Exercise 11A: Geologic Mysteries of the Grand Canyon

Geologic Mystery #1

Geologic Mystery #2

Geologic Mystery #3

In preparation for reconstructing the geologic history of the Grand Canyon, you will use your
computer to observe photographs of landscapes within the canyon that contain distinctive
geologic features.

Each of the photographs below contain an image of a geologic mystery, such as a dike, an
unconformity, or another geologic feature. Begin by drawing a cartoon (generalized sketch) of
each photograph. Identify on your sketch the geologic mystery contained in the photograph,
and then solve the relative sequence of events that formed the scene you observe. Click on the
image to open a larger view in a new window. Answer the question posed in the description for
each image, solving the geologic mystery that is visible in the photograph. Write your answers
in Worksheet 11A along with other interesting interpretations you make. For some mysteries,
you will need to apply the principles of relative dating you learned previously.

Geologic Mystery #1

Figure 11.1. Precambrian rocks in the bottom of the canyon at Hance Rapids, eastern Grand
Canyon. The lower red rock unit is Dox Sandstone, and the upper rock is Shinumo Quartzite.
There is also a dark-colored igneous rock. What are the relative ages of these three rocks?

Geologic Mystery #2

Exercise 11A

http://www4.nau.edu/geology/glg100/labs/lab11a.html

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2010-06-15 15:04

Figure 11.2. Paleozoic rocks at Stephen's Isle, central Grand Canyon. The bottom ledge is
Tapeats Sandstone, representing an ancient beach, overlain by slope-forming Bright Angel
Shale and Muav Limestone, with cliff-forming Redwall Limestone (containing corals) at the top
on the right. All four formations contain marine fossils. What changes in the environment could
account for this sequence of rocks?

Geologic Mystery #3

Figure 11.3. Inner Granite Gorge, upstream from Phantom Ranch, central Grand Canyon. The
lower unit is dark Precambrian metamorphic rocks (Vishnu Schist) cut by pinkish granite dikes;
the upper unit is Cambrian Tapeats Sandstone. What is the sequence of events represented
here, and what is the contact at the base of the sandstone?

Exercise 11A

http://www4.nau.edu/geology/glg100/labs/lab11a.html

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2010-06-15 15:04

Exercise 11A

http://www4.nau.edu/geology/glg100/labs/lab11a.html

3 z 3

2010-06-15 15:04