READING THE EARTH
-Jack R. Holt
A MAP OF MY COLLEGIATE LIFE
Pilfered away, by what the Bard who sang
Of the Enchanter Indolence hath called
“Good-natured lounging,” and behold a map
Of my collegiate life. -William Wordsworth
There are few maps that are as interesting as geological maps, multi-colored mosaics
and sweeping lines. I found them to be beautiful; so, naturally, when I was a student I
decorated my dorm room with a geological map of Oklahoma. Its “psychedelic”
appearance complimented my roommate’s nearly equal-sized poster of Jimi Hendrix on
the other side of the room. I knew something of the geology of northeastern Oklahoma
because I had grown up there and hunted for fossils almost everywhere I went. When I
went to college on the southeastern side of the Ozarks in Arkansas, I discovered that
some of the things that I had learned in Oklahoma would help me in Arkansas. Both
areas were marine and mainly from the Carboniferous periods. Thus, I became interested
in the geology of Arkansas. I admit that I was not so interested in mapping the geology; I
just wanted to use an understanding of the distribution of rocks to help me to find fossils.
In my search, I ranged over the northern half of Arkansas seeking trilobites and nearly
complete crinoids. Truthfully, though, I was thrashing about blindly. The university had
no geological maps (or geologists for that matter); so, I had to discover what I could. At
first, I relied on word of mouth directions. Later, I learned to read the rock and stop at
likely spots. This distraction made me something of a hazard as I drove. Also, I
recognized likely fossil-bearing rocks in places where parking was not allowed. In the
process I had my car towed once, and the state police asked me to move my car many
times. The outcome was that I wore out two used cars, accumulated quite a nice fossil
collection, and learned something about stratigraphy in the process. The way that I
learned the relationships between sedimentary rocks, the arcane names for the various
systems, the relative ages, and the fossils that they contained was not very different from
the methods employed during the 18th and early 19th centuries in the history of geology.
LEARNING TO READ
Learn to read slow: all other graces
Will follow in their proper places. -William Walker
The dawn of understanding the sequence of rock began slowly. Nicolaus Steno
(1638-1686), a Danish natural philosopher who went to live and work in Italy in the 17th
Century, developed the first set of laws in Geology while he studied the strata of rock
around Rome. His Law of Superposition said that younger layers overlie older layers.
The Law of Horizontality stated that all strata were originally laid down horizontally and
any deviation from the horizontal came later during deformation of the rock. Although
these laws seem today to be statements of the obvious, at the time that Steno wrote them,
there was nothing obvious in Geology at all. Indeed, even the understanding that fossils
were the remains of once living organisms was not generally accepted. Robert Hooke
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(1635-1703) studied the strata on the Isle of Wight and concluded that the layers were
unique and could be distinguished by the fossils that they contained. Such insights as
those of Steno and Hooke languished because layers of rocks and their fossils could not
easily be reconciled with the Biblical accounts of Creation and the Flood.
FIGURE 1. A Geological Map of Oklahoma.
Abraham Gottlob Werner (1749-1817), one of the founders of the Neptunist school of
geology, said that rocks precipitated out of the global ocean created by the Biblical Flood.
Thus, he proposed a connection between the Biblical account and the observations of
strata and fossils. He theorized that the first rocks to precipitate were granites (primary
rocks). Then, a sequence of secondary rocks with an associated series of fossils formed
the remaining layers. Although his explanation seemed compelling, it did not match well
with observation.
The appreciation of sedimentary rocks increased throughout the 18th Century.
Giovanni Arduino (1714-1795; Italy), a provincial director of mines defined the rocks of
the Italian mountains as Primary (metal-containing strata, but no fossils), Secondary (well
defined strata with fossils), and Tertiary (strata of low mountains that contain fossils,
gravel, sand and clay). Similarly, Johann Gottlob Lehman (1719-1767), German
professor of mineralogy defined a similar series of strata and declared that all geological
strata were laid down in a sequence according to age. By the dawn of the 19th Century,
the different ages of rocks fell into the categories: Primary, Transitional, Secondary,
Tertiary, and Diluvial oldest to youngest, respectively.
By the beginning of the 19th Century, the study of Geology also led to a growing
understanding that the earth and its surface features were formed over long periods of
time. James Hutton (1726-1797), a Geologist in Edinburgh Scotland, presented the
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concept of a cycle of rock formation and erosion. This required immense amounts of
time, and suggested that different strata are removed from each other by time.
