin of
CHAPTER 14
the Craton 309
Life of the Mesozoic 4 1 7
E N R I C H M E N T The Wealth of Reefs 312
Climate Controls It All 418 429
To the East, A Clash of Continents 315 Mesozoic Invertebrates 421
Sedimentation and Orogeny in Mesozoic Vertebrates 426
Dinosaurs: “Terrifying Lizards”
the West 323
Europe During the Late Paleozoic 326 GEOLOGY OF NATIONAL PARKS AND MONUMENTS
Gondwana During the Late Paleozoic 327
Climates of the Late Paleozoic 327 Dinosaur National Monument 430
Mineral Products of the Late
Dinosaurs: Cold-blooded, Warm-blooded, or
Paleozoic 328 Both? 444

GEOLOGY OF NATIONAL PARKS AND MONUMENTS Dinosaur Parenting 445
Acadia National Park 329
E N R I C H M E N T Can We Bring Back the Dinosaurs? 445
CHAPTER 12
Flying Reptiles 446
Life of the Paleozoic 3 3 5 Dragons of the Seas 448
The Rise of Modern Birds 449
Animals with Shells Proliferate—and So Does
Preservation 337 E N R I C H M E N T The Archaeopteryx Controversy 450

The Cambrian Explosion of Life: Amazing Fossil The Mammalian Vanguard 451
Sites in Canada and China 338 Sea Plants and Phytoplankton 455
Land Plants 457
The Great Ordovician Biodiversification Late Cretaceous Catastrophe 459
Event 343
E N R I C H M E N T Bolides and Modern Day
A Variety of Living Strategies 343 Catastrophism 463
Protistans: Creatures of a Single Cell 343
Marine Invertebrates Populate the Seas 344 CHAPTER 15

E N R I C H M E N T The Eyes of Trilobites 360 Cenozoic Events 4 6 9

Advent of the Vertebrates 361 The Tectonics–Climate Connection 470
The Rise of Fishes 363 Stability and Erosion Along the North American
Conodonts: Valuable But Enigmatic
Eastern Margin 472
Fossils 370 Gulf Coast: Transgressing and Regressing Sea 473
Advent of Tetrapods 370 The Mighty Cordillera 473
Plants of the Paleozoic 374
E N R I C H M E N T Oil Shale 477
E N R I C H M E N T A Walk Through an Ancient Rainforest 377
Creating the Basin and Range Province 478
Mass Extinctions 377
GEOLOGY OF NATIONAL PARKS AND MONUMENTS
CHAPTER 13 Badlands National Park, South Dakota 479

Mesozoic Events 3 8 5 E N R I C H M E N T Hellish Conditions in the Basin and Range
Province 482
The Breakup of Pangea 386
The Mesozoic in Eastern North America 387 Colorado Plateau Uplift 482
The Mesozoic in Western North America 390 Columbia Plateau and Cascades

GEOLOGY OF NATIONAL PARKS AND MONUMENTS Volcanism 482
Zion National Park 394 Sierra Nevada and California 485
The New West Coast Tectonics 487
E N R I C H M E N T Did Seafloor Spreading Cause Cretaceous Meanwhile, Drama Overseas . . . 488
Epicontinental Seas? 401 Big Freeze: The Pleistocene Ice Age 491

The Tethys Sea in Europe 406

Contents b xiii

What Caused the Ice Age? 497 E N R I C H M E N T Being Upright: Good News, Bad News 558
Cenozoic Climates: Global Warming Then
E N R I C H M E N T Neandertal or Neanderthal? 559
Cooling 501
E N R I C H M E N T Neandertal Ritual 560 565
CHAPTER 16
Humans Arrive in the Americas 563
Life of the Cenozoic 5 0 5 Human Population: 7 Billion and Growing
What Lies Ahead? 566
Grasslands Expand, Mammals Respond 507
Plankton 508 APPENDIX A
Marine Invertebrates 509
Vertebrates 512 Classification of Living Things A1
Mammals 517
Monotremes 519 APPENDIX B
Marsupials 519
Placental Mammals 520 Physiographic Provinces of the United States A5

E N R I C H M E N T How the Elephant Got Its Trunk 536 APPENDIX C

Demise of the Pleistocene Giants 539 Periodic Table and Symbols for Chemical
Elements A6
CHAPTER 17
APPENDIX D
Human Origins 5 4 3
Convenient Conversion Factors A9
Primates 544
Modern Primates 546 APPENDIX E
Primate Beginnings 547
The Early Anthropoids 550 Exponential or Scientific Notation A10
The Australopithecine Stage and the Emergence of
APPENDIX F
Hominins 552
A Species in Transition: Australopithecus Rock Symbols A10

Sediba 554 APPENDIX G
The Homo Erectus Stage 556
Final Stages of Human Evolution 557 Bedrock Geology of North America A11

APPENDIX H

Common Rock-forming Silicate Minerals A12

G L O S S A R Y G1

I N D E X I1

1

Orange, brown, and white cross-bedded Navajo Sandstone
exposed in Checkerboard Mesa, Zion National Park,
Utah. The criss-cross pattern is the result of deposition by
wind. (Glen R. Osburn)

CHAPTER 1

The Science of
Historical Geology

A million years is nothing. This planet lives and Key Chapter Concepts
breathes on a much vaster scale.
The study of events in the Earth’s past can often be
—Michael Crichton, Jurassic Park used to predict future events.

OUTLINE The Earth and its inhabitants have undergone
c PART I—DISCOVERING TIME AND continuous change during the past 4.56 billion
years. (4,560,000,000 years).
DECIPHERING EARTH’S AMAZING HISTORY
c WHY STUDY EARTH HISTORY? Physical geology examines the structure,
c GEOLOGY LIVES IN THE PRESENT AND composition, and processes that affect the Earth
today. Historical geology considers all past events
THE PAST on Earth.
c A WAY TO SOLVE PROBLEMS:
The scientific method is a way to find answers to
THE SCIENTIFIC METHOD questions and solve problems. It involves
c THREE GREAT THEMES IN EARTH HISTORY collection of information through observation
c WHAT LIES AHEAD? and experimentation, formulation of answers,
c SUMMARY and validation by testing.
c KEY TERMS
c QUESTIONS FOR REVIEW AND DISCUSSION The three most pervasive themes in the history of
Earth are the immensity of geologic time, plate
tectonics, and biologic evolution.

Welcome to the amazing history of our planet! Here
you will discover many astonishing events of the
past and learn how we came to understand them.
You will learn the intriguing story of how life devel-
oped on Earth and how an extraordinary species
evolved that is the only one capable of reading books
like this: us.

Our planet formed about 4.56 billion years ago. Since
that time, it has circled the sun like a small spacecraft
observing a rather average star. Between about 300,000
and 150,000 years ago, a species of primate we call Homo
sapiens (Latin: wise human) evolved on Earth. Unlike
earlier animals, these creatures with oversized brains and
nimble fingers asked questions about themselves and
their surroundings. Their questioning has continued to
the present day: How did Earth form? Why do earthquakes
occur? How do we unlock the history that lies beneath the land
and below the ocean floor?

Even ancient people sought answers to these ques-
tions. In frail wooden ships, they probed the limits of
the known world, fearing that they might tumble from
its edge or be consumed by dragons. Their descend-
ants came to know the planet as an imperfect sphere,
and they began to examine every obscure recess of its
surface. In harsher regions, exploration proceeded
slowly. It has been only within the last 100 years

1

2 c Chapter 1. The Science of Historical Geology cGEOLOGY LIVES IN THE PRESENT
AND THE PAST
that humans have penetrated the deep interior of
Antarctica. Today, except for a few areas of great For convenience, we divide the body of knowledge
cold or dense tropical forest, the continents are well called geology into physical geology and historical
charted. New frontiers for exploration now lie beneath geology. The word “convenience” is appropriate here.
the ocean and outward into space. This is because many aspects of physical geology are
necessary to understand the Earth’s history.
cWHY STUDY EARTH HISTORY? Conversely, many events in our planet’s 4.6 billion-
Earth’s spectacular history deserves to be closely year history determine the Earth’s physical character-
examined, for it permits us to see the future. We istics. Topics such as weathering and soils, mass
expect that many events of the past will happen again. wasting, geologic resources, the behavior of streams,
We owe it to ourselves and to our home planet to look glaciers, winds, ground water, ocean waves and cur-
carefully at those events and attempt to understand rents, and geologic resources are typical subjects found
them. in physical geology textbooks. Historical geology
From the time of its origin to the present day, Earth addresses Earth’s origin and evolution, distribution
has undergone continuous modification. Continents of lands and seas through time, the growth and reduc-
have been flooded by vast inland seas. They also have tion of mountains, and the succession of animals and
ponderously drifted across the face of the globe and plants that lived in the ocean and on continents down
slowly collided with other landmasses to form lofty through the ages. The historical geologist sees the
mountain ranges (Fig. 1-1). Massive glaciers have results of past geologic events and works backward in
buried vast tracts of forest and prairie. Earth has time to find their cause. The process rather reminds us
witnessed recurrent earthquakes, rampant volcanism, of the “Crime Scene Investigator” who arrives on the
catastrophic impacts of meteorites and asteroids, and scene of a murder and must reconstruct what hap-
major changes in the chemistry of the ocean and pened from whatever clues he or she can find.
atmosphere. Along with these physical changes, life
on Earth has also undergone change; sometimes slow, Geology primarily studies Earth, but its view has
but occasionally swift and deadly. broadened to include other planets. This increase in
All of these events of the geologic past have rele- scope is appropriate, because geologic knowledge is
vance to our lives today. By discovering why they employed in interpreting the images of the surfaces of
occur, we can better predict the future. For example, other planets and their moons, in estimating the power
we are carefully examining climatic trends of the past of volcanoes on Venus, and in identifying rocks and
so we can better understand today’s climatic changes. minerals from Earth’s moon.
With knowledge of Earth’s history, we can plan ahead.
We can avoid further damage to this planetary haven Geologists examine the minerals in meteorites
in space that is our home. Aside from these concerns, (Fig. 1-2) to discover how Earth formed. With sophis-
an important reason to study Earth history is simply to ticated instruments, they scrutinize images of planets
better understand our favorite and unique planet and or interpret data transmitted by space probes and
its amazing forms of life. planetary exploration rovers (Fig. 1-3). Other geolo-
gists unravel the structure of mountain ranges, attempt

FIGURE 1-1 The magnificent
Canadian Rocky Mountains viewed
from Malign Lake, British Columbia.
These mountains were initially raised
over 80 million years ago. Their present
appearance results from further uplift
and erosional sculpting during
subsequent geologic periods down to
the present day. (Harold Levin)

A Way to Solve Problems: The Scientific Method b 3

mathematics, and biology. For example, a petroleum
geologist must understand the physics of moving
fluids, the chemistry of oil and gas, and the biology
of the fossils (Fig. 1-5) that are used to trace subsurface
rock layers.

Because geology incorporates information from so
many other scientific disciplines, it is an “eclectic”
science; it draws on information from many sources.
All sciences are eclectic to some degree, but geology is
decidedly more so.

cA WAY TO SOLVE PROBLEMS:
THE SCIENTIFIC METHOD

FIGURE 1-2 Stony meteorite resting on snow of the Geologists, both physical and historical, employ the
Antarctic Ice Cap. The contrast of dark meteorites on white same procedures used by scientists in other disciplines.
snow makes Antarctica a good place for collecting meteorites. Those procedures are called the scientific method.
The meteorite is composed of iron and magnesium silicates. It The scientific method is a systematic way to uncover
is about 8 cm. in diameter. (Harold Levin) answers to questions, solutions to problems, and evi-
dence to prove or disprove ideas and beliefs.
to predict hazards like earthquakes and volcanic erup-
tions (Fig. 1-4), or study the behavior of glaciers, A scientific investigation often begins with a question
streams, or underground water. (Fig. 1-6). It proceeds to the collection of data (facts
from observations and experiments), and is followed
Many “exploration geologists” search for fossil fuels by the development of a hypothesis that fits all the
and the metallic ores vital to our standard of living. data and is likely to account for observations in the
This requires knowledge of both physical and histori- future as well as the present. A hypothesis then is tested
cal geology. To understand where to find resources, and critically examined by other scientists, so it subse-
exploration geologists draw on their knowledge quently may be confirmed, modified, or discarded. In
of Earth history, astronomy, physics, chemistry, some cases, several hypotheses may be proposed to
explain the same set of data, and each is tested until
the “best” one emerges. For example, the origin of the
universe and the origin of life have each been the
subject of several hypotheses.

