Fossils, Geologic Time,
Absolute & Relative
Dating, and Natural
Resources
Chapters 5 & 6
How Do Fossils Form?



Fossils are found in sedimentary rocks like
sandstone, limestone, and shale
In Virginia, most fossils are marine in origin. Our state
fossil, Chesapecten jeffersonius, is a large extinct
species of scallop that dates to approximately 4.5
million years ago. It was the first fossil ever
described in North America and is named after Thomas
Jefferson, one of our founding fathers, and an amateur
paleontologist.
Special conditions need to exist for a fossil to form:
1. Must be protected from scavengers
2. Must be protected from bacteria or microorganisms that can speed up the decay process
3. Must have hard parts like teeth or bones
4. Be near water so they can be buried fast
Can You Name That Fossil?
1
4
Cast
Petrified
2
5
Carbonaceous
3
Amber
Mold
6
Trace
and/or
Mold
Geologic Time Scale
 All events in Earth’s geologic
history are recorded on the
geologic time scale
 Divisions of geologic time are
eons, eras, periods, and
epochs
 Fossils from each major era
(Paleozoic, Mesozoic, Cenozoic)
are found in the rocks of
Virginia.
 The end of each era is typically
marked by a major extinction
event.
 We are currently in the
Cenozoic Era, the
Quaternary Period, and the
Recent Epoch.
 By using relative and absolute
dating methods, we have been
able to roughly estimate the
age of the Earth to be about
4.6 billion years.
Virginia Fossils
 Paleozoic fossils: Cambrian, Ordovician, Silurian,
and Devonian sedimentary rocks crop out in the
Valley and Ridge and Appalachian Plateau provinces
and preserve abundant marine fossils, indicating
the presence of extensive, shallow seas. By the
Carboniferous (i.e., 350 million years ago), the
western part of the state was covered in lush, dense
forests of “scale trees” (lycophytes), horsetails, and
ferns. The accumulation of organic material in these
large coastal swamps eventually produced Virginia’s
coal seams.
 Mesozoic fossils: In the Piedmont province, fossils
of dinosaur footprints, freshwater fish, and insects
are found in rift basin deposits of the Triassic. In the
eastern part of the state, where the sea had not yet
retreated, fossil oysters and belemnites have been
recovered from a few Cretaceous outcrops.
 Cenozoic fossils: Dramatic changes in sea level
occurred throughout the Cenozoic. The rise and fall
of sea level is recorded in the richly fossiliferous
rocks of Virginia’s Coastal Plain. Amazing numbers
of fossil clams, snails, and sand dollars can be
found in these marine rocks, along with fossilized
whale bones and shark teeth. Mastodon and
mammoth fossils, while not common, can be found
in Quaternary sediments that were deposited along
rivers and lakes.
Relative Dating
 Placing events in the order in
which they occurred.
 The Law of Superposition
states that the oldest rocks are
on the bottom of a sequence.
 The Law of Horizontality
states that all rock layers are
originally deposited horizontally.
Then, they may be disturbed by
faulting, folding, tilting,
erosion, or intrusions.
 The Law of Cross-Cutting
states that the intrusion is
always younger than the
surrounding rock.
Absolute Dating
 Placing a numerical age on an
event.
 Radioactive isotopes are used
to determine the ages of rocks
and fossils.
 Radioactive isotopes (the
parent) decay, or change, at a
regular rate into elements that
are stable (the daughter).
 The rate of decay of the parent is
called its half-life. This is the
time it takes for half of the
parent to change into the
daughter.
 Common radioactive isotopes are
Carbon-14, Uranium-238,
Strontium-90, and Potassium-40
Number of
half-lives
elapsed
Fraction
remaining
0
1/
1
100
1
1/
2
50
2
1/
4
25
3
1/
8
12.5
4
1/
16
6.25
5
1/
32
3.125
6
1/
64
1.563
7
1/
128
0.781
Percentage
remaining
...
...
...
n
1/ n
2
100/(2n)
Natural Resources
 Renewable resources
are those that when
used up, are replenished
in a reasonable amount
of time
 Examples of renewable
resources are sunlight,
water, and wood
 Nonrenewable
resources are the fossil
fuels that, when used,
take millions of years to
be replenished
 Fossil fuels consist of
coal, petroleum, and
natural gas
Virginia Resources
 Coal for energy
 Gravel and crushed stone
for construction
 Silica for electronics
 Zirconium and titanium
for advanced metallurgy
(The scientific study and technology of
extracting metals from ores, refining
them for use, and creating alloys and
useful objects from them)
 Limestone for making
concrete