The study of the sequence of rock in a modern sense began with Georges Leopold
Chretien Frederic Dagobert Cuvier (1769-1832) and Alexandre Brongniart (1770-1847)
in Paris (see Figure 2). They examined the Tertiary rocks in the Paris basin and
correlated outcrops of strata according to the vertebrate fossils that they contained. The
Paris Basin was particularly good for this because its strata occurred in a nearly unbroken
sequence from the Secondary through the Tertiary periods. Cuvier and Brongniart
studied the strata and read their contents like leaves of a great book. Thus, they were able
to reconstruct the sequence of strata (see Figure 3), and, thereby, map the basin. They
published the map and their conclusions in 1802. In this case (and all cases of geological
mapping), the construction of the map allowed a four dimensional description of the area
(three dimensions of space and time). Thus, they concluded that the region around Paris
had been inundated by alternating periods of marine water and freshwater.
FIGURE 2. Georges Cuvier (left) and Alexandre Brongniart (right).
Because the Paris Basin appeared to present a succession of vertebrate types that
lived and were supplanted by other vertebrates in newer strata, Cuvier theorized that the
earth was destroyed by a series of global floods, each followed by a new creation.
The geology of the Paris Basin extended across the English Channel into eastern and
southern England. There, a man untrained in science and independently of Cuvier, also
taught himself to read the rock strata. William Smith (1769-1839; see Figure 4) became
interested in interpreting rock strata while he was employed as a surveyor for a canal
system around the city of Bath. He collected fossils and noted the strata in which each
type was found. Smith realized that the strata could be correlated according to the fossils
which they contained even if the types of rocks differed. He also pioneered methods of
mapping the ways that strata dip and rise. Based on his extensive field work, Smith
produced a geological map of the Bath area. Then, after he was fired 1 , he traversed the
country as a freelance surveyor with a dream to produce a geological map of England and
Wales. Throughout that time, he lived at the edge of poverty, and was committed to
debtor’s prison for a while. Furthermore, the founder of the fledgling Geological Society
of London attempted to steal the geological map from Smith and produce it
independently. Finally, Smith did produce his beautiful map in 1815 (see Figure 5);
however, he was so deep in debt that he had to sell his precious fossil collection at the
time that he was fighting for priority. To add insult to injury, Smith was not allowed to
1
In part, he was fired because he spent so much time on his mapping project.
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join the Geological Society because he was not a member of the gentry. Only much later
after suffering so much personal and professional disappointment was his work finally
recognized by the Geological Society in 1831 when it awarded him the first Wollason
Medal, the society’s highest award. At which time, Smith was called “The Father of
British Geology.”
FIGURE 3. A composite cliff face in the Cuvier-Brongniart map of the Paris Basin.
NAMING THE STRATA
Who hath not own’d, with rapture-smitten frame,
The power of grace, the magic of a name? -Thomas Campbell
The stratigraphic methods of Cuvier and Smith pointed to the importance of fossils as
more than geological curiosities. They served to connect concurrent strata, even if the
types of rocks differed. The importance of fossils can be seen in the modern names for
the Transitional Era, now called the Paleozoic (= ancient animals), and the Secondary
Era, now called the Mesozoic (= middle animals). At the beginning of the 19th Century
such empirical methods were necessary for the science of Geology to become modern.
With the ability to map strata over very large areas, there was a need to define particular
stratigraphic sequences and develop a formal set of names for them.
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FIGURE 4. William Smith.
FIGURE 5. William Smith’s Stratigraphic Map of England and Wales.
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TABLE 1. The formal names of the geological systems of the Phanerozoic Eon.
This includes the type locality, the author(s), date proposed and source of the name.
SYSTEM NAME
LOCALITY
AUTHOR(S) DATE SOURCE OF THE
NAME
Jules
Means the 4th
QUATERNARY
France
1829
Desnoyers
period
Giovanni
Means the 3rd
TERTIARY
Italy
1760
Arduino
period
From Creta, the
Omalius
Latin word for
CRETACEOUS
Paris Basin
1822
d’Halloy
chalk.
Alexander
Northern
Named after the
von
JURASSIC
1795
Switzerland
Jura Mountains
Humboldt
Name reflects the
Southern
Fredrick von
occurrence of 3
TRIASSIC
1843
Germany
Alberti
distinctive strata in
its type locality
Roderick I.
From Perm, the
PERMIAN
Perm, Russia
1841
Murchison
type locality
William
Name reflects the
Central
Conybeare &
CARBONIFEROUS
1822
abundance of coal
England
William
in this system
Phillips
Roderick I.
Devonshire,
From the type
Murchison &
DEVONIAN
1840
England
locality
Adam
Sedgwick
From the name of
Western
Roderick I.
SILURIAN
1835
an ancient Welsh
Wales
Murchison
tribe
Scotland and
From the name of
Charles
ORDOVICIAN
Western
1879
an ancient Welsh
Lapworth
Wales
tribe
From Cambria, the
Western
Adam
CAMBRIAN
1835
Latin name for
Wales
Sedgwick
Wales
Currently, the categories of the Geological Timeline are in hierarchical categories.
