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FIGURE 21.9 The Grand Canyon, Arizona, provides a majestic cross section of horizontal sedimentary rocks. According to the principle of
superposition, traveling deeper and deeper into the Grand Canyon means that you are moving into older and older rocks.

on which there was a lack of sediment deposition or where erosional, some of the record once present will have been lost.
active erosion may even have occurred for some period of An unconformity may occur within a sedimentary sequence
time. When the rocks are later examined, that time span will of the same kind or between different kinds of rocks. The most
not be represented in the record, and if the unconformity is obvious kind of unconformity to spot is an angular unconformity.
An angular unconformity, as illustrated in Figure 21.11, is one in
which the bedding planes above and below the unconformity
are not parallel. An angular unconformity usually implies
some kind of tilting or folding, followed by a significant period
of erosion, which in turn was followed by a period of deposition
(Figure 21.12).

CORRELATION
Granite
The principle of superposition, the principle of crosscutting
relationships, and the presence of an unconformity all have
meaning about the order of geologic events that have occurred in
the past. This order can be used to unravel a complex sequence
of events such as the one shown in Figure 21.13. The presence
Rocks adjacent to of fossils can help, too. The principle of faunal succession
intruding magma may recognizes that life-forms have changed through time. Old life-
also be metamorphosed forms disappear from the fossil record and new ones appear,
by its heat.
but the same form is never exactly duplicated independently at
FIGURE 21.10 A granite intrusion cutting across older rocks. two different times in history. This principle implies that the

21-7 CHAPTER 21 Geologic Time 527