A hypothesis that survives repeated challenges and
is supported by accumulating favorable evidence may
be elevated to a theory. A theory has survived such

FIGURE 1-3 (A) NASA’s robotic rover Opportunity on Mars in 2004. Its instruments photographed rock
outcrops and found evidence for the former presence of water on the red planet. (B) The Phoenix lander on
the arctic plains of Mars in 2008. The soil sampler in the foreground is digging a trench and will pass soil
samples to small chemical laboratories for analysis. (Renditions by artist Corby Waste of the Jet Propulsion
Laboratory. Courtesy of NASA and the Jet Propulsion Laboratory)

4 c Chapter 1. The Science of Historical Geology

FIGURE 1-4 Geologist studies a
disastrous mudflow resulting from
the eruption of Mount St. Helens,
Washington State, in 1980. Mount
St. Helens is in the background. The
eruption devastated nearly 600 square
kilometers and killed 57 people.
(USGS/CVO)

SCIENTIFIC LAW
Understanding of how

things happen

THEORY
Consistent
confirmation from
further testing

Further testing

HYPOTHESIS
Explanation of

data

Data collection,
observations, and experiments

QUESTIONS
What do we want to

know? Is a belief
correct?

FIGURE 1-5 Fossil shells of single-celled marine animals. FIGURE 1-6 Typical steps in the scientific method. An
These distinctive shells of foraminifera are widely used to initial question stimulates the collection of data. Scientists
identify rock formations when drilling for oil and natural gas. then study the data and build a hypothesis to answer the
(The shells average about .10 mm.) (Harold Levin) question. Through exhaustive testing, the hypothesis is
accepted, revised, or rejected. A hypothesis that consistently
answers questions rises to the level of a scientific theory. If
the theory “works” in every known case over a long period,
it can attain the status of a scientific law or principle.

A Way to Solve Problems: The Scientific Method b 5

ENRICHMENT

Scientific Discoveries Must be Tested on the wobble detected in the dwarf star believed to be
caused by the attraction of Gliese 581g as it orbited its dwarf
The scientific method tells us that any finding or hypothesis star. Another clue was a slight dimming of the light from the
made by scientists must stand the test of additional exper- parent star caused when the planet passes in front of it.
imentation or observation. An example of how science is
continuously tested began in September 2010 when astron- Exciting news, but the calculations used to identify Gliese
omers from the United States made headlines with the 581g were soon to be tested by Swiss scientists. They
announcement that they had detected the first planet out- analyzed all the old and new data and announced they could
side of the solar system with conditions suitable for life. They not confirm the presence of Gliese 581g. Would this be the
called the body a “Goldilocks Planet” because, like the end of the story? Not at all, for other scientists are continuing
porridge in the children’s story, it was “not too hot and to analyze the data. New technology is being employed that
not too cold.” The presumed planet orbited a dwarf star may refine the data. Ultimately, additional discoveries will
named Gliese 581 and was called Gliese 581g. Like many either refute or support the original scientific report. That is
planets too far away to be seen directly by telescope, the the way science works.
evidence that Gliese 581g actually existed was based largely

intense scrutiny that it can be accepted with more found off the coast of North Africa. What gave
confidence than a hypothesis. Examples are the theory a stream sufficient power to erode such canyon-like
of relativity, plate tectonics theory, evolutionary the- features?
ory, and atomic theory.
3. A hard layer of sedimentary rock, detected by
It is important to understand that the term “theory” seismic instruments, lies 100 meters or so
has very different meanings to scientists and to the below the present seafloor. What is the origin of
public. To a scientist, a theory represents knowledge this hard layer?
that has a very high probability of being correct.
The term “theory” does not imply a lack of knowledge 4. Domelike rock structures exist deep beneath the
or a guess. Mediterranean seafloor. They were detected
years earlier by echo-sounding instruments,
The search for scientific truth does not end with the but they had never been investigated by drilling.
formulation of a theory. Even after a theory has been Are they huge plumes of salt (called salt domes)
firmly established, it must continue to survive rigorous like the ones that are common along the U.S. Gulf
testing derived from advances in science and technol- Coast? If so, what caused the precipitation of so much
ogy that its author could not have foreseen. If a theory rock salt?
continues to triumph over every challenge, it can be
raised to the level of a scientific law, such as the law of In 1970, geologists Kenneth J. Hsu and William B. F.
gravitational attraction. Ryan boarded the oceanographic research vessel
Glomar Challenger to search for answers (Fig. 1-8).
An Example of the Scientific Method They drilled the Mediterranean seafloor to obtain
samples. As drilling progressed, they recovered a sam-
Applying the scientific method in historical geology, ple from the surface of the hard layer. It consisted of
consider the following research into some curious pebbles of hardened sediment that had once been soft,
features of the Mediterranean seafloor. deep-sea mud, plus granules of gypsum (a mineral
commonly formed by the evaporation of seawater).
The Questions. Several observations raised ques- However, not a single pebble was found to indicate
tions about the Mediterranean Sea’s history: that the sediment had been carried to the sea from
surrounding land areas.
1. Microscopic single-celled plants and animals
living in the Mediterranean changed abruptly In the days following, samples of solid gypsum were
about 6 million years ago. Most of the older repeatedly brought on deck as drilling penetrated
organisms were nearly wiped out. A few survived the hard layer—clearly, it was a bed of gypsum. The
by migrating into the Atlantic. Somewhat later, composition and texture of the gypsum suggested it
the migrants returned, bringing new species with had formed by evaporation on desert flats. But sedi-
them. What dramatic event happened? Why did the ment above and below the gypsum layer contained tiny
near extinction occur? Why did the migrants subse- marine fossils, indicating not a desert-like environ-
quently return? ment, but normal open-ocean conditions.

2. An enormous buried gorge extends seaward The Hypothesis. The time had come to formulate a
from the present course of the Rhone River hypothesis that the Mediterranean Sea was once a desert.
(Fig. 1-7). Similar buried gorges had been

6 c Chapter 1. The Science of Historical Geology

BRITISH ISLES NORWAY
SWEDEN

tic Sea
Bal

POLAND
GERMANY

FRANCE
SPAIN
PORTUGAL YUGOSLAVIA Black Sea
TURKEY
RRihvoerne

Strait of Mediterranean Sea FIGURE 1-7 Mediterranean region. When the
Gibraltar ALGERIA Mediterranean was a desert, the Rhone River no
longer entered at sea level, but flowed down a steep
MOROCCO slope, eroding a gorge over a kilometer deep that is
now buried beneath sediment.
LIBYA EGYPT

Hsu and Ryan proposed that about 20 million years so the water evaporated to precipitate sodium chloride
ago, the Mediterranean was a broad seaway linked to (table salt).
the Atlantic by narrow straits, like the present Strait of
Gibraltar. Tectonic movements of Earth’s crust closed The dried-up Mediterranean had become a vast
the straits. Turned into a giant salt lake, the Mediter- “Death Valley” 3000 meters deep. Streams entering
ranean began to evaporate and shrink. Evaporation the basin from Europe and Africa now had steep
concentrated the various salts that were dissolved in gradients, enabling them to erode spectacular gorges.
the water, and this increasing salinity exterminated Then, about 5.5 million years ago, new crustal move-
scores of marine species. ments caused the Strait of Gibraltar to open. A gigan-
tic torrent of water poured into the Mediterranean
As evaporation continued, the remaining brine basin at a velocity of 140 kilometers per hour (87 miles
became so saturated that minerals dissolved in it per hour). The deluge quickly eroded a rapidly deep-
were forced to precipitate—that is, they were forced ening gorge that was over seven kilometers wide (four
to separate from the solution. This formed the hard and a half miles wide). It would have been an aston-
layer of gypsum. Different salts precipitate at different ishing spectacle to observe, but humans were not yet
rates, and the remaining brine in the central, deeper on the scene. As the sill that once barred Atlantic
part of the basin was rich in sodium and chlorine ions, waters from flowing into the Mediterranean basin

FIGURE 1-8 The Deep Sea Drilling
Vessel Glomar Challenger. The prominent
derrick at midship allows roughnecks on the
drilling floor to hoist lengths of pipe upward,
and then add that new length of pipe to the
pipe already “in the hole.” In 1970, while
operating in the Mediterranean, the Glomar
Challenger brought up drill cores indicating
the Mediterranean was once a desert.)
(# Corbis)

was being eroded, the rate of water flow increased Three Great Themes in Earth History b 7
enormously. The Mediterranean would have been
filled in only about two years. In that same period comprehend. It is not surprising, therefore, that our
of time, the global sea level would have dropped about ancestors believed hills and valleys were changeless
9.5 meters (35 feet). and eternal and that the planet originated only a few
thousand years ago.
Evidence for the erosion of the sill that had barred
Atlantic waters from entering the Mediterranean basin Eventually, we came to realize that the slow and
was found in cores drilled in the seafloor in preparation relentless work of erosion reduces mountains to plains,
for the proposed Africa–Europe tunnel project. The that valleys are the result of long periods of erosion,
cores reveal a deep, 200 kilometer long channel filled and that sands and gravels produced by erosion have
with the loose sediment. been turned to rock. These changes clearly required
vast amounts of time.
The turbulent currents racing down the slope tore
into the hardened salt flats, grinding them into But how much time? And which event preceded—
the pebbles observed in the first sample taken by the or followed—another? To answer these questions, it
Glomar Challenger. As the basin refilled, marine orga- was necessary to find the absolute age (actual age) of
nisms returned, mostly migrants from the Atlantic. rocks in years. A way to do so was enabled by the
Soon layers of oceanic ooze were deposited above the discovery of radioactivity in 1896.
old hard layer.
Certain atoms are unstable, causing them to be
Long after the Mediterranean Basin was refilled, radioactive. This means they decay by expelling parti-
pressure from the weight of overlying sediments cles of their nuclei, converting themselves into stable
forced the salt to flow plastically upward to form “daughter” atoms, mostly of different elements. A
salt domes like those on the U.S. Gulf Coast. well-known example is uranium, which is radioactive
and continually expels particles. The rate of this decay
The questions about the faunal changes, the salt and can be accurately measured. The term half-life
gypsum deposits, the unusual pebbly sediment, and expresses the rate of decay. Half-life is the time
the deeply buried gorges were now answered. The required for one-half of the original quantity of radio-
scientific method had worked well, and the hypothesis active atoms to decay.
that the Mediterranean Sea was once a desert could now be
critically examined by other geologists. This example As an example, the radioactive element uranium-
of how questions can be solved by the scientific 235 has a half-life of approximately 704 million years.
method demonstrates how geologists are like detec- This means that after 704 million years pass, only half
tives probing the rocks for clues to the complex past of (50%) of the uranium-235 in a mineral will remain.
our planet. After a second 704 million years pass, half of that half
(25%) will have decayed. Thus, a rock that contains
The Theory. The hypothesis has survived critical only 25% uranium-235 and 75% of the daughter
examination and is on its way to being accepted as a atoms must be 1,408 million years old (704 þ 704).
theory. It is important to understand that the term theory, Using this method, some Earth rocks have been cal-
as used in science, does not mean a guess or hunch. As culated to be 4.03 billion years old (Fig. 1-9), and some
spelled out by the National Academy of Sciences, a minerals are as old as 4.38 billion years.
scientific theory is a well-established explanation of
some aspect of the natural world that is based on a
body of facts that have been repeatedly confirmed.

cTHREE GREAT THEMES
IN EARTH HISTORY

Earth’s history is like a novel, with grand, sweeping
themes. The three major themes—intensely interact-
ing themes—are deep time, plate tectonics, and the
evolution of life.

