Chapter 2
Early Geologists Tackle
History's Mysteries
Fossils
Fossils are the remains or traces of
prehistoric life. They can tell us much
about the history of the Earth and life.
Early Interpretation of Fossils
Around 450 BC, the Greek philosopher,
Herodotus, noticed fossil seashells in
outcrops of sedimentary rocks that were
far from the sea, and high above sea level.
He concluded that the area had once been
beneath the sea.
Seas had once covered the continents.
Early Interpretation of Fossils
To some, the seashells suggested the Biblical
flood of Noah.
• But why would animals that lived in the water be
killed by a flood?
• Why do we see evidence of many floods
occurring at different times and in different
places?
• Why are different types of fossil animals and
plants found in the different sedimentary layers?
As geologists acquired a better understanding of
the immensity of geologic time, they were able to
use relative dating to produce a more valid
interpretation of fossils and sedimentary rocks.
The fossils are the record of plants and animals
which lived and died over an immense span of
time.
How do fossils form?
Sediment
Sediment is deposited worldwide. Sediment
includes:
• Gravel, sand, silt, mud and clay from rivers and
streams
• Clay and organic debris settling to the seafloor
• Dust and volcanic ash carried by the wind and
deposited
Sedimentary Rocks
Most sedimentary rocks occur in the form
of layers called beds or strata.
Each layer is the result of the deposition of
sediment during some natural event (such
as a flood or storm).
When plants and animals are covered by
sediment and buried, they may become
preserved as fossils.
Fossil Preservation
These characteristics are favorable for fossil
preservation:
• Rapid burial with sediment to prevent
destruction of the dead organism by
scavenging or bacterial decay.
• Presence of hard parts like bones, teeth,
or shell.
Preservation of Soft Parts
Soft parts may be preserved as fossils by:
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Freezing (wooly mammoths)
Desiccation (drying or mummification)
Preservation in tree sap (amber)
Preservation in tar (LaBrea tar pits)
Preservation in peat bogs (Lindow man –
England; Tollund man – Denmark)
• Carbonized imprints in fine-grained sediment.
Steno’s Laws
1. Principle of Superposition - Oldest rocks
on the bottom Younger rocks on top
2. Principle of Original Horizontality Sediments are deposited in flat, horizontal
layers.
3. Principle of Original Lateral Continuity
Sediments are deposited over a large
area in a continuous sheet.
Stratigraphy
Steno’s Laws are the basis of stratigraphy,
the study of layered rocks.
Stratigraphy allows geologists place rock
units into a sequence, to help interpret
Earth history.
Principle of Cross-Cutting
Relationships - Faults
Where a fault cuts across a sequence of sedimentary
rock, the fault is younger than the rocks it cuts.
The sedimentary rocks are older than the fault which
cuts them, because they had to be there first, before
they could be faulted.
Principle of Cross-Cutting
Relationships - Intrusions
Where an igneous intrusion cuts across a
sequence of sedimentary rock, the sedimentary
rocks are older than the igneous rock which
intrudes them.
The intrusion is younger than the rocks it cuts.
Principle of Cross-Cutting
Relationships - Unconformities
The irregular erosional surface is an unconformity.
The unconformity is younger than the rocks that have
been eroded.
The Principle of Inclusions –
Sedimentary Rocks
Fragments of eroded rock overlie the unconformity.
These are gravel clasts or inclusions.
The pieces of gravel are older than the bed in which
they are found.
The Principle of Inclusions –
Igneous Rocks
A xenolith is a fragment of the surrounding rock
which has broken off during an intrusion and
fallen into the magma.
The xenolith is older than the igneous rock which
contains it.
Comparison of inclusions in a sedimentary rock (A)
with inclusions in an igneous rock (B).
Which are gravel clasts and which are xenoliths?
Interpreting a Sequence of Events
Determine the order in which the geologic events
occurred.
European Researchers Unravel the
Succession of Strata
1. John Strachey (1671-1743) used
superposition and lateral continuity to
interpret the stratigraphy of coal-bearing
rocks in England. He also described an
unconformity.
