1. Rock Layers & Fossils
March 2018

2. Learning Target
I can explain the relationship between dating rock layers and the age of fossils.

3. About Fossils Found in sedimentary rocks
Sedimentary rock typically is layered
Layers derived from different periods of history

4. About the Methods
Scientists combine several well-tested techniques to find out the ages of fossils.
Relative Age
Superposition
Index Fossils
Absolute Age 

5. Relative Age
Think about a layer cake, you know that the layer at the bottom was the first one the baker put on the plate, and the upper ones were added later.
In the same way, geologists figure out the relative ages of fossils and sedimentary rock layers; rock layers, and the fossils they contain, toward the bottom of a stack of sediments are older than those found higher in the stack

6. Superposition law of superposition.
a basic scientific law of telling Earth-time
It states that in any undisturbed sequence of rocks deposited in layers, the youngest layer is on top and the oldest on bottom, each layer being younger than the one beneath it and older than the one above it.

8. Index Fossils Also known as guide fossils or indicator fossils
Used to define and identify geologic periods
Index fossils must have a short vertical range, wide geographic distribution and rapid evolutionary trends.
Think of them as geologic post-its

10. Absolute Age
Geologists find absolute ages by measuring the amount of certain radioactive elements in the rock.
As time passes, the "parent" radioactive elements change at a regular rate into non-radioactive "daughter" elements.
A.K.A. Radiometric Dating
You may have heard of Carbon-14