Directions (14–23): Record your answers on a separate answer sheet. Some questions may require the
use of material in the Appendix. Base your answers to questions 14-17 on the reading passage below.
Idaho During the Pliocene Epoch
The bluff that rises above the Snake River in Idaho
comprises the Hagerman Fossil Beds. The sediment layers
date from 3.7 million years old at river level to 3.15 million
years old atop the bluff. The bluff’s sediments include river
sands, thin shale layers deposited in ponds, clay flood
deposits, and occasional volcanic deposits. Radioactive
elements such as potassium-40 in the volcanic ashes allow
scientists to determine the age of the fossils. The beds
preserve the large numbers of fossils needed to study past
climates and ancient ecosystems. No other site preserves
such varied species from the Pliocene Epoch.
14. How many years worth of strata are in the bluff?
15. How do scientists determine the age of the
fossils at this site? What other methods could
they use to date the fossils?
16. What major climatic event occurred during the
epoch following the Pliocene, and how might it
have affected the environment at this site?
17. How might the changed climate have caused
species in this area to migrate or become extinct?
When significant environmental change occurs, a
population may adapt, migrate, or become extinct.
Hagerman fossils illustrate each response as the environment
changed from a wetter grassland savanna (with over twice
today’s ten inches of yearly precipitation) to today’s drier
high-desert conditions. Hagerman’s beaver and muskrat
adapted; they are similar to today’s species. Other fossils
indicate that llamas migrated from this area to South
America, and camels and horses crossed the Bering Land
Bridge to Eurasia. Large herbivores such as ground sloths and
mastodons became extinct, as did the animals that preyed
on them, the saber-toothed cats and hyena-like dogs.
21. Describe the location and identify the type of
unconformity shown in the illustration.
22. Can the relative ages of layers D, E, F, and G be
determined with certainty? Why or why not?
23. Suppose that after examining these strata you
travel several miles away and find strata that
appear to match layers B, C, and D. What is this
correlation of strata sequences called? What
techniques might you use to correlate the strata?
Base your answers to questions 18 through 23 on the
illustration to the right, which shows a series of rock
layers. Each layer is identified by a letter. The types of
rock are identified in the key.
18. Which layer is the oldest? Which layer is the
youngest? Explain your reasoning.
19. Describe the principle of cross-cutting
relationships. Which layer or layers demonstrate
that principle?
20. Identify the rocks that make up layers D, E, and
F, and order them from oldest to youngest. What
is the relative grain size of the layers? What can
you infer about geological events in this area
from the rock types and their sequence?
Standardized Test Practice
693