Deep Time FIGURE 1-9 Geologist Samuel Bowring stands before
Earth’s oldest known rocks. Named the Acasta Gneiss
Recognition of the immensity of geologic time is the single (pronounced “nice”), this most ancient of rocks outcrops in
most important contribution to human knowledge made by Canada’s Northwest Territories. Radioactive dating
geology. Geologists look back across 4.56 billion years methods indicate it is 4.03 billion years old, having survived
of Earth history, from our planet’s chaotic birth to the 87% of Earth’s 4.56-billion-year history. (Courtesy Samuel
present. Compared to the average duration of a human Bowring)
life, it is a span of time so huge as to be difficult to

8 c Chapter 1. The Science of Historical Geology convergent boundaries occur. Where plates grind
past one another, transform boundaries occur. You
Before the discovery of radioactivity, geologists will meet these terms again in Chapter 7, where we
were able to determine only if particular layers or examine plate tectonics in more detail.
bodies of rock were older or younger than others.
This determined a rock’s relative age. Relative age Evolution of Life (Biologic Evolution)
determinations provided a framework in which to
place events of the geologic past. Through relative Plate tectonics is the “great unifying theory” that
dating, geologists developed a geologic time scale, and explains many physical phenomena in geology. In
with dating through radioactive decay, that time scale biology, evolution is the “great unifying theory” for
was calibrated in actual years. We will examine how understanding the history of life. Because of evolution,
this was accomplished in Chapter 3. animals and plants living today are different from
their ancestors. They have changed in appearance,
Plate Tectonics in genetic characteristics, in the way they function,
and in their body chemistry, apparently in response to
A significant number of events in both physical and changes in the environment and competition for food.
historical geology are related to a grand unifying Fortunately, fossils record these changes for us to
concept termed plate tectonics. “Tectonics,” from study. Fossils are also valuable indicators of the age
the same Greek word as “architecture,” refers to large- of rocks.
scale deformation of rocks in Earth’s outer layers.
The term “plate” is given to large slabs of Earth’s Although Charles Darwin is credited for the con-
lithosphere. The lithosphere is the rigid outer layer of cept of evolution, the idea began as early as 2600
Earth (roughly 100 km thick) that includes the crust as years ago, in the seventh century BCE. We find it in
well as the uppermost part of the mantle (Fig. 1-10). the writings of the Greek philosopher Anaximander.
But Darwin and his colleague Alfred R. Wallace were
Earth’s surface consists of seven large lithospheric the first scientists to propose a hypothesis with
plates and about twenty smaller ones. The plates rest convincing evidence. They also proposed a working
on a plastic, easily deformed layer of the mantle called mechanism for evolution, which Darwin called natu-
the asthenosphere. Probably because of heat-driven ral selection.
convectional flow in the asthenosphere, the plates
move. They move almost imperceptibly, only milli- Natural selection is based on several important
meters per year. observations: that any given species produces more
organisms than can survive to maturity; that variations
Tectonic plates have well-defined edges or exist among offspring; that offspring must compete for
“margins.” Where two or more plates move apart food and habitat; and that those individuals with the
(diverge) from one another, the plate margins form
divergent boundaries. Where plates converge,

FIGURE 1-10 The lithosphere is Earth’s rigid outer shell. It lies above the asthenosphere,
and it includes the upper part of the mantle and both types of crust: continental and oceanic.

most favorable variations are most likely to survive and Summary b 9
pass their beneficial traits to the next generation.
Scientists eventually came to understand genetics as cWHAT LIES AHEAD?
the cause of these variations. Your book is divided into three parts:

Darwin provided many lines of evidence for evo- Part I—Discovering Time and Deciphering Earth’s
lution. He cited the direct evidence of changes seen Amazing History gives you a broad perspective on our
in fossils in successively younger strata. He noted subject, introducing the pioneers of historical geology
that certain organs were fundamentally similar from whose careful detective work and brilliant insights
species to species, but became modified to function enabled them to unravel the geologic record.
differently, apparently to make the species more
competitive. There were also useless organs in Part II—Rocks and What They Tell Us about
modern animals that clearly had a useful function Earth History illustrates how rocks and fossils
in ancestral species but were “evolving out.” (Human have recorded events in geologic history and how
examples include the appendix and tailbone.) He we have learned to read this wonderful archive. We
further noted that animals that looked quite different examine sedimentary rocks, because they often con-
as adults nevertheless had very similar-looking tain the ancient animal and plant debris we call
embryos. fossils. We study fossils because without them, noth-
ing would be known about our planet’s earlier inhab-
There were many other lines of evidence as well, itants. You will learn about the evidence for plate
but none as compelling as subsequent work in genet- tectonics and how this dynamic process has shaped
ics, biochemistry, and molecular biology. We now Earth and influenced life throughout geologic time.
know that the biochemistry of closely related orga-
nisms is similar to, but distinctly unlike, that of their Part III—History of Earth and Its Inhabitants gives
distant relatives. The sequence of amino acids in you our best chronology of physical and biological
proteins and the characteristics of the famous DNA events on Earth since its origin 4.56 billion years ago.
molecule are also most similar in closely related orga- We describe the planet’s birth as a primordial ball of
nisms. Discoveries such as these clearly indicate that cosmic debris and explain how our continents, oceans,
animals and plants of each geologic era arose from and the atmosphere evolved and interacted over time.
earlier species by the process we call biologic evolu- Here you can survey the vast panorama of animals and
tion. (“Biologic” refers to the way organisms change plants that populated bygone eras. Most recently,
through time, as opposed to the physical evolution of humans appeared on the scene, the first creatures
Earth rocks.) We will examine this important concept we know to discover Earth’s amazing history.
in Chapter 5.

SUMMARY

Earth is not a static ball of rock orbiting the sun. From the With sufficient testing, a hypothesis can become elevated
time of its origin 4.56 billion years ago to the present, it has to a theory or even a law.
been undergoing continuous change.
Three great themes in historical geology are deep time,
Knowledge of events in the geologic past have relevance to plate tectonics, and organic evolution.
conditions on Earth today and can be used to solve current
and future problems. Geology’s most important contribution to knowledge is
recognition of the immensity of geologic time. Realization
Geology is a science devoted to the study of Earth—its of deep time is fundamental to our understanding of Earth
origin, history, composition, and properties. Geology has and our place within the universe.
two interrelated branches. Physical geology focuses on
processes within and on the surface of Earth, as well as Plate tectonics describes large-scale movements and inter-
Earth’s chemical and physical features. Historical geology actions of rigid plates of Earth’s lithosphere. These move-
is the branch concerned with decoding the rock and fossil ments and interactions control major geologic events,
record of the planet’s long history. including mountain building, earthquakes, volcanism,
and the configuration of continents and ocean basins.
The scientific method is a procedure by which scientists
study problems and answer questions. The method usually Biologic evolution refers to changes that have occurred in
begins with one or more questions, then collection of data. organisms with the passage of time. It is fundamentally
From this, scientists formulate a hypothesis that is sup- important for understanding humanity’s relationship to
ported by the data. It can then be tested for its validity. Earth’s biological realm, and it provides a basis for deter-
mining the relative age of rocks.

10 c Chapter 1. The Science of Historical Geology

KEY TERMS

absolute age, p. 7 mantle, p. 8
asthenosphere, p. 8 natural selection, p. 8
convergent boundary, p. 8 physical geology, p. 2
crust, p. 8 plate tectonics, p. 8
divergent boundary, p. 8 relative age, p. 8
half-life, p. 7 scientific law, p. 5
historical geology, p. 2 scientific method, p. 3
hypothesis, p. 3 theory, p. 3
lithosphere, p. 8 transform boundary, p. 8

QUESTIONS FOR REVIEW AND DISCUSSION

1. Application of the scientific method may lead to the the collection of data through observation and
development of a hypothesis. The hypothesis may at a later experimentation, and the formulation and testing of
stage be elevated to a theory (like the theory of plate hypotheses.
tectonics). What must occur to validate a hypothesis in
this way? d. A method which seeks to understand the natural
world from accounts of mystical revelations, mythology,
2. What is the lithosphere? How does it differ from the or from information embedded in one’s traditions,
crust? language, or culture.

3. Define plate tectonics. Differentiate among convergent, 13. The Earth formed:
divergent, and transform plate boundaries. What is the
significance of plate tectonics in our lives today? a. 65 million years ago

4. What is the importance of fossils to historical geology? b. 4.56 billion years ago

5. Why are fossils useful as evidence for evolution? c. About 4,000 years ago

6. How old is our planet? What is the age of the oldest d. 2.5 billion years ago
rocks found so far? Why is it unlikely that geologists will find
rocks at Earth’s surface that are as old as Earth itself? e. 542 million years ago

7. Throughout geologic time, the climate and topography 14. Which of the statements below is not valid?
of particular regions have changed. Would these changes
affect the course of evolution? Provide an example. a. A theory is a commonly accepted guess.

8. In determining the age of Earth, why is it important to b. A theory is subjected to continuous testing.
determine the age of meteorites?
c. A theory has survived more scientific testing than a
9. How might knowledge of the cause and effects of ancient hypothesis.
episodes of global warming help in estimating hazards we
may face in the future? d. A theory may be based on experimentation, obser-
vation, or both.
10. What are the three great themes of Earth history?
15. With regard to the history of the Mediterranean Basin,
11. How is the absolute age of a rock body expressed? How which of the statements below are not valid?
does that differ from the way we express relative age?
a. The buried gorges beneath the seafloor where
12. Which of the following is not an example of the major rivers could have formed when sea level was either
scientific method? very low in the Mediterranean or when the basin was
empty of sea water.
a. A procedure for conducting research that states that
a testable hypothesis should be verifiable and the results b. The layer of hardened sediment rich in gypsum and
repeatable. salt beneath the floor of the Mediterranean suggest the
Basin was once dry.
b. A systematic approach to observing phenomena,
drawing conclusions, and repeatedly testing hypothesis. c. About 6 million years ago, fossils of marine plankton
in the Mediterranean differed from those in the adjoin-
c. A procedure for the systematic pursuit of knowledge ing Atlantic, indicating the Mediterranean was not
involving the recognition and formulation of a problem, connected to the Atlantic.

d. Pebbles recovered by the Glomar Challenger from the
hard layer 100 meters below the seafloor were dropped
by glaciers as they entered the sea and began to melt.



2

Strata at Siccar Point on the southeast coast of
Scotland reveal an angular unconformity. At this
location, James Hutton perceived that the steeply tilted
layers were formed when originally horizontal beds were
deformed by mountain building. These older strata were
then eroded, and were subsequently covered by younger,
flat-lying rocks. Hutton had clearly recognized the
significance of unconformities in Earth history.
(# Marli Bryant Miller)

CHAPTER 2

Early Geologists Tackle
History’s Mysteries

And some rin up hill and down dale, knapping the Key Chapter Concepts
chunky stanes to pieces wi’ hammers, like sae many
road makers run daft. They say it is to see how the Fossils are intriguing remnants of former life,
world was made. without which a history of Earth would be
incomplete.
—Sir Walter Scott, St. Ronan’s Well
Nicholas Steno provides the basic principles of
OUTLINE superposition, original horizontality, and original
c THE INTRIGUE OF FOSSILS lateral continuity.
c AN EARLY SCIENTIST DISCOVERS SOME
John Strachey, Giovanni Arduino, Johann
BASIC RULES Lehmann, Georg Fuchsel, and Peter Simon Pallas
c EUROPEAN RESEARCHERS UNRAVEL THE recognize the general age relationships and nature
of major groups of rock assemblages in Europe.
SUCCESSION OF STRATA
c NEPTUNISTS AND PLUTONISTS CLASH Abraham G. Werner publishes the first great
c UNIFORMITARIANISM: JAMES HUTTON textbook of mineralogy, but errs in his belief
that basalt was an oceanic deposit.
RECOGNIZES THAT THE PRESENT IS
KEY TO THE PAST James Hutton perceives the immensity of geologic
c THE PRINCIPLE OF FOSSIL SUCCESSION time and understands the relation between
c THE GREAT UNIFORMITARIANISM– processes of the present and of the geologic past.
CATASTROPHISM CONTROVERSY
c THE PRINCIPLE OF CROSS-CUTTING William Smith demonstrates the use of fossils for
RELATIONSHIPS correlating strata.
c EVOLUTION: HOW ORGANISMS CHANGE
THROUGH TIME Georges Cuvier conceives the theory of
c EARTH HISTORY IN AMERICA catastrophism and stipulates that life on Earth
c SUMMARY underwent periodic extermination, followed by
c KEY TERMS reappearance of entirely new life forms.
c QUESTIONS FOR REVIEW AND DISCUSSION
Charles Lyell explains the relative age of inclusions
of one rock in another, and how rocks that cut into
and across other rocks can also be used to
determine relative geologic age.

