Relative Time

Basic principles. As previously described in this book, geologists use some basic, simple principles to unravel “which came first”:

  • The law of superposition states that in an undisturbed sequence of sedimentary rocks or lava flows the overlying rock is younger than the underlying rock.
  • The law of original horizontality states that most sedimentary rocks (an exception is cross‐bedded sediment) formed as nearly horizontal layers. Any layered sequences that are now tilted were moved by later geologic processes.
  • Any rock that cross‐cuts another rock is younger than the rock it cross‐cuts. This rule applies also to mass wasting and erosion; whatever is eroded had to exist prior to the beginning of erosion.
  • The law of faunal succession states that fossil species succeed one another in undisturbed rocks in a definite and recognizable order around the world.
  • If fragments of one rock type are observed as inclusions within another rock type, the first rock type had to exist prior to the rock type that hosts its inclusions.
  • Unconformities are erosional surfaces that represent gaps in geologic time between the formation of the lower rock surface and the overlying sedimentary or volcanic layers.

If one applies these principles to Figure , the sequence of geologic events, from oldest to youngest, is as follows:

  1. The sequential deposition of layers A through J

  2. Tilting and erosion of these units

  3. Deposition of formation K, creating an unconformity

  4. Intrusion of granite

  5. Deposition of formation L

  6. Intrusion of cross‐cutting dike

  7. Erosion of formation L, followed by the deposition of formation M on the nonconformity

  8. Subsequent deposition of formations N and O

  9. Surface erosion, resulting in formation of drainage patterns

Figure 1

Sequence of Geologic Events