Laboratory Manual for Introductory Geology 4e

EXERCISE 12.5
Name:
Course:
Deciphering Geologic History Using Physical Principles of Relative Dating (continued)
Section:
Date:
Geologic cross section 4
I
E
A
H
B
Youngest
C
C
J
G
d
J
f
G
Oldest
We will see later in this chapter how it is possible to date the rocks above and
below an unconformity and therefore estimate the amount of geologic time missing
between their deposition.
12.3 Biological Methods for Relative Age Dating
In 1793, British canal builder and amateur naturalist William Smith noticed different
fossils in the layers of rock he was excavating. He discovered wherever he
worked that particular fossils were always found in rocks that lay above others, and
he suggested that fossils could be used to tell the relative ages of the rocks.
12.3.1 The Principle of Faunal and Floral Succession
Other geologists confirmed Smith’s hypothesis and formulated the principle of faunal
and floral succession, which states that fossil animals (fauna) and plants (flora)
throughout the world record an increasing complexity of life forms from older
rocks to younger ones. Not all fossils can be used to date rocks, however, because
some, such as blue-green algae, have existed over most of geologic time and are not
specific to any particular time span.
Index fossils are remains of plants or animals that were distributed widely
throughout the world but existed for only a short span of geologic time before
12.3 BIOLOGICAL METHODS FOR RELATIVE AGE DATING
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