Historical Geology/Introduction

The Earth from space: thunderstorms brew over the Pacific Ocean

In this introductory article I shall explain what this textbook contains and why I wrote it the way I did.

What is historical geology?Edit

Geology can roughly be divided into physical geology, which studies the materials of the Earth and the processes operating in it, and historical geology, which aims at a reconstruction of the history of the Earth.

Historical geology requires some knowledge of physical geology for its elucidation. (Imagine, by way of analogy, forensic scientists diagnosing cause of death as a gunshot wound, which is a historical question. It would obviously be necessary for them to know something about the behavior of guns, which would be a physical question.) However, the aim of historical geology is to understand the past, and knowledge of physical geology is merely an adjunct to this aim.

We may also speak of applied geology: for example, finding and extracting oil would fall under this category. This depends on both physical and historical geology: when petroleum geologists extract oil, it is certainly their understanding of the physical nature of rocks that allows them to extract it; but when they locate oil, it is their understanding of historical geology that makes them able to find oil with a success rate better than that which would be achieved just by guessing.

That is one application of historical geology. Here is another example: suppose someone wants to build a structure such as a major dam or a nuclear power plant at a certain site, and it is discovered that a geological fault runs under the site. It would then be crucial to discover when last there was movement along the fault: if it was a hundred or even a thousand years ago, then the proposed location is dangerous; if it was ten million years ago then it is probably safe.

However, historical geology is by no means confined to facts about the past that are presently useful: it is what is called a "pure" science, in which knowledge is sought for the sake of knowledge itself, whether it turns out to be useful or is merely interesting.

Pre-requisites for reading this bookEdit

This is an introductory work, and there are no pre-requisites for reading it except perhaps that the readers should be able to remember the chemistry they learned in high-school; for those who cannot, or who have not yet attended high school, an article on chemistry for geologists is provided in Appendix C.

In writing this textbook, I have not assumed that the reader knows any physical geology, and so have introduced concepts from physical geology where necessary. I have tried not to introduce more physical geology than is necessary, although occasionally I may have let my enthusiasm get the better of me.

The purpose of this bookEdit

This book has a single purpose: to explain how it is even possible to reconstruct the history of the Earth from data available in the present. The emphasis of this book is therefore methodological: rather than explaining what is known, we shall look at how it is known.

While these issues are sometimes mentioned in introductory geology textbooks, none that I know of addresses such questions systematically: one textbook will explain how geologists originally determined that granite is an igneous rock, but devote only a cursory page to radiometric methods; another, conversely, will provide us with lots of interesting information about the isochron method, but take the igneous nature of granite as read. I, on the other hand, have tried to be thorough in explaining how we know what is known.

There are a number of reasons why I have taken this approach.

Firstly, because other textbooks do not, and it seemed to me that this was a gap that I could usefully fill.

Secondly, because it seems to me that this is where the interest in geology really lies. Geology is not a set of dead facts: it is a detective story in which the application of reason takes a thousand diverse clues and unifies them into a single narrative that makes sense of it all. It is not, for example, particularly interesting that the core of the Earth is made mostly of iron — at least, it would be more interesting if it was made of cotton candy — but the fact that we can find out what it's made of should inspire awe in any reader not too jaded to feel that emotion.

Thirdly, because the lay reader may feel an understandable skepticism when hearing an expert expound on what happened 100 million years ago. Skepticism, when it is honest, is an admirable attitude, and deserves an honest reply.

Finally, the average reader with only a basic scientific education will probably have ended up with a better grasp of scientific facts than of the scientific method. This is probably an inevitable consequence of the demands of a broad science curriculum, and in making this observation I intend no criticism of science teachers; nonetheless, it appears to be the case. It is my hope that for readers in that situation, this textbook will serve as a worked example of the scientific method. Geology is ideally suited for this purpose, since it requires clarity of thought but not (at least at the introductory level) advanced mathematics.

The introductory nature of this bookEdit

This is an introductory work: it has been my aim to keep it short. When I inform the reader that the geologists Chilingar and Wolf managed to write a book of 808 pages on the single topic of the compaction of coarse-grained sediments, you will appreciate how much more I have left out than I have put in — and will probably be grateful.

