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Brianna Schneider Period #1 1-22-06 Word count:458 Our experiment was to make a mold of a fossil and then use plaster to turn it into a cast. We used a pastry dish and put sand in it. Then we took a fossil and made an imprint in the sand. After that we poured plaster into the cast. We let it dry for a day and then the next day we took the cast of the fossil out of the mold. Then we took a toothpick and scraped some of the sand off so that we could see the details of the fossil better. Absolute age of a rock is the number of years since the rock formed. Relative age of a rock is its age compared to the ages of other rocks. The difference between them is that one (absolute age) is the number of years since the rock formed and one (relative age) is the age compared to the ages of other rocks. Radioactive dating is when scientists first measure the amount of a radioactive element (the “parent” isotope) in the minerals that are in a rock. Then they measure the amount of the stable element into which the radioactive element decays (the “daughter” isotope). Radioactive dating is used by geologists to determine the absolute age of a rock. The geologic time scale is a record of the geologic events and the life forms in Earth’s history. The units of the geologic scale are eras, periods, and epochs. Eras are the three long units of time. They are the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. Periods are eras subdivided into units of geologic time. The Mesozoic Era has three periods the Cretaceous Period, Jurassic Period, and the Triassic Period. There is only one period in the Cenozoic Era. It is the Tertiary Period. On the other hand there are six periods in the Paleozoic Era. They are the Permian Period, Carboniferous Period, Devonian Period, Silurian Period, Ordovician Period, and the Cambrian Period. Epochs are further subdivided periods of the Cenozoic Era. I leaned a lot more about fossils and about rocks. I learned about the different eras, periods, and epochs. I learned that there are three eras and that there are 10 periods in all. I learned that there was one period in the Cenozoic Era, three in the Mesozoic Era, and six in the Paleozoic Era. I also learned how to make a cast using sand, a fossil, and some plaster. I also learned how scientists determine the age of a rock. They determine its age by using index fossils. Index fossils are a fossil that must be widely distributed and represent a type of organism that existed only for a short time.