European Researchers Unravel the
Succession of Strata
2. In Italy, Giovanni Arduino (1713-1795)
classified mountains as:
– "Primary mountains" composed of crystalline
rocks. These were interpreted to be oldest.
– "Secondary mountains" composed of
fossiliferous sedimentary rocks.
– "Tertiary mountains" composed of beds of
gravel, sand, and clay, that were youngest.
Neptunists and Plutonists Clash
Professor Abraham Gottlob Werner (17491817) was a prominent German geologist
who insisted that all rocks formed from a
great ocean. For this reason, he and his
followers were called Neptunists, after
Neptune, the Roman god of the sea.
He thought that the Earth had changed little
since its beginning.
Werner contributed to the geologic time scale.
He recognized:
• "Primitive rocks" (called "primary“ by Arduino). He
thought these rocks were deposited by a hot,
mineral-rich ocean. They formed the cores of
mountain ranges.
• "Transition rocks“ - Flat-lying sandstones, shales,
coals, limestones, and old lava flows. He thought
these rocks were deposited when Earth became
suitable for life.
• "Alluvium", - loose gravel, sand, and clay that
overlay the transition rocks.
Werner’s ideas were criticized because he
could not explain what had happened to
such an immense volume of water, and
because he insisted that lava flows were
precipitated from water.
Plutonists
Other geologists showed the volcanic
origin of lava flows. These were the
Plutonists, named for the Roman god of
the underworld. Plutonists said that "fire"
or heat, rather than water, was involved in
the origin of "primitive" igneous rocks.
James Hutton was a prominent Plutonist
who disagreed with Werner.
Uniformitarianism
James Hutton:
1. Saw Earth as a dynamic, ever changing place
where rocks and mountains form slowly, and
are slowly weathered and eroded.
2. Recognized that "the present is the key to the
past".
3. Recognized uniform natural laws govern
geologic processes, later called
uniformitarianism.
James Hutton - continued
4. Published Theory of the Earth in 1785.
5. Realized the immensity of geologic time. "No
vestige of a beginning, no prospect of an end."
6. First interpretation of sequence of events in an
unconformity.
7. Saw a world dominated by cycles.
Uniformitarianism
Uniformitarianism means that geologic
processes are uniform through time.
Physical and chemical laws that govern
nature are uniform.
Uniform natural laws govern weathering,
erosion, glacial movement, earthquakes,
volcanic eruptions, and the transport of
sediment by moving water.
Some events which occurred in the past, and left a
record in the rocks, ARE NOT OCCURRING
TODAY, or have not occurred in the human
lifespan.
These include:
– Huge meteorite impacts
– Extensive volcanism
– Large glacial ice sheets accompanied by
much lower sea levels
– Differences in atmospheric chemistry - Earth's
original atmosphere lacked oxygen, so
chemical process acting in the weathering
environment did not include oxidation
Actualism
• Many geologists prefer to use the term
actualism, to emphasize the importance of
natural laws to the concept of
uniformitarianism.
• Actualism is the principle that natural laws
governing past and present processes on
Earth have been the same.
Unconformities
An unconformity is an ancient surface of
erosion (or non-deposition), separating
older rocks from younger rocks.
If the older rocks are folded or tilted, the
unconformity is referred to as an angular
unconformity.
James Hutton recognized the
significance of the unconformity
at Siccar Point in Scotland
Principle of Fossil Succession
• William Smith (1769-1839) was an English
surveyor and civil engineer who was
working to site canals to transport coal in
England.
• He saw that layers of rocks occurred in a
definite order, and that rock units could be
differentiated on the basis of the fossils
they contain.
Principle of Fossil Succession
Fossils occur in a consistent vertical order in
sedimentary rocks all over the world. This
is the Principle of Fossil Succession.
Geologists interpret fossil succession to be
the result of evolution - the natural
appearance and disappearance of species
through time.
Fossil Succession Confirmed
Baron Georges Leopold Cuvier (17691832), a French vertebrate paleontology
expert (along with Alexander Brongniart),
confirmed William Smith's findings that
fossils display a definite vertical
succession in the rock record, and that the
succession is basically the same in widely
separated areas.
