Geologic Time


Terms to learn:

uniformitarianism         relative dating                     original horizontality             superposition         cross-cutting                                 inclusions
disconformity                 angular unconformity     nonconformity
isotope                                half-life


Topic outline:

  1. Geologic time
  2. Uniformitarianism
  3. Relative Dating methods
    1. Several important principles:
      1. Original Horizontality
      2. Superposition
      3. Cross-cutting relationships
      4. Inclusions
      5. Contact metamorphism
    2. Unconformities
      1. Disconformity
      2. Angular unconformity
      3. Nonconformity
  4. Absolute (Radiometric) Dating methods
    1. Isotopes - radioactive decay
    2. Half-life – parent isotope - daughter product
  5. Geologic Time Scale


Interesting places to visit:

Lots of info about various periods in geologic time:


A pictorial history of life through geologic time:


Fun dinosaur site with games:


USGS website on geologic time:


When you are finished with this topic, you should be able to answer the following questions:

  1. If you find 75% of a parent isotope in a rock and 25% of the daughter, how many half-lives have passed?
  2. Why is uniformitarianism important?
  3. What are examples of each of the principles of relative dating?
  4. What is an unconformity, how many types are there, and how are they formed?