Laboratory Manual for Introductory Geology 4e

EXERCISE 12.10
Cretaceous Paleogeography of North America
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After more than 200 years of study, geologists have gathered an enormous amount of data about the geologic history of
North America. This exercise presents some of those data and asks you to imagine what the geography of the continent
looked like toward the end of the Cretaceous Period, just before an event that led to extinction of most life forms—
including, most famously, the dinosaurs.
The map on the next page summarizes data from the distribution of sedimentary rocks that form in specific
environments (such as those you examined in Chapter 6); results from index fossil and numerical dating of sedimentary
and igneous rocks; and interpretations of the folding, faulting, and metamorphic history of Cretaceous and older rocks. We
saw earlier that index fossils are valuable for pinpointing the relative ages of the rocks in which they are found. Here, we
also use facies fossils, fossils of organisms that could live only in a narrowly restricted environment and therefore reveal
the surface geography at the time they lived.
On the map, draw boundaries delineating the following North American geographic settings in the late Cretaceous
Period:
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Continental interior
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Old, eroded mountain ranges
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Active tectonic zone
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Continental shoreline
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Shallow ocean (continental shelf and parts of the continent flooded by shallow seas)
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Deep ocean
Evidence for each setting is described just below the map. The facies fossils used as symbols on the map are, wherever
possible, Cretaceous organisms.
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What Do You Think We all have impressions of what the Earth may have looked like during the
Cretaceous from watching movies and television. Using the evidence from the Cretaceous map, describe on a
separate sheet of paper what you think the United States and Canada looked like at that time, by region—north, south,
central, and coastal areas. Then find your state or province, describe what you think it was like at that time, and suggest
what you might want to explore to learn more.
(continued)
12.5 CORRELATION: FITTING PIECES OF THE PUZZLE TOGETHER
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