Eons are divided into Eras, and Eras are divided into Periods. The formalization of the
categories, after recognition of the Paleozoic, Mesozoic, and Cenozoic Eras was the
definitions of the Periods or systems that they contained. See Table 1 for a list of the
periods or systems of the Phanerozoic Eon , the eon that includes multicelluar life. It
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indicates the place of discovery, the author(s), the date proposed, and the origin of the
name.
Particular stratigraphic sequences within the Mesozoic Era were developed first.
Mainly, these were the strata examined by Smith and to a lesser extent by Cuvier. Jean
Baptiste Julien d’Omalius d’Halloy (1783-1875) produced one of the earliest geological
maps of France, and through his study, defined the uppermost sequence of Mesozoic
strata as the Cretaceous Period. He derived the name from the chalk (Latin for chalk is
Creta) which defined most of the strata in western Europe. The sequence just beneath the
Cretaceous was defined on the basis of its marine fossils, particularly ammonites. Thus,
even though the rock types were very different across France, Switzerland, and England,
they were united on the basis of their fossils. his second sequence was defined by
Alexander von Humboldt 2 (1769-1859) in 1795 and called the Jurassic Period after the
Jura Mountains of Switzerland. The lower Mesozoic strata were grouped within the
Triassic Period and defined in 1834 by Frederick von Alberti in southern Germany.
The geology of the Paleozoic Era was older and much more complex than that of the
Mesozoic. However, one of the most distinctive and well-known systems contained the
economically-important coal-bearing strata. William Conybeare (1787-1857) and
William Phillips (1775-1828) named it the Carboniferous System in 1822. In the United
States the extensive Carboniferous has been separated into the younger Pennsylvanian
and older Mississippian Periods.
In 1831 two very different men set out to unravel the lower Paleozoic geology of
Wales. Adam Sedgwick (1785-1873; see Figure 6) was trained as a mathematician, but
became the Woodwardian Professor of Geology at Cambridge. Largely self-taught as a
geologist, he dove into the subject with great enthusiasm. In fact, he took special interest
in a young student named Charles Darwin who accompanied Sedgwick on one of his
forays into the Welsh countryside.
FIGURE 6. Adam Sedgwick (left) and Roderick Murchison (right).
Roderick Impy Murchison (1792-1871; see Figure 6) came to geology in a very
different way. A Scot, he joined the British army at 15 and served until after the
Napoleonic Wars. Then, at home he led an empty life with an interest only in
foxhunting. At 32, however, he struck up a friendship with the scientist, Humphry Davy,
who introduced him to the natural sciences. Murchison became fascinated with geology
2
In this way, von Humboldt anticipated one of the primary methods of Cuvier and Smith.
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and discovered that he had quite an aptitude for stratigraphic work. He threw all of his
energies into geology from that point on.
Murchison attacked the stratigraphy of Wales from the south where he began
mapping the lower “Old Red Sandstone”, a series below the Carboniferous. He then
worked into a system below that. He named it the Silurian after an ancient Welsh tribe
and identified it on the basis of the rocks and fossils. During the same time Sedgwick
worked on a much older series that was deeply folded and so altered that fossils were not
very much help. He called this the Cambrian after Cambria, the old Roman name for
Wales. They published their descriptions of the Silurian and Cambrian systems in 1835.
Then, the two men began to study the sequence of strata around Devonshire in
southern England. Eventually they recognized that the sequence of rock lay between the
distinctive Carboniferous above and the Silurian below. The new system had distinctive
fossil assemblage, and they named it the Devonian after Devonshire.
Murchison, who had made quite a name for himself, was enticed to travel to Russia
and work on the geology there. While working in the Perm region of Russia, he
identified a series above the Carboniferous, which he called the Permian in 1841.
Murchison took advantage of his travels in Russia and Scandinavia to map his beloved
Silurian system everywhere he could. He became convinced that life appeared in the
Silurian and any of the strata assigned to the Cambrian that had fossils had to be part of
his Silurian. At first the friends disagreed in a friendly way. After a time they began to
fight over the boundary strata of their respective systems. Murchison especially viewed
the Silurian as his empire and was loath to give up any territory to Sedgwick’s Cambrian.
The solution came much later (1879) when Charles Lapworth (1842-1920), a Scottish
geologist studied the Cambrian-Silurian boundary strata in Scotland. There, the fossils
and rocks were much clearer in defining an intervening series that he called the
Ordovician after another ancient Welsh tribe and settled the Murchison-Sedgwick
controversy, but it was after both men had died.
