Name:______________________________________________________  Period:___________

Relative and Absolute Dating


The Law of Superposition

 In any undisturbed sequence of strata, the oldest layer is 

 at the bottom of the sequence, and the

youngest layer is at the top of the sequence.


The Cross-Cutting Law

Any feature that cuts across a body of sediment or rock is younger than the body of sediment or rock that it cuts across.









The Law of Inclusions

If one rock body contains fragments of another rock body it must be younger than the fragments of rock it contains. OR…The inclusions are older than the rocks which contain them.



Telling Relative Time

Use the laws of superposition, inclusions and cross-cutting relationships to determine the relative ages of the following cross-sections. Determine the OLDEST bed FIR

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Outline the sequence of events in the cross sections below by numbering each rock unit or event in the order in which it occurred or was deposited.


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Refer to the cross-section below. For each of the following pairs of rock layers, identify the relative dating law that would be used to determine which bed was older and which was younger. Circle the letter of the OLDER bed.

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A & B ________________________ D & M ________________________


D & E ________________________ J & K ________________________

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Refer to the cross-section above. For each of the following pairs of rock layers identify the relative dating law that you used to determine which bed was older and which was younger. Circle the letter of the YOUNGER bed.

M & A _________________________________________________________

J & N __________________________________________________________

H & L __________________________________________________________


D & M _________________________________________________________

Absolute Dating: Estimating the Age of Fossils

Radiocarbon dating can be used to date organic material from less than 1000 years to as much as 50,000 years in age. Carbon is found in two forms in the atmosphere, carbon-14 and carbon-12. Carbon-14 is unstable and breaks down to form nitrogen-14 at a fixed fate. The half-life of carbon-14 is 5,730 years. This means it takes 5,730 years for half of the carbon-14 sample to break down and form nitrogen-14. Scientists can measure the amount of radioactive decay that has taken place be calculating the ratio of carbon-14:carbon-12 in the fossil and comparing it to the atmosphere.


Calculate the ages of each of the fossils below using the following steps:

a.      Determine how many half-lives have passed by using the table below:


Amount of C-14 remaining

# of half-lives that have passed

½ C-14


¼ C-14


1/8 C-14


1/16 C-14


1/32 C-14



b.      Multiple the number of years in the half-life (5,730) by the number of half-lives that have passed.



9. 1/16 of the C-14 remained


                                       AGE: _____________________________________


10. 1/8 of the C-14 remained


                                        AGE: _____________________________________

11. 1/32 of the C-14 remained


                                AGE: _____________________________________


12. ¼ of the C-14 remained


                                        AGE: _____________________________________

13. Which of the above fossils came from which rock layer in the diagram below? Write the latter of the fossil in the appropriate rock layer.