1. What is relative and absolute age? A rock layer’s relative age is its age compared to the ages of other rock layers. Generally (relative) versus specifically (absolute)… Example: you –11 – 13 years –birthday

2. What is the Law of Superposition? In horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it EX: (your experience, new question, or WU answered.)

3. What other clues help to find relative age? Clues from igneous rock: extrusions (hardens on surface) and intrusions (hardens beneath surface) are always younger. EX: (your experience, new question, or WU answered.)

4. Clues from faults: a fault is always younger than the rock it cuts through. EX: (your experience, new question, or WU answered.)

5. Clues from an unconformity: where an old, eroded rock surface is in contact with a newer rock layer. An unconformity is a gap in the geologic record where some rock layers have been lost because of erosion. EX: (your experience, new question, or WU answered.)

6. Clues from fossils: index fossils (widely distributed, existed only briefly) To date rock layers, geologists first give a relative age to a layer of rock at one location. Then they can give the same age to matching layers of rock at another location.