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21
21 Geologic

Time

These are modern-day stromatolites from Hamlin Pool in western Australia.
The dome-shaped structures shown in the photograph are composed of
layers of cyanobacteria and materials they secrete. They grow up to 60 cm
(about 2 ft) tall. Some of the oldest fossils are of ancient stromatolites that
developed in shallow marine environments about 3.5 billion years ago. When
samples from fossil stromatolites are cut into slices, microscopic images can
be produced that show the fossil remains of some of the world’s oldest cells.

CORE CONCEPT

Earth and its kinds of living things have changed greatly over billions of years.

OUTLINE

21.1 Fossils
Fossils Early Ideas About Fossils Reading Rocks
Fossils provide evidence for past Types of Fossilization The same geologic processes
life and show that the kinds of 21.2 Reading Rocks operate today as they did in
organisms on Earth have changed. Arranging Events in Order the past.
Shifting Sites of Erosion and Deposition
Correlation
21.3 Geologic Time
Early Attempts at Earth Dating
The Geologic Time Scale Modern Techniques
In the past, the positions of the The Geologic Time Scale The Geologic Time Scale
continents changed, the climate Geologic Periods and Typical Fossils Living things can be organized
changed, and the sea level Mass Extinctions into logical evolutionary
changed. People Behind the Science: Eduard Suess sequences.
Interpreting Geologic History—A Summary

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