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Chapter 5 Section 1 Geologic History • Objective • Contrast relative dating with absolute dating. • Relative Dating • Scientists use different methods to determine the age of objects in sedimentary rocks. • One method, Relative dating, examines a fossil’s position within rock layers to estimate its age. • The bottom layers of rock are usually the oldest, and the top layers are usually the youngest. • Scientists can use the order of rock layers to determine the relative age of objects within the layers. • For example, fossils in the bottom layers are usually older than fossils in the top layers. • The Geologic Column • To make relative dating easier, geologists combine data from all of the known rock sequences around the world. • From this information, geologists created the geologic column — an ideal sequence of rock layers that contains all of the known fossils and rock formations on Earth, arranging layers from oldest to youngest. • Paleontology • Paleontology is the science involved with the study of past life. Scientists who study past life are called paleontologists. • Paleontologists collect data by studying fossils. • Section 2 Looking at Fossils • Objective • Explain how fossils can be used to determine the history of changes in environments and organisms. • The remains or physical evidence of an organism preserved by geologic processes is called a fossil. • Trace fossils are any naturally preserved evidence of animal activity. • A cavity in rock where a plant or animal was buried is called a mold. • A cast is an object that is created when sediment fills a mold and becomes rock. • History of Changing Organisms • Scientists study the relationships between fossils to interpret how life has changed over time. • Since the fossil record is incomplete, paleontologists look for similarities between fossils over time to try to track change. • Using Fossils to Date Rocks • Scientists have found that particular types of fossils appear only in certain layers of rock. • By dating rock layers above and below these fossils, scientists can determine the time span in which the organism lived. • If the organism lived for a relatively short period of time, its fossils would show up in limited layers.