Charles Darwin and Alfred R. Wallace conceive
of natural selection as a mechanism for evolution.

Pioneers of American geology:

Louis Agassiz recognizes evidence for the
great Pleistocene Ice Age

James Hall understands the depositional and
mountain-building history of the Appalachian
Mountains

Ferdinand Hayden, John Powell, and Clarence
King complete geologic and topographic
surveys of the American West

Othniel C. Marsh and Edwin D. Cope collect
and describe extinct vertebrate animals that
once populated North America

13

14 c Chapter 2. Early Geologists Tackle History’s Mysteries

In 1795, William Smith, a surveyor in England, was pretation was offered about 450 BCE by the Greek
hired to determine the best route for a canal for coal philosopher Herodotus. While traveling in Egypt
barges. But before beginning work, Smith asked some and Libya, Herodotus saw fossil seashells in outcrops
preliminary questions: What kind of rocks would have of sedimentary rock far from the sea and high above
to be excavated? Could he predict what rocks would sea level. They were similar to those he had seen
be encountered along the route? Where would the along the shores of the Mediterranean. From this,
digging be easy and where difficult? he concluded that the land he stood on was once
beneath the sea.
As he walked along a creek, Smith studied every
exposed edge of the layered rocks. At one point, he In the centuries that followed, fossils of all
climbed down an embankment to closely examine a kinds were observed and collected across Europe.
protruding ledge of rock. He ran his hand across the Some were bones of ancient land animals, but the
rock surface. The shell of an ancient snail caught his majority were fossil shells of marine creatures similar
eye. With hammer and chisel, Smith chipped his dis- to those observed by Herodotus (Fig. 2-1). They
covery from the rock and placed it in a canvas bag. indicated that seas had once covered the continent.
But how could such a great flooding be reconciled with
He recognized this rock with its signature fossil. He religious doctrine of the day? To many the answer
knew what rock stratum he would find below it and seemed obvious. The fossil seashells and the rocks in
what lay above it. Only yesterday, he had seen this which they were embedded were an affirmation of the
same layer two miles to the north. There was remark- biblical story of a great flood and Noah’s Ark.
able consistency here. Smith perceived that each stra-
tum, known by its fossils and rock type, not only But why would ocean-dwelling creatures be exter-
extended invisibly beneath the farms in the distance, minated by a flood of water, the very medium in which
but also maintained its place in the vertical succession they lived? And how might we explain the evidence of
of rock layers. many floods at different times and in different places?
By the beginning of the 1800s, geologists had
William Smith’s discovery is just one of the great acquired a better understanding of the immensity
contributions to early geology that you will discover in of geologic time and were able to use methods of
this chapter. relative dating to derive a more valid interpretation of
fossils. They now realized that fossils were the record
cTHE INTRIGUE OF FOSSILS of entire dynasties of living creatures that preceded
The remains and traces of prehistoric life we call fossils the arrival of humans.
have sparked the interest and imagination of people
from before the advent of civilization to the present How Do Fossils Form?
day. We think that fossils were prized possessions of
Neandertals over 30,000 years ago, for fossils have Worldwide, sediment is continually deposited.
been found among the artifacts of these heavy-browed Sediment includes mud and silt from streams, debris
cave dwellers. settling from ocean water onto the seafloor, dust or
We suspect that Neandertals believed that fossils volcanic ash deposited by the wind, and chemical
held magical powers. But a more scientific inter- precipitates. Countless times in the geologic past,

FIGURE 2-1 Limestone containing
200-million-year-old fossil
ammonites. Ammonites are an extinct
group of cephalopods related to the
living chambered nautilus. (# Biophoto
Associates/Photo Researchers, Inc.)

FIGURE 2-2 Spider preserved in amber. (Courtesy An Early Scientist Discovers Some Basic Rules b 15
W. Bruce Saunders)
Principle of Superposition
plants and animals—dead or living—have been cov-
ered by sediment and preserved as fossils. The principle of superposition states that in any
sequence of undisturbed strata, the oldest layer is at the
For preservation, quick burial is needed. In the case bottom, and successively younger layers are successively
of animals, if the creature has a hard skeleton or shell, higher. This seems obvious, easily demonstrated by
all the better. After death, soft tissue will rot away as tossing sheets of paper on the floor: the one on the
flesh, and other soft tissue are consumed by bacteria bottom was “deposited” first and therefore is the old-
and scavengers. But shell and bone will be left to est. Yet Steno, based on his observations of strata in
petrify in the gradually hardening matrix of sediment. northern Italy, was the first to formally describe the
This is why they are plentiful as fossils. concept applied to rock layers. He recognized that, as
he went downward in examining a pile of strata, he was
However, occasionally we are rewarded with evi- going deeper into the past, and as he climbed upward,
dence of soft parts. Small animals, especially insects, he was seeing rocks of increasingly younger age.
are preserved in the hardened resin (“amber”) of
conifers (Fig. 2-2). Carbonized imprints of jellyfish The fact that superposition is self-evident does not
and worms can be seen in X-rays of rocks. Skin, hair, diminish the principle’s importance in determining the
and stomach contents of Ice Age mastodons have relative age of strata, oldest to youngest. The super-
been pickled in tar from oil seeps or frozen into positional relationship of strata is not always clear where
glacial ice. layers are steeply tilted or even overturned (Fig. 2-3). In
such instances, the geologist must examine the strata for
All of these preservations—as well as foot tracks, clues useful in recognizing their lowermost and upper-
trails, and even holes dug by some animals—permit us most layers. Useful clues in determining “which way
to discern the history of life. The story is not complete, was up” at the time of deposition are the way fossils lie in
for new fossil discoveries are made daily. Often only a the rock and the evidence of mud cracks and ripple
fragment of bone or shell remains, but therein lies the marks, which form on a surface.
fascination. We become detectives, using our reason,
imagination, and intuition to recreate the form and Principle of Original Horizontality
habits of a once-living thing.
The principle of original horizontality states that
cAN EARLY SCIENTIST DISCOVERS sediment is deposited in layers that are originally horizontal.
SOME BASIC RULES Steno reasoned this to be so because most sedimentary
particles settle out of water or air straight down, under
Niels Stensen (1638–1687), a Danish physician, was the influence of gravity (Fig. 2-4). This is obvious when
widely recognized for his studies in anatomy. He you pour a cup of sand into an aquarium filled with
moved to Italy, Latinized his name to Nicolaus Steno, water; the result is a horizontal layer of sand. Today, if
and became physician to the Grand Duke of Tuscany. we see flat-lying strata, it is probably in its original
The duke was a generous employer, giving Steno horizontal position, untilted and unfolded. But if we see
ample time to tramp the countryside, visit quarries, steeply inclined strata like those in Figure 2-3B, this
and examine strata. His observations of sedimentary indicates crustal disturbance that occurred after depo-
rocks led him to formulate three basic principles of sition, altering the original horizontality (Fig. 2-5).
historical geology: superposition, original horizontal-
ity, and original lateral continuity. All three are com- Principle of Original Lateral Continuity
mon sense but valuable, and are described below.
The principle of original lateral continuity states that
a rock layer extends continuously in all directions until it
thins out or encounters a barrier. Steno’s third principle
recognizes that, when sediment is deposited on the
floor of an ocean or a lake, it extends continuously in
all directions until thinning at the margin of the basin.
Again, pour a cup of sand into an aquarium, and this is
how it behaves. The layer may end abruptly against
some barrier to deposition, or grade laterally into a
different kind of sediment (Fig. 2-6).

The significance of this in geology is that, if you
observe an exposed cross-section of strata on a valley
wall, you know that the strata, as originally deposited,
will continue laterally on the other valley wall. Further,

16 c Chapter 2. Early Geologists Tackle History’s Mysteries

FIGURE 2-3 Principle of
superposition. (A) Example of a
superpositional sequence of
undisturbed Triassic and Jurassic
strata in Capital Reef National

Monument, Utah. ? Is the green

shale relatively younger or older than
the gray rock layer beneath it? (B)
Strongly folded strata in the
Himalayan Mountains of Tibet. It
is often difficult to recognize the
original tops and bottoms of rock
layers. Geologists look for bedding
features to help them make the

distinction. ? If a geologist was

unable to tell the top from the bottom of
beds in an outcrop of several beds, how
might that affect his interpretation of
the age of the strata from bottom to
top? (Answers to questions about
figures can be found in the Student
Study Guide). ((A) USGS, (B) Peter
L. Kresan Photography)

FIGURE 2-4 Undisturbed horizontal strata.
Erosion by the Colorado River in the
foreground has carved the canyon and exposed

the strata in this natural geologic laboratory. ?

Draw an arrow on the photograph to indicate where
you would find the youngest strata. (Farley Lewis/
Photo Researchers, Inc.)

European Researchers Unravel the Succession of Strata b 17

the layer will continue for some distance in all direc-
tions. This can be determined by observing other
valley walls, road cuts, quarries, mines, and drill cores
from wells. If you do not observe lateral continuity,
and the cause is not related to the reasons given earlier,
then there may have been displacement of strata by
faulting, or the layer may have been eroded away.

Today, we recognize that Steno’s three principles
are basic to the geologic specialty of stratigraphy,
which is the study of layered rocks, including their
texture, composition, arrangement, and correlation
from place to place. Because stratigraphy enables
geologists to sequence the events that are recorded
in rocks, it is the key to understanding Earthhistory.

cEUROPEAN RESEARCHERS UNRAVEL
THE SUCCESSION OF STRATA

FIGURE 2-5 Steeply dipping shale and sandstone strata. The stratigraphic principles formulated by Steno were
These are located in the Ouachita (WASH-i-tah) Mountains rediscovered several decades later by other European
of Oklahoma. (Harold Levin) scientists. John Strachey (1671–1743) used the princi-
ples of superposition and original lateral continuity to
decipher the strata of coal-bearing rock formations in
two English counties, one in northern England and the
other in the south. He clearly illustrated the sequence
of formations encountered at the surface and in the
coal mines. He described how horizontal strata rested

FIGURE 2-6 Principle of original lateral continuity. Cross-section A shows a sandstone
layer deposited within a low-lying area or sedimentary basin. It received sediment that was
eroded from surrounding uplands. B shows the same area after erosion has exposed the
sandstone on hillsides. Note that the stratum continues horizontally until it stops at the
margin of a basin or other obstruction, or until it grades into different sediment. Also, strata
may terminate abruptly as a result of erosion or faulting, but originally they were laterally

continuous. ? What conditions in the environment of deposition may have caused the sandstone layer

(stippled) to “pinch out” on the right side of section B?

