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105Lab09GeolTime - Geol 105 Lab#9 Geologic Time Week of...

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Geol 105 Lab #9 Geologic Time Week of March 19-23, 2012 You will be doing assigned problems from the lab manual, chapter 8, in addition to the exercise below. TRILOBITES AND RELATIVE AGE Relative Age The principles of superposition, original horizontality, and cross-cutting can be applied to deduce the relative order of the various rock layers and events in a given region. Once the order of deposition, erosion, and faulting have been determined it is then possible to use evolutionary changes in the fossils present within sedimentary rocks to establish relative ages for the rocks. Paleontologists use major evolutionary changes, usually the first appearance of a new and distinctive fossil, to determine the boundaries of the various time intervals (such as the Ordovician System or the Cretaceous System; see Figure 8.9 in the lab manual). Rocks that contain the same types of fossils are the same age. Some groups, such as the trilobites , are important for the process of assigning relative ages to
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