Laboratory Manual for Introductory Geology 4e

10.6 Reading Real Geologic Maps
You are now ready to apply what you’ve learned to interpreting the structure of
selected areas of North America using excerpts from published geologic maps.
Exercises 10.16 and 10.17 give you a sense of how to see “clues” in a map that help
you to picture the three-dimensional configuration of rocks underground.
We finish this chapter by talking about how geologists make geologic maps in
the first place. It isn’t easy! Students who want to learn the skill generally attend
a summer geology field camp, where they practice the art of mapping for several
weeks and gradually develop an eye for identifying rock types, contacts, folds, and
faults. Typically, in arid regions, not much soil forms, so bedrock may be abundantly
exposed; in such areas, geologists may actually see contacts and can walk out the
traces of contacts. Commonly, however, soil and vegetation cover much of the rock,
so outcrops are discontinuous and separated from one another by “covered intervals.”
In such cases, geologists must extrapolate contacts, using common sense and
an understanding of geologic structures. Exercise 10.18 provides the opportunity
for you to construct a map in an area where limited outcrop data are available. The
map shown is called an outcrop map because individual outcrops of rock are outlined.
To complete the map, you need to extrapolate contacts. In Exercise 10.19,
you will use all of your skills to interpret the structural geology of a region and present
your findings.
EXERCISE 10.16
The Observation Peak Quadrangle of Wyoming
Name:
Course:
Section:
Date:
Examine the geologic map of a portion of the Observation Peak quadrangle in Wyoming in FIGURE 10.14. This map area
contains a number of interesting geologic features, some of which you can understand based on the work you have done
earlier in this chapter. Each of the questions below refers to a specific feature on the map.
(a) In the northern part of the map, there is a large area of yellow (i.e., Quaternary deposits). What
kind of contact forms the boundary between these deposits and older
bedrock?
(b) The bright-red color represents igneous intrusions.
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What kind of intrusion is the larger round area?
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What kind of intrusions are the narrow bands?
(c) A thrust fault (a gently to moderately dipping reverse fault) is exposed in the southwestern quarter of the map.
The “teeth” of the thrust fault symbol lie on the hanging wall. In this locality, the fault dips about 40° in a westerly
direction.
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Where Indian Creek crosses the fault, what rock unit is in the hanging wall, and what rock unit is in the
footwall?
Is the older rock in the hanging wall or in the footwall?
Thinking about the direction of slip on a thrust fault, does this make sense?
(Explain your answer.)
(d) In the southeastern quarter of the map, you can see the trace of a fold.
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What type of fold is it?
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In which direction does it plunge?
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In the Ankareh Formation, what is the strike and dip of the strata on the western limb of
the fold?
10.6 READING REAL GEOLOGIC MAPS
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