Its brevity, however, means that it will probably leave some questions of some readers unanswered. It is my hope, however, that it will at least give readers the concepts and vocabulary to ask intelligent questions, and to understand the answers when they find them.

Selection of materialEdit

The material in this textbook tends to reflect my own interests, in that it dwells more on what can be learned from sedimentary rocks and fossils than from igneous and metamorphic rocks, and more on the history of the Earth from the origin of complex life onwards than on the history of the early Earth. This emphasis is a matter of personal preference: other authors have other preferences, and have written other books.

Except for a brief summary of the geological column in the article devoted to that concept, I have not attempted to give an account of what the history of the Earth actually is. Instead, as explained above, I have concentrated on methodological questions. It is not difficult, after all, for someone with internet access to discover what happened in (for example) the Ordovician period; but accounts of what happened will typically not explain how such things can be known: it is this gap that I have attempted to fill.

Readers who have read other textbooks on historical geology will notice one unusual omission. It is customary for such works to provide at least a brief account of the theory of evolution and the evidence for the fact of evolution. I have not done so. According to the plan of this book, with its emphasis on asking: "How do we know?" if I were to deal with evolution I would have to review evidence which is not only copious in quantity but also extraordinarily diverse in kind, more so than in any other branch of science. I estimate that to give even an outline sketch of the evidence would increase the length of this book by half again. A brief review would be too brief; an adequate review would be too long.

Even so, I would undertake this task if it was of critical importance, but it is not: all it would do for the reader is that, understanding evolution, the reader would understand why the principle of faunal succession ought to be true. But to practice geology, it is enough to know that it is true; and for the purposes of this book we need go no further than investigating the evidence that it is true, without an exceedingly lengthy digression explaining its underlying causes. In the end, in the hierarchy of ideas it is geology that supplies evidence for evolution, and not vice versa.

Arrangement of materialEdit

The material in this textbook is badly arranged. This is because the material in every geology textbook is necessarily badly arranged. The problem arises from the necessity of arranging it at all: of presenting it to the reader in a certain order. There is no right order because all the concepts in geology fit together to form a unified whole.

So with a few obvious exceptions it would be best if every article in this textbook, or any scientific textbook, came last in the book, so that the reader could read it in the light of all the other articles; but this is not possible. The reader does, however, have the option of reading through the whole book twice, and I recommend this course of action to any serious reader of this or any other similar textbook.

Vocabulary in this bookEdit

New terms will be marked by the use of boldface. This may not mark the absolute first use of a word, but the place in which it is first fully defined.

All words so introduced will also be defined in Appendix A.

Note on referencesEdit

I have thought it unnecessary to provide references to facts which can be found in all or most introductory works on geology, and which may be considered common knowledge among geologists. I have, however, provided references to support specific pieces of data which are not so widely known, and for which a practicing geologist might wish to see a reference.

A bibliography is provided in Appendix B.

Note to potential editorsEdit

By contributing this book to the Wikibooks project, I am making it possible for others to edit it. It would be courteous to contact me before making any major changes or additions to this book. I would particularly ask that no-one should try to expand this book in a way that digresses from its stated purpose, since that would increase the length of the work without helping to achieve its aim.

Minor corrections of fact, grammar, spelling, etc, are of course welcome. Note however that where I have consistently used some stylistic feature this is deliberate. In particular I have purposely used only standard scientific units such as meters, grams, degrees Celsius, etc, without translating them into feet, ounces, degrees Fahrenheit, etc, as I feel that any reader who wishes to study geology should get used to the standard units, and the sooner the better.


I would like to thank the people, too numerous and usually too anonymous to mention, who have pointed out errors or infelicities in early versions of this text.

I should particularly like to thank Louis Kirstein for his patience and diligence in reviewing the entire work. This was undertaken in his personal capacity and so should not be construed as endorsement or approval by the distinguished body that employs him.

Any remaining errors or omissions are of course my own fault.

Tim Hardcastle
Las Vegas
April 2013


Last modified on 13 May 2013, at 17:18