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Content Benchmark E.8.C.1
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Earth Science
Atmospheric Process and Water Cycle
Solar System and Universe
Earths Composition and Structure
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Nature of Science (NOS)
Life Science
Earth Science
Physical Science

Students know sedimentary rocks and fossils provide evidence for changing environments and the constancy of geologic processes. E/S

We as human have always been interested and curious about Earth’s history.  Our curiosity is only heightened with the examination of fossils.  We can examine life forms from millions of years ago by looking at the remnants of their imprints in the rocks.  Since sedimentary rocks accumulate over time we have a record of the past environments and ancient life of Earth.  This raises many questions such as:  Why are fossils found in sedimentary rocks?  What causes a fossil?  Is the rock record complete? How do we determine the age of rocks and the fossils they contain?

The Rock Cycle and Sedimentary Rocks
The rock cycle is a model of the transition of rock material throughout the Earth.  The cyclical pattern denotes that the material from one phase to another is constantly being recycled through geological processes.  Sedimentary rocks are primarily formed via two processes: chemically, and compaction and cementation.  Depending on the environment, chemically suspended material within a medium, like salt in water, can yield rocks when the dissolved minerals precipitate out of the liquid, or the liquid medium evaporates and leaves behind a crystal.  Compaction and cementation occur when fragments of rocks, clasts, are buried and compacted.  The process of compaction occurs when overlying sediments cause pressure, and thus squeeze the fragments together; if water is present in the process, then cementation can also occur.  The water carries dissolved minerals that may be left behind creating a cement to hold the clasts together.

For more information on the rock cycle go to

Preview of the Rock Cycle.

Figure 1. The rock cycle.

Sedimentary Rocks and Fossils
Sedimentary rocks are so very important to fossils because they provide the medium of preservation.  In clastic sedimentary rocks, formed from burial, compaction and cementation, fragments of rock can be as fine as clay and thus settle into the very fine details within a bone or other hard parts of dead organisms.  Typically only the hard parts of an organism are preserved, but fossils can come from a variety of sources.  Three main processes can produce a fossil; traces of organisms, petrified remains, and preservation of organic material.

Cathayornis yandica

Figure 2. Liaoning Fossil Bird, Cathayornis yandica.

Traces of organisms are most commonly referred to as molds and casts, but trace fossils, and coprolites, the fecal remains of organisms, are also produced through this method.  In traces of organisms, no part of the original organic material remains.  When an organism dies and is quickly buried the imprint it leaves behind after its decay is called a mold.  This empty cavity can then be filled in by sediment that will eventually harden to produce a cast.  The cast can be extremely detailed depending upon the grain size of the clasts that fills the mold.  A casts can provide great detail for the surface of the buried organism but the internal structure is not preserved.  A trace fossil is a fossil of the movement of ancient life.  Footprints and other traces of ancient life can be produce very similarly to a mold and cast.  The footprint can leave a cavity that sediment can fill and preserve.  Coprolites are produced from the preservation of dung and waste.  Petrified remains occur when minerals, carried by water, gradually replace organic material.  The slow process of mineralization often produces a near perfect mineral replica of the original organism.  This process most often occurs with downed trees and other plant parts. 

Unlike in the two types of fossilizations described above, it is possible for the remains of the organic material to be preserved.  Preservation of an organism’s organic material can occur if the organism is mummified or incased in amber, tar, or ice.  Mummification may occur in an arid environment that prevents bacteria from breaking down the remains due to the lack of water.  Amber, hardened tree sap, can incase and preserve small organisms.  Tar seeps and freezing can also preserve organisms by preventing decay.  When the original remains of the organism is preserved the structure and internal organs remain and often DNA can be extracted from the remains.

For more information about fossil visit

The Fossil Record
Fossils provide all of the evidence of past life, and due to the nature of their formation, they contain a record of past environmentsSedimentary rocks cover a large portion of Earth surface and are therefore more susceptible to weathering and erosion.  Because of this the rock record is like a book with pages, or chapters in some cases, missing.  Often paleontologists correlate rock layers (strata) between different locations in order to fill in the gaps of missing rock.  By examining the rock layers geologists are able to place them in relative date order to better understand Earth’s history.

