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Relative Dating—Key Principles
FIGURE 18.2 The Colorado River winding through Canyonlands National Park near Moab, Utah.
The strata exposed in the canyon walls contain clues to millions of years of Earth history.
(Photo by John T. Parkinson/Mira.com)
have been changed over and over again
by the same geological processes that
operate today.
During the late 1800s and early 1900s,
various attempts were made to determine
Earth’ s age. Although some of the methods
appeared promising at the time, none
proved to be reliable. What these scientists
were seeking was a numerical date.
Such dates specify the actual number of
years that have passed since an event
occurred. Today our understanding of
radioactivity allows us to accurately determine numerical dates for rocks that represent important events in Earth’ s distant
past. We will study radioactivity later in
this chapter. Prior to the discovery of
radioactivity, geologists had no accurate
and dependable method of numerical
dating and had to rely solely on relative
dating.