Thus, by the end of the 19th Century, all of the formal higher categories in the
Geological timeline from the abundant appearance animals to the present, an Eon called
the Phanerozoic, had been defined. The eras and periods had been arranged in their
relative positions, called relative chronology. With the discovery of radioactivity and the
recognition that radioactive decay provided a reliable clock for timing events and periods,
the relative chronologies could be given actual times before the present. Thus, geologists
have been able to add ranges of time to the formal descriptions of eras and periods.
QUESTIONS, & ANSWERS
I keep six honest serving-men
(They taught me all I knew);
Their names are What and Why and When
And How and Where and Who. -Rudyard Kipling
I wrote this essay as an answer to a student’s question last spring semester. I had
presented the Geological Timeline, and she asked where did all of whose names come
from, and how do we know they are in that order? I love such questions. They allow me
to connect the material that we are studying with the humanity of the science. Of course,
I have received my share of less than good questions. Indeed, despite the prevailing
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educational adage that there is no such thing as a stupid question, I have received many
stupid questions. To clarify my position, I might further define a stupid question as one
whose intent is to gain advantage rather than to gain knowledge. Such questions might
be applauded on the courtroom floor, but they have no place in a hall of learning.
Unfortunately, my response to the student was not as inspired as her question, but it did
lead me on a search for answers.
FIGURE 7. The current Geological Timeline with dates given in millions of years.
The most inspired questions, though, are the ones that we ask ourselves. These are
the ones that can lead to answers that don’t exist in books. Cuvier and Smith asked the
earth how strata were situated and then developed methods to answer those questions and
in the process produced beautiful maps that answered even more questions. The earth
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beckoned to others who used the methods of Cuvier and Smith to define the periods of
the Geological Timeline. Thus, slowly, geologists learned to read the earth’s surface.
Does that mean that there are no more questions if we have these answers? Of course
not. Geological Maps and ever more detailed Geological Timelines help to frame new
questions as they guide answers in the forms of explanations. Indeed, that the earth is
covered by strata that differ and is described by a history that can be divided into welldefined periods, points to many wonderful and subtle questions that are being asked now
and will be asked in the future.
Scientists do more than ask questions, though. They seek to answer them. Often, the
search for answers is lonely, and sometimes destructive as in the case of William Smith.
More often than not the search leads to a dead end because nature does not give up her
secrets easily. Sometimes dumb luck intervenes. With all of the problems of the search,
then, why do scientists bother? They do because there is a joy in the search itself. What
appeared as indolence to others who watched me leave on my fossil forays when I was a
student, was for me a complex mixture of fun, adventure, discipline, work, and education.
Although I am not a geologist, I do not regret a moment that I spent trying to read the
earth.
-August 2004
SOURCES USED TO WRITE THIS ESSAY
Adams, Frank D. 1938. The Birth and Development of the Geological Sciences. Dover
Publications, Inc. New York.
Boggs, Sam. 1987. Principles of Sedimentology and Stratigraphy. Merrill Publishing
Company. Columbus.
Bowler, Peter. 1992. The Fontana History of the Environmental Sciences. Fontana Press.
London.
Geikie, Andrew. The Founders of Geology. 2nd ed. Dover Publications, Inc. New York.
Lyell, Charles. 1990 (originally published 1830-33). Principles of Geology With a new
Introduction by Martin J. S. Rudwick. Vols. 1-2. The University of Chicago Press.
Chicago.
Mather, Kirtley F. and Shirley L. Mason. 1970. A Source Book in Geology, 1400-1900.
Source Books in the History of the Sciences. Harvard University Press. Cambridge,
Mass.
Rudwick, Martin J.S. 1997. Georges Cuvier, Fossil Bones, and Geological Catastrophes.
The University of Chicago Press. Chicago.
Trefil, James and Robert Hazen. 1998. The Sciences, An Integrated Approach. John
Wiley and Sons, Inc. New York.
White, John F., ed. 1962. Studyof the Earth, Readings in Geological Science. PrenticeHall, Inc. Englewood Cliffs, N.J.
Winchester, Simon. 2001. The Map the Changed the World. HarperCollins Publishers.
New York.
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QUESTIONS TO THINK ABOUT
1. I compared the development of our understanding of rock strata
to learning to read. What do you think of that comparison?
2. What was the contribution of (at least as far as geological
mapping goes) of Georges Cuvier and Alexandre Brongniart?
What was their conclusion?
3. What did William Smith do?
4. Why are fossils important in stratigraphic mapping?
5. What are the three Eras of the Phanerozoic Eon?
6. What are the Periods or Systems of the Phanerozoic? How were
they defined?
7. What were the contributions of Adam Sedgwick and Roderick
Murchison? Why did they begin as friends and colleagues and
end as enemies?
8. How have real times been applied to relative chronologies in the
Geological Time Line?
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