For more information about weathering and erosion see MS TIPS Benchmark E.8.C.2

For more information about the fossil record go to

For a database of fossils go to

Relative Age Dating
James Hutton’s principle of uniformitarianism is the basic principle that guides the relative age dating process.  The principle of uniformitarianism states that the geologic processes that occur today occurred in the past.  From this principle and observation geologists can examine rock strata and view them as pages in the book of Earth’s history.  The first law derived from the principle of uniformitarianism is the law of superposition.  The principle of original horizontality uses uniformitarianism to state that sedimentary rocks are originally deposited in horizontal layers.  The law of superposition states that, because all sedimentary rocks are originally deposited in horizontally beds, the oldest rock stratum is at the bottom with successive younger strata as you get to the surface.  The law of crosscutting relationships, another relative dating practice, states that any fault or intrusion within strata must be younger than the strata in which it cuts across.  The law of inclusion states that if a rock fragment is found within another layer the rock must be older than the layer in which it is found.

For more information about relative age dating go to and
Within time the strata can become distorted by uplifting crust, weathering and erosion can remove entire layers, and magma can rise and solidify.  These distortions over time can cause a break or gap in the rock record which is called an unconformity.  There are three types of unconformities; angular unconformity, disconformity, and nonconformity.  In an angular unconformity, strata is uplifted and tilted, then weathered and eroded.  New strata are then deposited on top of the tilted layers.  
Figure 3. Angular unconformity diagram.


A disconformity occurs when strata is uplifted, weathered, and eroded.  New sedimentary rock forms over the weathered surface, as the strata subside, causing an unconformity (missing layers of sedimentary rock) in the rock record.


Figure 4. Disconformity diagram.


Nonconformities occur when strata (sedimentary rock layers) rest in contact with igneous or metamorphic rock.  Because igneous and metamorphic rock do not stratify (form layers), determining if layers of sedimentary rock have been removed becomes difficult.  It would be impossible to tell if uplift, erosion and new stratification of sediment had occurred without correlation from other locations.


Figure 5. Nonconformity diagram.

For an on-line tutorial, with diagrams, of relative age dating and unconformity formation go to

Try It Out: Sample strata that can be used to practice dating the events.

 Figure 6. Sample geologic strata that can be used to test your
understanding of Relative Age Dating.

Which layers came first?

        1. Q (deposition)
        2. O (deposition)
        3. N (deposition)
        4. L (deposition)
        5. M (principle of inclusion, and contact metamorphism to produce a nonconformity)
        6. P (principle of cross-cutting relationships, with contact metamorphism)
        7. Tilt and Uplift
        8. Weathering and erosion
        9. H (deposition, produces an angular unconformity)
        10. I (deposition)
        11. J (deposition, possible disconformity)
        12. Weathering and erosion
        13. K (deposition)

Absolute Age Dating
The process of absolute age dating, radiometric age dating, is the use of radioactive isotopes to give an exact date or age of that material (rock or organic remains).  With the discovery that radioactive isotopes decay at constant rates, the age of rocks and other material can be found by comparing the amount of radioactive parent material to the stable daughter product.  The time it takes for half of the radioactive isotope to decay is referred to as the half-life.  For example, carbon 14 has a half-life of 5,730 years.  If a sample of organic material is found to have 50% daughter product (nitrogen 14 in this case) and 50% carbon 14, then the sample is 5,730 years old.  If two half-lives have gone by from the previous example then the sample (75% stable and 25% radioactive) then the sample is 11,460 years old.  Radioactive isotopes are not affected by weathering or erosion, heating and cooling processes (given that the sample does not melt).  Radiometric dating has produced strong evidence that the Earth is 4.5 to 4.6 billion years old.  The oldest discovered rocks on Earth (zircon crystals) were found to be 4.0-4.2 billion years old, the oldest known fossils were found to be 3.5 billion years old, and the most recent ice age was found to have ended 10,000 years ago. 

Figure 7. Graph comparing the number of parent atoms to daughter atoms in half lives.


