Rocks and
Rock Layers
Fossil Tell a Story
Let’s look at all of the interesting (and helpful) things they can tell us about
the past … because they have a lot to tell!
Clues to Past Climates
If you have gone
bike riding on
Slickrock trail in
Moab, you have
ridden on ancient
sand dunes. What
type of climate do
sand dunes suggest?
Clues to Past Weather Conditions
Sedimentary rocks
have clues to past
climates and
ecosystems. These
rocks were ancient
sand dunes, with
layers that formed
at different times, at
different angles,
depending on which
direction the wind
was blowing.
This is called cross-bedding and can be found all over Southern Utah.
Clues to Past Waterways
Ripple marks found near a river bed that
is still running with water.
Ancient ripple marks, now fossilized,
show where a river used to run.
Clues to Past Animal Life
What might have
made these
marks?
Most likely … worms moving along soft sand or mud.
Clues to Past Animal Behavior
Dinosaur footprints can
indicate areas of
gathering … either for
food or mating. These
prints that have been
preserved have taught
us a lot about the eating,
hunting and possible
migrating habits of these
animals.
Rock Formations Also Offer Clues
to the Past
Fossils are usually
buried in old rock
layers. These layers
occurred at different
times in the Earth’s
history. Each layer
has a time and a
story, and the next
few pages show what
information we can
gather from rock
layers.
Layering of Sedimentary Rocks
You have probably
noticed layers on
exposed mountain sides
before. These layers tell
a great deal about the
history of the area and
are great tools for
scientists.
Rock Layers
Sedimentary rocks form from layers
of sediment that collect in horizontal
layers. Over time, with pressure and
chemical cementing, rocks form.
Sometimes, the layers are exposed
by erosion, like these canyon walls in
Dinosaur National Park.
Uplift
These layers near Denver, Colorado show how horizontal layers
can be altered by uplift. In this case, the Rocky Mountains rose
and changed these layers. Can you tell which layer is oldest?
Answer: The layer that is on the far left is oldest.
Twisting
Sometimes layers of sedimentary rock are twisted by earthquake faults.
Slipping
Sometimes layers of sedimentary rock slip past each other
because of earthquake faults.
This picture shows Becks Spur which is in North Salt Lake City. The cliff is the Wasatch fault it was not made by people.
When the Wasatch Fault slips the mountain rises and the valley falls.
Slipping From a Different Angle
A Closer Look
The scratches were
formed as the rocks
slipped past each other.
The Importance of Order
Now, it isn’t always an easy
job to figure out which
layer came first. But if you
look at all the clues given, a
good guess can be made.
The key is to notice the
details.
Picking out the details
helps you to put things in
the order of their
occurrence.
Then you can put together
a history for that area,
including theories about
climate, animal life, seismic
activity and much, much
more.
Order
of
Events
Read the following list of events
carefully. Then determine the
correct order for the sentences.
I got out bread, peanut butter and jelly.
I ate the sandwich.
I put the two pieces of bread together.
I spread a layer of jelly on another piece of bread.
I was hungry.
I spread a layer of peanut butter on a piece bread.
How did you determine the order of the sentences? If you did it like many people, you based
your sentences on what you know normally happens when you make a lunch. Geologists do
the same thing to determine the order of past events.
Figuring Out the Order of Events
The process of putting things in a "correct order" based on experience is called Relative
Dating.
Relative Dating is determining whether an object is older or younger than other objects or
events. It doesn’t give an exact age but a relative (or comparison) age. This means that you
know which events came first, but not the exact time when they happened.
Clues to Help Figure Out Past Events
It’s a lot like what happens with the
photos someone takes over a lifetime.
Instead of putting them into a photo
album, they are packed away in a box.
Where are the oldest pictures going
to be in the stack? At the bottom or
the top? What about the newer
pictures? Where are they in the
stack?
Rock layers are a lot like the stack of
pictures. The layers at the bottom are
the oldest. Each layer will be
successively younger or more recent.
This is called Superposition.
Superposition is the principle that
says younger rocks lie above older
rocks in an undisturbed sequence.
Fossils Offer Important Clues
Scientists use the fossils of animals
to help determine relative age.
Certain groups of fossil animals and
plants occur in the geologic record
in a specific order.
If a scientist finds one of those
fossils they can then assume the
age of the rock based on the age of
the fossil.
Rock Layers Offer Clues,
Too something
But sometimes
will get in the way.
Igneous intrusions
-areas where igneous rock
cuts across layers of preexisting rock.
-the principle of crosscutting relationships states
that the intrusions are
younger than the rock
they cut.
With all that background knowledge you now have, it’s time for you to be the geologist. There
are five different pictures of one rock sequence throughout time.
Number the pictures from one to five with one being the oldest and five being the youngest.
Below each picture, choose the process that describes what has taken place and write that term
in the space provided.
Relative Dating Practice
More Relative Dating Practice
Examine the figure to
the right, then answer the
following questions:
1. Is Rock layer "A" younger or older than the other rocks? What evidence explains your answer?
2. Is the fault (dotted line labeled "L") older or younger than layer D? Explain your answer.
3. List the rock layers in order from oldest to youngest. (include the lines "K" and "L").
The End
Relative Dating Answers
Answer to the first question:
Answers to the next group of questions:
1. Younger - It represents an intrusion which is younger than the rock it cuts through.
2. Younger - the fault occurred before erosion and the sedimentation of layer D
3. E, F, G, H, I, J, L, K, D, C, B, A
The
Element
of Earthquakes
And sometimes
earthquakes
can get things a little
jumbled up … literally!
