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The understanding of geologic processes has been made
possible through the development of various means of measur-
ing ages and time spans in geologic systems. An understanding Water
of geologic time leads to an understanding of geologic processes,
which then leads to an understanding of the environmental
conditions that must have existed in the past. Thus, the mineral
composition, texture, and sedimentary structure of rocks are
clues to past events, events that make up the history of Earth.

A
ARRANGING EVENTS IN ORDER
The clues provided by thinking about geologic processes that
must have occurred in the past are interpreted within a logical Water
frame of reference that can be described by several basic princi-
ples. The following is a summary of these basic guiding principles
that are used to read a story of geologic events from the rocks.
Recall that the cornerstone of the logic used to guide thinking
about geologic time is the principle of uniformity. As described Irregular bottom
in chapter 19, this principle is sometimes stated as “the present is surface
the key to the past.” This means that the geologic features that you B
see today have been formed in the past by the same processes of
crustal movement, erosion, and deposition that are observed to- FIGURE 21.7 The principle of original horizontality. (A) Sedi-
day. By studying the processes now shaping Earth, you can under- ments tend to be deposited in horizontal layers. (B) Even where the
stand how it has evolved through time. This principle establishes sediments are draped over an irregular surface, they tend toward
the understanding that the surface of Earth has been continuously the horizontal.
and gradually modified over the immense span of geologic time.
The principle of original horizontality is a basic principle Top
that is applied to sedimentary rocks. It is based on the observa- (youngest)
tion that, on a large scale, sediments are commonly deposited in Sandstone
flat-lying layers. Old rocks are continually being changed to new
ones in the continuous processes of crustal movement, erosion,
and deposition. As sediments are deposited in a basin of depo- Shale
sition, such as a lake or ocean, they accumulate in essentially
flat-lying, approximately horizontal layers (Figure 21.7). Thus,
any layer of sedimentary rocks that is not horizontal has been
subjected to forces that have deformed Earth’s surface.
The principle of superposition is another logical and Limestone
obvious principle that is applied to sedimentary rocks. Layers of Bottom
sediments are usually deposited in succession in horizon tal lay- (oldest)
ers, which later are compacted and cemented into layers of sedi-
FIGURE 21.8 The principle of superposition. In an undisturbed
mentary rock. An undisturbed sequence of horizontal layers is
sedimentary sequence, the rocks on the bottom were deposited first,
thus arranged in chronological order with the oldest layers at and the depositional ages decrease as you progress to the top of
the bottom. Each consecutive layer will be younger than the one the pile.
below it (Figure 21.8). This is true, of course, only if the layers
have not been turned over by deforming forces (Figure 21.9).
The principle of crosscutting relationships is concerned Shifting Sites of Erosion and Deposition
with igneous and metamorphic rock, in addition to sedimentary The principle of uniformity states that Earth processes going
rock layers. Any geologic feature that cuts across or is intruded on today have always been occurring. This does not mean,
into a rock mass must be younger than the rock mass. Thus, if however, that they always occur in the same place. As erosion
a fault cuts across a layer of sedimentary rocks, the fault is the wears away the rock layers at a site, the sediments produced
youngest feature. Faults, folds, and igneous intrusions are always are deposited someplace else. Later, the sites of erosion and
younger than the rocks they originally occur in. Often, there is deposition may shift, and the sediments are deposited on top
a further clue to the correct sequence: The hotter igneous rock of the eroded area. When the new sediments later are formed
may have “baked,” or metamorphosed, the surrounding rock into new sedimentary rocks, there will be a time lapse between
immediately adjacent to it, so again the igneous rock must have the top of the eroded layer and the new layers. A time break in
come second (Figure 21.10). (For a worked example on this ma- the rock record is called an unconformity. The unconformity
terial, see the chapter 21 resources on www.mhhe.com/tillery.) is usually shown by a surface within a sedimentary sequence

526 CHAPTER 21 Geologic Time 21-6