been divided into the Paleogene period, the Neogene period, and the present Quaternary period.
DATING ROCKS All rocks look the same at first, but many in the same pile of random rocks you might find have come from vastly different time periods on the geologic scale. A lot has happened in these few billion years since the earth was formed. It would be nice to see how to tell when in time a rock was first develop. Fossils can help with this in some cases. You would need to know what the fossil is first and then find some relative way of seeing when the fossilized organism first died and became trapped in substances that later become rocks themselves. If you know when the organism died, you can see what else was laid down at the same time and possibly their relationships back in time. There are three major approaches to dating materials in geology. You can do relative dating once you know the exact date of at least some of the species in the rock. Think of sediment as it is laid down over a long period in time. What is the sediment? How long did the sediment take to get as thick as it is in the rock? What is the date of any known fossil in that sediment? What about the dates of other things above and below this level? The second approach involves actually dating the materials in the rock to a known date in time. This isn't always exact and the degree of exactness depends on your dating method. Carbon dating, for example, is only reliable for dating things from within the last 50,000 years and is most accurate when combined with tree-ring dating. The third approach involves magnetism. The magnetic field direction and location has changed, as you know. This helps determine many things with regard to the dating of rocks, just as it proved the theories on plate tectonics. Relative dating works best for sedimentary rock but it can be used for volcanic rock from volcanoes erupting more than once. Each layer or stratum is generally laid down horizontally. Exposed sedimentary rock is seen along the Grand Canyon or possibly in
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