18 c Chapter 2. Early Geologists Tackle History’s Mysteries

upon the eroded edges of inclined older formations was marked by deposition of fossil-bearing rock strata
(what we now call an unconformity, described later). that lie above the primitive rocks. He called these
“transition rocks,” suggesting they were deposited
Other naturalists developed a more global view of when Earth became suitable for life.
stratigraphy. In Italy, Giovanni Arduino (1713–1795)
classified mountains according to their most abundant Above his “transition rocks,” Werner observed flat-
rock type. He defined “primary mountains” as those lying sandstones, shales, coal beds, very fossiliferous
constructed of “crystalline rocks” (what we now call limestones, and old lava flows. He added a final term,
igneous and metamorphic). He recognized these “alluvium,” for the sand, gravel, and clay that usually
“primary mountains” as likely to be the oldest in a rested on the “transition rocks.”
mountain range. His “secondary mountains” were
constructed of layered consolidated fossiliferous rocks Werner’s ideas soon drew criticism. He failed to
(what we now call sedimentary). Arduino’s “tertiary explain what had become of the immense volume of
mountains” were gravel, sand, and clay beds. He made water that once covered Earth to a depth so great that
a sensible start at classifying mountains and rocks. all continents were submerged. An even greater
problem was his insistence that the lava flows were
The origin and history of mountain ranges deposited in the same manner as limestones and shales.
intrigued another geologist, Peter Simon Pallas. He Other geologists showed field evidence of features that
traveled throughout Asia and carefully studied the could have formed only in molten lava, demonstrating
Ural and Altai Mountains. Working in the Urals, their volcanic origin.
he observed the change in rock assemblages as he
traveled from the center to the flanks of the range. Geologists with this opposing view were called
These observations helped him produce a geologic Plutonists (after Pluto, the Roman god of the under-
history of the Ural Mountains. world). Plutonists said that “fire” (heat), rather than
water, was the key to the origin of “primitive” igneous
cNEPTUNISTS AND PLUTONISTS rocks. These rocks were not formed in Werner’s steamy
CLASH sea. James Hutton of Scotland, a prominent Plutonist,
correctly stated that rocks such as the lava and granite
Historical geology is a lively profession, filled with “formed in the bowels of the Earth of melted matter
controversy and conflicting ideas. Historical geolo- poured into rents and openings of the strata.”
gists debate the causes of extinctions (like the one that
killed off the dinosaurs). They investigate and debate cUNIFORMITARIANISM: JAMES
questions about how life first appeared on Earth, how HUTTON RECOGNIZES THAT THE
continents and ocean basins have changed through PRESENT IS KEY TO THE PAST
time, and the cause of ancient episodes of global
warming or cooling. Vigorous debates about geology James Hutton (1726–1797), an Edinburgh physician
began even when the science was in its infancy. An and geologist (Fig. 2-7), saw Earth as a dynamic, ever-
example is a controversy about the fundamental ques- changing globe where new rocks, lands, and mountains
tion of the origin of rocks. arise slowly but continuously to balance their loss to
erosion and weathering. His dynamic view opposed
Professor Abraham Gottlob Werner (1749–1817) Werner’s static concept of an Earth that had changed
was an influential, eloquent, and enthusiastic lecturer very little from its beginning. Hutton also believed
at the Freiberg Mining Academy in Saxony that “the past history of our globe must be explained
(Germany). He published the first great mineralogy by what can be seen to be happening now.” In other
textbook, and many geologists used his methods to words, the present is key to the past.
identify minerals and ores.
Uniformitarianism
Werner insisted that all rocks were deposited or
precipitated from a great ocean that once enveloped This simple yet powerful idea was later named uni-
the entire planet. Because of this, Werner and his formitarianism because it says that geologic processes
many followers were called Neptunists (after Neptune, are uniform through all time. Specifically, what is
the Roman god of the sea). “uniform” are the physical and chemical laws that govern
nature. Hence, uniformitarianism says that Earth’s
Werner envisioned his universal ocean as hot, history may be deciphered by observing events today
steamy, and saturated with all the dissolved minerals and assuming that the same phenomena have operated
needed to form the rocks of his oldest division, in the same way throughout geologic time.
“primitive” rocks (the ones Arduino called “primary”).
Werner thought that most of these rocks formed the These natural laws summarize all of our observa-
cores of mountain ranges. tional and experimental knowledge. They let us pre-
dict the conditions under which water becomes ice, the
Werner’s second stage of Earth history involved behavior of a volcanic gas when it is expelled at Earth’s
subsidence and cooling of the primitive ocean to
resemble the ocean of today. Werner said this change

Uniformitarianism: James Hutton Recognizes That the Present is Key to the Past b 19

FIGURE 2-7 The present is key to the past. James Hutton, primordial, oxygenless atmosphere under conditions
Scottish physician, farmer, and geologist, recognized that that have no present-day counterpart. Yet another
studying present processes (weathering, erosion, deposition of example is meteorite bombardment of Earth’s surface.
sediment, volcanism, etc.) provides the means of interpreting Today, large meteorites that reach the surface are rare.
ancient rocks. The idea became established as the principle of But 3 billion years ago, large meteorites bombarded
uniformitarianism. (The Granger Collection, New York) the surface mercilessly.

surface, or the effect of gravity on a grain of sand Many times in the geologic past, parts of continents
settling to the ocean floor. Uniform natural laws stood higher above the oceans than they do today.
govern geologic processes such as weathering, erosion, This higher land elevation allowed for faster rates of
glacial movement, earthquakes, volcanic eruptions, erosion and harsher climates than were possible during
and the transport of sediment by streams. the periods when lands were lower and partially
covered with inland seas. Similarly, at one time or
Hutton’s understanding of uniformitarianism was another in the geologic past, volcanism was much
simple and logical. By observing geologic processes more extensive than it is today.
around him, he was able to infer the origin of features
he discovered in rocks. For example, Hutton watched Nevertheless, ancient volcanoes disgorged gases
how waves wash onto the shore and produce ripple and deposited lava and ash just as present-day volca-
marks in the sand. From this, he inferred that strata noes do. Modern glaciers are more limited in area than
near Edinburgh that bore similar markings must have those of the recent geologic past, yet they still form
been shoreline deposits of an ancient sea. Those strata erosional and depositional features that resemble those
lay far inland from a coast, but Hutton was able to of their more ancient counterparts. All of this suggests
recognize that a sea once existed where Scottish sheep that present events do indeed give us clues to the past.
now graze.
At the same time, we must be constantly aware that
Uniform, But . . . in the past, the rates of change and intensity of pro-
cesses often varied from those we are accustomed to
The uniformitarianism concept works well for solving seeing today, and that some events of long ago simply
many geologic problems. However, the geologic past have no modern counterpart.
actually was sometimes quite unlike the present. For
example, before Earth evolved its present oxygen-rich Actualism
atmosphere, oxidation was not the major rock-weath-
ering agent that it is today. Other chemical reactions To emphasize the importance of natural laws in the
weathered rocks instead. Also, life originated in a concept of uniformitarianism, many geologists prefer
the term actualism. Actualism is the principle that
natural laws governing both past and present processes
on Earth have been the same. Hutton’s friend John
Playfair (1748–1819) provided an eloquent statement
of actualism in 1802:

Amid all the revolutions of the globe, the economy of
Nature has been uniform, and her laws are the only thing
that have resisted the general movement. The rivers and
rocks, the seas and the continents have changed in all
their parts; but the laws that describe those changes, and
the rules to which they are subject, have remained
invariably the same.

In addition to uniformitarianism, Hutton’s Theory of
the Earth (1785) brought together many concepts from
naturalists who preceded him. He showed that rocks
recorded events that had occurred over immense peri-
ods of time. He also demonstrated that Earth had
experienced many episodes of mountain-raising
upheaval, separated by quieter times of erosion and
sedimentation. As he put it, there had been a
“succession of former worlds.”

Hutton saw a world of cycles. In these cycles, water
sculpted Earth’s surface and carried erosional detritus
from the land, down the network of streams and rivers,

20 c Chapter 2. Early Geologists Tackle History’s Mysteries

and into the sea. The sediment in the sea became THE PRINCIPLE OF FOSSIL

ccompacted into stratified rocks, and then, through SUCCESSION

the action of enormous forces, these sea-bottom You have already met William Smith (Fig. 2-8). He was
rock layers were forced upward to form new lands. the Englishman employed to locate routes of canals, to
In this endless process, Hutton found “no vestige of a design drainage for marshes, and to restore springs. As
beginning, no prospect of an end.” No longer could he worked, he came to understand the principles of
geologists compress all of Earth history into the short stratigraphy, for they were of immediate use to him. He
span suggested by the Old Testament of the Bible. knew that different types of stratified rocks occur in a

Unconformities definite order. He knew they can be identified by their
lithology, the soils they form, and the fossils they

In his homeland of Scotland, at Siccar Point on the contain. Thus, he predicted the types and thicknesses
North Sea coast, Hutton observed a remarkable of rock that needed to be excavated for canals used to
positioning of the rocks. As you can see in the chap- transport coal from mines to markets. It is small wonder
ter-opening photo, steeply inclined older strata are that he acquired the nickname “Strata Smith.”
covered by flat-lying layers. It was clear to Hutton that
the lower rocks had been deposited, then tilted, then Smith’s use of fossils was particularly significant.
partly removed by erosion, and finally the younger Prior to his time, collectors rarely noted the precise
rocks were deposited on top. rock layer from which a fossil was taken. But Smith
carefully recorded the occurrence of fossils and quickly
The erosional surface separating the lower, slop- became aware that certain rock units could be identi-
ing layers from the overlying horizontal beds indi- fied by their fossil assemblages. He explained this
cated a time gap (hiatus) in the rock record. Because relationship in Strata Identified by Organized Fossils.
the flat-lying upper rocks did not “conform” to the Smith used his knowledge to trace strata from place
tilted lower rocks, another geologist named the rela- to place. Even if the layer he was tracing changed to a
tionship an unconformity. Hutton was the first to different composition (for example, from a very coarse
understand and explain the geological significance of sandstone to a finer siltstone), he knew the layers were
this feature. “correlative” because of their similar fossils.

An unconformity, or erosional gap in the rock Ultimately, this knowledge led to formulation of
record, can occur either between two flat-lying rock the principle of fossil succession. This principle
sequences or between two that lie at different angles, as stipulates that the life of each age in the Earth’s
at Siccar Point. Today, we call Hutton’s site an angular long history is unique for particular intervals of geo-
unconformity because the lower beds are tilted at an logic time. Thus, extinct animals and plants have

angle to the upper beds. This and other unconform-

ities provided Hutton with evidence for periods of

erosion in his “succession of worlds.”

During most of his career, Hutton’s published

reports attracted only modest attention, partly because

his writing often was complex and difficult to follow.

Much of the attention he did receive came from

opponents who preferred Werner’s views. But British

scientists who valued Hutton’s ideas convinced John

Playfair, Hutton’s friend and a professor of mathemat-

ics and natural philosophy, to publish a summary and

commentary on Hutton’s Theory of the Earth. By con-

trast, Playfair’s text was lucid, engaging, and persua-

sive. Later geologists came to understand Hutton’s

ideas not through his own publications, but through

Playfair’s more “reader-friendly” versions.

Hutton’s life was absorbed in investigating Earth.

He scrutinized every rock exposure and soon became

so familiar with certain strata that he could recognize

them at different localities. But what he could not do

was determine if dissimilar-looking strata at different FIGURE 2-8 William “Strata” Smith (1769–1839),
locations were roughly equivalent in age. He had not “Father of English geology.” Smith discovered that
discovered how to correlate beds that differed in successive rock formations have distinctive fossils that can be
composition and/or texture (lithology). This problem used in correlation as indicators of relative age of rocks.
was soon to be resolved by William Smith. (Science Photo Library/Photo Researchers, Inc.)