·  Parent



Useful Range

Type of Material



4.5 b.y

>10 million years 

Igneous Rocks and Minerals



710 m.y



14 b.y


40Ar & 40Ca

1.3 b.y

>10,000 years



47 b.y

>10 million years



5,730 y

100 - 70,000 years

Organic Material

Figure 8.Radioactive half-life data

For more information about absolute age dating go to

To learn more about the age of Earth go to

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Content Benchmark E.8.C.1

Students know sedimentary rocks and fossils provide evidence for changing environments and the constancy of geologic processes. E/S

Common misconceptions associated with this benchmark

1. Students have difficulties with the scale of time.

Some geological phenomena are so different from everyday reality that their understanding can be problematic.  The scale of geological time, the rate of mountain building or erosion, the architecture of an oil field or aquifer and the pressures at depth in the earth’s crust are a few such phenomena.  Students must be allowed the time to model geophysical process and make analogies between their model and the actual process.  Without doing so, their preconceptions and intuition may prevent them from fully grasping the 4.5 billion years of Earth’s existence. 

For an explanation about how to overcome geological misconceptions with hands-on activities that model Earth’s processes go to

2. Students have great difficulty in conceptualizing times that stretch into millions or billions of years.  

This problem stems from the lack of understanding of the magnitude. Using analogies is a great way to get students to start thinking of magnitudes of time accurately, and producing accurate scales for comparison yields great results.  For example, if a human blinks 10,000,000 times a year. Only the last 4,000 blinks (just over 4 hour’s worth) would represent the time humans have been on the Earth.   

For further explanation and a lesson plan for scaling geologic time, go to

3. Students incorrectly believe that fossils are pieces of dead animals or plants. 

Students must be made aware that fossils are typically made of rock.  Minerals fill in the spaces left behind by animals or plants, and that no part of the organism remains.  Only under very specific circumstances; freezing, insects trapped in tree sap, tar seeps and mummification, are there actual organic remains. 

For more information and activities go to

4. Students incorrectly believe fossils of tropical plants can not be found in deserts.

Students often incorrectly believe that the Earth is sedentary and that the environments of today’s Earth are constant.  With this misconception students can not understand why fossils of marine organisms could be found at the top of mountains, like Mt. Charleston for example.  The continents have shifted greatly over time and oceans have come and gone.  Uplift and subsidence have produced great changes in the topography of the landscape, but the rock record remains intact.  The fossil record provides clues about past environments of local areas.  In fact, tropical plant fossils have even been found on Antarctica.      

To learn more about the dynamic Earth go to  

To learn more about fossils from Antarctica go to

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Content Benchmark E.8.C.1

Students know sedimentary rocks and fossils provide evidence for changing environments and the constancy of geologic processes. E/S

Sample Test Questions

1st Item Specification: Understand why most fossils are found in
sedimentary rocks.

Depth of Knowledge Level 1

  1. Nearly all of the fossils found to date have been found in
    1. igneous rocks.
    2. metamorphic rocks.
    3. sedimentary rocks.
    4. volcanic rocks.
  1. Shells and bones of animals fossilize well because they
    1. contain hard parts.
    2. never decompose.
    3. have a crystal structure.
    4. will melt at high temperatures. 
  1. Use the diagram of the rock cycle below to answer the question that follows.


  1. What process must happen to rocks and sediment in order for a sedimentary rock to form?
    1. Melting and solidification
    2. Solidification and uplift
    3. Heat and pressure
    4. Compaction and cementation

Depth of Knowledge Level 2

  1. Which statement best explains why fossils are found nearly exclusively in sedimentary rock?
    1. As sediment forms it heats the surrounding environment, causing local organisms to die.
    2. As organisms die they settle into the layers of the sediment and become buried.
    3. As sediment settles in the ocean it collides with and traps many organisms, thus burying them.
    4. As organisms die they are quickly broken down into sediment that forms fossils.
  1. A student finds a fossil of a fish and concludes that the fossil must be made of sedimentary rock.  What evidence supports the student’s conclusion?
    1. Only marine fossils form in sedimentary rock.
    2. Sedimentary rocks are soft and fish bones will fossilize well in rock.
    3. Nearly all fossils form in sedimentary rock.
    4. Fish fossils only form on the shore in sedimentary rocks.

