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Finally, there are many kinds of fossils that are not the pre-
served remains of an organism but are preserved indications of
the activities of organisms. Some of the most interesting of such
Mold fossils are the footprints of various kinds of animals. There are
examples of footprints of dinosaurs and various kinds of extinct
mammals, including ancestors of humans. If an animal walked
Rock split to
reveal mold through mud or other soft substrates and the substrates were
covered with silt or volcanic ash, the pattern of the footprints
Shell Shell dissolved can be preserved. The tunnels of burrowing animals, the inden-
buried (void space)
tations left by crawling worms, and the nests of dinosaurs have
been preserved in a similar manner. Even the eggs of dinosaurs
A
and other animals have been preserved in a mineralized form.
As you can see, there are many different ways in which a
Cast fossil can be formed, but it must be found and studied if it is
to reveal its part in the history of life. This means the rocks in
which the fossil formed must now somehow make it back to the
surface of Earth. This usually involves movement and uplift of
Rock split to the rock and weathering and erosion of the surrounding rock
reveal cast
Shell Shell dissolved Sediment to release or reveal the fossil. Most fossils are found in recently
buried (void space) deposited eroded sedimentary rocks—sometimes atop mountains that
into void were under the ocean a long, long time ago. The complete record
B space of what has happened in the past is not found in the fossil alone
but requires an understanding of the layers of rocks present, the
FIGURE 21.5 Origin of molds and casts. (A) Formation of a
relationship of the layers to one another, and their age.
mold. (B) Formation of a cast.
CONCEPTS Applied
decayed, the original wood was replaced by mineral matter an
ion at a time. Over time, the “mix” of minerals being deposited Find Fossils
changed, and the various resulting colors appear to preserve the Collect fossils from roadcuts and old quarries in your area.
texture of the wood. Since the mineralization or replacement Make an exhibit showing the fossils with sketches of what
processes take place an ion at a time, a great amount of detail the animals or plants were like. What do these particular
can be preserved in such fossils. The size and shape of the cells animals and plants tell you about the history of your area?
and growth rings in petrified wood are preserved well enough
that they can be compared to modern plants. The skeletons or
shells of many extinct organisms are also typically preserved in
this way. The “fossilized bones” of dinosaurs or the “fossilized Myths, Mistakes, & Misunderstandings
shells” of many invertebrates are examples.
Turned to Stone?
It is a mistake to say that petrified trees are trees that have been
“turned to stone.” Petrified trees are stony replicas that were
formed by groundwater filling pore spaces with minerals. At the
same time, wood is dissolved and replaced by mineral deposits,
preserving details such as tree ring growth.

21.2 READING ROCKS

Reading history from the rocks of Earth’s crust requires both a
feel for the immensity of geologic time and an understanding
of geologic processes. By “geologic time,” we mean the age of
Earth, the very long span of Earth’s history. This span of time is
difficult for most of us to comprehend. Human history is mea-
sured in units of hundreds and thousands of years, and even
the events that can take place in a thousand years are hard to
FIGURE 21.6 Petrified fossils are the result of an ion-for-ion
replacement of the buried organisms with mineral material. These logs imagine. Geologic time, on the other hand, is measured in units
of petrified wood are in the Petrified Forest National Park, Arizona. of millions and billions of years.

21-5 CHAPTER 21 Geologic Time 525