The Principle of Cross-Cutting Relationships b 21

definite duration causing those living at one time to surmised that other apparent breaks in the fossil record
differ from those of another. This condition permits were not sudden and that each animal group’s fossil
geologists to assemble scattered fragments of the rock ancestors would be found in underlying beds.
record into a chronological sequence.
Today, geologists accept that both uniformitarian-
Interestingly, Smith did not know why each rock ism and catastrophism operate on our planet.
unit had a particular fauna. It was over 40 years before Uniformity prevails in day-to-day geologic processes.
Charles Darwin would publish On the Origin of Species. But occasionally, catastrophic events punctuate the
Today, we recognize that different animals and plants daily routine. Geologists recognize that rampant vol-
succeed one another over time because life evolves canism, asteroid impacts, or the onslaught of excep-
continuously. Because of this continuous change, or tionally harsh climatic conditions have indeed caused
evolution, only rocks that formed during the same age mass extinctions at various times in the geologic past.
can contain similar fossil assemblages.
cTHE PRINCIPLE OF CROSS-CUTTING
Smith’s success as a surveyor led him to all parts of RELATIONSHIPS
England for consultation. On his many trips, he care- In the early 1800s, the English geologist Sir Charles
fully recorded the types of rocks he saw and the fossils Lyell (1797–1875) authored a classic five-volume
they contained. Armed with these notes, in 1815 he work, Principles of Geology. The first volume, published
prepared a geologic map of England and Wales that in 1830, expanded on Hutton’s ideas, presented
is substantially accurate even today. In the 1830s, important geologic concepts of the day, and became
“Strata” Smith was declared the “father of English an indispensible reference for every English student of
geology.” geology. It would go through eleven volumes within
Lyell’s lifetime. Charles Darwin carried a copy of the
cTHE GREAT UNIFORMITARIANISM– first volume with him on his famous voyage aboard
CATASTROPHISM CONTROVERSY the H.M.S. Beagle.
In his famous book, Lyell (Fig. 2-9) described how a
As Smith made his observations in England, scientists geologic feature—like a mineral vein or a molten rock
across the English Channel in France also were
advancing the study of fossils. Baron Georges L eopold FIGURE 2-9 Sir Charles Lyell. His five-volume work,
Cuvier (KYOO-vee-yay, 1769–1832), an expert in com- Principles of Geology, expanded on Hutton’s idea, presented
parative anatomy, became the most respected verte- important geologic concepts of the day, and became an
brate paleontologist of his day. Cuvier and his indispensable handbook for English geologist. (The Natural
colleague Alexander Brongniart validated Smith’s History Museum/The Image Works)
findings that fossils display a definite succession of
types within a sequence of strata, and that this succes-
sion remains more or less constant even in widely
separated locations.

Cuvier recognized that certain large groupings of
strata were often separated by unconformities. Scan-
ning from a lower group of strata across an
unconformity to an overlying group, he often observed
a dramatic change in animal fossils. From this, Cuvier
concluded that the history of life was marked by
frightful catastrophes involving flooding of the con-
tinents and crustal upheavals. He wrote that each
calamity completely extinguished life and was followed
by the appearance of new animals and plants. This
theory was soon to be labeled catastrophism.

But many geologists of the time, including the
eminent Charles Lyell (more on him below), did
not buy Cuvier’s hypothesis. They thought that
Earth’s record of past life reflected continuous
uniform change down through the ages (Hutton’s
uniformitarianism view). Thus began a controversy
between catastrophism and uniformitarianism that
rivaled the earlier Neptunist–Plutonist debates.
Uniformitarians argued that seemingly abrupt changes
in fossil fauna were actually caused by missing strata or
other imperfections in the geologic record. They

22 c Chapter 2. Early Geologists Tackle History’s Mysteries

injected into other rocks—cuts across existing rock. FIGURE 2-10 The principle of
This means that the penetrating rock must be younger cross-cutting relationships. The
than the rock that is being penetrated (Fig. 2-10). Put dark rock (basalt) cuts across the
another way, the feature that is cut is older than the lighter-colored rock. The basalt was
feature that does the cutting. This principle of cross- once molten, and in the molten state
cutting relationships applies not only to rock bodies, it penetrated the existing light-
but also to geologic structures like faults and colored rock. Thus, the lighter-
unconformities. colored rock is older, and the basalt
is younger. (Harold Levin)
Using the principle of cross-cutting relationships,
let us reconstruct how the cross-section in Figure 2-11 pattern) is dramatically displaced by hundreds
came to be. We’ll start with the oldest rock and build of feet diagonally. If it occurred suddenly, such a
events toward the present. fault would generate a major earthquake. The
fault obviously is younger than the original rocks
1. First, the rocks in 1 were deposited. Note their it broke.
“original horizontality.”
3. The orange mass of molten magma 3 intruded
2. Then, Earth movements broke the rocks at fault next, cutting across the earlier fault. So, 3 is
2. Note how the layer of limestone (brick younger than rocks 1 and fault 2.

5 Unconformity 4. Next, a long period of erosion occurred, creating
the unconformity 4. The unconformity is youn-
ger than 1, 2, and 3.

5. New deposition began, forming rock layers 5.
The youngest of these layers, of course, is on top.

6. Many additional strata may have been deposited
above 5, but they have been eroded away, bring-
ing us to the present erosional surface 6.

6
Present
erosional
surface

4
Shale

Sandstone
1
3 Limestone

2 Igneous
rock

FIGURE 2-11 Determining the sequence of geologic events from cross-cutting relationships and
superposition. From first to last, the sequence indicated in the cross-section is deposition of 1, then
faulting 2, then intrusion of igneous rock mass 3, then erosion to produce the unconformity 4, then
deposition of 5, and finally erosion 6. Strata labeled 1 are oldest, and strata labeled 5 are youngest.

Evolution: How Organisms Change Through Time b 23
Limestone

Shale

Sandstone

FIGURE 2-12 Inclusions. (A)
Granite inclusions in sandstone
indicate that granite is the older unit.
(B) Inclusions of sandstone in the
granite indicate that sandstone is the

older unit. ? If the granite in (A) was

found to be 150 million years old, and the
shale above the sandstone 100 million
years old, what can be stated about the age
of the sandstone?

We have just applied the basic principles of stratig- Beagle, he carried with him a copy of the first volume
raphy in the same manner that a geologist does. of Lyell’s Principles of Geology. Before the Beagle
dropped anchor at its first stop (the volcanic island
Another of Lyell’s Principles regards rock frag- of St. Jago in the Cape Verde island chain), Darwin
ments. In Figure 2-12, along the granite-sandstone had carefully read the entire book. By the time Darwin
rock contact line, fragments of one rock appear in the returned from the five-year voyage in 1836, he had
other. These fragments are called inclusions. Lyell amassed volumes of notes to support his hypothesis
discerned that fragments within larger rock masses of how organisms have changed through time by
are older than the rock masses in which they are natural selection.
enclosed. Thus, whenever two rock masses are in
contact, the one containing pieces of the other will be Darwin’s hypothesis was based on logical observa-
the younger of the two. tions and conclusions. He observed that all living
things tend to reproduce at prodigious rates. Yet,
In Figure 2-12A, there are pebbles of granite (a despite their reproductive potential to do so, no single
coarse-grained igneous rock) within the sandstone group of organisms has overwhelmed Earth’s surface.
(a fine-grained sedimentary rock). This tells us that In fact, the size of any given animal or plant population
the older, eroded granite fragments were incorporated
into the younger sandstone as it formed. In contrast, in FIGURE 2-13 Charles Darwin as a young man. Darwin is
Figure 2-12B, the granite is younger, and it intruded shown shortly after he returned to England from his world
as a melt into the older sandstone. Because there are voyage on the H.M.S. Beagle. Observations made during this
sandstone inclusions in the granite, we know that the voyage helped him formulate the concept of evolution by
granite is the younger rock. natural selection. (Bridgeman Art Library/New York)

cEVOLUTION: HOW ORGANISMS
CHANGE THROUGH TIME

William Smith and others recognized that strata often
contain specific fossils and that a general progression
toward more modern-looking shells occurs in higher
(younger) strata. But why? Charles Darwin (1809–
1882) provided a hypothesis that accounted for the
changes seen in the fossil record.

We think of Charles Darwin (Fig. 2-13) as a
biologist. However, he also had a good understanding
of geology. He received instruction in the correct
interpretation of rock exposures in Wales from
Cambridge University geologist Adam Sedgwick
(1785–1873). We will meet with Adam Sedgwick again
in the next chapter. When Darwin had secured an
unpaid position as a naturalist aboard the H.M.S.

24 c Chapter 2. Early Geologists Tackle History’s Mysteries

remains fairly constant over long periods of time. Louis Agassiz on Glaciers
Because of this, Darwin concluded that not all the
individuals in a given generation can survive. Louis Agassiz (AGG-uh-see, 1807–1873), a Swiss pale-
ontologist, arrived in North America in 1846. He
In addition, Darwin recognized that individuals of became a Harvard University professor, founding
the same kind differ from one another in various the Harvard Museum of Comparative Zoology.
features of morphology (form and structure) and phys- Agassiz published important works on fossil fishes,
iology (organs and functions). He concluded that mollusks, and echinoids. By 1859, he had become a
individuals with variations that are most favorable well-known American scientist.
for survival and reproduction in a given environment
would have the best chance of surviving and trans- Before coming to America, Agassiz studied glaciers
mitting these same favorable traits to the next genera- in the Alps. He proposed that immense ice sheets once
tion. Hence the term natural selection. covered much of North America and Eurasia. In his
1840 Studies of Glaciers, he wrote:
In those pioneering days, even the most brilliant
scientists were handicapped by not knowing key facts The surface of Europe, adorned before by tropical vege-
or hearing about discoveries of others. In Darwin’s tation and inhabited by troops of large elephants, enor-
case, he had no knowledge of genetics. Therefore, he mous hippopotami, and gigantic carnivora, was suddenly
did not know the cause of the variations in plants and buried under a vast mantle of ice, covering alike plains,
animals that were so critical to his hypothesis. Gregor lakes, seas, and plateaus.
Mendel’s 1865 report of experiments in heredity had
escaped Darwin’s attention. Nevertheless, in the dec- Agassiz also found ample evidence in America for
ades following Darwin’s death, geneticists established his “Ice Age hypothesis.” Along the shores of Lake
that the variability essential to Darwin’s hypothesis of Superior, he showed his students bedrock with aligned
natural selection derives from new gene combinations scratches called glacial striations. These scratches were
that occur during reproduction and from genetic scoured by rocks frozen into the base of advancing ice,
mutation. which “sandpapered” the rocks over which it flowed.
He observed huge boulders transported by the ice
Darwin was reluctant to face the controversy that from locations far to the north and deposited on the
his hypothesis would provoke, so he did not publish his landscape, looking very out-of-place, as glacial erratics.
findings immediately. He did, however, confide in On American prairies, Agassiz recorded glacial
Charles Lyell and the great botanist Joseph Hooker, moraines, rock debris deposited by a melting glacier.
and these friends urged him to publish quickly, before And he correctly interpreted New York’s elongated
someone else did. But Darwin procrastinated. Then, Finger Lakes as large depressions gouged by glacial
in 1858, a comparatively unknown young naturalist, erosion.
Alfred Russel Wallace, sent Darwin a manuscript for
review that contained the basic concepts of natural His “Ice Age” became recognized as the main event
selection. Wallace had conceived of natural selection of the Pleistocene Epoch, beginning about 1.5 million
while on a biologic expedition to Indonesia. The idea years ago and ending only 8,000–10,000 years ago.
came to him shortly after he read an essay on human During this frigid period, an estimated 40 million
overpopulation by Thomas Malthus. cubic miles of ice and snow blanketed one-third of
the planet’s land area.
Lyell and Hooker wanted Darwin to receive credit
for his discovery and long years of research, so they James Hall’s 12 Kilometers of Strata
arranged for immediate presentation of Darwin’s work
and Wallace’s paper before the Linnaean Society, a James Hall (1811–1898), a brilliant geologist and
prestigious group of natural scientists. Thus, the idea paleontologist, was appointed director of New York’s
of natural selection was credited simultaneously to first geological survey. Hall had plenty of strata to
both scientists. Darwin now hastened to complete examine, for the total thickness of the stratigraphic
his book, On the Origin of Species, published in 1859. sequence in New York, scattered from place to place, is
a remarkable 40,000 feet (12 kilometers or 7.5 miles).
By the time Darwin died in 1882, geologists every- Based on fossils, Hall knew from the fossils they
where were using their knowledge of evolution, bio- contained that these rocks had been deposited in
logic succession, superposition, and cross-cutting shallow water. He correctly reasoned that the seafloor
relationships to unravel Earth’s extraordinary history. had subsided at the same time that the sediment that
formed the rocks was deposited, and that subsequently
cEARTH HISTORY IN AMERICA the Appalachian Mountains were raised from what
Geologists soon applied the knowledge gained in were once marine basins. Hall gained world fame
Europe on other continents. America proved to be for his knowledge of stratigraphy and his eight-volume
an exciting natural laboratory in which to study Earth’s The Paleontology of New York.
history.

