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This limestone
and this shale
can be correlated
by the fossil they
contain;
so can
this shale
and this sandstone.

OUTCROP
1

(The two outcrops may be
separated by long distances OUTCROP
with no exposed rocks 2
in between.)

FIGURE 21.14 Similarity of fossils suggests similarity of ages, even in different rocks widely separated in space.

and arrived at the conclusion that Earth was created at 9 a.m.
on Tuesday, October 26, in the year 4004 b.c. On the authority
of biblical scholars, this date was generally accepted for the next
century or so, even though some people thought that the geol-
ogy of Earth seemed to require far longer to develop. The date
of 4004 b.c. meant that Earth and all of the surface features had
formed over a period of about 6,000 years. This required a model
of great cataclysmic catastrophes to explain how all Earth’s fea-
tures could possibly have formed over a span of 6,000 years.
Near the end of the eighteenth century, James Hutton
reasoned out the principle of uniformity, and people began
to assume a much older Earth. The problem then became one
of finding some uniform change or process that could serve
as a geologic clock to measure the age of Earth. To serve as a
geologic clock, a process or change would need to meet three
FIGURE 21.15 This dinosaur footprint is in shale near Tuba
City, Arizona. It tells you something about the relative age of the shale, criteria: (1) the process must have been operating since Earth
since it must have been soft mud when the dinosaur stepped here. began, (2) the process must be uniform or at least subject to
averaging, and (3) the process must be measurable.
During the nineteenth century, many attempts were made
21.3 GEOLOGIC TIME to find Earth processes that would meet the criteria to serve as
How do you measure and track time intervals for something as a geologic clock. Among others, the processes explored were
old as Earth? First, you would need to know the age of Earth; (1) the rate that salt is being added to the ocean, (2) the rate
then you would need some consistent, measurable events to that sediments are being deposited, and (3) the rate that Earth
divide the overall age into intervals. Questions about the age of is cooling. Comparing the load of salts being delivered to the
Earth have puzzled people for thousands of years, dating back ocean by all the rivers, and assuming the ocean was initially
at least to the time of the ancient Greek philosophers. Many pure water, it was calculated that about 100 million years would
people have attempted to answer this question and understand be required for the present salinity to be reached. The calcula-
geo logic time but with little success until the last few decades. tions did not consider the amount of materials being removed
from the ocean by organisms and by chemical sedimentation,
however, so this technique was considered to be unacceptable.
EARLY ATTEMPTS AT EARTH DATING Even if the amount of materials removed were known, it would
One early estimate of the age of Earth was attempted by actually result in the age of the ocean, not the age of Earth.
Archbishop Ussher of Ireland in the seventeenth century. He A number of separate and independent attempts were
painstakingly counted up the generations of people mentioned made to measure the rate of sediment deposition, then com-
in biblical history, added some numerological considerations, pare that rate to the thickness of sedimentary rocks found on

21-9 CHAPTER 21 Geologic Time 529