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Fossils > Unit Outline
Print and Cast
Students make a mold of a "fossil print" and then create
a cast from it. This activity
has been adapted from Badlands National Park's Making
a Fossil Cast.
available in PDF format
Fossils in Time
Guiding Questions: What
are casts and molds? How do you create a cast?
Fossils are imbedded
in rocks. What are the techniques for excavating a fossil
Group size: individuals
Fossils are not always the actual remains of the living organisms.
Many fossils are just copies called imprints, molds or casts.
Imprints are impressions made by organisms in soft mud that
were preserved when the mud solidified. Imprints can be traces
of an animalís activity, rather than its actual remains. The
hardened tracks of animals or the burrows of prehistoric worms in
solidified mud are examples of fossil imprints.
Molds are made when organisms
are totally or partially buried in mud that hardens into rock. Over
time ground water may dissolve the organisms, leaving cavities shaped
like their bodies. Both imprints and molds are mirror images
of the organisms.
If a mold was later filled
with mud or mineral material, the hardened filling is called a cast.
It is a reproduction that has the same outer shape as the
organism. A cast looks like the organism itself, not like
its imprint. Paleontologists make casts of fossil molds by
filling them with liquids, such as plaster, that harden.
Object to cast, a distinctly shaped object representing
a plant or animal presence such as a small seashell, a pinecone,
a student's finger, hand, or toe.
Modeling clay, enough so that students
can create a form about twice the size of the seashell, finger or
other object to be cast
Paper plate for each
Paper cup, 7oz size
Plaster of Paris (available from craft or hardware
each student one of each type of cookie, two paper towels, and six
the students excavating in the hard cookies to extract the chocolate
chips or raisins using only the toothpicks. If they break
a toothpick so that it is no longer sharp, they can no longer use
it. It must be discarded.
3 to 5 minutes, stop and find out if anyone was successful in extracting
anything. As a class, review the Discussion Questions, below.
them excavate with the soft cookies using the same constraints and
3 to 5 minutes, stop and find out if the students were more or less
successful in extracting chips or raisins. Review the Discussion
Questions again. How did the results differ between the hard
and the soft cookies?
The imprint in the clay and
the plaster cast are both examples of how fossils form. Pressing
the shell into the clay represents burying the shell in mud. In
nature, the mud would have hardened into rock around the shell.
Removing the shell from the clay represents how the shell
dissolves over long periods of time, leaving a cavity called a mold
in the rock. The mold produced is a mirror-image imprint of
the shellís outside surface. In nature, this mold would have
been filled with sediment, or small particles or rock and minerals
that are deposited by water, wind, or ice that hardened into rock.
The Plaster of Paris hardened, like the sediment, but in a
much shorter period of time. The plaster is a replacement
of the shell and is called a cast.
Might a paleontologist find a fossil mold or a fossil cast?
Which is created by the original plant or animal?
Which looks most like the original plant or animal?