Relative Time

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					Relative Time
             Relative Time
• When looking at exposed rock, the layers
  are always sequenced from oldest to

• Relative dating does not tell how old the
  rocks are, only the order in which they
         Original Horizontality
• Sediment is deposited in horizontal or
  nearly horizontal layers.

• If the layers are tilted or folded, it took
  place after the horizontal layers were
  already formed.
Original Horizontality
• Within a sequence
  of undisturbed
  sedimentary rock,
  the layers are older
  on the bottom, and
  younger on the top.
    Cross-cutting or Intrusions

• A disrupted pattern is older than the
  cause of the disruption.

• Ex) Faults, dykes, canyons,
      batholiths, laccoliths, sills.
         Faunal Succession
• Fossil species succeed (follow) one
  another in a definite and recognizable

• Because we can track the changes of a
  particular species through the fossil
  record, faunal succession is evidence
  supporting the Theory of Evolution.

• Unconformities are the exceptions to
  the four principles of relative time.

• There are three different types of
• Older rocks have eroded away, and a new
  layer has formed on top. This is the
  hardest type of unconformity to detect.

• It is usually found only by studying fossils
  from the layers above and below the
           Angular Unconformity
•    A new layer of rock is deposited over a
     layer that has been tilted or folded and
     then eroded.
    a.   New sedimentary rock is formed.
    b.   Something folds or tilts the layers.
    c.   Erosion occurs.
    d.   A new layer of rock forms over the erosion
• An erosion surface on metamorphic rock
  has been covered by younger sedimentary

• This shows that erosion has occurred for a
  very long time before a new rock layer
  formed because metamorphic rock is
  usually found deep in Earth’s crust.