Learning About Our Past

1
Relative Dating(not to be confused with dating
your relatives!)
2
Fossil Record

• Fossils help to determine a rock layers relative
  age

• An index fossil is used to help
  determine relative age

3 overall characteristics 1. Easily
recognizable 2. Must be found
everywhere 3. Lived for a short time period
3
Determining the fossil record by

• Relative Age
• Absolute Age

4
Relative Age

• Not an exact age way of telling one layer of
  rock is older than another
• Laws and rules govern how you determine the
  relative ages of rock layers and things that you
  find around them.

5
Laws and Rules

• Law of Horizontality
• Rock layers are laid down horizontally never on
  a slant
• Law of Superposition
• each sedimentary rock layer is older than the
  overlying younger layer

6
Superposition is well illustrated by the strata
in the Grand Canyon
7
Laws and Rules

• Sometimes due to crustal movement sedimentary
  layers arent always horizontal
• Law of Cross-cutting Relationships
• States that a fault or intrusion is younger than
  rock layer it cuts through

8
Laws and Rules

• Igneous Intrusion
• occurs when magma squeezes into or between layers
  of pre-existing rock.

9
Recognizing laws rules
10
Laws and Rules

• Included Fragments
• Sometimes other things found in the rock layer
  give you clues about its age
• pieces of rock found IN another rock must be
  OLDER (formed first).

11
Laws and Rules

• Unconformity
• a buried surface of erosion separating two rock
  masses representing a gap in geologic time...

12
Horizontality,Included Fragments, ANDan
UNCOMFORMITY
13
Practice what happened here?
14
(No Transcript)