1. Announcements ALL late work due by 3:10 on 12/6. Final exam review will be given in class next week, following quiz, 12/11. Final essay on 12/19 & 12/20

2. Geologic Dating Index Fossils

3. Relative Dating Not exact dating Just determining relationships. –Older than... –Younger than... Recall, Law of superposition: Older rock layers are on the bottom, younger rock layers are on the top.

4. Index fossils Organisms that live during a short period of time. Can use them to determine the relative ages of rock layers. (We’ll look at exact dating in class 3rd/4th small.) Can correlate rock layers. If rock layers in different areas have the same index fossils, then the layers are the same age.

5. Index Fossil Characteristics Common & Easy to Identify - widespread means they will be found in more than one area or large areas. –Too narrow of an ecosystem decreases the usefulness of the fossil. (Unless you are looking for that ecosystem!) Found in all kinds of rock –Sedimentary most common –Means that the fossil is not limited to the type of rock, but the TIME of formation. Short period of time –Develop & go extinct quickly –Higher level of accuracy and precision of dating

6. Correlations Find the index fossils in layers 4 & 6 on the left. These layers are the same age as those layers on the right, with an additional layer, 5, in between.

7. Correlations Notice that the markings for the kinds of rock on either side are not necessarily the same. It is the fossils that are significant and help us date the rock.