Catastrophism
Cuvier also noted that the fossils changed
across unconformities. He concluded that
there had been a series of catastrophic
floods.
This viewpoint became known as
catastrophism.
Uniformitarianism vs.
Catastrophism
English geologist, Sir Charles Lyell (17971875) held the opposing viewpoint, that of
uniformitarianism.
Seemingly abrupt changes in the fossil
record were interpreted to result from
missing strata that were eroded along the
unconformity.
The ancestors of the "new" fossil groups
were actually present in the underlying
strata.
Uniformitarianism vs.
Catastrophism
Both uniformitarianism and catastrophism
probably operate jointly.
Uniformitarianism dominates the day-today processes.
Catastrophism comes into play
episodically with events such as asteroid
impacts, or sudden, severe climatic
changes that cause extinctions, and other
unusual events.
It has been documented that fossils
appear in a particular sequence in the rock
record. That is without question.
But why is this so?
Evolution: How Organisms
Change Through Time
English biologist and geologist, Charles Darwin
(1809-1882) provided a hypothesis to account
for the observed fossil succession.
He served as a naturalist in a 5-year mapping
expedition around the world on the H.M.S.
Beagle. He gathered volumes of data to support
his hypothesis of the evolution of organisms by
natural selection.
Natural selection is based on the
following observations:
1. A given species produces more offspring than
can survive to maturity.
2. Variations in morphology (form and structure)
and physiology (organs and functions) exist
among individuals of a species.
3. The individuals of a species must compete
with one another for food and habitat.
4. Individuals with the most favorable traits are
more likely to survive to reproduce.
5. Beneficial traits are passed on to the next
generation.
Cause of Variation
Darwin did not know the cause of the
variations among individuals in a species.
Many years later, after Darwin's death,
scientists determined that variations within
a species are caused by new gene
combinations that occur during
reproduction, and from genetic mutation.
Earth History in America
Ice Age Hypothesis
Louis Agassiz (1807-1873) was a Swiss
paleontologist who arrived in North
America in 1846 and founded the Harvard
Museum of Comparative Zoology. He
studied glaciers and proposed that ice
sheets once covered much of North
America and Europe ("Ice Age
hypothesis").
Ice Age Hypothesis
Evidence includes:
• Glacial striations (scratches on rock)
• Glacial erratics (huge boulders transported
by ice)
• Glacial moraines (mounds of rock debris
deposited by melting glaciers)
• Lakes scoured by glacial erosion
Ice Age Hypothesis
The "Ice Age" was recognized as the main
event of the Pleistocene Epoch (1.5 million
years ago to 8000-10,000 years ago). Ice
blanketed one third of Earth's land area.
Appalachian Mountain
Sedimentary Deposition
James Hall (1811-1898) was the director
of New York's first geological survey.
He recognized that fossils in 40,000 ft (7.5
mi) thick sedimentary rock sequence in NY
were deposited in shallow water, and
concluded that the seafloor had subsided
during deposition. The Appalachian
Mountains were later raised from a marine
basin.
Geology in the Western US
Ferdinand V. Hayden (1829-1887) mapped geology of Badlands of South
Dakota and other areas in the west.
Helped convince Congress to establish
Yellowstone National Park, the oldest
national park in the US.
Geology in the Western US
John Wesley Powell (1834-1902) journeyed by boat through Grand Canyon
on Colorado River to map geology despite
having lost an arm in Civil War. Directed
the U.S. Geological Survey.
The Dinosaur Rush
• Cope and Marsh were rivals who competed to
hire professional collectors to discover, describe,
and name dinosaur bones in the western U.S.
• O.C. Marsh (1831-1899) - first professor of
paleontology at Yale University, and later
founded the Peabody Museum of Natural
History.
• Edwin D. Cope (1850-1897) - wealthy Quaker
who taught at the University of Pennsylvania.
The Dinosaur Rush
Results of the work of Cope and Marsh:
• Thousands of specimens of dinosaurs
were collected for study and museum
exhibits.
• Enhanced our understanding of life in the
Mesozoic Era.
• Provided evidence for evolution.
• Established paleontology as a science
with a spirit of discovery.