2nd Item Specification: Given examples, identify how fossils and sedimentary rocks provide evidence of changing environments.

Depth of Knowledge Level 1

  1. The presence of limestone rock indicates what past environment?
    1. Ocean
    2. Mountain
    3. Desert
    4. Forest
  1. Fossils and sedimentary rocks form under specific conditions, so their presence in certain locations provides evidence for
    1. past environments.
    2. stationary continents.
    3. constant temperatures.
    4. magnetic pole reversal.

Depth of Knowledge Level 2

  1. The presence of coral fossils in some of Nevada’s bedrock indicates that
    1. coral lived on land environments.
    2. coral has lived on Earth only in recent times.
    3. most of the state of Nevada was once a mountainous region.
    4. areas of the state of Nevada were once covered with seas.
  1. What would be the most likely explanation if an amphibian fossil were found in Antarctica?
    1. The amphibian swam from its habitat to fossilize in the cold environment.
    2. The area must have been a moist and warm environment at one point in time.
    3. Amphibians can live in extremely cold environments.
    4. The area must be composed of newly formed rock.

3rd Item Specification: Understand rocks are dated by several methods (e.g., the law of superposition, radiometric dating, and index fossils).

Depth of Knowledge Level 1

  1. Several layers of the Earth are exposed. The oldest rock layer is
    likely to be
    1. at the bottom layer.
    2. the thickest layer.
    3. the layer with the most fossils.
    4. igneous intrusive rock.
  1. Use the diagram below showing a geologic cross section of rock layers to answer the following question.


  1. Which layer is most likely the oldest?
    1. C
    2. G
    3. P
    4. W
  1. The term used to identify a fossil that is produced from an animal that was geographically widespread, abundant, and lived for a relatively short period of time is a(n)
    1. marine fossil.
    2. determining fossil.
    3. index fossil.
    4. absolute fossil.

Depth of Knowledge Level 2

  1. A dolphin bone that originally contained 60 grams of radioactive carbon-14 now contains 15 grams of carbon-14.  How many carbon-14 half-lives have passed since this whale was alive?
    1. 1
    2. 2
    3. 3
    4. 4
  1. Use the diagram below showing a geologic cross section of rock layers to answer the following question.


  1. What has most likely occurred to the strata after the erosion at point I?
    1. Fault A
    2. Deposition of layer G
    3. Deposition of layer C
    4. Intrusion N

Constructed Response E.8.C.1

1. The image of the trilobite fossil below was taken from a rock outcrop. Trilobites were aquatic animals that became extinct about 260 million years ago.  Use the image and your knowledge of age dating to describe characteristics of the fossil and give examples of how the fossil could be dated.

Trilobite image of Corynexochida (from

  1. List two ways that the age of the fossil could be determined?
  2. Why is this fossil found exclusively in sedimentary rock?
  3. Explain how this fossil, of an aquatic animal, could be found in a layer of rock through a mountain above sea level.  

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Content Benchmark E.8.C.1

Students know sedimentary rocks and fossils provide evidence for changing environments and the constancy of geologic processes. E/S

Answers to Sample Question:

  1. C, DOK level 1
  2. A, DOK level 1
  3. D, DOK level 1
  4. B, DOK level 2
  5. C, DOK level 2
  6. A, DOK level 1
  7. A, DOK level 1
  8. D, DOK level 2
  9. B, DOK level 2
  10. A, DOK level 1
  11. A, DOK level 1
  12. C, DOK level 1
  13. B, DOK level 2
  14. D, DOK level 2

Constructed Response E.8.C.1 Score Rubric:

3 points

Response addresses all parts of the question clearly and correctly.