Earth History in America b 25

FIGURE 2-14 The Hayden field party in the summer of 1870. In this photo, taken near Red Buttes,
Wyoming, Ferdinand Hayden is the bearded geologist seated at the center of the table. (USGS)

Western Geology During the Civil War, he served as a battlefield
surgeon in a Union cavalry regiment. In South
After the Civil War, geologic work in the United Dakota, Hayden surveyed the badlands, the Black
States shifted westward, often through surveys of Hills, and territories along several major rivers.
the Western Territories mandated by Congress. A Subsequently he was ordered to work westward
prominent leader in these expeditions was Ferdinand into the Territory of Wyoming (Fig. 2-14). Indians
V. Hayden (1829–1887). A remarkable scientist, viewing Hayden in his geologic explorations named
Hayden was a physician before becoming a geologist. him “he who picks up rocks running.” It was Ferdi-
nand V. Hayden who convinced Congress to pass a
bill authorizing establishment of the first U.S.
National Park, Yellowstone.

Like Hayden, John Wesley Powell (1834–1902)
saw service during the Civil War. He lost his right
arm as a result of a wound received during the Battle
of Shiloh. That handicap, however, did not curtail
his geologic work in the slightest. Powell led several
geologic and geographic sureys of the West and
directed the U.S. Geologic Survey. His greatest feat
was a journey by boat through the Grand Canyon of
the Colorado River in 1869 (Fig. 2-15). The Grand
Canyon is among the most magnificent natural labo-
ratories for stratigraphy on our planet, with nearly
2,000 meters of rock layers exposed for study. These
layers contain twelve major rock units representing 2
billion years (roughly one-third) of Earth’s history.

Hayden, Powell, and others surveyed and drew
maps, recording the geology, biology, archaeology,
and paleontology of the country. Among their dis-
coveries was a wealth of dinosaur and giant mammal
bones in the rock formations of the western states.

FIGURE 2-15 The Grand Canyon of the Colorado River. The Dinosaur Rush
You come face-to-face with the immensity of geologic
time when viewing this most magnificent of natural features. Two paleontologists who exploited these bony trea-
(R. F. Dymek) sures were Othniel C. Marsh (1831–1899) and
Edwin D. Cope (1850–1897). Marsh became the first

26 c Chapter 2. Early Geologists Tackle History’s Mysteries

professor of paleontology at Yale University and later describing newly discovered vertebrates. Both made
founded the Peabody Museum of Natural History. mistakes in their haste to be first in publishing descrip-
Cope was a wealthy Quaker and taught at the Univer- tions of newly discovered fossil beasts.
sity of Pennsylvania. To hasten their description of
the abundant fossils in the West, both employed Nevertheless, the great dinosaur rush led by Marsh
professional bone-hunters who excavated fossils with and Cope provided thousands of specimens for study
pick and shovel and shipped the bones back to Marsh and museum exhibits, motivated research worldwide,
in Connecticut and Cope in Philadelphia. enhanced our understanding of life during the
Mesozoic and Cenozoic Eras, provided evidence for
Unfortunately, Marsh and Cope were egotistical, evolution, and established paleontology as a science
jealous, and competitive. A bitter feud developed as with a spirit of discovery.
each tried to surpass the other in naming and

SUMMARY

Fossils are the remains or traces of organisms from the Based on his studies of the order of strata near Bath,
geologic past preserved in rocks. England, William Smith published Strata Identified by
Organized Fossils. This work formed the basis for using
During the 1600s and 1700s, pioneering geologists for- fossils in correlation. Smith also produced the first geo-
mulated principles that are the foundation of the science of logic map of England and Wales.
historical geology.
Georges Cuvier established vertebrate paleontology as a
Nicholas Steno formulated three important principles science. He favored the theory of catastrophism, which
used in the interpretation of sedimentary rocks. The postulates periodic mass extinctions followed by the
principle of superposition states that, in layered sedimentary appearance of new faunas and floras.
rocks, the age of individual strata decreases from bottom to
top. The principle of original horizontality stipulates that Charles Lyell was a great teacher and interpreter of
because most sedimentary rocks are deposited more or less geology. He is particularly known for Principles of Geology,
horizontally, where they are found sloping or folded, they in which he described the use of cross-cutting relation-
have been deformed by strong crustal forces after deposi- ships and fragments (inclusions) for determining the rela-
tion. The principle of original lateral continuity reminds us tive age of rock bodies.
that strata extend laterally in all directions until they thin
out because of non-deposition, encounter a barrier (such Supported by his extensive research, Charles Darwin
as a coastline) or merge into another stratum. proposed that life on Earth has continuously evolved,
and that new forms of life result from natural selection,
In the 1700s, geologists in Europe classified mountains which provides for the survival and reproduction of orga-
according to their most characteristic kind of rock. The nisms best suited to their environment.
term primary was assigned to mountains composed mostly
of igneous and metamorphic rocks. Secondary mountains After observing glaciers in the Alps, Louis Agassiz presented
were mainly constructed of fossiliferous stratified sedi- his hypothesis that continental ice sheets covered much of
mentary rocks. Unconsolidated sediments such as sands North America and Eurasia in the recent geologic past.
and gravels were given the name tertiary.
James Hall proposed that the thick sequences of folded,
Abraham G. Werner wrote the first great mineralogy text- shallow marine sedimentary rocks in the Appalachian
book. He supported the erroneous concept of Neptunism, Mountains indicate subsidence of the basin floor as sedi-
proposing that all rocks were from an ancient sea. ment was added, followed by uplift and deformation that
formed mountains.
James Hutton recognized the immensity of geologic time.
He explained the concept that was later to be named Ferdinand Hayden and John Powell were influential lead-
the principle of uniformitarianism. In opposition to ers in the geologic surveys of the Western United States.
Neptunism, Hutton was a Plutonist who recognized Powell is particularly remembered for his amazing 1869
that layers of lava and granite solidified from molten exploration of the Grand Canyon of the Colorado River.
rock. Although he did not invent the term unconformity,
he recognized unconformities and understood the geo- Othniel Marsh and Edwin Cope organized and conducted
logic events that produced them. expeditions into the American West to recover, classify,
and describe hundreds of extinct vertebrates.

KEY TERMS

actualism, p. 19 principle of original lateral continuity, p. 15
catastrophism, p. 21 principle of original horizontality, p. 15
inclusions, p. 23 principle of superposition, p. 15
principle of cross-cutting relationships, p. 22 stratigraphy, p. 17
principle of fossil succession, p. 20 unconformity, p. 20

Questions for Review and Discussion b 27

QUESTIONS FOR REVIEW AND DISCUSSION

1. Which of Steno’s principles were used by William Smith 6. From the photograph of the unconformity at Siccar
in his studies of strata in England? Point at the beginning of this chapter, sketch five cross-
sections indicating the geologic events that occurred:
2. Smith found that there were different fossil groups in
different rock layers. What are two or more reasons that 1. deposition of the older Silurian strata,
might account for these differences?
2. deformation (folding) of those older strata,
3. What interpretation did Steno make when he observed
strata that were sloping steeply into the ground or even 3. erosion to produce the surface of unconformity,
vertical rather than horizontal? What interpretation would
you make about a sequence of strata with older strata on top 4. deposition of the younger Devonian strata, and
and younger strata on the bottom?
5. further deformation to provide for slope (dip) in
4. James Hutton visited an exposure of layered rocks in younger beds.
which the igneous rock basalt had been squeezed when
molten between two beds of sedimentary rock. What fea- 7. Why did James Hall insist that the thick sequence of
tures or evidence might Hutton have looked for that would sedimentary rocks in the Appalachian Mountains required
indicate the once-molten origin of the basalt, as opposed to it that they were deposited in a crustal basin that was subsiding
having been deposited as sediment? as sediment was being added.

5. Match the description at right with the geologist at left. 8. A vein of once molten rock cuts at a high angle across
layered horizontal rocks. Would the vein be younger or older
than the horizontal rocks into which it has been intruded?

___ John Powell A. Pioneer mineralogist and
___ Charles Lyell Neptunist
___ Abraham Werner
___ Georges Cuvier B. Proponent of catastrophism
___ Louis Agassiz C. Uniformitarianism
___ James Hutton D. Original horizontality
___ Charles Darwin E. Cross-cutting relationships
___ Nicholas Steno F. Natural selection
___ James Hall G. Recognition of Ice Age
___ Othniel Marsh H. Explorer of Grand Canyon
I. From depositional basins to

mountains
J. Fossil vertebrates of

Western U.S.
K. Fossil-based correlation

3

Flat-lying Permian, Triassic, and Jurassic strata
grandly exposed along the Colorado River at Dead
Horse Point, Canyonlands National Park,Utah.
Steno’s Principle of Superposition is easily understood in
locations such as this. (Russell Kord/minnowimages.
com/Aurora Photos) ? Where on the photograph would
you be most likely to find rocks of Permian age?

CHAPTER 3

Time and Geology

Each grain of sand, each minute crystal in the rocks Key Chapter Concepts
about us is a tiny clock, ticking off the years since it
was formed. It is not always easy to read them, and Relative geologic time places events in the order in
we need complex instruments to do it, but they are which they occurred without reference to actual time
true clocks. The story they tell numbers the pages of in years. Actual geologic time provides ages
Earth history. measured in years.

—Patrick M. Hurley, noted geophysicist, 1959 Geochronologic units are units of pure time.
Chronostratigraphic units are all the rocks
OUTLINE deposited during a given time interval.
c FINDING THE AGE OF ROCKS: RELATIVE
The geologic time scale was constructed bit-by-
VERSUS ACTUAL TIME bit from fossil-bearing stratigraphic sections in
c A SCALE OF GEOLOGIC TIME Europe. These became standard sections to
c ACTUAL GEOLOGIC TIME: CLOCKS IN THE which fossil-bearing strata in other locations
could be compared worldwide.
ROCKS
c RADIOACTIVITY PROVIDES A WAY TO We are curious about the great age of rocks and fossils.
For this reason, newscasters unfailingly report that a
DATE ROCKS recently discovered fossil or rock is “umpteen-zillion
c WHAT OCCURS WHEN ATOMS DECAY? years old.” And for this reason, geologists often are
c THE PRINCIPAL RADIOACTIVE TIMEKEEPERS asked the age of rocks and fossils brought to them by
c HOW OLD IS EARTH? students and collectors. When told that the samples
c SUMMARY have ages of ten-millions or even hundred-millions of
c KEY TERMS years, people are pleased, but perplexed: “How can this
c QUESTIONS FOR REVIEW AND DISCUSSION geologist know the age of this specimen just by looking
at it?” If you want the answer, read on.

