You'll probably still need to wear a watch, but there's an incredible bonus for paying attention during fifth-period geology: You can use fossils to tell time, too.
A fossil is a remnant of a long-ago animal or plant preserved in the earth's crust. And the type of fossil can help determine how long ago the animal lived. For example, if scientists know a now-extinct fish was swimming around 50 million years ago, finding a fossil of that fish offers clues as to how old the soil is. This is known as an "index" fossil, because it helps scientists organize the different ages of soil.