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Water

Deposition

Erosional surface

FIGURE 21.12 A time break in the rock record in the Grand
Canyon, Arizona. The horizontal sedimentary rock layers overlie
Rocks tilted, eroded almost vertically foliated metamorphic rocks. Metamorphic rocks
form deep in Earth, so they must have been uplifted, and the
overlying material eroded away before being buried again.

Water Lava flow

Angular unconformity established Sandstones

Subsequent deposition Shales
FIGURE 21.11 Angular unconformity. Development involves Limestones
some deformation and erosion before sedimentation is resumed. Basalt

same type of fossil organisms that lived only a brief geologic
time should occur only in rocks that are the same age. Accord-
Granite
ing to the principle of faunal succession, then, once the basic
sequence of fossil forms in the rock record is determined, rocks
can be placed in their correct relative chronological position
on the basis of the fossils contained in them. The principle also FIGURE 21.13 Deciphering a complex rock sequence. The
means that if the same type of fossil organism is preserved in limestones must be oldest (law of superposition), followed by the
two different rocks, the rocks should be the same age. This is shales. The granite and basalt must both be younger than the
logical even if the two rocks have very different compositions limestone they crosscut (note the metamorphosed zone around the
and are from places far, far apart (Figure 21.14). granite). It is not possible to tell whether the igneous rocks predate
or postdate the shales or to determine whether the sedimentary
Distinctive fossils of plant or animal species that were
rocks were tilted before or after the igneous rocks were emplaced.
distributed widely over Earth but lived only a brief time with a com- After the limestones and shales were tilted, they were eroded, and
mon extinction time are called index fossils. Index fossils, together then the sandstones were deposited on top. Finally, the lava flow
with the other principles used in reading rocks, make it possible covered the entire sequence.
to compare the ages of rocks exposed in two different locations.
This is called age correlation between rock units. Correlations of
exposed rock units separated by a few kilometers are easier to do, can be applied only to rocks in which fossils are well preserved,
but correlations have been done with exposed rock units that are which are almost exclusively sedimentary rocks. Correlation
separated by an ocean. Correlation allows the ordering of geologic also can be based on the occurrence of unusual rock types,
events according to age. Since this process is only able to determine distinctive rock sequences, or other geologic similarities. All
the age of a rock unit or geologic event relative to some other unit this is useful in clarifying relative age relationships among rock
or event, it is called relative dating (Figure 21.15). Dates with nu- units. It is not useful in answering questions about the age of
merical ages are determined by means different from correlation. rocks or the time required for certain events, such as the erup-
The usefulness of correlation and relative dating through tion of a volcano, to occur. Questions requiring numerical
the concept of faunal succession is limited because the prin ciples answers went unanswered until the twentieth century.

528 CHAPTER 21 Geologic Time 21-8