The science of determining the age of rocks is called
geochronology. It began over 400 years ago when
Nicholas Steno described how the position of strata in
a sequence could show the relative geologic age of the
layers, usually with the youngest layer on top. As
described in Chapter 2, this simple but important
idea was expanded and refined by geologists in the
1700s and 1800s, who correlated strata from different
locations that bore similar fossils. Outcrop by outcrop,
they pieced together the rock sequences with their
fossils, one above the other, until a standard geologic
time scale based on relative ages was constructed.

cFINDING THE AGE OF ROCKS:
RELATIVE VERSUS ACTUAL TIME

There are two different frames of reference when
dealing with geologic time. Relative geologic dating
places geologic events, and the rocks representing
those events, in sequence (what happened first, second,

29

30 c Chapter 3. Time and Geology and birds were the most recent to evolve, very roughly
200 million years ago.
and so on). This is done without specifying the actual
age in years. Relative geologic time simply tells us Divisions in the Geologic Time Scale
which event preceded or followed another, or which
rock mass is older or younger than another. This is a book about the history of Earth. Any history
needs a time chart to which particular events or
With the discovery of radioactivity, actual conditions can be referred. That is why we have a
geologic dating (or absolute dating) became possible. geologic time scale.
It expresses, in years, the approximate actual age of
rocks or geologic events. These actual ages usually In history classes, boundaries on the time chart are
are determined from the rate of decay of radioactive placed at times of great changes in culture and gov-
elements in the rocks, and are typically expressed in ernment. On the geologic time scale, boundaries are
hundred-thousands or millions of years. The actual placed where the fossil record reveals important
dates quantify the relative dates, showing the dura- changes in animals and plants. The divisions are
tion of time periods and specifically when events somewhat like our calendar, which divides years into
occurred. months, months into weeks, and weeks into days.
The geologic time scale divides eons into eras, eras
cA SCALE OF GEOLOGIC TIME into periods, and periods into epochs.
As early geologists were discovering the relationships
among rocks, they had no way of knowing how many Eons. The eons span the entire sweep of Earth
time units eventually would be in the completed geo- history. The Archean Eon began about 4.0 billion
logic time scale. Nor could they know which fossils years ago. It was preceded by a unit informally desig-
would be useful in correlation, or which new strata nated the “Hadean Eon.” The Hadean refers to the
might be discovered at a future time in some distant span of time from the planet’s origin to the beginning
part of the world. Consequently, the time scale grew of the Archean, an interval for which we have no
piece-by-piece, in an uncoordinated manner. It was adequate rock record. The Proterozoic Eon spans
like a scavenger hunt to find jigsaw puzzle pieces one at the time interval from 2500 to 542 million years ago,
a time, and then to assemble the pieces. and the Phanerozoic Eon covers the past 542 million
Consequently, time units were named incon- years. For convenience, geologists often refer to the
sistently as they were discovered. Sometimes a unit’s time before the Phanerozoic as the Precambrian (all
name was borrowed from local geography or from a of time prior to the Cambrian Period), which encom-
mountain range in which rocks of a particular age were passes 87% of all geologic time.
well exposed. Some units were named for ancient
tribes of Welshmen, which seems odd until you learn Eras, Periods, Epochs, and Ages. Each eon is
that the rocks were studied in Wales. Sometimes divided into eras. Their names give us a good oppor-
the name was suggested by the kind of rock that tunity to introduce Greek prefixes that are used in
predominated. historical geology. Note on the time scale that some
time interval names begin with paleo-, meso-, and so on.
Overviewing the Time Scale The prefixes mean: eo ¼ dawn, paleo ¼ ancient, meso ¼
middle, neo ¼ new, and ceno ¼ recent.
Refer to the Geologic Time Scale, Figure 3-1. The
only name familiar to you might be “Jurassic,” because The eras of the Phanerozoic Eon are the Paleozoic
of the classic dinosaur film, Jurassic Park. The other (ancient life), Mesozoic (middle life), and Cenozoic
names may seem intimidating, but they will soon (recent life). The paleo- and -zoon (Gr. Palaeos, ancient
become just as familiar. þ zoo, life) of the Paleozoic refer to now-extinct
ancient invertebrates, primitive fish, amphibians,
The column of rock layers on the left is hypothetical and reptiles. The Mesozoic was dominated by dino-
and generic; no such sequence literally exists. But its saurs. And the most recent era, the Cenozoic, is
purpose is to convey the idea that the geologic time dominated by mammals and modern plants. We
scale generally represents rock layers at various loca- live in the Cenozoic, which began about 65 million
tions on Earth. years ago.

The scale itself is divided into Eons, Eras, Periods, Eras are divided into shorter units called periods,
and Epochs, which will be explained in the next and periods are divided into epochs. These divisions
section. also are based on changes in life forms recorded by
fossils. The terms represent increments of time called
To the right of the scale are the time ranges for geochronologic units, which for convenience we will
major plant and animal groups. Note that multicellular simply call time units.
animals lacking backbones—the invertebrates—did
not appear until around 1 billion years ago. Then The actual rocks formed or deposited during a
came the fishes, land plants, and so on. Mammals specific time interval are called chronostratigraphic units
or more simply time-rock units. Table 3-1 shows the

A Scale of Geologic Time b 31

FIGURE 3-1 Geologic Time Scale. Following the principle of superposition, the scale is shown
with the oldest time period at the bottom and the youngest at the top. Note the time ranges
shown for plant and animal groups. See text for a walk-through of the time scale. (The time scale
is based on the 2009 International Stratigraphic Chart of the International Commission, on
Stratigraphy. Following the Commission’s recommendation, the Hadean is considered an
informal unit and the status of the Quaternary is not yet resolved.)

32 c Chapter 3. Time and Geology

TABLE 3 -1 Hierarchy of Time (Geochronologic) and Evolution of the Geologic Time Scale
Time-Rock (Chronostratigraphic) Terms
As we discussed earlier, the geologic time scale was not
Time Equivalent Time-Rock originally conceived as a coherent whole, but rather
Divisions Divisions evolved part-by-part as a result of research by many
geologists. As they labored to understand the confus-
Eon Eonothem ing relationships among the strata and fossils they
Era Erathem observed, trial-and-error built the time scale. Widely
Period System scattered local sections became correlated to standard
Epoch Series sections (Fig. 3-2).
Age Stage
Chron Zone (or chronozone) Let us briefly walk through the periods of the
Paleozoic, Mesozoic, and Cenozoic Eras, for these
time units that correspond to time-rock units. A sys- encompass the eon of “visible life” (Phanerozoic)
tem refers to all of the actual rock units of a given that is the focus of this book.
period, whereas a series is the chronostratigraphic
equivalent of an epoch, and a stage represents the rocks THE CAMBRIAN SYSTEM In the 1830s, Adam Sedgwick
formed during an age. (Fig. 3-3) named a sparsely fossiliferous sequence of
rocks the Cambrian System (for Cambria, the Latin
As an example of the way these terms are used, we name for Wales). Exposures of these strata (Fig. 3-4)
might say that climatic changes during the Cambrian provided a standard section against which other rocks on
Period are indicated by fossils found in the rocks of the Europe and other continents could be correlated.
Cambrian System. Note that time units bear the same Worldwide, all rocks deposited during the same time
names as the corresponding time-rock units. as the rocks in Wales (542–488 million years ago) are

FIGURE 3-2 The standard
geologic time scale for the
Paleozoic and other eras
developed without benefit of a
grand plan. Instead, it developed by
the compilation of “type sections”
from various European locations for

each of the periods. ? What criteria

at Devonshire demonstrated that these
strata were younger than those in
Wales?

A Scale of Geologic Time b 33

FIGURE 3-3 Adam Sedgwick (1785–1873) was one of
the foremost British geologists of the 1800s. A geology
professor at Cambridge University, Sedgwick defined the
highly deformed rocks of Northwestern Wales as the
Cambrian System. (World History Archive, Alamy)

FIGURE 3-4 Outcrop areas for
some Paleozoic strata in Great
Britain. The systems shown are
Cambrian (A), Ordovician (B),
Silurian (C), and Devonian (D).

34 c Chapter 3. Time and Geology

recognized as “Cambrian” through comparison to the Permia, an ancient Russian kingdom. His establishment
standard section in Wales. of the Permian System provides a fine example of the
logic employed by early geologists in assembling
THE ORDOVICIAN AND SILURIAN SYSTEMS While the puzzle of the standard time scale. The Permian is
Sedgwick labored over his Cambrian rocks in north- the youngest period in the “ancient life” Paleozoic Era.
ern Wales, a former student, Sir Roderick Impey
Murchison (1785–1871), studied fossil-bearing strata THE TRIASSIC, JURASSIC, AND CRETACEOUS SYSTEMS
in southern Wales. Murchison named these rocks The “middle life” Mesozoic Era includes three
the Silurian System, for early inhabitants called periods—Triassic, Jurassic, and Cretaceous.
the Silures. In 1835, the two men presented a paper,
On the Silurian and Cambrian Systems, Exhibiting the The Triassic System was named in 1834 by a
Order in Which the Older Sedimentary Strata Succeed German geologist for the trifold division of rocks of
Each Other in England and Wales. This publication this age in Germany. The Triassic saw the rise of early
initiated the early Paleozoic time scale. mammals.

Sedgwick had not described distinctive fossils in the Another German scientist, Alexander von
Cambrian, so it could not be recognized in other Humboldt (1769–1859), proposed the term Jurassic
countries. So, Murchison said that Sedgwick’s for strata of the Jura Mountains between France and
Cambrian was not a valid system. Murchison main- Switzerland. Reptiles prevailed in the Jurassic, notably
tained that all fossiliferous strata between the the huge plant-eating dinosaurs called sauropods and
Precambrian and the Devonian belonged to his their flesh-eating theropod foes. Flying reptiles and
Silurian System. Sedgwick vehemently objected and the earliest known birds also appeared.
nursed a deep wrath against his former friend. In 1879,
six years after Sedgwick’s death, the geologist Charles A Belgian geologist proposed the term Cretaceous
Lapworth ended the dispute by establishing a new (from the Latin creta, meaning “chalk”) for rock out-
geologic period to encompass Sedgwick’s Upper crops in France, Belgium, and Holland. The famous
Cambrian and Murchison’s Lower Silurian. Lapworth White Cliffs of Dover, England, are composed of
named the new system the Ordovician System. The chalk. Although chalk beds are prevalent in some
name was taken from a tribe known as the Ordovices Cretaceous successions, the system is actually recog-
that lived in the Welsh hills during the Roman con- nized on the basis of fossils. Dinosaurs still held center
quest ot England. stage during the Cretaceous Period but were com-
pletely exterminated at the close of Mesozoic Era.
THE DEVONIAN SYSTEM In 1839 Sedgwick and
Murchison proposed the Devonian System for out- DIVISIONS OF THE CENOZOIC The “recent life” era, the
crops near Devonshire, England. They based their Cenozoic, includes three geologic systems. The older
proposal on differences between fossils in rocks between Paleogene marks three beginnings of the “Age of
the Carboniferous System and the underlying Silurian. Mammals” when small horses, hornless rhinoceroses,
and early rodents and primates made their appearance.
THE CARBONIFEROUS SYSTEM (MISSISSIPPIAN AND
PENNSYLVANIAN) The term Carboniferous System The Neogene System includes the Miocene and
was coined in 1822 for strata that included coal beds in Pliocene Series. Youngest of all is the Quaternary
north-central England. The name refers to the fact that System, which includes rocks and fossils of the
coal consists largely of carbon. Europeans divided the Pleistocene ice age. During the Pleistocene, mam-
system into the Lower Carboniferous and the Upper moths, modern horses, camels, and saber-toothed cats
Carboniferous, the latter containing most of the roamed across Europe and North America. The most
workable coal seams. In North America, the broadly recent Holocene Series of the Quaternary is com-
comparable systems are the Mississippian (Lower posed of sediments and rocks formed after the retreat
Carboniferous strata that are extensively exposed in of Pleistocene glaciers.
the upper Mississippi River Valley) and Pennsylvanian
(Upper Carboniferous strata that have been heavily cACTUAL GEOLOGIC TIME:
mined for coal in Pennsylvania). CLOCKS IN THE ROCKS

THE PERMIAN SYSTEM In the 1840s, Murchison trav- Early Attempts to Determine Earth’s Age
eled across western Russia and was delighted to recog-
nize fossils that matched England’s Silurian, Devonian, Geologists are never satisfied knowing only the relative
and Carboniferous rocks. He recognized strata above geologic age of a rock or fossil. Scientific curiosity
Carboniferous rocks and decided they represented demands that we know its actual age in years. Numer-
a new system. He named it the Permian System, after ous attempts were made to calculate Earth’s age, some
quite ingenious. Here is a brief summary.

Biblical Calculation. An early, unscientific attempt
at quantitative dating was conducted in 1658 by