2. A Little Bit of History… The history of geologic time begins with the human interest in mining. Interests in rock units began to flourish with commercial mining in the 1500s and 1600s People began to identify strata (rock layers) by the types of fossils that were found within them. Two scientists were credited with contributing the most research to this area -- Nicolaus Steno and James Hutton.

3. Nicolaus Steno adapted two geologic principles Law of Superposition - sedimentary rocks are found with the oldest on the bottom and youngest on top. E.x. Paint layers Principle of Original Horizontality- If left undisturbed rock layers will stay in horizontal order adapted two geologic principles Law of Superposition - sedimentary rocks are found with the oldest on the bottom and youngest on top. E.x. Paint layers Principle of Original Horizontality- If left undisturbed rock layers will stay in horizontal order

4. James Hutton 18th-century Scottish physician and farmer  Studied his farm land for geologic changes  Principle of Uniformitarianism- geologic processes from the past could explain the current geologic processes. E.x. -what happened then happens now 18th-century Scottish physician and farmer  Studied his farm land for geologic changes  Principle of Uniformitarianism- geologic processes from the past could explain the current geologic processes. E.x. -what happened then happens now

5. Relative Age- gives the age of a rock layer based on the ages of layers around it (younger or older) What sort of events can change the order of rock layers?

6. Absolute Age  By dating the trace fossils within a rock we are able to identify the specific rock layer’s age.  Such aging techniques like carbon dating can be used as well  By dating the trace fossils within a rock we are able to identify the specific rock layer’s age.  Such aging techniques like carbon dating can be used as well

8. Geologic Column  Not one place contains a complete record of geologic time.  Geologic Column- ordered arrangement of rock layers based on relative ages of the rock  Not one place contains a complete record of geologic time.  Geologic Column- ordered arrangement of rock layers based on relative ages of the rock

10. How to use this…  Scientists estimate the ages of rock layers by averaging sediment deposits, radiometric dating and fossils.  Once we have dated a majority of layers the unknown ones can easily be dated.  Scientists estimate the ages of rock layers by averaging sediment deposits, radiometric dating and fossils.  Once we have dated a majority of layers the unknown ones can easily be dated.

11. Divisions of Geologic Time  Geologic time is typically divided by distinct changes (extinctions and dominant fossils)  BYO- billions of years old  MYO- millions of years old GGeologic time is typically divided by distinct changes (extinctions and dominant fossils) BBYO- billions of years old MMYO- millions of years old