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45. What era is also called the age of mammals? 8. Describe how the principles of superposition, horizontality, and
a. Cenozoic faunal succession are used in the relative dating of sedimentary
b. Mesozoic rock layers.
c. Paleozoic 9. How are the numbers of the ages of eras and other divisions of
d. Phanerozoic the geologic time scale determined?
46. The oxygen in our atmosphere is the result of 10. Describe the three basic categories of fossilization methods.
a. ultraviolet radiation.
11. Describe some of the things that fossils can tell you about Earth’s
b. photosynthesis.
history.
c. evolution of mammals.
d. cooling climates.
FOR FURTHER ANALYSIS
47. Earth is thought to be
a. 225 million years old.
1. Analyze the significant reasons that a pot chard of an ancient
b. 2.5 billion years old.
tribe is not considered to be a fossil.
c. 3.8 billion years old.
2. Considering the ways fossils are formed, describe what percent
d. 4.5 billion years old.
you would expect to be (1) preservation or alteration of hard
Answers parts, (2) preservation of shape, and (3) preservation of signs of
1. b 2. c 3. b 4. d 5. a 6. c 7. a 8. c 9. b 10. a 11. c 12. a 13. c 14. c activity. Give reasons for your selections.
15. b 16. a 17. d 18. d 19. a 20. b 21. b 22. c 23. d 24. b 25. a 26. c 3. Suppose you believe the principle of uniformity to be correct.
27. a 28. d 29. c 30. d 31. a 32. d 33. c 34. b 35. d 36. a 37. d 38. c Describe a conversation between yourself and another person
39. d 40. d 41. b 42. c 43. c 44. b 45. a 46. b 47. d who feels strongly that all geologic features were formed at the
same time in the recent past.
QUESTIONS FOR THOUGHT 4. What are the significant differences between the yearly time scale
and the geologic time scale?
1. What is the principle of uniformity? What are the underlying 5. Summarize why the different blocks of time (eon, era, periods)
assumptions of this principle? exist in the geologic time scale.
2. What is the geologic time scale? What is the meaning of the eras? 6. The history of Earth has been one of many changes over a long
3. Why does the rock record go back only 3.8 billion years? If this period of time, so why do some people today expect no change
missing record were available, what do you think it would show? in Earth and its atmosphere?
Explain.
INVITATION TO INQUIRY
4. Do igneous, metamorphic, or sedimentary rocks provide the
most information about Earth’s history? Explain.
Look for Clues
5. What major event marked the end of the Paleozoic and Mesozoic
eras according to the fossil record? Describe one theory that Visit a cliff or place where sedimentary rock has been exposed in a hill
proposes to account for this. by the building of a road. Make a drawing of the layers and measure
6. Briefly describe the principles and assumptions that form the their thickness. If it took 100 years to form a millimeter of sediment, how
basis of interpreting Earth’s history from the rocks. many years are represented by each different layer? Look for clues about
7. Describe the sequence of geologic events represented by an the type of sediment and how it was deposited. Look for fossils and other
angular unconformity. Begin with the deposition of the signs about past events. Make up a description about conditions in the
sediments that formed the oldest rocks represented. region at the time the sediments were being deposited.

PARALLEL EXERCISES
For additional Parallel Exercises, see the chapter 21 resources on www.mhhe.com/tillery.
The exercises in groups A and B cover the same concepts. Solutions to group A exercises are located in appendix E.
Group A Group B
1. A sequence of limestone that spans the Tertiary period is 1. A sequence of deep marine shale that spans the Silurian period
1,100 m thick. What was the average rate of deposition of this is 355 m thick. What was the average rate of deposition of this
limestone in mm/yr? (Assume no break in deposition.) sedimentary rock in mm/yr? (Assume no break in deposition.)
2. An angular unconformity exists between two sequences of 2. An unconformity exists between metamorphic rocks and
sedimentary rocks. The youngest rock beneath the unconformity overlying sedimentary rocks. The metamorphic rock is
is Early Permian age (280 million years), and the first Proterozoic age (1.1 billion years), and the sedimentary rock on
sedimentary rock on top of the unconformity is Middle Jurassic top of the unconformity is Early Cambrian (549 million years).
(176 million years). How long is the time break? How long is the time break?
3. At one locality there is a continuous sedimentary rock sequence 3. At one locality there is a continuous sedimentary sequence
ranging from Early Cambrian to Upper Pennsylvanian age. At ranging from Cretaceous to Recent age. At another locality 45 km
another locality 50 km to the west, this sequence has an to the northwest, this sequence has an unconformity and the
unconformity and the Devonian rocks are absent. How long, in Miocene and Oligocene sediments are absent. How long, in
years, is the time break of this unconformity? years, is the time break of this unconformity?

538 CHAPTER 21 Geologic Time 21-18