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11.2: Relative Ages of Rock

Difficulty Level: At Grade Created by: CK-12
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Lesson 11.2: True or False

Name___________________ Class______________ Date________

Write true if the statement is true or false if the statement is false.

_____ 1. Earth’s geologic processes have changed over time.

_____ 2. Extinction occurs when a species completely dies out.

_____ 3. Layers of sedimentary rock are called strata.

_____ 4. The relative age of a rock is its approximate age in years.

_____ 5. Rock layers on opposite sides of the Grand Canyon show lateral continuity.

_____ 6. Key beds are rock layers that have unconformities.

_____ 7. More than one type of index fossil provides stronger evidence that rock layers are the same age.

_____ 8. The Cretaceous Period ended when the first dinosaurs appeared.

_____ 9. The earliest geologic time scale showed how many years ago each era began.

_____ 10. Fish were common organisms during the Paleozoic Era.

Lesson 11.2: Critical Reading

Name___________________ Class______________ Date________

Read this passage based on the text and answer the questions that follow.


Geologists can learn a lot about Earth’s history by studying sedimentary rock layers. However, in some places, there’s a gap in rock layers. For some period of time in the past, no rock layers are present. A gap in a sequence of rock layers is called an unconformity.

In the 1700s, a geologist named James Hutton discovered an unconformity in Scotland. Hutton saw that the lower rock layers in an area were very old. The upper rock layers in the same area were much younger. However, there were no rock layers in between the older and younger layers. Hutton concluded that intermediate rock layers had once existed. What happened to them? They must have eroded away before the younger rock layers were deposited.

Hutton's discovery of this unconformity was a very important event in geology. Hutton knew that deposition and erosion are very slow processes. He understood that for both processes to occur would take an extremely long time. He concluded that Earth must be much older than people of his time thought. This was a really big discovery. It meant that there was enough time for life on Earth to gradually evolve. Hutton’s discovery was an important influence on the development of the theory of evolution.


  1. What is an unconformity?
  2. How might an unconformity cause confusion about the ages of rock layers?
  3. What was Hutton’s unconformity? What did Hutton conclude after discovering it?
  4. Why was Hutton’s discovery so important?

Lesson 11.2: Multiple Choice

Name___________________ Class______________ Date________

Circle the letter of the correct choice.

  1. If sedimentary rock layers are tilted, they must have
    1. formed at an angle.
    2. moved after they formed.
    3. been cross-cut by igneous rock.
    4. formed from deposits on a mountainside.
  2. A key bed of clay from around the time the dinosaurs went extinct led to the hypothesis that the extinction was caused by a
    1. large flood.
    2. huge volcano.
    3. giant asteroid.
    4. none of the above
  3. Evidence shows that Earth is about
    1. 1.9 million years old.
    2. 2.8 million years old.
    3. 3.8 billion years old.
    4. 4.5 billion years old.
  4. Eons of the geologic time scale are divided first into
    1. years.
    2. periods.
    3. eras.
    4. epochs.
  5. The Cenozoic Era is called the age of
    1. dinosaurs.
    2. mammals.
    3. reptiles.
    4. life.
  6. What does the term paleozoic mean?
    1. fossil life
    2. ancient rock
    3. rock strata
    4. old life
  7. Many of the divisions of the geologic time scale mark major events in the history of
    1. life.
    2. science.
    3. astronomy.
    4. Earth science.

Lesson 11.2: Matching

Name___________________ Class______________ Date________

Match each definition with the correct term.


_____ 1. whether a rock is older or younger than other rocks

_____ 2. law stating that rock layers are deposited in horizontal layers

_____ 3. law stating that rock layers closer to the surface are younger than deeper rock layers

_____ 4. law stating that rock layers are older than any rocks that cut across them

_____ 5. gap in a sequence of rock layers

_____ 6. law stating that matching nearby rock layers are the same age

_____ 7. study of rock layers


a. stratigraphy

b. superposition

c. relative age

d. lateral continuity

e. original horizontality

f. cross-cutting relationships

g. unconformity

Lesson 11.2: Fill in the Blank

Name___________________ Class______________ Date________

Fill in the blank with the appropriate term.

  1. A Danish geologist named __________ developed the laws of stratigraphy.
  2. A Scottish geologist named __________ determined that Earth must be very old.
  3. A(n) __________ is a thin layer of rock that is unique and widespread.
  4. Rock layers containing the same index fossils are about the same __________.
  5. The longest blocks of time on the geologic time scale are called __________.
  6. The era of the geologic period when dinosaurs lived was the __________ Era.
  7. The current epoch of the geologic time scale is named the __________ Epoch.

Lesson 11.2: Critical Writing

Name___________________ Class______________ Date________

Thoroughly answer the question below. Use appropriate academic vocabulary and clear and complete sentences.

Explain how geologists use stratigraphy to learn about Earth’s past.

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