Laboratory Manual for Introductory Geology 4e

FIGURE 12.1 Original horizontality in sedimentary and igneous rocks.
(a) Horizontal sedimentary rocks in the Grand Canyon, Arizona.
(b) Horizontal lava flows, Columbia Plateau, Washington State.
12.2.2 The Principle of Cross-Cutting
Relationships
The principle of superposition cannot be used if layers of
sedimentary or igneous rock have been tilted or folded
from their original horizontal attitudes, nor for most metamorphic
rocks and igneous rocks that weren’t originally
formed in horizontal layers. The principle of cross-cutting
relationships helps to determine the relative ages of such
rocks. Like the principle of superposition, it is based
on common sense: if one rock cuts across another, it must be
younger than the rock that it cuts (FIG. 12.2).
We use the term cross-cutting broadly here, not only to refer (c) Horizontal ash fall deposits, Long Valley, California.
to features that physically cut others, but also for the effects
of other geologic processes on pre-existing rocks. For example, the folding shown in
the bottom photo of Exercise 12.2 doesn’t cut across the sedimentary layers, but it
certainly affected them and must have occurred after they were deposited. Contact
FIGURE 12.2 Cross-bedded
sandstones in Zion National
Park, Utah. The person in
the photograph is pointing to a
contact that cuts across several
inclined beds.
12.2 PHYSICAL CRITERIA FOR DETERMINING RELATIVE AGE
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