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Working either individually or in small groups, obtain some seashells or snail shells. Coat one side of each shell with petroleum jelly, and press the coated side into a block of modeling clay about twice as large as the shell. Next, mold a thin rim around the top edge of each block of clay. Remove the shell, leaving a mold.

Now, following the directions on the package (or adding water until the mixture is thick) prepare some plaster of Paris. Using small brushes, paint petroleum jelly on the top of the block and inside the mold. (The petroleum jelly will prevent the plaster from sticking to the clay.)

Fill the mold to the top with plaster, and let it dry for twenty-four hours. Gently peel off the rim, remove the block of clay from the plaster, and you will have a good cast of one side of each shell. (The edges of the plaster may have to be trimmed with a dull knife.)

In a similar way marine organisms with shells are buried on the bottom of oceans and pressed into the mud, silt, and ooze. When their soft bodies decay and their shells dissolve, a hole or mold is left showing the exact shape of the outside of the organism. The mold is sometimes filled with other materials that form a cast (like the plaster) that is the exact shape of the original organism. Both the mold and the cast are fossils.

As an alternate or supplementary activity, you can make a complete fossil shell by pressing the shell between two blocks of clay. (Be sure to coat the entire surface of the shell with petroleum jelly, as well as all surfaces of the blocks.)

When pressing the blocks together, try to have only half the shell impression in each block. Separate the blocks, remove the shell, then hold the blocks tightly together with rubber
bands. Cut a funnel-shaped hole into the top of the blocks so that you can pour plaster into the mold. Prepare some

so that you can pour plaster into the mold. Prepare some plaster of Paris. Fill the mold with plaster, tap the mold gently several times to remove 1any trapped air, and add more plaster. If any plaster leaks from the mold, press the blocks together where the leak occurred. When the plaster has hardened, open the mold, and trim the fossil's edges to produce a good model.

Fossils provide knowledge of past ocean life, and fossils in layers of sediments help scientists order sequences of past events over geological periods of time. It is interesting to note that many sea-type fossils are found in mountain sides or tops, many miles inland. There is a hillside area near Waterford, California, where there are many oceanic fossils.

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