  1. The age of a fossil could be determined by relative age dating and/or absolute age dating (radiometric age dating).
  2. Fossils form through a number of ways, but almost exclusively they form in sedimentary rock.  This particular fossil is a mold and cast fossil that will only form in sedimentary rocks.  Also, the response could be related to, “this is a marine organism and the primary location for sedimentary rock formation occurs in large bodies of water?”
  3. At the time of the trilobite’s death he must have settled into layers of sediment at the bottom of the ocean.  As time passed and the land masses moved the rock layers that contained the trilobite must have been uplifted to form the mountain.  Finally, through weathering and erosion, the fossil was exposed to be seen.  The ultimate point is that the environment must have changed from a seabed to a mountain. 

2 points

Response addresses all parts of the question and includes only minor errors.

1 point

Response does not address all parts of the question.

0 points

Response is totally incorrect or no response provided.

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Content Benchmark E.8.C.1

Students know sedimentary rocks and fossils provide evidence for changing environments and the constancy of geologic processes. E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Interactive Rock Cycle

Annenberg Media produced a site with interactive rock cycle and classification information.  The site ends with a printable assessment about the rock cycle that would be a great follow up assignment after the rock cycle lesson. 

To learn more, go to

2. Relative Age Dating Lab

Pamela J. W. Gore from the Department of Geology, Georgia Perimeter College has a site with relative age dating information and instructions about ordering rock strata.  The site also connects to a lab activity for relative age dating.  The lab and information is written at a beginning college level, however the resource could easily be altered for student work or for a group discussion about relative age dating techniques. 

For relative age dating information go to

For the lab activity go to

3. Relative Age Dating activities and worksheets

Relative age dating is often difficult for students to fully understand because of the time it takes for sedimentary rocks to form and produce strata.  This site uses an analogy to explain relative age dating.  Using the analog of events in a student’s life, the students are then able to better connect the concept of relatively dating geologic time by relative dating events in their life.  The site offers teacher information and student worksheets. 

To access this information go to

4. Relative Age Dating pictures and clips

Class zone has a site that explains relative age dating with pictures of rock strata and animated clips.  The following link takes you to a page (Step 1) containing images of our dynamic Earth. Below the pictures are questions for discussion. “Steps” at the bottom of the page link to additional topics directly related to this benchmark such as; 2) story in the rock, 3) principle of superposition, 4) principle of original horizontality, 5) folded layers, 6) principle of cross-cutting relationships, 7) faulted layers, 8) unconformity, 9) analyzing sequences, 10) formation of an unconformity, and 11) stratigraphy and structural geology.

To access this resource go to

5. Fossils found in specific Geologic Times

“Here you can journey through the history of the Earth, with stops at particular points in time to examine the fossil record and stratigraphy.”

To examine portions of geologic time and examine fossils found within specific time periods go to,

6. Information about geologic history on a middle school level

The “Classroom of the Future” site has great information about geologic history written at a middle-school level.  The site is full of articles about the age of the Earth, geologic time, and the fossil record with activities to match. 

For more information go to

7. Rock Cycle Information

The University Corporation for Atmospheric Research (UCAR) produced a wonderful site with rock cycle information at three levels (Beginner, Intermediate, and Advance).  The site is rich with animation and teaching materials in both English and Spanish.

For more information go to

8.Relative Age Dating Activities

The University Corporation for Atmospheric Research (UCAR) also has a site about relative age dating.  In this site the students work in groups to understand relative age dating through modeling sedimentary rock formation and layers. 

For more information and a student handout go to

9. Activities that explain the importance of fossils

“Learning from the Fossil Record” is a web site with teaching materials and student activities.  The site explains the importance of fossils and the need for science literacy.

The site also links activities to the National Science Standards for each grade level.  To begin exploring this site visit

10. Radioactive Decay activities

The Lesson Plans Page has an activity for grades 7-9 to model radioactive decay with paper and scissors.  The site offers an analogy that students can easily understand and use to understand radiometric (absolute age) dating. 

For further information go to

11. A computer based activity on Radioactive Decay

An applet (computer modeling program) for modeling radioactive decay can be a useful tool.  The model shows students how the radioactive decay curve and graph are produced and how the half-life affects the time it takes for the parent material to breakdown. 

To use the applet go to

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Benchmark Related Vocabulary

Geological time scale
Sedimentary rock