Laboratory Manual for Introductory Geology 4e

FIGURE 1.8 An example of the principle of uniformitarianism.
(a) Asymmetric ripple marks in modern sand on the shore of Cape
Cod, Massachusetts.
(b) Asymmetric ripple marks in 145-million-year-old sandstone at
Dinosaur Ridge, Colorado.
As noted above, geologists have amassed a large body of evidence showing
that the Earth is about 4.56 billion years old. This enormous span is referred to
as geologic time (FIG. 1.9), and understanding its vast scope is nearly impossible
for humans, who generally live less than 100 years. Our usual frame of reference
for time is based on human lifetimes: a war fought two centuries ago happened
two lifetimes ago, and 3,000-year-old monuments were built about 30 lifetimes ago.
FIGURE 1.9 Milestones in geologic time.
First land plants
First amphibians
First reptiles
First mammals
First dinosaurs
Dinosaur extinction
First humans
400 300 200
100 Today
Time (millions of years ago)
Origin of the Earth
Oldest rock
First evidence of water
Oldest life (bacteria)
Nitrogen-CO 2
atmosphere
Oxygen in oceans
Oxygen in
atmosphere
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 Today
Time (billions of years ago)
20 CHAPTER 1 SETTING THE STAGE FOR LEARNING ABOUT THE EARTH