Go to THIS PAGE for a lesson in
how faults can affect the
“order” of the Earth’s layers.
Do the investigation and
answer the analysis questions.
Label this assignment, “Move
Over, I’m Coming Through”
and file it in your Science
Journal.
Were You There?
What did North America look like 2 million
years ago? Were there rivers and lakes? Were
there mountains in the same places as modern
day? Was North America in the same location
as it is today?
To answer these questions, geologists
construct maps of the lands and seas, as they
existed in past ages. This process of
reconstructing ancient geography is called
paleogeography. Like a private detective,
geologists analyze the clues they find
preserved in the rocks. The types of fossils
contained in the rocks are clues for the
geologists. When interpreted, these clues can
give direct knowledge about the distribution of
the lands and the seas, but also about the
natural environment of the area. This helps the
geologists to infer where the mountains and
basins were located in the geologic past.
Were You There?
The distribution of fossils (skeletons,
shells, leaf impressions, footprints,
and dinosaur eggs) in rocks of a
certain age tells something about
the ancient distribution of lands
and seas on the Earth's surface.
The remains of coral found in landlocked Iowa indicate that this
region was once covered by a coral
reef. Clam shells found in old
limestone of Pennsylvania and
New York indicate a shallow sea
once covered the area. The
remains of ancestral horses and
camels in rocks of South Dakota
show that the area was then dry
land or that land was nearby.
Were You There?
The distribution of the
fossils will help to identify
the ancient areas of land
and sea and also the
approximate shoreline.
The distribution of living
forms shows that thickshelled fossil animals once
lived in shallow water
close to shore, while thinshelled, delicate fossil
animals probably lived in
deeper, calmer water
offshore.
Were You There?
Fossils can also be used to indicate the
former temperature of water. In order
to survive, certain types of present-day
coral must live in warm and shallow
tropical salt waters, such as in the seas
around Florida and the Bahamas. When
similar types of coral are found in the
ancient limestone, they provide a good
estimate of the marine environment
that must have existed when they were
alive.
All these factors -- depth, temperature
and currents -- that are revealed by
fossils are important to help clarify the
picture of ancient geography
Pretend You Were There
You are to be the paleontologist now, and use research to determine what fossil evidence you will see in your geologic column.
Choose one of the areas listed below to study. For the area you have chosen to study, you need to learn about its geologic history. Imagine that you are a paleontologist
and are trying to determine the evidence of change in your specific location. You might try to find pictures of the area as well as pictures of the fossil record found in
that area. Present your fossil findings as a newspaper article.
Areas to Choose From:
Freshwater aquatic
Semi arid desert
Temperate forest
Marine aquatic
Coastal desert
Hot and dry desert Arctic Tundra
Cold desert
Taiga (coniferous) forest
Tropical forest
Savanna
Temperate
Alpine Tundra
For information on ecosystems you might look in a library or search on the Internet.
Your newspaper article should answer the following questions:
1.
How do your fossil findings indicate the ecosystem which was once present?
2.
How will a change in ecosystems be evident in the fossil record?
3.
What specific fossils would you find in each layer of rock?
Title this assignment “If You Were There”, and file it in your Science Journal.
New Fossils on the Rock
Have you ever wondered about why fossils that are found in
more recently deposited rock layers resemble existing species?
Read on and see if you can discover the answer to this
question.
New Fossils on the Rock
The process of fossil formation begins as a
living thing dies and is covered by something,
so that there is no oxygen to decay, and
fossilization begins. With time, nature
will replace the bones, or hard parts of the
critters body, with minerals, which preserves
it within the rock.
More recent fossil remains can be found
closer to the surface, while older fossils
representing animals that died long ago
would be found deeper in the earth. For
example, more recently evolved animals, like
birds, mammals, and modern day lizards, are
likely to be closer to the earth's surface than
dinosaur or trilobite fossils.
New Fossils on the Rock
Therefore scientists may get
a general idea as to the age
of a fossil by the depth of the
fossil in the rock layers of
Earth. Also, they can tell the
age of the rock by the fossil
they find within that rock
layer. Because Earth's species
have been constantly
changing, more recently
deposited rock layers would
contain fossils resembling the
species of today.
New Fossils on the Rock
The following pictures represent an example of this concept:
This picture represents the surface of the earth now. The
lizard is an example of a living critter that lives on the
surface, of which some may become fossilized in time.
New Fossils on the Rock
This picture represents
the rock layer where
many ancient fossils
have been found dating
400 million years
ago. This layer use to be
the surface layer and the
fossil found here use to
live on the surface of the
land, just like the lizard
New Fossils on the Rock
This is a picture of a
trilobite. These critters
lived 400 million years ago
in a very shallow
sea. Scientist are able to
determine the age of
layers of rocks, plus the
environment in which they
lived, by studying fossils
and rock layers. Because
trilobites loved so long
ago, they are often found
in layers of rocks that are
much older!
New Fossils on the Rock
We started off with this
question: Have you ever
wondered about why fossils
that are found in more
recently deposited rock layers
resemble existing species?
Have you figured out the
answer?
Write a short paragraph
explaining your answer to
this initial question. Label
it “New Fossils on the
Rock” and file it in your
Science Journal.